combined effects of kink bands and hygrothermal conditioning on tensile strength of polyarylate...

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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Combined effects of kink bands andhygrothermal conditioning on tensilestrength of polyarylate liquid crystalco-polymer and aramid fibers

A Abu Obaid1, S Yarlagadda1 and JW Gillespie Jr1,2,3,4

Abstract

Translation of tensile properties from high-performance fibers to end-use fabrics is sensitive to weaving-induced filament

defects and environmental exposure. In this effort, isolated and combined effects of hygrothermal exposure and curva-

ture-induced kink bands on tensile strength of VectranTM HT (polyarylate liquid crystal polyester fiber) and Kevlar� KM2

are studied. Hygrothermal conditioning was conducted at temperatures ranging from 40�C to 100�C in water for

30 days. Curvature-induced defects were created by wrapping tows around stainless steel rods of different diameters

(0.25 mm to 5 mm) to create kink bands. Combined effects were evaluated by conditioning tows with kink bands at

100�C for 30 days. All conditioned samples were dried and tested at room temperature. Hygrothermal aging showed

that tensile properties for Vectran fibers were not appreciably affected below 100�C (�12% reduction), while KM2 fibers

dropped continuously with increasing temperatures (�48% at 100�C). The influence of curvature on kink band density

was established for each fiber type. The isolated effect of kink band density on residual strength was approximately 15%

for both Vectran 1670/600 and KM2-600. Combined effects of curvature-induced kink bands followed by hygrothermal

exposure showed significant reductions in tenacity up to �96% for KM2 and 60% for Vectran HT1670/600. Inspection of

the microstructure within the kink bands reveals extensive micro-cracking and fibril failure due to accelerated moisture

ingress.

Keywords

Liquid crystal co-polymer Vectran fibers, Hygrothermal resistance, 2D weaving and High tenacity fibers

Introduction

High tenacity fibers are used in a variety of commercialand military applications in fiber, tow and woven fabricforms. Commercially available fibers include glass(S and E-Glass), aramids (Kevlar�, Armos, etc.) andother types of organic polymer fibers (VectranTM,Spectra�, Dyneema� etc.). In high-performance appli-cations, it is desirable to retain the inherent tenacity ofthe materials after weaving and during use. Weaving oftows can degrade tenacity through abrasion and cre-ation of defects.1–3 In organic fibers that exhibit lowcompressive strength,4 kink bands can form when weav-ing-induced curvatures exceed a critical level. In servicethe effects of humidity and temperature can also have aneffect on durability and tenacity retention.5

The current effort focuses on polyarylate liquid crys-tal co-polyester fiber (VectranTM) and Kevlar�

KM2-600 shown in Figure 1. VectranTM exhibits simi-lar mechanical properties (see Table 1) to Kevlar�

KM2-600, which provides a good basis for comparison.Vectran fiber is a thermotropic liquid crystal

co-polymer (LCP) spun from a melt into highly

1Center for Composite Materials, University of Delaware, Newark,

DE, USA2Department of Materials Science and Engineering, University of

Delaware, Newark, DE, USA3Department of Civil and Environmental Engineering, University of

Delaware, Newark, DE, USA4Department of Mechanical Engineering, University of Delaware,

Newark, DE, USA

Corresponding author:

JW Gillespie Jr, University of Delaware, Newark, DE 19716, USA.

Email: [email protected]

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oriented fibers having exceptional high specific tenacity,low creep and moisture absorption, (<0.1% at RT) andhigh hydrolysis resistance.6–10 Despite these advan-tages, Vectran fibers exhibit some changes in their inter-nal structure that may have an impact on themechanical performance of the fibers under hot/humid environments. Previous research7,8 on Vectranfibers using the DMA test method showed that thefibers exhibited a-relaxation peak at approximately100�C. An additional relaxation peak was observed inthe temperature range of 50–60�C. The authors attrib-uted the peak at 100�C to a slipping mechanism of theoriented chains relative to each other. Saw et al.8 stu-died the molecular reorganization in Vectran fiberduring heat treatment using thermal analysis andX-ray diffraction (WAX and SAX) techniques. Theyfound that partial failure of the entanglements occurredin the vicinity of 50 to 60�C. The fiber also underwentcontraction with increasing temperature due to con-formational rotation occurring along the extendedpolymer chain. These effects on tenacity have notbeen fully quantified.

