Rheometers with square root of time–temperature programming for the cure of rubbers

Abstract

Abstract Because of the drawbacks resulting from using a constant temperature, calorimetry has been developed with scanning mode and temperature programming with a constant heating rate. Following this way, a theoretical approach was made with moving die rheometers, showing some advantages over the method done under isothermal conditions. However, if it usual to vary the temperature linearly with time, it is shown that a better way consists in increasing the temperature of the dies with the square root of time. The theoretical study of this new method is made enabling the evaluation of the torque–temperature curves. Some advantages are obtained with respect to the linear programming, coming from the principle of this new method. Thus, the rate of heating is faster at the beginning of the process when the rubber sample is heated up to a temperature where the reaction starts, and becomes lower within the temperature range where the reaction develops. A method has been elaborated for evaluating the kinetic parameters from these torque–temperature curves. The effect of the parameters of interest, with the value of the rate of heating and the thickness of the rubber sample, is determined. Thus the value of the heating rate should be between 10 and 50 K/min0.5. A slight change in the thickness of the sample over the average value of 0.2 cm is of little effect. The profiles of temperature developed through the thickness of the sample are flatter than those obtained with the isothermal rheometer, leading subsequently to more uniform profiles of the state of cure, as they are obtained by calculation. Moreover, shorter times of experiment are obtained over the method based on the linear programming temperature system. The kinetic parameters such as the activation energy and the pre-exponential factor, as well as the overall order of the reaction, are thus obtained from only one experiment.