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Filled thermoplastic materials
Filled thermoplastic materials Part I: Fillers and compounding N. A. WATERMAN, R. TRUBSHAW and A. M. PYE Fulmer Research Institute, Stoke Poges, Slough SL2 4QD
The practice o f adding inorganic fillers to thermoplastic moulding materials, to improve properties and reduce costs, has increased dramatically in the past five years. This paper reviews the commercially available fillers; outlines the necessary compounding and processing precautions; details the performance/cost characteristics o f the new materials and illustrates their application in new components and products. 1. I n t r o d u c t i o n Thermoplastic materials offer the p r o d u c t maker the a b i l i t y to manufacture lightweight, c o m p l e x shaped, nearly finished components in one operation. A disadvantage, p a r t i c u l a r l y w i t h the l o w e r cost materials such as p o l y e t h y l e n e s , p o l y p r o p y l e n e s and polystyrenes, is dimensional instab i l i t y . This arises f r o m a c o m b i n a t i o n o f l o w elastic modulus, p o o r creep resistance and post-mould d i s t o r t i o n and is sufficient to exclude the application o f these materials to m a n y engineering components where t h e y w o u l d otherwise be suitable. T o r e m o v d this disadvantage, considerable e f f o r t is being expended by many organizations to develop p o l y m e r / f i l l e r c o m b i n a t i o n s which improve the desired performance
properties without increasing raw material and processing sorts. Reinforcement of thermoplastics, most commonly with glass fibres, is of course a wen-established practice. The improvements in performance properties of strength, stiffness and heat
74
resistance are rarely achieved with a significant reduction in total cost. This paper is concerned with fillers (including air) which may be added to thermoplastics to improve properties but also reduce the cost in position of the manufactured component. Part I describes the available fillers, details the necessary processing precautions and discusses the results obtained from certain finer/polymer combinations. Part II, which will appear in the next issue of this journal, will illustrate the application potential of filled thermoplastics by reference to currently produced and scheduled materials and components.
2. Primary considerations A general s u m m a r y o f the effect o f filler is presented in Table 1. It must be remembered that p r o p e r t y i m p r o v e ments are critically dependent on the q u a l i t y of. the c o m p o u n d i n g process (see section 4 below). In a d d i t i o n certain p o l y m e r / f i l l e r c o m b i n a t i o n s w i l l not o b e y the general rules ( f o r
example filling polypropylene with surface treated calcium carbonate can improve impact strength - see section 5 below) and the possible synergctic effects (or the reverse) of using more than one filler remains largely unexplored. It may be deduced from Table l
Table 1. General summary of the effect of fillers on the performance properties and processing characteristics of thermoplastics
Advantages
Limitations
Improved dimensional stability, e.g. better creep resistance, higher elastic modulus
Reduced tensile and impact strength with some fillers
Lower shrinkage and post-mould distortion
Surface finish generally inferior in terms of smoothness and gloss, to unfilled polymer
Lower coefficient of thermal expansion Faster production cycle times (components can be ejected from the mould at higher temperatures due to improved hot strength and after shorter times due to improved thermal conductivity)
Higher wear rates of machinery caused by some fillers
Improved bearing properties and abrasion resistance with certain fillers
Increase in density with most solid fillers compared with unfilled polymer
Long, thin complex shapes can present moulding problems
Improved fire resistance due to decrease heat of combustion Lower total costs of components
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
that the ideal candidate component for filled thermoplastics is a part requiring a stiffer, more heat-resistant material than the unfilled polymer, which needs to be produced at faster production speeds and does not require a very smooth surface finish or long thin complex shaped features. A complete analysis of the product requirements is essential before any consideration is given to which polymer, which fillers and how much.
3. C o m m e r c i a l l y available fillers
3.1 Fillers General Fillers, in the context of this article, include finely divided particulate materials derived from mineral sources such as calcium carbonate, talc and silicon flour; organic fibres and pulps, glass spheres and beads. Brief mention is also made of air which should be considered as a filler and is deliberately introduced into components by means of a blowing agent. The important parameters of all fillers are size, shape, density, comparability with the polymer both during processing and in-service in the component, availability and cost. Although particle sizes of up to 40/am diameter can improve stiffness and glass transition temperature (Tg), particles below 10/am are more effective. An average size around 3/am appears to give the best results for talc and calcium carbonate in polypropylene and there is evidence t that very fine particle sizes (1/am) can cause severe embrittlement. Average particle size and the range of sizes present will in any case be determined by the economics of fine particle production. Filler morphology is very important. Polymers filled with spherical particles are easier to mould, cause less wear of equipment, and produce the most isotropic end properties. Plate-like particles (such as talc) give greater improvements in stiffness but sometimes at the cost of embrittlement. The greatest improvement in the properties of filled thermoplastics is achieved by surface treating the filler with a coupling agent to improve bonding with the polymer. Surfacetreated fillers are also easier to disperse and mould, cause less abrasive wear of moulding equipment and allow the highest percentage loading levels. Coupling agents do not improve rigidity but are imperative for optimum impact and tensile properties of filled thermoplastics. To be effective -
the coupling agent must react with both filler surface and the polymer. Typical coupling agents are silicones and stearates. Silicones are combustible with a high flash point and can cause skin" irritation. Hence special handling precautions are required. Stearates do not present the same hazards and are widely used for calcium carbonate with polypropylene. However it is doubtful if any positive coupling effect is achieved. Improved performance properties are probably the result of more homogeneous dispersion of the filler throughout the matrix.
