Correlating Zero-Shear Viscosity Drop with Antioxidant Depletion in Recycled Polypropylene
Zero-shear viscosity drops exponentially before MFR shifts, providing an early indicator of antioxidant depletion in recycled polypropylene.
Phosphite Stabilizer
Meaning
Thermal additives protect polymer chains from thermal-oxidative degradation during high-temperature processing. A phosphite stabilizer acts as a secondary antioxidant by decomposing hydroperoxides into non-radical, stable alcohols. This action prevents the chain scission and crosslinking that occur when the polymer melt is exposed to heat and oxygen.
Hydroperoxide Reduction
Hydroperoxides are highly reactive species that form when polymer radicals react with oxygen. The phosphite stabilizer reduces these hydroperoxides to stable alcohols, preventing them from initiating further radical propagation. In doing so, the stabilizer itself is oxidized to a phosphate compound.
Melt Protection
Multi-pass extrusion and recycling processes expose the polymer to repeated thermal cycles. Incorporating a phosphite stabilizer maintains the melt flow index of the polymer by preventing chain degradation. This protection is necessary for maintaining the processing behavior and mechanical properties of the recycled resin.
Synergistic Action
Primary antioxidants such as hindered phenols are frequently blended with secondary stabilizers to provide broad thermal protection. This combination is more effective than either additive used alone, as the primary antioxidant traps active radicals while the phosphite compound decomposes the hydroperoxides. The resulting synergistic effect extends the processing life of the polymer.
By choosing the correct ratio of these two additives, compounders can prevent yellowing and maintain the physical strength of the resin through several heat cycles.