materials

Article Mechanical Properties of Nonwoven Reinforced Thermoplastic Composites

Muhammad Tausif *, Achilles Pliakas, Tom O’Haire, Parikshit Goswami and Stephen J. Russell Technology Research Group, School of Design, University of Leeds, Leeds LS2 9JT, UK; [email protected] (A.P.); [email protected] (T.O.); [email protected] (P.G.); [email protected] (S.J.R.) * Correspondence: [email protected]; Tel.: +44-113-343-3799

Academic Editor: Thomas Fiedler Received: 4 May 2017; Accepted: 28 May 2017; Published: 5 June 2017

Abstract: Reinforcement of flexible fibre reinforced plastic (FRP) composites with standard textile fibres is a potential low cost solution to less critical loading applications. The mechanical behaviour of FRPs based on mechanically bonded nonwoven preforms composed of either low or high modulus fibres in a thermoplastic polyurethane (TPU) matrix were compared following compression moulding. Nonwoven preform fibre compositions were selected from lyocell, terephthalate (PET), polyamide (PA) as well as para-aramid fibres (polyphenylene terephthalamide; PPTA). Reinforcement with standard fibres manifold improved the tensile modulus and strength of the reinforced composites and the relationship between fibre, fabric and composite’s mechanical properties was studied. The linear density of fibres and the punch density, a key process variable used to consolidate the nonwoven preform, were varied to study the influence on resulting FRP mechanical properties. In summary, increasing the strength and degree of consolidation of nonwoven preforms did not translate to an increase in the strength of resulting fibre reinforced TPU-composites. The TPU composite strength was mainly dependent upon constituent fibre stress-strain behaviour and fibre segment orientation distribution.

Keywords: nonwovens; flexible; composites; fibre reinforced; mechanical properties; low cost

1. Introduction Composites combine two or more distinct materials with a discrete separating interface [1,2]. The two materials are typically heterogeneous but can consist of different physical forms of the same material [3]. In -matrix composites, polymer resin forms the matrix of the composite and also acts as a and a stress transfer medium for the reinforcement component, which typically consists of fibres or textile structures [1,2,4,5]. The matrix can consist of an elastomeric, thermoset or thermoplastic polymer depending on whether a flexible or structural composite is required [6,7]. Elastomeric matrix are capable of large deformations before failure compared to thermoset and thermoplastics making them particularly useful in the design of flexible composites. Flexible composite applications include hoses, tyres, coated fabrics, inflatables, conveyor belts, surgical devices, diaphragms and reinforced membranes [8,9]. Controlling the reaction of various polyols and isocyanates readily modifies the mechanical, chemical and thermal properties of [10–12]. In TPU, the hard segments are connected by polyether or -based soft segments that provide the characteristic flexibility in the polymer and the segments are phase separated, resulting in hard and soft segment domains [13]. Thermoplastic polyurethanes (TPU) are versatile materials which can be processed into moulded parts [4], films [14], fibres [15] and reinforced composites [14]. TPUs exhibit good strength, excellent resistance to abrasion, extremely high strain to fracture, durability and high toughness [11,16]. The chemistry of the polymer

Materials 2017, 10, 618; doi:10.3390/ma10060618 www.mdpi.com/journal/materials Materials 2017, 10, 618 2 of 13 as well as processing route allows it to attain a wide range of mechanical, thermal and performance properties. For example, the tensile strength/elongation at break of commercial grade Elastollan® C 88 A 10 and 1278 D 11 U (BASF Polyurethanes GmbH, Ludwigshafen, Germany) is 50 MPa/600% and 50 MPa/350%, respectively [17]. The toughness of TPU in film and fibre form was reported as 111 J g−1 [14] and 567 J g−1 [15], respectively. Two- and three-dimensional textile assemblies have been extensively used to reinforce polymer-matrix composites, in numerous applications [18]. However, the study of synthetic nonwoven reinforcements of different structural and compositional formats has been limited. Nonwoven fabrics are assembled from fibres or filaments by web formation and bonding methods. Mechanically bonded nonwovens produced by methods such as needlepunching and hydroentangling rely on fibre entanglement and frictional resistance to slippage for their strength [19]. Compared to preforms with planar fibre orientation, the out-of-plane deflections of fibre segments in mechanically bonded nonwovens can impart mechanical properties in three dimensions [20]. The absence of chemical binders or fibre fusion within the fabric structure provides for high porosity as well as a large accessible fibre surface area for matrix-fibre adhesion. Additionally, nonwoven fabric production involves fewer process steps than conventional or similar textile formation processes and the production is inherently faster in terms of linear output [19] and can offer relatively low cost reinforcement, compared to conventional textile preforms. Nonwoven fabrics have a high thickness to weight ratio, as dense fibre packing is difficult to achieve at weight fractions equivalent to woven, knitted and braided fabrics. High solid volume fractions are therefore challenging to produce in nonwoven reinforced composites, owing to lack of fibre alignment, but the ability to modulate fibre orientation offers tailoring of properties in different directions [21]. The modulus of polyurethane matrix composites is known to increase with higher fibre volume fractions. Epstein and Shishoo [22] used nonwovens to reinforce elastomeric matrix in structural reaction injection moulding (SRIM) and found a decrease in the resin flow as the volume fraction of the nonwoven reinforcement increased, and resin flow was also found to be affected by fibre orientation. Acar and Harper [23] employed hydroentangled nonwoven preforms, composed of viscose, polyester and blend of polyester/viscose/, in polyester resin. The mechanical properties of the nonwoven fabrics and composites were compared at different fibre volume fractions, and while improvements in tensile strength were limited but good energy absorbing properties of the reinforced composites were reported. Wang [21] compared nonwoven glass fibre reinforcements to that of woven reinforcements and concluded that a nonwoven can be an effective reinforcement for flat panels and moulded parts with single curvature. The correct selection of fibre chemical composition, fibre orientation, fibre dimensions and fibre surface properties is known to be critical in terms of influencing the reinforcement behaviour of an elastomeric composite [24,25]. The tensile strength and modulus of fibres depends on interatomic and intermolecular bonds. Consequently, reinforcing behaviour can be modulated by changing the type of fibre in the TPU matrix composite [26]. High modulus fibres are routinely selected for demanding composite applications, but elsewhere when the functional requirements are less demanding, standard textile fibres have potential to facilitate lower cost composite production, or weight-saving whilst still meeting target performance specifications. Polyethylene, polypropylene, polyester, polyamide and regenerated (viscose, lyocell) are the most commonly employed polymers for man-made fibre production, with polypropylene, polyester and viscose being particularly important for nonwoven applications [19]. Clearly, the selection of fibre type is also limited by the thermal properties of the matrix material and processing conditions in composite manufacturing. Owing to these limitations, polymers with relatively low melting points such as polyethylene, polypropylene were unsuitable for the present study. Hence fibres thermally ◦ ◦ stable at temperatures higher than 220 C were selected: polyamide 6,6 (Tm 240–265 C), polyethylene ◦ ◦ terephthalate (PET, Tm 245–265 C), Lyocell (starts to degrade above 300 C) and polyphenylene terephthalamide (PPTA, starts to degrade above 400 ◦C). PPTA (para-aramid) fibre is commonly employed for high performance applications and is marketed as ® (Dupont, Wilmington, DE, Materials 2017, 10, 618 3 of 13

