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Polymeric Materials and Properties
1 Polymeric Materials and Properties There are large differences among the testing methods, significance, interpretation and usage of the properties, processing methods, and applications of materials manufactured by different industries. There is not a single material that is suitable for use for all applications. Each new property developed for each material opens the doors for new applications, technologies, and innovations that will improve the efficiency and quality of life of the end users. Product designers cannot expect equivalent properties for the families of materials (steels, thermoplastics, aluminum alloys, rubber), because each material family has unique properties developed for specific applications and market needs. In addition, these materials are processed using different manufacturing processes and quality control methods. All materials have advantages and disadvantages (properties, processes, and quality), making it difficult to compare the cost of finished products made of different materials and by different processes. The selection of materials is directly related to the end-use applications, whether or not one material’s performance is better than another. To illustrate this point, a thermoplastic material cannot replace a structural steel beam used in building construction. The thermoplastic resins do not have the required mechanical strength, creep resistance, or sufficient melt flow to be extruded into thick walled shapes. The thermoplastic beams would be expected to warp under the load of a building structure. However, structural beams can be made of thermoset composite plastics, although these composite materials are costlier than metals. In less demanding applications in the housing industry, wood composite structural beams are replacing steel beams because of their performance, light weight, and ease to work with, and competitive price. A thermoplastic material cannot replace the steel in automotive disc or drum brake housings because the product requires dimensional stability, low thermal expansion, and high strength and rigidity at elevated temperatures. Thermoplastic resins do not meet the requirements. However, brake pads made of thermoset polyimide have been successfully used in airplanes. Metals cannot replace automotive rubber tires, bellows, diaphragms, or compression seals because
metals do not have the elasticity, fatigue endurance, wear resistance, and toughness of rubber or thermoplastic elastomers (TPEs). Metals are not used for light-weight and compact cellular phone housings because metals are electrical conductors, heavier, corrodible, and expensive. Automotive cast iron engines and aluminum intake manifolds are being replaced by fiberglass reinforced nylon to lower the weight and improve the engine efficiency. These substitutions require new manufacturing processes but present opportunities for cost reduction. Automotive bumpers, external side panels, doors, fenders, and hoods made from steel are being replaced by plastics such as thermoplastic elastomer olefins (TPOs), thermoset composites, and polycarbonate alloys. These tough plastic materials have reduced weight and lower manufacturing costs, while providing added freedom for design and styling. Portable electric tools and small kitchen appliance housings are no longer made of die cast iron, steel, or aluminum, but have been replaced with nylon and acrylonitrile-butadiene-styrene (ABS). These plastics improve the toughness, electrical insulation, design flexibility, and styling; they also weigh less, and provide efficient molding at lower costs. Water faucet valves made of die cast steel, brass, or copper are being replaced by new designs, updated styles, and colors, using acetal resins, which eliminates corrosion, reduces cost and presents new market opportunities. Large irrigation valves for high performance applications (1.50–3.0 in. diameter) and small valves (0.75 and 1.00 in. diameter) used to be made of die cast steel and brass. The large valves have been successfully replaced with glass reinforced nylon 6/12 and the small valves with glass reinforced nylon 6/6 or acetal. This improved performance and reliability eliminates corrosion and allows cost reduction. Other low performance commercial valves made of rigid polyvinyl chloride (PVC) (lower cost) have also been produced for the irrigation market. Toilet anti-siphon valves made of several brass and copper components have been replaced with a multifunctional design in acetal resin that improves performance, eliminates corrosion, and reduces cost. 1
2
Selection of Polymeric Materials
The valves made from acetal resin have shown excellent performance over a 30-year period. The comparison of properties for material selection is an effective tool when applied to materials in the same family group. However, comparing properties between different material families is not recommended. The characteristics and process requirements for each material are developed for a particular need of the application, and the properties are obtained using test methods and equipment specific for each family of materials. Figure 1.1 shows eight graphs using various generic property values to compare the strengths and weaknesses between families of materials. The ferrous metal families include cast iron, cold rolled steel, structural steel, alloy steel, stainless steel, and tool steel. The nonferrous metal families include magnesium, aluminum, copper, nickel, brass alloy, and titanium.
The thermoplastic families include ABS, acrylic, acetal, nylon, liquid crystal polymers (LCPs), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polystyrene (PS), polyethylene (PE), polypropylene (PP), polycarbonate (PC), polyphenylene oxide (PPO), polyetherimide (PEI), polyetherketone ketone (PEKK), polysulfone (PSU), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and styrene-acrylonitrile (SAN). The thermoset families include phenolics, silicones, alkyds, diallyl phthalate, polyimides, aminos, unsaturated polyesters, epoxies, and urethanes. The rubber families include butadiene, butyl, chloroprene, nitrile, silicone, urethane, ethylene-propylene diene monomer (EPDM), ethylene-propylene monomer (EPM), fluorinated elastomer, and natural rubber.
Ferrous Metals Nonferrous Metals
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 0
2
4
6
8
10
-460 0
Specific Gravity
Ferrous Metals Nonferrous Metals
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 0
50 100 150 200 250
0
Tensile Stress (kpsi)
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 40 80 120 160 200
0
Coefficient of Thermal Expansion (in/in/°F) x 10–6 Rubber Mica Laminations Glass Laminations Thermosets Thermoplastics
2,000
5 10 15 20 25 30
Modulus of Elasticity (Mpsi)
Ferrous Metals Nonferrous Metals
0
500 1,000
Service Temp. (°F)
0.5 1.0 1.5 2.0 2.5
Thermal Conductivity (BTU/hr/ft2/°F/in) x 103 Rubber Mica Laminations Glass Laminations Thermosets Thermoplastics
106 107 108 10111015101610171018
0
Electrical Volume Resistivity (Ohm-cm)
Dielectric Strength (volt/0.001 in) x 103
Figure 1.1. Comparison of properties between families of materials.
1.0
2.0
3.0
4.0
1: Polymeric Materials and Properties
The specific gravity graph shows the unit weight of a material compared to water and reveals that metals are two to eight times heavier than plastics. On a strength-to-weight basis, plastics have a more favorable position, as indicated by the specific gravity graph. The cost of metals is much higher than plastics. The continuous exposure temperature graph shows that metals have a wider temperature range than plastics; metals can be used at colder and at elevated temperatures. The property and testing methods used for plastic materials are extremely important and critical for their classification and use temperature range. The tensile strength graph shows that metals are much stronger than plastics; metals resist higher forces when they are pulled apart, before catastrophic failure. Time and temperature affect the tensile strength and cold flow (creep) properties of plastics; they decrease with increasing temperature over a much smaller temperature range, and exposure time. The modulus of elasticity graph shows that metals have a higher resistance to deflection for short-term, intermittent, or continuous loading than plastics. Metals have better dimensional stability at elevated temperatures than plastics. Since plastics deflect more than metals under the same load, it is important that plastic products be loaded using different techniques. For plastics the load should be intermittent and well distributed in compression mode. The coefficient of linear thermal expansion graph shows that increasing the temperature causes higher dimensional changes for plastics than for metals when plastics and metals are used together in the same application and exposed to the same temperatures. The plastic parts expand more than the metal parts, thus requiring design modifications to compensate for the dimensional change in plastics. The thermal conductivity graph shows that metals are good conductors of heat, while plastics are excellent insulators. Despite their relatively low use temperature range, plastics may be superior to metals as high temperature heat shields for short exposures. A plastic part exposed to a radiant heat source may suffer surface degradation. However, this heat is not transmitted to the opposite surface as rapidly as in metals. The electrical volume resistivity graph compares only the insulation materials used in electrical applications because metals are conductors. The dielectric breakdown strength graph shows the voltage gradient at which electrical failure or breakdown occurs as a continuous arc; the higher the value, the better the material. Plastics have excellent electrical resistance properties, while metals are conductors.
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1.1 Tensile Stress-Strain Comparison Graphs The tensile strength, strain, and modulus of elasticity are among the most important mechanical properties of a material. They represent the mechanical behavior of a material while exposed to a tensile force. As the tensile load is gradually increased at a constant cross head speed, the total elongation over the gauge length and the load are measured and recorded continuously at each increment of the load until the specimen breaks. The stress at a given point may be obtained by dividing the value of the tensile load at that point by the original cross-sectional area. The tensile strength properties are the most widely specified data used for plastic materials. Different types of materials are often compared based on the tensile stress-strain graph generated by the tensile test. Tensile stress, elongation, and tensile modulus of elasticity are developed by analyzing the test results, using the stress-strain graphs. The stress-strain curve for each type of material has its own shape because it represents the mechanical characteristics and behavior of the material. Therefore, it is difficult to compare the tensile strength properties of different types of materials, if the stressstrain curves have significantly different shapes. To illustrate this point, a comparison of the stress-strain curves for different families of generic materials to verify the tensile strength properties is used as a practical tool for the material selection task. Figure 1.2 represents the stress-strain curve for nonferrous alloys and cast iron—the original strain is large with low modulus, and the yield point and strength properties difficult to define. Figure 1.3 represents the stress-strain curve for a medium carbon steel—the proportional limit, yield point, elongation, and break are well defined; however, the original strain is very small with high modulus (very rigid). Figure 1.4 represents the stress-strain curve for an alloy steel—the original strain is very small with high modulus without a yield point. Figure 1.5 represents the stress-strain curve for a high carbon steel—similar to the previous case, except for its brittleness. Figure 1.6 represents the stress-strain curve for rubber elastomer materials—flexible, soft, high impact resistance and unbreakable, good rebound and compression set, and low tensile strength. Figure 1.7 represents the stress-strain curve for unreinforced resins such as PE, PP, TPE, acetal, and
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B
Tensile Stress
Y
B Y
PL
Tensile Stress
Tensile Stress
Selection of Polymeric Materials
B PL
Strain, (%) Strain, (%)
Strain, (%)
Figure 1.4. Stress-strain curve for alloyed steel.
Tensile Stress
O
Figure 1.3. Stress-strain curve for medium carbon steel.
Tensile Stress
Figure 1.2. Stress-strain curve for nonferrous alloys and cast iron.
Tensile Stress
B
PL
Strain, (%) Strain, (%)
Figure 1.5. Stressstrain curve for high carbon steel.
Figure 1.6. Stress-strain curve for elastomers.
nylon (at 50% relative humidity). These materials have lower tensile strength, good impact resistance, poor temperature and creep resistance, and the stressstrain curve yields gradually until break. Tensile tests for these plastics were conducted at room temperature using American Society for Testing and Materials (ASTM) methods. Figure 1.8 represents the stress-strain curve for fiberglass reinforced nylon (at 50% relative humidity (RH)), polycarbonate, and other compounded polymers that have limited elongation and impact resistance and moderated tensile strength properties. The tensile tests for these plastics were conducted at room temperature using ASTM methods. Figure 1.9 represents the stress-strain curve for fiberglass reinforced resins such as PET, PBT, LCP, phenol-formaldehyde (PF), polyamide-imide (PAI), polyether-imide (PEI), polyaryletherketone (PAEK), and dry as molded nylon. These materials are brittle and usually break before yielding occurs. They have excellent tensile strength, dimension stability, and
Y
B
PL
Strain, (%)
Figure 1.7. Stress-strain curve for unfilled resins.
thermal and chemical resistance properties. The tensile tests for these plastics were conducted at room temperature using ASTM methods.
1.2 Property Data Information for Polymeric Materials The American Society for Testing and Materials and Underwriters Laboratories (UL) in cooperation with resin suppliers, product designers, and other institutions have developed the current ASTM test procedures employed for testing the properties of thermoplastic resins. These test results are used to produce the product data sheets of thermoplastic resins. Product design engineers who have knowledge of the polymer molding process characteristics and who understand the complexity and significance of the resin properties are able to develop a special product geometry that satisfies the injection molding process and the end-use service requirements. All these
Y B PL
5
Tensile Stress
Tensile Stress
1: Polymeric Materials and Properties
B
PL
Strain, (%)
Strain, (%)
Figure 1.8. Stress-strain curve for reinforced resins.
Figure 1.9. Stress-strain curve for brittle resins.
factors should be taken into account to select the ideal thermoplastic materials to be used for injection molding the new product. The product data sheet shows the important properties of a plastic material that have been developed or used for a specific application. This document is used in design studies to compare properties with other materials. However, it does not provide the information for understanding the fundamental behavior, the viscoelasticity, or temperature sensitivity of plastic materials. Plastic injection molding resins are developed and commercialized for a specific market and application. The product data sheet typically contains the properties tested in compliance with the ASTM, ISO, or other test procedures. All the product data sheets are different for each plastic material, because the resin is manufactured to comply with the specifications of the application, by providing the minimum number of tests required for that market. When more properties of the resin are needed for a new application, the resin supplier’s representative may be able to provide the additional data required for the design analysis of the new product that has not been published.
material property variations; and molding process problems. Generally, most resins have different performance characteristics that could create molding problems and/or part failures. When additives, such as flame retardants and stabilizers, are compounded into the resin, the characteristics of the matrix polymers are modified, sometimes with a loss of some of the properties. In addition, when fiberglass or minerals are added to increase the mechanical strength properties, processing becomes more difficult. When the rheology or viscosity of the compounded product increases, higher injection pressures and melt/mold temperatures are required to decrease the melt flow rate. The economics involved in plastic resin selection are complex, because the resin price is not usually the most important factor. When a plastic resin is used for injection molding a close dimensional tolerance end product, the following engineering requirements are essential in reducing manufacturing costs:
1.3 Material Selection Guidelines To select the best material for an application, first, compare the different properties and processing characteristics of several plastic resins that may meet the application requirements. Product designers must make their resin selection based on other important characteristics of the application such as product design geometry effects caused by weld lines; stress concentrations, fiberglass orientation, temperature, humidity and creep; problems caused by mold design, construction, quality, and plastic
• Part geometry needs to be designed to produce molded components with maximum productivity. • A precision mold must be designed and constructed for fast running cycles and molding the maximum number of cavities automatically. • It is important to select the best type, capacity, and running conditions of the injection molding machine. • Then, it is necessary to set up efficient injection molding process conditions for the resin being used. • It is also essential that the molding and maintenance organizations are well trained using an up-to-date technical training program provided by a qualified instructor. The cost of plastic material becomes increasingly significant for molding products with higher volume
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and less critical end-use requirements. Resin cost represents a high percentage of the molded components’ finished cost. For example, common items such as business machine housings, plumbing (faucets, valves, tubing, and shower heads), kitchenware, and appliance components (refrigerator liners, washing machine impellers, and vacuum cleaner housings) are price sensitive products. For these applications, resin cost becomes a very competitive business aspect of manufacturing.
1.4 Polymeric Materials Specifications There are more than 26,000 grades of thermoplastic, thermoset, and elastomeric neat resins and compounds on the market. They have been developed by blending, alloying, and compounding the existing polymeric families. Very few new polymeric families have been introduced into the market in the last few years. Many compounders of plastics buy the basic polymers from primary manufacturers and enhance their performance by adding additives, modifiers, reinforcements, and fillers. There are nearly 300 domestic suppliers of plastic materials. Many companies offer everything from commodity plastics to high performance polymers. There is an overwhelming amount of product specifications available to describe the process characteristics and end-use performance that have been generated by the resin suppliers. However, the property data are not always completely accurate or reported in a uniform format, because the data have often been developed for a particular application. The data sheets can cause considerable confusion for new customers requiring other properties for their applications. Therefore, making a comparison among the available plastic compounds for the material selection during the development of a plastic product becomes a very difficult task. Fortunately, over the past several years the large multinational resin suppliers have made strides to unify the test methods and the reporting of plastic properties. The progress has been slow, but the results are encouraging. Plastic product designers should have the necessary tools to compete in a world market. To develop a plastic product, the designers have the freedom in the part design, yet this gives rise to the more complicated issue of which material will work best for the job. Among the thousands of commercially available
Selection of Polymeric Materials
plastic grades, how can one be sure that the most appropriate materials are being considered for the application? Several plastic materials should be selected as candidates at the beginning of a project. A prototype mold similar to the production mold should be constructed to produce prototype parts using the initial candidate materials. The prototype mold should also be used to develop the molding parameters (shrinkage, part dimension, cycle time, etc.) that are needed for commercial part production. The molded prototype parts using the selected resins should be tested under end-use conditions to assess the suitability of the product against the requirements. Many plastic product designers select materials by intuition. They rely on experience to determine the suitable material. For example, acetal resin has been the workhorse material for gears, bearings, springs, and housings; therefore, they are likely to consider the same acetal for a new application. This is a poor material selection technique and alternative materials should be considered. Perhaps ABS, polypropylene, or nylon, may do the job, thus improving product performance, producing better molding efficiency, and reducing manufacturing costs. Another common practice is to rely on the material selection assistance provided by a particular reliable resin supplier to help specify the material. This is the approach taken by a number of part manufacturers, where the objective is to reduce the number and types of plastic materials used and work closely with the resin supplier to control cost. However, this approach closes the doors for new opportunities and innovations that other suppliers may offer. There are two database categories of plastic property data sheets supplied by the resin suppliers: • Single-point database system • Functional, or multi-point database system
1.4.1 Single-Point Database Systems The single-point databases report properties that are a single value. For example, tensile stress at yield, modulus of elasticity, elongation at break, Izod impact, Vicat softening point, gard gloss, and other properties are reported as a single value. There are roughly one hundred different property values that may be reported for any grade of resin. This information is of great value for the material
1: Polymeric Materials and Properties
7
selection process. It is very important to know about the test methods, test conditions, and reported units; understand the significance and limitations of the tests; and determine whether the test results have any relationship with the properties required by the product end-use and design. The single-point property values reported by resin suppliers should be scrutinized very carefully. It is expensive to test all the available properties of a plastic material. Consequently, material suppliers test only the properties that are needed to meet the material requirements of the intended market. For example, commodity plastic materials (polypropylene, polyethylene, and polystyrene) are tested for only a very limited number of properties such as specific gravity, melt flow index, tensile stress, and modulus of elasticity. Table 1.1 shows the typical material property information for a commercial polystyrene resin. Engineering polymers (acetal, nylon, PET, LCP) are tested for a wide range of properties required by their target markets. The single-point values may not be representative of the requirements for an application, or specifications of the plastic product. Heat deflection temperature and tensile strength property values are often reported as a single-point value at room temperature. In actuality, the effects of type of loads, strain, creep,
humidity, and temperature play a big role in the properties, characteristics, and behavior of a plastic material and the molded product performance. Single-point database systems provide the only viable means of finding out what materials are available. Using these databases, material selection for non-critical applications is quite simple; thus it takes very little time. However, for a difficult and complex plastic product, a complete detailed analysis of the functional database system is necessary.
1.4.2 Functional (Multi-point) Database Systems Plastic material properties are intended to represent the characteristic behavior of a plastic material during processing and in a product end-use environment. If a plastic product is to be used in a demanding environment where temperature, humidity, chemical resistance, and creep are critical variables, it is important to match the environment requirements of the application with the plastic material’s tested properties. For these applications, more detailed information is necessary. Engineering polymer suppliers can provide detailed test results of mechanical, electrical, chemical resistance, flammability, weathering, and thermal properties over a range of temperatures, humidity, and
Table 1.1. Fina 625 Polystyrene, Property Data Sheet Properties
ASTM
Value (US)
Value (SI)
Test Conditions
D1238
13.0 (g/10 min)
13.0 (g/10 min)
Load 5.0 kg at 200°C
Tensile stress at yield
D638
3.70 (kpsi)
25.5 (Mpa)
at 73°F (23°C)
Tensile modulus
D638
310 (kpsi)
2,137 (Mpa)
at 73°F (23°C)
Elongation at break
D638
40.0 (%)
40.0 (%)
at 73°F (23°C)
Flexural stress at yield
D790
6.70 (kpsi)
46.18 (Mpa)
at 73°F (23°C)
Flexural modulus
D790
400 (kpsi)
2,757 (Mpa)
at 73°F (23°C)
D256
1.30 (ft-lb/in)
69.55 (J/m)
at 73°F (23°C)
D1525
196 (°F)
91.06 (°C)
D523
96.0 (%)
96.0 (%)
Physical Melt flow rate Mechanical
Toughness Izod impact (notched) Thermal Vicat softening point Optical Gard gloss
at 73°F (23°C)
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Selection of Polymeric Materials
creep. If this functional information meets the end-use requirements of the product, it can be effectively utilized to determine whether the tested plastic material is suitable for the application. For example, consider a plastic product that will be exposed intermittently to temperatures of 300°F (149°C) as found in an automotive under-the-hood application. The plastic product may also be exposed to an applied stress continuously. In this case, functional database information of the plastic material is required, because single-point data are not sufficient to select the proper plastic material. Table 1.2 shows an acetal property data sheet providing an extensive number of properties that are required for the molded product applications and markets where acetal is used. Additional functional database properties (e.g., continuous service short- and long-term temperatures, UL yellow cards, weather effects on the tensile and
impact strength properties, and chemical resistance to common solutions) required for a detailed analysis of an application are available from the resin’s suppliers. Figures 1.10 and 1.11, respectively, show the stressstrain graph for acetal homopolymer exposed to various temperatures and long periods of time (creep and thermal degradation). Figure 1.12 shows the relative viscosity-shear rate graph for unfilled PBT.
1.5 Testing Polymeric Materials Plastic materials cannot be subjected to the same tests developed for metals. They have their own special test requirements to comply with the demands of the plastics industry. While some standard testing methods may be applicable, in most cases the results from the analytical tests developed for metals have
Table 1.2. Delrin® 100, Property Data Sheet Properties
ASTM
Value (US)
Value (SI)
Test Conditions
Melt flow rate
D1238
1.0 (g/10 min)
1.0 (g/10 min)
Load 1.05 kg at 190°C
Specific gravity
D792
1.42
1.42
at 73°F (23°C)
Water absorption
D570
0.25 (%)
0.25 (%)
24 hr immersion
D570
0.22 (%)
0.22 (%)
Equilibrium 50% RH
D570
0.90 (%)
0.90 (%)
Equilibrium, immersion
Tensile stress at yield
D638
10 (kpsi)
68.94 (MPa)
(0.2 in./min)
Tensile modulus
D638
400 (kpsi)
2,757 (MPa)
(0.2 in./min)
Elongation at break
D638
75.0 (%)
75.0 (%)
(0.2 in./min)
Flexural stress at yield
D790
14.3 (kpsi)
98.58 (MPa)
(0.05 in./min)
Flexural modulus
D790
420 (kpsi)
2,895 (MPa)
(0.05 in./min)
Compression stress
D695
18 (kpsi)
124.1 (MPa)
(0.05 in./min)
Shear stress
D732
9.5 (kpsi)
65.49 (MPa)
—
Poisson ratio
E132
0.350
0.350
at 73°F (23°C)
D256
2.30 (ft-lb/in.)
123.0 (J/m)
at 73°F (23°C)
D256
1.80 (ft-lb/in.)
96.3 (J/m)
at –40°F (–40°C)
D256
No break
No break
at 73°F (23°C)
Physical
Mechanical
Toughness Izod impact (notched) Izod impact (unnotched) Tensile impact strength
D1822
170.0
(ft-lb/in.2)
(kJ/m2)
at 73°F (23°C)
M94, R120
at 73°F (23°C)
358
Hardness Rockwell
D785
M94, R120
(Continued)
1: Polymeric Materials and Properties
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Table 1.2. Delrin® 100, Property Data Sheet (cont’d) Properties
ASTM
Value (US)
Value (SI)
Test Conditions
Coefficient of linear expansion
D696
5.8 × 10–5 (1/°F)
10.4 × 10–5 (1/°C)
Heat deflection temperature
D648
257 (°F)
125 (°C)
at 264 psi stress
D648
336 (°F)
169 (°C)
at 66 psi stress
Melting point
D2117
347 (°F)
175 (°C)
—
Specific heat
C351
0.35 (BTU/lb-°F)
0.083 (kJ)
—
C177
2.60 (BTU-in./hr-ft2-°F)
0.4 (W/m-K)
—
Dielectric constant
D150
3.70
3.70
at 106 Hz
Dielectric strength
D149
500.00 (V/mil)
19.7 (MV/m)
Short time (90 mils)
Dissipation factor
D150
0.005
0.005
at 106 Hz
Volume resistivity
D257
1015 (Ohm-cm)
1015 (Ohm-cm)
at 2% water, 73°F (23°C)
Arc resistance
D495
200 (sec)
200 (sec)
—
No tracking
No tracking
Flame extinguishes when arcing stops (120 mils)
Thermal
Thermal conductivity Electrical
very little value in defining the properties and the process characteristics of the thermoplastic polymers. The tests commonly used in the plastics industry are established and used to describe properties and process characteristics of these polymers. This allows resin producers, product designers, molding processors, and end users to have a common understanding about what thermoplastic polymers are and what type of properties can be correlated to the end-use results representing the conditions found in the application. Tests are not ends in themselves, but rather a means of gathering knowledge about the plastic materials. The real test of a material is conducted during the actual service. Once a plastic end product is taken home and used by the consumer, it no longer matters whether the tensile strength is 35,000 or 26,000 psi. The finished product succeeds entirely or fails. To assure success in automotive components and industrial products, the properties of the plastic materials are studied by design engineers. Through experience and judgment, they balance material characteristics and service requirements against the geometries, sizes, functionality, and performance needed in a product to give an adequate safety margin in the application. Reliable, useful tests for plastics must be based on an understanding of the properties, processing
characteristics, and the performance of these materials in similar applications. The following ASTM-UL testing methods are used for plastic materials: • Mechanical, and creep analysis • Physical analysis • Thermal, and flammability analysis • Characterization of temperature resistance • Electrical analysis • Optical testing and characterization • Weather aging and radiation resistance • Fungi and bacteria resistance • Determination of chemical resistance • Statistical analysis of test data These tests may also differ for the four major classes of plastic materials: thermoplastics, thermoplastic elastomers, liquid injection molding silicone, and thermosets. The testing methods are used to characterize the chemical composition, physical and structural morphology, and ease of processing. An important aspect of mechanical testing is the interpretation of results. Satisfactory mechanical
10
1.0 hr
73°F. 10.0
68.9
55.1 122°F.
7.0 6.0
41.3
5.0 4.0
212°F.
27.5
3.0 2.0
Tensile Stress, (kpsi)
2.5
8.0
Tensile Stress, (MPa)
Tensile Stress, (kpsi)
9.0
1,000 hrs
11.0
10 hrs 100 hrs
Selection of Polymeric Materials
10,000 hrs 50,000 hrs
2.0 1.5 1.0 0.5
13.8
0
1.0
0
0 0
5.0
10.0 15.0 Strain, (%)
0 20.0
0.5
1.0 1.5 Strain, (%)
2.0
2.5
Figure 1.11. Acetal stress-strain creep curves.
Figure 1.10. Acetal stress-strain curves at three temperatures.
characterization of an engineering thermoplastic requires not only proper geometry of the test specimens and the testing method but also the correct interpretation of the test results. Characterization of plastic material’s temperature resistance includes such diverse concerns as dimensional stability of the molded product, temperature effects on mechanical and electric properties, and thermal decomposition. Flammability classification, ignition temperature, limited oxygen index, and smoke densities are some of the tests used to define the properties of plastic materials. Electrical testing of plastic materials defines the dielectric properties, arc resistance, tracking resistance, insulation, conductivity, and service end-use temperature. Weather exposure and radiation susceptibility are not strictly separate from chemical resistance and degradation. Different tests are used to measure the resistance of plastic materials to temperature, moisture, ultraviolet (UV) radiation, pollution, ozone, oxidation, and microbiological attack. Optical testing is used to characterize such factors as transparency, clarity, reflectance, hazes, color, gloss, and refraction. Many of these tests are identical to those used for other materials. However, some of the unique properties and applications of polymers have led to the design of special testing procedures.
Chemical resistance is a diverse area that requires many types of tests, most of which have been developed specifically for plastic materials. Aspects of chemical resistance include the absorption and transport of solvents, acids, and bases; the effects of many types of additives; and various types of degradation of the plastic materials. Degradation can result from various sources, including chemical attack, thermal decomposition, photo-oxidative deterioration and environmental damage. Statistical analysis of test data is another important aspect of testing and characterization. For any type of thermoplastics testing, neither the tests nor the resulting characterizations are valid unless an appropriate statistical analysis test program has been established and the data is correctly reported. Table 1.3. shows an extensive list of tests, methods, and standards for plastic materials; these tests were established by ASTM and Underwriters Laboratories.
1.6 The Need for Uniform Global Testing Standards The world is fast becoming a single global market. The plastics industry faces the challenge of how to participate and take advantage of the emerging international markets, and to satisfy the needs of customers within domestic markets.
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0.058
400
0.042
300
464°F (240°C) 0.029
200
482°F (250°C)
100
0.0145 0.010
70
500°F (260°C)
50
0.007
0.004 100
400
1,000
4,000
Relative Viscosity (Pa-sec)
Relative Viscosity (Ib-sec/in2)
1: Polymeric Materials and Properties
30 10,000
Shear Rate (sec-1)
Figure 1.12. Unfilled PBT, relative viscosity vs. shear rate graph.
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
Melt flow rate
D1238
g/10 min
g/10 min
Specific gravity
D792
—
—
Density
D1505
lb/ft3
g/cm3
Mold shrinkage (MFD)
D955
%
%
Mold shrinkage (TD)
D955
%
%
Water absorption
D570
%
%
24 hr Immersion
D570
%
%
Equilibrium 50% RH
D570
%
%
Equilibrium, immersion
Tensile stress at yield
D638
psi
MPa
(0.2 in./min)
Tensile modulus
D638
psi
MPa
(0.2 in./min)
Elongation at yield
D638
%
%
(0.2 in./min)
Elongation at break
D638
%
%
(0.2 in./min)
Flexural stress at yield
D790
psi
MPa
(0.05 in./min)
Flexural modulus
D790
psi
MPa
(0.05 in./min)
Compression stress
D695
psi
MPa
(0.05 in./min)
Shear stress
D732
psi
MPa
Poisson ratio
E132
—
—
Coefficient of friction
D1894
—
—
Plastic/plastic static
Coefficient of friction
D1894
—
—
Plastic/plastic dynamic
Coefficient of friction
D1894
—
—
Plastic/metal static
Physical
Mechanical
(Continued)
12
Selection of Polymeric Materials
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods (cont’d) Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
Coefficient of friction
D1894
—
—
Plastic/metal dynamic
Taber abrasion
D1044
mg/k cycles
mg/k cycles
Load and abrasive wheel
Izod impact (notched)
D256
ft-lb/in.
J/m
at 73°F (23°C)
Izod impact (notched)
D256
ft-lb/in.
J/m
at –40°F (–40°C)
Izod impact (unnotched)
D256
ft-lb/in.
