U2S2-4 - Processes of Textile Manufacturing

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Process analysis of textile manufacturing

Processes of textile manufacturing

Moustafa S. Moussa

UNESCO - IHE, Delft, The Netherlands

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CONTENTS

2. PROCESS ANALYSIS OF TEXTILE MANUFACTURING

2.1 INTRODUCTION 1 2.2 TEXTILE INDUSTRY OVERVIEW 2 2.3. PROCESSES OF TEXTILE MANUFACTURING 3

2.3.1 YARN FABRICATION 3 2.3.1.1 Natural fibers 4 2.3.1.2 Manmade fibers 6

2.3.2 FABRIC PRODUCTION 7 2.3.3 FABRIC PROCESSING (WET PROCESSING) 8

2.3.3.1 Pre-treatment 9 2.3.3.2 Dyeing 9 2.3.3.3 Printing 10 2.3.3.4 Finishing 11

2.3.4 GARMENT MANUFACTURING / FABRICATION 12 2.3.5 STOCK AND YARN FABRICATION 12 2.3.6 CARPET FABRICATION 13

2.4. ENVIRONMENTAL IMPACTS OF TEXTILE INDUSTRIES 14 2.4.1 AIR POLLUTION 16 2.4.2 WATER POLLUTION 18 2.4.3 SOLID WASTE POLLUTION 19

2.5. IMPACT OF THE MAJOR PROCESS CHEMICALS ON HUMAN HEALTH 21 2.5.1 CHEMICAL SUBSTANCES 21 2.5.2 DUST AND FUMES 21 2.5.3 NOISE 22

REFERENCES 24 APPENDICES 25

2.3. PROCESSES OF TEXTILE MANUFACTURING

Textile manufacturing begins with the production or harvesting of raw fiber. Fiber used in textiles can be harvested from natural sources (e.g. wool, cotton) or manufactured from regenerative cellulose materials (e.g. rayon, acetate), or it can be entirely synthetic (e.g. polyester, nylon). After the raw natural or manufactured fibers are shipped from the farm or the chemical plant, they pass through four main stages of processing (2):

• Yarn production

• Fabric production

• Wet process

• Garment Manufacturing

In addition to garment as final product of these process stages, stock yarn (final customer product) and carpets are also manufactured within these stages but in different ways (Figure2.1).

Fabric production

Wet processing

Garment Manufacturing

Fabric

Yarn

Colored fabric

Garment

Fiber

Carpet fabrication

Yarn fabrication

Finished CarpetFinished yarn or stock

A

B

C

Yarn fabrication

Fabric production

Wet processing

Garment Manufacturing

Fabric

Yarn

Colored fabric

Garment

Fiber

Carpet fabrication

Yarn fabrication

Finished CarpetFinished yarn or stock

A

B

C

Yarn fabrication

Figure 2.1. Textile manufacturing processes. A,B,C are the processes categories as presented in e-efficiency

2.3.1 YARN FABRICATION Yarn fabrication is the process, which converts raw fiber into yarn or thread. The fibers are prepared and then drawn out and twisted to form the yarn, which is then wound onto a bobbin or cone. The yarn fabrication is entirely dry, although some yarns maybe dyed and finished as a final customer product (2.4). (3)

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Carding

Combing

Drawing

Spinning

Opening /Blending

Cleaning

Carding

Combing

Drawing

Roving

Spinning

Yarn

Row Fiber

Yarn

Carding

Combing

Drawing

Spinning

Opening /Blending

Cleaning

Carding

Combing

Drawing

Roving

Spinning

Yarn

Row Fiber

Yarn

Figure 2.2 Main steps in yarn production of cotton and wool

2.3.1.1 Natural fibers Natural fiber must be opened, blended, carded and/or combed, and drafted before spinning. The main steps for processing wool and cotton are illustrated in figure 2.2. Although equipment used for the production of cotton is designed somewhat differently from that used for wool, the machinery operates in essentially the same fashion.

Opening/Blending

Row material (cotton and synthetic) are received in compact bales. Opening is the first operation required to carried out to open material from highly pressed cotton/synthetic bales. The second generation of machines is traveling type, which extract material from each bale and open into the smallest flock gently. The advantages of this traveling type are that, more bales can be processes at a time and thus have long term blending.

