THERMAL-PROPERTIES OF NATURAL-RUBBER VERSUS TEMPERATURE AND PRESSURE

The thermal conductivity λ and the heat capacity per unit volume ρcp were determined by the transient hot-wire method at pressures up to 1.0 GPa and in the temperature range 130-340 K. Specimens were cold-pressed pure rubber and hot-pressed rubber containing 3.7% sulphur. The vulcanized variety was found to have a higher glass transition temperature Tg at all pressures. Our value of Tg at atmospheric pressure is higher by 23 K than the previously reported values from quasistatic experiments, which can be accounted for by the relaxation time of the excitation mode. The derivative of the transition temperature ∂Tg/∂P was found to decrease with increasing pressure for both samples, as predicted by a recent theory. The thermal-expansion coefficient was determined under pressure from the temperature response to adiabatic pressurization using the heat-capacity data.