Characterisation of the Anisotropic Thermoelastic Properties of Natural Fibres for Composite Reinforcement

Article Characterisation of the Anisotropic Thermoelastic Properties of Natural Fibres for Composite Reinforcement

James Thomason * ID , Liu Yang and Fiona Gentles

Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, UK; [email protected] (L.Y.); ﬁ[email protected] (F.G.) * Correspondence: [email protected]; Tel.: +44-141-548-2691

Academic Editors: Monica Ardanuy and Josep Claramunt Received: 24 July 2017; Accepted: 11 September 2017; Published: 25 September 2017

Abstract: There has been a substantial increase in the investigation of the potential of natural fibres as a replacement reinforcement in the traditional fibre reinforced polymer composite application. However, many researchers often overlook the anisotropic properties of these fibres, and the estimation of the potential reinforcement performance. A full understanding of the thermoelastic anisotropy of natural fibres is important for realistically predicting their potential performance in composite applications. In this study, the thermoelastic properties of flax and sisal fibres were determined through a combination of experimental measurements and micromechanical modelling. The results confirm the high degree of anisotropy in properties of the flax and sisal fibres. The implications of these results on using natural fibres as an engineering composite reinforcement are discussed.

Keywords: natural ﬁbres; mechanical performance; modulus; thermal expansion; anisotropy

1. Introduction The use of fibre reinforced polymer materials has increased dramatically during the last half century, becoming the standard for high performance in automotive, aeronautical and many other applications. Glass fibres (GF) currently represent more than 95% of the reinforcement fibres used globally in the engineering composites industry [1]. However, the increasing pressure on natural resources and the large amounts of energy required in glass fibre production has led to an upsurge in the research of fibres derived from natural sustainable plant sources as potential reinforcements for high performance composite materials [2–11]. Many academic researchers argue that plant-based natural fibres (NF) can successfully compete with glass fibres in today’s market because of their attractive properties, which may include low cost, low density, good specific strength properties, renewable, carbon dioxide neutrality emissions, and sustainability [3–8]. Nevertheless, a certain level of reinforcement performance is still required from such fibres in order to succeed in engineering composite applications. In this context, many researchers refer to the respectable level of axial modulus exhibited by some natural fibres, which can be made to appear more attractive by comparing modulus/density ratios [2–11]. An interesting point about such claims is that both low density and specific performance are claimed to be advantages for NF. It should, of course, be noted that any use of specific performance comparisons precludes claiming low weight or density as an advantage for NF over GF, as this is already included in the specific performance calculation. The suggestion that NF may well be suitable as a good reinforcement fibre for composite materials is also dependent on the use of standard micromechanical models and test methods. These have been developed by the composites community, based on the use of well-defined, uniformly cylindrical,

Fibers 2017, 5, 36; doi:10.3390/fib5040036 www.mdpi.com/journal/fibers Fibers 2017, 5, 36 2 of 12 often isotropic, man-made fibres with very low levels of variability. The assumption that these models and methods can be applied to complex, anisotropic, non-cylindrical NF, which have a high level of batch-to-batch, inter-fibre and intra-fibre variability [2,6,12–17], is open to question. In order to effectively predict realistic values for the properties of natural fibre reinforced composites, it is essential to fully appreciate the relevant properties of the constituent materials. In the case of natural fibres, the simple relationships that characterise isotropic materials are not sufficient due to the significant structural anisotropy of natural fibres. Therefore, further investigation is needed to effectively measure their orthotropic thermomechanical behaviour. Unfortunately, the majority of research to date has focused on reasonably attractive longitudinal fibre properties and has tended to ignore the challenges resulting from the high degree of anisotropy of natural fibres [12–15]. Although the longitudinal modulus of the natural fibres can be directly measured, characterising the transverse and shear properties of the fibre is more challenging due to the complex shapes and structure of such fibres. Nevertheless, it has been shown that such fibre properties can be estimated by using a range of micromechanical and semi-empirical models to analyse and model the natural fibre composite properties [12]. In one of the first papers which fully explored the thermomechanical anisotropy of a natural fibre, Cichocki and Thomason [12] presented a methodology to characterise the five thermoelastic constants of jute fibres using experimental results from off-axis loading of unidirectional composites. The fibre properties were calculated from the composite modulus and thermal expansion coefficient data, using a range of micromechanical models which were examined for their suitability. It was reported that the well-known Voigt and Reuss models appeared to be as appropriate as many of the more complex micromechanical models in producing values for the longitudinal and transverse moduli of the jute fibres. It was noted that Young’s longitudinal modulus of the jute fibre exceeded the fibre transverse modulus by a factor of five, and the fibre shear modulus by a factor of ten. Thomason expanded on these results to show that the anisotropy of these jute fibres, and specifically their low transverse and shear modulus, could fully explain the apparent low performance levels observed in the injection moulded jute–polypropylene composites [13,15]. Using similar methods, Ntenga et al. [18] reported high levels of mechanical anisotropy in sisal fibres and reported values of 11.5 GPa, 1.4 GPa, and 0.1 GPa, respectively for the sisal fibre longitudinal, transverse and shear moduli. Baley et al. [19] reported that the transverse modulus of flax fibres was 8 GPa, compared to a longitudinal modulus of 59 GPa. However, Shah et al. reported that flax fibres in a twisted yarn had a transverse modulus of 3.9 GPa, a longitudinal modulus of 46 GPa and a shear modulus of 2 GPa [20]. The results emphasize the need for composite researchers to fully appreciate and acknowledge the mechanical anisotropy of natural fibres caused by their highly complex structure. Sisal and flax are good examples of the many types of natural fibre which have been identified as appearing to have some appropriate, mechanical properties for structural reinforcement purposes. In this study, the thermoelastic properties of flax and sisal fibres have been determined using a combination of experimental measurements and micromechanical modelling, following the basic methodology presented by Cichocki and Thomason [12]. Dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) techniques were employed to characterise the unidirectional natural fibre–epoxy composites over a range of loading angles. The results were put into a number of micromechanical models to determine the values for the fibres transverse, longitudinal, shear moduli and thermal expansion coefficients over a range of temperatures.

