Why is there always color stain (black spot) in polyester dyeing

Why is there always color stain (black spot) in polyester dyeing

The principle is actually very simple: after polyester knitted fabric is dyed at high temperature and high pressure, due to the high oil content of grey fabric during weaving, the oligomer produced by the condensation of fiber and dye molecules causes the problem of color stains, which seriously affects the quality of the product. In order to control the occurrence of this problem, the analysis, discussion and research of color stains were carried out, the specific methods of controlling the problem were found, and the preventive measures were put forward, which improved the product quality and increased the economic benefits of the company.

Moreover, it will increase the floating color and affect the color fastness. If the pretreatment process is not in place, and the oil agent on the fiber is not removed evenly, cloud colored flowers may be produced; If in the dyeing process, the dispersion stability of dyes in the dye solution is poor and condensation occurs, these oils will combine with the aggregates of dyes and adhere to the dyes to produce color spots.

  1. Disperse dye dyeing

a.  disperse dyes are carried out under high temperature, pressure and damp heat.

b.  the dye is dispersed in aqueous solution in the form of particles of multiple single crystal molecules through dispersant. Its dyeing rate is very slow within 100 ℃. Even if it is dyed in boiling dyeing bath, the dyeing rate and percentage are not high, so it must be pressurized at 2atm (2.02 × Below 105Pa), the dyeing bath temperature can be increased to 120 ~ 130 ℃. As the temperature increases, the chain segments of fiber molecules move violently, resulting in more and larger instantaneous pores. At this time, the diffusion of dye molecules also increases, increasing the diffusion rate of dye to the fiber, accelerating the dyeing rate until the dye is absorbed and the dyeing is completed.

c.  due to the low solubility of disperse dyes in water, the dyes in the dye solution need to be dispersed in the dye bath as a suspension through a large number of dispersants when dyeing polyester fibers. In order to achieve better dyeing effect.

d.  in order to achieve better dyeing effect, a certain amount of dyeing auxiliaries are usually added. These dyeing auxiliaries mainly play the following roles in the dyeing process:

Increase the solubility of disperse dyes, promote the adsorption of disperse dyes on the fiber surface, plasticize the fiber or increase the swelling degree, accelerate the diffusion speed of disperse dyes in the fiber, and improve the dispersion stability of dyes.

e.  the auxiliaries used in high-temperature and high-pressure dyeing of general polyester fiber include the carrier for fiber plasticization, the surfactant for solubilizing disperse dyes or stabilizing the dye suspension and other dyeing auxiliaries, which play a very important role in polyester fiber dyeing.

2.Types of stains (spots)

a.  color stain

Most of the things with the same color as the dyed ones have both sides. Sometimes there are dirty things on the color stains, which can be removed. Such color spots and stains account for more than 60% of the total. Most of them are irregular, ranging in size from mung bean to soybean, but in severe cases, they have large fingernails.

b.  black spot

It is mainly concentrated in brown, including light brown, light beige, coffee and dark brown. It is a small black sesame sized color dot, like a black oily pen on the cloth.

  1. color spot of dyed color

The color spot is darker than that in the first case, and it is not big. Sometimes it can be cut off. Generally, there are more at both ends of a cylinder of cloth and less in the middle.

These three categories can be found when the dye vat is out of the cloth. When it comes out, it is slightly colored with repair agent, and seriously blackened.

d.  dispersive color dots

There is also a black golden yellow dot. The black golden yellow dot does not exist when it is released from the cloth. The cloth has golden yellow dots after drying in the dryer. Such yellow dots can be removed with alkali + degreaser. In particular, it is easier to appear when it is blackened, which is scattered on the cloth.

