Acid/Base Properties of Fumed Silica Fillers Used in Silicone Elastomers *

Inverse gas chromatography (IGC) and photoacoustic infrared spectroscopy (PAS) were used to investigate the effects of thermal history and silane treatment on the surface energetics of various fumed silica fillers. The specific interaction parameter, I_sp, was a factor of 2.3 (THF probe) lower for a silane treated fumed silica (Degussa R972) relative to an untreated filler (Degussa A200), thus confirming the deactivating effects of silane treatment. Heat treatment of raw fumed silica (Cabot HS-5) at 150°C and 650°C decreased I_sp (THF probe) from 15.3 kJ/mol to 10.5 kJ/mol. This decrease was attributed to the loss of bound water and surface silanols. The dispersive interactions increased with heat treatment as a result of the formation of higher electron density sites such as strained Si─O─Si bonds. Silazane-treated fillers were prepared using heat treated and hydrated feedstocks. I_sp values were a factor of 2-30 lower, depending upon the probe, for silazane-treated fillers made from heat treated feedstock. The dispersive interactions increased from 27.2 mJ/m² to 57.8 mJ/m² as a result of using heat treated feedstock, thus suggesting some of the strained Si─O─Si bonds formed upon drying remained intact even after silazane treatment.