J/m
at 73°F (23°C)
Gardner impact
D3029
in.-lb
mm-g
at 73°F (23°C)
Instrumented dart impact
D3763
in.-lb
mm-g
at 73°F (23°C)
D1822
ft-lb/in.2
kJ/m2
at 73°F (23°C)
Rockwell
D785
—
—
M, R, L, K, E scales
Durometer
D2240
—
—
Shore A, D scales
Barcol
D2583
—
—
Brittle temperature
D746
°F
°C
Vicat softening point
D1525
°F
°C
Glass transition temp
E1356
°F
°C
Continuous service temp
D794
°F
°C
Coefficient of linear expansion
D696
in./(in.-°F)
Heat deflection temp
D648
°F
°C
at 264 psi stress
D648
°F
°C
at 66 psi stress
Melting point
D2117
°F
°C
Specific heat
C351
BTU/(lb-°F)
J/g °C
Thermal conductivity
C177
BTU-in./hr-ft2-°F
W/m-K
Flammability rating
UL 94
HB, V2, V0, 5V
Relative thermal index
UL 746A
°F
°C
RTI mechanical without Impact
UL 746A
°F
°C
RTI mechanical with Impact
UL 746A
°F
°C
Dielectric constant
D150
—
—
at 106 Hz
Dielectric strength
D149
V/mil
kV/mm
Short time (90 mils)
Dissipation factor
D150
—
—
at 106 Hz
Volume resistivity
D257
Ohm-cm
Ohm-cm
at 2% water, 73°F (23°C)
Toughness
Tensile impact strength Hardness
Thermal
Electrical
(Continued)
1: Polymeric Materials and Properties
13
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods (cont’d) Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
D495
sec
sec
—
D495
—
—
Flame extinguishes when arcing stops (120 mils)
Hot wire ignition
UL 746A
sec
sec
High ampere ignition
UL 746A
No. of arcs
No. of arcs
High volt track rate
UL 746A
in./min
mm/sec
Competitive track index
UL 746A
Volts
Volts
D2864
%
%
Refractive index
D542
—
—
Haze
D1003
%
%
Arc resistance
No. of arcs
Flammability Limiting oxygen index Optical
Transmittance
D1003
%
%
Gardner gloss
D523
%
%
at 60°
Gloss
D2457
%
%
at 0°
The General Agreement on Tariffs and Trade (GATT), North American Free Trade Agreement (NAFTA), and European Union (EU) are all seeking positions that will eliminate barriers found in the global market. The benefits of uniform global test methods and data reporting formats enhance the business efficiency of the plastics industry. Comparable data will improve communication between the resin suppliers and their customers, and in the case of multinational companies, it will improve communication between manufacturing sites located around the world, and resin suppliers will have a greater opportunity to gain global competitiveness. Unfortunately, the debate over uniform global testing standards has taken place mostly without the participation of the most important members of the plastics industry, namely end users, processors, and resin suppliers.
1.6.1 Uniform Global Polymeric Materials Specifications The material selection process for new plastic applications, or substitution of the present material with a better resin to improve the efficiency of the plastic product, needs improvement to standardize manufacturing, quality control, uniform global testing, and reporting. The use of reliable and uniform global plastic material specifications, and the ability
to compare the properties of the plastic materials produced around the world, is ideal. Some of the multinational resin suppliers have been using this concept for many years. However, these same companies have expanded their manufacturing operations outside their home location (national or international), yet the small compounders, have only the basic testing necessities to support their internal production needs. These usually small operations are in business because they offer their commodity products at lower costs. After the plastic product design and specifications for the application requirements are defined, the material selection process involves screening all the available resin formulations to identify the most likely candidates at the lowest cost for the plastic product. The material selection process used around the world, which appears to be simple, often becomes a complicated task. This problem is caused by several factors such as global location, resin cost and quality, the use of different test methods, test specimens of various geometries, injection molding the specimens using different process parameters, specimen conditioning, specimen testing methods, and the lack of uniform reporting of the test results. For example, there are inconsistencies created by the use of test specimens with different thicknesses, various dimensions for the specimen test areas, specimen conditioning methods, and the use of different load speeds and temperatures for testing.
14
Wide variability makes it difficult to compare resins from different suppliers (if they have testing facilities) and sourced from various global manufacturing sites of a single supplier. Lot-to-lot variations in the quality of the resin and test data variations caused by the use of different testing laboratories may be encountered. There are over 26,000 plastic resin formulations in the USA alone. The bulk of available test data for these materials is taken from the resin supplier’s product information data sheet. Companies sometimes copy the property values for their products from their competitors, or from various plastic databases. Some resin suppliers’ manufacturing and marketing departments have different internal business objectives. Manufacturing departments want to operate without any production problems and with as few restrictions or limitations as possible. However, for plastic materials requiring property values with tight tolerances that are difficult to produce, the wider the specification values, the more favorable is the situation for the manufacturing organization. Consequently, they have a set of internal manufacturing specifications that are sometimes wider than those required for the successful marketing of the product. On the other hand, marketing departments want to sell the plastic materials with the highest property values (tightest specifications) and better quality, demanding a premium price for the resin. For example, for product properties that can be measured, each specification value has tolerances: lower limits for manufacturing and higher limits for marketing. However, for most resin suppliers, the internal property tolerances of the plastic materials are not reported in the product information sheet. Because of business and competitive pressure between resin suppliers, only the higher property values of the plastic materials are reported. For example, the notched Izod impact value reported on the plastic material product information sheet is 20 ft-lb/in. (1,070 J/m) (upper limit). But, the manufacturing laboratories tests this property of the running production material lot (lot size of 80–150 tons), and approves the plastic material if the value is 15 ft-lb/in. (802.5 J/m) (lower limit) or higher. In a production run of a plastic material, manufacturing only tests the minimum number of critical properties to meet quality assurance requirements. Some less critical properties are tested once a year, others every five or ten years depending on the product performance history and end-use applications. Standard methods, which have been developed with strong influence from resin suppliers, often condone these practices.
Selection of Polymeric Materials
The achievement of uniform global testing standards represents an uphill road to the plastics industry and resin suppliers, creating a new challenge in doing business. There are costs associated with test equipment investments, training personnel, and in retesting resins according to the new testing standards. The costs of separate or dual testing, where both standards are reported, must be taken into consideration. This additional work costs more, and the cost of plastic materials will increase, creating another obstacle for the global plastics industry.
1.6.2 Selection of Testing Standards There are several facilities for testing plastics standards at various locations around the world. The American Society for Testing and Materials and the Underwriters Laboratories in the USA, International Organization for Standardization (ISO) in Geneva, Switzerland, Deutsches Institut für Normung (DIN) in Germany, British Standards Institution (BSI) in Great Britain, Association Française de Normalisation (AFNOR) in France, and Japanese Industry standards (JIS) in Japan. Plastics testing standards developed by the Genevabased ISO are being promoted as a universal set of testing standards that could satisfy the needs for a single global standard. ISO was founded in 1947 and is a worldwide federation of national standards organizations from 100 countries. ISO promotes standardization to facilitate the international exchange of goods and services to develop cooperation in economical, intellectual, scientific, and technical activities. ISO encompasses standardization in all fields except electrical and electronics, which is the responsibility of the International Electrotechnical Commission (IEC). ISO and IEC procedures have almost totally replaced the national plastics standards in Europe. ISO 10350:1993 recommends a particular set of polymer properties and test procedures as a generally acceptable way to characterize and describe a plastic. Each property recommended in ISO 10350 has a property number and in most cases a recommended ISO or IEC test procedure. The EU and most European nations have adopted the ISO standards, replacing other national standards such as DIN, BSI, and AFNOR. Japan appears committed to converting to ISO test methods. Over 90 industrialized countries in the Pacific, Asian, and
1: Polymeric Materials and Properties
African regions have also adopted these universal standards. The USA had been alone in its reluctance to embrace ISO/IEC test standards, and to convert to the International System of Units (SI). ISO 10350:1993 categorizes the properties into five separate groups, and the number of test procedures is limited. The ISO recommended set of procedures does not meet the uniform global testing standard objectives, nor does it describe all the characteristics of the plastic materials required for the product design and material selection process. ISO does not produce truly comparable data with the existing ASTM/UL test methods. However, the testing procedures that are not included by ISO are replaced with ASTM/UL test methods. Specific details about the difference between ASTM/UL and ISO/IEC testing procedures, test conditions, reported units, and comparable properties of plastic materials are discussed in the subsequent chapters.
1.6.3 Issues and Concerns Several ISO/IEC global testing standards issues and concerns should be resolved before they are adopted by resin suppliers and customers in the USA. (1) The original target date of 1995 concerning the full conversion suggested by the Polymeric Materials Producers Division of the Society of the Plastics Industry raises questions regarding the urgency it implies. (2) The plastics industry manufacturers of durable goods, such as automobiles, computers and business machines, need to facilitate the conversion to ISO/IEC testing standards because of the global nature of their business. (3) The multinational engineering resin suppliers who also support the conversion, in order to participate in the global markets and achieve longterm cost savings and worldwide acceptance, need considerable time to implement the new standards. Resin suppliers recognize that the conversion to ISO/IEC test methods will not occur overnight. It is expected that ASTM/UL standards will continue to be used for many years. The acceptance of SI units is also expected to be a gradual transition process. It is important to recognize that separate testing for ASTM/UL and ISO standards will be carried out for some time. Although this puts a cost burden on resin
15
suppliers, their customers need to understand and to develop confidence in the ISO standards. (4) Accepting the costs of conversion to ISO/IEC test methods may involve a substantial initial financial commitment. This includes the cost for purchasing the ISO multipurpose test specimen mold and other mold inserts. It may also require the purchase of new molding equipment of the appropriate size along with suitable instrumentation and controls. Also involved are modifications to test equipment such as shorter support spans to facilitate flat wise testing for heat deflection temperature and new apparatus for Charpy impact tests. There are costs associated with training personnel and in retesting resins according to ISO requirements. The costs of separate or dual testing, where both standards are in broad effect, must be factored in as well. (5) There will be concerns about discarding an established historical property database in the conversion to ISO/IEC test standards. Though the concern is genuine, and the cost of recreating a reliable database is considerable, having to build such a database should not deter acceptance of ISO test protocols. (6) Some members of the US plastic industry argue that the standards development under ISO is a political process. The USA is at a disadvantage with only one vote in the process as opposed to 12 votes for EU nations. Critics also point to the fact that other countries are currently better organized and better able to influence the international standards setting process. Among the barriers to the implementation of ISO test standards in the USA, several key areas stand out including ready accessibility to standards information and documentation, the availability of molds and test equipment, and the continuing unfamiliarity with SI units. ASTM/UL standards are readily accessible. However, there are difficulties in accessing ISO/IEC test standards and the costs of those standards are a serious concern. The US plastic industry’s unfamiliarity with SI units is a formidable barrier to the acceptance of ISO test standards. Over 90% of ASTM D-20 standards specify the properties in SI units, while many companies continue to associate their data with the inch-pound (in.-lb) units. It will take time for the industry to become used to the new values involved.
16
1.6.4 Summary The current plastics testing practices for commodity plastics lack consistency. Multinational plastics companies would like to adopt uniform global testing standards. However, resins suppliers are clearly divided on the issue of the urgency to convert, accepting the costs of conversion, and the need for these new standards for high volume commodity resins used in the domestic markets. They believe that most customers will continue to use ASTM/UL standards with slow conversion to SI units, for sometime. Engineering resin suppliers serving the automotive, computer, and business machines market segments and other multinational customers feel the pressure to respond to their customers’ needs for uniform global testing standards. A plastic material’s properties tested and reported by ASTM/UL cannot be used directly to compare with the same plastic material’s properties tested by ISO. For example, the ASTM method for the Izod impact strength test is reported in foot-pounds/inch (ft-lb/in.), or impact energy per length. The ISO method for impact strength is the Charpy test and it is reported in kilojoules/square meter (kJ/m2), or impact energy per area. These two impact methods and the significance of the tests are different; therefore, these impact properties cannot be compared.
1.7 Origin and Applications of Polymeric Materials Plastic materials are the ultimate tribute to man’s creativity and inventiveness. Plastics are true manmade materials. Like any other material, they have their origin in nature. The structure of plastic materials is based on basic chemical elements such as carbon, oxygen, hydrogen, nitrogen, chlorine, and sulfur. These elements are extracted from oil, mineral, water, gas, coal, and even from living plants. It was man’s inspiration to take these elements and combine them through various chemical reactions in an almost unending series of combinations to produce the rich variety of polymers known today as plastics. It is possible to develop new families of polymers by compounding with other polymers, additives, reinforcements, and modifiers to create almost any property desired for a specific application or enduse product. These new polymers have similar properties to existing materials but offer greater design flexibility and cost incentives for manufacturing.
Selection of Polymeric Materials
There are some plastics with significant property improvements over existing materials, while other polymers can only be described as unique materials with exceptional properties previously unknown to the industrial world. Today, we have plastic materials that will melt at 200°F (93°C), while other plastic materials (Vespel®) can withstand up to 1,000°F (537°C). The heat shields that protect astronauts, satellites traveling in space, and airplanes that can fly to the boundaries of the universe are composed of plastic materials based on recent plastic technology. There are polymers used for shields that can stop a bullet such as polyaramides popularly known as Kevlar®. There are flexible plastic films that protect grocery products and there are rigid plastics that are rugged enough to serve as support beams in a building. Plastics are among the best electrical insulating materials known to mankind, and yet there are other types of special plastic materials compounded with graphite and metal fillers capable of conducting electricity, as used in satellite dishes. Plastic composite materials are used for airplanes, ships, cars, trucks, vessels, buildings, golf club shafts, while other flexible polymers are used as tires, seals, tubing, hoses, and as upholstery materials for furniture. There are impact resistant, weather resistant, hard, and transparent polymers used as windshields for airplanes, automobiles, and shower doors. There are also transparent packaging materials used to protect consumer items (storage containers, plastic wrap, beverage containers). It is this diversity that has made them so easily applicable to such a broad range of end uses today. This polymer diversity makes it difficult to grasp the idea of a single family of materials that can provide an infinite range of properties, characteristics, and transformation processes.
1.8 Modern History of Polymeric Materials Plastic materials have played an important role in the development of our modern civilization. These materials have an extensive range of properties and processing automation capability while offering cost advantages over metals, wood, rubber, and other building materials. It is surprising to realize that a little more than a century ago there were no manmade plastic materials anywhere in the world. The plastics industry dates back to 1868, when John Wesley Hyatt mixed pyroxylin made from cotton and
1: Polymeric Materials and Properties
nitric acid with camphor to create an entirely different and new product called “Celluloid.” The development of celluloid was in response to a competition sponsored by a manufacturer of billiard balls. Celluloid came about to overcome the shortage of ivory used to produce billiard balls. With the need for a new material and a production method for this application, celluloid was developed and the plastics industry was born. Celluloid quickly moved into other markets, including new applications such as shirt collars, cuffs and shirt fronts, dolls, combs, buttons, and window curtains used in early automobiles. However, the most important application of celluloid was the first photographic film used by Eastman to produce the first motion picture film in 1882. (In the late 1880s Hannibal Goodwin was awarded a patent for celluloid film.) This material is still in use by the motion picture industry today, under its chemical name cellulose nitrate. The plastics industry took its second major step 41 years later. Dr. Leo Hendrik Baekeland introduced the first phenol-formaldehyde “Phenolic” resin in 1909. This was the first plastic material to achieve world acceptance. What is more important, Baekeland also developed techniques for controlling and modifying the phenol-formaldehyde polymerization reaction. This technology made it possible to produce useful items such as marbleized clock bases and electric iron handles. The molding process for phenolic materials was based on heat and pressure. In this process the monomer is mixed in the liquid phase and formed into various shapes under heat and pressure. The part is then cooled in the mold. This is basically the same process that is still used in the industry to produce thermosetting plastic materials. The third major step in the development of plastics took place in the 1920s, with the introduction of cellulose acetate. This polymer had a similar structure to cellulose nitrate but was safer to use in processing and in applications. Urea-formaldehyde can be processed like phenolics, but it can also be molded into light colored articles that are more attractive than the black or brown colors of phenolics. PVC became the second largest selling plastic for such applications as flooring, upholstery, wire and cable insulation, tubing, hoses, and fittings. Polyamide or nylon, introduced by E. I. Du Pont, was first developed to produce synthetic yarns and fibers. Nylon represents one of the most important developments in the plastics industry. Research development under the leadership of Wallace Carothers in
17
the late 1920s through 1930s resulted in the invention of nylon 6/6. The tempo of plastics’ development picked up considerably in the 1930s and the 1940s. With each decade, newer, more exciting, and more versatile plastics came into existence. In the 1930s, the acrylic resins were introduced for signs and transparent articles. The introduction of polystyrene made this polymer the third largest selling plastic for house wares, toys, and for applications in the packaging industry. Melamine resins were also introduced for use in dinnerware, paints, and wet strength paper. Melamine later became a critical element (as a binder) in the development of decorative laminate kitchen counter tops, table tops, and panels. Polyester (PET) resins known as Dacron® were also introduced to produce synthetic yarns and fibers. During the World War II years of the 1940s, the demand for plastics accelerated as did research into new plastics that could aid in the defense effort of the war. Fluoropolymers (PTFE) and polyethylene were war-time developments that grew out of the need for a superior insulating material for the Manhattan Project and applications such as radar cables. Thermoset polyester resins were also introduced a decade later. Radical changes in the boat building industry were also a war-time development introduced for military use. ABS is best known today as the plastic material used for applications such as appliance housings, refrigerator liners, safety helmets, tubing, telephone handsets, and luggage. The original ABS research work was also a defense effort of World War II for the development of synthetic rubber materials. By the beginning of the 1950s, plastics were on their way to being accepted by designers and engineers as basic industrial materials. This decade also saw the introduction of polypropylene, as well as the development of acetal and polycarbonate—two plastics that, along with nylon, came to form the nucleus of a subgroup in the plastics family known as the “engineering thermoplastics.” Their outstanding impact strength and thermal and dimensional stability enabled engineering thermoplastic resins to compete directly with metals in many applications. The 1960s and 1970s also had their share of new polymer developments. The most important contribution was the thermoplastic polyesters used in exterior automotive parts, under-the-hood applications, and electrical and electronic components. Polyester bottles internally coated with high nitrile barrier
18
resins (outstanding resistance to gas permeation) inspired the new drink bottle packaging applications. During this time span, another subgroup of the plastics family called “high performance plastics” found new markets. This group includes such materials as polyimide, polyamide-imide, aromatic polyester, PPS, and polyether sulfone. These materials historically met their objectives in the demanding thermal needs of aerospace and aircraft applications, reinforcing the vision of the future that was plastics.
1.9 Polymeric Materials Families Plastic materials are the result of the combination of carbon elements reacting with oxygen, hydrogen, nitrogen, and other organic and inorganic elements. These polymers have the ability to change into a liquid (melt) and are capable of being formed into shapes by the application of heat and pressure. Plastic materials are composed of various polymeric families. Plastics include an extensive number of polymers and compounds with each kind of material having its own unique and special type of properties. Plastic materials are classified into the following families: thermoplastics, thermoplastic elastomers (TPEs), liquid injection molding silicones, thermosets, composites, and thermoset rubbers. Thermoplastic polymers consist of a long chain of molecules, either linear or branched, having side chains or unattached groups of molecules. Usually, the thermoplastic materials are commercially available in the forms of pellets, granules, or powders. These materials can be repeatedly melted by heat under pressure so they can be formed and then cooled and hardened into the final desired shape. Chemical changes do not take place, for the most part, during the transformation process. A simple analogy for a thermoplastic material is a wax candle that can be liquefied by heat and then solidify when cooled. TPE resins are rubbery materials with the characteristics of a thermoplastic and the performance characteristics of a thermoset rubber. TPEs are processed using the same thermoplastic equipment and methods such as extrusion, injection molding, compression molding, and blow molding. Liquid injection molding silicone is a polymeric family with unique characteristics. Generally, these materials consist of two liquid formulations in a 1:1 ratio which polymerize during injection molding. These compounds produce precision elastomeric molded parts efficiently. They use a liquid metering, mixing, and
Selection of Polymeric Materials
delivery system, a specially modified injection molding machine, and a high temperature precision mold. Thermoset polymers are created by a chemical reaction between the chain cross-link and the long molecule’s network during polymerization. Heat causes the linear polymer chains to bond together to form a three-dimensional network. Once polymerized or hardened, the material cannot be reprocessed by heating without degrading the bonds cross-linking the polymer molecules. One analogy for the thermoset polymers is the chemical transformation of concrete. When the cement powder blends with water and sand, the mixture becomes a thick paste. This mixture is then transferred to a cavity for curing and finally hardens to become a solid object (concrete). The chemical reaction transforms the product into concrete. The transformation processes of the concrete items are irreversible. Reprocessing concrete to extract the original components of cement, sand, and water is not possible. The concrete becomes a new, different, and strong material. Thermosetting polymers are not reprocessable or recyclable. Thermoset polymers are supplied in commercial form as resins, powders, and liquid monomer mixtures, or as partially polymerized molding compounds. In this uncured condition, they conform to the finished shape with or without pressure and can be polymerized with chemicals or heat.
1.10 Classification of Polymeric Materials by Performance Plastic materials classifications divide polymers into four family groups based on their application performance. The first is the “commodity plastics.” These materials have large consumption volume, extensive application end uses, low material cost, and limited property and performance. The commodity plastics include polystyrene, polyethylene, styreneacrylonitrile (SAN) copolymer, cellulose nitrate, polybutene, bismaleimide, unsaturated polyester, and polyvinyl chloride (PVC). The second group is the “intermediate plastics.” These materials have higher mechanical, thermal, chemical, and electrical properties than the commodity plastics. The basic polymer matrix properties remain constant, however, modifications can be made to change specific properties of the compound. Intermediate plastics include acrylic (polymethyl methacrylate (PMMA)), polyolefin thermoplastic
1: Polymeric Materials and Properties
elastomer (TPO), modified polyphenylene oxide (PPO), thermoplastic vulcanizate (TPV), melt processible rubber (MPR), high impact polystyrene (HIPS), ionomers, polypropylene, glass fiber reinforced polypropylene, ABS, styrenic block copolymer thermoplastic elastomers (TPEs), and ultra-high molecular weight polyethylene. The third group is the “engineering thermoplastics.” The level of mechanical properties that qualify as engineering grade is somewhat arbitrary; a tensile strength that is not lower than 7,000 psi and a minimum modulus of elasticity of 350,000 psi are reasonable criteria. Engineering thermoplastic resins include polyacetal, polyamide (nylon), polycarbonate, polybutadiene terephthalate (PBT), polyethylene terephthalate (PET), block copolyester TPEs, polyamide TPEs, and liquid injection molding silicone. The fourth group is the “high performance engineering materials.” These materials have the highest resistance and retain a high percentage of their useful mechanical properties at high temperatures, providing a longer service life of the product. They also maintain properties at higher electrical frequencies without sacrificing their chemical resistance properties, when exposed to corrosive elements. These resins are also inherently flame retardant, with UL 94 flammability ratings of V0 and 5V. The high performance engineering resins include high temperature nylon (polyamide), liquid crystal polymers (LCPs), polysulfone (PSU), fluoropolymers, polyetherimide (PEI), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), silicone polymers, and polyimide. Competition among resin producers to capture markets has created a resin supply of more than 26,000 resin grades in the USA alone. The result is that there is usually more than one choice, and often several choices, available to meet the end-use performance requirements.
1.11 Types of Thermoplastic Molecular Structures Polymeric thermoplastic materials are an aggregate of long-chained molecular structures. There are two different states, one is known as “crystalline polymers” and the other is known as “amorphous polymers.” However, there is no crystalline plastic material exhibiting solely a crystalline structure, but rather they have a mixed structure in which crystalline sections and amorphous sections coexist. This molecular structure is known as “semi-crystalline polymers.”
19
1.12 Manufacturing of Polymers The basic compositions of polymers are derived from organic materials primarily comprised of atoms of carbon and hydrogen (hydrocarbons). Resin suppliers use chemical reactions to transform basic feedstocks or monomers, which have been derived from natural gas, crude oil, and coal, into polymers. Petroleum is the principal source of alkanes and cycloalkanes. An oil refinery uses a distillation process, where the crude oil is first separated into fractions by distillation to produce gas, petroleum ether, gasoline, kerosene, fuel oil, lubricating oil, greases, paraffin wax, and asphalt. Some higher-boiling point fractions are subjected to cracking processes in which large molecules are broken down into smaller molecules, thus increasing the yield of the most valuable products such as gasoline. In addition, small molecules are converted into large ones by a process known as “alkylation.” Gases that are a by-product of these processes, together with the petroleum fractions and substances obtained from the fractions, form the basic feedstock to manufacture plastic materials. Natural gas and crude oil are the most common and economical feedstocks in use today. Coal is another excellent source for the manufacture of feedstock for plastics. Products such as castor oil or tung oil derived from plants are also adaptable. From these basic hydrocarbons come the feedstocks known as monomers (small, single molecules). The monomer is then subjected to a chemical reaction known as polymerization, which causes the small molecules to link together into much longer molecules called polymers. Chemically, the polymerization reaction transforms the monomer into a polymer. A single monomer can contribute to the manufacture of a variety of different polymers, each with its own distinct characteristics. A polymer may be defined as a high molecular weight compound that contains comparatively simple recurring units. Basically, there is a great deal of flexibility in the plastics manufacturing process for creating a wide range of polymers. The most important factors that determine the properties of the polymers are the chemical processes by which the small molecules link together into large molecules and the structural arrangement of the molecular chains formed. Another factor is the length of molecules in the polymer chain. Polymerizing two or more different monomers together (a process known as copolymerization) leads to the production of other polymer grades.
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Figure 1.13 shows the basic chemical feedstocks and simplified manufacturing flow charts for the most common polymeric materials.
1.13 Polymeric Materials Compounding Process The compounding process has been the major contributor in the development of new formulations for the plastics industry. A virtually endless array of additives, modifiers, colorants, fillers, and reinforcements permits resin suppliers to impart specific properties to the basic polymers, resulting in expanding opportunities for new applications and cost reductions. Compounding relies on polymerization chemistry to combine a base polymer with modifiers, additives, reinforcements, and other polymers to create new alloys and molecular structures. Compounded polymers can offer better performance, greater strength, and smoother processing at a competitive cost. Compounding is a complex, precise manufacturing process. The basic function of the compounding equipment is to mix two or more different polymers, additives, modifiers, fillers, and reinforcements into a uniform, continuous, and homogeneous mixture. Several types of compounding equipment are used: single screw and twin screw extruders that include tangential, intermeshing or self-wiping, and co-rotating and counter-rotating machines. These extruders differ in terms of their lengths (working segments), vacuum section, and extrusion die. Batch mixers are used to blend polymers and certain additives. Various types of material feeders (single and twin screws, gravity, vibration, liquid) are used to monitor the ratio of polymers, additives, and reinforcements. The quench tank, pelletizer, blender/lubricator, dryer, and packaging equipment are among the other accessories required for the compounding process. A basic polymer must be compounded for use by the processor who will transform it into a product. In some limited circumstances, it is possible to use the polymer as it comes out of the polymerization reaction. However, these polymers could be unstable or dangerous to process because of marginal thermal stability. More often, other manufacturing steps are required to modify the polymers into a form that can be easily handled by the plastic processors. The most popular solid forms of industrial polymers are pellets, granules, flakes, or powder. In the hands of the processor, these solid forms are generally
Selection of Polymeric Materials
subjected to heat and pressure, melted, forced into the desired shape, and then allowed to cure and set into a finished product. They are also available as semi-solids (pastes). Liquid polymers can also be used to impregnate fibrous materials that can then be allowed to harden into a variety of configurations (composites).
1.13.1 Chemical Additives for Polymeric Materials Compounding A variety of additives are used to overcome some of the limitations in plastic materials. Some of the common additives used in compounding and their functions are given below: • Antifogging agents: Prevent fogging that obscures viewing through clear plastic films or sheets. • Antioxidants: Prevent oxidative degradation of the polymers. • Antistatic agents: Prevent electrostatic buildup on the plastic product surfaces. • Biocides: Prevent growth of microorganisms that could cause odor, mildew, and stains on plastic surfaces. • Blowing agents: Release a gas that dissolves in plastics to form cellular or foamed structures. • Coupling agents: Provide improved bonding between the basic polymers and the reinforcements. • Cross-linking agents: Improve the physical properties of thermoplastics by cross-linking. • Curing agents: Catalyze or initiate curing of the plastic materials. • Flame retardants: Improve the polymers’ flameburning ignition levels. • Heat stabilizers: Prevent the polymers from heat-induced breakdown, and degradation, when the plastic product is exposed to high temperatures. • Internal lubricants: Improve the processing ability of the polymer by lowering melt viscosity. • Surface lubricants: Prevent the polymer melt from sticking to metal surfaces in the processing equipment (plastifying unit and mold cavities).
Nylon 6 Nylon 66 Polysulfone Polycarbonate PPS
Caprolactam Sulfuric acid
Cyclohaxanano oxime
Dichlorodephenyl Sulfone
Hexamethylenediamine
Diphenol A
Hydroxyl amine
Adiptic Acid
Tionyl Chloride
Phenol
Chlorobenzene
Cyclohexanone
Chlorine
Cyclohexane
Monochlorobenzene
Chlorine
Chlorine
Chlorobenzene
Methyl Chloride
Chlorine
Chlorine
BENZENE
METHANE
METHANOL
Hydrochloric Acid
Chlorosilane (Hydrochloric Acid + Silicone Metal
Silicon Metal
Terephthalic Acid
Ethylbenzene
Chloroform
Hydrogen Fluoride
Hydrogen Fluoride
Dimethyl Terephthalate
Chlorotrifluoroethane
Chlorotrifluoroethylene
Phthalic Anhydride + Maleic Anhydride + Propylene Glycol
Styrene
1: Polymeric Materials and Properties
Chlorodifluoromethane
Formaldehyde
Methyl Chlorosilane
Phenyl Chlorosilane
Acetone
Diphenol A
Sulpur
Phosgene
Water
1, 4 - Butanedoil
Silicone PBT 1, 2 - Ethylene Glycol
Tetrafluoroethylene
Tetrafluoroethylene
Alkyd
Chlorotrifluoroethylene
Hexafluoromethane
Melamine + Paper Pulp
Urea + Paper Pulp
PET Urea M. C. Melamine M. C. Acetal TFE Fluorocarbon FEP Fluorocarbon CTFE Fluorocarbon E-CTFE Fluorocarbon Alkyd Polyester (TS) Polystyrene
Figure 1.13. Simplified manufacturing flow chart for common polymeric materials.(Continued)
ETHYLENE
ETHANE
NATURAL GAS
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ABS Acrylonitrile (Propylene + Ammonia) + Butadiene
Styrene
Ethylene Methacrylic Acid Copolymer
Acrylonitrile (Propylene + Ammonia)
Metal Ion
Acetic Acid
Cumene
Ethylbenzene
Methacrylic Acid
Chloride
Isopropyl Alcohol
ETHANE
Benzene
Acetone
Acetaldehyde
Hydrogen Cyanide
PROPANE
Chloride
ETHYLENE
Acetone Cyanohydrin
Allyl Chloride
PROPYLENE
Propylene Oxide
BUTANE
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Styrene Acrylonitrile
Ionomer Polyethylene
Acetic Anhydride
Allyl Alcohol
Polypropylene Oxide Polyol
Epichlorohydrin
Sulfuric Acid
Ethylene Dichloride
Bisphenol A
Methacrylamide
Phenol
Vinyl Acetate
Chemical Cellulose (Wood Pulp)
Ethylene
Cellulose Acetate
EVA Vinyl Chloride
Methanol
1-Butene
MDI + TDI + Polymeric Isocyanate
Phthalic Anhydride
Chlorohydrin Intermediate
Methyl Methacrylate
Formaldehyde
PVC Polypropylene Phenolic Acrylic Epoxy Diallyl Phthalate Polyurethane Polybutylene
Figure 1.13. Simplified manufacturing flow chart for common polymeric materials. (Cont’d)
NATURAL GAS
Selection of Polymeric Materials
1: Polymeric Materials and Properties
• Plasticizers: Enhance the polymer flexibility, resiliency, and melt flow. • UV stabilizers: Prevent polymer deterioration when the plastic product is exposed to UV light.
