Indian Journal of Fibre & Textile ResearchVol. 20, March 1995, pp. 43-59

Review Article

Some developments in poly(ethylene terephthalate) fibre production andstructure-property relationships"

VBGuptaDepartment of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India

Received 7 October 1994; accepted 26 October 1994

Poly(ethylene terephthalate) (PET) fibre occupies the top place amongst the man-made fibres inthe world today. During the past few years, very significant developments have taken place in thetechnology of its production. The resulting.' products like partially-oriented yarns and spin-drawyarns are already well established, while the yarns spun at very high speeds and the micro-denier fi-bres are showing promise as fibres of the future. These developments have been accompanied by aparallel upsurge in the research and development efforts on PET and its products due to which amore clear understanding of its structure and the underlying fundamentals of structure-property rela-tionships are becoming available. As a result; products to meet specific functional requirements canbe designed with increasing confidence. The various developments that have taken place in theseareas in the past few years cover a very wide canvas and, therefore, only a few selected develop-ments related to some physical aspects of PET fibre production and structure-property relationshipswill be considered in this article, the emphasis being on fundamental principles as far as possible.·

Keywords: Poly(ethylene terephthalate) fibre, Fibre production, Fibre properties, Structure-propertyrelationships

I IntroductionPoly(ethylene terephthalate) (PET) fibre is today

the most widely used manufactured fibre, havingovertaken polyamide fibres' in 1972. Its fifty ye-ars of existence have recently been reviewed in acommemorative book-, Though the factors whichhave contributed to its success are numerous, thenature of the molecule is without doubt of utmostimportance.'. This is because the basic repeat unitof PET confers on the resulting semi-crystalline,oriented fibre a number of desirable features.These include, amongst others, thermal transitionswhich occur at the right temperatures and are ofthe right mangitude, and a relatively slow rate ofcrystallization. As a result, fibres can be producedwith a combination of properties which makethem suitable for a wide spectrum of applications.The present article deals with some of theseaspects, the focus being on developments inspinning processes, fibrous products, structureand properties and their inter-relationships. In ad-

-A condensed version of this paper was presented at a two-day seminar on Recent Developments in Man-made Fibres,held at the Indian Institute of Technology, New Delhi, India,on 20 & 21 January 1994.

dition, the fundamentals involved in the extrusionor spinning process are also briefly consideredand reference is made to some emerging applica-tion areas. The section on structure-property cor-relations deals with such important industrial pro-cesses as heat-setting and dyeing and with manyimportant properties, e.g. mechanical and thermalproperties.

2 Fibre SpinningA schematic sketch illustrating the principal fea-

tures of the melt-spinning process is shown in Fig.1 for a single filament. The die-swell effect is as-sumed to be small and, therefore, neglected. Themolten polymer is extruded at a specific ratethrough the spinneret hole into a transversely di-rected quench air stream and the spin line solidi-fies at some distance from the spinneret, indicatedin Fig. 1 as the freeze line. A spin finish is thenapplied (not shown in the figure) and the solidifi-ed filament is finally wound up at a speed signifi-cantly greater than the extrusion velocity so thatthe cross-sectional area is about 100-200 timessmaller than the initial area close to the spinneretexit.

44 INDIAN J. FIBRE TEXT. RES., MARCH 1995

Freeze line•

~~

r.Coolin'.! air..---

..•---L ----

.•..•

Take - up

Fig. I-Schematic diagram of melt-spinning

2.1 Fundamentals Involved in the Spinning Process2.1.1 Spinnability ofthe Fluid

One of the essential requirements for a fibre-forming polymer is that in the fluid form, it mustbe capable of assuming large irreversible defor-mation when subjected to a uniaxial stress field.This characteristic is called spinnability and is anecessary but not the sole condition for fibre for-mation.

The maximum thread length of the fluid is be-lieved to be a function of two mechanisms ofbreakage". The first is a cohesive and brittle frac-ture occurring when the tensile stress in a visco-elastic jet exceeds its tensile strength and the sec-ond possible process of breaking fluid threads isassociated with surface tension Polycondensateslike PET are very stable at take-up velocities ofup to 4000 m/rnin and melt draw-ratios of up to1000. On the other hand, melt-spinning of highmolecular weight polyolefins is associated withserious limitations due to cohesive instability astheir relaxation time may reach one second. It isessential for the spinning process that no spinna-bility limit be exceeded. When too Iowa viscosityof spinning fluid is used (e.g. too high a melt tem-perature), capillary break-up into individual drop-lets occurs due to materials with too long a relax-ation time, the cohesive mechanism appears andonly slow spinning is possible.

2.1.2 Role of Spinning ParametersThe spinning- parameters are of paramount im-

portance as they determine the fibre-line proces-sability and the final product quality. The orienta-tion that develops in the spin-line as a result of

the tensile deformation under stress, is a functionof the relative rates of extensional flow on the onehand and the disorienting thermal relaxationphenomena on the other. These rates, in turn, aregoverned by the velocity and temperature var-iations along the spin-line. Since the relaxationtime in the case of polyester melt is very small(may be as small as 0.0001 s), the rate of molecu-lar straightening must be very high for orientationto prevail.

The results of a study on computer simulationof melt spinning of PET up to wind-up speeds of3000 rnImin provide' some insight into the roleof spinning variables like melt temperature, intrin-sic viscosity, polymer throughput rate, spinneret.capillary diameter and temperature of cooling airon freeze line distance, the stress at freeze, initialforce and uptake force. Melt temperature fol-lowed by the intrinsic viscosity of the polymerturn out to be the most critical process parame-ters. It is noteworthy that both these parameterscontrol the melt viscosity and it may, therefore,be concluded that polymer rheology plays a cru-cial role in controlling the properties of the spunfilament. The role of polymer rheology will there-fore be discussed in sub-section 2.1.3. It is imper-ative to monitor closely and accurately the meltcharacteristics, more so for a multifilament mul-tiposition melt-spinning plant. These parametersbecome even more critical at higher speeds ofspinning and demand greater attention in ensuringuniform. melt properties. From the simulationstudies", the other parameters could be ranked inorder of decreasing importance as follows: uptakevelocity, throughput rate, spinneret dimensionsand cooling air temperature and its velocity.

One of the conclusions from the above analysisis that the cooling air conditions do not signifi-cantly affect the spinline force, which is a pre-dominant factor affecting the structure of the as-spun fibre. However, from the processing point ofview, quenching conditions are of paramount im-portance as far as the uniformity of the spun fila-ment is concerned. This point becomes clearwhen the simulation work is extended to gain aninsight into filament acceleration and cooling rateduring the spinning process", This work showsthat for a wind- up speed of 1000 rnImin, the fila-ment acceleration peaks to 50000 cm/s (ref. 2)af-ter 0.25 second of leaving the spinneret while thecooling rate shows a peak value of 4500°C/s after0.27 second of leaving the spinneret. The corre-sponding figures for wind-up speed of 3000rnImin are 450,000 cm/s (ref. 2) after 0.23 sec-ond and 12,000°C/s after 0.23 second. The accel-eration and cooling rate peaks are seen to lie

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 45

close to one another for any particular set ofspinning conditions. The closeness clearly bringsout the inter-relationship of the two importantphysical phenomena of melt-spinning, viz. heattransfer and mass flow. It also indicates the pos-sibility of controlling the deformation rate by ad-justing the cooling conditions, and thereby makingattempts to minimize filament breakages duringspinning.

