12.7 Synthetic fibres

12.7.1 Introduction polymeric materials is specified using the count; in other words the mass of the thread relative to a given Obtaining more valuable products from cheap length of it. The mass in grammes of 9,000 m (1,000 materials has always been one of mankind’s goals. yards) of thread is known as the denier count or Enormous interest was therefore aroused when Hilaire denier; another way of describing the thickness of a Bernigaud de Chardonnet presented the first samples continuous thread is to specify the tex, in other words of his at the International Exhibition in Paris in the weight in grammes of 1,000 m of yarn (or the 1889; as shiny and silky as natural silk, this was the decitex, dtex, referring to 10,000 m of yarn). In first artificial fibre ever made by man. addition to the fracture load and (percentage) Although as early as 1913, 1931 and 1932 deformation at break, other important mechanical processes to obtain filaments from poly(vinyl properties of fibres are their tenacity, or better, the chloride) and threads from poly(vinyl alcohol) and maximum energy that they can absorb without polystyrene were patented in Germany, the era of breaking, and resilience, or the maximum energy that synthetic fibres began in 1935 at the laboratories of they can absorb without suffering permanent the DuPont Experimental Station, Pure Science deformation. Section, Wilmington, Delaware, (USA). Here, Gerard The development of synthetic fibres progressed Berchet, one of Wallace Hume Carothers’ assistants, side by side with that of organic chemistry, and obtained just over 10 g of polyhexamethylene especially petrochemistry, which, with some extremely adipamide, subsequently commercialized in 1938 with rare exceptions, provides the base compounds for the the name , the first totally synthetic industrial synthesis of monomers. In 1936, ICI (Imperial fibre. Chemical Industries) patented the manufacture of In theory, all organic polymeric materials fibres from polyethylene in Great Britain; in 1937 the consisting of linear macromolecules with a sufficiently first polyurethane bristles were made; in 1938 Paul high molecular weight can be turned into fibres, in Schlack of IG Farbenindustrie (Germany) synthesized other words long filaments whose axial ratio tends to poly(e-caprolactam), whose fibres were infinity. However, only a small number of these commercialized in 1943 with the name Perlon; in provide filaments with physical and mechanical 1941, John Rex Whinfield and James Tennant Dickson properties allowing for practical applications. The of the Calico Printers Association of Manchester arrangements in which the fibres are found, (Great Britain), synthesized polyethylene independently of their origin, are the single fibre, a terephthalate, whose fibres were commercialized with group of several single fibres to form a thread, and the the names Terylene (ICI, Great Britain), Dacron interlacing of numerous threads to form textiles. A (DuPont, USA), Terital (Montecatini, Italy); in 1942 it characteristic property of any fibre, in addition to its was discovered that N-dimethylformamide was a chemical, physical and mechanical properties, is its solvent of polyacrylonitrile, thus making it possible to size perpendicular to its axis or to that of a group of obtain fibres commercialized by E.I. DuPont de fibres forming a thread. In practice, the radial Nemours (USA) with the name Orlon; in 1960 the dimension, and thus the thickness, of all continuous same company commercialized the new elastomeric fibres including silk and those made of synthetic polyurethane fibre with the trade name Lycra. Also in

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1960, Polymer of Terni (Montecatini group) perfected not unlikely that third generation fibres, given their the fibres obtained from isotactic polypropylene better mechanical performance, will replace metals in (Meraklon); in 1971 (E. I. DuPont de Nemours, many of their applications in the not too distant future. USA) entered the market, the first of the polyaramids, Synthetic fibres can be classified according to the obtained with the interfacial condensation of polar functional group repeated in their chain (for terephthalic acid and p-phenylenediamine, giving those made by polycondensation) or on the basis of the fibres with exceptional mechanical and thermal structural unit for those made by addition properties. polymerization. They are listed below in chronological Today, synthetic fibres are not merely an order of synthesis: alternative to natural and artificial fibres, but form • Polyamide fibres, obtained by condensation classes of high performance materials which play an polymerization and characterized by the regular extremely important role in the field of high recurrence along the macromolecular chain of the technology. The 5 denier of the first nylon filaments amide group NHCO. These include aliphatic have fallen to today’s 0.5 for polyamide or polyamides such as 66 (Nylon 66), 6 (Nylon 6, monofilaments; fabrics made from these are superior Perlon) 11 (Nylon 11, Rilsan) and others of minor in appearance, softness and sheen to those in natural interest, and aromatic polyamides (polyaramids). silk (2 denier). These fibres are known as high added • Polyester fibres, obtained by condensation value fibres, as are those used to make special purpose polymerization and characterized by the regular fabrics (thermochromic fabrics which change colour recurrence along the macromolecular chain of the depending on temperature, photochromic fabrics ester group COO. In addition to polyethylene which change colour depending on light, iridescent terephthalate (Terylene or Dacron), they include fabrics which change colour depending on how they the fibres obtained from wholly aromatic are hit by light, etc.). Also worth remembering are the . high technology fibres deriving from the application • Polyvinyl fibres, obtained by addition of the latest developments in the science and polymerization and characterized by a structural technology of fibre manufacturing (biodegradable unit deriving from vinylic or vinylidenic fibres for sutures; fibres which absorb and accumulate monomers; of these fibres the most important are solar energy such as Solar-a of 1988, made by the those made from acrylonitrile (Orlon), from vinyl Japanese firm Unitika, and similar products made by chloride (Leavin, Thermovil, Movil, etc.) and from Descente of Japan, widely used for sports wear; fibres tetrafluoroethane. for haemodialysis, fibres for the oxygenation of blood • Polyolefin fibres, obtained by addition in the artificial respiration machines known as polymerization, such as polyethylenes from mechanical lungs; fibres used for fabrics to make ethylene and polypropylenes (Meraklon) from space suits for extra-vehicular activities, fibres which propylene. absorb humidity and sweat for sports wear, fibres with • Polyurethane fibres, formally obtained by low friction with air, etc.) and finally superfibres, in condensation polymerization and characterized by other words fibres with exceptional mechanical the regular recurrence along the macromolecular properties (elasticity coefficient over 55 GPa and chain of the urethane group OCONH (Lycra). tenacity above 2.5 GPa), such as those made of high • Carbon fibres, included in this classification as tenacity polyethylene, para-aramids, and derivatives of polyacrylonitrile. polyacrylonitrile carbon. Natural fibres (and their derivatives) can be considered first generation fibres. Synthetic fibres 12.7.2 Polyamide fibres (aliphatic polyamides, polyesters, polyacrylonitrile, etc.), which appeared between the 1930s and 1960s, Polyamide fibres are generally known as nylon, the are second generation fibres, created to replace first trade name of the first wholly synthetic textile fibre of generation fibres. Today’s high performance fibres industrial importance. They are obtained with the (polyethylene, polyaramide, polyarylate, carbon fibres, condensation polymerization of aliphatic or aromatic etc.), which do not represent an alternative to natural diamines and aliphatic or aromatic organic diacids, or fibres as did those of the second generation, can be with the ring-opening polymerization of w-aminoacids classified as third generation fibres. These are used (Nylon 11 and Nylon 6). Polyamides are when fibres with low density, excellent mechanical conventionally named according to the number of performance and heat resistance are required (in carbon atoms in the diamine and the diacid, or the sectors such as sport, transportation, space technology w-aminoacid alone, and appear as corneous solids, etc.) or to reinforce other materials (composites). It is non-transparent, with a melting temperature over

