After scouring, most of the natural and man-made impurities on the fabric have been removed, and the capillary effect has been significantly improved, which has been able to meet the processing requirements of some varieties. However, for bleached fabrics and brightly colored light colored fabrics and colored fabrics, it is also necessary to increase the whiteness, so it is necessary to further remove the pigment on the fabric to make the fabric more white. Then, although the fabric has been scoured, especially at atmospheric pressure steaming, some impurities such as cottonseed hulls have not been completely removed, and these impurities can be completely removed by the action of the bleaching agent.
Oxidative bleaching agents such as hypochlorite and hydrogen peroxide are widely used in cotton printing and dyeing factories. For cotton and cotton fabric bleaching, peracid compounds such as sodium perborate, peracetic acid, sodium percarbonate, etc. are also occasionally used, sodium chlorite is mostly used for the bleaching of synthetic fibers and their blended fabrics. Hypochlorite bleaching is generally referred to as chlorine bleaching, hydrogen peroxide bleaching is referred to as oxygen bleaching, and sodium chlorite bleaching is referred to as sub-bleaching.
(1) Hypochlorite bleaching
1 Common hypochlorites are bleaching powder and sodium hypochlorite.
The bleaching powder is formed by the action of chlorine gas and slaked lime. For example, if chlorine gas is used in the lime milk, the bleaching powder containing twice the active ingredient can be prepared to pass chlorine gas in the caustic soda solution, and sodium hypochlorite can be obtained. The active ingredient in bleaching powder and bleaching powder is calcium hypochlorite, and the overall effect is not as good as sodium hypochlorite. At present, large-scale cotton printing and dyeing factories often produce sodium hypochlorite by themselves, or purchase sodium hypochlorite for bleaching in nearby chemical plants. Bleaching powder is used in small printing and dyeing factories in small cities or rural areas where sodium hypochlorite is not easy to obtain. Bleaching powder is more expensive and less used.
The sodium hypochlorite bleaching process and equipment are relatively simple, and are mostly used for bleaching cotton fabrics and cotton blended fabrics, and sometimes for bleaching of polyester-cotton blended fabrics. However, it cannot be used for bleaching of protein fibers such as silk and wool, because sodium hypochlorite has a destructive effect on protein fibers and yellowes the fibers.
Due to insufficient research on the molecular structure of natural pigments in fibers, it is still not fully understood. From the basic knowledge of known pigments, the natural pigment structure is destroyed during the bleaching process, achieving the purpose of eliminating pigmentation.
In the bleaching process, in addition to the destruction of natural pigments, the cotton fibers themselves may be damaged. Therefore, the bleaching process conditions must be controlled to achieve acceptable quality and inherent quality.
The two-time sodium chlorate bleaching method mainly includes two types of showering and continuous rolling.
The drenching is to evenly stack the fabric in the drenching box, and the circulating liquid is continuously sprayed on the fabric by a pump, and is circulated at room temperature for 1 to 1.5 hours, and then washed with water, leached with acid, and washed with water. Dripping is a discontinuous production that is rarely used today. Continuous rolling and bleaching is a padding and bleaching liquid on a rope continuous bleaching and bleaching machine. After being stacked in a stacking box, it is washed with water, rolled with acid, and washed in a piled pool. Waiting for opening, rolling and drying. dry.
The process of cotton fabric sodium hypochlorite rope continuous rolling and bleaching is as follows:
Bleaching liquid â†’ stacking (â†’ rolling liquid â†’ stacking) â†’ washing â†’ rolling acid â†’ stacking â†’ washing.
The amount of sodium hypochlorite in the bleaching solution is calculated by the effective chlorine. The fabric scoured in the cooking pot is usually padded with 1.5~2g/l of effective chlorine, and the flattened layer is placed at room temperature for about 1h. The bleaching solution of the steamed scouring fabric contains effective chlorine. 3g / l, after the bleaching liquid is stacked for about 1h. The low-grade cotton fabric has a high impurity content, and the effective chlorine should be increased by 0.5g/l during padding. The pickling agent uses sulfuric acid, the rope fabric has a sulfuric acid concentration of 1 to 3 g/l, the flat fabric is 2 to 3 g/l, and after the acid is rolled, it is stacked at 30 to 40 Â° C for 10 to 15 minutes. Small factories can also be piled up on the cobblestone floor after the bleaching liquid is piled up. Pay attention to labor protection when stacking.
