State of Dispersion of Reinforcing Silica in a Silicone Elastomer, as Investigated by Transmission Electron Microscopy
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State of Dispersion of Reinforcing Silica in a Silicone Elastomer, as Investigated by Transmission Electron Microscopy Rex J Field1, Jeanne Cambray2, Joachim Floess1, Stephane Rouanet1 1 Cabot Corporation, Nanogel Group, 700 E US Hwy 36, Tuscola IL 61953, USA 2 Cabot Corporation, Analytical Technical Centre, 157 Concord Road, Billerica MA 01821, USA ABSTRACT 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. INTRODUCTION Commercial silicone rubbers typically contain sub-micron particles dispersed within them which are responsible for the mechanical properties of the silicone elastomer required for commercial success. Fumed silica such as Cab-O-Sil® [1] has long been used for the reinforcement of higher-perfomance silicone rubber compositions. The state of dispersion of the reinforcing fumed silica in silicone elastomers has been the object of a number of studies (eg [2, 3]). From such work, the fumed silica is seen to have been broken down to pieces that are similar in size to the primary aggregates, 200 nm or so in diameter, formed during the synthesis by a flame process of the silica itself . A new class of materials becoming available commercially is that of the silica aerogels (otherwise known as high-porosity xerogels). These have been shown to give excellent reinforcement [4]. The particles can easily be prepared in forms which contain vinyl-groups covalently bonded to the surface of the silica [5]. It is the high porosity of the aerogels that allows them to perform efficiently as reinforcing agents. Historically, aerogels have been obtained using super-critical drying processes [6]. But commercial availability has been constrained by the technical difficulties of the drying. More recently methods have been developed which obviate the need for super-critical drying. The irreversibility of the collapse is prevented by end-capping the silanols, so that after the liquid has evaporated, the silica can "spring-back", as reported by Deshpande et al [
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