3.2 Filler details 3.2.1. Blowing agents. Blowing agents reduce material usage, improve rigidity and strength to weight ratio in moulded components and affect other properties such as thermal and electrical insulation, acoustic response and impact resistance. In addition to being used to produce structural foam mouldings, small concentrations ( 0 . 5 1.0%) are used to prevent sink marks in thick sections, ribs, bosses, etc, of mouldings. Although the blowing agents are expensive, as 1 5 - 3 0 % (by weight) of base polymer can be saved by use of 0 . 3 - 1 % of blowing agent, cost reduction is a prime reason for their usage. Two main types exist: physical blowing agents such as volatile liquids, gases such as nitrogen and carbon dioxide and soluble solids and chemical blowing agents such as azodicarbonamide which are compounded (preblended) with the resin; chemical blowing agents, which are more commonly used, decompose when the resin is heated with the evolution of gas. No one chemical blowing agent is suitable for all polymers and it is necessary to match polymer melt temperature and the thermal decomposition temperature of the blowing agent.
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
I I1 III IV
V
VI VII
Blowing agents are often combined with other fillers in thermoplastics. Short fibres, glass beads, and mineral fillers have all been tried but reduce the effectiveness of the weight reduction achieved by foaming. To maximize this benefit, hollow microspheres can be used as an additional filler. This filler cannot be used in conventional injection moulding as the higher pressures (compared with foam moulding) will collapse the spheres. 3.2.2. Calcium carbonates Forms: Limestone (dry or wet ground); chalk whiting, marble, precipitated chalk. Density: Minerals 2.71 ; Precipitates 2.6. Particle size: Minerals 0 . 5 - 1 0 / a m ; precipitates 0 . 0 5 - 1 0 / a m (typically 0.5/am). Effect on properties: Increase in stiffness but less so than clays and talc (silicates). Decrease in tensile strength, more so than for similar quantities of silicates. Stearate coated calcium (up to 30% by weight) carbonate can increase impact strength and change mode of failure from brittle to ductile. Untreated CaCO3 reduces impact strength but less so than talc. White mouldings can be achieved with good surface finish. Effect on processing: No compounding, processing or machinability problems. Known suitable polymers: Data has been recorded for up to 50% filler bonding with polypropylenes and polyethylenes. Common filler for PVC. Chalk has been used with acetals. 3.2.3. Silicates. S i O 2 . Silicon sand; silica flour; quartz; diatomaceous earth; kieselguhr; tripoli; precipitated amorphous silica; aerogel; pyrogenic; talc (see 3.2.3). Effect on properties: Silica flour does not excessively increase viscosity. Reduction in tensile and impact properties and in shrinkage. Silicates are heat and chemical resistant, have good thermal conductivity and electrical insulation properties. Density
Partlc'lesize
Silica sand Silica flour Diatomaceous earth (Kieselguhr or Tripoli) Precipitated amorphous silica Aerogel
2.7 2.7 2.0--2.3 Not known
8S% 200 mesh 98% 325 mesh 1-10/am ! -4/~m
Pyrogenic Talc
2.1 --2.2
80%, 7 #m 0 . 0 1 - 0 . 0 2 #m
2.65--2.8
Typically 95%
2.0--2.1 2.0 --2.1
325 mesh
75
Effect on processing: No special difficulties, but can cause wear to machines and moulds. Suitable polymers: Probably most thermoplastics. Silicates are widely used as fillers for paints and coatings (for talc see below).
3.2.4. Talc. Mga Si40 l0 .(OH)2. Powder Density: 2.65-2.8. Particle size: c. 95% 325 mesh. Effects on properties: Stiffness increased appreciably, even at small percentage loadings. Some reduction in tensile and impact properties together with reduced shrinkage. Flexural properties can show improvements. Improvements in electrical insulation, heat and moisture resistance and abrasion resistance. Effect on processing: Talc is one of the easiest mineral fillers to compound or process. Polypropylene master batches of 80% talc are available (e.g. TBA Arpylene), which is a higher percentage than can be readily attained with other fillers. Improvements in stiffness mean shorter cooling cycles. Talc-filled polypropylene sheet has been developed .for vacuum forming, as the improved toughness at melt temperatures overcomes the problems of forming pure polypropylene. Suitable polymers: Probably all thermoplastics. Talc-filled polypropylene and acetal are available commercially. Additional information: Some sources of talc can contain asbestiform needle-shaped particles which have the possibility of causing talcosis. Such grades are obviously unsuitable for use in food containers.
