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Material Data and Measurements
Chapter 9
Material Data and Measurements In this chapter we will go through those properties of thermoplastics that are often requested by designers and product developers when they are looking for a material in a new product or when they must meet different industry or regulatory requirements, such as electrical or fire classification. When plastic producers develop a new plastic grade they usually also publish a data sheet of material properties. Sometimes this is made as a “preliminary data sheet” with only a few properties. If then the product will be a standard grade, a more complete data sheet will be published. Many suppliers publish their material grades in the CAMPUS or Prospector materials databases on the Internet, which can be used to some extent free of charge (see next chapter).
Figure 9.1 What are the different requirements from authorities on a so unremarkable product as an electrical outlet that must be fulfilled to be sold on the market?
CAMPUS is very comprehensive and can describe a material with over 60 different data types, and at the same time you can get graphs (e. g. stress-strain curves) and chemical resistance to many chemicals. The most requested data when it comes to thermoplastics and that are usually in the “preliminary data sheet” are:
88
■■
Tensile or flexural modulus
■■
Flame resistant classification
■■
Tensile strength
■■
Electrical properties
■■
Elongation
■■
Rheology (flow properties)
■■
Impact strength
■■
Shrinkage
■■
Maximum service temperature
■■
Density
9.1 Tensile Strength and Stiffness
9.1 Tensile Strength and Stiffness Stiffness, tensile strength, and toughness in terms of elongation can be obtained by the curves in tensile testing of test bars.
Figure 9.2 The picture shows a test bar in a tensile tester. All plastic producers measure the mechanical properties on specimens manufactured according to various ISO standards, which makes it possible to compare data between different manufacturers. [Photo: DuPont]
Figure 9.3 In the CAMPUS materials database on the Internet, you can pick up thousands of datasheets. Depending on the type of plastic, different characteristics are shown. Often the following properties are displayed: Mechanical Thermal Rheological Electrical Fire classification Other (e. g., chemical)
89
D C
σt
B A
εt
A
Linear range
B
Elas c range
C
At yield
D
Max stress
PA66 + 30% GF
σ = Stress [ MPa ]
σ = Stress [ MPa ]
Chapter 9 — Material Data and Measurements
POM
ε = Elonga on [ % ]
ε = Strain [ % ] Figure 9.4 The picture shows a typical stress-strain curve for unconditioned PA66 obtained by tensile testing. The curve can be divided into the following segments: (A) Linear range (B) Elastic range (C) At yield (D) Maximal stress/strain
PA66 DAM
Figure 9.5 In this figure you can see tensile curves for different plastics. Note that stress at break for the acetal curve (red) is lower than the maximum stress. This is due to necking. The reason why the unconditioned PA66 gets increased stress at the end of the curve depends on molecule orientation, which gives a hardening effect. If you compare the green curve (PA66 with 30% glass fiber) and the blue (unreinforced PA66) you can see the effect of glass fiber reinforcements on stress and strain. You can get significantly stronger but more brittle materials. Higher elongation at break means tougher material.
In the linear region, it’s easy to make a strength calculation because you can use Hooke’s law, i. e. σ = Force/Area (MPa).
Figure 9.6 As long as you are in the range of (A) or (B) in the graph in Figure 9.4, a test bar regains its original shape after unloading (picture on the left here). If you exceed the so-called elongation at yield at (C), necking occurs (middle image) that will extend until the breakage at (D) (the test bar on the right).
From the stress-strain curve (Figure 9.4) you can get the following mechanical properties: 1. Stress at yield σy i. e. (C) in the curve 2. Elongation at yield εy i. e. (C) in the curve 3. Stress at break σB i. e. (D) in the curve 4. Elongation at break εB i. e. (D) in the curve 5. The stiffness of the material Et is specified as the tensile modulus and can be calculated in the linear region with the following equation: Et = Σt/εt If the tensile curve is nonlinear you need an approximation and calculate the tangent or secant modulus.
90
9.1 Tensile Strength and Stiffness Tension is measured in units of MPa (megapascals) and elongation in percentage. In Figure 9.9 the relationship between the various units is presented. In addition to the specification of stiffness as tensile modulus Et you can also specify it as flexural modulus ES. At present, the tensile modulus is much more common than the flexural modulus in the plastic raw material suppliers’ data sheets. Figure 9.7 and Figure 9.8 show the curve for bending stress-strain.
