Meaning
Presence of foreign polymers, unreacted additives, particulate matter or degraded gel inclusions within high density polyethylene resin impairs physical properties and melt processing. Occurring in virgin production streams or post-consumer recycling feeds, high density polyethylene contamination alters melt flow index and stress crack resistance. Defect propagation causes surface blemishes, pinholes in extruded films or structural failure in blow-moulded containers.
Measurement applies to bulk polymer inventory and reprocessed flake prior to final conversion.
Cross-Polymer Inclusion
Polypropylene or PET particles within high density polyethylene flake create phase boundaries that act as mechanical weak points. Even small amounts of high density polyethylene contamination involving immiscible polymers reduce environmental stress crack resistance in blow-moulded bottles. Entrained polypropylene raises the melting point, causing unmelted particles during low-temperature extrusion.
These unmelted grains plug melt filters and break film bubbles during blown film production.
Particulate Degradation
Carbonized polymer specks and metallic fragments originate from overheated extruder barrels or worn processing equipment. Detecting high density polyethylene contamination from degraded gels requires optical particulate counters and screen pack pressure monitors. Degraded gels create optical defects in thin packaging films and induce pinhole leaks.
Higher screen pack differential pressures force frequent production stops to replace clogged filter mesh.
Viscosity Variation
Chemical crosslinking or presence of low molecular weight waxes destabilizes melt rheology during moulding. Severe high density polyethylene contamination causes erratic cavity filling, severe flash, short shots and wall thickness variation.
