Studies on multilayer film coextrusion II. Interfacial instability in flat film coextrusion

Abstract

An experimental study was carried out to investigate the phenomenon of interfacial instability in multilayer flat‐film coextrusion. For the study, a sheet‐forming die with a feed block was used to coextrude three‐and five‐layer flat films. Polymers coextruded were: (a) low‐density polyethylene with polystyrene, and (b) high‐density polyethylene with polystyrene. It was observed that, for a given polymer system, there is a critical value of wall shear stress at which an irregular (i.e., unstable) interface between the layers sets in, giving rise to a pattern similar to that usually found in a wood panel. Once the instability sets in, the severity of interfacial instability is found to depend on both the total volumetric flow rate (hence wall shear stress) of the combined streams and the ratio of the individual layer thicknesses. An attempt is made to correlate the critical conditions for the onset of interfacial instability in terms of the layer thickness ratio, and the viscosity and elasticity ratios of the two polymers being coextruded.