The causes of stains in polyester dyeing and how to deal with them

The causes of stains in polyester dyeing and how to deal with them

Knowing the principle is actually quite simple:

After polyester knitted fabric is dyed at high temperature and high pressure, due to the large oil content of grey fabric during weaving, the oligomers produced by the condensation of fibers and dye molecules cause color stains and seriously affect the quality of the product. In order to control the occurrence of this problem, the analysis and research of stains were carried out, and specific methods to control the problem were found, preventive measures were proposed, product quality was improved, and the company’s economic benefits were increased.

Polyester knitted fabrics:

Polyester belongs to hydrophobic fiber, the fiber lacks a group that can bind to the dye, can not be dyed with water-soluble dyes, can only use small molecular weight, does not contain strong ionic water-soluble groups, low solubility of non-ionic disperse dyes dyeing. Polyester structure is compact, atmospheric pressure boiling dyeing, dye is difficult to diffuse into the fiber to dye the fiber through, so high temperature and high pressure dyeing is used.

One. Disperse dye staining

1. Disperse dyes are

In a hot and humid state with high temperature and pressure

under.

2. The dye is in the particle state of multiple single crystal molecules dispersed in aqueous solution by dispersant, it is very slow to dye within 100 ° C, even if it is stained in a boiling dyeing bath, the dyeing rate and dyeing percentage are not high, so

It must be pressurized below 2atm (2.02×105Pa), and the dyeing bath temperature can be increased to 120~130 °C

Due to the increase in temperature, the chain segment of the fiber molecule moves violently, and the instantaneous pores produced are also more and larger, and the diffusion of the dye molecules also increases, increasing the diffusion rate of the dye to the inside of the fiber, so that the dyeing rate is accelerated until the dye is exhausted and the dyeing is completed.

3. Due to the low solubility of disperse dyes in water, the dyes in the dyeing solution

It is necessary to disperse a large amount of dispersant in the dyeing bath in a suspension when dyeing polyester fibers.

In order to achieve a better dyeing effect.

4. In order to achieve a better dyeing effect, it is usually necessary to add a certain amount of dyeing auxiliaries, which mainly play a role in the dyeing process:

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

5. The additives used in the general polyester fiber high temperature and high pressure dyeing contain dyeing aids such as carriers for fiber plasticization, surfactants for solubilization of disperse dyes or stabilization of dye suspensions, which play a very important role in polyester fiber dyeing.

Two. The type of color stain (color spot).

1

Stains

The same color as dyeing, darker, print-like things, most of them have front and back, and sometimes there are dirty things on the stains, which can be picked off. Such color spots and stains account for more than 60% of the total. Most of them appear irregularly, ranging in size from mung bean to soybean size, but in severe cases have large fingernails.

2

Black spots

Mainly concentrated in brown, including light brown, light beige, coffee, dark brown colors, black sesame size small color dots, as if a black oily pen on the cloth clicked.

3

The dot of the dyed color

It is darker than the color point in the first case, and not large, sometimes it can be cut off, generally more at both ends of a cylinder of cloth, less in the middle.

These three categories can be found when the dye vat is out of the cloth. Came out with a slight color correction with patch, severe blackening.

4

Scattered color spots

There is also a black golden spot, the black golden dot is not out of the cloth, the cloth has golden yellow color spots after drying by the dryer, such yellow dots can be removed with alkali + degreaser. Especially when it is changed black, it is more likely to appear, and it appears scattered on the cloth surface.

Three. Causes of stains

Choice of auxiliaries

The emulsification stability of the extruder is not good, and there is oil precipitation at high temperature, and color spots appear when the dye condenses. The oil stain on the fabric has not been cleaned, and the puffing agent is encountered during dyeing and frees out, condensing the dye. It is recommended to add a high-temperature resistant dispersant.

Uneven chemical materials and defoamers are generated after generating oil points at high temperatures

The dye is not homogeneous and unfiltered directly into the dyeing machine; When the temperature of the grey cloth reaches 80 °C in the dyeing vat, the foam hanging cloth is foamed, and the operating temperature is too high with a defoamer.

