This article discusses four key technologies in the continuous scouring and bleaching of cotton and its blended knitted fabrics, and proposes specific control methods.
The four key technologies are: how to control easy stretch and deformation knitted fabrics during continuous scouring and bleaching processing The degree of relaxation during the process; how to ensure the best cloth surface effect of the fabric without wrinkles, scratches, friction hairiness, etc.; how to ensure that the degree of oxidative damage suffered by cotton fibers is the lowest; how to remove various impurities and their decomposition products and Clean the chemicals used.
Traditional printing and dyeing processing mode
Knitted fabrics are prone to stretch, deformation, wrinkles, curling, and unstable size and shape. These properties require dyeing and finishing processes, especially pre-treatment (also known as bleaching) and dyeing processes, to be as thorough as possible. Reduce the tension and pressure on the fabric during processing, that is, the higher the degree of relaxation of the fabric, the better.
Ordinary rope dyeing machines, jet overflow dyeing machines, airflow dyeing machines, etc. can realize loose processing of bleaching, dyeing, washing and other processes, and can be adapted to cylinders. It can knitted fabrics in various states such as shape, open width, seam edge and tube, etc. Therefore, it is not only the necessary equipment for dyeing knitted fabrics nowadays, but also a commonly used scouring and bleaching equipment.
Loose processing and adaptability to a wide range of fabrics (cylindrical, open width, seamed into tubes) are the outstanding advantages of the “dyeing machine bleaching method” commonly used in the industry. , but there are many shortcomings such as poor scouring and bleaching quality, poor cloth surface effect such as easy wrinkles, large energy consumption of water, electricity, steam, etc., large amounts of chemicals and pollutant emissions, low production efficiency, high labor intensity, etc. .
The fundamental way to overcome these shortcomings is to use continuous scouring and bleaching technology. However, the characteristics of knitted fabrics such as easy stretch and deformation increase the difficulty of scouring and bleaching technology. The main manifestations are as follows: ：
How to control the degree of relaxation of knitted fabrics during the continuous scouring and bleaching process. It is best to ensure that the fabric does not stretch or even shrink;
How to ensure the best cloth surface effect and avoid wrinkles, scratches, friction and hairiness;
How to ensure whiteness and “hair effect” Under the premise, the degree of cotton fiber damage is the lowest;
How to clean or remove various impurities, their decomposition products and chemicals remaining in the fabric.
How to control the relaxation degree of knitted fabrics during continuous scouring and bleaching processing
Tensile deformation process of knitted fabrics
The elongation phenomenon of knitted fabrics under the action of longitudinal tension is divided into three situations: changes in tissue structure and changes in yarn twist. and fiber elongation.
1. Changes in the organizational structure – changes in the spatial structure of the yarn are a phenomenon in which the length of the knitted fabric coil increases and the width decreases under the action of a small tensile external force. This kind of fabric deformation is very likely to cause It is easy to shrink the fabric through mechanical shrinkage finishing, such as: overfeed tentering, overfeed drying, overfeed calendering, felt blanket preshrinking and rubber blanket preshrinking, etc.
2. The change in yarn twist is caused by a large external stretching force or a small external force that stretches continuously for a long time, causing the fabric to have no chance to relax and shrink. The phenomenon of yarn elongation. This kind of change in fabric shape caused by yarn deformation is difficult to retract if only through overfeed tentering, overfeed drying, rubber blanket pre-shrinking and other finishing methods, but it can be processed through washing machines, loose dyeing machines, etc. Retracted due to water immersion relaxation treatment.
3. Fiber elongation is the phenomenon in which fibers are stretched under the action of a large external tensile force, or under the action of a large external force that stretches continuously for a long time. The changes in fabric shape caused by this reason cannot be restored through mechanical shrinkage finishing and water immersion relaxation treatment. Regenerated cellulose fiber fabrics, silk fabrics, and fabrics with smaller twists are more prone to fiber stretching caused by excessive external force during continuous processing.
It can be seen from the three conditions of tensile deformation of knitted fabrics that cotton and its blended knitted fabrics can withstand a certain amount of external force stretching. Therefore, the stress on knitted fabrics during continuous scouring and bleaching should be taken seriously. Of course, the smaller the tensile force, the better, and the higher the degree of relaxation, the better.
Relaxation effect and reasons of intermittent dyeing machine
In knitting Among intermittent dyeing machines, in addition to beam dyeing machines used for special processing requirements, ordinary rope dyeing machines, jet overflow dyeing machines and air flow dyeing machines are all loose rope dyeing machines. Their relaxation effects are It is the “relaxation zone” where the fabric is continuously fully relaxed during operation and the “low-tension stretch zone” where only small tension is applied to ensure that the fabric runs quickly.
The “relaxation zone + low tension stretching zone” structure of the dyeing machine is easy to stretch fibers (such as viscose) in the wet state for thermoplastic fiber (such as polyester, nylon, acrylic) fabrics. , modal) fabrics, filament (such as silk) fabrics and fabrics with smaller yarn twist (such as knitted fabrics), it is especially important.
