Now You Know Whitening Agent: Properties, Function, Mechanism and Usages (Part-5)

Whitening Agent: Properties, Function, Mechanism and Usages (Part-5)

Authors: Md. Mosharaf Hossain
Kiriti Kingkar Mondal
Tawhidul Islam
Dept. of Textile Engineering
Primeasia University, Dhaka

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Method of whitening agent
There are two methods,which are generally used for this purpose, By using a blue tinting agent , which absorbs the yellow part of the light and reflected light appears to be of bluish tint. The total light reflected by this mean is less than the total incident light.

By using fluorescent optical brightening agents:- The OBA s (optical brightening agents ) are most widely used in textiles , paper, detergents and plastics . The optical brightening effect is obtained by the addition of light, which means that the amount of light reflected by the Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. The OBAs are effective only when the incident light has a significance proportion (such as daylight) of UV rays. When material treated with OBAs are exposed to UV black light source, it glows in the dark. Anionic OBA’s exhaust on cotton, wool and silk, cationic OBA’s exhaust on acrylic and certain polyesters and nonionic OBA’s are exhaust on all synthetics.

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 color measurement, since different hues of whiteness can be measured like any other color.

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.

Storage
OBAs are extremely sensitive to light and should not be left standing unprotected for long. Though standard solutions may be kept in dark for several days, fresh ones are to be prepared during application.

Exhaustion of OBAs
The exhaustion of fluorescent brighteners is dependent on several factors: