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.
Autocatalysis
Meaning
Chemical reaction pathways where a reaction product itself accelerates the rate of that same reaction characterize many degradation mechanisms. In thermoplastic processing, autocatalysis occurs when the thermal breakdown of a polymer like polyvinyl chloride produces acidic byproducts that subsequently speed up the rate of further dehydrochlorination. This self-accelerating behaviour leads to rapid, localized deterioration of the polymer chains within the melt.
Chemical Mechanism
Acidic compounds generated during the initial cleavage of polymer backbones often act as proton donors that facilitate subsequent chain scission. During autocatalysis, the concentration of these active acidic species rises exponentially rather than linearly, which creates a highly corrosive environment for both the melt and the metal tool surface. Standard stabilizers must capture these initial degradation products to prevent the reaction from entering this runaway phase.
Processing Defect
Severe discoloration and a sudden drop in melt viscosity occur when a polymer undergoes this self-reinforcing degradation during moulding. The rapid reduction in molecular weight diminishes the mechanical strength of the final moulded part, which leads to embrittlement and premature component failure under load. In extreme cases, the liberated gases form gas pockets and voids that ruin the surface finish and structural integrity of the component.
Mitigation Strategy
Minimizing the residence time of the material in the barrel and lower barrel temperatures prevent the initiation of these run-away reactions. High shear zones must be avoided to limit localized frictional heating that could trigger the onset of autocatalysis.