Meaning
Linear polymer chains that have undergone hydrolytic degradation during processing can be coupled together to restore their original molecular weight and melt viscosity. This restorative reaction, known as chain extension, typically employs bi-functional or multi-functional additives that react with the chain end groups of polyesters or polyamides. Extrusion compounding represents the primary stage where these additives are introduced to counteract the effects of regrind thermal degradation.
Reaction Chemistry
Difunctional additives such as dianhydrides or diepoxides link two polymer chains together by reacting with active carboxyl or hydroxyl terminal groups. When a multi-functional chain extender is used, it creates branched structures that measurably increase melt strength. These covalent bonds hold up under subsequent thermal cycles, distinguishing the modified resin from untreated material.
The reaction occurs rapidly in the extruder melt zone, requiring precise feeding of the additive to prevent localized gelation and ensure a uniform molecular weight distribution throughout the polymer batch.
Process Control
Compounding conditions must be carefully balanced because the reaction kinetics of chain extension require adequate residence time and a specific temperature window. If the melt temperature is too low, the reaction does not complete, leading to unreacted additive residues. Conversely, excessive temperatures trigger secondary degradation reactions that undo the benefits of the molecular weight build.
Moulding Outcome
Moulders run a lower risk of part failure from brittle behavior when processed regrind is treated with a chain extension agent. This chemical modification prevents issues like parison sag in blow moulding and sheet drawing instabilities in extrusion. It allows higher regrind usage ratios without sacrificing mechanical properties.