1.13.2 Fillers and Reinforcements for Polymeric Materials Compounding Fillers and reinforcements are used to improve the mechanical properties, temperature resistance, and environmental resistance, and to reduce the cost (fillers) of the plastic materials. Some of the common fillers and reinforcement materials used for compounding, and their functions are given below: • Alumina trihydrate: Used as an extender, flame retardant, smoke suppressant. • Barium sulfate: Filler; white pigment; increases specific gravity, chemical resistance, and decreases the coefficient of friction. • Baron fibers: Increases tensile and compressive strength capacity; expensive. • Calcium carbonate: Most widely used as an extender, pigment, filler. • Calcium sulfate: Extender; enhances mechanical properties; increases impact resistance, tensile strength, and compressive strength. • Carbon black: Filler; black pigment; antistatic agent; aids in cross-linking; conductive; UV stabilizer. • Carbon, graphite fibers: Reinforcement; high modulus of elasticity and tensile strength; low density, decreases the coefficient of thermal expansion and friction; conductive. • Ceramic fibers: Reinforcement; very high temperature resistance; expensive. • Glass fibers: Largest volume reinforcement in use; increases mechanical strength, product dimensional stability; improves heat resistance and chemical resistance. • Kaolin: Second largest volume expander and pigment in use; used mostly in wires and cables, sheet and bulk molding compounds, and vinyl floorings. • Mica: Flake reinforcement; improves dielectric strength, thermal resistance, and mechanical properties.
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• Microspheres, hollow: Reduce weight; improves polymer melt flow, stiffness, and impact resistance. • Microspheres, solid: Improve polymer melt flow properties, uniform stress distribution. • Organic fillers: Extenders; e.g., wood flour, nutshell, corn cobs, rice, peanut hulls. • Polymer fibers: Reinforcement; lightweight; improves mechanical strength. • Silica: Filler; extender; reinforcement; thickens liquid systems, makes them thixotropic; helps to avoid plate-out in PVC; acts as flattening agent. • Talc: Extender; filler; improves stiffness, tensile strength, and creep resistance. • Wollastonite: Reinforcement; high loadings possible; improves mechanical and dielectric strength, heat resistance, and dimensional stability; lowers moisture absorption; decreases the impact strength.
1.13.3 Impact Modifiers for Polymeric Materials Compounding Impact modifiers improve the impact strength of polymers; stiffness, tensile strength, compressive strength, and temperature resistance can also be reduced. The impact modified polymer must be made heterogeneous with two distinct phases, one rigid and the other flexible. The following impact modifiers are used: • Olefinic thermoplastic elastomers: Improve impact resistance (toughness) of semi-crystalline thermoplastics. • Rubber and thermoplastic elastomers: Improve impact resistance of polyolefin polymers. • Styrene-butadiene multi-blocks: Improve impact resistance of HIPS. • ABS: Improves flexibility and cost reduction of polycarbonate. • Polyurethane: Improves impact resistance of thermoplastic polyester and polysulfone; reduces stiffness of epoxy resins. • Elastomers: Improve flexibility of unsaturated polyester. • EPDM and EPM: Improve impact resistance of nylon, polypropylene, and high density polyethylene (HDPE).
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1.13.4 Colorants for Polymeric Materials Compounding Pigments and dyes are used to color plastic products. A color master batch is the most common technique used for coloring polymers. The color concentrate is dispersed into a carrier or compatible (universal) polymer such as nylon 6, polyolefin, ethylene vinyl acetate (EVA), or polyethylene. The concentrate and carrier are mixed using intensive mixers (two rollers), extruded, and cut into pellets. Other color concentrates are produced in liquid forms or as fine loose powder; this causes severe dusting problems during the compounding process. Organic and inorganic pigments are the primary colorants in the plastic industry. There are general differences in chemical structure, coloring characteristics, and physical properties between the two types. Organic pigments are complex chemical compounds that contain one or more benzene rings and exhibit small particle size, greater transparency, and color strength. Inorganic pigments are metallic compounds that are larger in size, smaller in surface area, and denser. They absorb and scatter light and exhibit more opacity and lower tinting strength. They have better heat and UV stability, but their chemical resistance varies.
1.14 Families of Thermoplastic Polymers Plastics are a family of materials, not a single kind of material. Most fall into a particular group or family, which are specified as thermoplastics, thermoplastic elastomers, liquid injection molding elastomers, thermosets, and thermoset rubbers. Popular thermoplastics are discussed in this section.
1.14.1 Acrylonitrile-ButadieneStyrene (ABS) Terpolymer ABS resins have a well-balanced set of properties for molding tight dimensional control articles with outstanding surface finishing, good impact resistance, and metal plating characteristics. ABS resins belong to a versatile family of thermoplastic polymers. ABS is produced by combining three monomers: acrylonitrile, butadiene, and styrene. The chemical structure of these monomers requires each monomer to be an important component of the ABS resins. Acrylonitrile
Selection of Polymeric Materials
contributes heat resistance, chemical resistance, and surface hardness to the system. The butadiene component contributes toughness and impact resistance, while the styrene component contributes processibility, rigidity, and strength. ABS plastics are two-phase systems. Styreneacrylonitrile (SAN) forms the continuous matrix phase. The second phase is composed of dispersed polybutadiene particles, which has a layer of SAN grafted onto their surface. The binding matrix layer of SAN makes the two phases of the polymer compatible.
1.14.2 Acetal (Polyoxymethylene, POM, Polyacetal) Acetal resins provide a well-balanced set of properties including a hard self-lubricated surface and excellent chemical resistance, strength, stiffness, and toughness over a broad temperature range. The acetal homopolymer was first introduced in 1956 as a semi-crystalline form of polymerized formaldehyde forming a linear chain of molecules of oxymethylene. In the homopolymerization process, formaldehyde is separated from water and purified to CH2O gas, which is then polymerized to the polyoxymethylene molecule. In this case, the molecule is stabilized by a reaction with acetic anhydride to give acetate end groups. The acetate capped homopolymer is less resistant to attack by a base, but it has a higher melting point and mechanical advantages of strength, stiffness, toughness, hardness, creep, and fatigue than the acetal copolymer. In the acetal copolymerization process, formaldehyde is first converted into a cyclic structure of three formaldehyde molecules called trioxane. The trioxane is separated, purified, and reacted with a comonomer (ethylene oxide) to prepare polyoxymethylene which has randomly distributed –CH2–CH2– groups in the chain. This resultant raw polymer is then heated and treated with a base to degrade the ends of the molecules back to a –CH2–CH2– “block” point at each end. This results in a molecule that is resistant to further degradation by basic environments. The end-capping of homopolymer and copolymer chains is necessary to prevent the irreversible depolymerization of the polymer backbone during melt processing. The thermal energy causes “unzipping” of the –H–O–CH2–O–CH2– end group to a formaldehyde monomer.
1: Polymeric Materials and Properties
The outstanding characteristics of this polymer include stiffness, which permits the design of parts with large areas and thin cross sections; high tensile strength and creep resistance under a wide range of temperatures and humidity conditions; high fatigue resistance and resilience for applications requiring springiness and toughness. Acetal has achieved importance in applications because of a good balance of properties. Two types of acetals are available: one is a homopolymer resin with higher mechanical properties, higher end-use temperatures, and higher melt flow index, and the other is a copolymer resin with better processing characteristics and impact resistance.
1.14.3 Polymethyl Methacrylate (Acrylic, PMMA) PMMA (acrylic) polymers have outstanding optical properties, weatherability, and a full range of transparent, translucent, and opaque colors. Acrylics are composed of polymers and copolymers in which the major monomerics belong to two families of ester-acrylates and methacrylates. Hard, clear acrylic sheets are made from methyl methacrylate; molding and extrusion resins are made in a continuous solution from methyl methacrylate copolymerized with small percentages of other acrylates or methacrylates. The low strain optic coefficient of acrylics, coupled with their ability to be molded with low stress, makes them an ideal material for video disks. Sheets extruded from an acrylic base impact-modified grade have excellent thermoforming characteristics and can be stiffened by applying glass reinforced polyester to the inside surface with a spray gun to produce bathroom whirlpool tubs. The high flow grade has the best transparency, as it does not contain acrylonitrile, making it suitable for medical applications in which transparency is of prime importance. Acrylic plastics can be cleaned with solutions of inorganic acids, alkalis, and aliphatic hydrocarbons. However, chlorinated and aromatic hydrocarbons, esters, and ketones will attack the acrylic plastics.
1.14.4 High Temperature Nylon (HTN) HTN belongs to the aliphatic-aromatic polyamide family. It is produced either as a semi-crystalline or as an amorphous type of polymer.
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HTNs require higher melt temperatures for processing, resulting in a higher degree of crystallinity in the polymer. This polymer characteristic provides a better dimensional stability, improved chemical resistance, and higher end-use temperatures to the HTN molded products. HTN service temperature provides an additional 50°F (27.75°C) of higher stability than standard nylon 6/6 resins. HTNs are produced using various chemical additives, glass fiber reinforcements, mineral fillers, flame retardants, and impact modifiers, depending on the market needs. Injection molders familiar with nylon 6, 6/6, or 6/12 will not have to change molds or molding machines to process HTN. However, there are some processing differences between these two nylon families. Close attention must be paid to drying where conventional nylons require a moisture content of no more than 0.2%. A sealed HTN bag comes predried, but with the recommendation to predry at 175°F (80°C) for 2–16 hours or more before processing. As with all nylons, part dimensioning must take into account both shrinkage and ambient moisture absorption by the molded parts. The mold shrinkage rates of these semi-crystalline HTN polymers are similar to the conventional nylons. Shrinkage rates range from 0.18 to 0.22 in./in. (or %) for the unreinforced grades, and from 0.02 to 0.06 in./in. (or %) for the reinforced grades. Moisture absorption will reduce shrinkage. There is an increase in melt viscosity for HTN; it does not run like water as the standard easy flow nylons do, and the advantage is less mold flashing with lower temperature molds (180°F, or 82°C). One of the characteristics of the HTN resins is the fast molding cycle. These materials set very quickly in the mold cavity before the molded parts are ejected. This family of materials may carry the name nylon, but their high performance drastically exceeds the characteristics of traditional nylon materials. There is a specially formulated HTN series of polyphthalamides that has a melt temperature of 570°F (298°C) and provides excellent heat resistance for heat soldering owing to its partial aromatic nature. The flame retardant grade resins have achieved UL 94-V0 rating for a 0.031 in. (0.78 mm) wall thickness.
1.14.5 Liquid Crystal Polymer (LCP) LCPs are a family of diverse polymers that lack the molecular structure (amorphous and crystalline
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mixture) and homogeneity found in HTNs. Most commercial LCPs are copolyesters, copolyamides, or polyester-amides, although many other chemistries are possible. LCP structures range from partially aliphatic to wholly aromatic polymers. The melting point characteristics, the end-use temperature properties, the chemical and solvent resistance, flammability, processibility, and cost depend on individual products. LCPs are processed in the liquid crystalline state. All thermoplastic LCPs show one-dimensional order, which results from the semi-rigid, essentially linear architecture of the molecules. They are found in either the solid or the nematic states (parallel to a common axis). Because melt viscosity increases by a factor of two to ten when the polymer passes from the nematic to the isotropic state, processing is extremely difficult as melt temperature approaches the clearing point. Also important is that melt point is a low enthalpy transition, thus there is little crystallization, allowing fast cycling and low mold shrinkage. The chemical structure common to all melt processed LCPs is para-hydroxy benzoic acid. There are three classes of LCPs. The first is a general purpose class, which exhibits exceptional ease of processing, dimensional stability, molded part repeatability, chemical resistance, flame resistance, strength, and stiffness (e.g., computer multiple pin connectors). The second is a more temperature resistant variant, which is not quite as easy to process, but has good dimensional stability, good chemical resistance, excellent flame resistance, and very good strength and stiffness (e.g., ABS brake sensors). The third, a lower temperature performance class, is less expensive and less easily processed, but has good dimensional stability, with good strength and stiffness, less chemical resistance, and moderate flame resistance (e.g., computer hardware connectors).
Selection of Polymeric Materials
is equal to 0.96. The melting points range from 410°F to 491°F (210°C–255°C). Nylon 6/6 and nylon 6 are the most important commercial products. Other nylons are 6/9, 6/10, 6/12, 11, and 12. The higher the number of carbon atoms (i.e., the lower the concentration of amide groups), the lower the melting point. Nylons are modified by use of monomer mixtures leading to copolymers. These are normally less crystalline, more flexible, and more soluble than the homopolymers. Additives are compounded in nylons to improve thermal and photolytic stability, facilitate processing, increase flammability resistance, enhance hydrolytic resistance, and increase lubricity. Modification is an important asset. Fiber and mineral reinforcement are widely used. Blending with elastomeric modifiers has yielded nylons with improved toughness. Toughened nylon alloys (50% RH) have a notched Izod impact strength ranging from 2.5 to 4.5 ft-lb/in.; super tough nylon 6/6 (Zytel® ST-801) exhibits Izod impact values between 15 and 20 ft-lb/in. (50% RH). Nylon is resistant to oils, greases, solvents, and bases. Nylon is resistant to fatigue, repeated impact toughness and abrasion resistance, plus has a low coefficient of friction. Nylon has high tensile strength properties, creep resistance, and retains most of its mechanical and electrical properties over a wide temperature range. One of its limitations is high moisture pickup, which results in changes in dimensional and mechanical properties. Unreinforced nylon 6/6 has a temperature rating for continuous service ranges from 195°F to 265°F (90.5°C–129°C). The available types of nylon compounds include lubricated, nucleated, heat stabilized, UV stabilized, hydrolytically stabilized, flame retarded, glass reinforced, Kevlar® fiber reinforced, mineral reinforced, toughened, melt flow modified, electrically conductive, and several nylon alloys.
1.14.6 Polyamide (PA, Nylon) Polyamides are synthetic polymers that contain an amide group –CONH– as a recurring part of the chain. Nylons are made from (a) diamines and dibasic acids, (b) ω-amino acids, or caprolactam. The most common types of resins are nylon 6, nylon 6/6, and nylon 6/12. They are made from hexamethylene diamine and the 12-carbon acid, dodecanedioic acid, or HOOC(CH2)10COOH. The molecular weights of nylons range from 900 to 31,000. On average, for linear products of melt polymerization, the molecular weight per molecular number ratio is equal to 1.85, and the molecular viscosity per molecular weight ratio
1.14.7 Polyetherimide (PEI) PEIs are amorphous, high performance thermoplastic polymers. Their chemical structures consist of repeating aromatic imide and ether units. PEIs are characterized by high strength and rigidity at room and elevated temperatures, long-term high heat resistance, highly stable dimensional and electrical properties, and broad chemical resistance. Unmodified PEI is amber in color and transparent and exhibits inherent flame resistance and low smoke generation without the use of halogenated or other types of flame retarding additives.
1: Polymeric Materials and Properties
The amorphous structure of PEIs contributes to their excellent dimensional stability, low shrinkage, and highly isotropic mechanical properties. The high glass transition temperature (Tg) of 420°F (215°C) and high performance strength and modulus characteristics at elevated temperatures are provided by the very rigid imide groups in their chemical structure. The high Tg allows PEI to be used intermittently at 392°F (200°C) and permits short-term excursions to even higher temperatures. Higher strength and stiffness at elevated temperatures are achieved with glass or carbon fiber reinforcement. Unreinforced PEI resins are rated as having continuous end-use temperatures from 338°F to 356°F (170°C–180°C), and are listed as UL 94-V0, down to 0.010 in. (0.25 mm) wall thickness. PEI resins are available either in unmodified form or reinforced with 10%, 20%, 30%, and 40% glass fiber. Grades with carbon reinforcement for high strength and static dissipation, and a series of products with internal lubricants are also available. Other grades and blends are offered for blow molding, structural foam, and extrusion processes. PEI (Ultem®) has been formulated to meet specific needs such as electromagnetic interference shielding capability and aircraft cabin interior flammability (and heat release) requirements.
1.14.8 Polyetherether Ketone (PEEK) PEEK is a subgroup of the ketone polymer groups which are similar to polyarylether ketone (PAEK) and polyetherketone ketone (PEKK). Because of their semi-crystalline nature, PEEK resins demonstrate an excellent balance of physical properties, including strength at elevated temperature, chemical resistance, and hydrolytic and thermal stability. They offer the highest level of thermal resistance together with thermoplastic processing capability. PEEK resins are produced as unreinforced resins as well as glass and carbon fiber reinforced resins. PEEK materials retain their mechanical properties at high continuous end-use temperatures (higher than 480°F, or 248°C). The retention of flexural modulus and tensile strength properties is excellent, especially for the fiber reinforced resins. Another characteristic of PEEK-based resins is the UL 94-V0 rating without the addition of any flame retardant additives. PEEK resins have good electrical property values. Chemical resistance at elevated temperatures in various aggressive environments shows that reinforced PAEK is a chemically resistant material. The reten-
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tion of tensile properties for unreinforced PAEK resins is greater than 75%. Fiberglass reinforced PAEK resins exhibit a higher retention rate of tensile properties, but show some loss of tensile strength properties. Unreinforced PAEK resins react chemically with acids and bases.
1.14.9 Polycarbonate (PC) Polycarbonate is produced by reacting bisphenol A and carbonyl chloride in an interfacial process. This reaction is carried out under basic conditions in the presence of an aqueous and an organic phase. Molecular weight is controlled by a phenolic chain stopper. Trifunctional monomers are added for increased melt strength for extrusion and blow molding applications. Polycarbonate is an amorphous engineering thermoplastic material with exceptionally high impact strength, transparency, high dimensional stability, and moderate temperature resistance. Melt mass-flow rate is one of the most difficult properties to regulate without affecting the impact strength of polymer. Polycarbonate is characterized by an exceptionally high notched Izod impact strength between 12 and 17 ft-lb/in. only for 0.125 in. wall thickness. The notched Izod impact is drastically reduced when wall thickness is greater than 0.125 in. Polycarbonate has a glass transition temperature (Tg) of 300°F (148°C), high gloss finishing, low mold shrinkage, and low moisture absorption properties. Polycarbonate has high corona resistance and insulation resistance properties, and a dielectric constant that is independent of temperature. Polycarbonate can be compounded to produce resins for sterilizability, flame retardance, stain resistance, and fiberglass and mineral reinforcements with various additives. These ingredients enhance the thermal stability, UV stability, tensile strength, stiffness, and flame retardants.
1.14.10 Modified Polyphenylene Oxide (PPO) The chemical composition of PPO homopolymer is poly (2,6-dimethyl-1,4-phenylene ether) or poly [oxy-(2,6-dimethyl-1,4-phenylene)]. PPOs are rigid, amorphous, tough, and dimensionally stable plastics over a wide range of elevated temperatures. Modified PPO resins are the result of compounding PPO with polystyrene and various additives. An unmodified PPO has a glass transition temperature of 302°F (150°C). PPO resins are very difficult to process
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because of viscosity or fillers and their amorphous molecular structure. Modified PPO permits tailoring of the viscosity and toughness properties to meet specific requirements. The addition of rubber impact modified HIPS provides modified PPO with an improved Izod impact value. Nylon 6/6 improves the chemical resistance and thermal property of PPO. To increase flexural modulus and reduce mold shrinkage, glass and mineral filled modified PPO resins are available. Dimensional stability of modified PPO at moderated temperatures and stresses can be maintained in a number of environments such as strong alkalis and bases, detergents, and hot water. Filled and flame retardant grades are available. Modified PPO blends make excellent insulators because they have low loss and dissipation factors over broad temperature and frequency ranges. These plastics also have inherently low dielectric constants and high dielectric strengths. High humidity conditions have little effect on these properties.
1.14.11 Polybutylene Terephthalate (PBT) PBT is produced by the transesterification of dimethyl terephthalate with butanediol. This reaction takes place by a catalyzed melt poly-condensation, resulting in a repetition of the molecular units. PBT is a semi-crystalline and tough engineering thermoplastic. It is characterized by low moisture absorption, excellent electrical properties, and good chemical resistance, mechanical strength, and heat resistance. The continuous service temperature for unfilled PBT is 194°F (90°C) and for fiberglass reinforced PBT is 240°F (115°C). The thermal resistance of PBT is improved by compounding with nylon 6/6 (10–30%). Moisture absorption can be reduced and process characteristics and mechanical properties of PBT are improved by compounding with low density polyethylene (15–25%). An enhanced gloss surface also can be achieved when PBT is compounded with PET. Molten PBT is subject to hydrolytic degradation and so it must be rigorously dried before melt processing. A maximum moisture content of 0.02% is recommended.
1.14.12 Polyethylene Terephthalate (PET) PET is produced by a reaction of either purified terephthalic acid or dimethyl terephthalate with
Selection of Polymeric Materials
ethylene glycol. The high viscosity melt is converted into amorphous clear pellets by rapid quenching and cutting in water. PET polymers are long chain, unbranched molecules that are produced by the condensation reaction between a dibasic organic acid or ester and a glycol. PET injection molding grades include a proprietary crystallization system that provides rapid crystallization needed for good moldability at normal molding temperatures. PET has the stiffest polymer chain possible for thermoplastic polyesters with an outstanding combination of strength, stiffness, and high melt mass-flow properties. Basic PET applications are electrical and electronic components, industrial fibers and films, and beverage bottles. PET is a semi-crystalline engineering thermoplastic produced by a proprietary crystallization technology. Reinforced PET resins are only available in the following grades: general purpose, high melt massflow, low warp, toughened, and flame retardant. Most of the PET compounded resins are glass fiber/mineral reinforced. Glass fiber and mica are used for low warpage electrical applications. Reinforced grade resins are available with fiberglass contents ranging from 15 to 55%. There are several flame retardant resins meeting UL 94-V0 and UL 94-5V requirements, down to 0.031 in. (0.78 mm) wall thickness. Molten PET is subject to hydrolytic degradation and so it must be rigorously dried before melt processing. A maximum moisture content of 0.02% is recommended.
1.14.13 Polyethylene (PE) Polyethylene is manufactured using three basic chemical processes: slurry particle reactor process, gas phase process, and the new metallocene catalyst technology. The last process may also be used with conventional catalysts to combine the advantages offered by both polyolefin polymerization catalysts. A new class of catalysts for polyolefins based on nickel and palladium can produce a very broad range of molecular weights and branching. A highly efficient catalyst system aids in the polymerization of ethylene and allows the use of lower temperatures and pressures than required for manufacturing conventional low density polyethylene (LDPE). Polyethylene is available in a range of flexibilities, depending on the production process. High density polyethylene (HDPE) is the most rigid of the three basic types of polyethylene resins, which also include LDPE and linear low density polyethylene (LLDPE).
1: Polymeric Materials and Properties
HDPE can be processed by a wide variety of thermoplastic processing methods and is particularly useful where moisture resistance and low cost are required. Polyethylene is limited by low end-use temperature characteristics. Blow molding consumes the largest amount of HDPE. About 35% is used to make blow molded products. Extruded products consume about 30% and injection molding accounts for about 20%. High molecular weight HDPE resins have high strength; they are used in packaging films, sheets, pipes, and large blow and rotation molding items. LDPE is the second largest polyethylene in use by capacity in the USA. Extrusion is the dominant process used with LDPE resins. Extruded products, principally films, account for 75% of its use. High molecular weight LDPE film grade resins produce high-gloss, high-clarity films that exhibit good toughness and heat sealing capability.
1.14.14 Polytetrafluoroethylene (PTFE) These fluorocarbon resins are produced by the polymerization of tetrafluoroethylene. PTFE is composed of long, straight chains of fluorinated carbons (CF2 groups). The chemistry of PTFE provides the unique characteristics of the polymer. The helical backbone of carbon atoms symmetrically surrounded by fluorine atoms provides the resin with its unique chemical, electrical, and thermal properties. Continuous service temperatures range from near absolute zero (–460°F, or –273°C) to 500°F (260°C). This polymer has an initial melting point of 648°F (342°C) and although it does not char, the depolymerization temperature depends on the atmospheric conditions or whether oxygen is present. Accelerated degradation begins at 720°F (382°C). When exposed to flame, PTFE will burn, but it does not continue to burn when the flame is removed because it has an exceptionally high limiting oxygen index and will not support combustion in air. Like most fluoropolymers, PTFE has outstanding electrical insulation properties. Its dielectric constant and loss factor are low and uniform across a wide temperature and frequency range. PTFE is also known for its low coefficient of friction, which is the lowest of all materials. PTFE is flexible, strong, and tough at temperatures as low as –460°F (–273°C). It has an opaque white color, and can be formed into very thin and transparent films.
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PTFE polymers are available in granular, fine powder, and water base dispersion forms. PTFE cannot be processed by melt techniques because of the lack of flow (i.e., extremely high viscosity) of its molten form. PTFE has a high melt viscosity property and the polymer never becomes fluid. At temperatures above 620°F (327°C) it becomes a self-supporting gel. Because of these unusual melt characteristics, traditional thermoplastic processing equipment cannot be used. PTFE starts its transformation at ambient conditions, at pressures between 2,000 and 5,000 psi. The self-supporting gel is then sintered at temperatures ranging between 690°F and 710°F (365°C and 377°C) under a controlled temperature cycle. To make tapes or sheets of PTFE (up to 0.250 in. (6.35 mm) thick), a cylinder is molded and veneered. Free sintering (outside the mold) always follows compression of powder, whether the process is used to mold PTFE into sheets or billets through simple compression molding. To form multiple, simply shaped items, using an automatic process, isostatic and automatic molding methods are used. A flexible membrane and hydraulic pressure are used to make complicated shapes. PTFE is also highly resistant to blending with fillers and reinforcers. However, one or more fillers can be added to PTFE in some circumstances to prevent creep or cold flow from occurring when a load is applied to the soft material. Fillers are particularly desirable when PTFE is used for parts in a dynamic operation where a high rate of wear may occur. The fillers include fiberglass, carbon, graphite, bronze, and molybdenum disulfide. There are copolymers of tetrafluoroethylene that can be processed by traditional melt processing technologies and fluoropolymers that need special types of processing techniques. Fluoropolymer resins include fluorinated perfluoroethylene-propylene copolymer, perfluoroalkoxy alkane, ethylenetetrafluoroethylene, polyvinylidene fluoride, chlorotrifluoroethylene, ethylene chlorotrifluoroethylene, polyvinyl fluoride, and other less important polymers.
1.14.15 Polyphenylene Sulfide (PPS) PPS is produced commercially by the reaction of 1,4-dichlorobenzene with a suitable sulfur source, such as sodium sulfide. PPS is a semi-crystalline, aromatic polymer composed structurally of a series of alternating para-substituted phenylene rings and divalent sulfide moieties. PPS is an engineering thermoplastic
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material with an excellent combination of properties: thermal stability and unusual insolubility, chemical resistance, and inherent flame resistance. PPS has good thermal stability, so the isothermal weight losses as a function of time were measured at several temperatures. These results indicate good resistance to thermal degradation up to 700°F (370°C). PPS possesses excellent solvent resistance, being almost totally insoluble in organic solvents below 390°F (198°C). PPS is affected only by high temperature exposure to a few organic solvents, strong mineral acids, and strong oxidizing environments. Oxidizing agents such as peracetic acid and aqueous sodium hypochlorite oxidize the sulfide moiety to sulfoxide and/or sulfone groups. Because of its chemical structure, PPS can char when exposed to an external flame, but PPS is inherently flame resistant. It possesses a high limiting oxygen index and a low radiant flame spread index and it is classified as UL 94-V0 and UL 94-5V, down to 0.058 in. (1.47 mm) wall thickness. The continuous service temperature classification is between 330°F and 438°F (165°C and 225°C) depending on the PPS compound formulations. The auto-ignition temperature is 1,000°F (537°C). Glass fiber reinforcement produces injection and compression molding compounds that exhibit high tensile strength, good flexural strength, high heat deflection temperature, low elongation, and very low impact strength. Both filled and unfilled PPS grades exhibit the inherent flame resistance and excellent chemical resistance characteristics of the base resin. Results of long-term heat aging of moldings are consistent with the good thermal stability of the polymer. Glass fiber and mineral fiber PPS compounds have good retention of tensile properties under long-term temperature and load exposure. PPS and its compounds possess good overall electrical (insulating) properties. The glass filled compound has a low dielectric constant and dissipation factors are retained over a broad frequency range. In addition, the dissipation factor remains low at temperatures up to 390°F (198°C). These properties are also retained after exposure to high humidity environments.