Kevlar� KM2 fiber belongs to a family of polyp-phenyleneterephthalamide (PPTA) fibers with achemical structure shown in Figure 1. It was reportedfor PPTA fiber that short-range molecular motionsoccur at room temperature and above. These motionsinclude rotational motion of the free diamine rings thatare responsible for the diffusion of small polar mol-ecules (i.e. water) into the defect region of the interiorcrystal structure.11–13 In addition, PPTA fibers arehighly susceptible to water diffusion due to the presenceof amide groups.14,15 Water molecule and amide group

interaction results in tenacity degradation. There areseveral previous studies on hydrolysis resistance ofPPTA fibers against water at elevated15,16 and relativelylow temperatures.5 As reported in the literature,15

Kevlar� 49 fibers treated in water at 180�C using anautoclave cell for 1 day exhibited a 75% decrease intenacity. Abu Obaid et al.5 recently studied the degrad-ation in tenacity of Kevlar� KM2 fibers after treatmentin water at temperatures ranging from 40�C to 100�Cfor 17 and 34 days, and found the tenacity of thesefibers begins to decrease significantly for samples con-ditioned at 80�C for 17 days and the effect becomesmore pronounced at 100�C for 34 days, where thedecrease in tenacity was approximately 58%.

During weaving of fabric layers, tows of fibers aresubjected to a variety of loading conditions that canalso lead to tenacity degradation.1,2,17 Tows are sub-jected to combinations of tension, bending and abra-sion between adjacent tows and machine elementsduring weaving. It has also been reported in the litera-ture1 that Vectran fibers experience kink band forma-tion during the weaving process when fiber curvature istoo great. These kink bands were also observed in othertypes of synthetic fibers such as Armos (copolyaramidfiber),18 Dyneema,1 PBO and M5.4 Kink bands form onthe compression surface of the fiber, where this effect isdominated by fibrillar instability at a lower lengthscale.19,20 For these highly oriented fibers formationof kink bands is often due to weak interactions betweenthe microcrystalline domains leading to chain slippageand micro-buckling at low compressive strains. Asreported in the literature,4 a threshold level of flexuraldeformation or curvature is required to nucleate kinkband formation. Further increases in curvaturewill result in higher kink band density (i.e. number ofkink band per unit length of fiber). In general,kink bands represent defects that induce misalignmentof crystalline microfibrils, generate stress concentra-tions at boundaries of micro-buckled regions and areduction in tensile tenacity.5 Kink bands also openpathways for moisture ingress that can further reducethe tenacity.

This study focuses on the effects of hygrothermalaging, and the effects of kink bands before and afterhygrothermal aging on fiber tensile properties of

Figure 1. Chemical formula for VectranTM and Kevlar� KM2 fiber.

Table 1. Mechanical properties of various VectranTM fibers and

Kevlar� KM2-600 tows.1,2

Fiber type

Tenacity

(g/den)

Modulus

(g/den)

Strain at

Failure (%)

VectranTM HT1670/3002 25.90 600 3.80

Kevlar� KM2-6001,a 21.41 723 2.85

aProperties measured for dry, untwisted tows at cross-head speed of

308 mm/min and 254 mm gauge length.

2 Journal of Composite Materials 0(0)




Vectran HT and Kevlar KM2. Hygrothermal agingwas investigated by comparing tensile properties offiber tows before and after submersion in water for30 days at temperatures of 40, 60, 80 and 100�C.The effects of kink band density on tensile propertiesof both Vectran and KM2 fibers were measuredbefore and after wrapping of fiber tows around stain-less steel rods with different nominal diameters.Combined effects of hygrothermal conditioning offibers with kink bands were also assessed by condi-tioning fibers that were wrapped around cylinders ofdifferent radii, at 100�C for 30 days. All tenacityresults reported in this study are for tows that havebeen dried and subsequently tested in tension at roomtemperature.

Materials

VectranTM HT 1670/600 tows manufactured byKuraray Corporation and Kevlar� KM2-600 towsmanufactured by DuPont were evaluated to comparehygrothermal aging, kink band density and combinedeffects on tow tensile performance. The nominal char-acteristics for both Vectran and Kevlar tows as given bythe manufacturer are shown in Table 2.

Experimentation

Tensile testing

Tensile testing was conducted on the tows after thetows were exposed to different types of conditions.Prior to tenacity testing, all tows are dried at 50�Cfor 2.5 days in vacuum. All tension tests are conductedat room temperature. Tensile tests were performedaccording to ASTM standard D 2256-02 on all fibertows. All measurements were carried out under ambienttemperatures at a crosshead speed of 304.8mm/minuteusing a mechanical test frame with 100 lb. load cell. Tentows (each of them was 381mm long) were evaluatedfor each case of condition. Each tow was weighed usinga scale with an accuracy of 1mg, end-tabbed usingcardboard material and mounted in the clamps of thetesting machine with an effective gauge length of

254mm. From load-displacement data, tenacity in g/linear density was calculated using Equation 1:

Tenacity ¼ P=LD ð1Þ

where P is the failure load in gram-force and LD is thelinear density measured in denier. All fiber tows weretested in an untwisted state.

