In view of the problems that cotton knitted fabrics have poor dimensional stability and are prone to wrinkles and creases during the dyeing and finishing process, this article explains the reasons for shrinkage of cotton knitted fabrics and wrinkles and creases on the cloth surface, and proposes ways to reduce cotton knitted fabrics from both equipment and technology. The shrinkage rate of knitted fabrics and specific measures to prevent wrinkles and creases in cotton knitted fabrics provide a reference for dimensional stability and cloth surface effect during the dyeing and finishing process of cotton knitted fabrics.
Common quality problems in cotton dyeing and finishing
Quality problems that often occur in cotton knitted fabrics during the dyeing and finishing process are manifested in four aspects:
1. Problems such as color flowers, color difference, and poor color fastness are prone to occur in dyeing and printing;
2. The degree of fiber damage is relatively large;
3. Fabric shrinkage The rate is high and the dimensional stability is poor;
4. The cloth surface has serious wrinkles and creases and more friction hairiness, that is, the cloth surface effect is poor.
Through the joint efforts of dye and auxiliary chemical workers and dyeing and finishing workers, the problems related to dyeing and the problem of serious fiber damage have been better solved, but the size of cotton knitted fabrics The problems of poor stability and poor fabric surface effect (shown as wrinkles, creases and hairiness) have not been well solved for a long time. This article will explain the reasons for shrinkage of cotton knitted fabrics, anti-shrinkage technology and the reasons for wrinkles and creases in cotton knitted fabrics. Discuss ways to overcome it.
Causes of shrinkage and fabric wrinkles
1. Causes of shrinkage of cotton knitted fabrics
Shrinkage rate that measures the degree of fabric shrinkage , is the result of washing with a drum-type washing machine, that is, the fabric shrinks freely in a completely relaxed state in water. Not only the plain gray fabrics and ready-made garments after dyeing and finishing will shrink during loose washing, but the knitted fabrics that have just been knitted will shrink when completely relaxed. When immersed in water under normal conditions, dimensional shrinkage, that is, shrinkage, will also occur. The shrinkage process of knitted fabrics when washed with water or relaxed in water is completed by fiber shrinkage, yarn shrinkage and fabric structure shrinkage.
1.1 Fiber shrinkage
In spinning, spinning (chemical fiber), weaving and dyeing and finishing processes with large tensile forces, fiber shrinkage It will elongate due to external force stretching and form internal stress in the fiber. However, the drying effect in the stretched state, the cohesion force between fibers, and the interweaving resistance between yarns will temporarily cause the elongation state and internal stress. Fixed; when treated with water or steam, water molecules will weaken the force between the macromolecular segments of the fiber and reduce the retraction barrier of the macromolecular segments. Under the action of internal stress, the fiber will shrink.
Filament fabrics, yarn fabrics with smaller twist and regenerated cellulose fiber fabrics, during wet dyeing and finishing processes (such as scouring, bleaching and dyeing), the fibers are easily stretched and elongated by external forces, which will cause Large shrinkage; cotton knitted fabrics, if the loose processing method is used in the scouring, bleaching and dyeing processes, the fibers will not be stretched. The shrinkage caused by the fibers is very small, but if the stretching process is used Processing methods with greater tension will produce greater fiber shrinkage. It must be noted that the fiber deformation caused by external force stretching will be difficult to recover due to the cohesion force between fibers and the interweaving (coil knotting points) resistance between yarns. Therefore, attention should be paid to the scouring and bleaching of cotton knitted fabrics. Control of tensile external force during dyeing process.
1.2 Yarn shrinkage
Yarn shrinkage comes from two aspects: the water absorption swelling of the fiber and the elongation of the yarn due to external force.
First of all, water-absorbing swelling of fibers in water will cause the length of the yarn to shrink. The greater the swelling of the fiber fabric, the greater the shrinkage caused by this. The swelling properties of several cellulose fibers are shown in Table 1.
