DESIGN IN PLASTICS: PART 3

Thermal, Electrical And Chemical Properties Of Plastics

Previous articles examined design considerations for injection-molded parts (August 2000), and the physical and mechanical properties of plastics (November 2000)

By Frank Jaarsma
Ticona Corporation, Summit, NJ

Plastics often must meet multiple demands in any one application. Beyond the need for specific physical and mechanical properties such as strength and stiffness (discussed in the previous article in this series), plastics often encounter temperature, electrical and environmental stresses. In selecting the right plastic for an end use, designers must understand how key properties in these areas are measured. This article offers a compendium of the most significant thermal, electrical and chemical parameters as a guide.

Thermal properties

Higher temperatures make plastics more sensitive to mechanical stresses and vulnerable to chemical attack, while lower ones generally make them less ductile. In designing a part, it is important to understand what thermal conditions it will meet during processing, assembly, finishing, shipping, and end use to ensure it retains its integrity. The main thermal properties of plastics include:

* Melting point -- a property important for molding and assembly. Crystalline thermoplastics have sharp, well-defined melting points, while amorphous ones soften and grow more fluid over a wider range.

* Melt index -- measures how much plastic is extruded through a heated apparatus in 10 minutes (reported as g/10 min.). Greater flow rates indicate greater viscosity. The results guide the selection of molding conditions.

* Vicat softening point -- the temperature at which a small, circular, heated and lightly loaded probe penetrates a set distance into a specimen. It indicates a material's ability to withstand short-term contact with a heated surface. It is useful for crystalline plastics, but of limited value for other types that can creep during the test.

* Coefficient of linear thermal expansion (CLTE) -- measures the change in one dimension against the original dimension per unit change in temperature (in./in./°F·10^-5 or cm/cm/°C·10^-5) (Table 1). This is important in gauging stresses when assembling dissimilar materials. CLTE can vary with temperature, and anisotropic materials have different CLTEs in different directions.

TABLE 1

Coefficient of Linear Thermal Expansion
for Thermoplastics and Other Materials *

* Deflection temperature under load (formerly called heat deflection temperature) -- the temperature a test bar under a given load deflects a set amount. It is a relative measure of how well a plastic performs under load at elevated temperatures.

* Thermal conductivity -- the rate heat energy travels along or through a plastic. This is important in heat-generating applications and where heat dissipation is needed.

* Aging at elevated temperatures -- looks at how a plastic's physical, mechanical, thermal or electrical properties change over time when samples are stored at high temperature. Elongation increases at higher temperatures and the effects of strain manifest more quickly. At lower temperatures, plastics tend to lose impact strength.

* Flammability tests -- looks at a material's resistance to combustion. UL 94 is the most widely used flammability test for electrical devices. Ratings, which are V-0, V-1, V-2, 5V and HB, define how well a material combusts after a flame is removed.

* Limiting oxygen index -- measures how much oxygen is needed to sustain combustion. A plastic needing less than 21% oxygen will burn in air.

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