The invention relates to a process aid for use in rigid thermoplastic
compositions, and especially in rigid polyvinylchloride (PVC)
compositions. The process aid is an acrylic terpolymer of
methylmethacrylate, butyl acrylate and styrene at an optimal ratio to
produce a significant reduction in melt process viscosity. The resulting
reduction in extrusion melt torques enables manufacturers of rigid PVC
products to have higher throughput production rates.