12.7.1 Introduction

Obtaining more valuable products from cheapmaterials has always been one of mankinds goals.Enormous interest was therefore aroused when HilaireBernigaud de Chardonnet presented the first samplesof his rayon at the International Exhibition in Paris in1889; as shiny and silky as natural silk, this was thefirst artificial fibre ever made by man.

Although as early as 1913, 1931 and 1932processes to obtain filaments from poly(vinylchloride) and threads from poly(vinyl alcohol) andpolystyrene were patented in Germany, the era ofsynthetic fibres began in 1935 at the laboratories ofthe DuPont Experimental Station, Pure ScienceSection, Wilmington, Delaware, (USA). Here, GerardBerchet, one of Wallace Hume Carothers assistants,obtained just over 10 g of polyhexamethyleneadipamide, subsequently commercialized in 1938 withthe name nylon, the first totally synthetic industrialfibre.

In theory, all organic polymeric materialsconsisting of linear macromolecules with a sufficientlyhigh molecular weight can be turned into fibres, inother words long filaments whose axial ratio tends toinfinity. However, only a small number of theseprovide filaments with physical and mechanicalproperties allowing for practical applications. Thearrangements in which the fibres are found,independently of their origin, are the single fibre, agroup of several single fibres to form a thread, and theinterlacing of numerous threads to form textiles. Acharacteristic property of any fibre, in addition to itschemical, physical and mechanical properties, is itssize perpendicular to its axis or to that of a group offibres forming a thread. In practice, the radialdimension, and thus the thickness, of all continuousfibres including silk and those made of synthetic

polymeric materials is specified using the count; inother words the mass of the thread relative to a givenlength of it. The mass in grammes of 9,000 m (1,000yards) of thread is known as the denier count ordenier; another way of describing the thickness of acontinuous thread is to specify the tex, in other wordsthe weight in grammes of 1,000 m of yarn (or thedecitex, dtex, referring to 10,000 m of yarn). Inaddition to the fracture load and (percentage)deformation at break, other important mechanicalproperties of fibres are their tenacity, or better, themaximum energy that they can absorb withoutbreaking, and resilience, or the maximum energy thatthey can absorb without suffering permanentdeformation.

The development of synthetic fibres progressedside by side with that of organic chemistry, andespecially petrochemistry, which, with some extremelyrare exceptions, provides the base compounds for thesynthesis of monomers. In 1936, ICI (ImperialChemical Industries) patented the manufacture offibres from polyethylene in Great Britain; in 1937 thefirst polyurethane bristles were made; in 1938 PaulSchlack of IG Farbenindustrie (Germany) synthesizedpoly(e-caprolactam), whose fibres werecommercialized in 1943 with the name Perlon; in1941, John Rex Whinfield and James Tennant Dicksonof the Calico Printers Association of Manchester(Great Britain), synthesized polyethyleneterephthalate, whose fibres were commercialized withthe names Terylene (ICI, Great Britain), Dacron(DuPont, USA), Terital (Montecatini, Italy); in 1942 itwas discovered that N-dimethylformamide was asolvent of polyacrylonitrile, thus making it possible toobtain fibres commercialized by E.I. DuPont deNemours (USA) with the name Orlon; in 1960 thesame company commercialized the new elastomericpolyurethane fibre with the trade name Lycra. Also in

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Synthetic fibres

1960, Polymer of Terni (Montecatini group) perfectedthe fibres obtained from isotactic polypropylene(Meraklon); in 1971 Kevlar (E. I. DuPont de Nemours,USA) entered the market, the first of the polyaramids,obtained with the interfacial condensation ofterephthalic acid and p-phenylenediamine, givingfibres with exceptional mechanical and thermalproperties.

Today, synthetic fibres are not merely analternative to natural and artificial fibres, but formclasses of high performance materials which play anextremely important role in the field of hightechnology. The 5 denier of the first nylon filamentshave fallen to todays 0.5 for polyamide or polyestermonofilaments; fabrics made from these are superiorin appearance, softness and sheen to those in naturalsilk (2 denier). These fibres are known as high addedvalue fibres, as are those used to make special purposefabrics (thermochromic fabrics which change colourdepending on temperature, photochromic fabricswhich change colour depending on light, iridescentfabrics which change colour depending on how theyare hit by light, etc.). Also worth remembering are thehigh technology fibres deriving from the applicationof the latest developments in the science andtechnology of fibre manufacturing (biodegradablefibres for sutures; fibres which absorb and accumulatesolar energy such as Solar-a of 1988, made by theJapanese firm Unitika, and similar products made byDescente of Japan, widely used for sports wear; fibresfor haemodialysis, fibres for the oxygenation of bloodin the artificial respiration machines known asmechanical lungs; fibres used for fabrics to makespace suits for extra-vehicular activities, fibres whichabsorb humidity and sweat for sports wear, fibres withlow friction with air, etc.) and finally superfibres, inother words fibres with exceptional mechanicalproperties (elasticity coefficient over 55 GPa andtenacity above 2.5 GPa), such as those made of hightenacity polyethylene, para-aramids, andpolyacrylonitrile carbon.

Natural fibres (and their derivatives) can beconsidered first generation fibres. Synthetic fibres(aliphatic polyamides, polyesters, polyacrylonitrile,etc.), which appeared between the 1930s and 1960s,are second generation fibres, created to replace firstgeneration fibres. Todays high performance fibres(polyethylene, polyaramide, polyarylate, carbon fibres,etc.), which do not represent an alternative to naturalfibres as did those of the second generation, can beclassified as third generation fibres. These are usedwhen fibres with low density, excellent mechanicalperformance and heat resistance are required (insectors such as sport, transportation, space technologyetc.) or to reinforce other materials (composites). It is

not unlikely that third generation fibres, given theirbetter mechanical performance, will replace metals inmany of their applications in the not too distant future.

Synthetic fibres can be classified according to thepolar functional group repeated in their chain (forthose made by polycondensation) or on the basis of thestructural unit for those made by additionpolymerization. They are listed below in chronologicalorder of synthesis: Polyamide fibres, obtained by condensation

polymerization and characterized by the regularrecurrence along the macromolecular chain of theamide group NHCO. These include aliphaticpolyamides such as 66 (Nylon 66), 6 (Nylon 6,Perlon) 11 (Nylon 11, Rilsan) and others of minorinterest, and aromatic polyamides (polyaramids).

Polyester fibres, obtained by condensationpolymerization and characterized by the regularrecurrence along the macromolecular chain of theester group COO. In addition to polyethyleneterephthalate (Terylene or Dacron), they includethe fibres obtained from wholly aromaticpolyesters.

Polyvinyl fibres, obtained by additionpolymerization and characterized by a structuralunit deriving from vinylic or vinylidenicmonomers; of these fibres the most important arethose made from acrylonitrile (Orlon), from vinylchloride (Leavin, Thermovil, Movil, etc.) and fromtetrafluoroethane.

Polyolefin fibres, obtained by additionpolymerization, such as polyethylenes fromethylene and polypropylenes (Meraklon) frompropylene.

Polyurethane fibres, formally obtained bycondensation polymerization and characterized bythe regular recurrence along the macromolecularchain of the urethane group OCONH (Lycra).

Carbon fibres, included in this classification asderivatives of polyacrylonitrile.

