Why is the whiteness of the fabric decreased when the amount of optical brightener is large?

Since the intensity of the yellow light on the fabric is limited, the complementary color of the yellow light emitted by the fluorescent whitening agent absorbing ultraviolet light required to offset the yellow light (the dominant wavelength is about 570nm) is also limited. of. As the concentration of the fluorescent whitening agent on the fabric increases, the intensity of the blue-violet light emitted by it also increases, thereby offsetting part of the yellow light on the fabric, so that the intensity of the yellow light 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 blue-violet light emitted by it is exactly equal to the intensity of the yellow light on the fabric, and they exactly cancel each other. Also the whitest, the whitening effect is the best.

When the intensity of the blue-violet light is greater than the intensity of the yellow light on the offset fabric, the blue-violet light reflected by the whitening agent is extremely obvious.At the same time, due to the different constituent groups of the whitening agent, the different tones (such as blue Light purple, red light blue, etc.) are also more obvious. The combined effect of the above two factors increases the gray tone of the fabric, and its effect increases with the increase in the concentration of the whitening agent, thereby reducing the whitening effect. At this time, there are no traces of yellow light on the fabric, but it no longer looks white, bright and dazzling.

For this reason, 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 that they can be used reasonably to obtain the best brightening effect.