Reactive dyes are one of the main dyes used for dyeing cotton fabrics. The market is mainly dominated by reactive dyes, which are prone to hydrolysis. In traditional processes, there are problems with low dye uptake and fixation rates. The utilization rate of dyes is low, resulting in a large amount of colored wastewater. In addition, when dyeing with reactive dyes, in order to meet the fixation needs, alkali agents need to be added during the fixation stage. Dyes are easily hydrolyzed under alkaline conditions, leading to a decrease in dye utilization rate. In order to remove hydrolyzed and unfixed dyes, time-consuming and expensive water washing processes are often required. Water washing processes not only bring a large amount of wastewater, but also increase the difficulty of subsequent wastewater treatment, and greatly increase the cost of wastewater treatment. Based on this, in recent years, reactive dye dyeing and fixation of cotton fabrics have become a hot research topic.

01

Advantages, disadvantages, and dyeing mechanism of reactive dyes

  1. Structure of reactive dyes

Reactive dyes are water-soluble dyes with active groups in their molecular structure, which can covalently bond with hydroxyl groups on cellulose fibers and amino groups on protein fibers.

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The structure of reactive dyes can be represented by the general formula W-D-B-R, where W is a water-soluble group, D is the chromophore, B is the connecting group between the active group and the chromophore, and R is the active group. Dyeing involves the reaction of dyes with fibers to form covalent bonds. The structure of dyes determines the reaction activity, fixation rate, color fastness, and application conditions of reactive dyes.

Single reactive dyes can be divided into X type, K type, and KN type based on their reactivity. X type reactive dyes have strong reactivity, low stability, and are easy to hydrolyze, but have a fast dyeing rate, making them suitable for low-temperature dyeing; K-type dyes have weak reactivity and high stability, making them suitable for dyeing at higher temperatures; The performance and dyeing conditions of KN type reactive dyes are between the two. The main types of overflow and airflow dyeing machines in the market are KN type.

  1. Advantages and disadvantages of reactive dyes

2.1 Advantages of reactive dyes

(1) The covalent bond between reactive dyes and fibers generally does not dissociate, so once reactive dyes are dyed on fibers, they have good dyeing fastness, especially to wet treatment.

⑵ The brightness and brightness of the color are particularly good;

⑶ The cost is lower than that of vat dyes;

The chromatography is complete.

2.2 Disadvantages of reactive dyes

⑴ Poor resistance to chlorine and sunlight;

⑵ It is easy to hydrolyze in water and has low utilization rate, resulting in a large amount of colored wastewater. In order to meet the fixation needs of reactive dyes, an alkaline agent needs to be added during the dyeing stage, and the dyes are easily hydrolyzed under alkaline conditions, leading to a decrease in dye utilization rate.

02

Dyeing mechanism of reactive dyes

During dyeing, reactive dyes first dissolve in water, and the dyes and water molecules enter the interior of the fibers and are adsorbed by the fibers. The degree of dye uptake in the dye solution depends on the affinity of the dye and the concentration of electrolytes in the dye solution. During the dyeing process, the dye diffuses into the fiber and can also undergo chemical reactions with the fiber:

Cell OH+D-R-X → Cell O-R-D+HX

In addition, some dyes will undergo hydrolysis, and the hydrolyzed dyes can also adsorb onto the fibers. Cellulose fibers are not reactive in neutral media, and like reactive dyes and other dyes, they are only an adsorption relationship and cannot form strong chemical binding. Only when cellulose fibers are in alkaline media can they form covalent binding. This is because cellulose fibers form negative ions at this time, and their ionization concentration increases with the increase of pH.

Its reactivity is as follows:

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The dyeing process of reactive dyes includes three steps:

(1) Dye is applied, adsorbed onto the surface of the fiber, and diffused internally;

⑵ Fixation, chemical reactions occur between dyes and fibers, resulting in covalent bonding;

⑶ After washing, wash the unfixed floating dyes and hydrolytic dyes from the fibers.

