The whitening principle and important applications of fluorescent whitening agent
In the textile industry, the whiteness of the fiber itself often fails to meet the aesthetic requirements of people, especially natural fibers, which vary greatly in whiteness due to different growth environments and growth cycles. White material generally has a slight absorption of blue light from 450 to 480nm in the visible light, which causes a lack of blue, making it slightly yellow and gives a sense of old. For this reason, people have taken different measures to make items whiter and more colorful. Before the emergence of fluorescent whitening agent, there are two main whitening methods usually used.
1, add blue whitening method. This method can play a whitening effect, but the effect is limited, and because the total amount of reflected light is reduced, so that the items darken.
2, chemical bleaching method. Mainly through the redox reaction and the material fading, but the cellulose will cause some damage, and the bleached fabric often with yellow, but affect the whitening effect.
Fluorescent whitening agent can make up for the shortcomings of the traditional whitening method, and shows great superiority. Fluorescent whitening agent can absorb the higher energy near-ultraviolet light, so that its molecules into the excited state, and then be excited molecules leap to the lower energy ground state, and emit fluorescence. The fluorescent wavelength of the radiation is about 450nm blue light, the yellow color of the yellowish items can be compensated by the blue light reflected from the fluorescent whitening agent, thus increasing the apparent whiteness of the items. Because the intensity of the emitted light exceeds the intensity of the original visible light projected on the treated fabric, a slightly chromatic whitening effect is produced.
The use of fluorescent whitening agent is very wide, from the initial use only for textiles, to now widely used in paper, detergents, plastics, coatings, inks, leather and many other fields, the use of fluorescent whitening agent and the amount is still expanding. Fluorescent whitening agent with practical value, in addition to absorbing ultraviolet light and emitting violet-blue fluorescence and high fluorescence efficiency, itself must be close to colorless or slightly yellow, with the characteristics of ordinary dyes, good affinity for whitened fabrics such as fibers, good solubility or dispersion properties and better fastness to washing, sunlight and ironing properties. In countries around the world, the proportion of fluorescent brighteners used in different industries differ, but the use of the proportion of the order is basically the same: that is, mainly used in detergents, followed by paper, textile third, plastics and other areas of the amount of smaller. Fluorescent brighteners can also be used in combination with UV absorbers to solve the problem of weathering whitening of some polymer products.
Fluorescence reaction refers to the substance to accept the invisible UV light irradiation is radicalized, and then the energy of the radicalized into the visible light release process, which is a photoluminescence of cold luminescence phenomenon. Fluorescents can produce fluorescent reactions, but the things that produce fluorescent reactions, not only fluorescent agents. In the fluorescent light will show fluorescent reaction of the substance is not all fluorescent whitening agent. Some high-quality daily chemical products with natural ingredients such as vitamin E and glycerin will also be exposed to 365nm violet light to show a fluorescent reaction. Therefore, through the purple light irradiation can only prove the presence of fluorescent reaction, but can not identify whether it is a fluorescent whitening agent.
Internationally, fluorescent whitening agent is considered a white dye, and each structure of fluorescent whitening agent has its corresponding dye index number. In China, fluorescent brighteners are usually considered an important functional finishing aid. According to the type of chemical structure, fluorescent brighteners used in the textile industry mainly include six categories: 1, bis(triazinylamino)stilbene type; 2, stilbene biphenyl type; 3, bis(benzo)azole type; 4, stilbene type; 5, pyrazoline type; 6, coumarin type. When using fluorescent whitening agent, the appropriate fluorescent whitening agent must be selected according to the chemical composition and physical properties of the fiber, so as to obtain satisfactory whitening effect.
As mentioned above, the first major use of fluorescent brighteners is in the washing. Fabric detergent is a necessity in the daily life of residents. Surfactants and additives are the main components of fabric detergents, which play a vital role in the performance of detergents and the effect of washing clothes cleanliness. Textile printing and dyeing factories on the factory fabric fluorescent whitening treatment is a widespread and common technology, but it does not achieve permanent washing fastness, that is, as people wear and wash, fluorescent whitening agent will fall off, clothing will be old, even yellowing, etc.. Adding a certain amount of fluorescent whitening agent in the detergent with the right variety can not only increase the whiteness or brightness of the fabric being washed and improve the washing effect, but also improve the appearance of the detergent. At present, 50% of the world’s consumption of fluorescent brighteners is used in the detergent industry. Therefore, it can be said that fluorescent brighteners have become one of the important additives indispensable in the formulation of fabric detergents. When washing clothes, a detergent will generally face various fibers of fabrics at the same time, such as cotton, polyester-cotton, pure polyester, wool, silk, viscose, nylon, etc.. To achieve a better whitening and brightening effect on different fibers at the same time, there are requirements for the selection of fluorescent whitening agents. Bis-styryl biphenyl class, bis-triazine aminostilbene class of fluorescent brighteners are two widely used on the textile structure of the product, the textile fiber whitening effect is good, more importantly, it is applicable to a wide range of fiber varieties.
After years of scientific experiments and practical applications show that fluorescent whitening agent is a safe and harmless chemical products, fluorescent whitening agent is also difficult to absorb into the human body through the skin and cause toxic risks. Such substances are generally accepted internationally as conventional additives for paper, plastic products, textiles, clothing detergents, etc.
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