Assessment of Mechanical Property Variation of As-Processed Bast Fibers

sustainability

Article Assessment of Mechanical Property Variation of As-Processed Bast Fibers

Bryan Feigel 1, Hanami Robles 1, Jared W. Nelson 1,* , James M.S. Whaley 2 and Lydia J. Bright 3

1 Division of Engineering Programs, State University of New York at New Paltz, New Paltz, NY 12561, USA; [email protected] (B.F.); [email protected] (H.R.) 2 Sunstrand LLC, Louisville, KY 40206, USA; [email protected] 3 Department of Biology, State University of New York at New Paltz, New Paltz, NY 12561, USA; [email protected] * Correspondence: [email protected]

Received: 30 March 2019; Accepted: 7 May 2019; Published: 9 May 2019

Abstract: Hemp, flax, and kenaf are bast fibers with promising material characteristics to sustainably displace synthetic fibers used in composites; however, their use in composite applications is hindered by high material property variability. More widespread adoption and application, as well as improved quality methods, of fibers is contingent on the reduction of this variability. Efforts made herein to assess variability in as-processed fibers and methods were found to identify key sources of variability by investigating four areas: cross-sectional area approximation, physical defects, color and stem diameter, and fiber composition. Using fiber gage lengths closer to those found in composites, different geometric approximations of cross-sectional areas resulted in mean elliptical approximation showing the lowest variability across all fiber types. Next, by removing fibers exhibiting physical defects, maximum variation in tested flax fibers was reduced from 66% to 49% for ultimate tensile strength and 74% to 36% for elastic modulus. Additionally, fibers of darker color were found to have lower mechanical property variation than lighter or spotted fibers, and those coming from smaller stem diameters were found to be stronger than fibers from large stem diameters. Finally, contrary to previous findings with other lignocellulosics, clear trends between the lignin content in a fiber and its mechanical properties were not readily evident. Overall, these factors combined to significantly reduce mechanical property variation, while identifying the underlying contributing parameters.

Keywords: bast ﬁbers; mechanical properties; variation; modulus; tensile strength

1. Introduction Bast fibers, a subset of natural fibers, have the potential to sustainably displace synthetic fibers in polymer matrix composites [1–4]. Growth of natural fiber use in plastic is forecast at 15–20% annually, while in other areas, such as automotive applications and construction applications, this is forecast to increase by 15–20% and 50%, respectively [5]. Demand for natural fibers is driven by their competitive specific mechanical properties and low production cost [6,7]. A variety of additional factors, such as low energy consumption during production and sound absorbing efficiency, add additional momentum to the natural fiber demand [5]. However, product design for composite applications typically require material properties to be benchmarked values with a small, concretely defined variance. More importantly, industrial-scale manufacturing at rates of tons of fiber per hour requires identification of key parameters of variation to ensure that quality practices may be established, as current methodologies (e.g., single fiber testing) are not feasible at such high throughputs. Thus,

