Several Bleaching Methods for Notebooks and Cotton Fabrics Introduction - Did You Know Several?

Cotton bleaching and dyeing plants widely use oxidizing bleaches such as hypochlorite and hydrogen peroxide. For the bleaching of cotton and cotton fabrics, peracid compounds such as sodium perborate, peracetic acid, and sodium percarbonate are also occasionally used, and sodium chlorite is used for the bleaching of synthetic fibers and their blended fabrics. Hypochlorite bleaching is generally referred to as chlorine bleaching, hydrogen peroxide bleaching as oxygen bleaching, and sodium chlorite bleaching as subhalf bleaching.

(1) Hypochlorite bleaching

1 commonly used hypochlorite bleaching powder and sodium hypochlorite two.

The bleaching powder is formed by the action of chlorine gas and slaked lime. If the chlorine gas is used in the milk of lime, it can be obtained that the bleaching powder containing the active ingredient is proficient in the chlorine in the caustic soda solution and the 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 manufacture sodium hypochlorite by themselves, or purchase sodium hypochlorite for bleaching from nearby chemical plants, and bleaching powder is used by small-scale printing and dyeing plants in small cities or rural areas where it is not easy to obtain sodium hypochlorite. The bleaching powder is more expensive and less used.

Sodium hypochlorite bleaching process and equipment is relatively simple, mostly used for bleaching cotton fabrics and textile blends, and sometimes used for the bleaching of polyester-cotton blended fabrics. However, it cannot be used for the bleaching of protein fibers such as silk and wool, because sodium hypochlorite has a destructive effect on protein fibers and makes fibers yellow.

Since the molecular structure of natural pigments in fibers has not been sufficiently studied, it is still not very clear. From the basic knowledge of known pigments, the structure of natural pigments is destroyed in the bleaching process and the purpose of eliminating pigments is achieved.

In the bleaching process, in addition to destruction of natural pigments, the cotton fibers themselves may also be damaged. Therefore, it is necessary to control the bleaching process conditions so that both the appearance quality and the intrinsic quality are qualified.

2 Hypochlorite bleaching methods mainly include leaching and continuous rolling.

Dripping is to uniformly pile the fabrics in the showering tank, and the liquid is continuously sprayed on the fabrics with a pump, circulating at room temperature for 1 to 1.5 hours, followed by washing, acid rinsing, and water washing. Drifting is a non-continuous production that is rarely used at present.

The continuous rolling and padding is padding in a rope continuous bleaching and bleaching combined machine. After being piled in a stacking tank, it is washed with water, rolled and piled with acid, washed in a stacking pond, and waits for open width, water rolling, and drying. dry.

The continuous flow of cotton sodium hypochlorite rope rolling process is as follows:

Rolling liquid → stacking (→ rolling liquid → stacking) → washing → rolling acid → stacking → washing.

The amount of sodium hypochlorite in bleach solution is calculated as available chlorine. The cloth scouring in the cooking pot is generally padding with 1.5~2g/l of available chlorine, about 1h at room temperature and steaming scouring fabric containing bleaching solution containing available chlorine 2~ 3g/l, stacking about 1h after rolling liquid. Low-grade cotton fabrics have higher impurities, and effective chlorine should be increased by 0.5g/l during padding. Acid pickling agent with sulfuric acid, rope fabric sulfuric acid concentration of 1 ~ 3g / l, flat fabric 2 ~ 3g / l, rolling acid at 30 ~ 40 °C stacked 10 ~ 15min. Small factories can also use artificial piles after rolling liquids, piled on washed and pebbled floors, and pay attention to labor protection when stacking.

3 Factors that affect sodium hypochlorite bleaching:

(a) Effect of pH value: When the ph value of the bleaching liquid is 7, the bleaching effect is the worst. When the fiber damage is severe, the PH value is 2-4 and 9-11, the whiteness is better, and the fiber damage is also smaller. However, in the acidic solution, sodium hypochlorite releases chlorine gas, which severely pollutes the air in the workshop, affects the health of workers, and also corrodes equipment. The actual ph value in production is between 9.5 and 10.5.

(b) Effect of temperature : The temperature is high, and the bleaching speed is increased, but the temperature exceeds a certain limit, and at the same time it also accelerates the oxidative failure of cellulose, so it is generally controlled at 20 to 30°C. When the summer temperature exceeds 35°C, cooling measures should be taken or other process parameters such as concentration and time should be adjusted to protect the fiber.

(c) Effect of concentration: It depends on the fabric structure and scouring conditions. The concentration of bleaching liquor is calculated as available chlorine, because when the hypochlorite is produced, the resulting product is a mixture. For example, sodium hypochlorite is mixed with sodium chloride, and chlorine in sodium chloride is not bleached. The available chlorine content in hypochlorite decreases with the prolonged storage time. Therefore, when preparing the bleaching solution, the hypochlorite analysis should be performed to determine the available chlorine content so that the available chlorine content in the bleaching solution can be accurate to control the production process. After the amount of available chlorine in the bleach reaches a certain value, the fabric whiteness no longer increases. The effective chlorine content of the bleaching liquid is too high, which will affect the fabric strength. Printing and dyeing factories generally adopt methods to reduce the chlorine concentration of bleaching liquor and extend the bleaching time, so as to avoid excessive loss of fiber strength.

