Pretreatment of printing and dyeing

Undyed and dyed fabrics are collectively called grey cloth or grey cloth, and only a small amount is supplied to the market. Most of the original fabrics need to be further processed into bleached fabrics, colored fabrics or printed fabrics in printing and dyeing factories for consumers to use. Grey cloth often contains a considerable amount of impurities, including cotton fiber associated organisms and impurities, sizing agent on warp yarn during weaving, oil agent on chemical fiber, oil stain attached during weaving, etc. If these impurities and dirt are not removed, it will not only affect the color and feel of the fabric, but also affect the moisture absorption of the fabric, making the color of the fabric uneven and not bright, and also affect the dyeing fastness.

Pretreatment is the preparation process of printing and dyeing, and its purpose is to remove all kinds of impurities from the fabric without causing little damage to the grey cloth, so that the treated grey cloth becomes a white, soft and good wetting semi-finished product. Different kinds of fabrics have different requirements for pretreatment, and the production conditions of factories in different regions are also different, so the fabric processing sequence (process) and technological conditions in the pretreatment workshop are often different.

The conventional pretreatment process is generally as follows: singeing → desizing → scouring → bleaching → {mercerization (cellulose fiber), relaxation → alkali deweighting (polyester fiber)} → (whitening) → pre-setting.

Singeing is to quickly pass the open-width fabric through a flame or brush to burn the red metal surface and burn off the fluff on the fabric surface. Its purpose is to (1) remove the fluff on the fabric surface, make the fabric surface bright and beautiful, and prevent the uneven printing and dyeing defects caused by fluff during printing and dyeing; (2) The singeing of synthetic fiber blended fabric can avoid or reduce pilling during wearing.

Desizing is the process of removing the size from the warp of woven fabric, and at the same time, it can remove a small amount of natural impurities, which is beneficial to scouring and bleaching in the future and obtains satisfactory dyeing effect. The common desizing method is that starch or chemical pulp will swell violently under the action of hot caustic soda, which will reduce the adhesion between pulp and fabric, and then remove it with hot water and mechanical scrubbing.

Boiling can remove most natural impurities, such as waxy substances, pectin substances and nitrogen-containing substances. The purpose of scouring is to remove most of the natural impurities in the fabric, so that the fabric has a certain degree of water absorption, which is beneficial to the subsequent dyeing and finishing (convenient for the adsorption and diffusion of dyes in the printing and dyeing process).

Bleaching is the process of removing pigment from fiber with bleaching agent to make fiber have necessary whiteness. The purpose of bleaching is to remove the pigment from the fiber and give the fabric the necessary and stable whiteness, so as to improve the appearance and facilitate the subsequent processing, and the fiber itself will not be obviously damaged.

Mercerization refers to the processing of cotton and linen fabrics treated with concentrated caustic soda solution under certain tension. The strength, softness, luster, dyeability and water absorption of mercerized cotton and linen fabrics will be improved to some extent. When cotton fiber is treated with liquid ammonia, it will get the same change as mercerizing with concentrated caustic soda solution. The swelling change of cotton fiber is not as drastic as that of alkali mercerization, but the treatment effect is uniform, especially the elasticity and hand feel of the fabric will be improved to some extent.

The purpose of relaxation treatment is to eliminate the torsion and internal stress produced in fiber spinning, twisting and weaving, untwist the twisted fabric, form crepe effect and improve the fabric feel. Its principle of action is that under the action of high-temperature hydrolysate, auxiliaries and mechanical friction, the internal torsion and internal stress of twisted weft yarn are relaxed without tension, and the weft yarn is fully contracted, showing irregular wavy buckling in weft direction and regular wavy buckling in warp direction, resulting in uneven buckling effect on the silk surface.

The purpose of alkali deweighting finishing is to reduce the weight of the fiber itself and obviously change the bending and shearing properties of the fabric, so as to obtain a silky soft feel, soft luster, good drape and water retention, smoothness and elasticity. The principle is that alkali treatment hydrolyzes the polyester surface fiber, absorbs a lot of carboxyl groups and enhances the hydrophilicity of the fiber. At the same time, hydrolysis makes the fiber thinner and the surface gloss changes, thus giving silk style.

Whitening is to whiten the fabric with fluorescent whitening agent. The principle of whitening is that the blue-purple light emitted by fluorescent whitening agent is mixed with the yellow light reflected by fabric to form white light. When whitening, different brighteners should be selected for different fibers, because different brighteners have different affinities and fastness to different fibers.

