The effect of fluorescent whitening agent on fabric whiteness

There are many varieties of fluorescent brighteners, and certain brands of brighteners are used for the brightening of various fibers. Although the chemical structure and properties of various brighteners are different, the principle of brightening fibers or fabrics is the same. The whitening principle is mainly due to the conjugated double bond system in the molecule of the whitening agent, which has good planarity. This extraordinary molecular structure can absorb the ultraviolet rays (wavelength 300 ~ 400nm) under the sunlight. The blue-violet finish (wavelength is 420-500nm), the blue-violet finish mixes with the yellow finish on the fiber or fabric to become white finish, which makes the fiber or fabric obviously white. The brightening effect of fluorescent whitening agent is only the brightening and supplementing color in the whole school, and can not replace chemical bleaching. Therefore, if the fabric without bleaching is directly brightened with fluorescent whitening agent, the whitening effect is not ideal. of.
From the principle of the fluorescent brightener, the whitening effect of the brightener mainly depends on the content of ultraviolet rays in the irradiation day and the concentration of the fluorescent brightener on the fiber or fabric. When the content of ultraviolet light in the irradiating day is sufficient and the concentration of fluorescent brightener on the fabric changes within a certain range, the whitening effect will increase with the increase of the concentration of brightener on the fabric; but when the concentration of brightener on the fabric increases; When the concentration is increased to an appropriate concentration, the whitening effect is the best, and the highest whiteness value can be obtained. If the amount of the whitening agent exceeds the optimal concentration, not only the whitening effect on the fabric is not improved, but on the contrary, it decreases. (That is to say, the fabric is yellowed, but the whiteness decreases). At this time, the optimal concentration of the brightener is called the yellowing point of the brightener.
So, why does the whitening effect no longer increase as the concentration of fluorescent whitening agent on the fabric increases? The main reason is: because the intensity of the yellow finish on the fabric is limited, the yellow finish is offset (the dominant wavelength is about 570nm) ) The complementary color of the yellow finish which is emitted by the fluorescent brightener to absorb ultraviolet light-blue purple finish is also limited. As the concentration of the fluorescent whitening agent on the fabric increases, the intensity of the blue-violet finish emitted by the fabric also increases, thereby offsetting part of the yellow finish on the fabric, so that the intensity of the yellow finish gradually decreases, and the fabric becomes whiter and whiter. When the concentration of the fluorescent whitening agent on the fabric is increased to an appropriate concentration (that is, the yellowing point of the whitening agent), the intensity of the emitted blue-violet finish is exactly equal to the intensity of the yellow finish on the fabric, which exactly cancels each other. When the fabric is the whitest, the whitening effect is the best. When the intensity of the blue-violet finish is greater than the intensity that offsets the yellow finish on the fabric, the blue-violet finish reflected by the whitening agent is extremely obvious. At the same time, due to the different constituent groups of the whitening agent, different tones (such as Blue finish purple, red finish blue, etc.) are also more obvious, the result of the combined effect of the above two factors; increase the gray tone of the fabric, and its effect will increase with the increase of the whitening agent concentration, thereby reducing the whitening effect. Although there are no traces of yellow finish on the fabric at this time, it no longer looks white, bright and dazzling. Therefore, when choosing a brightener, in addition to considering its brightening effect, it is also necessary to look for the yellowing points of different brighteners on the fabric through a sample test, so as to use it reasonably to obtain the best brightening effect.