12.7.2 Polyamide fibres

Polyamide fibres are generally known as nylon, thetrade name of the first wholly synthetic textile fibre ofindustrial importance. They are obtained with thecondensation polymerization of aliphatic or aromaticdiamines and aliphatic or aromatic organic diacids, orwith the ring-opening polymerization of w-aminoacids(Nylon 11 and Nylon 6). Polyamides areconventionally named according to the number ofcarbon atoms in the diamine and the diacid, or thew-aminoacid alone, and appear as corneous solids,non-transparent, with a melting temperature over

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200C. Their density ranges between 1.00 and 1.20g/cm3. They are all insoluble in water, and can bedissolved cold in anhydrous acids (formic, sulphuric,glacial acetic acid) and phenols (phenol, p-cresol) at80C and above. They can be hydrolysed by hotmineral acids in an aqueous solution. The highstability of aliphatic polyamide fibres, obtained bycold drawing, and their insolubility are due to theformation of hydrogen links between the amide groupsof the polymeric chains. Introducing aromatic rings(polyaramids) to these leads to a raising of the meltingpoint.

Among the numerous polyamide fibres on themarket, we will mention only those of greatest interest,and specifically polyhexamethylene adipamide orNylon 66, poly(e-caprolactam) or Nylon 6 (Perlon),and that obtained from 11-aminoundecanoic acid orNylon 11 (Rilsan). It is important to note that only thefirst of these has withstood the emergence of newfibres on the market.

Nylon 66The story of Nylon 66s discovery by the E.I.

DuPont de Nemours research group headed byCarothers, and the various stages of itsindustrialization (synthesis of the monomers and theirpolymerization, spinning, finishing and weaving of thefibres) marked the beginning of the era of synthetictextile fibres.

Early in 1928, Carothers was called to leadDuPonts base organic chemistry research group, andgiven absolute freedom in terms of which inquiries topursue. During base research on polyesters, variousstrategies to obtain polymers with molecular weightsabove 4,000 using condensation polymerization hadbeen identified. By pure chance, it was discovered thatthese could be used to obtain flexible fibres which,after cold drawing, turned into extremely robust longthreads. However, these fibres had no potential forcommercial success due to their solubility in thesolvents used for dry cleaning and excessively lowmelting point, which would have made it impossible toiron any textiles made from them. This led to the ideaof studying less soluble materials, with better physicaland mechanical properties, and in particularpolyamides, which had already been the subject ofprevious research.

In the summer of 1934, polypentamethylenesebacamide (polyamide 5-10) was made, melt spunusing the needle of a syringe for injections as aspinneret. In October of the same year, a new synthesismethod was developed, based on the principle that ifdiamine and diacid are mixed in equimolecular ratio,the amine salt of the diacid is obtained even withoutheating; this in turn polymerizes when heated, creating

a polyamide. In the salt, the two monomers are foundin the exact ratio of 1 to 1, an essential condition toobtain high degrees of polymerization. Since thepolymers made using this method sometimes had adegree of polymerization too high for them to be spun,at the beginning of 1935 it was discovered that smallquantities of acetic acid could be added to thepolymerization mixture, regulating it as desired.

According to Carothers, among the diamine-dicarboxylic acid pairings which could producepolyamides of interest, was pairing 66. Again at thebeginning of 1935, Berchet was charged with thepreparation of the two monomers and the polymer. Thepreparation of polyamide 66 was completed (12.5 g ofpolymer, yield 90%) on 1 March 1935. Thepolyhexamethylene adipamide appeared as a solidcorneous mass which melted at 252-254C from whichfibres were obtained; however, its high melting point ledto fears that it might decompose during melt spinning.

In the spring of 1935, the first polyamide 5-10filaments were obtained using a stainless steelspinneret and the melt spinning process (a completelyinnovative technique); these were twisted to turn theminto threads, each consisting of 24 filaments and witha count of 123 denier, in other words about 5 denierper filament (compared to 2 denier per filament ofnatural silk). Using these threads, the first fabric incompletely synthetic fibre was made.

From the summer of 1935 onwards, Carothersgroup devoted itself exclusively to polyamide 66, forwhich industrial production seemed possible, whereaswork on polyamide 5-10 was abandoned; this productcould not be industrialized since the availability on themarket of castor oil, from which sebacic acid wasobtained, was limited. In the case of polyamide 66, onthe contrary, adipic acid could be synthesized frombenzene, obtainable from the petrochemical industryin practically unlimited quantities, even though thetechnique for the industrial synthesis ofhexamethylene diamine was still unknown.

Since polyamide 66 had the potential forcommercial success as a textile fibre, it was decided touse it to produce high quality yarns which couldcompete with those in natural silk. The use ofpolyamide 66 to make a fibre very similar to wool(Fiber W), on the other hand, was abandoned foreconomic reasons. The industrial synthesis of themonomers turned out to be well-suited to thelarge-scale production of the polymer, which did notpresent particular problems on the industrial scale.

To transform the polymer into fibres, melt spinningwas chosen. However, at the process temperature(above 260C), a small quantity of the polymerdecomposed in the melting chamber, with theformation of gaseous products which caused the

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interruption of the filaments. Pressurizing the meltingchamber allowed melt spinning to become anindustrial process. A pilot plant, about a tenth of thesize of a commercial plant (with a production capacityof about 125 kg of yarn per day) came into operationon 11 July 1938.

From the day of the first synthesis of polyamide66, DuPont had maintained absolute secrecy regardingthe new fibre and its developments, but word did getout since as early as 1937 bristles made frompolyamide 66 scraps and rejects had been marketed(without divulging their chemical composition), soldwith the trade name of Exton Bristles and used tomake toothbrushes (this was the first use of what waslater to become nylon).

In September 1938 the first patents wereregistered. The New York Times reported the new fibre,and wrote in an editorial: A new kind of rayon hasbeen produced []. Because of its impact on the silktrade [] Japan has reason to worry. In October ofthe same year, during the New York World Fair, in thesection of The World of Tomorrow, Charles Stine, vicepresident of DuPont, presented the new fibre withthese words: I am making the first announcement of abrand new chemical fiber. This textile fiber is the firstman-made organic textile fiber prepared wholly fromnew materials from the mineral kingdom. I refer to thefiber produced from nylon []. Though whollyfabricated from such common raw materials as coal,water and air, nylon can be fashioned into filaments asstrong as steel, as fine as a spiders web, yet moreelastic than any of the common natural fibers. Tenyears earlier, he himself had promoted the coreresearch programme which led to its creation, and hadwarmly supported Carothers employment.

Nylon was an instant commercial success, andDuPont had to nearly triple the planned plant, whichwas not yet fully operational, to meet the requestsarriving from all over the world. Womens stockings innylon (Nylons) were presented by DuPont for the firsttime at the International Exhibition in San Francisco inFebruary 1939. The following year, more than 1,300tonnes of nylon were produced (most of which wereturned into womens stockings) with a value of about 9million dollars and providing a profit of about 3million dollars, amply repaying the costs of researchand development. Within the space of two years, morethan 30% of the womens stocking market belonged toDuPont Nylons.

When the United States entered the war in 1941,nylon became a strategic material, used to reinforcethe tires of lorries, cables and parachute silk, inaddition to nets, cables and coverings for antiaircraftbarrages, items of military apparel (replacing wool andcotton), haulage cables, etc.

Industrial production of the polymerWeighed in stoichiometric quantities,

hexamethylene diamine and adipic acid are sent to aboiler where hexamethylene diamine adipate (salt 66)is formed. It is dissolved in water and sent to anevaporator, where the water is removed andpolymerization begins. The contents are then sent to aboiler where polymerization proceeds at low pressure,first at 180C and then at 250-275C. The polymerizedmass is extruded from the lower part of the boiler,passed over a moulding roller, cooled with jets ofwater and reduced to small chips.

