Before understanding the reasons, we must first know what is called "fastness to sunlight".

       Light fastness: refers to the ability of dyed products to maintain their original color under sunlight. According to general regulations, the determination of fastness to sunlight is based on sunlight. In order to facilitate control in the laboratory, artificial light sources are generally used, and corrections are made when necessary. The most commonly used artificial light source is a hernia lamp, but also a carbon arc lamp. When the dyed product is irradiated by light, the dye absorbs light energy, the energy level is increased, and the molecule is in an excited state, and the color system of the dye molecule changes or is destroyed, causing the dye to decompose and cause discoloration or fading.

1. The effect of light on dyes

       When a dye molecule absorbs the energy of a photon, it will cause the outer valence electron 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 (single 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.

       In the process of excitation, dye molecules are excited into electronic excited states of various vibrational energy levels. Their vibrational energy levels will be rapidly reduced, and the energy will be converted into heat and dissipated. This process of reducing energy levels is called vibration passivation. During the vibration passivation process, the S2 excited state with a low vibration energy level will also be transformed into an S1 excited state with a higher vibration energy level, and vibration passivation will continue to occur. In this way, the S2 excited state with the original higher energy level is quickly transformed into the S1 excited state with low vibration energy level.

       The transformation 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 transformation. There will also be a transformation between the singlet state and the triplet state, from S1 to T1 excited state. This kind of electronic energy state transformation under the condition of equal energy intersection with electron spin multiplicity is called intersystem crossing. Due to the "prohibition" of the electron spin selection law, the speed of intersystem crossing is generally relatively low.

        A photochemical reaction occurs between the excited dye molecules and other molecules, resulting in the light fading of the dye and the light brittleness of the fiber.

2. Factors affecting the light fastness of dyes

        1. The wavelength of the light source and the illuminating 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 influence of artificial sweat on the light fading of the dye;

        8. The influence of additives.

3. Methods to improve the fastness of dyes to sunlight

        1. Improve the structure of the dye so that it can consume light energy while minimizing the impact on the dye color system, thereby maintaining the original color; that is, the high light fastness dyes often referred to as high light fastness dyes. The price of such dyes is generally higher than that of ordinary dyes. For fabrics with high solar requirements, the first step should be to choose dyes.

        2. If the fabric has been dyed and the light fastness is not up to the requirement, it can also be improved by additives. In the dyeing process or after dyeing, add suitable auxiliary agents to make the light react before the dye when it is exposed to light, and consume the light energy, so as to protect the dye molecules. Generally divided into ultraviolet absorbers and anti-ultraviolet agents, collectively referred to as light fastness enhancers.

Lightfastness of light-colored fabrics dyed with reactive dyes

       The light fading of reactive dyes is a very complex photooxychlorination reaction. After knowing the light fading mechanism, the molecular structure of the dye is designed to consciously create some obstacles to the photooxidation reaction to delay the light fading. For example, yellow dyes containing Dole sulfonic acid group and pyrazolone, blue dyes containing methane phthalocyanine and bis-azo trichelate ring, and red dyes containing metal complexes, but still lack the bright red color and resistance to sunlight. Reactive dyes for light fastness.

       The light fastness of dyed products varies with the change of dyeing concentration. For fabrics dyed with the same dye on the same fiber, the light fastness increases with the increase of dyeing concentration. Light-colored fabrics have low dyeing concentration and are fast to the sun. The degree drops accordingly. However, the light fastness of common dyes on the printed dye color card is measured under the standard depth of 1/1 (that is, 1% owf or 20-30g/l dye concentration). If the dyeing concentration is 1/ In the case of 6, 1/12 or 1/25, the light fastness will be greatly reduced.

       Some people have proposed using ultraviolet absorbers to improve the fastness to light. This is not a desirable method. Ultraviolet rays need to be used a lot, and it can only be improved by half a level, and the cost will be greatly improved. Therefore, only fair selection of dyes can solve the light fastness.