Article Surface Modification of Flax Fibers for Manufacture of Engineering Thermoplastic Biocomposites

Madina Shamsuyeva 1,*, Boon Peng Chang 2 , Natalie Vellguth 3, Manjusri Misra 2,4 , Amar Mohanty 2,4 and Hans-Josef Endres 1

1 Institute of Plastics and Circular Economy IKK, Leibniz University Hannover, 30823 Garbsen, Germany; [email protected] 2 Bioproducts Discovery and Development Centre (BDDC), Department of Plant Agriculture, University of Guelph, Guelph, ON N1G 2W1, Canada; [email protected] (B.P.C.); [email protected] (M.M.); [email protected] (A.M.) 3 Fraunhofer Application Center HOFZET, Fraunhofer Institute for Wood Research WKI, 30453 Hanover, Germany; [email protected] 4 School of Engineering, Thornbrough Building, University of Guelph, Guelph, ON N1G 2W1, Canada * Correspondence: [email protected]



Received: 22 April 2020; Accepted: 28 May 2020; Published: 1 June 2020


Abstract: The aim of this feasibility study is to develop application-oriented natural fiber-reinforced biocomposites with improved mechanical and durability performance. The biocomposites were manufactured via a film-stacking process of epoxy-coated flax textiles and polyamide 6 (PA6). The fabricated biocomposites were subjected to thermo-oxidative ageing for 250, 500 and 1000 h and tested with regard to tensile properties. The results show that the biocomposites with epoxy-coated flax fibers possess considerably higher tensile properties compared with the reference specimens under all tested conditions.

Keywords: biocomposites; natural fibers; surface treatment; thermo-oxidative ageing; polyamide; flax

1. Introduction Natural fiber composites have already proven their beneficial material properties in various lightweight applications, like e.g., automotive (interior), sport and leisure or building construction [1–3]. The well-known advantages of the natural fibers such as low density, high damping and strain associated with further economic and ecological benefits like renewable origin, high availability and broad assortment of fiber types, biological degradation and recycling potential promote their commercial use in composite applications [1–4]. Although both thermoset and thermoplastic natural fiber composites find industrial applications, recycling possibilities of the thermoplastic composites represent a valuable advantage compared with the thermoset composites [3]. Among the standard thermoplastics like, for example, polyvinyl chloride, and polystyrene, polyethylene and polypropylene are used most commonly for the manufacture of the natural fiber composites [3,5]. Due to the lower processing temperature of these matrices, which is generally lower than 200 ◦C, the natural fibers do not undergo thermal degradation, which is associated with emission, color change and strong decrease of mechanical performance [6]. At the same time, effective use of engineering thermoplastics like, for example, polyamide would enable manufacture of natural fiber-reinforced composites with considerably higher mechanical performance, durability and thermal resistance than those with standard thermoplastics. This would open new application fields for the resulting biocomposites. The review on short cellulose fiber-reinforced polyamides for composite applications has already been published previously [7]. One of the simplest application-oriented approaches is to use polyamides

J. Compos. Sci. 2020, 4, 64; doi:10.3390/jcs4020064 www.mdpi.com/journal/jcs J. Compos. Sci. 2020, 4, 64 2 of 12 with low melting temperature like, for example, PA11 or PA12. In addition, further approaches like modification of the short cellulose fibers or adjustment of the compounding process parameters are used, in order to reduce the temperature gap between the thermal stability of cellulose-based fibers and processing temperature of the polyamides [7]. Due to this temperature gap, natural fiber textile-reinforced PA6 composites are rarely manufactured [8]. There are only few attempts reported on the improvement of thermal stability of the weaved jute [9] and flax [10] textiles. Both of the studies use epoxy resin coating of the textiles for an effective integration into polyamide. Epoxy coating is a thermoset acting as an isolating interphase between natural fibers and molten polyamide. Furthermore, PA6 and epoxy resins have good compatibility [11], i.e., epoxy resins have been used as a reactive coupler for the manufacture of PA6 blends with polycarbonate [12], polybutylene terephthalate [13,14], and poly-(phenylene ether) [15] or as crosslinking agent for the manufacture of PA6 foam [16]. Both of the aforementioned studies report that the resulting biocomposites with epoxy coated natural fibers possess increased bending, tensile and thermal properties [9,10]. However, in order to further develop this technology, the effect of temperature on the coated textiles and ageing behavior of the manufactured biocomposites have to be investigated. The aim of this study is to investigate the effect of the epoxy-based coating of the flax textiles on the tensile properties of the textiles and the corresponding PA6 biocomposites after thermo-oxidative ageing.

