DESIGN IN PLASTICS: PART 3
Thermal, Electrical And Chemical Properties Of Plastics

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Electrical properties

As good insulators, plastics provide essential dielectric properties. This quality is affected by temperature, moisture, time in service and other factors. Common electrical properties of plastics include:

* Dielectric strength -- measures the highest current that can be applied to a plastic before it allows a current to pass. It is expressed as the voltage just before this happens divided by the thickness of the sample (in volts/mil). It is affected by temperature, thickness, how the sample was conditioned, rate of voltage increase, test duration and contamination.

* Dielectric constant (or permittivity) -- looks at how easily a plastic can be polarized relative to a vacuum (Table 2). The test imposes an electrical field across an insulator, reverses the field and measures how long it takes to equilibrate. This dimensionless number, which is important in high-frequency applications, varies with temperature, moisture, frequency and thickness.

Table 2
Dielectric Constant and Dissipation Factor
for Various Thermoplastics at Room Temperature

* Dissipation factor -- measures the energy dissipated during rapid polarization reversals, as with an alternating current (Table 2). It can be seen as the ratio of energy lost as heat to current transmitted. It is usually measured at 1 MHz. This factor should be low when plastics are used as insulators in high-frequency applications such as radar and microwave equipment.

* Arc resistance -- measures time (in seconds) for an electrical arc imposed on a plastic to create a conductive path. It was developed for thermosets in which a path forms from decomposition products due to localized heating. A higher quantity is better where arcing is a possibility, as in switches, circuit breakers and auto ignitions.

* Comparative tracking index (CTI) -- an Underwriters' Laboratories test that measures how much voltage is needed to create a conductive path between electrodes on a surface after an electrolyte (0.1% ammonium chloride solution) is placed on that surface. CTI relates to the arc resistance of real-world equipment with surface contamination.

* Volume resistivity -- a standard measure of conductivity when a direct current potential is applied across a material (measured as ohms x area of the smaller electrode/specimen thickness). Materials measuring above 10⁸ ohm-cm are insulators.

* Surface resistivity -- expresses how well a current flows over the surface of a material between electrodes placed on the same side of a specimen. While volume resistivity is a property of the material, surface resistivity measures how susceptible a plastic is to surface contamination, especially moisture. It is useful when surface leakage may be a problem, but, since it is not exact, it should be used with wide margin of safety.

Environmental stresses

Chemicals (acids, bases, solvents and fuels), ultraviolet radiation, humidity and other environmental stresses can cause plastics to craze, crack, discolor, lose properties, soften or dissolve. Data from suppliers on how their products react to such stresses generally include exposure time, concentration and temperature. While this is helpful, the real world usually combines stresses that affect resins in different ways than one stress acting alone.

When evaluating a resin, it is best to expose it to the environments they will meet during assembly and end use. A resin's compatibility to an environment is often determined by time of exposure; concentration of adverse substances; whether these substances are solids, liquids or gases; radiation level and intensity; and the use of coatings and/or other protective barriers.

A chemical may interact in different ways to different plastics and even to different grades in the same family. It may react with the polymer to reduce molecular weight and alter short-term mechanical properties. It may dissolve the plastic, showing up as a shift in weight, dimension (swelling) and property loss. Or it can be absorbed into the polymer and cause plasticization, reducing strength, stiffness, and creep and impact resistance. A chemical may not look like it's affecting a plastic when there is no stress is applied, but may lead to failure under stress. And plastics are more susceptible to environmental stresses when they are under load or when temperature increases.

Chemical compatibility is measured by placing test bars of a plastic in a chemical for a set time at a set temperature and evaluating them for mechanical, appearance and other properties. End use testing is essential if chemical exposure is an issue.

All plastics degrade (fade, chalk or grow brittle) when exposed to the UV found in sunlight. UV testing exposes samples for set times either directly to the sun, or to high-intensity xenon or carbon-arc lamps in special test cabinets that accelerate testing.

A final word about properties

Resin screening should consider all factors a plastic will meet during handling, assembly, finishing and end use. In using published test data, keep in mind that many standard tests have a narrow focus and offer comparison data rather than absolute values. Since standard tests are done under controlled laboratory conditions, they usually do not include all factors that influence a part in actual use.

Some characteristics, like adhesion or wear cannot be predicted accurately by standard laboratory tests. Other properties, such as dimensional stability, strength, and rigidity, combine the effects of several factors and are hard to measure in the lab. For example, dimensional stability might combine coefficient of thermal expansion, moisture absorption, post-mold shrinkage, and relaxation of molded-in stresses. In nearly all cases, parts should be tested under the conditions they will encounter during their life cycle.

The next installment of this series will continue to look at how plastics are processed by injection molding.

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