What Is Nylon?A manufactured fiber in which the fiber forming substance is any long chain synthetic polyamide having recurring amide groups (-NH-CO-) as an integral part of the polymer chain (FTC definition). The two principal nylons are nylon 66, which is polyhexamethylenedianime adipamide, and nylon 6, which is polycaprolactam. Nylon 66 is so designated because each of the raw materials, hexamethylenediamine and adipic acid, contains six carbon atoms. In the manufacture of nylon 66 fiber, these materials are combined, and the resultant monomer is then polymerized. After polymerization, the material is hardened into a translucent ivory-white solid that is cut or broken into fine chips, flakes, or pellets. This material is melted and extruded through a spinneret while in the molten state to form filaments that solidify quickly as they reach the cooler air. The filaments are then drawn, or stretched, to orient the long molecules from a random arrangement to an orderly one in the direction of the fiber axis. This drawing process gives elasticity and strength to the filaments. Nylon 6 was developed in Germany where the raw material, caprolactam, had been known for some time. It was not until nylon 66 was developed in the United States that work was initiated to convert caprolactam into a fiber. The process for nylon 6 is simpler in some respects than that for nylon 66. Although nylon 6 has a much lower melting point than nylon 66 (a disadvantage for a few applications), it has superior resistance to light degradation and better dyeability, elastic recovery, fatigue resistance, and thermal stability. Two other nylons are: (1) nylon 11, a polyamide made from 11-aminoundecanoic acid; and (2) nylon 610, made from the condensation product of hexamethylenediamine and sebacic acid. Nylon 610 has a lower melting point than nylon 66 and the materials for its manufacture are not as readily available as those for nylon 66. Experimental work has been conducted on other. Nylon was the first truly synthetic fiber to be commercialized (1939). Nylon was developed in the 1930s by scientists at Du Pont, headed by an American chemist Wallace Hume Caruthers (1896-1937). It is a polyamide fiber, derived

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from a diamine and a dicarboxylic acid, because a variety of diamines and dicarboxylic acids can be produced, there are a very large number of polyamide materials available to produce nylon fibers. The two most common versions are nylon 66 (polyhexamethylene adiamide) and nylon 6 (Polycaprolactam, a cyclic nylon intermediate). Raw materials for these are variable and sources used commercially are benzene (from coke production or oil refining), furfural (from oat hulls or corn cobs) or 1,4-butadiene (from oil refining).

Federal Trade Commission Definition for Nylon Fiber:A manufactured fiber in which the fiber forming substance is a long-chain synthetic polyamide in which less than 85% of the amide-linkages are attached directly (-CO-NH-) to two aliphatic groups.

Nylon The Miracle FiberIn September 1931, American chemist Wallace Carothers reported on research carried out in the laboratories of the DuPont Company on giant molecules called polymers. He focused his work on a fiber referred to simply as 66, a number derived from its molecular structure. Nylon, the miracle fiber, was born. The Chemical Heritage Foundation is currently featuring an exhibit on the history of nylon. By 1938, Paul Schlack of the I.G. Farben Company in Germany, polymerized caprolactam and created a different form of the polymer, identified simply as nylon 6. Nylon's advent created a revolution in the fiber industry. Rayon and acetate had been derived from plant cellulose, but nylon was synthesized completely from petrochemicals. It established the basis for the ensuing discovery of an entire new world of manufactured fibers. An American Romance: DuPont began commercial production of nylon in 1939. The first experimental testing used nylon as sewing thread, in parachute fabric, and in women's hosiery. Nylon stockings were shown in February 1939 at the San Francisco Exposition and the most exciting fashion innovation of the age was underway.

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American women had only a sampling of the beauty and durability of their first pairs of nylon hose when their romance with the new fabric was cut short. The United States entered World War II in December 1941 and the War Production Board allocated all production of nylon for military use. Nylon hose, which sold for $ 1.25 a pair before the War, moved in the black market at $10. Wartime pinups and movie stars, like Betty Grable, auctioned nylon hose for as much as $40,000 a pair in war-effort drives. During the War, nylon replaced Asian silk in parachutes. It also found use in tires, tents, ropes, ponchos, and other military supplies, and even was used in the production of a high-grade paper for U.S. currency. At the outset of the War, cotton was king of fibers, accounting for more than 80% of all fibers used. Manufactured and wool fibers shared the remaining 20%. By the end of the War in August 1945, cotton stood at 75% of the fiber market. Manufactured fibers had risen to 15%.

A Short History of Manufactured FibersFor thousands of years, the use of fiber was limited by the inherent qualities available in the natural world. Cotton and linen wrinkled from wear and washings. Silk required delicate handling. Wool shrank, was irritating to the touch, and was eaten by moths. Then, a mere century ago, rayon the first manufactured fiber was developed. The secrets of fiber chemistry for countless applications had begun to emerge. Manufactured fibers now are put to work in modern apparel, home furnishings, medicine, aeronautics, energy, industry, and more. Fiber engineers can combine, modify and tailor fibers in ways far beyond the performance limits of fiber drawn from the silkworm cocoon, grown in the fields, or spun from the fleece of animals. The Early Attempts The earliest published record of an attempt to create an artificial fiber took place in 1664. English naturalist Robert Hooke suggested the possibility of producing a fiber that would be if not fully as good, nay better than silk. His goal remained unachieved for more than two centuries.

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The first patent for artificial silk was granted in England in 1855 to a Swiss chemist named Audemars. He dissolved the fibrous inner bark of a mulberry tree, chemically modifying it to produce cellulose. He formed threads by dipping needles into this solution and drawing them out - but it never occurred to him to emulate the silkworm by extruding the cellulosic liquid through a small hole. In the early 1880's, Sir Joseph W. Swan, an English chemist and electrician, was spurred to action by Thomas Edison's new incandescent electric lamp. He experimented with forcing a liquid similar to Audemars solution through fine holes into a coagulating bath. His fibers worked like carbon filament, and they found early use in Edison's invention. It also occurred to Swan that his filament could be used to make textiles. In 1885 he exhibited in London some fabrics crocheted by his wife from his new fiber. But electrical lamps remained his main interest, and he soon abandoned work on textile applications.

First Commercial ProductionThe first commercial scale production of a manufactured fiber was achieved by French chemist Count Hilaire de Chardonnet. In 1889, his fabrics of artificial silk caused a sensation at the Paris Exhibition. Two years later he built the first commercial rayon plant at Besancon, France, and secured his fame as the father of the rayon industry. Several attempts to produce artificial silk in the United States were made during the early 1900's but none were commercially successful until the American Viscose Company, formed by Samuel Courtaulds and Co., Ltd., began production its production of rayon in 1910. In 1893, Arthur D. Little of Boston, invented yet another cellulosic product acetate and developed it as a film. By 1910, Camille and Henry Dreyfus were making acetate motion picture film and toilet articles in Basel, Switzerland. During World War I, they built a plant in England to produce cellulose acetate dope for airplane wings and other commercial products. Upon entering the War, the United States government invited the Dreyfus brothers to build a plant in Maryland to make the product for American warplanes. The first commercial textile uses for acetate in fiber form were developed by the Celanese Company in 1924.

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In the meantime, U.S. rayon production was growing to meet increasing demand. By the mid-1920's, textile manufacturers could purchase the fiber for half the price of raw silk. So began manufactured fibers' gradual conquest of the American fiber market. This modest start in the 1920's grew to nearly 70% of the national market for fiber by the last decade of the century.

The Post-War Industry After the war, GI's came home, families were reunited, industrial America gathered its peacetime forces, and economic growth surged. The conversion of nylon production to civilian uses started and when the first small quantities of postwar nylon stockings were advertised, thousands of frenzied women lined up at New York department stores to buy. In the immediate post-war period, most nylon production was used to satisfy this enormous pent up demand for hosiery. But by the end of the 1940's, it was also being used in carpeting and automobile upholstery. At the same time, three new generic manufactured fibers started production. Dow Badische Company (today, BASF Corporation) introduced metalized fibers; Union Carbide Corporation developed modacrylic fiber; and Hercules, Inc. added olefin fiber. Manufactured fibers continued their steady march. By the 1950's, the industry was supplying more than 20% of the fiber needs of textile mills. A new fiber, acrylic, was added to the list of generic names, as DuPont began production of this wool-like product. Meanwhile, polyester, first examined as part of the Wallace Carothers early research, was attracting new interest at the Calico Printers Association in Great Britain. There, J. T. Dickson and J. R. Whinfield produced a polyester fiber by condensation polymerization of ethylene glycol with terephthalic acid. DuPont subsequently acquired the patent rights for the United States and Imperial Chemical Industries for the rest of the world. A host of other producers soon joined in. Today

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Innovation is the hallmark of the manufactured fiber industry. Fibers more numerous and diverse than any found in nature are now routinely created in the industry's laboratories. Nylon variants, polyester, and olefin are used to produce carpets that easily can be rinsed clean even 24 hours after they've been stained. Stretchable spandex and machine-washable, silk-like polyesters occupy solid places in the U.S. apparel market. The finest microfibers are remaking the world of fashion.For industrial uses, manufactured fibers relentlessly replace traditional materials in applications from super-absorbent diapers, to artificial organs, to construction materials for moon-based space stations. Engineered non-woven products of manufactured fibers are found in applications from surgical gowns and apparel interfacing to roofing materials, road bed stabilizers, and floppy disk envelopes and liners. Non-woven fabrics, stiff as paper or as soft and comfortable as limp cloth, are made without knitting or weaving. As they always have, manufactured fibers continue to mean, life made better.

