Meaning
Intramolecular reaction mechanisms in active polymer chains drive intramolecular chain transfer to yield cyclic species or shorter linear chains. In melt processing of aliphatic polyesters, back-biting kinetics govern the rate at which thermal cleavage reduces molecular weight during extrusion. The boundary of this kinetic behavior stops at solid-state polymerization temperatures where chain mobility drops below the threshold required for ring closure.
Melt Mechanism
Nucleophilic attack of a terminal hydroxyl group onto an internal ester carbonyl along the same chain cleaves the backbone. When processing temperature increases above processing limits, back-biting kinetics accelerate dramatically and convert linear polymer into volatile cyclic oligomers. This degradation mechanism lowers melt viscosity rapidly during injection moulding operations.
Regrind materials exhibit higher reaction rates due to accumulated hydroperoxides and unpassivated catalyst residues from prior heat cycles.
Viscosity Loss
Molecular weight reduction directly impairs mechanical toughness in moulded parts. A datasheet melt flow index measured on virgin resin fails to capture the rapid viscosity loss experienced when regrind undergoes extended barrel residence. Uncontrolled back-biting kinetics cause variable cavity filling and flash in thin-wall components.
Moulding engineers adjust barrel temperature profiles and residence times to suppress cyclic monomer formation.
Processing Limit
Drying virgin and recycled resin below fifty parts per million moisture mitigates hydrolysis but leaves thermal cleavage pathways active. High catalyst concentration acts as a primary promoter of intra-chain transfer reactions. Additives such as end-cap agents or phosphite stabilizers deactivate reactive chain ends to alter the effective back-biting kinetics during compounding.
Lowering melt temperature by ten degrees Celsius can halve the rate of cyclic ester formation in polyesters.
