Meaning
Chemical reactions that occur between polymers and atmospheric oxygen lead to gradual degradation of the material’s properties. Exposure of stored resin or molded parts to heat and oxygen triggers autoxidation, which breaks down polymer chains over time. This process reduces the mechanical strength, flexibility, and color stability of plastic parts during service.
Preventative antioxidant packages must be compounded into the resin to ensure that long-term oxidation does not compromise the molecular weight during processing and storage.
Radical Initiation
Radical generation initiates the process when heat or ultraviolet light breaks carbon-hydrogen bonds in the polymer backbone. During autoxidation, these unstable polymer radicals react with oxygen to form peroxy radicals. These reactive intermediates then abstract hydrogen atoms from neighboring polymer chains, continuing the degradation sequence.
Chain Propagation
Chain reactions accelerate as the concentration of free radicals increases, leading to widespread molecular breakdown. The addition of antioxidant additives in polymer compounding stops these cycles by scavenging free radicals before they can attack the polymer chains. Without stabilization, the reaction continues until the radical concentrations become high enough to terminate each other.
Polymer Degradation
Part failure is the eventual outcome of uninhibited chemical degradation. Molders observe autoxidation as yellowing, surface cracking, and a loss of impact resistance in molded parts. In compounding operations, this degradation reduces the melt flow rate and produces a brittle material that is unsuitable for structural applications.
