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Problems And Preventive Measures That May Arise During The Processes of Setting, Pre-shrinking, Mercerizing, Dyeing, And Finishing of Fabrics Containing Polyurethane Elastic Fibers.
Views: 0 Author: Site Editor Publish Time: 2025-12-15 Origin: Site
Problems and preventive measures that may arise during the processes of setting, pre-shrinking, mercerizing, dyeing, and finishing of fabrics containing polyurethane elastic fibers.
Due to the tension exerted during the weaving process, polyurethane elastic knits experience significant stretching deformation. Although they possess good elasticity, they still cannot return to their natural state. The fabric's width and thickness are prone to instability, which can greatly affect dyeing and finishing processes. The purpose of relaxation is to thoroughly pre-shrink the fabric and eliminate internal residual stress. In the dyeing and finishing of cotton knits, efforts should be made to minimize tension, allowing the fabric to remain in a relaxed state to avoid fiber elongation. This article briefly discusses the considerations that need to be addressed in the dyeing and finishing process, common defects, as well as an analysis and prevention of finishing wrinkles.
01. Considerations in Dyeing and Finishing Production
Selection and Inspection of Greige Fabric
(1) First, check the width and warp and weft density of the incoming greige fabric, and test the shrinkage elasticity (take a piece of the full fabric, treat it in boiling water for 30 minutes, then relax and dry before testing the elastic shrinkage rate, which is generally around 30%). For weft elastic fabrics, the width of the greige fabric should be adjusted proportionally according to the finished product's width and the amount of weft elongation.
(2) When accepting production orders, clarify the customer's quality requirements, especially for elasticity, shrinkage rate, width, and length. If processing incoming greige, understand the specifications of the greige fabric, the type of sizing agent used, and the elongation and shrinkage rates in both warp and weft directions, and test the elasticity of the greige; if the difference between the greige width and the finished product width is no more than 20%, and the fabric's elasticity is over 30%, the weft elongation and shrinkage performance of the finished product will be better.
(3) Due to the varying compactness of the fabric structure, uneven stress during weaving, and the discrepancy between edge and main structure, fabrics are prone to curling during dyeing and finishing. Therefore, the edge structure of 2/1 and 3/1 twill and satin fabrics should be densified, requiring a wider and denser edge for longer floats.
Pre-Shrinking of Elastic Fabrics
For elastic fabrics, especially weft elastic ones, the tension applied to the weft yarn during weaving must first be eliminated to restore its proper elasticity. Thus, pre-shrinking should be arranged at the beginning of the pre-treatment process. This can be scheduled before singeing, through hot water relaxation washing or combined with de-sizing (the more thorough the de-sizing, the more complete the pre-shrinking). Use a relaxed flat hot washing machine or a rolling dyeing machine. To avoid rapid shrinkage during high-temperature washing that can cause wrinkles or curling, a gradual temperature increase (60, 70, 80, 90°C) should be applied, allowing the fabric to shrink gradually.
Singeing
Before singeing, ensure that the seams are straight and the width difference is minimized. Sewing edges at both ends of the fabric reduces fraying, curling, and wrinkling in subsequent processes. As elastic fabrics do not withstand high temperatures, use a process with high speed and low flame temperature, ensuring the fabric surface temperature does not exceed a certain limit to avoid affecting elasticity. Singeing should be uniform, with a clean fabric surface and no scorch marks; it should quickly proceed to the next process, prohibiting prolonged storage.
De-sizing
The key to effective de-sizing is thorough washing with sufficient water. A gradual temperature increase and backflow method can be used to not only allow the fabric to shrink gradually but also swiftly remove swollen, oxidized sizing agents and impurities.
Pre-treatment and De-boiling Process
The elastic yarns in weft elastic fabrics should not be subjected to humid heat for extended periods, nor are they suitable for rope-like treatments. Weft elastic fabrics (such as satin and straight-textured styles) and double-elastic fabrics are prone to curling and wrinkling during dyeing and finishing processes and are not resistant to strong alkalis or high temperatures. In steaming processes, they are also susceptible to defects like wrinkling and curling. Therefore, the ideal process utilizes cold rolling pre-treatment conditions and employs low alkali concentration at room temperature.
Setting and Mercerizing Process
Setting and mercerizing are key processes for stabilizing the shape of elastic fabrics. The sequence of these procedures should be determined based on actual conditions. If the width of the semi-finished product is less than that of the finished product, mercerizing should be performed first; if the semi-finished width is greater, setting should be done first.
Mercerizing
For cotton-elastane fabrics, a direct roller mercerizing machine is recommended. Although the elastane fibers do not require mercerizing, the outer cotton fibers and warp yarns do. Mercerizing not only stabilizes the weft dimension of the fabric, improves dye uptake, and ensures warp shrinkage but also regulates elasticity and shrinkage rates. The temperature in the mercerizing box should not be too high to avoid wrinkling, ideally maintained at 85–90°C. Since mercerizing does not affect elastane fibers, the main factor influencing the width is the shrinkage effect of the elastane fibers. Thus, through mercerizing, cotton/elastane weft elastic fabrics should achieve high elastic stretch without excessive shrinkage.
