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It is one of the major stream in Textile Engineering refers to Textile Chemical Processing Engineering and Applied Science. The four streams in Textile Engineering are
Yarn Manufacturing Engineering
Fabric Manufacturing Engineering
Wet Processing Engineering
Apparel Manufacturing Engineering
Wet process is usually done on the manufactured assembly of interlacing fibers, filaments, and/or yarns having substantial surface (planar) area in relation to its thickness, and adequate mechanical strength to give it a cohesive structure. In other words, simply wet process is done on manufactured fabric. The processes of this stream is involved or carried out in aqueous stage and thus it is called wet process which usually covers the following area :
Pretreatment
Dyeing
Printing &
Finishing
All these stages are required aqueous medium and this aqueous medium is created by water. A massive amount of water is required in these processes per day. It is estimated that, on an average, almost 100 liter of water is used to process only 1 Kg of textile goods. Water can be of various quality & attributes. Not all water can be used in the textile process. Water must have some certain properties, quality, color & attributes for being used in textile processes. That is why, water is prime concerned in Wet Processing Engineering and for this water is discussed here first before the wet processes.
Water
Water having the chemical formula H2O and chemical name Dihydrogen Oxide.
Water is obtained from various sources like surface water, sub-soil water, deep well water, rain water and sea water. Most water used in the textile from deep well water which situates 800 ft below the surface level. The main problem which is concerned in using water in textile processes is Water Hardness. Hardness is caused by the presence of soluble salts of metals mainly calcium and magnesium. Iron, aluminum and copper salts may also contribute to the hardness, but their effects are much less. The soluble salts of magnesium and calcium that may be present in water are bicarbonates, chlorides, sulphates or nitrates. Hardness caused by bicarbonates is called temporary hardness, because this can be remove simply by boiling the water. Hardness caused by other salt is called permanent hardness. The sum of this two is called total hardness and expressed as PPM.
Problems of using hardwater in wet processes
Formation of scale in boiler
Reaction with soaps and detergents
Reaction with dyes
Problem due to Iron (Fe)
Water hardness both temporary and permanent can be removed by several processes such as-
Boiling Process
Liming process
Sodalime process
Base exchange process(Zeolite/Permutit)
Synthetic ion exchange process
Harvesting rain water is state of art idea developed recently and many factories like Envoy Textile and ACS Textile having them in the process of installing a facility to harvest rain water within factory premises and use this rain water in wet processes since it is a most pure water source.
Pretreatment
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
The process of singeing is carried out for the purpose of removing the loose hairy fibres 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 of course printing to obtain best results from these processes.
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.
Singeing machine can be of three types, 1. Plate Singeing, 2. Roller Singeing, 3. Gas Singeing.
Among them Gas Singeing Machine is widely used in the textile industry like Unifill Textile, Bangladesh, Tania Textile Division, Bangladesh.
In gas singeing machine, fire flame comes into direct contact to the fabric and burn the the protruding fibre. 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 "biopolishing" where enzyme is used to remove the protruding fibres.
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
The main purpose of scouring of cotton fabric is to remove natural impurities as well as added impurities of essentially hydrophobic character as completely as possible and leave the fabric in a highly absorptive condition without undergoing chemical or physical damage significantly
Impurities that can present in a cotton fabric are natural, added and picked up impurities.
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.
Three main processes involved in the scouring are 1. Saponification 2. Emulsification amd 3. Detergency
Main chemical reagent used in the cotton scouring is NaOH which performs following actions-
converts saponifiable fats and oils into soaps
dissolves mineral matter
converts pectose and pectin into their soluble salts
Another scouring chemical is detergent which is am emulsifying agent and removes dust and dirt particles from the fabric.
Since damage caused to the cotton substrate by NaOH & in order to reduce alkali content in the effluent; 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.
To know the history of dyeing, visit here Dyeing
Stages of Dyeing
Solution Dyeing
Solution dyeing, also known as dope or spun dyeing, is the process of adding pigments or insoluble dyes to the spinning solution before the solution is extruded through the spinneret. Only manufactured fibers can be solution dyed. Solution dyeing is used for difficult-to-dye fibers such as olefin fibers, and for dyeing fibers for end uses that require excellent colorfastness properties. Because the color pigments become a part of the fiber, solution dyed materials have excellent colorfastness to light, washing, crocking (rubbing), perspiration, and bleach. Dyeing at the solution stage is more expensive since the equipment has to be cleaned thoroughly each time a different color is produced. Thus, the variety of colors and shades produced are limited. In addition, it is difficult to stock the inventory for each color. Decisions regarding color have to be made very early in the manufacturing process. Thus, this stage of dyeing is usually not used for apparel fabrics.
Filament fibers that are produced using the wet spinning method can be dyed while the fibers are still in the coagulating bath. The dye penetration at this stage is high as the fibers are still soft. This method is known as gel dyeing.
