Recycled Polyolefin Oligomer Identification for Food Contact Clearance
Clearing recycled polyolefins for food contact demands HPLC-GC-FID and GCxGC-MS identification of oligomeric fractions evaluated against TTC thresholds.
Polyolefin Chain Scission
Meaning
Thermal and mechanical degradation represents the breaking of polymer backbone bonds during high-temperature extrusion or injection moulding. This polyolefin chain scission reduces the average molecular weight of the resin, leading to a higher melt flow index and decreased mechanical performance in the finished part. It is a critical concern when processing polypropylene, where degradation typically occurs under high shear and temperature.
Molecular Degradation
Free radical reactions drive the breakdown of long polymer chains into shorter segments during processing. Mechanical shear in the extruder screw combines with high temperatures to sever the covalent carbon-carbon bonds. This reaction is self-propagating in the presence of trace oxygen, leading to rapid degradation of the resin.
The resulting shorter chains cannot form the entanglements needed for high mechanical strength.
Melt Flow Impact
Rheological changes occur quickly as the molecular weight distribution shifts toward smaller molecules. Moulders observe a substantial increase in the melt flow rate of the resin, which can cause flashing at the mould parting line and inconsistent part weight. This behavior is particularly challenging when using recycled polyolefins, which have already undergone multiple heating cycles.
To maintain consistent part dimensions, process engineers must monitor viscosity and adjust the injection pressure. When viscosity drops too low, the physical properties of the moulded part fall below the specified structural limits.
Additive Stabilization
Antioxidant packages are added to the resin to intercept free radicals and prevent degradation. These stabilizers, such as hindered phenols and phosphites, sacrifice themselves to protect the polymer backbone during moulding. Adequate stabilizer levels are essential when incorporating regrind into the production process.