Evaluating Thermal and Oxidative Polyolefin Flake Degradation
Thermal and oxidative degradation alters polyolefin flake viscosity and residual stability, demanding MFR, OIT, and Carbonyl Index verification before processing.
Phosphite Antioxidants
Meaning
Secondary stabilizers decompose hydroperoxides into non-reactive compounds during melt compounding and injection moulding to prevent the initiation of thermal oxidation. These phosphorus-based chemical additives, known as phosphite antioxidants, protect the polymer melt from degradation and help maintain consistent rheological properties during high-shear processing steps. They work synergistically with primary hindered phenol stabilizers to extend polymer life.
Stabilizing Mechanism
Phosphite additives react with unstable hydroperoxide groups in the polymer matrix to convert them into stable alcohols while being oxidized into phosphates themselves. This sacrifice of the additive prevents the hydroperoxides from undergoing homolytic cleavage into highly reactive free radicals. This protective action is necessary when the polymer melt is subjected to high processing temperatures.
It halts the chain-reaction degradation that would otherwise cause a rapid drop in melt viscosity.
Melt Protection
In the extruder, high temperature and mechanical shear create conditions that accelerate thermal degradation. Adding these phosphorus compounds to the formulation prevents molecular weight loss and maintains the original melt flow index of the resin. This stability prevents defects like yellowing or gel formation in the produced parts.
Recycling Utility
Regrind materials have already experienced thermal history that has depleted their initial additive package. Incorporating fresh secondary stabilizers into the regrind restores the processing stability of the material, allowing for higher recycling ratios. This restoration reduces the risk of part failure and lowers the overall raw material costs.
Evaluating Degradation Mechanisms in Post-Consumer Polyolefin Flake
Polyolefin degradation evaluation requires coupling multi-load melt flow testing and OIT analysis to quantify chain scission and restabilize flake lots.