So, how should continuous scouring and bleaching equipment achieve this relaxing effect?
Continuous refining and bleaching equipment and relaxation effect control
1. Traditional continuous scouring and bleaching equipment and its relaxation degree control In the 1980s, the continuous scouring and bleaching technology of knitted fabrics had a good development history in China for a period of time. There are many types of equipment and processes, such as: continuous Cook + Continuous��The shape changes of the fabric are uniform and orderly. The “open-width, low-tension continuous boiling, bleaching and one-bath rolling and bleaching process” uses this method to prevent wrinkles.
Method 3 is to subject the fabric to irreversible swelling of fibers under the action of regular external forces before it is subjected to irregular tension and pressure. This process is mercerization. The mercerized cotton knitted fabric will not produce so-called “dead pleats” during the rope bleaching and dyeing processes. This is also the reason why the mercerized cotton fabric has high dimensional stability.
3. How to ensure that the degree of oxidative damage suffered by cotton fibers is minimized. The degree of oxidative damage suffered by cotton fibers during scouring and bleaching will not only affect the strength of the fabric, but also affect the dyeing performance of the fiber. It is one of the important reasons leading to color flowers and color differences in dyeing, and must be taken seriously.
The fundamental measure to minimize the degree of oxidative damage to cotton fibers is to control the decomposition reaction of hydrogen peroxide in a direction more conducive to reacting with impurities such as pigments, and to avoid the ineffectiveness of hydrogen peroxide as much as possible. Decompose (the reaction that generates 02) and inhibit the decomposition reaction that has a bleaching effect but also has a strong oxidizing effect on cotton fibers (such as the reaction that generates HO.).
In particular, the hydrogen peroxide scouring and bleaching effect and fiber damage degree are not only related to the amount of hydrogen peroxide, alkali agent and pH value, scouring and bleaching temperature and time, stabilizer selection and other factors, but also the scouring and bleaching equipment. The selection and formulation of related processes are also effective measures to control the degree of oxidative damage to cotton fibers. Cold press stack scouring and bleaching, hydrogen peroxide decomposes slowly at low temperatures, and cotton fiber damage is small; the chemical reaction zone of the multi-functional, fully loose continuous scouring and bleaching machine – the steam box, is set up with two temperature zones, from low temperature to The high-temperature stage insulation steaming process can more effectively control the decomposition of hydrogen peroxide and minimize the oxidative damage suffered by cotton fibers.
4. How to ensure that various impurities, their decomposition products and the chemicals used are washed away. Generally speaking, various impurities on cotton fabrics, such as waxy substances, pectin substances, etc., will be removed after scouring and bleaching. The products generated after the reaction are easy to wash, but the caustic soda adsorbed in the cotton fibers must be removed through acid neutralization. If the acid neutralization is not complete, it will cause color flowers and color differences in dyeing, and bleaching. The varieties show local yellowing, and the dyed varieties show local discoloration and decrease in dyeing fastness. Therefore, the process used in acid neutralization, such as the selection of acid agent, the temperature and time of treatment, etc., as well as the equipment for implementing this process are very important.
Improving the continuous refining and bleaching technology of knitted fabrics
It is common in the current industry The dyeing machine scouring and bleaching method used has many shortcomings such as unstable scouring and bleaching quality, poor cloth surface effect, and large water, electricity and steam consumption. It can no longer meet the requirements of the development of high-end knitted fabrics and energy-saving and emission-reduced production. However, it is loose. It has the advantages of good chisel and adaptability to a wide range of fabrics (cylindrical, open-width and seam-edged fabrics), making it more suitable for the characteristics of knitted fabrics and certain processing requirements. This is why the dyeing machine bleaching method is commonly used in the industry today. important reasons.
The continuous scouring and bleaching method has outstanding advantages such as good product quality and greatly reduced energy consumption. However, knitted fabrics are easy to stretch, deform, wrinkle, curl, and have multi-fiber composition, and require a variety of The characteristics of operation in various states (cylindrical, open width and seamed into a cylinder) greatly increase the technical difficulty of continuous scouring and bleaching. The purpose of studying the key technologies of continuous scouring and bleaching of knitted fabrics is to explore ways to solve technical problems from various aspects such as equipment, technology, and additives, and comprehensively improve the continuous scouring and bleaching technology of knitted fabrics.
References:
[1] Wang Jusheng, Sun Kai ． Principles of Dyeing and Finishing Technology (Volume 2) [M]. Beijing: Textile Industry Press, 1984
[2] Yan Kelu. Dyeing and Finishing Technology and Principles (Volume 1) [M]. Beijing; China Textile Publishing House, 2009.
[3] Fan Xuerong, Wang Qiang. Knitted fabric dyeing and finishing technology[M]. Beijing: China Textile Publishing House, 2004.
[4] Xiyun. Dyeing and finishing of medium and long fiber fabrics[M]. Beijing: Textile Industry Press, 1989.</p
