A composite slip model for molten polymers over metallic walls partially covered with low-surface-energy coatings

Molten polymers exhibit distinct slip characteristics over high-surface-energy metallic dies and low-surface-energy polymer processing aid (PPA) coatings. In this work, a methodology is developed to quantify the composite slip of molten polymers flowing over partially coated surfaces, where slip varies spatially between coated and bare regions. The proposed model is derived from a fundamental force balance governing pressure-driven flow and incorporates a recently formulated PPA coating dynamics framework describing the adsorption and desorption of PPA particles on the die wall [Jia et al., J. Rheol. 69, 495–505 (2025)]. The methodology is validated using capillary extrusion data for a linear low-density polyethylene melt, accurately capturing the evolution of wall shear stress and slip velocity associated with dynamic PPA coverage. The results demonstrate that the composite slip model provides a physically consistent description of the coupling between interfacial coating kinetics and macroscopic slip behavior in polymer flow.