Industrial production of fibresNylon 66 fibres are made by melt spinning. The

polymer is melted on a silver grid in the presence ofvery pure nitrogen and the molten mass is then pushedthrough a spinneret in perfectly controlled quantitiesby a metering pump. The yarn, soaked in antistaticoils, is cooled in a cold air column and wound aroundbobbins. It is then subjected to cold drawing (drawingratio of 1 to 4 or above, depending on the tenacityrequired) to confer on it the desired mechanicalproperties. Most of the continuous filament issubjected to a texturizing process. If, rather than acontinuous thread, it is desired to produce staple, thetow is drawn, crimped and then cut into the desiredlengths.

Nylon 66 is commercialized as multifilament,monofilament and staple, with a vast range of counts.The fibres may be opaque, semi-opaque or shiny, andtheir section may be either circular or a variety ofother shapes (star-shaped, trilobed etc.) whenever it isdesired to give the textiles a particular appearance.

Properties and uses of the fibresThe tenacity of the fibres when dry ranges from

3.7-4.1 g/dtex (for staple; g refers to the grammeforce) to 4.2-5.3 g/dtex (for continuous filament), andreaches 8.1-8.5 g/dtex for high tenacity fibres. Whendamp, tenacity decreases by 10-20%. The elongationat break ranges between 19 and 32% for continuousfilament and reaches 40% for staple. Elasticity is highand the recovery from deformation is total as long asthe limit of elasticity is not exceeded. Resistance toabrasion is extremely good, and hygroscopicity (4%) ishigh compared to other synthetic fibres(polyacrylonitrile 2%, polyesters 0.4%). The fibres canbe used at up to 150C (after 6 hours at thistemperature they begin to yellow) and are flammable(though less so than those of cotton or artificial silk).

Nylon 66 fibres are resistant to alkalis, soluble in cold formic acid and sulphuric acid, are not damagedby the solvents used in dry cleaning and have a goodaffinity with numerous classes of dyes (acid,

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premetallized, chrome, etc.), which provide colourswhich remain fast despite light and washing.

Among the numerous finishing operations,thermosetting is important to shape and set the fabrics;by eliminating any residual retraction, it ensures thattheir size remains stable with use and washing. Nylon66 fabrics are very resistant to crumpling and, wherenecessary, can be ironed at temperatures lower than150C.

Given their high elasticity, appearance, feel,resistance to abrasion and repeated folding, and lowhygroscopicity, Nylon 66 fibres are ideal for durableand comfortable fabrics for windproof jackets andsports wear, womens wear, underwear, hosiery andsocks made of blends with wool or cotton.

Nylon 6From 1930 onwards, Carothers had researched the

polymerization of e-aminocaproic acid with Berchet,and together they had obtained, alongside a polymerwith a low molecular weight, a cyclic compoundwhich was named lactam. Considering their researchcomplete, Carothers and Berchet published a studyconcluding that e-caprolactam did not polymerize. Inthe spring of 1937, DuPont, in negotiations toexchange patents, informed IG Farbenindustrie aboutobtaining polyamide 66 (the future nylon), certain thatits patents would be unassailable. IG Farbenindustrieresearchers, and especially Schlack, immediatelyreread Carothers publications on e-caprolactam, andin 1938 heated it obtaining poly(e-caprolactam), orNylon 6, later known as Perlon in Germany.

The polymerization of e-caprolactam takes placeby heating at 250-270C in an autoclave in an aqueoussolution, in the presence of Nylon 66 salt ore-aminocaproic acid as initiators. Acetic acid or otherprocess terminators may be added to control thedegree of polymerization. In continuouspolymerization, a concentrated aqueous solution,which may contain an initiator, is sent into a tubereaction chamber, consisting of a single tube or abattery of tubes, heated to 250-270C. In this way, thereagents are heated gradually as they run through thetubes. The polyamide thus produced is then turned intogranules.

Nylon 6 melts at 215-217C and is more stablethan Nylon 66 when exposed to heat; melt spinningthus presents fewer difficulties. In general terms, theprocess is identical to that used for Nylon 66. Anothermelt spinning process involves melting in an extruder,whose screw channels the melt into the spinning head,where a metering pump pushes it through thespinneret. The filaments are cooled in air to 18-20C(relative humidity 45-55%) and then cold drawn (ratioof 1 to 4) at 15C (relative humidity 60-70%). The tow

for staple, on the other hand, is hot drawn, workedwith warm water to eliminate any traces of monomer,crimped and cut into the required lengths. Thetexturing of continuous filaments and product typesare similar to those of Nylon 66.

The properties of Nylon 6 fibres and textiles, andtheir uses, are basically identical to those of Nylon 66,with the difference that the former are easier to dyeand that, since they melt at lower temperatures, greatercaution is required when ironing. Nylon 6 acquiredcommercial importance (especially in Germany) in theearly 1950s, and later vanished almost completelyfrom the market.

Nylon 11This polyamide, known as Rilsan, was created by

Organico SA (France) in collaboration with SNIAViscosa and produced, as well as in France, in India,Brazil, and the ex-Soviet Union.

The polymerization of 11-aminoundecanoic acid,made from castor oil, is similar to that ofe-caprolactam, and it is melt spun at 215-220C.Usually, polymerization and spinning are continuous,without an intermediate granule stage.

Nylon 11 melts at 189-190C, and this representsits major drawback in many applications, since itcannot be ironed. It is produced as a continuousfilament, as staple and as monofilament. The fibresare similar in appearance to the other polyamides; thesame can be said of their mechanical properties. Nylon11s resistance to oxidation is higher than that of theother Nylons (it yellows at 150C in air). Given its lowabsorption of water, Nylon 11 is not easy to dye; thismay also be done in the bulk during spinning. Like thepolyamides examined above, Nylon 11 is used forknitwear.

PolyaramidsPolyaramids (or aramids) are totally aromatic

polyamides, obtained from diamines and aromaticdiacids. The introduction of aromatic rings into thepolymeric chains of polyamides leads to a raising oftheir melting point (polyaramids usually decomposebefore melting); they can therefore only be spun fromsolutions in strong acids (sulphuric, nitric) and thefibres obtained are strongly heat-resistant.

The first polyaramid fibre, poly(m-phenyleneisophthalamide) was commercialized in 1967 by E.I.DuPont de Nemours with the name Nomex. Nomex isobtained by the interfacial polymerization ofm-phenylene diamine with isophthalic acid dichloride.The fibres, which can resist temperatures of 300C fora considerable time, are used for fire-fightingequipment (to provide flame resistance) or to makeclothes which must resist high temperatures. In the

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early 1970s, Teijin in Japan marketed Conex or Konex,a fibre with a composition similar to Nomex. Duringthe same years, the Soviet Union began to produceFenilon, similar to Nomex, for civilian and militarypurposes and as a material suitable for the aerospaceindustry.

The studies by E.I. DuPont de Nemoursresearchers continued, and in 1970 another fibre witha very high elasticity modulus was introduced on themarket, to be used as a reinforcement for tires:poly( p-benzamide), initially given the trade nameFiber B. In 1975 poly(p-phenylene terephthalamide)was commercialized, with mechanical propertiessimilar to Fiber B; from this it was possible to obtainfibres for use in rigid compounds. Poly( p-phenyleneterephthalamide), obtained by the interfacialpolymerization of p-phenylene diamine withterephthalic acid dichloride, was sold with the tradename Kevlar.

