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Concept
Photo-oxidation of polymers
Summary
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break (chain scission), resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics. In textiles the process is called phototendering.
Technologies have been developed to both accelerate and inhibit this process. For example, plastic building components like doors, window frames and gutters are expected to last for decades, requiring the use of advanced UV-polymer stabilizers. Conversely, single-use plastics can be treated with biodegradable additives to accelerate their fragmentation.
Many pigments and dyes can similarly have effects due to their ability to absorb UV-energy.
Susceptibility to photo-oxidation varies depending on the chemical structure of the polymer. Some materials have excellent stability, such as fluoropolymers, polyimides, silicones and certain acrylate polymers. However, global polymer production is dominated by a range of commodity plastics which account for the majority of plastic waste. Of these polyethylene terephthalate (PET) has only moderate UV resistance and the others, which include polystyrene, polyvinyl chloride (PVC) and polyolefins like polypropylene (PP) and polyethylene (PE) are all highly susceptible.
Photo-oxidation is a form of photodegradation and begins with formation of free radicals on the polymer chain, which then react with oxygen in chain reactions. For many polymers the general autoxidation mechanism is a reasonable approximation of the underlying chemistry. The process is autocatalytic, generating increasing numbers of radicals and reactive oxygen species. These reactions result in changes to the molecular weight (and molecular weight distribution) of the polymer and as a consequence the material becomes more brittle.
Official source
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