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.
Polyester knitted fabrics include polyester-covered cotton and polyester-cotton blended knitted fabrics
In actual production, problems such as color spots and stains 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 the properties of cellulose fiber or protein fiber. Reactive groups that can bind to dyes.
On the other hand, polyester molecules are arranged more closely. There are only small gaps in the fiber. When the temperature is low, the thermal movement of molecules changes their position to a smaller extent. Under humid conditions, polyester fiber does not increase the gap through violent swelling like cotton fiber. It is difficult for dye molecules to penetrate into the fiber.
Therefore, disperse dye dyeing of polyester knitted fabrics requires high temperature and high pressure dyeing.
Disperse dye dyeing
During high temperature and high pressure dyeing. The process of dyeing polyester fibers with disperse dyes is divided into four stages
1. Disperse dyes migrate from the dye solution to the fiber surface due to concentration differences:
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 good leveling effect. And in the process of these 4 stages. The morphology of disperse dyes in the dye liquor and fiber has undergone several changes:
First of all, disperse dyes are in the form of particles (multiple single crystal dye molecules) dispersed through dispersants. in aqueous solution. form a decentralized system.
Secondly, as the temperature increases, the thermal motion of the dye molecules intensifies and gradually differentiates into a single crystal state.
Finally, the disperse dye in the single crystal state penetrates into the fiber, transfers and reaches equilibrium inside the fiber.
The dye molecules in the dye liquor continue to enter the interior of the fiber, and at the same time, a certain proportion of the disperse dye inside the fiber is transferred from the interior of the fiber to the dye liquor.
Dyeing reaches equilibrium at various stages of the dyeing process with disperse dyes. 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. The formation of dye spots or stains can increase the degree of plasticization of the fibers and 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 effects, 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 speed 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.
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 reasons why oligomers produce color spots and stains
Oligomer, also known as 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% oligomers. Most of the oligomers are cyclic compounds formed by three ethyl terephthalates. When the temperature exceeds At 120°C, oligomers can dissolve in the dye bath and crystallize out of the solution, combining with the condensed dye.
During cooling, it deposits 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 dyeing depth and fastness. therefore.
For light colors, you can choose to keep it at 120℃ for 30 minutes. Dark colors must be pre-treated before dyeing.
In addition, dyeing under alkaline conditions is also an effective method to solve oligomers.
2. Dye condensation productsMore dispersants must be added for recrystallization;
3. Some allium quinone red disperse dyes such as C.I. Disperse Red 53, 60, 92, 121, 132, 159, etc. Near the end of dyeing, even if its concentration is well below the saturation level, recrystallization is prone to occur, especially when dyeing darker colors.
Especially when dyeing with hard water, it is easy to chelate with metal ions. The resulting chelates are poorly soluble under dyeing conditions and can leave blue spots or streaks on the fabric.
The factors that cause recrystallization include additives added during spinning, winding oil, alkaline residues, etc.
Refining before dyeing or adding chelating agents to the dye bath can avoid these problems. Once stains occur, they can be eliminated with alkaline reduction cleaning or acid treatment.
3. Aggregation and focus of thinness
Causing factors
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
①Disperse the dye at 40°C and use a concentrated dispersion;
②The temperature control is best when the dye liquor is heated;
③Use a dispersant with a protective colloidal effect;
④Do not use auxiliaries that have cloud points at high temperatures;
⑤Wash away all dyes and yarn auxiliaries including emulsifiers before dyeing;
⑥When dyeing at high temperature, carriers and non-ionic leveling agents must not be added before most of the dyes have been dyed on the fabric;
⑦No need For salts, only use acetic acid to adjust the PH value;
⑧Yarns or piece-dyed fabrics should be properly pre-shaped, and laboratory tests should be done to ensure the dispersion stability of disperse dyes. </p