Hygrothermal aging of tows

Hygrothermal effects on Vectran and KM2 fibers wereevaluated by measuring mechanical properties of thetows after hygrothermal aging by water immersion at40, 60, 80 and 100�C temperatures for 30 days. Forhygrothermal aging, fiber tows (each type was 10mlong) were cut from control spools and dried at 50�Cunder vacuum for 2.5 days. It is noteworthy that thislow temperature was chosen for drying in order to min-imize the effect of possible temperature inducedchanges in molecular reorganization on mechanicalproperties5. It has been also shown in same cited litera-ture that this drying procedure of KM2 fibers does notaffect the short duration water desorption from thefiber. Therefore, the changes measured in this studyare due to immersion in water for 30 days.

After the drying process, tows were wound aroundTeflon� cylinder with a length of 15 cm and a diameterof 6.5 cm). The cylinder was covered with a cotton clothto ensure that the wound fiber surfaces would be fullyimpregnated with water. Manual winding procedurewas carried out, where care was taken to ensure thatthe tows were uniformly wound without inducing fiber/fiber abrasion and with negligible constraints toprovide a stress free exposure of the fibers within thetow to water. The tow assemblies were then submersedin distilled water using Nalgene containers and placedin an oven equipped with a temperature control,where the temperature of the sample was alwaysconfirmed using an external calibrated thermometer.It is noteworthy that small holes were made in thecaps of the water containers to minimize the waterevaporation.

Winding procedure

The resistance of fibers to curvature was evaluated bytensile testing before and after winding the tows aroundstainless steel rods with different diameters. Tows werecut carefully from the spool and wound around thesteel in a way such that the fibers were kept close toeach other resulting in a circular helix with a pitch of2pb (b is the fiber diameter; see Figure 2). The radius of

Table 2. Nominal characteristics of evaluated tows.

Tow

Density

(g/cm3)

Filament

diameter

(mm)

Linear

density

(denier)

Filaments

count

VectranTM HT1670/600 1.41 15.8 1500 600

Kevlar� KM2-600 1.44 12.1 600 400

Abu Obaid et al. 3




curvature for a circular helix with radius of r can begiven as21

Radius of curvature Rð Þ ¼ rþ b2=r ð2Þ

In this study, the quantity b2 can be estimated as�10–6mm2, which is much smaller than the radii ofthe rods (it ranges from 0.25mm to 5mm) used.Therefore, the radius of curvature can be estimated asR–r. Table 3 lists the values of radii of curvatureapplied to the tows. It is noteworthy that winding pro-cedure was performed at negligible tension and withoutfiber/fiber abrasion.

Two sets of samples were prepared using the windingprocedure described above. The tows from the first setwere unwrapped and tested in tension, while the secondset was used for the study of combined effect of curva-ture/hygrothermal aging.

In order to investigate the combined effect of hygro-thermal conditioning after kink band formation, fibertows were first dried (50�C for 2.5 days under vacuum),then wrapped around steel rods with different radii ofcurvature, listed in Table 3, to create kink bands dens-ity. Prior wrapping procedure, the steel rod was

wrapped with a thin film made from Teflon� to protectthe fiber from the metal surface during wrapping pro-cedure and hygrothermal treatment. Specimens werethen conditioned at 100�C for 30 days (see Table 3).After conditioning, samples were dried at 50�C for 2.5days under vacuum and tenacity was measured.

Kink band density

Kink bands after formation that appeared on fibers dueto curvature at different radii were imaged using a cali-brated light microscope and the final kink band densitywas measured. It is noteworthy that, the fiber wasplaced on a glass slide without twisting prior imagingprocedure. It was observed that that filaments withinthe tow are being overlapped and mixed after theunwinding procedure which makes measurement ofthe kink band density on the monofilaments very diffi-cult. Moreover, kink band density was measuredfor both single fiber and tows wound around steelrods with r¼ 0.25mm (which is the smallest radius ofcurvature in this study). For tow method, kink banddensity was measured over at least 12 filaments takenfrom inter diameter (DI) and outer (OD) region. Toidentify the filaments taken from inner and outerregions, the surface of the rod was dyed in red colorprior to the tow winding. So, the fibers taken frominner regions are colored while fibers from outer diam-eter are not.

Results indicate that the kink band density obtainedfrom both the single fiber tow methods exhibit insignifi-cant differences for the two types of fibers (see Table 4).Thus, winding of the single fiber procedure wasadopted for the kink band density study. For eachtype of fiber, single fibers �10 cm long were taken care-fully from the tow and wound around steel rods atdifferent radii of curvature, given in Table 4. Then,the number of kink bands which microscopicallyappeared on the surface of the monofilament werecounted per unit fiber length (mm) and defined askink band density (ND).