1.14.16 Polypropylene (PP) Polypropylene is manufactured by polymerizing propylene monomer with a titanium based catalyst; a
Selection of Polymeric Materials
co-catalyst (triethylaluminum) is added to initiate the polymerization reaction and hydrogen is used in the reactor to control polymer molecular weight. This reaction is produced using a slurry or gas phase type of process. Polypropylene is the fastest growing commodity thermoplastic in the world. It is a versatile polymer used in a wide range of applications from fibers, flat filaments used in industrial bags and outdoor carpets, packaging films, home appliances, electrical and medical devices to automobile bumpers, air management systems, interior panel and under-the-hood components. There are three polypropylene structures produced during polymerization: isotactic, syndiotactic, and atactic. The principal structure of polypropylene is isotactic, which is a semi-crystalline polymer in a helical form. This structure has good mechanical properties that can be further improved with fiberglass reinforcements and mineral fillers (talc, calcium carbonate). Syndiotactic polypropylene is produced by the monomer units inserted alternately head-to-tail. This structure is more flexible with better impact resistance and clarity than the isotactic structure. Atactic polypropylene (hard wax amorphous monomer) is a by-product of the manufacturing process. The atactic monomer affects the process efficiency. This product is used in roofing tars and adhesives for the shoe industry. All forms of polypropylene are susceptible to oxidation due to the presence of a tertiary hydrogen. Polypropylene is stabilized against thermal degradation by the addition of primary and secondary antioxidants. Neutralizing agents are also added to stabilize the low levels of chloride ash generated during manufacturing. Other special additives are used such as antistatic agents, slip agents, and UV stabilizers. Polypropylene is produced as homopolymers and copolymers. The physical properties range from good tensile strength and stiffness to a tough, flexible, and low strength polymer. Polypropylene homopolymer has the highest melting point with a wide range of melt flow rates and stiffness. Polypropylene copolymers incorporate small amounts of ethylene which lower the crystallinity rate, producing higher impact strength even at low temperatures, and more flexibility, but a lower melting point and low melt mass-flow rate properties.
1: Polymeric Materials and Properties
1.14.17 Polystyrene (PS) A polymerization process using a thermal or catalyzed reaction of styrene monomer is used to produce an amorphous polystyrene polymer. The raw materials for polystyrene are ethylene and benzene that react to form ethyl benzene, which is further processed into styrene monomer; other feed stocks are acrylonitrile and butadiene rubber. Other additives such as plasticizers, release agents, and stabilizers are added to the process to give the polymers the desired characteristics. The formulation may also include colorants, flame retardants, UV stabilizers, and impact modifiers. Polystyrene is one of the most popular commodity amorphous thermoplastic resins. It has a broad range of balanced properties and an attractive price. Polystyrene is produced in the following grades: general purpose (GPPS), which is selected for its clarity, rigidity, and suitability for many applications; rubber modified medium impact (MIPS), which is selected where more flexibility is required; rubber modified high impact (HIPS), which is selected where high impact resistance is required; and expandable polystyrene (EPS), which is selected where insulation is required. MIPS and HIPS contain butadiene rubber as the comonomer to increase toughness. However, these impact resistance grades are opaque in color.
1.14.18 Polysulfone (PSU) PSU is produced from bisphenol A and 4,4-dichlorodiphenylsulfone by a nucleophilic process. PSU is an amorphous engineering thermoplastic material with exceptionally high temperature resistance and high rigidity; it is transparent and dimensionally stable. It exhibits a significant reduction in notched Izod impact strength. Its continuous service temperature is 320°F (160°C) with a glass transition temperature of 374°F (190°C) and it is classified as UL 94-V0, down to 0.062 in. (1.57 mm) wall thickness. PSU has been approved for use in several medical and laboratories applications. It exhibits good chemical resistance, but is sensitive to polar solvents and other solutions. PSU complies with the gamma sterilizability requirements. The melt viscosity of PSU is relatively insensitive to shear forces. The low degree of molecular orientation during injection molding affects the physical properties of the molded products slightly, independent of the melt flow direction.
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PSU can be plated by an electrolytic process which imparts added strength to the polymer. Both nickel and copper electroless processes can be used. All molded parts must be annealed before plating. PSUs can also be vacuum-metallized using standard equipment and techniques.
1.14.19 Polyvinyl Chloride (PVC) PVC is produced as a white powder containing coarse, porous particles. Several additives are used during processing to give the polymers the desired characteristics. The two major categories of PVC compounds available are: the homopolymer suspension powder blend and the dispersion materials. PVC suspension compounds account for more than 90% of the total PVC market. Suspension compounds can be produced as either rigid thermoplastics for nonplasticized applications or flexible thermoplastics for plasticized applications. The suspension ingredients may be blended using two types of mixers: the high intensity mixer and the ribbon blender. High intensity blenders and twin screw extruders are used for producing the rigid compounds. Ribbon blenders and single screw extruders are used for producing the flexible compounds. PVC dispersion compounds account for 7% of the total market. They have a fine particle size of about 1 µm. When dispersed in plasticizers and other liquid ingredients, they form plastisols and organosols that are applied as liquid, paste, coatings, and fused with heat. All PVC compounds require heat stabilizers to allow processing without degrading and discoloring the polymer. Plasticizers are added to increase the flexibility of the compound. They can also improve the heat stability or improve the flame retardancy of the compound. Fillers are used to reduce the cost, improve dimensional stability, stiffness, and impact strength. PVC is a recyclable commodity thermoplastic material of large consumption by the building and construction industry. PVC is popular because of its excellent impact, wear, chemical, and UV resistance. PVC is used in a large variety of end products such as flooring, garage doors, windows frames and profiles, siding, tubing, and connectors. These products are commonly available in standard sizes and shapes, low cost, and easy to work with (weld, repair, and paint).
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1.15 Families of Thermoplastic Elastomers (TPEs) TPEs combine the properties of thermoplastic and vulcanized rubber. TPEs are rubbery materials with the characteristics of a thermoplastic and the performance of a thermoset vulcanized rubber. The properties of TPEs are based on their chemical structure and the morphology of their matrix. In thermoset rubbers, carbon black is used as a filler to modify the hardness or stiffness of the rubber materials. In TPEs, the molecular structure of the polymer itself provides the flexibility, impact strength, and stiffness. Sometimes, it is necessary to modify the ramifications of the soft segments of the structure’s molecular chain to obtain the required properties (stiffness, toughness).
1.15.1 Thermoplastic Polyurethane (TPU) Elastomer Polyurethane chemistry is based on the reaction of isocyanates with various active hydrogen elements present in the compounds. The basic ingredients of TPUs are diisocyanates and long-chain and shortchain diols. The diisocyanates and short-chain diols form the basis of the hard segment structure, while the long-chain diols provide the basis of the soft segments. Because the hard and soft segments do not mix, TPU exhibits a two-phase structure. TPUs consist of an amorphous phase (soft block) and a crystalline phase (hard block). The hard segments are generally dispersed in the amorphous phase (continuous phase). The hard segments determine the hardness, stiffness, impact or tear strength, and upper end-use temperature. The soft segments determine the elastic characteristics and lower end-use temperature properties. TPUs can be categorized into polyester and polyether types. Polyester-based TPUs generally have better physical properties such as thermo-oxidative stability and oil resistance. At a similar hardness, polyether-based TPUs exhibit better low temperature properties such as hydrolytic stability and resistance to microbial attack. TPUs are known for their excellent resistance to abrasion. However, the abrasive wear of a TPU is considerably affected by the surface heat buildup during the test, coefficient of friction, stress loading, and contact areas. The compression set property of a polymer is a measure of its elastic recovery behavior under a
Selection of Polymeric Materials
specific load, at various times and temperatures. Typical compression set values of annealed TPU range between 25% and 50% of recovery to original position after load is removed, and between 60% and 80% of recovery without annealing. A compression set value under 25% is obtained with a deflection at 158°F (70°C) and curing for 22 hours. The thermal stability of TPU is dependent on the isocyanates structures and chain extenders. TPUs decompose slowly between 398°F (203°C) and 482°F (250°C). They exhibit a loss of mechanical properties and discoloration upon exposure to sunlight. The hydrophilic property of TPU has prevented its use in applications where consistently high electrical insulation resistance is required.
1.15.2 Styrenic Block Copolymer (SBS) Thermoplastic Elastomers Styrenic block copolymer TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization. At least one phase is a styrenic polymer that is hard at room temperature, but becomes fluid when the polymer is heated. Whereas another phase is a softer material that is rubber-like at room temperature. These polymers have many of the physical properties of vulcanized rubbers (softness, flexibility, resilience), and process characteristics and properties similar to the thermoplastics because of their molecular structures. Most of the polymer molecules have their end polystyrene segments in different domains. At room temperature, these polystyrene domains are hard and act as cross-links, tying the elastomeric chains together in a three-dimensional network. But in styrenic block copolymer TPEs the domains lose their strength when the material is heated or dissolved in solvents, allowing the polymer to flow. When the melt is cooled or the solvent is evaporated, the domains become hard again and the network regains its original integrity. These materials must be protected against oxidative degradation and in some cases against sunlight also, depending on their end use. Many types of styrenic block copolymer TPEs have been produced to meet specific end- use requirements. General purpose styrenic block copolymers, which are soluble when compounded under improved stability, are used for wire and cable coatings and medical applications.
1: Polymeric Materials and Properties
Like most thermoset vulcanized rubbers, the styrenic block copolymer TPEs have no commercial applications when the product is just a pure polymer. Depending on the particular requirements of each end use, they are compounded with other polymers, oils, fillers, and additives. In almost all cases, the products contain less than 50% of the styrenic block copolymer.
1.15.3 Polyolefin Thermoplastic Elastomer (TPO) TPO materials are defined as compounds of various polyolefin polymers, semi-crystalline thermoplastics, and amorphous elastomers. The most common types of TPOs are composed of polypropylene and ethylene-propylene rubber (EPR). EPR may be a copolymer of either only ethylene and propylene monomers or a third monomer. The diene monomer provides a small amount of unsaturation in the polymer chain for sulfur cross-linking. This is called EPDM rubber. Like most thermoplastic elastomers, TPO products are composed of hard and soft segments. The exact size and shape of these segments determines the properties of the compounds. Higher impact strength at low temperature is achieved by increasing the amount of ethylene present in the copolymer. Ethylene also reduces the rigidity of the copolymer. The soft segments of the polymer chain are composed of EPR or EPDM rubber. Rubber materials with nearly equal amounts of ethylene and propylene are totally amorphous. The softest EPDM rubber grades are the most efficient impact modifier additives used in TPOs. Some of the other additives used in TPO compounds function by modifying the rubber phase. These ingredients may include fillers, reinforcements, lubricants, heat stabilizers, antioxidants, UV stabilizers, colorants, and processing aids. All TPO compounds are unaffected by water and exhibit fair chemical resistance to acids and bases. Hot hydrocarbon solvents tend to soften and swell severely for the softer formulations and slightly for the harder TPO products. The chemically inactive surface of a TPO molded product makes bonding to other materials difficult. Most TPO compounds are good electrical insulating materials. They have good dielectric strength properties and they do not absorb moisture, because
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they are not hygroscopic. However, when a TPO product is subjected to weather (UV) and pollution, special TPO formulations with several UV and heat stabilizers must be selected for the application (higher cost). Many TPO parts that are used in automotive applications (bumpers) must be painted with an automotive finish to match or accent the painted finish of the other body panels. Special paint systems are available that will give excellent adhesion of the primer and paint to the TPO surface. The TPO compound materials do not have a molded surface that chemically reacts with the primer paint system to develop a strong and durable bond. To achieve adequate adhesion of the primer, the TPO molded surface must be modified (plasma, flame, electric treatment) to obtain a reactive surface.
1.15.4 Elastomeric AlloyThermoplastic Vulcanizate (EA-TPV) Vulcanized elastomeric alloys are thermoplastic elastomers composed of mixtures of two or more polymers that have received a proprietary treatment. EA-TPVs are a category of TPEs made of a rubber and plastic polymer mixture in which the rubber phase is highly vulcanized. The plastic phase of an EA-TPV is a polypropylene, and the rubber phase is an ethylene-propylene elastomer. The vulcanization of the rubber phase of an EATPV results in various property improvements. EATPVs are insoluble in rubber solvents and exhibits less swelling in some solvents than TPO. The vulcanization offers several improvements in the properties: increase in tensile strength and modulus, decrease in compression set, and decrease in swelling caused by oils. Vulcanization of the rubber phase also improves the retention of properties at temperatures below 200°F (93°C). The tear strength and abrasion resistance of EA-TPVs is good but not outstanding—the tear strength increases progressively with hardness. The desirability of low or high resilience (hysteresis) will depend on the end use. An outstanding dynamic property of EA-TPVs is their fatigue resistance. EA-TPVs have excellent electrical insulation properties, and excellent dielectric strength 400 volts/mil at a thickness of 0.080 in. (2.0 mm). The volume and surface resistivity of EPDM-TPVs are sufficiently high to justify their consideration for
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use as primary electrical insulators as well as jacketing material. The dielectric constant (specific inductive capacitance) and power factors are basic limits in the selection of a primary electrical insulation material.
1.15.5 Melt Processible Rubber (MPR) MPR is an amorphous polymer, providing a very low flexural modulus, good tensile strength, and linearly proportional stress-strain curve. Halogenated polyolefin polymers are intermixed with a large number of structurally coupled ethylene interpolymers. They contain functional groups that are strongly proton-accepting to promote hydrogen bonding with the alpha hydrogen atoms of the halogenated polyolefin over the entire composition range. MPR alloys are composed of proprietary ethylene interpolymers and chlorinated polyolefins, in which the ethylene interpolymer component has been partially cross-linked. Plasticizers, stabilizers, antioxidants, curing agents, and fillers can be incorporated into the resin to provide flexibility and reinforcement. MPR polymer blends are single-phase systems because they have a single glass transition temperature. As an amorphous material, MPR exhibits no crystalline melting point. It gradually softens with increasing temperature, but does not flow at any temperature unless also subjected to shear. By increasing the shear rate of the MPR, the melt viscosity property of the polymer increases proportionally. The polymer melt only becomes softer, but not a liquid melt. MPR is a thermoplastic elastomer and not a thermoset rubber material. The MPR resin produces parts that look, feel, and perform like vulcanized rubber. The MPR resin has been recognized as the most rubber-like product among all the TPE families. MPR resins can be processed in equipment used for thermoplastics or thermosets. It also offers the combination of resistance to heat, oil, chemicals, and weathering. Because of its rheology and its amorphous singlephase nature, MPR is obviously not a plastic. It has a fractional melt index, does not melt, and exhibits thixotropic behavior. It has a useful plastic processing property in flowing like a thermoplastic on application of shear. Unlike plastics, it displays minimum draw-down because of elastic recovery, at temperatures under 200°F (93°C).
Selection of Polymeric Materials
MPR has outstanding oil resistance, heat aging, weatherability, and is recyclable. It has mechanical properties, tear and abrasion resistance, and a continuous surface temperature similar to mid-performance rubbers. MPR is used as an energy absorbing polymer, but has limitations in applications requiring very dynamic motion and load carrying capabilities.
1.15.6 Block Copolyester Thermoplastic Elastomer Block copolyester TPEs are based on 1,4-butanediol, terephthalic acid, and polytetramethylene ether glycol or polypropylene glycol. Because the building blocks behave like high tensile strength engineering plastics and elastomers, the product combines these characteristics in proportion to the ratio of hard and soft molecular segments. The fast crystallization rate of the hard segment, combined with exceptional melt stability of the backbone, allows these polymers to be processed like a thermoplastic material. Block copolyester TPEs have an excellent combination of mechanical properties such as tensile strength, elasticity, creep resistance, and dynamic properties. These polymers are operational over a broad service temperature range without significant changes in properties. They also have excellent chemical, heat, and oil resistance properties. A distinguishing characteristic of block copolyester TPE resins versus other flexible materials is their excellent dynamic performance, which makes these resins suitable for applications requiring long-term spring properties and flex life. Operating within their elastic limits, block copolyester TPEs are creep resistant, supporting loads for a long time without stress relaxation. They can be subjected to repeated cycles of tension and compression without significant loss of mechanical strength. Block copolyester TPEs demonstrate low hysteresis loss in dynamic applications. Product components working at low strain levels exhibit complete recovery in cyclic applications with little heat buildup. Resistance to cut growth during flexing is outstanding, mainly because of high resilience and low heat buildup. Block copolyester TPEs rank high among thermoplastics for impact resistance; most of the compounded grades cannot be broken by conventional notched Izod impact testing equipment. Block copolyester TPEs are used in electrical applications below 600 volts. The following characteristics make them attractive for electrical and
1: Polymeric Materials and Properties
electronic applications: good dielectric properties, high mechanical strength, creep resistance, spring properties, flex fatigue resistance, high impact strength, abrasion resistance, and end-use service temperatures ranging between –40°F and 300°F (–40°C to 148°C). Block copolyester TPE compounds have excellent oil resistance; the stiffer grades offer the best performance in hot hydrocarbon environments, with many resins being suitable for use with hot oil, grease, fuels, and hydraulic fluids. Block copolyester TPE compounds do not contain plasticizer additives in their formulations. The inherent chemical purity of block copolyester TPEs makes them an excellent choice for food contact and medical applications.
1.16 Families of Thermoset Polymers Thermoset compounds are organic polymers that cure to a solid and infusible mass by forming an irreversible three-dimensional network of covalent chemical bonds. Thermoset compounds are polymers with a combination of mechanical, thermal, electrical, and chemical resistance properties that allow them to compete with metals, ceramic, and thermoplastic materials. Thermoset compounds are used in many applications. Construction represents the largest single market area, consuming about half of the compounds produced. Other applications include adhesives for plywood and particle board, binders for insulation, coatings, matrix resins for laminates, and electrical molding products. Because they lack the strength and stiffness of metals, nearly all thermoset compounds contain particulates or fibrous reinforcements. Fillers such as calcium carbonate, glass flakes, and wood flour are added to reduce cost and increase the rigidity of the cured product. Fibers such as glass, carbon, and polyaramid increase its strength, stiffness, and cost. The amount of fillers compounded ranges between 45% and 75%.
1.16.1 Polyester Alkyd (PAK) Polyester alkyds are formed by the reaction of polyhydroxy compounds with unsaturated maleic
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acids. The resulting unsaturation in the polymer backbone can be utilized to cross-link the polymer and form a thermoset compound. A small percentage of high viscosity monomer compounded in the resin yields a relatively high melt flow rate, necessary for large and complex parts. This benefit, together with excellent electrical properties, dimensional stability, and low cost, makes it suitable for a variety of applications in the automotive and electrical industries. Polyester alkyd thermoset compounds are used for injection, transfer, and compression molding. Polyester alkyd compounds can be cured without additional molding pressure and do not release water when cured. Therefore, they can be used in a variety of coating applications. Polyester alkyds are primarily electrical materials. They combine good insulating properties at temperatures up to 300°F (148°C) for intermittent use, and up to 250°F (120°C) for continuous use. They also offer low resin cost and good insert molding characteristics for delicate and complex applications. Polyester alkyd compounds are also reinforced with fiberglass and minerals to provide substantial improvements in physical strength and impact resistance, and facilitate the molding process.
1.16.2 Diallyl Phthalate/ Isophthalate (DAP, DAIP) Diallyl phthalate resins are products of the reaction of allyl alcohol and an organic acid or anhydride. The monomer diallyl phthalate can be prepared by direct esterification of allyl alcohol and phthalic anhydride. The solution is then partially polymerized to a fusible resin or prepolymer by heating with a free radical initiator. This diallyl phthalate prepolymer, combined with a free radical initiator and various fillers, constitutes a diallyl phthalate molding compound. There are two molding compound types of diallyl phthalate resins: the orthoresin diallyl phthalate and the metaresin diallyl isophthalate. The orthoresin is the most commonly used; it provides excellent electrical properties, while the metaresin provides superior heat resistance characteristics. Diallyl phthalate offers a balance of electrical insulating properties, volume resistivity, dielectric strength, and arc resistance. It retains these properties even under long-term exposure to high heat and humidity. The most frequently used compounds are short glass fiber reinforced, which represent approximately
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70% of the market. The remaining 30% is divided fairly evenly between long glass fiber reinforced and mineral filled compounds. Diallyl phthalate molding compounds are available only as a filled system and are supplied complete with catalyst, pigment, and mold release. Its physical form varies with the type of reinforcement used. The mechanical properties of diallyl phthalate vary widely, depending on filler or reinforcement type and quantity. Because of its extremely stable carbon-tocarbon linkage and its tight knit three-dimensional structures, fully cured diallyl phthalate is extremely resistant to creep or cold flow, moisture, strong and weak acids, alkalis, and organic solvents. Moisture has little effect on the dielectric strength of the molded parts. Diallyl isophthalate has better heat resistance than diallyl phthalate, with use temperatures approximately 20°F higher than the latter.
1.16.3 Epoxy (EP) Epoxy is a thermoset plastic that has an epoxy ring consisting of two carbon atoms that are single-bonded to a common oxygen atom. There are two types of epoxies. Those polymers made by a reaction with epichlorohydrin are known as glycidyls, while those made by peroxidizing olefins are known as cycloaliphatics. Epoxidized phenols, or phenol glycidyl ethers, are the most commercially important resins, particularly epoxidized bisphenol A, known as the diglycidyl ether of bisphenol A (DGEBA). This resin provides an excellent balance of physical, chemical, and electrical properties. Epoxidized phenol novolacs are also available in a range of viscosities. Epoxies are used in combination with a coreactant, or curing agent. Therefore, the final composite properties are influenced by the choice of coreactant. The large number of coreactants available include amines, anhydrides, acids, phenolics, and amides, thereby providing a broad spectrum of performance possibilities. After aromatic glycidyl ethers, the epoxidized alcohols, glycols and polyols, or aliphatic glycidyl ethers are considered most valuable. They are typically used in combination with DGEBA resin to allow better processing. These materials are often very low in viscosity, which makes them highly attractive as dilutents for the more viscous phenolic base products. They can also improve flexibility and toughness properties, although this is generally at the
Selection of Polymeric Materials
expense of thermal and chemical resistance, particularly if the dilutent is monofunctional. The most common members of this group are epoxidized butanol, known as butyl glycidyl ether, and the epoxidized long chain monoalcohols. Physical properties can be varied over a wide range of rigidity and flexibility. This group of resins created the structure for the adhesive industry, as well as being used extensively in the fiber reinforced composite industries. Performance properties of epoxy composites have ratings from good to excellent for electrical resistance, good chemical resistance, and reasonably high glass transition temperatures. The use of fillers can often raise the glass transition temperature, reduce shrinkage, and increase the thermal conductivity and thermal resistance of the composite. The amount of filler used depends on the rheology of the system, the particle size, and the oil absorption tendency of the particular filler. A major weakness of epoxy resins is poor UV resistance and weathering. Processing of high performance composites is quite complex, particularly if a multiple-step cure schedule is required to achieve the highest possible thermal resistance in the composite structure. Epoxies used in composites are cured by an addition crosslinking mechanism, which does not generate volatile by-products. Part dimensions can be influenced by the cure, particularly if the cure temperature is too high, because a very vigorous exotherm can be generated, causing excessive shrinkage and changing the composite structure.
1.16.4 Phenol-Formaldehyde (Phenolic, PF) Phenolic resins are products of the condensation reaction of phenol and formaldehyde. Water is the by-product of this reaction. Substituted phenols and higher aldehydes may be incorporated to achieve specific properties such as reactivity and flexibility. Phenolic molding materials are high performance thermoset compounds, and are considered to be the workhorse of the plastics industry. With the heat and pressure of the molding process, phenolics react to form a three-dimensionally cross-linked molecular structure. This structure yields excellent dimensional and thermal stability with high load-bearing capability at elevated temperatures. Phenolics are used for
1: Polymeric Materials and Properties
close tolerance precision molded components that must function in hostile environments. The unreinforced phenolic polymer is a brittle material. However, a wide range of properties and molding process conditions can be obtained by using a variety of fillers and reinforcements (45%–65%). Lubricants, colorants, and other modifiers are also used. Wood flour or cellulosic filler yields a molding material with well-balanced properties and cost effectiveness. It has a continuous service temperature of 300°F (148°C). Modification with mineral fillers up to 45% yields rigidity with improved dimensional and thermal stability characteristics. Reduced water absorption and a lower coefficient of thermal expansion can also be obtained. Glass fiber reinforcement yields substantial improvement in dimensional stability, rigidity, and mechanical properties. Glass fiber reinforcement can be tailored to match the thermal expansion of metals. The continuous service temperature is 355°F (179°C). Specialty grades can be formulated with graphite, polytetrafluoroethylene, and impact modifiers to improve toughness and reduce coefficient of friction. Phenolic materials are available in black and brown colors. Phenolics are not stable under UV radiation. They are inert to most common solvents and weak acids and they have excellent resistance to natural oils, fats, greases, petroleum products, and automotive fluids, but poor chemical resistance to strong acids and alkaline solvents.
1.16.5 Unsaturated Polyester (UP) Unsaturated polyester polymers are manufactured by the condensation and polymerization of dibasic acids (or anhydrides) with dihydric alcohols, with the dibasic acid or anhydride being partially or completely composed of a 1,2-ethylenically unsaturated material such as maleic anhydride or fumaric acid. The resultant polymer can vary from a high viscosity liquid (brittle) to a low melt solid material. The polymer is then dissolved in a liquid reactive vinyl (1,2-ethylenically unsaturated) monomer, such as styrene, vinyl toluene, diallyl phthalate, or methyl methacrylate, to give a solution with a viscosity in the range between 2.0 and 20.0 poise. Unsaturated polyester reinforced with fiberglass was developed to produce protective housings for
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radar equipment. It is used in a wide variety of markets, including automotive, construction, marine, transportation, industrial, electrical, and sanitary, and as replacements for wood, concrete, marble, steel, and aluminum. These polymers are mostly chosen because of their ease of fabrication, lower weight, higher strength, corrosion resistance, and lower cost. However, they have pronounced styrenic odors. The use of a reinforcing fiber to produce unsaturated polyester composites dramatically improves the tensile and flexural properties. Inorganic fillers are used to improve stiffness and to reduce cost. The rigid and high glass transition temperature for bisphenaol A fumarate or chlorendic polyesters retains flexural strength up to 250°F (121°C). They are used in elevated temperature electrical and corrosion resistant applications. Chlorendics are used in strong acids at high temperatures, while bisphenaol A fumarate is better in strong basic solutions. However, isophthalic polyesters are more widely used because they have better thermal and chemical resistance, higher mechanical properties, and are easier to process.
1.16.6 Polyimide (PI) Polyimides are members of a class referred to as heteroaromatics. They have excellent mechanical strength properties (seven times stronger than steel) and excellent electrical and thermal insulation properties; the high-temperature specialty grades have a temperature resistance ranging between –270°F and 740°F (–168°C and 393°C) for extended exposure times, and up to 900°F (482°C) short-term. Polyimides resist higher temperatures better than any other unfilled polymer. They are produced either in cross-linked molecular structures as thermosets or in linear forms as thermoplastics. Their high glass transition temperatures require special processing methods. Several approaches have been used to allow the thermosetting reaction to proceed by addition polymerization. Polyimides are suitable for high end-use temperatures; they exhibit excellent fire, chemical, and solvent resistance; low coefficient of friction; low wear, abrasion, thermal, and creep resistance. The presence of the imide ring in the structure causes some hydrolytic instability, particularly towards alkalis. Polyimides are difficult to process, therefore, DuPont manufactures the basic polyimide polymer,
38
transforms the polyimide powder into a film form (the film has the trade name Kepton®) for the electrical industry, and laminates the film with a fluoropolymer. DuPont also processes the powder into a fiber form known as Kevlar®. Polyimide powder compounded with low coefficient of friction additives is known as Vespel® and it is transformed into standard solid bars, plates, balls, and rods. Vespel® is also produced as custom finished parts.
1.16.7 Polyurethane (PUR) The general structure of polyurethane is R–(NCO)n, where typically n = 2–4, or even higher, and R is an aromatic or aliphatic group. The isocyanate group (R–N=C=O) reacts easily with hydroxyl groups (HO-R'). The resulting link between the two residues R and R' is the urethane group. Polyurethane thermosetting resins are used in coatings, adhesives, binders, sizings, flexible foams, fibers, sealants, and biomedical and external automotive structural applications. The production of filled polyurethanes by the reaction injection molding (RIM) process requires premixing the filler with the liquid components (polyol). Milled glass fibers are very common and can be used in concentrations up to 75% in the polyol. The effect of fillers on mechanical properties of the final foams follows the same order as observed in their influence on the viscosity of the raw materials. Flake materials have a more favorable effect on these properties and improve the dimensional stability at higher temperatures. With 20% mica, for example, the Young’s modulus can be increased by a factor of 3, but tensile strength and elongation at break are reduced. An example of reinforced polyurethane is the product of the structural RIM (SRIM) process. SRIM compounds are composites that contain random, unidirectional, or multidirectional glass fiber mats. These mats are put into the mold which is then closed and filled with the reactive mixture of raw materials. Because of its extremely low viscosity, the resin penetrates the reinforcing mat and solidifies the structure. This procedure allows the use of glass fiber contents of up to 70%.
1.16.8 Silicone (Si) The structure of silicone resembles that of silicon dioxide (SiO2) glass, which provides high temperature
Selection of Polymeric Materials
properties and resistance to radiation, ozone, and chemicals, and is different from that of organic polymers. Specifically, the polymer chain backbone consists of repeat units, whereas the polymer chain backbone of organic polymers is some configuration of repeat units. A SiO2 bond is stronger and more flexible than a C–C or C=C bond, which makes silicone polymers chemically stronger and more flexible than organic polymers. Silicone polymers resemble the three-dimensional network structures of sand (SiO2). The organic functionality of silicone allows light cross-linking between polymer chains, creating rubber elasticity. The outstanding characteristics of silicone include its ability to maintain properties at both low (down to −150°F (−101°C)) and high (up to 600°F (315°C)) temperatures, and its weathering and chemical resistance, physiological inertness, lubricity, excellent electrical properties, low surface tension, and compression set resistance.
