Heat Resistance and Thermal Stability of Polymers

the ability of polymers to retain their useful properties at elevated temperatures. Heat resistance sets an upper limit to the temperature region in which a polymeric material can support a mechanical load without undergoing a change in shape. The loss of heat resistance is a consequence of physical processes, such as the transition of glassy polymers to a high-elastic state or the melting of crystalline polymers. Thermal stability sets an upper limit to the working temperature in cases where the suitability of a polymer is governed by chemical resistance, that is, resistance to reactions usually leading to the degradation of polymers in inert or oxidizing media. The performance properties of rubbers, as well as of many solid polymers with high glass-transition and melting temperatures, depend on thermal stability; this stability is of great importance in processing and shaping articles of polymeric materials.

Depending on the type of article (coatings, fibers, construction materials) and the intended use, various methods are used for determining heat resistance. The heat resistance of solid construction materials is determined from changes in rigidity. The indicator here is the deformational heat resistance—the temperature at which a prohibitively large change in shape in a sample begins when subjected to a given load and heated at a given rate. In the USSR, standard methods for determining the strain heat resistance differ in the technique used for measuring deformation, the acceptable amount of deformation, the magnitude of the load, and the rate of heating. Thermal stability is measured through the change in weight of a sample while being heated at a given rate. Knowledge of heat resistance and thermal stability makes it possible to establish upper temperature limits for the use of polymers during short exposures to heat. When longer periods are involved, these temperatures are usually tens of degrees lower.