Environmental and Ageing Properties

The properties of elastomers are changed by their environment in that a physical or chemical process may occur. There are usually three environmental effects considered particularly important with polyurethane elastomers the effects of heat, hydrolytic resistance and swelling by immersion in certain fluids. 

High temperature At temperatures above ambient there is a temporary reduction of properties due to a weakening of the physical bonds within the polymer. All properties are affected to some degree. The effect of increasing temperature on the tear strength of a typical urethane is shown in Fig. 13.8 the properties at 70-80°C are only 50% of their value at room temperature. 

Low temperature Low temperature environments affect the properties of polyurethane elastomers, but no degradation occurs and the effect is completely reversible. An increase in Young s modulus occurs below with a corresponding increase in hardness, tensile strength, tear strength and torsional stiffness, and a decrease in resilience. 

In the use of polyester urethanes at low temperatures a crystallization phenomenon may be observed when the polyurethane is held at moderately low temperatures for some period of time. When crystallized the polymers are considerably harder and their stiffness characteristics are similar to those normally observed at much lower temperatures. The effect is reversible and can be removed by the application of heat or by the generation of internal heat by flexing. Again the use of mixed polyesters reduces this tendency to crystallization. 

It is possible to make some general comments on the relative hydrolytic stability of different polyurethane elastomer structures. The three principal linear polyol series used in synthesizing urethane elastomers can, for hydrolytic stability, be ranked in the following order  

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