1 Common quality problems in disperse dyeing
Polyester knitted fabrics are dyed under high temperature and high pressure, and color spots, stains, uneven dyeing, recrystallization, agglomeration and focus often occur. And other issues. 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-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. . Minor ones can be repaired by stripping and re-dying, but serious ones cannot be repaired and can only be picked and cut at the back, 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, the polyester fiber is hydrophobic. On the other hand, polyester molecules are arranged relatively tightly, and there are only small gaps in the fibers. When the temperature is low, the thermal motion of the molecules changes their positions to a smaller extent. Under humid conditions, polyester fibers do not swell violently like cotton fibers. This increases the voids and makes 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.
The morphology of disperse dyes in the dye liquor and fiber has undergone several changes:
①First of all, disperse dyes are in the form of particles The state (multiple single crystal dye molecules) is dispersed in the aqueous solution through a dispersant to form a dispersion system.
② Secondly, as the temperature increases, the thermal motion of the dye molecules intensifies and gradually differentiates into a single crystal state.
③Finally, 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.
Generally, the auxiliaries used in high-temperature and high-pressure dyeing of polyester fibers contain carriers that plasticize the fiber, solubilize disperse dyes, or stabilize the dye suspension. Dyeing auxiliaries such as surfactants play a very important role in the dyeing of polyester fibers.
3 Analysis of the causes of color spots and stains
High temperature There are two types of color spots and stains produced when 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 a repair agent or re-dyed after stripping; 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% 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.
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 reason why dye agglomeration produces color spots and stains
①Pretreatment
It is generally believed that the pretreatment of cotton knitted fabrics has a great impact on the dyeing quality, but not enough attention is paid to the pretreatment 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, polyester by-products in raw material production and oils in the weaving process can easily cause dyeing quality problems, such as color flowers, color differences, color spots, and color defects. spots, etc., so they must be treated before dyeing. Choose a suitable degreaser and heat it to 90°C to protect it.It can be eliminated by permanent 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
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