2.1.3 Melt RheologyIt was earlier seen that one of the most critical

parameters in melt-spinning is the melt viscosityof the polymer. Viscosity shows a strong depend-ence on the molecular weight and its distribution.In industry, the molecular weight is most conveni-ently and quickly assessed by measuring the limit-ing solution viscosity of the polymer in a suitablesolvent. But it is the melt viscosity of the polymerwhich determines its flow behaviour and is ofgreater practical importance. However, in additionto molecular weight, the melt rheology is also de-pendent on the shear rate. Therefore, it is usefulto relate the solution viscosity (intrinsic viscosity)of the polymer to its melt viscosity. A number ofempirical relationships of the following type havebeen proposed for PET:

1'/0= KV1')p,

where 1'/0 is the melt viscosity at zero shear rate;K and p, the constants; and./1', the intrinsic vis-cosity of the polymer. The following equation forzero shear melt viscosity (1'/0) was found"

r8286 ]l1o=vr)s.14sexp t+273 -4.6434-0.159 (DEG)

where t is the temperature of the melt in °Cand DEG, the weight percentage of diethy-lene glycol in PET. The analysis also shows"that PET melt flowing through a capillary is New-tonian close to the capillary axis and pseudo-plas-tic close to the wall of the capillary.

2.1.4 Filament UniformityThe non-uniformity of linear density or diame-

ter of the filament along the spinline has beenshown in the literature to arise from a number offactors, some of which are internal (char, gel, pig-ment agglomerates, catalyst or stabilizer residues)or external (fibre waste and other substances)contamination, local variation in temperature ormolecular weight leading to variation in polymerviscosity, instability of flow within the spinneretchannels, non-uniform dimensions of the chan-nels, temporary superheating of the spinneretwhich leads to formation of drips at low extension

velocity, variation of extrusion velocity, tempera-ture, take-up velocity, ambient temperature,equipment vibration, yam slippage, etc. Studies onfilament non-uniformity in PET have revealedthat the presence of low molecular weight tail inthe molecular weight distribution curve might beresponsible for the thick and thin places in the fi-lament, thus stressing the importance of a propermolecular weight distribution of the polymer usedfor spinning.

2.2 Developments in Melt-SpinningMelt-spinning of polymeric fibres has witnessed

a step change with the winding speeds increasingby an order of magnitude. The demand for highquality of the melts fed to the spinnerettes in-creases at high speeds. This increased quality re-quirement applies not only to polymer feed withrespect to viscosity, homogeneity, freedom fromdust, etc. but also to the processing conditions, in-volving proper drying and optimum melting con-.ditions in the extruder. The propulsion and dis-tribution of the melt to the individual spinningpositions, including mixing and filtering, requiremore attention. To deliver a perfectly homoge-nous melt, static mixers for distributive mixing areinstalled between the extruder and the meteringpumps. The mixing elements consist of a latticemade up of cross-wise interlocking webs whichare at an oblique angle to the pipe axis and areincorporated in the pipework of the spinning sys-tem. Quenching of the filaments needs special at-tention. Between 1937 and 1940, the first melt-spun polymeric fibre, viz. nylon 66, was extrudedthrough spinneret holes at 285°C and cooled bynatural convection down to 40-80°C beforereaching the first contact points (finish applicationrolls, godets, etc.). A distance of 5-6 m at take-upspeeds of 600-800 m/min was necessary for satis-factory cooling, corresponding to a cooling timeof 0.7 second. The air boundary layer formed bythe air friction on the filament caused poor heatconvection and a certain instability in the threadpath. A DuPont patent in 1939 stated that theboundary layer can be made as small as possibleby cross flowing air, reducing the thread coolingpath to 1.5 m, corresponding to a cooling time of0.5 second. Many important developments in lateryears like inflow and outflow quench systemshave reduced the cooling time to 0.05 second forfine yams.

Control of thread tension in the wind-Up zonebecomes more difficult with increasing spinningspeed. In contrast to conventional spinning, wind-up in high-speed spinning is carried out almost

46 INDIAN J. FIBRE TEXT. RES., MARCH 1995

exclusively without godets. In the 1000-2000m/min range, the spinning dynamics is consider-ably affected by the rheological force while athigher take-up velocities, the dominant role isplayed by air drag force.

In high-speed spinning, the orienting influenceof the uniaxial stress field overcomes to a signifi-cant extent the disorienting influence of thermalrelaxation process. During melt-spinning, the tem-perature at which -crystal formation can comm-ence is considerably enhanced at higher spinningspeeds. However, molecular relaxation in the am-orphous regions cannot be prevented. This hasbeen a cause of disappointment to the fibre prod-ucers as oriented, crystalline fibres having the de-sired properties cannot be prepared by one-stepprocess". Efforts are on to develop one-step tech-nology (SUb-section 2.3) as in addition to in-creased productivity, high-speed process also of-fers the benefit of reduction of energy and labourand reduction of total production cost besides, ofcourse, being the simplest process.

The spinline crystallization of PET at highspeeds involves significantly large undercooling;the melt temperature may decrease from 280°Cto 180°C in 0.005 second. The nucleation rate is,therefore, very high and the concentration of nuc-lei is very dense. The crystal size of the nucleusfor crystallization to proceed is considerably re-duced and approaches the unit cell dimension ofPETJ. This occurs because in an oriented melt,the entropy change on crystallization is small. Ata given supercooling, the free energy change iscorrespondingly increased. This has been termedas nucleative collapse or as spinodal defect de-composition and is also referred to as regime IIIof crystallization. The large number of small nucleicome together to form a crystallite and the crystal-lization is distinctly different compared to theusual nucleation and growth process that domi-nates the crystallization of a melt in the stress-freestate or at low stresses. For materials crystallizingfast and to a high degree (for example polypropy-lene), the increased crystallization rates at highspin stresses lead to well-oriented' lamellar struc-tures which form epitaxially on the originally fi-brillar nuclei and fibrillar crystallites. For a slowcrystallizer' (such as PET), high spin-stress extru-sion leads to an oriented, crystalline fibrillar sys-tem'".

2.3 One-step Melt-spinning TechnologyA one-step process based on high-speed spinn-

ing is very attractive for fibre producers. Thougha patent, claiming that PET fully-oriented yarn(FOY) can be obtained by a one-step process

with no drawing if spinning speeds exceed 4750rnimin, was filed in the U.S. as early as 1952,commercially available high-speed winders work-ing in the 6000 m/min range were disclosed onlyin 1978 after which considerable activity was no-ticed in this area. Though high-speed spinning at6000 mlmin yields an oriented, crystalline fibre,the end-product is not equivalent to the two-stepcommercial fibre. This is because the relaxationtime for molecules in the amorphous regions isvery small and in spite of the spinning being veryrapid, they relax to a significant extent with theresult that the amorphous orientation developedis low and the mechanical properties are not to-tally adequate. Another problem is the pro-nounced skin-core structure that develops at thesehigh speeds and affects the mechanical propertiesadversely. Commercial success has already beenobtained in designing "spinline heating spinning"(SHS technique), which makes use of a hot tubebelow the freezeline!' to induce the slight addi-tional stretch so that the conventional drawingstep can be omitted. Control of the thread line dy-namics by placing a hydraulic thread bath, whichincreases the tensile stress below the freezeline,has also been reported'? as an alternate route.

