01. What is shrinkage
The fabric is a fibrous fabric, and after the fibers themselves absorb water, they will experience a certain degree of swelling, that is, a reduction in length and an increase in diameter. The percentage difference between the length of a fabric before and after being immersed in water and its original length is usually referred to as the shrinkage rate. The stronger the water absorption ability, the more severe the swelling, the higher the shrinkage rate, and the poorer 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 between the two is usually represented by the weaving shrinkage.
Shrinkage rate (%)=[yarn (silk) thread length - fabric length]/fabric length
After being immersed in water, due to the swelling of the fibers themselves, the length of the fabric is further shortened, resulting in shrinkage. The shrinkage rate of a fabric varies depending on its weaving shrinkage rate. The weaving shrinkage rate varies depending on the organizational structure and weaving tension of the fabric itself. When the weaving tension is low, the fabric is tight and thick, and the weaving shrinkage rate is high, the shrinkage rate of the fabric is small; When the weaving tension is high, the fabric becomes loose, lightweight, and the shrinkage rate is low, resulting in a high shrinkage rate of the fabric. In dyeing and finishing, in order to reduce the shrinkage rate of fabrics, pre shrinkage finishing is often used to increase the weft density, pre increase the fabric shrinkage rate, and thus reduce the shrinkage rate of the fabric.
02. Reasons for fabric shrinkage
The reasons for fabric shrinkage include:
During spinning, weaving, and dyeing, the yarn fibers in the fabric elongate or deform due to external forces. At the same time, the yarn fibers and fabric structure generate internal stress. In the static dry relaxation state, static wet relaxation state, or dynamic wet relaxation state, different degrees of internal stress are released to restore the yarn fibers and fabric to their initial state.
Different fibers and their fabrics have different degrees of shrinkage, mainly depending on the characteristics of their fibers – hydrophilic fibers have a greater degree of shrinkage, such as cotton, linen, viscose and other fibers; However, hydrophobic fibers have less shrinkage, such as synthetic fibers.
When fibers are in a wet state, they swell under the action of immersion, causing the diameter of the fibers to increase. For example, on fabrics, this forces the curvature radius of the fibers at the interweaving points of the fabric to increase, resulting in a shortened length of the fabric. For example, cotton fibers swell under the action of water, increasing their cross-sectional area by 40-50% and length by 1-2%, while synthetic fibers generally exhibit thermal shrinkage, such as boiling water shrinkage, at around 5%.
Under heating conditions, the shape and size of textile fibers change and shrink, but they cannot return to their initial state after cooling, which is called fiber thermal shrinkage. The percentage of length before and after thermal shrinkage is called the thermal shrinkage rate, which is generally expressed as the percentage of fiber length shrinkage in boiling water at 100 ℃; It is also possible to measure the percentage of shrinkage in hot air above 100 ℃ using the hot air method, or to measure the percentage of shrinkage in steam above 100 ℃ using the steam method. The performance of fibers varies under different conditions such as internal structure, heating temperature, and time. For example, when processing polyester staple fibers, the boiling water shrinkage rate is 1%, the boiling water shrinkage rate of vinylon is 5%, and the hot air shrinkage rate of chloroprene is 50%. The dimensional stability of fibers in textile processing and fabrics is closely related, providing some basis for the design of subsequent processes.
03. Shrinkage rate of different fabrics
From the perspective of shrinkage rate, the smallest are synthetic fibers and blended fabrics, followed by wool and linen fabrics, cotton fabrics in the middle, silk fabrics with larger shrinkage, and the largest are viscose fibers, artificial cotton, and artificial wool fabrics.
The shrinkage rate of general fabrics is:
Cotton 4% -10%;
Chemical fiber 4% -8%;
Cotton polyester 3.5% -55%;
3% for natural white cloth;
3% -4% for woolen blue cloth;
Poplin is 3-4%;
Flower cloth is 3-3.5%;
Twill fabric is 4%;
Labor cloth is 10%;
Artificial cotton is 10%
04. Factors affecting shrinkage rate
Raw materials: The shrinkage rate of fabrics varies depending on the raw materials used. Generally speaking, fibers with high moisture absorption will expand, increase in diameter, shorten in length, and have a higher shrinkage rate after being immersed in water. If some viscose fibers have a water absorption rate of up to 13%, while synthetic fiber fabrics have poor moisture absorption, their shrinkage rate is small.