3.2.5. Magnesium carbonate. MgCO3 with CaCO3. Dolomite; magnesite; precipitated (heavy and light grades). Density: mineral and heavy precip. grades 0.6. Apparent density: Light precipitated grades 0 . 1 5 - 0 . 2 0 . Effect on properties: Reductions in tensile and impact properties together with increased stiffness and reduced shrinkage. Effect on processing: No special difficulties with compounding or processing. Suitable polymers: Polyethylenes, polypropylenes, PVC, ABS.
76
Additional information: Can be used as an auxiliary fire retardant in conjunction with antimony trioxide. Availability : Microdol grades available from Norwegian Talc. 3.2.6. Clays and aluminosilicates. Kaolins; AI2Si2Os(OH)4; SiO2 4 0 60%, A1203 2 5 - 4 0 % . Forms: China clay; kaolin clay; calcined clay; bentonite; fuller's earth; slate flour; mica; muscovite; vermiculite; pumice. Density
Particle size 20/am 8.0-0.3/am 5.0- i.0/am 0.5/am
Fuller's earth
2.6 2.5-2.6 2.5-2.63 2.6-2.7 1.75--2.5
Slate flour
2.8-3.1
99%, 400
Mica
2.7-2.9 2.2 -2.3 2.2-2.3
20/am Coarse Mostly 200
China clay Kaolin clay Calclined clay Bentonite
Mostly 30 mesh mesh
Vermiculite
Pumice
mesh
Effect on properties: Reductions in tensile and impact properties with improvements in stiffness and shrinkage. Surface modified mica is claimed to improve tensile, flexural and impact strengths as well as the heat deflection temperature. Effect on processing: Difficult to get good bonding of filler and polymer, especially when coupling agents are not used. China clays and kaolin are used to impart high surface gloss and smoothness to PVC mouldings and extrusions. Fuller's earth increases melt viscosity dramatically which makes screw rotation difficult. Suitable polymers: PVC (with china clays, kaolin, fuller's earth), ABS, PP (with surface modified mica). Additional information: Mostly used with thermosets, due to poor wetting with thermoplastics and because they absorb exothermic heat during curing and act as thixotropic agents. 3.2.7. Celluloses. Forms: Wood flour; nutshell flour; wood pulps; cotton fillers; cork; residues (sawdust, cereal, straws, jute, hemp, sisal, flax). Density: Generally low (c. 1.3). Fibrous form with much variation of size and strength. Woodflour 5 0 200/am with ratio of approx. 2.5: 1; nutshell flour approx. 40 mesh. Effect on properties: Cellulose materials have a number of advantages, notably their fibrous form which
greatly aids cohesion with the polymer and low density. Nutshell flour has high lignin and cutin wax content, giving greater hydrophobic properties. Improvements in the impact strength of polypropylene are reported when peanut hull flours are added. 2 The great variation in quality of these fillers, from batch to batch, makes close control of properties difficult, and with the rougher particles (e.g. residues) poor surface finish and low tensile and impact properties result. In general, electrical properties are poor. Cotton fillers increase impact and flexural properties, but their high cost does not make them of interest purely as cost reduction additives. Effect on processing: Care must be taken during thermal processing not to char or degrade these organic fillers. Suitable polymers: ABS, HDPE, PP and UPVC reported suitable with peanut hull flour. Calendered and extruded PP with wood flour or pulp. Extruded wood pulp filled polypropylene sheet (up to 50% loading) is commercially available. Material can be compression moulded and surface finished with PVC or carpet type, or other decorative finishes in one operation. Additional information: All these fillers are familiar additives to thermosets, where they are important in improving impact properties. Insufficient data is available on their effects with thermoplastics, although their generally low cost and low density, together with a fibrous nature, make them potentially attractive. 3.2.8. Glass. Forms. Fibres; solid beads; hollow spheres. Density: 2.5 (solid). Apparent density: 0 . 2 - 0 . 8 (hollow spheres). Effect on properties: Well-established value for increasing stiffness and improving shrinkage without seriously affecting other properties. Fibres will increase tensile strength while beads tend to lower it. Fibres greatly increase heat deflection temperature. From the cost reduction point of view, consideration could be given to a glass-reinforced grade of cheap polymer that may offer properties comparable to higher priced high-performance unfilled polymers. Effect on processing: Can cause wear to machines and moulds. Reduced cycle times because of higher heat distortion temperature.