3.Cause of color stain

a.  Selection of additives

The emulsifying stability of the expanding agent is not good. At high temperature, oily substances are separated out, and dye condensation appears color spots. The greasy dirt on the fabric has not been cleaned, and the swelling agent is free when dyeing, condensing the dye. It is recommended to add high temperature resistant dispersant.

b. The uneven chemical material and defoamer are generated after oil spots are generated at high temperature

The dye is directly fed into the dyeing machine without homogenization or filtration; When the temperature of grey cloth in the dye vat reaches 80 ℃, the cloth is foam suspended, and the operation temperature of defoaming agent is too high.

c.  improper control of dyeing heating rate

The dispersity of some dyes is not good, and improper or too fast control of dyeing temperature rise will cause dye spots;

The dispersant or dispersant is not resistant to high temperature, resulting in the secondary segregation of dyes to form dye spots;

Some additives are required to be thinned and added separately (high solid content), and the suspension point is not combined with the dye to form the dye point;

Some of the color dots are very small and can only be found by careful observation, and are different from the color light of the dyed cloth. That is because the dyes themselves contain insoluble substances, and the dispersity of these dyes is damaged during the manufacturing process and cannot enter the fiber. Most of these cases occur in the light color system or the bright color system;

The diffusion effect of red dye may be poor, which needs to be solved by adding dispersant.

If leveling agent is added to the dye sheet without dispersant, its dispersibility will not be enough, the dyes will gather and the color spots will be generated.

4.Prevention and treatment methods

A. Pretreatment

For natural fibers, pretreatment is mainly to remove their symbionts.

For polyester and other synthetic fibers, it is to remove the artificial “impurities”, including the oiling agent added in the spinning or weaving process and the contaminated oil dirt, dust, pigment, etc. Among them, the most noteworthy is the oiling agent.

The oil contains lubricant, emulsifier, antistatic agent, etc. Oiling agent is necessary for spinning and weaving of polyester and other synthetic fibers. However, the oil must be removed during dyeing and finishing. If the oil is not washed before dyeing and dyed with these oil, the oil will form a “dye resistance film” on the surface of polyester, preventing the dye from evenly diffusing and penetrating into the fiber. As a result, it is easy to cause uneven coloring, color streaks, color spots and other dyeing defects.

Moreover, it will increase the floating color and affect the color fastness. If the pretreatment process is not in place, and the oil agent on the fiber is not removed evenly, cloud colored flowers may be produced; If in the dyeing process, the dispersion stability of dyes in the dye solution is poor and condensation occurs, these oils will combine with the aggregates of dyes and adhere to the dyes to produce color spots.

Countermeasures: strengthen the purification treatment before dyeing

Before dyeing, the polyester dye shall be treated at 85 ℃ for 20 min in a light alkali solution (add an appropriate amount of high temperature resistant and non foaming surfactant if necessary) (note that the alkali concentration shall not be too high to avoid polyester hydrolysis). Wash once after draining, neutralize with acid if necessary, and then dye.

There are two purposes:

a. most oligomers in the fiber are extracted and discharged out of the machine in the dissolved state, which can significantly reduce the content of oligomers in the dyeing bath;

b. remove the oil agent applied to the fiber or fabric during spinning or weaving, as well as the contaminated oil dirt, dust, pigment, flower coat, etc.

B. Dye selection

Disperse dyes do not contain hydrophilic groups such as sulfonic acid group (-s03na), carboxylic acid group (-coona), but only some polar groups such as hydroxyl, amino, ethanol, azo and substituted amino groups. Therefore, they are very hydrophilic and almost insoluble in water. The dye particles can only be coated with anionic dispersant and dispersed in water. However, the “entrapment energy” between the dispersant molecules and the dye particles will decrease under the effect of the high temperature above 100 ℃ and the rapid flow of the dye solution. If the coated dye particles dissociate, they will regroup into larger dye particles. This phenomenon is the “high temperature condensation” of disperse dyes. The thermal condensation of disperse dyes with different structures is different.

There are three types:

a.  to prevent thermal condensation. These dyes will not agglomerate due to heating, cooling or collision. Generally, dyeing with such dyes will not cause uneven dyeing or stain;

b.  in order to easily generate thermal condensation, but with the diffusion of dye molecules to the fiber, the aggregation of dyes will be depolymerized again. As long as the dyes are evenly adsorbed on the fiber surface, generally, there will be no color streaks and stains;

c.  it is easy to produce thermal condensation and difficult to depolymerize in the dyeing process. When this kind of dye is used for dyeing, especially when the dispersant or leveling agent used has poor high-temperature dispersion ability, it is easy to cause uneven color due to the thermal condensation of the dye. Even tar compounds are produced due to the combination of dye aggregates and impurities such as oligomers, greases and fiber chips in the dye solution. Due to the high temperature and high pressure dyeing method, the dye solution has strong filtration, and the dispersion uniformity of the dye solution is highly required.

