Bound rubber and “crepe hardening” in silicone rubber

Some recent results of the research aimed to correlate properties of filled silicone rubber compounds with surface properties of fumed silica fillers are reported. It is shown that the specific interaction between silica surface silanol groups and the siloxane chain of silicone rubber plays the main role in “crepe hardening” and bound rubber formation in compounds. The silanol coverage of silica determines the tightness of polymer–filler network in bound rubber. The experimental data fit Meissner's theory of bound rubber quite well. Remilling of “crepe hardened” compounds was also studied, and the structure of the remilled compounds is proposed. The activation energy of bound rubber formation on mixing was found to be 16.8 kj/mole; this suggests that physical rather than chemical processes are involved in the interaction between silica and silicone rubber.