USA) and Twaron (Teijin, Osaka, Japan). The selected fibres also offer a range of moduli with values for PPTA, lyocell, PET and PA6 being 58–95, 22–31 [27], 6–11, 3.0–6.5 GPa, respectively [28]. Whilst both consist of regenerated cellulose, the modulus of lyocell fibre is at least double that of viscose [27]. Following preliminary results [29], the purpose of the present work was to understand the degree to which nonwoven reinforcements containing standard textile fibres can deliver satisfactory mechanical performance. The mechanical properties of nonwoven preform reinforced flexible TPU composites were compared with those reinforced with polyphenylene terephthalamide (aramid). Additionally, the role of fibre dimensions and the degree of fibre entanglement in the needlepunched preform on prepared composites were investigated.

2. Materials and Methods Lyocell (1.7 dtex, 38 mm), polyphenylene terephthalamide (PPTA) (1.7 dtex, 58 mm), polyethlyene terephthalate (PET) (1.6 dtex, 38 mm, 6.7 dtex, 60 mm and 16.7 dtex hollow fibre, 64 mm respectively), and polyamide 6,6 (3.3 dtex, 50 mm) fibres were carded (nominal mass area density of 70 g m−2, parallel-laid) and pre-needled (42 punches cm−2) to form nonwoven preforms. Thermoplastic polyurethane (TPU, Elastollan® A C 88 A 12, Shore hardness 88 A, BASF Polyurethanes GmbH) was pressed in to flat sheets (≈315 g m−2) and employed as a matrix material. Elastollan® A C 88 A is a polyester based TPU and exhibits good tensile strength and excellent tear strength properties. The respective nonwoven webs were sandwiched between two layers of TPU sheet and compression moulded at 200 ◦C and 20 kg cm−2 (1.96 MPa) to achieve a nominal fibre content of 10 wt % in a composite of dimensions 15 × 20 cm2. The fibre volume fraction was calculated by [30]. The mean fibre volume fraction was 9% ± 1%. The estimated volume fractions for the lyocell, PPTA, PET and PA nonwoven reinforced TPU composites were 8.1%, 8.5%, 8.8% and 10.5% respectively.

1 = Vf  ρ   , (1) 1 + f 1 − 1 ρr w f where, ρf = density of fibre; ρr = density of resin; wf = fibre weight fraction. To determine fibre mechanical properties, single fibres were mounted on card frames, conditioned for 24 h, at 20 ◦C and 65% relative humidity, and tensile strength was evaluated on a universal testing machine (5540, Instron, Norwood, MA, USA) with pneumatic fibre grips and 5 N load cell, according to ASTM D3822-14 [31] (20 mm gauge length, Constant rate of extension, CRE: 12 mm min−1). The nonwoven webs (NWSP 110.4-15 [32], 100 mm gauge length and 50 mm wide, CRE: 100 mm min−1) and dog-bone shaped tensile bars from nonwoven-reinforced composites (ISO 527-4:1997 [33], 55 mm gauge length and 10 mm wide, CRE: 10 mm min−1) were tested to determine tensile strength on a universal testing machine (Z010, Zwick Roell, Kennesaw, GA, USA) with mechanical jaws. The reported stress is actually engineering stress based on in the initial cross-sectional area of the composite. For scanning electron microscope (SEM) characterisation, the prepared composites were frozen to −20 ◦C, cross-sectioned, and cut with a fresh razor blade. The samples were sputter coated with gold and imaged on a tungsten source scanning electron microscope S-2600N, Hitachi, Tokyo, Japan). To study the effect of needlepunching density on the mechanical properties of the preforms and resulting composites, PET staple fibres (1.6 dtex, 38 mm) were parallel-laid to produce a nominal mass area density of 70 g m−2 webs. The prepared webs were bonded with three different needlepunching densities (42, 84 and 168 punches cm−2). The tensile strength of the preforms and composites were carried out according to aforementioned methods.