σ = Stress [ MPa ]
Figure 9.7 This picture shows that the test bar is fixed horizontally on two supports and loaded in the middle.
σs Es =
σs εs
Es
εs ε = Elongaon [ % ]
Figure 9.8 While making a bending load the curve becomes nonlinear. You must approximate and use a secant (diagonal) to calculate the flexural modulus ES = σs/εs. Figure 9.9 If you hang a weight of 1 kg on a string, the string will be loaded by the force F = 10 N (Newton). The stress in the string depends on area A and will be σ = F/A, i. e. if the string area is 1 mm2 the stress will be 10 N/mm2 = 10 MPa. The pressure P is also specified in MPa. Earlier it was specified in bar. The locking force of injection-molding machines is often specified in tons. The correct unit is however MPa and 1 ton = 10 MPa.
91
Chapter 9 — Material Data and Measurements
9.2 Impact Strength The impact test that is dominant today is the “Charpy.” You fix the test bar at both ends in a horizontal position and allow the pendulum to hit it in the middle. The unit of the Charpy test is kJ/m2. In the past another impact test according to “Izod” was commonly used. Here you need to fix the lower half of the test bar in a vertical position and hit the upper half of it. The unit for the Izod test is J/m, and there is no factor that allows you to convert the values between the two test methods.
Figure 9.10 This picture shows a test bar with a milled notch to be used in impact tests.
Figure 9.11 The picture shows a pendulum impact tester. The pendulum is locked in its starting position. When it passes and hits the test bar it loses energy. The energy loss value indicates the impact resistance. Usually you measure the impact at 23°C or at −30 °C with or without notch.
92
9.3 Maximum Service Temperature The impact test is a sensitive and good quality control method. Many molders are using a self-built drop weight tester. You can e. g. drill holes with about 5 cm distance in a 50 mm plastic drain pipe. A cylindrical weight with a ball-shaped bottom is then hoisted up in the pipe to a certain height and fixed with a pin. When the pin is pulled out the weight falls and hits the part that has been fixed under the pipe. If the part passes the required height with no damage the impact strength is OK. If it breaks something is wrong with the material or the process. View from above
View from the side
Impact Impact
Charpy notched impact kJ/m² ISO 179
Izod notched impact J/m ISO 180
Figure 9.12 The picture shows the attachment of the test bars according to Charpy to the left and Izod to the right.
9.3 Maximum Service Temperature 9.3.1 UL Service Temperature It is easy to get confused when trying to determine a material’s maximum service temperature as this can be specified differently. A leading international test institute called Underwriters Laboratories has developed the specification of the maximum continuous service temperature and calls it “UL service temperature.” To do this you have to put test bars in ovens at different temperatures and wait 60,000 hours (i. e. almost 7 years). Then you bring out the test bars and test them. The temperature that has affected the test bars so much that they have lost 50% of their initial values is specified as the maximum continuous service temperature (UL service temperature). It is specified both for mechanical and electrical properties.
9.3.2 Heat Deflection Temperature In most plastic data sheets you will find values of the material’s heat deflection temperature at different loads. Heat deflection temperature is abbreviated to HDT.
93
Chapter 9 — Material Data and Measurements
Figure 9.13 When measuring HDT you have to fix a test bar horizontally at both ends. Then you put it into an oven and load it in the middle with either 0.45 or 1.8 MPa. You let the oven temperature rise by 2 °C per minute and record the temperature at which the sample bar has bent down 0.25 mm as the HDT.
