What is mercerization
Cotton products (yarns, fabrics) are treated with concentrated caustic soda solution under tension, and then washed away under tension. process.
1 The purpose of mercerizing
1 , the gloss and brightness are improved after mercerization;
2. The adsorption capacity and chemical reaction ability are enhanced, and the dye color yield is increased;
3. Shrinkage, size The stability and fabric flatness are improved;
4. The mechanical properties of clothing such as strength and extensibility are changed.
2 Mercerizing process
Classification by process:
1. Bleach first and then silk: The mercerization effect is good and the waste alkali is cleaner, but the whiteness is poor and it is easy to stain. It is suitable for colored fabrics, especially heavy fabrics.
2. Silk first and then bleach: The whiteness is good, but the gloss is poor. The fabric is easily damaged during bleaching. It is suitable for bleached cloth and printed cloth.
3. Mercerization after dyeing: 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 quality of the fabric when dyeing dark colors. Surface effect and dye fastness, as well as some varieties that require high gloss, can also be mercerized after dyeing.
4. Semi-mercerizing before dyeing and regular mercerizing after dyeing: in order to improve the adsorption and chemical reactivity of the dye.
3 Evaluation of mercerized effect
1. Gloss: the main indicator to measure the appearance effect of mercerized fabrics one.
Variable angle photometry, polarization light method, etc. can be used to measure, but there is no unified ideal testing method. At present, visual evaluation is mostly used.
2. Microscopic sectioning to observe changes in fiber morphology
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 staining test method: combine different barium values (100~ 160) The sample is treated with a certain concentration of iodine solution h or direct blue 2B dye solution to make a color card. Then the iodine contamination and dyeing depth of the unknown sample are 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. However, some varieties with higher warp density produce negative shrinkage (increased door width).
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%), and the cross-section changes from kidney deformation to It is oval or even circular, and the cell cavity shrinks to a point. If appropriate tension is applied, the roundness of the fiber increases, the original wrinkles on the surface disappear, the surface smoothness and optical properties are improved, the intensity of reflected light is increased, and the fabric shows Silky luster.
2. Microstructure
Crystallinity ↓(70%→ 50%), the amorphous area ↑ makes the hydroxyl groups that were originally inaccessible in water accessible, so the dye adsorption performance and chemical reaction performance of the fiber are improved. In addition, due to the change of fiber morphology after mercerization, the surface and The internal light scattering is reduced, so when dyeing with the same concentration of dye, 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, thereby reducing fractures caused by stress concentration. It also reduces the factors causing breakage due to macromolecule slip.
3. Changes in molecular structure
Cotton fiber in concentrated alkali After swelling occurs in the liquid, the hydrogen bonds between macromolecule chains are broken up, which relieves the internal stress stored in the fabric. Through stretching, the macromolecules are oriented and arranged, and new molecular bonds are established at new positions, and the molecules The interstitial force is greater than before swelling.
Finally, the hydrogen bonds between the aligned fibers are fixed by removing the base under tension. At this time, the fibers are in a lower energy state and are therefore dimensionally stable.
5 Main factors of mercerization process
1. Alkali solution concentration: the main factor affecting the mercerizing effect
(1) Changes in length and diameter of cotton fibers after being treated with caustic soda solutions of different concentrations:
Concentration>8%, the diameter increases and the length shortens to the maximum.
(2) Shrinkage and dyeing strength of cotton yarn in NaOH solutions of different concentrations
(3) Concentration requirements for cotton mercerization ( The relationship between C and shrinkage and barium value) When C=177g/L, the barium value is 150, C=245g/l, the barium value is the highest. C=240~280g/l, the shrinkage tends to be stable.
In actual production, it can be selected according to the quality of semi-finished products and the quality requirements of finished products.
2. Tension
(1) The effect of tension on the gloss of the fabric Impact: When cotton fabrics are treated with concentrated alkali, only by adding appropriate tension can they show good luster. From the effect of tension on the performance of cotton yarn after mercerization, it can be seen that the greater the tension, the better the luster.
(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 be further improved, but the gloss will increase. There are not many, and the elongation at break and adsorption performance have declined.
(3) The influence of tension on fabric shrinkage: During mercerization, the warp and weft tension plays an extremely important role in fabric shrinkage.
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, thereby reducing the mercerizing effect, which is reflected in the decrease in shrinkage and barium value, so Low temperature is better for mercerizing alkali solution, but in actual production, economic benefits are taken into consideration, and the viscosity of the alkali solution increases when the temperature is too low, making it difficult for the mercerizing solution to penetrate into the yarn and fabric, and it is difficult to expand, so rolling is usually used. Pass cold water into the interlayer of the tank to cool the alkali liquid.
4. Time
The mercerizing effect is to make the caustic soda penetrate into the cotton yarn or fabric quickly, evenly and fully The inside interacts with 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 can reach the maximum value in 20 seconds. Extending the time will improve the mercerization. The effect is not significant.
5. Remove alkali
Remove Alkali has a great influence on the setting effect of mercerization. If there is more than 5% alkali on the fabric after relaxing the tension, the fabric will still shrink, thus affecting the gloss and 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 , wash and steam with light alkali. </p
