High Temperature Degradation of Silicone Rubber Compounds in a Silicone Oil Environment

Abstract

Abstract We have found that silicone rubber compounds undergo a reduction in effective chain concentration when heated in an environment of high temperature silicone oil and that this rate of reduction is dependent upon the effective chain concentration to the 3/2 power. The 3/2 power relationship results from a slower than expected reaction rate due to the dilution influence of further reactions of already ineffective network chains. Based on our experimental findings, we believe that below 204°C, in a silicone oil environment, the predominate degradation of the siloxane network is due to the siloxane rearrangement reaction between silicone oil and the crosslinked network. The resultant bond breaking and reforming have an experimentally determined average activation energy of about 60 kJ/mole. In addition, based on our experiments, we have concluded that above 204°C, the higher activation energy of 105–125 kJ/mole observed for the two compounds containing little or no iron oxide is caused by a depolymerization mechanism which involves breaking of the network and formation of low molecular weight cyclic siloxane materials. With significantly more iron oxide present, as is evidenced in the RTV (Fe2O3 + Al2O3) material, the lower activation energy seen even above 204°C is believed to be caused by the continued catalysis of the siloxane rearrangment reaction by the iron oxide.