(d) Dechlorination: Pickling after chlorine bleaching does not completely wash out the decomposed chlorine. A small amount of chlorine is still adsorbed on the fabric. Residual chlorine adsorbed during storage of the fabric will cause the fabric strength decreased, yellowing, will also affect the dye sensitive to chlorine. If necessary, chemical reagents and chlorine should be used to completely dechlorinate. The dechlorinating agent is preferably hydrogen peroxide. Hydrogen peroxide is also a bleaching agent in addition to reacting with chlorine, which can increase the bleaching effect. However, it is generally treated with reducing agents such as sodium bisulfite, sodium bicarbonate, and the like.

(2) Hydrogen peroxide bleaching

Hydrogen peroxide is also called hydrogen peroxide. The fabrics bleached with hydrogen peroxide are good in whiteness, pure in shade and yellow in storage, and are widely used in the bleaching of cotton fabrics. Oxygen bleaching is more adaptable 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 bleaching.

1 hydrogen peroxide bleaching process

Hydrogen peroxide bleaching method is more flexible, and it can be produced continuously or on a batch of equipment. It can be bleached by steaming method, and can also be used for cold-drifting and can also be used for flat width. At present, most of the printing and dyeing plants use a flat steam steam bleaching method. This method has a high degree of continuousness, automation, and high production efficiency. The process flow is simple and does not cause environmental pollution.

The hydrogen peroxide bleaching process is as follows:

Rolled hydrogen peroxide solution → steamed → washed.

Bleach containing hydrogen peroxide (100%) 2 ~ 5g / l, with caustic soda to adjust the pH value to 10.5 ~ 10.8, adding the appropriate amount of stabilizer and wetting agent, padding at room temperature padding, steaming 95 ~ 100 °C 45 ~ 60min, then wash out the cloth.

2 Factors affecting hydrogen peroxide bleaching:

(a) Effect of concentration: Similar to chlorine bleach, when the concentration of hydrogen peroxide in the bleaching liquid reaches 5 g/l, the bleaching requirement can be met, and the concentration increases again, the whiteness does not increase, but it causes the cotton fiber to become brittle. Thin fabrics can also properly reduce the hydrogen peroxide concentration in the rinse.

(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 to 100°C. Whiteness is also best. The cold-drift method should increase the hydrogen peroxide concentration and extend the bleaching time.

(c) Effect of pH: Hydrogen peroxide is relatively stable in acidic baths. The concentration of concentrated hydrogen peroxide solution for industrial use is approximately 30% to 35%. 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 pH value of the solution. 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 best level. Most of the actual production also adjusts the ph value of the liquid to about 10.

(d) Effects of metal ions in bleaching and stabilizers: Iron salts, copper salts and iron filings, copper shavings, and dust in water all have a catalytic decomposition effect on hydrogen peroxide, and hydrogen peroxide is decomposed into water and oxygen. Loss of bleaching. Oxygen permeates into the interior of the fabric, causing severe degradation of the cellulosic fibers during bleaching at high temperatures and alkaline conditions, often creating holes in the fabric. In order to prevent the above-mentioned rickets, an appropriate amount of stabilizer may be added to the bleaching liquid to reduce the decomposition rate of hydrogen peroxide. Stabilizers are used earlier in water glass, and the stable mechanism of water glass is not yet very clear. It is speculated that calcium silicate or magnesium silicate colloids can adsorb metal ions that catalyze the decomposition of hydrogen peroxide. Water glass is cheap and easy to obtain, but it has good stabilization effect. However, long-term use of water glass can easily form silicon scale that is difficult to remove at guide rollers and other places, affecting fabric quality. At present, the use of non-silicate stabilizers is studied at home and abroad, and most of them belong to organic phosphonates. The effect is also good. No scale is produced, but the price is higher than that of silicates, so water glass is still used in production. Phosphorus compounds are sometimes used in combination with sodium silicate to achieve better results.

3 Hydrogen peroxide other bleaching methods

In addition to the widespread use of steam bleaching, hydrogen peroxide bleaching methods are available in the following categories:

(a) Chlorine-oxygen double-bleaching method: Oxygen bleaching after chlorine bleaching and oxygen bleaching combined with dechlorination and bleaching. This method can reduce the concentration of hydrogen peroxide in the bleaching liquor. The process flow is:

Rolled sodium hypochlorite bleaching solution → stacking → washing → hydrogen peroxide bleaching → steaming → washing.

The sodium hypochlorite bleach contains 1 to 2 g/l of available chlorine, and the hydrogen peroxide bleach contains 1 to 3 g/l of hydrogen peroxide, and the process conditions are the same as those for bleaching.

(b) Cold piling method: In order to adapt to the requirements of multiple varieties, small batches, and multiple changes, especially small-scale printing and dyeing plants, cold-drifting can be used in the absence of oxygen-bleaching equipment. The concentration of hydrogen peroxide in the bleaching liquor of this method is relatively high, and persulfate is added. After the fabric is rolled and rinsed, the roll 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 relatively flexible. Drifting solution containing hydrogen peroxide (100%) 10 ~ 12g / l, water glass 25 ~ 3og / l, persulfate 7 ~ 1og / l, with caustic soda to adjust the bleaching liquid ph value to 10.5 ~ 10.8, at room temperature padding, Stack 6 to 16 hours, fully washed.

Concentrated hydrogen peroxide solution has a severe burn effect on the skin. Care should be taken when using it.

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