2. Materials and Methods

2.1. Materials The natural fibres used for the investigation were untreated, twist-free flax and sisal. The flax fibres were sourced from Wigglesworth fibres, and were grown in Germany. The sisal fibres were sourced from the same company, but were grown in Brazil. The longitudinal Young’s moduli for these Fibers 2017, 5, 36 3 of 12 particular fibres, obtained by single fibre tensile testing at multiple gauge lengths, were previously 70.8 GPa for flax and 30.4 GPa for sisal [16]. For the composite production, the epoxy matrix used Araldite 506 combined with a room temperature curing agent (triethylenetetramine), both sourced from Sigma Aldrich (Irvine, United Kingdom).

2.2. Composite Production Unidirectional natural fibre composites were manufactured by a vacuum infusion process, as described by Symington [21]. Unidirectional fibre mats were layered, by hand, into a 15 cm × 15 cm mould. A vacuum pump via a pressure pot was used to achieve a smooth and adjustable filling of resin in a mould. The ideal infusion rate of approximately 5 mm per second can be controlled by adjusting the inlet and outlet valves. This relatively slow infusion rate ensures the minimisation of trapped voids. The high compaction of the fibres, without significant damage to the microtubules of each fibre, was achieved through the sandwiching of aluminium and acrylic outer plates using a number of bolt fixings and G-clamps [12]. The infused composite was left in the mould to cure at room temperature for 12 h, and then post cured at 120 ◦C for 12 h. A Struers Accutom high speed precision cutting wheel with an aluminium oxide blade, was used to accurately machine samples for thermomechanical testing. Samples for DMA and TMA were cut to approximately 55 mm × 14 mm × 2 mm and 4 mm × 4 mm × 1 mm, respectively. The composite and resin samples were dried to achieve a constant weight in an oven at 120 ◦C and then stored in a desiccator until they were required for testing, to avoid the reabsorption of moisture.

2.3. Thermal and Mechanical Testing Thermal analysis was used on the natural fibre composites to investigate their glass transition temperature (Tg), the temperature dependence of their modulus and the coefficient of thermal expansion. A TA Instruments Q800 dynamic mechanical analyser (TA Instruments, New Castle, DE, USA) was used to determine the storage modulus, loss modulus and the Tan Delta of the composites over a range of temperatures. Samples were tested in the deformation mode using a three-point bending clamp (TA Instruments, New Castle, DE, USA) with a preload of 0.1 N and a frequency of 1 Hz. Samples were heated from −50 ◦C to 150 ◦C at a rate of 3 ◦C/min. A TA Instruments Q400E thermomechanical analysis machine (TA Instruments, New Castle, DE, USA) was used to determine the matrix glass transition temperature and the thermal expansion, coefficient of the natural fibre composites. A macro expansion probe was used to allow a large contact area of the sample to be tested. The composites were tested over a range of different fibre orientations (0◦, 25◦, 45◦, 65◦, 90◦).

2.4. Composite Fibre Volume Fraction All the composite samples that were tested using DMA and TMA were subsequently examined to determine their fibre volume fraction. Small samples of these composites were cut perpendicular to the fibre orientation using a Struers Discotom abrasive cutter (Struers, Ballerup, Danmark). The cut sections were then embedded in Epofix resin, and after a 24 h room temperature cure, the samples were ground using progressively finer grinding papers (120 grade, 1200 grade, 2400 grade and 4000 grade) and polished using 3 micron diamond paste. The samples were then photographed using an Olympus G51 microscope (Olympus, Essex, United Kingdom) at × 50 magnification. The pictures were then imported into Image Pro Plus image analysis software (Media Cybernetics, Rockville, MD, USA) and formatted to binary. The binary image allowed the software to count the number of pixels in the picture to calculate the fractional cross-sectional area taken up by the fibre. Once the volume fraction had been determined for each picture, an average fibre volume fraction for each composite was calculated. Fibers 2017, 5, 36 4 of 12 Fibers 2017, 5, 36 4 of 12

3.3. Results Results 3.1. Composite Fibre Volume Fraction 3.1. Composite Fibre Volume Fraction TypicalTypical optical optical micrographs micrographs ofof polishedpolished cross sections sections of of the the flax flax and and sisal sisal composites composites are are shown shown inin Figure Figure1. 1. Using Using the the methods methods described described above,above, th thee average average fibre fibre volume volume fraction fraction for for the the flax flax and and sisalsisal composites composites were were found found toto bebe 0.310.31 and 0.4, respectively. respectively.

(a) (b)