M A T E R I A L S IN ENGINEERING APPLICATIONS. Vol. 1, December 1978
Suitable polymers: Almost all thermoplastics. Additional information: Although on a weight for weight basis, glass is much cheaper than common plastics, the cost of surface treatment and compounding seem t o account for commercial glass-reinforced grades of polymer being slightly more expensive than unfilled grades. Solid beads axe available in grade sizes from 20 to 100/am dia., but 50/am has been found to be most suitable for thermoplastics applications. Hollow spheres cannot be used in high-pressure injection moulding processes and have found their main use as impact modifiers for rigid reaction injection moulded polyurethane mouldings. Silica-alumina hollow microspheres, with a density of 0.8 have been introduced under the trade name Fillite. Coupling agents, usually organosilanes or chrome complexes, are an essential part of glass-fibre fillers improving the glass resin bond. Solid glass beads are ' A ' type glass otherwise 'E' glass is dominant. 3.2.9. Calcium silicate. CaSiO3. Forms : Wollastonite. Density: 2.8, fibrous structure. Effect on properties: Confers heat, chemical and moisture resistance. Improves shrinkage, dimensional stability, stiffness, hardness, abrasion resistance, and gives outstanding electrical and thermal insulation properties. Effects on processing: Silicone coupling between Wollastonite and polymer essential if optimum properties are to be developed. Suitable polymers: PVC, polyolefins, nylons. Additional information: Much used in alkyd and vinyl type paints, and also with thermoset mouldings.
3.2. ! O. Processed mineral fibre. Forms: Mineral wool. Density: 2.7. Effect on properties: Seen as a possible alternative to asbestos and also as a means of improving impact strength. In most cases produces allround property enhancement. Use of coupling agents recommended. Effect on processing: No problems with compounding up to 50% weight. No information about moulding properties. Suitable polymers: Nylons, thermoplastic polyesters (PBT), acetal, polystyrene and p o l y p r o p y l e n e .
Additional information: The fibrous nature, high strength and excellent heat resistance of this filler make it useful as a property improver irrespective of cost reduction possibilities. 3.2.11. Asbestos. Forms: Mineral fibres; chrysotile MgSi20s (OH)3 - White Asbestos. Density: 2.55. Most commonly employed form. Amosite (FeMg)6SisO22(OH)2 brown asbestos Density: 3 . 4 - 3 . 5 . More heat and acid resistant than chrysotile. Crocodolite Na2 Fes Sis 022 (OH)2 blue asbestos. Density: 3 . 3 - 3 . 4 . The manufacture of products conraining crocidolite is banned in the UK by the Health and Safety Inspectorate. Anthophynite (FeMg)~ Sis 022 (OH)2. Limited application in plastics. Effect on properties: Large improvements in fire resistance, heat distortion temperature, and temperature. Low coefficient of thermal expansion. Low mould and machine -
Wear.
Suitable polymers: Polypropylene, polystyrene, styrene acrylonitrile (SAN) and nylons. Additional information: Asbestos has received a great deal of unfavourable publicity recently due to the health hazards associated with airborne asbestos fibres. The hazards do not apply to the moulding of fully compounded asbestos reinforced polymer pellets. Care must be exercised if mouldings are to be machined or ground or are likely to be subject to abrasion in use. Suppliers of asbestosfilled grades of materials offer a comprehensive information service on the safe processing of these materials. NB. Whilst only the use of crocidolite asbestos is banned in the UK, in certain countries around the world (e.g. Sweden) the use of asbestos is banned completely except for reinforcement of cement in pressure pipes and for friction materials, normally phenolic based.
4. Processing of filled thermoplastics While the c o m p o u n d i n g and m o u l d i n g of filled thermoplastics may be considered by the design engineer to be
the responsibility of the materials suppliers and the custom moulder, appreciation of the factors affecting the final component properties is
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
essential if trouble-free performance is to be achieved. 4.1. Compounding There is currently rapid development in the availability of fully compounded filled thermoplastics - a complete list will be given in Part II of this paper. Prototype moulding quantities of these materials can normally be obtained. However, the full potential of filled thermoplastics will only be realized for most applications if materials are tailored to individual end uses. In such cases during proving trials materials may be compounded by dry tumbling. Larger quantities may be melt compounded on special purpose equipment when some indication of property levels has been obtained. Even distribution of filler is essential for optimum properties and polymer in powder form is more suitable for dry tumbling than pellets. Mineral oil or glycol can be used as a wetting agent for granules to assist bleaching. Surface treatment of the filler with coupling agents such as silicones are essential for maximum property improvement. Stearate coatings improve dispersion of the filler in the polymer, reduce machinery wear and improve component properties in certain areas. 4.2. Moulding The problems of moulding heavily filled thermoplastics have two main causes. Fillers generally increase melt viscosity and can cause increased machine wear. Melt viscosity is sensitive to particle size and small particles give higher viscosity (but also normally better properties in the final component). If problems are encountered in moulding then raising the barrel temperature on the moulding machine, adding more lubricant (e.g. zinc stearate) and increasing filler particle size are possible remedies. Wear of machinery is an economic problem and the cost savings obtained by the use of filler has to be balanced against increased processing costs. Such costs will be difficult to estimate prior to full-scale production. The problem should not be overstated. In many cases, especially with surfacetreated fillers, no increase in wear is recorded. In the case of foamed plastics it is
77
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MATERIALS
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IN E N G I N E E R I N G
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APPLICATIONS,
V o l . 1, D e c e m b e r 1978
most important that suppliers' recommendations for physical and chemical blowing agents are followed. Use of the incorrect chemical blowing agent will not only fail to produce foaming
but degradation of the base polymer can occur. A summary of the effect of fillers on various thermoplastics is given in Table 2. In Part lI of this paper
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
comprehensive properties data on the commercially available grades of filled thermoplastics will be given together with typical applications, present and planned.
