Fabric wet processing techniques

The wet processing is a term that involves the mechanical and chemical treatment to improve the aesthetic value of the fabric, yarn, fiber.

The wet processing sector can be divided into three distinct sections.

• Preparation process or preparatory process.
• Coloration process.
• Finishing process.

The general process sequence followed for the fabric wet processing is shown below:

• Grey Stitching
• Shearing and Cropping
• Singeing
• Desizing
• Scouring
• Bleaching
• Mercerisation
• Dyeing and Printing
• Printing

Singeing

The aim of singeing is to bum-off the protruding fibers/hairiness of yarn in the fabric surface. The amount of protruding fibers in the fabric surface varies according to yarn counts (Higher the yarn count lower the protruding fibers level & vice versa).

If a fabric is to have a smooth finish, singeing is essential. Singeing is a dry process used on woven goods that removes fibers protruding from yarns or fabrics. These are burned off by passing the fibers over a flame or heated copper plates.

Singeing improves the surface appearance of woven goods and reduces pilling. It is especially useful for fabrics that are to be printed or where a smooth finish is desired. Pollutant outputs associated with singeing include relatively small amounts of exhaust gases from the burners.

Importance of singeing

• Singeing improves the end use and wearing properties of textiles.
• The burning-off of protruding fibers results in a clean surface of a fabric.
• Dyed fabric appearance becomes brighter as singeing reduces the fogginess caused by differing reflection of light by the protruding fiber.
• Singeing is an effective means of reducing pilling in blended fabrics.
• Un-singeined fabrics soil more easily than singed fabrics.
• A closely singed fabric is essential for effective printing.

Types of singeing:

• Hotplate singeing
• Roller singeing
• Gas singeing

Hot Plate Singeing

Dried Fabric is made to pass over the heated bright red plates at speeds up to 200 yards per minute, according to quality which burns out the protruding fibers and provided clear fabric surface.

Roller Singeing

In this type of singeing machine, the cloth passes over a hollow cylinder which revolves slowly in the opposite direction of the goods. The hollow cast iron or copper cylinder is fired internally. The fabric gets singed and runs through a water bath for quenching and plaited.

Gas Singeing

The most common singer is a row of gas burners arranged so that the material passes rapidly through the open flame and it burns out the protruding fibers from the fabric surface. While singeing is a simple process, care must be taken to not damage the fabric.

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Desizing

Desizing is the process of removing the size material from the warp yarns in woven fabrics. It is the first chemical action in the wet process sequence. It is the process of removing starch materials present in the gray fabric. Generally, enzymes are used to degrade and remove the starch present in the grey fabric. If those size materials are not removed from the grey fabrics, then the subsequent chemical treatments on the fabric will be irregular, which will cause the defective dyeing and printing.

Desizing is an important preparation step used to remove size materials applied prior to weaving. Manmade fibers are generally sized with water-soluble sizes that are easily removed by a hot-water wash or in the scouring process. Natural fibers such as cotton are most often sized with water-insoluble starches or mixtures of starch and other materials.

Enzymes are used to break these starches into water-soluble sugars, which are then removed by washing before the cloth is scoured. Removing starches before scouring is necessary because they can react and cause color changes when exposed to sodium hydroxide in scouring.

Importance of Desizing

Scouring

Scouring is a cleaning process that removes impurities from fibers, yarns, or cloth through washing. Alkaline solutions are typically used for scouring; however, in some cases, solvent solutions may also be used. Scouring uses alkali, typically sodium hydroxide, to break down natural oils and surfactants and to emulsify and suspend remaining impurities in the scouring bath. The specific scouring procedures, chemicals, temperature, and time vary with the type of fiber, yarn, and cloth construction.

Importance of Scouring

Souring is an important pre-treatment operation in the processing of cotton and cotton blended materials. The main objective of the process is to remove the non-cellulosic constituents in cotton, which make the fiber non-absorbent posing serious technical problems during the subsequent stages of wet processing. The extent of souring has a strong bearing on the ultimate quality of the finished product.

Impurities may include lubricants, dirt and other natural materials, water-soluble sizes, antistatic agents, and residual tints used for yarn identification. Typically, scouring wastes contribute a large portion of biological oxygen demand (BOD) loads from preparation processes. Desizing and scouring operations are often combined.

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Bleaching

Bleaching is a chemical process that eliminates unwanted colored matter from fibers, yarns, or cloth. Bleaching decolorizes colored impurities that are not removed by scouring and prepares the cloth for further finishing processes such as dyeing or printing. Several different types of chemicals are used as bleaching agents, and selection depends on the type of fiber present in the yarn, cloth, or finished product and the subsequent finishing that the product will receive.

Importance of Bleaching

The aim of bleaching is to remove any unwanted color from the fibers and to bring whiteness to fabric thereby it improves the absorbency of the material for dyeing and printing. The most common bleaching agents are hydrogen peroxide & Hypochlorite.

The most common bleaching agents include hydrogen peroxide, sodium hypochlorite, sodium chlorite, and sulfur dioxide gas. Hydrogen peroxide is by far the most commonly used bleaching agent for cotton and cotton blends, accounting for over 90 percent of the bleach used in textile operations, and is typically used with caustic solutions. Bleaching contributes less than 5 percent of the total textile mill BOD load (NC DEHNR, 1986).

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The bleaching process generally involves the following steps:

• The cloth is saturated with the bleaching agent, activator, stabilizer, and other necessary chemicals;
• The temperature is raised to the recommended level for that particular fiber or blend and held for the amount of time needed to complete the bleaching action; and
• The cloth is thoroughly washed and dried.

Mercerizing

Mercerization is a continuous chemical process used for cotton and cotton/polyester goods to increase dyeability, luster, and appearance. This process, which is carried out at room temperature, causes the flat, twisted ribbon-like cotton fiber to swell into a round shape and to contract in length. This causes the fiber to become more lustrous than the original fiber, increase in strength by as much as 20 percent, and increase its affinity for dyes. Mercerizing typically follows singeing and may either precede or follow bleaching