The chemical reactionsThe chemical reactions are as follows.

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Nylons are also called polyamides, because of the characteristic amide groups in the backbone chain. Proteins, such as the silk nylon was made to replace, are also polyamides. These amide groups are very polar, and can hydrogen bond with each other. Because of this, and because the nylon backbone is so regular and symmetrical, nylons are often crystalline, and make very good fibers.

The nylon in the pictures on this page is called nylon 6,6, because each repeat unit of the polymer chain has two stretches of carbon atoms, each being six carbon atoms long. Other nylons can have different numbers of carbon atoms in these stretches. Nylons can be made from diacid chlorides and diamines. Nylon 6,6 is made from the monomers adipoyl chloride and hexamethylene diamine.

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This is one way of making nylon 6,6 in the laboratory. But in a nylon plant, it's usually made by reacting adipic acid with hexamethylene diamine:

Another kind of nylon is nylon 6. It's a lot like nylon 6,6 except that it only has one kind of carbon chain, which is six atoms long.

It's made by a ring opening polymerization form the monomer caprolactam. Nylon 6 doesn't behave much differently from nylon 6,6. The only reason both are made is because DuPont patented nylon 6,6, so other companies had to invent nylon 6 in order to get in on the nylon business.

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MODEL OF MOLECULAR ARRANGEMENT:

In essence, cold drawing stretches chains in amorphous regions, but molecular folds are restricted and the molecules orient themselves along the fiber axis direction, resulting in enhanced orientation and high crystallinity. In the case of nylons, which have sheet-like crystal structures, drawing may enable the hydrogen-bonded polyamide sheets to slip past each other and form more oriented structure [4]. Hot drawing is a procedure using high temperature during drawing and annealing under restraint after drawing. Exposure to high temperature helps to

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increase the draw ratio, and higher moduli and tenacity can be achieved.. Ultra drawing of solidified crystalline material induces a high degree of chain extension (Figure 2), which leads to very high tensile strength and modulus. This results in a so-called high-performance fiber.

THE MODEL OF MOLECULAR CHAINS:

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A skin-core structure, mostly depending on spinning speed, is generally formed within melt-spun fibers. At a constant feeding rate, higher spinning speeds will produce more extended chains in the melt and form a finer filament. Therefore, the finer fiber usually has higher modulus and tenacity. Fine filament cannot be drawn as much as a coarse filament, because partial orientation on the outer parts of the filaments is formed when the molten fluid is drawn over the sides of the orifice. As a result, finer filaments have a greater proportion of 'skin' to bulk,

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i.e., better orientation has already been formed. Naturally, there is not much space for an improvement by cold drawing within fine filaments. The filaments become lustrous and strong.

NON-CONVENTIONAL SPINNING TECHNIQUESAlternative to conventional melt spinning, various solution-spinning techniques have been introduced. Solution spinning techniques (gel, wet, dry) enable the spinning of high molecular weight polyamides, leading to high tenacity filaments (tenacity 100cN/tex). As an innovation on fiber formation, new technologies producing micro fibers have been developed and reported. Primarily direct spinning and mechanical and solvent splitting produce micro fibers. Electro spinning represents another approach to fiber spinning, when electrical forces on polymer melt or solution surface overcome the surface tension and cause an ejection from an electrically charged jet. The diameter of the fibers produced by this technique is of the order of nanometers. Frequently, there are produced fibers that are electrically charged.

CRYSTALLINE STRUCTUREBoth nylon 6 and nylon 66 are semi-crystalline polymers. These linear aliphatic polyamides are able to crystallize mostly because of strong intermolecular hydrogen bonds through the amide groups (Figure. 3), and because of Vander walls forces between the methylene chains. Since these unique structural and thermo-mechanical properties of nylons are dominated by the hydrogen bonds in these polyamides, quantum chemistry can be used to determine the hydrogen bond potential . The left side of the figure shows hydrogen-bonding planes, and the right side shows the view down the chain axis. For the -form of nylon 6, adjacent chains are ant parallel and the hydrogen bonding is between adjacent chains within the same sheet (bisecting the CH2 angles). For the -form of nylon 6, the chains are parallel and the hydrogen bonding is between chains in adjacent sheets. . In nylon 66, the chains have no directionality. Research results have shown that the stable crystalline structure is the -form comprised of stacks of planar sheets of hydrogenbonded extended chains. It also appears that Young's modulus of the -form is higher than the -form.

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Mechanical, thermal and optical properties of fibers are strongly affected by orientation and crystallinity. Basically, higher fiber orientation and crystallinity will produce better properties. Crystallinity of nylons can be controlled by nucleation, i.e., seeding the molten polymer to produce uniform sized smaller spherulites. This results in increased tensile yield strength, flexural modulus, creep resistance, and hardness, but some loss in elongation and impact resistance. Another important benefit obtained from nucleation is decrease of setup time during processing .

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PROPERTIES OF NYLON 66-Tenacity-elongation at break ranges from 8.8g/d-18% to 4.3 g/d-45%. Its tensile strength is higher than that of wool, silk, rayon, or cotton. - 100% elastic under 8% of extension -Specific gravity of 1.14 -Melting point of 263oC -Extremely chemically stable -No mildew or bacterial effects -4 - 4.5% of moisture regain -Degraded by light as natural fibers -Permanent set by heat and steam -Abrasion resistant -Lustrous- Nylon fibers have the luster of silk -Easy to wash -Can be pre colored or dyed in wide range of colors; dyes are applied to the molten mass of nylon or to the yarn or finished fabric. -Resilient -Filament yarn provides smooth, soft, long lasting fabrics -Spun yarn lend fabrics light weight and warmth

PROPERTIES OF NYLON 6The main difference between nylon 6 and nylon 6,6 is nylon 6 has a much lower melting point than nylon 66. This is a serious disadvantage, as garments made from it must be ironed with considerable care.

Fibre ProductionPolymerisationThe term polymerisation defines the process of macromolecules formation through repetition of basic units: it of course applies only to synthesis fibres. In general, polymerisation reactions are activated and controlled during the process by various parameters, as temperature, pressure, catalysers, reaction stabilizers. The number of repetitive units is termed degree of polymerisation and is a parameter of great significance for fibre properties setting. As the length

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of the single molecules is not constant, but varies according to a statistical model, the degree of polymerisation or the correspondent molecular weight has to be considered as an average value. Depending on the various fibre typologies, the degrees of polymerisation may ran ge from some hundred units in the case of polymers obtained through condensation (PA, PES) to someUnder a production and application point of view, the degree of polymerisation is controlled by measuring following parameters: There are basically two mechanisms of chemical reaction available for the synthesis of linear Polymers:

A.Poly-condensation:

B.Poly-addition:

Fig. Poly-addition

Polymerisation techniquesFrom a processing point of view, the polymerisation can be carried out by mass treatment, solution or dispersion (suspension, emulsion). From the engineering point of view, the process can be:

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discontinuous, where reagents are entirely pre-loaded into the reactor and, as soon as the polymerisation is completed, the products are completely unloaded. The batch technique is used in particular for the production of small lots or of specialty items. continuous, where reagents are introduced from one end and reaction products come out from the other (this process is used especially for large productions). The reaction can also take place within a stationary phase (as typical for poly-additions) or at subsequent stages (as in poly-condensations). Whichever polymerisation method is applied, the reaction products (polymers) can appear as follows: n form of a solution to be conveyed to the spinning department; i in form of a melted polymer to be conveyed directly to the spinning department or to be transformed into grains (chips) for subsequent use ; in form of a suspension, from which the polymer is separated and conveyed to the spinning department; Along with the chemical reactants (monomers and possible catalysts) during the polymerisation stage or anyway in a stage preceding spinning, other additives can be added in order to provide the fibre with certain properties: a product of particular importance is a white dulling agent (titanium dioxide in grains), which is added in small quantities in order to give the fibres a dull appearance, which distinguishes them from the untreated fibres which, owing to their brighter and synthetic appearance, are named bright. Under this point of view, the fibre is termed on the basis of the added quantity of titanium dioxide (dullness degree) as follows: bright fibre: a fibre without or with minimal quantities of titanium dioxide;19 semi-bright fibre: a slightly delustred fibre semi-dull fibre: usually terms delustred fibres with 0,25-0.5% titanium dioxide contents dull fibre: fibre with 0,5-1% titanium dioxide superdull fibre: fibre with 1-3% titanium dioxide

Making nylon-6,6 industriallyNylon-6,6 is made by polymerising hexanedioic acid and 1,6diaminohexane. Because the acid is acidic and the amine is basic, they first react together to form a salt. That is then converted into nylon-6,6 by heating it under pressure at 350C.

