Key points of mercerization control
1. Tension
1. The effect of tension on fabric gloss
When cotton fabrics are treated with concentrated alkali, only by adding appropriate tension can they show good luster. From the influence of tension on the properties of cotton yarn after mercerization, we can It can be seen: high tension and 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 If appropriate tension is applied, the strength can be further improved, 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.
2. 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 is manifested in the decrease in shrinkage and barium value, so mercerizing lye at low temperature is better. 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. Internally, it is difficult to expand, so cold running water is usually passed through the rolling groove interlayer to cool the alkali liquid.
3. 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 interact 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 could reach 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.
4. Alkali removal
Alkali removal has a great influence on the setting effect of mercerization. If the tension is relaxed, there will still be more than 5% of alkali on the fabric. Alkali, 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;
②After relaxing the weft tension, enter the alkali removal box and wash and steam with light alkali.
Principle of mercerization
Concentrated caustic soda solution (about 18~25%) is used to treat cotton, fiber/cotton or polyester/cotton fabrics, so it can be obtained The fundamental reason for good mercerizing effect is that concentrated alkali solution can cause irreversible and violent swelling of cotton fibers. In addition, it must be matched with appropriate tension.
When the cotton fiber swells violently in concentrated alkali solution, the cross-section of the fiber changes from a flat waist shape or an ear shape to a round shape, and the cell cavity also shrinks. For completely mercerized fibers, Said it was almost reduced to a point.
And the natural torsion in the longitudinal direction disappears. If appropriate tension is applied to stretch the fiber or prevent it from shrinking, the wrinkles on the surface of the fiber disappear and become a very smooth cylinder, which is regular to the light. reflection, showing luster.
After the cotton fiber is subjected to the action of concentrated alkali, it will swell violently, which is not only reflected in the change of the lattice parameters, but also the transformation of some crystal areas into amorphous areas. After being washed with water to remove the alkali and dried, Although there will still be a certain degree of shrinkage in the diameter direction of the fiber, it cannot be restored to its original state. Instead, the shape during swelling is basically preserved and becomes irreversible swelling, so that the luster obtained is durable. At the same time, due to The content of the amorphous zone increases, and the adsorption properties of the fiber are therefore improved.
As for the reason why cotton fiber will swell irreversibly in concentrated alkali solution, it is mainly believed that cotton fiber can react with hydrogen oxidation in concentrated alkali solution Sodium forms cellulose sodium salt (cellulose-ONa). Since sodium ions are ions with strong hydration ability, when it combines with cellulose, a large amount of water will be brought into the interior of the fiber, causing the fiber to Severe swelling, and can also provide an appropriate explanation for why the concentration of alkali solution is too high or the alkali solution contains electrolytes such as salt, which causes the fiber to swell less. In fact, in addition to this explanation, there are some other explanations. For example, applying the membrane balance principle can also provide a satisfactory explanation for the fiber swelling performance. A brief explanation is as follows.
Using membrane balance to illustrate the principle of mercerization is to regard the inside of the fiber as the intramembrane system and the external alkali solution as the extramembrane system. When fiber is combined with alkali, there will be cellulose-O-, Na+, OH- and other ions. If there is salt, there will also be Cl-. Among them, cellulose-O- can only be inside the membrane and cannot diffuse outside the membrane, while other ions are mobile and establish a balance inside and outside the membrane according to certain conditions. If the volumes inside and outside the membrane are equal and unchanged during the equilibrium process, when equilibrium is reached, the concentration of ions inside (I) and outside (O) the membrane can be expressed as follows:
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(2)Bendingrollermercerizingmachine
Bendingrollermercerizingmachinecangenerallyprocesstwolayersoffabrics,anditscomponentsarebasicallythesameasclothmercerizing.Thebendingmercerizingmachineissimilartothemachine,butthestretchingandalkaliwashingdevicesaredifferent.Thebendingrollermercerizingmachinereliesonbendingspokesforstretching.Therefore,inthissection,thebendingrollstretchingpartisbrieflyintroduced.
Therollbendingextensionpartoftherollbendingmercerizingmachineiscomposedofashallowflatsteppedirontroughand10to12pairsofbendingrolls.Thefronthalfoftheirontroughislocatedatahigherposition.Therearetworowsof5to6pairsofhardrubberbendingrollersarrangedupanddown.Theyarealldrivenbythefabric.Theirfunctionistoexpandandextendthefabricthathasbeenpaddedwithconcentratedalkalisolution.Whenthefabriciscoveredwithconcentratedalkali;attherearoftheirontrough,atalowerposition,therearetworowsof5to6pairsofactivecastironbendingrollers,mostofwhichareimmersedinthehotdilutealkaliliquidflowingbackfromthealkaliremovalbox.Fabricsarewashedawayundertensionbytheactionoflye.
The axis of the bending roller is fixed. There are many fixed bushings on it. There is a cast iron sleeve that can rotate outside each bushing. , there is a ball bearing between the shaft sleeve and the sleeve. The outside of the sleeve can be covered with alkali-resistant rubber and has circumferential grooves on the surface. The stretching function of the bending roller relies on the weft component force generated when the fabric is wound around the arc-shaped inclined surface of the bending roller sleeve to widen the fabric. The bending roller mercerizing machine occupies a small area and has a poor stretching effect. It is easy to turn the weft yarn into an arc shape, and the warp density is unevenly distributed, with the middle being lower and the sides higher. At the same time, because the two fabrics are stacked together for processing, the washing The alkali efficiency is not as high as the shower and suction device of the cloth mercerizing machine, and it is rarely used in China.
(3) Straight-roll mercerizing machine
Two layers of fabric can generally be processed on the straight-roll mercerizing machine, but the main components are the cloth clips and the The bending roller mercerizing machine is quite different. It consists of a cloth feeding device, an alkali padding tank, a heavy-duty pad roller, an alkali removal tank, an alkali removal box and a flat washing tank.
After the fabric passes through the cloth feeding device and the bending roller for expansion, it enters the alkali padding tank of the mercerizing machine. The alkali padding tank is composed of a shallow iron tank containing concentrated mercerizing alkali solution and many straight rollers that alternate up and down and press each other. The upper row of straight rollers is covered with alkali-resistant rubber and is a passive roller that can be lifted up when threading cloth and pressed against the lower row of straight rollers during operation. The lower row is an automatic cast iron straight roller, which is immersed in concentrated alkali solution. After the fabric passes through the straight rollers impregnated with alkali solution, it passes through a heavy-duty nip roller to remove excess alkali solution, and then enters the alkali removal tank. The alkali removal tank is similar to the alkali padding tank. It is also composed of an iron tank and straight rollers. The difference is that the lower row of cast iron rollers is immersed in dilute alkali washing liquid to wash away a large amount of alkali liquid on the fabric. After the fabric goes out of the alkali tank and then goes through the alkali removal tank and the flat washing tank to wash away the residual caustic soda, the mercerization process is completed.
Whether it is the straight roller in the alkali padding tank or the alkali removal tank, there is no expansion effect, but the fabric is used to closely adhere to the straight roller. The surface of the roller relies on the friction between them to prevent the shrinkage of the fabric width. However, relying solely on the frictional resistance between the straight roller and the fabric is often not enough to completely prevent the shrinkage of the fabric, which is the main disadvantage of the straight roller mercerizing machine. </p
