Alteration of the materials by using light is known as photodegradation. Basically, photodegradation involves sunlight and air to act together. Photodegradation involves the process of oxidation (loss of electrons, increase in oxidation state) and hydrolysis (split of chemical bonds by adding water).
Photodegradation can destroy artefacts and the paintings and hence it is often not preferred. Photodegradation does not talk about the ageing or degradation of materials with the use of heat or infrared light. However, it does include the degradation that occurs in all of the ultraviolet light wavebands.
MECHANISM OF PHOTODEGRADATION
Photochemical reactions start off with the absorption of a photon that typically lies in the wavelength range of 290-700 nm at the Earth’s surface. The molecules change their configuration (the molecule goes from the ground state to an excited state) briefly when the absorbed photon transfers its energy to the electrons.
The excited state basically shows off a newer molecule. But, they are unstable and can easily oxidise or hydrolyze. Pollutants can be removed by direct photolysis or the indirect photolysis.
A lot of organic chemicals show thermodynamic instability when they come in the presence of oxygen but the rate at which the spontaneous oxidation takes place is slower at room temperature. Basically, such reactions are kinetically limited.
As a result of this kinetic stability, complex structures build up in the environment. When the light gets absorbed, triplet oxygen breaks down into mono oxygen which is highly reactive and affects the spin oxidations. Water and ozone produce hydroxyl radicals which degrade the organic compounds in the atmosphere.
APPLICATIONS OF PHOTODEGRADATION
- Food needs to be protected against the photodegradation. For example, beer can get degraded when exposed to sunlight and to avoid this, amber glass is used in making of the beer bottles since amber glass is a good absorber of UV radiation.
- Photodegradation can occur to paints, dyes and inks. Ceramics, on the other hand, are coloured with non-organic materials for preventing photodegradation and allowing the material to sustain the colour.
- The photodegradation of pharmaceuticals needs to be prevented from saving the aquatic organism.
- Owing to the large scale of agriculture and chemicals, the photodegradation of the pesticides is of primary interest. Pesticides have to perform biocidal acts and hence they are made to not succumb to photodegradation easily.
- Photosensitizers, photocatalysts and hydrogen peroxide are added for generation of hydroxyl radicals that attack the pesticides. These are added for intensifying their photodegradation.