Continuous and intermittent stress relaxation studies on foamed polysiloxane rubber

Stress relaxation studies of foamed polysiloxane rubber samples have been conducted using a thermomechanical analyser (TMA). The data were assessed using the principle of time-temperature superposition and also by a kinetic study of the experimental results. The time-temperature superposition technique produced acceleration factors that showed good adherence to the Arrhenius relationship and showed two distinct regions where processes with different activation energies dominate. Our kinetic studies showed that the stress relaxation isotherms between 21 °C and 100 °C showed poor correlation to both first- and second-order rate equations. In comparison, the stress relaxation isotherms at elevated temperatures between 120 °C and 175 °C showed good conformity to second-order kinetics. The second-order rate constants for the isotherms at elevated temperatures showed good adherence to the Arrhenius relationship yielding an apparent activation energy of 63 ± 4 kJ/mol. These values are in close agreement to that derived using time-temperature superposition (65 ± 5 kJ/mol). The simple kinetic approach adopted here could not be used to assess the stress relaxation data below 120 °C because a suitable rate function could not be identified. Intermittent stress relaxation studies allowed compression set data to be obtained as a function of time and temperature. Comparison of intermittent and continuous stress relaxation profiles at a number of different temperatures shows that the foamed polysiloxane rubber exhibits significant physical relaxation or spring back during the recovery process. To assess the physical contribution to compression set, samples showing set were swollen in toluene for 1 day and then dried in air to constant weight. All samples with set showed significant solvent-induced recovery suggesting reversible physical relaxation processes contribute significantly to compression set in these polymers. This suggests that possible compression set damage in real-time aged foamed polysiloxane rubbers may be partially reversible using the toluene swell procedure reported here, opening up the possibility of component re-use.