Table 3. Test matrix and conditions for combined curvature/

hygrothermal aging study of tows.

Material

Conditioning in

water for 30 daysaRadius of

curvature (mm)

VectranTM HT1670/600

Tow-Spool

100�C 0.25, 0.40, 0.60,

0.70, 0.91, 2, 5

Kevlar� KM2-600

Tow-Spool

100 �C 0.25,0.40, 0.60,

1, 2, 3, 5

aSamples were dried at 50�C for 2.5 days under vacuum before and after

water conditioning.

Table 4. Kink band density (Number of kinks per unit length,

mm) obtained from tow and single fiber method at r¼ 0.25 mm.

For tow method, kink band density was measured over at least

12 filaments taken from inter diameter (DI) and outer (OD)

region.

Radius of curvature

(mm)

Tow methodSingle fiber

methodID OD

Vectran 1670/600 10.1� 2.6 10.0� 2.1 9.0� 2

KM2-600 20.6� 3 20.6� 3.4 18.2� 2

Figure 2. Typical image for a part of a circular helix generated

by winding the Vectran fibers around a steel rod featuring kink

bands due to winding procedure.





Fourier transform infrared spectroscopy

In order to detect chemical changes in the fiber struc-ture due to conditioning in water, Fourier transforminfrared (FTIR) measurements were performed onsingle monofilaments, before and after conditioning inwater at 100�C for 30 days. FTIR tests were conductedin transmission mode at 4 cm–1 resolution using aNicolet Nic-PlanTM IR microscope with a liquid nitro-gen-cooled Mercury Cadmium Telluride (MCT-A)detector. Spectra were the result of 65 scans with aresolution of four wave numbers. Same FTIR test pro-cedure was used in literature.5

Results and discussions

Hygrothermal aging of fibers

Tenacity of fibers. Hygrothermal aging of VectranHT1670/600 and KM2 tows was performed in water for30 days at varying temperatures. For each type of fiber,tenacity at ambient conditions was measured for base-line samples (samples were dried in a vacuum at 50�C)and conditioned fibers in water. The results are shownin Figure 3. From this figure, tenacity of Vectran fibersexhibits relatively small changes due to conditioninguntil 80�C, with a maximum drop of 12% at 100�C.KM2 fibers show a clear decreasing trend evident at80�C with approximately a 50% reduction in tenacityat 100�C.

In order to investigate whether the hygrothermalconditioning in water induces thermal oxidation and/or chemical changes in the fiber structure, linear densi-ties and FTIR of spectra were studied for hygrother-mally conditioned fibers.

Linear density in denier for specimens was measuredafter drying at 50�C for 2.5 days under vacuum.

Average linear densities for both types of fibers (KM2and Vectran HT1670/600) are shown in Figure 4. Thelinear density of the conditioned fibers in water for30 days at 100�C exhibit insignificant differences fromtheir baseline samples. This indicates that there was nomass loss and the absorption/desorption of water is areversible process at all temperatures. Prior work hasdocumented that equilibrium water uptake in Kevlarfibers is on the order of 5% by weight under ambientconditions at room temperature.5 Weight change forVectran due to conditioning was not measurable, dueto its very low water uptake. Overall, results indicatethere was no thermal oxidation or degradation due tohygrothermal treatments.

FTIR absorbance bands of fibers. FTIR measurements wereperformed on Vectran HT 1670/600 fiber specimens.Specimens were dried at 50�C (baseline) and condi-tioned in water at 100�C for 30 days after drying at50�C. FTIR spectra for both baseline and conditionedVectran fibers in water are shown in Figure 5. For base-line, the major absorbance of bands of stretchingvibrational modes include aromatic phenyl-carboxyl(aryl-COOH) at 3462 cm–1, aromatic phenyl-hydroxyl(aryl-OH) at 3080 cm–1, and aryl-C (¼O)-O-aryl (esterlinkage) at 1745 cm–1. Baseline and conditioned fibersexhibit similar absorbance bands, especially thosebands located at 3462, 3080 and 1745 cm–1. This revealsthat the hygrothermal conditioning did not induce achemical degradation and/or a thermal oxidation ofthe fiber. For example, if there was a chemical hydroly-sis due to hygrothermal conditioning, the intensity ofthe absorbance band at 1745 cm–1 should exhibit a sig-nificant decrease if ester linkage is chemically attackedby water.

Figure 3. Tenacity at room temperature for Vectran HT 1670/

600 and KM2 tows. All samples were dried in vacuum at 50�C for

2.5 days before and after conditioning.

Figure 4. Linear density (den) values measured for Vectran

HT 1670/600 and KM2-600 fibers after conditioning in water

for 30 days and drying at 50�C for 2.5 days in vacuum.