Kevlar fibres are obtained by dry or wet spinningfrom solutions in sulphuric acid, in which theextremely rigid chains of the polymer (all fully in thetrans-conformation, since the cis-conformation issterically hindered) form a lyotropic mesophase inwhich the liquid crystals orient themselves parallel tothe axis of the fibre when they are extruded, makingdrawing to increase tenacity pointless. Kevlar fibresdecompose without melting at over 500C and haveextremely high resilience and tenacity (25 g/denier,seven times stronger than a steel wire, with theadvantage that their specific weight is 1/5 that ofsteel). They are used to reinforce radial tires and aboveall for advanced composite materials, widely used inaeronautical and space technologies. Other usesmainly concern special clothing (bullet-proof vests,helmets, protective gloves), sports wear, etc.

The fibres previously known as Arenka andsubsequently as Twaron, made in the Netherlands byAkzo NV, are identical to Kevlar in terms of chemicalstructure and properties.

12.7.3 Polyester fibres

These are polymeric synthetic fibres consisting at least85% of an ester of terephthalic acid with a glycol.Despite enormous amounts of research, polyethyleneterephthalate (PET) fibres are still unequalled, and aremore widely used than those in Nylon.

The history of PET fibres also begins withCarothers research on these polymers immediatelyfollowing his employment by E.I. DuPont deNemours. One of the very first polyesters synthesizedby Carothers as early as 1929 was that bycondensation between phthalic acid and ethylene

glycol. The results were not encouraging; a vitreousresin with low molecular weight (about 4,000) wasobtained, leading to the abandonment of this researchafter publication of the results. Even today, it isunclear why Carothers and his assistants did not tryreplacing phthalic acid (carboxylic group in orthoposition) with terephthalic acid (carboxylic group inpara position).

During this research, by reacting an organicdicarboxylic acid with 16 carbon atoms withpropylene glycol, a polyester had been obtained with amolecular weight of about 12,000, from whichextremely robust long filaments could be made bycold drawing. Although possessing far bettermechanical properties, especially elasticity, than rayonand natural silk, these fibres were soluble in all liquidsused for dry cleaning, deteriorated if treated withwater, and had a melting point below 100C. For thesereasons, they could not be used to make textiles, sincethese would have been neither washable nor ironable.

As early as October 1934, Edgar Spanagel, also amember of Carothers group, had prepared PET, butthis research was not pursued for various reasons.First, polyamides seemed far more promising asfibres; second, the melting point of PET wasconsidered too low for potential use as a fibre; andfinally, it seemed too easily hydrolysable. In 1940, theEnglishmen Whinfield and Dickson of Calico inManchester made PET from terephthalic acid andethylene glycol, patenting the procedure the followingyear. This research was then continued by ICI, whichhad purchased the patent from Calico with exclusiveworldwide rights for twenty years (with the exceptionof the United States). In 1945, ICI researcherscompleted the development of the process for makingPET and its fibres, which were marketed in 1947 withthe trade name Terylene.

Fibres identical to Terylene were produced almostimmediately in the major industrialized countries onlicence from ICI; these were known as Trevira inGermany (Hoechst), Terital in Italy (Montecatini) andTergal in France (Rhodiaceta) to mention onlyEuropean countries.

Although a Patents and Processes Agreement wasin force between E.I. DuPont de Nemours and ICI, thelatter, due to the safety measures called for by the war,had not informed DuPont about Whinfield andDicksons discovery. In 1944, rumours had reachedDuPont regarding the development in Great Britain ofa new fibre, Terrylite. Within a few weeks, DuPonttechnicians had obtained a sample which, based on thename, they assumed to be PET. In October 1944, withgreat scepticism but hoping that Carothers patents of1930 would cover its expenses, DuPont resumedresearch on PET. Using Carothers polymerization

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technique, a polymer with high intrinsic viscosity andhigh melting temperature was synthesized, whichcould easily be turned into fibres with excellentmechanical properties after cold drawing.

In early 1945 a meeting took place betweenEnglish and American patents experts, called by ICIand DuPont. At the end of this meeting, the Englishagreed that Carothers basic patents covered those forTerylene, but that this had not stopped Whinfield andDickson obtaining their own patents. As a result, ICIspatents were not valid in the USA, and DuPont wasable to commercialize its PET with the name Dacron.

Thanks to the experience matured in the field ofsynthetic fibres with Nylon and Orlon, DuPont actedfar more decisively than ICI in developing this newfibre, Fiber V. Although the two giants continued toexchange information about it, DuPont developedFiber V at far higher speed than Terylene; thisincreased further after 1948, the year in which thePatents and Processes Agreement ceased to berenewed.

The use of Fiber V to reinforce the frameworks ofcar tires did not provide good results, though it had allthe mechanical prerequisites. In March 1947 it wasdecided to abandon this market, and Fiber V wasdeveloped as a textile fibre, potentially in competitionwith Nylon and Orlon. Given the numeroussimilarities between the properties of Fiber V andthose of wool, a small amount of a fibre identical tothe latter was produced. Through external companies,in February 1948, a gabardine was made withextremely encouraging results, since it had propertiesidentical to those of wool, and was very resistant tocreasing thanks to the extremely high resilience ofPET fibres.

Having resolved the problems linked to the supplyof raw materials and monomers (in particular thoserelating to the production of dimethyl terephthalate),to polymerization and spinning (the continuousprocess, without the intermediate granule stage, turnedout to be economically preferable), to the reduction ofstatic electricity in the articles made and finally topilling (in other words the formation of small bobbleson the surface of articles due to friction, resolved byusing filaments with a non-circular section oranti-pilling fibres), Fiber V was put on the market in1953 with the trade name Dacron.

Industrial productionThe dimethyl ester of terephthalic acid is made to

react with an excess of ethylene glycol in the presenceof very small quantities (0.01-0.015%) of lithium ormagnesium salts in a nitrogen stream to entrain themethanol formed, heating to 200C until the latter iscompletely eliminated. The product of the reaction is

transferred to an autoclave heated to 290C where,under vacuum, the glycol gradually formed aspolymerization takes place is eliminated. The processends when the PET has reached a molecular weight ofabout 18,000. At this point the polymer can beextruded, cooled with water and reduced to chips. Meltspinning is used, heating the polymer chips until theymelt and channelling the melt into the spinnerets(holes about 0.3 mm in diameter) using a meteringpump.

Another way of carrying out the polymerizationinvolves the elimination of water, starting directlyfrom terephthalic acid and ethylene glycol in cascadereactors up until the final condensation, after whichthe molten polymer passes directly on to spinning(continuous process). This type of process, as well asavoiding the need to melt the polymer a second time,presents numerous other advantages, both technicaland economical. As they exit the spinneret, thefilaments are solidified by cooling in air, gathered onrollers at extremely high speed (1,000-1,500 m/min)and then subjected to hot drawing (70-90C). Forproduction as staple, after cooling, the filamentsexiting the spinnerets are gathered in tows and placedin purpose-built containers; they are then removedfrom these to be subjected to steam drawing. Thedrawn tow is then crimped and finally cut into thedesired lengths.

Properties and uses of the fibresIf cooled suddenly, molten PET does not

crystallize (density 1.34 g/cm3) and can easily beoriented by drawing. It crystallizes when heated totemperatures above 80C, with a maximumcrystallization velocity at 180C. Consequently,reheating immediately after drawing has the effect ofcreating highly crystalline fibres (crystallinity above40%, density 1.38-1.39 g/cm3) with high tenacity(from 5.8-7.2 g/dtex for continuous high tenacityfilaments to 2.3-5.0 g/dtex for staple). The possibilityof intervening during the transformation into fibresmakes it possible to obtain yarn with differentproperties: these range from high tenacity continuousfilament for tire coverings or other industrial uses, to astaple very similar to wool for use in knitwear, fromfibres for crease-proof textiles to those used forcurtaining, etc.

