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.
Carboxyl Acid End Groups
Meaning
Chemical end groups that represent the terminating carboxylic acid functionality on polyester chains function as reactive sites and indicators of polymer degradation. The concentration of carboxyl acid end groups in materials such as polyethylene terephthalate influences both the hydrolysis rate and the thermal stability of the resin during processing. This concentration is measured in equivalents per million grams of polymer.
Hydrolysis Acceleration
Moisture present in the melt reactivates these acidic sites to catalytically cleave the adjacent ester bonds. High levels of carboxyl acid end groups accelerate this hydrolytic degradation, which reduces the molecular weight and melt viscosity of the resin during the moulding run. This loss of viscosity can lead to flashing at the parting line of the tool.
Material Quality
Solid state polymerization is frequently used by resin manufacturers to reduce the concentration of these terminal acid groups while increasing the molecular weight. Moulders who use regrind material face a higher concentration of carboxyl acid end groups because each successive thermal cycle creates more chain ends. This difference in concentration determines the maximum allowable blend ratio of recycled material to virgin resin.
Processing Control
Strict drying protocols that keep the moisture content of the polymer below fifty parts per million are necessary to prevent the activation of these acidic end groups. Restricting the processing temperature and melt residence time further limits the hydrolytic degradation driven by these reactive sites.