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Popular Science | Why do color stains (black spots) appear when polyester is dyed?

After polyester knitted fabrics are dyed under high temperature and high pressure, due to the large oil content of the gray fabric during weaving, the agglomeration of fibers and d…

After polyester knitted fabrics are dyed under high temperature and high pressure, due to the large oil content of the gray fabric during weaving, the agglomeration of fibers and dye molecules produces oligomers, causing color stains. Seriously affects the quality of the product. In order to control the occurrence of this problem, we conducted analysis, discussion and research on color stains, found specific methods to control the problem, proposed preventive measures, improved product quality, and increased the company’s economic benefits.

Polyester knitted fabric

Polyester is a hydrophobic fiber. The fiber lacks groups that can combine with dyes. Water-soluble dyes cannot be used for dyeing. Only small molecular weight, non-containing fibers can be used. Dyeing with non-ionic disperse dyes with strong ionic water-soluble groups and low solubility. Polyester has a tight structure and is boiled and dyed under normal pressure. It is difficult for the dye to diffuse into the fiber and dye it through, so high-temperature and high-pressure dyeing is used.

Disperse dye dyeing

1. Disperse dyeing is carried out under high temperature and pressure under humid heat.

2. This dye is in the form of particles with multiple single crystal molecules dispersed in an aqueous solution through a dispersant. Its dyeing rate is very slow within 100℃. Even when dyed in a boiling dye bath, the dyeing rate is very slow. The rate and dyeing percentage are not high, so the pressure must be below 2atm (2.02×105Pa). The dye bath temperature can be increased to 120~130°C. As the temperature increases, the chain segments of the fiber molecules move violently, and the instantaneous pores generated are also The more and the larger the fiber, the faster the diffusion of dye molecules will be, which will increase the diffusion rate of the dye into the interior of the fiber and speed up the dyeing rate until the dye is absorbed and the dyeing is completed.

3. Since the solubility of disperse dyes in water is very low, 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 effect.

4. In order to achieve better dyeing results, it is usually necessary to add a certain amount of dyeing auxiliaries. The main functions of these dyeing auxiliaries in the dyeing process are:

Increase dispersion The solubility of dyes promotes the adsorption of disperse dyes to the fiber surface, plasticizes the fiber or increases the degree of swelling, accelerates the diffusion rate of disperse dyes in the fiber, and improves the dispersion stability of the dye.

5. 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 and other dyeing auxiliaries. Fiber dyeing hasplays a very important role.

Type of color stain (color point)

1. Color stain

Same color as the dye, darker Some things have the same mark, and most of them have front and back sides. Sometimes there are dirty things on the stains, which can be picked out. Such color spots and stains account for more than 60% of the total. Most of them appear irregularly, ranging in size from mung beans to soybeans, but in severe cases they can be as big as a fingernail.

2. Black dots

Mainly concentrated in brown, including light brown , light beige, coffee, dark brown colors, small black sesame-sized dots, as if a black oil-based pen was tapped on the cloth.

3. Color points of the dyed color

It’s better than the first In this case, the color spots should be deep and not big. Sometimes they can be picked out. Generally, there should be more at the two ends of a vat of cloth and less in the middle. These three categories can be found when the cloth comes out of the dye vat. After it came out, I used a touch-up agent to slightly correct the color, and seriously changed it to black.

4. Dispersed color points

There are also black golden spots. There are no black golden spots when the cloth comes out. After the cloth is dried in the dryer, there will be golden spots. Such yellow spots can be removed with alkali + degreaser, especially those that are changed to black. It is easier to appear dispersed on the canvas.

Causes of color stains

1. Selection of additives

The emulsification stability of the puffing agent is not good. Oily substances will precipitate at high temperatures, and dyes will agglomerate and appear color spots. If there is oil stain on the fabric that has not been cleaned, the swelling agent will come out during dyeing and condense the dye. It is recommended to add high temperature resistant dispersant.

