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.
Susceptible polymers
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. The process can be divided into four stages:
Initiation the process of generating the initial free radical. Propagation the conversion of one active species to another Chain branching steps which end with more than one active species being produced. The photolysis of hydroperoxides is the main example. Termination steps in which active species are removed, for instance by radical disproportionation Photo-oxidation can occur simultaneously with other processes like thermal degradation, and each of these can accelerate the other.
Polyolefins Polyolefins such as polyethylene and polypropylene are susceptible to photo-oxidation and around 70% of light stabilizers produced world-wide are used in their protection, despite them representing only around 50% of global plastic production. Aliphatic hydrocarbons can only adsorb high energy UV-rays with a wavelength below ~250 nm, however the Earth's atmosphere and ozone layer screen out such rays, with the normal minimum wavelength being 280–290 nm. The bulk of the polymer is therefore photo-inert and degradation is instead attributed to the presence of various impurities, which are introduced during the manufacturing or processing stages. These include hydroperoxide and carbonyl groups, as well as metal salts such as catalyst residues. All of these species act as photoinitiators. The organic hydroperoxide and carbonyl groups are able to absorb UV light above 290 nm whereupon they undergo photolysis to generate radicals. Metal impurities act as photocatalysts, although such reactions can be complex. It has also been suggested that polymer-O2 charge-transfer complexes are involved. Initiation generates radical-carbons on the polymer chain, sometimes called macroradicals (P•).
Chain initiation
Polymer ⟶ P ∙ + P ∙ {\displaystyle {\ce {Polymer->P\bullet +\ P\bullet }}}
Chain propagation
P ∙ + O 2 ⟶ POO ∙ {\displaystyle {\ce {P\bullet +\ O2->POO\bullet }}}
POO ∙ + PH ⟶ POOH + P ∙ {\displaystyle {\ce {POO\bullet +\ PH->{POOH}+\ P\bullet }}}
Chain branching
POOH ⟶ PO ∙ + OH ∙ {\displaystyle {\ce {POOH->PO\bullet +\ OH\bullet }}}
PH + OH ∙ ⟶ P ∙ + H 2 O {\displaystyle {\ce {{PH}+OH\bullet ->P\bullet +\ H2O}}}
PO ∙ ⟶ Chain scission reactions {\displaystyle {\ce {PO\bullet ->Chain\ scission\ reactions}}}
Termination
POO ∙ + POO ∙ ⟶ cross linking reaction to non − radical product {\displaystyle {\ce {POO\bullet +\ POO\bullet ->cross\ linking\ reaction\ to\ non-radical\ product}}}
POO ∙ + P ∙ ⟶ cross linking reaction to non − radical product {\displaystyle {\ce {POO\bullet +\ P\bullet ->cross\ linking\ reaction\ to\ non-radical\ product}}}
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