Evaluating Thermal and Oxidative Polyolefin Flake Degradation
Thermal and oxidative degradation alters polyolefin flake viscosity and residual stability, demanding MFR, OIT, and Carbonyl Index verification before processing.
Peroxyl Radicals
Meaning
Reactive oxygenated intermediates form when carbon-centred polymer radicals combine with atmospheric oxygen during thermal processing or weathering. These peroxyl radicals act as primary propagation agents in the autoxidation cycle of polyolefins, abstracting hydrogen from adjacent chains to create hydroperoxides and new carbon radicals. This self-accelerating cycle degrades the polymer melt during extrusion unless interrupted by primary antioxidants.
Stabilizers like hindered phenols scavenge these reactive species to protect the molecular weight of the resin.
Degradation Mechanism
Chain reactions driven by these radical species lead to rapid reduction in polymer performance. Hydroperoxide decomposition creates further alkoxy and hydroxyl radicals, multiplying the degradation rate. This multiplication causes yellowing and loss of mechanical properties in the final product.
Process Stability
High temperatures during compounding accelerate the rate of radical generation and subsequent chain scission. Extrusion of polypropylene without sufficient stabilizer packages results in a major drop in melt viscosity. This viscosity change causes erratic flow and flash during subsequent injection moulding.
Stabilizer Action
Primary antioxidants neutralize the radical intermediates by donating a hydrogen atom to form a stable hydroperoxide. Secondary stabilizers then decompose the hydroperoxide into non-reactive alcohols before it can generate further radicals. This cooperative stabilization system ensures the resin survives multiple heat histories.