OPTICAL BRIGHTENING AGENT — A Source of Whitener
Description:-
The Optical Brightening or Optical bleaches/dyes are ‘Finishes’ that gives an effect of ‘Whiteness and brightness’ because of the way in which they reflect light. These brightening compounds contain fluorescent colourless dyes causing Blue light to be reflected. This reflectance of Blue light depends on the dosage of Optical Brightening Agent.
TYPES OF Brightening Agents:-
There are two types of Brightening Agents viz., the Optical Brightening Agent (OBA) and Fluorescent Whitening Agent (FWA):-
1) In Optical Brightening Agents (OBA) — the Blue-tinting agent absorbs the yellow part of the light, so the reflected light appears to be of a Bluish Tint. The total light reflected by this way, is less than the total incident light. This OBAs used in Fabrics viz., Cotton, Polyesters, Acrylic, Coatings, Inks, etc.
2) In Fluorescent Whitening Agents (FWA) — This FWA is a complete range of neutral and shaded whiteners with the ability to create local shades versions and tailor-made. The FWAs are mostly used in Polyamides, wool, silk and Paper, in order to remove the yellowness and bring bright whiter shade. The FWA is also called as Optical Brighteners (OBA) as both are having similar properties of whiteners.
USES OF OBA:-
The reason for using Optical Brightening Agents is to compensate the yellowish color by improving whiteness and enhancing the brightness of the substance. By absorbing the ultraviolet radiation, Optical Whiteners function by re-emitting the blue light. The blue light that is reflected reduces yellowish tint and gives a Bright whiter-than-white appearance. The OBAs are used on different substrates, with a view to enhance bright white color i.e.:-
- Textile/ Fabric whitening — I.e. Cotton, Nylon, Silk, Wool, Acrylic, Polyester and Synthetic materials.
- Laundry Detergents & Soaps.
- Cosmetics — Nail polish, Eyeshadows, etc
- Papers, Plastics, Paint — Cellulose, Printing Inks, Coatings and Polymers.
The Optical Brighteners are most commonly used for Laundry detergents in order to make the clothes appear bright whiter, cleaner and to reduce the yellowishness. In Textile manufacturing, the finishing refers to the processes that convert the woven cloth into a usable material. The grey cotton fabric in its raw state, has lots of impurities and requires further treatment to develop the Full Finishing Textile.
EFFECTS OF OBA:-
The Optical Brightening effect is achieved by radiation of light. The light that is reflected by the whitening agents. by absorbing greater energy radiation of ultraviolet to violet region on the part of characteristic molecules, emitting lower energy radiation in the blue region in visible spectrum reacting with the appearance of yellow color.
PRECAUTIONARY MEASURES.
Before treatment of fabrics with Optical Brighteners, due care is taken during the Pre-treatment, Scouring and Removal of Sizing Chemicals, as these hinder the effects of OBA on the substrate and may result in uneven Brightening effects on the fabric. The following are the Finishing process applied in Textile Bleaching:-
*Singeing. This process is designed to burn-off surface fibers from fabrics to produce smoothness. The fabric is then passed over a plate heated by Gas flames.
*De-sizing. Depending on size of cloth used, the cloth is steeped in a dilute acid and rinsed.
*Scouring. This is a washing process that is chemically carried out on Cotton Fabrics to remove the Natural wax/ starch and non-fibrous particles, soiling or dirt from the fibers. This process is carried out in Iron vessels called Kiers. The Fabric is then boiled in an Akali which forms soap with free fatty acids if appropriate reagents are used like Bleaching to improve whiteness.
*Bleaching. This process removes the remaining traces of impurities in Cotton and improves whiteness by natural coloration.
CHEMICAL PROCESS OF FABRICS.
The washing process chemically is carried out on Cotton fabrics with a view to remove natural wax/ starch, non-fibrous particles, soiling or dirt from the fibers. The Fabric is subsequently, boiled in an Alkali which forms soapy along with free fatty acids using appropriate re-agents like bleaching This process improves the color into whiteness. In the Whitening Process i.e. where Bleaching or Brightening is concerned, preparation of fabrics, where the commercial value depends on highest possible whiteness, in bleaching. The Textile Houses are concerned with removal of colored impurities and their conversion into colorless substances.
TYPES OF BLEACHING.
1) Chemical Bleaching. In this process, impurities are oxidized and reduced to colorless products.
2) Physical Bleaching. In this process, an introduction of a complementary color where the undesired color is made invisible to the eye in an optical manner, for e.g. in Blueing the yellow cast of substrates such as Textiles, Papers, etc, is eliminated by means of Blue-violet dyes through color compensation. The treated product thus, looking brighter and whiter to the eye, but it is actually greyer than the untreated material. The yellow cast is produced by absorption of short-wavelength light i.e. violet-to-blue. With Optical Brightening Agents, this Lost Light is partly replaced, thereby a complete whiteness is achieved without loss of light.
WHY DO WE USE OBA.
The Textile Fibers do not appear perfectly White due to the presence of certain Colored impurities. Hence, Whitening with Optical Brightening Agents is used to “eliminate the yellowish tint” in the materials. This Optical Brightening Agents reacts with the yellowish color in the fabric and increases the reflection of Blue light radiation. This reflection of blue light converts the invisible short-wave ultraviolet rays of sunlight into a visible blue light and effectively neutralizing the yellow colour of the white materials, thereby producing a considerable amount of Whiteness.
INDUSTRY USING OBA IN LARGE SCALE.
The largest users of Optical Brighteners are mostly found in Detergents in the proportion of 0.05%-0.3%.
BENEFITS OF OBA IN TEXTILES
The OBAs used in Textiles are divided into Three different categories viz;
Products containing -
*Sulphunic acid groups corresponding to acid dyes for cotton and wool.
*Polyamides Cationic Whiteners reacts in the same way as basic dyes for Polyacrylonitraile Fibers.
*Whiteners that do not contain solubilising groups corresponding to disperse dyes for Polyester and secondary acetate fibers.
*The textile subtrates of natural or synthetic fibers are contaminated in a raw state by substances of varying degrees of Yellowness. With the increased use of synthetic fibers and Modernisation of application processes, the techniques of Textile Whitening has improved considerably to a larger extent. The manufacturers of OBA are facing difficulties in manifolds amids the fall of current Global Economic system, but they still find ideas of Innovative production of new products in the market, which is economic, safe and environmentally sound.
CONCLUSION.
It is concluded that there is every reason to believe that “Optical Brightening Agents” will have a demanding Bright future ahead for requirements of whitening agents, in all the fields where Brighteners are constantly used.