Cleaning

Cotton fiber must be cleaned to remove foreigen matter such as plant parts, seed hulls, dirt, etc, from the fibers. Cleaning is performed in successive steps from coarse to fine. Coarse cleaning remove heavy trash particles and fine opener removes the smaller trash particles. Cotton is passed through a series of machines (beaters) where opening and claening take place simultinusly. Speed of beater and clearance setting may be varied to achive the desired level of cleaning

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xtile.org 4

Carding

The blow room only opens the row material to flock whereas the card opens the flock into individual fibers. During this individualization process, coarse trash particles are removed in the licker-in zone and fine trash particles/ dust sucked away. The sheet of carded fibers is removed through a funnel into a loose rope like strand called a sliver.

Combing

Combing process serves to improve the row material in production of medium, medium to fine and fine yarns. This in turn reflects into better yarn evenness, strength, cleanliness, smoothness and visual appearance. In the wool system, combed sliver is used to make worsted yarn, while cards sliver is used for woolen yarn. In cotton system, the term combed cotton applies to the yarn mad from combed sliver. The function of comber is:

• To eliminate pre-determine quantity of short fiber.

• To eliminate remaining impurities.

• To eliminate large proportion of naps.

• To form a sliver of maximum possible evenness.

Drawing

The task of drawing process is to improve evenness over short, medium and especially long term level by straightening and paralyzing the fiber. In addition to this it also for blending of different fibers (Blending of Polyester / Cotton, Polyester / Viscose, Polyester / Wool Etc.) The drawing frame contains several pairs of rollers that rotate at successively faster speed. As the slivers pass through, they are further drawn out and lengthened, to the point where they may be five to six times as long as they were originally. In this process 4 to 8 carded or combed slivers are fed to the drafting arrangement.

Rove formation (Roving)

The main function of the roving is to attenuate the sliver. In this process draw frame sliver is fed to the drafting arrangement, which attenuate the sliver with a draft of between 5-20. The delivered strand is too thin to hold, which is strengthened by imparting twist immediately at the exit form the drafting arrangement. The twisted (rove) is finally wound on the bobbin, so that it is easy to transport to next process, stored and crelled on ring frame.


Spinning

In this process rove is fed to drafting arrangement, which further attenuate to spin into final yarn. The delivered stand of fine fiber from the drafting arrangement, is strengthened by inserting twist in it. This twist is generated by the spindle, which rotate at a higher speed. Each revolution of spindle import one turn of twist to the strand. Finally the yarn (twisted fiber strand) is wound on bobbin mounted on spindle with the help of traveler.

2.3.1.2 Manmade fibers Manmade fibers are often shipped as staple (similar in length to natural fibers), which is ready for spinning, or as filament yarn, which may be used directly or following further shaping or texturising. Both synthetic and cellulose are manufactured by processes that simulate or resemble the manufacture of silk (i.e., forcing a liquid through a small opening where the liquid solidifies to form a continuous filament). The three main methods of fiber manufacture are described below:

Wet spinning

In wet spinning, the polymer used to form the fiber is dissolved in solution. The solution is forced under pressure through an opening into a liquid bath in which the polymer is insoluble. As the solvent is dissipated in the bath, the fiber forms. Wet spinning produces rayon, acrylic, and modacrylic.

Dry spinning

Dry spinning uses a solvent that evaporates in air. The dissolthrough the spinnerete into a chamber of heated air or gas; recovered for reuse. Acrylic is produced by dissolving the polymbefore dry spinning. Other fibers formed by dry spinning incspandex, and aramid.

Melt spinning

Some polymeric fibers are spun by melting the polymer to a lforced through the spinner opening under pressure and cooled bfilament. Melt spinning requires no chemical reactions and no so

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ved polymer is extruded the solvent is generally er in dimethyl formamide lude acetate, triacetate,

iquid state. The liquid is y a jet of air to form the lvent recovery system.

-textile.org 6

2.3.2 FABRIC PRODUCTION

Fabric production, the second step, involves weaving, knitting, tufting and non-woven. Tufting and non-woven are used in the fabric manufacturing but not as widely as weaving and knitting.

Weaving Weaving is the most common method used for producing fabrics. The process is carried out of two sets of threads, which interlaces lengthwise yarns (warp yarns) with widthwise ones (weft or filling yarns).

