company news about 【Technical Discussions】Why polyester dyeing color stains (little black) always appear?

Why polyester dyeing color stains (little black) always appear?

 

After the high-temperature and high-pressure dyeing of the polyester fabric, the high oil content of the grey fabric during weaving, the oligomer generated by the agglomeration of the fibers and the dye molecules, causes the color stain problem, which seriously affects the quality of the product. In order to control the occurrence of this problem, our company and the dyeing factory technicians carried out the analysis and research of color stains, found the specific methods of controlling the problems, proposed preventive measures, improved the product quality, and increased the company's economic benefits.

 

【About Polyester Fabric:】

Polyester is a hydrophobic fiber. The fiber lacks a group capable of binding to the dye. It cannot be dyed with a water-soluble dye. It can only be dyed with a nonionic disperse dye with a small molecular weight, no strong ionic water-soluble groups and low solubility. The polyester structure is compact, and it is boiled at atmospheric pressure. It is difficult for the dye to diffuse into the fiber to dye the fiber, so high temperature and high pressure dyeing is used.

 

1 Disperse dyeing

      1. The disperse dye is carried out under a hot and humid state under high temperature and pressure.

      2. The dye is dispersed in an aqueous solution by a plurality of single crystal molecules in a particle state through a dispersing agent, and the dyeing rate is slow at 100 ° C, and the dyeing rate is not high even in a dyeing bath. Therefore, it must be pressurized below 2 atm (2.02×105 Pa), and the dye bath temperature can be increased to 120-130 ° C. Due to the increase in temperature, the segment of the fiber molecules moves vigorously, and the instantaneous pores generated are larger and larger. The diffusion of the dye molecules also increases, increasing the rate of diffusion of the dye into the interior of the fiber, increasing the rate of dyeing until the dye is exhausted to complete the dyeing.

      3. Since the solubility of the disperse dye in water is very low, the dye in the dye solution needs to be dispersed in the dye bath by a large amount of dispersant in the dyeing bath to dye the polyester fiber. 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 auxiliary.

The role of these dyeing auxiliaries in the dyeing process is mainly:

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

 

      5. Generally, the additives used in high temperature and high pressure dyeing of polyester fiber contain dyeing aids such as a fiber plasticizing carrier, a dispersing dye solubilizing or a dye suspension, and the like, which is very important for dyeing polyester fibers.

 

2 Type of color stain (color point)

     1. Color stain

Like the dyed color, deeper, the same things as the print, most of them have the front and back, and sometimes the stains on the stains are dirty, you can get rid of it. Such color points and color stains account for more than 60% of the total. Most of them appear irregular, ranging in size from mung beans to soy beans, but in severe cases there are large fingernails.

 

      2. Black spot

 

Mainly concentrated in brown, including light brown, light beige, coffee, dark brown color, black sesame size small color point, like a black oily pen on the cloth.

 

      3. Color point of the dyed color

 

It is deeper than the color point of the first case, and it is not big. Sometimes it can be smashed. Generally, there are more two ends of a cylinder cloth, and there are fewer in the middle.

These three types can be found when the dyeing cylinder is clothed. A slight repair with a patching agent was made, and the black color was severely changed.

 

      4. Scattered color point

 

There is also a black golden point. The black golden dots are not present when the cloth is clothed. After the cloth is dried by the dryer, there are golden yellow dots. These yellow spots can be removed with alkali + degreaser. Especially when it is black, it is more likely to appear, and it is spread on the cloth surface.

 

3 Cause of color stain formation

      1. Auxiliary choice

The emulsifying agent has poor emulsifying stability, and an oily substance precipitates at a high temperature, and the color point of the dye agglomerates. The oil on the fabric is not treated cleanly, and the expansion agent is released when dyeing, and the dye is condensed. It is recommended to add a high temperature dispersant.

 

      2. Uneven material and defoamer produce oil after high temperature

 

The dye is not homogenized and is not filtered directly into the dyeing machine; when the temperature of the grey cloth reaches 80 ° C in the dyeing tank, the foam is suspended, and the operating temperature of the defoaming agent is too high.

 

      3. Improper control of dyeing temperature

 

Some dyes have poor dispersibility, and improper control of dyeing temperature and too fast will cause dye spots;

When the additive is added, some requirements are required to be separately diluted (high solid content), and the suspension point is not combined with the dye to form a dye point;

 

4 Prevention and treatment

 

- Pretreatment

Pre-dye treatment is mainly for the removal of its commensal from natural fibers.

For synthetic fibers such as polyester, the artificial addition of "impurities" includes oils added during spinning or weaving, as well as contaminated grease, dust, pigments, and the like. Among them, the most notable is the oil agent.

