When designing with high-performance polymers,
it’s important to understand the nature of plastic materials, their properties and testing methods. Only then can you evaluate the advantages and limitations of a particular resin and determine if it is suitable for your application.
Design engineers new to plastics may find the following discussion helpful in understanding the role this information plays in material selection. This discussion is not exhaustive and serves only as a starting point.
Reliable performance at elevated temperature is often a critical factor to consider. Thermal properties provide guidelines for two important aspects of thermal performance. The first is the immediate softening effect that heat has on the polymer. This limits the ambient temperature to which a plastic component should be exposed, even for a short time. The second is the material’s long-term thermal stability. Since exposure to high temperature tends to cause property loss over time, it is essential to understand how long-term exposure affects the properties that are critical to your application.
Heat deflection temperature (HDT) is a relative measure of a polymer’s ability to perform at an elevated temperature while supporting a load. At this temperature, a test bar shows a specified deformation under a load of 264 psi (1.8 MPa). It is generally acceptable to use 5-10°C (9-18°F) below the HDT as the maximum operating temperature.
Relative thermal index (RTI) is a relative measure of a polymer’s ability to perform at an elevated temperature over time. It is defined as the temperature that can be endured for 100,000 hours in air with 50% retention of a specified property. The values in this brochure are based upon retention of tensile strength. The RTI can be used as a conservative guideline for maximum continuous use temperature. Many applications have shorter life requirements and RTI values for 5,000 hours and 10,000 hours may be available on individual data sheets.
Glass transition temperature (Tg) is the temperature at which a polymer undergoes a significant change in properties, transitioning from a glassy state to a rubbery state. For amorphous polymers, the temperature is usually about 10°C (18°F) above the HDT and generally represents the upper limit for short-term use. Semi-crystalline polymers lose some of their stiffness at this temperature, but can still have useful properties until reaching the material’s melting point.
Melting point (Tm) indicates the temperature at which the crystalline regions of a semi-crystalline polymer soften. It typically represents the absolute upper temperature limit for semi-crystalline polymers.
Since most applications have some degree of mechanical loading, it is important to understand how a material responds to load. Design engineers can often modify the load bearing ability or deformation under load of a component by adjusting section thickness. Tensile strength is measured by fixing one end of a test specimen and applying a load to the other end at a specified rate until it yields or breaks.
Tensile elongation is a measure of how much the specimen stretched before it yielded or broke. High tensile elongation indicates a tough, ductile material. Low tensile elongation often indicates a rigid, brittle material. Glass-fiber-reinforced materials generally have low elongation due to the glass fiber, so low values do not always signify brittleness.
Flexural modulus is measured by applying a load to the center of a specimen supported at two points. The modulus is defined as the slope of the stress/strain curve and gives a good indication of stiffness or rigidity.
When comparing materials, the one with the higher tensile strength will require a smaller section thickness to achieve the same load bearing ability. Similarly, the one with the higher flexural modulus will require a smaller section thickness for the same deformation. For some applications, the section may already be at the minimum thickness practical for molding and relative strength may not be a consideration.
Impact resistance can be loosely defined as the ability to resist breakage when struck by an object or dropped onto a hard surface. Izod impact is the most commonly used test to evaluate this property and it can be run both with and without a notch in the test bar.
The unnotched test gives a good indication of practical impact resistance. A value of NB indicates that the test specimen did not break under the test conditions. The notched test indicates the material’s tendency to break when a scratch or notch is present. A high unnotched value and a low notched value indicate a ductile material with a high level of notch sensitivity. When designing with this type of material, it is important to allow generous radii on all corners.
Most plastics are good electrical insulators. The proper-ties reported here – dielectric strength, volume resistivity and surface resistivity – provide basic information about a material’s ability to function as an electric insulator. Grades that contain high levels of carbon fiber or powder generally are not suitable for these types of applications. When designing a plastic component whose primary function is electrical insulation, you must consider several other important electrical properties before finalizing your material selection.
Weight reduction is a primary driver for many metal-to plastic conversions. Specific gravity, which is determined by dividing the density of the resin by the density of water, can be used to estimate the weight of a component. The material with the lowest specific gravity will produce the lightest component. Specific gravity can also impact the cost of a component. A material with a lower specific gravity will produce more parts per unit weight than one having a higher value.
Water absorption is measured by weighing parts before and after exposure to water for 24 hours. Water absorption can cause dimensional and property changes and all polymers are not affected in the same way. Although low water absorption is generally desirable, special consideration must be given to understanding the effects of water absorption rather than the absolute amount of water absorbed.
Exposure to chemicals can be detrimental to a material’s performance and each application should be tested for compatibility with the specific chemicals encountered in use. The ratings listed in this brochure are intended to give a general sense of what types of chemicals are compatible with certain materials and which ones are likely to be incompatible. These ratings are based upon significant exposure and some materials that have a poor rating may be suitable if exposure is brief. Some combinations that are rated excellent may not be suitable for a particular combination of reagent, temperature, stress level and material.
Processing & Fabrication
The properties listed indicate the processing temperature range required for each category. Melt temperature and mold temperature may help in the selection of processing equipment. The mold shrinkage values reported were measured using standard test methods and may not relate well to actual components. However, these values are useful for material comparison and can help in determining if a mold used for one material can produce the same size component from another material.
Melt flow rates are included for our amorphous resins and indicate how easily the materials flow. When comparing these values to other amorphous materials, it is important to check that the tests were run under the same temperature and load.
Typical processing techniques are shown for categories within each product family. Most of our products are processed by injection molding, but several grades can be extruded into sheet, profiles and other shapes. Extruded sheet can be thermoformed. Solution processing is used to make coatings and membranes.
The table below lists the test methods typically used by Solvay Advanced Polymers to generate typical property value ranges. Because properties are often referenced by different names, a list of the names most commonly used is also included. For more detailed information on test methods, please consult either the American Society for Testing and Materials (ASTM), the International Organization for Standardization (ISO), Underwriters Laboratories (UL) or International Electrotechnical Commission (IEC).
|Typical Test Methods|
|Heat Deflection Temperature
||Deflection Temperature Under Load, Temperature of Deflection Under Load, Heat Distortion Temperature
||ASTM D 648, ISO 75/Af at 284 psi/1.8MPa|
|Relative Thermal Index
||Continuous Use Temperature
||UL 746B, ASTM D 3045|
|Glass Transition Temperature
||ISO 11357-2, ASTM D 3418|
||ISO 11357-3, ASTM D 3418|
||Stress at Break
||ASTM D 638, ISO 527-1|
||Stress at Break
||ASTM D 638, ISO 527-1|
||ASTM D 790, ISO 178|
|Izod Impact Strength
||ASTM D 256, ISO 180, type A|
||ASTM D 149, IEC 60243-1|
||ASTM D 257, IEC 60093|
||ASTM D 257, IEC 60093|
||ASTM D 792, ISO 1183A|
||ASTM D 570, ISO 62 for 24 hours at 23°C (73°F)|
||Molding Shrinkage, Shrink Rate
||ASTM D 955, ISO 294-4|
|Melt Flow Rate
||Melt Mass Flow Rate, Melt Index
||ASTM D 1238, ISO 1133|