79
The practice o f adding inorganic fillers to thermoplastic moulding materials, to improve properties and reduce costs, has increased dramatically in the past five years. This paper reviews the commercially available fillers; outlines the necessary compounding and processing precautions; details the performance/cost characteristics o f the new materials and illustrates their application in new components and products. 1. I n t r o d u c t i o n Thermoplastic materials offer the p r o d u c t maker the a b i l i t y to manufacture lightweight, c o m p l e x shaped, nearly finished components in one operation. A disadvantage, p a r t i c u l a r l y w i t h the l o w e r cost materials such as p o l y e t h y l e n e s , p o l y p r o p y l e n e s and polystyrenes, is dimensional instab i l i t y . This arises f r o m a c o m b i n a t i o n o f l o w elastic modulus, p o o r creep resistance and post-mould d i s t o r t i o n and is sufficient to exclude the application o f these materials to m a n y engineering components where t h e y w o u l d otherwise be suitable. T o r e m o v d this disadvantage, considerable e f f o r t is being expended by many organizations to develop p o l y m e r / f i l l e r c o m b i n a t i o n s which improve the desired performance
properties without increasing raw material and processing sorts. Reinforcement of thermoplastics, most commonly with glass fibres, is of course a wen-established practice. The improvements in performance properties of strength, stiffness and heat
74
resistance are rarely achieved with a significant reduction in total cost. This paper is concerned with fillers (including air) which may be added to thermoplastics to improve properties but also reduce the cost in position of the manufactured component. Part I describes the available fillers, details the necessary processing precautions and discusses the results obtained from certain finer/polymer combinations. Part II, which will appear in the next issue of this journal, will illustrate the application potential of filled thermoplastics by reference to currently produced and scheduled materials and components.
2. Primary considerations A general s u m m a r y o f the effect o f filler is presented in Table 1. It must be remembered that p r o p e r t y i m p r o v e ments are critically dependent on the q u a l i t y of. the c o m p o u n d i n g process (see section 4 below). In a d d i t i o n certain p o l y m e r / f i l l e r c o m b i n a t i o n s w i l l not o b e y the general rules ( f o r
example filling polypropylene with surface treated calcium carbonate can improve impact strength - see section 5 below) and the possible synergctic effects (or the reverse) of using more than one filler remains largely unexplored. It may be deduced from Table l
Table 1. General summary of the effect of fillers on the performance properties and processing characteristics of thermoplastics
Advantages
Limitations
Improved dimensional stability, e.g. better creep resistance, higher elastic modulus
Reduced tensile and impact strength with some fillers
Lower shrinkage and post-mould distortion
Surface finish generally inferior in terms of smoothness and gloss, to unfilled polymer
Lower coefficient of thermal expansion Faster production cycle times (components can be ejected from the mould at higher temperatures due to improved hot strength and after shorter times due to improved thermal conductivity)
Higher wear rates of machinery caused by some fillers
Improved bearing properties and abrasion resistance with certain fillers
Increase in density with most solid fillers compared with unfilled polymer
Long, thin complex shapes can present moulding problems
Improved fire resistance due to decrease heat of combustion Lower total costs of components
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
that the ideal candidate component for filled thermoplastics is a part requiring a stiffer, more heat-resistant material than the unfilled polymer, which needs to be produced at faster production speeds and does not require a very smooth surface finish or long thin complex shaped features. A complete analysis of the product requirements is essential before any consideration is given to which polymer, which fillers and how much.
3. C o m m e r c i a l l y available fillers
3.1 Fillers General Fillers, in the context of this article, include finely divided particulate materials derived from mineral sources such as calcium carbonate, talc and silicon flour; organic fibres and pulps, glass spheres and beads. Brief mention is also made of air which should be considered as a filler and is deliberately introduced into components by means of a blowing agent. The important parameters of all fillers are size, shape, density, comparability with the polymer both during processing and in-service in the component, availability and cost. Although particle sizes of up to 40/am diameter can improve stiffness and glass transition temperature (Tg), particles below 10/am are more effective. An average size around 3/am appears to give the best results for talc and calcium carbonate in polypropylene and there is evidence t that very fine particle sizes (1/am) can cause severe embrittlement. Average particle size and the range of sizes present will in any case be determined by the economics of fine particle production. Filler morphology is very important. Polymers filled with spherical particles are easier to mould, cause less wear of equipment, and produce the most isotropic end properties. Plate-like particles (such as talc) give greater improvements in stiffness but sometimes at the cost of embrittlement. The greatest improvement in the properties of filled thermoplastics is achieved by surface treating the filler with a coupling agent to improve bonding with the polymer. Surfacetreated fillers are also easier to disperse and mould, cause less abrasive wear of moulding equipment and allow the highest percentage loading levels. Coupling agents do not improve rigidity but are imperative for optimum impact and tensile properties of filled thermoplastics. To be effective -
the coupling agent must react with both filler surface and the polymer. Typical coupling agents are silicones and stearates. Silicones are combustible with a high flash point and can cause skin" irritation. Hence special handling precautions are required. Stearates do not present the same hazards and are widely used for calcium carbonate with polypropylene. However it is doubtful if any positive coupling effect is achieved. Improved performance properties are probably the result of more homogeneous dispersion of the filler throughout the matrix.
