23 July 1971, Volume 173, Number 3994

Polyester Fiber: From Its Inventto Its Present Posit

The structure and properties of this versatile syntfiber have led to remarkable growth in many end

Alfred E. Brown and Kenneth A. Re

During the past several decades, sci-ence Land technology have contributedgreatly to the development, commer-ciailization, and continuing growth of ahost of polymeric products, which areused as fibers, plastics, or films. Amongthese products, polyester fibers hatveattracted particular attention becaLusetheir consumption in the United Statesin 1970 exceeded for the first time thatof nylon fibers. In addition, 1970 wasthe second successive year in which theconsumption of man-made fibers ex-ceeded that of the natural fibers cottonand wool.

There are many reasons for the ra,pidgrowth and broad use of polyester: forexa.mple, the achievements of polymerchemists and physicists in matnipulaltillgchemical structure and morphology toachieve a wide range of properties re-quired for various uses, the significantbreiakthroughs in organic synthesis andprocess chemistry which led to low-costraw materials, and the very favorableecononmics of large-scale malnLufactur-ing. This article presents an overviewof developments in polyester fiberssince they were invented 30 years ago,with emphasis on polyester chemistry.molecul[ar structure, properties. aind enduses.

Man-made fibers comprise two cate-

The atithoi-s are, respectively, presidenit andgrouLp leader at the Celanese Research Compalny,Sunmmit, Ncw Jersey 07901.

*-- Circle No. 2 on Readers' Service Card

gories: synthetic andPolyester fibers, likepolypropylene fibers,thetic category of m-

cause they are ma

small molecules, or r

large molecules, or p

ical reaction and thsynthetic polymers iRayon, acetate, andregenerated fibers, w

by chemically treatinlmer cellulose, and r

fibrous form. It is t1first made on a comn

e,arly 1940's, that havtbor the enormous grsuLmlption of man-ma

During the 1960'sester increased steadspOctacularly, in suchahle press garments,tured knit fabrics. Inon a very large scalfiber in automobile tition of polyester in 1

billion pounds, compbillion pounds of nyk

0.45 kilogram) (1). Iportunities for futurefavorable, since pol~economical, but thethe many synthetic fit

All fibers are bamolecules that haveweight and are forn

SCIE:NCE

small molecules into long chain mole-cules. The stringing together of severalthousand monomer units to form apolymer chain is accomplished by po-

* lymerization. In this process, chemists11on use individual monomers, or mixtures

of two or more, and control their spa-10on tial arrangement to produce polymers

that are converted to fibers with thedesired qualities of strength, durability,

thetic elasticity, dyeability, and chemical re-sistance. Herman Mark (2), a pioneer

uses. in the field of polymers described threeprinciples that determine the properties

sinbart of polymers and their tisefulness asstrong, durable materials in either fi-brous, plastic, film, or rubbery forms:

1) The capability of the chains toarrange themselves in a crystalline or

regenerated fibers. ordered structure;ny.on, acrylic, and 2) The degree of stiffness, flexiblebelong to the syn- or inflexible, of the chain; andan-made fibers be- 3) Whether or not the chains arede by converting chemically cross-linked.nonomers, to very By using combinations of these threeolymers, by chem- principles, it is possible to achieve vari-ien shaping these ous properties with polymers, as shownLnto fibrous form. schematically in Fig. 1. Each cornertriacetate are the of the triangle represents one of the,hich are prepared basic principles, and the sides and cen-g the natuLral poly- ter of the triangle indicate variousegenerating it into combinations. The chart outlines ex-ie synthetic fibers, amples based on the polymer charac-tiercial scale in tne teristics.le been responsible It was during the early 1930's thatrowth in the con- Wallace Carothers and his co-workersde fibers. at Du Pont discovered the scientificthe use of poly- principles that laid the foundation for

lily, and at times all development of synthetic fibers andmajor areas as dur- that resulted in the invention of nyloncarpets, and tex- 66 in 1935 (3). His studies established1970 it was used the following principles for a useful

e as a reinforcing synthetic fiber:ires. Total produc- 1 ) The molecular weight of the970 was about 1.7 polymer should be high, not less than)ared to about 1.5 12,000.Dn (1 pound equals 2) The polymer should be capablePredictions of op- of being crystallized, or oriented, ore growth are very both (these properties are generallyyester is not only possessed by linear polymers, providedmost versatile of there are no side groups to destroy

bers now available. linear symmetry and the close packingLsed on polymers, of chains).a high molecular 3) Three-dimensional polymers arened by combining unsuitable.