Figure 1. Optical micrographs of polished cross sections of natural fibre epoxy composites: (a) Flax Figure 1. Optical micrographs of polished cross sections of natural fibre epoxy composites: (a) Flax fibre composites; (b) Sisal fibre composites. fibre composites; (b) Sisal fibre composites. It can be observed that, unlike conventional composite fibers, the cross-sectional shapes of naturalIt can fibers be observedvary widely. that, Thomason unlike conventionalhas discussed the composite influence fibers, of the noncircularity the cross-sectional of the natural shapes of naturalfibres in fibers terms vary of the widely. challenges Thomason of obtaining has discussed accurate thevalues influence from single of the fibre noncircularity micromechanical of the tests; natural fibressuchin as, terms single of fibre the challengestensile testing of obtaining and single accurate fibre adhesion values fromtests for single determining fibre micromechanical interfacial shear tests; suchstrength as, single [16,17]. fibre The tensile images testing in Figure and 1 also single highlight fibre adhesion the complex tests in forternal determining structure of interfacial these natural shear strengthfibres. The [16, 17‘technical’]. The images fibres in are Figure actually1 also composit highlighte structures the complex consisting internal of structure an assembly of these of many natural fibres.elementary The ‘technical’ fibres, which fibres have are a polygonal actually compositecross-section, structures allowing consistingthem to fit closely of an assemblytogether. Due of many to elementarythe low cost fibres, requirements which have of many a polygonal of the applicat cross-section,ions, where allowing natural them fibres to fitare closely being together.considered Due as to thereinforcement, low cost requirements these fibre bundles of many are of the the most applications, prevalent wheremorphology natural observed. fibres are In the being case considered of flax, asthe reinforcement, technical fibres these are located fibre bundles aroundare the thestem most of the prevalent plant and morphology tend to contain observed. between In 10 the and case 40 of flax,elementary the technical fibres. fibresSisal technical are located fibres around originate the from stem within of the the plant large andleaves tend of the to plant, contain and between contain 10 andbetween 40 elementary 50 and 200 fibres. elementary Sisal technical fibres, which fibres tend originate to be slightly from within smaller the than large the leaves flax elementary of the plant, andfibres. contain Each betweenelementary 50 fibre and contains 200 elementary multiple fibres,concentric which cell tendwalls, to with be a slightly void in smallerthe middle, than known the flax as the lumen. It is clear from the micrographs, that the structure of these fibres reveals many potential elementary fibres. Each elementary fibre contains multiple concentric cell walls, with a void in the weak interfaces. Indeed, Figure 1 reveals that the flax fibre has been damaged during the sample middle, known as the lumen. It is clear from the micrographs, that the structure of these fibres reveals preparation and has failed at a number of internal interfaces. The images in Figure 1 highlight the many potential weak interfaces. Indeed, Figure1 reveals that the flax fibre has been damaged during need for the determination of the transverse properties of such fibres, as it is clearly unreasonable to the sample preparation and has failed at a number of internal interfaces. The images in Figure1 expect that the complex structures can be anything other than highly anisotropic. highlight the need for the determination of the transverse properties of such fibres, as it is clearly unreasonable3.2. Composite to and expect Fibre thatModulus the complex structures can be anything other than highly anisotropic.

3.2. CompositeA selection and of Fibre the ModulusDMA storage modulus results for fibre and epoxy are plotted as a function of temperature in Figure 2. A large reduction in stiffness, due to the epoxy matrix glass transition A selection of the DMA storage modulus results for fibre and epoxy are plotted as a function temperature (Tg), is clearly visible between 70 °C–90 °C. It can be seen over the whole temperature ofrange, temperature that the incomposites Figure2. of A Young’s large reduction modulus in decrea stiffness,se as duethe off-axis to the epoxyloading matrix angle increases. glass transition It is ◦ ◦ temperaturealso interesting (Tg), to is note clearly that visible the transverse between stiffne 70 C–90ss of someC. It canof the be composites seen over theloaded whole at higher temperature off- range,axis angles that the is lower composites than the of epoxy Young’s resin modulus at temperatures decrease below as the the off-axis matrix loading Tg. This angleis an indication increases. of It is alsothe interesting likely probability to note thatof low the transverse transverse stiffness stiffness of of the some flax of and the compositessisal fibre. The loaded longitudinal at higher fibre off-axis anglesmodulus is lower Ef1 can than be theobtained epoxy at resin any attemperature temperatures from below the composite the matrix longitudinal Tg. This is anmoduli indication (EC0) and of the likely probability of low transverse stiffness of the flax and sisal fibre. The longitudinal fibre modulus Fibers 2017, 5, 36 5 of 12

Fibers 2017, 5, 36 5 of 12 FibersEf1 can 2017 be, 5, obtained36 at any temperature from the composite longitudinal moduli (EC0) and the epoxy5 of 12 thematrix epoxy modulus matrix (Emodulusm) using (E them) using well-known the well-known Voigt model Voigt (also model known (also as known the linear as the rule linear of mixtures) rule of themixtures)using epoxy the equationmatrixusing the modulus below:equation (E mbelow:) using the well-known Voigt model (also known as the linear rule of mixtures) using the equation below: Ec0 − EmVm Ef1 = E − E V (1) = c0 Vf m m Ef1 − (1) = Ec0 EmVm E Vf (1) f1 V 10.5 10.5f 10.5 10.5 10.0 10.0 10.0 10.0 9.5 9.5 9.5 9.5 9.0 9.0 9.0 Flax 0 9.0 Sisal 0 Flax 10 Sisal 10 8.5 FlaxFlax 0 20 8.5 SisalSisal 0 20 FlaxFlax 10 30 SisalSisal 10 30

Log Modulus (Pa) (Pa) Modulus Log (Pa) Modulus Log Sisal 50 8.5 FlaxFlax 20 50 8.5 Sisal 20 Sisal 90 8.0 FlaxFlax 30 90 8.0 Sisal 30

Log Modulus (Pa) (Pa) Modulus Log (Pa) Modulus Log SisalResin 50 FlaxResin 50 Sisal 90 8.0 Flax 90 8.0 Resin 7.5 Resin 7.5 -50 0 50 100 150 -50 0 50 100 150 7.5 o 7.5 o -50 0 Temperature( 50 C) 100 150 -50 0 Temperature( 50 C) 100 150 Temperature((a) oC) Temperature((b) oC) (a) (b) FigureFigure 2. 2. ResultsResults of of dynamic dynamic mechanical mechanical analysis analysis (DMA) (DMA) of of epoxy epoxy matrix matrix and and composites composites at at various various Figure 2. Results of dynamic mechanical analysis (DMA) of epoxy matrix and composites at various loadingloading angles: angles: ( (aa)) Flax Flax fibre ﬁbre composites; composites; ( (bb)) Sisal Sisal fibre ﬁbre composites. loading angles: (a) Flax fibre composites; (b) Sisal fibre composites.

