Complexities associated with moisture in foamed polysiloxane composites

An understanding of mechanisms of moisture outgassing from silicones and the impact on material mechanical properties is important for compatibility and life prediction in sealed systems containing these materials. A series of thermomechanical (TMA) stress relaxation experiments have been performed to provide information on the important load bearing properties of these materials as a function of time and temperature. Two different silica reinforced foamed polysiloxane materials were tested, a peroxide cured rubber (M97) and a condensation-cured elastomer (S5370). The M97 foam showed unexpectedly complex stress relaxation profiles at temperatures around 100 °C, whereas the S5370 samples showed the expected smooth stress decay behaviour. Dried M97 foam samples show different stress relaxation behaviour to the non-dried materials. Furthermore, stress relaxation studies performed in controlled humidity environments showed that moisture has a significant accelerating influence on the underlying relaxation process. In dry regimes, a reduced stress relaxation rate was observed, with an increase in the force required to maintain a given amount of compression on the sample. To further develop our understanding of the effects of moisture, we have exposed samples to water enriched to 40% in ¹⁷O and used ¹⁷O nuclear magnetic resonance (NMR) spectroscopy to assess labelled hydrolysis reaction products. Our studies show that Si-¹⁷O-Si hydrolysis products are readily incorporated in the polymer and the degradation is enhanced by the influence of gamma radiation and/or heat. In addition, the polysiloxane foams showed different age related trends in sealed (where moisture is retained) and ventilated (open-to-air) regimes. Our observations have been explained by moisture influencing both physical and chemical degradation processes. Our findings on moisture induced changes in silicone stress relaxation rates are novel and demonstrate the importance of controlling humidity in service applications involving these materials.