Why do dyes fade under the sun?

    Why do dyes fade under the sun? Before understanding the reason, we first need to know what "sun fastness" is.

    Sun fastness: refers to the ability of a dye to maintain its original color under sunlight exposure. According to general regulations, the determination of sun fastness is based on sunlight. In order to facilitate control in the laboratory, artificial light sources are generally used and calibrated if necessary. The commonly used artificial light sources are hernia lights and charcoal arc lights. Under the irradiation of light, the dye absorbs light energy, and the energy level of the dye molecule is in an excited state. The coloring system of the dye molecule changes or is destroyed, resulting in the decomposition of the dye and discoloration or fading.

    1、 The influence of light on dye production

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

    According to their 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 → excited state, and ground state → second excited state. The ground state of the singlet state is written as S0, and the second excited singlet state is written as S1 and S2, respectively. The corresponding triplet state is represented by T0, T1, and T2.

    During the excitation process, dye molecules are excited into electron excited states with various vibrational energy levels. Their vibrational energy levels rapidly decrease, converting energy into heat and dissipating it. This process of lowering energy levels is called vibrational passivation. During the vibration passivation process, the S2 excited state with a low vibration energy level will also transform into the S1 excited state with a higher vibration energy level, and continue to undergo vibration passivation. In this way, the S2 excited state with a higher energy level quickly transforms into the S1 excited state with a lower vibrational energy level.

    The conversion between S2 and S1 electronic states under the condition of equal energy intersection does not involve changes in electron spin multiplicity, and is called internal conversion. There is also a transition between singlet and triplet states, from S1 to T1 excited state. This type of electronic energy state transition accompanied by electron spin multiplicity changes under equal energy intersection conditions is called intersystem crossing. Due to the "forbidden" effect of electron spin selectivity, the rate of intersystem crossing is generally relatively low.

    The intensified dye molecules undergo photochemical reactions with other molecules, resulting in the fading of the dye and the optical brittleness of the fibers.

    2、 Factors affecting the light fastness of dyes

    1. The wavelength of the light source and the illuminating light;

    2. Environmental factors;

    3. Chemical properties and organizational structure of fibers;

    4. Bond strength between dye and fiber;

    5. Chemical structure of dyes;

    6. Dye concentration and aggregation state;

    7. The influence of artificial sweat on dye fading;

    8. The impact of additives.

    3、 Methods for improving the sun fastness of dyes

    1. Improve the dye structure to minimize the impact on the dye coloring system while consuming light energy, thereby maintaining the original color; Commonly known as high sun fastness dyes. This type of dye is generally more expensive than ordinary dyes, and for fabrics with high sun exposure requirements, the first step should be to choose the dye.

    2. If the fabric has already been dyed but the sun fastness does not meet the requirements, additives can also be used to improve it. Add appropriate additives during or after the dyeing process, so that they undergo a light reaction before the dye when exposed to light, consuming light energy to protect the dye molecules. Generally divided into ultraviolet absorbers and anti ultraviolet agents, collectively referred to as sun fastness enhancers.

    The sun fastness of light colored dyed fabrics with reactive dyes

    The light induced fading of reactive dyes is a very complex photo oxidative chlorination reaction. After understanding the mechanism of light induced fading, we consciously create obstacles to the photo oxidation reaction in the molecular structure design of dyes to delay light induced fading. For example, yellow dyes containing Dole sulfonic acid groups and pyrazolone, blue dyes containing phthalocyanine methyl zan and diazo trichelding rings, and red dyes containing metal complexes, but still lack vibrant red reactive dyes with strong sun fastness.

    The sun fastness of dyed fabrics varies with the change of dyeing concentration. For fabrics dyed with the same dye on the same fiber, the sun fastness increases with the increase of dyeing concentration, while for light colored fabrics, the sun fastness decreases correspondingly when the dyeing concentration is low. However, the sun fastness of common dyes on printed dye color cards is measured at a dyeing concentration of 1/1 of the standard depth (i.e. 1% owf or 20-30g/l dye concentration). If the dyeing concentration is 1/6, 1/12, or 1/25, the sun fastness will significantly decrease.

    Some people have proposed using ultraviolet absorbers to improve sun fastness, but this is not an advisable method. Ultraviolet radiation requires a lot and can only improve by half a level, and the cost needs to be improved significantly. Therefore, only fair selection of dyes can solve the problem of sun fastness.