Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low‐density polyethylene, high‐density polyethylene, and tetratetracontane heated in air

Abstract

Samples of low‐density polyethylene (LDPE), high‐density polyethylene (HDPE), and tetratetracontane (n‐C44H90) free from additives were heated in air at temperatures between 120 and 180°C. As a comparison, “as received” HDPE containing unspecified additives has also been included. The structural changes have been studied with gel chromatography, viscometry, infrared spectroscopy, differential scanning calorimetry, and gravimetric measurements. LDPE, HDPE, and n‐C44H90 follow the same course of thermooxidative degradation when they are free from additives and present in the molten state. Both molecular‐diminishing and enlargement reactions occur. At temperatures below 150°C molecular enlargement is not observed until after rather long exposure times, whereas at higher temperatures enlargement occurs immediately. The difference is because “peroxide curing” becomes increasingly important above 150°C, whereas ester formation is operating at all temperature levels. Degradation below Tm is restricted to the amorphous phase that results in a different degradation pattern. In accelerated testing work extrapolations of the Arrhenius type in the prediction of structural change are thus not justified, even within the actual narrow temperature range. Neither are changes in commonly used standards like carbonyl content justified as a measure of the changes; for example, in mechanical properties. The stabilizer in the unpurified HDPE not only influences the induction period but also the course of the thermooxidative degradation.

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

“…This reaction is mainly found during processing under oxygen deficient conditions. Another way to introduce unsaturation in the polymer chain is by disproportionation of alkyl radicals [73,74], but this reaction does not have an influence on the molecular weight distribution.…”

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

“…In fact, models have been developed describing thermal degradation as obtained by FTIR spectroscopy [Gugumus (1996b); Salvalaggio et al (2006)]. In this context, differential scanning calorimetry (DSC) has been applied to obtain the heat of fusion and the melting points [Holmstr€ om and S€ orvik (1978); Volke-Sep ulveda et al (1999)], and gel permeation chromatography (GPC) to identify the variations in structure and molecular weight distribution (MWD) [Holmstr€ om and S€ orvik (1978); Iring et al (1980); Kumar et al (2002)].…”

Two-step degradation of high-density polyethylene during multiple extrusion

“…Although existing mechanisms of polyolefin degradation differ slightly from each other, they are all based upon the radical chain mechanism of oxidation. 3,4,[27][28][29][30][31][32][33][34][35][36] This scheme cannot be applied here and does not explain the differences observed between the first and subsequent processing steps. The amount of oxygen present in the two stages is definitely different; the powder adsorbs much more oxygen than the granules.…”