David Levy of the Madrid institute of materials science (ICMM-CSIC) and the national institute for aerospace technology (INTA), Spain, explores thin-film coatings that drastically reduce UV damage to materials

Ultraviolet (UV) light, either natural or artificial, causes organic compounds to decompose and degrade, because the energy of the photons in UV light is high enough to break chemical bonds. Organic materials, such as polymers, paints, pigments and dyes, are used in everything from car parts to fine art. 

 

Solar UV levels vary across the globe, and depend on time of day and season. The highest levels are shown in dark red

 

Polymers exposed to UV light can lose mechanical strength and integrity, while UV light causes the cellulose and lignin in wood to degrade, discolouring the wood and eventually causing fractures and cracking. The dyes in paintings and photographs progressively fade under UV light and paper becomes yellowed and brittle. UV light is the main factor responsible for the degradation of wooden furniture, plastic parts used in the car industry and artwork in museums, which are all exposed to natural or artificial lighting for long periods of time. These factors make efficient UV-protective coatings a necessary and valuable tool. 

Reducing or eliminating the damaging effects of solar and artificial UV radiation is a major challenge for materials scientists. One of the most widely used methods of UV protection is the dispersion of UV-absorbing molecules into a material. UV-absorbers must be colourless (or nearly colourless) compounds that show good photostability and can transform the absorbed UV energy into less harmful energy before reaching the substrate. Inorganic materials, based mainly on mixed metal-oxide films or particles, or organic molecules, such as phenolic molecules, can be used to absorb or scatter light. A well-known example of an inorganic UV-protector is titania (TiO₂), which is commonly used in sunscreens.

Coating photosensitive materials with UV-absorbing inorganic or polymer-based films has been studied extensively. Inorganic coatings, however, can only be used on heat resistant substrates, due to the high curing temperature required for their preparation. And polymer films have proven to have low photostability - the film itself degrading after prolonged irradiation with UV light.

Recently developed UV-protective coatings are based on a dispersion of UV-absorber molecules in a hybrid organic-inorganic sol-gel matrix. The sol-gel method allows the preparation of transparent, solid and porous inorganic material at low temperatures, and the incorporation of large amounts of organic UV-absorbing molecules in its pores, giving excellent UV protection. 

These coatings are just one micrometre thick, but can reduce the UV light reaching the substrate to less than seven per cent of the incident light. The coatings are also highly stable upon prolonged exposure to UV light and are fully transparent in the visible region of the spectrum. This means they can be used to coat a wide range of materials, without affecting the way they look. 

The lifetime of photosensitive materials can be increased from the scale of months to years, making the protective coatings very attractive indeed for commercial applications.

Read David Levy's review on 'Preventing UV-light damage of light sensitive materials using a highly protective UV-absorbing coating' in issue 8, 2007 of Chem. Soc. Rev.