Plastic has become a fundamental material in all aspects of our modern life. It has replaced many materials, such as glass, metal, paper and wood, and has made a complete difference to our everyday lives.
Despite the many benefits of its use, plastic often ages rapidly in the presence of light, oxygen and heat. This ageing causes plastic parts to lose their strength, hardness or flexibility, and they can discolour, lose their lustre or turn yellow.
Do you remember those appliances and furniture from the 1980s that turned completely yellow after five years? This is caused by oxidation of the polymer.
Most commercial plastics are manufactured through processes involving chain polymerisation, polyaddition or polycondensation reactions, which are generally controlled to produce individual polymer molecules with a defined chemical composition and molecular weight.
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Chemical reactions begin to occur in polymers when they are exposed to shear stress, heat, light, air, water, radiation or mechanical loading. As a result, its chemical composition and the molecular weight of the polymer will change. These reactions will alter the physical and optical properties of the polymer.
The thermal-autoxidation reactions of polymers can be summarised as follows.
Once thermal oxidation has started, a chain reaction occurs, accelerating degradation unless polymer stabilisers are used to interrupt the oxidation cycle.
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Exposure to sunlight, ultraviolet light and some artificial light can also have a detrimental effect on the life of plastic products. When UV radiation breaks down the chemical bonds in polymers, a process known as photodegradation occurs, eventually leading to cracking, chalking, colour changes and loss of physical properties.
In both cases, free radical molecules (R*) are produced which are highly reactive and will produce a carboxyl group (ROO*). This radical will react with another original polymeric chain to produce a carboxylic acid and a new polymeric radical. This cycle repeats itself, forming the first oxidation cycle.
A new reaction can then take place between this newly formed R* radical and the carboxylic acid to form 2 radicals, OH* and RO*. RO* will react with one of the original polymeric chains to form an alcohol and a new polymeric radical. This is the second oxidation cycle.
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Antioxidants - protecting polymers from thermal-oxidative degradation
Polymer antioxidants can be added to increase the shelf life of a product or to improve its high temperature stability. They also add a margin of stability during thermal processing.
Antioxidant additives can be classified as hindered phenols, metal deactivators, amines, phosphites, thioesters and binary mixtures.
Hindered phenols are used as primary antioxidants. Organophosphates are sometimes used as secondary antioxidants. When different antioxidants are used in a mixture, synergistic effects are created.
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.
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