Views: 1000 Author: Site Editor Publish Time: 2021-04-15 Origin: Site
The reason why fabrics need to be pre-shrunk is because the fabric has a shrinkage effect. According to the research of textile materials, the shrinkage effect can be divided into three categories: natural shrinkage of fiber elastic recovery; fiber hydrophilic swelling shrinkage and thermal shrinkage of fiber chemical structure . Chemical treatment of fiber shrinkage is not the scope of this article.
1. Natural shrinkage
The fabric is constantly under tension during the processing process, and the yarn and fiber have accumulated elongation deformation. The greater the tension, the more accumulated stress deformation. After the tension disappears, the elastic deformation in the accumulated deformation will be released, making the length of the fabric smaller and producing natural shrinkage. In production, some fabrics (such as fabrics with better elasticity) usually need to be placed for a period of time (usually 24 hours) after being laid, and the purpose is to give the fabric enough time to shrink naturally. Natural shrinkage is also a method of pre-shrinking.
2. Water absorption shrinkage
The hydrophilic fiber will produce anisotropic expansion after absorbing water, which is the main reason for the water absorption shrinkage (shrinkage rate) of the fabric. The better the moisture absorption of the hydrophilic fiber, the greater the shrinkage rate of the woven fabric. The experiment pointed out that the length and diameter of the hydrophilic fiber increase when it meets water and wet, but the increase in diameter is much larger than the increase in length.
Cellulose and protein fibers have better moisture absorption and swelling properties. In contrast, cotton fibers and viscose fibers are more prominent. After the fiber absorbs water, it swells and becomes thicker, and the yarn diameter becomes thicker and larger. After drying, the fiber and yarn basically return to their original thickness. The result of this "one piece and one relaxation" causes the accumulated elongation deformation in the fabric. (Also known as residual deformation) has been fully "released"; at the same time, the original warp and weft yarns in the fabric have changed and moved due to the expansion effect. Therefore, the area of the hydrophilic fabric is reduced, the thickness increases, and the surface is uneven after being dried in water.
The design idea of the damp heat shrinking machine is to utilize the water absorption and shrinkage characteristics of the fabric, and the shrinking process used is "wet spray drying".
In particular, it is pointed out that viscose fiber has short molecular chains, many amorphous areas, loose structure, large swelling rate and low wet modulus, and the shrinkage phenomenon is particularly serious. Even after mechanical preshrinking, sometimes it may not solve the problem.
3. Chemical heat shrinkage
Synthetic fibers (such as polyester, etc.) will undergo thermal shrinkage during processing and use. The main reason is that when the temperature of the fiber exceeds the glass transition temperature, the internal supramolecular orientation structure and crystalline structure change, which is macroscopically expressed as fiber length The shrinkage of the fabric is directly reflected in the change of fabric length and width. The "tentering and high-temperature steaming" pre-shrinking of synthetic fiber fabrics utilizes this property. When ironing synthetic fiber fabrics such as polyester, the temperature is strictly controlled to avoid heat shrinkage.
In particular, due to the characteristics of strong water absorption and felting of wool, the mechanical preshrinking of wool fabrics is usually carried out in a "tension-free state" using the method of "low temperature steaming and cooling". The washing of wool fabrics also requires Carry out at room temperature.