Shaping (Pre-Shaping)
The greige fabric can shrink by 40%–50% in width after being taken off the machine, turning it into a highly elastic narrow fabric; shaping processes are needed to control this shrinkage, balancing the specified width and shrinkage rate. In the shaping process, excessively high temperatures or prolonged duration can affect strength and cause yellowing, while too low temperatures or inadequate duration will not achieve desired results. The width of the fabric at the time of shaping directly affects the width of the finished product, and the weft shrinkage rate after shaping allows for predictions about the finished product's elasticity and shrinkage rate.
9. Dyeing
Cotton/Elastane Weft Elastic Fabric Dyeing，When dyeing cotton/elastane weft elastic fabrics, since the elastane fibers are encased in cotton fibers and the elastane content is only 3% to 5%, the dyeing can follow the procedures used for pure cotton fabrics. Typically, reactive and reductive dyes are used; for medium to dark colors, a two-phase method with reactive dyes is commonly employed. For thin elastic fabrics, continuous pad dyeing can lead to wrinkling between infrared drying and the first pre-drying stage, as well as during entry into the reductive steaming box. To overcome this, a widening device can be added to the equipment. Dyeing and printing may also use the steaming method for fixation. To prevent wrinkling during dyeing, infrared pre-drying and post-dyeing soap boiling are avoided.
During the dyeing of double-elastic fabrics, which are prone to wrinkling and curling, the continuous pad dyeing machine may be inadequate; therefore, a cold pile dyeing process is selected. In this process, the fabric is rolled immediately after being soaked in the dye liquor, preventing wrinkle marks from tension and other issues. Additionally, since there is no temperature increase during dyeing, this effectively prevents the damage to elasticity caused by high temperatures under tension. After dyeing, a relaxed washing machine is used, allowing the weft and warp elasticity to return to their desired state.
(1) Disperse dyes are suitable for dyeing under acidic conditions, while reactive dyes are suited for alkaline conditions.
(2) Disperse dyes are appropriate for high-temperature dyeing, whereas reactive dyes come in three types: low, medium, and high-temperature.
(3) When dyeing with reactive dyes, a significant amount of electrolyte must be added, but excess electrolyte can affect the stability of disperse dyes.
10. Post-Finishing of Elastane Fabrics
The after-finishing of elastic fabrics must meet the requirements for hand feel and appearance, and also ensure that the width further shrinks to match the finished product width. It is vital to prevent shrinkage during storage or when the fabric is wet. To address issues of poor dimensional stability and high shrinkage rates, post-finishing processes such as stretching and pre-shrinking must be carried out. During soft stretching, it is essential to use softeners that minimally affect the color, with appropriate dosage control; otherwise, there may be localized stretching and sliding of yarns in the finished product, especially in fabrics with a lower density of 4/1 and 3/1 weft and warp yarns. An automatic width adjustment device should be equipped on the hot air stretching machine to promptly correct any fabric bowing.
Before soft stretching, tests for shrinkage rates in both directions of the dyed fabric, as well as standards for diagonal fabric testing, should be conducted. Based on the shrinkage observed in width, the overfeeding percentage for the stretching machine and the shrinkage rate of the pre-shrinking machine should be determined. The fabric width at the time of laying should be 2.5 to 5 cm (1 to 2 inches) wider than the finished product width to ensure that it reaches the desired width after pre-shrinking.
Stretching Process
In the post-finishing processes, the stretching and pre-shrinking steps are crucial for controlling the dimensional stability and shrinkage rate of the elastic fabrics. Due to the tension experienced during multiple processing stages, the fabric may undergo significant shrinkage in the warp direction, so the post-finishing methods must account for changes in width during pre-treatment. Particular attention should be paid during the stretching phase; if the fabric width is uneven and elasticity is reduced, the stretching temperature should not exceed 160°C, and the duration should be less than 30 seconds, ensuring that the fabric has a certain level of moisture (about 80-90% dry) to keep the width and shrinkage rate within the required range during pre-shrinking.
Control of Overfeeding in Stretching
During the soft stretching process, overfeeding is essential. If tightening is performed after soft drying, the warp elongation rate may exceed 27%, making it impossible to meet the finished product's weft density requirements. Conversely, if soft drying is followed by overfeeding stretching, it can fully meet the requirements.
In the process of reforming, the fabric is treated with a softener and then subjected to short-loop soft drying before being threaded onto the machine. Without tension, it is impossible to engage the machine properly, which may lead to disengagement. Therefore, a certain amount of tension must be applied in the feeding section, but increasing tension can cause the fabric to elongate. Thus, overfeeding must be applied to allow elongation before recovering.