Density: The shrinkage rate varies depending on the density of the fabric. If the longitudinal and latitudinal densities are similar, their longitudinal and latitudinal shrinkage rates are also similar. A fabric with high warp density will experience greater warp shrinkage, while a fabric with higher weft density than warp density will experience greater weft shrinkage.
Yarn count thickness: The shrinkage rate of fabrics varies depending on the thickness of the yarn count. Clothes with coarse yarn count have a higher shrinkage rate, while fabrics with fine yarn count have a lower shrinkage rate.
Production process: Different fabric production processes result in different shrinkage rates. Generally speaking, during the weaving and dyeing and finishing process of fabrics, fibers need to be stretched multiple times, and the processing time is long. The shrinkage rate of fabrics with high applied tension is higher, and vice versa.
Fiber composition: Natural plant fibers (such as cotton and linen) and regenerated plant fibers (such as viscose) are more prone to moisture absorption and expansion compared to synthetic fibers (such as polyester and acrylic), resulting in a higher shrinkage rate. On the other hand, wool is prone to felting due to the scale structure on the fiber surface, which affects its dimensional stability.
Fabric structure: In general, 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 rate of plain weave fabrics is generally lower than that of flannel fabrics; In knitted fabrics, the shrinkage rate of plain knitted fabrics is lower than that of ribbed fabrics.
Production and processing process: Due to the inevitable stretching of the fabric by the machine during dyeing, printing, and finishing, tension exists on the fabric. However, fabrics can easily relieve tension when exposed to water, so we may notice shrinkage after washing. In practical processes, we usually use pre shrinkage to solve this problem.
Washing care process: Washing care includes washing, drying, and ironing, each of which will affect the shrinkage of the fabric. For example, hand washed samples have better dimensional stability than machine washed samples, and the washing temperature also affects their dimensional stability. Generally speaking, the higher the temperature, the poorer the stability.
The drying method of the sample also has a significant impact 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 least impact on the size of the fabric, while the rotary drum drying method has the greatest impact on the size of the fabric, with the other two being in the middle.
In addition, selecting an appropriate ironing temperature based on the composition of the fabric can also improve the shrinkage of the fabric. For example, cotton and linen fabrics can improve their size reduction rate through high-temperature ironing. But it’s not that higher temperatures are better. For synthetic fibers, high-temperature ironing not only cannot improve their shrinkage, but also can damage their performance, such as making the fabric hard and brittle.
The commonly used inspection methods for fabric shrinkage include dry steaming and washing.
Taking water washing inspection as an example, the shrinkage rate testing process and method are as follows:
Sampling: Take samples from the same batch of fabrics, at least 5 meters away from the fabric head. The selected fabric sample should not have any defects that affect the results. The sample should be suitable for water washing, with a width of 70cm to 80cm square blocks. After natural laying for 3 hours, place the 50cm * 50cm sample in the middle of the fabric, and then use a box head pen to draw lines around the edges.
Sample drawing: Place the sample on a flat surface, smooth out creases and irregularities, do not stretch, and do not use force when drawing lines to avoid displacement.
Water washed sample: To prevent discoloration of the marking position after washing, it is necessary to sew (double-layer knitted fabric, single-layer woven fabric). When sewing, only the warp side and latitude side of the knitted fabric should be sewn, and the woven fabric should be sewn on all four sides with appropriate elasticity. Coarse or easily scattered fabrics should be edged with three threads on all four sides. After the sample car is ready, place it in warm water at 30 degrees Celsius, wash it with a washing machine, dry it with a dryer or air dry it naturally, and cool it thoroughly for 30 minutes before conducting actual measurements.
Calculation: Shrinkage rate=(size before washing – size after washing)/size before washing x 100%. In general, the shrinkage rate of fabrics in both warp and weft directions needs to be measured.
Post time: Apr-09-2024