The difference between mercerization and alkali shrinkage
Mercerization: Cotton products (yarns, fabrics) are treated with concentrated caustic soda solution under tension, and then under tension The process of washing away caustic soda.
Alkali shrinkage: Cotton products are treated with concentrated caustic soda solution in a relaxed state to make the yarn or fabric shrink at will, and then the caustic soda is washed away, also called tension-free process Silky.
1 Purpose of fabric mercerization
1. Increase the gloss and brightness after mercerization; 2. Adsorption The ability, chemical reaction ability is enhanced, and the dye content is increased; 3. The shrinkage rate, dimensional stability, and fabric flatness are improved; 4. The mechanical properties of clothing such as strength and extensibility are changed.
2 Mercerization process
1. First bleach and then mercerize, the effect is good, the waste alkali is cleaner, but the whiteness is poor and easy to stain, so it is suitable Colored fabrics, especially heavy fabrics.
2. Silk first and then bleach. It has good degree, but poor gloss. The fabric is easily damaged during bleaching. It is suitable for bleached cloth and printed cloth.
3. Mercerizing after dyeing is suitable for varieties that are easy to scratch or difficult to level dye (after mercerization, the fabric feels harder and dyes faster). In order to improve the surface effect and dye fastness of the fabric when dyeing dark colors, And for some varieties that require high gloss, post-dye mercerization can also be used. (PS: Post-mercerization is selective for dyes)
4. Semi-mercerization before dyeing and regular mercerization after dyeing are to improve the adsorption and chemical reactivity of the dye.
3 Evaluation of mercerizing effect
1. Gloss is one of the main indicators to measure the appearance effect of mercerized fabrics.
Variable angle photometry, polarization light method, etc. can be used for testing, but there is no unified ideal testing method. At present, visual evaluation is mostly used.
2. Observe changes in fiber morphology through microscopic sections
3. Adsorption performance (1) Barium value method: It is a common method to test the mercerizing effect. The larger the barium value, the better the mercerizing effect. Barium value of cotton cloth = 100. Barium value > 150 means full mercerization, generally 135 to 150. (2) Iodine absorption method (3) Iodine contamination and dyeing test method: Samples with different barium values (100~160) are treated with a certain concentration of iodine solution h or direct blue 2B dye solution to make a color card, and then The iodine contamination and staining depth of the unknown sample were compared with the color card to quantitatively evaluate the mercerized barium value.
4. Dimensional stability: Use mechanical shrinkage method or immersion shrinkage method to measure the change in fabric length before and after treatment, and calculate the shrinkage rate through the formula. Generally, the shrinkage rate in the warp direction is often greater than that in the weft direction, but some fabrics with higher warp density The variety produces a negative shrinkage rate (the door width increases).
4 Changes in fibers after mercerization
1. Morphological structure
The fiber diameter increases and becomes rounded , the longitudinal natural twist rate changes (80% → 14.5%), the cross section changes from kidney to oval, or even circular, and the cell cavity shrinks to a point. If appropriate tension is applied, the roundness of the fiber increases and the original wrinkles on the surface disappear. The surface smoothness and optical properties are improved (the reflection of light changes from diffuse reflection to more directional reflection), the intensity of reflected light is increased, and the fabric shows a silky luster.
The change of fiber morphology in the fabric is the main reason for producing gloss, and tension is the main factor for improving gloss.
2. Microstructure
Crystallinity ↓ (70% → 50%), amorphous area ↑, so that the original The inaccessible hydroxyl groups in the water become accessible, so the dye adsorption performance and chemical reaction performance of the fiber are improved. In addition, due to the change of fiber shape after mercerization, the light scattering on the surface and inside is reduced, so when dyeing with the same concentration of dyes, , the dyeing depth also increases.
After the fiber swells, the hydrogen bonds between macromolecules are broken up. Under the action of tension, the arrangement of macromolecules tends to be neat, which improves the orientation. At the same time, the surface of the fiber is uneven. Deformations are eliminated and weak points are reduced. The fibers can evenly share external forces, thus reducing breakage caused by stress concentration. In addition, the fibers after puffing and rearrangement are in close contact with each other and the cohesion force also reduces the factors causing breakage due to the slippage of macromolecules.