Sustainability 2019, 11, 2655; doi:10.3390/su11092655 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 2655 2 of 15 overcoming such practical impediments are critical in achieving widespread adoption as a fiber reinforcement for use in structural or semistructural engineered composite applications. Various properties of natural fibers processed using a variety of methods have been studied previously [5,8,9]. However, and notably, the values of the generalized mechanical properties range widely, by as much as a factor of 3 for the strength of flax and kenaf [8]. When considering individual test groups, reported values vary for both tensile strength and elastic modulus [10–21]. A specific study of flax [22] resulted in more consistent mean strength values of 718–874 MPa; however, the standard deviation for these same cases ranged from 232 to 290 MPa. It is commonly noted that the most important variables are the structure, chemical composition, microfibrillar angle, and cell defects [5,23]; however, these do not entirely account for the variations noted. As such, there appear to be three main factors which account for the variability of all-natural fibers: agronomic factors, physical factors, and processing factors [5,24]. Agronomic factors refer to the natural fiber’s variety, growing conditions, and position within the stem of the selected fiber. Charlet et al. showed that flax fibers from the middle region of the stem exhibit higher mechanical properties than those extracted from the bottom or top of the plant [25]. Additional variation is shown to result between varieties of flax fiber [26]. Growth conditions such as plant age at harvest, soil quality, weather, and use of fertilizer have been shown to affect the mechanical properties and composition of various natural fibers [27,28]. Physical factors include the fiber’s composition, microfibrillar angle, and diameter. Most bast fibers are composed of an outer wall of pectin surrounding cellulose and hemicellulose strands, held together by lignin [8]. In flax fibers, the percent weight breakdown is roughly 71% cellulose, 2.2% lignin, 18.6–20.6% hemicellulose, 2.3% pectin, 10% moisture, and 1.2% wax [5,29–36]. The microfibrillar angle, or angle observed between the axis along the fiber length and the fibrils of the fiber, also impacts the mechanical properties. As this fibril angle decreases, the mechanical properties increase [37]. Strong dependence of mechanical properties on stem diameter has also been established: fiber from a low stem diameter leads to higher Young’s modulus and ultimate tensile strength [25,38]. Physical defects within a fiber also contribute to the mechanical properties. Lastly, processing factors consist of chemical and physical treatments intended to alter the initial fiber conditions. They often aim to prepare fibers for usage within a specific application. For example, fibers used within textiles may be chemically treated with bleach or acidic compounds to condition their physical softness and color. Alternatively, natural fibers intended for use in composites are chemically surface treated to improve adhesion [39–42]. Such treatment is necessitated by the hydrophilic nature of natural fibers and hydrophobic nature of most composites. This creates difficulties when forming the composite matrix [7,28,43,44]. As the degree of polymerization of cellulose increases, the mechanical properties will also increase. The degree of polymerization (DP) of natural fibers varies depending on the species of the fiber; for example, the cellulose of flax averages around 8000 DP [28]. Detailed studies on the effects of pretreatments on the composition of non-bast natural fibers have been performed through the use of fluorescence confocal microscopy. Coletta et al. utilized this confocal microscopy to investigate the effect of alkali and acid pretreatments on the delignification of sugarcane bagasse [45]. Other non-bast natural fibers, such as agave plant fibers and wood tracheids, have also been investigated through fluorescence confocal microscopy [44,46,47]. The methods utilized by Hilda et al. were replicated in this study and applied to bast fibers in order to characterize the lignin content in commercially available fibers and investigate its relationship to material properties and their variation [46]. As noted, fiber variability results in wide-ranging reports of material properties for natural fibers, leading designers to oftentimes overdesign end-products by adding extra material or choosing simpler (i.e., more consistent), albeit less environmentally friendly, synthetic materials. This is particularly true with engineered fiber-reinforced plastics, which commonly require very small variations between fibers to reliably estimate their performance. Significant work has been performed to establish natural fiber performance, including investigating the accuracy of cross-sectional area [47,48]. Methods utilized back-calculated constituent properties from composite testing and developed a correction factor to Sustainability 2019, 11, 2655 3 of 15 address for inconsistent fiber cross-sectional area. While this novel approach is successful for the material system analyzed, the review by Shah et al. identified that back-calculating fiber properties leads to significantly different values than fiber testing [12]. While it is recognized that work is being performed in these areas, fundamental to all is the identification of the key parameters affecting the variation of mechanical properties for large-scale as-processed fibers. Establishing a top-down knowledge base that further identifies and understands these key parameters will lead to characterization of the effect of variations due to plant variety, growing conditions, retting, harvesting, and decorticating for specific uses. Understanding the performance of these material systems for use in composite applications will create the foundation for new design paradigms as well as new manufacturing and quality approaches. Further, this will enable improvement and expansion of optimized, engineered properties for the use of natural fibers, increasing commercial viability. Thus, the work herein contributes to the identification and understanding of key parameters affecting fiber performance. Building upon the works noted above, the aim of this study is to identify and analyze the key parameters toward characterizing bast fibers for use in long (>20 mm) fiber-reinforced composites. Further, this work utilizes as-manufactured fibers taken directly from current commercially available flax, hemp, and kenaf fibers, which are investigated simultaneously. Contributing factors to the variation were then considered using individual fiber types in order to characterize their effect on variation. The materials and methods utilized to conduct each of the processes in the analysis are commonly accepted for investigating bast and other natural fibers. The results and discussion of four main areas are investigated: formulation for the approximation of cross-sectional area of a fiber; exploration of the relationship between physical defects and mechanical property variation; investigation of relationships between fiber diameter, fiber color, and ultimate tensile strength; and exploration into microstructure and the resulting mechanical properties. Findings in each of these areas of investigation are then summarized.