PET fibres can be made with sections of differentshapes, in addition to circular, producing yarns withdifferent appearances. The continuous filaments caneasily be texturized, making it possible to use these, inwool or cotton blends, in textile applications for whichstaple is indispensable.

The mechanical properties of PET fibres are notaffected by humidity. The elongation at break ranges

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from 8-11% for high tenacity filaments to 20-50% forstaple. The elasticity modulus one of thecharacteristics of PET fibres is high (from 100-130g/dtex for continuous high tenacity filaments to 30-70g/dtex for staple). The crystalline structure of PETprevents viscous flow under stress; creep is thereforeextremely limited and the fibres cannot be deformed.

The absorption of water by PET fibres is verylimited (0.4-0.5%), their melting temperature is 260Cand they retract at around 230C; as such, these fibrescan be ironed at temperatures up to 200C, more thansufficient to add or remove creases.

PET fibres have excellent resistance to chemicalagents, especially those used for dry cleaning. Sincetheir hydrolysis velocity is low, they resist well both towater and to acids and bases; they also present goodresistance (better than that of polyamides) to sunlightand can be dyed using dispersed dyes.

As already mentioned, PET fibres are used both ontheir own and in blends with wool or cotton. In somecases they may be prone to pilling, as is the case forother fibres. PETs strong points are the silk-liketextiles used for womens clothing and furnishings,sports fabrics (the use of microfibres makes it possibleto make items which are at once breathable andwaterproof), curtaining, safety belts for cars, padding,clothing of cotton type, etc.

Totally aromatic polyestersLike aromatic polyamides, aromatic polyesters

form liquid crystals. However, unlike the former,which form these in solution (lyotropic liquid crystalpolymers), the latter form liquid crystals when molten(thermotropic liquid crystal polymers); whereas theformer must be spun from a solution (their meltingpoint is close to that of decomposition), the latter canbe turned into fibres by melt spinning.

The first aromatic polyester to be developedcommercially was Xydar, by Dartcore, USA(December 1984), which derived from Carborundum,USAs EKKCEL (1972). Vectra (plastics and fibres)and Vectran (fibres only) are two totally aromaticpolyesters commercialized in 1986 by the thenCelanese, now Ticona GmbH (Germany). All thesematerials are prepared by condensation between p-hydroxybenzoic acid, terephthalic acid, and 4, 4-dihydroxy diphenyl. Given the low viscosity ofthe melt resulting from its liquid crystal structure,spinning does not present particular problems.Spinning is followed by thermal treatment at 300C orabove, thus obtaining an improvement of mechanicalproperties due in part to further polymerization.

Vectran fibres, like Kevlar fibres, have hightenacity and elasticity modulus; they do not absorbwater, their physical and mechanical properties are not

altered by humidity, they have excellent resistance toimpact (much appreciated when they are used foradvanced composite materials), excellent resistance toabrasion, good resistance to high temperatures (240Cfor constant use, 340C for brief periods) and highchemical stability (to acids in particular). For thesereasons they are widely used for cordage, work gloves,protective clothing, acid-resistant filters, materials forfriction, advanced composite materials, etc.

12.7.4 Polyvinyl fibres

These are synthetic fibres obtained by additionpolymerization from vinylic and vinylidenicmonomers. Poly(vinyl chloride) was the first vinylpolymer from which fibres were obtained (1913), butthese achieved little success. The most important arethe polyacrylic fibres obtained from polymers basedon acrylonitrile; of secondary importance are thoseobtained from polymers of vinyl chloride and vinylalcohol.

Polyacrylic fibresCurrently, not all polyacrylic fibres consist of

homopolymers of acrylonitrile, but of its copolymerswith other vinylic monomers, introduced into thepolymeric chains to avoid problems with productionand dyeing. If the content in structural units derivingfrom acrylonitrile is equal to at least 85% in weight,the fibres are described as acrylic, the most important;if the content is between 85% and 50% they are knownas modacrylic.

The first polyacrylonitrile (PAN) fibres appearedin 1942, when in DuPonts laboratories it wasdiscovered that this polymer is soluble indimethylacetamide (DMAC) and indimethylformamide (DMF). PAN could not be turnedinto fibres using melt spinning like nylon, since itdecomposes before melting.

The first PAN fibres (Fiber A) obtained fromsolutions in DMF by dry spinning from a solution orwet spinning from a solution immediately showed veryinteresting characteristics, such as a high melting pointand good chemical stability. When not drawn, or onlyslightly drawn, they had properties similar to those ofwool, whereas when drawn their properties weresimilar to silk; they also had excellent resistance tolight, chemical products and bacteria.

The significant drawback of Fiber A was that itcould not be dyed, and non-dyeable fibres have nocommercial potential. A further factor whichcontributed to slowing down its development was thatVinion N (a 50% acrylonitrile-50% vinyl chloridecopolymer) commercialized by the Carbide and

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Carbon, which seemed economically more viable, hadshown that DMF was strongly toxic. Due to the war, itwas also difficult to find the raw materials forproduction (those used for strategic materials such asnylon had precedence). It was pointed out to themilitary that Fiber As excellent resistance to light,chemical products and bacteria could resolve many oftheir problems in the jungle (such as tents, laces andtarpaulins in cotton, which deteriorated rapidly) andthis allowed DuPont to obtain raw materials moreeasily. Vinion N turned out to be inferior to Fiber Adue to its lower melting point and greater solubility insolvents. The problem of DMFs toxicity was resolvedby adopting the dry spinning from solution process,carried out in closed chambers. At the end of 1944 theproblem of dyeing remained unresolved; this isconsidered the Achilles heel of all hydrophobic fibres(nylon and polyesters).

After exhausting all attempts to find suitable dyes,it was decided to render Fiber A dyeable by modifyingits chemical structure. It was discovered that a 95%acrylonitrile-5% 2-vinylpyridine copolymer (OrlonA-3) could be dyed. However, 2-vinylpyridine was notavailable in large quantities, and was also extremelyexpensive. At the end of 1948, then, the problem of thedyeability of Orlon had not been fully resolved, and itwas unclear whether it was better to produce modifiedor non-modified Orlon, since the latter could bewidely used to replace cotton for tents and tarpaulins.

Since Orlon as a continuous filament did notbecome a successful product, attempts were made toproduce it as staple Orlon A-4, a copolymer ofacrylonitrile with 5-methyl-2-vinylpyridine, easier todye than Orlon A-3. Since the presence of cuprousions in the dye bath increased the reactivity of PANwith acidic dyes, and the problem of dyeing thusappeared to have been resolved without modifying thepolymer, the production of staple (Orlon Type 41)consisting of homo-PAN began. Orlon Type 41 was afailure due to the difficulty of weaving caused bystatic electricity, the non-uniformity of the fibrils andunpleasant odours when blended with wool.

A solution to the problems presented by PAN camefrom the rapid development of the 94%acrylonitrile-6% methyl acrylate copolymer, Orlon A-6(Orlon type 42 yarn). The staple turned out to be idealfor sweaters and wool blend worsteds; by the end of1951 sales had reached and superseded 50,000 tonnesper year, and Orlon became a great commercialsuccess.

