Different plastics are used to meet different gasket and seal requirements. An example is PTFE (polytetrafluoroethylene) that provides a virtually inert exposure to all kinds of elements and has outstanding high-temperature performance. Unreinforced PTFE can be vulnerable to creep and stress relaxation. However, reinforced PTFE modifies these limitations.

Gaskets are designed to meet different requirements such as retaining loads or meeting stress relaxation requirements, chemical or heat resistance, severe environments, and containment of liquids, greases, and so on. There are different industry tests and standards to meet many different service requirements. There are tests for applying compressive stress simulating the way many gaskets and seals are stressed in service (ASTM F 38). They measure the effects of such pertinent variables as stress relaxation in regard to time and environment.

Stress analysis is used to determine their capability to seal against leakage resulting from the pressure of a confined fluid. Generally high pressures or stress relates to increasing the tendency of a gasket to creep. Stress relaxation data to the designer provides a guide to developing a suitable design compromise, without overdesigning. These data show that the thinner the gasket, the less stress relaxation occurs. A gasket is redesign to be stronger and more expensive in construction. In material evaluations, stress relaxation can be related with geometric variables by means of a shape factor such as:

Shape factor = Annular area/total lateral area = (00 - /DJ/4f (4.23)

where OD = outside diameter, ID = inside diameter, and t = thickness.

The trend of this factor is generally consistent with plastics' behaviors as reviewed in Chapters 1 & 2. The relaxation-test data has to be determined for the plastic to be used. Individual behavior of one plastic is usually different when compared to another plastic.

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