Wet process engineering
Wet Processing Engineering is one of the major streams in Textile Engineering or Textile manufacturing which refers to the engineering of textile chemical processes and associated applied science. The other three streams in textile engineering are yarn engineering, fabric engineering, and apparel engineering. The processes of this stream are involved or carried out in an aqueous stage. Hence, it is called a wet process which usually covers pre-treatment, dyeing, printing, and finishing.
The wet process is usually done in the manufactured assembly of interlacing fibers, filaments and yarns, having a substantial surface area in relation to its thickness, and adequate mechanical strength giving it a cohesive structure. In other words, the wet process is done on manufactured fiber, yarn and fabric.
All of these stages require an aqueous medium which is created by water. A massive amount of water is required in these processes per day. It is estimated that, on an average, almost 50–100 liters of water is used to process only 1 kilogram of textile goods, depending on the process engineering and applications. Water can be of various qualities and attributes. Not all water can be used in the textile processes; it must have some certain properties, quality, color and attributes of being used. This is the reason why water is a prime concern in wet processing engineering.
Water
consumption and discharge of wastewater are the two major concerns. The textile industry uses a large amount of water in its varied processes especially in wet operations such as pre-treatment, dyeing, and printing. Water is required as a solvent of various dyes and chemicals and it is used in washing or rinsing baths in different steps. Water consumption depends upon the application methods, processes, dyestuffs, equipment/machines and technology which may vary mill to mill and material composition. Longer processing sequences, processing of extra dark colors and reprocessing lead to extra water consumption. And process optimization and right first-time production may save much water.- Fresh water: Most water used in the textile industry is from deep well water which is found 800 ft below the surface level. The main problem which is concerned with using water in textile processes is water hardness caused by the presence of soluble salts of metals including calcium and magnesium. Iron, aluminum, and copper salts may also contribute to the hardness, but their effects are much less. Using hard water in the wet process can cause problems such as the formation of scale in boilers, reactions with soap and detergents, reaction with dyes, and problems due to Iron.
- Wastewater:
Textile dyeing plants generate wastewater that contains synthetic and natural dyestuff, gum thickener and various wetting agents, pH buffers and dye retardants or accelerators. Following treatment with polymer-based flocculants and settling agents, typical monitoring parameters include BOD, COD, color, sulfide, oil and grease, phenol, TSS and heavy metals.
Pre-treatment
Wet process engineering is the most significant division in textile preparation and processing. It is a major stream in textile engineering, which is under the section of textile chemical processing and applied science. Textile manufacturing covers everything from fiber to apparel; covering with yarn, fabric, fabric dyeing, printing, finishing, garments, or apparel manufacturing. There are many variable processes available at the spinning and fabric-forming stages coupled with the complexities of the finishing and coloration processes to the production of a wide range of products.In the textile industry, wet process engineering plays a vital role in the area of pre-treatment, dyeing, printing, and finishing of both fabrics and apparel. Coloration in fiber stage or yarn stage is also included in the wet processing division.
All the processes of this stream are carried out in an aqueous state or aqueous medium. The main processes of this section include;
The process of singeing is carried out for the purpose of removing the loose hairy fibers protruding from the surface of the cloth, thereby giving it a smooth, even and clean looking face. Singeing is an essential process for the goods or textile material which will be subjected to mercerizing, dyeing and printing to obtain best results from these processes.
The fabric passes over the brushes to raise the fibers, then passes over a plate heated by gas flames. When done to fabrics containing cotton, this results in increased water affinity, 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 the removal of fluff and lint.
Singeing machines can be of three types: plate singeing, roller singeing, or gas singeing. Gas singeing is widely used in the textile industry. In gas singeing, a flame comes into direct contact to the fabric and burn the protruding fiber. Here, flame height and fabric speed is the main concern to minimize the fabric damage.
Singeing is performed only in the woven fabric. But in case of knit fabric, similar process of singeing is known as bio-polishing where enzyme is used to remove the protruding fibers.
Desizing
is the process of removing sizing materials from the fabric, which is applied in order to increase the strength of the yarn which can withstand with the friction of loom. Fabric which has not been desized is very stiff and causes difficulty in its treatment with a different solution in subsequent processes.After singeing operation the sizing material is removed by making it water-soluble and washing it with warm water. Desizing can be done by either the hydrolytic method or the oxidative method
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. Enzymes are applied in the desizing process if starch is used as sizing materials. Carboxymethyl cellulose and Poly vinyl alcohol are often used as sizing materials.
Scouring
is a chemical washing process carried out on cotton fabric to remove natural wax and non-fibrous impurities from the fibers 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. 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 fiber. 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. Even the most naturally white cotton fibers are yellowish at this stage, thus at the next process, bleaching, is required.The three main processes involved in the scouring are saponification, emulsification and detergency.
The main chemical reagent used in the cotton scouring is sodium hydroxide, which converts saponifiable fats and oils into soaps, dissolves mineral matter and converts pectose and pectin into their soluble salts.
Another scouring chemical is a detergent which is an emulsifying agent and removes dust and dirt particles from the fabric.
Since damage can be caused to the cotton substrate by sodium hydroxide. Due to this, and in order to reduce the alkali content in the effluent, Bio-scouring is introduced in the scouring process in which biological agent is used, such as an enzyme.
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 fiber 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. However, for white bedsheets and medical applications, the highest levels of whiteness and absorbency are essential.Reductive bleaching is also carried out, using sodium hydrosulphite. Fibers like polyamide, polyacrylics and polyacetates can be bleached using reductive bleaching technology.
After scouring and bleaching, optical brightening agents, are applied to make the textile material appear more white. These OBAs are available in different tints such as blue, violet and red.