Mercerization is a process in which cotton fabrics are treated with concentrated caustic soda solution under a certain tension and maintained at the required size, so that the fabrics obtain a silky luster.
1. Mercerizing process mechanism
When mercerized cotton fiber is treated with concentrated caustic soda, because of the small size of sodium ions, it can enter the amorphous region and crystalline region of the fiber at the same time, and the water molecules surrounding the sodium ions form a water Chemical layer, when sodium ions enter the interior of the fiber and combine with the fiber, water molecules are also brought in at the same time, causing the fiber to swell violently, forcing the diameter of the cotton fiber to increase, the length to shorten, the longitudinal curl of the fiber almost disappears, and the fiber cross-section is from the waist to the waist. The shape becomes nearly round, and the entire fiber changes from a flat ribbon to a cylindrical shape. At this time, a certain tension is applied. After the fiber is stretched, the uniformity and surface smoothness are improved. The fiber diffuses light before mercerization. The reflection changes to more directional reflection, which increases the intensity of the reflected light, causing the fabric to show a silky luster; the crystallinity of the cotton fiber after mercerization drops from about 70% before mercerization to 50-60%. Due to the fiber crystallinity The decrease results in good dyeing performance, while also improving the dyeing performance of dead cotton and improving the quality of dyed products. Improve the dimensional stability of the fabric, eliminate stress within the fabric and remove wrinkles.
The main agents used for mercerization are: caustic soda, alkali-resistant penetrant, acetic acid, etc.
The function of alkali-resistant penetrant: accelerate the wetting and penetration of mercerizing alkali solution, improve the mercerizing effect, and improve the dyeing depth, brightness, fullness and gloss.
Types of mercerizing machines: straight roller mercerizing machine, cloth clip mercerizing machine, straight roller cloth clip mercerizing machine, knitted fabric cylinder mercerizing machine, knitted fabric open width mercerizing machine, short Process Straight roller winding mercerizing machine, etc.
2. Mercerizing process procedure
1 Woven fabric mercerizing : Padding with concentrated alkali → Ventilation → Padding with concentrated alkali → Ventilation → (Preheating washing) → Expanding cloth clips to flush and absorb alkali → Straight roller flushing to remove alkali → Steam washing → Neutralization → Water washing → Drying
2 Knitted fabric mercerization: open width cloth feeding → photoelectric weft straightening device → straight roller groove → two-roller re-rolling car → five-roller large cylinder alkali rolling machine → two-roller re-rolling car → cloth clip expansion Width punch absorbs alkali → two-roll heavy rolling car → high-efficiency drum rinse box → two-roller light rolling car → high-efficiency drum rinse box → two-roller light rolling car → high-efficiency drum rinse box → two-roller light rolling car →High-efficiency drum rinsing box→Two-roller light rolling car→High-efficiency drum rinsing box→Two-roller light rolling truck→High-efficiency drum rinsing box→Two-roller heavy rolling car→Open-width drop cloth.
3. Mercerizing process conditions
The main process technical parameters of mercerizing Including: caustic soda concentration, rolling residue rate, alkali action time, warp tension, weft tension, number of alkali leaching, alkali removal temperature, drop cloth width, drop cloth pH value, etc.
4. Notes on mercerizing
1 The degree of dryness and wetness of the fabric before mercerization must be uniform. When mercerizing the wet fabric, the water-gathering rate is 60% for cotton fabrics and linen fabrics, and 50% for polyester-cotton fabrics. Ensure that the left, middle and right parts of the fabric are The liquid is evenly distributed.
2 When dry cloth is mercerized, the fabric must be fully cooled by a ventilation rack before padding with alkali solution to keep the temperature of the mercerizing concentrated alkali solution stable.
3 When the fabric is mercerized, it must bear a certain amount of tension in the warp and weft directions. The greater the tension, the better the luster of the fabric and the higher the size and temperature resistance.
4 During the mercerization process, the fabric must be kept flat to prevent curling, wrinkles, etc. Once tested, the mercerization must be repeated and repaired.