2.4 Other Spinning ProcessesThe melt-spinning technology described in the

earlier sections is the simplest and the most ele-gant available spinning technique and is quite ade-quate for producing filaments from fibre gradePET or from the still higher molecular weight tyrecord grade PET. However, melt viscosity rises inproportion to NP,,;4 and molecular weights higherthan the tyre cord grade PET are therefore notreadily amenable to melt spinning. Solution and/or gel spinning techniques are therefore resortedto. The crucial factor in this spinning technique isthe concentration of the polymer in the solventfor on this depends the density of molecular en-tanglements which, in turn, controls the drawabil-ity of the filament. Improved drawability allowsdraw ratios of 10-15 to be obtained, giving rise tohigher molecular orientation and more structuralcontinuity along the draw direction, as a result ofwhich fibres have improved mechanical propert-ies.

Another well-established production technologyis the spin-draw process in which the spinningand drawing operations are integrated in a singleunit.

The other spinning techniques like liquid crystalspinning and dry-jet wet spinning are used to pro-duce fibres from aromatic polyesters or copolyes-ters and are outside the scope of this article.

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 47

2.5 Molecular Theories of Fluid Flowl3

Molecular theories of flow have of late under-gone a sea change with the result that the basicconcept of how molecules move in a fluid has it-self been transformed.

Molecular theories of flow allow a better un-derstanding of the relationship between molecularparameters and rheological properties. In concen-trated polymer solutions and polymer melts, thereis a strong interaction between the molecularchains. Starting with the classical work of Eyringand his colleagues in the early forties, which in-volved a molecular segment as the unit of flow,the molecular theory was refined by Rouse totake into account the coordinated movement ofthe molecular chain segments. The innovativeidea of reptation, in which a molecular chain isconfined within a tube defined by its neighboursand moves by wriggling along a tortuous path,was conceived by De Gennes and translated intoa quantitative theory of polymer chain dynamicsof Doi and Edwards. Finally, Curtiss and Birddeveloped a kinetic theory for polymer melts andwere able to predict the correct dependence ofthe zero shear viscosity on molecular weight.

3 Fibre Products3.1 Introduction

The pattern for fibre production in all industrialnations is changing rapidly, moving towards lowervolume but much higher value-added products forindustrial markets. This has become necessary be-cause the commodity fibres, which are principallyused as apparel and household textiles, are domi-nated by acute pressure on price margins".Against this backdrop, the developing countriesare expanding their production capacities forcommodity fibres.

For the purpose of the present discussion, it isconvenient to divide the man-made fibres interms of their mechanical properties into the fpl-lowing three categories:(i) Standard textile fibres with average mechani-

cal properties: For this class of fibres, Atlasand Mark have laid down the following me-chanical property requirements 15: Tenacity,3-5 g/den; elongation-to-break, 35%; modu-lus of elasticity, 30-60 g/den; and completelyreversible elongation up to 5% strain.

(ii) Industrial fibres with intermediate mechanicalproperties: For this class of fibres, Atlas andMark have laid down the following propertyrequirements": Tenacity, 7-8 g/den; elonga-tion-to-break, 8-15%; modulus of elasticity,50-80 g/ den; and a high degree of toughness.

(iii) High performance fibres with excellent me-chanical properties: This class of fibres has atenacity of 15-50 g/den, elongation-to-breakof 0.5-5% and modulus of elasticity of 250-2500 g/den.

To the above three conventional categories offibre types, we may add a fourth category whichbridges the gap in the mechanical properties be-tween categories (ii) and (iii). The presence of thisgap in mechanical properties was recently high-lighted by Hearle!". The fourth category is as fol-lows:

(iv) Intermediate performance fibres with superiormechanical properties: This class of fibreswill have a tenacity of 8-20 g/den, elonga-tion-to-break of 5-15% and modulus of elas-ticity of 80-250 g/den.

These four fibre categories will be consideredin this section. However, before discussing themit would be appropriate to include a brief descrip-tion of the nature of the PET molecule and itslikely contributions to fibre properties. A part ofthe molecule is shown'? in Fig. 2. The aromaticring in the main chain with its attached carbonylgroups confers rigidity to the structure. It hasbeen pointed out by Daubeny et a/.l7 that in anoriented fibre which contains an aggregate of suchmolecules placed parallel to each other, the dis-tances between atoms in .neighbouring moleculesare all normal van der Waals distances; there is,therefore, no structural evidence for any usuallystrong forces between the molecules. The highglass transition temperature and melting point ofthe fibre thus arise mainly from the rigidity of thebackbone. A number of other properties, particu-larly the mechanical properties, derive from theoptimum combination of chain rigidity conferredby the aromatic ring and the flexibility derivedfrom the glycol residue. The choice of this repeatunit is thus indeed a tribute to the foresight of itsinventors. When many molecules come together,depending on the thermo-mechanical history towhich the sample is subjected, they may form am-orphous or semi-crystalline structures, which arerandom or with varying degrees of orientation. Inan oriented fibre, the structure is fibrillar in whicha single molecule gets trapped in several growingcrystals (Fig. 3). The detailed architecture of aPET fibre is still not known with certainly.

PET fibres can be spun over a broad range ofspinning speeds, viz.:

(i) Low spinning speed in the range of 500-'1500 m/min: The product is called low-oriented yarn (LOY) and PET staple fibre

48 INDIAN J. FIBRE TEXT. RES., MARCH 1995

Fig. 2- The chemical repeat unit of PET (between thedashed lines)

and industrial yams are produced throughthis route.

(ii) Medium speed in the range of 1500-4000m/min: The product is called partially-oriented yam (POY) and provides a feed-stock for draw-texturing or for drawn flatyams.

(iii) High speed in the range of 4000-6000m/min: The product is called highly-orient-ed yam (HOY).

(iv) Very high speed in the range above 6000m/min: The product is called fully-orientedyam (FOY). No commercial processes cur-rently operate in this speed range.

The present article will lay emphasis on deve-lopments in fibres produced at medium andhigher speeds.

3.2 Standard Textile FibresCategory (i), viz. standard textile fibres, forms a

very large application sector forPET fibres. It needs to be emphasised at this

Fig. 3-Schematic path of a single molecule in an orientedfibre

stage that though the bulk of fibres belonging tothis category are used for standard textile uses, anumber of non-woven products and also a varietyof industrial fabrics are produced from standardtextile fibres with average mechanical properties.

for convenience of discussion, the fibre pro-ducts in this category will be considered underthe following four headings with the emphasis be-ing on developments in:

• Partially-oriented yam• Fully-oriented yam• Speciality fibres, and• Micro-denier filaments

3.2.1 Partially-oriented Yarn (POY)The high-speed spinning process has been de-

scribed in an earlier section and the POY produc-tion technology forms a part of this broad area.

There are significant differences in structuredevelopment during spinning between the slow-crystallizing systems like PET, fast-crystallizingsystems like polyamides and very fast crystallizing

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 49

systems like polyolefins. In the case of PET,though an oriented, ordered mesophase appearsat spinning speeds of 1000 m/rnin and above, it isgenerally agreed 18 that up to speeds of 4000mlmin, as-spun PET fibre is predominantly amor-phous and is thought to be in the form of an en-tangled molecular network with an entanglementdensity of around 2.3 x 1026 per metre cube whichmeans that there are about 20 monomer units be-tween successive entanglements". However, someauthors believe that the structure may not behomogeneous as in addition to the rubber-like ne-twork, crystal nucIeFu.21 or extended chain mole-cules-? may also be present. As-spun nylon-6 alsodoes not crystallize on the spin-line at low spinn-ing speeds though crystallization occurs on condi-tioning in the package due to moisture pick-up.At speeds greater than 3000 mlmin, however,crystals are generated during the spinning processitself. Polyolefins are fast crystallizing and poly-propylene POY shows significant crystallinityeven at low speeds.

