UV absorbers include all organic and inorganic compounds which preferentially absorb UV radiation. These compounds have negligible absorption in the visible region and consequently a high light fastness. UV absorbers have to be distributed mono-molecularly in the substrate for maximum effect. Besides, they should meet other criteria such as :
- absorb effectively throughout the UV region (280-400nm)
- be UV stable itself
- dissipate the absorbed energy in such a way so as to cause no degradation or colour change in the medium it protects.
UV absorbers act on the substrate by several ways - by converting electronic excitation energy into thermal energy via a fast, reversible intramolecular proton transfer reaction; functioning as radical scavengers; and, functioning as singlet oxygen quenchers
Chemistry of UV absorbers
All organic UV absorbers applied industrially to textile materials in wet processing are derivatives of one of the three different structures given below :
a) o-hydroxybenzophenone
b) o-hydroxyphenylbenzotriazole
c) o-hydroxyphenyltriazine
Of the inorganic substances with UV absorbing characteristics, titanium dioxide deserves special mention
For all common fibres except acrylics, UV absorbers are now available that are like colourless dyes and are applicable with dyes by most of the usual methods. Separate application procedure is not needed. Care is needed only when they are applied with fluorescent whitening agents (FWAs)
Non reactive UV absorbers based on oxalic anilides, triazine or triazol compounds, salicylic acid esters, substituted acrylonitrile or nitrilo-hydrazones, emulsifying agents, water and polysiloxanes have also been reported