Abu Obaid et al. 5




It has also been reported in literature5 that that thetreatment of KM2 fibers in water at 100�C for 34 daysfollowed by drying at 50�C under vacuum for 2.5 daysdid not induce chemical degradation or thermal oxida-tion as observed in their FTIR spectra, or significantchanges in the mass of the fiber KM2 fibers as observedin their linear density measurements.

Degradation mechanism in tenacity of tows. As shown pre-viously, the hygrothermal treatments of KM2 fibers at100�C did not induce chemical degradation or thermaloxidation. Therefore, the degradation mechanism intenacity can be driven by the mechanisms of thewater diffusion into the fiber structure, molecularorganizations and the micro-cracking (i.e. mechanicaldamages). Mechanisms governing the diffusion of waterinto Aramid fibers have been well documented in theliterature.11–13,22,23 Transport of water molecules inaramid fibers is primarily governed by the flipping ofthe free phenylenediamine ring in the aramid backbone.It has been shown that this type of conformation asso-ciated with constrained molecules in the crystalline corehas a higher activation energy than the molecular con-formation associated with molecules found in the outershell of the fiber. When the ambient temperatureexceeds approximately 75�C, the rotation of these con-strained molecules is initiated, providing a pathway forwater molecules to penetrate into noncrystalline regionsand may interfere with the hydrogen bonds of molecu-lar chains, resulting in the distortion of the molecularpacking.11–13,22 It has been shown also in literature14

that the water molecules can irreversibly react withamide groups of Kevlar fiber, resulting in clusters ofwater-amide complex molecules. Chatzi et al.23

reported three different O-H stretching vibrations asso-ciated with absorbed water in Kevlar fibers. Two vibra-tions at 3640 cm–1 and 3560 cm–1 were associated withwater molecules that are weakly hydrogen-bonded toamide groups and the vibration at 3450 cm–1 was attrib-uted to free water molecules clustered between the crys-tallites (amorphous regions) and in the micro-voidsinside the fiber. Saijo et al.24 reported that thesorbed water in Kevlar fibers have a form of macro-water clusters. Therefore, the water diffusion into thefiber can weaken the structural performance of thefiber. Recently,5 dramatic loss in tensile strength ofKM2 fibers was observed in tows that have been con-ditioned in water at temperatures of 80�C and higher.Authors postulated that the ingress and subsequentremoval of water from the core of the fiber leads todamage in the fiber core, resulting in the observed ten-acity degradation of the tow.5 In this work, the fiberswere dried at 50�C after the hygrothermal treatments,where the removal of the bonded water moleculesduring the drying procedure can cause growth of dam-ages (micro-cracks). Li et al.25 observed micro-cracks inthe core of Kevlar 149 fiber after hygrothermal aging at90�C for 12 weeks. Similar observations were reportedby Allred and Roylance26 that the moisture facilitatesinternal cracking in the Kevlar filament through pre-existing defects in the network. Wide-angle X-ray dif-fraction (WAX) study performed on wet Kevlar fibersin previous work16,25 revealed that the crystallite size ofthe fiber exhibited an increase of �15% in lateral dir-ection and a decrease of 5% in the fiber direction(c-axis), suggesting a disturbance in the fine structureoccurred due to hygrothermal treatment. Additionally,70% of the N-H groups of the Kevlar fiber belong to

Figure 5. FTIR spectra for Vectran HT 1670/600 tows. Treated tows were dried at 50�C under vacuum prior testing.





highly ordered inaccessible material.27 Moreover,Kevlar fiber is composed of a highly oriented 3D crys-talline structure15,16,28 that is interspersed with highlyordered oriented amorphous regions. In particular, thecore section of the fiber has taut tie molecules thatbridge between the lamellae and noncrystalline regions.Thus, these taut tie molecules existed in the amorphouszones can degrade causing the micro-cracks to propa-gate during hydrolytic attacks.

The exact nature of the microstructure present in aVectran fiber is a matter of some debate. Generally, it isthought that the Vectran fiber has a complex micro-structure consisting of both a ‘‘quasi-three dimen-sional’’ orthorhombic structure and domainsconsisting of a pseudo hexagonal phase with longrange order in the two dimensions perpendicular tothe fiber axis and only short range order along thefiber axis. This is due to the fact that the x and y mono-mers are randomly distributed in the molecular back-bone, making it difficult for the molecules to form athree-dimensional molecular packing structure.However, both phases are highly oriented and have adense-packed molecular chain,8,29 which may inhibitthe penetration of moisture into the fiber.Additionally, Saw et al.,8 reported very large scalestructures in annealed Vectran fibers observed throughSmall Angle X-ray scattering, which were interpreted as‘‘clusters’’ of highly oriented molecular entanglements.It was postulated8 that these entanglements are respon-sible for the distribution of load throughout the fiber,and that annealing of the fibers increases the thermalstability of these entanglements.

