The fading and discoloration of dyed fabrics is a relatively complicated process. Under the action of sunlight, the dye absorbs light energy and the molecule is in an excited state. It is unstable. The energy obtained must be released in different forms to become a stable state. One of the forms is that the dye is directly decomposed and faded after receiving light energy. Different dyes have different fading mechanisms on different fibers. For example, the fading of azo dyes on cellulose fibers is an oxidation process, while the fading on protein fibers is the result of reduction.
The light fastness of the same dye on different fibers is very different. For example, the light fastness of the same dye on cotton and viscose fibers at the same concentration is different. The cotton is high. Another example is the light fastness of vat dyes on cellulose fibers, but poor on polyamide fibers. This is because the physical state of the dye on different fibers and the binding fastness to the fiber are different.
The light fastness of the dye is related to its molecular structure. For example, the light fastness of anthraquinone, phthalocyanine and metal complex dyes is generally higher, and the light fastness of insoluble azo dyes is quite different. Another example is that when the molecular structure of the dye contains more amino groups and hydroxyl groups, it is easy to absorb light energy and is not resistant to oxidation, resulting in low light fastness.
The light fastness also varies with the concentration of dyeing. When the same dye is dyed on the same fiber, the light fastness of low dyeing concentration is generally worse than that of high concentration. The light fastness is evaluated according to the "blue standard sample". "Blue standard" refers to the blue wool fabric dyed with the specified dye into the specified concentration. Under the specified conditions, the exposure time required for fading by the sun is roughly doubled step by step. When measuring, place the sample and the "blue standard sample" in a sunlight-exposed wooden frame at the same time, and cover the two ends of the sample and the "blue standard sample" with opaque black paper. 1/5 of the full length. Place the exposed wooden frame on the wooden frame and place it in the sun slanting to the south, and compare them to determine the grade of the sample.
The light fastness is divided into 8 grades, the 1st grade is the lowest, which is equivalent to the fading under the sun for 3h; the 8th grade is the highest, which is equivalent to the fading under the sun for more than 38h. In addition to natural light sources, artificial light sources, such as xenon lamps and carbon arc lamps, can also be used in the test. The instrument is equipped with exhaust and humidification devices to maintain certain exposure conditions. The light fastness tester can get rid of the limitation of climatic conditions, but the light wave composition of the light source of the light fastness tester is different from that of sunlight.