The dyeing of acrylic by cationic dyes is mainly due to the ionic bonding between the acidic groups of the acrylic molecules and the pigment cations of the cationic dyes. At the beginning of dyeing, since the surface of acrylic has a high negative potential, the dye and pigment cations are quickly adsorbed on the surface of the acrylic, neutralizing the negative charges. This adsorption only occurs on the surface of the fiber and does not reach the interior of the fiber.
As the temperature increases, when the temperature reaches the glass transition temperature of acrylic , due to the generation and intensification of the movement of molecular chain segments, the micro-gaps between molecular chains increase. At this time, the dye diffuses into the fiber, because the acrylic structure is relatively tight, and the dye and fiber have large electrical properties. Therefore, the diffusion resistance of the dye inside the fiber is relatively large, so some people think that the dye diffuses one by one. In short, the diffusion of cationic dyes in acrylic requires higher energy, so dye migration is more difficult. Temperature has a great influence on the diffusion rate of dyes.
1. Structure and properties of cationic dyes
Cationic dyes are used to adapt to the dyeing of acrylic fiber, on the basis of basic dyes A new type of dye developed through improvement. This type of dye can dissociate into positively charged pigment cations in aqueous solution, so it is called cationic dye.
2. Basic principles of cationic dyeing
The process of salt-forming combination between the colored cations of cationic dyes and the negatively charged groups on the fiber, that is:
Fiber-COOH+ CL-Dye-→ Fiber -COO-Dye + HCL
This combination process is a positioning adsorption process. The dyeing of acrylic fiber with cationic dyes includes three processes.
1.Adsorption- Acrylic fiber becomes negatively charged due to the dissociation of acidic groups in water, and it is easy to absorb dye cations, neutralizing the negative charge on the fiber surface. As the dye ions continue to diffuse into the fiber, the surface charge of the fiber increases, and the fiber absorbs the dye again.
2.Diffusion- The diffusion of dye into fibers is a complex process. The diffusion of dye molecules is usually difficult and often requires an increase in temperature.
3.Fixed- Dye fixation is the process of salt formation between dye cations and anionic groups on the fiber. It is basically an irreversible reaction.
3. Dyeing characteristics of cationic dyeing
When cationic dyes are used to dye acrylic, the adsorption is very fast and can easily lead to uneven adsorption and temperature rise. Excessive speed can easily causeUneven dyeing. Once the dyeing is uneven, it is difficult to rely on dye transfer to obtain even dyeing. To do this, the following factors must be noted:
1. Adjustment of pH value:In order to level dye, it is generally most appropriate to control the pH value at 4-5 for cationic dye dyeing.
2. Temperature control:The temperature is increased in stages, generally faster when it is below 60 degrees, and slower when it exceeds 60 degrees. It must be kept warm for 30-50 minutes after 100 degrees to ensure good penetration, otherwise the color fastness will be affected.
3. Application of retarder: In order to obtain a level dyeing effect, a retarder can be added to the dye bath. The retarder itself is a colorless cationic dye. It can also dye acrylic during cationic dyeing.
4. Electrolyte:Electrolyte can produce a retarding effect in dyeing acrylic fiber with cationic dyes.
