Polyester knitted fabrics are dyed under high temperature and high pressure, and problems such as color spots, stains, uneven dyeing, recrystallization, agglomeration and focus often occur. That is, oligomers, dye agglomeration, etc. are produced in fibers during dyeing processing. Specific measures to prevent color spots and stains are analyzed from each process, so as to reduce the occurrence of quality problems in production.
1. Polyester knitted fabrics include polyester Covering cotton and polyester-cotton blended knitted fabrics
In actual production, problems such as color spots and spots on the fabric surface after disperse dyes are dyed under high temperature and high pressure are often encountered. Slight cases can be repaired by stripping and re-dying, but serious cases cannot be repaired. Defects can only be picked and cut at the back end, causing great losses.
The raw material of polyester fiber is hydrophobic synthetic fiber: on the one hand, the molecular structure of polyester lacks active groups that can bind to dyes like cellulose fiber or protein fiber. On the other hand, polyester molecules are arranged more closely, and there are only smaller gaps in the fiber; when the temperature is lower, The thermal movement of molecules changes their position to a small extent. Under humid conditions, polyester fiber does not expand the gaps through violent swelling like cotton fiber, making it difficult for dye molecules to penetrate into the fiber.
Therefore, disperse dye dyeing of polyester knitted fabrics requires high temperature and high pressure dyeing.
2. Disperse dye dyeing
During high temperature and high pressure dyeing, The process of dyeing polyester fibers with disperse dyes.
Divided into four stages
1. Disperse dyes migrate from the dye solution to the fiber surface due to concentration differences:
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2. Disperse dyes are adsorbed to the fiber surface:
3. Disperse dyes penetrate into the fiber:
4. Disperse dyes migrate inside the fiber.
Thus achieving a good leveling effect in the process of these four stages.
The morphology of disperse dyes in dye liquor and fibers
Experience Several changes have been made:
First. Disperse dyes are in the form of particles (multiple single crystal dye molecules) dispersed in an aqueous solution through a dispersant. Form a dispersed system. Secondly. As the temperature increases. Thermal movement of dye molecules intensifies. Gradually differentiate into a single crystal state. Finally. The disperse dye in the single crystal state penetrates into the fiber. Transfer and reach balance within the fiber.
The dye molecules in the dye solution continue to enter the interior of the fiber. At the same time, a certain proportion of the disperse dye inside the fiber is transferred from the inside of the fiber to the dye liquor.
Dyeing reaches equilibrium at all stages of the disperse dye dyeing process. There will be single-crystal disperse dyes that get rid of the constraints of the dispersant when they obtain enough energy and combine with other single-crystal disperse dyes to form larger crystals (or recrystallization). Once the recrystallized crystals are large enough. Dye spots or color stains will be formed, which can increase the degree of plasticization of the fiber. This will help the dyeing process to be completed as quickly as possible.
In addition, the solubility of disperse dyes in water is very low, and the dyes in the dye bath need to be dispersed in suspension in the dye bath through a large amount of dispersant when dyeing polyester fibers; In order to achieve better dyeing results, it is usually necessary to add a certain amount of dyeing auxiliaries.
The role of dyeing auxiliaries in the dyeing process
a. Appropriately increase the solubility of disperse dyes: b. Promote the adsorption of disperse dyes to the fiber surface: c. Plasticize the fiber or increase the degree of swelling. Accelerate the diffusion rate of disperse dyes in fibers: d. Improve the dispersion stability of dyes.
Generally, the auxiliaries used in high-temperature and high-pressure dyeing of polyester fibers contain dyeing auxiliaries such as carriers that plasticize the fibers, solubilize disperse dyes or stabilize dye suspensions, etc. The agent plays a very important role in dyeing polyester fiber.
3. Analysis of the causes of color spots and stains
There are two types of color spots and stains produced when high-temperature and high-pressure dyeing machines dye polyester knitted fabrics.
One type is color spots and stains caused by the agglomeration of dyes. It can be repaired with repair agent or stripped and re-dyed.
The other is due to the production of oligomers in the fiber, which causes color spots and stains that are difficult to remove.
1. The reason why oligomers produce color spots and stains
Oligomers also Oligomer is a low molecular substance with the same chemical structure as the polyester fiber that exists inside the polyester fiber; it is a by-product in the polyester spinning process. Generally, polyester contains 1% to 3% of oligomers. Most of the oligomers are cyclic compounds formed by 3 ethyl terephthalates. When the temperature exceeds 120°C, the oligomers can dissolve in the dye bath and crystallize out of the solution, combining with the condensed dyes. . When cooled, it will deposit on the surface of machinery or fabrics, forming stains, color spots and other defects. Disperse dye dyeing is generally kept at 130°C for about 30 minutes to ensure the dyeing depth and fastness.
Light colors can be kept at 120℃ for 30 minutes, while dark colors must be pre-treated before dyeing. In addition, dyeing under alkaline conditions is also an effective method to resolve oligomers.
2. The reasons why dye agglomeration produces color spots and stains
a. Pre-treatment It is generally believed that the pre-treatment of cotton knitted fabrics has a great impact on the dyeing quality, but not enough attention is paid to the pre-treatment of polyester dyeing. Use low-quality degreasers to directly dye with slight treatment or no treatment at all. If dyeing quality problems occur, the operator may be suspected of improper operation. In fact, polyesterThings etc. These problems can be avoided well by refining before dyeing or by adding chelating agents to the dye bath. Once stains occur, they can be eliminated with alkaline reduction cleaning or acid treatment.
3. Aggregation and focus of thinness
Causing factors
It weakens the dissolving effect of the dispersant, reduces the electrostatic repulsion, increases the collision rate of the dye particles and increases their kinetic energy. Generally, the higher the dyeing concentration and temperature, and the longer the dyeing time, the greater the possibility of agglomeration and focusing. Dyeing auxiliaries such as carriers and leveling agents can easily replace the dispersant mixed in the dye, thereby reducing the dispersion stability.
Measures to improve stability during dyeing
① Disperse the dye at 40°C and use concentrated
②The temperature control is best when the dye liquor is heated;
③Use a dispersant with a protective colloidal effect;
④ No additives that have a cloud point at high temperatures;
⑤ Wash away all dyes and yarn auxiliaries before dyeing, including Emulsifier;
⑥ When dyeing at high temperature, carriers and non-ionic leveling agents must not be added before most of the dye has been dyed on the fabric;
⑦No salts are used, only acetic acid is used to adjust the PH value;
⑧Yarns or piece-dyed fabrics should be pre-shaped appropriately, and laboratory tests should be done to ensure Dispersion stability of disperse dyes. </p