918 ENCYCLOPAEDIA OF HYDROCARBONS SYNTHETIC FIBRES

200°C. Their density ranges between 1.00 and 1.20 a polyamide. In the salt, the two monomers are found g/cm3. They are all insoluble in water, and can be in the exact ratio of 1 to 1, an essential condition to dissolved cold in anhydrous acids (formic, sulphuric, obtain high degrees of polymerization. Since the glacial acetic acid) and phenols (phenol, p-cresol) at polymers made using this method sometimes had a 80°C and above. They can be hydrolysed by hot degree of polymerization too high for them to be spun, mineral acids in an aqueous solution. The high at the beginning of 1935 it was discovered that small stability of aliphatic polyamide fibres, obtained by quantities of acetic acid could be added to the cold drawing, and their insolubility are due to the polymerization mixture, regulating it as desired. formation of hydrogen links between the amide groups According to Carothers, among the diamine- of the polymeric chains. Introducing aromatic rings dicarboxylic acid pairings which could produce (polyaramids) to these leads to a raising of the melting polyamides of interest, was pairing 66. Again at the point. beginning of 1935, Berchet was charged with the Among the numerous polyamide fibres on the preparation of the two monomers and the polymer. The market, we will mention only those of greatest interest, preparation of polyamide 66 was completed (12.5 g of and specifically polyhexamethylene adipamide or polymer, yield 90%) on 1 March 1935. The Nylon 66, poly(e-caprolactam) or Nylon 6 (Perlon), polyhexamethylene adipamide appeared as a solid and that obtained from 11-aminoundecanoic acid or corneous mass which melted at 252-254°C from which Nylon 11 (Rilsan). It is important to note that only the fibres were obtained; however, its high melting point led first of these has withstood the emergence of new to fears that it might decompose during melt spinning. fibres on the market. In the spring of 1935, the first polyamide 5-10 filaments were obtained using a stainless steel Nylon 66 spinneret and the melt spinning process (a completely The story of Nylon 66’s discovery by the E.I. innovative technique); these were twisted to turn them DuPont de Nemours research group headed by into threads, each consisting of 24 filaments and with Carothers, and the various stages of its a count of 123 denier, in other words about 5 denier industrialization (synthesis of the monomers and their per filament (compared to 2 denier per filament of polymerization, spinning, finishing and weaving of the natural silk). Using these threads, the first fabric in fibres) marked the beginning of the era of synthetic completely synthetic fibre was made. textile fibres. From the summer of 1935 onwards, Carothers’ Early in 1928, Carothers was called to lead group devoted itself exclusively to polyamide 66, for DuPont’s base organic chemistry research group, and which industrial production seemed possible, whereas given absolute freedom in terms of which inquiries to work on polyamide 5-10 was abandoned; this product pursue. During base research on polyesters, various could not be industrialized since the availability on the strategies to obtain polymers with molecular weights market of castor oil, from which sebacic acid was above 4,000 using condensation polymerization had obtained, was limited. In the case of polyamide 66, on been identified. By pure chance, it was discovered that the contrary, adipic acid could be synthesized from these could be used to obtain flexible fibres which, benzene, obtainable from the petrochemical industry after cold drawing, turned into extremely robust long in practically unlimited quantities, even though the threads. However, these fibres had no potential for technique for the industrial synthesis of commercial success due to their solubility in the hexamethylene diamine was still unknown. solvents used for dry cleaning and excessively low Since polyamide 66 had the potential for melting point, which would have made it impossible to commercial success as a textile fibre, it was decided to iron any textiles made from them. This led to the idea use it to produce high quality yarns which could of studying less soluble materials, with better physical compete with those in natural silk. The use of and mechanical properties, and in particular polyamide 66 to make a fibre very similar to wool polyamides, which had already been the subject of (Fiber W), on the other hand, was abandoned for previous research. economic reasons. The industrial synthesis of the In the summer of 1934, polypentamethylene monomers turned out to be well-suited to the sebacamide (polyamide 5-10) was made, melt spun large-scale production of the polymer, which did not using the needle of a syringe for injections as a present particular problems on the industrial scale. spinneret. In October of the same year, a new synthesis To transform the polymer into fibres, melt spinning method was developed, based on the principle that if was chosen. However, at the process temperature diamine and diacid are mixed in equimolecular ratio, (above 260°C), a small quantity of the polymer the amine salt of the diacid is obtained even without decomposed in the melting chamber, with the heating; this in turn polymerizes when heated, creating formation of gaseous products which caused the

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interruption of the filaments. Pressurizing the melting Industrial production of the polymer chamber allowed melt spinning to become an Weighed in stoichiometric quantities, industrial process. A pilot plant, about a tenth of the hexamethylene diamine and adipic acid are sent to a size of a commercial plant (with a production capacity boiler where hexamethylene diamine adipate (salt 66) of about 125 kg of yarn per day) came into operation is formed. It is dissolved in water and sent to an on 11 July 1938. evaporator, where the water is removed and From the day of the first synthesis of polyamide polymerization begins. The contents are then sent to a 66, DuPont had maintained absolute secrecy regarding boiler where polymerization proceeds at low pressure, the new fibre and its developments, but word did get first at 180°C and then at 250-275°C. The polymerized out since as early as 1937 bristles made from mass is extruded from the lower part of the boiler, polyamide 66 scraps and rejects had been marketed passed over a moulding roller, cooled with jets of (without divulging their chemical composition), sold water and reduced to small chips. with the trade name of Exton Bristles and used to make toothbrushes (this was the first use of what was Industrial production of fibres later to become nylon). Nylon 66 fibres are made by melt spinning. The In September 1938 the first patents were polymer is melted on a silver grid in the presence of registered. The New York Times reported the new fibre, very pure nitrogen and the molten mass is then pushed and wrote in an editorial: “A new kind of rayon has through a spinneret in perfectly controlled quantities been produced […]. Because of its impact on the silk by a metering pump. The yarn, soaked in antistatic trade […] Japan has reason to worry”. In October of oils, is cooled in a cold air column and wound around the same year, during the New York World Fair, in the bobbins. It is then subjected to cold drawing (drawing section of The World of Tomorrow, Charles Stine, vice ratio of 1 to 4 or above, depending on the tenacity president of DuPont, presented the new fibre with required) to confer on it the desired mechanical these words: “I am making the first announcement of a properties. Most of the continuous filament is brand new chemical fiber. This textile fiber is the first subjected to a texturizing process. If, rather than a man-made organic textile fiber prepared wholly from continuous thread, it is desired to produce staple, the new materials from the mineral kingdom. I refer to the tow is drawn, crimped and then cut into the desired fiber produced from nylon […]. Though wholly lengths. fabricated from such common raw materials as coal, Nylon 66 is commercialized as multifilament, water and air, nylon can be fashioned into filaments as monofilament and staple, with a vast range of counts. strong as steel, as fine as a spider’s web, yet more The fibres may be opaque, semi-opaque or shiny, and elastic than any of the common natural fibers”. Ten their section may be either circular or a variety of years earlier, he himself had promoted the core other shapes (star-shaped, trilobed etc.) whenever it is research programme which led to its creation, and had desired to give the textiles a particular appearance. warmly supported Carothers’ employment. Nylon was an instant commercial success, and Properties and uses of the fibres DuPont had to nearly triple the planned plant, which The tenacity of the fibres when dry ranges from was not yet fully operational, to meet the requests 3.7-4.1 g/dtex (for staple; g refers to the gramme arriving from all over the world. Women’s stockings in force) to 4.2-5.3 g/dtex (for continuous filament), and nylon () were presented by DuPont for the first reaches 8.1-8.5 g/dtex for high tenacity fibres. When time at the International Exhibition in San Francisco in damp, tenacity decreases by 10-20%. The elongation February 1939. The following year, more than 1,300 at break ranges between 19 and 32% for continuous tonnes of nylon were produced (most of which were filament and reaches 40% for staple. Elasticity is high turned into women’s stockings) with a value of about 9 and the recovery from deformation is total as long as million dollars and providing a profit of about 3 the limit of elasticity is not exceeded. Resistance to million dollars, amply repaying the costs of research abrasion is extremely good, and hygroscopicity (4%) is and development. Within the space of two years, more high compared to other synthetic fibres than 30% of the women’s stocking market belonged to (polyacrylonitrile 2%, polyesters 0.4%). The fibres can DuPont Nylons. be used at up to 150°C (after 6 hours at this When the United States entered the war in 1941, temperature they begin to yellow) and are flammable nylon became a strategic material, used to reinforce (though less so than those of cotton or artificial silk). the tires of lorries, cables and parachute silk, in Nylon 66 fibres are resistant to alkalis, soluble addition to nets, cables and coverings for antiaircraft in cold formic acid and sulphuric acid, are not damaged barrages, items of military apparel (replacing wool and by the solvents used in dry cleaning and have a good cotton), haulage cables, etc. affinity with numerous classes of dyes (acid,