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The two monomers can both be made from cyclohexane.

Oxidation of the cyclohexane opens the ring of carbon atoms and produces a -COOH group at each end. That gives you the hexanedioic acid. Some of that can then be converted into the 1,6-diaminohexane.

The acid is treated with ammonia to produce the ammonium salt.

The ammonium salt is heated to 350C in the presence of hydrogen and a nickel catalyst. This both dehydrates the salt and reduces it to the 1,6-diaminohexane.

Making nylon-6,6 in the labIn the lab, it is easy to make nylon-6,6 at room temperature using an acyl chloride (acid chloride) rather than an acid. The 1,6-diaminohexane is used just as before, but hexanedioyl dichloride is used instead of hexanedioic acid.

If you compare the next diagram with the diagram further up the page for the formation of nylon-6,6, you will see that the only difference is that molecules of HCl are lost rather than molecules of water.

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In the lab, this reaction is the basis for the nylon rope trick. You make a solution of the hexanedioyl dichloride in an organic solvent, and a solution of 1,6-diaminohexane in water. You carefully float one solution on top of the other in a small beaker, taking care to get as little mixing as possible. Nylon-6,6 forms at the boundary between the two solutions. If you pick up the boundary layer with a pair of tweezers, you can pull out an amazingly long tube of nylon from the beaker.

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The term spinning defines the extrusion process through bored devices (spinnerets) of fluid polymer masses which are able to solidify in a continuous flow. The spinning process is sometimes designated as chemical or primary spinning to distinguish it from the textile or mechanical or secondary spinning. The polymer processing from the solid to the fluid state can take place with two methods: 1. By melting: This method can be applied on thermoplastic polymers which show stable performances at the processing temperatures (this method is used by 70% of the fibres) 2. By solution: The polymer is solved in variable concentrations according to the kind of polymer and of solvent, anyhow such as to produce a sufficiently viscous liquid (dope) (this method is used by 30% of the fibres). Spinning via melting is definitely preferable as it entails a simple transformation of the physical state, however it can be applied only to polymer having a melting temperature (PA 6 an Pa 6.6, PES, PP), whereas spinning by solution is used in case that the polymers attain a thermal degradation at a temperature lower than melting temperature (cellulose fibres, PAN). This last method is evidently more complicated than melt spinning, owing on one hand to the necessity of dissolving the polymer in a proper solvent, and on the other to the necessity of removing and recovering the polymer after extrusion. In the case of melt spinning, the extruded polymer, owing to its fast cooling, is transformed directly into a filament while keeping substantially unchanged the form of the cross-section resulting from the filament geometry; on the contrary, in the case of solution spinning the extruded filaments are subject to considerable structural changes brought about by the process for solvent extraction from the polymer mass. Solvent removal can take place in two ways:

Dry spinning:Solvent is removed through flows of warm gas suitably directed to the extruded filaments; gas temperature should be higher than the boiling temperature of the solvent, which will be extracted from the filaments, recovered and recycled. Filament solidification proceeds according to the extent of solvent evaporation; it takes place faster on the external yarn layers (thus creating a crust or skin), and successively slows down while proceeding towards the interior. As a consequence of the mass exchange, the original (round) cross-section of the filamentundergoes a contraction, thus generating cross-sections which characterize the various kinds of

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fibres and spinning processes.

Wet spinningThis spinning method is based on the introduction of an extruded polymeric viscose into coagulation baths where the liquor, usually water, behaves as a solvent towards the polymer solvent and as a non-solvent towards the polymer mass. Practically the solvent which is contained in the fibre in amorphous state (gel) is spread towards the liquor and at the same time the liquid of the bath is spread towards the interior of the fibre. The processing speeds are dependent on several parameters, as type and concentration of the polymeric solvent and of the liquor, which bring about structural variations in the fibre. In particular the formation of an outer, gardened and more compact cortex (skin), similarly to what happens in dry spinning, slows down the coagulation mechanism of the inner filament portion (core), thus creating unevenness with a more or less porous structure (voids formation). The fibre cross-sections result more or less modified, from the original round form to a lobated form, with a wrinkled surface.

General flowchart of the spinning processThe flowchart which applies to the various kinds of spinning methods is the following: The fluid polymer mass (melted or solution mass) is guided, through distribution lines, to the metering pumps (gear system), which guarantee a constant flow rate to the spinning positions, composed of a series of filters which purify and distribute the polymer; these are coupled with perforated plates of stainless steel (for melt spinning), but also of precious metals or of vitreous material (for solution spinning). The holes (capillaries), the number of which on the plate varies depending on the kind of fibre and can reach several thousands, can have circular or special cross-sections (shaped or hollow sections).The filaments extruded from the spinnerets, after being converted back to their original state of solid polymer, are interrupted and taken up in suitable packages (bobbins, cans) or conveyed directly to subsequent processing phases. In the case of melt spinning, if the polymer does not derive already in melted state from polymerisation, the fluid polymer mass is obtained through melting of the solid polymer grains (chips). This operation was originally carried out inside containers (pipes) which were

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electrically heated and equipped with grids to separate solid grains from the polymer during melting (grid melting device). The use of such system is at present limited only to few applications and has been replaced by more reliable and efficient devices (screw extruder). The relations which connect some spinning parameters one another (and are calculated for melted polymers) are the following: Polymer flow rate: mF = VF Tsp/10,000 where: mF = polymer quantity for each yarn (g/min) VF = take-up speed (m/min) Tsp = linear mass of taken-up yarn (dtex) If we know the linear mass of the drawn yarn (Td) and the draw ratio R, the relation becomes: mF = VF Td R/10,000 Extrusion speed of the melted polymer VB = 4 mB/d2 VB = extrusion speed at spinneret hole (m/min) mB = polymer quantity per spinneret hole (g/min) d = hole diameter (mm) = density of melted polymer (g/cm3) Spinning ratio Q = VF/VB

Spinning systems for nylon fibres

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fig: Solution spinning

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Fig. Spinning systems for nylon fibres

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Round cross-section of melt-spu thermoplastic fibres

Multi-lobal cross-section of wet-spun viscose

Lobed (dog-bone-shaped) cross-section of dry-spun acrylic fibre

Melt spinning Lobed (kidney-shaped) cross-section of wet-spun acrylic fibre

Fig. 14 Typical cross-sections of fibres produced with different spinning processes.

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Solution spinningRound cross-section of melt-spu Multi-lobal cross-section of thermoplastic fibres wet-spun viscose Lobed (dog-bone-shaped) cross-section Lobed (kidney-shaped) cross-section of dry-spun acrylic fibre of wet-spun acrylic fibre

DrawingThe polymer extruded by the spinnerets in form of filaments has not yet the properties which are typical of a textile fibre: in fact the polymer mass (solidified through cooling or solvent removal) is characterized by a mass of disorderly placed molecular chains (in amorphous state) which provides the material with poor thermal and chemical stability, low resistance to ageing, high plasticity and deformability and consequently insufficient physical/textile properties. If we take natural fibres as models, we need to orientate the molecular chains (orientation phase) in the direction of the fibre axis and at the same time or successively activate or increase the ordered arrangement of the intermolecular structure (crystallization phase). This process can be partly activated during spinning by increasing the ratio between the take-up speed and the extrusion speed (spinning ratio) but, excepted the case of high speed spinning of continuous filament yarns, the process needs to be completed by an additional operation of mechanical drawing. The process entails winding the yarns on rollers or cylinders running at high speed and can be carried out continuously on filaments coming from the spinning room (single-phase process) or on filaments coming from a phase subsequent to spinning (two-phase process). The speed ratio between the delivery or drawing rollers and the feeding rollers is the draft ratio The mechanical configuration of the rotating devices and the filament path are designed in order to ensure the equivalence of fibre speed with the speed of contact organs.

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Pre-orientation and partial Drawing crystallization Fig. Molecule orientation during drawing Feed rollers Drawing zone R=V2/V1 Delivery roller or draft roller Heating plate (if available)

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Fig. 16 Drawing principle Draft ratio levels are variable and depend on the fibre typology, on the production process and on the end-use characteristics: they can fluctuate between values slightly higher than 1 ( 1,2 for traditional cellulose fibres) and max. 10 (for acrylic fibres). Usual ratios for thermoplastic fibres are situated between 3 and 5; higher values identify fibres for technical applications. Optimal conditions for fibre drawing are attained when the molecular chains show high mobility and creep; this result is in practice attained by increasing temperature to levels higher than glass transition and by introducing plasticizers which can make the structure more deformable and can reduce glass transition temperature (generally by acting upon the system water/humidity or using spinning solvents). From an operational point of view, the draft zone can operate at room temperature (cold drawing) or at heated conditions (warm drawing) and consists of rollers, contact plates, heated air chambers or steam chambers and of immersion baths. In order to provide the drawn fibres with thermal stability, usually these fibres undergo also a treatment at temperature higher than drawing temperature, under controlled tensions or in a free state, with the objective of eliminating internal tensions through readjustment of intermolecular chemical links and of the crystallization degree.