Industrial productionAmong the various ways of polymerizing

acrylonitrile (remembering that it is soluble in water,whilst polyacrylonitrile is insoluble both in water and

in its monomer, so that bulk polymerization createsenormous problems), the most widely used is that ofradical polymerization in a solution, using highlypolar organic compounds (DMF or dimethylsulphoxide) or aqueous solutions of inorganic salts(60% zinc chloride, 44-50% sodium thiocyanate,calcium thiocyanate and perchlorates) as solvents.The advantage of this process lies in the fact that thepolymer solution can be spun directly. In the dryspinning process (generally used to producecontinuous filaments) a 20-30% solution of thepolymer in DMF is extruded at 80-150C in a vertical tube, in which a stream of hot air (230-260C) causes the evaporation of the DMF(which is recovered) and the solidification of thefilaments which are gathered on bobbins. The fibres,which still contain 10% DMF, are then washed withwater and subjected to drawing at 80-110C in hotair, or at 70-100C in water.

In wet spinning (used to produce staple) thepolymer dissolved in DMAC or in DMF is extrudedinto water, in which the filaments coagulate forming abundle which is washed, drawn in hot water, crimpedand cut into the desired lengths.

Properties and uses of the fibresPAN fibres are essentially amorphous and soften

above 225C. They are very stable when heated andtend to yellow if exposed for long periods totemperatures over 130C; their tenacity decreases byless than 4% after 100 hours at 150C. Since theygenerally consist of copolymers of acrylonitrile inaddition to homo-PAN, they present a broad spectrumof properties, depending on their chemicalcomposition and the way they have been treated. Theshape of their section is circular for continuousfilaments and dog bone for staple.

Staple has a tenacity of between 2 and 3 g/dtexwhilst continuous filament, which is less used, is moretenacious (4-4.2 g/dtex). Elasticity is not high(recovery of 50-60% for a deformation of 10%). Theabsorption of humidity (above 2% at 20C) is amongthe highest for synthetic fibres. Despite this, it driesvery rapidly and can be used for wash and wearclothing; it also has good resistance to sunlight and tochemical agents (dilute acids and bases, liquids for drycleaning, etc.).

Dyeing is carried out using either basic dyes,which provide fibres with solid and brilliant colours,or with acid dyes. The articles produced cannot beironed, since they would become deformed.

Worldwide production of acrylic fibres isextremely high (millions of tonnes per year); they areused both alone and in blends with wool for outerknitwear, sports socks and exterior curtaining.

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Modacrylic fibresThese are acrylic fibres which contain less than

85% acrylonitrile. Among the numerous types, it isworth mentioning Verel and Dynel, both of which havegood flame-retardant properties.

Verel (Tennessee Eastman, USA) containsacrylonitrile (about 60%) and vinylidene chloride andis produced only as staple (including a high retractiontype) by wet spinning from a solution. Easy to dyewith basic, dispersed and premetallized dyes, it ismainly used in blends with other fibres (natural,artificial or synthetic); it has modest mechanicalproperties (tenacity 1.7-2.3 g/dtex, deformation atbreak 35-40%), softens at around 200C and resistsageing better than other synthetic fibres, wool andcotton. It dissolves in hot acetone.

Dynel (Union Carbide, USA) is a 40%acrylonitrile-60% vinyl chloride copolymer. The fibres(including a high retraction type), obtained by wetspinning from a solution, are thermoplastic and retracteven in boiling water; they are dyed at temperaturesbelow 100C in the presence of swelling agents. Theirtenacity is low (2-3.8 g/dtex) and their elongation atbreak ranges from 14-40%. They are used for carpets,industrial textiles, protective clothing and furnishingsin places covered by fire regulations (cinemas,theatres, etc.).

Poly(vinyl chloride) fibresPoly(vinyl chloride) (PVC) fibres were the first

synthetic fibres ever made, in 1913. In 1934 IGFarbenindustrie (Germany) presented the PC fibre(post-chlorinated PVC, to render it soluble in acetone)and in 1941 Rhodiaceta made PVC filaments spunfrom a mixture of carbon sulphide and acetone.Montefibre (Montedison, Italy) produced Leavin,crystalline fibres from syndiotactic PVC (spinningfrom solutions in cyclohexanone), far more resistant toheat than atactic PVC.

PVC fibres have low tenacity (3 g/dtex), areproduced as continuous filaments and as staple, arethermoplastic (they soften at around 80C) and are notattacked by acids and bases, but are particularlysensitive to ketones; they are dyed in baths with azoicdeveloping dyes or using dispersed or solution dyes (for black). Furthermore, these fibres are stronglyflame-retardant; they dehydrochlorinate and brownwhen exposed to heat. Their flame retardant propertiesmake them ideal for furnishings and industrial textiles,thanks in part also to their resistance to acids and bases.

Different types of more or less modified PVCfibres can be found on the market; these includethermoretracted fibres (Thermovil, Movil T), whichresist temperatures of up to 100C and those obtainedfrom chlorinated PVC (Clevil).

Poly(vinylidene chloride) fibresFibres of poly(vinylidene chloride), or more

accurately of copolymers of vinylidene chloride/vinylchloride (Saran, 20% vinyl chloride) are melt spun atabout 180C and are almost dyed in bulk. They arenon-flammable, have low tenacity, soften between 120and 160C and are basically used for industrialtextiles.

Poly(vinyl alcohol) fibresPoly(vinyl alcohol) fibres, obtained from the

partial hydrolysis of poly(vinyl acetate), are wet spunfrom aqueous solutions, coagulated in a sodiumsulphate solution and treated at 240C to render themmore compact. They are then made insoluble in waterby acetylation (30-40% in moles) and partialcrosslinked with formaldehyde.

Poly(vinyl alcohol) fibres are shiny and irregular insection, have high tenacity (up to 8 g/dtex), good heatresistance (they retract 10% at 220C), soften at250C, resist ageing well and can be dyed easily. Theyare used almost exclusively in Japan (KurashikiRayons Kuralon), in blends with cotton or rayon, tomake textiles for umbrellas, awnings, carpets, etc.

Polytetrafluoroethylene fibresPolytetrafluoroethylene (PTFE) is insoluble in all

solvents and decomposes before melting; its fibres aretherefore obtained by extruding a dispersion ofextremely tiny particles of the polymer in a coagulantbath of water and hydrochloric acid. The filamentsthus obtained, made of particles separate from oneanother, are then heated rapidly to 390C, so that theparticles syntherize to form a continuous filament.They can also be melt spun from copolymers oftetrafluoroethylene with propylene, perfluoropropylene or perfluorovinyl ethers.

PTFE fibres have an extremely low frictionalcoefficient, total resistance to all chemical agents andexcellent heat resistance (they can be used between70C and 280C), but have low tenacity (1.3 g/dtex).Their value lies in their high chemical inertia and heatresistance; for this reason they are used for industrialand space applications.

12.7.5 Polyolefin fibres

Polyolefin fibres derive from olefin (basically ethyleneand propylene) polymers, and include polyethyleneand polypropylene fibres.

Polyethylene fibresPolyethylene fibres have never had particular

commercial importance, given their poor mechanical

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and textile properties, which have led to their use onlyfor special applications. Even the synthesis, at thebeginning of the 1960s, of polyehtylene with a highmolecular weight (2106), named UHMWPE (UltraHigh Molecular Weight PolyEthylene) andcharacterized by physical and mechanical propertiesfar superior to those of conventional polyethylene(molecular weight 104) did not bring any changes,since the product could not be spun due to theextremely high viscosity of its melt. In 1976 the gelspinning process was finally discovered at the DSMlaboratories (Netherlands); this involved drawing(drawing ratio of 1 to 30) fibres from a dilutedsolution of UHMWPE in xylene or decaline, with atenacity and elasticity modulus close to the theoreticalvalues for planar zigzag chains.

