Using dynamic shear rheology to estimate branching induced in high- density polyethylene compositions by peroxide modification
Brenton Fletcher, Andrew Chryss, Sati N. Bhattacharya
CRC for Polymers / RMIT University
Australia

Keywords: reactive extrusion, HDPE, rheology


Dynamic shear rheology is used to assess the degree of branching present in polyethylene compositions subjected to peroxide modification. Combinations of high density polyethylene (HDPE, density 0.960), two linear low density polyethylenes (LLDPE, densities of 0.918 and 0.935) and a medium density polyethylene (MDPE, density 0.942) were reactively extruded with up to 0.20wt% addition of an organic peroxide to promote chain linking mechanisms within the composition. The aim of this was to improve the melt strength of the resin for processing in pipe applications. Increasing the density and length of branches in the composition through peroxide modification is theoretically a way of achieving this.

In the course of developing such a material it was deemed that a fast method of assessing the occurrence of any induced branching would be desirable. A dynamic shear temperature ramp test was used where the loss tangent, tan d, is plotted as a function of temperature. Comparing the slope of tan d versus temperature for polymer resins of different densities, that is, for polymers with different degrees of branching, a method of estimating the degree of branching for the peroxide modified compositions is proposed. Gel permeation chromatography (GPC) is used to quantify the degree of branching and differential scanning calorimetry (DSC) is used to support any discontinuities that occur due to melting points or other important morphological changes in the compositions.