Printing And Dyeing Technology Promotion: Bleaching Of Cotton Fabrics

< p > 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 already met the processing requirements of some varieties.

But for bleached fabrics and brightly coloured light coloured fabrics and coloured fabrics, it is also necessary to improve whiteness. Therefore, it is necessary to remove pigment from the fabric further and make the fabric more white.

Then, though the fabric has been scoured, especially the atmospheric steam cooking, some impurities such as cottonseed hull can not be completely removed, and these impurities can be completely removed through the action of bleaching agents.

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< p > cotton bleaching and dyeing plants are widely used as oxidant bleaching agents such as hypochlorite and hydrogen peroxide.

Bleaching of cotton and cotton fabrics is also occasionally applied to peracids such as sodium perborate, peracetic acid, sodium percarbonate and so on. Sodium chlorite is used for bleaching synthetic fibers and their blended fabrics.

Hypochlorite bleaching is usually referred to as chlorine bleaching. Hydrogen peroxide bleaching is referred to as oxygen bleaching. Sodium chlorite bleaching is referred to as sub bleaching.

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< p > strong > (1) < /strong > a href= "http://www.91se91.com/news/list.aspx Classid=101112108" > strong > hypochlorite bleaching < /strong > /a > /p >

< p > < strong > commonly used hypochlorite has two kinds of bleaching powder and sodium hypochlorite.

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< p > bleaching powder is made of chlorine and hydrated lime. If chlorine is used in lime milk, it can produce twice as much bleaching powder as active ingredient.

The effective composition of bleaching powder and bleaching powder is calcium hypochlorite, and the total effect is not as good as sodium hypochlorite.

At present, large scale cotton printing and dyeing factories often make sodium hypochlorite by themselves, or buy sodium hypochlorite for use in nearby chemical plants. Bleaching powder is used for small printing and dyeing factories in small cities or rural areas where sodium hypochlorite is not easy to get.

Bleaching powder is more expensive and less used.

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< p > sodium hypochlorite bleaching process and equipment are relatively simple, and are often used for bleaching cotton fabrics and cotton blended fabrics, and sometimes for bleaching of polyester cotton blended fabrics.

But it can not be used for bleaching of protein fiber such as silk and wool. Sodium hypochlorite can destroy protein fiber and yellowing fiber.

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< p > because the molecular structure of natural pigments in fiber is not enough, it is not clear yet.

Judging from the basic knowledge of known pigments, the structure of natural pigments is destroyed during bleaching, so as to achieve the purpose of eliminating pigment.

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< p > in the bleaching process, besides the destruction of natural pigments, the cotton fibers themselves may also be damaged. Therefore, the technological conditions of bleaching must be well controlled, so that both the appearance quality and the intrinsic quality are qualified.

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< p > < strong > 2. Sodium hypochlorite bleaching mainly consists of two types: bleaching and bleaching and continuous rolling.

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< p > drifting is to uniformly pile the fabric in the shower box, and continuously circulate the rinsing liquid circulation on the fabric with the pump, circulate 1 to 1.5h at room temperature, then wash, rinse and wash.

Drifting and bleaching are discontinuous production.

Continuous rolling and bleaching is carried out on a rope type continuous scouring and bleaching machine. After being stacked in the stacking box, it is washed by water, rolled by acid piles, washed and stacked in a stacking pool, waiting for opening, rolling and drying.

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< p > cotton fabric sodium hypochlorite rope continuous rolling process is as follows: < /p >

< p > rolling and bleaching liquid to stack, then to water washing, rolling acid, then stacking and washing.

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The amount of sodium hypochlorite in the P > bleach is calculated by effective chlorine. The fabric of the scouring pot is usually soaked with 1.5 ~ 2g/L bleaching liquid containing effective chlorine, 2 ~ 3g/L of effective chlorine in the bleach solution of the steam scouring fabric at normal temperature and 1H in the bleaching fluid.

The low grade cotton fabric is high in impurity, and the effective chlorine should be increased by 0.5g/L during the dip rolling.

Sulfuric acid is used in pickling agent, the sulphuric acid concentration of rope fabric is 1 ~ 3g/L, flat fabric is 2 to 3g/L, after rolling acid, it is stacked 10 to 15min at 30~40 C.

