Impact of thermal ageing on the tin catalyst species in room temperature vulcanised polysiloxane rubbers

Tin 2-ethylhexanoate is used as a catalyst in the synthesis of room temperature cured polysiloxane rubbers. The catalyst has the potential to influence the important load bearing and ageing properties of the rubber. In this study, the structure of the catalyst and the nature of the tin species in both fresh and thermally aged polysiloxane samples (RTV5370) have been investigated. Infra-red and C-13 NMR of the catalyst confirms the presence of 2-ethyl hexanoate ligands on the tin. Mossbauer Spectroscopy of the catalyst shows that the tin exists predominantly in the tin(II) oxidation state with some residual tin(IV) impurities. The tin(IV) phase predominates in the as-synthesised rubber, representing some 67% of the total tin signal, with the remainder being unreacted tin(II) species. The tin(IV) phase in the catalyst and that in the rubber are different. Overall, our observations suggest that the catalyst has taken part in the cure reaction and has been chemically modified as a result. The isomer shift (IS) and the quadrupole splitting (QS) from Mossbauer Spectroscopy suggests that the tin (IV) phase in the rubber is most probably SnO2 (stannic oxide). Toluene extraction studies show that this phase is not extractable from the rubber network, whereas the tin(II) species is easily removed. Both high temperature and room temperature ageing increase the proportion of the tin(IV) in the rubber. (C) 2003 Elsevier Ltd. All rights reserved.