To prevent the warp yarns from braking during weaving, the warp threads are coated with a size before weaving, to increase their tensile strength and smoothness. Natural starches are the most commonly used sizes, although compounds such as polyvinyl alcohol (PVA), resins, alkali-soluble cellulose derivatives, and gelatine glue have been used. The sizing compound is dried on the threads and remains a part of the cloth until it is removed in the subsequent processes. Other chemicals, such as lubricants, agents, and fillers, are often added to impart additional properties to a fabric (3).

Knitting In knitting fabric is formed through interlocking series of yarn loops. Rows of stitches are formed so that each row hangs on the row behind it, usually using sophisticated, high-speed machinery.

Tufting Tufting is the process of inserting additional yarns into fabric to create a pile fabric. The substrate fabric can range from a thin backing to heavy burlap-type material and may be woven, knitted, or web. In modern tufting machines, a set of hollow needles carries the yarn from a series of spools held in a creel and inserts the yarn through the substrate cloth. Patterns may be formed by varying the height of the tuft loops. Tufting is used for apparel fabrics, upholstery, and blankets, although most tufting machines are used for carpeting. Well over 90 percent of broadloom carpeting is made by tufting (2).

Non-woven Non-woven fabrics are comparatively new in the range of fabrics manufactured. Non-woven fabric has a strong appeal to both the manufacturer and the public for generally, it can be produced rapidly and cheaply and gives consumer satisfaction. Non-woven is described as a fiber products arranged in parallel, cross or randomly distributed, bonded either by mechanical means or by adhesive or thermoplastic fibers under the influence of heat and pressure. For the production of non-woven fabrics, initially a mixture of different type of fibers is made. One of the fibers which is evenly distributed within the mixture is a special type of fiber which can, at any suitable stage of processing, be brought into a tacky condition, enabling it to play the role of an adhesive or bonding substance. Then the fiber mixture is brought into the form of a comparatively thick layer or web of width corresponding


to the desired width of the fabric, which will ultimately be formed. In the final stage, the fiber layer is hot pressed, so that the special fibers within it partially melt and become securely bonded together. When the pressure is removed, the non-woven fabric is formed in which the fibers are simply held together by the bonding fibers (5).

2.3.3 FABRIC PROCESSING (WET PROCESSING)

The fabric produced from the weaving or knitting is in rough condition and is often termed 'grey' fabric. The material is rough to the touch and contains impurities, which are either natural in the fibers or added to facilitate the process of fabric manufacture. Fabric processing (wet processing) is done to improve the appearance and serviceability of the fabric in many ways. The main operations carried out in this step include pre-treatment, dyeing, printing and finishing (figures 2.3 and 2.4) (5,3).

As is evident from the description of the textile manufacturing process, the first two stages of manufacturing, that is, yarn manufacture and fabric preparation, involve mainly dry operations, which consume very little water and chemicals. The third stage of manufacturing, that is, wet processing, involves wet operations. The quantum of waste generated is relatively high in this stage (see e-efficiency r month 3,4,5,7).

Bleaching Dyeing and/or Printing FinishingKnitting Fabric

Finished fabricBleaching Dyeing and/or Printing FinishingKnitting

FabricFinished fabric

Figure 2.3 Wet processing of knitted cotton fabrics.

Singeing Mercerising Woven Fabric

Pre-treatment(desizing-scouring- bleaching)

Finishing Dyeing and/or Printing Dyeing and/or Printing Finished

fabric

Singeing Mercerising Woven FabricWoven Fabric



Finishing Dyeing and/or Printing Dyeing and/or Printing Finished

fabric

Figure 2.4 Wet processing of woven cotton fabrics.


2.3.3.1 Pre-treatment The pre-treatment process is carried out to prepare the textile material for subsequent processing, which includes dyeing, printing and finishing. The main operations include:

Desizing In this process, the sizing ingredients are removed from the grey fabric by dissolving them. Desizing, with acid or enzymes etc., then removes size from the fabric, so that chemical penetration of the fabric in later stages is not inhibited.

Scouring The scouring process is carried out to remove impurities such as wax, fatty acids, oils, etc, present in the fabric. Scouring is carried out in alkaline conditions (with sodium hydroxide) under high pressure and temperature (above 100oC).