Countermeasures: choose disperse dyes with small thermal coacervation.

C.heating rate

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

a. The dye was first run in a buffer bath composed of high-temperature dispersant and sodium acetate acetate to soak it and remove the air in the dye, and began to heat up at the same time;

b. Fully mix the dye with a mixer to make a dye dispersion, and add it at 50 ~ 60 ℃;

c. Temperature rise to L35 ℃ at the rate of 1 ~ 2 ℃ / min and dye for 30 min;

d. Lower the temperature at the rate of 2.5 ℃ / min, wash with water, and conduct reduction cleaning if necessary, but the temperature rise rate should not be too fast, otherwise it is easy to cause uneven dye coloring and color streaking, especially when dyeing medium light color and whitening (adding blue dye such as disperse red or disperse blue 2BLN inside), color streaking defects are most likely to occur

This is because the dyeing rate of disperse dyes is directly proportional to the dyeing temperature. With the increase of dyeing temperature, the expansion speed of polyester and the dyeing speed of dyes will be significantly accelerated.

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

a. It is related to the levelness of the dyes used. Those with good levelness can be faster, while those with poor levelness should be slower.

b. It is related to the dyeing depth. It can be dyed faster in dark colors and slower in light colors.

c. It is related to the heat resistance of polyester. The glass transition temperature of polyester is 67 ~ 81 ℃, the microstructure of polyester is glassy at 85 ℃, and the color absorption is very slow, so the temperature rise can be faster; At 90 ~ 110 ℃, polyester swells rapidly and its color absorption capacity is significantly enhanced, so the temperature rise should be slower; 110 ~ L35 ℃, because there is less dye in the dye solution, the temperature rise can be faster.

d. It is related to the circulation state of the dye solution. If the pressure of the dye solution is high and the penetration is strong, and the dye solution can quickly and closely contact with the fiber, the temperature rise can be faster, otherwise it will be slower.


To properly control the heating rate, when the dyeing temperature is below 85 ℃, the dye is rarely dyed and can be heated directly. 90 ~ 110 ℃ is the temperature area with the fastest increase of dyeing rate, and the heating rate should be strictly controlled below 1 ℃ / min. It can be slightly faster in the range of 115 ~ 135 ℃, but the heating rate needs to be controlled at about 3 ℃ / min.

  1. Circulation state of dye solution

The dyeing process of knitted fabric is the penetration process of dye solution.

Therefore, the requirements for the circulation state of dye liquor are high. The dye liquor must have strong penetration, which is the premise to achieve uniform dyeing. If the strength of the circulating pump is not enough or the valve is not properly mastered, the flow and flow rate of the dye liquor penetrating the dye will be insufficient and uneven. Obviously, it is easy to produce chromatic aberration, especially in the heating phase.

Countermeasures: the circulation state of the dye liquor must be good. It must be ensured that the flow and pressure of the dye liquor are large, because the flow and pressure of the dye liquor are large, which is conducive to the penetration and leveling of the dye liquor. To this end, circulation pumps should be carefully selected and used. The flow of the pump shall be 25 ~ 60 L / (kg · min).

Cleaning and inspection of dye vat

Disperse dyes are prone to recrystallization due to repeated heating and cooling in the high-temperature cylinder. The crystal adheres to the cylinder wall and the filter screen cover, resulting in poor rubbing fastness of the dye, and even blocking the filter of the high-temperature and high-pressure dyeing machine.


a. Once a month, wash the residue on the cylinder wall with a high-pressure water gun. The filter must be removed and cleaned every 24 hours;

b. The dyeing times of the dyeing VAT shall be balanced. If recrystallized oligomers are produced in the dyeing bath, more dispersants must be added.

As the dyeing process is a dynamic, complex and changeable process, which is affected by many factors (such as process, equipment, operation, water, electricity, steam, etc.), it is necessary to pay close attention to the influence of various factors and strictly operate the process in the production process.

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Post time: Jun-17-2022