3. Results and Discussion In line with aims of the study, this section is sub-divided to consider the effect of fibre type, fibre linear density and needlepunching density on the mechanical properties of the nonwoven reinforced elastomeric composites. Materials 2017, 10, 618 4 of 13

Materials 2017, 10, 618 4 of 13 3.1. Effect of Fibre Type and Linear Density Fibre tensile properties, length and fibre-matrix adhesion are key parameters affecting the final mechanicalFibre tensile behaviour properties, of the lengthreinforced and composites. fibre-matrix The adhesion critical are fibre key length parameters for matrix affecting-to-fibre the finalload mechanicaltransfer depends behaviour on the of fibre the reinforceddiameter, composites.tensile strength The and critical fibre fibre to matrix length bond for matrix-to-fibre strength. The fibre load transferlengths dependsselected in on the the fibrecurrent diameter, study were tensile substantially strength and higher fibre to than matrix the bond critical strength. fibre length, The fibre as lengthsreflected selected by the inresulting the current mechanical study were properties substantially [28,34 higher]. The fibre than- thematrix critical adhesion fibre length, can be as controlled reflected by thealtering resulting the surface mechanical energy properties of the fibr [28es,34 and/or]. The matrix fibre-matrix resin. adhesionThe effect can of fibre be controlled (including by PET altering and thePPTA) surface surface energy treatments of the fibres on and/ormechanical matrix properties resin. The ofeffect nonwoven of fibre (including reinforced PET polyurethane and PPTA) surfacecomposit treatmentses has been on previously mechanical reported properties [35] of. The nonwoven mean single reinforced fibre polyurethanestrength values composites for each fibre has beentype are previously given in reportedFigure 1 [and35]. representative The mean single tenacity fibre strength-strain curves values are for shown each fibrein Figure type 2 are. The given fibres in Figureshow a1 andrange representative of tensile strength tenacity-strain and strain curves % values are shown, with inPPTA Figure exhibiting2. The fibres premium show mechanical a range of tensileproperties strength compared and strain to the % rest values, of the with fibres PPTA. The exhibiting reported premium moduli mechanicalof PPTA, Lyocell, properties PET compared and PA6 toare the 58 rest–95, of22 the–31 fibres.[27], 6– The11, 3.0 reported–6.5 GPa, moduli respectively of PPTA, [28] Lyocell,, which PET is andconsistent PA6 are with 58–95, the 22–31[ tensile27 data], 6–11, in 3.0–6.5Figure 2 GPa,. respectively [28], which is consistent with the tensile data in Figure2.

Figure 1.1. Tensile strength of single fibres.fibres.

Figure 2.2. Representative tenacity tenacity-strain-strain curves of single fibrefibre tensile testing in FigureFigure1 1. .

The engineering stress and modulus of the pre-needled nonwoven preform, both in the The engineering stress and modulus modulus of the the pre-needled pre-needled nonwoven nonwoven preform, both in the machine- (MD) and cross-direction (CD), is shown in Figure 3,4, respectively. The apparent machine-machine- (MD)(MD) andand cross-direction cross-direction (CD), (CD) is, shown is shown in Figures in Figure3 and 3,44,, respectively.respectively. TheThe apparent apparent anisotropy in the preforms, reflected by the MD and CD values is associated with directional anisotropy in in the the preforms, preforms, reflected reflected by the by MD the and MD CD and values CD values is associated is associated with directional with directional variation variation in the fibre segment orientation, which largely depends upon the method of web formation invariation the fibre in segmentthe fibre segment orientation, orientation, which largely which largely depends depend upons theupon method the method of web of formationweb formation [36]. [36]. In the present study, the nonwoven preforms were based on parallel-laid webs with In[36] the. In present the present study, the study, nonwoven the nonwoven preforms were preforms based on were parallel-laid based on webs parallel with- preferentiallaid webs fibrewith preferential fibre orientation in the MD. Comparing the tensile strength data in Figures 1 and 3, it is orientationpreferential infibre the orientation MD. Comparing in the theMD. tensile Comparing strength the data tensile in Figuresstrength1 dataand3 in, it Figures is evident 1 and that 3, theit is evident that the trend in fibre tensile strength does not correspond to the nonwoven preform tensile strength. At a fixed needle type and needling density, in this instance of 42 punches cm−2, development of fibre entanglement and the associated increase in network strength depends on the

