Meaning
Secondary stabilizer depletion occurs when processing heat consumes the chemical additives meant to protect polymers from thermal breakdown. phosphite antioxidant degradation involves the conversion of active hydroperoxide decomposers into inactive phosphates during high-shear extrusion or molding, and it stops when the stabilizer is entirely converted or the melt cools.
Hydrolysis Reaction
Moisture in the resin can react with the phosphite molecules before they can perform their stabilizing function. This reaction produces acidic byproducts that can corrode the molding equipment or cause the polymer itself to break down. Keeping resin dry is essential for maintaining the effectiveness of the additive package.
Melt Stability
Active phosphites prevent the change in molecular weight that occurs when a polymer is melted multiple times. They react with oxygen-rich radicals to stop the chain-scission process that makes plastic brittle. As the concentration of active phosphite drops, the melt flow index of the resin begins to drift.
Recyclability Limit
Every heat cycle consumes a portion of the original antioxidant load. In regrind operations, the level of degraded phosphates increases while the protection for the polymer decreases. This depletion limits the number of times a material can be reprocessed before its physical properties fail to meet the specification.