3.2 Filler details 3.2.1. Blowing agents. Blowing agents reduce material usage, improve rigidity and strength to weight ratio in moulded components and affect other properties such as thermal and electrical insulation, acoustic response and impact resistance. In addition to being used to produce structural foam mouldings, small concentrations ( 0 . 5 1.0%) are used to prevent sink marks in thick sections, ribs, bosses, etc, of mouldings. Although the blowing agents are expensive, as 1 5 - 3 0 % (by weight) of base polymer can be saved by use of 0 . 3 - 1 % of blowing agent, cost reduction is a prime reason for their usage. Two main types exist: physical blowing agents such as volatile liquids, gases such as nitrogen and carbon dioxide and soluble solids and chemical blowing agents such as azodicarbonamide which are compounded (preblended) with the resin; chemical blowing agents, which are more commonly used, decompose when the resin is heated with the evolution of gas. No one chemical blowing agent is suitable for all polymers and it is necessary to match polymer melt temperature and the thermal decomposition temperature of the blowing agent.
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
I I1 III IV
V
VI VII
Blowing agents are often combined with other fillers in thermoplastics. Short fibres, glass beads, and mineral fillers have all been tried but reduce the effectiveness of the weight reduction achieved by foaming. To maximize this benefit, hollow microspheres can be used as an additional filler. This filler cannot be used in conventional injection moulding as the higher pressures (compared with foam moulding) will collapse the spheres. 3.2.2. Calcium carbonates Forms: Limestone (dry or wet ground); chalk whiting, marble, precipitated chalk. Density: Minerals 2.71 ; Precipitates 2.6. Particle size: Minerals 0 . 5 - 1 0 / a m ; precipitates 0 . 0 5 - 1 0 / a m (typically 0.5/am). Effect on properties: Increase in stiffness but less so than clays and talc (silicates). Decrease in tensile strength, more so than for similar quantities of silicates. Stearate coated calcium (up to 30% by weight) carbonate can increase impact strength and change mode of failure from brittle to ductile. Untreated CaCO3 reduces impact strength but less so than talc. White mouldings can be achieved with good surface finish. Effect on processing: No compounding, processing or machinability problems. Known suitable polymers: Data has been recorded for up to 50% filler bonding with polypropylenes and polyethylenes. Common filler for PVC. Chalk has been used with acetals. 3.2.3. Silicates. S i O 2 . Silicon sand; silica flour; quartz; diatomaceous earth; kieselguhr; tripoli; precipitated amorphous silica; aerogel; pyrogenic; talc (see 3.2.3). Effect on properties: Silica flour does not excessively increase viscosity. Reduction in tensile and impact properties and in shrinkage. Silicates are heat and chemical resistant, have good thermal conductivity and electrical insulation properties. Density
Partlc'lesize
Silica sand Silica flour Diatomaceous earth (Kieselguhr or Tripoli) Precipitated amorphous silica Aerogel
2.7 2.7 2.0--2.3 Not known
8S% 200 mesh 98% 325 mesh 1-10/am ! -4/~m
Pyrogenic Talc
2.1 --2.2
80%, 7 #m 0 . 0 1 - 0 . 0 2 #m
2.65--2.8
Typically 95%
2.0--2.1 2.0 --2.1
325 mesh
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Effect on processing: No special difficulties, but can cause wear to machines and moulds. Suitable polymers: Probably most thermoplastics. Silicates are widely used as fillers for paints and coatings (for talc see below).
3.2.4. Talc. Mga Si40 l0 .(OH)2. Powder Density: 2.65-2.8. Particle size: c. 95% 325 mesh. Effects on properties: Stiffness increased appreciably, even at small percentage loadings. Some reduction in tensile and impact properties together with reduced shrinkage. Flexural properties can show improvements. Improvements in electrical insulation, heat and moisture resistance and abrasion resistance. Effect on processing: Talc is one of the easiest mineral fillers to compound or process. Polypropylene master batches of 80% talc are available (e.g. TBA Arpylene), which is a higher percentage than can be readily attained with other fillers. Improvements in stiffness mean shorter cooling cycles. Talc-filled polypropylene sheet has been developed .for vacuum forming, as the improved toughness at melt temperatures overcomes the problems of forming pure polypropylene. Suitable polymers: Probably all thermoplastics. Talc-filled polypropylene and acetal are available commercially. Additional information: Some sources of talc can contain asbestiform needle-shaped particles which have the possibility of causing talcosis. Such grades are obviously unsuitable for use in food containers.