287

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Invention of Polyester

The useful fiber-forming propertiesof polyester were first recognized in1941, by J. R. Whinfield and J. T.Dickson (4). The crucial idea came tothe inventors from a study of the workof Carothers, who had turned his atten-tion to polyamides after having ex-

amined the aliphatic polyesters andfound them to have too low a meltingpoint and poor thermal and hydrolyticstabilities (5). Whinfield employed "in-telligent thought, reading of the litera-ture, and imagination" in proposingthat molecular symmetry has a pro-

found effect on the fiber-forming prop-

erties of polyester polymers (3, p. 161).He established the soundness of hisconcepts by concentrating on the sym-

metrical, aromatic polyesters. He foundthat the polymer derived from tereph-thalic acid (TPA) and ethylene glycol,poly(ethylene terephthalate) (Fig. 2),was fiber-forming and crystallizable.Poly(ethylene terephthalate), hereafterreferred to as polyester or PET, alsohad a high melting point and was re-

sistant to hydrolysis. In 1941, Whin-field and Dickson submitted a patentapplication in which they described thepolycondensation of ethylene glycol andterephthalic acid to obtain a stiff,straight chain polymer that could bealigned and crystallized by formingfibers from the molten polymer. Thisled to the dominant patent on PET(6).

Development Problems

Terephthalic acid was not a common

chemical substance in 1941, and a com-

mercially feasible synthesis had to bedeveloped. Such efforts are ordinarilythe province of the chemical industry,not textile firms. Thus, Calico Printers'

Association Ltd. entered into an agree-

ment with Imperial Chemical Industries(ICI) to develop PET in the UnitedKingdom and throughout the world,with the exception of the United States.American rights to PET were sold toDu Pont. The large increase in fundingfrom ICI accelerated the development.A commercial process for TPA was de-veloped by oxidation of para-xylene,which was available from coal tar andpetroleum oils. Ethylene glycol was

already in commercial production.Means were found to prepare the mate-rials in a state of high purity, in orderto prevent degradation during thecourse of polymerization, and means

of controlling the average molecularweight of the polymers were identified.

Although technical people today are

well aware of the relationships among

costs of research, development, andcommercialization activities in takingan invention from the laboratory to themarketplace, it is interesting to readWhinfield's own account of the PETdevelopments (7):

While it is difficult to estimate the actualcost of the discovery itself, it may benoted that this was made in a small tex-tile laboratory which certainly requiredless than the equivalent of $50,000 a

year to maintain. To take this discoveryfrom the laboratory through the phase ofpilot plants and the appraisal of theirproducts involved expenditure amountingto more than 100 times this sum, whilethe capital investment for commercialproduction represented a multiplication ofthe estimated discovery cost by a factorof at least 600. This is a very high rateof progression, and illustrates clearly thefact that while ideas and the acquisitionof new knowledge may be cheap enough,the conversion of these ideas and thisknowledge to practical use is a rathercostly undertaking.

The funding for PET development inBritain up to the end of 1950 was inexcess of 6 million dollars. An addi-

tional 30 million dollars were spent toconstruct plants that were to produce11 million pounds of yarn and fiberannually by the end of 1954. In theUnited States, Du Pont pursued develop-ment work independently, introducedexperimental quantities of PET in1950, and established the first full-scale production in early 1953. Produc-tion has expanded at an ever-increas-ing rate, and many new domestic andforeign producers have emerged. Worldproduction was estimated in excess of3 billion pounds in 1970.

Manufacture

PET is the dominant linear homo-polymer polyester structure. Othershave been investigated extensively (8),but the only one to achieve commercialsignificance is Eastman Kodak Com-pany's poly(1,4-cyclohexanedimethyl-ene terephthalate), in which ethyleneglycol is replaced by 1,4-cyclohexane-dimethanol. This polymer has a highermelting point than PET does.The raw materials used in the prep-

aration of PET are TPA prepared fromalkyl benzenes, and ethylene glycol or

ethylene oxide. Increased production ofpolyester has been accompanied by in-creased capacity for the raw materialsand lower bulk cost. Ethylene glycoland TPA are availab;le in large supplyat 61/2 to 71/2 cents per pound and 14to 15 cents per pound, respectively. Asa result of these prices and the low cost

of manufacture, polyester fiber is eco-

nomically very competitive with otherfibers in the uses to which PET issuited.

Purity of raw materials is essentialto producing a polymer with a highmolecular weight and no color-formingbodies. Normally, TPA was purified byconverting it to dimethyl terephthalate

Crystallization

3

Chain-stiffening

2 I

Cross-!inking

Fig. 1. Principles of polymer structure.

