PET Drying Windows and the Hydrolysis a Moulder Absorbs
Excess residual moisture causes instant ester chain hydrolysis in molten PET, dropping intrinsic viscosity and destroying mechanical part strength.
Hydrolysis
Meaning
Chemical reaction involving the breaking of molecular bonds by the addition of water causes the degradation of many engineering plastics. During the melt phase, hydrolysis rapidly reduces the molecular weight of polyesters and polyamides. This process is the most common cause of brittle failure in moulded parts.
Degradation Pathway
Water molecules react with the functional groups in the polymer backbone to split the chains into shorter segments. This specific pathway for hydrolysis is thermally activated, meaning the damage happens almost instantly once the wet resin enters the heated barrel of the injection machine. The resulting melt becomes very thin and loses its structural strength.
Drying Prevention
Prevention of this chemical attack requires the removal of almost all moisture from the resin before processing. Because hydrolysis can occur at water levels as low as 0.02 percent, a standard desiccant dryer is necessary for many materials. Monitoring the dew point of the drying air ensures the environment is dry enough to stop the reaction.
Processing Limit
Excessively high temperatures accelerate the rate of bond breaking even if the moisture level is low. A moulder must balance the need for flow with the risk of hydrolysis by keeping the melt temperature within the range recommended by the material supplier. Stable production depends on a perfect dry and a controlled heat.