Technology | How does the continuous scouring and bleaching technology of knitted fabrics ensure the best fabric surface effect?

This article discusses the 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 the degree of relaxation of easily stretchable knitted fabrics in the continuous scouring and bleaching process; how to ensure the best cloth surface effect of the fabric without wrinkles, scratches, friction, and hairiness; how to ensure cotton The degree of oxidative damage suffered by the fiber is the lowest; how to clean all kinds of impurities and their decomposition products as well as the chemicals used.

Traditional printing and dyeing processing mode

Knitted fabrics are easy to stretch and deform, easy to wrinkle, easy to curl, and size and shape are not easy to stabilize, etc., which require dyeing and finishing, especially pre-treatment (also known as scouring and bleaching) and dyeing procedures. The size of the tension and pressure, that is, the higher the degree of relaxation of the fabric, the better.

Ordinary rope dyeing machine, jet overflow dyeing machine, airflow dyeing machine, etc., can realize loose processing of scouring, bleaching, dyeing, washing, etc., and can be adapted to various states such as tube shape, open width, and seam into a tube. Knitted fabrics are therefore not only the necessary equipment for dyeing knitted fabrics, but also the commonly used scouring and bleaching equipment.

The loose processing and the wide range of adaptable fabrics (tube, open width, seam into a tube) are the outstanding advantages of the "dyeing machine scouring and bleaching method" commonly used in the industry, but there are poor scouring and bleaching quality and poor cloth surface effect. Such as prone to wrinkles, high energy consumption such as water, electricity and gas, large amount of chemicals and large pollutant emissions, as well as many shortcomings such as low production efficiency and high labor intensity.

The fundamental way to overcome these shortcomings is to adopt continuous scouring and bleaching technology. However, the easy stretching and deformation of knitted fabrics increase the difficulty of scouring and bleaching technology. The main manifestation is: how to control the knitted fabric in the continuous scouring and bleaching process. The degree of slack, it is best that the fabric does not stretch or even shrink; how to ensure the best cloth surface effect, avoid wrinkles, scratches, and friction hairiness; how to ensure whiteness and "hair effect" , The degree of cotton fiber damage is the lowest; how to wash or remove various impurities and their decomposition products and chemicals remaining in the fabric.

How to control the degree of relaxation of knitted fabrics in the continuous scouring and bleaching process?

1. Stretching deformation process of knitted fabric

The elongation phenomenon of knitted fabrics under the action of longitudinal tension is divided into three situations: changes in organizational structure, changes in yarn twist, and fiber elongation.

1. Changes in organizational structure-changes in yarn spatial structure

It is a phenomenon in which the length of the knitted fabric loop increases and the width decreases under the action of a small external stretching force. This kind of fabric deformation can easily shrink the fabric through mechanical preshrinking, such as: overfeeding tentering, overfeeding drying, overfeeding calendering, blanket preshrinking and rubber blanket preshrinking.

2. Changes in yarn twist

It is the phenomenon that the yarn is stretched due to the effect of large external force, or under the effect of small external force for a long time and continuous stretching, so that the fabric does not have the opportunity to relax and shrink. The change in fabric shape caused by this kind of yarn deformation is difficult to shrink if only through the finishing methods of overfeeding tentering, overfeeding drying, and rubber blanket preshrinking, but it can be processed by washing machine, loose dyeing machine, etc. The water immersion relaxes and shrinks.

3. Fiber elongation

It is a phenomenon in which the fiber is stretched under the action of a large external force, or under the action of a large external force continuously for a long time. The change in fabric shape caused by this reason cannot be recovered by mechanical preshrinking and water immersion relaxation treatment. Regenerated cellulose fiber fabrics, silk fabrics, and fabrics with less twist are more prone to fiber stretching due to excessive external force during continuous processing.

It can be seen from the three cases of knitted fabric stretching and deformation that cotton and its blended knitted fabric can withstand a certain amount of external force stretching. Therefore, we should face up to the stretching tension and the resulting tension of the knitted fabric during continuous scouring and bleaching. Of course, the smaller the tensile force, the better, and the higher the degree of relaxation, the better.

Second, the relaxation effect of intermittent dyeing machine and its reasons

Among the batch dyeing machines for knitted fabrics, in addition to the beam dyeing machines used for special processing requirements, ordinary rope dyeing machines, jet overflow dyeing machines and airflow dyeing machines are all loose rope dyeing machines. The effect is the "relaxation zone" where the fabric is continuously fully relaxed during operation and the "low tension stretch zone" where only a small tension is applied to ensure that the fabric runs quickly.

This kind of "relaxation zone + low tension stretching zone" structure of the dyeing machine is suitable for thermoplastic fiber (such as polyester, nylon, acrylic) fabrics, fibers (such as viscose, modal) fabrics, filaments that are easily stretched in the wet state. It is especially important for fabrics (such as silk) and fabrics with small yarn twists (such as knitted fabrics).