Pre-setting is mainly aimed at synthetic fibers and their blended fabrics. It is a process of keeping the fabric in a certain size, heating it at high temperature for a certain time, and then cooling it at an appropriate speed. The purpose of preheating and setting is to eliminate the existing wrinkles on the fabric, prevent the fabric from producing wrinkles that are difficult to remove under hot and humid conditions, and improve the thermal stability of the fabric.

For blended fabrics, if both fibers are needed and the same conditions can be used, they are combined (singeing and desizing); If there is only one need for two kinds of fibers, only the conditions that are useful to one side and harmless (or less harmful) to the other side can be adopted (high temperature alkali is not needed after alkali mercerization); If we need two kinds of fibers but can't use the same conditions, we can only carry out the process that is useful to one side and has little influence on the other, and then carry out the process that is useful to the other side and has little influence on this side (whitening and dyeing).

Pretreated water

Printing and dyeing factory is an enterprise that uses a lot of steam and water, and the water consumption of pretreatment process accounts for a large proportion in the whole printing and dyeing process. According to statistics, every time a printing and dyeing factory produces 1km cotton printing and dyeing fabric, it consumes nearly 20 tons of water, of which pretreatment water accounts for about 50%. The quality of water not only affects the product quality of pretreatment and other processes, but also affects the consumption of dyes and additives. Although water quality can be improved by various methods, due to the large water consumption of printing and dyeing plants, no matter which method is adopted to improve water quality, it will occupy equipment and venues, consume energy and chemicals, and lead to increased costs. Therefore, the printing and dyeing factory should be built in a place with sufficient water, good water quality and good sewage discharge conditions.

Calcium and magnesium salts in water are the most harmful to printing and dyeing. Insoluble calcium-magnesium soap will precipitate on the fabric when it reacts with soap, and insoluble scale will be generated in alkaline solution, which will attach to pretreatment equipment (such as the inner wall of the machine tank, valves, guide rollers, etc.) and hinder normal production. When the content of iron and manganese salts in water exceeds the specified limit, rust spots and catalytic oxidation of cotton fibers will occur during scouring. When bleaching with oxidant, iron and manganese salts also play a catalytic role in decomposing bleaching agent, making cotton fiber brittle. Boiler water must be soft water, otherwise the scale will be tightly attached to the boiler tube wall, reducing the thermal conductivity of the boiler wall and consuming more fuel; Scale deposition can also cause boiler explosion accidents. The water quality requirements of the printing and dyeing factory are as follows:

Transparency > 30? Chromaticity ≤ 10 (platinum cobalt degree), pH ≤ 6.5 ~ 8.5, iron content ≤0. 1mg/L, manganese content ≤0. 1mg/L, and total hardness: dye solution and soaping water < 0.36mg equivalent /L. ..

Pretreatment assistant

Commonly used auxiliaries for pretreatment include alkali, acid, oxidizing bleaching agent, surfactant, enzyme preparation for desizing starch pulp, etc.

1. Caustic soda is usually used as alkali. The main component of industrial caustic soda is sodium hydroxide, including solid caustic soda (purity over 95%) and liquid caustic soda (content 30% ~ 42%). Solid caustic soda is convenient for long-distance transportation and storage, and liquid caustic soda is convenient to use, but its volume is large. Because caustic soda is made by salt electrolysis, carbon dioxide in the air can easily react with caustic soda to generate sodium carbonate, so the content of soda ash in caustic soda should not be greater than 1% ~ 3%, and the content of sodium chloride should not be greater than 1.5% ~ 3%. Caustic soda is a kind of strong alkali, which is extremely corrosive to skin and mucosa. Pay attention to labor protection when using, especially when dissolving solid alkali, the operator must wear labor protection articles. Caustic soda is an important chemical reagent in cotton and cotton fabric printing and dyeing factory. It is often used for desizing, scouring and mercerizing in fabric pretreatment. The standard alkali consumption per 100 meter of fabric is 0.8 ~ 1.2 kg.

Soda Soda belongs to salt in chemical classification. It is alkaline after hydrolysis in aqueous solution, and its alkalinity is weak, so it cannot replace caustic soda. Generally used as water softener, scouring agent for colored fabrics and scouring agent for protein fibers. Ammonia is rarely used in cotton printing and dyeing plants, and is sometimes used for liquid ammonia mercerizing and scouring of acetate fiber fabrics. Other sodium silicate (also known as sodium silicate) is an assistant in scouring and hydrogen peroxide bleaching.

