Frequency and temperature dependent dynamic shear response of polyurethane foams

Abstract

Dynamic mechanical analysis (DMA) experiments have been performed on rigid polyurethane (PU) foams using a torsional rheometer. Circular disk samples were tested at several frequencies between 1-2,000 cpm and at temperatures between 25°C-150°C. The dynamic torsion tests were carried out in two phases. In the first phase, logarithmic frequency sweeps were carried out at each temperature with 0.05° angle of strain. In the second phase, linear strain sweeps in the range of 0.05°-5° angle of strain were carried out at a frequency of 100 cpm. The frequency sweep results allowed to obtain a master curve based on the principle of frequency temperature superposition at a reference temperature of 115°C. The predictions obtained from the master curve have been validated at selected frequencies. The shift factors were found to follow an Arrhenius relationship and the activation energy was found to be 264.46 kJ/mol. Results from the dynamic strain sweep tests indicate that samples go through predominantly viscoelastic behaviour. The strain sweep samples showed significant fracture near the ridges on the top die whereas the frequency sweep samples showed significant striations near the reference temperature.