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Technologies for the production of continuous filament yarns and of discontinuous fibres (staple fibres)As already mentioned, from a morphological point of view fibres can be divided into discontinuous fibres and into continuous filaments. This distinction applies also to natural fibres, although they have only one single case of continuous filament: silk, which is moreover available in nature only as monofilament and is available in limited quantities. Only with the coming of manmade fibres, continuous filaments took on a great importance by giving rise to innovative transformation processes and application sectors; at present the market of man-made fibres is roughly divided equally between the two forms of fibre. Theoretically, every fibre can originate continuous filaments or staple fibres; actually however production and application reasons have conditioned the use of one fibre form or of the other: elastane is produced exclusively as continuous filament, and nylon mostly in this form; polyester, polypropylene and viscose are produced in both forms, that is as continuous filament and as staple, whereas acrylic is produced almost exclusively in staple form. Although the production principles are identical for continuous filament and for staple fibre, the two processes differ considerably in terms of plant engineering.

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A) Continuous filament yarns B) Tow for staple fibre productio a) mixer b) extruder screw c) main extruder d) hopper e) chip feeding line

f) side extruder g) rotary gear pump h) continuous filter i) distribution line (manifold) k) spinning position l) cooling chamber m) take-up head n) tow for staple production

Fig. 17 Melt spinning lines for continuous filament and staple fibre

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EXTRUDER

ROTARY GEAR PUMP

SPINNING AGGREGATE

CRIMPING CHAMBER

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CRIMPING CHAMBER

Fig. Mechanical elements characterizing a production process

Continuous filament yarnsContinuous filament yarns can be composed of a single filament (monofilament yarns) or of several filaments (multifilament yarns) and are described through abbreviations, the first figure of which indicates the total linear mass (expressed in dtex or, less usually, in den), the second figure indicates the filament number and a third figure if any shows the twists per length unit (turns/m) imparted to the yarn. Monofilaments for traditional textile uses have linear masses ranging from 10 to 50 dtex approximately; monofilaments with larger linear masses find on the contrary use in technical applications and are identified with their thickness expressed by the diameter of the round crosssection (0.062 mm).

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Multifilaments have variable filament number (up to 300 filaments for traditional textile uses, up to 1000-2000 for technical uses and floorcovering) and the linear mass of each filament ranges from 0.4 to 5 dtex. A yarn can be declared as microfilament when its linear mass is lower than 1 dtex; as a rule, the number of filaments in a microfilament is higher than the linear mass of the yarn (e.g.: 200 dtex/220 filaments). The yarn extruded by the spinneret presents smooth and parallel filaments (flat and parallel yarn). Owing to processing and application requirements, parallel filaments are mostly tied together by means of entangling points (entangled yarns) or of twists (twisted yarns); on the other hand there are flat filaments, characterized by rigidity and poor covering power, which can be converted into curly or crimped yarns (textured yarns). The spinnerets which produce continuous filament yarns have usually a number of holes equal to the number of filaments composing the yarn; there are however also some cases in which the spinneret produces several yarns, which are successively wound on separated bobbins (multibundle spinnerets), or cases in which several spinnerets produce a single yarn which is wound on a single bobbin. The general scheme of a spinning line for thermoplastic polymers (Fig. 17A) consists of: one or more units, each composed of a screw extruder distribution system (manifold) spinning head or spinning position metering pumps spinnerets (up to 8 or 8x2 per position) spinning chimneys take-up units (winders for up to 8 bobbins) In a first zone placed upright under the spinnerets, the filaments are struck crosswise by flows of cold and controlled air (cooling zone), to be cooled and solidified; then a second zone follows, where the filaments are assembled, lubricated by contact or spray devices and, if necessary, linked one another through entanglement points (produced by air nozzles) and wound on cylindrical bobbins. The take-up speed plays a role of primary importance in establishing yarn haracteristics. As far as traditional spinning is concerned, spinning speeds vary, depending on the fibre, from 1000 to 1800 m/min; under these conditions, the polymer remains substantially amorphous, scarcely oriented, with high propensity to degradation and ageing, and requires consequently to be quickly (i.e. within few days) processed.

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By increasing spinning speed, the yarn is subjected to an increasing stress (due essentially to air resistance and to force of inertia) with consequent higher level of orientation and crystallization. Therefore, depending on the various speed levels, fibres with different characteristics can be produced; these are identified with English acronyms which are conventionally used to distinguish the single processes: Type of yarn Speed (basis PES) LOY (Low Oriented Yarn) 1000-1800 m/min MOY (Medium Oriented Yarn) 1800-2800 m/min POY (Partially or Pre-oriented Yarn) 2800-4000 m/min HOY (High Oriented Yarn) 4000-6000 m/min FOY (Fully Oriented Yarn) > 6000 m/min

Fig. 19: Basic configurations of different spinning technologies for continuous filament yarns Yarns produced with high speed spinning (HOY and FOY) show some qualitative and technological problems and found therefore up to now a limited diffusion at processing and application level. An advanced form of LOY, which needs to be submitted to a drawing process in order to be usable, is POY, a yarn characterized by about 100-120% elongation at break. This yarn is widely used thanks to its good stability to ageing and, although not directly usable in the production of textile items, to its suitability to intermediate processes which combine a specific process (warping, sizing, etc.)

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with a complementary drawing process; in fact this yarn finds wide use in drawtexturization, but also in draw-warping and in draw-sizing. Yarns originated by a specific drawing process performable either directly in spinning or successively in a separate phase, are named fully drawn yarns (FDY).

Drawing processFlat yarns resulting from a drawing process are named FDY (Fully Drawn Yarns). The drawing process can be carried out with two different techniques: Two-stage-process This technique can be applied to yarns which are not fully drawn (LOY, MOY, POY). With the old traditional plants, the bobbins of LOY yarns were drawn in a suitable department by machines named draw-twisters. On these machines the yarn runs along a vertical path composed by the feeding system (with bobbins in upper position), by the draft zone and by a winding device similar to the one of a ring twister (Fig. 22 a). During the winding on a stiff tube, the yarn is provided with a light twist originated by the rotation of the ring around the spindle. The speed of current machines can range between 600 and 1500 m/min depending on the yarn type, and the weight of the yarn packages (cops) can reach up to 4 kilos; in order to increase productivity and to reduce costs, the machines can be equipped with automatic doffing device. The 1980s recorded the development of a new type of drawing frame (draw-winder), in which the winding on spindles was replaced by a take-up system on bobbins with cross-winding (Fig. 22 b). This system permits a higher winding speed (up to 2000 m/min), the production of packages with higher weight (10-15 Kg) and, from the quality point of view, of a yarn with more uniform properties thanks to a more accurate control of the variations in the winding tensions (the winding frame with spindle winding can cause tension peaks). Yarns wound on spindle present a twist which binds together and protects the filaments; on the contrary the filaments of yarns wound on bobbins are parallel so that, to make up for this deficiency, an intermingling device (a nozzle with intermittent flow of compressed air) placed before the winding device can be envisaged. Polymer chips masterchips (e.g. dyestuffs) Extrusion module Individual metering system on each working position Melted polymer Individual spinning and take-up system 10 position module For some applications as technical uses, additional cylinders (some of which heated) are positioned after the main drawing zone, for the scope of stabilizing the yarn and of fixing a prearranged thermal retraction (Fig. 22 c).

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Fig. 22 Principles of two-phase drawing Single-phase process

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The increase of spinning speeds (4000-5000 m/min) set itself as additional goal, besides rising productivity, the drawing of the continuous filament yarn right during the spinning process (Fig. 19 FDY configurations). The spinning configuration may be modified on the basis of the various fibre typologies and technologies; in any case the yarn needs, before its take-up, to be submitted to a drawing unit Feeding Feeding roller Heating plate (if available) Drawing or delivery roller Cops Bobbins Drawing Stabilization Setting Intermingling nozzle composed of one or more drawing zones placed between godets and moreover of heating sources (the heated cylinders themselves, or steam/air heated tubes, or chambers positioned before or after the cylinders). The requested yarn characteristics (tenacity, elongation, tensile modulus, thermal retraction) can be modified to a large extent through various adjustment possibilities. Usually yarns coming from the same spinning position are drawn by just one drawing unit, and are then again separated during winding on the various packages composing the winding unit. A comparison between the two drawing processes highlights the fact that the single-phase process is an integrated process characterized by high productivity, low labour costs and reduced space requirements, whereas the twophase process presents a simpler technology and higher flexibility in terms of product range extension and of productivity.