Vf and Vm are the volume fractions of the fibre and the matrix, respectively. It should be noted V and V are the volume fractions of the fibre and the matrix, respectively. It should be noted that that Vthef fand properties Vmm are theof the volume composites fractions and of epoxy the fibre matr andix change the matrix, by orders respectively. of magnitude It should in the be regionnoted the properties of the composites and epoxy matrix change by orders of magnitude in the region of Tg. thatof T gthe. These properties large changes of the composites can give unexpected and epoxy matrresultixs changefrom the by micromechanical orders of magnitude equations in the used region in These large changes can give unexpected results from the micromechanical equations used in this work; ofthis T gwork;. These and large so, changeswe have can restricted give unexpected the temperat resulturess forfrom the the micromechanical micromechanical analysis equations to the used range in and so, we have restricted the temperatures for the micromechanical analysis to the range of −50 ◦C to thisof − 50work; °C to and +50 so, °C. we The have results restricted for the the longitudinal temperatures fibre for moduli the micromechanical of flax and sisal analysis fibres obtained to the range from +50 ◦C. The results for the longitudinal fibre moduli of flax and sisal fibres obtained from Equation (1) ofEquation −50 °C to(1) +50 are °C. compared The results with for the the epoxy longitudinal matrix modufibre modulilus in Figure of flax 3a. and It sisalcan befibres seen obtained that the fromaxial are compared with the epoxy matrix modulus in Figure3a. It can be seen that the axial stiffness of both Equationstiffness of(1) both are compared of these fibres with theis considerably epoxy matrix greater modu lusthan in theFigure stiffness 3a. It ofcan the be epoxyseen that matrix; the axial and of these fibres is considerably greater than the stiffness of the epoxy matrix; and hence, as observed in stiffnesshence, as of observed both of thesein the fibresintroduction, is considerably it can be greater expected than that the these stiffness fibres of will the giveepoxy a considerablematrix; and the introduction, it can be expected that these fibres will give a considerable reinforcement effect in the hence,reinforcement as observed effect in inthe the introduction, composite itlongitudinal can be expected direction. that theseIt is fibresfurther will noted give that a considerable the values composite longitudinal direction. It is further noted that the values obtained for the room temperature reinforcementobtained for the effect room in temperaturethe composite fibre longitudinal moduli of flaxdirection. and sisal It is are further in good noted agreement that the with values the fibre moduli of flax and sisal are in good agreement with the values previously reported from single obtainedvalues previously for the room reported temperature from single fibre fibre moduli tensile of testing. flax and sisal are in good agreement with the valuesfibre tensile previously testing. reported from single fibre tensile testing. 80 5 80 5 Flax 70 Sisal Flax 4 Resin 7060 Sisal 4 Resin 60 Flax

dulus (GPa) 50 Sisal 3 Flax

dulus (GPa) 50 Resin 40 Sisal 3 Resin 4030 2 2 3020 1 20 Transverse Modulus (GPa) Modulus Transverse Longitudinal Mo 10 1 Transverse Modulus (GPa) Modulus Transverse Longitudinal Mo 100 0 -50 -25 0 25 50 -50 -25 0 25 50 0 Temperature (oC) 0 Temperature (oC) -50 -25 0 25 50 -50 -25 0 25 50 Temperature(a) (oC) Temperature(b) (oC) (a) (b) Figure 3. Comparison of the fibre and resin moduli: (a) Fibre longitudinal modulus; (b) Fibre FiguretransverseFigure 3. 3. Comparison modulus.Comparison of of the the fibre ﬁbre and and resin resin moduli: moduli: (a (a) )Fibre Fibre longitudinal longitudinal modulus; modulus; (b (b) )Fibre Fibre transversetransverse modulus. modulus. The transverse fibre modulus Ef2 can be obtained at any temperature from the composite longitudinalThe transverse moduli fibre(Ec90) modulusand the epoxy Ef2 can matrix be obtained modulus at (E anym) using temperature the well-known from the Reuss composite model The transverse ﬁbre modulus Ef2 can be obtained at any temperature from the composite longitudinal(also known modulias the inverse (Ec90) and rule the of mixtures)epoxy matrix using modulus the equation (Em) below:using the well-known Reuss model longitudinal moduli (E ) and the epoxy matrix modulus (E ) using the well-known Reuss model (also known as the inversec90 rule of mixtures) using the equationm below: (also known as the inverse rule of mixtures)1 using1  the1 equationV  below: =  − m  (2) 1 = 1 1 − Vm Ef2 Vf  Ec90 Em  (2) Ef2 Vf  Ec90 Em  Fibers 2017, 5, 36 6 of 12

