Revising the Mechanism of Polyolefin, Degradation and Stabilisation: Insights from Chemiluminescence, Volatiles and Extractables

Alam, Sayed (2019) Revising the Mechanism of Polyolefin, Degradation and Stabilisation: Insights from Chemiluminescence, Volatiles and Extractables. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

In this study the thermal-oxidative degradation of the polyolefins: polypropylene (PP), high-density polyethylene (PE-HD), and linear low-density polyethylene (PE-LLD) have been evaluated after melt processing and multiple extrusion passes. Recent literature on the initiation of autoxidation in polyolefins suggests that oxygen-containing radicals that are already present in polyolefin reactor powders (ROO•, •OH) can abstract hydrogen, trigger -scission reactions and oxidative propagation. An overview of the literature also supports the premise that, at least in the first instance, abstraction of hydrogen occurs preferentially from allylic sites, since this process is thermodynamically more favoured. Chemiluminescence (CL) has been used to characterise the nature of species arising in the melt at 180oC in the range of 350-680 nm as expected, characteristic changes do occur in the CL spectra as a function of residence time in the melt and in air. Data points under the integral CL curve have been related to the formation of a transition-state cage cited by other workers1. Unlike the Russell mechanism, this follows the decomposition of primary, secondary and tertiary peroxides via a tetroxide that cleaves in an asymmetric manner. The CL results support this premise, suggesting that luminescence arises from cage recombination of peroxyl radicals and explains why some antioxidants are more effective than others in their participation in Hydrogen Atom Transfer (HAT). The CL data also questions the roles of secondary antioxidants as peroxide decomposers, suggesting instead that they scavenge alkoxyl radicals from cage decomposition. Because primary and secondary peroxyl from cage termination reactions give non-radical products (alcohols, ketones and aldehydes), whilst tertiary peroxyl cannot undergo this asymmetric cage reaction (instead escaping the cage as alkoxyl radicals) this has been used to explain the range of volatiles and extractables observed during polyolefin oxidation. The polyolefins investigated contain different amounts of chain branching and the data shows that although the types of volatiles are similar regardless of polyolefin type (hydrocarbons (branched, cyclic linear); aldehydes; carbon dioxide; ketone; carboxylic acids; esters; ethers and furans) the rate of thermal oxidation is faster for polyolefins with higher levels of branching. The results emphasise the fact that the Basic Auto-oxidation Scheme (BAS) which has been the accepted standard cycle for thermo-oxidative degradation, is not as simple as depicted. Therefore, revisions to the BAS of polymer thermal oxidation have been proposed based on this information.