The operating temperature of a seal is a vital factor because any substantial difference between this and normal ambient temperature will normally modify the material characteristics, particularly in the case of elastomers.

The changes which occur at low temperatures are quite different from those produced by elevated temperatures.

With decreasing temperature the tendency is for all elastomers to become progressively harder with loss of flexibility and slower recovery from deformation. Hardness/temperature curves in themselves do not give any particularly useful information as hardness may reach a nominal or actual maximum value while the material still retains good flexibility.

Direct measurement of flexibility or torsional stiffness is much more significant, and if this is plotted against temperature it will show a curve with a characteristic bend, for example T2 point. From this it can be determined the freeze point where a marked loss of flexibility starts.

Beyond the freeze point stiffness increases very rapidly with further decreases in temperature until the brittle point is reached, that is, the material becomes brittle and will break if flexed.For design purposes the freeze point can be determined as the temperature at which the original stiffness (at 20°C) is doubled (x2 freeze point).

The freeze point equivalent to an increase in stiffness to ten times the original may also be given (x10 freeze point) as a close indication of the temperature at which the material becomes quite unusable for flexing and is rapidly approaching the brittle condition. The x2 freeze point represents a safe minimum temperature for working.

With certain elastomers a decrease in temperature may promote definite crystallization of the material in addition to normal stiffening. This may build up slowly, or even be localized to give a flat spot on the seal. The material may well be still useable under such conditions, due to the fact that it is nowhere near its brittle point, when in such cases the necessary resilience can be provided by spring loading if there is no immediate or economic choice of an alternative elastomer.

For any basic elastomer, low temperature characteristics can be modified to some extent by compounding. Thus an increase in hardness will usually lower the brittle point but make the material less flexible generally, whilst improvements in chemical resistance will often raise the brittle point.

It should also be emphasised that laboratory tests on the material alone at low temperature will not necessarily be characteristic of the material performance in service as a seal.

This is largely because the fluid in contact with the seal can affect the degree of plasticization; it can be absorbed, for instance, and increase the effective degree of plasticization, or leach out a proportion of the original plasticizer. Control of these effects is largely a matter of compounding, although compatibility with the fluid may be a prior requirement, in which case it may be necessary to sacrifice some low temperature performance.

At elevated temperatures, all elastomers lose strength and thus tend to become softer and more flexible.

Normally recovery is complete on reduction of temperature, but if the temperature is high enough some changes may be permanent.

Also ageing characteristics are accelerated by heat, normally taking the form of a progressive increase in hardness and modulus with loss of elastomeric properties.

A further important effect which may have to be considered when the operating temperature of the seal differs substantially from normal room temperature is the relative thermal expansion or contraction of the seal and its surrounds. The thermal coefficient of expansion is much higher than that of metals (roughly ten times that of steel).

This is normally most significant at elevated temperatures where thermal expansion of the seal is substantially greater than that of its surrounds and actual volumetric expansion may be further increased by swelling in contact with the fluid.