Discontinuous fibres (tow and staple fibre)This production takes place in plants which have a conception completely different from those designed for the production of continuous filaments.The basic concept is to obtain from spinnerets a high number of parallel filaments (spinning tow or tow) to be delivered to subsequent textile processes (Fig. 17B). The tow must therefore be considered as an intermediate step in the production line of discontinuous fibres which are designed to feed subsequent textile processes (spinning and nonwoven sectors); it is characterized by a considerable linear mass (up to 150 ktex) and is composed of filaments with the same range of counts as the standard range for staple fibres (from a 0,4 dtex micro-fibre to 17 dtex staple fibre for carpeting). Production lines The transformation process is, excepted variations due to the different nature and typology of the fibre, essentially made up of following processes: spinning, drawing, heat-setting, oiling, crimping, drying, cutting into staple if necessary, baling (in form of tow or of staple fibre). The processes can be carried out by means of production lines which transform without break the fibre delivered by the spinnerets as far as the end-phases (continuous single-phase lines) or by means of lines in which the flow of the fibrous material is interrupted after

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spinning and is fed in a subsequent step (as from drawing) as far as the lines of final conversion (discontinuous two-phase lines). Continuous single-phase lines Continuous single-phase lines are used for the production of fibres, in which the flow speed of the material can be balanced through the various transformation phases; a typical application of these lines is the wet spinning process (for acrylic and viscose fibre spinning), but their use has also been recently extended to the compact spinning process for thermoplastic polymers.The limited spinning speeds of these processes are compensated by the high flow rates of the spinnerets. The final speeds of the lines (composed of 1 or more tows) can reach 50-200 m/min. Discontinuous two-phase lines These lines are used when there are differences of operating speeds between the different phases. This occurs in processes in which spinning speeds are higher than in subsequent processing stages. Such is the case of processes traditionally used for thermoplastic fibres and dry spinning processes, in which spinning speeds (over 1000 m/min for hermoplastic polymers, 400-600 m/min for PAN dry spinning) are higher than speeds of downstream processes (final speeds, resulting from initial speeds and from drawing ratio of about 200-300 m/min). One of the most specific technological differences between staple fibre spinning and filament spinning is to be found in the characteristics of their spinnerets. In fact, in the case of continuous filaments the number of holes per spinneret is relatively low and closely connected with the number of filaments in the yarn, whereas in the case of tow production the spinnerets are bigger and have a higher number of holes. The nature of the material and the configuration of the spinnerets must anyway take into account the rheological properties (viscosity) of the polymer mass as well as the spinning typology. Spinning parameters must ensure an even solidification of the polymer mass after extrusion, without varying textile properties and originating physical imperfections (drops, badly drawn yarn pieces, stuck together fibres, broken filaments, etc.) in the filaments. Concerning wet spinning (PAN, CV), owing to the low extrusion speed and to the coagulation process, spinnerets with round cross-section are usually employed; these spinnerets, which can be made of different materials, show thousands of holes (even more than 100.000). As to spinning from melted polymer, considering the high speeds and the necessity of a quick and uniform cooling of the extruded filaments, different technologies are used which, as far as traditional discontinuous processes, fall within following criteria (Fig. 23): Rectangular spinnerets with lateral cooling flow This structure shows holes positioned on parallel rows of rectangular plates made of special steel, designed in a way that the flow of cooled air comes only from one direction and consequently

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does not hit uniformly the various filament rows; this fact involves a limit in increasing the number of holes; moreover the system does not ensure an efficient removal of spinning steams (monomers). Ring spinnerets (round spinnerets with a large central hole) Cooling is carried out by adjusting air flow (coming from the bottom of the chimney) on the whole filament bundle by means of a flow which is addressed from the outside to the inside of the filament bundle or from its inside to its outside. This system guarantees a more uniform solidification and a more effective removal of steams and impurities, thus enabling a higher production capacity (up to 4,5-5 kg/min). In the case of mass productions, the holes per spinneret attain a number of 5,0006,000 (5,250 holes for standard fibres, 6,000 holes for micro-fibres). The bundles of cooled yarns are successively oiled (by means of spray or roller devices), conveyed in horizontal direction by diverting pulleys, combined with other yarn bundles in order to obtain a tow of larger size (or sub-tow) which, in the case of the two-stage process, is collected into cans; these are placed in a certain number on the creel to feed the drawing line.

Fig. 23 Technologies of staple fibre spinning

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Fabric ProductionFabric Production With Circular Knitting MachinesIntroductionAll over the world, the majority of knit fabrics are manufactured on circular knitting machines. The high performance level of these machines, the different materials and the range of yarn counts that they are able to process, the wide variety of designs and stitches are some of the reasons which have granted circular machines the market leadership in the knitting sector. The variety of knit fabrics that can be manufactured with these machines can meet the needs of a very large end user market; from the traditional outwear and underwear sectors to hosiery, household and car interiors, without forgetting technical textile applications.

Basic Structure of a Large-diameter Circular Knitting MachineCircular knitting machines include a number of fundamental elements, based on similar mechanical principles with some small changes according to the different models:- The machine base. The most recent trend among circular machine manufacturers is to build knitting machines with a solid yet smaller base and an architecture that facilitates access to the machine components for routine operations to be carried out during setting up procedures and production; - The core of the machine, which includes the needle-bed area and all the systems operating during the knitting process. The feed systems are placed along the circumference of the circular needle-bed. Circular knitting machines can be divided into two basic models: in the first one the needle-beds rotate and the cam frame stands still, while in the other one the needle-beds stand still and the cam frame revolves. - The yarn spools holder. If the system is attached to the upper part of the machine, it is a circular rack; if arranged at the machine side, it is a lateral creel. - The yarn feeding system, made up by the yarn feeding unit which must ensure a smooth and steady yarn feeding, and a thread guide system which provides the needles with the yarn necessary for the stitch formation

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.- The fabric take-down and winding system housed in the lower part of the machine; depending on the machine model, the fabric take-down and winding motion can rotate together with the needle-beds or stand still. - The drive, usually an inverter drive, i.e. a motor with electronic variation of speed for optimum acceleration and slow-down ramps and optimum throughput speed in all conditions. Lastly, several machine manufacturers (above all of single-bed machines) tend to build convertible machines, which have the same basic structure and are equipped with special conversion kits which allow the change of stitch formation motions such as cams, cam frames and thread guides to generate different styles. The cost of these machines is extremely advantageous compared to the investment costs necessary for buying a complete series of machines each one producing a single article.

Picture Overall view of a circular knitting machine

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The Yarn Feeding SystemYarn FeedersYarn feeders can be divided into positive or negative types depending on the possibility of controlling the yarn feeding speed and uniformity. A yarn feeder is the negative type when the needle takes the yarn directly from the ackage during the stitch formation step, and the feeding tension of the yarn cannot be controlled.

Picture A negative yarn feeding device

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This feeding technique can generate differences in the yarn length used for stitch formation. This is due to the variable tension of the yarn since a new spool has a certain diameter which gradually reduces as more yarn is unwound and fed into the machine. In addition, the spool can be too hard or too soft. Circular knitting machine manufacturers have eliminated this problem by implementing two distinct solutions: 1. The motorised yarn accumulator levels off the yarn tension since when rotating, it accumulates a certain quantity of yarn on a constant-diameter pulley and then stops. The yarn wound on the accumulator is then conveyed to the thread guide always maintaining the same tension. The machine takes up the yarn, gradually emptying the accumulator, which is then restarted automatically to replenish its yarn reserve. This solution is particularly indicated when the same type of feeding technique cannot to be applied to all the feed systems due to the structure of the knit stitches. Therefore, yarn accumulators are mainly used on machines for the manufacturing of fabrics of pre-set length, or also of continuous cloths with Jacquard patterns. 2. Positive feed systems control the tensions of the yarn fed by means of a drive wheel or a drive belt system. The drive wheel systems, which in the past were much more widespread than today, consist of two conical toothed wheels. The thread passes between the two wheels and the quantity of yarn can be adjusted by approaching or withdrawing the wheels. This positive system grants a smooth feeding of the yarn on all the feed systems. Today, the belt system has by far become the most common positive feeding system. The belt makes the spool rotate, and the number of rotating spools corresponds to the number of feed systems. By adjusting the belt RPM, the quantity of thread can be increased or reduced. This system grants an accurate control of the yarn tension (picture 80).

Stitch Formation MotionsCircular knitting machines, both the single or double-bed types according to our initial classification, can incorporate various stitch formation motions depending on the machines technical features.

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Take-down and Winding MotionsThe fabric take-down and winding motions have been designed to facilitate stitch knock-over and fabric take-down procedures. The take-down and winding functions are kept separated in order to allow a smooth running of the machine and avoid possible fabric distortions.

