Whitening of Textile materials
(Working mechanism of Whitening agents (Bleaching agent, Blue and OBA/FWA)
Materials that evenly reflect most of the light at all wavelengths striking their surface appear white to the human eye. Natural fibers, for example, generally absorb more light in the blue region of the visible spectrum (‘blue defect’) than in others because of impurities (natural pigments) they contain. As a result, natural fibers take on an unwanted, yellowish tone. Synthetic fibers also have this yellowish tone, although not as pronounced.
Whiteness in substrates can be improved by-
(1) Increasing reflection (reflectance) or
(2) Compensating the blue defect.
Bleaching has both of these effects to some extent, but invariably leaves behind part of the yellowish tone. Even the most careful bleach cannot remove all traces of a yellowish tone. Before the arrival of optical brightening agents (OBA), common practice was to apply small amounts of blue or violet dyes (called ‘bluing’) to boost the visual impression of whiteness. 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 dyes, OBAs counterbalance the yellowish tone 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. OBAs are virtually colorless compounds which, when present on a substrate, absorb primarily invisible ultraviolet light in the 300-400 nanometer (nm) range and re-emit in the visible violet-to-blue fluorescent light.
This ability of OBAs to absorb invisible short wavelength radiation and re-emit in the visible blue light which imparts a brilliant whiteness to the light reflected by a substrate, is the key to OBAs effectiveness.
See attachment for better understanding-