288

Loca- Polymer characteristics Examples Usestion

1 Flexible and crystallizable chains Polyethylene Plastics and films2 Cross-linked, amorphous Phenolformaldehyde Thermoset plastics

networks of flexible chains3 Rigid chains Polyimides High temperature

plasticsA Crystalline domains in a Polyester Fibers, plastics, and

viscous network filmsCellulose acetate Fibers, plastics, and

films

B Moderate cross-linking with Polychloroprene Oil-resistant rubbersome crystallinity Polyisoprene Resilient rubber

C Rigid chains, partly cross-linked Heat-resistant materials Jet and rocket enginesD Crystalline domains with rigid Materials of high Buildings and vehicles

chains between them and cross- strength and tempera-linking between chains ture resistance

SCIENCE, VOL. 173

(DMT) and purifying the ester. Pro-cesses for the purification of TPA itselfhave recently been developed. The pur-ified materials are reacted with glycolto make the monomer bis(/,-hydroxy-ethyl) terephthalate (BHET), which isformed by the reaction of DMT withglycol (with an appropriate catalystsuch as manganese acetate), or fromthe direct esterification of TPA withglycol. The BHET is polycondensed byremoving excess glycol, with an appro-priate caltalyst such as antimony tri-oxide. Molecular weight, or the degreeof polymerization, is determined frommelt viscosity. Methods for purifyingthe BHET monomer prior to polycon-densation have been described (9).This allows use of crude TPA for thesynthesis.

Batch polymerization is the mainsource of the world production of PETpolymer. However, large-scale opera-tions are finding advantages in con-tinuous polymerization and direct spin-ning systems. The elimination of solidpolymer, subsequent blending, and re-melting lead to cost savings in large-capacity operations. Continuous polym-erization and direct spinning alsominimize the competitive reactions thaItlead to the thermal degradation en-countered in batch processing, therebymaking it easier to achieve high-strength industrial yarns of a highmolecular weight for tire cord. A typi-cal system of continuous polymeriza-tion consists of a melter, ester exchangecolumn, glycol flash vessel, low po-lymerizer, high polymerizer, and amolten polymer manifold system thatfeeds several banks of spinning heads.

Polyester is converted to the fibrousform by melt spinning, the process inwhich a fiber is formed by the extru-sion of a melted polymer through aspinneret into a cooling zone. The spin-neret is a metallic cap, with micro-scopic holes in a flat surface. Moltenpolymer is forced through the spinneretand emerges as thin filaments. The pro-duction of fiber by melt spinning ispreferred over the other two principalcommercial methods, dry and wet spin-ning. In dry spinning, the polymer,which is dissolved in a solvent, ispassed through a spinneret and formedinto a fiber by the evaporation of thesolvent. In wet spinning, the polymersolution is extruded through a spin-neret into a coagulating bath.

Melt spinning allows great versatilityin control of fiber diameter, structure,and cross-sectional shape. The moltenpolymer (at about 290°C) is forced23 JULY 1971

T 0-CH2-CH2-O-C- Q-CjA

Fig. 2. PET structural formulla.

through a sand-bed filter to a stainlesssteel spinneret. The throughput of poly-

mer is controlled by a metering pumpthat generates the pressure required forforcing the molten polymer through thefilter-spinneret assembly (10). Sincehydrolytic degradation of PET is veryhigh under molten conditions, it isessential to the maintaining of PET'smolecuilar weight and desirable prop-erties that extremely dry conditions ob-tain when it is transferred to the spin-neret.The extruded filaments solidify in

air below the spinneret. Variables inpolymer molecular weight, melt viscos-

ity, polymer throughput, spinneret ge-ometry, tension on the molten fila-ments, quench rate, and drawdown(amount of stretching between thespinneret and the take-up reel) affectthe structural morphology of the ex-truded filaments. Normally, filamentsare amorphous at this stage, but showorientation (a degree of alignment ofthe macromolecular units) along thefiber axis because of the viscoelasticresponse of the poly.rer.

Drawing, relaxing, and stabilizationproduce the oriented crystalline struc-

0 10 20 30 40 50 6

A

7

B

6

5 -oC

4-E

3

2

0 10 20 30 40 50 60

Strain (percent elongation)

Fig. 3. Stress-strain behavior of polyesterfihers: (A) high tensile strength PET fila-ment: (B) high tensile strength PET sta-ple; (C) regular tensile strength PET fila-ment, (D) regLllar tensile strength PETstaple: (E) poly (1 ,4-cyclohexanedimethy-lenteeptat ).Ilene terephthalate).

ture that gives the fiber its character-istic properties of strength, modulus,recovery, toughness, dimensional sta-bility, and so on. Drawing is thestretching of fibers of low molecularorientation to several times their orig-inal length; relaxing is the releasing ofstrains and stresses in the drawn fibers;and stabilization is the treatment of thefibers with heat to enable them to resistfurther changes in dimensions.

In the drawing process, PET, as

spun yarn, is heated above its glasstransition temperature (- 80°C). Dur-ing stretching, the molecular chains inthe amorphous filament respond by a

viscous process, involving the relativemovement of chains, and by an elasticprocess, involving their uncoiling. Athigh stress, the resulting orientationinduces crystallization, reinforces thestructure, and impedes viscous flow.Normally, an ordered crystalline struc-ture is produced, the degree or orderdepending markedly on the tempera-ture, rate of stretching, and draw ratio.

