background and overview[1]
fluorescent whitening agents are called optical whitening agents, or whitening agents for short. organic substances that can improve the whiteness of fabrics, paper, etc. the whitening agent can absorb ultraviolet light with a wavelength of 330 to 360 nanometers, and the reflected light wavelength is blue-violet light with a wavelength of ~440 nanometers, which is complementary to the yellow light originally reflected by the fabric that is still yellowing after bleaching, so it is compatible with added to white color, making the fabric white. after whitening treatment, part of the ultraviolet light is converted into visible light and reflected, and the total amount of reflection is increased compared to that without treatment, so the brightness of the treated fabric also increases, making the white fabric whiter and the colored fabric increased in brightness. fluorescent whitening is different from chemical bleaching in that it will not damage the color and strength of the fabric. however, for fabrics and papers with dark background colors, if they are directly treated with a whitening agent without bleaching, good results will not be achieved. the organic compounds used as fluorescent whitening agents have at least 4 conjugated double bonds in their molecular structure, such as -c=c-c=c-c=c-c=c- or -n=c- c=c—c=n—c=c—. there are four common structures of chromophores: ①stilbene type, such as the whitening agent vbl. mainly used as cotton fiber whitening and whitening component in advanced synthetic detergents. ② coumarin type, such as whitening agent ws. it has strong blue fluorescence and is a whitening agent for polyamide, wool and silk. ③azole type. mainly used for whitening wool, polyamide and acrylic fiber. ④naphthalenedicarboximide type, such as apl. it can be used to whiten polyamide, polyester fiber and pvc plastics. whitening agents are mainly used in the textile printing and dyeing industry to improve the whiteness and luster of various natural and synthetic fiber fabrics. in recent years, its application in papermaking, plastics, leather, soap, synthetic detergents and other daily necessities industrial sectors has also grown rapidly. triazol-stilbene based optical brighteners, such as c.i. optical brightener 40 (blankophorg) or c.i. optical brightener 191 (blankhor cc), in addition to the above properties, are also stable against chlorine bleaching.
whitening principle[2]
white substances generally slightly absorb the blue light of 450~480nm in visible light, resulting in a lack of blue, making it slightly yellow and giving people an old feeling. for this reason, people have taken different measures to make items whiter and more colorful. the commonly used methods are: ① adding blue and whitening method. adding blue can whiten, but the effect is limited, and because the total amount of reflected light is reduced, the color of the item becomes darker. ②chemical bleaching method mainly causes substances to fade through redox reactions, so it will cause certain damage to cellulose, and the bleached objects are often yellowish, which affects the whitening effect. the fluorescent whitening agent discovered in the 1930s made up for the shortcomings of the above methods and showed great advantages. fluorescent whitening agent 191 absorbs higher-energy near-ultraviolet light to cause its molecules to enter an excited state, and then the excited molecules transition to a lower-energy ground state and emit fluorescence. due to the energy loss, the wavelength of the radiated fluorescence becomes longer, which is about 450nm blue light. the yellow color of the yellowed items can be compensated by the blue light reflected by the fluorescent whitening agent, thus increasing the apparent whiteness of the items. since the intensity of the emitted light exceeds the intensity of the original visible light projected on the object to be treated, a slightly colored whitening effect is produced. fluorescent whitening agent 191, which has practical value, in addition to absorbing ultraviolet light and emitting purple-blue fluorescence and having high fluorescence efficiency, must also be close to colorless or slightly yellow and have the characteristics of ordinary dyes. substances such as fibers have good affinity, good solubility or dispersion properties, and good fastness properties such as washing, light fastness and ironing resistance. the whitening properties of fluorescent whitening agent 191 are determined by the special structure of its molecule, and its intramolecular groups will affect the nature and intensity of fluorescence. some groups have little impact on the color development system, but can change the application performance and affinity of optical brighteners to fibers, plastics, etc.
main reference materials
[1] social dictionary of science and technology·chemistry
[2] application and development trends of optical brighteners