Bleaching Bleaching is used to whiten fabrics and yarns. Different chemicals such as hypochlorites, hydrogen peroxides, etc, are used as bleaching agents. The process conditions during bleaching vary with the type of agent used. Once bleaching is complete, the bleaching agent must be completely removed, either by thorough washing or through the use of enzymes.

Mercerizing Mercerizing increases the tensile strength, luster and dye uptake of the cotton fabric or yarn. In this process, the cotton yarn or fabric is treated with cold sodium hydroxide solution. This causes swelling of the fiber, which results in an increase in the dye intake. Excess sodium hydroxide is normally recovered for reuse in either the scouring or other mercerization stages.

2.3.3.2 Dyeing Dyeing is employed to give an all-over shade to the fabric. It basically involves diffusion of dye molecules into the textile fabric, which imparts the required color. In the dyeing process these dye particles quickly come into contact with the surface of each fiber, form a thin layer and diffuse into it (5). There are essentially two techniques available for dyeing the textile material. These are:

Batch technique: The liquor and the textile are placed together in a vessel and the required amount of dye is added.

Continuous technique: The dye is dissolved or dispersed in the liquor. A definite quantity of dye liquor is locally applied to the textile.


The major classes of dyestuffs used in the textile industry are as follows (3):

Acid Dyes: Mainly used on wool, silk and polyamide fibers. They give very bright colors, whose fastness ranges from very poor (allowing colors to run) to very good.

Basic Dyes: Usually applied to acrylics and polyesters to produce very bright colors.

Direct Dyes: Commonly applied to rayon and cotton.

Disperse Dyes: Applied to cellulose acetate, polyamide and polyester fibers.

Reactive Dyes: This group produces a range of bright shades, and commonly used for cellulose textiles.

Sulphur Dyes: Most commonly used for dyeing cotton, rayon and cotton-synthetic blends and produce strong, deep colors in the final fabric.

Vat Dyes: These cover an almost full range of shades and are particularly important in the dyeing of cellulose fibers (such as cotton).

Azoic Dyes: Produce deep shades of blue, violet, yellow, orange and reddish.

The detailed information on these dyes (descriptions; exhaustion/fixation levels) is given in appendix1.

2.3.3.3 Printing Printing is a process by which colored patterns are produced on the fabric. Unlike dyeing, it is usually only carried out on prepared fabric where it is applied to specific areas to achieve a planned design. The color is applied to the fabric and then treated with steam, heat or chemicals to fix the color on the fabric. The most commonly used printing techniques are:

Pigment printing: Commonly used for all fabric types.

Wet printing: Uses reactive dyes for cotton and generally has a softer feel than pigment printed fabrics.

Discharge printing: Creates patterns by first applying color to the fabric and then removing selected areas.

Final washing of the fabric is carried out to remove excess paste and leave a uniform color (3).


2.3.3.4 Finishing This stage includes the final operations necessary for making the textile presentable and attractive. It imparts the final aesthetic, chemical and mechanical properties to the fabric as per the end use requirements. The finishing operations include:

Drying Drying removes the moisture from the fabric using the drying machine.

Providing Dimensional Stability This is one of the most important finishing operations. The fabric, which is in a distorted condition, is brought to the required dimensions of width and length.

Calendering A kind of glossy skin is formed on the fabric surface during calendering. The damp fabric is pressed hard against a hot, polished metal surface until it dries.

Softening After calendering, the fabric becomes a little stiff. Breaking down this stiffness is called softening. The fabric is led through the softening machine so that it touches the studded rollers lightly and drags them around. In this way, the surface of the fabric is lightly disturbed making it much softer.

Apart from the above mentioned finishing operations many other special properties depending upon requirements such as protection from rain, wind, cold, sunlight, fire resistance, etc., are provided. The properties are done by passing the fabric through a trough of chemicals (for providing special finishes) followed by drying.

Depending upon the type of fabric to be processed and the final product, any or all of the above processing operations can be carried out. Each of these operations involves consumption of huge quantities of water and chemicals (5).

Table 2.2 Summary of the main operating condition in textiles manufacturing (5).

Process Conditions

Sizing • Size formulation depends on the type of yarn. Size concentration is governed by the yarn count (8-15%). Temperature ranges from 80-90 °C. Drying at 100-130 °C.

Singeing • Direct or indirect flames are used to remove fuzzy fibers, followed by quenching.