Structure

Drawn polyester fibers are composedof crystalline and noncrystalline re-

gions. The unit cell has been deducedfor both poly(ethylene terephthalate)(11) and poly(1,4-cyclohexanedi-methylene terephthalate) (12) bymeans of x-ray diffraction studies. ForPET, the unit cell is triclinic and con-tains one repeating unit. The moleculesare nearly planar in configuration, andtheir side-by-side arrangement is suchthat corresponding structural units lieon planes that are perpendicular to thefiber axis; thus aromatic rings, carbonylgroups, and so on face one another inexact register in the crystal.The physical properties of the fiber,

such as tensile strength and shrinkage,are related to the degree of crystallin-ity and the molecular orientation inpolyester fibers. The degree of crystal-linity can be measured by x-ray diffrac-tion (13). Measurements of overallmolecular orientation (both crysta lineand noncrystalline) are obtained byoptical methods. Combinations of x-

ray diffraction techniques and opticalmethods have been used to determinethe orientation in each phase (14).

Highly oriented (high draw ratio)yarns show the highest tensile strengthand initial modulus or stiffness. An in-crease in molecular weight increasesboth tensile strength and extensibility,whercas changes in draw ratio affect

289

Table 1. Physical properties of polyester fibers (mechanical properties at 210C, 65 percentrelative humidity, using 60 percent per minute strain rate).

Poly(ethylene terephthalate) Poly(cyclo-

Filament yarns Staple and tow hexanedi-Property Regular High Regular High terephtha-

tensile tensile tensile tensile late) staplestrength strength strength strength and tow

Breaking strength 2.8-5.6 6.0-9.5 2.2-6.0 5.8-6.0 2.5-3.0(g/denier) *

Breaking elongation 19-34 10-34 25-65 25-40 24-34(%)

Initial modulus 75-100 115-120 25-40 45-55 24-35(g/denier) *

Elastic recovery 88-93 90 75-85 85-95(%) (at 5% (at 5% (at 5% (at 2%

elongation) elongation) elongation) elongation)Moisture regaint(%) 0.4 0.4 0.4 0.4 0.34

Specific gravity 1.38 1.38 1.38 1.38 1.22Melting temperature

(°C) 265 265 265 265 290

* Grams per denier-grams of force per denier. Denier is linear density, the mass for 9000 metersof fiber. t The amount of moisture in the fiber at 21°C, 65 percent relative humidity.

one at the expense of the other. Fibermodulus increases with increasing mo-

lecular weight and increasing drawratio. The high fiber modulus achiev-able in PET'is usually attributed to thestiffness in the chain of the polymermolecule and alignment of the poly-mer chains into a highly oriented (inthe fiber direction) structure (15).

Like other synthetic fibers, PET isavailable commercially in three forms:

- Filament yam-a yarn composedof one or more filaments that run thewhole length of the yarn.

- Tow-a large number of continu-ous fibers collected into a loose strandthat is substantially without twist.> Staple-fiber prepared by cutting

tow to short lengths for conversion intoyarn by the conventional methods usedfor most natural and synthetic fibers.

Continuous-filament textile yarns are

usually made on a draw twister. Thedraw, or stretch, is achieved by passingthe yarn over a heated surface betweenrolls of different speeds. The drawnyarn is wound up on a cylindricaltube; the package size is usually 2 to 6pounds of yarn. In spin-draw processes,which have been developed more re-

cently (16), the extruded filaments pass

directly over a series of heated stretchrolls and are wound up at high speeds.Tire cord is normally made by such a

spin-draw process.In the manufacture of staple, very

large tows of undrawn yarn are madeby combining several smaller tows (upto 250,000 filaments). They are thendrawn between pairs of rolls operatingat different speeds, with the tow heated

290

above the glass transition temperature.The tow is crimped, dried, and heatstabilized, and is either packaged as towor cut to the desired staple length.

Physical Properties

The different stress-strain curves ofstaple and filament polyester fibers areillustrated in Fig. 3. The unique "tailor-ability" of the polyester fibers to en-compass a wide range of propertiesmakes it possible to produce manymechanical property variants for a

Table 2. Consumption of polyester fibers inthe United States. [Source: Textile EconomicsBureau, Incorporated]

Estimated con-sumption of

polyester fibersEnd use (millions of

pounds)1968 1969 1974

StapleMen's and boys' wear 425 510 850Women's and

children's wear 130 160 345Home furnishings

Bedding (sheets, pil-lowcases, spreads) 110 135 275

Fiberfill 40 45 85Fabrics 30 35 80

Rugs and carpets 75 110 195Miscellaneous 20 25 70

Total 830 1020 1900Filament

Men's and boys' wear -15 30 55Women's and

children's wear 65 125 225Home furnishings 15 20 65Tire cord 100 135 290Miscellaneous 15 10 45

Total 210 320 680

large variety of textile and industrialend uses. The mechanical propertiesof the variants shown in Fig. 3 can berelated to some areas of end use asfollows:

1) Tire cord and high-strength, high-modulus industrial yams.

2) High-modulus staple fiber forblending with cotton, to give optimumprocessability to durable press fabrics.

3) Textile filament yarn for wovenand knit fabrics.

