Meaning
The standardized testing protocol designated as ISO 1628-5 establishes procedures for measuring the viscosity number of thermoplastic polyester homopolymers and copolymers in dilute solution. Laboratory technicians apply this standard to polymer sourcing and moulding operations by dissolving resin pellets in a specified solvent mixture and measuring efflux times through a capillary viscometer at a controlled temperature. Viscosity values derived from this method quantify average molecular mass, providing compounders with a direct numerical metric for batch uniformity before materials reach the processing floor.
Raw material suppliers use the standard to certify resin lots against commercial datasheets, while incoming quality control teams reject out-of-specification polymer shipments that would otherwise cause flow instabilities during production. The applicability boundary stops at linear condensation polymers specified within the scope, excluding branched architectures that exhibit non-Newtonian flow behavior under identical dilution conditions.
Solution Viscosity
Solution viscosity dictates how a polymer melt behaves inside the barrel of an injection moulding machine or the extrusion die. Operators set screw rotation speeds and temperature profiles based on the molecular weight distribution calculated through this standard, ensuring steady pressure transmission during the filling phase. Low molecular mass batches flow excessively, producing flashing around parting lines and degrading mechanical performance in the finished article.
High molecular mass polymer creates excessive back pressure, leading to localized thermal degradation and shear heating defects within the plasticizing unit. Processing engineers correlate laboratory viscosity numbers directly with melt flow rates to predict how pellets will perform under high shear rates typical of thin-walled packaging production.
Thermal Degradation
Thermal degradation occurs when polymer chains break down during compounding or moulding, altering the intrinsic viscosity measured by the standard. Extrusion temperatures set too high cause polymer scission, resulting in a measurable drop in viscosity and a corresponding loss of tensile strength in moulded components. Moisture trapped inside hygroscopic pellets accelerates polymer hydrolysis during processing, destroying molecular weight before the resin reaches the cavity.
Mould setters mitigate these failures by drying resin feedstocks to moisture thresholds below regulatory limits prior to processing, protecting the integrity of the polymer structure.
Regrind Economics
Regrind economics depend on maintaining strict viscosity control when scrap parts are shredded and blended back into virgin resin streams. Multiple thermal cycles induce chain branching or chain scission, shifting the viscosity number away from the baseline established by the raw material manufacturer. Moulders limit regrind addition percentages to prevent cumulative viscosity drift, which causes unpredictable shrinkage rates and warpage in dimensionally critical parts.
Virgin material commands a premium price because its molecular weight distribution remains tightly bounded, whereas recycled polymer requires extensive laboratory testing to verify that dilution properties still comply with processing windows.