The backbone structure of Vectran fiber has esterlinkages that do not form hydrogen bonds with watermolecules, as is the case for aramid fibers, where watermolecules bond strongly to the amid groups.14 As aresult, one would expect these fibers to be less suscep-tible to hygrothermal attack, than KM2 fibers based onAramid fiber chemistry.

From Figure 3 it can be seen that the hygrothermaltreatment of Vectran fiber has a relatively small butclear effect on the Vectran material, in that fiber ten-acity decreased by �12% at 100�C. This drop in ten-acity cannot be explained by thermal oxidation orchemical degradation mechanisms, since the linear den-sities of conditioned fibers did not exhibit changes attemperatures and FTIR spectrum did not change afterconditioning in water at 100�C for 30 days. It has beenshown in literature7,8 that the thermal treatments ofdried Vectran fibers induce molecular reorganizationswithin the fine structure. It is possible that the disturb-ances in the molecular structure that can be induced byhygrothermal treatment weaken the integrity of themolecular structure, resulting in some tenacity loss.Thus, the degradation in tenacity observed for

Vectran fibers is most likely a mechanical-basedmechanism.

Curvature effect on tenacity of tows

Tenacity for Vectran HT and KM2 tows was evaluatedafter winding them around steel rods with differentradii listed in Table 3. Figure 6 shows the measuredtenacity values for KM2-600 and Vectran HT Towsas a function of radius of curvature and their controlvalues (they are �22 g/den for KM2-600 and �26 g/denfor Vectran HT). KM2 shows a decreasing trend below1mm radius of curvature and insignificant changesabove 1mm radius of curvature, where the highestloss in tenacity of 15% was obtained for the smallestradius of curvature. It should be noted that KevlarKM2 tows in woven fabrics typically see a radius ofcurvature that is �0.7mm. This is a similar range asreported by Deteresa et al.30 for Kevlar 49 fibers wherea 10% loss in tenacity was seen due to kink band for-mation in single fibers. Like KM2 fibers, Vectran HT1670/600 fibers exhibit more or less the same behaviorfor tenacity as a function of radius of curvature.

In order to correlate the degradation in tenacity dueto curvature shown in Figure 6, the kink band densitywas measured as a function of radius of curvature forboth types of fibers (Vectran and KM2) using thesingle-fiber winding method. It is noteworthy that thefiber was inspected under light microscope to insurethat it is free of damages or kinks prior winding pro-cedure around the rod. Also, care was taken that neg-ligible abrasion was induced during winding. For eachradius of curvature, the kink band density (counts perunit fiber length) was measured using a light micro-scope. Kink bands on single fibers during curvature atlow radii were also imaged at high magnification usingscanning electron microscope (SEM). Figure 7 showsthat the kink band density for Vectran HT1670/600 andKM2 fibers exhibit decreasing trends as radius ofcurvature increases. This indicates that the propagationof the kinks become more pronounced at lower radii ofcurvature. This effect can be also seen in Figure 8,where the kink bands that appeared as dark regionsalong the Vectran filament became denser due to thepropagation mechanism after winding at lower radii ofcurvature. KM2 fibers show higher kink band densitycompared to Vectran fibers, for a given radius of curva-ture, due to the difference in the fine structure betweenVectran and Aramid fibers.

Trends of kink band density for KM2 and HT 1670/600 fibers correlate well with their results of tenacityversus radius of curvature (see Figures 6 and 7). Thiscorrelation indicates a decreasing trend with increasedkink band density. These results suggest that the reduc-tion in tenacity for both types of fibers are due to the

Abu Obaid et al. 7




bending deformation. The fibrils are peeled from thefiber surface which results in a micro-cracking alongthe fiber as observed from SEM images for the fibersduring the bending procedure (see Figure 9). The fail-ure of these fibrils can be due to tension stresses onouter surface that are induced by bending of the fiber.

Curvature/hygrothermal aging combined effects. Tenacityand kink band density of tows were measured after

hygrothermal treatments at 100�C for 30 days ontows that were wrapped around steel rods with differentradii. Figures 10 and 11 show the tenacity versus radiusof curvature after curvature/hygrothermal combinedtreatment for KM2 and Vectran fibers. Detailed deg-radation mechanisms due to hygrothermal treatmentson the tenacity for KM2 and Vectran fibers were pre-viously discussed. It is clear from these figures that thecombined curvature/hygrothermal treatment signifi-cantly amplifies these effects with smaller curvatures

Figure 6. Tenacity versus radius of curvature measured for KM2 and Vectran tows after winding procedures and their control

values. Note that tows were dried at 50�C for 2.5 days prior to tensile testing at room temperature.

