Following the fabrication of the fabric, it is taken through a number of treatments before it is ready for the market. These processes are in the main meant to give the fabric aesthetic appeal, to prolong the life span of the product and to make the fabric suitable for certain applications. The processes are organized into three basic operations, namely: (i) preparation; (ii) coloration and (iii) finishing. All three operations are carried out in the form of wet or chemical treatment. This step of fabric/textile production is different from the hitherto considered processes of fabric production in that whilst all the processes until now have been purely mechanically based, the wet processes are mostly chemically based. The only exception is the formation of syntheticfibers, which is also a chemical process.
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3.6.1 Preparation Processes
The main objective of the preparation processes is to ensure that the textile has the right physical and chemical properties to enable it to be colored orfinished. Often, thefiber from which the yarns of the fabric are made contains impurities that are natural to thefiber or are picked up from earlier processing stages. It is important, in order to dye or print on the fabric, tofirst remove these impurities. It is the removal of the impurities that constitute the preparation of the fabric. Although there are some preparation steps that are general to allfibers, for instance scouring, most of the preparation procedures are applicable to specific groups offibers, such as cel- lulosicfibers or man-madefibers. Even for those operations that are common to all fabrics, the material needed and the conditions under which the operation is carried out will differ for different fabrics from differentfibers. Accordingly, these proce- dures will be discussed in subsequent sections based on thefiber from which the fabric was made.
3.6.1.1 Preparation of Cotton and Cellulosic Fabrics
A woven cotton fabric may be prepared according to theflowchart in Fig.3.7.
Singe is the process whereby surface hair on woven fabric is removed.This is thefirst operation in the preparation of a fabric for dyeing or printing. This process is accomplished in a singeing machine. In the machine, the fabric passes at very great speed over a naked gasflame. In the process, the surface hairs are burnt away.
Singeing may be done on one side or on both sides of the fabric. Unlike cellulosic fibers that form a dusty ash upon burning this method is not suitable for protein and otherfibers that leaves out hard black residues upon burning. There are three main reasons for singeing a fabric. Thefirst reason is to improve the wettability of the fabric. Surface hair helps to trap air into a fabric, as such when immersed in water it takes long for water to enter into the fabric. Second, the formation of pills on blended fabrics can be minimized with singeing. Pills are little balls offiber that form on some fabrics as a result of abrasion that occurs during usage. Finally, singeing creates a smooth surface for printing and dyeing. Displaced or moved hair from the surface of a printed fabric will cause details of the print to be distorted or get fuzzy. This can be avoided by singeing the fabric surface. It is noteworthy that knitted fabrics are not singed.
Following the singeing process, the fabric is sized to facilitate weaving yarns by treatment with starch and/or other substances, for instance, polyvinyl alcohols and polyacrylic acids. For cotton and other cellulosicfibers, starch-based sizes are the ones often used. Sizes enable the yarn to withstand the stresses encountered in the loom. These substances, however, must be removed from the fabric before dyeing or printing. The starch and other substances applied for this purpose are calledsizes and the process of removing them from the fabric is calleddesizing. Sizes are not applied to yarns used for knitting as the high stresses found in weaving are absent during knitting. Removal of starch-based sizes is carried out by the application of
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bacteria, acids, enzymes or oxidizing agents. Irrespective of the approach, the objective is to break down the insoluble starch into soluble components followed by dissolution and rinsing.
Desizing by use of bacteria
In this approach, bacteria are used to break down the starch in the fabric. This is achieved by soaking the fabric in water and placing it in a damped place for several hours. Bacteria from the atmosphere break down the starch in the fabric, which is then rinsed away. The process is very difficult to control as the bacteria could degrade the cellulose of the fabric itself if allowed to act too long on the fabric.
Desizing by use of acids
In this process, a 0.5–1.0% (wt) (on weight offiber) cold solution of sulfuric acid or hydrochloric acid is used. The fabric is soaked in the acid and left for 2–3 h at room temperature. The method has the advantage that it removes any metal contamina- tion from the fabric. However, the disadvantage is that the acid reacts with the cellulose of the fiber just as much as it does with the starch. This method is therefore not popular as a desizing agent.
