Polymers break down because of autoxidation caused by things like heat, light, heavy metals, and physical stress (friction or bending). To stop this autoxidation, we use additives like thermo-oxidation blockers (against heat), photo-oxidation blockers (against UV light), and metal deactivators (against heavy metals). Even though these are all called antioxidants, the term “antioxidants” usually refers specifically to thermo-oxidation blockers. Thermo-oxidation inhibitors include phenolic antioxidants, phosphorous antioxidants, Sulfur antioxidants. 

Phenolic antioxidants 

Phenolic antioxidants are substances that help get rid of harmful particles called peroxy radicals that appear when things are heated and oxidized.  These antioxidants target different kinds of radicals, mainly peroxy radicals, that form when oxidation happens, turning them into hydroperoxide while the antioxidant itself changes into phenoxy radicals. The hydroperoxide that forms can be broken down into a stable type of alcohol using other substances known as peroxide decomposers, like phosphorous or sulfur antioxidants. 

Because of this, phenolic antioxidants are considered primary antioxidants, while phosphorous and sulfur antioxidants are labeled as secondary antioxidants. Without phenolic antioxidants, the peroxy radicals can transform into hydroperoxide by taking hydrogen from the chains of polymers, which leads to the creation of polymer radicals that start to break down the polymer. When these polymer radicals keep forming, it speeds up the oxidation process, causing the polymer to break down faster. 

Phosphorous antioxidants 

Phosphorous antioxidants are special molecules that break down hydroperoxides created during the heating and oxidation of materials. They work well to keep plastics steady while they are being processed because they react quickly with hydroperoxides. Making plastics involves mixing different ingredients into the plastic during a compounding phase and then shaping the plastic by melting it in a conversion phase. In these processes, the plastic is exposed to very high temperatures, which can easily damage it through thermal oxidation.

These phases are known to create a lot of hydroperoxide due to the breakdown of the polymers. To protect the plastic, hydroperoxide needs to be quickly turned into a safe alcohol. Overall, phosphorous antioxidants are great at stabilizing polymers, and the quicker they react with hydroperoxide, the better they work in keeping the plastic stable. 

Sulfur antioxidants 

Sulfur antioxidants also help break down hydroperoxides. While they aren’t as quick to react with hydroperoxides as phosphorus antioxidants, they can still break down a lot of hydroperoxide molecules. This quality makes them good for making materials last longer in certain environments. 

Just like phosphorus antioxidants, sulfur antioxidants help in breaking down hydroperoxides too. Phosphorus antioxidants work well for short bursts of heat to keep plastics stable at high temperatures, but sulfur antioxidants are better for keeping plastics stable over a longer time at lower temperatures. Generally, phosphorus antioxidants are used to keep plastics stable while they’re being made, while sulfur antioxidants help extend the life of plastics after they have been processed. 

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