Figure 7. Kink band density (counts per unit fiber length, Nd)

measured for untreated Vectran HT and KM2 fibers (they were

not treated in water) after wrapping procedure at different radii.

Kink

(a)

(b)

(c)

(d)

Figure 8. Image for baseline Vectran fiber (a) and after

wrapping around steel rods with radius of (b) 5 mm, (c) 1 mm and

(d) 0.25 mm. Spacing of kink bands (appear on the fiber as dark

regions) decreases (i.e. kink band density increases) as radius of

curvature of wrapping is reduced.





(and the resultant increased stress). This should alsoresult in an increase in kink band density, and conse-quently more reduction in tenacity. During the curva-ture/hygrothermal combined treatment, the waterdiffusion into the fiber structure can disturb andweaken the molecular interactions between the chains(i.e. hydrogen bonds) increasing kink bands density.Leal19 studied the effect of hydrogen bonding and mois-ture cycling on compressive strength of M5 fibers andfound that moisture cycling decreased the degree ofhydrogen bonding by 53% and compressive strengthby 25%.

Figure 12 shows kink band density versus radius ofcurvature for fibers after curvature/hygrothermal treat-ment at 100�C for 30 days. Compared to kink banddensity results generated from curvature study ofuntreated fibers (see Figure 6), the curvature/hygrother-mal (see Figure 12) treatment resulted in a significant

increase in the kink band density, especially at lowerradii. Below 1-mm radius of curvature, this increasewas estimated to be a factor of �4.5 for Vectran HT1670/600 and �15 for KM2 fibers. This remarkableincrease in the kink band density for KM2 fibers atlower radii can be due to significant distortions in themicrostructure due to compressive stresses. Bendingeffects on morphology of Kevlar fiber using loop testmethod were well documented in literature,4,19,31 wherethe looped fiber exhibits kink bands due to the plasticdeformation. Also, it was shown that the compressivestress for looped fiber is proportional to 1/R (R is theradius of curvature).4,19 Therefore, the wound fiber atlower radius of curvature can experience higher com-pressive stresses resulting in higher kink bands density.It was discussed previously that for KM2 fibers, thehygrothermal treatment can induce micro-cracks inthe core, disturb and weaken the molecular interactions

Figure 9. SEM photographs for single fiber of Vectran 1670/600 fibers during bending at r¼ 0.25 mm and 0.4 mm, featuring kink

bands. Note the peeled off fibrils from the surface.

Figure 10. Tenacity versus radius of curvature measured for

KM2-600 fibers after combined curvature/hygrothermal treat-

ment at 100�C for 30 days.

Figure 11. Tenacity versus radius of curvature measured for

Vectran HT1670 fibers after combined curvature/hygrothermal

treatment at 100�C for 30 days.

Abu Obaid et al. 9




between the chains. So, the combined curvature/hygro-thermal treatment significantly amplifies these effectswith smaller curvatures (and the resultant increasedstress), resulting in higher frequency of kinks alongthe fiber axis (i.e. higher kink band density).

A comparison between KM2 and Vectran fibersshows that Vectran fibers tend to have a lower kinkband density for a given radius of curvature. This canbe attributed to higher resistance to hygrothermal agingof Vectran compared to KM2, as discussed previously.

From Figures 10 and 11, the degradation in tenacitydue to curvature/hygrothermal combined treatment(DCH) was calculated and summarized in Table 5.DCH values are also compared in this Table with deg-radation in tenacity values due to curvature only (DC)calculated from Figure 6 and hygrothermal aging only(DH). Both Vectran HT and Kevlar KM2 show signifi-cantly higher DCH values than DC and DH values.These results confirm that curvature of fibers amplifiesthe impact of hygrothermal treatment on tenacity deg-radation of these fibers. This is likely because the kink-ing and subsequent fibrillation of the filaments provideswater with direct access to the crystalline core, acceler-ating the diffusion of water into the grain boundariesand interstitial spaces.

A comparison between KM2 and Vectran fibers(Table 5) reveals that for a given radius of curvature,the tenacity degradation level (DCH) due to curvature/hygrothermal combined treatment is significantlyhigher for KM2 compared to that of VectranHT1670/600. These results correlate well with thebehavior of the kink band density observed by fibers,especially at low radii of curvature (Figure 12).

