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Textile testing expertise: shrinkage

2021-09-01 14:54
1. Reasons for shrinkage of textiles:
The textiles become smaller in size after washing, generally due to the following reasons:
The first is thermal shrinkage, which is mainly manifested in thermoplastic chemical fibers such as polyester and nylon. The improvement method is to do heat setting during dyeing and finishing, and obtain good dimensional stability through high temperature setting.
The second is the felting of wool and other fiber textiles with a scale structure. Under the mechanical action of the water, the unidirectional and irreversible positioning of the wool scales makes the wool fabric continue to be felted. The improvement method is addition (closed scale layer) And subtraction (remove the scale layer by chemical method) two kinds.
The third category is the shrinkage of cellulose fiber textiles. The reason for this is relatively complicated. There is usually a misunderstanding that the reason for shrinkage is that the fiber or yarn is stretched and stretched and the length of the The shrinkage rate of linen fabric sometimes far exceeds this value, so it must have other more important reasons.
As shown in the figure below, due to the interweaving structure and the cylindrical structure of the yarn itself, the yarn is not completely straight in the fabric, but has a certain curved curve. We call this degree of bending weave shrinkage, and the yarn is in The length of the fabric interwoven structure to be extended is called the winding.
Cellulose fibers such as cotton, linen and rayon will swell when they meet with water, and this swelling is anisotropic, that is, the swelling is large in diameter, and the elongation in the length direction is small. The yarn becomes thicker but does not become too long. The thickening of the yarn leads to an increase in the winding stroke, but the yarn cannot be stretched much. Therefore, the yarn can only become more curved when the shrinkage becomes larger, which leads to a smaller size of the fabric. This is the main reason for the shrinkage of cotton and linen fabrics.
The swelling of rayon fiber is greater than that of cotton and linen fiber, so the shrinkage rate will be greater at this level. Another reason for the greater shrinkage rate of rayon is that the wet modulus of rayon is very small, and it is easily stretched and elongated in the wet state, resulting in a much larger shrinkage rate than cotton and linen.
The warp shrinkage rate of cotton and linen fabrics is mainly improved by mechanical preshrinking methods, such as rubber blanket preshrinking and setting machine overfeeding. The weft shrinkage rate is mainly controlled by mercerizing, and the internal stress is eliminated by mercerizing. Stable width.
There is also a chemical method, which is resin cross-linking. Resin cross-linking reduces the fiber swelling performance, and for rayon fiber with low wet modulus, it has the effect of making it cross-linked and not easy to be stretched, thereby improving the fabric Shrinkage rate. This is currently the most mature and reliable method to significantly improve the shrinkage rate of rayon, and it has higher dimensional stability than the mechanical pre-shrinking method and the advantages of long-wearing and not easy to stretch.
2. Inference and discussion:
2.1. For cotton and linen non-stretch woven fabrics, the control process of the shrinkage in the weft direction is the control of the mercerized width, not the heat setting. Cotton and linen fiber does not have the same thermoplasticity as polyester and nylon fiber (or in other words, the glass transition temperature of cotton and linen fiber is too high, and the current setting machine can't reach it. Even if it can reach it, it will be scorched instead of softened, so it is meaningless. ), so heat setting does not make it like chemical fiber fabrics to obtain dimensional stability, only through the elimination of the internal stress of mercerizing, in order to obtain good dimensional stability.
2.2. In actual operation, for some cellulose fiber non-stretch fabrics, the weft direction is pulled to the extreme first, and then the width is retracted by the water again, which may also partially improve the weft shrinkage rate. There are two guessed reasons:
One is that when the weft is pulled to the extreme, the weft yarns are partially untwisted, resulting in improved weft shrinkage. From this perspective, this method is only effective for light and thin fabrics, because the tension provided by the setting machine is not enough to untwist the weft yarns of heavy fabrics; it will be more effective for thick and hard yarns with low twist, because this type of yarn is more effective. Untwisting easily occurs.
Another reason may be because of the rigidity of the fiber itself, which is similar to ironing a fold on clothes at high temperature, which cannot be easily eliminated. When the weft is drawn at high temperature, the yarn is ironed in the current bending state, and the stronger its rigidity, the more likely it is to obtain a "mechanical setting" effect (different from the setting of the glass transition temperature of chemical fibers). From this From a perspective, hemp is greater than cotton, and cotton is greater than rayon, because in terms of fiber rigidity, hemp is greater than cotton, and cotton is greater than rayon.
This kind of improvement is only a partial improvement, and the shrinkage rate improvement data is usually only 1-3%, and it is adaptable to the fabric, which is a last resort. The normal approach should be to improve the mercerizing process.
2.3. The warp shrinkage rate of cotton-polyester or cotton-polyamide blended fabric is usually not as large as that of pure cotton fabric. The reason is that as described in 8.1, it only shrinks due to "dry setting" deformation, and there is no weft fiber swelling that causes it. The shrinkage of the circumnavigation increases. People in cotton long cart factories are usually surprised that interlaced fabric dyeing factories never do pre-shrinking machines, because the necessity is not as strong as pure cotton fabrics.
2.4. Elastic fabrics containing spandex must be heat-set to obtain a good shrinkage rate. There is a time-temperature equivalent effect, higher temperature x shorter setting time and lower temperature x longer setting time It is equivalent in obtaining good shrinkage of the fabric, but the former is better in terms of economy, and the rebound effect of the fabric is better in the latter.
2.5. Shrinkage rate and shrinkage rate are two different concepts. Shrinkage rate refers to the rate of change in the size of the fabric after washing, while shrinkage refers to the warp elongation or shorten. Shrinkage rate = (grey fabric weft density-finished weft density)/grey fabric weft density. A large dyeing and finishing shrinkage rate does not mean a large shrinkage rate of the fabric. The dyeing and finishing shrinkage rate of the fabric is the "material consumption" of the fabric in order to obtain a specific style under a specific dyeing and finishing process, and the shrinkage rate is a physical property of the fabric itself. For example, for warp stretch fabric, we can use different heat setting processes to make it have a variety of shrinkage rates, but the shrinkage rate can be maintained at about 5%.
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