The oil agent contains a lubricant, an emulsifier, an antistatic agent, and the like. The application of an oil agent is necessary for spinning and weaving synthetic fibers such as polyester. However, the oil must be washed out during dyeing and finishing. If it is not washed before dyeing, it will be dyed with these oils. The oil will form a "blocking film" on the surface of the polyester, which will prevent the dye from spreading evenly into the fiber. . Therefore, it is easy to cause uneven coloring, and coloring, staining, and the like are caused.

Moreover, it also increases the floating color and affects the color fastness. If the pre-dyeing treatment process is not in place and the oil agent on the fiber is not uniformly removed, cloud-like color flowers may be produced; if the dye dispersion in the dyeing process is poor in dispersion stability and cohesion occurs, these oil agents may Aggregates of the dye bind and stain the dye to produce stains.

 

Response measures: Strengthen pre-dye purification treatment:

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

There are two purposes:

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 dye bath;

2. Remove the oil applied to the fibers or fabric during spinning or weaving, as well as contaminated grease, dust, pigments, floral coats, and the like.

 

 

- Dye selection

Disperse dyes are poorly hydrophilic and hardly soluble in water. The dye particles can only be coated by means of an anionic dispersant and dispersed in water. However, under the dual factors of high temperature above 100 ° C and fast flow of dye liquor, the "envelope energy" between the dispersant molecules and the dye particles will be reduced. If the coated dye particles are freed, they will re-aggregate into larger dye particles. This phenomenon is a disperse dye of different structures of "high-temperature cohesiveness" of disperse dyes, and its thermal cohesion phenomenon is different.

Generally divided into three types:

1. In order to prevent heat condensation. These dyes do not coalesce due to temperature rise, temperature drop, or collision. Dyeing with such dyes generally does not cause uneven dyeing or staining;

2. In order to easily cause thermal coagulation, as the dye molecules diffuse into the interior of the fiber, the aggregate of the dye will be depolymerized again, as long as the dye is uniformly adsorbed on the surface of the fiber, generally no color flower or color stain will be caused;

3. In order to easily cause thermal coagulation, it is difficult to depolymerize during the dyeing process. When dyeing with such dyes, especially when the dispersing agent or leveling agent used is poor in high-temperature dispersing ability, it is easy to cause uneven color due to thermal coagulation of the dye. Even due to the combination of dye aggregates and impurities such as oligomers, oils, and lint in the dye liquor, the tar formation is highly filtered by the high temperature and high pressure dyeing method, and the dispersion uniformity of the dye liquor is strong. high expectation.

 

Response measures: Select a disperse dye with low thermal cohesion.

 

- Heating rate

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

a. It is related to the levelness of the dye used. The leveling property is good, it can be faster, and the worse is slower.

b. It is related to the depth of dyeing. It can be faster when dyeing dark color and slower when dyeing light color.

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

d. It is related to the circulation state of the dyeing liquid. If the pressure of the dyeing liquid is large and the penetrating power is strong, and the dyeing liquid can quickly contact with the fiber, the temperature rises faster, otherwise it is slower.

 

 

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

 

- Dyeing state of dye liquor

In the dyeing process, the knit fabric is the penetration process of the dye liquor.

Therefore, the circulation state of the dye solution is required to be high. The dyeing liquid 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 not properly grasped, the flow rate and flow rate of the dyeing liquid penetrating the dyeing agent will be insufficient and uneven. Obviously, it is easy to produce chromatic aberrations, especially during the warming phase.

 

Response measures: The circulating state of the dyeing liquid must be good. It must be ensured that the flow rate and pressure of the dyeing liquid are large because the flow rate of the dyeing liquid is large and the pressure is high, which is beneficial to the penetration and leveling of the dyeing liquid. To do this, carefully select and use a circulating pump. The pump flow should be 25 to 60 L/(kg•min).

 

- Dyeing cylinder cleaning inspection

Disperse dyes are often re-crystallized due to repeated heating and cooling in high-temperature cylinders. The crystals adhere to the cylinder wall and the filter cover, causing poor dyeing fastness and even filters for high-temperature and high-pressure dyeing machines. Blocked.

 

Response measures: a. The dyeing tank flushes the cylinder wall residue with a high-pressure water gun once a month, and the filter must be removed and washed every 24 hours;

b. The dyeing cylinder needs to be dyed in a small number of times. If a recrystallized oligomer is produced in the dye bath, more dispersing agent must be added.

Since the dyeing process is a dynamic, complex and variable process, affected by many factors (such as process, equipment, operation, water, electricity, steam, etc.), it is necessary to pay close attention to various factors in the production process. Impact, strict operation process.