1. Test method for color fastness to light
The light fastness test refers to placing the textile samples together with a set of blue wool standard samples under the artificial light source and exposing them to the specified conditions, and then comparing the two colors to evaluate the color fastness.
2. Ways to improve color fastness to light
The photobleaching mechanism of dyes is very complicated, but the main reason is that the dyes are excited after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The light fastness of textiles mainly depends on the chemical structure of the dye, as well as its aggregation state, bonding state and mixed color combination.
3. Select dyes according to fiber properties and textile uses
For cellulose fiber textiles, dyes with better oxidation resistance should be selected; for protein fibers, dyes with better reduction resistance or containing weak oxidative additives should be selected; other fibers should be selected according to the effect on fading. In order to enhance the photo-oxidative stability of the azo group in the dye molecular structure, some strong electron-withdrawing groups are usually introduced in the ortho position of the azo group during the dye synthesis process, thereby reducing the electron cloud density of the azo gas atom.
In addition, hydroxyl groups can also be introduced at the two ortho positions of the azo group, and its coordination ability can be used to complex with heavy metals, thereby reducing the electron cloud density of the hydrogen atom of the azo group, and shielding the azo group. Color fastness to light of dyes.
4. The dye should be selected according to the color depth
A large number of tests have proved that the color fastness to light of reactive dyes on cellulose fibers is proportional to the depth of the dyed luster, that is, the darker the color and luster, the better the color fastness to light. This is because the higher the concentration of the dye on the fiber, the greater the aggregation of the dye molecules, the smaller the surface area of the same amount of dye exposed to air, moisture and light, and the lower the probability of the dye being oxidized by light.
Conversely, the lighter the color, the more highly dispersed the dyes are on the fibers, the higher the probability of being exposed to light, and the final decrease in the color fastness to light. Therefore, when dyeing light-colored varieties, dyes with higher light fastness should be selected. In addition, many post-finishing agents such as softeners and anti-wrinkle finishing agents are added to the fabric, which will also reduce the light fastness of the product. Therefore, dyes that are not sensitive to these finishing agents should be selected.
5. Dyes with good light resistance stability and compatibility should be selected for color matching
Different dyes have different fading properties and even different photofading mechanisms. Sometimes the presence of one dye sensitizes the fading of another dye. When color matching, dyes that will not sensitize each other and can even improve light resistance should be selected, which is especially important when dyeing dark varieties such as black.
If one of the three primary colors fades too quickly, the dyed fiber or fabric will soon be discolored, and the faded dye residue will also affect the light stability of the other two dyes that are not faded. Reasonable control of the dyeing process, to fully combine the dyes with the fibers, and to avoid the hydrolyzed dyes and unfixed dyes remaining on the fibers is an important way to obtain higher color fastness to light.