Fibers 2017, 5, 36 1 1 1 V 6 of 12 = − m (2) Ef2 Vf Ec90 Em Results for the transverse fibre moduli of flax and sisal fibres that were obtained from Equation (2) areResults compared for the with transverse the epoxy fibre matrix moduli modulus of flax and over sisal the fibres temperature that were range obtained −50 from °C to Equation +50 °C (2)in ◦ ◦ Figureare compared 3b. It can with immediately the epoxy be matrixobserved modulus that the over values the of temperature Ef2 for both fibres range are−50 significantlyC to +50 lowerC in thanFigure the3b. modulus It can immediately of the epoxy be matrix observed across that the the whole values temperature of E f2 for both range. fibres The are transverse significantly modulus lower ofthan the the sisal modulus fibres is of only the approximately epoxy matrix across 50% of the th wholeat of the temperature matrix. The range. flax fibre The transverse transverse modulus modulus isof even the sisal lower fibres with is values onlyapproximately of only 30% of that 50% of of the that epoxy of the matrix. matrix. Figure The flax 3b clearly fibre transverse illustrates modulus a major weaknessis even lower in the with application values ofof onlythese 30%natural of thatfibres of as the a composite epoxy matrix. reinforcement, Figure3b clearlyin that they illustrates provide a nomajor reinforcement weakness inof thethe applicationpolymer matrix of these in the natural transverse fibres direction. as a composite On the reinforcement,contrary, these infibres that have they anprovide anti-reinforcement no reinforcement effect of thein the polymer transverse matrix direction in the transverse and result direction. in a composite On the contrary,with a lower these transversefibres have modulus an anti-reinforcement than the matrix effect polymer in the alone. transverse direction and result in a composite with a lowerThe transverse mechanical modulus anisotropy than the of matrixthese polymernatural fibr alone.es is further visualised in Figure 4, which presentsThe mechanicalthe ratio of anisotropy Ef1/Ef2 for of the these flax natural and sisal fibres fibres, is further and visualised compares in them Figure with4, which some presents other reinforcementthe ratio of Ef1 fibres/Ef2 for [22]. the It flax can and be sisalseen fibres, that both and comparessisal and themflax are with highly some anisotropic other reinforcement in their mechanicalfibres [22]. Itperformance. can be seen that The both flax sisal fibres and have flax area modulus highly anisotropic ratio from in 55–80 their mechanicalacross the temperature performance. rangeThe flax studied, fibres haveand can a modulus be seen ratioto have from a level 55–80 of across anisotropy the temperature comparable range with studied, pitch carbon and can fibre. be seenThe sisalto have fibres a level have of a anisotropyconsiderably comparable lower level with of anisotropy, pitch carbon with fibre. a modulus The sisal ratio fibres of have approximately a considerably 17. Thislower is levellower of than anisotropy, most carbon with a and modulus aramid ratio fibres, of approximately but still highly 17. anisotropic This is lower in comparison than most carbon with glassand aramidfibres. fibres, but still highly anisotropic in comparison with glass fibres.

FigureFigure 4. 4. ComparisonComparison of of the the modulus modulus anisotropy anisotropy of of natu naturalral fibres ﬁbres with with other other reinforcement reinforcement fibres. ﬁbres.

The values obtained for the off-axis of Young’s moduli (ECθ) from Figures 1 and 2 can also be The values obtained for the off-axis of Young’s moduli (ECθ) from Figures1 and2 can also be used to obtain values for the composite shear modulus GC12 , which can then be used to obtain a value used to obtain values for the composite shear modulus GC12 , which can then be used to obtain a value for the fibre shear modulus Gf12. The mechanics concerning the coordinate system transformations for the ﬁbre shear modulus G . The mechanics concerning the coordinate system transformations can be applied, leading to the f12well-known relationship between off-axis modulus and the principal can be applied, leading to the well-known relationship between off-axis modulus and the principal properties of a unidirectional composite ply [12]: properties of a unidirectional composite ply [12]: 4 4 1 cos θ sin θ  1 2νC12  2 2 1 =cos4 θ +sin4 θ + 1 −2ν cos θ.sin θ (3) = + +  − C12  2 θ · 2θ ECθ EC1 EC2 GC12 EC1 cos sin (3) ECθ EC1 EC2 GC12 EC1 

Hence,Hence, by by measuring measuring the the composite composite modulus modulus as as a a function function of of the the loading loading angle, angle, it it is is possible possible to to obtain a value for GC12 by using the curve fitting Equation (3) with the experimental results taken obtain a value for GC12 by using the curve ﬁtting Equation (3) with the experimental results taken from fromFigure Figure2 at various 2 at various temperatures. temperatures. An example An example is shown is inshown Figure in5 forFigure the ﬂax5 for and the sisal flax composite and sisal compositemoduli obtained moduli at obtained 25 ◦C. at 25 °C. FibersFibers2017 2017, 5,, 5 36, 36 77 of of 12 12 Fibers 2017, 5, 36 7 of 12

Figure 5. The composite modulus at various loading angles compared to values fitted using Equation Figure 5. The composite modulus at various loading angles compared to values fitted using Equation Figure(3). 5. The composite modulus at various loading angles compared to values fitted using Equation (3). (3).

TheThe ﬁbre fibre shear shear modulus modulus G Gcanf12 can then then be obtained be obtained at any at temperature any temperature from the from composite the composite moduli The fibre shear modulus f12Gf12 can then be obtained at any temperature from the composite moduli (GC12) and the epoxy matrix shear modulus (Gm), also using the well-known Reuss model: (GC12) and the epoxy matrix shear modulus (Gm), also using the well-known Reuss model: moduli (GC12) and the epoxy matrix shear modulus (Gm), also using the well-known Reuss model:

111 Vm 1 1111= 1 V−Vm (4) GVGG= = −− m (4) Gf12 f12Vf G fC12 C12Gm m (4) GVGGf12 f C12 m ResultsResults for for the the shear shear moduli moduli of of flax flax and and sisal sisal fibres fibres obtained obtained from from Equation Equation (2) (2) are are compared compared Results for the shear moduli of flax and sisal fibres obtained from Equation (2) are compared withwith the the epoxy epoxy matrix matrix shear shear modulus modulus in in Figure Figure6. 6. It It can can be be seen seen that that the the shear shear modulus modulus of of the the sisal sisal with the epoxy matrix shear modulus in Figure 6. It can be seen that the shear modulus of the sisal fibresfibres differs differs a a little little from from the the shear shear modulus modulus of of the the epoxy epoxy polymer polymer and and is is actually actually lower, lower, at at lower lower fibres differs a little from the shear modulus of the epoxy polymer and is actually lower, at lower temperatures.temperatures. The The shear shear modulus modulus of of the the flax flax fibres fibres (1.0 (1.0 GPa–2.1 GPa–2.1 GPa) GPa) is is significantly significantly higher higher than than both both temperatures. The shear modulus of the flax fibres (1.0 GPa–2.1 GPa) is significantly higher than both thethe sisal sisal fibres fibres and and the the epoxy epoxy matrix. matrix. However, However, as as a a reference, reference, it it can can be be noted noted that that the the shear shear modulus modulus the sisal fibres and the epoxy matrix. However, as a reference, it can be noted that the shear modulus ofof E-glass E-glass fibres fibres is is more more than than an an order order of of magnitude magnitude greater, greater, at at around around 30 30 GPa. GPa. of E-glass fibres is more than an order of magnitude greater, at around 30 GPa.