4) Regular polyester staple for 100percent polyester fabrics, carpet yams,and blending with cotton or wool. Poly-ester tow for fiberfill.

5) Regular poly(l ,4-cyclohexanedi-methylene terephthalate) staple for car-pet yams.The toughness of a fiber is measured

by the total area under the stress-straincurve. Toughness is the amount of workor energy required to break the fiber,as well as a measure of the work orenergy that can be absorbed by thefiber as it is loaded to any given stressor level of elongation. All of the poly-ester variants described in Fig. 3 re-quire a great deal of work before theycan be broken and are capable of ab-sorbing considerable energy under nor-mal conditions in the specific applica-tions for which they are manufactured.The physical properties of the variouspolyester fibers shown in Fig. 3 aresummarized in Table 1.

Polyester fibers, like other organicfibers, show nonlinear and time-de-pendent elastic behavior. Under load,creep will occur, and there will be de-layed recovery on removal of the load.The elasticity of PET fiber at low levelsof strain increases with increased struc-tural orientation, which is the degree ofalignment of the molecules in the fiberdirection.

Uses of PET

The volume of polyester fibers in alarge number of end uses is expandingat a rapid rate. Many modifications orvariants of PET have been developedfor a wide variety of applications inmen's, women's, and children's apparel,carpeting and home fumishings, tirecord, and other industrial markets.A marked increase in the growth

rate of PET occurred in the early1960's, with the introduction and wideconsumer acceptance of the concept of"easy-care" fabrics (also referred toas durable press, permanent press, and

SCIENCE, VOL. 173

no-iron). These terms are all usedto indicate that fabrics containing PETcan be set or stabilized so that theresulting configuration is maintainedduring wear or use, with little or noironing after laundering. Thus, trousercreases or skirt pleats are there for thelife of the garment. For these uses,PET staple fiber is blended with cottonin a 50: 50 or 65: 35 ratio of PET tocotton. Blends with rayon are also com-mon. The cotton or rayon is alwaystreated with resin so that it too is set.During the past decade, the concepthas spread to encompass women's dressgoods, blouses, and sportswear; men'sdress shirts, sportswear, and slacks;children's clothing; and durable presssheets, pillowcases, and bedspreads, aswell as drapery fabrics and tablecloths.Polyester staple fiber is also blendedwith wool to produce fabrics for light-weight suits, enabling them to retaincreases and resist wrinkles.The recent development of textured

PET yarn for knit fabrics is a majorachievement. Such knit garments arewrinkle resistant and are used in wom-en's and men's garments. This has be-come the major end use of polyestertextile filaments. Textured yarns arecontinuous filament yarns, in which thefilaments have been crimped or havebeen looped at random to give the yarngreater volume or bulk. Polyester fila-ment yarn is easily textured and givesgood dimensional stability to knits thatare set with heat.

Polyester is used both in washablescatter rugs as well as in conventionalcarpets, particularly shag rugs. This enduse consumed 110 million pounds in1969. Polyester is used for other homefurnishings such as drapery and uphol-stery fabrics.Rayon and nylon cords were used to

reinforce rubber tires prior to 1965, theyear that PET was commercially intro-duced in this use. High performancecaused the rapid expansion in sales ofPET tire yarns to the 200 millionpounds per year level in 1970 (17). Tirecord made of PET is equal in strengthto nylon, but whereas nylon tires "flat-spot," PET tires do not. When drivingon tires with flatspots, which developafter the tire has been standing in thecold overnight, the consumer feelsthumps for the brief period before therotating tire warms up and all tire andcord segments are fully round again(18). Dimensional stability of tirecords is an extremely important prop-erty. The factors of shrinkage at ele-23 JULY 1971

Table 3. Estimated U.S. capacity for polyester fiber in 1970.

Producer Brand name Pounds (millions)

Allied Chemical 10 filamentAmerican Enka Encron 40 staple and filamentBeaunit Vycron 80 staple and filamentDow Badische Anavor 30 staple and filamentDu Pont Dacron 500 staple, 100 filamentEastman Chemical Kodel 250 stapleFiber Industries* Fortrel 350 staple, 175 filamentFMC (American V7iscose) Avlin 25 staple, 15 filamentGoodyear Tire and Rubber 6 filamentHystron Fibers Trevira 60 staple, 12 filamentIRC Fibers (American Cyanamid) 25 filamentMonsanto Blue C 60 staplePhillips Fibers Quintess 25 staple and filament

Total 1,760

* Owned jointly by Celanese Corporation and Imperial Chemical Industries Ltd.

vated temperature, growth under load,and sensitivity to moisture combine todetermine (i) how the fabric reacts todipping for adhesion, calendering, tirebuilding, and curing; (ii) uniformity oftire dimensions and tire balance; and(iii) performance of the tire with re-spect to growth, flatspotting, and ridecharacteristics. The all-around stabilityof PET is better than that of eitherrayon or nylon (19). Adhesion of thePET cords to rubber was a probleminitially, but it was solved by develop-ing new adhesive systems. Other areasfor high tensile strength PET yarns areV-belts, sewing threads, and industrialfabrics.

