Meaning
Degradation of secondary antioxidants during melt processing limits the thermal protection available to a polymer during subsequent exposures to heat and oxygen. The rate of phosphite stabilizer depletion increases with each extrusion cycle, as these sacrificial additives react with hydroperoxides to prevent polymer chain scission and cross-linking.
Reaction Mechanism
Secondary phosphites function by reducing highly reactive hydroperoxides to stable alcohols, being oxidized to phosphates in the process. This chemical conversion consumes the active stabilizer, leaving the polymer vulnerable to thermo-oxidative degradation once the reserve of unreacted phosphite is exhausted. High melt temperatures and high shear rates within the extruder barrel accelerate this consumption rate, especially in the presence of oxygen or moisture.
Consequently, monitoring the conversion of phosphite to phosphate provides a reliable measure of the thermal stress experienced by the resin during compounding.
Processing Impact
Analytical techniques such as high-performance liquid chromatography or Fourier-transform infrared spectroscopy measure the concentration of active stabilizer remaining in the resin. A significant drop in the stabilizer level is accompanied by a rise in the melt flow index of polypropylene due to chain scission, or a drop in the melt flow index of polyethylene due to chain branching. Molders observe these shifts as erratic filling behavior and increased part brittleness.
Recycling Limitation
Reclaiming scrap or processing regrind resin compounds the loss of stabilizer, as each heat history consumes a portion of the original additive package. Unless fresh stabilizer is added during compounding, the regrind blend will suffer from rapid discoloration and gel formation during subsequent molding.