2.5 2.5 Flax FlaxSisal 2.0 SisalResin 2.0 Resin 1.5 1.5

1.0 1.0

Shear Modulus(GPa) 0.5 Shear Modulus(GPa) 0.5

0.0 0.0 -50 -25 0 25 50 -50 -25 0 25 50 Temperature (°C) Temperature (°C)

FigureFigure 6. 6.Shear Shear modulus modulus of of flax flax and and sisal sisal fibres fibres compared compared with with the the shear shear modulus modulus of of the the matrix. matrix. Figure 6. Shear modulus of flax and sisal fibres compared with the shear modulus of the matrix. 3.3.3.3. Composite Composite and and Fibre Fibre Thermal Thermal Expansion Expansion 3.3. Composite and Fibre Thermal Expansion FigureFigure7 illustrates7 illustrates the the TMA TMA measured measured thermal thermal strain strain of of the the epoxy epoxy matrix matrix and and the the natural natural fibre fibre Figure 7 illustrates the TMA measured thermal strain of the epoxy matrix and the natural fibre compositescomposites with with different different fibre fibre orientation. orientation. The The data data exhibits exhibits some some typical typical elements elements of of TMA TMA analysis analysis composites with different fibre orientation. The data exhibits some typical elements of TMA analysis ofof polymer polymer materials. materials. WhenWhen below the glass glass transition transition temperature temperature (T (Tg),g ),the the thermal thermal strain strain is low is low but of polymer materials. When below the glass transition temperature (Tg), the thermal strain is low but butas the as the temperature temperature increases increases towards towards Tg, Ttheg, thethermal thermal strain strain increases, increases, and andabove above Tg, the Tg ,thermal the thermal strain as the temperature increases towards Tg, the thermal strain increases, and above Tg, the thermal strain strainis significantly is significantly high. high. TheThe temperature temperature region region of ofthis this change change of of slope slope is is indicative ofof thethe polymer polymer is significantly high. The temperature region of this change of slope◦ is indicative◦ of the polymer (matrix)(matrix) T gTgwhich, which, in in both both cases, cases, can can be be seen seen to to be be in in the the range range of of 70 70 C°C to to 90 90 C°C (similar (similar to to the the values values (matrix) Tg which, in both cases, can be seen to be in the range of 70 °C to 90 °C (similar◦ to the values obtainedobtained by by DMA). DMA). It It can can further further be be noted noted from from these these figures, figures, that that when when loading loading at at 90 90°,, the the thermal thermal obtained by DMA). It can further be noted from these figures, that when loading at 90°, the thermal strainstrain of of the the composite composite is is approximately approximately the the same same as as the the resin resin thermal thermal strain. strain. At At fibre fibre orientation orientation strain of the composite is approximately the same as the resin thermal strain. At fibre orientation angles, the composite thermal strain is very low and a change in slope at Tg becomes quite indistinct. angles, the composite thermal strain is very low and a change in slope at Tg becomes quite indistinct. Fibers 2017, 5, 36 8 of 12

Fibers 2017, 5, 36 8 of 12 angles, the composite thermal strain is very low and a change in slope at Tg becomes quite indistinct. Interestingly,Interestingly, when when the the flax flax composites composites are are at at 0° 0◦ fibrefibre orientation, orientation, the the thermal thermal strain strain turns turns negative, negative, aboveabove the the matrix matrix T Tgg temperature.temperature. From From the the results, results, it it ca cann be be concluded concluded that that the the composite composite thermal thermal strainstrain decreases decreases as as the the fibre fibre orientation orientation decreases decreases from from 90° 90◦ toto 0°. 0◦.

(a) (b)

FigureFigure 7. 7. ResultsResults of of TMA TMA of of epoxy epoxy matrix matrix comp compositesosites at at various various loading loading angles: angles: (a (a) )Flax Flax fibre ﬁbre composites;composites; ( (bb)) Sisal Sisal fibre ﬁbre composites. composites.

In a similar manner to the analysis of the composite and fibre modulus, the coefficient of thermal In a similar manner to the analysis of the composite and fibre modulus, the coefficient of thermal expansion of the composites determined by TMA can be used to establish the coefficient of thermal expansion of the composites determined by TMA can be used to establish the coefficient of thermal expansion of the fibre using micromechanical models. The longitudinal coefficient of thermal expansion of the fibre using micromechanical models. The longitudinal coefficient of thermal expansion expansion of the fibre (αf1) can be obtained using a rearrangement of the well-established Schapery of the fibre (α ) can be obtained using a rearrangement of the well-established Schapery [23] equation: [23] equation: f1

