Photochemical fundamentals of dye fading
The effect of light on dyes during sun exposure can be explained in two ways.
First of all, as far as the structure of the dye is concerned, under the action of ultraviolet light, the chemical bonds in the dye are changed or even broken, so that the structure of the chromophore and auxochromic groups in the dye is destroyed, thereby losing color; Under irradiation, the three-dimensional structure of the dye changes, resulting in a change in color, which is manifested as color change.
On the other hand, it is explained from the change of the electronic structure and its energy in the dye molecule. Under the irradiation of a large amount of ultraviolet light with higher energy, some of the bonding electrons obtain higher energy and transition to the anti-bonding orbitals of higher energy levels, or electrons in the bonding orbits of lower energy levels transition to anti-bonding orbitals . When these electrons return to their original orbits, they release different energies and exhibit different colors and thus different light fastnesses.
Influencing factors of light fastness
1. It is related to the structure of the dye: generally speaking, the light fastness of anthraquinone dyes, phthalocyanine dyes, sulfur blue in vulcanization and metal complex dyes is relatively good; the light fastness of most insoluble azo dyes is also relatively good. High; while the benzidine type azo dyes have low light fastness; triarylmethane dyes are not light fast.
2. It is related to the type of fiber: ①Indigo has a low light fastness on wool but low light fastness on cellulose; ②Vanlarmin blue salt B has a higher light fastness on viscose than on cotton many.
3. It is related to the influence of external conditions: the level of moisture content in the air and the level of temperature have a great influence on the light fastness. Generally, in the case of high moisture content, the light fastness is lower. For example: Vanillamin blue is more prone to reddening and fading in the south than in the north.
4. Related to auxiliaries: There are many auxiliaries that can effectively improve the light fastness.