Capillary Flow of Polypropylene Melt

I. Non-Newtonian Behaviour and End Correction Effect

Abstract

Melt flow behaviors of commercial polyproplylene (viscosity average molecular weight M_v=2.39×105, n-Heptane insolubles, 98.5%) were investigated with a capillary viscometer, Koka Flow Tester, using 8 straight-type dies ranging in the ratio of the length to the radius from 2 to 30 and 5 tapered-dies ranging in the inlet angle from 5° to 90°, at 250°C. Melt flow can be approximately expressed by the two parameter equation, τ=KDⁿ (where, τ, shear stress, D, shear rate, n, constant depending on the non-Newtonian behavior, K, constant). End correction factor ν, determined by Bagley's method, increases linearly from 3 to 12 with increasing ln D from 2.7 to 4 (sec^-1). A method to estimate the effect of end loss at the tapered inletwas proposed. The end loss at tapered inlet shows minimum for 20° tapered die. Constant critical shear stress τ_c=1.1×10⁶ dyne/cm² for straight dies was obtained if the end effect was taken into account.τ_c for tapered die shows maximum at 20°. Bagley's method of estimating the shear modulus of the melt i. e., plots of ν vs.τ, is inapplicable for polypropylene.