The heating rate of dyeing is improperly controlled

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

The dispersant or penghua agent is not resistant to high temperature, causing the dye to produce secondary separation to form dye points;

When the additive is added, some of them are required to be added separately (high solid content), and the suspension point is not operated according to the requirements to combine with the dye to form a dye point;

Some color spots are small and dot-like and can only be found by careful observation, and are different from the color light of dyed cloth. That’s because the dyes themselves contain insoluble substances that are destroyed during the manufacturing process and cannot enter the fiber. This mostly happens in light or bright colors;

The diffusion effect of red dyes may not be good, and it needs to be solved by adding such diffusers as a diffuser.

If the dyeing single adds leveling agent and does not add dispersant, then its dispersibility is not enough, the dye is aggregated, and the color spot is produced.

Four. Prevention and treatment methods

Pre-treatment

Pre-dyeing treatment is mainly about removing their symbiotics for natural fibers.

For synthetic fibers such as polyester, it is to remove their artificial “impurities” including oil additives added in the spinning or weaving process, as well as dirt, dust, pigments, etc. Among them, the most notable is the oil agent.

The oil contains lubricants, emulsifiers, antistatic agents, etc. The application of oil agent is necessary for the spinning and weaving of synthetic fibers such as polyester. However, the oil agent must be washed off during dyeing and finishing processing, if it is not washed before dyeing, dyeing with these oil agents, the oil agent will form a “dye barrier film” on the surface of polyester, preventing the uniform diffusion and penetration of the dye into the fiber. Therefore, it is easy to cause uneven coloring, resulting in color flowers, stains and other stains.

Moreover, it will also increase the floating color and affect the color fastness. If the dyeing pre-treatment process is not in place, the oil agent on the fiber is not evenly removed, and cloud-like color flowers may be produced; If during the dyeing process, the dye in the dyeing solution has poor dispersion stability and agglomeration occurs, these oil agents will combine with the aggregate of the dye and adhere to the staining substance to produce stains.

Countermeasures: Strengthen pre-dyeing purification treatment

Before dyeing, the polyester dye is first treated in light alkali solution (if necessary, an appropriate amount of high-temperature-resistant, non-foaming surfactant can be added) at 85 °C for 20 minutes (note, ‘alkali concentration should not be too high, so as not to hydrolyze polyester). After draining, it is washed once, neutralized by acid if necessary, and then stained.

The purpose is twofold:

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

2. Remove the oil agent applied to the fiber or fabric during the spinning or weaving process, as well as the dirt, dust, pigment, flower clothes, etc. Dye selection

Disperse dyes do not contain hydrophilic groups such as sulfonic acid (-S03Na) and carboxylic acid (-COONa), and only contain some polar groups such as hydroxyl groups, amino groups, ethanol groups, azo groups, and substituted amino groups, so the hydrophilicity is very poor and almost insoluble in water. The dye particles can only be coated with the help of an anionic dispersant and dispersed in water. However, under the dual factors of high temperature above 100 °C and rapid flow of dye solution, the “enveloping energy” between the dispersant molecules and the dye particles will be reduced. If the enveloped dye particles are free, they will re-aggregate into larger dye particles. This phenomenon is that the “high temperature cohesion” of the disperse dye is different for disperse dyes with different structures, and their thermal agglomeration phenomena are different.

It can be roughly divided into three types:

1. It is not easy to heat agglomeration. 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. In order to be prone to thermal agglomeration but as the dye molecules diffuse to the inside of the fiber, the aggregate of the dye will be depolymerized again, as long as the dye is evenly adsorbed on the surface of the fiber, it will generally not cause color flowers and stains;

3. It is easy to heat agglomerate, and it is difficult to depolymerize during the dyeing process. When dyeing with such dyes, especially when the dispersant or leveling agent used has poor high-temperature dispersion ability, it is easy to cause uneven color due to thermal agglomeration of the dye. Even tar compounds are produced due to the combination of dye aggregates with impurities such as oligomers, oil agents, and fiber chips in the dyeing solution, and the dyeing solution has strong filterability due to the high temperature and high pressure dyeing method, and the dispersion uniformity of the dye solution is very high.