1.16.9 Vinyl Ester (BPA) Vinyl ester resins are unsaturated esters of epoxy compounds. The most common versions are the reaction products of methacrylic acid and bisphenol A, an epoxy compound dissolved in a styrene monomer. Corrosion resistant reinforced resins based on vinyl esters and other compounds have successfully replaced traditional materials such as glass, steel, aluminum, concrete, and brick. Vinyl ester resins have many properties of epoxies combined with the processibility of polyesters. The addition of the methacrylate group allows vinyl esters to be cured in ways similar to the curing of unsaturated polyester compounds. The use of styrene or other reactive monomers allows low viscosities to be obtained at ambient temperature. Because of curing and material handling similarities, vinyl esters are often classified with unsaturated polyesters. One of the advantages of vinyl ester resins reinforced with fiber materials is that the reinforcement can be oriented in the direction requiring strength. The properties of vinyl ester resins are fatigue resistance, retention of properties at moderate temperatures, impact resistance, and creep resistance. Their excellent corrosion resistance is attributed to three basic factors: (1) the corrosion susceptible ester linkage is shielded by a methyl group;
1: Polymeric Materials and Properties
(2) the vinyl groups are very reactive and a complete cure of the backbone is easily accomplished; (3) the epoxy backbone is very resistant to chemical attacks. Processing can be based on either ambient temperature or elevated temperature cure systems. Vinyl ester resins are cured by peroxide initiated free-radical polymerization of the reactive unsaturations of
39
styrene and the methacrylate groups. Using these ambient temperature cure systems and additives gives vinyl ester resins a wide range of gel times. Gel times from less than one minute to over three hours are possible. Some of the processes used to fabricate parts with vinyl ester resins at ambient temperature include open molding techniques such as hands lay-up and sprayup, filament winding, and centrifugal casting.
metals do not have the elasticity, fatigue endurance, wear resistance, and toughness of rubber or thermoplastic elastomers (TPEs). Metals are not used for light-weight and compact cellular phone housings because metals are electrical conductors, heavier, corrodible, and expensive. Automotive cast iron engines and aluminum intake manifolds are being replaced by fiberglass reinforced nylon to lower the weight and improve the engine efficiency. These substitutions require new manufacturing processes but present opportunities for cost reduction. Automotive bumpers, external side panels, doors, fenders, and hoods made from steel are being replaced by plastics such as thermoplastic elastomer olefins (TPOs), thermoset composites, and polycarbonate alloys. These tough plastic materials have reduced weight and lower manufacturing costs, while providing added freedom for design and styling. Portable electric tools and small kitchen appliance housings are no longer made of die cast iron, steel, or aluminum, but have been replaced with nylon and acrylonitrile-butadiene-styrene (ABS). These plastics improve the toughness, electrical insulation, design flexibility, and styling; they also weigh less, and provide efficient molding at lower costs. Water faucet valves made of die cast steel, brass, or copper are being replaced by new designs, updated styles, and colors, using acetal resins, which eliminates corrosion, reduces cost and presents new market opportunities. Large irrigation valves for high performance applications (1.50–3.0 in. diameter) and small valves (0.75 and 1.00 in. diameter) used to be made of die cast steel and brass. The large valves have been successfully replaced with glass reinforced nylon 6/12 and the small valves with glass reinforced nylon 6/6 or acetal. This improved performance and reliability eliminates corrosion and allows cost reduction. Other low performance commercial valves made of rigid polyvinyl chloride (PVC) (lower cost) have also been produced for the irrigation market. Toilet anti-siphon valves made of several brass and copper components have been replaced with a multifunctional design in acetal resin that improves performance, eliminates corrosion, and reduces cost. 1
2
Selection of Polymeric Materials
The valves made from acetal resin have shown excellent performance over a 30-year period. The comparison of properties for material selection is an effective tool when applied to materials in the same family group. However, comparing properties between different material families is not recommended. The characteristics and process requirements for each material are developed for a particular need of the application, and the properties are obtained using test methods and equipment specific for each family of materials. Figure 1.1 shows eight graphs using various generic property values to compare the strengths and weaknesses between families of materials. The ferrous metal families include cast iron, cold rolled steel, structural steel, alloy steel, stainless steel, and tool steel. The nonferrous metal families include magnesium, aluminum, copper, nickel, brass alloy, and titanium.
The thermoplastic families include ABS, acrylic, acetal, nylon, liquid crystal polymers (LCPs), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polystyrene (PS), polyethylene (PE), polypropylene (PP), polycarbonate (PC), polyphenylene oxide (PPO), polyetherimide (PEI), polyetherketone ketone (PEKK), polysulfone (PSU), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and styrene-acrylonitrile (SAN). The thermoset families include phenolics, silicones, alkyds, diallyl phthalate, polyimides, aminos, unsaturated polyesters, epoxies, and urethanes. The rubber families include butadiene, butyl, chloroprene, nitrile, silicone, urethane, ethylene-propylene diene monomer (EPDM), ethylene-propylene monomer (EPM), fluorinated elastomer, and natural rubber.
Ferrous Metals Nonferrous Metals
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 0
2
4
6
8
10
-460 0
Specific Gravity
Ferrous Metals Nonferrous Metals
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 0
50 100 150 200 250
0
Tensile Stress (kpsi)
Ferrous Metals Nonferrous Metals
Thermosets Thermoplastics
Thermosets Thermoplastics 40 80 120 160 200
0
Coefficient of Thermal Expansion (in/in/°F) x 10–6 Rubber Mica Laminations Glass Laminations Thermosets Thermoplastics
2,000
5 10 15 20 25 30
Modulus of Elasticity (Mpsi)
Ferrous Metals Nonferrous Metals
0
500 1,000
Service Temp. (°F)
0.5 1.0 1.5 2.0 2.5
Thermal Conductivity (BTU/hr/ft2/°F/in) x 103 Rubber Mica Laminations Glass Laminations Thermosets Thermoplastics
106 107 108 10111015101610171018
0
Electrical Volume Resistivity (Ohm-cm)
Dielectric Strength (volt/0.001 in) x 103
Figure 1.1. Comparison of properties between families of materials.
1.0
2.0
3.0
4.0
1: Polymeric Materials and Properties
The specific gravity graph shows the unit weight of a material compared to water and reveals that metals are two to eight times heavier than plastics. On a strength-to-weight basis, plastics have a more favorable position, as indicated by the specific gravity graph. The cost of metals is much higher than plastics. The continuous exposure temperature graph shows that metals have a wider temperature range than plastics; metals can be used at colder and at elevated temperatures. The property and testing methods used for plastic materials are extremely important and critical for their classification and use temperature range. The tensile strength graph shows that metals are much stronger than plastics; metals resist higher forces when they are pulled apart, before catastrophic failure. Time and temperature affect the tensile strength and cold flow (creep) properties of plastics; they decrease with increasing temperature over a much smaller temperature range, and exposure time. The modulus of elasticity graph shows that metals have a higher resistance to deflection for short-term, intermittent, or continuous loading than plastics. Metals have better dimensional stability at elevated temperatures than plastics. Since plastics deflect more than metals under the same load, it is important that plastic products be loaded using different techniques. For plastics the load should be intermittent and well distributed in compression mode. The coefficient of linear thermal expansion graph shows that increasing the temperature causes higher dimensional changes for plastics than for metals when plastics and metals are used together in the same application and exposed to the same temperatures. The plastic parts expand more than the metal parts, thus requiring design modifications to compensate for the dimensional change in plastics. The thermal conductivity graph shows that metals are good conductors of heat, while plastics are excellent insulators. Despite their relatively low use temperature range, plastics may be superior to metals as high temperature heat shields for short exposures. A plastic part exposed to a radiant heat source may suffer surface degradation. However, this heat is not transmitted to the opposite surface as rapidly as in metals. The electrical volume resistivity graph compares only the insulation materials used in electrical applications because metals are conductors. The dielectric breakdown strength graph shows the voltage gradient at which electrical failure or breakdown occurs as a continuous arc; the higher the value, the better the material. Plastics have excellent electrical resistance properties, while metals are conductors.
3
1.1 Tensile Stress-Strain Comparison Graphs The tensile strength, strain, and modulus of elasticity are among the most important mechanical properties of a material. They represent the mechanical behavior of a material while exposed to a tensile force. As the tensile load is gradually increased at a constant cross head speed, the total elongation over the gauge length and the load are measured and recorded continuously at each increment of the load until the specimen breaks. The stress at a given point may be obtained by dividing the value of the tensile load at that point by the original cross-sectional area. The tensile strength properties are the most widely specified data used for plastic materials. Different types of materials are often compared based on the tensile stress-strain graph generated by the tensile test. Tensile stress, elongation, and tensile modulus of elasticity are developed by analyzing the test results, using the stress-strain graphs. The stress-strain curve for each type of material has its own shape because it represents the mechanical characteristics and behavior of the material. Therefore, it is difficult to compare the tensile strength properties of different types of materials, if the stressstrain curves have significantly different shapes. To illustrate this point, a comparison of the stress-strain curves for different families of generic materials to verify the tensile strength properties is used as a practical tool for the material selection task. Figure 1.2 represents the stress-strain curve for nonferrous alloys and cast iron—the original strain is large with low modulus, and the yield point and strength properties difficult to define. Figure 1.3 represents the stress-strain curve for a medium carbon steel—the proportional limit, yield point, elongation, and break are well defined; however, the original strain is very small with high modulus (very rigid). Figure 1.4 represents the stress-strain curve for an alloy steel—the original strain is very small with high modulus without a yield point. Figure 1.5 represents the stress-strain curve for a high carbon steel—similar to the previous case, except for its brittleness. Figure 1.6 represents the stress-strain curve for rubber elastomer materials—flexible, soft, high impact resistance and unbreakable, good rebound and compression set, and low tensile strength. Figure 1.7 represents the stress-strain curve for unreinforced resins such as PE, PP, TPE, acetal, and
4
B
Tensile Stress
Y
B Y
PL
Tensile Stress
Tensile Stress
Selection of Polymeric Materials
B PL
Strain, (%) Strain, (%)
Strain, (%)
Figure 1.4. Stress-strain curve for alloyed steel.
Tensile Stress
O
Figure 1.3. Stress-strain curve for medium carbon steel.
Tensile Stress
Figure 1.2. Stress-strain curve for nonferrous alloys and cast iron.
Tensile Stress
B
PL
Strain, (%) Strain, (%)
Figure 1.5. Stressstrain curve for high carbon steel.
Figure 1.6. Stress-strain curve for elastomers.
nylon (at 50% relative humidity). These materials have lower tensile strength, good impact resistance, poor temperature and creep resistance, and the stressstrain curve yields gradually until break. Tensile tests for these plastics were conducted at room temperature using American Society for Testing and Materials (ASTM) methods. Figure 1.8 represents the stress-strain curve for fiberglass reinforced nylon (at 50% relative humidity (RH)), polycarbonate, and other compounded polymers that have limited elongation and impact resistance and moderated tensile strength properties. The tensile tests for these plastics were conducted at room temperature using ASTM methods. Figure 1.9 represents the stress-strain curve for fiberglass reinforced resins such as PET, PBT, LCP, phenol-formaldehyde (PF), polyamide-imide (PAI), polyether-imide (PEI), polyaryletherketone (PAEK), and dry as molded nylon. These materials are brittle and usually break before yielding occurs. They have excellent tensile strength, dimension stability, and
Y
B
PL
Strain, (%)
Figure 1.7. Stress-strain curve for unfilled resins.
thermal and chemical resistance properties. The tensile tests for these plastics were conducted at room temperature using ASTM methods.
1.2 Property Data Information for Polymeric Materials The American Society for Testing and Materials and Underwriters Laboratories (UL) in cooperation with resin suppliers, product designers, and other institutions have developed the current ASTM test procedures employed for testing the properties of thermoplastic resins. These test results are used to produce the product data sheets of thermoplastic resins. Product design engineers who have knowledge of the polymer molding process characteristics and who understand the complexity and significance of the resin properties are able to develop a special product geometry that satisfies the injection molding process and the end-use service requirements. All these
Y B PL
5
Tensile Stress
Tensile Stress
1: Polymeric Materials and Properties
B
PL
Strain, (%)
Strain, (%)
Figure 1.8. Stress-strain curve for reinforced resins.
Figure 1.9. Stress-strain curve for brittle resins.
factors should be taken into account to select the ideal thermoplastic materials to be used for injection molding the new product. The product data sheet shows the important properties of a plastic material that have been developed or used for a specific application. This document is used in design studies to compare properties with other materials. However, it does not provide the information for understanding the fundamental behavior, the viscoelasticity, or temperature sensitivity of plastic materials. Plastic injection molding resins are developed and commercialized for a specific market and application. The product data sheet typically contains the properties tested in compliance with the ASTM, ISO, or other test procedures. All the product data sheets are different for each plastic material, because the resin is manufactured to comply with the specifications of the application, by providing the minimum number of tests required for that market. When more properties of the resin are needed for a new application, the resin supplier’s representative may be able to provide the additional data required for the design analysis of the new product that has not been published.
material property variations; and molding process problems. Generally, most resins have different performance characteristics that could create molding problems and/or part failures. When additives, such as flame retardants and stabilizers, are compounded into the resin, the characteristics of the matrix polymers are modified, sometimes with a loss of some of the properties. In addition, when fiberglass or minerals are added to increase the mechanical strength properties, processing becomes more difficult. When the rheology or viscosity of the compounded product increases, higher injection pressures and melt/mold temperatures are required to decrease the melt flow rate. The economics involved in plastic resin selection are complex, because the resin price is not usually the most important factor. When a plastic resin is used for injection molding a close dimensional tolerance end product, the following engineering requirements are essential in reducing manufacturing costs:
1.3 Material Selection Guidelines To select the best material for an application, first, compare the different properties and processing characteristics of several plastic resins that may meet the application requirements. Product designers must make their resin selection based on other important characteristics of the application such as product design geometry effects caused by weld lines; stress concentrations, fiberglass orientation, temperature, humidity and creep; problems caused by mold design, construction, quality, and plastic
• Part geometry needs to be designed to produce molded components with maximum productivity. • A precision mold must be designed and constructed for fast running cycles and molding the maximum number of cavities automatically. • It is important to select the best type, capacity, and running conditions of the injection molding machine. • Then, it is necessary to set up efficient injection molding process conditions for the resin being used. • It is also essential that the molding and maintenance organizations are well trained using an up-to-date technical training program provided by a qualified instructor. The cost of plastic material becomes increasingly significant for molding products with higher volume
6
and less critical end-use requirements. Resin cost represents a high percentage of the molded components’ finished cost. For example, common items such as business machine housings, plumbing (faucets, valves, tubing, and shower heads), kitchenware, and appliance components (refrigerator liners, washing machine impellers, and vacuum cleaner housings) are price sensitive products. For these applications, resin cost becomes a very competitive business aspect of manufacturing.
1.4 Polymeric Materials Specifications There are more than 26,000 grades of thermoplastic, thermoset, and elastomeric neat resins and compounds on the market. They have been developed by blending, alloying, and compounding the existing polymeric families. Very few new polymeric families have been introduced into the market in the last few years. Many compounders of plastics buy the basic polymers from primary manufacturers and enhance their performance by adding additives, modifiers, reinforcements, and fillers. There are nearly 300 domestic suppliers of plastic materials. Many companies offer everything from commodity plastics to high performance polymers. There is an overwhelming amount of product specifications available to describe the process characteristics and end-use performance that have been generated by the resin suppliers. However, the property data are not always completely accurate or reported in a uniform format, because the data have often been developed for a particular application. The data sheets can cause considerable confusion for new customers requiring other properties for their applications. Therefore, making a comparison among the available plastic compounds for the material selection during the development of a plastic product becomes a very difficult task. Fortunately, over the past several years the large multinational resin suppliers have made strides to unify the test methods and the reporting of plastic properties. The progress has been slow, but the results are encouraging. Plastic product designers should have the necessary tools to compete in a world market. To develop a plastic product, the designers have the freedom in the part design, yet this gives rise to the more complicated issue of which material will work best for the job. Among the thousands of commercially available
Selection of Polymeric Materials
plastic grades, how can one be sure that the most appropriate materials are being considered for the application? Several plastic materials should be selected as candidates at the beginning of a project. A prototype mold similar to the production mold should be constructed to produce prototype parts using the initial candidate materials. The prototype mold should also be used to develop the molding parameters (shrinkage, part dimension, cycle time, etc.) that are needed for commercial part production. The molded prototype parts using the selected resins should be tested under end-use conditions to assess the suitability of the product against the requirements. Many plastic product designers select materials by intuition. They rely on experience to determine the suitable material. For example, acetal resin has been the workhorse material for gears, bearings, springs, and housings; therefore, they are likely to consider the same acetal for a new application. This is a poor material selection technique and alternative materials should be considered. Perhaps ABS, polypropylene, or nylon, may do the job, thus improving product performance, producing better molding efficiency, and reducing manufacturing costs. Another common practice is to rely on the material selection assistance provided by a particular reliable resin supplier to help specify the material. This is the approach taken by a number of part manufacturers, where the objective is to reduce the number and types of plastic materials used and work closely with the resin supplier to control cost. However, this approach closes the doors for new opportunities and innovations that other suppliers may offer. There are two database categories of plastic property data sheets supplied by the resin suppliers: • Single-point database system • Functional, or multi-point database system
1.4.1 Single-Point Database Systems The single-point databases report properties that are a single value. For example, tensile stress at yield, modulus of elasticity, elongation at break, Izod impact, Vicat softening point, gard gloss, and other properties are reported as a single value. There are roughly one hundred different property values that may be reported for any grade of resin. This information is of great value for the material
1: Polymeric Materials and Properties
7
selection process. It is very important to know about the test methods, test conditions, and reported units; understand the significance and limitations of the tests; and determine whether the test results have any relationship with the properties required by the product end-use and design. The single-point property values reported by resin suppliers should be scrutinized very carefully. It is expensive to test all the available properties of a plastic material. Consequently, material suppliers test only the properties that are needed to meet the material requirements of the intended market. For example, commodity plastic materials (polypropylene, polyethylene, and polystyrene) are tested for only a very limited number of properties such as specific gravity, melt flow index, tensile stress, and modulus of elasticity. Table 1.1 shows the typical material property information for a commercial polystyrene resin. Engineering polymers (acetal, nylon, PET, LCP) are tested for a wide range of properties required by their target markets. The single-point values may not be representative of the requirements for an application, or specifications of the plastic product. Heat deflection temperature and tensile strength property values are often reported as a single-point value at room temperature. In actuality, the effects of type of loads, strain, creep,
humidity, and temperature play a big role in the properties, characteristics, and behavior of a plastic material and the molded product performance. Single-point database systems provide the only viable means of finding out what materials are available. Using these databases, material selection for non-critical applications is quite simple; thus it takes very little time. However, for a difficult and complex plastic product, a complete detailed analysis of the functional database system is necessary.
1.4.2 Functional (Multi-point) Database Systems Plastic material properties are intended to represent the characteristic behavior of a plastic material during processing and in a product end-use environment. If a plastic product is to be used in a demanding environment where temperature, humidity, chemical resistance, and creep are critical variables, it is important to match the environment requirements of the application with the plastic material’s tested properties. For these applications, more detailed information is necessary. Engineering polymer suppliers can provide detailed test results of mechanical, electrical, chemical resistance, flammability, weathering, and thermal properties over a range of temperatures, humidity, and
Table 1.1. Fina 625 Polystyrene, Property Data Sheet Properties
ASTM
Value (US)
Value (SI)
Test Conditions
D1238
13.0 (g/10 min)
13.0 (g/10 min)
Load 5.0 kg at 200°C
Tensile stress at yield
D638
3.70 (kpsi)
25.5 (Mpa)
at 73°F (23°C)
Tensile modulus
D638
310 (kpsi)
2,137 (Mpa)
at 73°F (23°C)
Elongation at break
D638
40.0 (%)
40.0 (%)
at 73°F (23°C)
Flexural stress at yield
D790
6.70 (kpsi)
46.18 (Mpa)
at 73°F (23°C)
Flexural modulus
D790
400 (kpsi)
2,757 (Mpa)
at 73°F (23°C)
D256
1.30 (ft-lb/in)
69.55 (J/m)
at 73°F (23°C)
D1525
196 (°F)
91.06 (°C)
D523
96.0 (%)
96.0 (%)
Physical Melt flow rate Mechanical
Toughness Izod impact (notched) Thermal Vicat softening point Optical Gard gloss
at 73°F (23°C)
8
Selection of Polymeric Materials
creep. If this functional information meets the end-use requirements of the product, it can be effectively utilized to determine whether the tested plastic material is suitable for the application. For example, consider a plastic product that will be exposed intermittently to temperatures of 300°F (149°C) as found in an automotive under-the-hood application. The plastic product may also be exposed to an applied stress continuously. In this case, functional database information of the plastic material is required, because single-point data are not sufficient to select the proper plastic material. Table 1.2 shows an acetal property data sheet providing an extensive number of properties that are required for the molded product applications and markets where acetal is used. Additional functional database properties (e.g., continuous service short- and long-term temperatures, UL yellow cards, weather effects on the tensile and
impact strength properties, and chemical resistance to common solutions) required for a detailed analysis of an application are available from the resin’s suppliers. Figures 1.10 and 1.11, respectively, show the stressstrain graph for acetal homopolymer exposed to various temperatures and long periods of time (creep and thermal degradation). Figure 1.12 shows the relative viscosity-shear rate graph for unfilled PBT.
1.5 Testing Polymeric Materials Plastic materials cannot be subjected to the same tests developed for metals. They have their own special test requirements to comply with the demands of the plastics industry. While some standard testing methods may be applicable, in most cases the results from the analytical tests developed for metals have
Table 1.2. Delrin® 100, Property Data Sheet Properties
ASTM
Value (US)
Value (SI)
Test Conditions
Melt flow rate
D1238
1.0 (g/10 min)
1.0 (g/10 min)
Load 1.05 kg at 190°C
Specific gravity
D792
1.42
1.42
at 73°F (23°C)
Water absorption
D570
0.25 (%)
0.25 (%)
24 hr immersion
D570
0.22 (%)
0.22 (%)
Equilibrium 50% RH
D570
0.90 (%)
0.90 (%)
Equilibrium, immersion
Tensile stress at yield
D638
10 (kpsi)
68.94 (MPa)
(0.2 in./min)
Tensile modulus
D638
400 (kpsi)
2,757 (MPa)
(0.2 in./min)
Elongation at break
D638
75.0 (%)
75.0 (%)
(0.2 in./min)
Flexural stress at yield
D790
14.3 (kpsi)
98.58 (MPa)
(0.05 in./min)
Flexural modulus
D790
420 (kpsi)
2,895 (MPa)
(0.05 in./min)
Compression stress
D695
18 (kpsi)
124.1 (MPa)
(0.05 in./min)
Shear stress
D732
9.5 (kpsi)
65.49 (MPa)
—
Poisson ratio
E132
0.350
0.350
at 73°F (23°C)
D256
2.30 (ft-lb/in.)
123.0 (J/m)
at 73°F (23°C)
D256
1.80 (ft-lb/in.)
96.3 (J/m)
at –40°F (–40°C)
D256
No break
No break
at 73°F (23°C)
Physical
Mechanical
Toughness Izod impact (notched) Izod impact (unnotched) Tensile impact strength
D1822
170.0
(ft-lb/in.2)
(kJ/m2)
at 73°F (23°C)
M94, R120
at 73°F (23°C)
358
Hardness Rockwell
D785
M94, R120
(Continued)
1: Polymeric Materials and Properties
9
Table 1.2. Delrin® 100, Property Data Sheet (cont’d) Properties
ASTM
Value (US)
Value (SI)
Test Conditions
Coefficient of linear expansion
D696
5.8 × 10–5 (1/°F)
10.4 × 10–5 (1/°C)
Heat deflection temperature
D648
257 (°F)
125 (°C)
at 264 psi stress
D648
336 (°F)
169 (°C)
at 66 psi stress
Melting point
D2117
347 (°F)
175 (°C)
—
Specific heat
C351
0.35 (BTU/lb-°F)
0.083 (kJ)
—
C177
2.60 (BTU-in./hr-ft2-°F)
0.4 (W/m-K)
—
Dielectric constant
D150
3.70
3.70
at 106 Hz
Dielectric strength
D149
500.00 (V/mil)
19.7 (MV/m)
Short time (90 mils)
Dissipation factor
D150
0.005
0.005
at 106 Hz
Volume resistivity
D257
1015 (Ohm-cm)
1015 (Ohm-cm)
at 2% water, 73°F (23°C)
Arc resistance
D495
200 (sec)
200 (sec)
—
No tracking
No tracking
Flame extinguishes when arcing stops (120 mils)
Thermal
Thermal conductivity Electrical
very little value in defining the properties and the process characteristics of the thermoplastic polymers. The tests commonly used in the plastics industry are established and used to describe properties and process characteristics of these polymers. This allows resin producers, product designers, molding processors, and end users to have a common understanding about what thermoplastic polymers are and what type of properties can be correlated to the end-use results representing the conditions found in the application. Tests are not ends in themselves, but rather a means of gathering knowledge about the plastic materials. The real test of a material is conducted during the actual service. Once a plastic end product is taken home and used by the consumer, it no longer matters whether the tensile strength is 35,000 or 26,000 psi. The finished product succeeds entirely or fails. To assure success in automotive components and industrial products, the properties of the plastic materials are studied by design engineers. Through experience and judgment, they balance material characteristics and service requirements against the geometries, sizes, functionality, and performance needed in a product to give an adequate safety margin in the application. Reliable, useful tests for plastics must be based on an understanding of the properties, processing
characteristics, and the performance of these materials in similar applications. The following ASTM-UL testing methods are used for plastic materials: • Mechanical, and creep analysis • Physical analysis • Thermal, and flammability analysis • Characterization of temperature resistance • Electrical analysis • Optical testing and characterization • Weather aging and radiation resistance • Fungi and bacteria resistance • Determination of chemical resistance • Statistical analysis of test data These tests may also differ for the four major classes of plastic materials: thermoplastics, thermoplastic elastomers, liquid injection molding silicone, and thermosets. The testing methods are used to characterize the chemical composition, physical and structural morphology, and ease of processing. An important aspect of mechanical testing is the interpretation of results. Satisfactory mechanical
10
1.0 hr
73°F. 10.0
68.9
55.1 122°F.
7.0 6.0
41.3
5.0 4.0
212°F.
27.5
3.0 2.0
Tensile Stress, (kpsi)
2.5
8.0
Tensile Stress, (MPa)
Tensile Stress, (kpsi)
9.0
1,000 hrs
11.0
10 hrs 100 hrs
Selection of Polymeric Materials
10,000 hrs 50,000 hrs
2.0 1.5 1.0 0.5
13.8
0
1.0
0
0 0
5.0
10.0 15.0 Strain, (%)
0 20.0
0.5
1.0 1.5 Strain, (%)
2.0
2.5
Figure 1.11. Acetal stress-strain creep curves.
Figure 1.10. Acetal stress-strain curves at three temperatures.
characterization of an engineering thermoplastic requires not only proper geometry of the test specimens and the testing method but also the correct interpretation of the test results. Characterization of plastic material’s temperature resistance includes such diverse concerns as dimensional stability of the molded product, temperature effects on mechanical and electric properties, and thermal decomposition. Flammability classification, ignition temperature, limited oxygen index, and smoke densities are some of the tests used to define the properties of plastic materials. Electrical testing of plastic materials defines the dielectric properties, arc resistance, tracking resistance, insulation, conductivity, and service end-use temperature. Weather exposure and radiation susceptibility are not strictly separate from chemical resistance and degradation. Different tests are used to measure the resistance of plastic materials to temperature, moisture, ultraviolet (UV) radiation, pollution, ozone, oxidation, and microbiological attack. Optical testing is used to characterize such factors as transparency, clarity, reflectance, hazes, color, gloss, and refraction. Many of these tests are identical to those used for other materials. However, some of the unique properties and applications of polymers have led to the design of special testing procedures.
Chemical resistance is a diverse area that requires many types of tests, most of which have been developed specifically for plastic materials. Aspects of chemical resistance include the absorption and transport of solvents, acids, and bases; the effects of many types of additives; and various types of degradation of the plastic materials. Degradation can result from various sources, including chemical attack, thermal decomposition, photo-oxidative deterioration and environmental damage. Statistical analysis of test data is another important aspect of testing and characterization. For any type of thermoplastics testing, neither the tests nor the resulting characterizations are valid unless an appropriate statistical analysis test program has been established and the data is correctly reported. Table 1.3. shows an extensive list of tests, methods, and standards for plastic materials; these tests were established by ASTM and Underwriters Laboratories.
1.6 The Need for Uniform Global Testing Standards The world is fast becoming a single global market. The plastics industry faces the challenge of how to participate and take advantage of the emerging international markets, and to satisfy the needs of customers within domestic markets.
11
0.058
400
0.042
300
464°F (240°C) 0.029
200
482°F (250°C)
100
0.0145 0.010
70
500°F (260°C)
50
0.007
0.004 100
400
1,000
4,000
Relative Viscosity (Pa-sec)
Relative Viscosity (Ib-sec/in2)
1: Polymeric Materials and Properties
30 10,000
Shear Rate (sec-1)
Figure 1.12. Unfilled PBT, relative viscosity vs. shear rate graph.
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
Melt flow rate
D1238
g/10 min
g/10 min
Specific gravity
D792
—
—
Density
D1505
lb/ft3
g/cm3
Mold shrinkage (MFD)
D955
%
%
Mold shrinkage (TD)
D955
%
%
Water absorption
D570
%
%
24 hr Immersion
D570
%
%
Equilibrium 50% RH
D570
%
%
Equilibrium, immersion
Tensile stress at yield
D638
psi
MPa
(0.2 in./min)
Tensile modulus
D638
psi
MPa
(0.2 in./min)
Elongation at yield
D638
%
%
(0.2 in./min)
Elongation at break
D638
%
%
(0.2 in./min)
Flexural stress at yield
D790
psi
MPa
(0.05 in./min)
Flexural modulus
D790
psi
MPa
(0.05 in./min)
Compression stress
D695
psi
MPa
(0.05 in./min)
Shear stress
D732
psi
MPa
Poisson ratio
E132
—
—
Coefficient of friction
D1894
—
—
Plastic/plastic static
Coefficient of friction
D1894
—
—
Plastic/plastic dynamic
Coefficient of friction
D1894
—
—
Plastic/metal static
Physical
Mechanical
(Continued)
12
Selection of Polymeric Materials
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods (cont’d) Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
Coefficient of friction
D1894
—
—
Plastic/metal dynamic
Taber abrasion
D1044
mg/k cycles
mg/k cycles
Load and abrasive wheel
Izod impact (notched)
D256
ft-lb/in.
J/m
at 73°F (23°C)
Izod impact (notched)
D256
ft-lb/in.
J/m
at –40°F (–40°C)
Izod impact (unnotched)
D256
ft-lb/in.
