Rheological Study of Tyre Tread Compound (Part II): A Method to Study the Relaxation Time and Viscosity Index Using Parallel Plate Rheometer

The object of this study was to investigate the relaxation time (lambda,k) and the viscosity index (etak) of a tyre tread compound. The rheological behaviour of the sample was studied using a parallel plate rheometer and the rheological material functions, such as complex shear viscosity (etak), elastic shear modulus (G'), and viscous shear modulus (G") were measured in frequency sweep test (FST). The study has focused on a method for calculating lambdak, and the etak using generalized Maxwell model (GMM) over a limited range of frequency and temperature. Time sweep test, stress sweep test and frequency sweep test were carried out using a parallel plate rheometer. The stable zone (or steady state region) which is suitable for measuring the rheological material functions was obtained from time sweep test results at various temperatures and frequencies. Stress sweep test was then carried out to determine linear viscoelastic zone of the compound at a given temperature and frequency. The critical stress, which determines the linear-nonlinear transition point, was obtained from stress sweep test results at each tem perature and frequency. The complex shear viscosity (d*H*), elastic shear modulus (G'), and viscous shear modulus (G") were measured in frequency sweep test and the relaxation time was then calculated. The results showed that the rheological behaviour of the compound followed the GMM. The relaxation time was also found to be a decreasing function of both temperature and frequency.