Machinery's Handbook, 31st Edition
ELECTRICAL PROPERTIES OF PLASTICS 577 internal voids, rate of voltage increase, and duration of test. Although the use of volts/mil units in test results suggests that the same dielectric strength value should apply to dif - ferent film thicknesses, tests show that thinner films fail at lower V/mil than thicker ones. Dielectric constant or permittivity is a dimensionless constant that indicates how eas- ily a material can become polarized by imposition of an electrical field on an insulator. Polarization occurs because of charge-carrying atoms or functional groups (dipoles) in materials. When the field is applied, these groups try to align their positive and negative ends with the field. Reversal of the direction of the electric field results in reversal of the polarization. The dielectric constant is the ratio of the permittivity of the material in normal ambient conditions to the permittivity of a vacuum. Permittivity is important when plastics are used as insulating materials in capacitors and high-frequency electrical apparatus. Changes in temperature, humidity, electrical frequency, and part thickness may affect the dielectric constant. Dielectric constants for a dozen various engineering thermoplastics are listed in Table 3, page 577. Many others are included in the main table of plastics properties, Typical Properties of Plastics Materials on page 387. If a polar material is subjected to an alternating electric field, its dipoles reverse their orientation twice per cycle. This movement encounters frictional resistance at the mo - lecular level, and at high frequencies that friction can generate considerable heat, as de - tected by the electrical phase angle between voltage and current. This effect is called the heat dissipation factor . The heat dissipation factor is a measure of heat energy dis- sipated by rapidly repeated reversals of polarization, as with an alternating current. The dissipation factor can also be thought of as the ratio of electrical energy lost to that transmitted at a representative frequency, such as 1 MHz (10 6 cycles/s). This high- frequency heat generation, undesired in many insulation uses, can be used to preheat thermosetting materials just before molding. It is also the basis for electronic heat-seal- ing, widely used in the fabrication of articles from plasticized-PVC films. Some heat dis - sipation factor values are shown in Table 3. Table 3. Typical Values of Dielectric Constants and Heat Dissipation Factors for Several Thermoplastics at Room Temperature
Heat Dissipation Factor
Heat Dissipation Factor 0.003-0.014
Dielectric Constant 3.7-3.9 2.1-3.9 2.9-3.4
Dielectric Constant 2.3-2.9
Material
Material
Acetal Acrylic ABS
0.001-0.007 Polypropylene 0.001-0.060 Polysulfone 0.006-0.021 Modified PPO 0.006-0.190 Polyphenylene sulfide
2.7-3.8 0.0008-0.009 2.4-3.1 0.0002-0.005
3.1-8.3
2.9-4.5 2.6-3.1 3.7-10
0.001-0.002 0.001-0.022
Nylon 6 / 6 Polycarbonate TP Polyester
2.9-3.8 0.0006-0.026 Polyarylate 3.0-4.5 0.0012-0.022 Liquid crystal
0.010-0.060 Arc resistance as determined by ASTM D495 is the length of time (seconds) required for an electric arc applied on the surface of an insulating material to develop a conductive path. Materials that resist such a development are preferred for switchgear parts and other high-voltage apparatus. This test is used mostly for thermosetting materials because, un - like thermoplastics which tend to melt in the arc, thermosets decompose and do produce conductive paths. The Comparative Tracking Index (CTI) is a UL test with an ASTM counterpart (D3638) that is similar to the arc resistance test except that the surface to be tested is precoated with an ammonium chloride electrolyte. The test measures the voltage required to cause a conductive path to form between the electrodes and indicates the arc resistance of a contaminated surface, a condition often found in electrical and electronic equipment. End-Use Environmental Considerations.— The environment that will be encountered by the product is a prime consideration at the design stage. Problems with cracking, craz ing, discoloration, loss of properties, melting, or dissolving can be encountered in the
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