Fig. 3.7 A generalized flowchart for the preparation of cotton fabric
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Desizing with oxidizing agents
Hydrogen peroxide is an example of oxidizing agents used to desize starch from cellulosicfibers. The fabric is soaked in a hot solution of the oxidizer for several hours or steamed at 20 min or so at 100–150 °C. Unlike the previous methods, this one does not cause serious damage to thefiber. An advantage of this method is that the peroxide could be used also as a bleaching agent so that the desizing and the bleaching could be combined into one.
Desizing using enzymes
Amylases that are known to break down starch under the right conditions are used in this process. In this case, electrolytes are added to the medium containing the enzymes to improve on the activity of the enzymes. The enzyme solution is pre- pared using hard water. This method is the safest with regard tofiber damage as the enzymes act only on the starch.
After desizing, the fabric is ready for scouring. This is the process that removes wax, fat, partially broken down seeds or husks still remaining in the fabric and generally remove any remaining water soluble impurities from the fabric. Scouring may be done by the use of alkali solutions. An example is the use of sodium hydroxide solution at high temperatures. The alkali reacts with the fats and waxes and turns them into soap. The soap thus produced improves the detergency of the solution and facilitates the removal of dirt and the solubilization of the previously water-insoluble impurities. An 8.0% (w/v) alkali solution at 120 °C may take 2– 20 min to complete the process. At temperatures above 100 °C, the process is carried out in the absence of air, otherwise the air would oxidize thefiber. Scouring may also be done by the use of organic solvents in which the fats and waxes are soluble. An example of such solvents is trichloroethylene. The disadvantage of scouring using solvents is that the solvents are unable to digest and remove seed and husk fragments in the fabrics.
Subsequent to the scouring, the fabric is bleached. Bleaching the fabric at this stage does two things. It destroys from the fabric any coloring matter and removes any residual seed or husk matter in the fabric. Bleaching is done by the use of oxidizing agents. The oxidizing agents commonly used are hydrogen peroxide (H2O2), sodium hypochlorite (NaOCl) and sodium chlorite (NaClO2). The choice of the oxidizing agent depends on the nature of thefiber and the equipment to be used.
For cotton, the most commonly used bleach is hydrogen peroxide. It is preferable to hypochlorite because it does not produce any toxic by-products and does not hurt the environment. It is applied at high temperatures and in the pH regime of 10–11.
Its major disadvantage is its relatively poor stability. The use of sodium chlorite is not popular as it produces chlorine dioxide gas which is highly toxic.
The final step in the preparation of cotton fabrics is mercerization. It is the process of getting rid of little bundles (also calledneps) that form on the surface of fabrics. The bundles are formed from immature fibers (commonly referred to as dead fibers) in the fabric. Mercerization is achieved by applying an appropriate medium to swell the immature fibers and give them a rounder shape and greater
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reflectivity. The method is only applicable to cellulose-based fabrics or fabric blends containing cotton. Woven as well as knitted fabrics may be mercerized.
However, mercerizing knitted fabrics is much more difficult and requires special- ized techniques. Mercerization changes the physical and chemical properties of a fabric by making it more lustrous, stronger, greater capacity to absorb more dyes, softer handle and more extensible. During mercerization the fabric, under tension, is soaked in 18–30% sodium hydroxide at 13–15 °C. Sufficient time is allowed for the sodium hydroxide to adequately absorb into the substrate. Thereafter, the alkali is washed out with the substrate still under tension. Both yarns and fabrics can be mercerized.
3.6.1.2 Preparation of Wool Fabrics
Wool fabrics are normally prepared according to the steps outlined in Fig.3.8.
Cropping
Thefirst step in the preparation of wool fabrics is removing surface hair from the fabric. This process is called cropping or shearing. Cropping is a mechanical process that is achieved in a cropping machine. The cropping machine isfitted with
Fig. 3.8 Flowchart for the preparation of wool fabric
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a series of helical blades, which rotate at very high speeds. As the fabric passes beneath the blades, the hair is removed by the cutting action created between the blade and a stationary ledger blade beneath the fabric. Cropping is much slower than singeing, but it makes possible a more controlled removal of hair. The method is applicable to anyfiber type.
Scouring
Scouring of wool fabric is done to remove impurities from the fabric prior to printing or dyeing. Impurities such as lubricants, anti-static agents, oils and waxes applied to the yarn orfiber to improve processing and weaving, or knitting are all removed at this stage. Natural impurities such as wool grease (lanolin), suint (sweat), vegetable matter, parasites, branding fluids, pesticides and veterinary products are expected to have been removed by the time the fabric is formed.