Fiber Dyeing
Stock dyeing, top dyeing, and tow dyeing are used to dye fibers at various stages of the manufacturing process prior to the fibers being spun into yarns. The names refer to the stage at which the fiber is when it is dyed. All three are included under the broad category of fiber dyeing.
Stock dyeing
Stock dyeing is dyeing raw fibers, also called stock, before they are aligned, blended, and spun into yarns.
Top dyeing
Top dyeing is dyeing worsted wool fibers after they have been combed to straighten and remove the short fibers. The wool fiber at this stage is known as top. Top dyeing is preferred for worsted wools as the dye does not have to be wasted on the short fibers that are removed during the combing process.
Tow dyeing
Tow dyeing is dyeing filament fibers before they are cut into short staple fibers. The filament fibers at this stage are known as tow.
The dye penetration is excellent in fiber dyeing; thus, the amount of dye used to dye at this stage is also higher. Fiber dyeing is comparatively more costly than yarn, fabric, and product dyeing. The decision regarding the selection of colors has to be made early in the manufacturing process. Fiber dyeing is typically used to dye wool and other fibers that are used to produce yarns with two or more colors. Fibers for tweeds and fabrics with a “heather” look are often fiber dyed.
Yarn Dyeing
Yarn dyeing adds color at the yarn stage. Skein, package, beam, and space dyeing methods are used to dye yarns.
skein dyeing
In skein dyeing the yarns are loosely wound into hanks or skein and then dyed. The yarns have good dye penetration, but the process is slow and comparatively more expensive.
package dyeing
In package dyeing yarns that have been wound on perforated spools are dyed in a pressurized tank. The process is comparatively faster, but the dye uniformity may not be as good as that of skein dyed yarn.
beam dyeing
In beam dyeing a perforated warp beam is used instead of the spools used in package dyeing.
Space dyeing is used to produce yarns with multiple colors.
In general, yarn dyeing provides adequate color absorption and penetration for most materials. Thick and highly twisted yarns may not have good dye penetration. This process is typically used when different colored yarns are used in the construction of fabrics (e.g. plaids, checks, iridescent fabrics).
Fabric Dyeing
Fabric dyeing, also known as piece dyeing, is dyeing fabric after it has been constructed. It is economical and the most common method of dyeing solid colored fabrics. The decision regarding color can be made after the fabric has been manufactured. Thus, it is suitable for “quick response” orders. Dye penetration may not be good in thicker fabrics; thus, yarn dyeing is sometimes used to dye thick fabrics in solid colors. Various types of dyeing machines are used for piece dyeing. The selection of the equipment is based on factors such as dye and fabric characteristics, cost, and the intended end use.
Union dyeing
Union dyeing is “a method of dyeing a fabric containing two or more types of fibers or yarns to the same shade so as to achieve the appearance of a solid colored fabric” (Source- Dictionary of Fiber and Textile Technology). Fabrics can be dyed using a single or multiple step process. Union dyeing is used to dye solid colored blends and combination fabrics commonly used for apparel and home furnishings.
Cross dyeing
Cross dyeing is “a method of dyeing blend or combination fabrics to two or more shades by the use of dyes with different affinities for the different fibers” (Source- Dictionary of Fiber and Textile Technology). The cross dyeing process can be used to create heather effects, and plaid, check, or striped fabrics. Cross dyed fabrics may be mistaken for fiber or yarn dyed materials as the fabric is not a solid color, a characteristic considered typical of piece dyed fabrics. It is not possible to visually differentiate between cross dyed fabrics and those dyed at the fiber or yarn stage. An interesting example is cross dyeing blue worsted wool fabric with polyester pin stripes. When dyed, the wool yarns are dyed blue, whereas the polyester yarns remain white.
Cross dyeing is commonly used with piece or fabric dyed materials. However, the same concept is applicable to yarn and product dyeing. For example, silk fabric embroidered with white yarn can be embroidered prior to dyeing and product dyed when an order is placed.
Product Dyeing
Product dyeing, also known as garment dyeing, is the process of dyeing products such as hosiery, sweaters, and carpet after they have been produced. This stage of dyeing is suitable when all components dye the same shade (including threads). This method is used to dye sheer hosiery since it is knitted using tubular knitting machines and then stitched prior to dyeing. Tufted carpets, with the exception of carpets produced using solution dyed fibers, are often dyed after they have been tufted. This method is not suitable for apparel with many components such as lining, zippers, and sewing thread, as each component may dye differently. The exception is tinting jeans with pigments for a “vintage” look. In tinting, color is used, whereas in other treatments such as acid-wash and stone-wash, chemical or mechanical processes are used. After garment construction, these products are given the "faded" or "used" look by finishing methods as opposed to dyeing.
Dyeing at this stage is ideal for quick response. Many T-shirts, sweaters, and other types of casual clothing are product dyed for maximum response to fashion’s demand for certain popular colors. Thousands of garments are constructed from prepared-for-dye (PFD) fabric, and then dyed to colors that sell best.
