The difference between mercerization and alkali shrinkage

Mercerizing: Process in which cotton products (yarns, fabrics) are treated with a concentrated caustic soda solution under conditions of tension and then washed off the caustic soda under tension.

Alkali shrinkage: cotton products in a relaxed state with concentrated burning lye treatment, so that yarn or fabric arbitrary contraction, and then wash the process of caustic soda, also known as tension-free mercerization.

1. Purpose of fabric mercerization

1, after mercerizing luster, brightness increase;

2. Enhanced adsorption capacity, chemical reaction capacity and dye load;

3, shrinkage rate, dimensional stability, fabric smoothness;

4, strength, elongation and other mechanical properties have changed.

2. Process of mercerizing

1. Float the silk first

Mercerizing effect is good, waste alkali is relatively clean, but the whiteness is poor, easy to stain, suitable for color cloth, especially heavy fabric.

2. Silk before bleaching

Good whiteness, but poor luster, fabric is easy to be damaged during bleaching, suitable for bleached cloth, printed cloth.

3, after dyeing mercerization

It is suitable for varieties that are easy to be bruised or not easy to be evenly dyed (the fabric feels hard after mercerizing, and dyeing faster). In order to improve the surface effect and dyeing fastness of the fabric when dyeing dark color, and some varieties with high requirements for luster, it can also use mercerizing after dyeing. (PS: post mercerization is dye selective)

4, dyeing the first half of the mercerizing, dyeing after the conventional mercerizing

In order to improve the adsorption and chemical reactivity of dyes.

3. Evaluation of mercerizing effect

1. Luster

One of the main indexes to measure the appearance effect of mercerized fabrics.

Variable Angle photometry and polarized light can be used to measure, but there is no uniform ideal test means, visual evaluation is often used at present.

2. Morphological changes of fibers were observed by microsectioning

3, adsorption performance

(1) Barium value method: It is a common method to test the mercerization effect. Large barium value means good mercerization effect. Barium value of cotton cloth =100, barium value > 150 means full mercerization, usually 135 ~ 150.

(2) iodine absorption method

(3) Iodine stain and staining test method: Samples with different barium values (100 ~ 160) are treated with iodine solution h or direct blue 2B dye solution of a certain concentration to make color cards. Then, the depth of iodine stain and dyeing of unknown samples is compared with the color cards to quantitatively assess the mercerized barium value.

4. Dimensional stability

Mechanical shrinkage method or dip shrinkage method was used to measure the length change of the fabric before and after treatment. The shrinkage rate was calculated by the formula. Generally, the warp shrinkage rate was often greater than the zonal shrinkage rate, but some varieties with higher warp density produced negative shrinkage rate (increased door width).

4. Changes of fibers after mercerization

1. Morphological structure

The fiber diameter increases and becomes round, the longitudinal natural distortion rate changes (80%→14.5%), the cross section changes from kidney to oval or even round, 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 (the reflection of light changes from diffuse reflection to more directional reflection). By increasing the intensity of the reflected light, the fabric shows a silky luster.

The change of fiber shape in fabric is the main reason to produce luster, and the tension is the main factor to enhance luster.

2. Microstructure

Crystallinity ↓(70%→50%), amorphous region ↑, so that the original inaccessible hydroxyl group in water becomes accessible, so the adsorption performance and chemical reaction performance of the fiber have been improved, in addition, due to the change of fiber shape after mercerization, the surface and internal light scattering is reduced, so the dyeing depth is also increased when dyeing with the same concentration of dye.

After fiber swelling, the hydrogen bonds between macromolecules are separated. Under the action of tension, the arrangement of macromolecules tends to be orderly, which improves the orientation. Meanwhile, the uneven deformation of fiber surface is eliminated and the weak links are reduced. The fiber can evenly share the external force, thus reducing the fracture caused by stress concentration. In addition, the bulked and rearranged fibers are close to each other, and the adhesion force also reduces the fracture factors caused by the slip of macromolecules.

