Why Is The Amount Of Optical Brightening Agent Too Much, Instead, The Whiteness Of The Fabric Decreases?

There are many types of optical brightening agents, and there are certain types of whitening agents for whitening of various fibers.

Although the chemical structure 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 conjugated double bond system in the molecule of the whitening agent, which has good planarity. This special molecular structure can absorb ultraviolet rays (wavelength 300-400nm) in sunlight under sunlight and emit. Blue-violet light (wavelength 420-500 nm), blue-violet light is mixed with yellow light on the fiber or fabric to become white light, so that the fiber or fabric is obviously whitened.

Reflection curve of a sample containing a brightener

The brightening effect of the fluorescent whitening agent is only optical brightening and color supplementation, and it cannot replace chemical bleaching. For this reason, the whitening effect of the fluorescent whitening agent on the fabric without bleaching is not ideal. From the principle of the fluorescent whitening agent, the whitening effect of the whitening agent is mainly determined by the content of ultraviolet rays in sunlight and the concentration of the fluorescent whitening agent on the fiber or fabric.

When the content of ultraviolet light in the sunlight is sufficient, when the concentration of the fluorescent whitening agent on the fabric changes within a certain range, its whitening effect is enhanced with the increase of the concentration of the whitening agent on the fabric;

However, when the concentration of the whitening agent is increased to a proper 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, not only the whitening effect of the fabric is not It is increased, but decreased somewhat (that is, the so-called yellowing of the fabric, but the whiteness decreases). At this time, the optimal concentration of the brightener is called the yellowing point of the brightener.

The yellowing point of various fluorescent whitening agents is different. For example, the fluorescent whitening agent DT (for polyester) has a yellowing point of 0.8%. That is, when the polyester is whitened, when the amount of DT whitening agent exceeds 0.8%, its whiteness value decreases with increasing concentration.

                                                                Yellow point

The fluorescent whitening agent VBL (for cotton fabrics) has a yellowing point of 0.5%, the fluorescent whitening agent DCB (for acrylic whitening) has a yellowing point of 0.8%, and the fluorescent whitening agent CH ( For acrylic expanded yarn) Its yellowing point is 3.3%, Blankophor BBU (liquid) (Bayer company product, for cotton fabrics) Its yellowing point is 1.26%, Uvitex (ERN-P CGY company Products, for polyester fabrics) has a yellowing point of 0.8%, UvitexEBF 250% (paste) (CGY company products, 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 brightener on the fabric, the main reason is: because the intensity of the yellow light on the fabric is limited, the yellow light is canceled (the main wavelength is about 570nm) The complementary color light of yellow light emitted by the fluorescent whitening agent that absorbs ultraviolet light and emits blue-violet light is also limited.

As the concentration of the fluorescent whitening agent on the fabric increases, the intensity of the emitted blue-violet light also increases, thereby offsetting part of the yellow light on the fabric, which gradually reduces the intensity of the yellow light, and the fabric becomes more and more white.

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 is exactly equal to the intensity of the yellow light on the fabric, which exactly cancels each other. At this time, the fabric It is also the whitest and has the best whitening effect.

When the intensity of the blue-violet light is greater than the intensity of the yellow light on the fabric, the blue-violet light reflected by the brightener is extremely obvious.At the same time, due to the different composition groups of the brightener, different shades (such as Light purple, red light blue, etc.) are also more obvious. As a result of the combined effect of the above two factors, the gray tone of the fabric is increased, and its effect is strengthened with the increase of the whitening agent concentration, thereby reducing the whitening effect. There was no yellow light on the fabric, but it no longer looked white, bright and dazzling.

For this reason, when selecting a whitening agent, in addition to considering its whitening effect, it is necessary to pass a sample test to find the yellowing point of different whitening agents on the fabric for reasonable use to obtain the best whitening effect.