An investigation of the effects of transition metal compounds on the heat stabilisation of polyorganosiloxanes

The heat stabilising effects of ferric oxide and ferric octoate in polyorganosiloxanes have been examined by measurement of weight loss, hardness change, compression stress relaxation, reduced viscosity and cross-link density along with infra-red and differential thermal analysis. Systems involving gum stock both with and without stabiliser and in the vulcanised and unvulcanised states have been examined over a temperature range 150-300°C in air. In addition, a limited amount of work has been carried out on weight loss with vulcanised systems in nitrogen at 250°C and on reduced viscosity with unvulcanised systems similarly at 250°C.

Thermal degradation curves in terms of weight loss have indicated that stabilisation is only apparent in polyorganosiloxanes containing stabiliser in both the vulcanised and unvulcanised states at temperatures in excess of 200°C. In this respect, reduced viscosity and crosslink density measurements on aged samples have indicated that the onset of stabilisation is coincident with thermal scission of the polymer chain. These measurements have suggested that during thermal scission the reactive ends so formed react with the stabiliser. The observed decrease in weight loss in stabilised systems compared with their non-stabilised counterparts may be interpreted as being the result of this latter reaction, this being particularly evident at temperatures in excess of 200°C.

Hardness measurements have indicated that stabilised systems were not prone to embrittlement or high hardness changes as were the unstabilised ones. The high and low hardness changes recorded with unstabilised and stabilised systems respectively are similarly reflected in the cross-link density measurements recorded.

Spectroscopic evidence along with physical measurements suggests that during thermal ageing a chemical reaction occurs between the polymer and the stabiliser resulting in the formation of a new macromolecular polymer containing Si-O-Fe bonds. The resultant polymer appears to be more resistant to thermal oxidative degradation.

The work has been of considerable practical benefit in the development of silicone rubber for high temperature applications.