If not handled properly during dyeing, almost all fabrics will produce varying degrees of “rain stripe” defects, especially when processing thin cotton and nylon-cotton fabrics on open-width equipment such as jig dyeing and pad dyeing. protrude.
The causes of “rain-like streaks” are roughly the following 6 aspects:
1) The yarn (silk) used in the fabric is unevenly dry, resulting in uneven density in parts of the cloth surface or even in the entire piece.
2) The dye molecules selected have larger particles, especially disperse and vat dyes, which are prone to migration.
3) There is a lack of control and maintenance of processing equipment. For example, the guide rollers and expanders of jigger dyeing machines are entangled with yarn and other obstacles when used, the padding rate of the padding car is too high, the infrared temperature is too high during pre-baking, and hot air drying Excessive air volume when dry, etc.
4) The selection and use of dyeing materials are not appropriate. It is better to choose dyes with smaller molecules that are not easy to migrate. Otherwise, the prescription should be adjusted, such as adding leveling agents, anti-migration agents, etc.
5) Strengthen the inspection of gray fabrics so that relevant measures can be taken in a timely manner.
6) The pre-treatment is not in place, such as the slurry, grease and wax are not removed, so that the fabric cannot obtain good and uniform hygroscopicity.
All of the above will cause “rain-like strips” to appear on the fabric during the dyeing process.
Avoid or reduce “rain bars” The methods are:
1) Strengthen the inspection of gray fabrics and try to find problems before dyeing. If it is discovered that there is uneven density in the weaving of cotton gray fabrics, on the one hand, it will be explained to the customer in advance; on the other hand, all links from pre-processing to drying will be strictly monitored to reduce the defects to a minimum. In practice, 100% cotton fabrics can avoid the occurrence of “rain stripes”, while polyester-cotton or nylon-cotton blended fabrics are relatively difficult to produce.
2) Enhance mercerization. The dosage of sodium hydroxide can be increased to 230-250 g/L, and the vehicle speed is controlled at 20-25 m/min, so that the cotton fibers can fully expand under a certain tension and the fabric structure becomes loose. However, it must be pointed out that during pre-treatment, various impurities on the fabric, such as slurry, grease and wax, must be removed, and the high heat treatment must not be excessive. Otherwise, the impurities will form a waterproof film around the fiber and reduce the hygroscopicity of the fabric. (Gross effect), which increases the migration tendency of the dye liquor during baking, making it easier to produce “rain streaks” defects.
3) Mild pre-drying should be used after fabric dyeing. The liquid carrying rate after pre-drying should be controlled at about 30%. During hot air drying, the moisture inside and outside the fabric can be evaporated evenly to prevent dye liquid migration. Generally, the speed of hot air circulation should be from slow to fast, and the temperature should be from low to high.
4) Minimize dry heat treatments such as tentering, pre-shaping or drying before dyeing. Frequent heat treatment before dyeing will increase the density of the fabric, which will have an impact that is difficult to eliminate during dyeing and subsequent processing. In particular, the pre-shaping process of polyester cotton and nylon cotton will often aggravate the formation of “rain-like strips”. Therefore, this type of blended interwoven fabric should be processed on open-width equipment. Due to the spinning of nylon (nylon), its evenness is uneven, so it is not suitable to heat treat nylon/cotton. Practice has proven that nylon is alkali-resistant, so although nylon-cotton fabrics do not need to be mercerized like cotton fabrics, it is necessary to carry out alkali treatment before dyeing. The NaOH concentration is controlled at 150-180 g/L to improve uneven dryness. of tows to reduce the formation of “rain streaks”. After polyester is treated with alkali weight reduction, the “peeling” effect will cause the fiber sliver to become uneven in thickness and increase the occurrence of defects. Therefore, when formulating the processing technology, the process must be implemented differently according to the fabric components.
5) For fabrics that are prone to “rain streaks”, in addition to screening dyes in the prescription, leveling agent SFH (acrylamide and acrylic acid binary copolymer) can also be used to improve the dyeing effect. The dosage is: pad dyeing ⒛g/L; jig dyeing 3~5 g/L; overflow dyeing 1~2 g/L. Generally speaking, the dosage can be appropriately increased for dyes that are prone to migration, equipment that is difficult to operate, and fabrics with obviously different density.
6) For polyester-cotton or nylon-cotton blended fabrics, in order to avoid “rain streaks”, one-step dyeing in the same bath is more effective than the two-bath dyeing method, and the probability of defects can be reduced by 50%. The reflectivity of synthetic fibers such as polyester and nylon is greater than that of cotton fiber, and the refractive index is smaller than that of cotton. Therefore, sometimes it looks like “rain-like strips”, but in fact it is the reflection and refraction of light when the two fibers are juxtaposed. An illusion caused by differences. In order to overcome this illusion, try to make the color light of the two component fibers consistent. In fact, for two different fibers or dyes, it is almost impossible to achieve the same color and light in the true sense. The only way is to adjust the color and light on polyester, brocade and other synthetic fibers.It must be about 20% darker than cotton and slightly darker, so that the visual color light between the two can be relatively balanced, and the degree of “rain-like streaks” can be greatly reduced.
Extended reading: https://www.alltextile.cn/product/product-36-6 .html
Extended reading: https://www.alltextile.cn/product/ product-61-527.html
Extended reading: https://www .yingjietex.com/product/DE-28-Memory-Fabric.html
Extended reading: https://www.alltextile.cn/product/product-44-465.html
Extended reading: https://www.alltextile.cn/product/product-73-928.htm