Materials 2017, 10, 618 5 of 13 trend in fibre tensile strength does not correspond to the nonwoven preform tensile strength. At a MaterialsMaterials 2017 2017, ,10 10, ,618 618 55 of of 13 13 fixed needle type and needling density, in this instance of 42 punches cm−2, development of fibre modulus,entanglementmodulus, interinter- and-fibrfibre thee frictionfriction associated andand bendingbending increase rigidityrigidity in network of of thethe strength fibres.fibres. TheThe depends oscillationoscillation on the of of modulus, the the barbedbarbed inter-fibre needlesneedles duringfrictionduring andneedlepunching needlepunching bending rigidity induces induces of the fibres.fibre fibre entanglement Theentanglement oscillation andof and the increased increased barbed needles structural structural during integrity integrity needlepunching in in the the preform.inducespreform. fibre The The entanglement two two contributing contributing and increased effects effects relate relate structural to to the the integrity capstan capstan in effect theeffect preform. and and the the The fibre fibre two--fibrefibre contributing contact contact pressures.effectspressures. relate The The to capstanthe capstan capstan effect effect effect at at and fibre fibre the crossovers crossovers fibre-fibre and and contact the the pressures. contact contact pressure pressure The capstan are are the theeffect result result at fibre of of intercrossoversinter--loopinglooping and of of the fibre fibre contact segments. segments. pressure This This are restricts restricts the result the the of displacement displacementinter-looping of of fibre fibres, fibres, segments. increasing increasing This resistance resistance restricts the to to slippagedisplacementslippage [37][37]. . Deflection ofDeflection fibres, increasing ofof fibre fibre segmentssegments resistance occursoccurs to slippage asas needleneedle [37 barbs].barbs Deflection carrycarry thethe of fibre fibrefibre segments segmentssegments and occursand theirtheir as abilityneedleability to barbs to deflect deflect carry will will the dependfibre depend segments upon upon the and the fibre theirfibre modulus.ability modulus. to deflect For For a a will fibre fibre depend of of circular circular upon cross thecross fibre section, section, modulus. the the bendingForbending a fibre stiffnessstiffness of circular isis linearlylinearly cross proportionalproportional section, the bendingtoto thethe Young’sYoung’s stiffness modulusmodulus is linearly ofof thethe proportional fibre,fibre, isis affectedaffected to the by Young’sby fibrefibre lengthmoduluslength and andof increases increases the fibre, as as isaa function function affected of of by the the fibre four four lengththth power power and of of increases fibre fibre diameter diameter as afunction [38,39 [38,39]]. .Therefore, Therefore, of the fourth the the l powerlyocellyocell fibresoffibres fibre produced produced diameter the the[38 , strongest39 strongest]. Therefore, nonwoven nonwoven the lyocell preforms preforms fibres (Figure (Figure produced 2) 2) despitethe despite strongest the the fact fact nonwoven that that PPTA PPTA preforms fibre fibre exhibited(Figureexhibited2) despite the the highest highest the fact strengthstrength that PPTA and and fibre modulus modulus exhibited (Figure(Figure the highestss 11 and and strength 3 3).). Furthermore,Furthermore, and modulus pre pre (Figures--needlneedl1ed edand PETPET3). preformsFurthermore,preforms containing containing pre-needled a a range range PET ofpreforms of fibre fibre linear linearcontaining densities densities a range exhibitedexhibited of fibre linear in inferiorferior densities mechanical mechanical exhibited properties properties inferior, , especiallymechanicalespecially coarse coarse properties, (16.7 (16.7 dtex dtex especially)) PET PET fibres coarsefibres did did (16.7 not not dtex) markedly markedly PET fibres influence influence did not results. results. markedly The The 16.7 influence16.7 dtex dtex fibres results.fibres w w Theereere the16.7the coarsestcoarsest dtex fibres amongamong were allall the fibresfibres coarsest suchsuchamong thatthat fewerfewer all fibres fibrefibre such segmentssegments that fewer couldcould fibre bebe carriedcarried segments byby the couldthe needleneedle be carried barbbarbss tobyto effecttheeffect needle entanglement entanglement barbs to effectof of the the entanglementweb. web. The The tensile tensile of themodulus modulus web. The (Figure (Figure tensile 4) 4) of modulusof the the nonwoven nonwoven (Figure 4 preformspreforms) of the nonwoven exhibited exhibited highpreformshigh variationvariation exhibited andand one highone--wayway variation ANOVAANOVA and one-wayandand postpost- ANOVA-hochoc analysisanalysis and post-hoc revealedrevealed analysis thatthat onlyonly revealed thethe PPTAPPTA that preformpreform only the werePPTAwere significantly significantly preform were ( (pp- significantly-valuevalue 0.000) 0.000) different, (different,p-value 0.000) in in the the different, MD MD, ,from from in the the the rest rest MD, of of from the the samples. samples. the rest of the samples.

FigureFigure 3 3.3. . TensileTensileTensile strength strength strength of of of pre prepre-needled--needledneedled nonwoven nonwoven preforms preforms preforms in in in machine machinemachine--- andand cross crosscross-direction.--direction.direction. (Typical(Typical(Typical stress stress-strainstress--strainstrain curvecurve forfor eacheach sample sample is is given given in in Figure Figure S1SS11 ofof SupplementarySupplementary data).data)data). .

Figure 4. Tensile modulus of pre-needled nonwoven-reinforcements in machine- and cross-direction. FFigureigure 4 4.. TensileTensile mo modulusdulus of of pre pre-needled-needled nonwoven nonwoven-reinforcements-reinforcements in in machine machine-- and and cross cross-direction.-direction.

TThehe maximum maximum stress stress of of the the nonwoven nonwoven--reinforcedreinforced TPUTPU composites composites in in the the MD MD andand CDCD is is shown shown inin Figure Figure 5 5. .A A critical critical volume volume fraction fraction is is required required before before fibres fibres contribute contribute to to composite composite s strengthtrength [40] [40], , andand inin thethe presentpresent study,study, withwith aa lowlow solidsolid volumevolume fractionfraction ofof 99%% ±± 1%,1%, inclusioninclusion ofof aa nonwovennonwoven preformpreform was was found found to to improve improve the the tensile tensile strength strength and and modulus modulus of of the the composites composites compared compared to to that that