3. Changes in molecular structure
After cotton fiber swells in concentrated alkali solution, the hydrogen bonds between macromolecular chains are broken down and the The internal stress stored in the fabric is relieved. Through stretching, the macromolecules are oriented and arranged, and new molecular bonds are established at new positions, and the intermolecular force is greater than before swelling. Finally, under tension, the hydrogen bonds between the aligned fibers are fixed (fixed in a more natural and stable state). At this time, the fibers are in a lower energy state, so their dimensions are stable.
5 Main factors of mercerizing process
The basic conditions of mercerizing process are the concentration, temperature, action time, tension and de-alkali of NaOH solution.
1. Alkali solution concentration
The main factors affecting the mercerizing effect ①After the cotton fiber is treated with caustic soda solutions of different concentrations, the changes in length and diameter: Concentration >8%, the diameter increases , the length is shortened to the maximum;
②Shrinkage and dyeing strength of cotton yarn in NaOH solutions of different concentrations; ③Concentration requirements for cotton mercerization (relationship between C, shrinkage and barium value): When C=177g/L, The barium value is 150, C=245g/l, and the barium value is the highest. C=240~280g/l, the shrinkage tends to be stable.
Therefore, the most suitable caustic soda concentration for mercerization is about 245g/l. Taking into account factors such as the alkali absorption of the fabric itself and the alkali consumption of acidic gases in the air, the alkali concentration for cotton mercerization is 260-280g/l. In actual production, the selection can be based on the quality of semi-finished products and the quality requirements of finished products.
2. Tension
1) The effect of tension on fabric gloss
When cotton fabrics are treated with concentrated alkali, only adding Appropriate tension can showProduce good luster. It can be seen from the influence of tension on the performance of cotton yarn after mercerization: high tension means good gloss.
2) The influence of tension on the mechanical properties and adsorption properties of fabrics
That is, under weak conditions, the strength of cotton yarn has been improved. If appropriate tension is applied, its strength can still be Further improvement, but the gloss does not increase much, and the elongation at break and adsorption performance decrease.
3) The influence of tension on fabric shrinkage
During mercerization, the warp and weft tension plays an extremely important role in fabric shrinkage. In actual production, the warp and weft shrinkage rates of fabrics of various specifications are unbalanced. For fabrics with higher warp density, such as khaki and Fu Tiao, the shrinkage rate in the warp direction greatly exceeds the shrinkage rate in the weft direction, so the warp tension is given priority; while for thin fabrics such as plain cloth, the opposite is true.
3. Temperature The effect of caustic soda and cellulose fiber is an exothermic reaction, so increasing the temperature of the alkali solution will weaken the swelling of the fiber, thus reducing the mercerizing effect, which is reflected in the shrinkage rate And the barium value decreases, so low temperature is better for mercerizing lye. However, in actual production, economic benefits are taken into consideration, and the viscosity of the lye increases when the temperature is too low, making it difficult for the mercerizing lye to penetrate into the yarn and fabric, and the expansion is relatively large. It is difficult, so usually the cold water is passed through the interlayer of the rolling groove to cool the alkali liquid.
4. Time The mercerizing effect is to allow the caustic soda to quickly, evenly and fully penetrate into the interior of the cotton yarn or fabric and act on the fibers, so a certain amount of time must be guaranteed. The cotton yarn was mercerized with 280g/l NaOH under no tension. It was found that the yarn shrinkage and dye absorption rate reached the maximum value in 20 seconds. Extending the time did not significantly improve the mercerizing effect. In addition, the time is related to the alkali concentration and temperature. When the concentration is low, the action time should be appropriately extended; generally 50 to 60 seconds are used.
5. Alkali removal has a great influence on the setting effect of mercerization. If after relaxing the tension, there is still more than 5% alkali on the fabric, the fabric will still shrink, thus affecting the gloss. , weft shrinkage. Alkali removal is carried out in two steps: ① In the case of expansion, use a flushing and suction device to rinse the fabric with hot dilute alkali; ② After relaxing the weft tension, enter the alkali removal box and wash and steam it with light alkali.
Alkali shrinkage:
The process of treating cotton knitted fabrics with concentrated caustic soda solution in a relaxed state. The purpose of alkali shrinkage and mercerization is different. Its main purpose is not to obtain luster, but to increase the tissue density and elasticity of knitted fabrics.
1 Problems that are prone to alkali shrinkage
There are density differences, fine wrinkles, holes, contamination, etc.