2. Materials and Methods

2.1. Materials Following materials were used for the manufacture of the epoxy-coated flax fiber-reinforced PA6 composites: twill (1/3) weaved flax fabric manufactured using a double rapier weaving machine as described in the prior publication [17]. Bisphenol A-based epoxy resin with the trade name H543 and an amine-based hardener with the trade name R131 were purchased from Hacotech and used for the surface coating of the weaved fabrics. The density of the cured epoxy resin is 0.92 0.02 g/cm3 and the ± Tg is 60.2 ◦C. PA6 in the form of 100 µm thick film was supplied by Jura-Plast.

2.2. Coating of the Weaved Textiles and Composite Manufacture

Weaved flax textiles were pre-dried at 80 ◦C for 24 h. Afterwards, the textiles were fixed in a frame with the dimensions of 30 40 cm, sprayed manually using a spray pistol with the epoxy resin 2 × (1 g/cm ) and dried at 80 ◦C for 30 min in an air circulation oven. Since PA6 is hygroscopic with a water absorption of 2.8–3.2% [18], the PA6 films were dried before composite manufacture at 80 ◦C for at least 24 h. In order to manufacture the composites, 4 layers of the flax textiles were stacked alternatively with PA6 films. The use of the 4 textile layers enables manufacture of both types of specimens with an optimal thickness required for the testing according to the used testing standard (ISO 527). The stacked textiles and films were also stored in air-circulating oven at 80 ◦C prior to hot pressing, in order to prevent moisture absorption. The composites were fabricated via film stacking process using a hot press KV 322 manufactured by Rucks. The stacked sequence with dimensions of 20 30 cm was × pressed with a force of 100 kN for 5 min at 225 4 C, cooled down to 100 C under pressure using ± ◦ ◦ water cooling at a rate of 10 K/min. Finally, the pressed specimens were removed out of the hot press and cooled down to room temperature under a metal weight with a force of approximately 0.48 kN. The summary of the investigation steps as well as the nomenclature of the manufactured samples is represented in Figure1. This figure shows that the following parameters were held constant during the study: type of the used weave art, textile spraying procedure and composite manufacturing process. Variation of these parameters influences considerably the mechanical performance of the resulting composites [8,17,19]. The fiber weight fraction of the resulting reference composite (FLA-PA6) and the composite with the coated textile (FLA-EP-PA6), were 43 wt.% and 63 wt.%, respectively. The overall weight of the reference was 160 g and of the composite with coated textile 230 g. The varied parameters J. Compos. Sci. 2020, 4, x 3 of 13 J. Compos. Sci. 2020, 4, 64 3 of 12 the textiles, change of the temperature during textile testing and duration of the thermo-oxidative ageing.J. Compos. Sci. 2020, 4, x 3 of 13 include application of the coating onto the textiles, change of the temperature during textile testing the textiles, change of the temperature during textile testing and duration of the thermo-oxidative and duration of the thermo-oxidative ageing. ageing.

FigureFigure 1. 1. 1. The TheThe main main main steps steps steps of the of thestudy study and the theand nomenclature nomenclature the nomenclature of the the manufactured manufactured of the manufactured samples: flaxsamples: flax reference flax reference(FLA-Ref),reference (FLA (FLA epoxy-coated-Ref),-Ref), epoxy epoxy- flax,coated-coated (FLA-EP), flax, flax, (FLA flax-reinforced (FLA-EP), - flaxEP),-reinforced flax polyamide-reinforced polyamide 6 polyamide (FLA-PA6) 6 (FLA- PA6) 6 and (FLA and polyamide- PA6) and 6 polyamidereinforcedpolyamide 6 with 6reinforced reinforced epoxy-coated with with epoxy epoxy flax-coated (FLA-EP-PA6).-coated flax (FLA flax -(FLAEP-PA6).-EP -PA6).

2.3.2.3. Accelerated Accelerated Thermo Thermo Thermo-Oxidative-O-xidativeOxidative Ageing Ageing TheThe accelerated accelerated thermo thermo-oxidativethermo-oxidative-oxidative ageing ageing of the of t ensile the tensilet ensilespecimens specimens was performed was performed according according to to ISO 188-2011 in a cabinet ageing oven EB10 at 110 °C for 250 h, 500 h and 1000 h. These specimens ISO 188-2011188-2011 in a cabinet ageing oven EB10 at 110 ◦°CC for 250250 h,h, 500500 hh andand 10001000 h.h. These specimens were evaluated in comparison with the specimens stored for the same time at standard laboratory were evaluatedevaluated in comparison with the specimens stored for the same time at standard laboratory conditions (23 °C with 40%–50% humidity in a seal Ziploc bag) as according to the ASTM D618 conditions (23 C with 40–50% humidity in a seal Ziploc bag) as according to the ASTM D618 standard. conditionsstandard. Figure (23◦ °C2 represents with 40% air– 50%flow schematic humidity and in aphoto seal images Ziploc of bag) the performed as according experiment. to the ASTM D618 Figurestandard.2 represents Figure 2 represents air flow schematic air flow andschematic photo imagesand photo of the images performed of the performed experiment. experiment.

