Evaluating Hydrocarbon Oligomer Migration in Flexible Polyethylene Packaging
Evaluating oligomer migration in polyethylene packaging requires measuring C12-C45 POSH fractions via HPLC-GC-FID against specific migration limits.
Thermal Degradation Scission
Meaning
Polymer chain cleavage driven by excessive thermal energy represents the exact molecular breakdown known as thermal degradation scission. This chemical alteration severs backbone covalent bonds within thermoplastic resins when processing temperatures exceed safe operational limits. Uncontrolled molecular fracturing lowers the average molecular weight, destroying the mechanical strength required for structural moulded components.
Polymeric materials subjected to prolonged high-barrel temperatures during injection moulding experience this backbone fracture, leading to melt viscosity drops and structural failure in the finished part.
Thermal Window
Extrusion barrels operate within strict temperature boundaries specified by resin manufacturers to prevent molecular breakdown. Barrel heaters transfer heat energy to pellets, but excessive residence time inside the cylinder compounds the risk of polymer cleavage. Residence time calculations balance screw speed against shot size to ensure the melt remains below the threshold where backbone bonds rupture.
Thermoplastic polyesters and polyamides exhibit high sensitivity to heat history, demanding precise heater band calibration to avoid degradation during processing cycles.
Regrind Economy
Secondary processing cycles subject virgin polymers to repeated thermal exposures, accumulating chain scission events with each melt phase. Scrap parts ground into regrind pellets carry altered rheological profiles compared to pristine resin grades. Datasheet values assume single-pass virgin material, whereas parts moulded from high percentages of regrind exhibit reduced tensile strength due to prior molecular weight reduction.
Moulders blend regrind with virgin pellets to dilute the cumulative thermal history, protecting the final part specification from brittleness and dimensional warp.
Defect Mechanics
Melt fracture originating from degraded polymer chains generates surface blemishes and structural weakness in injected components. Gas evolution from broken molecular fragments creates voids and silvery streaks along flow fronts during cavity filling. Dimensional stability deteriorates because altered melt flow rates change volumetric shrinkage inside the mould tooling.
Molecular weight distribution narrows during scission events, lowering impact resistance and rendering the moulded part unfit for load-bearing applications.