Standard Karl Fischer Oven Desorption Limits for Polyolefin Flakes
Oven Karl Fischer testing at 180°C to 200°C verifies polyolefin flake moisture against the 300 to 500 ppm operational processing ceiling.
Thermal Decomposition
Meaning
Irreversible chemical breakdown of polymer macromolecular chains, functional additives, and processing aids occurs under excessive heat without requiring the presence of atmospheric oxygen. Moulding operations and extrusion compounding systems monitor thermal decomposition because exceeding a resin’s technical processing window shears covalent bonds, lowering molecular weight and destroying structural properties. The degradation cascade releases corrosive off-gases, volatile oligomers, and carbonaceous char, ending only when temperatures drop below the activation threshold for polymer bond scission.
Chain Cleavage
Heat inputs above the ceiling temperature of a polymer initiate random chain scission, end-chain unzipping, or side-group elimination across the backbones. Exposing polyoxymethylene to elevated melt temperatures triggers rapid unzipping that unleashes formaldehyde gas, posing health hazards and foaming the melt. In polyvinyl chloride processing, thermal decomposition drives dehydrochlorination, releasing gaseous hydrochloric acid that etches expensive tool steel and pits mirror-polished cavity surfaces.
Polyolefins undergo cross-linking or chain fragmentation depending on the presence of tertiary carbon bonds.
Moulding Defects
Degradation products trapped within the injection barrel cause splay marks, silver streaks, and severe burn marks across moulded part surfaces. As thermal decomposition proceeds, the melt flow rate climbs dramatically, leading to mould flashing, short shots, and inconsistent part weights across production runs. Visual yellowing and mechanical embrittlement render technical components unusable in structural automotive or aerospace environments.
Operators face prolonged downtime to strip screw assemblies and mechanically polish scorched barrels coated with degraded carbon crusts.
Monitoring Strategy
Thermogravimetric analysis establishes the precise mass loss onset temperature for virgin resins, filled compounds, and regrind blends. Stabilisation packages containing primary hindered phenols and secondary phosphites defer thermal decomposition by intercepting carbon radicals and decomposing hydroperoxides. Converters processing regrind must incorporate fresh processing stabilisers to survive subsequent thermal passes without property degradation.
Managing residence times inside the barrel prevents stationary polymer pools from reaching critical decomposition thresholds during unplanned production halts.