Among the various high tenacity fibres inUHMWPE of commercial interest spun from a gel areAllieds Spectra 900 and 1000 (USA, 1984) andDyneemas Dyneema SK90 (Netherlands, 1986). Thesefibres are chemically stable, have melting pointsbetween 145 and 155C (depending on the drawingratio) and preserve their mechanical properties up totemperatures close to their melting point. They are tentimes more robust (free breaking length 336 km) thanthose of steel and are used either as such, or incomposites, or to reinforce other polymers (in loudspeaker cones, bows, bows for musical instruments,helmets, etc.). Due to their low density (1 g/cm3),high tenacity, high resistance to impact andatmospheric agents, resistance to abrasion, highstability in the presence of ultraviolet light andexcellent insulating and water-repellent properties,high tenacity UHMWPE fibres are used for cordageand cables, bullet-proof vests, protective clothing,parachutes and construction materials.

Polypropylene fibresThe discovery of isotactic polypropylene by Giulio

Natta (1954) led to research on the potential formaking fibres from it; these were produced on anindustrial scale from 1960 onwards by the Polymer(Montecatini Group, Terni, Italy, now Meraklon) withthe trade name Meraklon.

Before spinning, antioxidants and sometimes, giventhe difficulties presented by dyeing, pigments, colorantsand granules (granules are more stable than powder) areadded to the isotactic polypropylene in powder form.For melt spinning, special types of spinnerets are usedto avoid the rupture and deformation of the fibres, dueto their high tixotropy when molten. The fibres (with acircular section) are produced either as continuousfilaments (drawn at 120C through a die) or as staple(the tow is drawn in steam, crimped, thermoset and cutinto the desired lengths).

The fibres are highly crystalline, melt at between170 and 174C, and have low density (0.90 g/cm3).They can be prepared with varying degrees of tenacity(up to 9 g/dtex), have good mechanical properties andresistance to viscous flow, present good resistance toacids and alkalis and many chemical products (theyswell in hydrocarbons), are not easy to dye (dyeing, asmentioned above, is done in solution withthermostable pigments) and have low resistance tolight (stabilizers are also used for purposes which donot require long exposure to sunlight). Given their lackof sensitivity to water, resistance to abrasion andcyclical bending stresses, lightness and tenacity, thesefibres are used for manufacturing fishing nets, cords,industrial filters, in the automobile industry, as ageotextile (synthetic grass for sports fields), carpetsand rugs (characterized by their ease of cleaning,brilliant colours, resilience and resistance to decay).On their own or in wool blends they are used forknitwear, underwear and sports wear (double layeredgarments with an inner layer in polypropylene andouter layer in natural fibre, ensuring the rapid removalof sweat by capillary action, keeping the skin dry).

12.7.6 Polyurethane fibres

Today, polyurethane fibres are the elastomeric fibres(Spandex) commercialized by E.I. DuPont de Nemours(USA) in 1962 (Lycra) after twenty years of researchaimed at obtaining synthetic elastomeric fibres toreplace those in natural rubber covered with cotton.

As early as 1942, DuPonts researchers hadobserved that N-substituted (Type 8 Nylon) Nyloncould be turned into filaments with elastomericproperties. During this research, it emerged that toobtain elastomeric fibres it was necessary to makeblock copolymers consisting of flexible segments (togive elasticity) and rigid segments (to confer highmelting temperatures).

The preparation of these block copolymers wasmade possible by the discovery of the interfacialpolymerization method. At the beginning of 1954,by extruding a prepolymer of Adiprene (anelastomeric polyurethane made by Orchem, USA)in a diamine solution, fibres were obtained withexcellent elastomeric properties. Later, apoly(ether-urea-urethane) was obtained (Type 80),with even better properties, which could be dryspun or wet spun from solutions indimethylformamide or dimethylacetamide. Afterresolving the problems relating to their sensitivityto ultraviolet light and proneness to yellowing,these polyurethane fibres were commercialized(1962) with the names Lycra for multifilaments

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and Vyrene for monofilaments. Within the space oftwo years, these fibres had become one ofDuPonts most profitable products, with a return oninvestments of over 30% net of taxes.

Synthesizing Spandex is fairly complex, andinvolves preparing a prepolymer (polyester orpolyether with a low degree of polymerization withhydroxy terminals which later form the rigid blocks inthe polymeric chain), reacting this with a diisocyanate(forming isocyanic chain terminals), and then with adiamine or a glycol which form the flexible blocks.Spinning is done using the methods outlined abovebefore the final crosslinking which can also beobtained by simple heating (to be such, an elastomermust be partially crosslinked).

Lycra has allowed for the preparation of veryfine elastic textiles with a high compression force(not obtainable using rubber fibres); they are easy todye using acid or plastosoluble dyes. Unlike rubberfibres (minimum count 150 denier), Spandex fibresmay have far lower counts (down to 40 denier);Lycras do not break when sewn (unlikemonofilaments of rubber), since the needle passeseasily between filaments. Given an identicalelongation, Spandex has a higher tenacity andelasticity modulus than rubber fibres; this meansthat, for an identical count, they have a higher (3-6 times) compression force or, in other words,that given an equal compression force they can havea lower count. This property represents the key totheir success. Their elastic properties are also good,only slightly inferior to those of rubber.

Polyurethane fibres are never used on their own,but always in combination with other fibres (especiallypolyamides) to confer elasticity on the articlesproduced (elastic tights, swimsuit, stockings etc.).

12.7.7 Polyacetal fibres

The only known polyacetal fibres are those derivedfrom Delrin (the polyoxymethylene obtained by E.I.DuPont de Nemours by polymerizing formaldehyde,commercialized in early 1960), whose physical andmechanical properties allow them to be used assubstitutes for non-ferrous materials (brass,aluminium) in many of their applications.

Filaments of Delrin (Tenac SD, Asahi ChemicalIndustry, Japan), obtained by superdrawing at highpressure, consist of perfectly oriented chains whichconfer upon them excellent mechanical properties(better than those of steel wire): excellent resistance toheat, chemical stability and resistance to atmosphericagents. Furthermore, these fibres have high creepresistance and do not absorb water: they are used for

fishing nets and lines, as a reinforcement for cement,as cords for tennis rackets, etc.

12.7.8 Carbon fibres

This paragraph will deal with carbon fibres; thoughnot synthetic, they do derive from polyacrylonitrile(PAN). The first carbon fibres were made in the USAby pyrolysis of viscose rayon, but were short-lived;they were soon supplanted by those obtained from thepyrolysis of PAN, which appear in the form ofcontinuous filaments and consist of graphitic carbon.Their high crystallinity confers upon them exceptionalqualities: they are extremely tenacious, highly heatresistant and very light (density 1.8-2 g/cm3) and arewidely used as a reinforcement in advancedcomposites. Among the major producers of carbonfibres is Courtaulds (Great Britain).

Carbon fibres are made by graphitizing a precursorfibre which may be either rayon viscose or PAN. Thecontinuous fibres of PAN are heated in air (oxidation)at about 300C so as to form ladder structures which,when further heated to temperatures up to 3,000C,loose ammonia and hydrocyanic acid, creatingstructures containing over 95% carbon. These are thenturned into graphitic carbon by treatment in an argonstream at 2,200-2,300C. Carbon fibres have excellentmechanical properties and are practicallyincombustible (they can be heated until they are cherryred, about 800C, without suffering damage). They areblack in colour and are basically used in compositematerials, especially when high mechanicalperformance and low density are required, alongsidegood fatigue resistance and dimensional stability. Theaerospace industry (missiles, artificial satellites,civilian and military aircraft, helicopters etc.) is one ofthe major markets for carbon fibres and theircomposites. This is because their use leads to asignificant reduction in weight which translates intolower fuel consumption or a larger paying cargo.Carbon fibres and their composites are widely used inthe nautical industry (hulls, sails, masts etc.) and in theautomobile industry, various sports sectors (tennis,fishing rods, cycling), for conductive materials(electrodes), in textiles (Orlon black) for protectionagainst high temperatures, etc.

