Inpolymer chemistry,photo-oxidation (sometimes: oxidativephotodegradation) is thedegradation of a polymer surface due to the combined action of light and oxygen.^[1] It is the most significant factor in the weathering of plastics.^[2] Photo-oxidation causes the polymer chains to break (chain scission), resulting in the material becoming increasingly brittle. This leads tomechanical failure and, at an advanced stage, the formation ofmicroplastics. Intextiles, the process is calledphototendering.

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 withbiodegradable additives to accelerate their fragmentation.Manypigments anddyes 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 asfluoropolymers,polyimides,silicones and certainacrylate polymers. However, global polymer production is dominated by a range ofcommodity plastics which account for the majority ofplastic waste. Of thesepolyethylene terephthalate (PET) has only moderate UV resistance and the others, which includepolystyrene,polyvinyl chloride (PVC) andpolyolefins likepolypropylene (PP) andpolyethylene (PE) are all highly susceptible.

Photo-oxidation is a form ofphotodegradation and begins with formation offree radicals on the polymer chain, which then react with oxygen inchain reactions. For many polymers the generalautoxidation mechanism is a reasonable approximation of the underlying chemistry. The process isautocatalytic, generating increasing numbers of radicals and reactive oxygen species. These reactions result in changes to themolecular weight (andmolecular 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. Thephotolysis ofhydroperoxides is the main example.

Termination steps in which active species are removed, for instance byradical disproportionation

Photo-oxidation can occur simultaneously with other processes likethermal degradation, and each of these can accelerate the other.

Polyolefins such aspolyethylene andpolypropylene 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.^[1] Aliphatic hydrocarbons can only adsorb high energy UV-rays with a wavelength below ~250 nm, however the Earth's atmosphere andozone layer screen out such rays, with the normal minimum wavelength being 280–290 nm.^[3]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 includehydroperoxide andcarbonyl groups, as well as metal salts such as catalyst residues.

All of these species act asphotoinitiators.^[4]The organic hydroperoxide and carbonyl groups are able to absorb UV light above 290 nm whereupon they undergo photolysis to generate radicals.^[5] Metal impurities act asphotocatalysts,^[6] although such reactions can be complex.^[7][8] It has also been suggested that polymer-O₂charge-transfer complexes are involved.^[9][10] Initiation generates radical-carbons on the polymer chain, sometimes called macroradicals (P•).

Chain initiation

${\displaystyle \text{Polymer}⟶\text{P}\bullet +\text{P}\bullet }$

Chain propagation

${\displaystyle \text{P}\bullet +{\text{O}}_2^{}⟶\text{POO}\bullet }$

${\displaystyle \text{POO}\bullet +\text{PH}⟶\text{POOH}+\text{P}\bullet }$

Chain branching

${\displaystyle \text{POOH}⟶\text{PO}\bullet +\text{OH}\bullet }$

${\displaystyle \text{PH}+\text{OH}\bullet ⟶\text{P}\bullet +{\text{H}}_2^{}\text{O}}$

${\displaystyle \text{PO}\bullet ⟶\text{Chain}\text{scission}\text{reactions}}$

Termination

${\displaystyle \text{POO}\bullet +\text{POO}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{POO}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{P}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

Classically the carbon-centred macroradicals (P•) rapidly react with oxygen to form hydroperoxyl radicals (POO•), which in turn abstract an H atom from the polymer chain to give a hydroperoxide (POOH) and a fresh macroradical. Hydroperoxides readily undergophotolysis to give an alkoxyl macroradical radical (PO•) and ahydroxyl radical (HO•), both of which may go on to form new polymer radicals via hydrogen abstraction. Non-classical alternatives to these steps have been proposed.^[11] The alkoxyl radical may also undergobeta scission,^[12] generating an acyl-ketone and macroradical. This is considered to be the main cause of chain breaking in polypropylene.^[13]

Secondary hydroperoxides can also undergo an intramolecular reaction to give a ketone group, although this is limited to polyethylene.^{[1][14][15][16]}

The ketones generated by these processes are themselves photo-active, although much more weakly. At ambient temperatures they undergoType II Norrish reactions with chain scission.^[17] They may also absorb UV-energy, which they can then transfer to O₂, causing it to enter its highly reactivesinglet state.^[18] Singlet oxygen is a potent oxidising agent and can go on to cause further degradation.

