Polyester knitted fabric are dyed by high-temperature and high-pressure, often have problems such as color point, uneven dyeing, recrystallization, agglomeration. Oligomers, dye agglomeration are generated in the fiber during dyeing processing.

Polyester fabric including polyester cover cotton and polyester/cotton blended fabric

In actual production, will appear problems such as color point and color stain on the surface of disperse dyeing after high-temperature and high-pressure. Minor problems can be stripped and re-repaired, serious cannot be repaired, only be cut in the finishing process, causes a huge loss.

Polyester fiber raw materials are hydrophobic synthetic fibers:

1. A lack of cellulose fibers or protein fibers in the molecular structure of polyester, an active group capable of binding to dyes.
2. Polyester molecules are arranged closely.

There are only small gaps in the fiber, and when the temperature is low, the thermal motion of the molecule only changes its position to a small extent. Under wet conditions, polyester fiber does not cause the void to increase as the cotton fiber can swell vigorously. Dye molecules are difficult to penetrate into the interior of the fiber.

Therefore, the polyester knitted fabric disperse dyeing needs to be dyed by high-temperature and high-pressure.

Disperse dyeing

When dyeing at high-temperature and high-pressure, the disperse dyeing on polyester fiber is divided into 4 stages.

1. Disperse dyes due to poor concentration, migration from the dye solution to the fiber surface;
2. Disperse dye is absorbed onto the fiber surface;
3. Disperse dye penetrate into the fiber;
4. Disperse dye migrates the fiber inside;

Several changes in the morphology of disperse dyes on dye liquors and fibers.

1. The disperse dye is dispersed in an aqueous solution in a state of particles (a plurality of single crystal dye molecules) through a dispersing agent to form a dispersion system.
2. As the temperature increasing, the thermal motion of the dye molecules intensifies and gradually differentiates into a single crystal state.
3. The disperse dye in the single crystal state penetrates into the fiber inside and transfers and balances inside the fiber.

The dye molecules in the dye solution continuously enter the fiber inside and a certain proportion is transferred from the fiber inside to the dye solution. The solubility of disperse dyes in the water is very low, and it is necessary to disperse a large amount of dispersing agent in the dye solution and then dye the polyester fiber.

In order to meet a better dyeing effect, it is necessary to add a certain amount dyeing auxiliary.

The role of dyeing auxiliaries in the dyeing process:

1. Increasing the solubility of disperse dyes;
2. Promoting the absorption of disperse dyes on the fiber surface;
3. Increasing the degree of swelling of the fiber and accelerate the diffusion rate of the disperse dye in the fiber;
4. Increasing the dispersion stability of the dye;

Dyeing auxiliaries such as carriers for plasticizing fibers, solubilizing disperse dyes or stabilizing dye suspensions in high-temperature and high-pressure dyeing play an important role in dyeing polyester fibers.

Caused of color points and stains

1. The cause of color points and stains of the oligomer

The oligomer is a by-product of the polyester spinning process, generally, the polyester contains 1%-3% of oligomer. When the temperature exceeds 120℃, the oligomer can be dissolved in the dye solution and precipitated crystals, which are combined with the coagulated dye.

Upon cooling, the oligomer deposits on the machine and fabric surface to form color points and stains. Disperse dyeing is kept at 130℃ for 30min to ensure the dyeing depth and fastness. Therefore, the light color can be kept at 120℃ for 30min and the dark color should be pre-finishing before dyeing. Dyeing under alkaline conditions is also an effective method for solving oligomers.

2. The cause of color points and stains of the condensation of dyes
2. 1 Pre-finishing

The by-product of polyester in the production of raw materials and the oil agent in the weaving process are easy to cause dyeing quality problems. Therefore it is necessary to pre-finishing before dyeing. Using a suitable degreaser and heating to 90℃ for 10min and washing.

1. 2 Selection of dyes and auxiliaries

Improper selection of dyes

When dyeing dark color, the thermal collision of the dye particles under high-temperature dyeing conditions increases; Re-agglomeration of dye molecules and improper use of fillers in dyes can make the color points and stains.

Improper selection of auxiliaries

The poor quality dispersant condenses on the surface of fabric at high-temperature with impurities inside the fiber and in the dyeing chamber; it is easy to react with different ionic auxiliaries to form demulsification, forming precipitates on the fabric, making color points and stains.

Measures to prevent color points and stains

1. The grey fabric need to treat with 3%NaOH₃ and 100% surface active detergent at 130℃ for 60min, liquor ratio 1:10-1:15 has a certain erosion effect on the polyester fiber, but help to remove oligomers and can improve the pilling for the medium and short fibers.
2. Controlling the dyeing temperature below 120℃, using the carrier dyeing method can reduce the oligomers and get the same dyeing depth.
3. Adding the dispersing gum auxiliaries during dyeing, can get the leveling effect and prevent the oligomers from precipitating on the fabric.
4. After dyeing, the dye solution is quickly discharged from the machine at high-temperature and drain time max. 5min. Because the oligomers are uniformly distributed in the dye solution at 100-120℃ and below 100℃ easy to accumulate and precipitate on the dye, but it is easy to form folds for thick fabric.
5. Dyeing under alkaline conditions can reduce the formation of oligomers and remove the oil residue on the fabric surface.
6. After dyeing, using reducing agents to washing. Adding 32.5% NaOH₃ 5ml/L and sodium hydrosulfite 3-4g/L at 70℃ for 30min, then cool washing-hot washing-cool washing and neutralized with acetic acid.
7. For heavy knitted fabric the liquor ratio should more than 1:10 and choose the overflow dyeing machines and ensure the fabric running speed.
8. For dyes sensitive to metal ions such as RED 60 must be use soft water for production.
9. Strengthen production site management, strict using procedures, auxiliaries and dyes should be added separately, mixing dyes and auxiliaries with dilution is forbidden and dyes need to be filtered before entering the dyes tank.