Commercial silicone rubbers typically contain submicron particles dispersed within them, the particles being responsible for the mechanical properties required for commercial success. Fumed silica has long been used for the reinforcement of higher-perfomance silicone rubber compositions, but high-porosity aerogels can function as well. The object of the work here was to compare the state of dispersion of some high-porosity aerogels with that of a fumed silica.
Model silicone HCR (“heat-cured rubber”) compositions were prepared, and their mechanical properties characterized. Thin sections of the rubbers were then examined by TEM.
Much of the fumed silica had been dispersed to give sub-micron sized features, although a number of larger features were present. The hydrophobic aerogel, in contrast, had been dispersed to give even finer features in the rubber, with very few super-micron fragments. The state of dispersion of the hydrophilic aerogel was quite different, showing many poorly-broken down large fragments up to 5 µm or more in diameter. The visual appearance of the compound reflected this poorer state of dispersion.