New technology for reactive dye dyeing: reactive dye wet short steam dyeing
Pad dyeing is one of the important dyeing methods of reactive dyes, but usually in the pad dyeing process, the fabric needs to be dried intermediately after passing through the pad dyeing solution, so as to increase the processing speed and reduce the cost during subsequent steaming or baking to fix the color. The dye is hydrolyzed to obtain high color fixation rate and color fastness. However, intermediate drying also brings many problems: firstly, it consumes energy. Evaporating water when drying wet fabrics consumes a lot of heat energy; secondly, dyes are prone to migration during drying, causing color difference and reducing color fastness and dyeing reproducibility. In addition, drying after padding the dyeing solution not only adds a processing step, which is inconvenient to manage, but also when the dry fabric is steamed, the dyes and chemicals have to absorb water and dissolve again, and the dry fabric absorbs moisture and will be released. Heat produces overheating, which is detrimental to dye adsorption and color fixation. Therefore, wet steaming is a long-term goal.
It is very difficult to steam dyed fabrics. First, the wet fabric is steamed directly. Due to the heat absorption and evaporation of the water, the heating rate of the fabric is greatly slowed down, which prolongs the steaming and color fixing time. Secondly, the fabric contains a large amount of moisture (usually the liquid carrying rate after padding is 60%~ 70%), during the steaming and heating process, the reactive dyes on the fabric will undergo a large amount of hydrolysis, reducing the color fixation rate and color fastness. There are many states of moisture on fabrics, which can be roughly divided into two categories: water absorbed by fibers and free water on fabrics. Absorbed water can be divided into chemically bound water (mainly bound to the fiber molecular chain through hydrogen bonds) or non-freezing water (its freezing point is well below 0°C) and bound water. The content of chemically bound water is not much, and the chance of reaction with dyes is also low, because it cannot move freely; a considerable part of the absorbed water is in the fiber pores, and the fiber pores are very thin. This part of water is not easy to flow freely, and the reaction speed with dyes is very low. Also lower, called bound water. Free water outside the fiber is generally easy to move. A small part is in the capillary tubes between the fabric fibers. Although it is not easy to flow due to the capillary effect, its nature is basically the same as that of water in the solution, and most of it is free to flow. Water, they can also flow due to their own gravity. Both states of water outside the fiber are easy to react with dyes and are the main factors for dye hydrolysis. Since the fabric usually contains a large amount of free moisture outside the fiber during wet steaming (or baking), the dye hydrolysis rate is high.
The wet short steaming process is to use special steaming equipment on the premise of selecting appropriate dyes and color-fixing alkali agents to heat the fabric as quickly as possible, and the moisture on the fabric will quickly drop from 60% to 70%. After the appropriate level, wet steaming or steaming is performed to quickly fix the dye. The moisture content of cotton fabrics quickly drops to about 30%, and that of viscose fabrics to about 35%. At this time, the moisture on the fabrics is basically bound. Water and chemically bound water have very little free water. Therefore, at this time, it can not only ensure that the pores in the fiber are filled with water, but also facilitate the dissolution, diffusion, adsorption and fixation of the dye in the pores, and reduce the hydrolyzed dye.
In order to quickly evaporate the moisture on the fabric and maintain it at a suitable level, in addition to supplying normal pressure saturated steam, the wet short steaming steamer also needs to have additional equipment to quickly heat the steam. It often uses steam/air mixed gas or high-temperature superheated steam as the heating medium. The former has a steaming temperature of 120-130°C and a time of 2-3 minutes; the latter has a steaming temperature of around 180°C and a time of only 20-75 s. , under this condition the fixation rate is higher.
There are many kinds of reactive dyes suitable for wet short steaming process. Lower temperature steaming and baking is suitable for highly reactive dyes, such as dichloro-s-triazine and fluorochloropyrimidines; high-temperature steaming and baking is suitable for slightly less reactive dyes, including commonly used dual-reactive dyes. Regardless of the type of dye, it is required that when the moisture content of the wet fabric is high during steaming, the dye will not undergo a large amount of hydrolysis, and a rapid color fixation reaction will occur only after reaching a sufficiently high temperature. Therefore, a weakly alkaline alkali agent should be used to fix the color, or the alkali cannot be strong when the moisture content of the fabric is high. This includes a mixed alkali of baking soda or soda ash and some alkali agents. If low-alkali or neutral color-fixing is performed, The effect will be better. Studies have found that using neutral color fixing agents to fix colors has good effects whether at 120-130°C or around 180°C. In short, the wet short steam dyeing process has many advantages, and the research and development of dyes, auxiliaries, equipment and processes should be strengthened in the future.
Small liquor ratio dyeing with reactive dyes
The dyeing liquor ratio is directly related to the concentration of dyes and chemicals in the dye liquor, as well as the dye uptake rate and fixation rate of the dye. , dye solution stability, levelness and reproducibility are all affected. The dye uptake rate and dye fixation rate of dyes increase with the decrease of liquor ratio within a certain range. Different dyes increase at different rates, and the fixation rate and dye uptake rate increase at different rates. Generally, the dye uptake rate increases with the decrease of liquor ratio. The rate increases faster than the fixation rate.
It can be seen that small liquor ratio dyeing can not only speed up the dyeing rate, but also improve the color fixation rate to a certain extent. Therefore, the amount of salt can be greatly reduced, low-salt dyeing can be performed, and the dye and alkali can be increased. agent utilization. For example, when the dye concentration is 6% (o.w.f) and the dyeing bath ratio is 40:1, the salt concentration can be as high as about 540% (o.w.f); when the bath ratio drops to 3.5:1, the salt concentration can be reduced to 6% (o.w.f), the same dyeing depth can be obtained.��To achieve the above goals, it is required to strictly control the entire process from dye selection to dyeing, color fixation and post-dye washing, and cooperate with scientific management to carry out so-called controlled dyeing.
“Right the first time” and controlled dyeing must first select dyes reasonably. According to different dyes, different dyeing processes are formulated and strictly controlled. Most of the time, the entire process is automatically controlled by computers. In order to be able to precisely control the dyeing process, it is necessary to understand the dyeing thermodynamics and kinetics as well as the reaction kinetics of dyes. If these characteristics are calculated according to conventional dyeing thermodynamics and kinetics, as well as the relevant equations of reaction kinetics, it is neither convenient nor difficult to quickly guide production. Therefore, some dye companies have adopted the method of determining dyeing characteristic values under specific conditions. These dyeing characteristic values are used to represent the dyeing characteristics of dyes and serve as the basis for selecting and controlling the dyeing process. The main dyeing characteristic values include S, E, R or T50, F and MI, LDF, etc., which are used to represent the directness, reactivity, color fixation, migration and leveling properties of the dye respectively. According to the dyeing characteristic value of the dye, the dyeing adding material (dye, salt and alkali agent, etc.) working curve can be designed and automatically metered and added; a reasonable working curve of heating and heat preservation can be designed to ensure that the dyeing and color fixation of the dye are consistent It is carried out at a reasonable speed, which is fast and even dyeing; it can also be washed quickly and efficiently after dyeing, saving energy and water, and reducing sewage.
The dyes produced by some large dye companies are equipped with dyeing characteristic values, which can be used as a reference for users. The advanced dyeing equipment produced in recent years is also equipped with controlled devices, including not only automatic metering and feeding systems, but also control computers and software. Therefore, the first-time success rate of reactive dye dyeing is very high. It can be predicted that with the increasing demand for clean dyeing and the development of dyes and equipment, “right first time” and controlled dyeing technology will develop faster. AA
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