[[]αC1(()Ef1VVf ++EEmVVm) −−ααmEEmVVm] ααf1 == C1 f1 f m m m m m (5) f1 Ef1Vf (5) Ef1Vf where αC1 and αm are the longitudinal coefficient of thermal expansion (CTE) of the composite and where αC1 and αm are the longitudinal coefficient of thermal expansion (CTE) of the composite and the linear CTE of matrix [17]. There is not, as yet, a universally applicable micromechanical model or the linear CTE of matrix [17]. There is not, as yet, a universally applicable micromechanical model or equation for predicting the transverse CTE of composites; and so, we have compared the results from equation for predicting the transverse CTE of composites; and so, we have compared the results from the published methods of both Chamberlain [24] and Chamis [25]. The rearrangement of the equation the published methods of both Chamberlain [24] and Chamis [25]. The rearrangement of the equation from Chamis gives the following expression for the transverse CTE (αf2) of the fibres: from Chamis gives the following expression for the transverse CTE (αf2) of the fibres: √ Ef1 αC2 − 1 − Vf 1 + νfVm E )α α − (1− V )(1+ ν V E1f1 )αm αf2 = C2 f√ f m m (6) V E (6) α = f 1 f2 V Whereas, the equation from Chamberlain results inf the following expression: Whereas, the equation from Chamberlaih n results in the following expression: i Em (α − αm) νm(F − 1 + Vm) + (F + V ) + (1 − ν )(F + 1 − Vm) C2 f Ef1 f12 α = αm + (7) f2 −−++++−+−() E m (ααC2 m) ν mF1V m (FV)2V ff (1ν f12)(F 1 V m ) E f1 (7) αα=+ where αC2f2is the m transverse CTE of the composite and F is the fibre packing factor with a value of 0.785 2Vf for the square packing [24]. whereThe αC2 longitudinal is the transverse and transverse CTE of the CTEs composite of the flaxand and F is sisal the fibresfibre packing obtained factor from with thisanalysis a value areof 0.785compared for the with square the packing epoxy matrix [24]. CTE in Figure8 respectively. It can be seen that the longitudinal CTEsThe for longitudinal both natural and fibres transverse are not onlyCTEs an of order the flax of magnitudeand sisal fibres smaller obtained than theirfrom transversethis analysis CTEs, are comparedbut also negative with the in epoxy both cases.matrix In CTE other in words,Figure flax8 resp andectively. sisal fibres It can shrink be seen along that their the longitudinal length when CTEsheated for up. both This natural is in agreement fibres are not with only the resultsan order of of Cichocki magnitude and Thomasonsmaller than who their found transverse that jute CTEs, fibre butalso also has negative a negative in longitudinalboth cases. In CTE other [12 ].words, flax and sisal fibres shrink along their length when heated up. This is in agreement with the results of Cichocki and Thomason who found that jute fibre also has a negative longitudinal CTE [12]. Fibers 2017, 5, 36 9 of 12 Fibers 2017, 5, 36 9 of 12

(a) (b)

FigureFigure 8. 8. ComparisonComparison of of resin resin and and fibre fibre coefficients coefficients of of thermal thermal expansion: expansion: ( (aa)) Flax Flax fibres; fibres; ( (bb)) Sisal Sisal fibres. fibres.

The two equations for the fibre transverse CTE give similar values in the case of both flax and The two equations for the fibre transverse CTE give similar values in the case of both flax and sisal, although the Chamberlain equation results in systematically higher values for αf2 in comparison sisal, although the Chamberlain equation results in systematically higher values for α in comparison to the Chamis equation. The sisal fibres appear to have a slightly smaller transversef2 CTE compared to the Chamis equation. The sisal fibres appear to have a slightly smaller transverse CTE compared to flax fibres over the −50 °C to +50 °C temperature range. In both cases, the fibre transverse CTE is to flax fibres over the −50 ◦C to +50 ◦C temperature range. In both cases, the fibre transverse CTE of the same order of magnitude as the epoxy matrix CTE, which is probably a reflection of the thermal is of the same order of magnitude as the epoxy matrix CTE, which is probably a reflection of the response of the amorphous polymeric components of these fibres. An overall summary of the thermal response of the amorphous polymeric components of these fibres. An overall summary of the temperature dependence of the various thermomechanical parameters obtained for sisal and flax temperature dependence of the various thermomechanical parameters obtained for sisal and flax fibres fibres in this study is shown in Table 1. It should be noted that the values of αf2 in Table 1 are an in this study is shown in Table1. It should be noted that the values of α in Table1 are an average of average of the values obtained using Equations (6) and (7). f2 the values obtained using Equations (6) and (7). Table 1. Summary of flax and sisal fibre thermoelastic properties at different temperatures. Table 1. Summary of flax and sisal fibre thermoelastic properties at different temperatures. Flax Sisal Flax Sisal −50 °C −25 °C 0 °C 25 °C 50 °C −50 °C −25 °C 0 °C 25 °C 50 °C −50 ◦C −25 ◦C 0 ◦C 25 ◦C 50 ◦C −50 ◦C −25 ◦C 0 ◦C 25 ◦C 50 ◦C E1f (GPa) 69.8 67.6 65.7 62.5 60.2 25.0 24.6 23.3 21.9 21.1 E (GPa) 69.8 67.6 65.7 62.5 60.2 25.0 24.6 23.3 21.9 21.1 E2f (GPa)1f 1.3 1.2 1.1 1.0 0.75 1.9 1.8 1.7 1.6 1.3 E2f (GPa) 1.3 1.2 1.1 1.0 0.75 1.9 1.8 1.7 1.6 1.3 12 G G(GPa)12 (GPa)2.1 2.1 1.8 1.81.5 1.5 1.4 1.4 1.1 1.1 1.0 1.0 1.1 1.1 1.11.1 1.11.1 α1fα 1f ◦ −6.0−6.0 −6.9− 6.9 −7.1−7.1 −−8.08.0 −6.96.9 −3.33.3 −−4.64.6 −7.47.4 −−3.93.9 −4.0−4.0 (µm/m(µm/m °C)C) α 2f 48.0 68.1 68.4 82.7 65.2 33.2 54.7 70.0 79.1 76.8 µα2f ◦ ( m/m C) 48.0 68.1 68.4 82.7 65.2 33.2 54.7 70.0 79.1 76.8 (µm/m °C) 3.4. Implications for Natural Fibre Performance as a Composite Reinforcement 3.4. Implications for Natural Fibre Performance as a Composite Reinforcement The results presented in Table1 illustrate the high levels of thermomechanical anisotropy present in theThe natural results fibres. presented Although in thisTable anisotropy 1 illustrate does the not, high in itself, levels present of thermomechanical insurmountable challenges anisotropy to presentthe further in the application natural fibres. of such Although fibres as reinforcementsthis anisotropy fordoes engineering not, in itself, composites, present insurmountable the implications challengesdo bear some to the further further discussion. application Table of such2 compares fibres as values reinforcements of the thermomechanical for engineering composites, properties [ the22] implicationsof some other do well-known bear some reinforcementfurther discussion. fibres Table at 25 2◦ C,compares with those values obtained of the in thermomechanical this work for flax propertiesand sisal. [22] of some other well-known reinforcement fibres at 25 °C, with those obtained in this work for flax and sisal. Table 2. Comparison of reinforcement fibre thermoelastic properties at room temperatures [22]. Table 2. Comparison of reinforcement fibre thermoelastic properties at room temperatures [22]. E-Glass Carbon Aramid Flax Sisal E-Glass Carbon Aramid Flax Sisal Ef1 (GPa) 77 220 152 62.5 21.9 Ef1 (GPa) 77 220 152 62.5 21.9 Ef2 (GPa) 68 14 4.2 1.0 1.6 EGf2f12 (GPa)(GPa) 68 30 14 14 2.9 4.2 1.4 1.0 1.61.1 ◦ αGf1f12(µ (GPa)m/m C) 30 5 − 140.4 3.6 2.9 −8.0 1.4 − 1.13.9 α (µm/m ◦C) 5 18 77 83 80 αf1f2 (µm/m °C) 5 −0.4 3.6 −8.0 −3.9 αf2 (µm/m °C) 5 18 77 83 80 Fibers 2017, 5, 36 10 of 12

