Determining Hydrolytic Degradation Thresholds in Virgin Resin Feedstocks
Hydrolytic degradation thresholds require drying virgin polyesters below 0.02 percent and polyamides below 0.10 percent water before melt processing.
Ester Linkage
Meaning
Chemical bonds formed by the condensation reaction between a carboxylic acid and an alcohol group constitute the repeating connections in polyester resins. The stability of the ester linkage is a governing factor in the durability and processing window of materials such as polyethylene terephthalate and polybutylene terephthalate. These bonds are susceptible to cleaving when exposed to water at elevated temperatures.
Hydrolytic Scission
Moisture trapped in the polymer resin during melting acts as a reactant that breaks this specific bond. This cleavage of the ester linkage reduces the length of the polymer chains, which is known as depolymerization, and causes a sharp decline in the mechanical properties of the moulded part. The reaction generates a carboxyl end group and a hydroxyl end group, which further catalyzes subsequent degradation.
Processing Consequence
Moulders observe a dramatic increase in melt flow index and a loss of melt strength when this bond cleavage occurs during the cycle. This degradation results in parts with low impact resistance and brittle failure, even if the cosmetic appearance of the part remains unchanged. Dehumidifying dryers must be used to reduce the resin moisture content below thirty parts per million to protect the polymer from this degradation.
Material Recovery
Recycled polyester resins must undergo solid-stating to rebuild these cleaved bonds and restore the lost molecular weight. This thermal treatment in a vacuum or inert gas atmosphere drives the reaction in reverse, which increases the average chain length and the intrinsic viscosity of the polymer.