The thermal‐mechanical degradation of high density polyethylene

Abstract

AbstractThe changes in molecular structure of high density polyethylene which can take place during processing have been studied using a nitrogen blanketed Brabender plastograph to simulate processing conditions. At high melt temperatures (> 290°C) decreases in melt viscosity and a narrowing of molecular weight distribution were observed, while at lower melt temperatures an increase in melt viscosity was observed. The increase in melt viscosity arises from a molecular enlargement reaction which is mainly attributable to the formation of long chain branches (LCB) which were detected using intrinsic viscosity measurements as well as by a novel method involving the measurement of melt elasticity on a Weissenberg rheogoniometer. The scission and enlargement reactions are not mutually exclusive but competitive, and a basic reaction scheme is proposed to explain the experimental results. The effect on this basic reaction scheme of mechanical shear, polymer unsaturation, and oxygen content has been investigated. The presence of excess oxygen promoted the scission reaction, the extent of molecular weight reduction increased with decreasing temperature as would be expected for a shear induced reaction. However, it was found that under nitrogen the effect of mechanical shear at low melt temperature (<290°C) was to increase the melt viscosity and extent of LCB when compared to the corresponding reactions performed at the same melt temperatures but in the absence of shear; an explanation of this effect which involves the breakdown by shear of a radical cage is proposed. The observation that polymers having a high vinyl content undergo extensive increases in melt viscosity and LCB at all melt temperatures and in the presence or absence of shear has also been explained on the basis of the proposed reaction scheme.