12.7.9 Synthetic fibres for medical use

Threads for sutures, meshes to reinforce the abdominalcavity, vascular replacements and hollow fibres for thetreatment of blood (artificial kidneys, mechanical

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lungs) represent the main uses of synthetic fibres in thefield of medicine. For these purposes, in addition topossessing suitable physical and mechanical properties,the fibres must be compatible with the tissues of thehuman body and with blood, have extremely lowhistotoxicity, must not be carcinogenic and mustbehave suitably while they remain in the human body.

Threads for suturesTheir specific function is to join the tissues of the

human body until healing has occurred, after suturingfollowing surgery, wounds or trauma. They must havegood resistance to traction, a low frictional coefficientwith tissues and sufficient flexibility and elasticity(since the resistance of the knot depends on this).

Non-absorbable suture threads (which remainintact indefinitely in the human body) are obtainedfrom the commonest synthetic fibres (polypropylene,polyethylene and their copolymers, Nylon 66,polyethylene terephthalate).

Absorbable suture threads (monofilament andmultifilaments) were initially made of poly(glycolicacid) fibres (1970) and later of glycolic acid-dilactidecopolymers, poly(p-dioxanone), and glycolic acid-trimethylene carbonate copolymers. After about a month the threads dissolve, since they degrade tomonomers by hydrolysis of the ester bonds.

Before use, absorbable and non-absorbable suturethreads are sterilized with ethylene oxide or withionizing radiation.

Meshes for the reinforcement of the abdominalcavity

From 1995 it has become routine to use meshes inpolymeric materials to reinforce the abdominal cavity(for example in the case of hernias); these are obtainedby weaving a wide variety of synthetic fibres, such aspolyamides, polyesters, polypropylene or absorbablefibres such as those of poly(glycolic acid) or lacticacid-glycolic acid copolymers.

Polyamide (Nylon 66) meshes, the first to beemployed (as early as 1944), are now rarely used, sincethey cause acute inflammatory responses. Of all thematerials used to reinforce the abdominal cavity, themost frequently used today are polypropylene andpolyethylene terephthalate (Dacron) meshes. Thelatter, developed simultaneously with the former, areless widely used since they cause a greaterinflammatory response and a more significant reactionto the foreign body than those in polypropylene.

Polypropylene and Dacron meshes are obtained byweaving a single filament, a pair of filaments ormultiple filaments. Meshes in Dacron are sterilizedwith ionizing radiation, and those in polypropylenewith ethylene oxide.

Vascular replacementsVascular replacements in polyethylene terephthalate

fabric (Dacron) or polytetrafluoroethylene (Teflon) arewidely used to replace defective veins or arteries (forexample in the case of aneurisms) of 6, 8, 10 mm indiameter. Despite their good biocompatibility, theanticoagulant properties of those in Dacron are nothigh, whereas those in Teflon have good anticoagulantproperties. Currently, there are no vascularreplacements for blood vessels less than 3 mm indiameter; in these cases it is therefore preferred toreplace them with blood vessels taken from other partsof the body.

Fibres for the treatment of bloodThe filter membrane in haemodialysis machines

(artificial kidneys) consists of bundles of hollowpolyacrylonitrile fibres, through which the moleculescontained in blood with a molecular weight up to20,000 can permeate, but not those (includingalbumin) with a molecular weight of around 70,000,which are retained.

The artificial lung is a gas exchanger which servesto supply O2 to blood and remove CO2 from it usingmembranes consisting of bundles of hollowmicroporous polypropylene fibres; due to thehydrophobicity of the fibres, their pores are freelypermeated by gas but not by blood.

12.7.10 Microfibres and nanofibres

MicrofibresThe development of sophisticated and complex

processes to obtain synthetic fibres (especiallypolyamides and polyesters) with increasingly lowcounts has led to ultrafine fibres (microfibres) withdiameters in the order of micrometres (down to single0.01 denier fibres, with a diameter of about 0.4 mm)used to make fabric and non-fabrics with specificproperties. Ultrafine fibre technology was initiallydeveloped in Japan to manufacture artificial leatherssuch as Torays Ecsaine (Alcantara in Europe,Ultrasuede in the USA), similar to suede (easily dyed,widely used for car seats, to cover sofas andarmchairs, etc.), consisting of monofilaments as smallas 0.1 denier. A further development of microfibreswas the creation of high density water-repellenttextiles and non-fabrics, used for clothing and sport(so-called microfibre clothing) and cloths suitable forcleaning lenses, jewels, crystals etc.

NanofibresThe main characteristics of a polymeric

nanofibre are a diameter in the order of

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nanometres, and consequently a high surface areaand superior mechanical properties. Among thevarious techniques for preparing nanofibres(spinning, template synthesis, phase separation,autoassembly), electrospinning is the mostcommonly used for its efficiency and the greatsimplicity of the equipment required. This processconsists in applying a potential difference either toa molten polymer (polyethylene, polypropylene,polyethylene terephthalate etc.) or to a solution ofit (polycarbonates, polystyrene, polyurethanes,polyacrylonitrile, polyethylene terephthalate,Nylon 66, poly(vinyl chloride), etc.), causing theformation of a jet of material which thensubdivides itself into extremely thin fibres.

Nanofibres are used in composite materials(elasticity modulus and resistance of the matrixmaterial higher than those obtained with commonfibres, including carbon fibres and Kevlar), intextiles for protection from chemical agents(thanks to the considerable absorbent capacity dueto the high surface area of the nanofibres), inmembranes (which can be used as high efficiencyfilters, given the limited volume occupied by thefibres), in bandages sprayed directly onto woundsby electrospinning (avoiding the formation ofscars), to help the regrowth of human tissues in theevent of disease (nanofibres, having a diameterlower than that of cells, act as frameworks for theregeneration of the tissue), as replacements for softhuman tissues, to transport drugs inside the humanbody, and cosmetic masks.

It is important to stress that hitherto syntheticpolymeric nanofibres have not been commercialized;these should therefore still be considered subjects forlaboratory research, with a view to promisingapplications in the future.

Bibliography

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Hearle J.W.S. (edited by) (2001) High-performance fibres,Boca Raton (FL), CRC; Cambridge, Woodhead.

Hermes M.E. (1996) Enough for one lifetime. Wallace Carothersinventor of nylon, Washington (D.C.), American ChemicalSociety and Chemical Heritage Foundation.

Holmes D.F. (1983) History of the DuPont companys textilefibers department, Wilmington (DE), DuPont, Textile FibersDepartment.

Hongu T., Phillips G.O. (1990) New fibers, New York, EllisHorwood.

Hounshell D., Kenly Smith J. (1988) Science and corporatestrategy. DuPont R&D 1902-1980, Cambridge, CambridgeUniversity Press.

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Klein W. (1994) Man-made fibres and their processing,Manchester, The textile institute.

Mark H.F. et al. (editorial board), Kroschwitz J.I. (editor inchief) (1985-1990) Encyclopedia of polymer science andengineering, New York, John Wiley, 24v.

Mishra S.P. (2000) A text book of fibre science and technology,New Delhi, New Age International.

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Luigi TrossarelliValentina Brunella

Dipartimento di Chimica Inorganica, Fisica e dei Materiali

Universit degli Studi di TorinoTorino, Italy

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