Photo-oxidation of polymers
In
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-
Susceptible polymers
Susceptibility to photo-oxidation varies depending on the chemical structure of the polymer. Some materials have excellent stability, such as
Photo-oxidation is a form of
- 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 hydroperoxidesis 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
All of these species act as photoinitiators.[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 as photocatalysts,[6] although such reactions can be complex.[7][8] It has also been suggested that polymer-O2 charge-transfer complexes are involved.[9][10] Initiation generates radical-carbons on the polymer chain, sometimes called macroradicals (P•).
Chain initiation
Chain propagation
Chain branching
Termination
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 undergo
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 undergo Type 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 reactive singlet state.[18] Singlet oxygen is a potent oxidising agent can go on to form cause further degradation.
Polystyrene
For polystyrene 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 polystyrene phenyl groups absorb light to form singlet oxygen, which acts as a radical initiator. [23] Carbonyl impurities in the polymer (c.f. 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,
Polystyrene is observed to yellow during photo-oxidation, which is attributed to the formation of polyenes from these terminal alkenes.[25]
Polyvinyl chloride (PVC)
Pure
Propagation steps involve the hydroperoxyl radical, which can abstract hydrogen from both hydrocarbon (-CH2-) and organochloride (-CH2Cl-) sites in the polymer at comparable rates.
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 being
Poly(ethylene terephthalate) - (PET)
Unlike most other commodity plastics
Photodissociation involves the formation of an excited terephthalic acid unit which undergoes Norrish 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 to acetaldehyde 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 aromatic terephthalic 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]
Secondary factors
Environment
Perhaps surprisingly, the effect of temperature is often greater than the effect of UV exposure.[5] This can be seen in terms of the Arrhenius 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 land plastic pollution in the marine environment degrades more slowly.[40][41] Materials buried in landfill do not degrade by photo-oxidation at all, though they may gradually decay by other processes.
Effects of dyes and other additives
Additives to enhance degradation
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 of microplastics.[47] Oxo-plastics would be difficult to distinguish from untreated plastic but their inclusion during plastic 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]
Prevention
UV attack by sunlight can be ameliorated or prevented by adding anti-UV
Frequently, glass can be a better alternative to polymers when it comes to UV degradation. Most of the commonly used glass 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 on polycarbonate (PC) to decrease the oxidation and photo-yellowing rate caused by solar radiation.[52]
Analysis
Weather testing of polymers
The photo-oxidation of polymers can be investigated by either natural or accelerated weather testing.
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 the stress–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 the Young’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 the yield strain, fracture strain, and toughness, 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 resulting from degradation is directly proportional to the surface area SA of the specimen:[57]
Here, is the density and kd 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.
Detection
Degradation can be detected before serious cracks are seen in a product by using infrared spectroscopy,[58] which is able to detect chemical species formed by photo-oxidation. In particular, peroxy-species and carbonyl 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 a
The effects of degradation can also be characterized through scanning 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.
See also
- Forensic polymer engineering
- Photodegradation
- Polymer degradation
- Stress corrosion cracking
- Thermal degradation of polymers
References
- ^ ISBN 978-3-446-40801-2.
- S2CID 92300829.
- ISBN 9783900734220.
- .
- ^ S2CID 225243217.
- .
- .
- .
- .
- .
- S2CID 51679950.
- .
- .
- ]
- .
- .
- .
- .
- ^ PMID 25674392.
- .
- .
- .
- ^ .
- ^ .
- ^ .
- .
- .
- .
- ^ .
- .
- ^ .
- .
- .
- .
- ^ .
- ^ .
- .
- S2CID 225595988.
- PMID 12448533.
- .
- .
- S2CID 97316912.
- ^ "THE PHOTO-OXIDATION OF POLYMERS - A comparison with low molecular weight compounds" (PDF). Pergamon Press Ltd. 1979 - Pure & Appi. Chem., Vol. 51, pp.233—240. Retrieved 9 February 2011.
- .
- .
- .
- ^ "on the impact of the use of oxo-degradable plastic, including oxo-degradable plastic" (PDF). EUROPEAN. Retrieved 11 November 2020.
- S2CID 209432804.
- .
- ^ the EU directive 2019/904 (Article 5), EU directive 5 June 2019
- ^ L. Guedri-Knani, J. L. Gardette, M. Jacquet, A. Rivaton, Photoprotection of poly(ethylene-naphthalate) by zinc oxide coating, Surface and Coatings Technology, Volumes 180-181, 1 March 2004, Pages 71-75
- ^ A. Moustaghfir, E. Tomasella, A. Rivaton, B. Mailhot, M. Jacquet, J. L. Gardette, J. Cellier, Sputtered zinc oxide coatings: structural study and application to the photoprotection of the polycarbonate, Surface and Coatings Technology, Volumes 180-181, 1 March 2004, Pages 642-645.
- .
- PMID 35054761.
- S2CID 233537494.
- ^ S2CID 249937870.
- S2CID 212404939.
- S2CID 233639741.