Hindered phenols are primary antioxidants that function by scavenging peroxy radical intermediates in the oxidation process. They are effective over a wide range of temperatures and provide protection during melt processing, end use, and long-term storage. As processing stabilizers, hindered phenols serve to minimize changes in melt viscosity and reduce discoloration. During end use at elevated temperatures, hindered phenols can reduce discoloration and also improve the retention of useful mechanical properties. Some hindered phenols (such as AO-20, AO-50) also contribute to the light stability of plastics.
Partially-hindered phenols (such as AO-30, AO-40, AO-80) have the highest reactivity with peroxy radicals. These antioxidants are recommended for the stabilization of oxidatively-sensitive materials such as unsaturated elastomers. Combinations of partially-hindered and conventional hindered phenols sometimes provide synergistic performance.
Combinations of hindered phenols with secondary antioxidants (such as phosphites and thioethers) and hindered amine light stabilizers (HALS) often show a synergistic performance.