Conclusions on Gum Rubbers

Thermal transport properties of rubber depend not only on the temperature but also on the type of rubber and the type and quantity of filler. It is generally thought that they are not greatly affected by vulcanization [5], 

The effect of temperature on the thermal conductivity of rubber is similar to that for amorphous plastics. Below the glass transition tanperature 7 the conductivity increases with increasing temperature, and above 2 it decreases. At around room temperature, the thermal conductivity becomes nearly temperature independent. However, the conductivity of some rubbers, for example butyl, increases with temperature from room temperature. 

The addition of carbon black to natural rubber iuCTeases the conductivity at room temperature, and this conductivity dcCTcases with inCTeasing temperature in aU cases. The diffusivity for the same material passes through aminimum at about 90°C. Because specific heat exhibits a small increase with temperature over this tanperatuie range, the increase in diffusivity reflects the dcCTcase in density with rising tanperature. 

Carbon, whether in amorphous or graphitic form, has a much higher conductivity and diffusivity than rubber, and the addition of carbon black to a rubber almost invariably increases the values of the thermal transport properties at room tanperature. However, at higher temperatures, values for the gum in some cases exceed those of the filled compounds. Unlike the heat capacity, the resultant conductivity and diffusivity of a filled material cannot be calculated on a simple volume additivity basis. 

The addition of a plasticizer to a polymer lowers the glass transition tanpera-ture and also extends the transition temperature range. Above 7 of the unplasticized material, the conductivity decreases with increasing plasticizer concentration, because of the increasing free volume. On the other hand, below 7 of the plasticized material, the conductivity increases with increasing plasticizer concentration. 

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