Stress relaxation and thermogravimetric studies on room temperature vulcanised polysiloxane rubbers

Abstract

The stress relaxation properties of foamed polysiloxane rubbers are important as they can influence the useful life of components made from such a polymer system. Of particular interest is understanding the changes in properties with time and temperature and whether the mechanisms responsible for stress relaxation are able to induce other material property changes. The stress relaxation properties of Dow Corning S5370 polysiloxane samples, aged under controlled conditions, have been measured using a Thermomechanical analyser (TMA) at a number of different temperatures. The results were assessed using the principle of Time-Temperature Superposition. Derived acceleration factors showed good adherence to the Arrhenius relationship and showed two regions where processes with different activation energies dominate. A transition region where there is a change in the predominant degradation process is evident at around 120 °C. Thermogravimetric analysis (TGA) studies were used to provide an improved understanding of the degradation at elevated temperatures (>120 °C). Time-temperature superposition and Arrhenius treatment of the TGA results reveal an activation energy (75±6 kJ/mol) which correlates closely to that derived from stress relaxation (65±5 kJ/mol). Overall, these observations suggest that the degradation processes at elevated temperatures which influence stress relaxation also induce significant weight loss. The dominant degradation process at elevated temperatures(>120 °C) is most probably silicone head to tail unzipping reactions resulting in the production of volatile cyclic species.