Melt Shear History and Contaminant Catalyzed Scission in Post Consumer Polyolefin Silo Blends
Deactivate metal catalysts and neutralize acid traces with targeted additives to stabilize viscosity drift and optimize landed cost per good recycled part.
Tertiary Carbon Scission
Meaning
A polymer degradation reaction occurs preferentially at tertiary carbon atoms along the polymer chain because of the lower bond dissociation energy of these sites. This chemical pathway, known as tertiary carbon scission, represents the dominant degradation mechanism in branched polyolefins such as polypropylene during melt processing and thermal aging. It leads to a rapid reduction in molecular weight and a corresponding increase in the melt flow index of the resin.
Molecular Pathway
The presence of methyl side groups in polypropylene creates tertiary carbon atoms that are highly susceptible to radical attack. Radical generation from thermal or mechanical shear results in the formation of tertiary alkyl radicals on the polymer backbone. These radicals then undergo beta-scission, which cleaves the carbon-carbon backbone bonds and generates a shorter polymer chain and a new radical.
This reaction occurs preferentially over other scission pathways because the resulting radicals are highly stabilized by the neighboring alkyl groups.
Melt Rheology
The reduction in chain length and molecular weight caused by this cleavage alters the melt flow behavior of the resin. Moulders observe an increase in the melt flow rate during subsequent processing runs of the same material. This change in flow behavior indicates that the polymer has degraded, which will reduce the impact strength and toughness of the final moulded part.
Sourcing Mitigation
Recycled polypropylene has often undergone multiple heat cycles, causing extensive chain breakage via this mechanism. Moulders compounding this regrind must utilize primary and secondary antioxidants to arrest the radical cycle before beta-scission can occur. This stabilization is necessary to maintain the physical properties of the recycled resin across multiple moulding cycles.
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.