Fiber swelling causes the effective diameter of the yarn to increase. Fibers that cannot stretch or even become shorter (measured by the stroke of the fiber around the yarn axis) are unlikely to untwist the yarn. In this case, the position and posture in the yarn can only be adjusted by reducing the stroke along the yarn axis (from Ll to L2) to shorten the yarn.
In addition, during the wet processing of dyeing and finishing, excessive tensile external force or the tensile external force is not too large but acts continuously on the fabric for a long time, resulting in the fabric having no chance to relax and shrink, and it will inevitably cause This causes yarn elongation and yarn twist reduction, and the water in wet processing will play a twist-setting role in cellulose fiber yarns.
For yarns in a free state, this kind of stretching can be completely retracted, but for yarns in fabrics, it is difficult to recover due to the cohesion force between fibers and the interweaving resistance between yarns. If It is difficult to retract only through finishing methods such as overfeed tentering and rubber blanket preshrinking, but in a drum washing machine or in a water treatment process that allows the fabric to be completely relaxed, the yarn that has been stretched through fiber swelling can be retracted. shrink.
1.3 Fabric shrinkage
Cotton fiber absorbs water and swells in water. In addition to causing changes in the yarn structure, it also causes changes in the loop structure of the knitted fabric. First, fiber swelling increases the effective diameter of the yarn. When the yarn cannot be untwisted and the yarn itself is shortened, at the coil sleeve node, the only way to increase the coil width, shorten the coil height or reduce the distance between coils is to Adjusting the three-dimensional spatial state of the knitted loops, this process may cause the fabric to shrink together in the longitudinal and transverse directions (b1 < b, a1 < a), or it may cause the fabric to shrink in one direction in the longitudinal and transverse directions, and there may even be some contraction in the other direction. Increase, that is, negative shrinkageAlthough the relevant equipment has been installed, the ideal effect has not been obtained. There are still phenomena such as insufficient size stability and serious wrinkles and creases on the fabric surface.
Judging from the related technologies in the past, more attention was paid to loose processing, such as using intermittent loose dyeing machines (mostly rope jet dyeing machines) for bleaching, dyeing and washing. Although the dimensional stability of the fabric has been improved to a certain extent through the entire process flow and the use of loose hot air dryers to dry fabrics, the problems caused are wrinkles and creases on the fabric, aggravation of friction hairiness, and water, electricity, Gas consumption has increased significantly and product quality has become unstable; flat-width continuous scouring and bleaching technology and flat-width dyeing technology have received attention in recent years, but most of the equipment used is a compact continuous method. Although it can obtain smooth Cloth surface effect, but there is a phenomenon that the fabric shrinkage is larger or the weight per unit area (gram mass) is lighter.
It can be seen that in order to obtain the most ideal dimensional stability and cloth surface effect, it is not enough to just pay attention to the above “loose processing” and “open width processing”, you must also pay attention to the following three Points:
First, pay attention to the deformation of the fabric shape caused by the fiber swelling due to water absorption during the first wet treatment of the cotton knitted gray fabric, and the resulting impact on the dimensional stability and surface effect of the fabric. Influence. The best method is to place the fabric in a completely relaxed state in water, and allow the fabric to shrink freely and evenly through fiber swelling. This can not only achieve good dimensional stability but also ensure a smooth cloth surface.
Second, pay attention to the “shaping” effect of the mercerization process on cotton knitted fabrics, which can cause irreversible swelling of cotton fibers. The mercerized cotton knitted fabric not only has high dimensional stability, but also will not cause wrinkles or creases in the fabric no matter how large the external force (pressure or tension) is.
Third, pay attention to shrink-proof finishing technology. “Over-feeding and anti-shrinking technology” (over-feeding expansion, over-feeding drying, over-feeding calendering, etc.) is not the entire content of anti-shrinking technology. Some mature anti-shrinking technologies for cotton woven fabrics such as rubber blanket pre-shrinking machine anti-shrinking technology etc., should be promoted and applied in anti-shrinkage finishing of knitted fabrics.
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