Machinery's Handbook, 31st Edition
Thermal Properties of Plastics 575 Flammability ratings also are produced by Underwriters Laboratories. UL tests mea- sure the ability to continue burning after a flame is removed, and the percentage of oxygen needed for the material to continue burning. Other tests measure combustibility, ignition temperatures, and smoke generation. The importance of flammability testing for plastics is reflected by ASTM’s listing of ten different tests under the index subject “Flammabil - ity—Plastics.” The high carbon and hydrogen content of plastics means they are usually flammable. Some exceptions are fluorocarbons and chlorofluoroplastics, which are not flammable. Silicones and rigid PVC will not sustain fire by themselves. Compounders can significantly reduce any plastic’s flammability by judiciously choosing from among hundreds of available fire-retarding additives. Effect of Temperature on Mechanical Properties.— The equivalence of high strain rates and low temperatures (and vice versa) must be kept in mind when designing with plastics materials. Stress-strain curves for tests performed with one strain rate at sev - eral temperatures are similar to those for tests with one temperature and several strain rates. That is, very high strain rates and very low temperatures produce similar responses in materials. Conversely, the effects of very low strain rates, that is, creep effects, can be determined more quickly by testing at elevated temperatures and employing time- temperature superposition methods to estimate creep at lower temperatures and longer times. End-use testing at temperatures near or above the highest values expected in every - day use of a product helps the designer estimate long-term performance of components. Strength, modulus, and elongation behavior are similar for tensile, compressive, flex ural, and shear properties. Generally, strength and modulus decrease with increasing tem perature. The effect of temperature increase is shown by the curves in Fig. 11 for crystalline and amorphous materials, where a gradual drop in modulus is seen as the glass-transition temperature T g is approached. The deflection temperature marks a flexural modulus of 35,200 psi (242.7 MPa) at a test stress of 66 psi (455 kPa), giving rise to the horizontal dashed line in Fig. 11 . Above the glass-transition temperature, amorphous materials suffer rapid diminution of modulus, and, even with glass-fiber and other reinforcements, modulus drops rapidly above T g . Some crystalline resins maintain usable moduli at temperatures approaching the crystalline melting point ( T m ). Glass-fiber reinforcement can substantially improve the modulus of crystalline materials between the glass-transition and melting temperatures. Generally, strength-versus-temperature curves exhibit shapes like modulus curves, while, as expected, elongations increase with rising temperatures.
Amorphous unfilled reinforced
Semi-Crystalline unfilled reinforced
= T g = Melt DTUL
Temperature
Fig. 11. Schematic of Modulus Behavior of Semi-Crystalline and Amorphous Plastics Showing T g and T m and the Beneficial Effect of Reinforcement on Deflection Temperature Under Load (DTUL) As temperatures drop significantly below normal ambient values, most plastics materials become brittle and lose much of their room-temperature impact strength, although a few
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