A Study Of Interactions In Silica-Silicone Polymer Systems.

Reviews are given of the preparation and properties of pyrogenic silicas and poly(organosiloxane) elastomers and rubbers, and also of reinforcement and elastomer-filler interactions in silicone rubbers. A series of silica fillers with varying specific hydroxylated surface area (SHSA) were prepared by the hexamethyldisilazane solution treatment of high surface area, high purity (>99.8% Si02), pyrogenic silica fillers (Aerosils). The quantitative and qualitative characterisation of the silicas demonstrated the gradual replacement of the silica surface hydroxyls (highly interactive) with trimethylsilyl groups (highly inactive). The effects of silica surface activity/ inactivity were investigated in systems involving either a high molecular weight poly(dimethylvinylsiloxane) (PMVS) elastomer, or low molecular weight fluid poly(dimethylsiloxane)s (PDMSs). In unvulcanised PMVS/silica systems (milled and non-milled), elastomer-silica interactions were studied through the measurement of non-extracted elastomer-filler complex (NEFC) and inter-related swelling parameters. The silica SHSA strongly affected the initial development of NEFC, though most PMVS/silica systems developed substantial NEFC levels on storage. Initial interaction levels are postulated as being dependent upon the amount of silica networking formed (due to interaction between particles ruptured during milling, and to inter particle hydrogen-bonding and condensation reactions), and on the amount of elastomer adsorbed onto the silica. For fully-treated silicas, interactions are considered to be dependent on weak elastomer-silica networks formed during milling. NEFC developed in non-milled systems if some silica surface activity existed but no NEFC was developed with fully-treated silicas. The PDMS/silica systems were studied rheologically. PDMSs with trimethylsilyl (inactive) or hydroxy (active) end-groups were used. In the former, viscosity increased with SHSA indicating filler aggregation and networking; in the latter, the hydroxy end-groups interfered with the networking, leading to reduced interaction. Similar behaviour was found in the development of NEFC in milled PDMS/silica systems. In vulcanised PMVS/silica systems, the effect of silica networking was demonstrated by analysis of the Young's moduli of systems containing various concentrations of silicas and subjected to a variety of treatments. The properties of non-milled rubbers were also investigated and they demonstrated the dependence of Young's modulus on milling and silica networking. Other rubber physical properties were investigated and were found to be dependent on elastomer-silica interactions arising from silica networking.