Optical brightening agents
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.
On the other hand, if both fabrics are seen side by side in a room at a distance of several yards from the window where there will be lower proportion of ultra violet light, the reddish shade will appear to be stronger. These strong variations will not be observed in the case of a neutral shade, ie, in sunlight the neutral white shade appears slightly superior to the reddish and slightly inferior to the greenish shade, where as the opposite effect is obtained in reduced day light. The same effect can be observed when comparing optically brightened material having a rough surface with such material which has a smooth surface. While an optically brightened smooth material of a reddish shade may appear equal in brilliance to that of a similar material with a greenish shade, it will be seen that in the case of a rough surface the reddish material will appear to be more brilliant.
Classification of Optical Brightening Agents:
The classification of of OBA can be based either on the chemical structure of the brightener or on its method of application. They can be broadly classified primarily in to two large groups. Direct {Substantive} brighteners and Disperse brighteners. Direct optical brightening agents are predominantly water soluble substances used for the brightening of natural fibers and occasionally for synthetic materials such as polyamide. Disperse optical brightening agents are mainly water insoluble and as with disperse dyes they are applied either to colored from an aqueous dispersion or they can be used for mass colouration. They are used for synthetic materials such as polyamide, polyester, acetate and occasionally on paper. From the chemical point of view they are classified according to their chemical structure. Chemical optical brightening agents are classified in to derivatives of stilbene, coumarin, 1,3 diphenylpyrazoline, derivatives of naphthalene dicarboxylic acid, derivatives of heterocyclic dicarboxylic acids, derivatives of cinnamic acid and substances belonging to other chemical systems.
Chemistry of Optical Brighteners:
About 80% of all OBAs produced are derived from stilbene derivatives, the latter absorbing in the ultra violet regions at (α) = 342 nm. All optical brighteners are dyestuffs, but in place of the chromophoric system which is the characteristic for dyes, it contains a fluorescening system and like a normal dye certain substituents which promote the affinity, depending on the type of fiber on which it is applied. In this manner, brighteners which are suitable for cotton are more or less substantive derivatives of diaminostilbene disulphonic acid. The stilbene derivatives can be present in two isomeric forms, ie in the Cis configuration and in the Trans configuration .Optical brighteners in the Trans form can be made both in the powder and liquid form.The Cis form, which is rapidly formed under the action of light from the trans form will not go on cotton and for this reason, the solutions of this whitener is protected against light. Many of the optical brighteners are derived from the heterocyclic compounds containing nitrogen atoms.
Measurement of Whiteness and Evaluation of OBA:
OBA are evaluated in the same way as dyes. Their concentration in powder and liquid form is determined by subjective {visual } comparison of the samples in daylight or under an ultra violet lamp, by titration with cetylpyridiniumchloride or spectrophotometrically against a standard of known concentration. Objects can only be seen as colored objects when they are illuminated by light. Since light is an electro magnetic radiation, which is either absorbed or reflected by the object which appears, colored due to the action of electro magnetic radiation on the human eye. Fluorescers or optical brighteners, as they are also called, are to improve the whiteness of textiles. Objective measurement of the whiteness and of the change in whiteness can be accomplished by colour measurement, since different hues of whiteness can be measured like any other colour.
To assist in a correct interpretation of such measurements, knowledge of the fundamentals of colorimetry is required, that is color measurement comprising the systematic compilation and evaluation of physical data supplied by suitable measurements. The wavelengths of the colors perceived in the range from violet { 400- 430 nm}{ shorter wavelengths are called ultra violet }, to 430- 485 nm blue, 485 to 570nm green, 570- 585 nm yellow, 585 to 610 nm orange, and above 610 nm red. There are many cases in practice where it is interesting to know the degree to which an optically brightened fabric has been bleached, as it is often important to know the original whiteness and the increase in whiteness achieved by the optical brightener. Besides, it is often interesting to know whether a shading dyestuff was used in addition to the brightener to enhance the whiteness. It is necessary to use a special spectrophotometer for the measurement of optically brightened specimens. Although the reflectance curve will not be in accordance with actual conditions up to a wave length of approximately 420nm {i.e. in the excitation range of the brightener}, the reflectance curve of the substrate will be reproduced accurately at higher wavelengths where fluorescence is no longer observed.