Through the joint efforts of dye and auxiliary chemical workers and dyeing and finishing workers, problems related to dyeing and serious fiber damage have been better solved. How to improve the dimensional stability of cotton knitted fabrics and obtain good fabric surface effects (no wrinkles, no friction hairiness, etc.) has long been a topic of concern in the knitting industry. For this reason, many process technologies have been researched and established and developed. Related equipment, but has never achieved ideal results. There are still phenomena such as insufficient size stability and serious wrinkles and creases on the fabric.
Reasons for shrinkage of cotton knitted fabrics
Reasons for shrinkage of cotton knitted fabrics
The shrinkage rate, which measures the degree of fabric shrinkage, is the result of washing in 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 that have been dyed and finished will shrink during loose washing, but also the fabrics that have just been woven will shrink. When the knitted blank is soaked in water in a completely relaxed state, it will also shrink in size, that is, shrink. 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.
In spinning, spinning (chemical fiber), weaving and dyeing and finishing processes where the tensile force is large, the fiber will elongate due to external force stretching and form internal stress in the fiber, and drying in the stretched state The elongation state and internal stress will be temporarily fixed due to the cohesion force between fibers and the interweaving resistance between yarns. When treated with water or steam, water molecules will weaken the force between fiber macromolecule segments. , lowering the retraction barrier of macromolecular segments, and the fiber will shrink under the action of internal stress.
In filament fabrics, yarn fabrics with smaller twists and regenerated cellulose fiber fabrics, during the wet processing of dyeing and finishing (such as scouring, bleaching and dyeing), the fibers are easily stretched and elongated by external forces, which will produce a large shrinkage; cotton For knitted fabrics, if the loose processing method is used in the scouring, bleaching and dyeing process, the fibers will not be stretched, and the shrinkage caused by the fibers will be very small. However, if the processing method is used with a large stretching tension, A greater fiber shrinkage will occur. 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.
Yarn shrinkage comes from two aspects: the water-absorbing swelling of the fiber and the elongation of the yarn caused by external forces.
First of all, the fiber absorbs water and swells in water, which will cause the length of the yarn to shrink. The greater the swelling of the fiber fabric, the greater the shrinkage caused by this.
△Swelling properties of several cellulose fibers
△Principle diagram of yarn shrinking due to fiber swelling
Fiber swelling causes the effective diameter of the yarn to increase. Fibers that cannot stretch or even become shortened (measured by the fiber’s stroke around the yarn axis) can only be reduced by reducing the stroke along the yarn axis (when untwisting of the yarn is impossible). (from L1 to L1′) to adjust the position and posture in the yarn 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, which will inevitably lead to yarn elongation and yarn elongation. The phenomenon of reduced thread twist, and the water in wet processing will play a role in setting the twist of cellulose fiber yarn.
△Yarn stretching state diagram
For yarns in a free state, this kind of stretching can be completely retracted, but yarns in fabrics are difficult to recover due to the cohesion between fibers and the interweaving resistance between yarns. If only through overfeeding, tentering, Finishing methods such as rubber blanket shrinking are difficult to retract, but yarns that have been stretched by fiber swelling can be retracted in a drum washing machine or in a water treatment that allows the fabric to be completely relaxed.
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. Fiber swelling causes the knitted fabric to shrink, as shown in the figure below.
Note: a and a′ are the coil widths before and after water swelling.Anti-wrinkle and crease technology for cotton knitted fabrics
An effective step to prevent wrinkles and creases (called dead pleats) that cannot be eliminated by post-finishing on single-sided cotton knitted fabrics is the first wet treatment process for cotton knitted gray fabrics. There are three specific methods.
Before the fabric is subjected to irregular tension and pressure, it is immersed in water in a completely relaxed state, and the fabric shrinks freely and evenly through fiber swelling; a dense fabric that shrinks evenly will shrink due to the cohesion force between fibers and the interweaving resistance between yarns. It is relatively stably fixed by the action of the method, and no dead folds will be produced due to pressure or tension (appropriate size) in subsequent processing. For example, in the multifunctional fully loose continuous scouring and bleaching machine, the first unit that the gray fabric passes through is the relaxation and anti-please machine that can immerse the fabric in water in a completely relaxed state, where the fabric shrinks and becomes dense freely and evenly through fiber swelling.
△The working principle diagram of the relaxation and anti-please machine
Before the fabric is subjected to irregular tension and pressure, it is wet processed under regular and appropriate external forces (including transverse and longitudinal directions). In this case, the change in the shape of the fabric caused by the fiber swelling due to water absorption is uniform and Ordered. There are some low-tension continuous scouring and bleaching machines used for open-width processing of open-width knitted fabrics, which use this method to prevent wrinkles.
Before the fabric is subjected to irregular tension and pressure, the fibers are irreversibly swollen under the action of regular external forces. This process is mercerization. The mercerized cotton knitted fabric will not produce so-called dead pleats during the rope bleaching and dyeing processes. The mercerized cotton fabric has higher dimensional stability.
During the first wet treatment of cotton knitted gray fabrics, the shape of the fabric is deformed due to water absorption and swelling of the fibers, and the resulting impact on the dimensional stability and surface effect of the fabric. 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.
Pay attention to the shaping effect of the mercerization process on cotton knitted fabrics that 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.
Pay attention to anti-shrinkage 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 cotton woven fabric anti-shrinking technologies such as rubber blanket pre-shrinking machine anti-shrinking technology, etc. It should be promoted and applied in anti-shrinkage finishing of knitted fabrics.