J/m
at 73°F (23°C)
Gardner impact
D3029
in.-lb
mm-g
at 73°F (23°C)
Instrumented dart impact
D3763
in.-lb
mm-g
at 73°F (23°C)
D1822
ft-lb/in.2
kJ/m2
at 73°F (23°C)
Rockwell
D785
—
—
M, R, L, K, E scales
Durometer
D2240
—
—
Shore A, D scales
Barcol
D2583
—
—
Brittle temperature
D746
°F
°C
Vicat softening point
D1525
°F
°C
Glass transition temp
E1356
°F
°C
Continuous service temp
D794
°F
°C
Coefficient of linear expansion
D696
in./(in.-°F)
Heat deflection temp
D648
°F
°C
at 264 psi stress
D648
°F
°C
at 66 psi stress
Melting point
D2117
°F
°C
Specific heat
C351
BTU/(lb-°F)
J/g °C
Thermal conductivity
C177
BTU-in./hr-ft2-°F
W/m-K
Flammability rating
UL 94
HB, V2, V0, 5V
Relative thermal index
UL 746A
°F
°C
RTI mechanical without Impact
UL 746A
°F
°C
RTI mechanical with Impact
UL 746A
°F
°C
Dielectric constant
D150
—
—
at 106 Hz
Dielectric strength
D149
V/mil
kV/mm
Short time (90 mils)
Dissipation factor
D150
—
—
at 106 Hz
Volume resistivity
D257
Ohm-cm
Ohm-cm
at 2% water, 73°F (23°C)
Toughness
Tensile impact strength Hardness
Thermal
Electrical
(Continued)
1: Polymeric Materials and Properties
13
Table 1.3. Polymeric Materials Properties Database per ASTM/UL Test Methods (cont’d) Properties
ASTM/UL
Units (US)
Units (SI)
Test Conditions
D495
sec
sec
—
D495
—
—
Flame extinguishes when arcing stops (120 mils)
Hot wire ignition
UL 746A
sec
sec
High ampere ignition
UL 746A
No. of arcs
No. of arcs
High volt track rate
UL 746A
in./min
mm/sec
Competitive track index
UL 746A
Volts
Volts
D2864
%
%
Refractive index
D542
—
—
Haze
D1003
%
%
Arc resistance
No. of arcs
Flammability Limiting oxygen index Optical
Transmittance
D1003
%
%
Gardner gloss
D523
%
%
at 60°
Gloss
D2457
%
%
at 0°
The General Agreement on Tariffs and Trade (GATT), North American Free Trade Agreement (NAFTA), and European Union (EU) are all seeking positions that will eliminate barriers found in the global market. The benefits of uniform global test methods and data reporting formats enhance the business efficiency of the plastics industry. Comparable data will improve communication between the resin suppliers and their customers, and in the case of multinational companies, it will improve communication between manufacturing sites located around the world, and resin suppliers will have a greater opportunity to gain global competitiveness. Unfortunately, the debate over uniform global testing standards has taken place mostly without the participation of the most important members of the plastics industry, namely end users, processors, and resin suppliers.
1.6.1 Uniform Global Polymeric Materials Specifications The material selection process for new plastic applications, or substitution of the present material with a better resin to improve the efficiency of the plastic product, needs improvement to standardize manufacturing, quality control, uniform global testing, and reporting. The use of reliable and uniform global plastic material specifications, and the ability
to compare the properties of the plastic materials produced around the world, is ideal. Some of the multinational resin suppliers have been using this concept for many years. However, these same companies have expanded their manufacturing operations outside their home location (national or international), yet the small compounders, have only the basic testing necessities to support their internal production needs. These usually small operations are in business because they offer their commodity products at lower costs. After the plastic product design and specifications for the application requirements are defined, the material selection process involves screening all the available resin formulations to identify the most likely candidates at the lowest cost for the plastic product. The material selection process used around the world, which appears to be simple, often becomes a complicated task. This problem is caused by several factors such as global location, resin cost and quality, the use of different test methods, test specimens of various geometries, injection molding the specimens using different process parameters, specimen conditioning, specimen testing methods, and the lack of uniform reporting of the test results. For example, there are inconsistencies created by the use of test specimens with different thicknesses, various dimensions for the specimen test areas, specimen conditioning methods, and the use of different load speeds and temperatures for testing.
14
Wide variability makes it difficult to compare resins from different suppliers (if they have testing facilities) and sourced from various global manufacturing sites of a single supplier. Lot-to-lot variations in the quality of the resin and test data variations caused by the use of different testing laboratories may be encountered. There are over 26,000 plastic resin formulations in the USA alone. The bulk of available test data for these materials is taken from the resin supplier’s product information data sheet. Companies sometimes copy the property values for their products from their competitors, or from various plastic databases. Some resin suppliers’ manufacturing and marketing departments have different internal business objectives. Manufacturing departments want to operate without any production problems and with as few restrictions or limitations as possible. However, for plastic materials requiring property values with tight tolerances that are difficult to produce, the wider the specification values, the more favorable is the situation for the manufacturing organization. Consequently, they have a set of internal manufacturing specifications that are sometimes wider than those required for the successful marketing of the product. On the other hand, marketing departments want to sell the plastic materials with the highest property values (tightest specifications) and better quality, demanding a premium price for the resin. For example, for product properties that can be measured, each specification value has tolerances: lower limits for manufacturing and higher limits for marketing. However, for most resin suppliers, the internal property tolerances of the plastic materials are not reported in the product information sheet. Because of business and competitive pressure between resin suppliers, only the higher property values of the plastic materials are reported. For example, the notched Izod impact value reported on the plastic material product information sheet is 20 ft-lb/in. (1,070 J/m) (upper limit). But, the manufacturing laboratories tests this property of the running production material lot (lot size of 80–150 tons), and approves the plastic material if the value is 15 ft-lb/in. (802.5 J/m) (lower limit) or higher. In a production run of a plastic material, manufacturing only tests the minimum number of critical properties to meet quality assurance requirements. Some less critical properties are tested once a year, others every five or ten years depending on the product performance history and end-use applications. Standard methods, which have been developed with strong influence from resin suppliers, often condone these practices.
Selection of Polymeric Materials
The achievement of uniform global testing standards represents an uphill road to the plastics industry and resin suppliers, creating a new challenge in doing business. There are costs associated with test equipment investments, training personnel, and in retesting resins according to the new testing standards. The costs of separate or dual testing, where both standards are reported, must be taken into consideration. This additional work costs more, and the cost of plastic materials will increase, creating another obstacle for the global plastics industry.
1.6.2 Selection of Testing Standards There are several facilities for testing plastics standards at various locations around the world. The American Society for Testing and Materials and the Underwriters Laboratories in the USA, International Organization for Standardization (ISO) in Geneva, Switzerland, Deutsches Institut für Normung (DIN) in Germany, British Standards Institution (BSI) in Great Britain, Association Française de Normalisation (AFNOR) in France, and Japanese Industry standards (JIS) in Japan. Plastics testing standards developed by the Genevabased ISO are being promoted as a universal set of testing standards that could satisfy the needs for a single global standard. ISO was founded in 1947 and is a worldwide federation of national standards organizations from 100 countries. ISO promotes standardization to facilitate the international exchange of goods and services to develop cooperation in economical, intellectual, scientific, and technical activities. ISO encompasses standardization in all fields except electrical and electronics, which is the responsibility of the International Electrotechnical Commission (IEC). ISO and IEC procedures have almost totally replaced the national plastics standards in Europe. ISO 10350:1993 recommends a particular set of polymer properties and test procedures as a generally acceptable way to characterize and describe a plastic. Each property recommended in ISO 10350 has a property number and in most cases a recommended ISO or IEC test procedure. The EU and most European nations have adopted the ISO standards, replacing other national standards such as DIN, BSI, and AFNOR. Japan appears committed to converting to ISO test methods. Over 90 industrialized countries in the Pacific, Asian, and
1: Polymeric Materials and Properties
African regions have also adopted these universal standards. The USA had been alone in its reluctance to embrace ISO/IEC test standards, and to convert to the International System of Units (SI). ISO 10350:1993 categorizes the properties into five separate groups, and the number of test procedures is limited. The ISO recommended set of procedures does not meet the uniform global testing standard objectives, nor does it describe all the characteristics of the plastic materials required for the product design and material selection process. ISO does not produce truly comparable data with the existing ASTM/UL test methods. However, the testing procedures that are not included by ISO are replaced with ASTM/UL test methods. Specific details about the difference between ASTM/UL and ISO/IEC testing procedures, test conditions, reported units, and comparable properties of plastic materials are discussed in the subsequent chapters.
1.6.3 Issues and Concerns Several ISO/IEC global testing standards issues and concerns should be resolved before they are adopted by resin suppliers and customers in the USA. (1) The original target date of 1995 concerning the full conversion suggested by the Polymeric Materials Producers Division of the Society of the Plastics Industry raises questions regarding the urgency it implies. (2) The plastics industry manufacturers of durable goods, such as automobiles, computers and business machines, need to facilitate the conversion to ISO/IEC testing standards because of the global nature of their business. (3) The multinational engineering resin suppliers who also support the conversion, in order to participate in the global markets and achieve longterm cost savings and worldwide acceptance, need considerable time to implement the new standards. Resin suppliers recognize that the conversion to ISO/IEC test methods will not occur overnight. It is expected that ASTM/UL standards will continue to be used for many years. The acceptance of SI units is also expected to be a gradual transition process. It is important to recognize that separate testing for ASTM/UL and ISO standards will be carried out for some time. Although this puts a cost burden on resin
15
suppliers, their customers need to understand and to develop confidence in the ISO standards. (4) Accepting the costs of conversion to ISO/IEC test methods may involve a substantial initial financial commitment. This includes the cost for purchasing the ISO multipurpose test specimen mold and other mold inserts. It may also require the purchase of new molding equipment of the appropriate size along with suitable instrumentation and controls. Also involved are modifications to test equipment such as shorter support spans to facilitate flat wise testing for heat deflection temperature and new apparatus for Charpy impact tests. There are costs associated with training personnel and in retesting resins according to ISO requirements. The costs of separate or dual testing, where both standards are in broad effect, must be factored in as well. (5) There will be concerns about discarding an established historical property database in the conversion to ISO/IEC test standards. Though the concern is genuine, and the cost of recreating a reliable database is considerable, having to build such a database should not deter acceptance of ISO test protocols. (6) Some members of the US plastic industry argue that the standards development under ISO is a political process. The USA is at a disadvantage with only one vote in the process as opposed to 12 votes for EU nations. Critics also point to the fact that other countries are currently better organized and better able to influence the international standards setting process. Among the barriers to the implementation of ISO test standards in the USA, several key areas stand out including ready accessibility to standards information and documentation, the availability of molds and test equipment, and the continuing unfamiliarity with SI units. ASTM/UL standards are readily accessible. However, there are difficulties in accessing ISO/IEC test standards and the costs of those standards are a serious concern. The US plastic industry’s unfamiliarity with SI units is a formidable barrier to the acceptance of ISO test standards. Over 90% of ASTM D-20 standards specify the properties in SI units, while many companies continue to associate their data with the inch-pound (in.-lb) units. It will take time for the industry to become used to the new values involved.
16
1.6.4 Summary The current plastics testing practices for commodity plastics lack consistency. Multinational plastics companies would like to adopt uniform global testing standards. However, resins suppliers are clearly divided on the issue of the urgency to convert, accepting the costs of conversion, and the need for these new standards for high volume commodity resins used in the domestic markets. They believe that most customers will continue to use ASTM/UL standards with slow conversion to SI units, for sometime. Engineering resin suppliers serving the automotive, computer, and business machines market segments and other multinational customers feel the pressure to respond to their customers’ needs for uniform global testing standards. A plastic material’s properties tested and reported by ASTM/UL cannot be used directly to compare with the same plastic material’s properties tested by ISO. For example, the ASTM method for the Izod impact strength test is reported in foot-pounds/inch (ft-lb/in.), or impact energy per length. The ISO method for impact strength is the Charpy test and it is reported in kilojoules/square meter (kJ/m2), or impact energy per area. These two impact methods and the significance of the tests are different; therefore, these impact properties cannot be compared.
1.7 Origin and Applications of Polymeric Materials Plastic materials are the ultimate tribute to man’s creativity and inventiveness. Plastics are true manmade materials. Like any other material, they have their origin in nature. The structure of plastic materials is based on basic chemical elements such as carbon, oxygen, hydrogen, nitrogen, chlorine, and sulfur. These elements are extracted from oil, mineral, water, gas, coal, and even from living plants. It was man’s inspiration to take these elements and combine them through various chemical reactions in an almost unending series of combinations to produce the rich variety of polymers known today as plastics. It is possible to develop new families of polymers by compounding with other polymers, additives, reinforcements, and modifiers to create almost any property desired for a specific application or enduse product. These new polymers have similar properties to existing materials but offer greater design flexibility and cost incentives for manufacturing.
Selection of Polymeric Materials
There are some plastics with significant property improvements over existing materials, while other polymers can only be described as unique materials with exceptional properties previously unknown to the industrial world. Today, we have plastic materials that will melt at 200°F (93°C), while other plastic materials (Vespel®) can withstand up to 1,000°F (537°C). The heat shields that protect astronauts, satellites traveling in space, and airplanes that can fly to the boundaries of the universe are composed of plastic materials based on recent plastic technology. There are polymers used for shields that can stop a bullet such as polyaramides popularly known as Kevlar®. There are flexible plastic films that protect grocery products and there are rigid plastics that are rugged enough to serve as support beams in a building. Plastics are among the best electrical insulating materials known to mankind, and yet there are other types of special plastic materials compounded with graphite and metal fillers capable of conducting electricity, as used in satellite dishes. Plastic composite materials are used for airplanes, ships, cars, trucks, vessels, buildings, golf club shafts, while other flexible polymers are used as tires, seals, tubing, hoses, and as upholstery materials for furniture. There are impact resistant, weather resistant, hard, and transparent polymers used as windshields for airplanes, automobiles, and shower doors. There are also transparent packaging materials used to protect consumer items (storage containers, plastic wrap, beverage containers). It is this diversity that has made them so easily applicable to such a broad range of end uses today. This polymer diversity makes it difficult to grasp the idea of a single family of materials that can provide an infinite range of properties, characteristics, and transformation processes.
1.8 Modern History of Polymeric Materials Plastic materials have played an important role in the development of our modern civilization. These materials have an extensive range of properties and processing automation capability while offering cost advantages over metals, wood, rubber, and other building materials. It is surprising to realize that a little more than a century ago there were no manmade plastic materials anywhere in the world. The plastics industry dates back to 1868, when John Wesley Hyatt mixed pyroxylin made from cotton and
1: Polymeric Materials and Properties
nitric acid with camphor to create an entirely different and new product called “Celluloid.” The development of celluloid was in response to a competition sponsored by a manufacturer of billiard balls. Celluloid came about to overcome the shortage of ivory used to produce billiard balls. With the need for a new material and a production method for this application, celluloid was developed and the plastics industry was born. Celluloid quickly moved into other markets, including new applications such as shirt collars, cuffs and shirt fronts, dolls, combs, buttons, and window curtains used in early automobiles. However, the most important application of celluloid was the first photographic film used by Eastman to produce the first motion picture film in 1882. (In the late 1880s Hannibal Goodwin was awarded a patent for celluloid film.) This material is still in use by the motion picture industry today, under its chemical name cellulose nitrate. The plastics industry took its second major step 41 years later. Dr. Leo Hendrik Baekeland introduced the first phenol-formaldehyde “Phenolic” resin in 1909. This was the first plastic material to achieve world acceptance. What is more important, Baekeland also developed techniques for controlling and modifying the phenol-formaldehyde polymerization reaction. This technology made it possible to produce useful items such as marbleized clock bases and electric iron handles. The molding process for phenolic materials was based on heat and pressure. In this process the monomer is mixed in the liquid phase and formed into various shapes under heat and pressure. The part is then cooled in the mold. This is basically the same process that is still used in the industry to produce thermosetting plastic materials. The third major step in the development of plastics took place in the 1920s, with the introduction of cellulose acetate. This polymer had a similar structure to cellulose nitrate but was safer to use in processing and in applications. Urea-formaldehyde can be processed like phenolics, but it can also be molded into light colored articles that are more attractive than the black or brown colors of phenolics. PVC became the second largest selling plastic for such applications as flooring, upholstery, wire and cable insulation, tubing, hoses, and fittings. Polyamide or nylon, introduced by E. I. Du Pont, was first developed to produce synthetic yarns and fibers. Nylon represents one of the most important developments in the plastics industry. Research development under the leadership of Wallace Carothers in
17
the late 1920s through 1930s resulted in the invention of nylon 6/6. The tempo of plastics’ development picked up considerably in the 1930s and the 1940s. With each decade, newer, more exciting, and more versatile plastics came into existence. In the 1930s, the acrylic resins were introduced for signs and transparent articles. The introduction of polystyrene made this polymer the third largest selling plastic for house wares, toys, and for applications in the packaging industry. Melamine resins were also introduced for use in dinnerware, paints, and wet strength paper. Melamine later became a critical element (as a binder) in the development of decorative laminate kitchen counter tops, table tops, and panels. Polyester (PET) resins known as Dacron® were also introduced to produce synthetic yarns and fibers. During the World War II years of the 1940s, the demand for plastics accelerated as did research into new plastics that could aid in the defense effort of the war. Fluoropolymers (PTFE) and polyethylene were war-time developments that grew out of the need for a superior insulating material for the Manhattan Project and applications such as radar cables. Thermoset polyester resins were also introduced a decade later. Radical changes in the boat building industry were also a war-time development introduced for military use. ABS is best known today as the plastic material used for applications such as appliance housings, refrigerator liners, safety helmets, tubing, telephone handsets, and luggage. The original ABS research work was also a defense effort of World War II for the development of synthetic rubber materials. By the beginning of the 1950s, plastics were on their way to being accepted by designers and engineers as basic industrial materials. This decade also saw the introduction of polypropylene, as well as the development of acetal and polycarbonate—two plastics that, along with nylon, came to form the nucleus of a subgroup in the plastics family known as the “engineering thermoplastics.” Their outstanding impact strength and thermal and dimensional stability enabled engineering thermoplastic resins to compete directly with metals in many applications. The 1960s and 1970s also had their share of new polymer developments. The most important contribution was the thermoplastic polyesters used in exterior automotive parts, under-the-hood applications, and electrical and electronic components. Polyester bottles internally coated with high nitrile barrier
18
resins (outstanding resistance to gas permeation) inspired the new drink bottle packaging applications. During this time span, another subgroup of the plastics family called “high performance plastics” found new markets. This group includes such materials as polyimide, polyamide-imide, aromatic polyester, PPS, and polyether sulfone. These materials historically met their objectives in the demanding thermal needs of aerospace and aircraft applications, reinforcing the vision of the future that was plastics.
1.9 Polymeric Materials Families Plastic materials are the result of the combination of carbon elements reacting with oxygen, hydrogen, nitrogen, and other organic and inorganic elements. These polymers have the ability to change into a liquid (melt) and are capable of being formed into shapes by the application of heat and pressure. Plastic materials are composed of various polymeric families. Plastics include an extensive number of polymers and compounds with each kind of material having its own unique and special type of properties. Plastic materials are classified into the following families: thermoplastics, thermoplastic elastomers (TPEs), liquid injection molding silicones, thermosets, composites, and thermoset rubbers. Thermoplastic polymers consist of a long chain of molecules, either linear or branched, having side chains or unattached groups of molecules. Usually, the thermoplastic materials are commercially available in the forms of pellets, granules, or powders. These materials can be repeatedly melted by heat under pressure so they can be formed and then cooled and hardened into the final desired shape. Chemical changes do not take place, for the most part, during the transformation process. A simple analogy for a thermoplastic material is a wax candle that can be liquefied by heat and then solidify when cooled. TPE resins are rubbery materials with the characteristics of a thermoplastic and the performance characteristics of a thermoset rubber. TPEs are processed using the same thermoplastic equipment and methods such as extrusion, injection molding, compression molding, and blow molding. Liquid injection molding silicone is a polymeric family with unique characteristics. Generally, these materials consist of two liquid formulations in a 1:1 ratio which polymerize during injection molding. These compounds produce precision elastomeric molded parts efficiently. They use a liquid metering, mixing, and
Selection of Polymeric Materials
delivery system, a specially modified injection molding machine, and a high temperature precision mold. Thermoset polymers are created by a chemical reaction between the chain cross-link and the long molecule’s network during polymerization. Heat causes the linear polymer chains to bond together to form a three-dimensional network. Once polymerized or hardened, the material cannot be reprocessed by heating without degrading the bonds cross-linking the polymer molecules. One analogy for the thermoset polymers is the chemical transformation of concrete. When the cement powder blends with water and sand, the mixture becomes a thick paste. This mixture is then transferred to a cavity for curing and finally hardens to become a solid object (concrete). The chemical reaction transforms the product into concrete. The transformation processes of the concrete items are irreversible. Reprocessing concrete to extract the original components of cement, sand, and water is not possible. The concrete becomes a new, different, and strong material. Thermosetting polymers are not reprocessable or recyclable. Thermoset polymers are supplied in commercial form as resins, powders, and liquid monomer mixtures, or as partially polymerized molding compounds. In this uncured condition, they conform to the finished shape with or without pressure and can be polymerized with chemicals or heat.
1.10 Classification of Polymeric Materials by Performance Plastic materials classifications divide polymers into four family groups based on their application performance. The first is the “commodity plastics.” These materials have large consumption volume, extensive application end uses, low material cost, and limited property and performance. The commodity plastics include polystyrene, polyethylene, styreneacrylonitrile (SAN) copolymer, cellulose nitrate, polybutene, bismaleimide, unsaturated polyester, and polyvinyl chloride (PVC). The second group is the “intermediate plastics.” These materials have higher mechanical, thermal, chemical, and electrical properties than the commodity plastics. The basic polymer matrix properties remain constant, however, modifications can be made to change specific properties of the compound. Intermediate plastics include acrylic (polymethyl methacrylate (PMMA)), polyolefin thermoplastic
1: Polymeric Materials and Properties
elastomer (TPO), modified polyphenylene oxide (PPO), thermoplastic vulcanizate (TPV), melt processible rubber (MPR), high impact polystyrene (HIPS), ionomers, polypropylene, glass fiber reinforced polypropylene, ABS, styrenic block copolymer thermoplastic elastomers (TPEs), and ultra-high molecular weight polyethylene. The third group is the “engineering thermoplastics.” The level of mechanical properties that qualify as engineering grade is somewhat arbitrary; a tensile strength that is not lower than 7,000 psi and a minimum modulus of elasticity of 350,000 psi are reasonable criteria. Engineering thermoplastic resins include polyacetal, polyamide (nylon), polycarbonate, polybutadiene terephthalate (PBT), polyethylene terephthalate (PET), block copolyester TPEs, polyamide TPEs, and liquid injection molding silicone. The fourth group is the “high performance engineering materials.” These materials have the highest resistance and retain a high percentage of their useful mechanical properties at high temperatures, providing a longer service life of the product. They also maintain properties at higher electrical frequencies without sacrificing their chemical resistance properties, when exposed to corrosive elements. These resins are also inherently flame retardant, with UL 94 flammability ratings of V0 and 5V. The high performance engineering resins include high temperature nylon (polyamide), liquid crystal polymers (LCPs), polysulfone (PSU), fluoropolymers, polyetherimide (PEI), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), silicone polymers, and polyimide. Competition among resin producers to capture markets has created a resin supply of more than 26,000 resin grades in the USA alone. The result is that there is usually more than one choice, and often several choices, available to meet the end-use performance requirements.
1.11 Types of Thermoplastic Molecular Structures Polymeric thermoplastic materials are an aggregate of long-chained molecular structures. There are two different states, one is known as “crystalline polymers” and the other is known as “amorphous polymers.” However, there is no crystalline plastic material exhibiting solely a crystalline structure, but rather they have a mixed structure in which crystalline sections and amorphous sections coexist. This molecular structure is known as “semi-crystalline polymers.”
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1.12 Manufacturing of Polymers The basic compositions of polymers are derived from organic materials primarily comprised of atoms of carbon and hydrogen (hydrocarbons). Resin suppliers use chemical reactions to transform basic feedstocks or monomers, which have been derived from natural gas, crude oil, and coal, into polymers. Petroleum is the principal source of alkanes and cycloalkanes. An oil refinery uses a distillation process, where the crude oil is first separated into fractions by distillation to produce gas, petroleum ether, gasoline, kerosene, fuel oil, lubricating oil, greases, paraffin wax, and asphalt. Some higher-boiling point fractions are subjected to cracking processes in which large molecules are broken down into smaller molecules, thus increasing the yield of the most valuable products such as gasoline. In addition, small molecules are converted into large ones by a process known as “alkylation.” Gases that are a by-product of these processes, together with the petroleum fractions and substances obtained from the fractions, form the basic feedstock to manufacture plastic materials. Natural gas and crude oil are the most common and economical feedstocks in use today. Coal is another excellent source for the manufacture of feedstock for plastics. Products such as castor oil or tung oil derived from plants are also adaptable. From these basic hydrocarbons come the feedstocks known as monomers (small, single molecules). The monomer is then subjected to a chemical reaction known as polymerization, which causes the small molecules to link together into much longer molecules called polymers. Chemically, the polymerization reaction transforms the monomer into a polymer. A single monomer can contribute to the manufacture of a variety of different polymers, each with its own distinct characteristics. A polymer may be defined as a high molecular weight compound that contains comparatively simple recurring units. Basically, there is a great deal of flexibility in the plastics manufacturing process for creating a wide range of polymers. The most important factors that determine the properties of the polymers are the chemical processes by which the small molecules link together into large molecules and the structural arrangement of the molecular chains formed. Another factor is the length of molecules in the polymer chain. Polymerizing two or more different monomers together (a process known as copolymerization) leads to the production of other polymer grades.
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Figure 1.13 shows the basic chemical feedstocks and simplified manufacturing flow charts for the most common polymeric materials.
1.13 Polymeric Materials Compounding Process The compounding process has been the major contributor in the development of new formulations for the plastics industry. A virtually endless array of additives, modifiers, colorants, fillers, and reinforcements permits resin suppliers to impart specific properties to the basic polymers, resulting in expanding opportunities for new applications and cost reductions. Compounding relies on polymerization chemistry to combine a base polymer with modifiers, additives, reinforcements, and other polymers to create new alloys and molecular structures. Compounded polymers can offer better performance, greater strength, and smoother processing at a competitive cost. Compounding is a complex, precise manufacturing process. The basic function of the compounding equipment is to mix two or more different polymers, additives, modifiers, fillers, and reinforcements into a uniform, continuous, and homogeneous mixture. Several types of compounding equipment are used: single screw and twin screw extruders that include tangential, intermeshing or self-wiping, and co-rotating and counter-rotating machines. These extruders differ in terms of their lengths (working segments), vacuum section, and extrusion die. Batch mixers are used to blend polymers and certain additives. Various types of material feeders (single and twin screws, gravity, vibration, liquid) are used to monitor the ratio of polymers, additives, and reinforcements. The quench tank, pelletizer, blender/lubricator, dryer, and packaging equipment are among the other accessories required for the compounding process. A basic polymer must be compounded for use by the processor who will transform it into a product. In some limited circumstances, it is possible to use the polymer as it comes out of the polymerization reaction. However, these polymers could be unstable or dangerous to process because of marginal thermal stability. More often, other manufacturing steps are required to modify the polymers into a form that can be easily handled by the plastic processors. The most popular solid forms of industrial polymers are pellets, granules, flakes, or powder. In the hands of the processor, these solid forms are generally
Selection of Polymeric Materials
subjected to heat and pressure, melted, forced into the desired shape, and then allowed to cure and set into a finished product. They are also available as semi-solids (pastes). Liquid polymers can also be used to impregnate fibrous materials that can then be allowed to harden into a variety of configurations (composites).
1.13.1 Chemical Additives for Polymeric Materials Compounding A variety of additives are used to overcome some of the limitations in plastic materials. Some of the common additives used in compounding and their functions are given below: • Antifogging agents: Prevent fogging that obscures viewing through clear plastic films or sheets. • Antioxidants: Prevent oxidative degradation of the polymers. • Antistatic agents: Prevent electrostatic buildup on the plastic product surfaces. • Biocides: Prevent growth of microorganisms that could cause odor, mildew, and stains on plastic surfaces. • Blowing agents: Release a gas that dissolves in plastics to form cellular or foamed structures. • Coupling agents: Provide improved bonding between the basic polymers and the reinforcements. • Cross-linking agents: Improve the physical properties of thermoplastics by cross-linking. • Curing agents: Catalyze or initiate curing of the plastic materials. • Flame retardants: Improve the polymers’ flameburning ignition levels. • Heat stabilizers: Prevent the polymers from heat-induced breakdown, and degradation, when the plastic product is exposed to high temperatures. • Internal lubricants: Improve the processing ability of the polymer by lowering melt viscosity. • Surface lubricants: Prevent the polymer melt from sticking to metal surfaces in the processing equipment (plastifying unit and mold cavities).
Nylon 6 Nylon 66 Polysulfone Polycarbonate PPS
Caprolactam Sulfuric acid
Cyclohaxanano oxime
Dichlorodephenyl Sulfone
Hexamethylenediamine
Diphenol A
Hydroxyl amine
Adiptic Acid
Tionyl Chloride
Phenol
Chlorobenzene
Cyclohexanone
Chlorine
Cyclohexane
Monochlorobenzene
Chlorine
Chlorine
Chlorobenzene
Methyl Chloride
Chlorine
Chlorine
BENZENE
METHANE
METHANOL
Hydrochloric Acid
Chlorosilane (Hydrochloric Acid + Silicone Metal
Silicon Metal
Terephthalic Acid
Ethylbenzene
Chloroform
Hydrogen Fluoride
Hydrogen Fluoride
Dimethyl Terephthalate
Chlorotrifluoroethane
Chlorotrifluoroethylene
Phthalic Anhydride + Maleic Anhydride + Propylene Glycol
Styrene
1: Polymeric Materials and Properties
Chlorodifluoromethane
Formaldehyde
Methyl Chlorosilane
Phenyl Chlorosilane
Acetone
Diphenol A
Sulpur
Phosgene
Water
1, 4 - Butanedoil
Silicone PBT 1, 2 - Ethylene Glycol
Tetrafluoroethylene
Tetrafluoroethylene
Alkyd
Chlorotrifluoroethylene
Hexafluoromethane
Melamine + Paper Pulp
Urea + Paper Pulp
PET Urea M. C. Melamine M. C. Acetal TFE Fluorocarbon FEP Fluorocarbon CTFE Fluorocarbon E-CTFE Fluorocarbon Alkyd Polyester (TS) Polystyrene
Figure 1.13. Simplified manufacturing flow chart for common polymeric materials.(Continued)
ETHYLENE
ETHANE
NATURAL GAS
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ABS Acrylonitrile (Propylene + Ammonia) + Butadiene
Styrene
Ethylene Methacrylic Acid Copolymer
Acrylonitrile (Propylene + Ammonia)
Metal Ion
Acetic Acid
Cumene
Ethylbenzene
Methacrylic Acid
Chloride
Isopropyl Alcohol
ETHANE
Benzene
Acetone
Acetaldehyde
Hydrogen Cyanide
PROPANE
Chloride
ETHYLENE
Acetone Cyanohydrin
Allyl Chloride
PROPYLENE
Propylene Oxide
BUTANE
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Styrene Acrylonitrile
Ionomer Polyethylene
Acetic Anhydride
Allyl Alcohol
Polypropylene Oxide Polyol
Epichlorohydrin
Sulfuric Acid
Ethylene Dichloride
Bisphenol A
Methacrylamide
Phenol
Vinyl Acetate
Chemical Cellulose (Wood Pulp)
Ethylene
Cellulose Acetate
EVA Vinyl Chloride
Methanol
1-Butene
MDI + TDI + Polymeric Isocyanate
Phthalic Anhydride
Chlorohydrin Intermediate
Methyl Methacrylate
Formaldehyde
PVC Polypropylene Phenolic Acrylic Epoxy Diallyl Phthalate Polyurethane Polybutylene
Figure 1.13. Simplified manufacturing flow chart for common polymeric materials. (Cont’d)
NATURAL GAS
Selection of Polymeric Materials
1: Polymeric Materials and Properties
• Plasticizers: Enhance the polymer flexibility, resiliency, and melt flow. • UV stabilizers: Prevent polymer deterioration when the plastic product is exposed to UV light.