1. Reasons for the density difference of gray cloth after alkali shrinkage
①The longitudinal (vertical) density difference of gray cloth is too large. ② During alkali shrinkage, the concentration of alkali solution, the alkalinity and temperature of the hot washing tank are not uniform. Preventive measures ① Blank fabrics with different longitudinal densities should be processed in batches. ② Operate according to process requirements.
2. Fine wrinkles
Reduce the roller pressure; high-end products that are difficult to dye can be The process involves first scouring, then drying and then alkali shrinkage.
3. Holes
There is a difference in the linear speed of the front and rear rollers. The gray cloth is worn by the rollers and causes holes; adjust the linear speed between the front and rear rollers. There may be hard objects such as knitting needles embedded in the rollers (generally causing regular holes); the rubber rollers must be inspected before driving, and the operator must remove the broken needles from the gray fabric.
4. Contamination
The blank cloth before alkali shrinkage is stained with stains; clean the floor, stacking trucks, etc. before work. The cloth feed turntable, milk roller, cloth storage box, etc. are not properly cleaned; the equipment must be cleaned before driving.
2 Factors affecting alkali shrinkage
During the alkali shrinkage process, some process conditions, such as Alkali solution concentration, alkali solution temperature, alkali shrinkage time, alkali removal effect, etc. have a great influence on the alkali shrinkage effect. Therefore, the alkali shrinkage time must be strictly controlled to obtain the ideal effect.
1. Alkali concentration
The concentration of cotton fiber in caustic soda The puffing effect is closely related to the concentration of alkali solution. Only when the alkali solution concentration reaches a certain value, the fiber will produce significant puffing effect. Therefore, the alkali solution concentration is the main factor affecting alkali shrinkage. Tests show that when the alkali concentration reaches about 245g/L, the shrinkage rate of the fabric reaches the maximum. In actual production, maximum shrinkage is not required, so the alkali concentration is usually controlled within the range of 140~200g/L. The specific treatment concentration depends on the tissue density of the knitted fabric.
2. Alkaline temperature
The temperature of the lye will also Affect the alkali shrinkage effect. The reaction of alkali on cellulose is an exothermic reaction, so increasing the temperature is detrimental to alkali shrinkage. However, if the temperature of the alkali solution is too low, the penetration of the alkali solution into the fiber will be affected due to the high viscosity of the alkali solution. Taking into account the two factors of permeability and treatment effect during production, too low temperature cannot be used. The temperature of alkali solution should be 15~20℃. Since the reaction between alkali and cellulose is an exothermic reaction, the temperature of the alkali solution will increase as the process proceeds. In order to prevent the temperature of the alkali solution from rising, an alkali solution tank with an interlayer can be used. Run cold water through people to achieve the purpose of cooling down.
3. Alkali shrinkage time
The alkali shrinkage effect not only depends on the concentration and temperature of the alkali solution , also depends on the action time of caustic soda and fabric, that is, alkali shrinkage time. Alkali shrinkage time refers to the entire time from immersing the fabric in alkali solution to washing with alkali. From the perspective of the reaction between alkali and cellulose, the time required is very short, but it takes a certain amount of time for the alkali solution to penetrate into the fiber, and it also takes time to ensure that the fabric shrinks fully. Therefore, during alkali shrinkage, if the fabric is washed to remove alkali immediately after being padded with alkali solution, the fabric will not have time to shrink, which will affect the alkali shrinkage effect. After padding the alkali solution, it must be stacked to extend the alkali shrinkage time and improve the alkali shrinkage effect. Since the specific conditions of the alkali shrinkage equipment used in each factory are different, the length of the stacking time is also different, generally between 5 and 20 minutes. In addition, if the concentration of alkali solution is increased, the alkali shrinkage time can be shortened accordingly.
The time is very short, but it takes a certain amount of time for the alkali solution to penetrate into the fiber, and it also takes time to ensure that the fabric shrinks fully. Therefore, during alkali shrinkage, if the fabric is washed to remove alkali immediately after being padded with alkali solution, the fabric will not have time to shrink, which will affect the alkali shrinkage effect. After padding the alkali solution, it must be stacked to extend the alkali shrinkage time and improve the alkali shrinkage effect. Since the specific conditions of the alkali shrinkage equipment used in each factory are different, the length of the stacking time is also different, generally between 5 and 20 minutes. In addition, if the concentration of alkali solution is increased, the alkali shrinkage time can be shortened accordingly. </p
