The use of vinyl functional aerogels for reinforcement of silicone rubbers

Silane-treated silicate-based aerogels are excellent reinforcing fillers for silicone rubbers. The physical properties of the aerogel filler can be readily altered by controlling the properties of the hydrogel, and the vinyl functionality of the aerogel surface can be easily and precisely controlled in the silylation step of the aerogel manufacturing process. By changing the vinyl content on the surface of the aerogel, the physical properties of the rubber can be markedly changed without the use of any in situ treating agents during compounding or alteration in the formulation. The silicone rubber hardness and modulus of elasticity exhibit a sensitive response to the vinyl content, while the plasticity of the uncured base shows no effect of vinyl loading. In contrast, the viscosity (plasticity) of the uncured rubber is a strong function of the aerogel surface area, whereas the rubber modulus and strength are only slightly affected by particle properties such as surface area. In addition, the high porosity of aerogels allows for good fracture characteristics and dispersion of the filler during compounding, while the large pore size provides good accessibility to and utilization of the vinyl sites within the porous structure. The ability to easily control the aerogel structure, to completely hydrophobize the silica surface, and to precisely control the vinyl loading make aerogels a potentially useful new particle for rubber reinforcement. A brief summary of the treatment chemistry and the vinyl surface chemistry is also presented in the paper.