920 ENCYCLOPAEDIA OF HYDROCARBONS SYNTHETIC FIBRES

premetallized, chrome, etc.), which provide colours for staple, on the other hand, is hot drawn, worked which remain fast despite light and washing. with warm water to eliminate any traces of monomer, Among the numerous finishing operations, crimped and cut into the required lengths. The thermosetting is important to shape and set the fabrics; texturing of continuous filaments and product types by eliminating any residual retraction, it ensures that are similar to those of Nylon 66. their size remains stable with use and washing. Nylon The properties of Nylon 6 fibres and textiles, and 66 fabrics are very resistant to crumpling and, where their uses, are basically identical to those of Nylon 66, necessary, can be ironed at temperatures lower than with the difference that the former are easier to dye 150°C. and that, since they melt at lower temperatures, greater Given their high elasticity, appearance, feel, caution is required when ironing. Nylon 6 acquired resistance to abrasion and repeated folding, and low commercial importance (especially in Germany) in the hygroscopicity, Nylon 66 fibres are ideal for durable early 1950s, and later vanished almost completely and comfortable fabrics for windproof jackets and from the market. sports wear, women’s wear, underwear, hosiery and socks made of blends with wool or cotton. Nylon 11 This polyamide, known as Rilsan, was created by Nylon 6 Organico SA (France) in collaboration with SNIA From 1930 onwards, Carothers had researched the Viscosa and produced, as well as in France, in India, polymerization of e-aminocaproic acid with Berchet, Brazil, and the ex-Soviet Union. and together they had obtained, alongside a polymer The polymerization of 11-aminoundecanoic acid, with a low molecular weight, a cyclic compound made from castor oil, is similar to that of which was named lactam. Considering their research e-caprolactam, and it is melt spun at 215-220°C. complete, Carothers and Berchet published a study Usually, polymerization and spinning are continuous, concluding that e-caprolactam did not polymerize. In without an intermediate granule stage. the spring of 1937, DuPont, in negotiations to Nylon 11 melts at 189-190°C, and this represents exchange patents, informed IG Farbenindustrie about its major drawback in many applications, since it obtaining polyamide 66 (the future nylon), certain that cannot be ironed. It is produced as a continuous its patents would be unassailable. IG Farbenindustrie filament, as staple and as monofilament. The fibres researchers, and especially Schlack, immediately are similar in appearance to the other polyamides; the reread Carothers’ publications on e-caprolactam, and same can be said of their mechanical properties. Nylon in 1938 heated it obtaining poly(e-caprolactam), or 11’s resistance to oxidation is higher than that of the Nylon 6, later known as Perlon in Germany. other Nylons (it yellows at 150°C in air). Given its low The polymerization of e-caprolactam takes place absorption of water, Nylon 11 is not easy to dye; this by heating at 250-270°C in an autoclave in an aqueous may also be done in the bulk during spinning. Like the solution, in the presence of Nylon 66 salt or polyamides examined above, Nylon 11 is used for e-aminocaproic acid as initiators. Acetic acid or other knitwear. process terminators may be added to control the degree of polymerization. In continuous Polyaramids polymerization, a concentrated aqueous solution, Polyaramids (or aramids) are totally aromatic which may contain an initiator, is sent into a tube polyamides, obtained from diamines and aromatic reaction chamber, consisting of a single tube or a diacids. The introduction of aromatic rings into the battery of tubes, heated to 250-270°C. In this way, the polymeric chains of polyamides leads to a raising of reagents are heated gradually as they run through the their melting point (polyaramids usually decompose tubes. The polyamide thus produced is then turned into before melting); they can therefore only be spun from granules. solutions in strong acids (sulphuric, nitric) and the Nylon 6 melts at 215-217°C and is more stable fibres obtained are strongly heat-resistant. than Nylon 66 when exposed to heat; melt spinning The first polyaramid fibre, poly(m-phenylene thus presents fewer difficulties. In general terms, the isophthalamide) was commercialized in 1967 by E.I. process is identical to that used for Nylon 66. Another DuPont de Nemours with the name . Nomex is melt spinning process involves melting in an extruder, obtained by the interfacial polymerization of whose screw channels the melt into the spinning head, m-phenylene diamine with isophthalic acid dichloride. where a metering pump pushes it through the The fibres, which can resist temperatures of 300°C for spinneret. The filaments are cooled in air to 18-20°C a considerable time, are used for fire-fighting (relative humidity 45-55%) and then cold drawn (ratio equipment (to provide flame resistance) or to make of 1 to 4) at 15°C (relative humidity 60-70%). The tow clothes which must resist high temperatures. In the