2. Sulfuric acid is often used for desizing, scouring and bleaching, and neutralization after mercerization. The content of H2S04 in industrial sulfuric acid is generally 92.5% ~ 98%, and the product is brown due to a small amount of iron, sulfur dioxide and organic matter. Concentrated sulfuric acid is corrosive, so handle it carefully. Concentrated sulfuric acid with water can generate a lot of heat energy. When diluting concentrated sulfuric acid, the acid can only be slowly poured into cold water and stirred properly. Never inject water into concentrated sulfuric acid, because sulfuric acid is a poor conductor of heat and has strong water absorption. If the high heat generated by water can't be emitted immediately, some water will boil quickly, causing sulfuric acid to splash with steam, hurting people and corroding skin and clothes. Concentrated sulfuric acid should be stored in a ventilated and dry place to avoid being heated or rain water soaking into the container, causing explosion. Hydrochloric acid has the same function as sulfuric acid and can also be used to neutralize alkaline reagents. However, sulfuric acid is cheap, high in acid content, strong in neutralization ability, low in industrial hydrochloric acid concentration and inferior to concentrated sulfuric acid, so sulfuric acid is still the main pretreatment.

3. Bleach: Both cotton and cotton fabrics are bleached with oxidizing bleach. There are several commonly used oxidative bleaches.

? A. Hydrogen peroxide, commonly known as hydrogen peroxide, is used for bleaching cotton fabrics and polyester-cotton blended fabrics with good results. Industrial hydrogen peroxide contains 30% hydrogen peroxide and a small amount of sulfuric acid as stabilizer. 30% hydrogen peroxide solution has a strong irritating effect on the skin, and it will decompose and explode when it comes into contact with metal objects such as iron, copper and chromium. Store in a cool place. Long-term storage of active ingredients will reduce, so it is not suitable for long-term storage.

B. hypochlorite, which is formed by the reaction of chlorine and alkali. The product of reaction with lime is called bleaching powder (it is a double salt of calcium hypochlorite and calcium chloride, and its molecular formula is generally represented by CaOCl2). Excellent bleaching powder contains about 30% ~ 35% available chlorine. Bleaching powder essence with higher available chlorine content than ordinary bleaching powder (available chlorine refers to the effective component of bleaching powder, that is, the amount of chlorine produced after a certain amount of bleaching powder reacts with acid, expressed in percentage). Because bleaching powder is calcium salt, it is not very convenient to use. Except for small factories, almost all of them are using sodium hypochlorite. Sodium hypochlorite is the product of the reaction between chlorine and caustic soda, and it is a mixture of sodium hypochlorite and sodium chloride. Sodium hypochlorite can be manufactured by printing and dyeing plants or supplied by chemical plants. Commercial sodium hypochlorite is a colorless or yellowish liquid containing available chlorine100 ~140 mg/l. Sodium hypochlorite has the same properties and uses as bleaching powder, and is generally used to bleach low-grade cotton fabrics and Victoria cotton blended fabrics.

C. Sodium chlorite, used for bleaching high-grade fabrics, has better effect than hypochlorite and stable whiteness. Due to the high price of sodium chlorite, the corrosion resistance of equipment is high, and chlorine dioxide, the decomposition product of sodium chlorite, is highly toxic, so it has not been widely used in China at present.

4. Surfactant: Almost all printing and dyeing processes are carried out in aqueous solution. Because of the large surface tension of water, the aqueous solution can not quickly and well wet and penetrate the fiber, which is not conducive to the printing and dyeing process. Therefore, a substance that can reduce the surface tension of water is often added to water, which is called surfactant. Usually, adding a small amount of surfactant to water can significantly reduce the surface tension of water. There are many such substances, such as ordinary soap, red oil, peregal and so on. Surfactants can be divided into wetting penetrant, emulsifier, dispersant and detergent according to their properties. Although the properties and uses of surfactants are different, their molecular structures have a common feature, which is composed of hydrophilic groups and hydrophobic groups.

Surfactants are usually classified into ionic and nonionic types according to their charges after being dissolved in water, and ionic surfactants can be further classified into cationic, anionic and amphoteric types, as shown in the following figure: anion surfactant, cationic surfactants, amphoteric surfactants and nonionic surfactants. In general dyeing and finishing, anionic and nonionic surfactants are mainly used.

Surfactant can significantly reduce the surface tension of water, and the degree of reduction is within a certain concentration range, which is related to the concentration of surfactant. When the concentration reaches a certain value, the surface tension of the solution will no longer decrease. The minimum concentration required for the solution surface tension to reach the minimum value is called critical micelle concentration (CMC). Different surfactants have different critical micelle concentrations, and the surfactant concentration used should be slightly greater than the critical micelle concentration in order to give full play to its role.