background and overview[1]
fluorescent whitening agent is a colorless organic compound that can produce fluorescence, which can improve the whiteness and gloss of substances. it is mainly used in textile, papermaking, plastics, synthetic detergent and other industries. the whitening effect of fluorescent whitening agents is based on the principle of optical complementary colors. after being treated with fluorescent whitening agents, yellowing substances can not only reflect visible light, but also absorb and convert ultraviolet light (wavelength 300~nm) other than visible light. it is reflected as purple-blue visible light (wavelength ~500nm). yellow and blue are complementary colors, thus canceling out the original yellow color of a substance, making it white and pleasing to the eye (see dyes). there are five main categories of optical brighteners classified according to their chemical structure. ①stilbene type: the most widely used type of fluorescent whitening agent, mainly used in cotton fiber, paper making, soap making and other industries. such as optical brightener optical brightener vba. ② coumarin type: it has the basic structure of coumarone and is used in celluloid, pvc plastics, etc., such as fluorescent whitening agent ws (c.i. fluorescent whitening agent 52), which has strong blue fluorescence. ③ pyrazoline type: used for wool, polyamide, acrylic and other fibers. for example, optical brightener wg (c.i. optical brightener 54) has green fluorescence. ④benzoxazepine: used in polyester, nylon and other fibers and polyvinyl chloride, polystyrene and other plastics. for example, the optical brightener dt (c.i. optical brightener 135) has red fluorescence. ⑤naphthalimide type: used for polyester, acrylic, nylon and other fibers, such as optical brightener at (c.i. optical brightener 162), which has blue fluorescence. the fluorescent whitening agent vba has good water solubility. the aqueous solution displays blue fluorescence and is anionic. the fluorescent whitening agent vba can not be bathed with cationic surfactants and cationic dyes, but can be mixed with almost all other types of chemicals. the additive is used in the same bath and has a good whitening effect on cotton fabrics under strong alkali and strong acid conditions.
whitening mechanism[2]
the whitening mechanism of the fluorescent whitening agent vba is summarized as follows: 1) the substance adsorbs the fluorescent whitening agent and can emit invisible light in the form of visible light. the object’s emissivity to light is enhanced, and the reflected light intensity exceeds the intensity projected on the the visible light on the object being processed is strong, so the object is visually whitened; 2) using the principle that blue and yellow are complementary colors in optics, the absorption wavelength range of the fluorescent whitening agent vba molecules under sunlight is 320 ~ the invisible light of nm excites the electrons in the chromophoric group from the ground state s0 to the transition state s1. since the excited state is unstable, the molecular energy state decreases. when it returns to the ground state s0 by radiating photons, it changes from the first excited singleton to the ground state s0. the lowest vibration energy level of the state returns to the ground state, emitting blue-violet fluorescence in the visible light band, thus making up for the yellow light on white objects, making people feel that the whiteness and brightness of white objects are significantly improved, and the colors of colored objects are more vivid and brighter .
main reference materials
[1] encyclopedia of china (chemical engineering volume)
[2] research progress on fluorescent whitening agents