Forpolystyrene the complete mechanism of photo-oxidation is still a matter of debate, as different pathways may operate concurrently^[20] and vary according to the wavelength of the incident light.^[21][22]Regardless, there is agreement on the major steps.^[19]

Pure polystyrene should not be able to absorb light with a wavelength below ~280 nm and initiation is explained though photo-labile impurities (hydroperoxides) and charge transfer complexes,^[23] all of which are able to absorb normal sunlight.^[24]Charge-transfer complexes of oxygen and polystyrenephenyl groups absorb light to formsinglet oxygen, which acts as a radical initiator.^[23] Carbonyl impurities in the polymer (cf.acetophenone) also absorb light in the near ultraviolet range (300 to 400 nm), forming excited ketones able to abstract hydrogen atoms directly from the polymer.^[24] Hyroperoxide undergoes photolysis to form hydroxyl and alkoxyl radicals.

These initiation steps generate macroradicals at tertiary sites, as these are more stabilised. The propagation steps are essentially identical to those seen for polyolefins; with oxidation,hydrogen abstraction and photolysis leading tobeta scission reactions and increasing numbers of radicals. These steps account for the majority of chain-breaking, however in a minor pathway the hydroperoxide reacts directly with polymer to form a ketone group (acetophenone) and aterminal alkene without the formation of additional radicals.^[25]

Polystyrene is observed to yellow during photo-oxidation, which is attributed to the formation ofpolyenes from these terminal alkenes.^[25]

Pureorganochlorides likepolyvinyl chloride (PVC) do not absorb any light above 220 nm. The initiation of photo-oxidation is instead caused by various irregularities in the polymer chain, such as structural defects^[26][27] as well as hydroperoxides, carbonyl groups, and double bonds.^[28] Hydroperoxides formed during processing are the most important initiator to begin with,^[29] however their concentration decreases during photo-oxidation whereas carbonyl concentration increases,^[30] as such carbonyls may become the primary initiator over time.^[29][31][32]

Propagation steps involve the hydroperoxyl radical, which can abstract hydrogen from both hydrocarbon (-CH₂-) and organochloride (-CH₂Cl-) sites in the polymer at comparable rates.^[29][31] Radicals formed at hydrocarbon sites rapidly convert to alkenes with loss of radical chlorine. This formsallylic hydrogens (shown in red) which are more susceptible to hydrogen abstraction leading to the formation ofpolyenes in zipper-like reactions.

When the polyenes contain at least eight conjugated double bonds they become coloured, leading to yellowing and eventual browning of the material. This is off-set slightly by longer polyenes beingphotobleached with atmospheric oxygen,^[33] however PVC does eventually discolour unlesspolymer stabilisers are present. Reactions at organochloride sites proceed via the usual hydroperoxyl and hydroperoxide before photolysis yields the α-chloro-alkoxyl radical. This species can undergo various reactions to give carbonyls, peroxidecross-links and beta scission products.^[34]

Unlike most other commodity plasticspolyethylene terephthalate (PET) is able to absorb thenear ultraviolet rays in sunlight. Absorption begins at 360 nm, becoming stronger below 320 nm and is very significant below 300 nm.^[1][35][36] Despite this PET has better resistance to photo-oxidation than othercommodity plastics, this is due to a poorquantum yield or the absorption.^[37] The degradation chemistry is complicated due to simultaneousphotodissociation (i.e. not involving oxygen) and photo-oxidation reactions of both the aromatic and aliphatic parts of the molecule. Chain scission is the dominant process, with chain branching and the formation of coloured impurities being less common. Carbon monoxide, carbon dioxide, and carboxylic acids are the main products.^[35][36]The photo-oxidation of other linear polyesters such aspolybutylene terephthalate andpolyethylene naphthalate proceeds similarly.

