Correlation between processability and properties of a high density polyethylene by a rheological approach

Abstract

The thermal‐oxidative and thermal‐mechanical stability of high density polyethylene (chromium based catalyst technology) was examined at many different temperatures using a rheological approach. The changes in molecular structure, which take place during processing, have been studied using a Clextral co‐rotating twin‐screw extruder in comparison with dynamic measurements performed with a rotational rheometer under definite conditions of temperature, strain and frequency and in presence of air. In order to evaluate the degradation response, an investigation of elastic modulus G′ as a function of frequency ω on the residual sample after ageing has been carried out. The molecular weight increase, probably due to the formation of small amounts of long chain branching, is clearly observed through the growth of the elastic properties, mainly at low frequencies (i.e. high relaxation times). The stabilised polymer shows a less pronounced tendency towards degradation, even if a critical temperature (240 °C) has been found at which antioxidant has not any effect in avoiding degradation.

Detergent impurity effect on recycled HDPE: Properties after repetitive processing

“…Rheological measurements in Figures and show an increase in the zero shear viscosity with increasing number of extrusion cycles. According to the literature, long chain branching is the most probable cause during the first processing cycles – . Also crosslinking has been suggested as one explanation .…”

Review on the thermo-oxidative degradation of polymers during processing and in service

“…In PE both chain branching and scission reactions play a role. In most instances branching dominates [76], although this is dependant on many factors such as temperature, amount of unsaturation and stabilizers [69,70,71]. Recently it was postulated that during processing the degradation begins with chain scission in the longer chains, due to their higher probability of entanglements, forming macro radicals.…”

Increase of long-chain branching by thermo-oxidative treatment of LDPE: Chromatographic, spectroscopic, and rheological evidence

“…Rheology is well known to detect with high sensitivity variations of the polymer structure [Gahleitner (2001);M€ unstedt (2011)], although as far as the influence of thermo-oxidative degradation is concerned, the studies have been focused mainly on the linear-viscoelastic regime both experimentally [Mariani et al (2002); Shangguan et al (2010)] and theoretically [Drozdov (2007)]. In the context of the present contribution, low-density polyethylene (LDPE) was treated thermally and thermo-oxidatively, and it was reported recently that strain hardening in uniaxial elongation is the stronger, the longer the oxidative time is, which was explained as the effect of an enhanced LCB content induced by thermo-oxidative degradation [Rol on-Garrido et al (2011a)].…”