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early 1970s, Teijin in Japan marketed Conex or Konex, glycol. The results were not encouraging; a vitreous a fibre with a composition similar to Nomex. During resin with low molecular weight (about 4,000) was the same years, the Soviet Union began to produce obtained, leading to the abandonment of this research Fenilon, similar to Nomex, for civilian and military after publication of the results. Even today, it is purposes and as a material suitable for the aerospace unclear why Carothers and his assistants did not try industry. replacing phthalic acid (carboxylic group in ortho The studies by E.I. DuPont de Nemours’ position) with terephthalic acid (carboxylic group in researchers continued, and in 1970 another fibre with para position). a very high elasticity modulus was introduced on the During this research, by reacting an organic market, to be used as a reinforcement for tires: dicarboxylic acid with 16 carbon atoms with poly( p-benzamide), initially given the trade name propylene glycol, a polyester had been obtained with a Fiber B. In 1975 poly(p-phenylene terephthalamide) molecular weight of about 12,000, from which was commercialized, with mechanical properties extremely robust long filaments could be made by similar to Fiber B; from this it was possible to obtain cold drawing. Although possessing far better fibres for use in rigid compounds. Poly( p-phenylene mechanical properties, especially elasticity, than rayon terephthalamide), obtained by the interfacial and natural silk, these fibres were soluble in all liquids polymerization of p-phenylene diamine with used for dry cleaning, deteriorated if treated with terephthalic acid dichloride, was sold with the trade water, and had a melting point below 100°C. For these name Kevlar. reasons, they could not be used to make textiles, since Kevlar fibres are obtained by dry or wet spinning these would have been neither washable nor ironable. from solutions in sulphuric acid, in which the As early as October 1934, Edgar Spanagel, also a extremely rigid chains of the polymer (all fully in the member of Carothers’ group, had prepared PET, but trans-conformation, since the cis-conformation is this research was not pursued for various reasons. sterically hindered) form a lyotropic mesophase in First, polyamides seemed far more promising as which the liquid crystals orient themselves parallel to fibres; second, the melting point of PET was the axis of the fibre when they are extruded, making considered too low for potential use as a fibre; and drawing to increase tenacity pointless. Kevlar fibres finally, it seemed too easily hydrolysable. In 1940, the decompose without melting at over 500°C and have Englishmen Whinfield and Dickson of Calico in extremely high resilience and tenacity (25 g/denier, Manchester made PET from terephthalic acid and seven times stronger than a steel wire, with the ethylene glycol, patenting the procedure the following advantage that their specific weight is 1/5 that of year. This research was then continued by ICI, which steel). They are used to reinforce radial tires and above had purchased the patent from Calico with exclusive all for advanced composite materials, widely used in worldwide rights for twenty years (with the exception aeronautical and space technologies. Other uses of the United States). In 1945, ICI researchers mainly concern special clothing (bullet-proof vests, completed the development of the process for making helmets, protective gloves), sports wear, etc. PET and its fibres, which were marketed in 1947 with The fibres previously known as Arenka and the trade name Terylene. subsequently as Twaron, made in the Netherlands by Fibres identical to Terylene were produced almost Akzo NV, are identical to Kevlar in terms of chemical immediately in the major industrialized countries on structure and properties. licence from ICI; these were known as Trevira in Germany (Hoechst), Terital in Italy (Montecatini) and Tergal in France (Rhodiaceta) to mention only 12.7.3 Polyester fibres European countries. Although a Patents and Processes Agreement was These are polymeric synthetic fibres consisting at least in force between E.I. DuPont de Nemours and ICI, the 85% of an ester of terephthalic acid with a glycol. latter, due to the safety measures called for by the war, Despite enormous amounts of research, polyethylene had not informed DuPont about Whinfield and terephthalate (PET) fibres are still unequalled, and are Dickson’s discovery. In 1944, rumours had reached more widely used than those in Nylon. DuPont regarding the development in Great Britain of The history of PET fibres also begins with a new fibre, Terrylite. Within a few weeks, DuPont Carothers’ research on these polymers immediately technicians had obtained a sample which, based on the following his employment by E.I. DuPont de name, they assumed to be PET. In October 1944, with Nemours. One of the very first polyesters synthesized great scepticism but hoping that Carothers’ patents of by Carothers as early as 1929 was that by 1930 would cover its expenses, DuPont resumed condensation between phthalic acid and ethylene research on PET. Using Carothers’ polymerization

922 ENCYCLOPAEDIA OF HYDROCARBONS SYNTHETIC FIBRES

technique, a polymer with high intrinsic viscosity and transferred to an autoclave heated to 290°C where, high melting temperature was synthesized, which under vacuum, the glycol gradually formed as could easily be turned into fibres with excellent polymerization takes place is eliminated. The process mechanical properties after cold drawing. ends when the PET has reached a molecular weight of In early 1945 a meeting took place between about 18,000. At this point the polymer can be English and American patents experts, called by ICI extruded, cooled with water and reduced to chips. Melt and DuPont. At the end of this meeting, the English spinning is used, heating the polymer chips until they agreed that Carothers’ basic patents covered those for melt and channelling the melt into the spinnerets Terylene, but that this had not stopped Whinfield and (holes about 0.3 mm in diameter) using a metering Dickson obtaining their own patents. As a result, ICI’s pump. patents were not valid in the USA, and DuPont was Another way of carrying out the polymerization able to commercialize its PET with the name Dacron. involves the elimination of water, starting directly Thanks to the experience matured in the field of from terephthalic acid and ethylene glycol in cascade synthetic fibres with Nylon and Orlon, DuPont acted reactors up until the final condensation, after which far more decisively than ICI in developing this new the molten polymer passes directly on to spinning fibre, Fiber V. Although the two giants continued to (continuous process). This type of process, as well as exchange information about it, DuPont developed avoiding the need to melt the polymer a second time, Fiber V at far higher speed than Terylene; this presents numerous other advantages, both technical increased further after 1948, the year in which the and economical. As they exit the spinneret, the Patents and Processes Agreement ceased to be filaments are solidified by cooling in air, gathered on renewed. rollers at extremely high speed (1,000-1,500 m/min) The use of Fiber V to reinforce the frameworks of and then subjected to hot drawing (70-90°C). For car tires did not provide good results, though it had all production as staple, after cooling, the filaments the mechanical prerequisites. In March 1947 it was exiting the spinnerets are gathered in tows and placed decided to abandon this market, and Fiber V was in purpose-built containers; they are then removed developed as a textile fibre, potentially in competition from these to be subjected to steam drawing. The with Nylon and Orlon. Given the numerous drawn tow is then crimped and finally cut into the similarities between the properties of Fiber V and desired lengths. those of wool, a small amount of a fibre identical to the latter was produced. Through external companies, Properties and uses of the fibres in February 1948, a gabardine was made with If cooled suddenly, molten PET does not extremely encouraging results, since it had properties crystallize (density 1.34 g/cm3) and can easily be identical to those of wool, and was very resistant to oriented by drawing. It crystallizes when heated to creasing thanks to the extremely high resilience of temperatures above 80°C, with a maximum PET fibres. crystallization velocity at 180°C. Consequently, Having resolved the problems linked to the supply reheating immediately after drawing has the effect of of raw materials and monomers (in particular those creating highly crystalline fibres (crystallinity above relating to the production of dimethyl terephthalate), 40%, density 1.38-1.39 g/cm3) with high tenacity to polymerization and spinning (the continuous (from 5.8-7.2 g/dtex for continuous high tenacity process, without the intermediate granule stage, turned filaments to 2.3-5.0 g/dtex for staple). The possibility out to be economically preferable), to the reduction of of intervening during the transformation into fibres static electricity in the articles made and finally to makes it possible to obtain yarn with different pilling (in other words the formation of small bobbles properties: these range from high tenacity continuous on the surface of articles due to friction, resolved by filament for tire coverings or other industrial uses, to a using filaments with a non-circular section or staple very similar to wool for use in knitwear, from anti-pilling fibres), Fiber V was put on the market in fibres for crease-proof textiles to those used for 1953 with the trade name Dacron. curtaining, etc. PET fibres can be made with sections of different Industrial production shapes, in addition to circular, producing yarns with The dimethyl ester of terephthalic acid is made to different appearances. The continuous filaments can react with an excess of ethylene glycol in the presence easily be texturized, making it possible to use these, in of very small quantities (0.01-0.015%) of lithium or wool or cotton blends, in textile applications for which magnesium salts in a nitrogen stream to entrain the staple is indispensable. methanol formed, heating to 200°C until the latter is The mechanical properties of PET fibres are not completely eliminated. The product of the reaction is affected by humidity. The elongation at break ranges