3. Changes in molecular structure

When cotton fibers swell in the concentrated lye, the hydrogen bonds between the macromolecular chains are separated, relieving the internal stress stored in the fabric. Through stretching, the macromolecules are oriented to establish a new molecular bond at a new position, and the intermolecular force is greater than before swelling.

Finally, under tension, the hydrogen bonds between the aligned fibers are fixed (in a more natural, stable state), and the fibers are in a lower energy state and thus stable in size.

5 Main factors of mercerizing process

The basic conditions of mercerizing process are the concentration of NaOH solution, temperature, acting time, tension and alkali removal.

1. Concentration of lye

The main factors affecting mercerizing effect

① The change of length and diameter of cotton fiber treated with different concentrations of caustic soda solution: concentration > 8%, the diameter increased, length shortened to the maximum;

② Shrinkage and dyeing power of cotton yarn in NaOH solution of different concentration;

(3) Concentration requirements of cotton mercerizing (relationship between C and shrinkage and barium value) :

When C=177g/L, barium value is 150, and when C=245g/l, barium value is the highest.

C=240~280g/l, the contraction tends to be stable.

Therefore, the most suitable concentration of caustic soda for mercerizing is about 245g/l. Considering the alkali absorption of the fabric itself and the alkali consumption of acid gas in the air, the concentration of mercerizing alkali in cotton cloth is 260 ~ 280g/l. In actual production, it can be selected according to the quality of semi-products and the quality requirements of finished products.

2. Tension

1) The influence of tension on fabric luster

When cotton fabric is treated with concentrated alkali, only with appropriate tension can it show good luster. It can be seen from the influence of tension on the performance of cotton yarn after mercerization that the tension is large and the luster is good.

2) Influence of tension on mechanical properties and adsorption properties of fabrics

That is, under weak conditions, the strength of cotton yarn has been improved, if the appropriate tension is applied, its strength can be further improved, but the luster increase is not much, and the fracture elongation and adsorption properties are reduced.

3) Influence of tension on shrinkage rate of fabric

The warp and weft tension plays an important role in the shrinkage of fabric during mercerizing.

In actual production, the warp and weft shrinkage rate of fabrics of various specifications is not balanced. For fabrics with high warp density such as khaki and Fuqiao, the warp shrinkage rate is much higher than the zonal shrinkage rate, so the warp tension is given priority. A thin fabric such as plain cloth is the opposite.

3. Temperature

The effect of caustic soda and cellulose fibers is exothermic reaction, so increasing the temperature of lye can weaken the swelling of fibers and thus reduce the mercerizing effect, which is reflected in the decrease of shrinkage rate and barium value. Therefore, low temperature is better for mercerizing lye. However, considering the economic benefits and the increase of the viscosity of lye at too low temperature in actual production, it is difficult for the reducing liquid to penetrate into yarns and fabrics, and it is difficult to expand. Therefore, it is usually used to pass cold water into the sandwich of rolling groove to cool the lye.

4. Time

Mercerization is to make caustic soda quickly and evenly and fully infiltrate into the cotton yarn or fabric and fiber action, so a certain time must be guaranteed.

The cotton yarn was mercerized with 280g/lNaOH under no tension. It was found that the yarn shrinkage rate and dye absorption rate reached the maximum within 20 seconds, and the mercerizing effect was not significant with the extension of time. In addition, the time is related to alkali concentration and temperature, low concentration, should be appropriate to extend the action time; Generally, 50 ~ 60s is used.

5. Remove alkali

Alkali removal has a great impact on the shaping effect of mercerizing. If there is more than 5% alkali on the fabric after relaxing the tension, the fabric will still shrink, which affects the gloss and zonal shrinkage. Dealkalization is carried out in two steps:

(1) In the case of expanding, the hot dilute alkali is washed on the fabric with a flushing device;

② After relaxing the zonal tension, enter the alkali removal box and wash and steam with light alkali.

