A brief analysis of the low-water, energy-saving, emission-reducing, high-efficiency short-process bleaching and dyeing process of cotton knitted fabrics, mainly expounding the intermittent series pre-treatment high-efficiency short-process process and the series dyeing short-process process, and comparing the quality and quality with the corresponding traditional processes. The effectiveness of energy conservation and emission reduction and the reduction of pollution indicators, etc.
The current situation of bleaching and dyeing of knitted fabrics
Knitted fabrics have many fiber types and weaving methods, and the coil structure is easy to deform. For a long time, intermittent vat dyeing has been used. The bleaching and dyeing process is long, low in efficiency and high in energy consumption. As energy resources become increasingly scarce, the country’s requirements for energy conservation and environmental protection become more stringent, processing costs increase, and corporate profits are low. Knitted dyeing and finishing processes are developing towards efficient short-process, water-less and energy-saving bleaching and dyeing processes. First, through equipment upgrading and transformation, the use of small liquor ratio equipment plays a key role in energy conservation and emission reduction; secondly, through the development and application of efficient multi-functional additives, the process flow can be shortened while the product quality meets the requirements, so that the overall energy consumption can be reduced. Reduce, reduce the concentration of pollutants to a certain extent; thirdly, currently there are mature process equipment for open-width pre-treatment and open-width washing of knitted fabrics. Through open-width refining and bleaching, energy consumption and emissions can be greatly reduced. The pre-treatment, dyeing and post-printing washing of outer fabrics account for 80% of the total water consumption and produce a large amount of waste water. The following focuses on the high-speed process technology of intermittent bleaching and dyeing of knitted fabrics.
High-efficiency short-flow pretreatment process
1. The traditional alkali-oxygen one-bath pretreatment process began in the 1980s. The alkali-oxygen one-bath pretreatment process is highly efficient. The short-flow process has been rapidly promoted and applied in the pre-treatment of woven and knitted fabrics due to its obvious energy saving and emission reduction effects. The successful application of this process relies on an alkali-oxygen stabilizer with good alkali resistance, which can effectively decompose the hydrogen peroxide in the caustic soda solution. And the concentration of free radicals (HOO-) is reduced to minimize damage to the fabric. HO-2 is mainly produced to decompose the slurry and impurities on the fabric.
For woven fabrics, there is slurry on the fabric and the structure is tight. The concentration of caustic soda in the pad-steaming process is high, usually reaching 35g/L~60g/L, and the requirements for stabilizers are high, so the earliest developed The alkali-oxygen one-bath process has been successfully applied to polyester-cotton fabrics. On pure cotton woven fabrics, the first cold stack and then short steaming process is more commonly used. Although it cannot be completed in one step, it has achieved energy saving and emission reduction to a certain extent. Because the structure of knitted fabrics is prone to deformation and hemming, the bath method is still widely used for pre-treatment. The bath ratio is generally 1:8 to 1:10, and the caustic soda concentration is 2 to 4g/L. However, the requirements for stabilizers are reduced, so alkali oxygen is used. The one-bath process has been very successfully applied to knitted fabrics and yarns.
The earliest alkali oxygen process prescription is generally: high-efficiency refining agent 1-2g/L, alkali oxygen stabilizer 0.5-lg/L, 30% hydrogen peroxide 6-8g/L, caustic soda 2-4g/L , with the help of the synergistic effect of alkali oxygen stabilizer, high-efficiency refining agent, caustic soda and oxidant, the impurities on the cotton fiber are oxidized, hydrolyzed and metathesized, saponified, emulsified, solubilized, washed and other physical and chemical effects, so that the fabric becomes “even” , translucent, clean” processing effect. After rinsing, pickling and deoxygenating, HAc or NaAc 0.5g/L, deoxidizing enzyme 0.08-0.lg/L. The working fluid contains at least 4 kinds of additives. Improper ingredients will cause the working fluid to fail, reduce the strength of the fabric, and have a high pH value in the residual liquid. Usually, the water consumption per ton of product is 32 to 40 tons.
2. New alkali-oxygen one-bath pretreatment process. The so-called new alkali-oxygen process mainly relies on a multi-functional additive. In fact, it is a three-in-one multifunctional scouring agent, which mainly includes alkaline salt, penetrating agent, stabilizer, and chelating dispersant. It can be bathed with hydrogen peroxide and act synergistically on cotton fibers to make the fabric “even, transparent, and clean” processing effect.
The prescription is multifunctional additive 2-3g/L, 30% hydrogen peroxide 4-6g/L. For gray fabrics with many impurities, caustic soda can be added appropriately. After refining and bleaching, it can be neutralized and deoxidized with traditional alkali. The oxygen process is the same.
The difference between the two is that the new alkaline oxygen formula is simpler, the pH value of the working solution is lower, the utilization rate of hydrogen peroxide is higher, the fabric is less damaged by strong force, and the pH value of the boiling and bleaching residual liquid is low. The author of this article found in the study of the linen pre-treatment process that the whiteness of the fabric treated by this new alkali oxygen process is higher than that of the traditional alkali oxygen process. This is related to the fact that the alkaline salt in the multifunctional additive is δ-layered sodium metasilicate. It has strong ability to absorb impurities and good washing power.