Figure 2. Schematic representation of the air flow (left), ageing oven (middle), specimens during ageing testing (right). Figure 2.2. Schematic representation of of the air flow flow (left), ageing oven ((middle),), specimensspecimens during during ageing testing (right).).

J. Compos. Sci. 2020, 4, 64 4 of 12

2.4. Mechanical Testing Both coated textiles and the manufactured biocomposites were subjected to tensile testing according to ISO 527-4. The textiles were tested using Zwick/Roell Z100 TEW from ZwickRoell (Ulm, Germany) at the HOFZET and the biocomposites using Instron 3382 at the BDDC. Both tests were conducted using a 100 kN load cell with a speed of 2 mm/min. The coated textiles were tested at 23 ◦C, 40 ◦C and 60 ◦C without prior thermo-oxidative ageing in order to evaluate the strength of the reinforcing fibers as a function of temperature. Textile specimens and composites were conditioned prior to the testing at 23 ◦C and 50% relative humidity for 88 h. The average value of five tested specimens was taken for evaluation of the results. The composite specimens were manufactured using a diamond saw.

2.5. Digital Microscope Imaging Optical imaging was performed at the Institute of Bioplastics and Biocomposites (IfBB) using a 3D digital microscope VHX 5000 Keyence (Neu-Isenburg, Germany) without further specimen preparation.

2.6. Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy Analysis The surface of both ambient and oven aged conditions of the flax-PA6 biocomposites were examined using a Phenom ProX desktop scanning electron microscope (SEM) from Phenom World (Eindhoven, The Netherlands) at an accelerating voltage of 10 kV under a high vacuum atmosphere. The samples were prior coated with a thin gold layer with a Cressington Sputter Coater 108 auto vacuum sputter chamber. Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted on several zones of the flax-PA6 biocomposites to scan the oxygen content qualitatively. The oxygen concentration in atomic weight percentage for each sample under laboratory and thermo-oxidative ageing conditions was recorded. At least nine different areas of the sample surface were measured and the average and standard deviation were reported.

3. Results and Discussion

3.1. Tensile Strength of Fabrics Tensile properties of the weaved flax fabrics tested at different temperatures are represented in Figure3. The aim of this test is to evaluate the e ffect of the epoxy coating on the strength of the textiles and to predict the behavior of the coating during hot pressing. The results show that the epoxy-coated flax textiles possess considerably higher and more stable tensile properties compared with the references at all tested temperatures. The tensile strength of the reference sample, i.e., flax textile without coating is 18.7 MPa. It is important to consider, that there is a large number of factors affecting mechanical properties of weaved textiles like, for example, yarn fineness, twisting grade and counts, textile density and crimp, twill weave type, yarn finish, staple fiber strength, etc. [20] Consequently, it is not possible to compare accurately the achieved values with the literature. The mean value of the tensile strength of the reference sample decreases with the increase of the temperature. This tendency is observed both in the case of the reference and coated textile samples. According to the literature the tensile strength of the elementary flax does not change in this temperature region [21]. Possibly, this change could be induced by the effect of temperature and drying on the textile structure, similarly, as it has already been reported for the apparent tensile strength of cotton and rayon cords [22]. To the best of authors’ knowledge there is no previously published work on effect of temperature on the tensile strength of flax yarn structure. So, this latter assumption should be checked in a separate study. Tensile strength of the epoxy coated flax textiles at room temperature is more than twice as high as of the reference sample. At the same time, the strength decreases with the increasing temperature faster than in the case of the reference, i.e., at 60 ◦C the difference between the uncoated and coated textile is 49%. The storage modulus of epoxy resin and flax fiber-reinforced epoxy drops considerably J.J. Compos.Compos. Sci. 20202020,, 44, ,x 64 5 of 135 of 12

in this temperature region [23]. Therefore, the faster decrease rate is also associated with the inincreasing this temperature flexibility region of the coating. [23]. Therefore, Basically, the at fasterhigher decrease temperature rate the is also reinforcing associated effect with of the epoxy increasing flexibilityis reduced of and the the coating. textile strength Basically, of at the higher flax remains, temperature so the the values reinforcing are close e fftoect those of the of the epoxy reference. is reduced and the textile strength of the flax remains, so the values are close to those of the reference.

Figure 3. Tensile strength of flax textiles at 23 ◦C, 40 ◦C and 60 ◦C.

