Fracture of Polymer Melts at High Shear Stress

Abstract

An analysis of the flow of polymer melts as well as of melt fracture at high shear rates has been carried out. On the basis of the flow properties of polyethylene, polypropylene, polystyrene, polymethylmetacrylate melts, and silicone rubbers measured over a wide range of shear rates from 10−3 up to 103 sec−1, the following conclusions have been drawn: Under low rates of shear the behavior of polymer melts is of Newtonian nature, whereas at high rates the character of flow depends on the value of the dimensionless criterion κ=τ/ηNγ̇=κ3τ·R/ηN·V, where τ is shear stress γ̇ is shear rate, ηN is Newtonian viscosity, R is characteristic dimension, and V is mean velocity of flow. The fracture of polymer melts occurs above a critical constant value of the number κ: κcrit=0.025. Available flow data, analysed by the method of reduced variables, show that most previously suggested criteria are essentially equivalent. This treatment eliminates their restricted validity and technological applicability and brings the description of fracture close to molecular theories of non-Newtonian flow.