Whitening Agent: Properties, Function, Mechanism and Usages (Part-2)

Whitening Agent: Properties, Function, Mechanism and Usages (Part-2)
Authors: Md. Mosharaf Hossain
Kiriti Kingkar Mondal
Tawhidul Islam

Dept. of Textile Engineering
Primeasia University, Dhaka

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Chemistry of whitening agent:
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.

Fluorescence is produced by the absorption of radiation having a high energy on the part of the molecule, which re – emits this radiation of lower energy i.e. of longer wave length, 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 structure principles. For example Anthranilic acid has very strong blue violet fluorescence in the aqueous solution, but nevertheless unsuitable as a brightener. Most of the brightener 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 depended on the structure of the molecule but also on its condition. Whether a fluorescent substance is suitable as 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 comparering different textile fabrics treated with different brighteners and processing approximately the same brightness difference in hue can be deleted, since the human eye is particularly sensitive to difference in whiteness. If an optically brightened fabric with radish 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 then the radish one in bright sunlight.
Example of an optical brightener's structure
Fig: Example of an optical brightener's structure
Textile materials like cotton or cotton/polyester blends are almost always pre-brightened when manufactured. This is because the printing and colors will be brighter and more attractive if applied to bright fabric. Moreover, the washing agents and commercial detergents available nowadays commonly have optical brighteners combined in them and while washing the fabric gets whiter. It is known that, in most cases, the presence of an optical brightener causes a decrease in the light fastness of a dyed fiber. These compounds can also have a direct photochemical effect on the fiber in the absence of dyes, as in the case of wool. Optically whitened wool will yellow on exposure to light much faster than untreated wool by a photocatalytic process. However, in colored textiles, sometimes a difference on hue is detected already in the first domestic wash, even in the case of solid colors. This undesirable effect has been a considerable obstacle for several textile industries, with clients and consumers becoming more and more demanding. Therefore, it’s important to study the effect in order to avoid, as much as possible, similar situations.

Commercial name of whitening agent in textile

Product Name
Strength
Application
Shade
Area of application
Kolorcron BA Powder
450 E Value
Cellulose-Exhaust in H2O2 bleach. Polyami- de/Wool/ Silk –Exhaust Single bath scouring bleaching at high Temp. with Hydro.
Neutral to Bluish White
Cellulose, Polyam- ide, Wool/ Silk
Kolorcron WHN
180 E Value
Cellulose-Exhaust in H2O2 bleach. Polyami- de/Wool/ Silk –Exhaust Single bath scouring bleaching at high Temp. with Hydro.
Neutral to Bluish White
Cellulose, Polyam- ide, Wool/ Silk
Kolorcron 4BB
125 E Value
No Padding application Exhaust at 80* C & above
Bluish Violet White
Cellulose Fibers
Kolorcron BA Liquid
125 E Value
Cellulose-Exhaust in H2O2 bleach. Polyami- de/Wool/ Silk –Exhaust Single bath scouring bleaching at high Temp. with Hydro.
Neutral to Bluish White
Cellulose, Polyam- ide, Wool/ Silk
Kolorcron CXT Liquid
100 E Value
Padding & Low Temp Exhaust application. Single bath scouring and Bleaching at high Temp.
Bluish Violate White
Cellulose Fiber & Cellulose Blend
Kolorcron S
70 E Value
No padding application Exhaust 40* - 60* C & regenerate
Bluish White
Cellulose Fiber & Cellulose Blend

Chemical Constitution of Optical Brighteners
Optical brighteners are usually derivatives of:
  • Triazine-stilbenes (di-, tetra- or hexa-sulfonated)
  • Coumarins
  • Imidazolines
  • Diazoles
  • Triazoles
  • Benzoxazolines
  • Biphenyl-stilbenes
Brighteners can be "boosted" by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.

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.

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