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.
Secondary Antioxidant
Meaning
Chemical additive formulations protect organic polymers from oxidative degradation during melt processing. A secondary antioxidant functions by reacting with hydroperoxides to convert them into stable, non-reactive alcohols. This preventive action is critical for maintaining the molecular weight and color of the resin during compounding and moulding.
Thermal Stabilization
High temperatures and mechanical shear during extrusion generate polymer radicals that quickly react with oxygen. The secondary antioxidant acts as a hydroperoxide decomposer, interrupting the autoxidation cycle before it can cause chain scission. This protection is necessary to prevent the polymer from becoming brittle.
Melt Processing
Melt flow instability and yellowing are typical signs of thermal degradation in polymers such as polypropylene. Incorporating a secondary antioxidant together with a primary stabilizer helps to keep the viscosity of the melt within the specified limits. This melt stability ensures that the moulding process remains consistent across different production lots.
Synergistic Performance
Secondary stabilizers are often combined with primary hindered phenols to achieve maximum protection. This combination is highly effective because the primary antioxidant scavenges the active radicals while the secondary agent decomposes the hydroperoxides. The resulting synergism allows the compound to withstand the high shear of twin-screw compounding.
By optimizing the blend of these two additive types, manufacturers can ensure that the polymer retains its structural integrity and cosmetic appearance even after multiple extrusion passes.