Optical brightener, Optical Brightening Agent (OBA), Fluorescent Brightening Agent (FBA) or Fluorescent Whitening Agent (FWA) is a compound which, when applied to a textile material, absorbs the short wavelength electromagnetic radiation (300-400 nm) which is invisible to the human eye, and converts it into visible light of longer wavelength (400-500 nm), which is emitted either as violet, pure blue or greenish blue.
Numerous materials especially textiles, both natural and synthetics are not completely white and efforts have been made since ancient times to free them from their yellowish tinges. This yellowish tinge from fabric is generally decolourized by different bleaching methods. However, even if bleaching processes are carried to the technically acceptable limits of damage to the fibres, they never succeed in completely removing this intrinsic colour. Whiteness in these substrates can be improved by the action of products named Optical Brightening Agents (OBAs), by intensifying the whiteness level or the colour of the material. OBA increases the apparent reflectance of the article in the blue‐violet region of the spectrum. Treated material remits more light in the visible region than does an untreated white sample and thus appears ‘whiter than white’.
When the electromagnetic radiation is combined with the more yellowish self-colour of a textile material, a brilliant white is produced. In coloured textiles, the presence of an optical brightener or fluorescent whitening agent will intensify the colours. These dyes absorb light in the green-yellow region of the spectrum, thereby reducing lightness. But, since at the same time they shift the shade of the yellowish material towards blue, the human eye perceives an increase of whiteness. Unlike blue dyes, optical brighteners offset the yellowish cast and at the same time improve lightness because their bluing effect is not based on subtracting yellow-green light, but rather on adding blue light.
Mechanism of Fluorescent / Optical brightening agent
The Fluorescent brightening agents operate by the phenomena of fluorescence. In order to understand the mechanism of fluorescent brightening agents, it is necessary to understand “Fluorescence”. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength and therefore lower energy than the absorbed radiation. 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.
Brightening is neither bleaching nor blueing. There are some types of brighteners, which will neither fluoresce in powder form nor in solution form and will only show this property after they have been applied on the fibre. 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 fibre. 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. To the trained observer, even bleached are white textile material has a slight yellow tinge. This small amount of yellow can give the impression of slight soiling and may detract from their aesthetic appeal. The presence of a slight amount of blue gives the impression that the textile material is whiter.
Classification of Optical Brightening Agents
OBAs can be broadly classified into two large groups; Direct (Substantive) brighteners and Disperse brighteners.
Direct optical brightening agents: These are predominantly water-soluble substances used for the brightening of natural fibres and occasionally for synthetic materials such as polyamide. Disperse optical brightening agents: These are mainly water-insoluble and as with disperse dyes, they are applied either to coloured 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 into derivatives of stilbene, coumarin, 1,3 diphenylpyraline, derivatives of naphthalene dicarboxylic acid, derivatives of heterocyclic dicarboxylic acids, derivatives of cinnamic acid and substances belonging to other chemical systems.
Measurement of Whiteness
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. There are many cases in practice where it is interesting to know the degree to which an optically brightened fabric has been breached, as it is often important to know the original whiteness and the increase in whiteness achieved by the optical brightener. Also, it is required 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.
Role of OBAs in Textile Wet-Processing
A fluorescent whitener should be optically colourless 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 coloured, i.e., blue-to-violet, the use of excess whitener always gives either a blue-to-violet or a bluish-green cast.
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 coloured impurities or their conversion into colourless substances. In chemical bleaching, impurities are oxidized or reduced to colourless products. Physical bleaching involves the introduction of a complementary colour whereby the undesired colour is made invisible to the eye in an optimal manner. Through colour compensation, the treated product appears whiter to the eye; however, it is actually grayer than the untreated material. With the aid of Optical brightening agents (OBAs), optical compensation of the yellow cast may be obtained.
Desired properties of OBAs for textiles use
• Should not have its own colour
• Good solubility and good substantivity for the textile substrate under OBA application.
• Should have good light as well as wet fastness properties.
• Effect of temperature on the exhaustion and build up properties.
• Application pH range and sensitivity towards change in pH.
• Should have good levelling and penetrating properties.
• Should not decompose to coloured products on exposure to atmospheric conditions as well as storage, and it should not absorb light in the visible region.
• Should be compatible and stable with electrolytes, finishing chemicals, auxiliary and process such as heat and temperature.
• Should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.
There has been a considerable increase in the use of optical brighteners. In addition, the number of fibre types and fibre mixtures has increased tremendously, which means that the processor must now cope with a greater variety of application methods. The development thus has led to a 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. Furthermore, 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. Sarex has developed various optical brightening agents for polyamide and acrylic. These OBAs imparts excellent whiteness to the substrates. They can be applied by both exhausts as well as pad application.