Take-down MotionThe take-down motion consists of 2 or 3 rollers placed beneath the cylinder. In the simplest system configuration (i.e. the two-roller) the fabric passes between two rollers that stretch it by rotating in opposite directions. Anyway, the best system is the three-roller take-down motion which pulls the fabric without slipping and without exerting too much pressure that could damage the fabric. From a mechanical point of view, a take-down system can be either equipped with a swivelling arm or with a lever and spring mechanism. Modern take-down systems are motorised and the latest models also incorporate an electronic control.

Picture 97 A 2-roller take-down motion

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Circular knitting machines pose some problems as regards the winding of the fabric, as the fabric itself is delivered in tubular form and must be spread flat prior to winding. The spreading of the tubular fabric generates some distortions because of the different distances between the various zones of the tubular fabric emerging from the take-down system and the same zones wound on the fabric roll. These differences reflect into uneven winding tensions (the tension is lower in the fabric centre and higher at its edges). To avoid these problems, a metal frame called spreader has been incorporated before the fabric winding system. The spreader increases the width of the tubular fabric by giving it an almost circular shape, equalising the distances between the various zones of the fabric and the nip line of the winding system.

Picture Take-up and winding motion with fabric spreader A special take-down system has been designed for variable needle-bed machines since these machines do not use the whole needle-bed. This special take-down motion features independent rollers to adjust the tension during the knitting process and differentiate the tension between the central part of the cloth and the edges. On the most recent machines, it is possible to set up to 99 different tension values.

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Winding MotionThe fabric winding motion is provided with a clutch. In this way, to grant a steady peripheral speed, the angular speed of the winding roller can be gradually reduced as the diameter of the fabric roll increases.

Picture 99 Clutch connection between the winding roller and the take-down motion rollers At the ITMA 99 fair in Paris, an Italian manufacturer of circular knitting machines exhibited an innovative winding system mounted on a 30-inch circular knitting machine. In practice, this machine features an open base that allows the fabric cutting and opening on only one side prior to winding. Obviously, in order

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to allow the take-up of the open fabric, the width of the winding roller must be twice the width of a standard one. The take-up step is carried out on the already opened fabric, and the edges of the fabric are kept tensioned by means of two rollers with worm-screw profiles. Thanks to this innovative solution, no further rollers squeeze the knit fabric. This avoids the problem of central marks which is particularly serious on elastane fibres.

Control and Monitoring SystemModern circular knitting machines feature on-board computers (CPU) complete with a display and a keyboard to control and monitor the most important functions: - speed - number of machine revolutions - working hours - causes of machine stops - detector of the yarn length fed into the machine On modern microprocessor-controlled machines, the LCD display is equipped with an alphanumeric keyboard for entering the operators settings. The whole system is controlled by an electronic circuit which signals the status of the machine and the possible causes of machine stops by means of flashing lights. All the electronic control components are accommodated in a cubicle linked with the machine by special connectors. Sometimes, together with these functions, the machine can also carry out needle selection procedures by retrieving the information saved on floppy disks or by means of a direct connection to a dedicated CAD system.

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Picture Control unit

Stitch Formation on Dial-cylinder Knitting MachinesThe stitch formation cycle on double-bed dial-cylinder machines is similar to that of flat knitting machines. The needles start rising from their lowest position; the previous stitch slips along the needle stem and opens the latch; when the needle reaches its highest position the previous course is on the stem, beyond the open latch. The needle starts lowering and the thread guide feeds the thread for the new stitch which is seized by the hook; at the same time the previous one slips forward on the stem and closes the latch. Once the previous course has been knocked over on the new course, the cycle is completed. The same movements are carried out by the needles in the dial. Here, however, the dial needles move on a horizontal plane, so instead of raising and lowering movements, we will have forward and backward needle movements.

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Picture The various steps of stitch formation on dial-cylinder knitting machines Several machine models allow the variation of the stitch knocking over in order to have either a simultaneous or a differentiated knocking-over operation. In the former case, the needles of the cylinder and the needles of the dial form the stitch simultaneously; in the latter case, by varying some controls or by racking the dial by five of six needles with respect to the cylinder, it is possible to knock over first the needles of the cylinder and then the needles of the dial. With the simultaneous knocking-over technique, the resulting fabric will be more consistent, soft and stretchable since the two series of needles can take up the quantity of thread necessary to form the stitch. On the contrary, with the differentiated knocking-over technique, in order to take up the quantity of thread necessary for the stitch formation, the needles in the dial have to make the thread slip with respect to the needles of the cylinder that have already been lowered. In fact, it is easier to take up part of the thread from the stitches already formed on the cylinder.

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In this way, the fabric formation will require less thread, resulting in a denser and less stretchable construction. On dial-cylinder circular knitting machines it is also possible to transfer the stitches from a needle-bed to the opposite one to create complicated design patterns. The stitch transfer is carried out usually from the cylinder to the dial to exploit the wider possibilities for selection offered by the cylinder needle bed. Obviously, the stitch transfer in the opposite direction is also possible. To carry out the stitch transfer on mechanically controlled machines, it is necessary to replace some of the knitting feed systems with special feed systems equipped with transfer cams on the cylinder and receiving cams on the dial. In general, there is one transfer system for every two knitting feed systems. Obviously, the replacement of these knitting feed systems with transfer systems causes a reduction of output rates. Besides the transfer cams, it is also necessary to provide the machine with special needles with opening spring, like that of flat-bed knitting machines.

Rib-stitch MachinesRib-stitch machines constitute the most typical category of double-bed machines. The dial needles of rib-stitch machines are arranged in staggered position with respect to the cylinder needles. These machines are mainly used for manufacturing continuous tubular fabric with rib-stitch or derived patterns. Manufacturers offer rib-stitch machines in a wide range of models with diameters up to 40 inches; the most common models are however the 30, 34 and 36 inches, with gauge from E 10 to 28. The models with multi-track selection feature up to 5 tracks on the cylinder and up to 2 tracks on the dial.

Interlock MachinesInterlock machines are dial-cylinder machines of special design. In fact, the cylinder needles and the dial needles are arranged in front of each other. Obviously, in order to achieve different needle motions, the needles themselves must be of different types: on interlock machines, needles are generally arranged in such a way that a short needle is alternated with a long one in the cylinder, and a long needle is alternated with a short one in the dial. To drive short and long needles, two cam tracks are necessary on both the cylinder and the dial. On one feed system, the short needles of a needle-bed and the long needles of the opposite one operate alternately and form a half-course of rib stitches; in the next feed system, the needles operate inversely and form a second half-course of rib stitches

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interknitted with the previous one. In interlock fabrics, a knit stitch course is therefore made up by two interknitted halfcourses of rib stitches. Interlock machines, built mainly in the 30-inch diameter version, and E 18 to E 32 gauges, feature a huge number of feed systems (up to 108) and are designed mainly for the production of cloths with interlock patterns.

Picture Arrangement of needles and tracks on interlock machines

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Fabric Production With Projectile weaving machineweaving Pretreatments: WINDINGUnevenness in traditionally spun staple yarns is a natural phenomenon usually induced by the process of manufacturing (spinning). Although with modern process controls and machines many imperfections in the spun yarns can be controlled, some still remain in the final yarns. Most common of all imperfections are thin or weak places, thick places, slubs, neps, and wild fibers, as shown schematically in. During the subsequent processes of winding, warping, and slashing, not all but some of these imperfections create obstacles to steady and smooth working. Therefore, it is important to classify, quantify, and remove those imperfections which may cause the interruption of the operation In other words, only objectionable faults need to be removed for trouble-free processing of the yarns. The ring-spinning operation produces a ring bobbin containing just a few grams of yarn which is unsuitable for the efficiency of further processing, such as warping, twisting, and quilling. This necessitates the preparation of a dense and uniform yarn package of sufficiently large size which can unwind in the subsequent operations without interruptions. The packages prepared for warping are normally cross-wound, containing several kilograms of yarns. This implies that a number of knots or splices are introduced within each final package. Bear in mind, each knot or splice itself is an artificially introduced imperfection; therefore, the size of this knot or splice must be precisely controlled to avoid an unacceptable fault in the final fabric. In modern winding machines, knots and splices are tested photoelectrically for size, and only acceptable knots and splices are allowed to pass on to the winding package. In modern spinning processes, such as open end, friction and air jet, the spinning process itself produces a large cross-wound package, thus eliminating the winding operation. Nowadays splices are used on all the spinning systems including ring, open end, and air jet for repairing ends down during spinning. In the absence of winding, it is pertinent to note that the yarn spun on such modern spinning systems must have no or only a small number of

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objectionble imperfections. In most modern spinning machines, the manufacturers have incorporated devices that continuously monitor the quality of the yarn being spun, thus assuring fault-free spun yarns.