Photodissociation involves the formation of an excitedterephthalic acid unit which undergoesNorrish reactions. The type I reaction dominates, which cause chain scission at the carbonyl unit to give a range of products.^[1][38]

Type II Norrish reactions are less common but give rise toacetaldehyde by way of vinyl alcohol esters.^[36] This has an exceedingly low odour and taste threshold and can cause an off-taste in bottled water.^[39]

Radicals formed by photolysis may initiate the photo-oxidation in PET. Photo-oxidation of the aromaticterephthalic acid core results in its step-wise oxidation to 2,5-dihydroxyterephthalic acid. The photo-oxidation process at aliphatic sites is similar to that seen for polyolefins, with the formation of hydroperoxide species eventually leading to beta-scission of the polymer chain.^[1]

Perhaps surprisingly, the effect of temperature is often greater than the effect of UV exposure.^[5] This can be seen in terms of theArrhenius equation, which shows that reaction rates have an exponential dependence on temperature. By comparison the dependence of degradation rate on UV exposure and the availability of oxygen is broadly linear. As the oceans are cooler than landplastic pollution in the marine environment degrades more slowly.^[40][41] Materials buried inlandfill do not degrade by photo-oxidation at all, though they may gradually decay by other processes.

Mechanical stress can effect the rate of photo-oxidation^[42] and may also accelerate the physical breakup of plastic objects. Stress can be caused by mechanical load (tensile andshear stresses) or even bytemperature cycling, particularly in composite systems consisting of materials with differingtemperature coefficients of expansion. Similarly, suddenrainfall can causethermal stress.

Dyes andpigments are used in polymer materials to provide colour, however they can also affect the rate of photo-oxidation. Many absorb UV rays and in so doing protect the polymer, however absorption can cause the dyes to enter an excited state where they may attack the polymer or transfer energy to O₂ to form damagingsinglet oxygen.Cu-phthalocyanine is an example, it strongly absorbs UV light however the excited Cu-phthalocyanine may act as aphotoinitiator by abstracting hydrogen atoms from the polymer.^[43] Its interactions may become even more complicated when other additives are present.^[44]Fillers such ascarbon black can screen out UV light, effectively stabilisers the polymer, whereasflame retardants tend to cause increased levels of photo-oxidation.^[45]

Biodegradable additives may be added to polymers to accelerate their degradation. In the case of photo-oxidationOXO-biodegradation additives are used.^[46] These aretransition metal salts such asiron (Fe),manganese (Mn), andcobalt (Co). Fe complexes increase the rate of photooxidation by promoting thehomolysis of hydroperoxides viaFenton reactions.

The use of such additives has been controversial due to concerns that treated plastics do not fully biodegrade and instead result in the accelerated formation ofmicroplastics.^[47] Oxo-plastics would be difficult to distinguish from untreated plastic but their inclusion duringplastic recycling can create a destabilised product with fewer potential uses,^[48][49] potentially jeopardising the business case for recycling any plastic. OXO-biodegradation additives were banned in the EU in 2019^[50]

Bisoctrizole: Aphenolic benzotriazole based UV absorber used to protect polymers

Active principle of the ultraviolet absorption via aphotochromic transition

UV attack by sunlight can be ameliorated or prevented by adding anti-UVpolymer stabilizers, usually prior to shaping the product byinjection moulding. UV stabilizers in plastics usually act by absorbing the UV radiation preferentially, and dissipating the energy as low-level heat. The chemicals used are similar to those insunscreen products, which protect skin from UV attack. They are used frequently inplastics, includingcosmetics andfilms. Different UV stabilizers are utilized depending upon the substrate, intended functional life, and sensitivity to UV degradation. UV stabilizers, such asbenzophenones, work by absorbing the UV radiation and preventing the formation offree radicals. Depending upon substitution, the UVabsorption spectrum is changed to match the application. Concentrations normally range from 0.05% to 2%, with some applications up to 5%.

Frequently, glass can be a better alternative to polymers when it comes to UV degradation. Most of the commonly usedglass types are highly resistant to UV radiation. Explosion protection lamps for oil rigs for example can be made either from polymer or glass. Here, the UV radiation and rough weathers belabor the polymer so much, that the material has to be replaced frequently.