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from 8-11% for high tenacity filaments to 20-50% for altered by humidity, they have excellent resistance to staple. The elasticity modulus – one of the impact (much appreciated when they are used for characteristics of PET fibres – is high (from 100-130 advanced composite materials), excellent resistance to g/dtex for continuous high tenacity filaments to 30-70 abrasion, good resistance to high temperatures (240°C g/dtex for staple). The crystalline structure of PET for constant use, 340°C for brief periods) and high prevents viscous flow under stress; creep is therefore chemical stability (to acids in particular). For these extremely limited and the fibres cannot be deformed. reasons they are widely used for cordage, work gloves, The absorption of water by PET fibres is very protective clothing, acid-resistant filters, materials for limited (0.4-0.5%), their melting temperature is 260°C friction, advanced composite materials, etc. and they retract at around 230°C; as such, these fibres can be ironed at temperatures up to 200°C, more than sufficient to add or remove creases. 12.7.4 Polyvinyl fibres PET fibres have excellent resistance to chemical agents, especially those used for dry cleaning. Since These are synthetic fibres obtained by addition their hydrolysis velocity is low, they resist well both to polymerization from vinylic and vinylidenic water and to acids and bases; they also present good monomers. Poly(vinyl chloride) was the first vinyl resistance (better than that of polyamides) to sunlight polymer from which fibres were obtained (1913), but and can be dyed using dispersed dyes. these achieved little success. The most important are As already mentioned, PET fibres are used both on the polyacrylic fibres obtained from polymers based their own and in blends with wool or cotton. In some on acrylonitrile; of secondary importance are those cases they may be prone to pilling, as is the case for obtained from polymers of vinyl chloride and vinyl other fibres. PET’s strong points are the silk-like alcohol. textiles used for women’s clothing and furnishings, sports fabrics (the use of microfibres makes it possible Polyacrylic fibres to make items which are at once breathable and Currently, not all polyacrylic fibres consist of waterproof), curtaining, safety belts for cars, padding, homopolymers of acrylonitrile, but of its copolymers clothing of cotton type, etc. with other vinylic monomers, introduced into the polymeric chains to avoid problems with production Totally aromatic polyesters and dyeing. If the content in structural units deriving Like aromatic polyamides, aromatic polyesters from acrylonitrile is equal to at least 85% in weight, form liquid crystals. However, unlike the former, the fibres are described as acrylic, the most important; which form these in solution (lyotropic liquid crystal if the content is between 85% and 50% they are known polymers), the latter form liquid crystals when molten as modacrylic. (thermotropic liquid crystal polymers); whereas the The first polyacrylonitrile (PAN) fibres appeared former must be spun from a solution (their melting in 1942, when in DuPont’s laboratories it was point is close to that of decomposition), the latter can discovered that this polymer is soluble in be turned into fibres by melt spinning. dimethylacetamide (DMAC) and in The first aromatic polyester to be developed dimethylformamide (DMF). PAN could not be turned commercially was Xydar, by Dartcore, USA into fibres using melt spinning like nylon, since it (December 1984), which derived from Carborundum, decomposes before melting. USA’s EKKCEL (1972). Vectra (plastics and fibres) The first PAN fibres (Fiber A) obtained from and Vectran (fibres only) are two totally aromatic solutions in DMF by dry spinning from a solution or polyesters commercialized in 1986 by the then wet spinning from a solution immediately showed very Celanese, now Ticona GmbH (Germany). All these interesting characteristics, such as a high melting point materials are prepared by condensation between and good chemical stability. When not drawn, or only p-hydroxybenzoic acid, terephthalic acid, and slightly drawn, they had properties similar to those of 4, 4-dihydroxy diphenyl. Given the low viscosity of wool, whereas when drawn their properties were the melt resulting from its liquid crystal structure, similar to silk; they also had excellent resistance to spinning does not present particular problems. light, chemical products and bacteria. Spinning is followed by thermal treatment at 300°C or The significant drawback of Fiber A was that it above, thus obtaining an improvement of mechanical could not be dyed, and non-dyeable fibres have no properties due in part to further polymerization. commercial potential. A further factor which Vectran fibres, like Kevlar fibres, have high contributed to slowing down its development was that tenacity and elasticity modulus; they do not absorb Vinion N (a 50% acrylonitrile-50% vinyl chloride water, their physical and mechanical properties are not copolymer) commercialized by the Carbide and

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Carbon, which seemed economically more viable, had in its monomer, so that bulk polymerization creates shown that DMF was strongly toxic. Due to the war, it enormous problems), the most widely used is that of was also difficult to find the raw materials for radical polymerization in a solution, using highly production (those used for strategic materials such as polar organic compounds (DMF or dimethyl nylon had precedence). It was pointed out to the sulphoxide) or aqueous solutions of inorganic salts military that Fiber A’s excellent resistance to light, (60% zinc chloride, 44-50% sodium thiocyanate, chemical products and bacteria could resolve many of calcium thiocyanate and perchlorates) as solvents. their problems in the jungle (such as tents, laces and The advantage of this process lies in the fact that the tarpaulins in cotton, which deteriorated rapidly) and polymer solution can be spun directly. In the dry this allowed DuPont to obtain raw materials more spinning process (generally used to produce easily. Vinion N turned out to be inferior to Fiber A continuous filaments) a 20-30% solution of the due to its lower melting point and greater solubility in polymer in DMF is extruded at 80-150°C solvents. The problem of DMF’s toxicity was resolved in a vertical tube, in which a stream of hot air by adopting the dry spinning from solution process, (230-260°C) causes the evaporation of the DMF carried out in closed chambers. At the end of 1944 the (which is recovered) and the solidification of the problem of dyeing remained unresolved; this is filaments which are gathered on bobbins. The fibres, considered the Achilles heel of all hydrophobic fibres which still contain 10% DMF, are then washed with (nylon and polyesters). water and subjected to drawing at 80-110°C in hot After exhausting all attempts to find suitable dyes, air, or at 70-100°C in water. it was decided to render Fiber A dyeable by modifying In wet spinning (used to produce staple) the its chemical structure. It was discovered that a 95% polymer dissolved in DMAC or in DMF is extruded acrylonitrile-5% 2-vinylpyridine copolymer (Orlon into water, in which the filaments coagulate forming a A-3) could be dyed. However, 2-vinylpyridine was not bundle which is washed, drawn in hot water, crimped available in large quantities, and was also extremely and cut into the desired lengths. expensive. At the end of 1948, then, the problem of the dyeability of Orlon had not been fully resolved, and it Properties and uses of the fibres was unclear whether it was better to produce modified PAN fibres are essentially amorphous and soften or non-modified Orlon, since the latter could be above 225°C. They are very stable when heated and widely used to replace cotton for tents and tarpaulins. tend to yellow if exposed for long periods to Since Orlon as a continuous filament did not temperatures over 130°C; their tenacity decreases by become a successful product, attempts were made to less than 4% after 100 hours at 150°C. Since they produce it as staple Orlon A-4, a copolymer of generally consist of copolymers of acrylonitrile in acrylonitrile with 5-methyl-2-vinylpyridine, easier to addition to homo-PAN, they present a broad spectrum dye than Orlon A-3. Since the presence of cuprous of properties, depending on their chemical ions in the dye bath increased the reactivity of PAN composition and the way they have been treated. The with acidic dyes, and the problem of dyeing thus shape of their section is circular for continuous appeared to have been resolved without modifying the filaments and ‘dog bone’ for staple. polymer, the production of staple (Orlon Type 41) Staple has a tenacity of between 2 and 3 g/dtex consisting of homo-PAN began. Orlon Type 41 was a whilst continuous filament, which is less used, is more failure due to the difficulty of weaving caused by tenacious (4-4.2 g/dtex). Elasticity is not high static electricity, the non-uniformity of the fibrils and (recovery of 50-60% for a deformation of 10%). The unpleasant odours when blended with wool. absorption of humidity (above 2% at 20°C) is among A solution to the problems presented by PAN came the highest for synthetic fibres. Despite this, it dries from the rapid development of the 94% very rapidly and can be used for ‘wash and wear’ acrylonitrile-6% methyl acrylate copolymer, Orlon A-6 clothing; it also has good resistance to sunlight and to (Orlon type 42 yarn). The staple turned out to be ideal chemical agents (dilute acids and bases, liquids for dry for sweaters and wool blend worsteds; by the end of cleaning, etc.). 1951 sales had reached and superseded 50,000 tonnes Dyeing is carried out using either basic dyes, per year, and Orlon became a great commercial which provide fibres with solid and brilliant colours, success. or with acid dyes. The articles produced cannot be ironed, since they would become deformed. Industrial production Worldwide production of acrylic fibres is Among the various ways of polymerizing extremely high (millions of tonnes per year); they are acrylonitrile (remembering that it is soluble in water, used both alone and in blends with wool for outer whilst polyacrylonitrile is insoluble both in water and knitwear, sports socks and exterior curtaining.

