Meaning
Hydrolytic degradation involving ester scission breaks polyester polymer chains by water attack at carbonyl carbon bonds during high temperature processing. Polyethylene terephthalate and polybutylene terephthalate degrade through this mechanism when moisture meets molten polyester inside the barrel of an injection moulding machine. The chemical cleavage shortens the average molecular weight and causes intrinsic viscosity drop, which destroys mechanical strength in moulded components.
Molecular Weight
Polymer chains break into shorter segments when residual moisture reacts with ester linkages under thermal stress. Tensile strength and impact resistance decline rapidly as chain length diminishes. Moulders prevent this damage by drying pellets below zero point zero two percent moisture before feeding them to the hopper.
Recycled regrind carries higher moisture affinity than virgin resin, demanding longer residence times in desiccant dryers to avoid excessive degradation.
Viscosity Degradation
Intrinsic viscosity drops during processing when water molecules cleave polymer backbones at active ester sites. Molten material loses melt strength, causing drool at the nozzle and dimensional instability across finished parts. Quality control laboratories measure melt flow rate to detect molecular weight reduction before production lots ship to customers.
Datasheet values assume dry resin processed under strict parameters, whereas shop floor reality often introduces moisture variations that alter final part performance.
Thermal Defect
Molecular chain scission generates carboxyl end groups that accelerate autocatalytic degradation during subsequent thermal cycles. Surface splay, silver streaks, and brittle zones appear on moulded parts when degraded polymer fills the cavity. Processing temperatures must remain below three hundred degrees Celsius to limit thermal breakdown rates.
Material specifications demand virgin resin blending with regrind to maintain acceptable mechanical tolerances in structural applications.
