Novel Wet Process Silica Prepared from Alkyl Silicates. Part III: Use in Silicone Elastomers for Optical Applications

Abstract

The first two papers of this three-part series of papers dealing with a novel wet-process hydrophobic (WPH) silica technology covered the silica synthesis (Part I) and reinforcement performance (Part II) respectively. The WPH silica was highly reinforcing and provided optically clear compositions as well. The first optically clear, high consistency silicone elastomer was developed in the midnineteen fifties by Polmanteer et al., using different technology from that described in this paper. Optical clarity was accomplished in the earlier work by matching the refractive indices of the filler and polymer at 25°C, making them isorefractive at this specific temperature. This paper will discuss the unique features of this new silica technology as it relates to optically clear silicone elastomers. The term, optically clear, in the context of this paper refers to an elastomeric material, 2.54 mm in thickness, that exhibits less than 4% haze, and more than 85% transmission. It will be shown that, when using the older technology based on isorefractive compositions, optical clarity is reduced with temperature change from the isorefractive temperature, due primarily to an increase in haze. The new silica technology eliminates this problem and effectively broadens the application temperatures for optically clear, thermally stable silicone elastomers.