Small factories can also use artificial stacking cloth after rolling and bleaching liquid, pile up on the ground where the pebble is washed, and pay attention to labor protection when stacking cloth.

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< p > < strong > the factors affecting the bleaching of sodium hypochlorite: < /strong > /p >

< p > (A) pH value: when the pH value of the bleaching liquid is 7, the bleaching effect is the worst, and the fiber damage is serious. When the pH value is 2~4 and 9~11, the whiteness is better, and the fiber damage is also smaller.

However, in acid solution, the decomposition of sodium hypochlorite releases chlorine, which seriously pollute the air of workshops, affects workers' health, and also corrodes equipment.

The actual pH value is between 9.5 and 10.5 in production.

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< p > (B): the effect of temperature is high, and the rate of bleaching is accelerated. However, the temperature exceeds a certain limit and at the same time accelerates the oxidation brittleness of cellulose, so it is generally controlled at 20~30 C.

When the summer temperature exceeds 35 C, cooling measures should be taken or other technological parameters such as concentration and time should be adjusted to protect fibers.

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The effect of < p > (C) concentration depends on fabric structure and scouring condition.

The concentration of the bleaching solution is calculated with effective chlorine, because the products obtained are hypochlorite when mixed with sodium chloride, but chlorine in sodium chloride is not bleached.

The content of effective chlorine in hypochlorite decreases with the prolongation of storage time. Therefore, when preparing bleaching liquor, the effective chlorine content in the bleaching solution should be determined by hypochlorite analysis, so that the effective chlorine content in the bleach liquor can be accurately controlled in order to control the production process.

The whiteness of the fabric is no longer increased when the effective chlorine content in the bleach solution reaches a certain value.

Excessive chlorine content in the bleaching liquor affects the strength of the fabric.

Printing and dyeing factories generally adopt a way to reduce the concentration of effective chlorine and prolong the time of bleaching, so as to avoid excessive loss of fiber strength.

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< p > (D) the problem of dechlorination: acid washing after fabric bleaching can not completely remove chlorine from decomposition, but still a small amount of chlorine is adsorbed on the fabric.

The adsorption of residual chlorine, < a href= "http://www.91se91.com/news/list.aspx? Classid=101112102" > fabric > /a >, will result in fabric strength decline and yellowing during storage, and will also affect the dyeing of chlorine sensitive dyes.

If necessary, chemicals should be used to react with chlorine to remove chlorine completely.

Hydrogen peroxide is the best dechlorination agent. Hydrogen peroxide, in addition to chlorine, is also a bleach. It can increase bleaching effect.

But generally use reducing agents such as Sodium Bisulfite, baking soda and so on.

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< p > < strong > (2) hydrogen peroxide bleaching < /strong > < /p >

< p > hydrogen peroxide is also called hydrogen peroxide.

The fabric bleached with hydrogen peroxide has good whiteness, pure color and no yellowing during storage. It is widely used in cotton fabric bleaching.

Oxygen bleaching is more adaptable than chlorine bleaching, but hydrogen peroxide is more expensive than sodium hypochlorite, and oxygen bleaching needs stainless steel equipment, which has higher energy consumption and higher cost than chlorine bleaching.

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< p > < strong > hydrogen peroxide bleaching process < /strong > /p >

< p > hydrogen peroxide bleaching is more flexible, not only for continuous production, but also for intermittent equipment. Steam bleaching can also be used, or cold drift can be used for rope or flat width.

At present, the use of flat width steam bleaching is more widely used in printing and dyeing factories. This method has a higher degree of continuity, automation and production efficiency. The process is simple, and there is no environmental pollution.

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< p > hydrogen peroxide bleaching process is as follows: < /p >

< p > hydrogen peroxide bleaching liquid, steam evaporation and water washing.

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< p > bleach solution contains hydrogen peroxide (100%) 2 to 5g/L, adjust pH to 10.5 ~ 10.8 with caustic soda, add stabilizer and wetting agent, soak and bleach liquid at room temperature, steam 45 to 60min at 95~100 degrees, then wash out cloth.