Dye types
Acid dyes
Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic fibers using neutral to acid dye baths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category.
Basic dyes
Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some use for wool and silk. Usually acetic acid is added to the dyebath to help the uptake of the dye onto the fiber. Basic dyes are also used in the coloration of paper.
Direct dyes
Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, with the addition of either sodium chloride (NaCl) or sodium sulfate (Na2SO4) or sodium carbonate (Na2CO3). Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.
Mordant dyes
Mordant dyes require a mordant, which improves the fastness of the dye against water, light and perspiration. The choice of mordant is very important as different mordants can change the final color significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques. The most important mordant dyes are the synthetic mordant dyes, or chrome dyes, used for wool; these comprise some 30% of dyes used for wool, and are especially useful for black and navy shades. The mordant, potassium dichromate, is applied as an after-treatment. It is important to note that many mordants, particularly those in the heavy metal category, can be hazardous to health and extreme care must be taken in using them.
Vat dyes
Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye, which, in this leuco form, has an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye. The color of denim is due to indigo, the original vat dye.
Reactive dyes
Reactive dyes utilize a chromophore attached to a substituent that is capable of directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion MX, Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at room temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers at home or in the art studio.
Disperse dyes
Disperse dyes were originally developed for the dyeing of cellulose acetate, and are water insoluble. The dyes are finely ground in the presence of a dispersing agent and sold as a paste, or spray-dried and sold as a powder. Their main use is to dye polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic fibres. In some cases, a dyeing temperature of 130 °C is required, and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.
Azoic dyeing
Azoic dyeing is a technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by treating a fibre with both diazoic and coupling components. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique, in that the final color is controlled by the choice of the diazoic and coupling components. This method of dyeing cotton is declining in importance due to the toxic nature of the chemicals used.
Sulfur dyes
Sulfur dyes are two part "developed" dyes used to dye cotton with dark colors. The initial bath imparts a yellow or pale chartreuse color, This is aftertreated with a sulfur compound in place to produce the dark black we are familiar with in socks for instance. Sulfur Black 1 is the largest selling dye by volume.
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.
Screen Printing
Hand screen printing
Semi-autimatric screen printing
Fully automatic screen printing
Rotary screen printing
Finishing
Textile finishing in a broad sense, covers all the processes which the fabric undergoes after leaving the loom or the knitting machine to the stage at which it enters the market. But in a restricted sense, it is the term used for a series of processes to which all bleached, dyed, printed and certain grey fabrics are subjected before they put on the market.
The object of textile finishing is to render textile goods fit for their purpose or end-use and/or improve serviceability of the fabric.
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
~ Moth proofing
~ Mildew-proofing
~ Flame retardant
~ Anti-static
~ soil resistance & many more !
Finishing processes can be divided into-
Mechanical finishing
Calendering
Calendering is an operation carried out on a fabric to improve its aesthetics. In this process, fabric passes between heavily loaded rollers. At the nip, the high pressure exerted on the fabric, flattens and smoothes its surface. By changing the parameters of the process many other effects can be achieved.
Purposes of calendering are to upgrade the fabric hand and to impart a smooth, silky touch to the fabric, to compress the fabric and reduce it's thickness, to improve the opacity of the fabric, to reduce the air permeability of the fabric by changing it's porosity, to impart different degree of luster of the fabric and to reduce the yarn slippage.
In calendering, the fabric passes through a series of calender rollers by wrapping, the face in contact with a roller alternates from one roller to the next. An ordinary calender consists of a series of hard and soft(resilient) bowls(rollers) placed in a definite order. The soft roller may be compressed with either cotton or wool-paper, linen paper or flax paper. The hard metal bowl is either of chilled iron or cast iron or steel. The calender may consists of 3,5,6,7 and 10 rollers. The sequence of the rollers is that no two hard rollers are in contact with each other. Pressure may be applied by compound levers and weights, or hydraulic pressure may be used as an alternative. The pressure and heat applied in calendering depends on the type of the finish required.
Raising
An important and oldest textile finishing is brushing or raising. by this process a wide variety of fabrics including blankets, flannelettes, industrial fabrics etc. can be produced. The process of raising consists of lifting from the body of the fabric a layer of fibers which stands out from the surface which is termed as "pile". The formation of pile on a fabric results in a "lofty" handle and may also subdue the weave or pattern and color of the cloth.
There are to types of raising machine- 1. Teasel machine and 2. Card-wire machine.
The speed of the card-wire raising machine varies from 12-15 yards per minute, which is 20-30% higher than that of teasel-raising. That is why the card-wire raising machine is widely used.
Beetling
Compressing Shrinkage
Heat Setting
Chemical finishing
Softening
Easy care finishing
Water repellency
Soil Release finishing
Flame retardant
UV Protection finishing
Antimicrobial finishing
Anti-pilling finishing
Non-slip finishing
Anti-static
Elastomeric finishing
Anti order and fragrance finishing
or
Temporary
semi-parmanent
permanent