3.2.5. Magnesium carbonate. MgCO3 with CaCO3. Dolomite; magnesite; precipitated (heavy and light grades). Density: mineral and heavy precip. grades 0.6. Apparent density: Light precipitated grades 0 . 1 5 - 0 . 2 0 . Effect on properties: Reductions in tensile and impact properties together with increased stiffness and reduced shrinkage. Effect on processing: No special difficulties with compounding or processing. Suitable polymers: Polyethylenes, polypropylenes, PVC, ABS.
76
Additional information: Can be used as an auxiliary fire retardant in conjunction with antimony trioxide. Availability : Microdol grades available from Norwegian Talc. 3.2.6. Clays and aluminosilicates. Kaolins; AI2Si2Os(OH)4; SiO2 4 0 60%, A1203 2 5 - 4 0 % . Forms: China clay; kaolin clay; calcined clay; bentonite; fuller's earth; slate flour; mica; muscovite; vermiculite; pumice. Density
Particle size 20/am 8.0-0.3/am 5.0- i.0/am 0.5/am
Fuller's earth
2.6 2.5-2.6 2.5-2.63 2.6-2.7 1.75--2.5
Slate flour
2.8-3.1
99%, 400
Mica
2.7-2.9 2.2 -2.3 2.2-2.3
20/am Coarse Mostly 200
China clay Kaolin clay Calclined clay Bentonite
Mostly 30 mesh mesh
Vermiculite
Pumice
mesh
Effect on properties: Reductions in tensile and impact properties with improvements in stiffness and shrinkage. Surface modified mica is claimed to improve tensile, flexural and impact strengths as well as the heat deflection temperature. Effect on processing: Difficult to get good bonding of filler and polymer, especially when coupling agents are not used. China clays and kaolin are used to impart high surface gloss and smoothness to PVC mouldings and extrusions. Fuller's earth increases melt viscosity dramatically which makes screw rotation difficult. Suitable polymers: PVC (with china clays, kaolin, fuller's earth), ABS, PP (with surface modified mica). Additional information: Mostly used with thermosets, due to poor wetting with thermoplastics and because they absorb exothermic heat during curing and act as thixotropic agents. 3.2.7. Celluloses. Forms: Wood flour; nutshell flour; wood pulps; cotton fillers; cork; residues (sawdust, cereal, straws, jute, hemp, sisal, flax). Density: Generally low (c. 1.3). Fibrous form with much variation of size and strength. Woodflour 5 0 200/am with ratio of approx. 2.5: 1; nutshell flour approx. 40 mesh. Effect on properties: Cellulose materials have a number of advantages, notably their fibrous form which
greatly aids cohesion with the polymer and low density. Nutshell flour has high lignin and cutin wax content, giving greater hydrophobic properties. Improvements in the impact strength of polypropylene are reported when peanut hull flours are added. 2 The great variation in quality of these fillers, from batch to batch, makes close control of properties difficult, and with the rougher particles (e.g. residues) poor surface finish and low tensile and impact properties result. In general, electrical properties are poor. Cotton fillers increase impact and flexural properties, but their high cost does not make them of interest purely as cost reduction additives. Effect on processing: Care must be taken during thermal processing not to char or degrade these organic fillers. Suitable polymers: ABS, HDPE, PP and UPVC reported suitable with peanut hull flour. Calendered and extruded PP with wood flour or pulp. Extruded wood pulp filled polypropylene sheet (up to 50% loading) is commercially available. Material can be compression moulded and surface finished with PVC or carpet type, or other decorative finishes in one operation. Additional information: All these fillers are familiar additives to thermosets, where they are important in improving impact properties. Insufficient data is available on their effects with thermoplastics, although their generally low cost and low density, together with a fibrous nature, make them potentially attractive. 3.2.8. Glass. Forms. Fibres; solid beads; hollow spheres. Density: 2.5 (solid). Apparent density: 0 . 2 - 0 . 8 (hollow spheres). Effect on properties: Well-established value for increasing stiffness and improving shrinkage without seriously affecting other properties. Fibres will increase tensile strength while beads tend to lower it. Fibres greatly increase heat deflection temperature. From the cost reduction point of view, consideration could be given to a glass-reinforced grade of cheap polymer that may offer properties comparable to higher priced high-performance unfilled polymers. Effect on processing: Can cause wear to machines and moulds. Reduced cycle times because of higher heat distortion temperature.
M A T E R I A L S IN ENGINEERING APPLICATIONS. Vol. 1, December 1978
Suitable polymers: Almost all thermoplastics. Additional information: Although on a weight for weight basis, glass is much cheaper than common plastics, the cost of surface treatment and compounding seem t o account for commercial glass-reinforced grades of polymer being slightly more expensive than unfilled grades. Solid beads axe available in grade sizes from 20 to 100/am dia., but 50/am has been found to be most suitable for thermoplastics applications. Hollow spheres cannot be used in high-pressure injection moulding processes and have found their main use as impact modifiers for rigid reaction injection moulded polyurethane mouldings. Silica-alumina hollow microspheres, with a density of 0.8 have been introduced under the trade name Fillite. Coupling agents, usually organosilanes or chrome complexes, are an essential part of glass-fibre fillers improving the glass resin bond. Solid glass beads are ' A ' type glass otherwise 'E' glass is dominant. 3.2.9. Calcium silicate. CaSiO3. Forms : Wollastonite. Density: 2.8, fibrous structure. Effect on properties: Confers heat, chemical and moisture resistance. Improves shrinkage, dimensional stability, stiffness, hardness, abrasion resistance, and gives outstanding electrical and thermal insulation properties. Effects on processing: Silicone coupling between Wollastonite and polymer essential if optimum properties are to be developed. Suitable polymers: PVC, polyolefins, nylons. Additional information: Much used in alkyd and vinyl type paints, and also with thermoset mouldings.
