Cotton Knowledge: Cotton bulk fiber dyeing

Cotton loose fiber dyeing can make cotton yarns get special effects and increase added value. The article introduced the dyeing equipment and process flow of cotton loose fiber, as well as the effective way to improve the dyeing quality and the corresponding measures.

Cotton knitted garments are increasingly favored by consumers for their good moisture absorption, breathability and wearing comfort. According to the product style, production cost and market demand, cotton knitwear can be dyed at various stages of its production, such as loose fibers, slivers, yarns, fabrics, clothing dyeing and so on. In order to obtain a certain effect of cotton yarn, such as a richer color gradation, a feeling of blemishes, or to reduce the chromatic aberration of a yarn, a loose fiber dyeing method may be used. After dyeing cotton with different colors and mixing spinning and weaving in a specific ratio, colorful and unique mixed yarns can be obtained, which greatly increases the added value of the yarn.

In the early 1990s, Zhejiang Yuyao Jiufeng Dyeing Factory first proposed the cotton loose fiber processing technology, first of all, to first bleaching and dyeing the bulk cotton, and to work closely with Ningbo Bailun Group Co., Ltd. and the former Ninghai Cotton Textile Factory. R&D has greatly promoted the implementation of this technology and has achieved great success, driving the rapid development of the entire yarn spinning industry. At the China International Patent Technology Conference in 2005, the patent “Cotton Loose Fiber Processing Method” obtained by Jiufeng Dyeing Factory and Ningbo Haide Knitting Dyeing Co., Ltd. won the gold medal.

The domestic bulk fiber dyeing machine is mainly an open dyeing machine. Its appearance is similar to the cheese dyeing machine. It is composed of a master cylinder, a dyeing cage, and a main pump. There are no cylinder heads and auxiliary cylinders. Such dyeing cylinders are inexpensive, easy to operate, and structural. Compact, small footprint (only to space development) and so on. Before dyeing, the loose fiber is loaded into the dyeing cage. After compaction, the dyeing cage is hung into the master cylinder, and the circulating main pump is directly started. The dyeing liquid is discharged from the porous mandrel of the dyeing cage through the fiber layer under the action of the main pump. Return to the circulating main pump and continue to dye from the inside out. The dyeing temperature and time are controlled by the operator. After the dyeing is completed, the residual liquid is discharged, and multi-channel clean water is added for washing or soaping to remove the floating color on the fibers. Then remove the fiber from the dyeing cage and remove it for dehydration.

2 Disperse fiber dyeing process

The production workshop arranges according to the plan, evenly puts the dyed cotton or cotton net into the dyeing cage, hangs it into the bulk fiber dyeing machine, and processes the raw cotton into the required bleached cotton or colored cotton through the bleaching or dyeing process. The specific production process As shown in Fig. 2, the processes are: cotton distribution → cylinder loading → dyeing → dehydration → drying → packaging.

Fig. 2 Schematic diagram of the production process of loose fiber dyeing

2.1 Pre-processing

1) with cotton

According to the maturity, color, and quality of cotton, the cotton grade is divided into 7 grades, ie grades 1 to 7, and grade 3 is grade grade. Cotton used for bulk cotton dyeing is better than grade 3. For loose fiber dyeing, cotton with moderate micronaire value (maturity and fineness) should be selected, and the fiber maturity is high, and the natural twist is more, which helps the fibers to cohere with each other, so the strength, elasticity, dyeing uniformity, relative yarning Good quality. Cotton fibers with too low micronaire values ​​have poor maturation and are prone to deleterious defects and poor dyeability. When mixing cotton, mixed cotton batches with large differences in micronaire values ​​should be avoided.

Soft water must be used for bleaching, dyeing, and soaping. Hard water will play a catalytic role in bleaching, making cotton fiber brittle, increased cable. Hard water used for dyeing and soaping will produce insoluble iron, calcium, and magnesium salts and deposit on fibers to form stains or white powder. If the factory does not have a soft water system, adding water softener (such as sodium hexametaphosphate, chelating dispersant) during the process can effectively reduce the water hardness.

There are more waxes and pectins and other impurities in the loose cotton, the fibers are yellow, and the permeability is poor, so it is not suitable for dyeing. In order to make cotton have better water absorption, in order to facilitate the adsorption and diffusion of dyes in dyeing, but also not affect fiber spinnability as much as possible, can use scouring agent under high temperature and alkaline conditions for processing. The process is shown in Figure 3. After refining, the cotton surface is relatively clean, and in the subsequent processing, the dyeing liquid can penetrate into the fiber quickly and evenly, and the dyeing quality is improved, and the dyeing depth and the wet rubbing color fastness are improved.

4) Bleaching

After the loose cotton fiber is refined, some of the impurities are removed, and the water absorption is improved and can be directly used for dyeing. However, for high whiteness requirements and bright varieties, whiteness is not enough, and oxygen bleaching processing is also required. In the production operation, alkali and hydrogen peroxide stabilizers should be added first, and hydrogen peroxide should be added for a period of time to prevent the local alkali concentration from being too large, and the partial hydrogen peroxide solution can be decomposed too quickly, resulting in uneven whiteness. The process is shown in FIG. 4 . Oxygen bleaching after the need to use acetic acid wash, such as washing is not clean, the residual acid will make the fiber brittle. In addition, in order to prevent the residual hydrogen peroxide from affecting the subsequent dyeing, a small amount of deoxygenase may be added during the subsequent washing.

5) Whitening

For those who require a particularly white loose cotton variety, brighter brighteners with good fastness can be used for whitening. However, bleaching cannot replace bleaching. For example, the effect of fiber bleaching is not good, and brighteners alone cannot achieve optimum whiteness. In addition, when the whitening should pay attention to the use of soft water, and the amount of fluorescent whitening agent should not be too high, otherwise it will make cotton slightly yellow.