Functions of the Winding OperationImportant functions of the winding operation are 1. Clearing of yarn faults 2. Making larger wound packages 3. Preparing soft packages for dyeing

Knotting/SplicingThe process of piecing (joining) two yarn endsresulting from yarn breaks, removal of a yarn defect, or due to the end of the supply packagehas received considerable attention in the past two decades. An ideal yarn piecing would be one which can withstand the subsequent processes without interruption and which does not lead to any deterioration in the quality of the finished product . The yarn joining or piecing technique should be suitable for all fiber types irrespective of yarn structure and linear density. Earlier attempts in this area were directed to tying two ends by a weavers knot or fishermans knot such that the ends do not slip apart. However, the size of the knot, which depends on the type of knotter and the linear density of the yarns, would normally be two to three times the diameter of the single yarn, leading to a characteristic objectionable fault in the finished product. Knots have a detrimental effect on quality; they are obstructive because of their prominence and so frequently cause breaks due to catching in thread guides or even being sheared off. This leads to time-wasting stoppages of the machinery during warping, sizing, and weaving. Due to the above-mentioned drawbacks of knotted yarns, knotless yarn joining methods have received considerable attention by researchers

WARPINGWarping is a process of transferring yarn from a predetermined number of single-end packages, such as cones or cheeses, into a sheet of parallel yarns of a specified length and width. The individual warp yarns are uniformly spaced across the whole width of the beam. In warping, the sheet of parallel yarns is wound onto a flanged beam called a warpers beam . The function of warping is primarily to transfer large lengths of yarns from a number of large wound packages to a warpers beam containing a predetermined number of yarn ends (threads), so that it

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runs without interruption at a high speed. Removing faults from yarns during warping is not recommended because it affects the efficiency of the process. A single break makes several hundred other good warp yarns inoperative, thus affecting productivity

Warping SystemsThere are two basic systems of warping, namely, the direct system and the indirect system, as shown in. In the direct system the warp yarns from a creel are wound directly onto a flanged warpers beam. This system is most widely used for mass production of warpers beams containing only one type of warp yarns. Because of the difficulties involved in setting up a pattern of different types and colors of warp yarns, the direct system is not normally used for the preparation of a patterned warpers beam [1]. Any pattern that is required to be produced is adjusted by combining beams of different colors during the slashing operation. The cumbersome work of setting the pattern at the slashing stage is time intensive and inefficient. In the indirect

Fig. 4.12 Direct warping system. (Courtesy of West Point Foundry and Machine Company.)

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number of these beams is placed at the back of the sizing machine. The type of creel, tensioner, warp stop motion, head stock, and control devices on warping machines may vary depending on the manufacturer; however, their basic functions remain the same. Indirect System Unlike the direct system, warping and the preparation of a weavers beam takes place on the same machine but in two consecutive steps. In the first step, the warp is prepared in sections on a large drum with one conical end, as shown in Then the rewinding of the entire warp sheet from this drum to a weavers beam is done in the second step. The operations in the first and the second step are commonly known as warping and beaming, respectively. For preparing the sections on a drum, the warp yarn is withdrawn from the creel through a tension device and stop motion (similar to the one described in the previous section) and is in turn passed through a leasing reed.

Fig. 4.15 Sectional warping and beaming machine. (Courtesy of Sucker-MullerHacoba GmbH & Co., Germany.) Then all Jthe sectional warps are condensed into a section of the desired width by passing them through a V-shaped reed guide and over a measuring roller to the rum. The density of the warp in a given section is the same as the number of ends

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per unit width required in the weavers beam for producing a fabric with a given color pattern and specification. The length of each section is generally equal to the length of the warp required in a weavers beam plus due allowance for the waste in the process. One end of the warping drum is conical shaped, as shown in Fig. 4.15. This is necessary for providing support to the outside ends of the first section to prevent the yarns from sloughing off at the end of the drum.

SIZINGThe primary purpose of sizing is to produce warp yarns that will weave satisfactorily without suffering any consequential damage due to abrasion with the moving parts of the loom. The other objective, though not very common in modern practice, is to impart special properties to the fabric, such as weight, feel, softness, and handle. However, the aforementioned primary objective is of paramount technical significance and is discussed in detail herein. During the process of weaving, warp yarns are subjected to considerable tension together with an abrasive action. A warp yarn, during its passage from the weavers beam to the fell of the cloth, is subjected to intensive abrasion against the whip roll, drop wires, heddle eyes, adjacent heddles, reed wires, and the picking element [12], as shown in. The intensity of the abrasive action is especially high for heavy sett fabrics. The warp yarns may break during the process of weaving due to the complex mechanical actions consisting of cyclic extension, abrasion, and bending. To prevent warp yarns from excessive breakage under such weaving conditions, the threads are sized to impart better abrasion resistance and to improve yarn strength. The purpose of sizing is to increase the strength and abrasion resistance of the yarn byencapsulating the yarn with a smooth but tough size film. The coating of the size film around the yarn improves the abrasion resistance and protects the weak places in the yarns from the rigorous actions of the moving loom parts. The functions of the sizing operation are 1. To lay in the protruding fibers in the body of the yarn and to cover weak places by encapsulating the yarn by a protective coating of the size film. The thickness of the size film coating should be optimized. Too thick a coating will be susceptible to easy size shed-off on the loom.

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Fig. 4.17 Parts of the loom and major abrasion points.

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2. To increase the strength of the spun warp yarn without affecting its extensibility. This is achieved by allowing the penetration of the size into the yarn. The size in the yarn matrix will tend to bind all the fibers together, as shown in. The increase in strength due to sizing is normally expected to be about 10 to 15with respect to the strength of the unsized yarn. Excessive penetration of the size liquid into the core of the yarn is not desirable because it affects the flexibility of the yarn. 3. To make a weavers beam with the exact number of warp threads ready for weaving. illustrates various possible conditions that may occur in practice depending upon the properties of the size employed. This emphasizes the importance of an optimal balance between the penetration of the size into the yarn and providing a protective coating around the yarn, as shown in Fig. 4.19d. The flow properties of the size liquid and the application temperature have important effects on the distribution of the size within the yarn structure. More size at the periphery of the yarn will tend to shed off on the loom under the applied forces because the size is not well anchored on the fibers. Too much penetration, as shown in Fig. 4.19a, may leave too little size around the yarn surface to protect it against the abrasive action. To rectify such a condition, a higher size add-on is required to provide the required protective surface coating [13].

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Fig. Head end of a sizing machine. (Courtesy of West Point Foundry and Machine Company.)

Preparation of weaving machinesTo obtain satisfactory weaving performance, it is essential to have not only a correct yarn preparation, but also an efficient organization which permits to have warps available at the right moment, thus avoiding any dead time with style or beam change. All these prerequisites aim at ensuring to the weaving mills a sufficient flexibility and at permitting them to cope promptly with a variable market demand. Currently several weaving mills have installed weaving machines which enable to perform the quick style change (QSC), leading to a considerable reduction of the waiting time of the machine. The following chart presents the possible alternatives for the preparation of the weaving machine: Changing style means producing a new fabric style, weavers beam changing means going on weaving the same fabric style just replacing the empty beam with a full beam of same type. Drawing-in consists of threading the warp yarns through the drop

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wires, the healds and the reed (fig. 28). Depending on the styles of the produced fabrics and on the companys size, this operation can be carried out manually, by drawing-in female workers

Fig. 28 Drawing-in operating in pairs (a time consuming activity which requires also skill and care), or by using automatic drawing-in machines.

piecing-upThe piecing-up of the warp yarns (Fig. 32) permits to the weaving mills which are in a position to use it (not many mills at the moment) to simplify and speed up considerably the loom starting operations in case of warps which were drawn-in or tied-up outside the weaving machine. The warp threads are laid into a uniform layer by the brush roller of the piecing-up machine and successively pieced-up between two plastic sheets respectively about 5 cm and 140 cm wide, both covering the whole warp width. The plastic sheet can be inserted into the weaving machine simply and quickly, avoiding to group the threads together into bundles; the threads are then pieced-up on the tying cloth of the take-up roller. If a new drawing-in operation is not necessary (this expensive operation is avoided whenever possible) because no style change is needed, the warp is taken from the beam store and brought directly to the weaving room, where it is knotted on board the loom to the warp prepared with the knotting machine.

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Fig. -Piecing-up

Projectile weaving machinesThe projectile weaving machine made its appearance in the market at the beginning of the 50s and is today still used in the whole world. Thanks to its steady renovation and to the use of advanced electronic systems as well as of microprocessors for the supervision and the control of the various devices, this machine is characterized by a good productivity level (450 rpm and 1050 m/min of inserted weft) and by high operational reliability. It is established especially in the field of machines with high reed width.

