Photodegradation

(organic chemistry) Chemical changes resulting from the absorption of light that reduce the useful properties of materials, particularly polymers. The chemical changes can include bond scission (especially of the molecular backbone), color formation, crosslinking, and chemical rearrangements.

Reduction in the useful properties of materials because of chemical changes resulting from the absorption of light. The chemical changes can include bond scission (especially of the molecular backbone), color formation, cross-linking, and chemical rearrangements. All organic materials can photodegrade, but the process has greatest practical relevance for polymers where scission of the polymer backbone is particularly important. Photodegradations of polymers in the absence of oxygen (photolysis) or using wavelengths shorter (more energetic) than those at the Earth's surface (<280 nanometers) have been studied extensively, but only the more practical situation of polymers exposed to terrestrial sunlight (or its equivalent) in air is discussed in this article.

Although all organic polymers can be degraded by light, the rate of degradation varies enormously from polymer to polymer, and is also dependent on the incident wavelengths. Light containing ultraviolet (uv; shorter-wavelength) components is much more destructive than visible light, so that polymers exposed indoors, behind window glass (transmitting > 330 nm), will degrade much more slowly than samples exposed outdoors.

For many aromatic polymers, such as polyester and the aramids, in which the polymer itself is the chromophore (light-absorbing group), backbone scission results predominantly from this direct absorption of light energy. For many other polymers, including polyolefins, and polyvinyl chloride where only impurities absorb energy from sunlight, scission of a chemical bond by light to give free radicals is followed by reaction of these highly reactive free radicals with atmospheric oxygen. See also Free radical.

Although numerous organic materials will undergo photodegradation, hydrocarbon polymers are particularly vulnerable because their useful properties depend entirely on their high molecular weights, in the tens or hundreds of thousands. Anything that reduces the molecular weight of polymeric systems will alter the characteristics of these systems and limit their service life. In fact, the scission of as few as one carbon-carbon bond in a thousand in a polymer molecule can completely destroy its useful physical properties. This sensitivity is not observed in lower-molecular-weight substances such as liquid hydrocarbons.

A general approach to reducing the rates of photodegradation for all types of polymers is the use of low levels of additives. These additives, known as photostabilizers or uv stabilizers, are effective at fractions of a weight percent. See also Photochemistry; Polymer; Stabilizer (chemistry).

Photodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. However, other forms of electromagnetic radiation can cause photodegradation. Photodegradation includes photodissociation, the breakup of molecules into smaller pieces by photons. It also includes the change of a molecule's shape to make it irreversibly altered, such as the denaturing of proteins, and the addition of other atoms or molecules. A common photodegradation reaction is oxidation. This type of photodegradation is used by some drinking water and wastewater facilities to destroy pollutants. Photodegradation in the environment is part of the process by which ambergris evolves from its fatty precursor.

The US federal standard for testing plastic for photodegradation is 40 CFR Ch. I (7–1–03 Edition)PART 238

Photo degradation also destroys paintings and other artifacts.

Applications

Six-pack soda can rings have been photodegradable LDPE plastic since the 1990s.

See also: Biodegradable plastic

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