So, how should continuous scouring and bleaching equipment achieve this relaxation effect?

3. Continuous scouring and bleaching equipment and its relaxation effect control

1. Traditional continuous scouring and bleaching equipment and its slack 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 scouring + continuous hydrogen peroxide bleaching (rolling and bleaching).

The key equipment that affects the degree of fabric relaxation is a rope-like segmented pressurized washing machine that can operate continuously. Although a "relaxation zone" is set in the unit washing machine; but due to the backward automatic control technology and some unreasonable structures Due to the high tensile strength and mechanical friction of the fabric, the relaxation effect is unsatisfactory, so it has not been well promoted and applied.

2. Modern new technology continuous scouring and bleaching equipment and its slack control

The following types of new equipment for continuous scouring and bleaching (oxygen bleaching) of knitted fabrics have been adopted.

a. Rope continuous scouring and bleaching one-bath scouring and bleaching process

This equipment is rope-shaped processing and can be used for scouring and bleaching knitted fabrics in various states such as tube-shaped, open-width, and seam-to-tube. Both the scouring and bleaching machine and the washing machine adopt the "relaxation zone + low tension stretching zone" method, and the relaxation effect of the fabric can reach the relaxation level of the jet dyeing machine.

b. Cylindrical fabric open width continuous scouring and bleaching one-bath rolling and bleaching process scouring and bleaching machine

This equipment is a special equipment for scouring and bleaching tubular knitted fabrics, and cannot be used for open-width and seam-seaming tubular knitted fabrics. The continuous washing machine adopts the "relaxation zone + low tension stretching zone" method, which has a higher degree of relaxation.

c. Open-width fabric open-width low-tension continuous scouring and bleaching one-bath rolling and bleaching process scouring and bleaching machine

This equipment is a special equipment for open-width scouring and bleaching of open-width knitted fabrics, and cannot be used for tube-shaped and seamed knitted fabrics. There is no "relaxation zone" in the continuous washing machine, and the control system alone reduces the stretching tension of the fabric as much as possible, and the degree of relaxation is average.

d. Multifunctional, full-loose scouring and bleaching one-bath rolling and bleaching process scouring and bleaching machine

At present, more and more new chemical fibers are used in knitwear, which puts forward higher and newer requirements for dyeing and finishing technology. For this reason, the latest domestically developed "multifunctional, fully loose continuous scouring and bleaching One-bath rolling and bleaching process scouring and bleaching machine".

The general process flow is: cloth feeding→relaxation and anti-folding treatment→two washing steps (→water washing→dropping cloth)→padding scouring and bleaching liquid→first steaming zone→second steaming zone→three washings→padding acid liquid →Stacking→washing 3 times→dropping cloth.

The equipment can adopt two operation modes of cylindrical open width and rope shape, and can be used for the scouring and bleaching of knitted fabrics in various states of cylindrical, open width, and hemming; through a specially set "relaxation anti-folding machine", Rope processing can obtain the same fabric surface flatness as open width processing. The blade runner structure in the washing machine forms a "relaxation zone" with an excellent loosening effect, and the degree of relaxation reaches or exceeds the level of the jet dyeing machine.

Some of the above four continuous scouring and bleaching machines have the structure of "relaxation zone + low tension stretching zone", which can naturally achieve a good relaxation effect; some do not have the structure of "relaxation zone + low tension stretching zone" , But the stretching tension can be controlled to reduce the stretch of the fabric as much as possible, and good post-finishing and shrink-proof measures must be taken, which can also meet the processing requirements of the dimensional stability of cotton and its blended knitted fabrics.

Fourth, how to ensure the best cloth effect of the fabric

The best cloth surface effect of the fabric means that the fabric will not produce wrinkles that are difficult to eliminate due to mechanical external force or other factors, and the phenomenon of scratches and friction hairiness due to mechanical external force. Cotton knitted fabrics, especially single-sided varieties, are prone to wrinkles that are difficult to eliminate after finishing when they are rope-like scouring and bleaching. They are also called "dead wrinkles". This is a common product quality problem in the scouring and bleaching method of dyeing machines.

1.Causes of "dead wrinkles" (difficult to eliminate after finishing) of cotton knitted fabrics

Cotton fiber absorbs water and swells in water, which will inevitably cause changes in yarn structure and loop structure. The change of yarn structure is manifested by the thickening of yarn, the shortening of yarn length and the increase of yarn twist; the change of loop structure is manifested by the increase of loop arc and the shortening of loop column. The result of these two changes causes the fabric to shrink and become dense.

In the process of shrinking and densification, if the fabric is subjected to irregular external forces (tension or pressure), it will cause unevenness in the density of the fabric after shrinking. The more loose the fabric, the more prominent this phenomenon; due to the structure of the fabric Constraint, although the fiber swelling process is reversible, the deformed fabric shape will not be restored due to the recovery of the fiber shape, so the shape (such as wrinkles) formed by the irregular external force of the fabric is retained. This is why cotton knitted fabrics appear "dead folds" during rope scouring, bleaching and dyeing.

