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Wet Processing Engineering

Wet Processing Engineering is the most significant division in the textile preparation & processing. Wet Processing Engineering is a major stream in Textile Engineering & referred as WPE in short which is under the section of Textile Chemical Processing Engineering and Applied Science. Textile manufacturing is vast area starting from fiber to apparel; covering with yarn, fabric, fabric dyeing, printing, finishing, garments or apparel manufacturing etc. There are many variable processes available at the spinning and fabric-forming stages coupled with the complexities of the finishing and colouration processes to the production of a wide ranges of products.

In Bangladesh Textile manufacturing is a major industry. In this industry wet processing plays a vital role in the area of Pre-treatment, Dyeing, Printing & Finising of both fabrics & apparels. But coloration in fibre stage or yarn stage is also included in the wet processing division.

All the processes of this stream is carried out in an aqueous state or aqueous medium & that's why it is called Wet Processing. The main processes of this section is given below-

                                                          ♦ Singeing
                                                          ♦ Desizing
                                                          ♦ Scouring
                                                          ♦ Bleaching
                                                          ♦ Mercerizing 
                                                          ♦ Dyeing
                                                          ♦ Printing
                                                          ♦ Finishing

♦ Singeing

Singeing is designed to burn off the surface fibres from the fabric to produce smoothness. The fabric passes over brushes to raise the fibres, then passes over a plate heated by gas flames. In the textile industry, loose fibres protruding on the surface of textile goods are singed to remove them. When done to fabrics containing cotton, this results in increased wettability, better dyeing characteristics, improved reflection, no "frosty" appearance, a smoother surface, better clarity in printing, improved visibility of the fabric structure, less pilling and decreased contamination through removal of fluff and lint.

♦ Desizing

Depending on the sizing materials that has been used, the cloth may be steeped in a dilute acid and then rinsed, or enzymes may be used to break down the sizing material. Enzyme is applied in desizing process if starch is used as sizing materials. CMC(carboxy methyl cellulose) & PVA (Poly vinyl alcohol) is often used as sizing materials.

♦ Scouring

Scouring, is a chemical washing process carried out on cotton fabric to remove natural wax and non-fibrous impurities (eg the remains of seed fragments) from the fibres and any added soiling or dirt. Scouring is usually carried in iron vessels called kiers. The fabric is boiled in an alkali, which forms a soap with free fatty acids (saponification). A kier is usually enclosed, so the solution of sodium hydroxide can be boiled under pressure, excluding oxygen which would degrade the cellulose in the fibre. If the appropriate reagents are used, scouring will also remove size from the fabric although desizing often precedes scouring and is considered to be a separate process known as fabric preparation. Preparation and scouring are prerequisites to most of the other finishing processes. At this stage even the most naturally white cotton fibres are yellowish, and bleaching, the next process, is required.

Since damage caused to the cotton substrate by NaOH; Bio-scouring is introduced in the scouring process in which biological agent is used like enzyme.

♦ Bleaching

Bleaching improves whiteness by removing natural coloration and remaining trace impurities from the cotton; the degree of bleaching necessary is determined by the required whiteness and absorbency. Cotton being a vegetable fibre will be bleached using an oxidizing agent, such as dilute sodium hypochlorite or dilute hydrogen peroxide. If the fabric is to be dyed a deep shade, then lower levels of bleaching are acceptable, for example. However, for white bed sheetings and medical applications, the highest levels of whiteness and absorbency are essential. Bleaching of textiles can be classified in to oxidative bleaching and reductive bleaching.

Oxidative bleaching Generally oxidative bleachings are carried out using sodium hypochlorite, sodium chlorite or hydrogen peroxide. Natural fibres like cotton, ramie, jute, wool, bamboo are all generally bleached with oxidative methods.

Reductive bleaching Reductive method of bleaching is done with Sodium hydrosulphite, a powerful reducing agent. Fibres like Polyamide, Polyacrylics and Polyacetates can be bleached using reductive bleaching technology.

After scouring and bleaching, Optical Brightening Agents (OBA), are applied to make the textile material to appear more brilliant whites. These OBA are available in different tints such as blue, violet and red.

♦ Mercerizing

Mercerization is a treatment for cotton fabric and thread that gives fabric or yarns a lustrous appearance and strengthens them. The process is applied to cellulosic materials like cotton or hemp. A further possibility is mercerizing during which the fabric is treated with caustic soda solution to cause swelling of the fibres. This results in improved lustre, strength and dye affinity. Cotton is mercerized under tension, and all alkali must be washed out before the tension is released or shrinkage will take place. Mercerizing can take place directly on grey cloth, or after bleaching.

♦ Dyeing

Dyeing is the process of adding color to textile products like fibers, yarns, and fabrics. Dyeing is normally done in a special solution containing dyes and particular chemical material. After dyeing, dye molecules have uncut chemical bond with fiber molecules. The temperature and time controlling are two key factors in dyeing. There are mainly two classes of dye, natural and man-made.

♦ Printing

Printing is referred as localized dyeing. Printing, on the other hand, is the application of colour in the form of a paste or ink to the surface of a fabric, in a predetermined pattern.Printing designs on to already dyed fabric is also possible. Textile printing is the process of applying colour to fabric in definite patterns or designs. In properly printed fabrics the colour is bonded with the fiber, so as to resist washing and friction. Textile printing is related to dyeing but, whereas in dyeing proper the whole fabric is uniformly covered with one colour, in printing one or more colours are applied to it in certain parts only, and in sharply defined patterns. In printing, wooden blocks, stencils, engraved plates, rollers, or silkscreens can be used to place colours on the fabric. Colourants used in printing contain dyes thickened to prevent the colour from spreading by capillary attraction beyond the limits of the pattern or design.

♦ Finishing

Finishing on fabric is carried out for both aesthetic & functional purposes to improve the quality & look of a fabric. Fabric may receive considerable added value by applying one or more finishing processes. Some finishing processes are -

                                        ~ Raising
                                        ~ Calendering 
                                        ~ Crease resistance
                                        ~ Filling
                                        ~ Softening
                                        ~ Stiffening 
                                        ~ Water repellency 
                                        ~ Mildew-proofing
                                        ~ Flame retardant
                                        ~ Anti-static
                                        ~ soil resistance & many more !