Most important of these is conductivity. The reason is that the conductivity of rubber material is very small, so that when heat is generated in rubber material as a result of deformations, as explained in vicoelastic properties this is not readily dissipated. Some increase in the conductivity is possible by incorporation of suitable filling materials having inherently high conductivity, the possibilities of this procedure is limited and it must be accepted that any technically useful rubber material will have low heat conductivity.
The temperature developed in the rubber material depends also on its specific heat. This is an essentially additive property which can be calculate from the proportions of rubber material and other constituents. ; thus for a compounded rubber comprising 60 part(by weight) of china clay per 100 parts rubber material the specific heat would be {(60 x 0.21) + (100 x 0.50)} / (60+100) = 0.39 cal/g approximately. A similar procedure can be applied to thermal conductivities .
Another thermal property of practical importance is the coefficient of expansion ; this is much greater than for metal. An incidental consequence of the big difference in expansion between steel and rubber material is that the differential expansion must be taken into account in designing moulds for precision part.
All comercially available rubber material are organic meterials which become unserviceable at temperature much below those at which metal will operate. Nevertheless the range of operating temperatures is being rapidly extended upwards by the development of special polymers, especially the silicone rubbers and those containing fluorine.
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