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Modacrylic fibres Poly(vinylidene chloride) fibres These are acrylic fibres which contain less than Fibres of poly(vinylidene chloride), or more 85% acrylonitrile. Among the numerous types, it is accurately of copolymers of vinylidene chloride/vinyl worth mentioning Ve re l and Dynel, both of which have chloride (Saran, 20% vinyl chloride) are melt spun at good flame-retardant properties. about 180°C and are almost dyed in bulk. They are Verel (Tennessee Eastman, USA) contains non-flammable, have low tenacity, soften between 120 acrylonitrile (about 60%) and vinylidene chloride and and 160°C and are basically used for industrial is produced only as staple (including a high retraction textiles. type) by wet spinning from a solution. Easy to dye with basic, dispersed and premetallized dyes, it is Poly(vinyl alcohol) fibres mainly used in blends with other fibres (natural, Poly(vinyl alcohol) fibres, obtained from the artificial or synthetic); it has modest mechanical partial hydrolysis of poly(vinyl acetate), are wet spun properties (tenacity 1.7-2.3 g/dtex, deformation at from aqueous solutions, coagulated in a sodium break 35-40%), softens at around 200°C and resists sulphate solution and treated at 240°C to render them ageing better than other synthetic fibres, wool and more compact. They are then made insoluble in water cotton. It dissolves in hot acetone. by acetylation (30-40% in moles) and partial Dynel (Union Carbide, USA) is a 40% crosslinked with formaldehyde. acrylonitrile-60% vinyl chloride copolymer. The fibres Poly(vinyl alcohol) fibres are shiny and irregular in (including a high retraction type), obtained by wet section, have high tenacity (up to 8 g/dtex), good heat spinning from a solution, are thermoplastic and retract resistance (they retract 10% at 220°C), soften at even in boiling water; they are dyed at temperatures 250°C, resist ageing well and can be dyed easily. They below 100°C in the presence of swelling agents. Their are used almost exclusively in Japan (Kurashiki tenacity is low (2-3.8 g/dtex) and their elongation at Rayon’s Kuralon), in blends with cotton or rayon, to break ranges from 14-40%. They are used for carpets, make textiles for umbrellas, awnings, carpets, etc. industrial textiles, protective clothing and furnishings in places covered by fire regulations (cinemas, fibres theatres, etc.). Polytetrafluoroethylene (PTFE) is insoluble in all solvents and decomposes before melting; its fibres are Poly(vinyl chloride) fibres therefore obtained by extruding a dispersion of Poly(vinyl chloride) (PVC) fibres were the first extremely tiny particles of the polymer in a coagulant synthetic fibres ever made, in 1913. In 1934 IG bath of water and hydrochloric acid. The filaments Farbenindustrie (Germany) presented the PC fibre thus obtained, made of particles separate from one (post-chlorinated PVC, to render it soluble in acetone) another, are then heated rapidly to 390°C, so that the and in 1941 Rhodiaceta made PVC filaments spun particles syntherize to form a continuous filament. from a mixture of carbon sulphide and acetone. They can also be melt spun from copolymers of Montefibre (Montedison, Italy) produced Leavin, tetrafluoroethylene with propylene, perfluoro crystalline fibres from syndiotactic PVC (spinning propylene or perfluorovinyl ethers. from solutions in cyclohexanone), far more resistant to PTFE fibres have an extremely low frictional heat than atactic PVC. coefficient, total resistance to all chemical agents and PVC fibres have low tenacity (3 g/dtex), are excellent heat resistance (they can be used between produced as continuous filaments and as staple, are 70°C and 280°C), but have low tenacity (1.3 g/dtex). thermoplastic (they soften at around 80°C) and are not Their value lies in their high chemical inertia and heat attacked by acids and bases, but are particularly resistance; for this reason they are used for industrial sensitive to ketones; they are dyed in baths with azoic and space applications. developing dyes or using dispersed or solution dyes (for black). Furthermore, these fibres are strongly flame-retardant; they dehydrochlorinate and brown 12.7.5 Polyolefin fibres when exposed to heat. Their flame retardant properties make them ideal for furnishings and industrial textiles, Polyolefin fibres derive from olefin (basically ethylene thanks in part also to their resistance to acids and bases. and propylene) polymers, and include polyethylene Different types of more or less modified PVC and polypropylene fibres. fibres can be found on the market; these include thermoretracted fibres (Thermovil, Movil T), which Polyethylene fibres resist temperatures of up to 100°C and those obtained Polyethylene fibres have never had particular from chlorinated PVC (Clevil). commercial importance, given their poor mechanical

