Mercerizing is a process in which cotton fabric is treated with concentrated caustic soda solution under a certain tension condition and the required size is maintained, so that the fabric can obtain a silk like luster.
1. Mercerizing mechanism
When mercerized cotton fiber is treated with concentrated caustic soda, because of the small volume of sodium ion, it can enter into the amorphous and crystalline areas of the fiber at the same time, forming a hydration layer around the water molecules around the sodium ion. When the sodium ion enters into the fiber and combines with the fiber, the water molecules are also brought in at the same time, resulting in the violent swelling of the fiber, forcing the diameter of the cotton fiber to increase, the length to shorten, and the fiber's The longitudinal crimp almost disappears, the cross section of the fiber changes from waist shape to nearly round shape, and the whole fiber changes from flat strip shape to cylinder shape. At this time, a certain tension is applied. After stretching, the uniformity of the fiber increases, the surface smoothness increases, and the diffuse reflection of the fiber to the light before mercerizing changes into more directional reflection, which increases the intensity of the reflected light, so that the fabric shows silk one After mercerizing, the crystallinity of cotton fiber is reduced from about 70% before mercerizing to 50-60%. Due to the decrease of crystallinity of fiber, good dyeing performance is produced, meanwhile, the dyeing performance of dead cotton is improved, and the quality of dyeing products is improved. Improve the dimensional stability of the fabric, eliminate the internal stress and wrinkle.
The main mercerizing agents are: caustic soda, alkali resistant penetrant, acetic acid, etc.
Function of alkali resistant penetrant: accelerate the wetting and penetration of mercerizing alkali solution, improve mercerizing effect, and improve dyeing depth, brightness, fullness and luster.
Types of mercerizing machine: straight roller mercerizing machine, clip mercerizing machine, straight roller clip mercerizing machine, cylinder mercerizing machine for knitted fabrics, split width mercerizing machine for knitted fabrics, short process straight roller reeling mercerizing machine, etc
2. Mercerizing process
1. Mercerizing of woven fabrics: soaking in concentrated alkali → ventilating → soaking in concentrated alkali → ventilating → (preheating washing) → alkali removal by clip expanding and punching → alkali removal by straight roll washing → steaming washing → neutralizing → washing with water → drying
2. Mercerizing of knitted fabrics: open width feeding → photoelectric weft adjusting device → straight roll groove → two roll heavy rolling mill → five roll big cylinder alkali mill → two roll heavy rolling mill → clip expanding and absorbing alkali → two roll heavy rolling mill → high efficiency drum washing box → two roll light rolling mill → high efficiency drum washing box → two roll light rolling mill → high efficiency drum washing box → two roll light rolling mill → high efficiency drum washing box Drum washing box → two roll light rolling mill → high efficiency drum washing box → two roll heavy rolling mill → flat cloth falling.
3. Mercerizing process conditions
The main technological parameters of mercerizing include: caustic soda concentration, rolling surplus rate, alkali action time, warp tension, weft tension, times of alkali leaching and absorption, alkali removal temperature, width of cloth falling, pH value of cloth falling, etc.
4. Precautions for mercerizing
1. Before mercerizing, the degree of wetness and dryness of the fabric must be uniform. When mercerizing wet cloth, the rolling allowance of cotton fabric and hemp fabric is 60% and that of polyester cotton fabric is 50%, so as to ensure that the liquid in the left, middle and right parts of the fabric is uniform.
2. During mercerizing of dry cloth, the alkali liquor shall be fully cooled by the ventilating frame before the fabric is soaked and rolled to keep the temperature of concentrated alkali liquor stable.
When mercerizing, the fabric should bear certain tension in both warp and weft directions. The greater the tension, the better the luster of the fabric and the higher the dimension temperature.
4. During mercerizing, the fabric must be kept flat to prevent curling, wrinkling, etc. once tested, mercerizing repair must be repeated.
The mercerizing process of knitted fabric generally adopts grey fabric mercerizing or post singeing mercerizing. Due to the poor water absorption of the fabric, penetrant must be used to strengthen the permeability of concentrated caustic soda solution and improve the mercerizing effect.
6. The pH value of mercerized cloth is often detected with extensive indicator or test paper to keep the fabric neutral as much as possible, and neutralized with acid during washing if necessary.
7. Concentrated caustic soda (concentrated acid) has severe corrosive and burning effects on human skin. Personnel contacting concentrated caustic soda (concentrated acid) shall be equipped with necessary protective equipment. In case of splashing of caustic soda (or acid) on skin or eyes, they shall wash with plenty of water before going to hospital.
5. Evaluation of mercerizing effect
1) Luster: it is one of the main indexes to measure the appearance effect of mercerized fabric, but there is no uniform and ideal test method. At present, visual evaluation is widely used.
2) Microsection was used to observe the morphological changes.
3) Adsorption properties
A barium value method: it is a test method to measure the absorptivity of cotton fiber. It is often used to test the increase of absorptivity of fabric after mercerizing. Take a piece of mercerized and non mercerized cloth samples, wash them with hot water and cold water, dry them, dry them in 100-105 for 1h, take out the constant weight, weigh 2 g each accurately, cut them into small pieces, put them into 100 ml conical flask with cap, suck 0.25mol/l barium hydroxide 30ml into the bottle, immediately cover the bottle, shake it from time to time. After 2 hours, take 10ml, take phenolphthalein as indicator, titrate with 0.1mol/l hydrochloric acid standard solution, and calculate barium value.
B. iodine staining and dyeing method: the samples with different barium values (100150) are treated with iodine solution of a certain concentration or direct blue 2b to make a color card, and then the contamination and dyeing depth of the iodine solution of the unknown samples are compared with the known color card to quantitatively evaluate the barium mercerization value.
4) Dimensional stability of fabric: the method of shrinkage is used to test the change of warp and weft length of fabric before and after mercerizing. The smaller the shrinkage is, the better the mercerizing effect is.
6. Common quality problems of mercerizing and Solutions
1) Left middle right color difference
A. the difference of wool effect in the left, middle and right of pretreatment results in the difference of alkali content in each part of the fabric.
The alkali concentration of each part of b-rolling alkali tank is inconsistent.
Wear of C rolling mill and inflexible air cylinder lead to inconsistent liquid content in left, middle and right parts of the fabric
The difference between the left, the middle and the right of the drenching alkali caused the difference of the alkali in each part of the fabric.
E insufficient water washing.
2) The warp and weft shrinkage of the fabric is too large.
A alkali concentration is too high. (large weft shrinkage of thin fabric)
B. insufficient amplification (too much tension in the clip), resulting in the difficulty of weft amplification.
The warp tension of the tension roller in the middle of the two alkali rolling grooves is too small, the warp mercerizing effect of the fabric is poor, and the warp shrinkage of the fabric is large.
D. the effect of drenching alkali is poor, and the alkali content is too large when the fabric comes out of the clip. The shrinkage of the fabric is large when washed with high alkali and high temperature.
E. the tension of the equipment is too large, and the weft shrinkage of the fabric is large.
3) Broken edge, hole
A. the clip is worn or inflexible.
B. the clip track is out of alignment and the tension on the left and right of the fabric is inconsistent.
The door width of C clip outlet is too wide,
D. the amplitude is too large or too fast.
E. the edge detection or expansion device is not flexible.
4) auxiliaries
In the process of scouring and bleaching, the residual surfactant on the fabric is damaged in the presence of alkali, and the lipophilic group collects and contaminates the fabric, resulting in light spots on the part of the fabric, which are easy to form color spots after the soft treatment and direct repair.
