Why does excessive use of Optical brightener reduce the whiteness of the fabric?
Is there a standard for the dosage of various whitening agents?
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There are many kinds of Optical brightener, and all kinds of fiber whitening agents have certain trademarks.
Although the chemical structures and functions of various whitening agents are different, the whitening principles for fibers or fabrics are the same.
The whitening principle is mainly due to the presence of conjugated double bond systems in the molecules of the whitening agent, which has outstanding planarity. This special molecular structure can absorb ultraviolet light in sunlight (with a wavelength of 300-400nm) and declare blue purple light (with a wavelength of 420-500nm). Blue purple light mixes with yellow light on the fiber or fabric to become white light, resulting in a significant whitening of the fiber or fabric.
▲ Reflection curve of samples containing whitening agents
The whitening effect of Optical brightener is only optical brightening and color supplement, and cannot replace chemical bleaching. Therefore, the unbleached fabric can be directly whitened with Optical brightener, and its whitening effect is not ambitious. It can be seen from the principle of Optical brightener that the whitening effect of the whitening agent mainly depends on the content of ultraviolet light in the sunlight and the concentration of Optical brightener on the fiber or fabric.
When the content of ultraviolet ray in the sunlight is sufficient, and the concentration of Optical brightener on the fabric changes in a certain scale, its whitening effect increases with the addition of the concentration of whitening agent on the fabric;
But when the concentration of the whitening agent is increased to a certain appropriate concentration, its whitening effect is optimal, and the highest whiteness value can be achieved. If the dosage of the whitening agent exceeds the optimal concentration, not only does it not improve the whitening effect on the fabric, but it decreases on the contrary (generally speaking, the fabric turns yellow and the whiteness decreases). At this moment, the optimal concentration of the whitening agent is called the yellowing point of the whitening agent.
The yellowing point of various Optical brightener is different, for example, the yellowing point of Optical brightener DT (used for polyester) is 0.8%. When using DT whitening agent to whiten polyester, the whiteness value decreases with the addition of concentration when the dosage exceeds 0.8%.
Yellowing point
The yellowing point of Optical brightener VBL (used for pure cotton fabric) is 0.5%, that of Optical brightener DCB (used for acrylic fiber whitening) is 0.8%, that of Optical brightener CH (used for acrylic fiber bulk yarn) is 3.3%, that of Blankphor BBU (liquid) (used for pure cotton fabric by Bayer company) is 1.26%, and that of Uvitex (used for polyester fabric by ERN-P CGY company) is 0.8%, UvitexEBF 250% (slurry) (CGY company product, used for polyester fabrics) has a yellowing point of 3%.
Therefore, when selecting whitening agents, in addition to considering their whitening effect, small sample experiments should also be conducted to find the yellowing spots of different whitening agents on the fabric, in order to use them reasonably and achieve the best whitening effect. Then, why does the whitening effect no longer increase with the addition of Optical brightener concentration on the fabric? The main reason is that the intensity of yellow light on the fabric is limited, so the complementary light of yellow light emitted by Optical brightener absorbing ultraviolet light required to offset the yellow light (the main wavelength is about 570nm) – blue violet light is also limited.
With the addition of the concentration of Optical brightener on the fabric, the intensity of the blue violet light emitted from the fabric is also added, so as to offset some yellow light on the fabric, which gradually reduces the intensity of the yellow light, and the fabric becomes whiter and whiter.
When the concentration of the Optical brightener on the fabric is added to an appropriate concentration (namely the yellowing point of the whitening agent), the intensity of the blue violet light emitted from the fabric is just equal to the intensity of the yellow light on the fabric, which just offset each other. At this time, the fabric is also the whitest and has the best whitening effect.
When the intensity of blue-violet light is greater than the intensity of canceling out the yellow light on the fabric, the blue-violet light reflected by the whitening agent is extremely prominent. At the same time, due to the different composition groups of the whitening agent, the different colors (such as blue-violet, red-blue, etc.) that appear are also more prominent. The results of the above two factors summarize the effect of adding gray color to the fabric, which strengthens with the addition of the whitening agent concentration, resulting in a decrease in the whitening effect, Although there were no traces of yellow light on the fabric at this time, it no longer looked bright, bright, and dazzling.
Therefore, when selecting whitening agents, in addition to considering their whitening effect, small sample experiments should also be conducted to find the yellowing spots of different whitening agents on the fabric, in order to use them reasonably and achieve the best whitening effect.
Post time: Aug-04-2023