Most synthetic polymers are fairly stable for long periods provided that they are (a) not heated and (b) kept away from light. But they can be attacked very slowly by oxygen, and the process of oxidation is accelerated by either heat or light.
The condition for oxidative degradation can exist even before the plastic product is used, during normal polymer processing, when temperature can reach 300℃. Fortunately the processing time at high temperature is usually short and exposure to oxygen at this stage is limited, but protection is nevertheless advisable to guard against discoloration. Generally during polymer degradation free radicals are formed, resulting in uncontrolled chain reactions. Anti-oxidants are used to decrease the amount of free radicals by different mechanisms e.g. by avoiding the formation of free radicals using complexing agents or hydroperoxide decomposers, or by decreasing the amount of radicals using radical scavengers or hydrogen donors.
Primary and secondary antioxidants: antioxidants are generally divided into primary and secondary categories. The term primary antioxidants is used to denote those additives that suppress oxidation over the lifetime of the product, where as the main function of secondary antioxidants is to protect the polymer for the much shorter period when they are being processed.