Machinery's Handbook, 31st Edition
574 Thermal Properties of Plastics Vicat softening point is the temperature at which a small, circular, lightly gravity-loaded, heated probe penetrates a specific distance into a thermoplastics test specimen. This test (ASTM D1525) measures the ability of a thermoplastics material to withstand short-term contact with a heated surface and is most useful for crystalline plastics. Amorphous ther- moplastics tend to creep during the test, which reduces its usefulness for such materials. Deflection temperature under load (DTUL) is the temperature at which a test bar of 0.5 inch (12.7 mm) thickness, loaded to a specified bending stress, will deflect by 0.010 inch (0.25 mm). This ASTM test (D648) is run at bending stresses of 66 lb f /in 2 (0.455 MPa) or 264 lb f /in 2 (1.82 MPa) or both. The value obtained is referred to as the heat deflection temperature (HDT), also called heat distortion temperature, and is an indication of the ability of the material to perform at elevated temperatures under load. Both stress and deflection for a specific design of test bar are given, so the test may be regarded as establishing the temperature at which the flexural modulus is reduced to particular values—35,200 psi (242.7 MPa) at 66 psi stress, and 140,000 psi (965.3 MPa) at 264 psi stress. Tables 2a and 2b provides the range of HDTs for some common thermoplastics and thermosets. The addition of reinforcements, such as glass fiber (GF), can increase the HDT, so that the parts may be used at higher service temperatures than unreinforced grades. Like metals, thermoplastic materials expand when heated and contract when cooled. For a given temperature range, most plastics change dimensions much more than metals. The coefficient of linear thermal expansion ( a ) is the ratio of the fractional change in a linear dimension for a unit change of temperature and is expressed as in/in-F, or cm/cm-C. Typ- ical average values for common materials and selected plastics are shown in Table 8, page 372 . ASTM Method D696 directs that samples should be prepared so as to give a mini - mum of anisotropy, but if anisotropy is suspected, specimens shall be cut so as to measure a along the principal axes of anisotropy. The values in Table 8, page 372, may be presumed to have been measured according to those instructions. Thermal conductivity is the rate at which a material conducts heat energy along its length or through its thickness. For example, if a tightly covered plastic insulated cooler at 32 ° F (0°C) is filled with ice and chilled beverage cans and the outside is 80 ° F (27°C), heat from the surrounding warm air passes slowly through the box wall, melting the ice inside. A low thermal conductivity of the wall resists this heat flow. The property is defined by Fourier’s law of heat conduction as follows: Q k x T x ∆ = where Q x = flux, i.e., the rate of heat flow per unit area of the wall surface, k = thermal conductivity of the box-wall material, D T = temperature difference from the hot side to the cold side, and x = wall thickness. English units for k are Btu/h-in 2 ( ° F/in); SI units are J/s-m 2 (K/m). In sharp contrast to metals, plastics are poor heat conductors but excellent insulators, and plastic foams even more so. Plastics are also good electrical insulators. These insulat ing properties are valuable in many applications. Conductivities can be enhanced where needed by loading the plastics with copper or aluminum powder. In anisotropic materials, thermal conductivity may differ along major axes. Thermal conductivities for plastics are listed in the table Typical Properties of Plastics Materials on page 387. Aging at elevated temperatures may affect physical, mechanical, electrical, or thermal properties of plastics materials. Data from tests on specimens stored at specific tempera tures for suitable periods are presented as plots of properties versus aging time at various temperatures, and may be used as an indication of thermal stability of the material. Temperature index is a rating by Underwriters Laboratories (UL) of electrical and mechanical properties (with and without effects of impacts) of plastics materials used in electrical equipment for certain continuous operating conditions.
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