Fiberfill made of PET is widely usedin mattresses, sleeping bags, pillows,and so on. Fiberfill is crimped stapleor tow that is used as filling material.It replaces, or is blended with, feathersand down, and is formed into sheetsfor use as a lightweight filling materialin insulation. Filling materials of PETare resilient, stable, and can be laun-dered without losing these properties. Abreakdown of polyester staple and fila-ment in various end uses is given inTable 2.

Resistance to Degradation

Polyester fibers resist environmentalconditions well. The resistance of PETto sunlight is exceeded only by theacrylics. To improve PET's perform-ance, light stabilizers and antioxidantsare added prior to melting and extrud-ing.The most important chemical reac-

tion 'in polyester is hydrolysis, whichproduces breaks in the chains and re-duces the physical strength by reducing

molecular weight. The attack by wateritself is rapid if the temperature is suf-ficiently high. At 100°C in water, 20percent of the strength of polyester islost after 1 week, whereas at 70°Cfor several weeks there is no measur-able loss of strength (20). Basic sub-stances attack the fibers in two differ-ent ways: strong alkalies, such ascaustic soda, etch the surface of thefibers and reduce their strength; am-monia and other organic bases pene-trate the structure initially through thenoncrystalline regions, causing degra-dation and loss of strength.

Polyester fibers have excellent resist-ance to dry-cleaning solvents and con-ventional bleaches. The resistance isconditioned by the degree of crystallin-ity and molecular orientation presentin the fiber.

Dyeing and Finishing

Because PET does not have reactivedyesites, it is normally dyed with dis-perse dyes, which act primarily throughdiffusion and are bound in the fi.erthrough secondary bonds. The struc-tural orientation of the PET fibersgreatly influences the rate at which theycan be dyed. The greater the orienta-tion, the slower the dyeing rate. Therate can be accelerated by adding tothe dye bath chemicals that cause theI ET fibers to swell and allow morerapid penetration of the dyes.A continuous dyeing process called

Thermosol is widely used to apply dis-perse dyes to PET fabrics. The fabricsare padded with the dyes, dried, andthen heated (205° to 240°C for 30 to60 seconds) to diffuse and fix the dyesin the PET fibers. The fabrics most

291

commonly dyed by this procedure arethe 50: 50 and 65: 33 blends of PETand cellulosics. Several manufacturershave introduced polyester fibers thatare modified to have anionic dyesites.These fibers can be dyed with disperseand cationic dyes; when used in blendswith regular polyester, unique multi-color effects can be obtained. Ther-mally stable, melt-soluble dyes havealso been made to color the polymerprior to spinning (21).

Chemical finishing of polyester-cel-lulosic blends is a very important partof durable press. It is not a finish forPET, but a system whereby the cottonin the PET blends is given a resincross-linking treatment to improve itsrecovery power and wrinkle resistance.The best known durable press system(22) involves the following five steps:(i) the dyed PET and cotton blend isimpregnated with an aminoplast resinderivative, (ii) the fabric is dried, (iii)the gannents are cut and sewn, (iv)creases are pressed into the garments,and (v) the garments are placed in anoven and the resins cured. This is re-ferred to as a delayed or postcuredsystem. Curing resins before cuttingand sewing the fabric, as is the casewith durable press sheets, is referred toas precuring. The concept of using ablend with a higher percentage of PETto produce no-resin durable press hasbeen recently introduced. Durable pressis achieved in this polyester variant (80percent polyester to 20 percent cellu-losics) by hot head pressing at 170° to1950C (23).Many of the unique properties of

polyester are achieved by treating thedrawn fiber or fabric structure withheat. The relaxation of stress, causedby heating the filament above the glasstransition temperature, can be used toset a given length or shape, which isthen stable at lower temperatures. Fur-ther crystallization during this heattreatment also reinforces the structureand raises the transition temperature(24).Heat setting is used to develop max-

imum performance in polyester fibers,either alone or in blends with naturalfibers. It makes fabrics resistant toshrinkage during tumble drying, press-ing, or ironing. It also makes fabricsresistant to wrinkles during wear andhelps to produce the easy-care prop-erbies for which polyester fabrics arenoted.The success and rapid growth of

polyester fibers tend to overshadow the

292

0L-600 120

° 400 - 060&

200 0.40

1954 1958 1962 1966 1970Year

Fig. 4. Increase in total PET staple con-sumption with corresponding change inprice of 1.5 denier per filament staple;(A) total consumption; (B) price.[Source: (10, p. 36) and estimates byTextile Economics Bureau, Incorporated.]

