Meaning
A degradation pathway occurs when heat during compounding or injection moulding breaks the covalent bonds of a polymer backbone. In the processing of thermoplastic resins, thermal chain scission reduces the molecular weight of the polymer and increases its melt flow index.
Mechanical Breakdown
The process begins when temperatures exceed the thermal stability limit of the polymer, causing random breaks in the chain. During thermal chain scission, the shorter chains cannot form the same level of entanglements, which leads to a significant reduction in the mechanical strength of the moulded parts. This degradation is often accompanied by the release of volatile compounds.
Process Defect
Processors see a reduction in the melt strength of the polymer when executing extrusion or blow moulding. If thermal chain scission occurs in the extruder, the reduced viscosity can cause the parison or sheet to sag and distort, which increases the rate of scrap and limits the reuse of regrind material. This breakdown also causes a loss of toughness in the finished product.
Additive Protection
Stabilizers can be added to the resin to intercept free radicals and reduce the rate of degradation. Using these additives helps prevent thermal chain scission during recycling and extrusion steps.
