Whitening Agent: Properties, Function, Mechanism and Usages (Part-8)

Whitening Agent: Properties, Function, Mechanism and Usages (Part-8)
Authors: Md. Mosharaf Hossain
Kiriti Kingkar Mondal
Tawhidul Islam

Dept. of Textile Engineering
Primeasia University, Dhaka

Previous Part

End uses of optical brighteners include:

  1. Detergent whitener (instead of bluing agents)
  2. Paper brightening (internal or in a coating)
  3. Fiber whitening (internal, added to polymer melts)
  4. Textile whitening (external, added to fabric finishes)
  5. Color-correcting or brightening additive in advanced cosmetic formulas (shampoos, conditioners, eye makeup)
Role of Optical Brightening agents in Textile Wet-Processing:
The operation of whitening, i.e., bleaching or brightening, is concerned with the preparation of fabrics whose commercial value is dependent on the highest possible whiteness. In bleaching, textile process houses are concerned with the removal of colored impurities or their conversion into colorless substances. In chemical bleaching, impurities are oxidized or reduced to colorless products. Physical bleaching involves the introduction of a complementary color whereby the undesired color is made invisible to the eye in an optical manner, eg, in bluing the yellow cast of substrates such as textiles, paper, sugar, etc. is eliminated by means of blue or blue-violet dyes. Through color compensation the treated product appears whiter to the eye; however, it is actually grayer than the untreated material.
OBA on textile material
OBA on textile material
With the aid of Optical brightening agents (OBAs), also referred to as fluorescent whitening agents, optical compensation of the yellow cast may be obtained. The yellow cast is produced by the absorption of short-wavelength light (violet-to-blue). With OBAs, this lost light is in part replaced; thus a complete white is attained without loss of light. This additional light is produced by the whitener by means of fluorescence. Fluorescent whitening agents absorb the invisible UV portion of the daylight spectrum and convert this energy into the longer-wavelength visible portion of the spectrum, i.e., into blue to blue-violet light. Fluorescent whitening, therefore, is based on the addition of light, whereas the bluing method achieves its white effect through the removal of light.

A fluorescent whitener should be optically colorless on the substrate, and should not absorb in the visible part of the spectrum. In the application of OBAs, it is possible to replace the light lost through absorption, thereby attaining a neutral, complete white. Further, through the use of excess whitener, still more UV radiation can be converted into visible light, so that the whitest white is made more sparkling. Since the fluorescent light of a fluorescent whitener is itself colored, ie, blue-to-violet, the use of excess whitener always gives either a blue-to-violet or a bluish green cast.

Many chemical compounds have been described in the literature as fluorescent compounds that provide a suitable whitening effect. Collectively these materials are aromatic or heterocyclic compounds; many of them contain condensed ring systems. An important feature of these compounds is the presence of an uninterrupted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds are derivatives of stilbene or 4,4-diaminostilbene; biphenyl; 5-membered heterocycles such as triazoles, oxazoles, imidazoles, etc. or 6-membered heterocycles, e.g. coumarins, naphthalimide, s-triazine, etc.

Textile substrates of natural or synthetic fibers are contaminated in the raw state by substances of varying degrees of yellowness. Bleaching is required to remove the yellowish cast. Chemical bleaching agents destroy the yellow coloring matter in fibers. However, even if bleaching processes are carried to the technically acceptable limits of damage to the fibers, they never succeed in completely removing this intrinsic color. To produce the color white, it is necessary to dye with a fluorescent whitener.

OBAs used in textiles can be divided into three categories
  1. Products containing sulfonic acid groups, corresponding to acid dyes, for cotton, wool, and polyamides;
  2. Cationic whiteners that behave in the same way as basic dyes, for polyacrylonitrile fibers; and
  3. Whiteners containing no solubilizing groups, corresponding to disperse dyes, for polyester and secondary acetate fibers.
This is not a strict division since nonionic OBAs can whiten polyacrylonitrile and polyamide, and certain cationic OBAs produce effects on polyester. For dyeing fiber blends such as viscose–polyamide, polyamide–Spandex, or polyester–cotton, only compatible OBAs may be used that do not interfere with one another or have any detrimental effect on fastness properties.

Before treatment with optical brighteners, due care should be taken during pretreatment, scouring and removal of sizing chemicals. Otherwise these will hinder the effect of OBA onto the substrate and may result in uneven brightening effect on the fabric.

In an exhaust procedure the fluorescent whitener is exhausted from a long liquor onto the substrate until an approximate equilibrium is reached between whitener in the bath and whitener on the substrate. In this procedure the equilibrium is biased primarily toward OBA on the substrate, ie, the highest possible degree of exhaustion is desired. Exhaust procedures are used for loose stock, yarns, woven fabrics, and knit goods which give poor or unsatisfactory results in padding processes, and for garments and garment parts.

Padding methods, i.e. application from short liquors, are increasingly important for whitening piece-goods. Woven fabrics or knit goods are passed in an unfolded, open-width state through a small trough charged with treatment liquor containing OBA and subsequently between squeeze rollers to express the liquor to a precisely defined liquor pick-up.

In conjunction with the increased use of synthetic fibers and blends of synthetic and natural fibers, and the modernization of application processes which has taken place simultaneously, the technique of textile whitening has been improved considerably.

In case of polyester fibers brighteners are applied in the form of fine aqueous dispersions. The brightener particles penetrate into the fiber in a state of molecular dispersion and they are held in the fiber by Vander waals forces. A part of finely dispersed particle forms a molecular solution in the dye-bath. Some of the finely dispersed particles may diffuse directly into the fiber. In addition to the dispersion and diffusion of the brightener, stability is also of vital importance.

During the drying and, if required, the heat treatment that follows, the fluorescent whitener is fixed on the substrate. OBAs used in padding procedures must have low substantivity during the padding operation. This is an important prerequisite for level whitening with no tailing.

Furthermore, fluorescent whiteners are applied in combination with bleaching and finishing steps. Fluorescent whiteners used in such processes must be stable and should not interfere with the operation. Whitening in combination with the finishing process is used primarily for woven fabrics of cellulosic fibers and their blends with synthetic fibers.

We at Matex offer Optical brightening agents under the brand name Megawhite. Highest brightness and best fastness can be achieved by using Megawhite series of brighteners. It meets all the requirements with regard to whiteness, hues and fastness properties.

Despite the difficulties faced by OBA producers and customers alike in the midst of the current global economic downturn there is still room for innovative producers to develop and to introduce onto the market new products offering more cost-effective, safe and environmentally sound solutions for the customer. There is every reason to believe that OBAs have a bright future. 

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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