Mechanisms of wall coating and wall slippage during extrusion of polyethylene with fluoropolymer based Polymer Processing Aids (PPAs) are reviewed for various types of PPAs. It is shown that the spreading of PPA coating on die surface can be improved by using a wetting agent, like a polyethylene glycol. Wall slip is favored by lowering the entanglement density in the interdiffusion phase between PPA and polyethylene. Finally, the morphology of PPA coating is investigated. It can be explained by the balance between viscosity and interfacial energy. (C) 2014 Elsevier B.V. All rights reserved.