926 ENCYCLOPAEDIA OF HYDROCARBONS SYNTHETIC FIBRES

and textile properties, which have led to their use only The fibres are highly crystalline, melt at between for special applications. Even the synthesis, at the 170 and 174°C, and have low density (0.90 g/cm3). beginning of the 1960s, of polyehtylene with a high They can be prepared with varying degrees of tenacity molecular weight (2106), named UHMWPE (Ultra (up to 9 g/dtex), have good mechanical properties and High Molecular Weight PolyEthylene) and resistance to viscous flow, present good resistance to characterized by physical and mechanical properties acids and alkalis and many chemical products (they far superior to those of conventional polyethylene swell in hydrocarbons), are not easy to dye (dyeing, as (molecular weight 104) did not bring any changes, mentioned above, is done in solution with since the product could not be spun due to the thermostable pigments) and have low resistance to extremely high viscosity of its melt. In 1976 the gel light (stabilizers are also used for purposes which do spinning process was finally discovered at the DSM not require long exposure to sunlight). Given their lack laboratories (Netherlands); this involved drawing of sensitivity to water, resistance to abrasion and (drawing ratio of 1 to 30) fibres from a diluted cyclical bending stresses, lightness and tenacity, these solution of UHMWPE in xylene or decaline, with a fibres are used for manufacturing fishing nets, cords, tenacity and elasticity modulus close to the theoretical industrial filters, in the automobile industry, as a values for planar zigzag chains. geotextile (synthetic grass for sports fields), carpets Among the various high tenacity fibres in and rugs (characterized by their ease of cleaning, UHMWPE of commercial interest spun from a gel are brilliant colours, resilience and resistance to decay). Allied’s Spectra 900 and 1000 (USA, 1984) and On their own or in wool blends they are used for Dyneema’s Dyneema SK90 (Netherlands, 1986). These knitwear, underwear and sports wear (double layered fibres are chemically stable, have melting points garments with an inner layer in polypropylene and between 145 and 155°C (depending on the drawing outer layer in natural fibre, ensuring the rapid removal ratio) and preserve their mechanical properties up to of sweat by capillary action, keeping the skin dry). temperatures close to their melting point. They are ten times more robust (free breaking length 336 km) than those of steel and are used either as such, or in 12.7.6 Polyurethane fibres composites, or to reinforce other polymers (in loud speaker cones, bows, bows for musical instruments, Today, polyurethane fibres are the elastomeric fibres helmets, etc.). Due to their low density (1 g/cm3), (Spandex) commercialized by E.I. DuPont de Nemours high tenacity, high resistance to impact and (USA) in 1962 (Lycra) after twenty years of research atmospheric agents, resistance to abrasion, high aimed at obtaining synthetic elastomeric fibres to stability in the presence of ultraviolet light and replace those in natural rubber covered with cotton. excellent insulating and water-repellent properties, As early as 1942, DuPont’s researchers had high tenacity UHMWPE fibres are used for cordage observed that N-substituted (Type 8 Nylon) Nylon and cables, bullet-proof vests, protective clothing, could be turned into filaments with elastomeric parachutes and construction materials. properties. During this research, it emerged that to obtain elastomeric fibres it was necessary to make Polypropylene fibres block copolymers consisting of flexible segments (to The discovery of isotactic polypropylene by Giulio give elasticity) and rigid segments (to confer high Natta (1954) led to research on the potential for melting temperatures). making fibres from it; these were produced on an The preparation of these block copolymers was industrial scale from 1960 onwards by the Polymer made possible by the discovery of the interfacial (Montecatini Group, Terni, Italy, now Meraklon) with polymerization method. At the beginning of 1954, the trade name Meraklon. by extruding a prepolymer of Adiprene (an Before spinning, antioxidants and sometimes, given elastomeric polyurethane made by Orchem, USA) the difficulties presented by dyeing, pigments, colorants in a diamine solution, fibres were obtained with and granules (granules are more stable than powder) are excellent elastomeric properties. Later, a added to the isotactic polypropylene in powder form. poly(ether-urea-urethane) was obtained (Type 80), For melt spinning, special types of spinnerets are used with even better properties, which could be dry to avoid the rupture and deformation of the fibres, due spun or wet spun from solutions in to their high tixotropy when molten. The fibres (with a dimethylformamide or dimethylacetamide. After circular section) are produced either as continuous resolving the problems relating to their sensitivity filaments (drawn at 120°C through a die) or as staple to ultraviolet light and proneness to yellowing, (the tow is drawn in steam, crimped, thermoset and cut these polyurethane fibres were commercialized into the desired lengths). (1962) with the names Lycra for multifilaments

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and Vyrene for monofilaments. Within the space of fishing nets and lines, as a reinforcement for cement, two years, these fibres had become one of as cords for tennis rackets, etc. DuPont’s most profitable products, with a return on investments of over 30% net of taxes. Synthesizing Spandex is fairly complex, and 12.7.8 Carbon fibres involves preparing a prepolymer (polyester or polyether with a low degree of polymerization with This paragraph will deal with carbon fibres; though hydroxy terminals which later form the rigid blocks in not synthetic, they do derive from polyacrylonitrile the polymeric chain), reacting this with a diisocyanate (PAN). The first carbon fibres were made in the USA (forming isocyanic chain terminals), and then with a by pyrolysis of viscose rayon, but were short-lived; diamine or a glycol which form the flexible blocks. they were soon supplanted by those obtained from the Spinning is done using the methods outlined above pyrolysis of PAN, which appear in the form of before the final crosslinking which can also be continuous filaments and consist of graphitic carbon. obtained by simple heating (to be such, an elastomer Their high crystallinity confers upon them exceptional must be partially crosslinked). qualities: they are extremely tenacious, highly heat Lycra has allowed for the preparation of very resistant and very light (density 1.8-2 g/cm3) and are fine elastic textiles with a high compression force widely used as a reinforcement in advanced (not obtainable using rubber fibres); they are easy to composites. Among the major producers of carbon dye using acid or plastosoluble dyes. Unlike rubber fibres is Courtauld’s (Great Britain). fibres (minimum count 150 denier), Spandex fibres Carbon fibres are made by graphitizing a precursor may have far lower counts (down to 40 denier); fibre which may be either rayon viscose or PAN. The Lycras do not break when sewn (unlike continuous fibres of PAN are heated in air (oxidation) monofilaments of rubber), since the needle passes at about 300°C so as to form ladder structures which, easily between filaments. Given an identical when further heated to temperatures up to 3,000°C, elongation, Spandex has a higher tenacity and loose ammonia and hydrocyanic acid, creating elasticity modulus than rubber fibres; this means structures containing over 95% carbon. These are then that, for an identical count, they have a higher turned into graphitic carbon by treatment in an argon (3-6 times) compression force or, in other words, stream at 2,200-2,300°C. Carbon fibres have excellent that given an equal compression force they can have mechanical properties and are practically a lower count. This property represents the key to incombustible (they can be heated until they are cherry their success. Their elastic properties are also good, red, about 800°C, without suffering damage). They are only slightly inferior to those of rubber. black in colour and are basically used in composite Polyurethane fibres are never used on their own, materials, especially when high mechanical but always in combination with other fibres (especially performance and low density are required, alongside polyamides) to confer elasticity on the articles good fatigue resistance and dimensional stability. The produced (elastic tights, swimsuit, stockings etc.). aerospace industry (missiles, artificial satellites, civilian and military aircraft, helicopters etc.) is one of the major markets for carbon fibres and their 12.7.7 Polyacetal fibres composites. This is because their use leads to a significant reduction in weight which translates into The only known polyacetal fibres are those derived lower fuel consumption or a larger paying cargo. from Delrin (the polyoxymethylene obtained by E.I. Carbon fibres and their composites are widely used in DuPont de Nemours by polymerizing formaldehyde, the nautical industry (hulls, sails, masts etc.) and in the commercialized in early 1960), whose physical and automobile industry, various sports sectors (tennis, mechanical properties allow them to be used as fishing rods, cycling), for conductive materials substitutes for non-ferrous materials (brass, (electrodes), in textiles (Orlon black) for protection aluminium) in many of their applications. against high temperatures, etc. Filaments of Delrin (Tenac SD, Asahi Chemical Industry, Japan), obtained by superdrawing at high pressure, consist of perfectly oriented chains which 12.7.9 Synthetic fibres confer upon them excellent mechanical properties for medical use (better than those of steel wire): excellent resistance to heat, chemical stability and resistance to atmospheric Threads for sutures, meshes to reinforce the abdominal agents. Furthermore, these fibres have high creep cavity, vascular replacements and hollow fibres for the resistance and do not absorb water: they are used for treatment of blood (artificial kidneys, mechanical

