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Why Dyes Fade in The Sun, There Are Several Factors!

Light fastness: refers to the ability of the dyed product to maintain its original color under sunlight. According to general regulations, the determination of light fastness is based on sunlight. 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 xenon lamp, 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 intensified state, and the color system of the dye molecules changes or is destroyed, resulting in the decomposition of the dye and the phenomenon of discoloration or fading.

First,The effect of light on dye production
When a dye molecule absorbs the energy of a photon, it will cause the outer shell valence electrons of the molecule to transition from the ground state to the excited state.

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

During the excitation process, dye molecules are excited into electronically excited states of various vibrational energy levels, and their vibrational energy levels will be rapidly reduced, converting energy into heat and dissipating. This process of reducing energy levels is called vibrational passivation. During the vibration passivation process, the S2 excited state with low vibration energy level will also be converted into the S1 excited state with higher vibration energy level, and the vibration passivation will continue. In this way, the original S2 excited state with a higher energy level is rapidly transformed into the S1 excited state with the lowest vibrational energy level. The conversion between S2 and S1 electronic energy states under the condition of equal energy intersection does not include the change of electron spin multiplicity, which is called internal conversion. A transition also occurs between the singlet and triplet states, from the S1 to the T1 excited state. This kind of electronic energy state transformation accompanied by the change of electron spin multiplicity under the condition of equal energy intersection is called intersystem crossing. Due to the "forbidden" by the law of electron spin selection, the speed of crossing between systems is generally relatively low.

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

Second,Factors Affecting Light Fastness of Dye
1. The light source and the wavelength of the irradiated light;

2. Environmental factors;

3. The chemical properties and organizational structure of the fiber;

4. Bonding strength between dye and fiber;

5. The chemical structure of the dye;

6. Dye concentration and aggregation state;

7. The effect of artificial sweat on the photofading of dyes;

8. The influence of additives.

Third,Method for Improving Light Fastness of Dye
1. Improve the structure of the dye, so that it can consume light energy while minimizing the impact on the color system of the dye, so as to maintain the original color; that is, the dye with high light fastness that is often said. The price of such dyes is generally higher than that of ordinary dyes. For fabrics with high sun exposure requirements, the first thing to do is to choose dyes.

2. If the fabric has been dyed and the light fastness does not meet the requirements, it can also be improved with additives. During the dyeing process or after dyeing, add suitable auxiliaries to make it photoreact before the dye when it is illuminated, and consume light energy, so as to protect the dye molecules. Generally, it is divided into ultraviolet absorbers and anti-ultraviolet agents, collectively referred to as light fastness enhancers.

Forth,Light fastness of light-colored fabrics dyed with reactive dyes

Photofading of reactive dyes is a very complex photooxychlorination reaction. After knowing the mechanism of photofading, some obstacles to the photooxidation reaction are consciously created in the design of the molecular structure of the dye to delay photofading. For example, yellow dyes containing dolsulfonic acid groups and pyrazolones, blue dyes with phthalocyanine formazan and bis-azo trichelate ring, and red dyes with metal complexes, but still lack bright red sun resistance. Reactive dyes for light fastness.

The light fastness of the dyed product varies with the dyeing concentration. The light fastness of the fabric dyed on the same fiber by the same dye increases with the increase of the dyeing concentration. The light-colored fabric has a low dyeing concentration and is fast to light decrease accordingly. However, the light fastness of common dyes on the printed dye color card is measured when 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 1/1 6. In the case of 1/12 or 1/25, the light fastness will be greatly reduced.

Some people propose to use ultraviolet absorbers to improve the light fastness, which is not an advisable method. It takes a lot of ultraviolet rays, but it can only be improved by half a grade, and the cost will be much higher. Therefore, only a reasonable choice of dyes can solve the light fastness.



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