the invention relates to a process aid for use in rigid thermoplastic compositions , and especially in rigid polyvinylchloride 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 .