At low radii of curvature, KM2 fibers exhibit a sig-nificant degradation in tenacity, which can be explainedby bending/hygrothermal effects. Prior to water treat-ment, the winding procedure induces kink bands whichcan be plastically deformed regions. For aromatic

fibers, the formation of these kink bands is often dueto weak interactions between the molecular chains,which leads to chain slippage, resulting in micro-buck-ling at low compressive strains.7,8 Leal19 studied thekink band formation for M5 fibers and found that mis-aligned fibrils can fail in shear, generating stress con-centrations at the boundaries of the micro-buckledregions, through which the kinks propagate across thefiber as the compressive load increases. Thus, themicro-buckled regions of KM2, where the kink bandsare formed due to weakened molecular interactions(hydrogen bonding) between the layers, can bedescribed as ‘‘defected structural regions’’ that can con-tribute to tenacity degradation prior and throughhygrothermal /curvature treatment. Thus, these micro-structural defects (kink band) due to bending duringhygrothermal/curvature combined treatment make thefiber structure more susceptible to water diffusion lead-ing to an accelerated degradation in tenacity.

At larger radii of curvature (>2mm), both types offibers exhibit lower levels of tenacity loss, which can bedue to less frequency of kink bands experienced by thefiber. It is also noticeable that the differences betweenDCH and DCþDH values for Vectran HT 1670/600are greater than that for KM2 fibers. A comparison ofkink band density reveals a similar trend. Kink banddensities at large radii (>2mm) are similar betweencurvature only and curvature/hygrothermal for KM2,but for Vectran HT 1670/600 system, kink band densitymore than doubles at 2mm compared to curvature onlyand curvature/hygrothermal conditioning. This may bedue to bending stresses (curvature induced) underhygrothermal conditioning which enhances micro-crack propagation due to defects and overall disturb-ance in the solid state structure of the fiber (such as

Figure 12. Kink band density as a function of radius of

curvature measured for Vectran HT and KM2 fibers after

curvature/hygrothermal treatment at 100�C for 30 days.

Table 5. Comparison of degradation in tenacity results for

KM2 and Vectran fibers. DCH is the degradation level due to

curvature/hygrothermal combined treatment, DC is the degrad-

ation level due to additive curvature and DH is the degradation

level due to hygrothermal conditioning at 100�C.

Radius of

curvature

(mm)

Vectran 1670/600

DH¼ 7% KM2-600 DH¼ 48%

DC DCH DC DCH

0.25 16% 60% 15% 96%

0.4 17% 54% 11% 89%

0.71 9% 53% – 93%

0.81 – 52% 13% 88%

1 8% 45% 8% 84%

2 5% 44% 0% 62%

3 1% 39% 2% 59%

5 4% 37% 3% 52%





chain slippage and the failure of entanglements totransmit the load along the filament).7,8

Conclusions

The effect of hygrothermal aging, curvature and com-bined curvature/hygrothermal aging on tenacity ofVectran HT and KM2 was evaluated. From hygrother-mal aging study, tenacity of Vectran HT fibers showedsmall reductions due to conditioning in water for30 days at 100�C, where the maximum loss in tenacitywas 12%, while the tenacity of KM2 fibers exhibited aclear decreasing trend with increasing conditioningtemperature, and a loss of �50% at 100�C.Reduction in tenacity for both types of fibers cannotbe explained by chemical degradation or oxidationmechanism, as seen from linear density and FTIRresults for untreated and conditioned fibers that donot differ. The relatively small reduction in tenacityobserved by conditioned Vectran fiber can be attributedto the disturbances in the structure of the fiber (such asthe chain slippage effects and the failure of entangle-ments to transmit the load along the filament). Vectranfibers exhibit overall significantly higher hydrolysisresistance compared to KM2 fibers. This can be attrib-uted to the differences in chemical backbone and crys-talline structure between both types of fibers. Aninteresting future study would be to assess reversibilityof this loss in tenacity for both systems from hygro-thermal exposure, through a heat-treatment step.

Curvature effects on tenacity were measured afterwrapping tows at different radii, where Vectran HT1670/600 fibers were evaluated and compared toKM2-600. A comparable impact of curvature on thetenacity of Vectran 1670/600 and KM2 was obtained,however, KM2 fibers exhibited higher kink band dens-ity for a given curvature level, compared to Vectranfibers. Both types of fibers exhibited decreasing trendsof tenacity with smaller radii of curvatures, particularlybelow 1mm. The reduction in tenacity observed byboth types of fibers can be attributed to the kinkbands propagation associated with peeling of fibrilsand the micro-cracking process induced by bendingstresses.

A study on the effects of combined curvature hygro-thermal combined treatments on tenacity of the fiberswas performed, and clearly demonstrated an amplifiedeffect on the tenacity degradation mechanisms in bothVectran HT and KM2. A significant increase in kinkband formation was also measured. For example,below 1mm radius of curvature, this increase was esti-mated to be a factor of �4.5 for Vectran HT 1670/600and �15 for KM2 fibers. At all radii of curvatureapplied, the tenacity loss was significantly higher forKM2 fibers compared to Vectran HT 1670/600.

Funding

This research received no specific grant from any funding

agency in the public, commercial, or not-for-profit sectors.

Conflict of interest

None declared.

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