1.13.2 Fillers and Reinforcements for Polymeric Materials Compounding Fillers and reinforcements are used to improve the mechanical properties, temperature resistance, and environmental resistance, and to reduce the cost (fillers) of the plastic materials. Some of the common fillers and reinforcement materials used for compounding, and their functions are given below: • Alumina trihydrate: Used as an extender, flame retardant, smoke suppressant. • Barium sulfate: Filler; white pigment; increases specific gravity, chemical resistance, and decreases the coefficient of friction. • Baron fibers: Increases tensile and compressive strength capacity; expensive. • Calcium carbonate: Most widely used as an extender, pigment, filler. • Calcium sulfate: Extender; enhances mechanical properties; increases impact resistance, tensile strength, and compressive strength. • Carbon black: Filler; black pigment; antistatic agent; aids in cross-linking; conductive; UV stabilizer. • Carbon, graphite fibers: Reinforcement; high modulus of elasticity and tensile strength; low density, decreases the coefficient of thermal expansion and friction; conductive. • Ceramic fibers: Reinforcement; very high temperature resistance; expensive. • Glass fibers: Largest volume reinforcement in use; increases mechanical strength, product dimensional stability; improves heat resistance and chemical resistance. • Kaolin: Second largest volume expander and pigment in use; used mostly in wires and cables, sheet and bulk molding compounds, and vinyl floorings. • Mica: Flake reinforcement; improves dielectric strength, thermal resistance, and mechanical properties.
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• Microspheres, hollow: Reduce weight; improves polymer melt flow, stiffness, and impact resistance. • Microspheres, solid: Improve polymer melt flow properties, uniform stress distribution. • Organic fillers: Extenders; e.g., wood flour, nutshell, corn cobs, rice, peanut hulls. • Polymer fibers: Reinforcement; lightweight; improves mechanical strength. • Silica: Filler; extender; reinforcement; thickens liquid systems, makes them thixotropic; helps to avoid plate-out in PVC; acts as flattening agent. • Talc: Extender; filler; improves stiffness, tensile strength, and creep resistance. • Wollastonite: Reinforcement; high loadings possible; improves mechanical and dielectric strength, heat resistance, and dimensional stability; lowers moisture absorption; decreases the impact strength.
1.13.3 Impact Modifiers for Polymeric Materials Compounding Impact modifiers improve the impact strength of polymers; stiffness, tensile strength, compressive strength, and temperature resistance can also be reduced. The impact modified polymer must be made heterogeneous with two distinct phases, one rigid and the other flexible. The following impact modifiers are used: • Olefinic thermoplastic elastomers: Improve impact resistance (toughness) of semi-crystalline thermoplastics. • Rubber and thermoplastic elastomers: Improve impact resistance of polyolefin polymers. • Styrene-butadiene multi-blocks: Improve impact resistance of HIPS. • ABS: Improves flexibility and cost reduction of polycarbonate. • Polyurethane: Improves impact resistance of thermoplastic polyester and polysulfone; reduces stiffness of epoxy resins. • Elastomers: Improve flexibility of unsaturated polyester. • EPDM and EPM: Improve impact resistance of nylon, polypropylene, and high density polyethylene (HDPE).
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1.13.4 Colorants for Polymeric Materials Compounding Pigments and dyes are used to color plastic products. A color master batch is the most common technique used for coloring polymers. The color concentrate is dispersed into a carrier or compatible (universal) polymer such as nylon 6, polyolefin, ethylene vinyl acetate (EVA), or polyethylene. The concentrate and carrier are mixed using intensive mixers (two rollers), extruded, and cut into pellets. Other color concentrates are produced in liquid forms or as fine loose powder; this causes severe dusting problems during the compounding process. Organic and inorganic pigments are the primary colorants in the plastic industry. There are general differences in chemical structure, coloring characteristics, and physical properties between the two types. Organic pigments are complex chemical compounds that contain one or more benzene rings and exhibit small particle size, greater transparency, and color strength. Inorganic pigments are metallic compounds that are larger in size, smaller in surface area, and denser. They absorb and scatter light and exhibit more opacity and lower tinting strength. They have better heat and UV stability, but their chemical resistance varies.
1.14 Families of Thermoplastic Polymers Plastics are a family of materials, not a single kind of material. Most fall into a particular group or family, which are specified as thermoplastics, thermoplastic elastomers, liquid injection molding elastomers, thermosets, and thermoset rubbers. Popular thermoplastics are discussed in this section.
1.14.1 Acrylonitrile-ButadieneStyrene (ABS) Terpolymer ABS resins have a well-balanced set of properties for molding tight dimensional control articles with outstanding surface finishing, good impact resistance, and metal plating characteristics. ABS resins belong to a versatile family of thermoplastic polymers. ABS is produced by combining three monomers: acrylonitrile, butadiene, and styrene. The chemical structure of these monomers requires each monomer to be an important component of the ABS resins. Acrylonitrile
Selection of Polymeric Materials
contributes heat resistance, chemical resistance, and surface hardness to the system. The butadiene component contributes toughness and impact resistance, while the styrene component contributes processibility, rigidity, and strength. ABS plastics are two-phase systems. Styreneacrylonitrile (SAN) forms the continuous matrix phase. The second phase is composed of dispersed polybutadiene particles, which has a layer of SAN grafted onto their surface. The binding matrix layer of SAN makes the two phases of the polymer compatible.
1.14.2 Acetal (Polyoxymethylene, POM, Polyacetal) Acetal resins provide a well-balanced set of properties including a hard self-lubricated surface and excellent chemical resistance, strength, stiffness, and toughness over a broad temperature range. The acetal homopolymer was first introduced in 1956 as a semi-crystalline form of polymerized formaldehyde forming a linear chain of molecules of oxymethylene. In the homopolymerization process, formaldehyde is separated from water and purified to CH2O gas, which is then polymerized to the polyoxymethylene molecule. In this case, the molecule is stabilized by a reaction with acetic anhydride to give acetate end groups. The acetate capped homopolymer is less resistant to attack by a base, but it has a higher melting point and mechanical advantages of strength, stiffness, toughness, hardness, creep, and fatigue than the acetal copolymer. In the acetal copolymerization process, formaldehyde is first converted into a cyclic structure of three formaldehyde molecules called trioxane. The trioxane is separated, purified, and reacted with a comonomer (ethylene oxide) to prepare polyoxymethylene which has randomly distributed –CH2–CH2– groups in the chain. This resultant raw polymer is then heated and treated with a base to degrade the ends of the molecules back to a –CH2–CH2– “block” point at each end. This results in a molecule that is resistant to further degradation by basic environments. The end-capping of homopolymer and copolymer chains is necessary to prevent the irreversible depolymerization of the polymer backbone during melt processing. The thermal energy causes “unzipping” of the –H–O–CH2–O–CH2– end group to a formaldehyde monomer.
1: Polymeric Materials and Properties
The outstanding characteristics of this polymer include stiffness, which permits the design of parts with large areas and thin cross sections; high tensile strength and creep resistance under a wide range of temperatures and humidity conditions; high fatigue resistance and resilience for applications requiring springiness and toughness. Acetal has achieved importance in applications because of a good balance of properties. Two types of acetals are available: one is a homopolymer resin with higher mechanical properties, higher end-use temperatures, and higher melt flow index, and the other is a copolymer resin with better processing characteristics and impact resistance.
1.14.3 Polymethyl Methacrylate (Acrylic, PMMA) PMMA (acrylic) polymers have outstanding optical properties, weatherability, and a full range of transparent, translucent, and opaque colors. Acrylics are composed of polymers and copolymers in which the major monomerics belong to two families of ester-acrylates and methacrylates. Hard, clear acrylic sheets are made from methyl methacrylate; molding and extrusion resins are made in a continuous solution from methyl methacrylate copolymerized with small percentages of other acrylates or methacrylates. The low strain optic coefficient of acrylics, coupled with their ability to be molded with low stress, makes them an ideal material for video disks. Sheets extruded from an acrylic base impact-modified grade have excellent thermoforming characteristics and can be stiffened by applying glass reinforced polyester to the inside surface with a spray gun to produce bathroom whirlpool tubs. The high flow grade has the best transparency, as it does not contain acrylonitrile, making it suitable for medical applications in which transparency is of prime importance. Acrylic plastics can be cleaned with solutions of inorganic acids, alkalis, and aliphatic hydrocarbons. However, chlorinated and aromatic hydrocarbons, esters, and ketones will attack the acrylic plastics.
1.14.4 High Temperature Nylon (HTN) HTN belongs to the aliphatic-aromatic polyamide family. It is produced either as a semi-crystalline or as an amorphous type of polymer.
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HTNs require higher melt temperatures for processing, resulting in a higher degree of crystallinity in the polymer. This polymer characteristic provides a better dimensional stability, improved chemical resistance, and higher end-use temperatures to the HTN molded products. HTN service temperature provides an additional 50°F (27.75°C) of higher stability than standard nylon 6/6 resins. HTNs are produced using various chemical additives, glass fiber reinforcements, mineral fillers, flame retardants, and impact modifiers, depending on the market needs. Injection molders familiar with nylon 6, 6/6, or 6/12 will not have to change molds or molding machines to process HTN. However, there are some processing differences between these two nylon families. Close attention must be paid to drying where conventional nylons require a moisture content of no more than 0.2%. A sealed HTN bag comes predried, but with the recommendation to predry at 175°F (80°C) for 2–16 hours or more before processing. As with all nylons, part dimensioning must take into account both shrinkage and ambient moisture absorption by the molded parts. The mold shrinkage rates of these semi-crystalline HTN polymers are similar to the conventional nylons. Shrinkage rates range from 0.18 to 0.22 in./in. (or %) for the unreinforced grades, and from 0.02 to 0.06 in./in. (or %) for the reinforced grades. Moisture absorption will reduce shrinkage. There is an increase in melt viscosity for HTN; it does not run like water as the standard easy flow nylons do, and the advantage is less mold flashing with lower temperature molds (180°F, or 82°C). One of the characteristics of the HTN resins is the fast molding cycle. These materials set very quickly in the mold cavity before the molded parts are ejected. This family of materials may carry the name nylon, but their high performance drastically exceeds the characteristics of traditional nylon materials. There is a specially formulated HTN series of polyphthalamides that has a melt temperature of 570°F (298°C) and provides excellent heat resistance for heat soldering owing to its partial aromatic nature. The flame retardant grade resins have achieved UL 94-V0 rating for a 0.031 in. (0.78 mm) wall thickness.
1.14.5 Liquid Crystal Polymer (LCP) LCPs are a family of diverse polymers that lack the molecular structure (amorphous and crystalline
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mixture) and homogeneity found in HTNs. Most commercial LCPs are copolyesters, copolyamides, or polyester-amides, although many other chemistries are possible. LCP structures range from partially aliphatic to wholly aromatic polymers. The melting point characteristics, the end-use temperature properties, the chemical and solvent resistance, flammability, processibility, and cost depend on individual products. LCPs are processed in the liquid crystalline state. All thermoplastic LCPs show one-dimensional order, which results from the semi-rigid, essentially linear architecture of the molecules. They are found in either the solid or the nematic states (parallel to a common axis). Because melt viscosity increases by a factor of two to ten when the polymer passes from the nematic to the isotropic state, processing is extremely difficult as melt temperature approaches the clearing point. Also important is that melt point is a low enthalpy transition, thus there is little crystallization, allowing fast cycling and low mold shrinkage. The chemical structure common to all melt processed LCPs is para-hydroxy benzoic acid. There are three classes of LCPs. The first is a general purpose class, which exhibits exceptional ease of processing, dimensional stability, molded part repeatability, chemical resistance, flame resistance, strength, and stiffness (e.g., computer multiple pin connectors). The second is a more temperature resistant variant, which is not quite as easy to process, but has good dimensional stability, good chemical resistance, excellent flame resistance, and very good strength and stiffness (e.g., ABS brake sensors). The third, a lower temperature performance class, is less expensive and less easily processed, but has good dimensional stability, with good strength and stiffness, less chemical resistance, and moderate flame resistance (e.g., computer hardware connectors).
Selection of Polymeric Materials
is equal to 0.96. The melting points range from 410°F to 491°F (210°C–255°C). Nylon 6/6 and nylon 6 are the most important commercial products. Other nylons are 6/9, 6/10, 6/12, 11, and 12. The higher the number of carbon atoms (i.e., the lower the concentration of amide groups), the lower the melting point. Nylons are modified by use of monomer mixtures leading to copolymers. These are normally less crystalline, more flexible, and more soluble than the homopolymers. Additives are compounded in nylons to improve thermal and photolytic stability, facilitate processing, increase flammability resistance, enhance hydrolytic resistance, and increase lubricity. Modification is an important asset. Fiber and mineral reinforcement are widely used. Blending with elastomeric modifiers has yielded nylons with improved toughness. Toughened nylon alloys (50% RH) have a notched Izod impact strength ranging from 2.5 to 4.5 ft-lb/in.; super tough nylon 6/6 (Zytel® ST-801) exhibits Izod impact values between 15 and 20 ft-lb/in. (50% RH). Nylon is resistant to oils, greases, solvents, and bases. Nylon is resistant to fatigue, repeated impact toughness and abrasion resistance, plus has a low coefficient of friction. Nylon has high tensile strength properties, creep resistance, and retains most of its mechanical and electrical properties over a wide temperature range. One of its limitations is high moisture pickup, which results in changes in dimensional and mechanical properties. Unreinforced nylon 6/6 has a temperature rating for continuous service ranges from 195°F to 265°F (90.5°C–129°C). The available types of nylon compounds include lubricated, nucleated, heat stabilized, UV stabilized, hydrolytically stabilized, flame retarded, glass reinforced, Kevlar® fiber reinforced, mineral reinforced, toughened, melt flow modified, electrically conductive, and several nylon alloys.
1.14.6 Polyamide (PA, Nylon) Polyamides are synthetic polymers that contain an amide group –CONH– as a recurring part of the chain. Nylons are made from (a) diamines and dibasic acids, (b) ω-amino acids, or caprolactam. The most common types of resins are nylon 6, nylon 6/6, and nylon 6/12. They are made from hexamethylene diamine and the 12-carbon acid, dodecanedioic acid, or HOOC(CH2)10COOH. The molecular weights of nylons range from 900 to 31,000. On average, for linear products of melt polymerization, the molecular weight per molecular number ratio is equal to 1.85, and the molecular viscosity per molecular weight ratio
1.14.7 Polyetherimide (PEI) PEIs are amorphous, high performance thermoplastic polymers. Their chemical structures consist of repeating aromatic imide and ether units. PEIs are characterized by high strength and rigidity at room and elevated temperatures, long-term high heat resistance, highly stable dimensional and electrical properties, and broad chemical resistance. Unmodified PEI is amber in color and transparent and exhibits inherent flame resistance and low smoke generation without the use of halogenated or other types of flame retarding additives.
1: Polymeric Materials and Properties
The amorphous structure of PEIs contributes to their excellent dimensional stability, low shrinkage, and highly isotropic mechanical properties. The high glass transition temperature (Tg) of 420°F (215°C) and high performance strength and modulus characteristics at elevated temperatures are provided by the very rigid imide groups in their chemical structure. The high Tg allows PEI to be used intermittently at 392°F (200°C) and permits short-term excursions to even higher temperatures. Higher strength and stiffness at elevated temperatures are achieved with glass or carbon fiber reinforcement. Unreinforced PEI resins are rated as having continuous end-use temperatures from 338°F to 356°F (170°C–180°C), and are listed as UL 94-V0, down to 0.010 in. (0.25 mm) wall thickness. PEI resins are available either in unmodified form or reinforced with 10%, 20%, 30%, and 40% glass fiber. Grades with carbon reinforcement for high strength and static dissipation, and a series of products with internal lubricants are also available. Other grades and blends are offered for blow molding, structural foam, and extrusion processes. PEI (Ultem®) has been formulated to meet specific needs such as electromagnetic interference shielding capability and aircraft cabin interior flammability (and heat release) requirements.
1.14.8 Polyetherether Ketone (PEEK) PEEK is a subgroup of the ketone polymer groups which are similar to polyarylether ketone (PAEK) and polyetherketone ketone (PEKK). Because of their semi-crystalline nature, PEEK resins demonstrate an excellent balance of physical properties, including strength at elevated temperature, chemical resistance, and hydrolytic and thermal stability. They offer the highest level of thermal resistance together with thermoplastic processing capability. PEEK resins are produced as unreinforced resins as well as glass and carbon fiber reinforced resins. PEEK materials retain their mechanical properties at high continuous end-use temperatures (higher than 480°F, or 248°C). The retention of flexural modulus and tensile strength properties is excellent, especially for the fiber reinforced resins. Another characteristic of PEEK-based resins is the UL 94-V0 rating without the addition of any flame retardant additives. PEEK resins have good electrical property values. Chemical resistance at elevated temperatures in various aggressive environments shows that reinforced PAEK is a chemically resistant material. The reten-
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tion of tensile properties for unreinforced PAEK resins is greater than 75%. Fiberglass reinforced PAEK resins exhibit a higher retention rate of tensile properties, but show some loss of tensile strength properties. Unreinforced PAEK resins react chemically with acids and bases.
1.14.9 Polycarbonate (PC) Polycarbonate is produced by reacting bisphenol A and carbonyl chloride in an interfacial process. This reaction is carried out under basic conditions in the presence of an aqueous and an organic phase. Molecular weight is controlled by a phenolic chain stopper. Trifunctional monomers are added for increased melt strength for extrusion and blow molding applications. Polycarbonate is an amorphous engineering thermoplastic material with exceptionally high impact strength, transparency, high dimensional stability, and moderate temperature resistance. Melt mass-flow rate is one of the most difficult properties to regulate without affecting the impact strength of polymer. Polycarbonate is characterized by an exceptionally high notched Izod impact strength between 12 and 17 ft-lb/in. only for 0.125 in. wall thickness. The notched Izod impact is drastically reduced when wall thickness is greater than 0.125 in. Polycarbonate has a glass transition temperature (Tg) of 300°F (148°C), high gloss finishing, low mold shrinkage, and low moisture absorption properties. Polycarbonate has high corona resistance and insulation resistance properties, and a dielectric constant that is independent of temperature. Polycarbonate can be compounded to produce resins for sterilizability, flame retardance, stain resistance, and fiberglass and mineral reinforcements with various additives. These ingredients enhance the thermal stability, UV stability, tensile strength, stiffness, and flame retardants.
1.14.10 Modified Polyphenylene Oxide (PPO) The chemical composition of PPO homopolymer is poly (2,6-dimethyl-1,4-phenylene ether) or poly [oxy-(2,6-dimethyl-1,4-phenylene)]. PPOs are rigid, amorphous, tough, and dimensionally stable plastics over a wide range of elevated temperatures. Modified PPO resins are the result of compounding PPO with polystyrene and various additives. An unmodified PPO has a glass transition temperature of 302°F (150°C). PPO resins are very difficult to process
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because of viscosity or fillers and their amorphous molecular structure. Modified PPO permits tailoring of the viscosity and toughness properties to meet specific requirements. The addition of rubber impact modified HIPS provides modified PPO with an improved Izod impact value. Nylon 6/6 improves the chemical resistance and thermal property of PPO. To increase flexural modulus and reduce mold shrinkage, glass and mineral filled modified PPO resins are available. Dimensional stability of modified PPO at moderated temperatures and stresses can be maintained in a number of environments such as strong alkalis and bases, detergents, and hot water. Filled and flame retardant grades are available. Modified PPO blends make excellent insulators because they have low loss and dissipation factors over broad temperature and frequency ranges. These plastics also have inherently low dielectric constants and high dielectric strengths. High humidity conditions have little effect on these properties.
1.14.11 Polybutylene Terephthalate (PBT) PBT is produced by the transesterification of dimethyl terephthalate with butanediol. This reaction takes place by a catalyzed melt poly-condensation, resulting in a repetition of the molecular units. PBT is a semi-crystalline and tough engineering thermoplastic. It is characterized by low moisture absorption, excellent electrical properties, and good chemical resistance, mechanical strength, and heat resistance. The continuous service temperature for unfilled PBT is 194°F (90°C) and for fiberglass reinforced PBT is 240°F (115°C). The thermal resistance of PBT is improved by compounding with nylon 6/6 (10–30%). Moisture absorption can be reduced and process characteristics and mechanical properties of PBT are improved by compounding with low density polyethylene (15–25%). An enhanced gloss surface also can be achieved when PBT is compounded with PET. Molten PBT is subject to hydrolytic degradation and so it must be rigorously dried before melt processing. A maximum moisture content of 0.02% is recommended.
1.14.12 Polyethylene Terephthalate (PET) PET is produced by a reaction of either purified terephthalic acid or dimethyl terephthalate with
Selection of Polymeric Materials
ethylene glycol. The high viscosity melt is converted into amorphous clear pellets by rapid quenching and cutting in water. PET polymers are long chain, unbranched molecules that are produced by the condensation reaction between a dibasic organic acid or ester and a glycol. PET injection molding grades include a proprietary crystallization system that provides rapid crystallization needed for good moldability at normal molding temperatures. PET has the stiffest polymer chain possible for thermoplastic polyesters with an outstanding combination of strength, stiffness, and high melt mass-flow properties. Basic PET applications are electrical and electronic components, industrial fibers and films, and beverage bottles. PET is a semi-crystalline engineering thermoplastic produced by a proprietary crystallization technology. Reinforced PET resins are only available in the following grades: general purpose, high melt massflow, low warp, toughened, and flame retardant. Most of the PET compounded resins are glass fiber/mineral reinforced. Glass fiber and mica are used for low warpage electrical applications. Reinforced grade resins are available with fiberglass contents ranging from 15 to 55%. There are several flame retardant resins meeting UL 94-V0 and UL 94-5V requirements, down to 0.031 in. (0.78 mm) wall thickness. Molten PET is subject to hydrolytic degradation and so it must be rigorously dried before melt processing. A maximum moisture content of 0.02% is recommended.
1.14.13 Polyethylene (PE) Polyethylene is manufactured using three basic chemical processes: slurry particle reactor process, gas phase process, and the new metallocene catalyst technology. The last process may also be used with conventional catalysts to combine the advantages offered by both polyolefin polymerization catalysts. A new class of catalysts for polyolefins based on nickel and palladium can produce a very broad range of molecular weights and branching. A highly efficient catalyst system aids in the polymerization of ethylene and allows the use of lower temperatures and pressures than required for manufacturing conventional low density polyethylene (LDPE). Polyethylene is available in a range of flexibilities, depending on the production process. High density polyethylene (HDPE) is the most rigid of the three basic types of polyethylene resins, which also include LDPE and linear low density polyethylene (LLDPE).
1: Polymeric Materials and Properties
HDPE can be processed by a wide variety of thermoplastic processing methods and is particularly useful where moisture resistance and low cost are required. Polyethylene is limited by low end-use temperature characteristics. Blow molding consumes the largest amount of HDPE. About 35% is used to make blow molded products. Extruded products consume about 30% and injection molding accounts for about 20%. High molecular weight HDPE resins have high strength; they are used in packaging films, sheets, pipes, and large blow and rotation molding items. LDPE is the second largest polyethylene in use by capacity in the USA. Extrusion is the dominant process used with LDPE resins. Extruded products, principally films, account for 75% of its use. High molecular weight LDPE film grade resins produce high-gloss, high-clarity films that exhibit good toughness and heat sealing capability.
1.14.14 Polytetrafluoroethylene (PTFE) These fluorocarbon resins are produced by the polymerization of tetrafluoroethylene. PTFE is composed of long, straight chains of fluorinated carbons (CF2 groups). The chemistry of PTFE provides the unique characteristics of the polymer. The helical backbone of carbon atoms symmetrically surrounded by fluorine atoms provides the resin with its unique chemical, electrical, and thermal properties. Continuous service temperatures range from near absolute zero (–460°F, or –273°C) to 500°F (260°C). This polymer has an initial melting point of 648°F (342°C) and although it does not char, the depolymerization temperature depends on the atmospheric conditions or whether oxygen is present. Accelerated degradation begins at 720°F (382°C). When exposed to flame, PTFE will burn, but it does not continue to burn when the flame is removed because it has an exceptionally high limiting oxygen index and will not support combustion in air. Like most fluoropolymers, PTFE has outstanding electrical insulation properties. Its dielectric constant and loss factor are low and uniform across a wide temperature and frequency range. PTFE is also known for its low coefficient of friction, which is the lowest of all materials. PTFE is flexible, strong, and tough at temperatures as low as –460°F (–273°C). It has an opaque white color, and can be formed into very thin and transparent films.
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PTFE polymers are available in granular, fine powder, and water base dispersion forms. PTFE cannot be processed by melt techniques because of the lack of flow (i.e., extremely high viscosity) of its molten form. PTFE has a high melt viscosity property and the polymer never becomes fluid. At temperatures above 620°F (327°C) it becomes a self-supporting gel. Because of these unusual melt characteristics, traditional thermoplastic processing equipment cannot be used. PTFE starts its transformation at ambient conditions, at pressures between 2,000 and 5,000 psi. The self-supporting gel is then sintered at temperatures ranging between 690°F and 710°F (365°C and 377°C) under a controlled temperature cycle. To make tapes or sheets of PTFE (up to 0.250 in. (6.35 mm) thick), a cylinder is molded and veneered. Free sintering (outside the mold) always follows compression of powder, whether the process is used to mold PTFE into sheets or billets through simple compression molding. To form multiple, simply shaped items, using an automatic process, isostatic and automatic molding methods are used. A flexible membrane and hydraulic pressure are used to make complicated shapes. PTFE is also highly resistant to blending with fillers and reinforcers. However, one or more fillers can be added to PTFE in some circumstances to prevent creep or cold flow from occurring when a load is applied to the soft material. Fillers are particularly desirable when PTFE is used for parts in a dynamic operation where a high rate of wear may occur. The fillers include fiberglass, carbon, graphite, bronze, and molybdenum disulfide. There are copolymers of tetrafluoroethylene that can be processed by traditional melt processing technologies and fluoropolymers that need special types of processing techniques. Fluoropolymer resins include fluorinated perfluoroethylene-propylene copolymer, perfluoroalkoxy alkane, ethylenetetrafluoroethylene, polyvinylidene fluoride, chlorotrifluoroethylene, ethylene chlorotrifluoroethylene, polyvinyl fluoride, and other less important polymers.
1.14.15 Polyphenylene Sulfide (PPS) PPS is produced commercially by the reaction of 1,4-dichlorobenzene with a suitable sulfur source, such as sodium sulfide. PPS is a semi-crystalline, aromatic polymer composed structurally of a series of alternating para-substituted phenylene rings and divalent sulfide moieties. PPS is an engineering thermoplastic
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material with an excellent combination of properties: thermal stability and unusual insolubility, chemical resistance, and inherent flame resistance. PPS has good thermal stability, so the isothermal weight losses as a function of time were measured at several temperatures. These results indicate good resistance to thermal degradation up to 700°F (370°C). PPS possesses excellent solvent resistance, being almost totally insoluble in organic solvents below 390°F (198°C). PPS is affected only by high temperature exposure to a few organic solvents, strong mineral acids, and strong oxidizing environments. Oxidizing agents such as peracetic acid and aqueous sodium hypochlorite oxidize the sulfide moiety to sulfoxide and/or sulfone groups. Because of its chemical structure, PPS can char when exposed to an external flame, but PPS is inherently flame resistant. It possesses a high limiting oxygen index and a low radiant flame spread index and it is classified as UL 94-V0 and UL 94-5V, down to 0.058 in. (1.47 mm) wall thickness. The continuous service temperature classification is between 330°F and 438°F (165°C and 225°C) depending on the PPS compound formulations. The auto-ignition temperature is 1,000°F (537°C). Glass fiber reinforcement produces injection and compression molding compounds that exhibit high tensile strength, good flexural strength, high heat deflection temperature, low elongation, and very low impact strength. Both filled and unfilled PPS grades exhibit the inherent flame resistance and excellent chemical resistance characteristics of the base resin. Results of long-term heat aging of moldings are consistent with the good thermal stability of the polymer. Glass fiber and mineral fiber PPS compounds have good retention of tensile properties under long-term temperature and load exposure. PPS and its compounds possess good overall electrical (insulating) properties. The glass filled compound has a low dielectric constant and dissipation factors are retained over a broad frequency range. In addition, the dissipation factor remains low at temperatures up to 390°F (198°C). These properties are also retained after exposure to high humidity environments.
1.14.16 Polypropylene (PP) Polypropylene is manufactured by polymerizing propylene monomer with a titanium based catalyst; a
Selection of Polymeric Materials
co-catalyst (triethylaluminum) is added to initiate the polymerization reaction and hydrogen is used in the reactor to control polymer molecular weight. This reaction is produced using a slurry or gas phase type of process. Polypropylene is the fastest growing commodity thermoplastic in the world. It is a versatile polymer used in a wide range of applications from fibers, flat filaments used in industrial bags and outdoor carpets, packaging films, home appliances, electrical and medical devices to automobile bumpers, air management systems, interior panel and under-the-hood components. There are three polypropylene structures produced during polymerization: isotactic, syndiotactic, and atactic. The principal structure of polypropylene is isotactic, which is a semi-crystalline polymer in a helical form. This structure has good mechanical properties that can be further improved with fiberglass reinforcements and mineral fillers (talc, calcium carbonate). Syndiotactic polypropylene is produced by the monomer units inserted alternately head-to-tail. This structure is more flexible with better impact resistance and clarity than the isotactic structure. Atactic polypropylene (hard wax amorphous monomer) is a by-product of the manufacturing process. The atactic monomer affects the process efficiency. This product is used in roofing tars and adhesives for the shoe industry. All forms of polypropylene are susceptible to oxidation due to the presence of a tertiary hydrogen. Polypropylene is stabilized against thermal degradation by the addition of primary and secondary antioxidants. Neutralizing agents are also added to stabilize the low levels of chloride ash generated during manufacturing. Other special additives are used such as antistatic agents, slip agents, and UV stabilizers. Polypropylene is produced as homopolymers and copolymers. The physical properties range from good tensile strength and stiffness to a tough, flexible, and low strength polymer. Polypropylene homopolymer has the highest melting point with a wide range of melt flow rates and stiffness. Polypropylene copolymers incorporate small amounts of ethylene which lower the crystallinity rate, producing higher impact strength even at low temperatures, and more flexibility, but a lower melting point and low melt mass-flow rate properties.
