Alkali shrinking process
The basic shrinkage of cotton knitted fabrics has two processes, dry shrinkage and wet shrinkage. Dry shrinkage is generally used. Dry shrinking is directly alkali shrinking the grey cloth, and then bleaching. In this way, the process is short and can be continuous than production, but the wettability of the gray cloth is poor, and it is easy to produce uneven shrinkage. Adding penetrant can improve the permeability of lye. Wet shrinking is the basic shrinkage of the grey cloth after scouring. Although the fabric penetrates in this way! It is healthy, the alkali absorption is uniform, and the elasticity, gloss and leveling are better than dry shrinkage. However, because the cloth is wet, the water on the fabric will bring human lye, the lye will be diluted, and the temperature of the lye will rise, affecting the alkali shrinkage effect. Alkali shrinking of dark and high-end products in the dyeing process can be carried out after dewatering or drying and then alkali shrinking. After training and bleaching, the capillary effect of the fabric is good, the alkali absorption is uniform, and the alkali shrinkage quality is good, but the process is long and the energy consumption is large. The rough cloth is first impregnated with a penetrant, followed by an alkaline solution. After wetting the fabric, the alkali complex is beneficial to the uniform absorption of alkali by Shaansu, which can improve the leveling property. However, it takes a certain time for sodium hydroxide to replace the water in the fiber, and the lye will also be diluted. It is oval or even circular, the cell cavity shrinks to a point, and the moisture content of the entire fiber should not exceed 65% to 70% after being changed from a flat ribbon, and it should be uniform.
What are the problems of alkali shrinkage and how can they be prevented?
Problems with alkali shrinkage include fine wrinkles, holes, and stains.
(1) Fine wrinkles:
Reduce the pressure of rolls; high-end products with great difficulty in dyeing can adopt the process of cooking, drying and alkali shrinking.
(2) Hole:
The linear speed of the front and back rolls is different, and the grey fabric is damaged by the wear of the rolls; adjust the linear speed between the front and back rolls. Hard objects such as knitting needles may be embedded in the rollers (usually causing regular holes); the rubber rollers must be checked before driving, and the trolley stopper must remove the broken needles from the grey cloth.
(3) Contamination:
The rough cloth is stained before alkali shrinkage; do a good job of cleaning the ground and cloth stacker before shifts. The cleaning work of the cloth feed turntable, milk roller, storage box, etc. is not good; the equipment must be cleaned before driving.
What are the factors that affect alkali contraction?
During the alkali shrinking process, some process conditions, such as the concentration of the alkali solution, the temperature of the alkali solution, the time of the alkali shrinkage, and the effect of removing the alkali, have a great influence on the effect of the alkali shrinkage. Therefore, the alkali shrinkage must be strictly controlled to obtain the desired effect.
Lye concentration:
The swelling effect of cotton fiber in caustic soda is closely related to the concentration of lye. Only when the lye concentration reaches a certain value, the fiber will have a significant swelling effect, so the lye concentration is the main factor affecting the alkali shrinkage. The test shows that when the lye concentration reaches about 245g / L, the shrinkage of the fabric reaches the maximum level. In actual production, maximum shrinkage is not required, so the concentration of lye is usually controlled within the range of 140 ~ 200g / L, and the specific treatment concentration depends on the tissue density of the knitted fabric.
Lye temperature:
The temperature of the lye also affects the effect of alkali shrinkage. The reaction of alkali to cellulose is an exothermic reaction, so increasing the temperature is not good for alkali shrinkage. However, if the temperature of the lye is too low, the lye's viscosity will affect the penetration of the lye into the fiber. Considering the two factors of permeability and treatment effect in the production, too low temperature cannot be adopted, and the temperature of the lye should be 15 ~ 20 ℃. Since the reaction between alkali and cellulose is an exothermic reaction, the temperature of the lye will increase with the progress of the process. In order not to increase the temperature of the lye, an lye tank with an interlayer can be used. Pass in cold water to cool down.
Base shrink time:
The alkali shrinking effect depends not only on the concentration and temperature of the lye, but also on the action time of caustic soda and the fabric, that is, the alkali shrinking time. Alkali shrinking time refers to the total time of the fabric from immersion of lye to alkali washing. From the reaction of alkali and cellulose, the time required is very short, but it takes a certain time for the alkali to penetrate into the fiber, and it takes time to ensure that the fabric fully shrinks. Therefore, in the case of alkali shrinkage, if the fabric is washed and alkali-degraded immediately after being immersed in the alkali solution, the fabric will have no time to shrink, which will affect the alkali shrinkage effect. After padding the lye, it must be stacked to extend the alkali shrinking time and improve the alkali shrinking effect. Because the specific conditions of the alkali shrinking equipment used in each plant are different, the length of the stacking time also varies, generally between 5-20 minutes. In addition, if the lye concentration is increased, the alkali shrinkage time can be shortened accordingly.