areas in which limitations in the prop-

erties of the fibers inhibited marketpenetration and growth (25). Polyesterfibers faced several major problems.The hydrophobic nature of PET makesit difficult to dye. The strength of PETfibers leads to pilling, the formation offuzzy balls on the fabric surface bythe rubbing of loose ends too long andstrong to break away. Because of itshydrophobicity, polyester shows some

affinity for oily soil, at times makingit difficult to wash the soil out. Poly-ester fibers also tend to develop staticunder conditions of low humidity. Dur-ing recent years, these and other prob-lems were the subject of intensive re-

search and development efforts by themanufacturers of polyester. As a result,they have developed PET variants, al-ready commercially available, that are

more easily dyed, show less pilling, are

cleaner after laundering, and are anti-static.The world production capacity of

PET was increased to 4.7 billionpounds in 1970, of which the U.S.capacity was appproximately 1.8billion pounds (26) (Table 3). Majorfactors in the growth of polyester stapleand filament are (i) the expansion ofend uses such as durable press, tex-tured filament, and tire cord; (ii) theentry of new producers into the market;and (iii) competitive prices due ,to theincreased volumes. The price-volumerelationships for staple are shown in

Fig. 4, where the total consumptionfrom 1954 to 1970 is indicated, withcorresponding changes in price of 1.5denier per filament staple.

It has been estimated that the con-sumption of polyester fiber in theUnited States will almost double from1969 to 1974, reaching 2.6 billionpounds in 1974; it is expected to reach4 billion pounds by 1979 '(27). Theprojected increases show the majorgains in staple for durable press appli-cations, and in filament for both tex-tured knits and tire cord (Table 2).Half of all automobile tires being pro-duced in the United States are rein-forced with polyester cord, and 85percent of new automobile, original-equipment tires in the United Statescontain polyester cord.

Summary

The use of polyester fibers has grownmore rapidly than that of any otherman-made fiber. Many factors havecontributed to this growth. Polyester'sunique physical properties of strength,high modulus, elasticity, and durabilityare the basis for its success. The tailor-ability of the fiber makes it possible togenerate a whole family of propertyvariants for a wide variety of end uses.The ready availability and low cost ofthe raw materials, the continuing ad-vances in polymerization technology,and the versatility of the melt spinningprocess have also been major factors inestablishing polyester as the leadingman-made fiber.New end uses have had a major im-

pact on the growth of polyester. Mostnotable was the introduction of durablepress fabrics for clothing and homefurnishings. The consumer preferencefor easy-care fabrics and garmentsmakes the durable press area one ofcontinuing growth. Two relatively newareas where growth is expected to con-

tinue at a rapid rate are tire cord andtextured knits for women's and men'souterwear.

References and Notes

1. Dly. News Rec. 159, 1 (24 November 1970);Chem. Spotlight 25, 2 (8 January 1971).

2. H. F. Mark, Sci. Am. 217, 148 (September1967).

3. R. Hill, J. Soc. Dyers Colour 68, 160 (1952).4. J. R. Whinfield and J. T. Dickson were work-

ing in the laboratories of Calico Printers'Association Ltd. in England.

5. J. Jewkes, D. Sawers, R. Stillerman, TheSources of Invention (Norton, New York,ed. 2, 1970) pp. 310-312.

6. J. R. Whinfield and J. T. Dickson, BritishPatent No. 578,079 (29 August 1946).

SCIENCE, VOL. 173

7. J. R. wlhinfleld, Text Res. J. 23, 289 (1953).

8. I. Goodman and J. A. Rhys, Polyesters(Elsevier, New York, 1956), vol. 1.

9. Chem. Eng. News 48, 42 (23 November 1970).10. G. Farrow and E. S. Hill, in Encyclopedia of

Polymer Science and Technology, H. F. Mark,N. G. Gaylord, N. M. Bikales, Eds. (Inter-science, New York, 1969), vol. 11, p. 11.

11. R. Daubeny, C. W. Bunn, C. J. Brown,Proc. Roy. Soc. London Ser. A 226, 531(1954).

12. C. A. Boye, J. Polym. Scd. 55, 275 (1961).13. W. 0. Statton, J. Appi. Polym. Scd. 7, 803

(1963).14. G. Farrow and J. Bagley, Text. Res. J. 32,

587 (1962).

15. I. M. Ward, J. Macromol. Sci.-(Part B)Physics 1, 667 (1967).

16. J. G. Gillet, R. Lipscomb, R. B. Macleod,British Patent No. 874,652 (10 August 1961).

17. Estimate based on 1969 consumption of 172million pounds [Text. Organon 41, 199 (1970)1.

18. W. E. Claxton, M. J. Forster, J. J. Robert-son, G. R. Thurman, Text. Res. J. 36, 903(1966).

19. E. J. Kovac and T. M. Kersker, ibid. 34, 69(1964).

20. Chemical Properties of "Terylene": The Ef-fect of Water and Steam (Technical BulletinB3, ed. 1, Imperial Chemical Industries FibresLtd., Harrogate, Eigland, 1962).

21. Reports on the Progress of Applied Chemis-

try (Society of Chemical Industry, Gordon,London, 1969), vol. 54, p. 540.