2. Materials and Methods

2.1. Materials and Equipment This work utilized natural fibers as provided by Sunstrand LLC, located in Louisville, Kentucky, to investigate the variability of their fibers’ mechanical properties [49,50]. Technical fiber strands of greater than 25 micrometers in diameter were attached to plastic mounting tabs [51] using Loctite 3972 UV light curing adhesive (Henkel Corporation, Westlake, OH, USA). The diameters were measured using a Fiber Dimensional Analysis System and the supplied mounting tab system [51]. Common tweezers were utilized to aid the mounting process. Tensile testing was performed using these same mounted samples [51]. Confocal microscopy was performed using a Leica TCS SPE spectral confocal microscope (Leica Microsystems Inc., Buffalo Grove, IL, USA). Fibers were wetted for view on the confocal microscope using a standard micropipet.

2.2. Fiber Treatment The provided hemp, flax, and kenaf fibers were processed by Sunstrand before being supplied for testing. In all cases, fibers were grown and field-retted to their specification. Sunstrand’s large-scale proprietary processes of mechanically opening, cleaning, decorticating, and separating were utilized in all cases, producing fibers intended for use in long fiber composite materials (100–200 mm). This dry processing was complemented with chemical processing in the cases noted and bleached using a proprietary wet process to remove color and degum the fiber. All materials tested are commercially available from Sunstrand. Sunstrand’s proprietary process for the materials provided to this study follows a general three-step mechanical process, with an optional fourth cutting operation to reduce length, after field retting and before optional chemical processing. These steps start with bale opening, where stalks are mechanically Sustainability 2019, 11, x FOR PEER REVIEW 4 of 15 SustainabilitySustainability2019, 11, 2655 2019, 11, x FOR PEER REVIEW 4 of 15 4 of 15 The third step is referred to as refining and is where the remaining core is removed; fiber bundles are The third step is referred to as refining and is where the remaining core is removed; fiber bundles are unpackedopened and cut via belowpinned 600 cylinders mm. After which bale act opening, as combs, decortication, pulling and tearing where the fibers majority from of one core another. is opened via pinned cylinders which act as combs, pulling and tearing the fibers from one another. removedBleached from the fibers fiber move via the onward use of from beater primary drums mechanic and separational processing screens, to is a performed.two-step chemical The third processing Bleached fibers move onward from primary mechanical processing to a two-step chemical processing step is referredending towith as refiningfiber dewatering, and is where dryi theng, remaining and a final core pinned is removed; cylinder fiber to open bundles bundles are opened formed via as a result ending with fiber dewatering, drying, and a final pinned cylinder to open bundles formed as a result pinned cylindersof hydrogen which bonding. act as combs,The initial pulling chemical and tearingstep is scouring the fibers through from one the another. use of sodium hydroxide that of hydrogen bonding. The initial chemical step is scouring through the use of sodium hydroxide that Bleachedremoves fibers waxes, move oils, onward and pectin, from aiding primary the next mechanical process, processing the second tochemical a two-step step, chemicalwhere bleaching removes waxes, oils, and pectin, aiding the next process, the second chemical step, where bleaching processingis conducted ending with through fiber dewatering, hydrogen peroxide, drying, and changing a final pinned the color cylinder to varying to open degrees bundles depending formed on the is conducted through hydrogen peroxide, changing the color to varying degrees depending on the as a resultrequirements of hydrogen of bonding.the product. The initial chemical step is scouring through the use of sodium requirements of the product. hydroxide that removes waxes, oils, and pectin, aiding the next process, the second chemical step, where bleaching2.3. Fiber Characterization is conducted through hydrogen peroxide, changing the color to varying degrees 2.3. Fiber Characterization depending on the requirements of the product. 2.3.1. Preselection and Selection 2.3.1. Preselection and Selection 2.3. Fiber Characterization Single technical fibers were pulled from a processed bundle of loosely accumulated fibers Single technical fibers were pulled from a processed bundle of loosely accumulated fibers according to a standardized and documented internal lab procedure before observing the 2.3.1. Preselectionaccording andto Selectiona standardized and documented internal lab procedure before observing the morphology along the length of the specimen. Minimal additional separation took place at this point, Singlemorphology technical fibers along were the length pulled of from the aspecimen. processed Minimal bundle of additional loosely accumulated separation fiberstook place according at this point, and gage sections were chosen to be clear of unwanted strands and elemental fibers. A subset of fibers to a standardizedand gage andsections documented were chosen internal to be lab clear procedure of unwa beforented strands observing and theelemental morphology fibers. alongA subset the of fibers viewed under a digital binocular compound microscope at 5× to 20× magnification. The consistency length ofviewed the specimen. under a Minimal digital binocular additional compound separation microscope took place at at this 5× to point, 20× magnification. and gage sections The were consistency of the fiber’s shape was observed along its gage length and images were captured with the use of the chosen toof be the clear fiber’s of unwanted shape was strands observed and along elemental its gage fibers. leng Ath subsetand images of fibers were viewed captured under with a digital the use of the white light microscope. Fiber characteristics such as fraying, kinks, or inconsistencies, as defined in white light microscope. Fiber characteristics such as fraying, kinks, or inconsistencies, as defined in binocularTable compound 1, were microscoperecorded. While at 5 tothe 20 majoritymagnification. of fibers with The consistencythese characteristics of the fiber’s were shape tested, was in groups Table 1, were recorded. While× the majority× of fibers with these characteristics were tested, in groups observedwith along a white its gage light length microscopy and images selection were process, captured fibers with exhibiting the use of these the white characteristics light microscope. were discarded with a white light microscopy selection process, fibers exhibiting these characteristics were discarded Fiber characteristicsprior to testing. such as fraying, kinks, or inconsistencies, as defined in Table1, were recorded. While the majorityprior of to fibers testing. with these characteristics were tested, in groups with a white light microscopy selection process,Table fibers 1. Fiber exhibiting characteristics these characteristics with corresponding were discarded descriptions prior toand testing. representative white light Table 1. Fiber characteristics with corresponding descriptions and representative white light microscopy images (20× magnification) of flax. Table 1. Fibermicroscopy characteristics images with (20× correspondingmagnification) descriptions of flax. and representative white light microscopy images (20 magnification) of flax. × Characteristics Description Image Characteristics Description Image Characteristics Description Image