However, where they are not, the scouring process removes all of these. It must be noted that wool could also be scoured in loose stock or in yarn. Scouring of wool fabrics is done in scouring machines using soap or nonionic synthetic detergents and sodium carbonate solution (i.e. soap and soda scout). The temperature of the scouring medium is kept between 45 and 55 °C.
Carbonizing
This is a chemical treatment process during which any remaining vegetable impurity in the fabric is removed. To achieve this, the fabric is first impregnated with a 5–6% HCl or H2SO4or a salt which will produce a strong acid when the goods are dried. Examples of such salts are aluminum chloride and magnesium chloride. After the application of the acid or acid salt, the fabric is dried and baked.
The action of the acid or salt in the fabric is to hydrolyze the vegetable matter to dark, brittle hydrocellulose which is then removed mechanically by passing the fabric between franged rollers. As a final step of the process, the fabric is neu- tralized and rinsed. Carbonizing may also be carried out at the loose wool stage.
Bleaching
Wool may be bleached with both reducing and oxidizing agents, unlike cotton, which are bleached only with oxidizing agents. When reducing agents are used, the wool may be bleached by exposing the wool fabric to sulfur dioxide (SO2) or by treating the fabric in a liquor containing reducing agents such as sodium bisulfate (NaHSO3), sodium dithionite (Na2S2O4), sodium metabisulfite (Na2S2O5), or zinc formaldehyde-sulfoxylate Zn(HSO2CH2O)2 or sodium formaldehyde-sulfoxylate (NaHSO2CH2O). The use of reducing agents in the bleaching of wool however is limited for a number of reasons. First, the whiteness produced is not permanent as on exposure to air the original color of the wool returns. This is due to an oxidation process that reverses the reducing action. Second, the use of sulfur dioxide results in an uneven appearance in the fabric. Hydrogen peroxide is the only oxidizing agent used to bleach wool. The other bleaching agents previously mentioned all have detrimental effects on the wool and as such are not used. Bleaching with hydrogen peroxide has a number of advantages. First, the whiteness it produces is permanent.
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Second, it bleaches evenly. Bleaching with hydrogen peroxide is done at a pH of between 8.5 and 9.0 and at a temperature of 40–50 °C. Higher pH of the liquor will cause damage to the wool. It is important during the bleaching of wool that the pH is kept constant. This is achieved by adding sodium silicate to the liquor. The silicate acts as a buffer to keep the pH constant. An added advantage of sodium silicate is that it acts to stabilize the liquor against the catalyzing action of metals, which degrades the peroxide. After bleaching, the fabric is neutralized using acetic acid.
Felting
Felting is a characteristic of hairfibers only. It is the ability offibers to form matted entanglements such that a woven fabric, for example, can become so matted that the warp and welt yarns are no longer visible. This results in a fabric that is apparently composed of a mangled web offibers. This type of fabric is called a felt fabric.
Felting occurs when the cuticle scales on two fibers lying alongside each other interlock during the relative motions of thefibers.
Felting is done for two reasons: (i) felt fabric has a different appearance to an unfelted one. Therefore, felting is sometimes carried out as a design consideration only; (ii) during felting the air spaces betweenfibers within the fabric get smaller. It, therefore, becomes more difficult for air to pass through the fabric. Since air is a good thermal insulator, fabric that has smaller air spaces will be warmer to wear than those with bigger spaces. Felting is always accompanied by shrinkage of the fabric because thefibers move closer to each other. Industrially, felting is carried out in milling machines in a process calledmilling. During this process, the fabric that is wetted by soap (the soap acts as a lubricant) is continually passed through the machine until the desired felting is achieved. The process is controlled by the liquor temperature and pH, the fabric speed through the machine and the pressure exerted on the fabric by the machine.
An important operation during the preparation of wool fabrics is setting. Setting is a process of giving a fabric a sustainable shape or form. It is done to avoid distortions in the fabric structure, such as waviness (cockling) during processing.
The distortions are mostly brought about during the scouring process and if they are not resolved can adversely affect the subsequent processing of the fabric. The setting of the fabric during the preparation stage is non-permanent and is achieved through two techniques, namely, crabbing and blowing. During crabbing, the fabric isfirst soaked in hot water and subsequently wound around a roller under tension.
