Meaning
Polymeric hydrolysis inhibitors known as carbodiimides scavenge terminal carboxyl and hydroxyl groups generated by moisture and thermal degradation inside condensation plastics. Polyesters and polyurethanes undergo chain cleavage when exposed to processing temperatures above two hundred degrees Celsius in the presence of trace water, dropping molecular weight and destroying mechanical performance. These chemical additives react directly with the reactive hydrogen of free acid groups, forming stable urea or acylurea linkages that halt unzipping reactions.
The boundary of their efficacy lies in resin saturation levels, where exceeding standard loading thresholds causes screw slippage during extrusion and excessive mold plate-out during high-speed injection.
Additive Reactivity
Condensation polymers degrade through acidolysis during melt processing, driving a continuous reduction in intrinsic viscosity. Water splits the ester bonds, creating carboxylic acid chain ends that accelerate further cleavage in an autocatalytic loop. Carbodiimides interrupt this cycle by donating functional groups that bond permanently with those acid ends.
Granular masterbatches introduce these agents into the feed throat, ensuring uniform dispersion before the polymer enters the plastication zone. Thermal stability improves immediately, allowing recyclers to process post-consumer regrind without catastrophic molecular breakdown.
Resin Specification
Material datasheets define virgin polymer grades by melt flow index and moisture content limits, whereas processors must account for the cumulative thermal history of regrind streams. Virgin engineering thermoplastics arrive with strict water ppm boundaries, but blended regrind introduces unpredictable hydrolytic damage that standard drying struggles to reverse. Extrusion lines tolerate low additive concentrations without shifting output rates, but excessive dosages alter melt rheology and screw friction.
Moulders running thick-walled automotive components rely on these compounds to maintain impact strength through multiple thermal cycles, bypassing the rapid embrittlement typical of untreated recycled polyester.
Processing Defect
Moisture trapped in polyester pellets produces silver streaks, surface voids and severe molecular weight drops during the injection cycle. Hydrolytic scission reduces the tensile elongation at break by half within minutes of high-shear plastication if inhibitors are absent from the formulation. Processors measure success by monitoring melt pressure stability and final part notched impact resistance across sustained production runs.
Unmitigated degradation causes plate-out on steel mold surfaces, leaving particulate defects that force expensive tool cleaning shutdowns.
