Whitening principle of fluorescent whitening agent
1. Whitening principle
A large number of organic materials, natural or artificial textiles or other materials, such as paper, plastic, etc., do not look white. Usually they absorb more blue-violet light (400 ~ 480nm) than yellow light, so they may be more Or less yellowish.
Fluorescent whitening agents are dyed onto a substrate with a certain whiteness (such as textiles, plastics, paper, detergents, etc.) through a certain method. This substrate reflects when illuminated by sunlight (or a light source containing sufficient ultraviolet light). Visible light, while the molecules of the fluorescent whitening agent on the substrate absorb the invisible ultraviolet light in sunlight that is consistent with its characteristic frequency (about 340-380nm) energy, and then emits in the form of extremely bright and gorgeous fluorescence in the blue-violet (about 410 ~ 460nm) visible light region. This blue-violet fluorescence is mixed with the yellowish light on the substrate (yellow is the complementary color of blue-violet) to obtain white light, which has a whitening effect. Since the ability of the substrate whitened by the fluorescent whitening agent to reflect visible light is enhanced, the whiteness and brightness of the substrate are increased, making the whitened substrate look white, pleasing to the eye, and bright. It can also be seen that the whitening of fluorescent whitening agents is optical complementary color brightening.
Since the fluorescence of fluorescent whitening agents is generated by ultraviolet light, the ultraviolet light content of the irradiation light source has an impact on the fluorescence intensity and whitening effect of the fluorescent whitening agent. If the ultraviolet light content in the irradiation light is high, its whitening effect will be affected. It is particularly noticeable; but under light sources with low or no ultraviolet light content, the whitening effect is very small or no whitening effect can be seen at all.
2. Comparison between whitening with fluorescent whitening agents and whitening with chemical bleaching
After natural fiber and chemical fiber fabrics have been refined, most of the natural or artificial impurities have been removed, and the whiteness of the textiles can meet the general processing requirements, but they still have a slight yellow color. To eliminate this yellowish color and meet certain higher whiteness requirements, the following three methods are usually used: chemical bleaching, bluing and whitening, and fluorescent whitening agent whitening.
- Chemical bleaching
Chemical bleaching uses oxidants or reducing agents to destroy (remove) the color development system of pigments on textiles and other substrates to achieve the purpose of decolorizing and improving whiteness, making textiles and other substrates look whiter. Due to the oxidation or reduction effect of oxidants or reducing agents, in addition to the destruction of pigments, varying degrees of damage to textiles and other substrates will also occur, such as a decrease in strength.
- Top blue and white
Traditional bluing uses a blue inorganic pigment or organic dye as a bluing agent to treat textiles and other substrates. It absorbs yellow light in the spectrum and reflects more blue light. The blue light gives our eyes a whiter feeling. . However, because the compensating blue dye or pigment itself also absorbs part of the visible light, the total amount of reflected light is smaller than that of the non-blued substrate, so the brightness of the whitened substrate becomes smaller and looks dull or gray.
- Whitening with fluorescent whitening agents
It can be seen from the whitening principle of fluorescent whitening agents that the whitening of fluorescent whitening agents is optical complementary color and brightening, so it will not cause damage to the whitened substrate. It is currently the most effective and commonly used method to improve the whiteness of the substrate.
Obviously, the above methods are different from each other, but their purpose is to whiten slightly yellow textiles and other substrates; they are also related to each other. The whitening of fluorescent whitening agents cannot replace chemical bleaching. Textiles containing pigments If other substrates are treated with fluorescent whitening agents without chemical bleaching, the ideal whitening effect will not be obtained. Similarly, direct bluing treatment on substrates such as textiles that have not been bleached will not achieve a good whitening effect.