3.2. ! O. Processed mineral fibre. Forms: Mineral wool. Density: 2.7. Effect on properties: Seen as a possible alternative to asbestos and also as a means of improving impact strength. In most cases produces allround property enhancement. Use of coupling agents recommended. Effect on processing: No problems with compounding up to 50% weight. No information about moulding properties. Suitable polymers: Nylons, thermoplastic polyesters (PBT), acetal, polystyrene and p o l y p r o p y l e n e .
Additional information: The fibrous nature, high strength and excellent heat resistance of this filler make it useful as a property improver irrespective of cost reduction possibilities. 3.2.11. Asbestos. Forms: Mineral fibres; chrysotile MgSi20s (OH)3 - White Asbestos. Density: 2.55. Most commonly employed form. Amosite (FeMg)6SisO22(OH)2 brown asbestos Density: 3 . 4 - 3 . 5 . More heat and acid resistant than chrysotile. Crocodolite Na2 Fes Sis 022 (OH)2 blue asbestos. Density: 3 . 3 - 3 . 4 . The manufacture of products conraining crocidolite is banned in the UK by the Health and Safety Inspectorate. Anthophynite (FeMg)~ Sis 022 (OH)2. Limited application in plastics. Effect on properties: Large improvements in fire resistance, heat distortion temperature, and temperature. Low coefficient of thermal expansion. Low mould and machine -
Wear.
Suitable polymers: Polypropylene, polystyrene, styrene acrylonitrile (SAN) and nylons. Additional information: Asbestos has received a great deal of unfavourable publicity recently due to the health hazards associated with airborne asbestos fibres. The hazards do not apply to the moulding of fully compounded asbestos reinforced polymer pellets. Care must be exercised if mouldings are to be machined or ground or are likely to be subject to abrasion in use. Suppliers of asbestosfilled grades of materials offer a comprehensive information service on the safe processing of these materials. NB. Whilst only the use of crocidolite asbestos is banned in the UK, in certain countries around the world (e.g. Sweden) the use of asbestos is banned completely except for reinforcement of cement in pressure pipes and for friction materials, normally phenolic based.
4. Processing of filled thermoplastics While the c o m p o u n d i n g and m o u l d i n g of filled thermoplastics may be considered by the design engineer to be
the responsibility of the materials suppliers and the custom moulder, appreciation of the factors affecting the final component properties is
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
essential if trouble-free performance is to be achieved. 4.1. Compounding There is currently rapid development in the availability of fully compounded filled thermoplastics - a complete list will be given in Part II of this paper. Prototype moulding quantities of these materials can normally be obtained. However, the full potential of filled thermoplastics will only be realized for most applications if materials are tailored to individual end uses. In such cases during proving trials materials may be compounded by dry tumbling. Larger quantities may be melt compounded on special purpose equipment when some indication of property levels has been obtained. Even distribution of filler is essential for optimum properties and polymer in powder form is more suitable for dry tumbling than pellets. Mineral oil or glycol can be used as a wetting agent for granules to assist bleaching. Surface treatment of the filler with coupling agents such as silicones are essential for maximum property improvement. Stearate coatings improve dispersion of the filler in the polymer, reduce machinery wear and improve component properties in certain areas. 4.2. Moulding The problems of moulding heavily filled thermoplastics have two main causes. Fillers generally increase melt viscosity and can cause increased machine wear. Melt viscosity is sensitive to particle size and small particles give higher viscosity (but also normally better properties in the final component). If problems are encountered in moulding then raising the barrel temperature on the moulding machine, adding more lubricant (e.g. zinc stearate) and increasing filler particle size are possible remedies. Wear of machinery is an economic problem and the cost savings obtained by the use of filler has to be balanced against increased processing costs. Such costs will be difficult to estimate prior to full-scale production. The problem should not be overstated. In many cases, especially with surfacetreated fillers, no increase in wear is recorded. In the case of foamed plastics it is
77
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most important that suppliers' recommendations for physical and chemical blowing agents are followed. Use of the incorrect chemical blowing agent will not only fail to produce foaming
but degradation of the base polymer can occur. A summary of the effect of fillers on various thermoplastics is given in Table 2. In Part lI of this paper
MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, December 1978
comprehensive properties data on the commercially available grades of filled thermoplastics will be given together with typical applications, present and planned.
79