Machinery's Handbook, 31st Edition
576 Thermal Properties of Plastics materials show only a gradual decrease. Concern over failures at low temperatures led to the development of ASTM D746, “Brittleness Temperature of Plastics,” in 1943. In this test, small, unnotched test bars are set up as cantilever beams in groups of five or more, brought to a preset low temperature, and struck by a weighted, blunt-nosed bar falling at 2 m/s. The experiment is repeated at other temperatures, higher and lower, and the fraction of breaks is plotted versus temperature on normal-probability graph paper. From the plot, the temperature at which half the bars break is estimated and reported as the brittleness temperature. Plas tics reinforced with long glass fibers have relatively high impact values, both notched and unnotched, at room temperature. They retain much of these values at lower temperatures. Electrical Properties.— The most notable electrical property of plastics is that they are good insulators, but there are many other electrical properties that must be considered in plastics part design. Conductivity in solids depends on the availability and mobility of movable charge carri ers within the material. Metals are good conductors because the metal atom has a loosely held, outermost electron, and the close proximity of the atoms allows these outer electrons to break free and move within the lattice structure. These free electrons give metals the ability to conduct large currents, even at low voltages. Outer electrons in materials such as glass, porcelain, and plastics are tightly bound to the atoms or molecules so there are no free electrons. Electrical current cannot be conducted, and the materials act as insulators. Volume resistivity is the electrical resistance of a material when a steady electromotive force (emf) difference is applied to a sample of the material that is made part of an electric circuit. Ohm’s law governs the flow of direct current: I R V = where I = current, amperes (A) V = emf difference across the test element, volts (V) R = resistance, ohms ( W ) R is determined by the element’s dimensions and the material’s electrical resistivity: R = ρ A L where L = length of the element, cm A = cross-sectional area perpendicular to the current direction, cm 2 r = resistivity of the conductor material, ohm-cm Materials having resistivities above 10 8 ohm-cm are considered insulators, and materials with values between 10 8 and 10 3 ohm-cm are considered partial conductors. Most plastics have volume resistivities in the range of 10 12 to 10 18 ohm-cm. Surface resistivity is a measure of the susceptibility of an electrical insulating material to surface contamination, particularly moisture, which makes it electrically conductive. The tests use parallel electrodes that are placed on the same surface of the material. Measured resistance is multiplied by the width of the electrodes and divided by the distance between them and reported as ohms per square. Methods for determining both volume and surface resistivities of insulating materials are spelled out in ASTM D257. The results of such measurements are sometimes reported in terms of their reciprocals, vol- ume conductivity (siemens/cm) and surface conductivity (siemens). The unit siemens is the reciprocal of resistance, 1/ R (with units1/ W .) The siemens was formerly known as the mho. Dielectric strength is a measure of the voltage required to cause an insulator to break down and allow an electric current to pass through the resulting conductive path. It may be measured (ASTM Test D149) at any or several frequencies from 0 Hz (direct current) to 800 Hz; the results are expressed in volts per mil (0.001 inch, or 0.0254 mm) of thick - ness. High values of this property are desired in capacitor films and wire coverings. Vari - ables that may affect test results include temperature, frequency, sample contamination,
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