In the tables below with values from the CAMPUS materials database (see next chapter) you will find HDT for a number of thermoplastics. NOTE! Some deviation from the values below may occur depending on the viscosity and additives of the materials. Table 9.1 Table with common plastics heat deflection temperatures
Type of polymer
HDT at 0.45 MPa
HDT at 1.8 MPa
Melting point
ABS
100
90
–
Acetal copolymer
160
104
166
Acetal homopolymer
160
95
178
HDPE, polyethylene
75
44
130
PA 6
160
55
221
PA 6 + 30% glass fiber
220
205
220
PA 66
200
70
262
PA 66 + 30% glass fiber
250
260
263
Polyester PBT
180
60
225
PBT + 30% glass fiber
220
205
225
Polyester PET
75
70
255
PET + 30% glass fiber
245
224
252
PMMA (acrylic plastic)
120
110
–
Polycarbonate
138
125
–
Polystyrene
90
80
–
PP, polypropylene
100
55
163
PP + 30% glass fiber
160
145
163
Note: The amorphous materials have no melting point
94
9.4 Flammability Tests
9.4 Flammability Tests The international testing institute Underwriters Laboratories has developed various tests to specify a material’s fire resistance. You select test bars with different thickness and ignite them either horizontally or vertically. We specify this as HB (= horizontal burning) or V-2, V-1, or V-0 (V = vertical burning). For a material to be classified as fire resistant, it must be extinguished by itself within a certain distance (HB) and at a certain time. When testing a material for V-0 to V-2 you will also give attention to possible drops that ignite cotton (see below).
9.4.1 HB Rating
Figure 9.14 The flame is applied for 30 seconds before the ignition speed is measured. HB classification is obtained if the ignition speed measured between two points does not exceed: 1. 40 mm/min for 3–13 mm test bars 2. 75 mm/min for test bars < 3 mm 3. If the flame goes out before the first mark
9.4.2 V Rating
Figure 9.15 When testing a test bar in a vertical position you will apply the flame twice during each 10 seconds. The contact time of the second ignition begins immediately after extinguishing the test bar of the first flame.
95
Chapter 9 — Material Data and Measurements Table 9.2 This specifies the times that must be met for the test to be approved. Under the test bar there is cotton, and attention is given if resultant drops will ignite it. Finally, if any afterglow occurs, the time of this will be measured. [Source: Underwriters Laboratories]
Flame application
20 mm high Tirill burner flame
Flame application time
2 × 10 s
The second flame application time begins as soon as the ignited specimen is extinguished or immediately if the specimen does not ignite Flammability rating UL 94
V-0
V-1
V-2
Burning time after flame application (sec)
< 10
< 30
< 30
Total burning time (s) (10 flame applications)
< 50
< 250
< 250
Burning and afterglow times of specimens after second flame application (s)
< 30
< 60
< 60
Dripping of burning specimens (ignition of cotton batting)
No
No
Yes
Specimens completely burned
No
No
No
9.5 Electrical Properties There are a variety of test methods for electrical properties of plastics. Usually, you specify the material’s insulating capability to or resistance to creep currents on the surface. The following methods are often published in data sheets: 1. Dielectric strength 2. Volume resistivity 3. Arc resistance 4. Surface resistivity 5. “Tracking” resistance CTI (Comparative Tracking Index)
Figure 9.16 Tests according to methods (1) to (3) above are made in test equipment with the principle shown in the figure to the left, and tests according to (4) and (5) are made in test equipment with the principle shown in the figure to the right. If you want to know more about electrical test methods for plastics we can recommend this website: www.ul.com.
96
DC Voltage
DC Voltage
9.7 Shrinkage
9.6 Flow Properties: Melt Index You can measure the melt flow properties of thermoplastics by using a test method called the melt flow index, MFI. Another name for this method is melt flow rate, MFR. Below you can see the principle for melt index.
Figure 9.17 When you are testing the flow properties of a thermoplastic melt you start by heating up the granules in a cylinder. The temperature according to the standard will vary depending on the polymer. Once the material has reached the specified temperature you put a weight (also polymer dependent) on the piston and record the time it takes for the material to flow out of the cylinder. You specify the MFR in units of cm3/10 minutes. MFI is specified as the weight of the material flow after 10 minutes and the unit is g/10 minutes.
9.7 Shrinkage Mold shrinkage is the difference between the dimensions of the cavity and the dimensions of the molded part.
Shrinkage cross direction
Shrinkage flow direction
Fan gate
Figure 9.18 The mold shrinkage is measured after one day (at least 16 hours). Semi-crystalline materials undergo a post-crystallization that can last for months depending on the ambient temperature and type of polymer. This causes a shrinkage called post-shrinkage. The total shrinkage = mold shrinkage + post-shrinkage. Shrinkage is usually measured in both the flow and cross-flow directions.
