Antioxidants protect plastics and other materials against thermally induced oxidation. This process can lead to a number of undesirable effects, including discoloration, changes in melt viscosity, and deterioration of mechanical properties. As a result, the useful life of the polymer or final article is limited. Plastics are susceptible to oxidative degradation during high temperature melt processing operations and end use, as well as during long-term storage.
Oxidation is a chain reaction process involving free radical and hydroperoxide intermediates. Antioxidants inhibit this process by reacting with and decomposing these reactive species. Different families of antioxidants are reactive with different intermediates in the oxidation process and also present different levels of reactivity as a function of temperature.
Primary antioxidants, such as hindered phenols, function mainly by scavenging the peroxy radical intermediates in the oxidation process. In general they are effective over a wide temperature range, and are useful to improve both the processing and long-term thermal stability of plastics.
Secondary antioxidants, such as phosphites and thioethers, function by decomposition of hydroperoxides. Phosphites are most effective at the high temperatures of melt processing operations, while thioethers function best in the solid phase at long-term use temperatures.
Blends of primary and secondary antioxidants often show a synergistic performance superior to either type of antioxidant used alone. For example, blends of hindered phenols and phosphites represent the state-of-the-art for the melt process stabilization of many plastics.
Transition metal ions such as copper and iron can accelerate polymer degradation by catalysis of the oxidation process. Special additives known as metal deactivators are used to chelate with transition metal ions and render them inert as oxidation catalysts.
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