Virgin Resin Specification Gaps and Undeclared Regrind Content
Undeclared regrind in virgin polymer collapses oxidation stability and broadens viscosity bands, increasing landed cost per good part despite initial pellet discounts.
Primary Antioxidants
Meaning
Sterically hindered phenolic compounds function as chemical stabilizers to inhibit thermal oxidation in polymer materials. Formulators introduce primary antioxidants into resin recipes to scavenge free radicals generated during high-temperature melt processing. The additive preserves molecular weight distribution, preventing premature chain scission and polymer crosslinking.
Application covers processing and long-term thermal stabilization, ending where thermal exposure exceeds the decomposition temperature of the additive compound itself.
Scavenging Mechanism
Free radical propagation begins when heat and mechanical shear cleave carbon-hydrogen bonds along the polymer backbone. Reactive alkyl and alkylperoxyl radicals react with the active hydroxyl group on the hindered phenol molecule. The additive donates a hydrogen atom to convert the reactive radical into a stable hydroperoxide or polymer species.
The resulting phenoxy radical remains unreactive due to steric hindrance provided by bulky t-butyl side groups.
Synergistic Blend
Combining primary stabilizers with secondary phosphite hydroperoxide decomposers yields superior protection compared to single-component additives. Phosphites convert harmful hydroperoxides into inert alcohols, preserving phenolic molecules for radical scavenging duties. Blended packages optimize stabilizer efficiency while reducing overall additive concentration.
Processing Limit
High extrusion temperatures accelerate the consumption rate of phenolic hydrogen donors. Overheating leads to additive depletion, leaving the base resin unprotected during subsequent injection moulding stages. Degraded phenolic molecules form quinoid structures that impart yellow discoloration to clear or white parts.