Countermeasures: Use disperse dyes with low thermal cohesion.

Heating rate

HPHT dyeing is usually divided into four steps:

a. The staining material is first run in a buffer bath composed of high-temperature dispersant and sodium acetate, so that it is soaked and evenly removed from the air in the stain, and the temperature begins to rise at the same time;

b. Thoroughly beat the dye with a blender to make dye dispersion, and add it at 50~60 °C;

c. Heat up to l35 °C at a rate of 1~2 °C/min, and keep heat and dye for 30 min;

d. Cool down at a rate of 2.5 °C / min, wash with water, and do reduction cleaning if necessary, but the heating rate should not be too fast, otherwise it is easy to cause uneven dyeing and color flowers, especially when dyeing light colors and whitening (add dispersed red or dispersed blue 2BLN and other blue agents), color flower staining defects are the most likely to occur

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

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

a. It is related to the uniformity of the dye used, the uniformity is good, it can be faster, and the poor is slower.

B. Related to the dyeing depth, it can be faster when dyeing dark colors, and slower when dyeing light colors.

c. It is related to the heat resistance of polyester. The glass transition temperature of polyester is 67~81 °C, and the microstructure of polyester at 85 °C is glassy, and the color absorption is very slow, so the heating can be faster; 90~1 10 °C, polyester swells rapidly, and the color absorption ability is significantly enhanced, so the heating is slower; 110~l35 °C, because the dye in the dyeing solution is less, so the heating can be faster.

d. Related to the circulation state of the dyeing solution, if the pressure of the dye solution is large, the penetration is strong, and the dyeing liquid can be in close contact with the fiber quickly, the heating can be faster, otherwise it is slower.

Countermeasures

To correctly control the heating rate, when the dyeing temperature is below 85 °C, the dye is rarely stained and can be directly heated. 90~110 °C is the temperature area with the fastest increase in dyeing speed, and it is necessary to strictly control the heating rate below l°C/min. It can be slightly faster in the range of 115~135 °C, but it is necessary to control the heating speed at about 3 °C/min.

The circulating state of the dye solution

The dyeing process of knitted fabrics is the penetration process of dyeing solution.

Therefore, the circulation state of the dye solution is required to be high. The dye solution must have strong penetrating power, which is the premise of achieving uniform dyeing, if the strength of the circulating pump is not enough or the valve is improperly mastered, the flow rate of the dye liquid through the dye is insufficient and uneven. Obviously, this is prone to chromatic color difference flowers, especially during the heating phase.

Countermeasures: The circulation state of the dyeing solution must be good, and the flow and pressure of the dyeing solution must be large because the flow rate and pressure of the dyeing solution are large, which is conducive to the penetration and uniform dyeing of the dyeing solution. For this purpose, it is necessary to carefully select and use the circulation pump. The flow rate of the pump should be 25~60 L/(kg·min).

Cleaning inspection of the dye vat

Disperse dyes are often prone to recrystallization due to repeated heating and cooling in high-temperature cylinders, and the crystals are adhered to the cylinder wall and filter screen cover, resulting in poor friction fastness of the dyeing matter, and even blocking the filter of the high-temperature and high-pressure dyeing machine.

Preventive measures:

a. The dye tank is washed with a high-pressure water gun once a month, and the filter must be removed and washed every 24 h;

B. The number of dyeing times of the dyeing tank needs to be balanced, and if recrystallization oligomers are produced in the dyeing bath, more dispersants must be added.

Because the dyeing process is a dynamic, complex and changeable process, affected by a variety of factors (such as process, equipment, operation, water, electricity, steam, etc.), so in the production process, we should pay close attention to the influence of various factors and strictly operate the process.

Professional answers to textile and garment industry questions, welcome to pay attention to the public number: SCQY2018

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Professional answers to textile and garment industry questions, welcome to pay attention to the public number: SCQY2018

In order to answer your doubts, the above is what we have organized about 100% Polyester single dyed Pongee, hope it can help you.