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< p > < strong > the factors affecting hydrogen peroxide bleaching: < /strong > /p >

The effect of < p > (A) concentration is similar to that of chlorine bleaching. When the concentration of hydrogen peroxide in the bleaching liquor reaches 5g/L, the bleaching requirement has been achieved, and the whiteness is not increased with the increase of concentration.

Thinner fabrics can also reduce the concentration of hydrogen peroxide in the bleaching solution.

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< p > (B) the influence of temperature: the decomposition rate of hydrogen peroxide increases with the increase of temperature. Therefore, the temperature can be used to shorten the bleaching time. At 90~100, the decomposition rate of hydrogen peroxide can reach 90% and whiteness is the best.

The cold bleaching rule should increase the concentration of hydrogen peroxide and prolong the bleaching time.

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< p > (C) pH value: hydrogen peroxide is relatively stable in acid bath, industrial concentration of hydrogen peroxide solution is about 30% ~ 35%, and a small amount of sulphuric acid is added to maintain stability.

In alkaline bath, the decomposition rate of hydrogen peroxide increases with the increase of the pH value of the solution, and the pH value is between 3 and 13.5. However, when the pH value is 9~10, the whiteness of the fabric reaches the best level. In practice, the pH value of the bleaching solution is mostly adjusted to 10.

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< p > (D) the influence of metal ions and stabilizers in bleaching: the ferric salts, copper salts and iron chips, copper dust and dust in water can catalyze the decomposition of hydrogen peroxide and decompose hydrogen peroxide into water and oxygen, thereby losing the bleaching effect.

Oxygen penetrates into the fabric, which will seriously degrade cellulose fibers at high temperature and alkaline conditions during bleaching, and often produce holes in the fabric.

In order to prevent the above defects, appropriate stabilizers can be added to the bleaching liquid to reduce the decomposition rate of hydrogen peroxide.

In the stabilizer, water glass is used earlier, and the stabilization mechanism of sodium silicate is not very clear. It is presumed that calcium silicate or magnesium silicate colloid can adsorb metal ions that catalyze the decomposition of hydrogen peroxide.

Water glass is cheap and easy to obtain, and its stability is good. But for a long time, it is easy to form silicon scales which are difficult to remove at the guide rolls and affect the quality of fabrics.

At present, the use of non silicate stabilizer is mostly studied at home and abroad. Most of them belong to organic phosphonate, and the effect is also good. No fouling occurs, but the price is higher than < a href= "http://www.91se91.com/news/list.aspx Classid=101112107" > silicate < /a >, so water glass is still used in production.

Sometimes mixing phosphorus containing compounds with sodium silicate also achieves good results.

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< p > < strong > the other bleaching methods of hydrogen peroxide < /strong > < /p >

< p > except for the wide application of steam bleaching, there are several ways of hydrogen peroxide bleaching: < /p >

< p > (A) chlorine oxygen double bleaching method: after chlorine bleaching and oxygen bleaching, oxygen bleaching has both dechlorination and bleaching effects. This method can reduce the concentration of hydrogen peroxide in the bleaching liquor, and its technological process is: < /p >

< p > rolling sodium hypochlorite bleaching solution, stacking, washing, rolling, hydrogen peroxide bleaching, steaming and washing.

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< p > sodium hypochlorite bleaching liquor contains 1 to 2g/L of effective chlorine, and hydrogen peroxide bleaching liquid contains 1 to 3g/L of hydrogen peroxide.

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< p > (B) cold pad batch method: in order to meet the requirements of multiple varieties, small batch and multiple changes, especially in small printing and dyeing factories, cold bleaching can be used in the absence of oxygen bleaching equipment.

The concentration of H2O2 in the bleaching solution is high, and added to persulfate, then the fabric is rolled and bleached, then rolled immediately by plastic film to prevent evaporation and drying and then stacked at room temperature.

Although this method is long and inefficient, it is more flexible.

The bleaching liquor contains hydrogen peroxide (100%) 10 ~ 12g/L, sodium silicate 25 ~ 3Og/L, persulfate 7 to 1Og/L, and the pH value of the bleaching solution is adjusted to 10.5 to 10.8 by caustic soda, and is then rolled at room temperature, stacking 6 to 16h, and fully washed.

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< p > concentrated hydrogen peroxide solution has serious burns to the skin. Attention should be paid to labour protection when using it.

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