Why does the whiteness of fabrics decrease when too much fluorescent whitening agent is used? Are there standards for the dosage of various whitening agents?
There are many varieties of fluorescent whitening agents, and there are certain brands of whitening agents for whitening various fibers. Although the chemical structures and properties of various whitening agents are different, the principle of whitening fibers or fabrics is the same. The whitening principle is mainly due to the fact that the molecules of the whitening agent contain a conjugated double bond system and have good planarity. This special molecular structure can absorb ultraviolet rays in sunlight (wavelength is 300~400nm) and emit light when exposed to sunlight. Blue-violet light (wavelength is 420~500nm). The blue-violet light mixes with the yellow light on the fiber or fabric and turns into white light, thus making the fiber or fabric obviously white.
Reflection curve of sample containing whitening agent
Fluorescent whitening The whitening effect of the agent is only an optical brightening and complementary color, and cannot replace chemical bleaching. Therefore, if the fabric without bleaching is directly whitened with a fluorescent whitening agent, the whitening effect will not be ideal. It can be known from the principle of fluorescent whitening agents that the whitening effect of the whitening agent mainly depends on the content of ultraviolet rays in the sunlight and the concentration of the fluorescent whitening agent on the fiber or fabric.
When the content of ultraviolet rays in the sunlight is sufficient and the concentration of the fluorescent whitening agent on the fabric changes within a certain range, the whitening effect will be enhanced as the concentration of the whitening agent on the fabric increases;
But 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 dosage of the whitening agent exceeds the optimal concentration, not only will it The whitening effect of the fabric has not improved, but has decreased (that is, it is commonly said that the fabric turns yellow and the whiteness decreases). At this time, the optimal concentration of the whitening agent is called the yellowing point of the whitening agent.
The yellowing points of various optical brighteners are different. For example, the yellowing point of optical brightener DT (used for polyester) is 0.8%. That is, when whitening polyester, when the dosage of DT whitening agent exceeds 0.8%, its whiteness value decreases as the concentration increases.
The yellowing point of the fluorescent whitening agent VBL (used for pure cotton fabrics) is 0.5%, and the yellowing point of the fluorescent whitening agent DCB (used for whitening acrylic fibers) is 0.8%. Agent CH (used for acrylic bulked yarn) has a yellowing point of 3.3%, BlankophorBBU (liquid) (Bayer company product, used for pure cotton fabrics) has a yellowing point of 1.26%, Uvitex (ERN-PCGY company product, used Its yellowing point is 0.8% for polyester fabrics, and UvitexEBF250% (paste) (product of CGY company, used for polyester fabrics) has a yellowing point of 3%.
So, why does the whitening effect no longer increase with the increase of the concentration of fluorescent whitening agent on the fabric? The main reason is: since the intensity of yellow light on the fabric is limited, the yellow light is offset (The dominant wavelength is around 570nm) The required complementary color light of yellow light emitted by the fluorescent whitening agent absorbing ultraviolet rays – blue-violet light is also limited.
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, causing the yellow light intensity to gradually decrease, and the fabric will also become more and more bright. The whiter.
When the concentration of the fluorescent whitening agent on the fabric increases to an appropriate concentration (that is, the yellowing point of the whitening agent), the intensity of the blue-violet light emitted is exactly equal to the intensity of the yellow light on the fabric. They exactly offset each other. At this time, the fabric is the whitest and the whitening effect is the best.
When the intensity of blue-violet light is greater than the intensity that offsets the yellow light on the 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 cyan violet, 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 is strengthened as the concentration of the whitening agent increases, thus The whitening effect is reduced. Although there is no trace of yellow light on the fabric at this time, it no longer looks white, bright, and dazzling.
For this reason, when selecting a whitening agent, in addition to considering its whitening effect, sample tests should also be conducted to find the yellowing spots of different whitening agents on the fabric so that they can be used rationally to obtain the best results. Whitening effect. </p