It can be observed from the values present in Table2 that other reinforcements, such as carbon fibre and aramid fibre, used in engineering composite applications also have high levels of thermomechanical anisotropy. However, the challenge for those who wish to use natural fibres is not so much the anisotropy itself; rather it is the very low values of fibre transverse and shear modulus. These values for natural fibre are much lower than most polymer matrices; consequently, natural fibres perform poorly as reinforcement for the composite material in any off-axis loading scenario. In fact, natural fibres are not seriously considered by many as a replacement for carbon or aramid fibres, but they are regularly promoted as a possible replacement for glass fibres. In this case, the applications which are being considered are most often low performance, low cost, moulded composites; where a large proportion of the reinforcement fibres experience off-axis loading. In this situation, the isotropic nature of glass fibres still results in a significant level of reinforcement; whereas, the natural fibres will perform poorly. There are many articles to be found in the literature which strongly identify the fibre–matrix interface as a challenge for the further development of natural fibre composites. Many authors have seen this as an issue of chemical compatibility and adhesion. However, it has recently been shown that the residual compressive radial stresses at the fibre–matrix interface caused by the fibre–matrix mismatch in CTE can account for a large proportion of the interfacial stress transfer capability in many composites [15,26–30]. Thomason has published an analysis for jute–PP composites which indicates that this hypothesis can be used to explain the very low levels of apparent interfacial shear strength in these composites [13,15]. Clearly, the data presented above where the value of αf2 for flax and sisal is very close to that of the polymer matrix, implies a similar lack of any interfacial radial compressive stress to the interfacial stress transfer capability in composites using these fibres. In fact, if, as appears to be the case at some points in Figure8, αf2 is greater than the CTE of the polymer matrix, this could result in an interface under radial tension, which could be very weak. In further discussion of the interface in fibre reinforced composites, it is worth observing that the accepted stress transfer mechanism is shear along the interface. This has mainly been considered in terms of its effects on the interface and surrounding matrix, but there has been little attention paid to the effects of this interfacial shear stress on the reinforcing fibres. However, considering the very low values obtained for the natural fibre shear modulus in comparison to glass fibres, one must assume that the levels of shear strain on the fibre side of the interface must be at least an order of magnitude higher for natural fibres than for glass fibres. It is interesting to ask what effects such high levels of shear deformation might have on the internal structure of these natural fibres, how that might manifest itself in the apparent interfacial stress transfer capability, and the overall performance of the natural fibre composite. Charlet et al. [31] have published the results of a unique study of the properties on the internal interphase region between elementary fibres within technical flax fibres. They gave a value for the average shear strength of these interphases of 2.9 MPa and a value for the shear modulus of 0.02 MPa. It seems clear that there is a much higher probability of failure of these internal interphase regions due to high shear strains, rather than the failure of the fibre–matrix interface. Such a result would go a long way to explaining why the chemical modification of the interface has such little effect on improving the composite performance with natural fibres in comparison to glass fibres. It also suggests that the way to move forward to improve natural fibre composite performance should be more focused on the internal structure of the technical fibres.

4. Conclusions The thermoelastic behaviour of untreated flax and sisal epoxy composites has been investigated and quantified in this study, and both fibres were shown to exhibit high levels of thermoelastic anisotropy. The longitudinal and transverse properties of the fibres were found to be significantly different, highlighting the highly anisotropic structure of the natural fibres. The longitudinal and transverse modulus of flax at room temperature (25 ◦C) was found to be 65.7 GPa and 1.1 GPa, respectively. The room temperature longitudinal and transverse modulus of sisal was found to be Fibers 2017, 5, 36 11 of 12

23.3 GPa and 1.7 GPa, respectively. The shear modulus of both fibres was also low, with sisal 1.1 GPa and flax 1.5 GPa. The coefficient of the thermal expansion of the fibres exhibits low negative values in the longitudinal direction and high values (typical of polymers) in the transverse direction. The level of anisotropy in these fibres did not change significantly within the temperature range of −50 ◦C to +50 ◦C. These results confirm that overlooking the anisotropy of natural fibres will lead to a significant overestimation of their reinforcement potential in off-axis loading situations. Furthermore, it is suggested that the root cause of the apparent poor interfacial stress transfer performance of natural fibres may well lie in their thermoelastic anisotropy, and not primarily in the chemical interactions at the natural fibre–polymer interface.

Acknowledgments: The authors would like to express their gratitude to the Saudi Basic Industries Corporation for the financial support. The costs to publish in open access is self-covered. Author Contributions: James Thomason and Liu Yang conceived and designed the experiments; Fiona Gentles performed the experiments; James Thomason and Liu Yang analyzed the data; James Thomason and Liu Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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