Meaning
Reduction in the molecular weight of a polyester resin due to thermal and hydrolytic degradation during processing leads to a loss of mechanical performance. Polyethylene terephthalate is highly sensitive to moisture at high temperatures. This occurrence of intrinsic viscosity decay reduces the polymer chain length, making the finished part brittle.
Moulders monitor this reduction to prevent premature part failure.
Polymer Degradation
Chemical breaking of the ester bonds in the presence of water molecules at elevated temperatures causes a rapid decline in polymer chain length. When intrinsic viscosity decay is allowed to occur, the material loses its toughness and impact resistance. This hydrolysis reaction happens within seconds if the raw resin has not been dried to a moisture level below 0.005 percent before processing.
The resulting shorter chains cannot slide past each other without failing under mechanical load, which translates to a weak finished part.
Defect Appearance
Injected and blown parts made from degraded resin show distinct visual flaws and poor physical properties. When intrinsic viscosity decay is severe, it results in thin wall sections and reduced part weight due to changes in melt flow behavior. The degraded polymer has a lower melt viscosity, which can lead to flashing at the tool parting lines and increased cycle times.
These defects raise the cost of quality control because the moulder must reject entire batches that do not meet the minimum strength requirements.
Prevention Method
Avoiding the loss of molecular weight requires the strict maintenance of resin drying systems and the monitoring of dew point temperatures. Continuous desiccant dryers must reduce the moisture content of the polymer before it enters the extruder hopper. In addition, limiting the use of regrind material prevents the compounding effect of multiple heat histories.
By maintaining a stable polymer chain length, processors can ensure that the actual part matches the mechanical properties listed on the material datasheet. This strict control of moisture prevents hydrolytic scission and protects the mechanical limits of the molded part during its service life.