3. One-bath low-temperature refining and bleaching process for knitted fabrics. The one-bath low-temperature refining and bleaching process for knitted fabrics is accomplished with the help of low-temperature scouring enzyme. The low-temperature scouring enzyme is actually composed of δ-layered sodium metasilicate. It is composed of liquid Mn-L type (polymer coordination manganese compound) dispersion, powdery heavy metal chelating agent, and high-efficiency surfactant. It can reduce the refining and bleaching temperature from 100°C to about 85°C. Its special ligand has a strong effect on lignin. And pigments produce selective oxidation, which causes less damage to cellulose.
Low-temperature refining agent has the functions of penetration, emulsification, chelation dispersion and oxygen bleaching stabilization. The general prescription is: low-temperature refining agent 1-2g/L, 30% hydrogen peroxide 5-6g/L, caustic soda 2- 3 g/L, neutralization water washing and deaeration treatment are the same as before. This process has low boiling and bleaching temperature, short heating time, low product weight loss and little cotton fiber damage. Compared with the traditional alkali oxygen process, the energy consumption is reduced by 20 to 30%, and the processing time is saved by about 15%. It is more suitable for the refining and bleaching of multi-fiber knitted fabrics that are not resistant to high temperatures.
4. Low-alkali refining and bleaching process Low-alkali refining and bleaching of cotton knitted fabrics relies on low-alkali bleaching and refining additives. This additive is also a multifunctional surfactant, a blue transparent liquid, and has stable catalytic hydrogen peroxide And the functions of wetting and penetration allow the pure cotton knitted fabrics to be smelted and bleached under high temperature and low alkali. After scouring and bleaching, the pH of the residual liquid is close to neutral, and there is no need to neutralize it with acid. After scouring and bleaching, the liquid is drained and washed once.The higher the dyeing concentration of reactive dyes, the greater the degree of dye association and the more unbonded dyes. Therefore, when dyeing deep and dense colors, the amount of floating color is large, and post-processing is particularly important, which affects the dispersion and dispersion of soaping agents. The higher the requirements for decontamination ability and re-contamination ability.
When the dyeing concentration of conventional reactive dyes is above 5% (o.w.f.), in order to improve the color fastness of fabrics dyed with reactive dyes, companies generally perform two high-temperature soapings and then strengthen hot water cleaning. The soaping enzyme developed on the market can catalyze the decomposition of hydrolyzed reactive dyes and unfixed reactive dyes. The decomposition products have higher solubility. Lowering the washing temperature can also make floating colors on the fabric more clean, ensuring color fastness and Under the premise of maintaining the color, the number and time of cleaning can be reduced.
1. Post-soaping treatment and quality comparison of different processes
Dyeing uses reactive yellow M-3RE, reactive red M-3BE, reactive dark blue M-2GE, reactive black B-ED , 4 dyes were dyed separately, the dye concentration was 6% (o.w.f), salt 40g/L, soda ash 25g/L, fixed for 40 minutes, then soaped with traditional methods for post-processing. The comparison with soaping enzyme post-processing is shown in Table 3.
2. Comparison of the absorbance values of the residual liquid after different processes. The absorbance values of the washing residual liquid, the first cleaning and the second cleaning liquid of the soaping enzyme process and the traditional high-temperature soaping process. The dyeing concentration of the fabric is 6% (o.w.f). The residual liquid after soaping enzyme treatment and the traditional high-temperature soaping residual liquid of the same weight of fabrics are tested respectively, and their CODcr values are compared as shown in Table 4.
The second cleaning water after soaping and enzyme washing is basically colorless. However, the second cleaning solution of the traditional process still has a certain color. It can be inferred that only one cleaning is required after soaping enzyme washing. However, the traditional high-temperature soaping process requires strong cleaning with hot water after the second cleaning, and the amount of sewage discharge is big. The CODcr of the washing residue treated with soaping enzyme l is lower than that of the traditional high-temperature soaping process, indicating that soaping enzyme does have a digestion effect on floating colors on fabrics and is not just emulsification washing.
Editor’s Note
Printing and dyeing scientists have developed many high-efficiency auxiliaries for dyeing and finishing knitted fabrics, which can be used according to the company’s equipment and products. Characteristics selection, and pilot testing and sample application research, and then popularization and application to achieve energy saving and emission reduction results. Of course, in order to significantly reduce the comprehensive energy consumption of the company’s products, the production equipment needs to be updated and transformed into new low liquor ratio equipment, or boldly adopt flat-width refining and bleaching and flat-width washing post-processing equipment to achieve the goal.
References:
[1] Yue Shifang, Chen Xue. Application of phosphorus-free complexing dispersant in alkali-oxygen pad-steaming process [J], Printing and Dyeing, 2008(14). 19-20. </p