3.2. Digital MicroscopeFigure Imaging 3. Tensile of Flax strength Fabrics of andflax textiles Biocomposites at 23 °C, 40 °C and 60 °C. Mechanical properties of the coated textiles and the resulting composites are greatly influenced 3.2. Digital Microscope Imaging of Flax Fabrics and Biocomposites by the coating material and the corresponding spraying process. In order to evaluate the quality of the coatingMechanical and analyseproperties the of performance the coated textiles of the and epoxy the coatingresulting during composites heating, are thegreatly fracture influenced area of the textilesby the coating after tensile material testing and the was corresponding analyzed using spraying digital process. microscope. In order The to digitalevaluate microscope the quality imagesof ofthe the coating textiles and at analyse different the temperatures performance of are the represented epoxy coating at 100duringund heating, 200 themagnification fracture area in of Figure the 4. × × Thesetextiles images after tensile show testing that the was reference analyzed textileusing digital has same microscope. structure The at digital all tested microscope temperatures. images of These specimensthe textiles areat different associated temperatures with high fibrillation,are represented a small at 100 number× und of200 dust× magnification particles and in aFigure bright 4. color. TheThese epoxy-coated images show flax that textiles the reference are, in contrast, textile has darker same due structure to the coatingat all tested and showtemperatures. a splinted These fracture specimens are associated with high fibrillation, a small number of dust particles and a bright color. with numerous epoxy and dust particles on the fiber surface at 23 ◦C. The yarns of the specimen tested The epoxy-coated flax textiles are, in contrast, darker due to the coating and show a splinted fracture at 40 ◦C are glossy due to the softening of the epoxy coating. In the case of the specimen tested at with numerous epoxy and dust particles on the fiber surface at 23 °C. The yarns of the specimen 60 C the fibers of the individual yarns are glued together by the softened epoxy and single fibers tested◦ at 40 °C are glossy due to the softening of the epoxy coating. In the case of the specimen tested possess a small number of epoxy drops at the surface. These results show, that due to the use of spray at 60 °C the fibers of the individual yarns are glued together by the softened epoxy and single fibers coating and short curing cycle the epoxy-coating remains mainly at the outer layers of the textiles and possess a small number of epoxy drops at the surface. These results show, that due to the use of spray penetrationcoating and intoshort the curing yarns cycle takes the place epoxy during-coating the remains second heating,mainly at i.e., the hot outer pressing. layers of the textiles and penetrationThe spraying into process the yarns is one takes of place the most during common the second methods heating, used i.e., for hot textile pressing. finishing. Parameters like, forThe example,spraying process the rate is of one the of sprayingthe most common and process methods time used influence for textile the finishing. resulting Parameters properties and shouldlike, forbe example controlled, the rate during of the coating spraying [19 ].and In pro thecess case time of theinfluence flax fibers the resulting for composite properties application, and sprayingshould be technology controlled has during been coating used for [19] example. In the for case combined of the flax ultraviolet fibers for (UV) composite/water application, ageing [24] and asspraying an approach technology for the has enzymatic been used rettingfor example [25]. for In thiscombined study, ultraviolet the spraying (UV process)/water ageing used for [24] the and epoxy coatingas an approach of the textiles for the did enzymatic not cause retting any further [25]. In unexpected this study, the physical spraying changes, process i.e., used no changefor the ofepoxy the fiber structurecoating of could the textiles be observed did not usingcause any digital further microscopy unexpected or scanningphysical changes, electron i.e. microscopy., no change Potentially, of the suchfiber physical structure change could could be observed affect the using bonding digital mechanism microscopy between or scanning PA6 and electron coated microscopy. flax fabrics like, forPotentially, example, such promoting physical or change reducing could mechanical affect the interlockingbonding mechanism and, thus, between affect mechanicalPA6 and coated performance flax of the resulting composite.

J. Compos. Sci. 2020, 4, x 6 of 13 fabricsJ. Compos. like Sci., 2020for example,, 4, 64 promoting or reducing mechanical interlocking and, thus, affect mechanical6 of 12 performance of the resulting composite.

Figure 4. Digital microscope images of flax textiles after tensile testing. Epoxy drops are marked with Figure 4. Digital microscope images of flax textiles after tensile testing. Epoxy drops are marked with redred and and air air inclusions inclusions with white.