5 The mercerization process of knitted fabrics generally adopts mercerization of gray cloth or mercerization after singeing. Because the fabric has poor water absorption, penetrant must be used to enhance the penetration performance of concentrated caustic soda solution and improve the mercerization effect.
6 Frequently use a wide range of indicators or test paper to detect the pH value of mercerized fabric, try to keep the fabric neutral, and use acid to neutralize it during the washing process if necessary.
7 Concentrated caustic soda (concentrated acid) has severe corrosion and burning effects on human skin. Personnel who come into contact with concentrated alkali liquid (concentrated acid liquid) should be equipped with necessary protective equipment in case of emergency If lye (or acid) splashes on your skin or eyes, rinse with plenty of water before seeking medical advice.
5. Evaluation of mercerizing effect
1) Gloss: It is a measure of the appearance effect of mercerized fabrics One of the main indicators, but there is no unified ideal testing method, and visual evaluation is currently used.
2) Observe microscopic morphological changes through microscopic sections.
3) Adsorption performance
A Barium value method: It is a test method to measure the absorption performance of cotton fiber, commonly used to test The degree to which the absorbency of a fabric increases after it undergoes the mercerization process. Take one piece of mercerized and unmercerized cloth sample, wash it with hot water and cold water and dry it at 100-105 for 1 hour. Take out the constant weight, accurately weigh 2 grams of each, cut into small pieces, and place them in 100 ml. In a covered Erlenmeyer flask, absorb 30mL of 0.25mol/L barium hydroxide and inject it into the flask. Cover the lid immediately and shake it from time to time. After 2 hours, absorb 10Ml, use phenolphthalein as indicator, titrate with 0.1mol/L hydrochloric acid standard solution, and calculate the barium value.
B Iodine staining and staining method: Treat samples with different barium values (100 150) with a certain concentration of iodine solution or direct blue 2B to make a color card, and then Compare the iodine solution contamination and staining depth of the unknown sample with the known color card to quantitatively evaluate the mercerized barium value.
4) Fabric dimensional stability: Use the fabric shrinkage method to test the changes in the warp and weft length of the fabric before and after mercerization. The smaller the fabric dimensional shrinkage, the better the mercerization effect.
6. Common quality problems of mercerization and methods to overcome them
1) Color difference between left, middle and right
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A The left, middle and right wool effects of pre-treatment are inconsistent, resulting in inconsistent amounts of alkali in various parts of the fabric.
B The alkali concentration in various parts of the alkali rolling tank is inconsistent.
C The wear of the rolling mill and the inflexibility of the cylinder lead to inconsistent liquid amounts in the left, middle and right parts of the fabric
D Watering during expansion The alkali absorption is inconsistent on the left, middle and right sides, resulting in inconsistent alkali absorption in various parts of the fabric.
E Insufficient washing.
2) The shrinkage rate of the fabric in the warp and weft directions is too large.
A The alkali concentration is too high. (Thin fabrics have a large shrinkage in the weft direction)
B Insufficient expansion (the tension is too high when feeding the cloth clip), resulting in difficulty in weft expansion.
C The warp tension of the cloth roller between the two alkali rolling tanks is too small, the fabric’s warp mercerization setting effect is poor, and the fabric’s warp shrinkage is large.
D The effect of leaching alkali is poor. The amount of alkali contained in the fabric when it comes out of the cloth clip is too large. Fabrics shrink greatly when washed with high alkali and high temperatures.
E The equipment tension is too high and the fabric shrinks greatly in the weft direction.
3) Broken edges and holes
A The cloth clips are worn or inflexible.
B The cloth clip track is out of balance, and the tension on the left and right sides of the fabric is inconsistent.
C The width of the cloth outlet is too wide,
D The expansion is too large or too rapid.
E The edge detection or amplification device is not flexible.
4) Auxiliary spots
When boiling and bleaching, the surfactant remaining on the fabric is destroyed by alkali, and the lipophilic groups gather. Contamination of the fabric will cause local dye rejection and light spots on the fabric; direct repair after post-processing and softening will easily lead to color spots.
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