Deriving Cramer Class Thresholds for Unresolved Polyolefin Hydrocarbon Humps
Deriving polyolefin hydrocarbon hump compliance requires LC-GC-FID fraction isolation and Cramer Class derivation to set verifiable limits against migration test data.
Polyolefin Melt Degradation
Meaning
The irreversible change in polymer structure caused by heat and shear during processing alters the physical properties of the final part. During polyolefin melt degradation, the long polymer chains are broken into shorter fragments. This process occurs most frequently inside the barrel of an injection moulding machine or an extruder.
Monitoring the melt flow rate helps detect when the material has been compromised.
Thermal Stress
Excessive temperatures cause the chemical bonds in the polymer to vibrate and eventually break. Because polyolefin melt degradation is accelerated by high heat, cooling the feed zone is necessary.
Chain Scission
The breaking of carbon-to-carbon bonds reduces the average molecular weight of the resin. In polyolefin melt degradation, this scission leads to a decrease in viscosity and a higher melt flow index. Brittle parts and reduced impact resistance are the common outcomes of this structural change.
The presence of oxygen further speeds up the reaction by forming unstable radicals. Stabilizers like hindered phenols are added to the virgin resin to prevent this breakdown.
Product Performance
Parts made from degraded material may fail prematurely under mechanical load. Frequent testing of regrind ensures that polyolefin melt degradation has not reached a level that threatens the integrity of the product. Controlling the residence time in the machine is the primary way to minimize this defect.