Linear Temperature Programming Rheometers for the Cure of Rubbers. Effect of the Various Parameters

In spite of their drawbacks, rheometers are usually run under isothermal conditions for the cure of rubbers: At least three experiments are needed at different temperatures selected within a narrow temperature window to determine the kinetic parameters of the cure of rubbers, and it takes some time for thermal equilibrium between the sample, initially at room temperature, and the dies to be established. With the rheometer in scanning mode, the dies and the sample are heated from room temperature up to the selected final temperature at a constant rate. The heating rate is crucial in this new method. The rubber used in this study is an ethylene propylene diene monomer vulcanised with 20% sulphur and with a rather high cure enthalpy. The state-of-cure versus temperature curves obtained in scanning mode have been calculated using a numerical model that takes into account the heat transfer by conduction through the rubber, the coefficient of heat transfer at the die-rubber interface, and the kinetics of the heat generated by the cure reaction. The kinetic parameters were determined mathematically from these state-of-cure versus temperature curves, which are similar to torque-temperature curves, by obtaining straight lines from them or using a least squares method. The kinetic parameters calculated in these ways were similar to those introduced by obtaining the state-of-cure versus temperature curves using the numerical model. Whatever the heating rate, ranging from 2 to 10 K/min, and whatever the coefficient of heat transfer at the die-rubber interface which expresses the quality of the contact, the moving die rheometer can be used in scanning mode to determine the kinetics of the cure of rubbers, even when the enthalpy of cure is high.