3 factors affecting the bleaching of sodium hypochlorite:
(a) Effect of pH: When the pH value of the bleaching liquid is 7, the bleaching effect is the worst. When the fiber damage is severely ph value of 2 to 4 and 9 to 11, the whiteness is good and the fiber damage is small. However, in the acidic liquid, sodium hypochlorite liberates chlorine gas, which seriously pollutes the air in the workshop, affects the health of workers, and also corrodes equipment. The actual pH in production is between 9.5 and 10.5.
(b) Effect of temperature: The temperature is high, the bleaching speed is increased, but the temperature exceeds a certain limit, and the oxidation brittleness of cellulose is also accelerated, so it is generally controlled at 20 to 30 Â°C. When the summer temperature exceeds 35 Â°C, cooling measures should be taken or other process parameters such as concentration and time should be adjusted to protect the fibers.
(c) Effect of concentration: depending on fabric structure and scouring conditions. The concentration of the bleaching solution is calculated as available chlorine, because when the hypochlorite is prepared, the obtained product is a mixture, such as sodium hypochlorite mixed with sodium chloride, and the chlorine in the sodium chloride has no bleaching effect. The available chlorine content in hypochlorite decreases with the storage time. Therefore, in the preparation of bleach, the hypochlorite should be analyzed to determine the available chlorine content, so that the effective chlorine content in the bleach is accurate, so as to control the production process. After the effective chlorine content in the bleaching solution reaches a certain value, the whiteness of the fabric does not increase. The effective chlorine content of the bleaching liquid is too high, which in turn affects the strength of the fabric. Printing and dyeing factories generally adopt a method of reducing the effective chlorine concentration of the bleaching liquid and prolonging the bleaching time to avoid excessive loss of fiber strength.
(d) Dechlorination problem: The pickling of the fabric after chlorine bleaching does not completely remove the decomposed chlorine gas, and a small amount of chlorine gas is still adsorbed on the fabric. Fabrics with residual chlorine adsorbed will cause a strong drop in fabric and yellowing during storage and will also affect the dyeing of chlorine sensitive dyes. If necessary, chemical agents should be used to react with chlorine to completely remove chlorine. The dechlorination agent is preferably hydrogen peroxide. In addition to reacting with chlorine gas, hydrogen peroxide itself is also a bleaching agent, which can increase the bleaching effect. However, it is generally treated with a reducing agent such as sodium hydrogen sulfite or baking soda.
(2) Hydrogen peroxide bleaching
Hydrogen peroxide is also called hydrogen peroxide. The fabric bleached with hydrogen peroxide has good whiteness, pure color and is not easy to yellow when stored, and is widely used for bleaching cotton fabrics. Oxygen bleaching has greater adaptability than chlorine bleaching, but hydrogen peroxide is more expensive than sodium hypochlorite, and oxygen bleaching requires stainless steel equipment, which consumes more energy and costs more than chlorine bleach.
1 hydrogen peroxide bleaching process
The hydrogen peroxide bleaching method is flexible, and can be continuously produced or produced on batch equipment. It can be bleached by steaming, cold rope can be used as rope, or flat. At present, the printing and dyeing plant uses more flat steaming bleaching method. This method has higher degree of continuity, automation and production efficiency, simple process flow and no environmental pollution.
The process of hydrogen peroxide bleaching is as follows:
Rolled through hydrogen peroxide bleaching â†’ steaming â†’ water washing.
The bleaching solution contains hydrogen peroxide (100%) 2ï½ž5g/l, adjust the pH value to 10.5ï½ž10.8 with caustic soda, add proper amount of stabilizer and wetting agent, dip the bleaching liquid at room temperature, steaming at 95ï½ž100Â°C 45ï½ž 60 min, then wash the cloth out.
