Before we understand why, we first need to know what is called "sun fastness".

Fastness to sunlight: refers to the ability of the dye to maintain its original color under sunlight. According to the general provisions, the determination of fastness to sunlight is the standard. In order to facilitate control in the laboratory, artificial light sources are generally used and corrected if necessary. The most commonly used artificial light source is the hernia light, but also the carbon arc lamp. Under the irradiation of light, the dye absorbs light energy, the energy level increases, the molecules are in an active state, and the hair color system of the dye molecules changes or is destroyed, resulting in the discoloration or fading of the dye.

First, the impact of light on dyes

When a dye molecule absorbs the energy of a photon, it causes the outer valence electrons of the molecule to transition from the ground state to the excited state.

According to the different structure, the dye molecules can undergo different activation processes under the action of different wavelengths of light waves, such as π →π*, n →π*, CT (charge transfer), S →S (singlet), S → T (trilinear), ground state → first excited state and ground state → second excited state. The ground state of the singlet is written as S0, and the first and second excited singlets are written as S1 and S2, respectively. The corresponding trilinear states are represented by T0, T1, and T2.

In the process of activation, the dye molecules are excited into a variety of vibrational energy levels of the electron excited states, their vibrational energy levels will be rapidly reduced, the energy into heat and dissipated, this process of reducing the energy level is called vibration passivation. In the process of vibration passivation, the excited state of S2 with low vibrational energy level will also be transformed into the excited state of S1 with high vibrational energy level, and continue to undergo vibration passivation. In this way, the original S2 excited state with a higher energy level quickly transforms into the S1 excited state with the lowest vibrational energy level.

Under the condition of equal energy intersection, the transformation between S2 and S1 electron energy states does not include the change of electron spin multiplicity, which is called internal transformation. There is also a transition between singlet and trilinear states, from S1 to T1. This kind of electron energy state transformation under the condition of equal energy intersection accompanied by the change of electron spin multiplicity is called intersystem crossover. Due to the "prohibition" of the electron spin selection law, the rate of intersystem crossover is generally relatively low.

The photochemical reaction between the activated dye molecules and other molecules leads to the photofading of the dye and the photobrittleness of the fiber.

Second, the factors affecting the light fastness of dyes

1. The wavelength of light source and illumination;

2. Environmental factors;

3. Chemical properties and organizational structure of fibers;

4. Bonding strength of dye and fiber;

5. Chemical structure of dye;

6. Dye concentration and aggregation;

7. The influence of artificial sweat on the fading of dye light;

8. Effects of auxiliaries.

Methods for improving the fastness of dyes to sunlight

1, improve the structure of the dye, so that it can consume light energy while minimizing the impact of the dye color system, so as to maintain the original color; It is often said that high fastness dyes. Such dyes are generally higher in price than ordinary dyes, and for fabrics with high sun exposure requirements, dyes should first be selected.

2, if the fabric has been dyed, and the sun fastness does not meet the requirements of the case, it can also be improved by additives. In the dyeing process or after dyeing add appropriate additives, so that it is subjected to light reaction before the dye, consume light energy, in order to protect the dye molecules. Generally divided ultraviolet absorbers and anti-ultraviolet agents, collectively referred to as sun fastness enhancement agents.

Fastness to sunlight of light colored fabrics dyed with reactive dyes

The fading of reactive dyes by light is a very complex photooxychlorination reaction. After knowing the mechanism of photofading, we consciously create some obstacles to the photooxidation reaction in the design of the molecular structure of dyes to delay the photofading. Examples include yellow dyes containing dolesulfonic acid and pyrazolone, blue dyes containing phthalocyanine methylene and bisazotriechelate rings, and red dyes containing metal complexes, but still lack the vivid red fastness to sunlight.

The sun fastness of the dye varies with the change of dyeing concentration. For the fabric dyed with the same dye on the same fiber, the sun fastness improves with the increase of dyeing concentration. For the light-colored fabric, the dyeing concentration is low, and the sun fastness decreases correspondingly. However, the sun fastness of common dyes on the printed dye color card is determined at the dyeing concentration is 1/1 of the standard depth (that is, 1% owf or 20-30g/l dye concentration), if the dyeing concentration is 1/6, 1/12 or 1/25, the sun fastness will decrease significantly.

Some people put forward the use of ultraviolet absorbers to improve the fastness of the sun, this is not an advisable method, ultraviolet rays to use a lot, but also can only improve half a level, the cost has to improve a lot. Therefore, only reasonable selection of dyes can solve the sun fastness