Materials 2017, 10, 618 6 of 13

The maximum stress of the nonwoven-reinforced TPU composites in the MD and CD is shown in Figure5. A critical volume fraction is required before fibres contribute to composite strength [ 40], and in the present study, with a low solid volume fraction of 9% ± 1%, inclusion of a nonwoven preform was found to improve the tensile strength and modulus of the composites compared to that of the unreinforcedMaterialsMaterials 2017 2017, ,10 10 TPU, ,618 618 sheet. Previously, the composite strength has been linked to nonwoven66 of of preform 13 13 strength [23]. In the current study, the composite strength was strongly related to the mechanical of the unreinforced TPU sheet. Previously, the composite strength has been linked to nonwoven propertiesof the of unreinforced the reinforcing TPU fibre,sheet. consistentPreviously, with the composite the findings strength of Epstein has been and linked Shishoo to nonwoven who reported preform strength [23]. In the current study, the composite strength was strongly related to the that thepreform modulus strength of a[23] composite. In the current is influenced study, the by composite the single strength fibre modulus was strongly [35 ]. related This to is the further mechanicalmechanical propertiesproperties ofof thethe reinforcingreinforcing fibre,fibre, consistentconsistent withwith thethe findingsfindings ofof EpsteinEpstein andand ShishooShishoo evidenced by the stress-strain responses in Figure6. The tensile strength and modulus of the TPU whowho reportedreported thatthat thethe modulusmodulus ofof aa compositecomposite isis influencedinfluenced byby thethe singlesingle fibrefibre modulusmodulus [35][35].. ThisThis isis sheet substantially increases as a result of fibre reinforcement, according the intrinsic fibre properties. furtherfurther evidencedevidenced byby thethe stressstress--strainstrain responsesresponses inin FigureFigure 66.. TheThe tensiletensile strengthstrength andand modulusmodulus ofof thethe TwoTPU importantTPU sheet sheet substantiallyconsiderations substantially increases increases are the as as fibre a a result result mechanical of of fibre fibre properties reinforcement, reinforcement, as well according according as the matrix-fibrethethe intrinsic intrinsic fibre adhesion.fibre Amongpropertiesproperties all samples,.. Two Two the important important PPTA and considerations considerations lyocell fibre are are nonwoven the the f fibreibre reinforced mechanical mechanical samples properties properties exhibited as as well well a double as as the the peak in thematrixmatrix stress-strain--fibrefibre adhesion.adhesion. curve, AmongAmong where allall the samples,samples, first peak thethe PPTAPPTA may beandand associated lyocelllyocell fibrefibre with nonwovennonwoven de-bonding reinforcedreinforced of the samplessamples fibre and matrixexhibitedexhibited phase a[ a41 double double]. The peak peak de-bonding in in the the stress stress is likely--strainstrain to curve, curve, be associated where where the the with first first peak the peak low may may strain be be associated associated % at break with with of the PPTAdede and--bondingbonding lyocell ofof thefibres.the fibfibrere Previously, andand matrixmatrix phasephase it has [41][41] been.. TheThe reported dede--bondingbonding that isis a likelylikely typical toto be load-elongationbe associatedassociated withwith thecurvethe lowlow of the strain % at break of the PPTA and lyocell fibres. Previously, it has been reported that a typical hydroentangledstrain % at break (PET) of nonwoven the PPTA reinforcedand lyocell composite fibres. Previously, exhibits it many has been peaks reported and troughs, that a typical linked to load-elongation curve of the hydroentangled (PET) nonwoven reinforced composite exhibits many localload failure-elongation of matrix curve and of fibres, the hydroentangled at a comparable (PET) fibre nonwoven volume reinf fractionorced of composite 11% [23 ].exhibits However, many in the peakspeaks andand troughs,troughs, linkedlinked toto locallocal failurefailure ofof matrixmatrix andand fibres,fibres, atat aa comparablecomparable fibrefibre volumevolume fractionfraction current study, the stress-strain curves were smooth. ofof 11%11% [23][23].. However,However, inin thethe currentcurrent studystudy,, thethe stressstress--strainstrain curvescurves werewere smoothsmooth..

FigureFigureFigure 5. Tensile 5 5. .Tensile Tensile strength s strengthtrength of of of nonwoven-reinforced nonwoven nonwoven--reinforcedreinforced thermoplastic thermoplastic polyurethane polyurethane polyurethane ( (TPUTPU (TPU))) composites. composites. composites.

50 50 PPTA (1.7dtex/58mm) PPTA (1.7dtex/58mm) Lyocell (1.7dtex/38mm) Lyocell (1.7dtex/38mm) PET (1.6dtex/38mm) PET (1.6dtex/38mm) 40 PET (16.7dtex/64mm) 40 PET (16.7dtex/64mm) PET (6.7/60mm) PET (6.7/60mm) (3.3dtex/50mm) Nylon (3.3dtex/50mm) No Fibre - TPU Sheet No Fibre - TPU Sheet 30 30

20 20

Engineering Stress (KPa)

Engineering Stress (KPa)

10 10

Strain at break = 330% Strain at break = 330% 0 0 0 20 40 60 80 100 0 20 40 60 80 100 Strain (%) Strain (%)

FigureFigureFigure 6. Representative 66. . RepresentativeRepresentative stress-strain stress stress--strainstrain curves curve curve ofss nonwoven-reinforced ofof nonwovennonwoven--reinforcedreinforced thermoplastic thermoplastic thermoplastic polyurethane polyurethane polyurethane(TPU) composites(TPU)(TPU) composites composites reported in reportedreported Figure 5 in.in (The FigureFigure graph 5 5. . ((The isThe limited graph graph to is is 100% limited limited strain to to 100% 100% for clarity. strain strain Thefor for clarity. non-reinforcedclarity. The The non-reinforced TPU sample exhibits maximum stress of 5.7 KPa and strain at break of 330%). TPU samplenon-reinforced exhibits TPU maximum sample exhibits stress ofmaximum 5.7 KPa stress and strainof 5.7 KPa at break and strain of 330%). at break of 330%).

TheThe highhigh viscosityviscosity ofof elastomericelastomeric matrixmatrix cancan hinderhinder resinresin infiltrationinfiltration particularlyparticularly asas thethe fibrefibre volumevolume fraction fraction increasesincreases [22,42[22,42]].. Furthermore, Furthermore, void void formation formation during during manufacture manufacture is is a a known known problemproblem affectingaffecting thethe mechanicalmechanical propertiesproperties ofof compositescomposites [34][34].. FreedomFreedom fromfrom voidvoid formationformation waswas

