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.
Gels in Recycled Polymer
Meaning
Undissolved high-molecular-weight regions or cross-linked particles fail to homogenize with the surrounding melt during plastic processing. These localized defects, referred to as gels in recycled polymer, act as stress concentrators and disrupt the cosmetic appearance of moulded and extruded products. They originate from polymer degradation during previous thermal cycles or from foreign material contamination.
Defect Formation
High shear and localized overheating in the extruder generate radical species that cause polymer chains to cross-link. These cross-linked structures cannot melt or flow like the surrounding linear chains. When recycled feedstock is processed, these unmelted lumps remain intact within the melt stream.
They carry through to the final product as visible specks or raised bumps.
Processing Consequence
In thin-film extrusion, these unmelted particles cause localized thinning or pinholes that lead to bubble rupture. For injection moulders, they block hot runner nozzles and create surface blemishes on cosmetic parts. This blockage increases the reject rate of the produced batches, raising production costs.
Filtration Method
Melt filtration screens capture these solid particles before they reach the die or mould cavity. Finer mesh screens remove smaller particles but increase head pressure and melt temperature, which can accelerate the degradation of the resin. Compounding operations must balance the screen size against the throughput requirements of the production run.