PET POY became a commercial reality in theearly 1970's mainly because of the availability ofcommercial winders around that time and also be-cause of the introduction of simultaneous drawtexturing. The polymer yarn producers werequick to realise that POY overcomes both theshortcomings of LOY {low-oriented yarn), viz, theshelf-life problem and the difficulty of stringingup, mainly because of the increased orientation,which gives the fibre storage stability and whichallows heat-induced crystallization to occur at atemperature 30°C lower than for LOY. A furtheradvantage of high-speed spinning is the higherthread uniformity obtainable from the more stablethreadline provided by the higher draw downforce which means higher threadline tension andby the increased frictional drag of air". Subse-quently, draw-warping was developed as a way ofmaking flat yarns.

One of the most remarkable advantages ofhigh-speed spinning is the increase in throughputper spinneret. In the case of PET yarn of fixed fi-nal linear density it has been shown23.24 that thethroughput does not increase in proportion to thespinning speed as a consequence of the growingmolecular orientation in the spun yarn; a gain ofabout 22% is obtained in throughput in going from1500 to 2500 mlmin, but there is only another8% gain on going from 2500 to 3500 mlmin.This arises because filaments of superior molecu-lar orientation need a reduced draw ratio in asubsequent drawing process. This means that thelinear density of the undrawn filament must be

lowered provided the linear density of the drawnyarn is to be maintained constant. The followingequation defines this relationship-':

M=(L9000XDRXWUSXden)

where M = Mass throughput rate in g/minDR = Effective draw ratioWUS = Wind-up speed in mlmin, andden = Denier of drawn filament.

The degree of molecular orientation and theproductivity increase can be modified by otherfactors. Lower polymer molecular weight, throughlowering the viscosity, leads to lower orientationin the yarn; reducing the threadline cooling rateby using a shroud or a secondary source of heathas the same effect; and higher filament lineardensity gives slower cooling and so operates inthe same direction. Such modifications, therefore,lead to increased throughput at a given wind-upspeed.

3.2.2 Fully-oriented Yam (FOY)Fully-oriented yarns can be obtained through

different routes, the principal routes being as fol-lows:

The POY route: POY has become universallyaccepted as a feedstock for textured yarn wherefurther drawing and texturing get integrated inone process. Alternately, POY can be furtherdrawn with the help of a draw twister into afully-oriented flat yam. Still another possibility isthe use of the draw-warping method to achieve anFay.

High-speed spinning route: This route has al-ready been briefly referred to earlier. Though theyam spun at 6000 mlrnin or higher speed is quiteclose to a FOY, it still needs a draw of around20% to be equivalent to a two-step, commercialfibre. The spun fibre itself is used for a limitednumber of applications. Speeds higher than 6000mlmin are not very helpful as a relatively thickskin develops around the core of the filament andintroduces a level of inhomogeneity that is not ac-ceptable. Thus, one-step PET yams have yet tobecome a commercial success.

Hot tube method: The use of a hot tube belowthe freeze line may allow a further draw to oc-cur and a FaY of good quality can be producedusing this method.

Spin-draw method: Integration of spinning fol-lowed by drawing in the same machine is now awell-established technique of fibre production.

3.2.3 Speciality FibresSpeciality fibres include physically and cherni-

50 INDIAN J. FIBRE TEXT. RES., MARCH 1995

cally modified fibres, such as high-shrinkage fibres,modified dye· variant fibres, profile fibres,hollow fibres, hybrid fibres, dyed fibres, texturedyams, bicomponents, high tenacity fibres, etc. Theproduction volume of these fibres is presentlyvery low in India and in view of their value addi-tion role, it needs to be raised significantly.

3.2.4 Micro-denier FibresStrictly speaking, micro-denier fibres should

have been discussed under sub section 3.2.3 butin view of their current importance, they are be-ing taken up separately. It is believed'" that pol-yester microfibres may form up to 50% of thecontinuous filament market by 1995 and it is,therefore, not surprising that a very large numberof publications are currently appearing on thisproduct.

The first micro-denier was a PET fibre of 0.3dpf (denier per filament) released by Toray in Ja-pan in 1970. Teijin in Japan followed soon afterand in 1972 released a PET and a polyamide fi-bre each of 0.15 dpf. In 1989, Toray released aPET fibre of 0.05 dpf, perhaps the finest microfi-bre in commercial use".

By using the conventional spinning methodsand making use of specialized machines and goodquality polymers, it is now possible to economi-cally produce microfibres down to 0.2 dpf. Manu-facture of fibres below this range is possible viavarious bicomponent splitting approaches downto 0.01 dpf; the splitting of the components beingachieved either mechanically or through a solvent.A third method is based on dissolving one of thephases of a mixed melt-spun fibre containing twophases of an island-in-the-sea type. This gives ult-ra-fine fibres of about 0.001 or even lower denier.

The main advantages of microfibres can be il-lustrated by the following example. Compared toa PET yam containing 1.5 dpf, an equivalent yamcontaining 0.25 dpfwill show:

a reduction in flexural rigidity (which is pro-portional to the square of dpf') by a factor of40,

- an increase in number of fibres per unitweight by a factor of over 6, and

- an increase in surface area per unit weightby a factor of 2.75.

As a result, there is a sharp change in fibre aes-thetics. However, their production requires criticalprocessing parameters. The spinning of PET mic-rofibres through the conventional route has beenreported by Kiang and Cuculo " and the need forbetter filtration, proper static mixing and lowspinlinestress have been emphasized. In addition,

these authors made the following points: A po-lymer of lower intrinsic viscosity of around 0.56dl/g is better suited for microfibre production.When combined with higher temperature of ex-trusion, say between 295°C and 305°C, the elasticeffects are reduced and low die-swell is observed.The lower viscosity allows the desired spinlinestretch to be achieved at lower. stresses. A lowercapillary diameter ensures low draw down but itshould not be too low otherwise shear rates be-come too high and die-swell increases. The au-thors have also described a number of other pro-cessing requirements for producing these fibres.They conclude that spinline stress is the mostimportant parameter in determining the fine den-ier limit. The spinline stress is shown to be affect-ed by the cooling profile, the elongational flowproperties of the polymer as weU as by the airdrag generated in the spinline, which is the mostimportant term determining the spinline stress le-vel due to the very fast cooling of the fine denierfibres.

3.3 Industrial FibresCategory (ii), viz. industrial fibres, also forms a

very large application sector for PET fibres, parti-cularly in industrially advanced countries like theUSA where industrial polyester yarn consump-tion in 1991 was over 50% of the total polyesterfilament yarn consumptiorr'", with tyre cord aloneaccounting for 38%; the rest included seat belts,V belts, coated fabrics, cordage and other rubbergoods like hoses. PET filaments have the advan-tage of a higher glass transition temperature and amuch higher elastic modulus compared to fila-ments of nylon 66; these help in eliminating flat-spotting. However, the low strength and pooradhesion to rubber of PET filaments were twoproblems but they were soon overcome, the firstby using PET of higher molecular weight of in-trinsic viscosity 0.90 instead of the usual 0.61 dl/g. Multifilament yarn spun from this grade of po-lymer at a wind-up speed of 500 mlmin could bedrawn to a draw ratio of 6 to develop a tenacityof 8-10 glden. The second problem, viz. that ofadhesion, was solved by predipping the cord in abath of an epoxide resin and a diamine before therubber latex application. Today, the filaments areusually spun at a wind-up speed of 1500-3000mlmin and then drawn by a factor of about 2, thefinal tenacity and crystallinity being developed byheating and stretching during cord preparation.

