Absorption of dicumyl peroxide by extruded polyethylene: Difference between surface and bulk morphology

Abstract

AbstractUnder some conditions, the absorption of dicumyl peroxide (DICUP) at 70°C by extruded low density polyethylene (LDPE) displayed features which are characteristic of two‐stage sorption. For example, the initial slope of the DICUP absorption curve (plot of Mt/M∝ against \documentclass {article}\pagestyle{empty}\begin{document}$\sqrt t$\end{document} ) reflecting diffusion in the surface region was 2.5 times higher than the slope of the second stage (reflecting diffusion in the bulk) for 4‐mm‐diameter LDPE rods extruded at 130°C and 180 rpm (die pressure = 6000 psig). Only a single stage curve was evident in the same rod with its surface removed or in smaller diameter extrudate. Increasing the screw speed from 10 to 45 rpm at nearly constant extrusion pressure (1800–2380 psig) resulted in an approximately 50% decrease in the initial (first stage) slope with a negligible effect on the second stage slope. Increasing the die pressure (4900–6000 psig), by decreasing the extrusion temperature, at constant screw speed (125 rpm) resulted in an almost threefold decrease in the second stage slope without apparent effect on the first stage slope. Photomicrographs demonstrated the presence of distinct surface and bulk morphologies with evidence of a transcrystalline surface layer oriented prependicular to the surface at low screw speeds and larger spherulites in the bulk of the high pressure extrudate. Such morphological features and the observed dependence of the two stage sorption curves on extrusion conditions are consistent with the surface and bulk morphology of the extruded LDPE rod being dependent on the screw speed and die pressure, respectively. These uptake curves were determined by modifying the classical sorption technique to separate the measurement of the equilibrium uptake from the continuous recording of the slow changes in mass during absorption. This technique may be useful in the characterization of the migration process in other plastic/penetrant systems where diffusion is too slow to be measured by conventional means.