Materials 2017, 10, 618 7 of 13

The high viscosity of elastomeric matrix can hinder resin infiltration particularly as the fibre volume fraction increases [22,42]. Furthermore, void formation during manufacture is a known Materials 2017, 10, 618 7 of 13 problem affecting the mechanical properties of composites [34]. Freedom from void formation was confirmedconfirmed by assessment of SEM images, Figure 77.. TheThe highhigh numbernumber ofof circularcircular fibrefibre cross-sectionscross-sections in the SEM images confirms the preferential fibre orientations in the MD. The magnitude of in theMaterials SEM images 2017, 10, 618 confirms the preferential fibre orientations in the MD. The magnitude of7 ofuniaxial 13 nonwovenuniaxial nonwoven preform preform strength strength depends depends on the on fibre the segment fibre segment orientation orientation distribution distribution and fibre and entanglement.fibreconfirmed entanglement. by For assessment nonwoven For nonwoven of SEM reinforced images, reinforced composites, Figure composites, 7. The fibre high orientationnumber fibre orientation of circular plays fibre the plays majorcross the-sections role major as role the fibresas thein are fibresthe bonded SEM are imagesbonded in the confirms matrix in the thephase. matrix preferential This phase. is reflectedfibre This orientations is by reflected the lackin the byof MD. correlati the The lack magnitudeon of between correlation of uniaxialthe betweenpreform nonwoven preform strength depends on the fibre segment orientation distribution and fibre (theFigure preform 3) and (Figure composite’s3) and composite’s maximum engineering maximum engineering stress (Figure stress 6). (FigureThe directional6). The directional geometrical entanglement. For nonwoven reinforced composites, fibre orientation plays the major role as the arrangementgeometrical arrangement of fibres can of fibres therefore can therefore be tailored be tailored to modulate to modulate composite composite properties, properties, by by process-structuringfibres are bonded in thethe nonwovenmatrix phase. preform This is reflected during by its the manufacture. lack of correlation between the preform process(Figure-structuring 3) and composite’sthe nonwoven maximum preform engineering during its stress manufacture. (Figure 6 ). The directional geometrical arrangement of fibres can therefore be tailored to modulate composite properties, by process-structuring the nonwoven preform during its manufacture.

(a) (b) (a) (b) Figure 7.7. ScanningScanning electron electron microscope microscope ( (SEM)SEM) images of nonwoven-reinforcednonwoven-reinforced TPU composite Figure 7. Scanning electron microscope (SEM) images of nonwoven-reinforced TPU composite crosscross-sections-sections in the machine direction: Polyethylene Polyethylene terephthalate terephthalate fibres fibres (6.7 (6.7 dtex dtex)) ( a) and lyocell cross-sections in the machine direction: Polyethylene terephthalate fibres (6.7 dtex) (a) and lyocell fibres (b). fibresfibres (b). (b). The inclusion of the nonwoven reinforcement, at a mean solid volume fraction of 9% ± 1%, in a The inclusionThe inclusion of the of the nonwoven nonwoven reinforcement, reinforcement, at a mean solid solid volume volume fraction fraction of 9 of% 9%± 1%,± in1%, a in a TPU matrix results in an expected increase in Young’s Modulus of the composites (Figure 8). With TPU matrixTPU matrix results results in anin an expected expected increase increase inin Young’sYoung’s Modulus of of the the composites composites (Figure (Figure 8). With8). With referencereference to the to the values values obtained obtained for for PPTA, PPTA, the the crucial influence influence of of fibre fibre modulus modulus on the on the reference to the values obtained for PPTA, the crucial influence of fibre modulus on the corresponding correspondingcorresponding composite composite values values are are highlighted highlighted in FigureFigure 8.8 . Despite Despite the the fibre fibre modulus modulus being being composite values are highlighted in Figure8. Despite the fibre modulus being substantially lower than substantiallysubstantially lower lower than than PPTA, PPTA, reinforcement reinforcement withwith standard textile textile fibres fibres at ata solid a solid volume volume fraction fraction PPTA, reinforcement with standard textile fibres at a solid volume fraction of only 9% ± 1% is capable of onlyof only9% ± 9 1%% ± is1% capable is capable of ofmore more than than doubling doubling the modulusmodulus of of the the composite, composite, depend depending oning the on the of more than doubling the modulus of the composite, depending on the selected fibre type, (Figure8). selectedselected fibre fibre type type, (Figure, (Figure 8). 8).

Figure 8. Tensile modulus of nonwoven-reinforced TPU composites. Figure 8. Tensile modulus of nonwoven-reinforced TPU composites. The influenceFigure of 8 fibre. Tensile diameter modulus on theof nonwoven nonwoven-reinforced preform wasTPU exploredcomposites. by examining the properties of TPU composites produced from three different PET linear densities. The MD tensile Thestrength influence shown ofin Figure fibre diameter9 decreases o withn the increasing nonwoven fibre preform linear density, was explored whereas there by examiningis no clear the propertiestrend inof the TPU CD. composites Clearly, the producedhigher specific from surface three area different resulting PET from linear an increase densities. in fibre The fineness, MD tensile strength shown in Figure 9 decreases with increasing fibre linear density, whereas there is no clear trend in the CD. Clearly, the higher specific surface area resulting from an increase in fibre fineness,

Materials 2017, 10, 618 8 of 13

The influence of fibre diameter on the nonwoven preform was explored by examining the properties of TPU composites produced from three different PET linear densities. The MD tensile strengthMaterials 2017 shown, 10, 618 in Figure9 decreases with increasing fibre linear density, whereas there is no clear trend8 of 13 inMaterials the CD. 2017 Clearly,, 10, 618 the higher specific surface area resulting from an increase in fibre fineness, is likely8 of 13 tois increase likely to theincrease total interface the total surface interface for surface matrix to for fibre matrix adhesion to fibre aiding adhesion stress aidingtransfer. stress Similarly, transfer. the is likely to increase the total interface surface for matrix to fibre adhesion aiding stress transfer. stiffnessSimilarly, of the the stiffn TPU compositeess of the TPU decreases composite as fibre decreases fineness increasesas fibre fineness in the nonwoven increases preform, in the nonwoven Figure 10. Similarly, the stiffness of the TPU composite decreases as fibre fineness increases in the nonwoven Furthermore,preform, Figure the 10 strength. Furthermore, of the composite the strength samples of the containing composite fibres samples of different containing linear fibres density of different can be preform, Figure 10. Furthermore, the strength of the composite samples containing fibres of different relatedlinear density to differences can be related in single to fibre differences mechanical in single properties, fibre mechanical Figure1. properties, Figure 1. linear density can be related to differences in single fibre mechanical properties, Figure 1.

