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Talk about the whitening principle of fluorescent whitening agent

To understand the whitening principle of fluorescent whitening agents, we must first understand the relationship between light and color. The ancient Greek philosopher Aristotle believed that light is color, and only when there is light can there be color, because without it there would be no scene. Light is the only source of color for all objects. When light stimulates the human retina, it forms a color vision. Without light, there is no color. Light is an electromagnetic wave from the sun, other celestial bodies, flames, and artificial light sources. It has a wide range. When the wavelength is too long (infrared region) or too short (ultraviolet region), the human eye cannot distinguish it.

Only the narrow light region with a wavelength range between 380 and 760 nanometers is the visible light region of the human eye. It has been found that the longest wavelength of several evenly distributed colors in the visible light region is red, followed by orange, yellow, green, blue, and purple. We can see this effect in the rainbow after the rain, or when the light It can also be seen when passing through a prism to separate into a spectrum.
The type of light source, the way objects absorb and reflect light can determine the color of the objects we see. When light strikes an opaque object, the surface of the object absorbs most of the visible light and reflects part of the visible light. The color of the reflected light is the true color of the object. For example, a piece of lemon yellow and yellow fabric will absorb almost all colors in the light except yellow. White objects reflect almost all colors in light, while black objects absorb almost all colors. The inherent color of any object is only visible under white light. In fact, the light itself is not completely colorless.
Fluorescent brig


htener is a color conditioner with brightening and brightening effect, widely used in many fields such as papermaking, textile, detergent and so on. There are about 15 basic structural types of fluorescent whitening agents, and nearly 400 structural types. There are two types of fluorescent whitening agents allowed to be added to laundry detergents in China: stilbene biphenyls (such as CBS, etc.) and bistriazine amino stilbene (such as 33 #, etc.).
The principle of the fluorescent whitening agent is to absorb the invisible ultraviolet light in the light and emit a visible blue light. After superimposed with the yellow light emitted by the fabric, the white light is complemented to form white light, which increases the white light emitted by the fabric. Become brighter, this is the brightening effect. Fluorescent whitening agents can absorb invisible ultraviolet light (wavelength range is about 60-380 nm), converted into longer wavelength blue light or purple visible light, thus can compensate for the undesirable yellowish in the matrix, while reflecting The original incident wavelength is more visible light in the 400-600 nm range, thus making the product appear whiter, brighter and more vivid.
It can be seen that the fluorescent whitening agent is different from the blue whitening mechanism. The former is to add light and the latter is to reduce light. When the light is added strongly, the color reversal rate can greatly exceed 10%. In the past, people thought that the object that could reflect 100% of incident light should be the whitest. After the advent of optical brighteners, people obtained this whiteness. In terms of people’s visual consciousness, white with a blue hue appears whiter than white with a yellow hue. Therefore, within a certain limit, increasing the blue saturation can increase the feeling of white.

Since the fluorescent whitening agent has a whitening function, how does it work? The whitening properties of fluorescent whitening agents are determined by the specific structure of their molecules. Its chromophoric group has a conjugated system where π → π * transition can occur. The most common of these systems are benzene ring, naphthalene ring, triazine ring, vinyl group, five-membered heterocyclic ring and some other fused ring systems. Electronic systems with a small degree of conjugation generally absorb only very short wavelengths of light. As the conjugated system increases, the wavelength of light that can be absorbed increases, the easier the electrons are excited and the greater the fluorescent efficiency of the whitening agent, thus satisfying Requirements for optical brighteners.
In order to improve the comprehensive performance of fluorescent whitening agents, it is also necessary to introduce color-assisting groups, including electron-pushing groups (such as alkoxy, alkyl, substituted amino, etc.) and electron-pulling groups (such as sulfonic acid, cyano, carboxyl, etc.) ), These groups will affect the nature and intensity of fluorescence. Some groups have little effect on the color development system, but can change the application performance of the fluorescent whitening agent and its affinity for fibers, plastics, etc. The structure of fluorescent brighteners cannot contain nitro, nitroso or diazo groups. Although they can improve the fastness to sunlight, these groups will weaken or even quench the fluorescence completely.
So how does the fluorescent whitening agent work in the whitened substance? Fluorescent brighteners appear differently in different substances. In the field of detergents or textile printing and dyeing, the whitening agent is water-soluble. It is completely dissolved in the matrix in the molecular state, and then adsorbed on the clothing fibers during the washing process to play a whitening effect. Its state in the plastic matrix is ​​a process of dissolution and aggregation. It is dispersed and even dissolved in the matrix with various additives through shear mixing after the plastic matrix is ​​melted. Therefore, there are two possibilities for fluorescent whitening agents in plastics: as in fiber whitening or detergents, they are uniformly dissolved in the polymer matrix in the ideal state of single molecules, or exist in the form of fine particles or other forms.

Fluorescent brightener...
When actually using the fluorescent whitening agent, the biggest problem encountered is the yellowing phenomenon after the fluorescent whitening agent is added. This is caused by the poor solubility of the substrate to the fluorescent whitening agent. It is used in a matrix with low solubility At this time, because the amount of the fluorescent whitening agent added is difficult to control, and also has the effect of migration in the matrix, the whitening matrix of the fluorescent whitening agent tends to be yellowed easily. This article provides Credit  Chemicals.

Application of several water-soluble fluorescent whitening agents in papermaking

Post time: Apr-22-2020