A second roller presses down on the roll of cloth to apply the necessary smoothing pressure. During blowing, the fabric is wound under tension onto a hollow per- forated cylinder through which steam is then blown for about one or two minutes.
Setting can be best achieved if the fabric pH is about 9. This can be performed by first giving the fabric an appropriate wet treatment. The set achieved would nor- mally remain in the fabric provided it is not wetted again at a temperature equal to or higher than that at which it was previously set.
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3.6.1.3 Preparation of Man-Made Fiber Textiles
Man-made fibers by nature of their processing contain far less impurities than naturalfibers. As a result, man-made fibers require far less preparation steps than those discussed in the previous sections. Most of the impurities accompanying fabrics made form man-made fibers are acquired more from the processes that follow the preparation of thefiber rather than from thefiber formation processes.
Lubricants, anti-static agents, sizes that are added during yarn preparation and weaving/knitting must all be removed as impurities after the fabric has been formed. The principal preparation steps for fabrics made from man-madefibers are shown in Fig.3.9.
Desizing and scouring
Desizing and scouring are combined in one operation for fabrics from man-made fibers. The operation is performed using a non-ionic synthetic detergent with the addition of a little alkali at a temperature of between 50 and 60 °C. Subsequent rinsing after the detergent treatment is sufficient to desize and scour fabrics from man-madefibers. The same process is applicable to both weaved and knitted fabrics.
Setting
Fabrics from man-made fibers, particularly, synthetics and modified regenerated cellulosefibers are set by a process calledheat setting. Heat setting takes advantage of the ability of the fibers to soften and therefore can be molded when heated.
During heat setting, the fabric isfirst exposed to dry heat or steam at a temperature above their softening temperature whilst they are heldflat and to specified width.
Fig. 3.9 A generalized flowchart for the preparation fabrics from man-madefibers
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Subsequently, they are cooled whilst still heldflat. The set conferred on man-made fibers has more permanency than that on wool brought about by crabbing or blowing processes.
Fluorescent Brightening
This is a process by which whiteness is added to a fabric to enhance it to a level above that which is achieved by bleaching alone. This is achieved withfluorescent brightening agents (FBA). Thefluorescent brightening agents are substances that have the ability to absorb light (generally, radiation) and to re-radiate it at a longer wavelength. The FBA used in textiles often have the ability to absorb white light (typically with wavelengths of 300–400 nm in the non-visible area) plus the additional characteristics of being able to convert any ultra-violet radiation present into visible light at the end of the visible spectrum (at wavelengths of 400–500 nm).
FBA may be added to bleaching liquor and applied to the fabric or applied by methods similar to those used to dye fabrics.
The most common class of brightness for textiles are the stilbenes (di‐and tetra‐ sulfonated triazole‐stilbenes and di‐sulfonated stilbene‐biphenyl). However, long-term exposure to UV caused them to fade as a result of the formation of optimally inactive stilbene cis-isomers found at the center of the molecule. Other classes of FBAs are coumarins, imidazolines, benzoxalines naphthalimide, pyrazine and triazine. Stilbene derivatives are FBAs used for cellulosics, while distyryl and triazolyl stilbene derivatives are used for polyamides, and benzimidazole deriva- tives for acrylic fabrics. FBAs for polyesters are benzoxazole and styryl derivations.
Tinopal (a thiophenediyl benzoxazole class) is the most efficient brighteners for cotton and exhausts within a pH range of 8–11 or higher and work best when applied by padding method.
3.6.2 Coloration of Fabrics
Fabrics may be colored to enhance their aesthetic appeal or for functional reasons, for example, military camouflage and fluorescent jackets for road repair workers and the police. Coloration of fabrics is achieved with pigments or dyes. Dyes are water-soluble substances and are absorbed into the fabric when applied. Some dyes arefiber specific. On the other hand, pigments are water-insoluble substances that adhere to the surface of the fabric when applied. They are not fiber specific.
Processes used to add color to fabrics are dyeing and printing.
3.6.2.1 Dyeing
Dyeing completely covers the fabric with color (i.e. the substrate is literally, completely soaked in color during dyeing). Dyeing is achieved using dyes that may be of natural or synthetic origin. Dyes are organic chemicals that are able to absorb and reflect visible light. Such molecules are called chromophores. Chromophores contain one or two functional groups on the molecule namely, chromogen-color providing group and auxogens-color intensifying or deepening group. The dye is
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