TheThe infiltration infiltration of of PA6 PA6 strongly affects affects the mechanical performance performance of of the the resulting resulting composites. composites. InIn or orderder to evaluate evaluate the the infiltration infiltration quality quality of of the the manufactured manufactured composites, composites, the the cross cross-section-section area area of of thethe specimens specimens was was investigated investigated using using digital digital microscope microscope imaging. The The digital digital microscope microscope images images at at twotwo magnifications magnifications are are represented represented in in Figure 55.. TheThe referencereference specimenspecimen hashas goodgood qualityquality withoutwithout porosity showing thatthat thethe pre-dryingpre-drying of of the the biocomposite biocomposite constituents constituents was was eff ective.effective. The The images images of theof thecross-section cross-section show show that the that specimen the specimen with coated with coated textiles textiles possesses possesses high degree high of degree porosity. of However,porosity. However,visually both visually reference both compositesreference composites and those containingand those containing coated flax coated fabrics flax did notfabrics have did any not defects. have anyThe defects. observed The air observed inclusions air could inclusions be induced could during be induced the spraying during process.the spraying Some process. of these Some air inclusions of these aircan inclusions be observed can in be Figure observed4 at 23in FigureC/100 4 .at Since 23 °C/100 the presence×. Since the of thesepresence pores of leadsthese topores reduction leads to of ◦ × reductionmechanical of properties,mechanicalthe properties, optimization the optimization of the spraying of the process spraying should process be investigated, should be investigated, in order to inimprove order to tensile improve properties. tensile This properties. study represents This study merely represents a one merely of the first a one feasibility of the first studies feasibility in this studiesfield focused in this onfield the focused effectiveness on the ofeffectiveness the epoxy coating of the epoxy on the coating durability on the and durability mechanical and performance mechanical performanceof the resulting of biocomposites.the resulting biocomposites. Consequently, Consequently, the effect of the the spraying effect of process the spraying should process be investigated should beseparately, investigated including separately, detailed including evaluation detailed of evaluation process parameters of process like, parameters for example, like, for the example, rate of the the ratespraying, of the dropspraying, volume, drop distance volume, to distance the substrate, to the etc. substrate, on the infiltrationetc. on the qualityinfiltration and quality the mechanical and the mechanicalproperties of properties the elementary of the flaxelementary fibers, yarns, flax fibers, textiles yarns, and textiles the resulting and the biocomposites. resulting biocomposites.

J. Compos. Sci. 2020, 4, 64 7 of 12 J. Compos. Sci. 2020, 4, x 7 of 13

FigureFigure 5. 5. DigitalDigital microscope microscope images images of of the the cross cross-section-section of of FLA FLA-PA6-PA6 and and FLA FLA-EP-PA6-EP-PA6 biocomposites. biocomposites.

3.3.3.3. Tensile Tensile Properties Properties of of Biocomposites Biocomposites TensileTensile properties properties of of the the flax flax-reinforced-reinforced PA6 PA6 conditioned for 250, 500 500 and and 1000 1000 h h under under ambient ambient andand thermo thermo-oxidative-oxidative conditions areare presentedpresented in in this this part. part. The The tensile tensile strength strength of of the the biocomposites biocomposites at atambient ambient conditions conditions is shown is shown in Figure in Figure6. The 6. tensile The tensilestrength strength of the reference of the reference sample at sample the beginning at the beginningof the experiment of the experiment is 69.2 MPa. is This69.2 isMPa. comparable This is comparable with the literature with the values literature reported values for twillreported weaved for twillflax-PA6 weaved biocomposites, flax-PA6 biocomposites, which range from which 32.2 range to 90.2 from MPa 32.2 [ 8 to] depending 90.2 MPa [8] on thedepending manufacturing on the manufacturingparameters. The parameters. hot-pressing The parameters, hot-pressing especially, parameters, the especially, temperature the (225 temperature◦C) and force (225 (100°C) a kN)nd forceused (100 during kN) hot used pressing during aff ecthot mechanicalpressing affect properties mechanical of the properties resulting composites,of the resulting since composites, the thermal sincedegradation the thermal of some degradation flax constituents of some starts flax already constituents at 159–175 starts◦C already and of the at 159 hemicellulose–175 °C and at 200of the◦C. hemicelluloseThis process isat accompanied200 °C. This process by emission is accompanied of volatile by organic emission compounds of volatile and organic unpleasant compounds smells and as unpleasantwell as a decrease smells asin mechanical well as a decrease performance in mechanical [6]. The biocomposites performance [6] with. The coated biocomposites textiles show with at coatedthe beginning textiles ofshow the experimentat the beginning 22 % andof the after experiment 1000 h 16% 22 higher% and strengthafter 1000 compared h 16% higher with reference.strength comparedSeveral eff withects leadreference. to this Several improvement effects lead simultaneously: to this improvement reinforcing simultaneously: effect of the reinforcing epoxy coating effect as ofpresented the epoxy above, coating increased as presented fraction above, of the increased reinforcing fraction components of the includingreinforcing flax components fibers and epoxy,including and, flaxfinally, fibers chemical and epoxy, reaction and, between finally, amine chemical groups reaction of PA6 andbetween epoxide, amine which groups takes of place PA6 at and temperatures epoxide, whichabove 200 takes◦C[ place26]. In at the temperatures case of the reference above 200 composite, °C [26]. (FLA-PA6) In the case the of flax the fiber reference weight composite, fraction is (FLA63 wt.%-PA6) and the in flax the casefiber ofweight the FLA-EP-PA6 fraction is 63 43 wt.% wt.%. and The in latter the case specimen of the containsFLA-EP- approx.PA6 43 wt.%. 31 wt.% The of latterepoxy. specimen The weight contains fraction app ofrox. the 31 fibers wt.%decreased of epoxy. The due weight to the coating fraction with of the the fibers epoxy decreased resin, and due thus to theresulted coating also with in the the increase epoxy resin, of the and overall thus composite resulted also weight in the from increase 160 g of to the 230 overall g, i.e., approximately composite weight 44%. fromConsequently, 160 g to 230 although g, i.e., the approx compositesimately with 44%. coated Consequently, textiles possess although higher the tensile comp properties,osites with theycoated are textilesheavier. possess At the samehigher time tensile due toproperties, the observed they porosityare heavier. and theAt generalthe same known time due diffi cultiesto the regardingobserved porositydensity measurementand the general of known the natural difficulties fibers [regarding27], it is not density possible measurement to define quantitatively of the natural density fibers [27] and,, itthus, is not the possible fiber volume to define fraction quantitatively of the composite density constituents, and, thus, the which fiber would volume simplify fraction accurate of the composite evaluation constituents,of the mechanical which performance. would simplify Although accurate conditioning evaluation of of the the specimens mechanical at ambient performance. conditions Although shows conditioninga slight tendency of the towardsspecimens improvement at ambient conditions of the tensile shows strength a slight and tendency a pronounced towards increase improvement of the ofmodulus the tensile (Figure strength7) takes and place a pronounced due to the physicalincrease ageingof the modulus process [ 28(Figure,29]. 7) takes place due to the physical ageing process [28,29].

