While polymers have many advantages over metals (lower density, parts consolidation, lower energy consumption during processing, etc.), they behave differently from metals when subjected to mechanical loads or exposed to thermal or chemical environments. Some of the differences in the properties and behaviors of polymers and metals are briefly discussed below. They apply to both thermoplastic and thermoset polymers.
1. The principal advantage of polymers over metals is their low density.The density of different types of polymers used in automotive applications ranges from 0.9 g/cm3 (for PP) to 2.1 g/cm3 (for polytetrafluoroethylene (PTFE), commonly known as by one of its tradename Teflon). PP is used in instrument panels, bumper beams, heating ducts, etc. PTFE, because of its low friction coefficient, is used in bearings.
2. They have much lower modulus and strength than metals. For example, the modulus of steel is 207 GPa and the modulus of polymers is lower than 5 GPa. The modulus and strength of polymers can be enhanced by adding fibers, such as glass and carbon fibers; but even with fiber reinforcement, these two properties may not be as high as the modulus and strength of steel. However, with proper polymer selection, it is possible to achieve significant weight saving over metal counterparts in many applications.
3. Their mechanical properties, such as modulus, yield strength, strain-to-failure, and impact strength, are influenced by temperature and strain rate, and for some polymers (e.g., PA-6 and PA-6,6), also by humidity. Decreasing the temperature, for example, increases modulus and yield strength, but decreases strain-to-failure and impact strength. Similarly increasing the strain rate increases modulus and yield strength, but decreases strain-to-failure and impact strength.
4. They exhibit creep and stress relaxation. These are both time and temperature dependent behavior and must be taken into account for long-term application of polymers under load. Creep is manifested by increasing strain with time, even when the stress on the polymer is maintained at a constant value (Fig. 5.2A). Due to creep, a polymer beam under load will show increasing deflection with increasing time. Stress relaxation is manifested by decreasing stress with time even when the strain on the polymer is held constant (Fig. 5.2B). Due to stress relaxation, the joint stress in a bolted joint of a polymer part may reduce with increasing time, making the joint less effective. Both creep and stress relaxation increase with increasing temperature.
5. Increasing strain due to creep or decreasing stress due to stress relaxation are, in effect, equivalent to a reduction of the modulus of the polymer with time. When the creep strain exceeds a critical value (which depends on the polymer type and temperature), the polymer may fail even though the stress on the polymer is lower than its yield or tensile strength. This phenomenon is known as creep rupture.
6. Unlike low carbon steels, polymers, in general, do not exhibit an endurance limit when subjected to fatigue (cyclic) loading. The fatigue life of a polymer increases with decreasing stress level. In general, the fatigue performance of semicrystalline polymers is better than that of amorphous polymers. Among the commonly used semicrystalline polymers, polyoxymethylene (POM or acetal) shows the best fatigue performance, followed by polyamide-6 and PA-6,6. Another point to note is that, at high frequencies of cycling or at high fatigue stress levels, internal heat generated within the polymer during fatigue cycling may cause softening of the polymer, which causes a reduction in modulus of the polymer and ultimately, thermal failure.
7. In many automotive applications, the polymer parts may be exposed to chemicals such as gasoline, motor oil, antifreeze, paint, cleaning solvents, and road salt. In general, semicrystalline polymers, such as high-density polyethylene (HDPE), PP, and polyamides, have much higher chemical resistance than amorphous polymers, such as ABS and PC. Typically, semicrystalline polymers are selected for under the hood applications, such as windshield washing fluid bottles and radiator end caps.
8. The effect of chemicals on polymers is influenced by temperature, and for some polymers, also by the presence of tensile stress. The combination of tensile stress (Either applied or residual) and a chemical environment may cause stress cracking in the polymer. A measure of this behavior is called environmental stress crack resistance, which depends on a number of factors, including the chemical structure and molecular weight of the polymer, tensile stress level, types of chemicals to which it is exposed and also the time of exposure.
9. Many polymers with relatively high ductility and high impact resistance at room temperature can transform into a brittle material exhibiting low ductility and low impact resistance at temperatures lower than the room temperature. For example, PP exhibits up to 200% elongation-at-break at 23°C, but as the temperature is reduced to 0°C or lower, it exhibits only 1%-2% elongation-at-break and turns into a brittle polymer. If high elongation-at-break is desired at such low temperatures, either an ethylene propylene copolymer or an elastomer-blended PP is selected instead of PP.
10. The notch sensitivity of polymers also varies with the polymer type and affects the impact behavior. For example, both PC(polycarbonate) and ABS (Acrylonitrile butadiene styrene) are considered ductile polymers and exhibit high impact strength in standard Izod or Charpy impact tests. However, PC is more notch sensitive than ABS. PC changes from a ductile polymer to a brittle polymer if a sharp notch (e.g., a surface scratch) appears on its surface. The impact resistance of ABS also decreases with notch sharpness, but a transition from ductile to brittle behavior is not observed.
11. Long-term use of polymers should not only take into account the possibility of creep and stress relaxation but also the effect of aging on polymer properties. Aging occurs because of irreversible degradation of the polymer molecules due to oxidation, reduction in molecular weight, etc., and depending on the polymer type, is aggravated by heat, ultraviolet light, chemical environment, etc. Very often, aging creates surface embrittlement, but there may also be an overall deterioration of the polymer’s properties.
12. Polymers have a significantly higher coefficient of thermal expansion than metals. Thus, dimensional changes in polymer parts due to temperature changes are much higher. For a polymer part fitted next to a metal part, this can cause interference when the temperature is increased, or create gaps when the temperature is decreased. The differential thermal expansion or contraction between a metal part and a polymer part may also create thermal stresses between them.
13. The surface finish of polymer parts is controlled mainly by the surface finish of the mold surfaces. Polymers can be molded in a wide variety of colors using colorants or color master batches; however, uniform distribution of color on a large part and color matching with neighboring parts can sometimes be a challenge. Polymer surfaces can also be painted; however, the long-term adhesion between the paint and the polymer surface may be affected by environmental conditions, such as high humidity. Polyethylene and PP surfaces cannot be painted unless they are modified by plasma treatment or flame treatment. The painting process for steel body panels involves exposure to high temperatures (up to 220°C for 30 minutes or longer) for curing and drying. Many thermoplastic polymers will experience heat sagging and creep at such high temperatures and may not be suitable for painting in paint baking ovens.