Alkali contraction:

The process of treating cotton knitwear in a relaxed state with a concentrated caustic soda solution. The purpose of alkali shrinkage and mercerization is different, its main purpose is not to obtain luster, but to increase the density and elasticity of the knitted fabric.

1. Easy problems of alkali contraction

There are density differences, fine wrinkles, holes, stains and so on.

1. Difference in density of grey cloth after alkali shrinkage

cause

① The longitudinal (straight) density difference of blank cloth is too large.

(2) There is no uniform consistency between the concentration of alkali solution and the alkalinity and temperature of hot washing tank during alkali shrinkage.

preventive measure

① The blank cloth with different longitudinal density should be processed in batches.

② Operate according to the technological requirements.

2. Fine wrinkles

Reduce roll pressure; For high-grade products with great dyeing difficulty, the technological process of boiling, drying and alkali shrinkage can be adopted.

3. Holes are broken

The linear speed of the roll is different, and the grey cloth is worn out by the roll. Adjust the linear speed between the front and rear rollers. The roll may be embedded with hard materials such as knitting needles (usually causing regular holes); Before driving, the rubber roller should be checked, and the car guard must remove the broken needle from the grey cloth.

4. Stain

The blank cloth is stained before alkali shrinkage; Do a good job before the shift of the ground, pile up cloth car cleaning work. Cloth feeding turntable, milk roller, cloth storage box and other cleaning work is not good; The equipment must be cleaned before driving

2. Factors affecting alkali shrinkage

In the process of alkali shrinkage, some technological conditions, such as lye concentration, lye temperature, alkali shrinkage time, alkali removal effect and so on have a great impact on the effect of alkali shrinkage, therefore, alkali shrinkage must be strictly controlled in order to obtain the ideal effect.

1. Concentration of lye

The swelling effect of cotton fiber in caustic soda is closely related to the concentration of lye. Only when the concentration of lye reaches a certain value, the fiber will produce significant swelling effect, so the concentration of lye is the main factor affecting the alkali shrinkage. The test shows that when the concentration of lye reaches about 245g/L, the shrinkage rate of the fabric reaches the maximum. In actual production, maximum shrinkage is not required, so the lye concentration is usually controlled within the range of 140~200g/L, depending on the fabric density.

2. Lye temperature

The temperature of the lye also affects the effect of the shrinkage. The reaction of the base to the cellulose is an exothermic reaction, so raising the temperature is not good for the base shrinkage. However, if the temperature of the lye is too low, it will affect the penetration of the lye into the fiber due to its high viscosity. In the production, considering the permeability and treatment effect of two factors, can not use too low temperature, lye temperature should be 15~20℃. As the reaction between alkali and cellulose is an exothermic reaction, the temperature of lye will rise with the progress of the process. In order not to increase the temperature of lye, a lye tank with interlayer can be used to pass cold water through the interlayer to achieve the purpose of cooling down.

3, alkali shrinkage time

The effect of alkali shrinkage depends not only on the concentration and temperature of lye, but also on the action time of caustic soda and fabric, that is, alkali shrinkage time. Alkali shrinkage time is the total time from dipping lye to washing the fabric. In terms of the reaction between the alkali and the cellulose, the time required is very short, but it takes time for the lye to penetrate into the fiber and for the fabric to fully shrink. Therefore, when alkali shrinkage, if the fabric is washed immediately after alkali immersion, the fabric will not have time to shrink, which will affect the effect of alkali shrinkage. After dipping the lye, it is necessary to put it in pile to prolong the time of alkali shrinkage and improve the effect of alkali shrinkage. Due to the specific situation of the alkali shrinkage equipment used by each plant, the length of the storage time is also different, generally between 5 and 20 minutes. In addition, if the concentration of lye is increased, the alkali shrinkage time can be shortened accordingly.

Source: Yibu net