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Figure 6. Tensile strength of the biocomposites at ambient conditions.

FigureFigure 6. 6. TensileTensile strength strength of of the the biocomposites biocomposites at at ambient ambient conditions. conditions.

FigureFigure 7. 7. TensileTensile modulus modulus of of the the biocomposites biocomposites at at ambient ambient conditions. conditions.

Tensile strength of the biocomposite samples in thermo-oxidative ageing oven are represented in Tensile strength of the biocomposite samples in thermo-oxidative ageing oven are represented Figure8. The resultsFigure show 7. Tensile that independent modulus of the of biocomposites the time line, at the ambient biocomposites conditions. with epoxy-coated in Figure 8. The results show that independent of the time line, the biocomposites with epoxy-coated flax textile possess higher strength than those without coating. The highest difference of 48% is flax textileTensile possessstrength higher of the strengthbiocomposite than samples those without in thermo coating.-oxidative The highestageing oven difference are represented of 48% is observed after 250 h. At the same time the tensile strength of both biocomposites in general decreases. inobserved Figure 8.after The 250 results h. At show the same that timeindependent the tensile of strength the time of line, both the biocomposites biocomposites in with general epoxy decreases.-coated This decrease is induced by the simultaneous ageing effect of all composite constituents. Since the flaxThis textiledecrease possess is induced higher by strength the simultaneou than thoses ageing without effect coating. of all Thecomposite highest constituents. difference ofSince 48% the is investigated composite represents a complex system consisting of natural fibers, thermoplastic and observedinvestigated after composite 250 h. At therepresents same time a complex the tensile system strength consisting of both ofbiocomposites natural fibers, in thermoplasticgeneral decreases. and thermoset, separate study should be carried out, in order to evaluate the effect of the ageing on each of Thisthermoset, decrease separate is induced study by should the simultaneou be carried out,s ageing in order effect to evaluateof all composite the effect constituents. of the ageing Since on ea thech the constituents. However, even after 1000 h the composite with coated flax textiles (76.0 MPa) shows investigatedof the constituents. composite However, represents even a aftercomplex 1000 system h the compositeconsisting withof natural coated fibers, flax textilesthermoplastic (76.0 MPa) and approximately 10% higher strength than the reference specimen (69.0 MPa) at the beginning of the thermoset,shows approx separateimately study 10% should higher bestrength carried than out, thein order reference to evaluate specimen the (69.0effect MPa) of the at ageing the beginning on each experiment, consequently presenting the improved durability of the composites with epoxy-coated of the experimentconstituents., c However,onsequently even presenting after 1000 the h improved the composite durability with coatedof the compositesflax textiles with (76.0 epoxy MPa)- flax textiles. showscoated approxflax textiles.imately 10% higher strength than the reference specimen (69.0 MPa) at the beginning of the experiment, consequently presenting the improved durability of the composites with epoxy- coated flax textiles.