97
Material Data and Measurements In this chapter we will go through those properties of thermoplastics that are often requested by designers and product developers when they are looking for a material in a new product or when they must meet different industry or regulatory requirements, such as electrical or fire classification. When plastic producers develop a new plastic grade they usually also publish a data sheet of material properties. Sometimes this is made as a “preliminary data sheet” with only a few properties. If then the product will be a standard grade, a more complete data sheet will be published. Many suppliers publish their material grades in the CAMPUS or Prospector materials databases on the Internet, which can be used to some extent free of charge (see next chapter).
Figure 9.1 What are the different requirements from authorities on a so unremarkable product as an electrical outlet that must be fulfilled to be sold on the market?
CAMPUS is very comprehensive and can describe a material with over 60 different data types, and at the same time you can get graphs (e. g. stress-strain curves) and chemical resistance to many chemicals. The most requested data when it comes to thermoplastics and that are usually in the “preliminary data sheet” are:
88
■■
Tensile or flexural modulus
■■
Flame resistant classification
■■
Tensile strength
■■
Electrical properties
■■
Elongation
■■
Rheology (flow properties)
■■
Impact strength
■■
Shrinkage
■■
Maximum service temperature
■■
Density
9.1 Tensile Strength and Stiffness
9.1 Tensile Strength and Stiffness Stiffness, tensile strength, and toughness in terms of elongation can be obtained by the curves in tensile testing of test bars.
Figure 9.2 The picture shows a test bar in a tensile tester. All plastic producers measure the mechanical properties on specimens manufactured according to various ISO standards, which makes it possible to compare data between different manufacturers. [Photo: DuPont]
Figure 9.3 In the CAMPUS materials database on the Internet, you can pick up thousands of datasheets. Depending on the type of plastic, different characteristics are shown. Often the following properties are displayed: Mechanical Thermal Rheological Electrical Fire classification Other (e. g., chemical)
89
D C
σt
B A
εt
A
Linear range
B
Elas c range
C
At yield
D
Max stress
PA66 + 30% GF
σ = Stress [ MPa ]
σ = Stress [ MPa ]
Chapter 9 — Material Data and Measurements
POM
ε = Elonga on [ % ]
ε = Strain [ % ] Figure 9.4 The picture shows a typical stress-strain curve for unconditioned PA66 obtained by tensile testing. The curve can be divided into the following segments: (A) Linear range (B) Elastic range (C) At yield (D) Maximal stress/strain
PA66 DAM
Figure 9.5 In this figure you can see tensile curves for different plastics. Note that stress at break for the acetal curve (red) is lower than the maximum stress. This is due to necking. The reason why the unconditioned PA66 gets increased stress at the end of the curve depends on molecule orientation, which gives a hardening effect. If you compare the green curve (PA66 with 30% glass fiber) and the blue (unreinforced PA66) you can see the effect of glass fiber reinforcements on stress and strain. You can get significantly stronger but more brittle materials. Higher elongation at break means tougher material.
In the linear region, it’s easy to make a strength calculation because you can use Hooke’s law, i. e. σ = Force/Area (MPa).
Figure 9.6 As long as you are in the range of (A) or (B) in the graph in Figure 9.4, a test bar regains its original shape after unloading (picture on the left here). If you exceed the so-called elongation at yield at (C), necking occurs (middle image) that will extend until the breakage at (D) (the test bar on the right).
From the stress-strain curve (Figure 9.4) you can get the following mechanical properties: 1. Stress at yield σy i. e. (C) in the curve 2. Elongation at yield εy i. e. (C) in the curve 3. Stress at break σB i. e. (D) in the curve 4. Elongation at break εB i. e. (D) in the curve 5. The stiffness of the material Et is specified as the tensile modulus and can be calculated in the linear region with the following equation: Et = Σt/εt If the tensile curve is nonlinear you need an approximation and calculate the tangent or secant modulus.
90
9.1 Tensile Strength and Stiffness Tension is measured in units of MPa (megapascals) and elongation in percentage. In Figure 9.9 the relationship between the various units is presented. In addition to the specification of stiffness as tensile modulus Et you can also specify it as flexural modulus ES. At present, the tensile modulus is much more common than the flexural modulus in the plastic raw material suppliers’ data sheets. Figure 9.7 and Figure 9.8 show the curve for bending stress-strain.
