Why is the amount of fluorescent whitening agent used too much, but the whiteness of the fabric decreases?

There are many varieties of fluorescent whitening agents. Although the chemical structure and properties of various whitening agents are different, the whitening principles for fibers or fabrics are the same. The principle of whitening is mainly due to the conjugated double bond system in the molecule of the whitening agent, which has good planarity. This special molecular structure can absorb ultraviolet light (wavelength 300 ~ 400nm) in sunlight under sunlight. The blue-violet light (wavelength is 420-500 nm), the blue-violet light is mixed with the yellow light on the fiber or fabric to become white light, so that the fiber or fabric is obviously white. However, when the concentration of the whitening agent is increased to a certain 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, it will not only have no whitening effect on the fabric Increase, on the contrary, decrease (that is, the yellowness of the fabric is generally said, but the whiteness decreases). At this time, the optimal concentration of the whitening agent is called the yellowing point of the whitening agent.

The whitening effect is no longer enhanced with the increase in the concentration of the fluorescent whitening agent on the fabric. The main reasons are: Because the intensity of the yellow light on the fabric is limited, the fluorescence needed to offset the yellow light (the dominant wavelength is around 570nm) The whitening agent absorbs ultraviolet light and emits the complementary color light of the yellow light---blue-violet light is also limited. For this reason, when choosing a brightener, in addition to considering its brightening effect, it is also necessary to pass a sample test to find the yellowing points of different brighteners on the fabric for reasonable use to obtain the best brightening effect.