Common textile fabric shrinkage knowledge

The common knowledge about shrinkage of textile fabrics is that the shrinkage rate of synthetic fibers and blended fabrics is the lowest, followed by wool, hemp and cotton fabrics in the middle, and the shrinkage of silk fabrics is large, while the largest is viscose, rayon and rayon fabrics.
Objectively speaking, all cotton fabrics have shrinkage and fading problems. The key is the finishing. Therefore, the fabric of general home textiles is pre-shrunk.
It is worth noting that pre-shrinking treatment does not mean no shrinkage, but that the shrinkage rate is controlled within 3% - 4% of the national standard, especially the natural fiber clothing will shrink. Therefore, in addition to selecting the quality, color and pattern of the fabric, the shrinkage rate of the fabric should also be understood when purchasing clothes.
1、 Influence of fiber and shrinkage
After absorbing water, the fiber will swell to a certain extent. Generally, the swelling of fibers is anisotropic (except nylon), that is, the length is shortened and the diameter is increased. Generally, the percentage of the difference between the length of the fabric before and after water and its original length is called shrinkage. The stronger the water absorption capacity, the stronger the swelling, the higher the shrinkage rate, and the worse the dimensional stability of the fabric.
The length of the fabric itself is different from the length of the yarn (silk) used, and the difference is usually expressed by the shrinkage.
Fabric shrinkage (%)=[yarn (silk) length - fabric length]/fabric length
After the fabric is launched into the water, due to the swelling of the fiber itself, the length of the fabric is further shortened, resulting in shrinkage. The shrinkage of fabrics is different with different shrinkage. The fabric shrinkage varies with the fabric structure and weaving tension. When the weaving tension is small, the fabric is tight and thick, and the weaving shrinkage is large, the shrinkage of the fabric is small; If the weaving tension is large, the fabric will be loose and light. If the weaving shrinkage is small, the shrinkage of the fabric will be large. In the process of dyeing and finishing, in order to reduce the shrinkage of the fabric, pre-shrinking finishing is often used to increase the weft density and increase the shrinkage of the fabric in advance, thus reducing the shrinkage of the fabric.
2、 Causes of shrinkage
(1) When the fiber is spinning, or the yarn is weaving, dyeing and finishing, the yarn fiber in the fabric is stretched or deformed by external force, and the yarn fiber and fabric structure produce internal stress. In the static dry relaxation state, or static wet relaxation state, or in the dynamic wet relaxation state, or full relaxation state, the release of different degrees of internal stress will restore the yarn fiber and fabric to the original state.
(2) Different fibers and their fabrics have different degree of shrinkage, which mainly depends on the characteristics of their fibers - hydrophilic fibers have a large degree of shrinkage, such as cotton, hemp, viscose and other fibers; However, hydrophobic fibers have less shrinkage, such as synthetic fibers.
(3) When the fiber is in the wet state, it will expand under the action of the soaking solution, which will make the fiber diameter larger. For example, on the fabric, it will force the fiber curvature radius of the weaving point of the fabric to increase, resulting in the shortening of the fabric length. For example, cotton fiber expands under the action of water, the cross-sectional area increases by 40~50%, and the length increases by 1~2%, while synthetic fiber generally shrinks by about 5% due to heat, such as boiling water.
(4) When the textile fiber is heated, the shape and size of the fiber changes and shrinks, and it cannot return to the initial state after cooling, which is called fiber thermal shrinkage. The percentage of length before and after thermal shrinkage is called thermal shrinkage, which is generally expressed by the percentage of fiber length shrinkage in boiling water at 100 ℃; It is also useful to measure the percentage of shrinkage in hot air above 100 ℃ by hot air method, and it is also useful to measure the percentage of shrinkage in steam above 100 ℃ by steam method. The performance of the fiber is also different under different conditions such as internal structure, heating temperature and time. For example, the boiling water shrinkage of the processed polyester staple fiber is 1%, the boiling water shrinkage of the vinylon is 5%, and the hot air shrinkage of the chloroprene fiber is 50%. There is a close relationship between fiber processing and the dimensional stability of the fabric, which provides some basis for the design of the subsequent process.
3、 Shrinkage rate of general fabrics
Cotton 4% - 10%
Chemical fiber 4% - 8%
Cotton polyester 3.5% - 5%
3% for plain white cloth
3-4% for wool and blue cloth
3-4.5% for poplin
3-3.5% for calico
4% for twill
Labor cloth is 10%
10% for rayon
4、 Reasons for affecting shrinkage
1. Raw materials
The shrinkage rate of fabrics is different with different raw materials. Generally speaking, the fibers with high hygroscopicity will expand, increase in diameter, shorten in length and increase in shrinkage after immersion. For example, the water absorption of some viscose fibers is as high as 13%, while the water absorption of synthetic fiber fabrics is poor, and its shrinkage is small.
2. Density
The shrinkage of the fabric varies with its density. If the density in longitude and latitude is similar, the shrinkage in longitude and latitude is also close. The warp shrinkage of fabrics with high warp density will be large. On the contrary, the weft shrinkage of fabrics with higher weft density will be large.
3. Yarn count
The shrinkage rate of the fabric varies with the yarn count. The shrinkage rate of coarse count fabric is large, and that of fine count fabric is small.
4. Production process
The shrinkage of fabrics varies with the production process. Generally speaking, in the process of weaving, dyeing and finishing, the fiber needs to be stretched many times, and the processing time is long. The shrinkage of the fabric with large tension is large, and vice versa.
5. Fiber composition
Compared with synthetic fibers (such as polyester and acrylic), natural plant fibers (such as cotton and hemp) and regenerated plant fibers (such as viscose) are easier to absorb moisture and expand, so the shrinkage rate is higher, while wool is easy to felt due to the scale structure of the fiber surface, affecting its dimensional stability.
6. Fabric structure
Generally, the dimensional stability of woven fabrics is better than that of knitted fabrics; The dimensional stability of high-density fabrics is better than that of low-density fabrics. In woven fabrics, the shrinkage of plain fabrics is less than that of flannel fabrics; In knitted fabrics, the shrinkage rate of plain stitch is smaller than that of rib fabric.
7. Production and processing process
As the fabric is inevitably stretched by the machine during dyeing, printing and finishing, there is tension on the fabric. However, it is easy to release the tension of the fabric when it is exposed to water, so we will find that the fabric shrinks after washing. In the actual process, we usually use pre-shrinking to solve this problem.
8. Washing care process
Washing care includes washing, drying and ironing. Each of these three steps will affect the shrinkage of the fabric. For example, the dimensional stability of hand-washed samples is better than that of machine-washed samples, and the washing temperature will also affect its dimensional stability. In general, the higher the temperature, the worse the stability. The drying method of the sample also has a great influence on the shrinkage of the fabric.
The commonly used drying methods include: drip drying, metal mesh spreading, hanging drying and rotary drum drying. The drip-drying method has the smallest impact on the size of the fabric, while the tumbler-arch drying method has the largest impact on the size of the fabric, and the other two are in the middle.
In addition, selecting an appropriate ironing temperature according to the composition of the fabric can also improve the shrinkage of the fabric. For example, cotton and linen fabrics can be ironed at high temperature to improve their size shrinkage. However, the higher the temperature is, the better. For synthetic fiber, high temperature ironing can not improve its shrinkage, but will damage its performance, such as hard and brittle fabrics.