There has been a considerable increase in the use of Optical brighteners. In addition the number of fiber types and fiber mixtures has increased tremendously, which means that the processor must now cope with a greater variety of application methods. There is hardly a white textile, or a white paper or hardly a household detergent which does not contain a brightener. The development, thus has led to continually increasing demand made on these products both by processors and consumers. They are required to be used on a variety of finishing processes and they should be compatible with practically all chemicals and auxiliaries used at different stages. Further more, good all round fastness properties and a good yield are also desired. In addition to this, different shades of whites are desired, as white shades are subject to fashion trends.


Historical Development of Optical Brightening Agents:


Numerous materials especially textiles, both classical {Cotton, wool, linen and silk} and synthetic {mainly polyamide, polyester and polyacrylonitrile }, are not completely white and efforts have been made since ancient times to free from their yellowish tinges. Bleaching in the sun, blueing and later chemical bleaching of textile and other materials increased the brightness of the products and eliminated to a certain extent the yellowish tinge to greyish yellow hue or the local impurity of the original or industrially treated material. When Optical brighteners first came up they were regarded as bleaching auxiliaries, which enabled a shorter or a milder bleach when used in very small quantities {Approximately 0.001 to 0.05% }. They were also called as Optical Bleaching Agents. Cotton and linen bleachers knew 200 years ago the effect of bleaching could be improved with the help of horse chestnut extracts. This is due to the fact the inner bark of the horse chestnut contains aesculin or esculinic acid, a glucoside which is a derivative of coumarin and which has ultra violet fluorescence. Scientist recommended aesculin for improving the whiteness on the basis of theoretical considerations. An aqueous solution of aesculin proved more suitable, but had two major draw backs. Firstly it was not fast to washing and secondly aesculin on the fiber was very sensitive to light. Then came the introduction of organic products based on Diaminostilbine sulphonic acid derivatives.



Fluorescence of Optical Brighteners: {Mechanism}


Brightening is neither bleaching nor blueing. Fluorescent colours will reflect more light than they can absorb from the visible range of the spectrum. Whiteness can also be increased by using substances which would give colourless solutions but were strongly fluorescent. Fluorescence is produced by the absorption of radiation having a high energy on the part of the molecule, which re-emits this radiation as a radiation of lower energy i.e. of longer wavelength, the difference in energy being transformed in to kinetic energy. To enable a molecule to fulfill this function, it must be built according to certain structural principles. For example Anthranilic acid has very strong blue violet fluorescence in its aqueous solution, but nevertheless unsuitable as a brightener.


Most of the brighteners will hardly fluoresce in powder form; their fluorescence will only appear in solution. There are some types, which will not fluoresce in solution and will only show this property after they have been applied on the fiber. Thus, it can be concluded that fluorescence is not only dependent on the structure of the molecule but also on its condition. Whether a fluorescent substance is suitable as a brightener can only be determined after it has been applied to the textile fiber. Apart from this the product must meet certain demands in respect of properties such as fastness to washing and light etc. On comparing different textile fabrics treated with different brighteners and possessing approximately the same brightness, differences in hue can be detected, since the human eye is particularly sensitive to differences in whiteness. If an optically brightened fabric with reddish white shade is compared with another fabric having a greenish white shade, both of which appear to be equally brilliant if viewed in daylight which is incident from a northerly direction, it will be seen that the greenish shade will appear more brilliant than a reddish one in bright sunlight.