σ = Stress [ MPa ]
Figure 9.7 This picture shows that the test bar is fixed horizontally on two supports and loaded in the middle.
σs Es =
σs εs
Es
εs ε = Elongaon [ % ]
Figure 9.8 While making a bending load the curve becomes nonlinear. You must approximate and use a secant (diagonal) to calculate the flexural modulus ES = σs/εs. Figure 9.9 If you hang a weight of 1 kg on a string, the string will be loaded by the force F = 10 N (Newton). The stress in the string depends on area A and will be σ = F/A, i. e. if the string area is 1 mm2 the stress will be 10 N/mm2 = 10 MPa. The pressure P is also specified in MPa. Earlier it was specified in bar. The locking force of injection-molding machines is often specified in tons. The correct unit is however MPa and 1 ton = 10 MPa.
91
Chapter 9 — Material Data and Measurements
9.2 Impact Strength The impact test that is dominant today is the “Charpy.” You fix the test bar at both ends in a horizontal position and allow the pendulum to hit it in the middle. The unit of the Charpy test is kJ/m2. In the past another impact test according to “Izod” was commonly used. Here you need to fix the lower half of the test bar in a vertical position and hit the upper half of it. The unit for the Izod test is J/m, and there is no factor that allows you to convert the values between the two test methods.
Figure 9.10 This picture shows a test bar with a milled notch to be used in impact tests.
Figure 9.11 The picture shows a pendulum impact tester. The pendulum is locked in its starting position. When it passes and hits the test bar it loses energy. The energy loss value indicates the impact resistance. Usually you measure the impact at 23°C or at −30 °C with or without notch.
92
9.3 Maximum Service Temperature The impact test is a sensitive and good quality control method. Many molders are using a self-built drop weight tester. You can e. g. drill holes with about 5 cm distance in a 50 mm plastic drain pipe. A cylindrical weight with a ball-shaped bottom is then hoisted up in the pipe to a certain height and fixed with a pin. When the pin is pulled out the weight falls and hits the part that has been fixed under the pipe. If the part passes the required height with no damage the impact strength is OK. If it breaks something is wrong with the material or the process. View from above
View from the side
Impact Impact
Charpy notched impact kJ/m² ISO 179
Izod notched impact J/m ISO 180
Figure 9.12 The picture shows the attachment of the test bars according to Charpy to the left and Izod to the right.
9.3 Maximum Service Temperature 9.3.1 UL Service Temperature It is easy to get confused when trying to determine a material’s maximum service temperature as this can be specified differently. A leading international test institute called Underwriters Laboratories has developed the specification of the maximum continuous service temperature and calls it “UL service temperature.” To do this you have to put test bars in ovens at different temperatures and wait 60,000 hours (i. e. almost 7 years). Then you bring out the test bars and test them. The temperature that has affected the test bars so much that they have lost 50% of their initial values is specified as the maximum continuous service temperature (UL service temperature). It is specified both for mechanical and electrical properties.
9.3.2 Heat Deflection Temperature In most plastic data sheets you will find values of the material’s heat deflection temperature at different loads. Heat deflection temperature is abbreviated to HDT.
93
Chapter 9 — Material Data and Measurements
Figure 9.13 When measuring HDT you have to fix a test bar horizontally at both ends. Then you put it into an oven and load it in the middle with either 0.45 or 1.8 MPa. You let the oven temperature rise by 2 °C per minute and record the temperature at which the sample bar has bent down 0.25 mm as the HDT.
