Effect of die exit stress state, Deborah number, uniaxial and planar extensional rheology on the neck-in phenomenon in polymeric flat film production

Abstract

In this work, effect of the second to first normal stress difference ratio at the die exit, −N2/N1, uniaxial extensional strain hardening, ηE,U,max3η0, planar-to-uniaxial extensional viscosity ratio, ηE,PηE,U, and Deborah number, De, has been investigated via viscoelastic isothermal modeling utilizing 1D membrane model and a single-mode modified Leonov model as the constitutive equation. Based on the performed parametric study, it was found that there exists a threshold value for De and ηE,U,max3η0, above which, the neck-in starts to be strongly dependent on −N2/N1. It was found that such critical De decreases if −N2/N1, ηE,U,max3η0 increases and/or ηE,PηE,U decreases. Numerical solutions of the utilized model were successfully approximated by a dimensionless analytical equation relating the normalized maximum attainable neck-in with ηE,U,max3η0, ηE,PηE,U, −N2/N1 and De. Suggested equation was tested by using literature experimental data considering that −N2/N1 depends on die exit shear rate, temperature and utilized constitutive model parameters for given polymer melt. It was found that approximate model predictions are in a very good agreement with the corresponding experimental data for low as well as very high Deborah numbers, at which neck-in strongly depends on −N2/N1. It is believed that the obtained knowledge together with the suggested simple model can be used for optimization of the extrusion die design (influencing flow history and thus die exit stress state), molecular architecture of polymer melts and processing conditions to suppress neck-in phenomenon in production of very thin polymeric flat films.