Sensitivity of condensation cured polysiloxane rubbers to sealed and open-to air thermal ageing regimes

The results of a study investigating the thermal ageing properties in sealed and open to air regimes of a unfilled condensation curing (tin catalysed) organosiloxane polymer from Dow Corning (MS2420) and a room temperature vulcanising siloxane (RTV5370) from Rhodia silicones is presented. MS2420 samples were aged in air at 60 °C for 6 months and 90 °C for 1 month. Changes to the non-network species in aged MS2420 samples was assessed principally by GCMS and GPC of the solvent extractable matter. Closed system ageing generated cure reversion (thermally activated degradation processes involving chain scission) of the non-network chains whilst open system ageing results in loss of low molecular weight species. Suppressing volatile emission, as in sealed ageing, promotes cure reversion leading to the build-up of low molecular weight species in the rubber network. Excess residual catalyst levels cause postcure effects that reduce the amount of non-network siloxane species. In a separate set of experiments, cured RTV5370 siloxane rubbers have been thermally aged in open to air and sealed (inert atmosphere) regimes at temperatures up to 200 °C. Changes to the polymer network in aged RTV5370 samples was assessed by Instron compression tests, rubber hardness (IRHD) and segmental chain dynamics using solid-state nuclear magnetic resonance. The observed trend in sealed ageing regimes is that of softening of the rubber with loss of compressive strength and increased segmental chain dynamics. Studies on real-time aged RTV5370 rubbers derived from field trials suggest that a sealed polymer ageing methodology best represents the in-service ageing regime. In summary, our results show that the thermal ageing behaviour of condensation cured polysiloxane rubbers is sensitive to the ageing regime. These finding help define the best accelerated ageing methodology for the polymer system which in turn may lead to increased confidence in lifetime prediction studies.