Knowledge of textile testing

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 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.

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 textile testing equipment 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.

2.1. 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, 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.

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