Uniform thickness Consistent Uniform thickness Consistent UniformConsistent thickness alongalong gage gage length length along gage length

Microfibrils Microfibrils Microﬁbrils Fraying peeling Fraying Fraying peeling peeling along lengthalong length along length

Observable Observable Observable corner or sharp corner or sharpcorner or sharp Kink Kink Kink change in change in direction change in direction direction

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Table 1. Cont. SustainabilitySustainability 2019, 11, x 2019 FOR, 11PEER, x FOR REVIEW PEER REVIEW 5 of 15 5 of 15 Characteristics Description Image

ObservableObservable Observable variation in variationvariation in in InconsistencyInconsistencyInconsistency diameter alongdiameter along diameterlength along length length

2.3.2. Mounting 2.3.2. Mounting Once observed,observed,Once observed, eacheach fiber fiber each was was fiber mounted mounted was usingmounted using a UVa UVusing curable curable a UV resin resincurable onto onto disposable resin disposable onto plastic disposable plastic tabs withtabs plastic tabs awith V-shaped a V-shapedwith groove, a V-shaped groove, as seen asgroove, in seen Figure in as Figure1 seen. The in mounting1. Figure The mounting 1. boardThe mounting ensured board ensured a board gage length ensureda gage of length a 20 gage mm of length to 20 be mm used of to 20 mm to forbe used tensile befor testing.used tensile for Whiletesting. tensile others Whiletesting. have others While notably have others testednotably have fibers notablytested using fibers tested a shorter using fibers lengtha shorterusing (4–10 a lengthshorter mm) (4–10 length to reduce mm) (4–10 mm) variation,to reduceto thisvariation,reduce longer variation, this length longer wasthis length longer chosen was length as itchos represents wasen aschos it aenrepresents length as it represents more a length common a more length to common the more composites common to the to the industrycompositescomposites [12 industry]. Given industry the[12]. intent Given [12]. of the the Given intent investigation, the of intent the investigation, itof was the deemedinvestigation, it was important deemed it was to deemedimportant assess the important fiberto assess as close to the assess the tofiber the as intended fiberclose asto useclosethe stateintended to the as possible.intended use state Afteruse as statepossibl mounting, ase. possibl After the e.mounting, excess After fibermounting, the length excess the was fiber excess cut length and fiber saved was length cut for was cut additionaland savedand analysis,for saved additional for such additional analysis, as confocal analysis, such microscopy. as confocalsuch as confocal microscopy. microscopy.

Figure 1.Figure Fiber mounted1. Fiber mounted onto V-shaped onto V-shaped tab (left) tab that (left) is thenthen that measuredmeasured is then measured usingusing dimensionaldimensional using dimensional analysis analysis (middle) (middle) beforebefore tensiletensile before testingtesting tensile isis testing performedperformed is performed (right).(right). (right).

