Experimental studies of interfacial instabilities in multilayer flow of polymer melts

In the past decade the need for development of specifically engineered structures of polymers has resulted in proliferation of products that are manufactured using various coating and multilayer extrusion technologies. It was well known that this class of flows are susceptible to interfacial instabilities that manifest themselves in form of long, intermediate or short wavelength interfacial waves. Hence, to establish processing windows for stable operation of multilayer coating and extension processes a better understanding of interfacial instabilities associated with the multilayer flow of polymeric liquids is required.;In this study we have examined the effect of viscosity, elasticity, layer depth ratio and disturbance wavenumber on the interfacial stability of two layer (A-B), three layer symmetric (A-B-A) and asymmetric (A-B-C) plane Poiseuille flows of polymer melts. In our experiments, the instability is observed and measured with the aid of digital image processing techniques. Based on these experiments, we have prepared a number of stability contours and growth rate plots that clearly demonstrate the effect of various important parameters (i.e., number of layers and their arrangement, layer depth ratio, viscosity ratio and elasticity ratio) on the stability of the interface/interfaces as well as the growth or decay rate of various disturbances.;We have investigated the coupling between interfacial instabilities and mechanical interlocking in polymeric films consisting of the incompatible pair PP/HDPE and the compatible pair of LLDPE/PP. Our experimental results show that mechanical interlocking between the two phases can be achieved by inducing a controlled amount of interfacial instabilities by properly selecting the initial disturbance frequency and amplitude as well as the layer depth ratio. It has been shown that in our test geometry maximum strength enhancement can be achieved at dimensionless wavenumber near unity that correspond to the disturbances with the largest growth rates.