Evaluating Degradation Mechanisms in Post-Consumer Polyolefin Flake
Polyolefin degradation evaluation requires coupling multi-load melt flow testing and OIT analysis to quantify chain scission and restabilize flake lots.
Hydroperoxide Breakdown
Meaning
The chemical decomposition of unstable oxidized polymer intermediates yields highly reactive alkoxy and hydroxy radicals that propagate polymer degradation. This reaction step, known as hydroperoxide breakdown, represents the transition from slow induction to rapid auto-catalytic oxidation. It occurs under the influence of heat, ultraviolet light, or trace transition metal contaminants.
Chemical Mechanism
Hydroperoxides are fragile species that decompose readily at processing temperatures above one hundred and fifty degrees Celsius. The cleavage of the oxygen-oxygen single bond produces alkoxy and hydroxy radicals, which then abstract hydrogen from neighboring polymer chains. This creates further alkyl radicals, setting off a cascading chain reaction that accelerates the degradation of the resin.
The presence of transition metal residues from catalysts can accelerate this reaction by lowering the activation energy required for the bond cleavage.
Degradation Control
Primary antioxidants cannot prevent this decomposition but secondary stabilizers, such as phosphites and thioethers, react with hydroperoxides to convert them into stable, non-reactive alcohols. This preventive reaction breaks the degradation loop before it can generate new active radicals. Moulders rely on these secondary stabilizers to preserve molecular weight during melt processing.
Process Economics
When processing regrind, the depletion of hydroperoxide decomposers leads to rapid degradation and loss of physical properties. Adding these stabilizers to the polymer melt reduces the rate of scrap generation and enables higher recycle content in the raw feedstock. This addition lowers the overall material costs for the injection moulder.