In order to improve the whiteness of the production, we want to increase the amount of whitening agent to achieve the goal, but the result is not white but yellowing fabric, through checking the data and analyzing the reasons, after many experiments, we found the yellowing point and solution.
1 Principle of fluorescent whitening agent
There are varieties of fluorescent whitening agent, whitening of various fiber need different brand of whitening agent, although the chemical structure and properties of various whitening agent is different, but the whitening principle of fiber or fabric is the same. The principle of whitening is mainly due to the whitener molecules containing a total of conjugate double bond system, good flatness, the special molecular structure under the sunlight can absorb sunlight, ultraviolet light (wavelength of 300 ~ 400nm) and violet light (wavelength of 420 ~ 500nm), or violet light and fiber mixed with the yellow light on the fabric into a white light,Thus the fiber or fabric becomes noticeably white.
2 The reason why whiteness decreases
（1）Since the intensity of yellow light on the fabric is limited, the complementary color light -- blue and purple light -- emitted by fluorescent whitening agent absorbing ultraviolet light is also limited to offset the yellow light (the main wavelength is about 570nm).
（2）The whitening effect of fluorescent brightener is only optical brightening and complementing, and cannot replace chemical bleaching. Therefore, the whitening effect of fluorescent brightener is not ideal for the fabric without bleaching.
（3）According to the principle of fluorescent whitening agent, the whitening effect of whitening agent mainly depends on the UV content in the sunlight and the concentration of fluorescent whitening agent on fiber or fabric.When the UV content is sufficient, the fluorescence whitening agent concentration on the fabric changes within a certain range, and the whitening effect increases with the increasing of the whitening agent concentration on the fabric, and the blue violet light intensity also increases, so as to offset some yellow light on the fabric, and the fabric becomes whiter and whiter.When the concentration of whitening agent increases to a certain proper concentration, the intensity of blue violet light emitted by it is exactly the same as the intensity of yellow light on the fabric, which exactly cancels each other, and the whitening effect is the best, and the whiteness value is the highest.But if increasing the dosage of white agent than optimal concentration, the fabric on the absorption of violet light intensity degrees more than offset the fabric on the strength of the yellow light, is an obvious whitening agent blue violet light reflected by, at the same time due to the raised all sorts of white agent group into different groups, different shades of the rendered (such as green light purple, red, blue, etc.) are clearly shown, a result of the combined action of the above factors, the increase of gray fabric, its whiteness subsequently decline.
（4）When there is acid or chlorine gas in storage places of the bleached fabric , the moisture content is high, it is easy to absorb water, due to the water is liquid, Under the molecular attraction of hydrophilic groups ,It has the tendency that the moisture regain of cloth cover tends to balance, this is the reason why fabric yellowing and expand into pieces, fabric spot by point and bleeding in the upper and lower layers of fabrics.
（5）Impure water quality, high hardness of water or heavy metalions may affect the whitening effect.
3 The yellow spots of the whitening agent
The optimum concentration of whitening agent is called the yellowing point of whitening agent.The yellowing spots of various fluorescent brighteners are different.For example, the yellowing point of fluorescent whitening agent DT(used for polyester) is 0.8%, that is, when the amount of DT whitening agent exceeds 0.8%, its whiteness value decreases with the increase of the amount.The yellowing point of fluorescent whitening agent VBL (used for pure cotton fabric) was 0.5 %.The yellowing spot of fluorescent whitening agent DCB(used for whitening acrylic fiber) was 0.8%.The yellowing point of fluorescent whitening agent CH(used for acrylic bulked yarn) is 3.3 %. BlankophorBBU (liquid,Bayer products, for pure cotton fabrics) had a yellowing point of 1.26%; Uvite ERN -- P(a product of CGY company used in polyester fabric) has a yellowing point of 0.8%; Uvite EBF 250 %C pulp MCGY products, used in polyester fabrics) yellowing point of 3 %.
4 Solutions
（1）A series of corresponding measures should be taken in the production process, such as in the process of boiling and bleaching, the wash after pickling must be sufficient;Soaping to minimize the residue of soap; Soft water should be used to whiten and the pH value should be alkaline (pH8~9).The fluorescent whitening agent should choose products with good acid resistance, such as Uvitex RT, CF, BT(ciba -jiagi company);Tinopal2B, SP (ciba -jiaggi), Leucophor B (sandex); BlankophorBBU (Bayer, inc.), and so on.
（2）The traditional process is: scouring and bleaching - hot water washing - acid - deoxidation - water washing.Glacial acetic acid was washed after hot water.Glacial acetic acid is Volatile acid, poor permeability, can only neutralize the caustic soda on fiber surface, it is difficult to penetrate into the fiber interior.Especially thick fabrics, often leave residual caustic soda in the internal fiber .Therefore, neutralizing acid can not only neutralize caustic soda on the surface of the fiber, but also neutralize caustic soda inside the fiber, so that the effect of complete neutralization can be achieved. Meanwhile, the time and temperature of over-acidity should be strictly controlled so as to completely neutralize. Therefore, the time of over-acidity should be changed from 15 min to 30 min.This is especially important for bleaching heavy fabrics with a mass of 190 g/m2 or more.
（3）Cationic softener, weak cationic softener and ordinary silicone oil can cause different degrees of yellowing of white fabrics.Anion softener generally does not cause yellowing and is preferred for white fabrics.
（4）The fabric should avoid contact with acid or gases such as chlorine .
（5）Soft water should be used.High water hardness will decline whitening effect. Acid, free chlorine and heavy metal ions such as copper and iron in water can significantly affect the whitening effect.
（6）Control the dosage of fluorescent whitening agent