03

Methods for improving the dyeing fastness of reactive dyes

  1. Choose the correct dye

The color fastness is closely related to the structure of dyes, and the selection of dyes should consider:

(1) Strong reactivity between dyes and fibers;

⑵ High fixation rate;

(3) Good acid and alkali stability;

The bond energy of the dye fiber covalent bond should be high, the chemical stability should be good, and bond breakage is not easy to occur;

The ability of antioxidants and reducing agents is good;

The compatibility and compatibility of color matching dyes are good.

  1. Choose the correct dyeing process

The dyeing process has a significant impact on the color fastness of dyeing, and the selection of dyeing process can be considered from the following aspects:

  1. ① Try to use higher temperature dyeing, such as using medium temperature dye (KN type) dyeing, the dyeing temperature can be increased from 60-65 ℃ to 75-80 ℃, and then dropped to 60-65 ℃, and alkali fixation is added;

② Adopting low-temperature weak alkali fixation;

③ The fixation temperature is generally 60-65 ℃;

④ The fixing agent uses a single soda ash;

⑤ The pH is stable at around 11;

⑥ Sodium carbonate is added in batches;

⑦ Using the correct water washing and soap washing process, the purpose of soap washing is to remove floating colors, and the purpose of water washing is to remove residual alkaline agents on the fibers. Water washing and soap washing should prevent the already bonded dyes from breaking, hydrolyzing, and falling off the fibers.

For this reason, it is important to pay attention to the following points: before soaping, it is necessary to wash with water. Soaping agents with good washing ability, dispersing ability, emulsifying ability, and carrying capacity must be selected, and they must be soaped at a higher temperature (90 ℃) under neutral conditions.

  1. For special reactive turquoise blue dyes, which accelerate hydrolysis under salt conditions, R Brilliant Blue has high directness and is very sensitive to alkali. Special dyeing processes are provided for these two special dyes, which are different from conventional KN type dyes. Different dyeing temperature curves and the process of adding salt and alkali are introduced for special turquoise blue and Brilliant Blue to avoid color splashes and improve reproducibility, which is very necessary.

04

Examples of Reactive Dye Processes

  1. Cuilan Craft
  2. Brilliant blue staining
  3. Conventional staining

05

Fixation and Fixation Treatment Methods

After dyeing and fixing with reactive dyes, although covalent bonds are formed with the fibers, there are more or less unfixed dyes on the fibers, especially in the pores of the fibers. In addition, during the application stage, the dyes in the fibers may also break bonds and form hydrolytic dyes. In a humid state, these dyes will diffuse from the fibers, causing desorption and reducing color fastness, especially for some dark colored products. To this end, after dyeing with reactive dyes or thorough washing, cotton fabrics are treated with some fixing agents for fixation treatment.

  1. Requirements for fixing agents

⑴ Environmental protection, no pollution to the environment;

After being fixed with a fixing agent, it can comprehensively improve various color fastness, including soaping fastness, dry wet grinding fastness, and acid alkali sweat fastness;

(3) After being fixed with a fixing agent, the color change should be reduced, which does not affect the color light of the fabric and has a small impact on the feel, making it easy for production control;

(4) Color fixing agents require safety, are non-toxic, and do not produce toxic or harmful substances during storage and use.

  1. Fixation mechanism

(1) Utilize the cationic groups in the formaldehyde free fixing agent molecule and the anionic groups in the dye molecule to form ion bonds, forming a precipitate on the fiber, thereby reducing the water solubility of the dye and improving the soaping fastness and white base dyeing fastness of the dyed fabric. The more cationic groups in the fixing agent molecule, the stronger the cation, and the better the soaping fastness of the fixed fabric.

(2) Utilize the film forming performance of the fixing agent to form a solid film, which firmly covers the dye on the fiber surface, thereby improving the fastness of dyed fabrics, especially to friction.

⑶ Utilize the intermolecular attraction between the fixing agent and fibers and dyes to increase the fixing strength of the fixing agent, thereby improving dyeing fastness.

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