Figure 9. Tensile strength of three polyethlyene terephthalate (PET) nonwoven-reinforced TPU Figure 9.9. TensileTensile s strengthtrength of three polyethlyene polyethlyene terephthalate (PET)(PET) nonwoven-reinforcednonwoven-reinforced TPU composites (extracted from Figure 5). composites (extracted from Figure5 5).).

Figure 10. Tensile modulus of the PET nonwoven-reinforced TPU composites (extracted from Figure 8). Figure 10 10.. TensileTensile modulus of the PET nonwoven nonwoven-reinforced-reinforced TPU composites (extracted from FigureFigure8 8).). 3.2. Effect of Needlepunching Density 3.2. Effect of Needlepunching Density 3.2. Effect of Needlepunching Density Local fibre arrangement and fibre entanglement in the preform (PET-1.6 dtex, 38 mm) is Local fibre arrangement and fibre entanglement in the preform (PET-1.6 dtex, 38 mm) is adjustedLocal by fibre increasing arrangement the punch and fibre density entanglement during its in manufacture, the preform (PET-1.6 and the dtex,impact 38 mm)on its is resulting adjusted adjusted by increasing the punch density during its manufacture, and the impact on its resulting bystrength increasing is revealed the punch in density Figure during 11. Needlepunching its manufacture, ofand staple the impact fibre on webs its resulting initially strength increase iss strength is revealed in Figure 11. Needlepunching of staple fibre webs initially increases revealedconsolidation in Figure and the11. degree Needlepunching of fibre entanglement, of staple fibre increasing webs initially tensile increases strength consolidation and modulus, and until the a consolidation and the degree of fibre entanglement, increasing tensile strength and modulus, until a degreethreshold of fibre is reached entanglement, beyond whichincreasing out- tensileof-plane strength fibre segments and modulus, reduce until planar a threshold strength is and reached fibre threshold is reached beyond which out-of-plane fibre segments reduce planar strength and fibre beyondmobility which. Excessive out-of-plane needling fibrecan also segments cause reducefibre breakage planar strengthand holes and on fibrethe preform mobility. surface Excessive [43]. mobility. Excessive needling can also cause fibre breakage and holes on the preform surface [43]. needlingGiven the can relatively also cause low fibre needling breakage density, and holessuch a on threshold the preform was surface not reached [43]. Givenin the thecurrent relatively study, low as Given the relatively low needling density, such a threshold was not reached in the current study, as is evident by the systematic increase in tensile strength with increasing punch density from 42 to 168 is evident by the systematic increase in tensile strength with increasing punch density from 42 to 168 punches cm−2. Note that this was not accompanied by a large increase in preform density (Figure 12). punches cm−2. Note that this was not accompanied by a large increase in preform density (Figure 12). Fibres in parallel-laid nonwovens are preferentially oriented in the MD and the observed decrease in Fibres in parallel-laid nonwovens are preferentially oriented in the MD and the observed decrease in anisotropy, reflected by the MD:CD strength ratio in Figure 13, can be associated with local in-plane anisotropy, reflected by the MD:CD strength ratio in Figure 13, can be associated with local in-plane re-orientation and out-of-plane fibre reorientation of fibre segments as the punch density increases. re-orientation and out-of-plane fibre reorientation of fibre segments as the punch density increases. The strain at break %, Figure 14, increased with increasing punch density. The higher level of The strain at break %, Figure 14, increased with increasing punch density. The higher level of

Materials 2017, 10, 618 9 of 13 needling density, such a threshold was not reached in the current study, as is evident by the systematic increase in tensile strength with increasing punch density from 42 to 168 punches cm−2. Note that this was not accompanied by a large increase in preform density (Figure 12). Fibres in parallel-laid nonwovens are preferentially oriented in the MD and the observed decrease in anisotropy, reflected by the MD:CD strength ratio in Figure 13, can be associated with local in-plane re-orientation and Materials 2017, 10, 618 9 of 13 out-of-planeMaterials 2017, 10 fibre, 618 reorientation of fibre segments as the punch density increases. The strain at break9 of %,13 Figure 14, increased with increasing punch density. The higher level of entanglement may increase the entanglemententanglement may may increa increasese the the normal normal force force on on the the existing existing fibre fibre to to fibre fibre cross cross--overover points points facilitatnormaling force higher on the elongation existing fibre of theto fibre fibre cross-over segments points between facilitating bond points higher, and elongation increas ofed the tensile fibre segmentsfacilitating between higher bond elongation points, of and the increased fibre segments tensile strength. between bond points, and increased tensile strengthstrength..

Figure 11. Effect of punch density on tensile strength of PET (1.6 dtex, 38 mm) nonwoven preforms. Figure 11.11. Effect of punch density on tensile strength of PET (1.6 dtex dtex,, 38 mm) nonwoven preforms preforms.. (Typical stress-strain curve for each sample is given in Figure S2 of Supplementary data). (Typical(Typical stress-strainstress-strain curvecurve forfor eacheach samplesample isis givengiven inin FigureFigure S2S2 ofof SupplementarySupplementary data).data).