22. Koret Company, United States Patent No.2,974,432 (1961).

23. Fabrication of Durable Press Garments FromFabrics of 80% Fortrel Polyester/20% Cell.losic Staple (Technical Bulletin TBP-30, Cela-nese Fibers Marketing Co., Charlotte, N.C.,1970).

24. A. B. Thompson and D. W. Woods, Trans.Faraday Soc. 52, 1383 (1956).

25. E. V. Burnthall and J. Lomartire, Text.Chem. Color 2, 218 (1970).

26. Mod. Text. Mag. 51, 31 (March 1970).27. J. V. Sherman, Barrons 50, 11 (21 September

1970).

Enzymatic Modification ofTransfer RNA

Modified nucleosides form at the polynucleotidelevel, but their function is not established.

Dieter Soll

Transfer RNA (1) defines a class ofmacromolecules that are of key im-portance in cellular processes. Eachcell contains about 60 different tRNAspecies which can be separated by a

variety of techniques. Each tRNA spe-cies can be esterified with its specificamino acid by the cognate aminoacyl-tRNA synthetase (aminoacylation re-

action). The main and best understoodrole of tRNA is in ribosome-mediatedprotein synthesis where aminoacyl-tRNAensures the correct codon-directed in-sertion of its amino acid into thegrowing peptide chain (2). To dateonly a few other roles of tRNA areknown: it participates in the synthesisof bacterial cell walls and in the trans-fer of amino acids onto proteins andphospholipids (3). The involvement oftRNA in virus infection, cell differen-tiation, hormone action, and regulatorymechanisms has been suggested. How-ever, there is no conclusive evidencefor direct tRNA participation in anyof these processes (2).

Transfer RNA is unique among cel-lular RNA species because it containsa large number of modified nucleotides.The chain length of the known tRNAmolecules varies between 75 and 87nucleotides, and their content of modi-23 JULY 1971

fled bases can reach 20 percent (4). Allthe known primary sequences can bearranged in a cloverleaf model of sec-ondary structure. An example is givenin Fig. 1. The three-dimensional struc-ture of tRNA is not yet known. Withthe exception of the anticodon and theC-C-A-acceptor end (carrying theamino acid), no nucleotide sequencesor structural features can yet be as-

signed to the other functions of thetRNA molecule or designated as recog-nition sites for enzymes interacting withtRNA (5). The number of structuralvarieties of modified nucleosides foundin tRNA is surprisingly large (6).About 50 modified nucleosides havebeen isolated from tRNA of various or-

ganisms. The structure of 37 of thesenucleosides has been determined. Someexamples are given in Fig. 2. Most ofthese nucleosides are characterized by a

relatively simple modification of one ofthe four major nucleosides. The methyl-ated nucleosides can be divided intotwo classes in which the methylation ofthe parent nucleoside took place eitheron the heterocyclic moiety (for example,ribothymidine, Fig. 2) or on the ribosegroup (for example, 2'-O-methylguano-sine, Fig. 2). In the thionucleosides a

sulfur has replaced an oxygen (for ex-

ample, 4-thiouridine, Fig. 2). Hydro-genation of the 5-6 double bond of thepyrimidine ring. leads to the formationof reduced pyrimidine nucleosides (forexample, dihydrouridine, Fig. 2). TIhemost interesting simple modified nu-cleoside is pseudouridine (Fig. 2),which possesses a carbon-glycosidiclinkage instead of the common nitro-gen-glycosidic one. Recently, nucleo-sides with more complex modificationshave become known. These hypermodi-fled nucleosides contain a larger sidechain or an additional ring structurebearing functional groups such as acarboxyl, hydroxyl, or ester group oran allylic double bond. Threonylcar-bamoyladenosine (Fig. 2), the fluores-cent nucleoside Y (Fig. 2), 6-(3-methylbut-2-en-1-ylamino) purine ribo-side commonly called N6-(A2-isopent-enyl)adenosine (Fig. 2), and its 2-methylthio derivative, are the most wellknown.With the progress in the techniques

of tRNA purification and sequenceanalysis, the position of the modifiednucleosides in the primary sequencesof the tRNA molecules has becomeknown. Generally, there is a clusteringof modified nucleosides in the loop re-gions (Fig. 1). Ribothymidine in tRNAis always located 23 nucleotides fromthe acceptor end in the sequence G-T-&-C. The hypermodified nucleosidesare located adjacent to the 3' end ofthe anticodon. An interesting patternof their distribution in the various ac-ceptor RNA species has emerged. Theoccurrence of isopentenyladenosine orsome close derivatives of it in Escheri-chia coli, Lactobacillus, yeast, or ratliver tRNA are restricted to tRNA spe-cies which have an adenosine residuein the third position of the anticodonand correspondingly recognize codonsstarting with U (7-9) (first horizontal

The author is associate professor in the depart.ment of molecular biophysics and biochemistryat Yale University, New Haven, Connecticut.

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