Meaning
Polymer chain length reduction quantifies the loss of molecular weight in condensation polymers resulting from thermal or hydrolytic scission. Intrinsic viscosity degradation measures the decline in hydrodynamic volume of dissolved polymer chains, providing a direct measure of structural degradation during drying or extrusion processes. Moulding operations track viscosity loss in polyethylene terephthalate because reduced chain length impairs impact resistance and environmental stress crack performance in finished containers.
The measurement stops applying when resin structure undergoes cross-linking rather than chain scission.
Hydrolytic Scission
Water content above fifty parts per million in polyester melt induces rapid cleavage of ester linkages inside injection moulding barrels. Intrinsic viscosity degradation accelerates exponentially with melt temperature, reducing melt strength and causing parison sag in blow moulding lines. Thorough desiccant drying before processing prevents water-driven molecular weight reduction.
Melt Rheology
Lower intrinsic viscosity reduces polymer melt resistance to shear flow.
Recyclate Quality
Multiple heat cycles during mechanical recycling cause cumulative polymer chain scission in post-consumer resin streams. Regrind integration lowers overall resin viscosity, requiring process adjustment to maintain mould filling pressures and cycle times. Solid state polymerization restores intrinsic viscosity by reheating resin under vacuum, re-building polymer chain length before bottle preform molding.
Quality specifications establish acceptable viscosity loss limits between virgin resin input and final moulded part output.