Another growing use for PET industrial fibre isas a reinforcing fibre in hybrid composites ofglass fibre and unsaturated polyester resin where

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 51

the PET fibres add to the impact and flexuralstrength of the composite". The geotextile arearepresents another large outlet for industrial PETfibres where its higher creep resistance is an ad-vantage.

3.4 High Performance FibresThe estimated value of crystal modulus for PET

is 110 GPa while the highest measured value ofmodulus reported in the literature-? is 39 GPa fora solution-spun fibre from high molecular weightPET of intrinsic viscosity 2.6 dl/g drawn to a finaldraw ratio of 16.4 in three stages of. draw, the fi-nal stage being at 200°C. Thus, in the presentstage of development, PET can only provide a fi-bre in the lower range of mechanical propertiesexpected of a high performance fibre. Fully aro-matic polyesters or co-polyesters can give superi-or mechanical properties but they are outside thescope of this article.

3.5 Fibres Intermediate between Industrial and HighPerformance Fibres

The solution and/or gel spinning technique iscapable of providing this category of fibres usingsolutions with a range of polymer concentration.However, fibres belonging to this category can al-so be produced through the simpler melt-spinningroute using special drawing and annealing proce-dures.

The first such method makes use of the zonedrawing and zone annealing techniques-", An am-orphous or very low crystalline as-spun fibreforms the starting material. It is drawn at a relat-ively low temeprature to give a fibre of low crys-tallinity which is converted by zone annealing at ahigh tension into a highly oriented, highly crystal-line fibre with little backfolding. A fibre with amodulus of 21.4 GPa has been produced by thistechnique. A second method uses two-stage draw-ing of a conventional melt-spun yarn, each stagebeing followed by an annealing step". This allowsthe molecules to lie close to each other over long-er lengths in the crystalline phase and a modulusof 18.6 GPa has been obtained at a draw ratio ofaround 20.

4 Structure-Property Relations

4.1 IntroductionWith the growing commercial success of PET

fibres and the realization that its macroscopicproperties are predominantly related to its finestructure, R&D efforts on structure-property rel-ations have witnessed an upsurge all over theworld. The fine structure of a fibre produced

from a specific grade of a fibre-forming polymeris a function of a large number of processing par-ameters which can be grouped under three broadheadings, viz. spinning, drawing and heat setting.By varying these parameters, a very wide varietyof structures and morphologies becomes available,giving rise to a spectrum of properties.

The techniques used for characterizing fibresand fibre-forming polymers have also witnessedsignificant improvements and some new powerfultechniques have also been added32,33. In the earl-iest studies on fine structure of fibres, X-ray dif-fraction and electron microscopy made the majorcontributions. In the X-ray diffraction area, one ofthe important developments is the use of synchro-tron radiation which has made it possible to makein situ studies of molecular rearrangements duringmechanical deformation or heat-setting. Thesmall-angle X-ray scattering has also benefitedfrom synchrotron radiation whose brilliance is atleast three orders of magnitude higher than thatof radiation from a rotating anode, and which isproduced as a highly collimated beam. In the areaof electron microscopy, scanning electron micro-scope and the more recent scanning tunnellingmicroscope and atomic force microscope havemade it possible to have more detailed structuraland morphological information. The other tech-niques which are now providing significant'amount of structural information are neutronscattering, fourier transform infrared and Ramanspectroscopy.

4.2 Heat-setting and Its Effect on StructurePET fibres and fabrics contain residual stresses

whose magnitude is a function of the thermo-me-chanical history to which the product had beenearlier subjected. The presence of residual stressgives rise to thermal shrinkage which renders theproduct dimensionally unstable. Heat-setting stab-ilizes the structure of the fibre and the fabric byproviding thermal energy to the molecules so thatthey take up positions close to equilibrium, thusrelaxing the residual stresses. Fabrics may beheat set flat or pleated.

4.2.1 Crystallization on Heat-settingPET is generally heat-set between 190°C and

225°C and the setting effects are associated withchanges in crystalline morphology. Therefore, it isimportant to understand how crystallization oc-curs. There are three possibilities. The smaller,imperfect crystals can melt at the setting tempe-raturew followed by recrystallization into a newform. As opposed to this mechanism, solid state

52 INDIAN J. FIBRE TEXT. RES., MARCH 1995

transformation can take place in the following twoways. First, the molecules in the amorphous re-gions which are well-parallelized but not in regis-ter can, with the aid of thermal energy, gain en-ough mobility to reduce their free energy byforming small crystallites. Second, the larger exist-ing crystallites can become more perfect by ex-pelling the defects out of the crystal.

Crystallization is prohibitively slow below 1'gbecause of lack of molecular mobility and alsoabove Tm because of too much mobility. The rateof crystallization is maximum for PET around180°C. PET is not a fast crystallizing polymer; themaximum crystallization rate for PET is stated:"to be 0.016/s while it is 0.14/s for nylon 6 and1.66/s for nylon 66.

It is well known that polymers crystallize fasterwhen they are oriented and it has been shownthat the crystallization half time for PET reducesfrom 660 s for unoriented sample to 0.01 s forsample having a birefringence of 0.08.

4.2.2 Effect of Heat-setting o!l StructureThe effect of heat-setting on structure will be il-

lustrated with' some results obtained from exten-sive srudies+-" on commercial multifilament yarn(76/36/0), designated as control, which was producedfrom PET chips with a titanium dioxide contentof 0.4% and a melting point of 258°C. Its averagemolecular weight (Mn) was 19,040. The polymerchips were spun at 290°C at a pressure of 1400psi and quenched at 20°C in an atmosphere of97% relative humidity. The take-up speed was800 m/rnin and the average molecular weighteMn) of the spun filament was 17,820. The spunmultifilament yarn was stretched on a draw twis-ter with a top godet temperature of 95°C and aplate temperature of 140°C. The winding speedwas 642 m/min. The draw ratio was 3.92. Thedrawn yarn was heat set in a silicone oil bath attemperatures between 100°C and 250°C undertwo conditions, viz. when the yarn was held tautat constant length (taut-annealed or TA) andwhen the yarn was free to shrink (free-annealedor FA). The crystallinity, crystallite size and crys-tallite orientation factor are shown " in Fig. 4(a)while the birefringence and amorphous orienta-tion are shown in Fig. 4(b). The crystallinity andcrystal size for both sets of samples increase withincreasing heat-setting temperature. Crystalliteorientation and birefringence (average orientation)register an increase with increasing heat-settingtemperature for taut-annealed sample but dec-rease for free-annealed samples; The amorphousorientation factor decreases in both cases. The 1'g

90

" 80.t:!VI 70~E 60'"~ 50u

.7-oG.