In the tables below with values from the CAMPUS materials database (see next chapter) you will find HDT for a number of thermoplastics. NOTE! Some deviation from the values below may occur depending on the viscosity and additives of the materials. Table 9.1 Table with common plastics heat deflection temperatures
Type of polymer
HDT at 0.45 MPa
HDT at 1.8 MPa
Melting point
ABS
100
90
–
Acetal copolymer
160
104
166
Acetal homopolymer
160
95
178
HDPE, polyethylene
75
44
130
PA 6
160
55
221
PA 6 + 30% glass fiber
220
205
220
PA 66
200
70
262
PA 66 + 30% glass fiber
250
260
263
Polyester PBT
180
60
225
PBT + 30% glass fiber
220
205
225
Polyester PET
75
70
255
PET + 30% glass fiber
245
224
252
PMMA (acrylic plastic)
120
110
–
Polycarbonate
138
125
–
Polystyrene
90
80
–
PP, polypropylene
100
55
163
PP + 30% glass fiber
160
145
163
Note: The amorphous materials have no melting point
94
9.4 Flammability Tests
9.4 Flammability Tests The international testing institute Underwriters Laboratories has developed various tests to specify a material’s fire resistance. You select test bars with different thickness and ignite them either horizontally or vertically. We specify this as HB (= horizontal burning) or V-2, V-1, or V-0 (V = vertical burning). For a material to be classified as fire resistant, it must be extinguished by itself within a certain distance (HB) and at a certain time. When testing a material for V-0 to V-2 you will also give attention to possible drops that ignite cotton (see below).
9.4.1 HB Rating
Figure 9.14 The flame is applied for 30 seconds before the ignition speed is measured. HB classification is obtained if the ignition speed measured between two points does not exceed: 1. 40 mm/min for 3–13 mm test bars 2. 75 mm/min for test bars < 3 mm 3. If the flame goes out before the first mark
9.4.2 V Rating
Figure 9.15 When testing a test bar in a vertical position you will apply the flame twice during each 10 seconds. The contact time of the second ignition begins immediately after extinguishing the test bar of the first flame.
95
Chapter 9 — Material Data and Measurements Table 9.2 This specifies the times that must be met for the test to be approved. Under the test bar there is cotton, and attention is given if resultant drops will ignite it. Finally, if any afterglow occurs, the time of this will be measured. [Source: Underwriters Laboratories]
Flame application
20 mm high Tirill burner flame
Flame application time
2 × 10 s
The second flame application time begins as soon as the ignited specimen is extinguished or immediately if the specimen does not ignite Flammability rating UL 94
V-0
V-1
V-2
Burning time after flame application (sec)
< 10
< 30
< 30
Total burning time (s) (10 flame applications)
< 50
< 250
< 250
Burning and afterglow times of specimens after second flame application (s)
< 30
< 60
< 60
Dripping of burning specimens (ignition of cotton batting)
No
No
Yes
Specimens completely burned
No
No
No
9.5 Electrical Properties There are a variety of test methods for electrical properties of plastics. Usually, you specify the material’s insulating capability to or resistance to creep currents on the surface. The following methods are often published in data sheets: 1. Dielectric strength 2. Volume resistivity 3. Arc resistance 4. Surface resistivity 5. “Tracking” resistance CTI (Comparative Tracking Index)
Figure 9.16 Tests according to methods (1) to (3) above are made in test equipment with the principle shown in the figure to the left, and tests according to (4) and (5) are made in test equipment with the principle shown in the figure to the right. If you want to know more about electrical test methods for plastics we can recommend this website: www.ul.com.
96
DC Voltage
DC Voltage
9.7 Shrinkage
9.6 Flow Properties: Melt Index You can measure the melt flow properties of thermoplastics by using a test method called the melt flow index, MFI. Another name for this method is melt flow rate, MFR. Below you can see the principle for melt index.
Figure 9.17 When you are testing the flow properties of a thermoplastic melt you start by heating up the granules in a cylinder. The temperature according to the standard will vary depending on the polymer. Once the material has reached the specified temperature you put a weight (also polymer dependent) on the piston and record the time it takes for the material to flow out of the cylinder. You specify the MFR in units of cm3/10 minutes. MFI is specified as the weight of the material flow after 10 minutes and the unit is g/10 minutes.
9.7 Shrinkage Mold shrinkage is the difference between the dimensions of the cavity and the dimensions of the molded part.
Shrinkage cross direction
Shrinkage flow direction
Fan gate
Figure 9.18 The mold shrinkage is measured after one day (at least 16 hours). Semi-crystalline materials undergo a post-crystallization that can last for months depending on the ambient temperature and type of polymer. This causes a shrinkage called post-shrinkage. The total shrinkage = mold shrinkage + post-shrinkage. Shrinkage is usually measured in both the flow and cross-flow directions.
97