Why Does The Use of Fluorescent Brighteners Cause The Fabric To Become Less White, Even Though More of Them Are Used?

There are many types of fluorescent brighteners, and there is a specific model of brightener for whitening various types of fibers. Although the chemical structures and properties of various brighteners are different, the principle of whitening fibers or fabrics is the same.

Its whitening principle mainly lies in the fact that the molecules of the brightener contain conjugated double bond systems, which have good planarity. This special molecular structure can absorb ultraviolet light (wavelength of 300-400 nm) from sunlight and emit blue-purple light (wavelength of 420-500 nm) under sunlight. The blue-purple light mixes with the yellow light on the fiber or fabric to become white light, making the fiber or fabric appear much whiter.

The whitening effect of fluorescent brightener is only optical brightening and color compensation, and it cannot replace chemical bleaching. Therefore, if the fabric is not bleached, its whitening effect will not be ideal even if it is treated with a fluorescent brightener. From the principle of fluorescent brightener, it can be known that the whitening effect of the brightener mainly depends on the content of ultraviolet light in the sunlight and the concentration of fluorescent brightener on the fiber or fabric.

When the content of ultraviolet light in sunlight is sufficient, the whitening effect of fluorescent brightener on the fabric increases as the concentration of the brightener on the fabric increases within a certain range.

But when the concentration of the brightener is increased to a suitable concentration, its whitening effect is best, and the highest whiteness value can be obtained. If the amount of brightener exceeds the optimal concentration, not only will the whitening effect of the fabric not be improved, but it will actually decrease (i.e., the fabric will appear yellow, and the whiteness will actually decrease). In this case, the optimal concentration of the brightener is called the yellowing point of the brightener.

The yellowing points of various fluorescent brighteners are different, for example, the yellowing point of fluorescent brightener DT (used for polyester) is 0.8%. That is, when polyester is whitened with DT brightener, the whiteness value will decrease as the concentration increases when the amount of DT brightener exceeds 0.8%.

Fluorescent brightener VBL (used for pure cotton fabrics) has a yellowing point of 0.5%, fluorescent brightener DCB (used for acrylic brightening) has a yellowing point of 0.8%, fluorescent brightener CH (used for acrylic hollow fiber) has a yellowing point of 3.3%, Blankophor BBU (liquid) (Tiankun product, used for pure cotton fabrics) has a yellowing point of 1.26%, Uvitex (ERN-P CGY company product, used for polyester fabrics) has a yellowing point of 0.8%, and Uvitex EBF 250% (paste) (CGY company product, used for polyester fabrics) has a yellowing point of 3%.

So, why does the whitening effect no longer increase with the increase of fluorescent brightener concentration on the fabric? The main reason is that the intensity of yellow light on the fabric is limited, so the complementary color light needed to cancel out the yellow light (main wavelength of about 570 nm) emitted by the fluorescent brightener absorbing ultraviolet light is also limited.

As the concentration of fluorescent brightener on the fabric increases, the intensity of the blue-violet light emitted also increases, which counteracts the yellow light on the fabric and causes the yellow light intensity to gradually decrease. As a result, the fabric becomes whiter and whiter.

When the concentration of fluorescent brightener on the fabric reaches a suitable concentration (i.e. the yellowing point of the brightener), the intensity of the blue-violet light emitted by the brightener is exactly equal to the intensity of the yellow light on the fabric, canceling each other out, and at this point the fabric is the whitest and the whitening effect is the best.

When the intensity of blue-purple light is greater than the intensity of yellow light that is cancelled out by the fabric, the blue-purple light reflected by the brightener becomes very noticeable, and at the same time, the different tints (such as cyan-purple, red-blue) that are presented by the brightener due to its various constituent groups also become more noticeable. The result of these two factors working together is that the gray tone of the fabric increases, and the effect becomes stronger as the concentration of the brightener increases, thereby reducing the whitening effect. Although there is no trace of yellow light on the fabric, it no longer looks white, bright, or dazzling.

Therefore, when selecting brighteners, in addition to considering their brightening effect, it is necessary to conduct small-scale trials to find out the yellow spots on fabrics caused by

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