.6~'c5:=E

.4 ~•..u

0.3

u-1.00 i.98.E

~- .96.Ec.~

.94 ~•.~0.92 E

VI>-~

100 140 180 220 260Hl!at-sdtilg Temperature l·C)

Fig. 4(a)- X-ray crystallinity, crystallite size correspondingto (010) plane, and Hermans crystallite orientationfactor (fc) as a function of heat-setting temperature

-;;0.23<:I •••••

~...0.11 _~ ContRll@!,cO.1:E.~ 0.17co D FA

o TA0.1J~ 0.9 .o Contj 0.7.~120.5c:

.91<50.3i0.1E« 100 140 1BO Z20 2fJJ

H~t- setting Temperature (oC)

Fig. 4(b)- Birefringence (~n) and Hermans amorphousorientation factor U~"') as a function of heat-settingtemperature

values of these samples, as determined by differ-ential scanning calorimetry", are shown in Fig. 5as a function of heat-setting temperature. It isnoteworthy that at high temperatures of heat-sett-ing, the 1'g decreases. The data are replotted as a

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 53

140r---------~~--------------~

.-:::.120• Control• FA• TA

,!:' 100 • •80~-L----~-----J-- L_ ~

100 140 180 220 260Heat - setting Temperature ,·C)

Fig. 5-Variation of ~ with heat-setting temperature

1~0r-----------------------------~

:='100

• Controla "FAo TA.= 120

80~~------L------L------L-----~30 40 6050

Cr ystallinity ( % IFig. 6-Dependence of ~ on X-ray crystallinity

function of crystallinity in Fig. 6 and the highlycrystalline samples are seen to have low T.. Thisbehaviour may be explained as follows. Up toheat-setting temperatures of 160De, new crystalsare formed by coming together of well-parallel-ized chains in the amorphous regions which in-creases the number of small crystals in the fibre.As a result, the fibre set at 160De' contains a largenumber of small crystals so that the amorphousphase is highly constrained. Heat-setting at highertemperatures can result in the formation of crys-tals of larger size, sometimes by the fusion ofsmaller crystallites. The fibre consequently con-tains a small number of large crystals, thus reduc-ing the constraints on the amorphous phase.

The low-angle X-ray diffraction studies on thesesamples revealed " that in the as-drawn state, thePET multifilament yam did not have clear-cutboundaries between the crystalline and amor-phous phases. Heat-setting of the yam in the free-to-shrink condition introduces clear-cut separa-tion between the phases and also results in in-crease in chain folding. Under constant length an-nealing the phases are not so distinctly separatedand the folds are less, until the heat-setting tem-peratures are high.

4.3 Some Physical Characteristics ofthe Crystalline andAmorphous Phases

Some physical characteristics of PET fibre andits two phases are shown in Table 1. The higherdensity, melting point, modulus and orientation ofthe crystalline regions are noteworthy. Thus, if asemicrystalline polymer is considered as a two-

Table I-Some physical characteristics of PET

Characteristic Crystal Amorphous As-drawnfibre

1.45517

1.47738

1.51539

284

Density (g/cm ') 1.33 1.38

Melting point eC)

~(0C)

Notapplicable68 (Dry)58 (Wet)

1.0

264125 (Dry)110 (Wet)

0.4

Notapplicable

-0Moisture absorption(%)

Elastic modulus(1010 dynes/em")

Longitudinal

TransverseHermans orientationfactor (Infrared) 0.88

1373.68

121.82

182

70 0.94 0.58

phase system, it may be assumed that permeabil-ity in the crystalline phase is negligible comparedto that in the amorphous phase as the crystallinephase with a relatively higher density is muchmore compact compared to the amorphous phase.

4.4 Stress-strain Behaviour4.4.1 Yarns Spun at Different Speeds

The orientation that develops during the spinn-ing process has a predominant effect on the me-chanical properties of the as-spun yams. This isevident from the true stress-strain curves of PETyarns plotted in Fig. 7 for various spinningspeeds. The mechanical properties of the fila-m<::nts spun at high speeds are quite differentfrom those of conventionally drawn filaments.The yield stress rises and the natural draw ratiocharacteristic of low-oriented yarns first reducesand then disappears as the spinning speed in-creases.

4.4.2 Heat-set Multifilament YarnsThe samples used for this study were the same

as described in sub-section 4.2.2, which had beenprepared for structural studies, viz. the spun anddrawn sample (called control) heat-set at tempera-tures between 100De and 2500e in the free-to-shrink and constant length conditions. To illustr-ate the type of data obtained, the average stress-strain curves for the control and the free- andtaut-annealed samples heat-set at 100, 160 and2200e for 1 min are shown in Fig. 8. It is note-worthy that the stress-strain curves for samplesannealed at constant length are quite close to one

54 INDIAN J. FIBRE TEXT. RES., MARCH 1995

12005. . I .pinning speed ,m rrun1. 10002. 12003. 15004. 20005· 30006. 35007 40008. 50009. 5500

1000

800e0-L

600II'II'QI...

+-V\

400

200

Fig. 7- True stress-strain curves of PET yarns spun at different speeds shifted along thestrain axis to match their natural draw ratios

"IA-l60r~220 Tkl00 Control,.<r > ~e-: ;;:?,FA-l00.s-: /FA-160, ,/ /'

I. . ,,-

//

/ .// "",,-

/ // .r:. /' ,/---- -,-r>:.'/

4.5

"0Cl.

"" 3.0

'"'"~+-VI

1.5

,/FA-22D------ ,.

nealed sample. The main morphological differ-ence is that the two phases are more distinctlyseparated in free-annealed sample and they arepredominantly connected in series. Both thesefactors lead to lower modulus, lower strength andhigher extensibility of the free-annealed samples.

It is worth pointing out that the stress-straincharacteristics of taut-annealed samples are closerto the regular PET filaments since most of the in-dustrial heat-setting is under tension. A further in-teresting observation is that the stress-strain char-acteristics of the FA-160 yarn are close to thoseshown by a regular PET staple fibre.

O~--~~--~~--~~--~~--~~--~10 20 30 40 SO . 60 4.4.3 Structural Changes During Tensile Elongation

Strain (% I The earlier data showed that there are consid-erable differences in the stress-strain behavioursof yarns which have been prepared by heat-sett-ing the commercial drawn yam at 220°C in thetaut (TA 220) and free (FA 220) conditions. Togain an understanding of the structural changesoccurring in these fibres during the tensile test,rheo-optical Fourier-transform infrared (FTIR)studies were made on PET films which were pre-pared from 'Mylar' PET film obtained from DuPonr". The film was semi-crystalline and wasstretched in the Instron tensile tester at 140°C toa draw ratio- of 2.5. The stretched film was heat-set in silicone oil at 220°C for 15 min in the tautand free conditions. Both samples were about55% crystalline and their other structural charac-

Fig. R-Stress-strain curves for PET fibres heat-set for I minat lOO,160and220°Candforcontrolsample(FA-Free-

annealed, TA- Taut-annealed)

another and to the control. The curves for thefree-annealed samples, on the other hand, showconsiderable variation; in particular, they show amore distinct yield behaviour and their ductilityincreases with increase in heat-setting tempera-ture. These differences in tensile properties arisefrom structural and morphological differences be-tween these two fibre types which have essentiallysimilar crystallinity. The main structural differenceis that the orientation of the molecules in the am-orphous regions is much lower in the free-an-

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 55

teristics were also close to those of the TA-220and FA-220 yarn samples. The samples were exa-mined in the rapid-scan made on an FfIR and the fol-lowing data were derived from the polarizationspectra series acquired during elongation: (i)changes in the trans/gauche conformations of theethylene glycol segments, (ii) the transient changesin crystalline and amorphous orientation, and (iii)chain unfolding.

In the FA sample, the orientation of the mole-cules in the amorphous phase showed a gradualimprovement throughout the test, while chain un-folding occurred in the sample above 20% strain.This indicates that the predominant mechanism ofdeformation in this sample could be chain uncoil-ing in the amorphous phase followed by longitudi-nal slip processes within the sample. In the TAsample, chain unfolding occurred at low strainsaccompanied by slight improvement in amor-phous and crystalline orientation. Thus, longitudi-nal slip appeared to be the main deformation me-chanism in this sample.