2. Oil spots are generated due to uneven chemical composition and defoaming agent at high temperatures

The dye is not homogenized and filtered but enters the dyeing machine directly; the gray fabric foams when the temperature in the dye vat reaches 80°C, which is caused by the use of defoaming agent and the operating temperature is too high.

3. Improper control of dyeing heating speed

Dispersion of some dyes Not good, the dyeing temperature rise is not controlled properly. Too fast will cause dye spots; the dispersant or softening agent is not resistant to high temperatures, causing secondary separation of the dye to form dye spots; when adding additives,There is a requirement that the dye needs to be diluted and added separately (high solid content). If the operation is not carried out according to the requirements, suspended points will be combined with the dye to form dye points; some color points are very small and present a point-like distribution and can only be discovered by careful observation, and are different from Dyeing cloth with colored light. That’s because the dyes themselves contain insoluble substances. The dispersion of these dyes is destroyed during the manufacturing process and they cannot enter the fiber. This situation mostly occurs in light or bright colors; the diffusion effect of red dye may not be good, and a diffusing agent needs to be added to solve the problem.

If a leveling agent is added to the dyeing sheet without a dispersant, the dispersion will not be enough, the dye will aggregate, and color spots will occur.

Prevention and treatment methods


Pre-dyeing treatment of natural fibers is mainly to remove their symbionts. For synthetic fibers such as polyester, artificial “impurities” are removed, including oil added during spinning or weaving, as well as contaminated oil, dust, pigments, etc. Among them, the most noteworthy is oil.

Oil contains lubricants, emulsifiers, antistatic agents, etc. Applying oil is necessary for spinning and weaving of synthetic fibers such as polyester. However, the oil must be washed away during dyeing and finishing. If the oil is not washed before dyeing and is dyed with the oil, the oil will form a “dye-resisting film” on the surface of the polyester, preventing the dye from evenly diffusing and penetrating into the fiber. . Therefore, it is easy to cause uneven coloring, color flowers, stains and other dyeing defects. Moreover, it will also increase the floating color and affect the color fastness. If the pre-dyeing treatment process is not in place and the oil on the fiber is unevenly removed, cloud-like color flowers may occur;In the process, the dispersion stability of the dyes in the dye liquor is poor and agglomeration occurs. These oils will combine with the dye aggregates and adhere to the dyed materials to produce stains.

Countermeasures: Strengthen purification before dyeing

Before dyeing, first put the polyester dye in light alkali solution (if necessary, add an appropriate amount of high temperature resistant, non-foaming surfactant) at 85°C for 20 minutes (note that the alkali concentration should not be too high to avoid hydrolysis of the polyester). After draining, wash once, neutralize with acid if necessary, and then dye.

There are two purposes:

1. Extract most of the oligomers in the fiber and discharge them out of the machine in a dissolved state. This can significantly reduce the oligomers in the dye bath. content.

2. Remove the oil applied to the fibers or fabrics during the spinning or weaving process, as well as the contaminated oil, dust, pigments, florals, etc.

Dye selection:

Disperse dyes do not contain sulfonic acid groups (-S03Na), Hydrophilic groups such as carboxylic acid group (-COONa) only contain some polar groups such as hydroxyl groups, amino groups, ethanol groups, azo groups, and substituted amino groups, so they have poor hydrophilicity and are almost insoluble in water. The dye particles can only be coated with anionic dispersants and then dispersed in water.�. However, under the dual factors of high temperature above 100°C and rapid flow of dye liquor, the “coating energy” between dispersant molecules and dye particles will decrease. If the coated dye particles are freed, they will reaggregate into larger dye particles. This phenomenon is the “high temperature cohesion” of disperse dyes. Disperse dyes with different structures have different thermal cohesion phenomena.

Can be roughly divided into three types:

1. It is not prone to thermal condensation. These dyes do not agglomerate due to heating, cooling or collision with each other. Dyeing with such dyes generally does not cause uneven dyeing or stains.