928 ENCYCLOPAEDIA OF HYDROCARBONS SYNTHETIC FIBRES

lungs) represent the main uses of synthetic fibres in the Vascular replacements field of medicine. For these purposes, in addition to Vascular replacements in polyethylene terephthalate possessing suitable physical and mechanical properties, fabric (Dacron) or polytetrafluoroethylene (Teflon) are the fibres must be compatible with the tissues of the widely used to replace defective veins or arteries (for human body and with blood, have extremely low example in the case of aneurisms) of 6, 8, 10 mm in histotoxicity, must not be carcinogenic and must diameter. Despite their good biocompatibility, the behave suitably while they remain in the human body. anticoagulant properties of those in Dacron are not high, whereas those in Teflon have good anticoagulant Threads for sutures properties. Currently, there are no vascular Their specific function is to join the tissues of the replacements for blood vessels less than 3 mm in human body until healing has occurred, after suturing diameter; in these cases it is therefore preferred to following surgery, wounds or trauma. They must have replace them with blood vessels taken from other parts good resistance to traction, a low frictional coefficient of the body. with tissues and sufficient flexibility and elasticity (since the resistance of the knot depends on this). Fibres for the treatment of blood Non-absorbable suture threads (which remain The filter membrane in haemodialysis machines intact indefinitely in the human body) are obtained (artificial kidneys) consists of bundles of hollow from the commonest synthetic fibres (polypropylene, polyacrylonitrile fibres, through which the molecules polyethylene and their copolymers, Nylon 66, contained in blood with a molecular weight up to polyethylene terephthalate). 20,000 can permeate, but not those (including Absorbable suture threads (monofilament and albumin) with a molecular weight of around 70,000, multifilaments) were initially made of poly(glycolic which are retained. acid) fibres (1970) and later of glycolic acid-dilactide The artificial lung is a gas exchanger which serves

copolymers, poly(p-dioxanone), and glycolic to supply O2 to blood and remove CO2 from it using acid-trimethylene carbonate copolymers. After about membranes consisting of bundles of hollow a month the threads dissolve, since they degrade to microporous polypropylene fibres; due to the monomers by hydrolysis of the ester bonds. hydrophobicity of the fibres, their pores are freely Before use, absorbable and non-absorbable suture permeated by gas but not by blood. threads are sterilized with ethylene oxide or with ionizing radiation. 12.7.10 Microfibres and nanofibres Meshes for the reinforcement of the abdominal cavity Microfibres From 1995 it has become routine to use meshes in The development of sophisticated and complex polymeric materials to reinforce the abdominal cavity processes to obtain synthetic fibres (especially (for example in the case of hernias); these are obtained polyamides and polyesters) with increasingly low by weaving a wide variety of synthetic fibres, such as counts has led to ultrafine fibres (microfibres) with polyamides, polyesters, polypropylene or absorbable diameters in the order of micrometres (down to single fibres such as those of poly(glycolic acid) or lactic 0.01 denier fibres, with a diameter of about 0.4 mm) acid-glycolic acid copolymers. used to make fabric and ‘non-fabrics’ with specific Polyamide (Nylon 66) meshes, the first to be properties. Ultrafine fibre technology was initially employed (as early as 1944), are now rarely used, since developed in Japan to manufacture artificial leathers they cause acute inflammatory responses. Of all the such as Toray’s Ecsaine (Alcantara in Europe, materials used to reinforce the abdominal cavity, the Ultrasuede in the USA), similar to suede (easily dyed, most frequently used today are polypropylene and widely used for car seats, to cover sofas and polyethylene terephthalate (Dacron) meshes. The armchairs, etc.), consisting of monofilaments as small latter, developed simultaneously with the former, are as 0.1 denier. A further development of microfibres less widely used since they cause a greater was the creation of high density water-repellent inflammatory response and a more significant reaction textiles and ‘non-fabrics’, used for clothing and sport to the foreign body than those in polypropylene. (so-called microfibre clothing) and cloths suitable for Polypropylene and Dacron meshes are obtained by cleaning lenses, jewels, crystals etc. weaving a single filament, a pair of filaments or multiple filaments. Meshes in Dacron are sterilized Nanofibres with ionizing radiation, and those in polypropylene The main characteristics of a polymeric with ethylene oxide. nanofibre are a diameter in the order of

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nanometres, and consequently a high surface area Bibliography and superior mechanical properties. Among the various techniques for preparing nanofibres Cook J.F. (1984) Handbook of textile fibres, Shildon, Merrow, (spinning, template synthesis, phase separation, 2v.; v.II. autoassembly), electrospinning is the most Hearle J.W.S. (edited by) (2001) High-performance fibres, commonly used for its efficiency and the great Boca Raton (FL), CRC; Cambridge, Woodhead. simplicity of the equipment required. This process Hermes M.E. (1996) Enough for one lifetime. consists in applying a potential difference either to inventor of nylon, Washington (D.C.), American Chemical Society and Chemical Heritage Foundation. a molten polymer (polyethylene, polypropylene, Holmes D.F. polyethylene terephthalate etc.) or to a solution of (1983) History of the DuPont company’s textile fibers department, Wilmington (DE), DuPont, Textile Fibers it (polycarbonates, polystyrene, polyurethanes, Department. polyacrylonitrile, polyethylene terephthalate, Hongu T., Phillips G.O. (1990) New fibers, New York, Ellis Nylon 66, poly(vinyl chloride), etc.), causing the Horwood. formation of a jet of material which then Hounshell D., Kenly Smith J. (1988) Science and corporate subdivides itself into extremely thin fibres. strategy. DuPont R&D 1902-1980, Cambridge, Cambridge Nanofibres are used in composite materials University Press. (elasticity modulus and resistance of the matrix Huang Z.-M. et al. (2003) A review on polymer nanofibers material higher than those obtained with common by electrospinning and their applications in nanocomposites, «Composites Science and Technology», 63, 2223-2253. fibres, including carbon fibres and Kevlar), in Klein W. (1994) Man-made fibres and their processing, textiles for protection from chemical agents Manchester, The textile institute. (thanks to the considerable absorbent capacity due Mark H.F. et al. (editorial board), Kroschwitz J.I. (editor in to the high surface area of the nanofibres), in chief) (1985-1990) Encyclopedia of polymer science and membranes (which can be used as high efficiency engineering, New York, John Wiley, 24v. filters, given the limited volume occupied by the Mishra S.P. (2000) A text book of fibre science and technology, fibres), in ‘bandages’ sprayed directly onto wounds New Delhi, New Age International. by electrospinning (avoiding the formation of Reader W.J. (1975) Imperial chemical industries. A history, scars), to help the regrowth of human tissues in the London, Oxford University Press. event of disease (nanofibres, having a diameter Trossarelli L., Brunella V. (2002) L’invenzione del nylon lower than that of cells, act as frameworks for the tra realtà, leggende e misteri, «Memorie dell’Accademia delle Scienze di Torino. Classe Scienze Fisiche», 26, regeneration of the tissue), as replacements for soft 117-159. human tissues, to transport drugs inside the human body, and cosmetic masks. Luigi Trossarelli It is important to stress that hitherto synthetic Valentina Brunella polymeric nanofibres have not been commercialized; these should therefore still be considered subjects for Dipartimento di Chimica Inorganica, Fisica e dei Materiali laboratory research, with a view to promising Università degli Studi di Torino applications in the future. Torino, Italy

930 ENCYCLOPAEDIA OF HYDROCARBONS