4.5 Recovery BehaviourTextile fibres are subjected to considerable

strains during their life time and therefore to en-sure dimensional stability, it is necessary to designfibres with good recovery characteristics. Thestress-strain curves for PET fibres were shown toexhibit a pronounced yield region where the ten-sile modulus falls quite rapidly. At strains belowthe yield point, the behaviour is more nearlyHookean so that their elastic recovery on releaseof the stress is high. At strains above the yield region,viscous flow plays a larger part in the process ofextension, and the recovery on release of stressbecomes progressively poorer.

The elastic recovery and work recovery of PETfibres are excellent at strains up to about 5% inhighly oriented fibres and up to 3% in relaxed fi-bres and in this range they are superior to thosefor nylon-6 fibres!". At higher strains, they deteri-orate sharply and become inferior to nylon-e. Ty-pically, the immediate elastic recovery falls fromabout 98% at 1% extension to 65% at 5% exten-sion. These values relate to instantaneous re-covery at room temperature. At temperaturesabove ~, the measured recoveries can be poor.

There have been only a limited number of stud-ies to gain an understanding of the structural fea-tures which influence recovery. The recovery be-haviour of PET (2 GT) has been shown to be re-latively poor compared to 3 GT and 4 GTfibres". This was attributed to the dominance ofthe amorphous phase in controlling recovery in 2

GT fibres while in 3 GT and 4 GT fibres, thecrystallites also play an important role. Brody"analyzed the recovery data for a number of fibresincluding PET and concluded that recovery couldbe attributed to the elastomeric nature of the am-orphous phase.

Some studies on PET yarns heat-set in the re-laxed state and at constant length have shown thatthe samples heat-set at constant length have supe-rior recovery behaviour. It has been suggested"that at low strains the recovery behaviour is dom-inated by the amorphous phase in both the sam-ples; since the taut-annealed samples have higheramorphous orientation, they show better recovery.At higher strains, in taut-annealed samples, boththe crystalline and amorphous phases influencethe recovery behaviour and this is mainly becauseof the parallel coupling between them. In the free-annealed samples, the amorphous phase domi-nates the recovery behaviour.

4.6 Dynamic Mechanical PropertiesWhile the tensile stress-strain experiments pro-

vide information of considerable value, they pro-vide little information on the lossy nature of thefibre. A convenient way of studying the transitionsshown by PET is by making dynamic mechanicalmeasurements. Typically, PET fibres show twotransitions, one at about - 40°C which is calledthe p-transition and the other at about no°c,

the a-transition or the glass transition. Thea-relaxation occurs in the temperature range 80-150°C at a frequency of 1 Hz, the exact positionbeing very dependent on structural factors such ascrystallinity and orientation. For an unorientedand amorphous sample (low speed-spun PET), itis close to 80°C (ref. 44). When this sample is heat set,it develops crystallinity but is without orientationand the a-transition is now seen to occur at aboutl LO'C. If the as-spun fibre is drawn and thenheat-set, the fibre is both crystalline and orientedand the ~ is seen at 130°e. The data for thecommercial PET yarn before heat-setting (control)and after heat-setting at 220°C in free condition(FA 220) and at constant length (TA 220) areshowrr" in Fig. 9. The storage modulus is higherfor the control sample and is quite close to thatfor the TA-220 sample. However, when heat-setin the relaxed state, the amorphous phase under-goes considerable disorientation and the storagemodulus is not only lower but also shows consid-erable temperature dependence. The tan <5 valuesare also relatively high for this sample and are in-dicative of the faster rate of decrease of modulusin this sample. The loosely packed molecules in

56 INDIAN 1. FIBRE TEXT. RES., MARCH 1995

r-----------------------------~.~

0·15

..05 t:,... CI

s ~-- 0.10c:>-

"0 2,

•.•...

0.05

o 025 100 150 ~O

Tempernture ("C)

Fig. 9- Temperature-dependence of storage modulus (F. ')and tan bfor control,FA-220 and TA-220 samples heat set

for 30 min

the free-annealed sample account for considerableenergy dissipation at ~.

4.7 Dyeability CharacteristicsSince standard, unmodified PET fibres normally

contain neither basic nor strongly acidic groups,they are not generally dyeable with ionic or ca-tionic dyestuffs. The most important class of dy-estuff used for their colouration is that of dispersedyes!",

Disperse dyestuffs are absorbed and retainedthrough a process of solution in the less crystal-line regions and are commonly applied as disper-sion in water. Slight solubility in water is advanta-geous as it permits diffusion to the fibre surfacebut substantial solubility can lead to poor dyebath exhaustion. However, only a few dispersedyes produce even, moderate shades at the boil.Due to the high ~ of PET, the diffusion rate islow and an adequate shade is not built up. Thereare various ways in which this problem has beenovercome" and has led to the development ofcarrier-free dyeable PET and cationic dyeablepolyester. The latter is produced by incorporatingan additive containing suiphonic acid groups inthe PET chain during the polymerization process.

The earliest studies on dyeability of heat-setPET filaments with disperse dye were reported byMarvirr" who observed minimum in dye uptake ata heat-setting temperature of about 180°C. A si-milar minimum has also been observed for free-

42~ Dyeing Temperature J 130"C-0.•....•.. 380 c FAO'l

..lC 0 TA- 34<U>--0

.•...300

C]'I

26<U

oX:

tlCi 22::J

~>.1'0Cl

14

Fig. lO-Dye uptake for heat-set samples

annealed and taut-annealed samples, as shown inFig. 10 for 24 h dyeing at 130°C using C.I. Dis-perse Red 11. The higher dye uptake of slack-an-nealed sample is believed to be due to the availa-bility of larger amount of free volume. The dyecan either be sorbed into microvoids in the fibrefollowing a Langmuir isotherm or can be molecu-larly dissolved into the free volume created by seg-mental motion.

The minimum in dye-uptake may be explainedon the basis of fibre morphology as follows. Atthe lowest temperatures of heat-setting, new crys-tals are formed in the control sample by comingtogether of parallel chain segments in the amor-phous regions which increases the number ofsmall crystallites, At 180°C, their number is verylarge and so the path through the amorphous re-gions is very tortuous. In the later stages of crys-tallization, the crystallites increase in size and theamorphous volume per crystal increases. The fi-bre has now a smaller number of large crystallites.The amorphous volume per crystal has been mea-sured and shows a minimum at 180°C. It may beadded that the decrease of segmental motion at180°C due to the constraints imposed by a largenumber of small crystallites is also distinctly seenin the ~ data presented earlier.

4.8 Optical PropertiesA single crystal of PET will be characterized by

three principal polarizabilities in the three princi-pal directions. It will thus have three principal re-fractive indices and therefore two birefringences.Textile fibres are generally axially symmetric so

GUPTA: DEVELOPMENTS IN PET FIBRE PRODUCTION 57

that the refractive indices perpendicular to the fi-bre axis are indistinguishable. The principal re-fractive indices for a fibre are therefore ~I forlight polarized parallel to the fibre axis and n 1. forlight polarized perpendicular to the fibre axis.PET filaments, whose draw ratio is generallyaround 4, have a birefringence = (~I - n1.) ofaround 0.16 or above.

The birefringence is made up of three compo-nents, viz. orientational, distortional and formbirefringence; the principal contribution comingfrom orientational birefringence.