2. It is easy to cause thermal condensation, but as the dye molecules diffuse into the fiber, the dye aggregates will deaggregate again. As long as the dye is evenly adsorbed on the fiber surface, it will generally not cause color flowers or colors. stains.

3. It is prone to thermal condensation and is difficult to depolymerize during the dyeing process. When dyeing with this type of dye, especially when the dispersant or leveling agent used has poor high-temperature dispersion capabilities, it is easy to cause uneven color due to thermal condensation of the dye. Even tar compounds are produced due to the combination of dye aggregates with impurities such as oligomers, oils, and fiber scraps in the dye liquor. The high-temperature and high-pressure dyeing method has strong filterability for the dye liquor and affects the uniformity of the dye liquor dispersion. High expectations.

Countermeasures: Use disperse dyes with low thermal cohesion.

Heating rate:

High temperature and high pressure dyeing is usually divided into four steps:

1. The dyeing material is first run in a buffer bath composed of high-temperature dispersant and acetic acid-sodium acetate to soak it evenly and eliminate the air in the dyeing material. At the same time, the temperature begins to rise.

2. Mix the dye thoroughly with a mixer to make a dye dispersion and add it at 50~60℃.

3. Raise the temperature to l35℃ at a rate of 1~2℃/min , and incubated for 30 min.

4. Cool down at a rate of 2.5℃/min, wash with water, and perform reduction cleaning if necessary. But the heating rate should not be too fast, otherwise it will easily cause uneven coloring of the dye and color blooming, especially When dyeing medium to light colors and whitening (with the addition of bluing agents such as Disperse Red or Disperse Blue 2BLN), dyeing defects are most likely to occur.

This is because the dyeing rate of disperse dyes is directly proportional to the dyeing temperature. As the dyeing temperature increases, the expansion speed of polyester and the coloring speed of dye will be significantly accelerated.

According to experience, the speed of heating rate is related to the following aspects:

1. It is related to the levelness of the dye used. If the dyeability is good, it can be done faster, but if the stainability is poor, it can be done slowly.

2. It is related to the dyeing depth. When dyeing dark colors, it can be faster, and when dyeing light colors, it should be slower.

3. It is related to the heat resistance of polyester. The glass transition temperature of polyester is 67 to 81°C. At 85°C, the microstructure of polyester is in a glassy state and absorbs color very slowly, so the temperature can be raised faster. At 90 to 110°C, polyester swells rapidly., the color absorption capacity is significantly enhanced, so the temperature rises slower; 110 ~ 135 ℃, because there is less dye in the dye liquor, the temperature rises faster.

4. It is related to the circulation state of the dye solution. If the pressure of the dye solution is large and the penetration is strong, and the dye solution can quickly and closely contact the fibers, the temperature can be raised faster, otherwise it must be slower.


To correctly control the heating speed, when the dyeing temperature Below 85°C, the dye rarely stains and can be heated directly. 90~110℃ is the temperature area where the dyeing speed increases the fastest, and the temperature rise rate needs to be strictly controlled below 1℃/min. It can be slightly faster within the range of 115~135℃, but the temperature rise rate needs to be controlled at about 3℃/min.

Dye liquid circulation status:

The dyeing process of knitted fabrics is the penetration process of dye liquor. Therefore, the requirements for the circulation state of the dye liquor are relatively high. The dye liquor must have strong penetrating power, which is the prerequisite for uniform dyeing. If the circulation pump is not strong enough or the valve is not properly controlled, the flow rate and flow rate of the dye liquor through the dyed material will be insufficient and uneven. Obviously, this can easily produce chromatic flowers, especially during the warming phase.


The circulation state of the dye solution must be good and must be ensured The flow and pressure of the dye liquor should be large because the flow and pressure of the dye liquor are large, which is conducive to the penetration and leveling of the dye liquor. For this reason, circulation pumps must be carefully selected and used. The flow rate of the pump should be 25~60 L/(kg·min).

Dye vat cleaning inspection

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Author: clsrich