Reactive dyes dye cellulose fibers to form chemical bonds with the fibers in the form of covalent bonds. Therefore, theoretically, the covalent bond between the dye and the fiber can give the dyed fabric excellent dye fastness. But in fact, its dyed products often fade, change color or stain during testing, use, washing, and even storage. Especially the wet rubbing fastness and soaping staining fastness when dyeing dark colors, and the light fastness and chlorine bleaching fastness when dyeing light colors are not satisfactory.
There are two main reasons for its poor color fastness: one is the problem of the dye itself; the other is the problem of the dyeing process.
Reactive dyes dye cellulose fibers, although they are chemically combined with the fibers in the form of covalent bonds, but when the dyes are subjected to moisture, heat, light, acid, alkali and oxidants, certain parts of the matrix structure will occur change. For example, the decomposition of the azo group, the oxidation of the amino group, the detachment of the complex metal ion, or the hydrolysis and fracture of the covalent bond of the dye-fiber, etc., will result in discoloration and lower fastness of some dyed products. For another example, the covalent bond established between reactive dyes and cellulose macromolecules will be hydrolyzed and broken under certain conditions, become hydrolyzed dyes and fall off, making the wet fastness and light fastness of the dyed product poor.
In the process of using reactive dyes, whether the dyeing method is correct or not directly affects the fastness of the dyed material. For example: improper chemical materials, hydrolysis of dyes, too hard water, too much calcium and magnesium ions, causing dye aggregation and precipitation; improper fixation conditions, low fixation rate; poor washing after dyeing, poor soaping, and unfixed dyes are not removed Clean; the finished product contains acid and alkali, and the increase of dye hydrolysis and chain scission will cause the increase of floating color.
The so-called "floating color" refers to the dye attached to the fiber but not bonded to the fiber. They include: dyes that are partially or completely hydrolyzed; dyes that are adsorbed on the fiber but have not participated in the reaction; the ethylene vitriol sulfate group has been eliminated, and the sulfate group has fallen off, but has not bonded to the fiber. Since floating dyes have different degrees of affinity to fibers, this brings difficulties to the ease of dyeing. Therefore, in the actual use process, it is impossible to completely remove the floating dye on the fiber by washing and soaping.
Dyeing fastness is closely related to the chemical structure and dyeing performance of dyes. In order to improve dyeing fastness, you must first choose a good dye. Pay attention to the following points: ① The chemical reactivity between the dye and the fiber should be strong, so that the two The combination of covalent bonds easily occurs. ② The fixing rate should be high. ③Good resistance to acid and alkali hydrolysis. ④ Dyestuffs-the bond energy of the fiber bond should be high, and the chemical stability should be good, and bond breakage is not easy to occur under acid, alkali, humidity and heat conditions. ⑤The antioxidant capacity should be strong, so that it is not easy to be oxidized, discolored and faded. ⑥Dyeing and coloring are better, and compatibility is better.
Reactive dyes combine with calcium and magnesium ions or other heavy metal ions to form insoluble or poorly soluble metal dyes. In the presence of a higher concentration of electrolyte, these dyes will form aggregates of different sizes, which will gradually adsorb on the surface of the fiber. The more serious ones will produce color spots, and the lighter ones will form floating colors. These floating colors will hinder the penetration and diffusion of the dye into the fiber, reduce the dye uptake rate, make the color lighter, the cloth surface shade will change, the vividness will decrease, and the dyeing fastness will be significantly reduced. Therefore, it must be in water before mixing. Add chelating and dispersing agent (2g/L) with strong chelating and dispersing ability (sodium hexametaphosphate can also be used) to purify the dyeing water.
① Try to dye with higher humidity. If dyeing with medium-wet normal-wet dyes (KN type) is used, the dyeing humidity can be increased from 60℃~65℃ to 75℃~80℃, and then reduced to 60℃~65℃, and alkali fixation is added.
②Using alkali fixing process. In the process of adding alkali to fix color, the bonding reaction between dye and fiber will also occur hydrolysis reaction with water to produce floating dye. The speed of the hydrolysis reaction is directly related to the strength of the fixing conditions. The more severe the fixing conditions, the stronger the alkalinity, and the faster the dye hydrolysis. Generally, 60℃~65℃ is used. Single soda ash is used as the fixing alkali agent, and the pH value is stable at about 11. The soda ash is added in batches.
③Water washing and soaping: The purpose of washing after dyeing is to remove the residual alkali agent on the fiber to prevent the dyes that have been bonded and fixed due to high temperature and alkali from being broken and hydrolyzed from the fiber during high temperature soaping.
First, wash with clean water before soaping.
Second, choose a soaping agent with good washing ability, dispersing ability, emulsifying ability and dirt-carrying ability.
Third, it is best to add 1~2g/L chelating dispersant to the soap lotion.
Fourth, soaping must be performed under neutral conditions (pH=6~7). Fifth, it must be soaped at high temperature.