Poly(ethylene-naphthalate) (PEN) can be protected by applying a zinc oxide coating, which acts as protective film reducing the diffusion of oxygen.^[51] Zinc oxide can also be used onpolycarbonate (PC) to decrease the oxidation and photo-yellowing rate caused by solar radiation.^[52]

The photo-oxidation of polymers can be investigated by either natural or accelerated weather testing.^[53] Such testing is important in determining the expected service-life of plastic items as well as the fate ofwaste plastic.

In natural weather testing, polymer samples are directly exposed to open weather for a continuous period of time,^[54] while accelerated weather testing uses a specialized test chamber which simulates weathering by sending a controlled amount of UV light and water at a sample. A test chamber may be advantageous in that the exact weathering conditions can be controlled, and the UV or moisture conditions can be made more intense than in natural weathering. Thus, degradation is accelerated and the test is less time-consuming.

Through weather testing, the impact of photooxidative processes on the mechanical properties and lifetimes of polymer samples can be determined. For example, the tensile behavior can be elucidated through measuring thestress–strain curve for a specimen. This stress–strain curve is created by applying a tensile stress (which is measured as the force per area applied to a sample face) and measuring the corresponding strain (the fractional change in length). Stress is usually applied until the material fractures, and from this stress–strain curve, mechanical properties such as theYoung's modulus can be determined. Overall, weathering weakens the sample, and as it becomes more brittle, it fractures more easily. This is observed as a decrease in theyield strain,fracture strain, andtoughness, as well as an increase in the Young's modulus and break stress (the stress at which the material fractures).^[55]

Aside from measuring the impact of degradation on mechanical properties, the degradation rate of plastic samples can also be quantified by measuring the change in mass of a sample over time, as microplastic fragments can break off from the bulk material as degradation progresses and the material becomes more brittle through chain-scission. Thus, the percentage change in mass is often measured in experiments to quantify degradation.^[56]

Mathematical models can also be created to predict the change in mass of a polymer sample over the weathering process. Because mass loss occurs at the surface of the polymer sample, the degradation rate is dependent on surface area. Thus, a model for the dependence of degradation on surface area can be made by assuming that the rate of change in mass${\displaystyle -\frac{\mathrm{d}m}{\mathrm{d}t}}$ resulting from degradation is directly proportional to the surface area SA of the specimen:^[57]

${\displaystyle -\frac{\mathrm{d}m}{\mathrm{d}t}=k_d\rho SA}$

Here,${\displaystyle \rho }$ is the density and k_d is known as the specific surface degradation rate (SSDR), which changes depending on the polymer sample's chemical composition and weathering environment. Furthermore, for a microplastic sample, SA is often approximated as the surface area of a cylinder or sphere. Such an equation can be solved to determine the mass of a polymer sample as a function of time.

Degradation can be detected before serious cracks are seen in a product by usinginfrared spectroscopy,^[58] which is able to detect chemical species formed by photo-oxidation. In particular, peroxy-species andcarbonyl groups have distinct absorption bands.

In the example shown at left, carbonyl groups were easily detected by IR spectroscopy from a cast thin film. The product was aroad cone made byrotational moulding inLDPE, which had cracked prematurely in service. Many similar cones also failed because an anti-UV additive had not been used during processing. Other plastic products which failed included polypropylene mancabs used at roadworks which cracked after service of only a few months.

The effects of degradation can also be characterized throughscanning electron microscopy (SEM). For example, through SEM, defects like cracks and pits can be directly visualized, as shown at right. These samples were exposed to 840 hours of exposure to UV light and moisture using a test chamber.^[56] Crack formation is often associated with degradation, such that materials that do not display significant cracking behavior, such as HDPE in the right example, are more likely to be stable against photooxidation compared to other materials like LDPE and PP. However, some plastics that have undergone photooxidation may also appear smoother in an SEM image, with some defects like grooves having disappeared afterwards. This is seen in polystyrene in the right example.

• Forensic polymer engineering
• Photodegradation
• Polymer degradation
• Stress corrosion cracking
• Thermal degradation of polymers

• Polymers
• Materials degradation
• Plastics and the environment
• Ultraviolet radiation

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