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Figure 8. Tensile strength of the biocomposites under ageing conditions.

The tensile modulus of the samples after ageing is represented in Figure 9. The results show that the thermo-oxidativeFigureFigure conditioning 8. 8.Tensile Tensile strength strength improves of the tensile biocomposites biocomposites modulus under of under the ageing both ageing conditions.biocomposites. conditions. Thermal and thermo-oxidative ageing of PA6 and synthetic fiber-reinforced PA6 composites has been broadly TheThe tensile tensile modulus modulus of of the the samples samples after ageing ageing is is represented represented in Figure in Figure 9. The9. Theresults results show show that that described in previous studies [8,30,31]. Chain scissoring and crosslinking of the polymer molecules the thermo-oxidative conditioning improves tensile modulus of the both biocomposites. Thermal and theduring thermo-oxidative thermal ageing conditioning leads to the observed improves change tensile of the modulus mechanical of the properties. both biocomposites. [32] As a result, Thermal the andthermo thermo-oxidative-oxidative ageing ageing of of PA6 PA6 and and synthetic synthetic fiber fiber-reinforced-reinforced PA6 PA6 composites composites has been has beenbroadly broadly tensiledescribed strength in previous of the flax studies biocomposites [8,30,31]. Chain is reduced scissoring while and their crosslinking tensile modulus of the polymerincreases. molecules This is the described in previous studies [8,30,31]. Chain scissoring and crosslinking of the polymer molecules resultduring of thetherm thermalal ageing-embrittlement leads to the observed and oxidative change -ofembrittlement the mechanical mechanism properties. [32] associated As a result, with the the during thermal ageing leads to the observed change of the mechanical properties [32]. As a result, crosslinkingtensile strength of the of molecules the flax biocomposites [30] and thermo is reduced-chemical while ageing their tensileof flax. modulus increases. This is the the tensileresult of strength the thermal of the-embrittlement flax biocomposites and oxidative is reduced-embrittlement while their mechanism tensile modulus associated increases. with the This is the resultcrosslinking of the of thermal-embrittlement the molecules [30] and thermo and oxidative-embrittlement-chemical ageing of flax. mechanism associated with the crosslinking of the molecules [30] and thermo-chemical ageing of flax.

FigureFigure 9. Tensile modulus modulus of of the the biocomposites biocomposites under under ageing ageing conditions. conditions. Figure 9. Tensile modulus of the biocomposites under ageing conditions. DueDue to to thethe presencepresence of didifferentfferent laminate laminate layer layerss and and flax flax fibers fibers on on the the surfaces surfaces it is it difficult is difficult to to observeobserve diDue differencefference to the regardingpresence regarding of fiber-matrix differentfiber-matrix laminate adhesionadhesion layer usingsusing and flaxscanningscanning fibers electrononelectron the surfaces microscopy it is difficult conducted to on theon fractured observe the fractured difference surface surface ofregarding the of aged the fiber FLA-PA6aged-matrix FLA and-adhesionPA6 FLA-EP-PA6 and usingFLA- EPscanning laminate-PA6 laminatelectron samples.e microscopy samples. However, However, conducted the degree the of oxidativeon the degradation fractured surface of the of samplesthe aged wasFLA- examinedPA6 and FLA qualitatively-EP-PA6 laminat usinge samples. EDS. It has However, been observed the that the oxygen concentration on the surface of each sample increases with the increase of ageing duration from 250 to 1000 h, Figure 10. Each of the oxygen content measurements is an average of nine areal scans of 400 to 500 µm wide of the designated spots (Figure 10 top right). The large standard deviation of the percentage oxygen content for all samples was due to the different laminate layers and flax fibers on the fracture surfaces. It was observed that the oxygen content of both biocomposites J. Compos. Sci. 2020, 4, x 10 of 13

