Verifying Food Contact Migration Test Reports against Upstream Resin Specifications
Verifying food contact reports against resin specifications requires matching batch numbers, validating simulants, and recalculating surface contact ratios.
Polyolefin Shear Degradation
Meaning
Polymer melt flow index values and molecular weight distribution curves benchmark the irreversible mechanical breakdown of high molecular weight chains during processing. Occurrence of polyolefin shear degradation results from excessive mechanical shear forces splitting polymer backbones inside extruders, injection nozzles, or hot runner channels. High shear stress ruptures carbon-carbon bonds in high molecular weight polypropylene or polyethylene fractions.
The boundary of this breakdown mechanism lies in mechanical chain scission caused by velocity gradients, distinct from purely thermal oxidation caused by static heat exposure.
Chain Scission
Shear stresses exceeding critical molecular thresholds snap long polymer chains into shorter fragments during high-speed injection. Susceptibility to polyolefin shear degradation increases when processing high molecular weight grades through restrictive gate geometries. Chain shortening broadens molecular weight distribution and lowers melt elasticity.
Fragmented chains reduce environmental stress crack resistance and tensile strength in finished components.
Process Rheology
Irreversible viscosity drops caused by mechanical chain breakage alter mold filling dynamics during continuous production. Molders observe polyolefin shear degradation as an unexpected increase in melt flow rate when processing regrind or reprocessed scrap. Higher flow rates cause mold flash at parting lines and unpredictable part weight variations.
Adjusting screw speed and opening gate dimensions reduces local shear rates below critical degradation thresholds.
Regrind Limit
Repetitive extrusion cycles accumulate mechanical chain damage, limiting regrind loading ratios in high-performance applications. Virgin resin maintains longer chain lengths and superior impact strength compared to reprocessed feedstocks. Exceeding recommended regrind content leads to brittle part failures.