2. Methods to prevent "dead wrinkles" (difficult to eliminate after finishing) of cotton knitted fabrics

The first method is to immerse the fabric in water in a completely loose state before being subjected to irregular tension and pressure, so that the fabric can shrink freely and uniformly through fiber swelling; a dense fabric that has been uniformly contracted will be constrained by the structure of the fabric. It is relatively stable and fixed, and there will be no "dead folds" due to pressure or tension (appropriate size) during subsequent processing. The anti-wrinkle effect of the "multifunctional, fully-loose continuous boiling, bleaching and one-bath scouring and bleaching process" is that the fabric is completely immersed in the water in a relaxed state through the "relaxation anti-folding machine", and water is absorbed by the fiber Swelling causes the fabric to shrink and shrink freely and uniformly.

The second method is to wet the fabric under regular and appropriate external forces (including horizontal and vertical) before the fabric is subjected to irregular tension and pressure. In this case, the fabric shape changes due to the swelling of the fiber due to water absorption. It is uniform and orderly. "Open-width, low-tension continuous scouring, bleaching, one-bath rolling and bleaching process scouring and bleaching machine" uses this method to prevent wrinkles.

The third method is to perform irreversible swelling of fibers under regular external forces before the fabric is subjected to irregular tension and pressure. This process is mercerizing. Mercerized cotton knitted fabrics will not produce so-called "dead folds" during the rope-like scouring, bleaching and dyeing process. This is also the reason for the high dimensional stability of mercerized cotton fabrics.

3. How to ensure that the degree of oxidative damage suffered by cotton fibers is the lowest

The degree of oxidative damage to cotton fiber during scouring and bleaching not only affects the strength of the fabric, but also affects the dyeing performance of the fiber. It is one of the important reasons for the color and color difference in dyeing, which must be paid attention to.

The fundamental measure to minimize the degree of oxidative damage of cotton fiber is to control the decomposition reaction of hydrogen peroxide to make it more conducive to the reaction of pigments and other impurities, and to avoid the ineffective decomposition of hydrogen peroxide (reaction to generate 02). And inhibit the decomposition reaction (such as the formation of HO.) which has a bleaching effect but at the same time has a strong oxidative effect on the cotton fiber.

In particular, the effect of hydrogen peroxide scouring and bleaching and the degree of fiber damage are related to factors such as the amount of hydrogen peroxide, alkaline agent and pH value, scouring and bleaching temperature and time, and the selection of stabilizers, as well as the selection of scouring and bleaching equipment and the formulation of related processes. It is also an effective measure to control the degree of oxidative damage of cotton fiber. Cold-pressed reactor scouring and bleaching, the decomposition rate of hydrogen peroxide is slow at low temperature, and the cotton fiber damage is small; The high-temperature stage insulation steaming process can more effectively control the decomposition of hydrogen peroxide and minimize the degree of oxidative damage suffered by the cotton fiber.

4. How to ensure that all impurities and their decomposition products and chemicals used are cleaned

Generally speaking, various impurities on cotton fabrics, such as waxy substances, pectin substances, etc., are easily cleaned after the scouring and bleaching reaction, but the caustic soda absorbed in the cotton fibers must be neutralized by acid It can only be removed. If the acid is not completely neutralized, it will cause color stains and chromatic aberrations in dyeing, local yellowing of bleached varieties, local discoloration of dyed varieties, and decreased color fastness. Therefore, the process used for acid neutralization, such as the selection of acid agent, the temperature and time of treatment, and the equipment to implement this process are very important.

Comprehensively improve the continuous scouring and bleaching technology of knitted fabrics

The dyeing machine scouring and bleaching method commonly used in the industry has many shortcomings, such as unstable scouring and bleaching quality, poor cloth surface effect, and high water, electricity and gas consumption. However, the advantages of good slack and wide range of fabrics (tube, open width and seam into tube fabrics) make it more suitable for the characteristics of knitted fabrics and certain processing requirements. This is also the current industry generally adopts dyeing machines. The important reason for the scouring and bleaching method; the continuous scouring and bleaching method has outstanding advantages such as good product quality and greatly reduced energy consumption, but the knitted fabric is easy to stretch and deform, easy to wrinkle, easy to curl and multi-fiber composition, so it needs more The characteristics of operation in different states (tube, open width and seam into a tube) greatly increase the technical difficulty of continuous scouring and bleaching. The purpose of studying the key technology of continuous scouring and bleaching of knitted fabrics is to start with equipment, technology, auxiliaries and other aspects, explore methods to solve technical problems, and comprehensively improve the technology of continuous scouring and bleaching of knitted fabrics.

Author: School of Textiles and Clothing, Qingdao University, Xu Weijing, Han Guangting, Zhang Yuanming, Shandong Kang