Meaning
Material degradation processes that occur when polymer resins are exposed to high temperatures for extended periods in the presence of oxygen affect resin properties. This specific degradation is caused by thermal oxidation over drying, which governs the loss of molecular weight in thermoplastic materials. The phenomenon does not apply to vacuum drying or nitrogen-purged drying systems.
Degradation Mechanism
Extended exposure to heated air breaks the polymer chains and changes the chemical structure of the resin. When a material undergoes thermal oxidation over drying, it often exhibits a distinct yellowing or darkening in color. This cosmetic change is accompanied by a reduction in mechanical strength and impact resistance.
The chemical bonds of the polymer are permanently broken, making the material brittle.
Process Control
Strict monitoring of the drying time and temperature prevents the resin from staying in the dryer too long. To avoid thermal oxidation over drying, drying hoppers should be sized to match the consumption rate of the extruder or injection molding machine. If the production line stops, the heating system should be turned down or switched to a standby mode.
This precaution prevents the resin from being baked during unexpected machine downtime.
Product Consequence
Molded parts made from degraded resin are highly likely to fail when subjected to mechanical stress. The occurrence of thermal oxidation over drying can lead to the rejection of entire production batches due to poor physical performance. This failure is expensive because it is often discovered only after the parts have been molded and tested.
Testing the resin before molding ensures that only healthy polymer enters the machine.
