Meaning
Reaction of dihydric phenol compounds with molecular oxygen at elevated temperatures generates colored quinoid derivatives and degrades the polymer backbone. In the life cycle of engineering plastics, bisphenol a oxidation represents the primary chemical degradation mechanism that occurs during the drying, compounding, and injection moulding of polycarbonate resins. The resulting breakdown of the monomer units causes yellowing of the polymer and limits the use of recycled material in optical applications.
Degradation Chemistry
Thermally initiated homolytic cleavage of the carbon-carbon or carbon-oxygen bonds in the polymer backbone generates highly reactive free radical intermediates. When bisphenol a oxidation proceeds in the presence of trace oxygen, these radicals react to form hydroperoxides that subsequently decompose into chromophores. The process is autocatalytic, meaning that the degradation products themselves accelerate further oxidative breakdown of the polymer matrix.
Mechanical Consequence
Breakdown of the polymer chains reduces the molecular weight and narrows the mechanical safety margin of the molded part. Polycarbonate loses its characteristic impact strength, transforming from a highly ductile material to a brittle one that fails under low stress. Moulders observe a drop in the spiral flow length of the melt, indicating that the degraded polymer has a lower melt viscosity than specified on the virgin resin datasheet.
The financial impact is significant because the molded parts fail tensile tests and require the discarding of entire production lots.
Protection Strategy
Addition of secondary phosphite antioxidants and hindered phenol stabilizers during compounding inhibits the free radical chain reactions. Minimizing the oxygen content in the resin dryer also prevents these oxidative reactions.