degree of oxidative degradation of the samples was examined qualitatively using EDS. It has been observed that the oxygen concentration on the surface of each sample increases with the increase of ageing duration from 250 to 1000 h, Figure 10. Each of the oxygen content measurements is an average J.of Compos. nine areal Sci. 2020 scans, 4, 64 of 400 to 500 µm wide of the designated spots (Figure 10 top right). The10 large of 12 standard deviation of the percentage oxygen content for all samples was due to the different laminate layers and flax fibers on the fracture surfaces. It was observed that the oxygen content of both wasbiocomposites different even was underdifferent the even lab conditions.under the lab The conditions. FLA-EP-PA6 The FLA exhibited-EP-PA6 lower exhibited oxygen lower content oxygen on thecontent fractured on the surfaces fractured as compared surfaces as to FLA-PA6. compared The to FL oxygenA-PA6. concentration The oxygen for concentration both biocomposites for both increasesbiocomposites with increasing increases ageingwith increasing duration. ageing Similar duration. results were Similar reported results in the were previous reported study in [33 the], whereprevious the study percentage [33], where oxygen the content percentage of the oxygen aged composites content of the were aged higher composites than the were lab-conditioned higher than compositethe lab-conditioned samples. Thecomposit oxygene samples. diffuses graduallyThe oxygen from diffuses edges ofgradually the samples from towards edges of the the core samples as the ageingtowards time the increasecore as the [29 ].ageing The FLA-EP-PA6time increase shows [29]. The in general FLA-EP lower-PA6 shows oxygen in concentration general lower than oxygen the FLA-PA6concentration laminates. than the The FLA presence-PA6 oflaminates. the epoxy The coating presence on the of fiberthe epoxy acts as coating additional on barrierthe fiber to hinderacts as theadditional oxygen di barrierffusion to within hinder the the biocomposite. oxygen diffusion This result within further the indicates biocomp thatosite. the This epoxy result coated further flax textilesindicates were that more the epoxy effective coated in retarding flax textiles the rate were of oxidationmore effective as compared in retarding to the the uncoated rate of samplesoxidation and as hencecompared improved to the theuncoated durability samples of the and biocomposites. hence improved the durability of the biocomposites.

FigureFigure 10.10. SchematicSchematic diagramdiagram ofof testedtested samplessamples inin ageingageing ovenoven andand oxygenoxygen concentrationconcentration analyzedanalyzed withwith scanningscanning electron electron microscopy–energy-dispersive microscopy–energy-dispersive X-ray X-ray spectroscopy spectroscopy (SEM–EDS) (SEM–EDS at the) at outer the outer layer oflayer the of aged the biocompositeaged biocomposite specimens. specimens. 4. Conclusions 4. Conclusions This study presents tensile properties of epoxy-coated flax textiles and PA6 reinforced with these This study presents tensile properties of epoxy-coated flax textiles and PA6 reinforced with these textiles before and after thermo-oxidative ageing. The results show that the coated textiles as well as textiles before and after thermo-oxidative ageing. The results show that the coated textiles as well as the corresponding biocomposites have considerably higher tensile properties than the reference under the corresponding biocomposites have considerably higher tensile properties than the reference all tested conditions. Several factors lead to this improvement simultaneously: reinforcing effect of the under all tested conditions. Several factors lead to this improvement simultaneously: reinforcing epoxy resin, increase of the reinforcement fraction in the biocomposites and chemical reaction between

PA6 and epoxy. Only a longer thermo-oxidative ageing leads to a slight decrease of the tensile strength and increase of modulus induced by thermal-embrittlement and oxidative-embrittlement mechanisms of the PA matrix and epoxy resin. J. Compos. Sci. 2020, 4, 64 11 of 12

Basically, this study shows that the approach followed has a great potential for the manufacture of biocomposites with improved mechanical and durability performance. Furthermore, it can contribute to the manufacture of the high-performance biocomposites based on renewable resources, since all of the used composite constituents are available commercially in a biobased or a partially biobased form. Since the used thermoset coating cannot be melted, its effect on the recyclability of the composites should be investigated, e.g., its effect on the cryogenic grinding and the reinforcing effect as a filler in extrusion. To sum up, this approach would broaden the application fields of biocomposites based on renewable resources. At the same time, there is still a great demand for further research in order to understand and further explore the individual factors leading to the improvement mechanical performance after the coating. The next studies will focus on: the optimization of the coating process, in order to reduce porosity and, thus, further improve the mechanical properties; investigation of the effect of ageing on the individual constituents of the composites, for example, using a design of experiments approach; and the effectiveness of the coating on the other woven arts like plain, satin or non-crimp textiles.

Author Contributions: Conceptualization, methodology, investigation and data curation M.S., B.P.C. and N.V.; writing—original draft preparation, M.S.; writing—review and editing, N.V., B.P.C., H.-J.E., A.M., M.M., M.S.; project administration, M.S.; resources and funding acquisition, H.-J.E., A.M., M.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially funded by the German Federal Ministry of Education and Research, grant number 031B0502 and from own institutional resources. Acknowledgments: The authors would like to thank Jana Winkelmann und Ricardo Wege for the manufacture of the weaved textiles for this study. A.K.M., M.M. and B.P.C. are thankful to the financial support from Ontario Research Fund, Research Excellence Program; Round-7 (ORF-RE07) from the Ontario Ministry of Research Innovation and Science (MRIS) (Project # 052644 and 052665); and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)-Canada/University of Guelph-Bioeconomy for Industrial Uses Research Program Theme (Project # 030251) to carry out this research. Conflicts of Interest: The authors declare no conflict of interest.

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