Figure 12. Effect of punch density on density of PET (1.6 dtex, 38 mm) nonwoven preforms (fabrics). Figure 12. Effect of punch density on density of PET (1.6 dtex, 38 mm) nonwoven preforms (fabrics). Figure 12. Effect of punch density on density of PET (1.6 dtex, 38 mm) nonwoven preforms (fabrics). While increasing the punch density increases the tensile strength of the preform (Figure 11), there WhileWhile increasingincreasing thethe punchpunch densitydensity increasesincreases thethe tensiletensile strengthstrength ofof thethe preformpreform (Figure(Figure 11),11), is a small but significant decrease in the tensile strength of the TPU composite, Figure 15. One-way therethere is is aa small small but but significant significant decrease decrease in in thethe tensiletensile strengthstrength of of the the TPU TPU composite, composite, FigureFigure 1515.. ANOVA analysis shows a significant (p-value 0.006) effect of different levels of punch density on tensile OneOne--wayway ANOVA ANOVA analysis analysis show showss aa significant significant ( (pp--valuevalue 0.006) 0.006) effect effect of of different different levels levels of of punch punch strength in the MD and no significant effect in the CD (p-value 0.0604). Further post-hoc analysis in the densitydensity onon tensiletensile strengthstrength inin thethe MDMD andand nono significantsignificant effecteffect inin thethe CDCD ((pp--valuevalue 0.0604).0.0604). FurtherFurther −2 −2 postMD- revealshoc analysis that only in 168the punchesMD reveal cms thatis significantly only 168 punches different cmfrom−2 samplesis significantly at 0 (p-value different 0.021) from and post-hoc analysis in the MD − reveals that only 168 punches cm is significantly different from samples42 (p-value at 0 0.006) (p-value punches 0.021) cmand 242. The (p-value small 0.006) decrease punches in tensile cm−2 stress. The small at the decrease highest punchin tensile densities stress samples at 0 (p-value 0.021) and 42 (p-value 0.006) punches cm−2. The small decrease in tensile stress may be attributed to out-of-plane fibre segment deflections and a slight increase in the preform (fabric) atat the the highest highest punch punch dens densitiesities may may be be attributed attributed to to out out--ofof--planeplane fibre fibre segment segment deflections deflections and and a a slightslight increaseincrease inin thethe preformpreform (fabric)(fabric) density,density, whichwhich maymay bebe expectedexpected toto increaseincrease thethe resistanceresistance toto TPUTPU flowflow infiltrationinfiltration duringduring compositecomposite manufacturemanufacture duedue toto aa decreasedecrease inin thethe intrinsintrinsicic permeability.permeability.

Materials 2017, 10, 618 10 of 13

density,Materials which 20172017,,may 1010,, 618618 be expected to increase the resistance to TPU flow infiltration during1010 composite ofof 1313 manufactureMaterials 2017 due, 10, to618 a decrease in the intrinsic permeability. 10 of 13

FigureFigure 1313.. Effect of punch densityy on on ((MD:CD)) tensiletensile strength strength ratio ratio of of PET PET (1.6 (1.6 dtex,, 38 38 mm) FigureFigurenonwoven 13. 13Effect. Effectpreforms of of punch .. punch density density on on (MD:CD)(MD:CD) tensile tensile strength strength ratio ratio of PET ofPET (1.6 (1.6dtex,dtex, 38 mm) 38 mm) nonwovennonwoven preforms. preforms.

FigureFigureFigure 14. Effect1414.. Effect of punchof punch density density on on strain strain (%) of of PET PET (1.6 (1.6 dtex dtex,,, 3838 38 mm) mm) nonwoven nonwoven preform preform... Figure 14. Effect of punch density on strain (%) of PET (1.6 dtex, 38 mm) nonwoven preform.

Figure 15. Effect of punch density on tensile strength of PET nonwoven (1.6 dtex/38 mm) reinforced flexible composites. (Typical stress-strain curve for each sample is given in Figure S3 of Supplementary data).

Materials 2017, 10, 618 11 of 13

4. Conclusions The mechanical properties of single fibres, nonwovens and reinforced composites were evaluated and the key findings can be summarised as follows:

• Conventional textile fibres such as PET and lyocell have the potential to increase, six and nine times respectively, the tensile modulus of compression moulded TPU composites when introduced as nonwoven preforms with solid volume fractions of <10%. In addition, reinforcement of TPU moulded composites with PET- and lyocell nonwovens led to a minimum 2.5 fold increase in tensile strength compared to unreinforced TPU. This is substantially lower than can be achieved with comparable nonwoven PPTA fibre preforms, but there may be cost benefits in selecting lower cost reinforcements for less demanding applications. • High strength fibres do not essentially produce high strength nonwoven preforms, with all other parameters constant. The tensile properties of nonwoven-reinforced composites are mainly dependent upon fibre tensile properties and fibre segment orientation distribution. Though the tensile strength of the nonwoven preform does not markedly contribute to the tensile strength of the composite, the structural and dimensional modifications associated with increased levels of bonding such as greater fibre entanglement, fibre re-orientation, fabric density and fibre volume fractions can influence resulting mechanical properties. • Cross-sectional analysis of nonwoven-reinforced composite parts, revealed void-free embedding of conventional fibres in the TPU matrix. • Modulating the degree of fibre entanglement and mechanical bonding in the nonwoven preform by adjusting the level of needling resulted in a small but significant decrease in tensile strength of the TPU composite. Based on a fixed fibre type and solid volume fraction, increasing fibre linear density reduced the strength of the nonwoven-reinforced TPU composite.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/10/6/618/s1, Figure S1: Typical stress-strain curves for tensile strength of pre-needled nonwoven preforms in machine- and cross-direction (reference to Figure3 of the main document), Figure S2: Typical stress-strain curves for effect of punch density on tensile strength of PET (1.6dtex, 38mm) nonwoven preforms (reference to Figure 11 of the main document), Figure S3: Typical stress-strain curves for effect of punch density on tensile strength of PET nonwoven (1.6dtex/38mm) reinforced flexible composites (reference to Figure 15 of the main document). Typical stress-strain curves for Figures3, 11 and 15 are provided. Acknowledgments: Sport Infinity has received funding from the European Union’s Horizon 2020 research and innovation programme 2014–2018 under grant agreement No 645987. It falls within ‘Materials solutions for use in the creative industry sector’. Author Contributions: Muhammad Tausif, Parikshit Goswami and Stephen Russell conceived and designed the study. The preparation of the manuscript and analysis of results was carried out by Muhammad Tausif, Parikshit Goswami and Stephen Russell. The experiments were performed by Muhammad Tausif with support from Achilles Pliakas and Tom O’Haire. All authors gave approval for the final version to be submitted. Conflicts of Interest: The authors declare no conflict of interest.

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