General operationIn this weaving machine the weft insertion is carried out by small clamp projectiles which number depends on the weaving width and which with their grippers take out the weft yarn from big cross-wound bobbins and insert it into the shed always in the same direction. The projectiles work in sequence, that is they are launched in succession. They run therefore one after the other, describing in the space a continuous, endless route, as if they would be stuck on a conveyor belt. The first projectile takes and holds in its back the weft in form of a tail; then, pushed by the release of the projectile thrower, it passes through the shed and deposits the weft inside the warp; subsequently the projectile falls and is collected by a device which, by passing under the array of the warp threads, takes it at reduced speed

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back to the starting point. Here the projectile goes up to take up a new weft; meanwhile the other projectiles have run after each other making the same operation. shows the projectile conveyor chain (shuttle return chain), the projectile (shuttle) with its back clamp to seize the yarn (thread grippers), the cutting tool (scissors) to separate the inserted weft from the bobbin and the strap which, through twisting, launches the projectiles (torsion rod).

Projectile guideThe limited weight and the reduced volume of the projectile make a projectile guide necessary. The projectiles therefore do not come into contact with the threads, but run inside a sort of channel composed of the thin prongs of a rake, which form reminds a semiclosed hand. This rake goes up from under the threads at the moment of the projectile launch and has of course to fall back lowering itself at the slay stroke. To enable this movement, the rake is secured on the slay and is positioned very close to the reed; the rakes laminas are not in contact with the warp, or touch it very lightly because the reed opens them the way. The latest models of the projectile machine have been equipped with new types of guide dents, which are divided and placed in alternate position, in order to reduce the stress on weft and warp threads. This permits to use in warp even very delicate yarns as for instance untwisted or entangled yarns and at the same time to cope with high quality requirements.

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Fig: Projectile guide

Projectile launching mechanismThe operational principle of the launching mechanism is the following (fig. 48 and 49): a torsion bar 2 is anchored, at one side, to the fixed point 1, whereas the free end is connected by a toothed groove to the percussion shaft 3. The percussion lever 9, which is fixed to the percussion shaft 3, follows per force the movements of this last and consequently of the free end of the torsion bar 2. During its rotation, the cam 8 shifts the knee-joint lever 4+5, so that the torsion bar 2 is put under tension by the percussion shaft 3 and the percussion lever 9 is put in launching position (the scheme shows the launching mechanism with the torsion bar in the phase of maximum tension). The torsion bar 2 remains under tension until the roller 7 slides along the bend of lever 5. The particular shape of this lever makes so that the roller, when leaving it, presses its end, thus giving the starting point to the torsion bar for the articulation of the knee-joint lever 4+5. Subsequently the torsion bar 2 returns suddenly to its rest position imparting a strong acceleration to the projectile 11 through the percussion shaft 3 , the percussion lever 9 and the percussion element 10.

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Fig. 48 - Projectile launching mechanism

Fig. 49 Loading of the torsion bar: a) torsion bar 2 in rest, knee-joint lever 4+5 in articulate position; b) loading phase; c) torsion bar in tension and kniejoint lever in stable position, before the launching control by roller 7.

The oil brake 6 serves to damp the stroke. The projectiles stroke time, that is the insertion time, is adjusted by modifying the torsion angle of the bar through an angular shift of the anchorage point, which has proper adjustment windows.

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Insertion cycle of the projectile machineThe schemes in Fig. 50 show the insertion cycle of the projectile machine: a) The projectile 1 is put in launching position; the weft is hold at its end by the weft carrier 2 and is controlled by the weft tensioner 3, by the weft brake 4 and by the eyelet 7 situated in proximity of the feeding bobbin 8; b) The weft carrier 2 gets open after the projectile clamp has got hold of the end of the weft thread; c) The projectile 1 is launched and crosses the shed dragging with itself the weft, while the weft tensioner 3 and the weft brake 4 operate in a way as to minimize the stress on the yarn (the critical phases are particularly the initial acceleration phase and the final stop phase in the collector box); d) The projectile 1 on the one hand and the weft carrier 2 on the other take up the right position to build up the selvedge, while the tensioner arm opens to adjust the weft tension; e) The weft carrier 2 closes while the selvedge clamps 5 get hold of the weft thread on both sides and the projectile clamp is opened to release the weft end; f) The thread is cut by the scissors 6 on the launching side, while the projectile 1 is placed in the transport chain;

Fig. 50 Insertion cycle of the projectile machine

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g) The weft is beaten by the reed, while the weft carrier 2 moves back to its initial position and the weft tensioner 3 opens further to recover the thread piece and to keep it under tension. The projectile is brought back to the launching zone; h) The selvedge needles 9 insert the weft ends into the subsequent shed (tuck-in selvedge), while a new projectile is placed in launching position.

Electronically controlled projectile brakeThe present machines have the projectile brake adjusted by a microprocessor, and this permitted to increase the efficiency rate and to reduce the maintenance costs. The electronically controlled brake has the function of stopping the projectiles in the correct position, without any need of manual intervention (contrarily to previous mechanism). This result is obtained by means of a controlled double upper brake lining and of a lower fixed brake lining (Fig. 51 and 52). The mechanism works as follows: the sensor 1 and 3 detect the position of projectile 4 inside the collector mechanism and communicates it to a microprocessor which, on the basis of the received information, transmits a corresponding order to the stepping magnet 14. This last operates on a wedge-shaped guide element 13 which, by shifting the upper bracket lining 8, modifies the braking intensity. The sensor 2 controls instead the timely arrival of the projectiles in the collector mechanism. Three cases are possible: A) Position I (normal projectile position): the control co-ordinates S of sensors 1 and 3 are covered by the projectile; B) Position II (projectile too far penetrated / insufficient braking): the control coordinate S of sensor III is not covered; C) Position III (projectile insufficiently penetrated / excessive braking): the control coordinate S of sensor 3 is not covered. In the first case the microprocessor does not answer; in the second and third case, it causes respectively the closing and opening of the brake, thus controlling the number of steps necessary to bring the projectile again to normal position.

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Fig. Projectile brake

Fig. Brake adjustment

Colour selectorIn the multi-colour weaving machine, the weft carrier is controlled by a turret loader (fig. 53); as it is a 4-colour system, 4 weft carriers will be mounted on the loader. The weft carrier loader is driven by a conical gear controlled by a toothed quadrant; when the quadrant moves upwards or downwards, the loader is pushed in the opposite direction and the extension of the oscillation defines which carrier to take position opposite the clamp of the projectile to be launched. A block ensures that the turret loader remains in the selected position until further order. In the case of cam-controlled heald frames, the colour is selected by a punched card special unit. If the heald frames are driven by a dobby or by a Jacquard machine, the gear which drives the toothed quadrant is integrated in the dobby or in the Jacquard machine. The modern selectors are controlled by servomotors and the weft sequence is programmable.

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Fig. Colour selector

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How to Dye Nylon?Dyes for Polyamide FibersWith the exception of the aramid fibers, the polyamides dye readily with a wide variety of dyes. Since the polyamides contain both acid carboxylic and basic amino end groups and have a reasonably high moisture regain, the fibers tend to dye like protein fibers such as wool and silk. Since the molecular structure is somewhat more hydrophobic, more regular, and more densely packed in the polyamides than in protein fibers, they also exhibit to some degree the dyeing characteristics of other synthetic fibers such as polyesters and acrylics. Due to their highly regular molecular structure and dense chain packing, the aramid fibers resemble polyester and are dyed only by small dye molecules such as disperse dyes. Polyamides such as nylon 6, 6,6, and Qiana can be readily dyed with dyes containing anionic groups, such as acid, metallized acid, mordant dyes, and reactive dyes and with dyes containing cationic groups such as basic dyes. Acid dyes on nylon can be mordanted effectively for additional fastness; however, the colorfastness of basic dyes is poorer and more difficult to stabilize by mordanting. Vat and azoic dyes can be applied to nylons by modified techniques, and polyamides can be readily dyed by disperse dyes at temperatures above 80C. Aramids can only be dyed effectively with disperse dyes under rigorous dyeing conditions. The biconstituent fiber of nylon and polyester can be effectively dyed by several dye types due to the nylon component, but for deep dyeings disperse dyes are preferred. Nylon 6 and 6,6 are produced in modifications that are light, medium, or deep dyeableby acid dyes or specially dyeable by cationic dyes.

PRE-DYEING TREATMENTSThe pre-dyeing stage includes a series of operations that prepare the textile product for subsequent finishing treatments such as dyeing, printing and finishing. These operations vary according to the type of fibre on which they have to be carried out, to the structure of the textile product (staple, top, sliver, yarn, fabric) and also depend on the subsequent treatments to be carried out, which may change according to various factors such as market demands, customer requirements, staff experience, and availability of machines. The pre-dyeing stage includes for example desizing, singeing, mercerising, scouring, and bleaching. Each process varies according to the processing conditions and the above-mentioned specific situations. Some of these processes (for example bleaching and mercerising) can

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be considered either preliminary operations or finishing treatments; this depends on the type of the downstream processes to be carried out on yarns or