Fillers 

Reinforcing fillers are added to improve the otherwise poor tensile strength of silicones. Silica in the form of silica fume with a particle size in the 10-40 nm range is the most preferred filler, although carbon black is used. Fillers do interact with the vulcanized rubber to form a pseudo-vulcanization. This can occur during mixing (creep hardening) or during storage (silo aging).