The origin of mercerization
Mercerization refers to the silky luster that appears on the surface of certain cotton fabrics after processing.
In 1844, when the British chemist Mercer was using cotton cloth to filter sawdust in concentrated caustic soda, he found that the cotton cloth became thicker, and he learned that concentrated caustic soda can make cotton fibers swell. He applied for a patent in 1850; in 1890, when Lower treated cotton with concentrated caustic soda, he discovered that applying tension could improve the luster of cotton; in 1895, mercerization began to be industrialized. In order to commemorate Messer, mercerization was called Messer treatment. (Mercerizing).
1. Refers to light. Chen Houzhu of the Southern Dynasty wrote a seven-rhyme poem on a boating trip to Xuanpu in spring: “The silk shines in the sun, and the colors of the flowers sink in the water.”
2. The luster of silk. Yu Shinan of the Tang Dynasty wrote in his “Federation and New Year’s Banquet for Palace Ministers”: “The mercerized light brings out the color of the willows, and the sun shines through the smoke of the locust trees.” The “Tampering Chapter” of Weizhuang in the former Shu Dynasty: “The fish eyes are woven with white robes, and the water chestnut ribbons are woven with clusters of mandarin ducks.”
3. The process of adding luster to cotton yarn or cotton fabrics through chemical effects. Cotton fabrics that have been treated with mercerization are called mercerized cloth, mercerized towels, etc.
The meaning of mercerization
Mercerization: Cotton products (yarns, fabrics) are treated with concentrated caustic soda solution under tension conditions, and then The process of washing away caustic soda under tension.
Alkali shrinkage: Cotton products are treated with concentrated caustic soda solution in a relaxed state to make the fibers shrink arbitrarily, and then the caustic soda is washed away. It is also called tension-free mercerization and is mainly used for cotton knitting. Product processing.
Characteristics of mercerization
After mercerization: the fabric undergoes the following changes
1. Increased gloss
2. The adsorption capacity and chemical reaction ability are enhanced
3. Shrinkage, dimensional stability, and fabric flatness are improved
4. The mechanical properties of clothing such as strength and extensibility are changed despite alkali shrinkage. It cannot improve the gloss of the fabric, but it can make the yarn tighter, improve the elasticity, and plump the hand. In addition, the strength and dye adsorption capacity can be improved.
Mercerized characteristics
Mercerized fiber properties
Changes of cotton fiber after the action of concentrated alkali
1. Morphological structure
The fiber diameter increases and becomes rounded, and 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, the original wrinkles on the surface disappear, the surface smoothness, and the optical properties are improved (reflection of light). The reflection changes from diffuse reflection to more directional reflection), increasing the intensity of reflected light, and the fabric shows a silky luster. Changes in fiber morphology within the fabric are the main cause of gloss, and tension is the main factor in improving gloss.
2. Microstructure
Crystallinity ↓ (70% → 50%), amorphous area ↑, making the hydroxyl groups that were originally inaccessible in water become Because it is accessible, 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 light scattering on the surface and inside 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 uneven deformation of the fiber surface is eliminated, reducing weak links. 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.
Principle of mercerization
Mercerization is a complex process. There are two theories to explain the reason why cotton fibers swell violently in concentrated alkali solution.
1. Hydration theory
(1) Caustic soda reacts with natural cellulose (cellulose) to generate alkali cellulose. There are two main types:
①Alcohol compounds:
②Molecular compounds (addition compounds): Both products are unstable. They are hydrolyzed into hydrated cellulose after washing with water, and then become mercerized after dehydration and drying.
(2) The reason why cotton fiber undergoes severe irreversible swelling when treated with concentrated NaOH is that sodium ions are small in size and can enter the crystal area of the fiber; at the same time, Na+ is an ion with strong hydration ability. , there are as many as 66 water molecules surrounding a Na+, forming a hydration layer. When Na+ enters the interior of the fiber and combines with the fiber, a large amount of water is also brought in, causing severe swelling. into the crystalline region, therefore, the swelling is irreversible.
(3) This swelling is affected by temperature.
(4) Swelling is also affected by NaOH concentration and neutral salt.
2. Donnen membrane equilibrium theory
Swelling is the result of osmotic pressure.
Assuming that cellulose is a weak base, it can form sodium salt in caustic soda solution. The sodium salt of cellulose ionizes to form the immovable cellulose anion Cell-O?, and there is still mobile Na+ in the solution. and OH-, if there is NaCl, and Cl?, the fiber surface is regarded as having properties similar to a semipermeable membrane, these ionsThe mercerizing effect is manifested in the decrease in shrinkage and barium value, so mercerizing alkali solution at low temperature is better. However, 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 the inside of the fabric, and it is difficult to expand, so the cold water is usually passed through the rolling groove interlayer to cool the alkali liquid.
Time
The mercerizing effect is to make the caustic soda quickly, evenly and fully penetrate into the cotton yarn or fabric and act on the fiber, so a certain 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.
Alkali removal
Alkali removal has a great influence on the setting effect of mercerization. If after relaxing the tension, there will still be more than 5% alkali on the fabric. 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 Loosen the weft After returning to tension, it enters the alkali removal box and is washed and steamed with light alkali.
Mercerizing equipment
Commonly used mercerizing machines include cloth clip mercerizing, bent roller mercerizing and straight roller mercerizing machines.
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