2 Factors affecting hydrogen peroxide bleaching:
(a) Effect of concentration: Similar to chlorine bleaching, when the concentration of hydrogen peroxide in the bleaching liquid reaches 5g/l, the bleaching requirement can be achieved, the concentration is increased, and the whiteness is not increased, which in turn causes the cotton fiber to be brittle. The thin fabric can also appropriately reduce the concentration of hydrogen peroxide in the bleaching liquid.
(b) Effect of temperature: The decomposition rate of hydrogen peroxide increases with increasing temperature, so the bleaching time can be shortened by increasing the temperature. Generally, the decomposition rate of hydrogen peroxide can reach 90% at 90-100 Â°C. Whiteness is also the best. The use of cold bleaching should increase the concentration of hydrogen peroxide and extend the bleaching time.
(c) Effect of pH: Hydrogen peroxide is relatively stable in an acidic bath, and the concentration of industrial hydrogen peroxide concentrated solution is about 30% to 35%, and a small amount of sulfuric acid is often added to maintain stability. The decomposition rate of hydrogen peroxide in the alkaline bath increases with the increase of the pH value of the solution, and the ph value has a bleaching effect between 3 and 13.5, but when the pH is 9-10, the whiteness of the fabric can reach the optimal level. In actual production, most of the bleaching liquid pH is adjusted to about 10.
(d) The influence of metal ions during bleaching and the role of stabilizers: iron salts, copper salts and iron filings, copper scraps and dust in water, which have catalytic decomposition of hydrogen peroxide, and decompose hydrogen peroxide into water and oxygen. Thereby losing the bleaching effect. Oxygen permeates into the interior of the fabric, which can cause severe degradation of the cellulosic fibers under high temperature alkaline conditions during bleaching, often creating holes in the fabric. In order to prevent the above mentioned diseases, an appropriate amount of stabilizer may be added to the bleaching liquid to reduce the decomposition rate of hydrogen peroxide. The use of water glass in stabilizers is relatively early, and the mechanism of stabilization of water glass is not fully understood. It is speculated that calcium silicate or magnesium silicate colloid adsorbs metal ions that catalyze the decomposition of hydrogen peroxide. Water glass is cheap and easy to obtain, and has a good stabilizing effect. However, for long-term use, it is easy to form silicon scale which is difficult to remove at the guide rolls and the like, which affects the quality of the fabric. At present, non-silicate stabilizers are being studied at home and abroad, most of which belong to organic phosphonates. The effect is also good, no scale is formed, but the price is higher than silicate, so water glass is still used in production. Sometimes a phosphorus-containing compound is used in combination with sodium silicate, and a good effect is also obtained.
3 other bleaching methods of hydrogen peroxide
In addition to the wide application of steam bleaching, there are several methods for hydrogen peroxide bleaching:
(a) Chlorine-oxygen double bleaching method: first chlorine bleaching, oxygen bleaching, oxygen bleaching combined with dechlorination and bleaching, this method can reduce the concentration of hydrogen peroxide in the bleaching liquid, the process flow is:
Rolling sodium hypochlorite bleach â†’ stacking â†’ washing â†’ rolling hydrogen peroxide bleaching â†’ steaming â†’ washing.
The sodium hypochlorite bleaching solution contains 1 to 2 g/l of effective chlorine, and the hydrogen peroxide bleaching solution contains hydrogen peroxide of 1 to 3 g/l. The process conditions are the same as those of the respective bleaching.
(b) Cold-rolled pile method: In order to meet the requirements of multi-variety, small batch, and many changes, especially in small printing and dyeing factories, cold bleaching method can be used in the absence of oxygen bleaching equipment. In this method, the concentration of hydrogen peroxide in the bleaching liquid is high, and persulfate is added. After the fabric is rinsed, the film is immediately wrapped with a plastic film to prevent evaporation and drying, and then stacked at room temperature. Although this method has a long time and low production efficiency, it is more flexible. The bleaching liquid contains hydrogen peroxide (100%) 10ï½ž12g/l, water glass 25ï½ž3og/l, persulfate 7ï½ž1og/l, and the pH value of the bleaching liquid is adjusted to 10.5ï½ž10.8 with caustic soda, and padded at room temperature. Stack for 6 ~ 16h, fully washed.
The concentrated hydrogen peroxide solution has a severe burn effect on the skin, and labor protection should be paid attention to when using it.
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