When Does Color Fade?
An article’s color fades when subjected to a chemical reaction in which oxygen is released. This oxygen combines with the natural coloring matter or with the dye to produce a colorless compound. Thus the color of the article becomes paler and is finally taken out altogether, or bleached.
The most famous bleaching agent is the sun, and the process by which it makes color fade is called photo degradation, which is the alteration of materials by light. Typically, the term refers to the combined action of sunlight and air.
Photo degradation is usually oxidation and hydrolysis and does not apply to how materials may be aged or degraded via infrared light or heat, but does include degradation in all of the ultraviolet light wavebands. There are light absorbing color bodies called chromophores that are present in dyes. The colors we see are based upon these chemical bonds and the amount of light that is absorbed in a particular wavelength.
Ultraviolet rays can break down the chemical bonds and thus fade the colors in an object – it is a bleaching effect. The old fashioned method of bleaching textiles by laying them out flat in the sunlight is still used in places. Some objects may be more prone to fading, such as dyed textiles and watercolors. Other objects may reflect the light more, which makes them less prone to fade. Paints, inks and dyes that are organic are more susceptible to photo degradation than those that are not.
Chemical bleaching is much quicker, but needs to be controlled with great care. In about 1790 it was discovered that chlorine gas and its compounds were good bleaching agents and chloride of lime (calcium hypochlorite), made by the action of chlorine on slaked lime came into use. This “bleaching powder”, as it was called, was dissolved in water.
It was removed when the bleaching was complete by washing the article or immersing it in neutralizing solutions. More modern bleaching agents are sodium hypochlorite, hydrogen peroxide and sulphur dioxide.
Photo degradation of plastics and other materials can be inhibited with additives, which are widely used. These additives include antioxidants, which interrupt degradation processes. Typical antioxidants are derivatives of aniline. Another type of additives are UV-absorbers. These agents capture the photon and convert it to heat. Typical UV-absorbers are hydroxy-substituted benzophenones, related to the chemicals used in sunscreen.