The crystalline and amorphous phase contribu-tions to orientational birefringence (~n) are ex-pressed as:

~n=(~n)cryst+ (~n)am

This equation can also be written as:~n= f3 ~nco!c + (1 - (3)~n.mo!amwhere f3 is the degree of crystallinity; fc and !am,the Hermans crystallite and amorphous orienta-tion factors; and ~nco and ~namo, the intrinsiccrystalline and amorphous birefringence respec-tively. The intrinsic birefringences of the crystal-line and amorphous phases are for an ideal unitwith all molecular chains well aligned. ~nco hasbeen reported." to be anywhere between 0.212and 0.31. It was observed:'? that the lower valuesarose from studies on cold-drawn fibres while thehigher values were based on studies on heat-setfibres. It was suggested'" that if a perfect crystalhad an intrinsic birefringence of ~nc,,' then a crys-tal with perfection index P; will have birefringenceP; ~nc,,' assuming that an ideally perfect crystalwill have P; = 1. The measured birefringence ~nof a fibre of crystallinity f3 and crystallite orienta-tion fc can then be written as:

~n = ~~nco f3 t; + Pam~namo (1 - (3)!am

where P.,m is the perfection index of the amor-phous phase; ~namo' its intrinsic birefringence;and !am' its orientation. Since fibres have differentdegrees of defect density, the reported depend-ence of intrinsic birefringence on morphology is.not surprising.

4.9 Thermal CharacteristicsThe thermal characteristics of PET include

such important features as melting and crystalliza-tion and are of practical utility in heat-setting andshrinkage.

Melting represents the transition of a polymerfrom an ordered state to the disordered state of aliquid melt and in this sense is the opposite of

crystallization which is a process of transition of apolymer from a disordered state to an orderedone. While low molecular bodies melt in a narrowtemperature range, polymers melt over a verywide range of up to 100°C. The process of gradu-al decrease in the degree of polymer crystallinityas it is heated is known as partial melting and thetemperature at which the last traces of crystallin-ity disappear is known as equilibrium meltingtemperature, t.;

At the melting point (Tm), equilibrium exists be-tween the liquid and crystal phases and Tm = ~HI~S, where ~H and ~S are the changes in en-thalpy and entropy at melting. ~H is related tothe difference in secondary bonding strengths be-tween the chains in the crystalline and liquidstate, while ~S represents the difference in orderbetween the two states and is generally large forpolymer chains which are flexible and hence cap-able of assuming a large number of conformationsthan stiff molecules.

If the PET fibre is constrained from shririkingin the DSC cell, Tm can rise by 6-9°C and it isthus necessary to consider the effect of molecularorientation, both in the crystalline and amorphousregions, on melting poinr".

The wide crystal size distribution in PET fibreand the presence of imperfections like chain endsin the crystallites result in the melting point notbeing sharp. In a fibre, crystallites form duringcooling. In fibre production, as the fibre coolsduring spinning, the difference of temperature be-tween the filament surface and the environmentcan be large and this can set temperature gra-dients of the order of 1O,000°C/cm to 20,000°CIcm. This corresponds to temperature differencesof 1O-20°C between the surface and filament axis.The crystallites that are formed are therefore notof uniform size.

As far as crystallization is concerned, thoughfully extended chain has minimum free energy,the PET chains are known to crystallize by fold-ing because in this way crystallization is most rap-id. Thus, chain folding is due to kinetic reasonsand the resulting crystals are not in their stableststate but will tend towards it whenever they havea chance, e.g. on subsequent heating. This ex-plains why crystal size becomes larger on heat-setting. When a PET fibre is taken to a highertemperature during heat-setting, different mechan-isms can become operative; one is the partialmelting and recrystallization of the fold surfaces,the other results from a longitudinal translation ofthe polymer chains within the crystalline lattice,which becomes sufficiently softened to permit

58 INDIAN J. FIBRE TEXT. RES., MARCH 1995

such a transinon. In addition to these two me-chanisms, small regions of parallel molecules inthe amorphous phase can also crystallize.

It has been observed that PET filaments, evenwhen heat-set at 255°C in the relaxed state, retainhigh crystallite orientation apparently because theBrownian motion in the surrounding viscous po-lymer melt is insufficient to permit high amountof disorientation. This speculation touches on ananalogy to liquid crystals.

Heat-setting or annealing of PET fibres is oftenused as a means of improving some fibre propert-ies, particularly its stability. Fibres are usually an-nealed as oriented structures after neck-drawing.Besides this, for operational reasons, fibres arerarely annealed in a completely relaxed state. Theindustrial heat-setting is performed mostly on fi-bres wound on bobbins, thus having only limitedfreedom of shrinkage. Occasionally, fibres are an-nealed in one continuous operation directly afterthe process of neck-drawing. In such cases, thetreatment is done under constant length or evenunder constant tension; a slight extension beingapplied during annealing. Heat-setting decreasesthe amount of shrinkage the fibre may display infurther use. The boiling shrinkage of fibres heat-set at different temperatures is shown in Fig. II.As expected, the free-annealed fibres show grea-ter dimensional stability.

In PET fibres, orientation-induced crystalliza-tion plays an important role as briefly discussedearlier.

The shrinkage of PET fibres is believed to arisefrom the tendency of the rubbery phase to coil upwhen subjected to temperatures above ~ and also

8D FA

Contrnl• TA

6

~0

01 4CI

·cs.lI:C•..

J::;2VI

o100 140 180 220 260

Heat-setting Temperature ,o()

Fig. Ll e-Shrinkage in boiling water as a function of heat-setting temperature for free-annealed and taut-annealed samples

from the folding of molecules at higher tempera-ture50.51. It has been reported '? that during isoth-ermal heat-treatment of PET fibres, there is acompetition between two processes: (i) recoilingof chains, and (ii) crystallization. In the earlystages of heat treatment, the recoiling of thechains prevails over crystallization, causing posi-tive shrinkage. Once crystallization begins, therate of shrinkage decreases and is ultimately su-perseded by elongation. It has recently beenshown>' that the speed of production of the fibrescan play an important role in determining the in-teraction between shrinkage and crystallization.References

1 Man-made fiber year book, edited by H J Koslowski, (Chemi-fasern Textilindustrie, Frankfurt), 1993, I.

2 Polyester: 50 years of achievement, edited by D' Brunsch-weiler and 1 W S Hearle (The Textile Institute, Manches-ter), 1993.

3 Gupta V B, Fibre structure: A molecular jigsaw puzzle-inPolyester. 50 years of achievement, edited by D Brunsch-weiler and J W S Hearle (The Textile Institute, Manches-ter), 1993, 170.

4 Ziabicki A, Fundamentals of fibre formation: The science offibre spinning and drawing (Wiley Interscience, London),1976.

5 Bhuvanesh Y C & Gupta V B, Indian J Fibre Text Res,15 (1990) 145-153.

6 Verma R K, Bhuvanesh Y C, Gupta V B, Manabe T &lalan R, Indian J Fibre Text Res, 16 (1991) 39-45.

7 Hedge N V, Some studies on the origin of nubs in spunand drawn filament yarns, M. tech. dissertation, Textile De-partment, lIT-Delhi, 1974.

8 Kawaguchi T, Industrial aspects of high-speed spinning, inHigh speed fibre spinning. Science and engg. aspects, edit-ed by A Ziabicki and H Kawai (lohn Wiley & Sons, NewYork) 1985,3-19.

9 George H H, Spinline crystallization of PET, in Highspeed fibre spinning. Science and engg. aspects, edited by AZiabicki and H Kawai (lohn Wiley & Sons, New York),1985,271-294.

10 Noether H D, Int J Polym Mater, 7 (1979) 57-82.11 Brody H, Hot tube spinning of polyester fibres, paper pre-

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