Influence of elastic effects on capillary flow of molten polymers

Capillary viscometer measurements for a number of polyethylenes and polystyrenes reveal deviations from linearity in plots of applied pressure versus capillary length to diameter ratio, at constant rates of shear, resulting in apparently negative end corrections. This is contrary to previous physical considerations. Some polymers obey Hooke's law in shear permitting the separation of the end correction into two components. A Couette correction, dependent upon the capillary dimensions, is constant for any given polymer, shear rate, and temperature; an elastic shear strain correction, dependent upon the transit time through the capillary, may be determined by post-extrusion measurements. Determination of these two correction leads to linear plots of pressure versus capillary length to diameter ratio and enables flow curves to be calculated independent of the capillary dimensions and the swell properties of the polymer.