2.2 Staining

Reactive dyes are widely used in impregnation dyeing of cotton loose fibers. They have complete chromatogram, bright color, low price, good level dyeing, and convenient operation and application. During the bulk fiber dyeing process, the cotton fiber is still in the dyeing tank. The dyeing liquid continuously penetrates the fiber from the inner layer of the dyeing cylinder to the outer layer through the main pump to uniformly dye the dye. After circulating for a period of time, add Yuan Mingfen in the dye solution of nearly neutrality to make the dye adhere to the cotton fiber as evenly as possible; when the dye-attached fiber is close to equilibrium, gradually increase the pH value of the dyeing agent by adding alkaline agent gradually. Accelerate the fixation reaction of dyes and fibers, so that the dyes are fixed on the fibers and achieve the purpose of coloration. The process is shown in Figure 5.

Fig. 5 Dyeing process of cotton loose fiber reactive dyes

2.3 Post-processing

After dyeing, a large amount of floating color adheres to the surface of the cotton fiber, and soaping and washing are required to remove the floating color. Further, fixing and softening are required to improve the color fastness, feel and spinnability of the fiber.

1) soaping

By thoroughly washing with water and boiling soap, a large amount of hydrolyzed and unreacted dye remaining on the fiber surface can be efficiently washed out. A small amount of chelating dispersant should be added during soaping to not only purify the water, but also prevent the secondary staining of the fiber in the floating color of the soap, thereby improving the color fastness. Dark colored cotton should be soaped once with a neutral soap lotion. The initial washing and soaping is very obvious for improving the fastness, but with the increase in the number of soaping and washing, the dyestuffs that have been dyed will be destroyed and broken. The effect on the fastness improvement will be weakened. After soaping, it needs to be neutralized with acetic acid. The process is shown in Figure 6.

2) Fix oil

Deep color cotton series, such as deep red, dark green, deep black, deep blue, etc., floating color dyes have high directness to the fiber, so it is difficult to wash completely from the fiber, so these color cotton washable The degree is not ideal, need to use fixing agent to be improved. Fixing agents are mostly polymer chelates, which can form a protective film on the surface of the fiber; or make dyes and fibers tightly and firmly connected to prevent the dye from falling off from the fibers and improve the color fastness.

In addition, after the pre-treatment and dyeing, the waxy and lipid substances on the fiber surface are destroyed, the hand feel becomes hard, and it is easy to cause more neps and broken ends in the spinning process. Therefore, it is necessary to use a softening agent (soft sheet or silicone oil) and an antistatic agent bathing treatment to improve the feel of the fiber and increase the spinnability of the colored cotton.

2.4 Dehydration, drying and packaging

1) Dehydration

Dehydration is the use of high-speed centrifugal dewatering machinery, the process of fleeing most of the water (free moisture) in the dyed or wet-finished cotton loose fiber away from the fiber surface, facilitating the processing of the next process, and improving the efficiency of the drying of the cotton, saving Electricity, steam and other energy sources. The dehydration of loose fibers includes two forms: one is manual loading of loose fibers, which is a bulky physical operation, and is urgently needed to be reformed; the other is suspended in a cake form, as shown in Fig. 7, which is a great dehydration method. The labor intensity is reduced, and the loss of loose fibers can be avoided, which is the development trend of the loose fiber dewatering machine in the future. Centrifugal dewatering machine is mainly composed of a rotating cage, a central shaft, a shell, a column, and the like. During the processing, the loose fibers are evenly placed in the rotating cage. When the rotating cage rotates at a high speed, centrifugal force is generated, so that the fibers in the rotating cage are pressed against the cage wall, and moisture in the fibers is thrown from the holes in the cage wall. The dehydration efficiency of the cage is related to the speed and diameter. The time required for dewatering depends on the speed of the central shaft, the diameter of the cage, and the density of the fiber cake. This type of dewatering machine is easy to operate and has high production efficiency. After the normal dehydration, the moisture content of the fiber is about 30 to 35%. The disadvantage is that it cannot be continuously dehydrated, and the equipment utilization is limited.

2) drying cotton packing

After the dehydrated colored cotton fiber, there will still be more residual moisture, and it is necessary to use hot steam to make a forced flow on the surface of the wet fiber to evaporate the moisture to reach a predetermined moisture content. The bulk fiber cotton drying equipment is mostly a rotary screen or a flat screen hot air drying type dryer, among which the rotary screen type drying machine is widely used for its high bake efficiency. The rotary screen dryer adopts a suction type rotary screen roller. Part of the hot air in each rotary screen roller flows forward to a rotary screen roller under the action of the exhaust fan. As the humidity gradually increases, it is finally discharged. outer. The purpose of this design is to make full use of thermal energy and improve the drying efficiency. By adjusting the temperature and speed of the dryer, the moisture regain of the colored cotton fiber can be controlled to be 6% to 10%. When baking cotton, wet cotton should be fully shredded, flattened and fed into the dryer to ensure uniform wet and dry. After the colored cotton is dried, it is necessary to use a hydraulic baler to compress the loose fibers and bundle them into packages of a certain density and specifications. Packaging is an indispensable last step in color cotton processing. By packing, the cotton fibers of various colors can be classified and compressed to a predetermined density (about 450 kg/m3 or so), and bundled, packaged, and brushed and put together to improve the volumetric efficiency of storage or transportation.

3 Conclusion

Cotton loose fiber dyeing is a rapidly developing and extremely vital dyeing method in recent years. It greatly improves the high added value and efficiency of dyed yarns

beneficial. With the continuous introduction of new dyeing equipment and high technology into this field, the bulk fiber dyeing method will continue to develop and improve.

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