Antioxidants are used to protect polymers from degradation, both during processing (=short term stabilization) and during use (=long term stabilization). Polymer degradation generally results in gel formation and changing melt flow index, loss of mechanical and esthetical properties (crazing, gloss reduction, chalking), and 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.
A complete range of anti-oxidants exist, starting from cheaper, general purpose anti-oxidants, up to anti-oxidants specially designed for high end applications like hot water pipes. High end anti-oxidants typically have better properties towards discoloration (yellowing/pinking), migration/chalking or hydrolisation.