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.
Transition Metal Redox Cycling
Meaning
Catalytic processes occur when trace transition metal ions repeatedly change their oxidation states to accelerate polymer degradation during processing or service. Through transition metal redox cycling, metal contaminants like copper and iron continuously generate free radicals by decomposing hydroperoxides. This ongoing cycle of reactions breaks down the polymer chains and reduces the mechanical properties of the moulded part.
Deactivating these metals is necessary for long-term polymer stability.
Radical Generation
Metal ions react with hydroperoxides to form radicals, switching between oxidation states. During transition metal redox cycling, the metal is not consumed but continues to drive the degradation.
Thermal Aging
Plastic components used in high-temperature environments such as car engines are vulnerable to rapid failure if metal ions are present. When transition metal redox cycling is active, the polymer loses its tensile strength and turns brittle much faster than expected. This degradation can lead to cracks and sudden failure of the part in service.
It poses a significant challenge for designers who need to ensure a long service life for these components.
Additive Synergy
To prevent this degradation, compounders use a combination of primary antioxidants and metal deactivators. While metal deactivators bind the metal ions to stop transition metal redox cycling, the primary antioxidants scavenge the remaining free radicals. This combined action provides a high level of protection against thermal oxidation.
It allows moulders to use recycled polymer that may contain metal contaminants without sacrificing the quality of the finished parts.