Desizing • Desize formulation depends on the nature of the sizing agents, i.e. enzymatic or oxidative treatments are used for starch sized fabrics, whilst CMC, CMS or acrylate sizes can be removed by hot water (80-90 °C). PVA can be removed by using hot water in the presence or absence of peroxygens.


Process Conditions

Scouring • Kier boiling: NaOH 30g/l, wetting agent (2g/1), temperature 120 °C, time (12 hours).

• Continuous scouring: NaOH 30-50g/1, wetting agent (2g/1), temperature 90 °C.

Bleaching • Hypochlorite bleaching: 1.5-2g active chlorine/1,2g Na2C03/l, room temperature, time (2 hours).

• Peroxide bleaching: 10-16g H2O2/l (100%), 2.5g NaOH/l, 2-5gNa2Si03/l, 4 g organic stabilizer/l, 2g non-ionic wetting agent/l, temperature 90-95 °C, time (45-60 min).

Mercerization • 20-30% NaOH, 1-2g non-ionic wetting agent/I, temperature 18 °C , time (20-40 seconds).

Dyeing • A range of different dyestuff classes can be used for 100% cellulose fibers (e.g. direct, reactive, sulphur, vat, indigo). Dye bath formulation and dyeing conditions depend on the class used.

Printing • The same classes of dyestuffs used in dyeing can also be used in printing. Pigments can also be used.

Finishing • Soft finishing: By using different types of softening agents (e.g. cationic, non-ionic, silicone elastomers) different types of application (exhaustion or padding techniques) can be used. Exhaustion formulation - 2-4% softening agent at 40-50 °C for 15-20 minutes at pH 6.

• Resin finishing: Using N-methylol finishing agents in the presence of an acid catalyst using the padding technique.

2.3.4 GARMENT MANUFACTURING / FABRICATION

The finished cloths are fabricated into a variety of apparel and household industrial products. The simpler of these products, such as bags, sheets, towels, blanket, and draperies often produced by the textile mills themselves, but apparel or more complex house-wares are usually fabricated by cutting trades (2).

2.3.5 STOCK AND YARN FABRICATION

Yarn dyeing and finishing are different from woven fabric finishing because there is no sizing and desizing operation. They are different from knit fabric finishing because of their mercerising operation and water use. The main processes involved are cleaning, scouring, bleaching, mercerizing, dyeing and special finishing. Sewing thread, textile and carpet yarn are typical product in this category. Several techniques are available for


processing raw yarn into the finished product. The most common process is probably package dyeing, but other processes, such as space dyeing, are widely used. In package dyeing, yarn wound on perforated tubes is placed in a large vessel, which is sealed. The dye solution, at an appropriate temperature, is circulated through the yarn. The dyed yarn is washed, rinsed and dried. Finally, the yarn is unraveled and then wound on cons for the subsequent use by other mills (1).

2.3.6 CARPET FABRICATION

Carpet mills use mostly manmade fibers (synthetic fibers: nylon, acrylic and polyester), but some wool and cotton is also processed. This category is characterized by any or all of the following operational units: bleaching, scouring, carbonizing, dyeing, printing resin treatment, water proofing, flam proofing, soil repellency, backing with foamed and unfoamed latex or jute. Carpet backing without other carpet manufacturing operation may be included in the dry processing mill category. Some carpet is backed with latex in a separate plant, other carpet mills do latexing in the same plant with the finishing.

Tufted carpet consist of face yarn that is looped through a woven mat backing (mostly polypropylene some jute) dyed or printed and then backed with either latex foam or coated with latex and a burlap-type woven fabric backing but over latex.

The dominant face yarn is nylon, followed by acrylic and polyester. The latter two groups taken together are about equal to nylon. Since dyeing of these fibers in carpet differs little from dyeing fabric, the dyeing description for these fibers is similar to the one described in (2.5). The yarn is tufted into woven or non-woven polypropylene or jute primary backing in a dry operation. Following this, the tufted carpet can be either printed or dyed. If printed, a semi continuous screen printing operation is performed, followed by wash and rinse in the same machine. If dyed, the most common method is beck dyeing, in manner quite similar to that described in previous categories for yarn and goods. After it is dyed the carpet is dried in a tunnel drier. The carpet is then ready for application of adhesive and secondary backing (1).


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U2S2-4 - Processes of Textile Manufacturing