1: Polymeric Materials and Properties
1.14.17 Polystyrene (PS) A polymerization process using a thermal or catalyzed reaction of styrene monomer is used to produce an amorphous polystyrene polymer. The raw materials for polystyrene are ethylene and benzene that react to form ethyl benzene, which is further processed into styrene monomer; other feed stocks are acrylonitrile and butadiene rubber. Other additives such as plasticizers, release agents, and stabilizers are added to the process to give the polymers the desired characteristics. The formulation may also include colorants, flame retardants, UV stabilizers, and impact modifiers. Polystyrene is one of the most popular commodity amorphous thermoplastic resins. It has a broad range of balanced properties and an attractive price. Polystyrene is produced in the following grades: general purpose (GPPS), which is selected for its clarity, rigidity, and suitability for many applications; rubber modified medium impact (MIPS), which is selected where more flexibility is required; rubber modified high impact (HIPS), which is selected where high impact resistance is required; and expandable polystyrene (EPS), which is selected where insulation is required. MIPS and HIPS contain butadiene rubber as the comonomer to increase toughness. However, these impact resistance grades are opaque in color.
1.14.18 Polysulfone (PSU) PSU is produced from bisphenol A and 4,4-dichlorodiphenylsulfone by a nucleophilic process. PSU is an amorphous engineering thermoplastic material with exceptionally high temperature resistance and high rigidity; it is transparent and dimensionally stable. It exhibits a significant reduction in notched Izod impact strength. Its continuous service temperature is 320°F (160°C) with a glass transition temperature of 374°F (190°C) and it is classified as UL 94-V0, down to 0.062 in. (1.57 mm) wall thickness. PSU has been approved for use in several medical and laboratories applications. It exhibits good chemical resistance, but is sensitive to polar solvents and other solutions. PSU complies with the gamma sterilizability requirements. The melt viscosity of PSU is relatively insensitive to shear forces. The low degree of molecular orientation during injection molding affects the physical properties of the molded products slightly, independent of the melt flow direction.
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PSU can be plated by an electrolytic process which imparts added strength to the polymer. Both nickel and copper electroless processes can be used. All molded parts must be annealed before plating. PSUs can also be vacuum-metallized using standard equipment and techniques.
1.14.19 Polyvinyl Chloride (PVC) PVC is produced as a white powder containing coarse, porous particles. Several additives are used during processing to give the polymers the desired characteristics. The two major categories of PVC compounds available are: the homopolymer suspension powder blend and the dispersion materials. PVC suspension compounds account for more than 90% of the total PVC market. Suspension compounds can be produced as either rigid thermoplastics for nonplasticized applications or flexible thermoplastics for plasticized applications. The suspension ingredients may be blended using two types of mixers: the high intensity mixer and the ribbon blender. High intensity blenders and twin screw extruders are used for producing the rigid compounds. Ribbon blenders and single screw extruders are used for producing the flexible compounds. PVC dispersion compounds account for 7% of the total market. They have a fine particle size of about 1 µm. When dispersed in plasticizers and other liquid ingredients, they form plastisols and organosols that are applied as liquid, paste, coatings, and fused with heat. All PVC compounds require heat stabilizers to allow processing without degrading and discoloring the polymer. Plasticizers are added to increase the flexibility of the compound. They can also improve the heat stability or improve the flame retardancy of the compound. Fillers are used to reduce the cost, improve dimensional stability, stiffness, and impact strength. PVC is a recyclable commodity thermoplastic material of large consumption by the building and construction industry. PVC is popular because of its excellent impact, wear, chemical, and UV resistance. PVC is used in a large variety of end products such as flooring, garage doors, windows frames and profiles, siding, tubing, and connectors. These products are commonly available in standard sizes and shapes, low cost, and easy to work with (weld, repair, and paint).
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1.15 Families of Thermoplastic Elastomers (TPEs) TPEs combine the properties of thermoplastic and vulcanized rubber. TPEs are rubbery materials with the characteristics of a thermoplastic and the performance of a thermoset vulcanized rubber. The properties of TPEs are based on their chemical structure and the morphology of their matrix. In thermoset rubbers, carbon black is used as a filler to modify the hardness or stiffness of the rubber materials. In TPEs, the molecular structure of the polymer itself provides the flexibility, impact strength, and stiffness. Sometimes, it is necessary to modify the ramifications of the soft segments of the structure’s molecular chain to obtain the required properties (stiffness, toughness).
1.15.1 Thermoplastic Polyurethane (TPU) Elastomer Polyurethane chemistry is based on the reaction of isocyanates with various active hydrogen elements present in the compounds. The basic ingredients of TPUs are diisocyanates and long-chain and shortchain diols. The diisocyanates and short-chain diols form the basis of the hard segment structure, while the long-chain diols provide the basis of the soft segments. Because the hard and soft segments do not mix, TPU exhibits a two-phase structure. TPUs consist of an amorphous phase (soft block) and a crystalline phase (hard block). The hard segments are generally dispersed in the amorphous phase (continuous phase). The hard segments determine the hardness, stiffness, impact or tear strength, and upper end-use temperature. The soft segments determine the elastic characteristics and lower end-use temperature properties. TPUs can be categorized into polyester and polyether types. Polyester-based TPUs generally have better physical properties such as thermo-oxidative stability and oil resistance. At a similar hardness, polyether-based TPUs exhibit better low temperature properties such as hydrolytic stability and resistance to microbial attack. TPUs are known for their excellent resistance to abrasion. However, the abrasive wear of a TPU is considerably affected by the surface heat buildup during the test, coefficient of friction, stress loading, and contact areas. The compression set property of a polymer is a measure of its elastic recovery behavior under a
Selection of Polymeric Materials
specific load, at various times and temperatures. Typical compression set values of annealed TPU range between 25% and 50% of recovery to original position after load is removed, and between 60% and 80% of recovery without annealing. A compression set value under 25% is obtained with a deflection at 158°F (70°C) and curing for 22 hours. The thermal stability of TPU is dependent on the isocyanates structures and chain extenders. TPUs decompose slowly between 398°F (203°C) and 482°F (250°C). They exhibit a loss of mechanical properties and discoloration upon exposure to sunlight. The hydrophilic property of TPU has prevented its use in applications where consistently high electrical insulation resistance is required.
1.15.2 Styrenic Block Copolymer (SBS) Thermoplastic Elastomers Styrenic block copolymer TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization. At least one phase is a styrenic polymer that is hard at room temperature, but becomes fluid when the polymer is heated. Whereas another phase is a softer material that is rubber-like at room temperature. These polymers have many of the physical properties of vulcanized rubbers (softness, flexibility, resilience), and process characteristics and properties similar to the thermoplastics because of their molecular structures. Most of the polymer molecules have their end polystyrene segments in different domains. At room temperature, these polystyrene domains are hard and act as cross-links, tying the elastomeric chains together in a three-dimensional network. But in styrenic block copolymer TPEs the domains lose their strength when the material is heated or dissolved in solvents, allowing the polymer to flow. When the melt is cooled or the solvent is evaporated, the domains become hard again and the network regains its original integrity. These materials must be protected against oxidative degradation and in some cases against sunlight also, depending on their end use. Many types of styrenic block copolymer TPEs have been produced to meet specific end- use requirements. General purpose styrenic block copolymers, which are soluble when compounded under improved stability, are used for wire and cable coatings and medical applications.
1: Polymeric Materials and Properties
Like most thermoset vulcanized rubbers, the styrenic block copolymer TPEs have no commercial applications when the product is just a pure polymer. Depending on the particular requirements of each end use, they are compounded with other polymers, oils, fillers, and additives. In almost all cases, the products contain less than 50% of the styrenic block copolymer.
1.15.3 Polyolefin Thermoplastic Elastomer (TPO) TPO materials are defined as compounds of various polyolefin polymers, semi-crystalline thermoplastics, and amorphous elastomers. The most common types of TPOs are composed of polypropylene and ethylene-propylene rubber (EPR). EPR may be a copolymer of either only ethylene and propylene monomers or a third monomer. The diene monomer provides a small amount of unsaturation in the polymer chain for sulfur cross-linking. This is called EPDM rubber. Like most thermoplastic elastomers, TPO products are composed of hard and soft segments. The exact size and shape of these segments determines the properties of the compounds. Higher impact strength at low temperature is achieved by increasing the amount of ethylene present in the copolymer. Ethylene also reduces the rigidity of the copolymer. The soft segments of the polymer chain are composed of EPR or EPDM rubber. Rubber materials with nearly equal amounts of ethylene and propylene are totally amorphous. The softest EPDM rubber grades are the most efficient impact modifier additives used in TPOs. Some of the other additives used in TPO compounds function by modifying the rubber phase. These ingredients may include fillers, reinforcements, lubricants, heat stabilizers, antioxidants, UV stabilizers, colorants, and processing aids. All TPO compounds are unaffected by water and exhibit fair chemical resistance to acids and bases. Hot hydrocarbon solvents tend to soften and swell severely for the softer formulations and slightly for the harder TPO products. The chemically inactive surface of a TPO molded product makes bonding to other materials difficult. Most TPO compounds are good electrical insulating materials. They have good dielectric strength properties and they do not absorb moisture, because
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they are not hygroscopic. However, when a TPO product is subjected to weather (UV) and pollution, special TPO formulations with several UV and heat stabilizers must be selected for the application (higher cost). Many TPO parts that are used in automotive applications (bumpers) must be painted with an automotive finish to match or accent the painted finish of the other body panels. Special paint systems are available that will give excellent adhesion of the primer and paint to the TPO surface. The TPO compound materials do not have a molded surface that chemically reacts with the primer paint system to develop a strong and durable bond. To achieve adequate adhesion of the primer, the TPO molded surface must be modified (plasma, flame, electric treatment) to obtain a reactive surface.
1.15.4 Elastomeric AlloyThermoplastic Vulcanizate (EA-TPV) Vulcanized elastomeric alloys are thermoplastic elastomers composed of mixtures of two or more polymers that have received a proprietary treatment. EA-TPVs are a category of TPEs made of a rubber and plastic polymer mixture in which the rubber phase is highly vulcanized. The plastic phase of an EA-TPV is a polypropylene, and the rubber phase is an ethylene-propylene elastomer. The vulcanization of the rubber phase of an EATPV results in various property improvements. EATPVs are insoluble in rubber solvents and exhibits less swelling in some solvents than TPO. The vulcanization offers several improvements in the properties: increase in tensile strength and modulus, decrease in compression set, and decrease in swelling caused by oils. Vulcanization of the rubber phase also improves the retention of properties at temperatures below 200°F (93°C). The tear strength and abrasion resistance of EA-TPVs is good but not outstanding—the tear strength increases progressively with hardness. The desirability of low or high resilience (hysteresis) will depend on the end use. An outstanding dynamic property of EA-TPVs is their fatigue resistance. EA-TPVs have excellent electrical insulation properties, and excellent dielectric strength 400 volts/mil at a thickness of 0.080 in. (2.0 mm). The volume and surface resistivity of EPDM-TPVs are sufficiently high to justify their consideration for
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use as primary electrical insulators as well as jacketing material. The dielectric constant (specific inductive capacitance) and power factors are basic limits in the selection of a primary electrical insulation material.
1.15.5 Melt Processible Rubber (MPR) MPR is an amorphous polymer, providing a very low flexural modulus, good tensile strength, and linearly proportional stress-strain curve. Halogenated polyolefin polymers are intermixed with a large number of structurally coupled ethylene interpolymers. They contain functional groups that are strongly proton-accepting to promote hydrogen bonding with the alpha hydrogen atoms of the halogenated polyolefin over the entire composition range. MPR alloys are composed of proprietary ethylene interpolymers and chlorinated polyolefins, in which the ethylene interpolymer component has been partially cross-linked. Plasticizers, stabilizers, antioxidants, curing agents, and fillers can be incorporated into the resin to provide flexibility and reinforcement. MPR polymer blends are single-phase systems because they have a single glass transition temperature. As an amorphous material, MPR exhibits no crystalline melting point. It gradually softens with increasing temperature, but does not flow at any temperature unless also subjected to shear. By increasing the shear rate of the MPR, the melt viscosity property of the polymer increases proportionally. The polymer melt only becomes softer, but not a liquid melt. MPR is a thermoplastic elastomer and not a thermoset rubber material. The MPR resin produces parts that look, feel, and perform like vulcanized rubber. The MPR resin has been recognized as the most rubber-like product among all the TPE families. MPR resins can be processed in equipment used for thermoplastics or thermosets. It also offers the combination of resistance to heat, oil, chemicals, and weathering. Because of its rheology and its amorphous singlephase nature, MPR is obviously not a plastic. It has a fractional melt index, does not melt, and exhibits thixotropic behavior. It has a useful plastic processing property in flowing like a thermoplastic on application of shear. Unlike plastics, it displays minimum draw-down because of elastic recovery, at temperatures under 200°F (93°C).
Selection of Polymeric Materials
MPR has outstanding oil resistance, heat aging, weatherability, and is recyclable. It has mechanical properties, tear and abrasion resistance, and a continuous surface temperature similar to mid-performance rubbers. MPR is used as an energy absorbing polymer, but has limitations in applications requiring very dynamic motion and load carrying capabilities.
1.15.6 Block Copolyester Thermoplastic Elastomer Block copolyester TPEs are based on 1,4-butanediol, terephthalic acid, and polytetramethylene ether glycol or polypropylene glycol. Because the building blocks behave like high tensile strength engineering plastics and elastomers, the product combines these characteristics in proportion to the ratio of hard and soft molecular segments. The fast crystallization rate of the hard segment, combined with exceptional melt stability of the backbone, allows these polymers to be processed like a thermoplastic material. Block copolyester TPEs have an excellent combination of mechanical properties such as tensile strength, elasticity, creep resistance, and dynamic properties. These polymers are operational over a broad service temperature range without significant changes in properties. They also have excellent chemical, heat, and oil resistance properties. A distinguishing characteristic of block copolyester TPE resins versus other flexible materials is their excellent dynamic performance, which makes these resins suitable for applications requiring long-term spring properties and flex life. Operating within their elastic limits, block copolyester TPEs are creep resistant, supporting loads for a long time without stress relaxation. They can be subjected to repeated cycles of tension and compression without significant loss of mechanical strength. Block copolyester TPEs demonstrate low hysteresis loss in dynamic applications. Product components working at low strain levels exhibit complete recovery in cyclic applications with little heat buildup. Resistance to cut growth during flexing is outstanding, mainly because of high resilience and low heat buildup. Block copolyester TPEs rank high among thermoplastics for impact resistance; most of the compounded grades cannot be broken by conventional notched Izod impact testing equipment. Block copolyester TPEs are used in electrical applications below 600 volts. The following characteristics make them attractive for electrical and
1: Polymeric Materials and Properties
electronic applications: good dielectric properties, high mechanical strength, creep resistance, spring properties, flex fatigue resistance, high impact strength, abrasion resistance, and end-use service temperatures ranging between –40°F and 300°F (–40°C to 148°C). Block copolyester TPE compounds have excellent oil resistance; the stiffer grades offer the best performance in hot hydrocarbon environments, with many resins being suitable for use with hot oil, grease, fuels, and hydraulic fluids. Block copolyester TPE compounds do not contain plasticizer additives in their formulations. The inherent chemical purity of block copolyester TPEs makes them an excellent choice for food contact and medical applications.
1.16 Families of Thermoset Polymers Thermoset compounds are organic polymers that cure to a solid and infusible mass by forming an irreversible three-dimensional network of covalent chemical bonds. Thermoset compounds are polymers with a combination of mechanical, thermal, electrical, and chemical resistance properties that allow them to compete with metals, ceramic, and thermoplastic materials. Thermoset compounds are used in many applications. Construction represents the largest single market area, consuming about half of the compounds produced. Other applications include adhesives for plywood and particle board, binders for insulation, coatings, matrix resins for laminates, and electrical molding products. Because they lack the strength and stiffness of metals, nearly all thermoset compounds contain particulates or fibrous reinforcements. Fillers such as calcium carbonate, glass flakes, and wood flour are added to reduce cost and increase the rigidity of the cured product. Fibers such as glass, carbon, and polyaramid increase its strength, stiffness, and cost. The amount of fillers compounded ranges between 45% and 75%.
1.16.1 Polyester Alkyd (PAK) Polyester alkyds are formed by the reaction of polyhydroxy compounds with unsaturated maleic
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acids. The resulting unsaturation in the polymer backbone can be utilized to cross-link the polymer and form a thermoset compound. A small percentage of high viscosity monomer compounded in the resin yields a relatively high melt flow rate, necessary for large and complex parts. This benefit, together with excellent electrical properties, dimensional stability, and low cost, makes it suitable for a variety of applications in the automotive and electrical industries. Polyester alkyd thermoset compounds are used for injection, transfer, and compression molding. Polyester alkyd compounds can be cured without additional molding pressure and do not release water when cured. Therefore, they can be used in a variety of coating applications. Polyester alkyds are primarily electrical materials. They combine good insulating properties at temperatures up to 300°F (148°C) for intermittent use, and up to 250°F (120°C) for continuous use. They also offer low resin cost and good insert molding characteristics for delicate and complex applications. Polyester alkyd compounds are also reinforced with fiberglass and minerals to provide substantial improvements in physical strength and impact resistance, and facilitate the molding process.
1.16.2 Diallyl Phthalate/ Isophthalate (DAP, DAIP) Diallyl phthalate resins are products of the reaction of allyl alcohol and an organic acid or anhydride. The monomer diallyl phthalate can be prepared by direct esterification of allyl alcohol and phthalic anhydride. The solution is then partially polymerized to a fusible resin or prepolymer by heating with a free radical initiator. This diallyl phthalate prepolymer, combined with a free radical initiator and various fillers, constitutes a diallyl phthalate molding compound. There are two molding compound types of diallyl phthalate resins: the orthoresin diallyl phthalate and the metaresin diallyl isophthalate. The orthoresin is the most commonly used; it provides excellent electrical properties, while the metaresin provides superior heat resistance characteristics. Diallyl phthalate offers a balance of electrical insulating properties, volume resistivity, dielectric strength, and arc resistance. It retains these properties even under long-term exposure to high heat and humidity. The most frequently used compounds are short glass fiber reinforced, which represent approximately
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70% of the market. The remaining 30% is divided fairly evenly between long glass fiber reinforced and mineral filled compounds. Diallyl phthalate molding compounds are available only as a filled system and are supplied complete with catalyst, pigment, and mold release. Its physical form varies with the type of reinforcement used. The mechanical properties of diallyl phthalate vary widely, depending on filler or reinforcement type and quantity. Because of its extremely stable carbon-tocarbon linkage and its tight knit three-dimensional structures, fully cured diallyl phthalate is extremely resistant to creep or cold flow, moisture, strong and weak acids, alkalis, and organic solvents. Moisture has little effect on the dielectric strength of the molded parts. Diallyl isophthalate has better heat resistance than diallyl phthalate, with use temperatures approximately 20°F higher than the latter.
1.16.3 Epoxy (EP) Epoxy is a thermoset plastic that has an epoxy ring consisting of two carbon atoms that are single-bonded to a common oxygen atom. There are two types of epoxies. Those polymers made by a reaction with epichlorohydrin are known as glycidyls, while those made by peroxidizing olefins are known as cycloaliphatics. Epoxidized phenols, or phenol glycidyl ethers, are the most commercially important resins, particularly epoxidized bisphenol A, known as the diglycidyl ether of bisphenol A (DGEBA). This resin provides an excellent balance of physical, chemical, and electrical properties. Epoxidized phenol novolacs are also available in a range of viscosities. Epoxies are used in combination with a coreactant, or curing agent. Therefore, the final composite properties are influenced by the choice of coreactant. The large number of coreactants available include amines, anhydrides, acids, phenolics, and amides, thereby providing a broad spectrum of performance possibilities. After aromatic glycidyl ethers, the epoxidized alcohols, glycols and polyols, or aliphatic glycidyl ethers are considered most valuable. They are typically used in combination with DGEBA resin to allow better processing. These materials are often very low in viscosity, which makes them highly attractive as dilutents for the more viscous phenolic base products. They can also improve flexibility and toughness properties, although this is generally at the
Selection of Polymeric Materials
expense of thermal and chemical resistance, particularly if the dilutent is monofunctional. The most common members of this group are epoxidized butanol, known as butyl glycidyl ether, and the epoxidized long chain monoalcohols. Physical properties can be varied over a wide range of rigidity and flexibility. This group of resins created the structure for the adhesive industry, as well as being used extensively in the fiber reinforced composite industries. Performance properties of epoxy composites have ratings from good to excellent for electrical resistance, good chemical resistance, and reasonably high glass transition temperatures. The use of fillers can often raise the glass transition temperature, reduce shrinkage, and increase the thermal conductivity and thermal resistance of the composite. The amount of filler used depends on the rheology of the system, the particle size, and the oil absorption tendency of the particular filler. A major weakness of epoxy resins is poor UV resistance and weathering. Processing of high performance composites is quite complex, particularly if a multiple-step cure schedule is required to achieve the highest possible thermal resistance in the composite structure. Epoxies used in composites are cured by an addition crosslinking mechanism, which does not generate volatile by-products. Part dimensions can be influenced by the cure, particularly if the cure temperature is too high, because a very vigorous exotherm can be generated, causing excessive shrinkage and changing the composite structure.
1.16.4 Phenol-Formaldehyde (Phenolic, PF) Phenolic resins are products of the condensation reaction of phenol and formaldehyde. Water is the by-product of this reaction. Substituted phenols and higher aldehydes may be incorporated to achieve specific properties such as reactivity and flexibility. Phenolic molding materials are high performance thermoset compounds, and are considered to be the workhorse of the plastics industry. With the heat and pressure of the molding process, phenolics react to form a three-dimensionally cross-linked molecular structure. This structure yields excellent dimensional and thermal stability with high load-bearing capability at elevated temperatures. Phenolics are used for
1: Polymeric Materials and Properties
close tolerance precision molded components that must function in hostile environments. The unreinforced phenolic polymer is a brittle material. However, a wide range of properties and molding process conditions can be obtained by using a variety of fillers and reinforcements (45%–65%). Lubricants, colorants, and other modifiers are also used. Wood flour or cellulosic filler yields a molding material with well-balanced properties and cost effectiveness. It has a continuous service temperature of 300°F (148°C). Modification with mineral fillers up to 45% yields rigidity with improved dimensional and thermal stability characteristics. Reduced water absorption and a lower coefficient of thermal expansion can also be obtained. Glass fiber reinforcement yields substantial improvement in dimensional stability, rigidity, and mechanical properties. Glass fiber reinforcement can be tailored to match the thermal expansion of metals. The continuous service temperature is 355°F (179°C). Specialty grades can be formulated with graphite, polytetrafluoroethylene, and impact modifiers to improve toughness and reduce coefficient of friction. Phenolic materials are available in black and brown colors. Phenolics are not stable under UV radiation. They are inert to most common solvents and weak acids and they have excellent resistance to natural oils, fats, greases, petroleum products, and automotive fluids, but poor chemical resistance to strong acids and alkaline solvents.
1.16.5 Unsaturated Polyester (UP) Unsaturated polyester polymers are manufactured by the condensation and polymerization of dibasic acids (or anhydrides) with dihydric alcohols, with the dibasic acid or anhydride being partially or completely composed of a 1,2-ethylenically unsaturated material such as maleic anhydride or fumaric acid. The resultant polymer can vary from a high viscosity liquid (brittle) to a low melt solid material. The polymer is then dissolved in a liquid reactive vinyl (1,2-ethylenically unsaturated) monomer, such as styrene, vinyl toluene, diallyl phthalate, or methyl methacrylate, to give a solution with a viscosity in the range between 2.0 and 20.0 poise. Unsaturated polyester reinforced with fiberglass was developed to produce protective housings for
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radar equipment. It is used in a wide variety of markets, including automotive, construction, marine, transportation, industrial, electrical, and sanitary, and as replacements for wood, concrete, marble, steel, and aluminum. These polymers are mostly chosen because of their ease of fabrication, lower weight, higher strength, corrosion resistance, and lower cost. However, they have pronounced styrenic odors. The use of a reinforcing fiber to produce unsaturated polyester composites dramatically improves the tensile and flexural properties. Inorganic fillers are used to improve stiffness and to reduce cost. The rigid and high glass transition temperature for bisphenaol A fumarate or chlorendic polyesters retains flexural strength up to 250°F (121°C). They are used in elevated temperature electrical and corrosion resistant applications. Chlorendics are used in strong acids at high temperatures, while bisphenaol A fumarate is better in strong basic solutions. However, isophthalic polyesters are more widely used because they have better thermal and chemical resistance, higher mechanical properties, and are easier to process.
1.16.6 Polyimide (PI) Polyimides are members of a class referred to as heteroaromatics. They have excellent mechanical strength properties (seven times stronger than steel) and excellent electrical and thermal insulation properties; the high-temperature specialty grades have a temperature resistance ranging between –270°F and 740°F (–168°C and 393°C) for extended exposure times, and up to 900°F (482°C) short-term. Polyimides resist higher temperatures better than any other unfilled polymer. They are produced either in cross-linked molecular structures as thermosets or in linear forms as thermoplastics. Their high glass transition temperatures require special processing methods. Several approaches have been used to allow the thermosetting reaction to proceed by addition polymerization. Polyimides are suitable for high end-use temperatures; they exhibit excellent fire, chemical, and solvent resistance; low coefficient of friction; low wear, abrasion, thermal, and creep resistance. The presence of the imide ring in the structure causes some hydrolytic instability, particularly towards alkalis. Polyimides are difficult to process, therefore, DuPont manufactures the basic polyimide polymer,
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transforms the polyimide powder into a film form (the film has the trade name Kepton®) for the electrical industry, and laminates the film with a fluoropolymer. DuPont also processes the powder into a fiber form known as Kevlar®. Polyimide powder compounded with low coefficient of friction additives is known as Vespel® and it is transformed into standard solid bars, plates, balls, and rods. Vespel® is also produced as custom finished parts.
1.16.7 Polyurethane (PUR) The general structure of polyurethane is R–(NCO)n, where typically n = 2–4, or even higher, and R is an aromatic or aliphatic group. The isocyanate group (R–N=C=O) reacts easily with hydroxyl groups (HO-R'). The resulting link between the two residues R and R' is the urethane group. Polyurethane thermosetting resins are used in coatings, adhesives, binders, sizings, flexible foams, fibers, sealants, and biomedical and external automotive structural applications. The production of filled polyurethanes by the reaction injection molding (RIM) process requires premixing the filler with the liquid components (polyol). Milled glass fibers are very common and can be used in concentrations up to 75% in the polyol. The effect of fillers on mechanical properties of the final foams follows the same order as observed in their influence on the viscosity of the raw materials. Flake materials have a more favorable effect on these properties and improve the dimensional stability at higher temperatures. With 20% mica, for example, the Young’s modulus can be increased by a factor of 3, but tensile strength and elongation at break are reduced. An example of reinforced polyurethane is the product of the structural RIM (SRIM) process. SRIM compounds are composites that contain random, unidirectional, or multidirectional glass fiber mats. These mats are put into the mold which is then closed and filled with the reactive mixture of raw materials. Because of its extremely low viscosity, the resin penetrates the reinforcing mat and solidifies the structure. This procedure allows the use of glass fiber contents of up to 70%.
1.16.8 Silicone (Si) The structure of silicone resembles that of silicon dioxide (SiO2) glass, which provides high temperature
Selection of Polymeric Materials
properties and resistance to radiation, ozone, and chemicals, and is different from that of organic polymers. Specifically, the polymer chain backbone consists of repeat units, whereas the polymer chain backbone of organic polymers is some configuration of repeat units. A SiO2 bond is stronger and more flexible than a C–C or C=C bond, which makes silicone polymers chemically stronger and more flexible than organic polymers. Silicone polymers resemble the three-dimensional network structures of sand (SiO2). The organic functionality of silicone allows light cross-linking between polymer chains, creating rubber elasticity. The outstanding characteristics of silicone include its ability to maintain properties at both low (down to −150°F (−101°C)) and high (up to 600°F (315°C)) temperatures, and its weathering and chemical resistance, physiological inertness, lubricity, excellent electrical properties, low surface tension, and compression set resistance.
1.16.9 Vinyl Ester (BPA) Vinyl ester resins are unsaturated esters of epoxy compounds. The most common versions are the reaction products of methacrylic acid and bisphenol A, an epoxy compound dissolved in a styrene monomer. Corrosion resistant reinforced resins based on vinyl esters and other compounds have successfully replaced traditional materials such as glass, steel, aluminum, concrete, and brick. Vinyl ester resins have many properties of epoxies combined with the processibility of polyesters. The addition of the methacrylate group allows vinyl esters to be cured in ways similar to the curing of unsaturated polyester compounds. The use of styrene or other reactive monomers allows low viscosities to be obtained at ambient temperature. Because of curing and material handling similarities, vinyl esters are often classified with unsaturated polyesters. One of the advantages of vinyl ester resins reinforced with fiber materials is that the reinforcement can be oriented in the direction requiring strength. The properties of vinyl ester resins are fatigue resistance, retention of properties at moderate temperatures, impact resistance, and creep resistance. Their excellent corrosion resistance is attributed to three basic factors: (1) the corrosion susceptible ester linkage is shielded by a methyl group;
1: Polymeric Materials and Properties
(2) the vinyl groups are very reactive and a complete cure of the backbone is easily accomplished; (3) the epoxy backbone is very resistant to chemical attacks. Processing can be based on either ambient temperature or elevated temperature cure systems. Vinyl ester resins are cured by peroxide initiated free-radical polymerization of the reactive unsaturations of
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styrene and the methacrylate groups. Using these ambient temperature cure systems and additives gives vinyl ester resins a wide range of gel times. Gel times from less than one minute to over three hours are possible. Some of the processes used to fabricate parts with vinyl ester resins at ambient temperature include open molding techniques such as hands lay-up and sprayup, filament winding, and centrifugal casting.