Abstract: Silica fume subject to a mechanical reduction in size by a milling process such as a fluid energy mill or jet mill is disclosed. The overall size reduction is at least 10%. The milled silica fume product should have not more than 0.5% by weight unable to pass through a 325 mesh screen. The milled silica fume can be used in thermoplastic resins, cementitious material and refactory material. The use of the milled silica fume with UPVC or PVC to make conduit with improved elastic modulus, improved impact resistance, decreased wall thickness and increased calcium carbonate content is specifically disclosed.

Abstract: A lubricating system composition for the extrusion of polyvinyl chloride resin conduits is provided by treating silica fume particles at least in part with a lubricant to form a premix additive for polyvinyl chloride extrusion compounds whereby the resulting product has unique and particularly advantageous physical characteristics which makes possible the reduction in wall thickness and an increase in loading of calcium carbonate to effect a material reduction in cost of manufacture.