Studies on multilayer film coextrusion III. The rheology of blown film coextrusion

Abstract

Abstract Multilayer blown film coextrusion was studied, both experimentally and theoretically. For the experimental study, an annular die with a feed‐port system was designed and multilayer blown films were produced by rotating the inner mandrel with a one horsepower variable‐speed drive at speeds from nearly 2 to 6 rpm, and by inflating the tubular molten film with air. The die has 16 feed slots and melt pressure transducers are mounted along the axial direction of the outer wall of the annular flow channel. The transducers were used to determine the pressure gradient in the annular flow channel, which then permitted determination of the reduction in pressure drop when different combinations of two polymer systems were coextruded. Polymers used for b own film coextrusion were: (1) low‐density polyethylene with ethylene‐vinyl acetate; (2) low‐density polyethylene with high‐density polyethylene; (3) low‐density polyethylene with polypropylene; (4) high‐density polyethylene with ethylene‐vinyl acetate. For the theoretical study, stratified helical flow was analyzed using a power‐law non‐Newtonian model. A computational procedure was developed to predict the number of layers, layer thickness, and the volumetric flow rate as functions of certain processing variables (namely, the pressure drop in the die, and the angular speed of rotation of the inner mandrel of the die) and the rheological parameters of the individual polymers concerned. Comparison was made of the theoretical prediction of volumetric flow rate with experimental ones. Some representative results are presented of the theoretically predicted axial and angular velocity distributions, shear stress profiles, and shear rate profiles.