Antioxidants
Antioxidants are used to terminate the oxidation reactions taking place due to different weathering conditions and reduce the degradation of organic materials. For example, synthetic polymers react with atmospheric oxygen. Organic materials undergo auto-oxidizations due to free radical chain reaction. Oxidatively sensitive substrates will react with atmospheric oxygen directly and produce free radicals. Free radicals are of different forms, consider organic material RH. This material reacts with oxygen to give free radicals such as R•, RO•, ROO•, HO•. These free radicals further react with atmospheric oxygen to produce more and more free radicals. For example, R• + O2 → ROO• ROO• + RH → ROOH + R• This can be terminated using the antioxidants. Then this reaction comes to, 2R• → R—R ROO• + R• → ROOR 2ROO• → Non-radical products Weathering of polymers is caused by absorption of UV lights, which results in, radical initiated auto-oxidation. This produces cleavage of hydro peroxides and carbonyl compounds. This is because of the weak bond in hydro peroxides which is the main source for the free radicals to initiate from. Homolytic decomposition of hydro peroxide increases the rate of free radicals production. Therefore it is important factor in determining oxidative stability. The conversion of peroxy and alkyl radicals to non-radical species terminates the chain reaction, thereby decreasing the kinetic chain length.
Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with organic substrate (RH) for peroxy radicals, thereby terminating the chain reaction and stabilizing the further oxidation reactions.
At K17, ROO• + AH -> ROOH + A• At K6, ROO• + RH -> ROOH + R•
Here K17 is larger than K6, therefore AH can be at low concentrations. At low concentrations AH are more effective because the usual concentration in saturated plastic polymer range from 0.01 to 0.05% based on the weight of the polymer.
Benzofuranones is another most effective antioxidant, which terminates the chain reaction by donating weakly bonded benzylic hydrogen atom and gets reduced to a stable benzofuranyl (lactone).
Antioxidants inhibit the formation of the free radicals thereby enhancing the stability of polymers against light and heat.
Read more about this topic: Stabilizers For Polymers