2.3.3. Fiber 2.3.3. Dimensional Fiber Dimensional Analysis Analysis DimensionalDimensional data on data the cross on the sections cross sections of the fibersfibers of the were fibers collectedcollected were collected using a FiberFiberusing DimensionalDimensionala Fiber Dimensional AnalysisAnalysis System (FDAS7200)System (FDAS7200) [51]. [51]. Typically, Typically, [51]. Typically, the diameter the diameter of the fiberfiber of the was fiber the was mean the of mean at least of threeat least three measurementsmeasurements alongalong the the along length length the of of thelength the fiber fiber of [11 the ,[11,25,525 fiber,52]. 2]. In[11,25,5 thisIn this case,2]. case,In the this diametersthe case, diameters the at fivediameters at positions five positions at five along positions thealong gage thealong length gage the length were gage recorded were length recorded withwere a recorded laserwith micrometer.a laser with mi a crometer.laser At eachmicrometer. positionAt each positionAt the each fiber positionthe was fiber rotated, the was fiber allowing rotated, was rotated, theallowing varyingallowing the diameter varying the ofvaryingdiameter each position diameter of each to beofposition recorded.each position to be The recorded. cross-sectionalto be recorded. The cross-sectional areas The atcross-sectional each positionareas at along areaseach at each theposition lengthposition along were the averagedalong length the and werelength then averaged were used averaged to and determine then and used stress. then to used Severaldetermine to didetermineff erentstress. approximations Severalstress. Severaldifferent of different theapproximations cross-sectionalapproximations of areathe cross-sectional based of the on cross-sectional diameter area measurementsbased area on based diameter on were diameter measurements compared: measurements minor were circle, compared: were major compared: minor circle, minor rectangular,circle, majorcircle, and circle,major mean-elliptical. rectangular,circle, rectangular, Similarand mean-elliptical. workand mean-elliptical. in this areaSimilar has Similar includedwork in work approximationsthis inarea this has area included such has asincluded convex-hullapproximationsapproximations and super-ellipsesuch as suchconvex-hull [ 48as]. convex-hull Convex and hullsuper- and measurementellipse super- [48].ellipse was Convex not[48]. feasible Convexhull measurement utilizing hull measurement the FDAS7200, was not was not alsofeasible making utilizingfeasible the utilizing super-ellipse the FDAS7200, the FDAS7200, approximation also making also infeasible.makingthe super-ellipse the super-ellipse approximation approximation infeasible. infeasible. DimensionsDimensions taken by taken the FDAS by the measured FDAS measured diametri diametrical diametrical components cal components along the the along fibers’ fibers’ the gage gage fibers’ length. length. gage length. ApproximationsApproximations of the cross-sectional of the cross-sectional area consistedconsis areated consis of threeted of geometrical three geometrical shapes basedshapes on based these on these diameters:diameters: circular,circular, rectangular,circular, rectangula rectangula andr, ellipticaland r,elliptical and (Figure elliptical 2(Figure). The minor (Figure2). andThe 2). majorminor The circular andminor approximationsmajor and circularmajor circular usedapproximations theapproximations minimum used and the maximumused minimum the minimum diameters,and maximum and respectively. maximum diameters, diam The respectively.eters, rectangular respectively. The used rectangular the The average rectangular used of the all used the average averageof all diameters of all diameters measured measured for an individual for an individual fiber, while fiber, the while elliptical the elliptical used both used the bothminimum the minimum and maximumand maximum diameters diameters for the forminor the andminor major and axismajor lengths, axis lengths,respectively. respectively. An image An ofimage a of a representativerepresentative bast fiber bast can fiber be seen can bein Figureseen in 2, Figure with an2, withestimated an estimated pentagonal pentagonal fiber shape fiber shown shape shown below it.below Each it.of Eachthe fiber of the area fiber approximations area approximations are then ar shown,e then visualizinshown, visualizing the associatedg the associated error of error of each method.each method. Sustainability 2019, 11, 2655 6 of 15 diameters measured for an individual fiber, while the elliptical used both the minimum and maximum diameters for the minor and major axis lengths, respectively. An image of a representative bast fiber can be seen in Figure2, with an estimated pentagonal fiber shape shown below it. Each of the fiber areaSustainability approximations 2019, 11, x FOR are PEER then REVIEW shown, visualizing the associated error of each method. 6 of 15

Figure 2. Figure 2. A representationrepresentation of the methodsmethods for calculatingcalculating fiberfiber cross-sectional area is comparedcompared to aa hexagonal approximation. Calculated cross section was assumed to be constant along the fiber length. hexagonal approximation. Calculated cross section was assumed to be constant along the fiber length. 2.3.4. Fiber Tensile Testing 2.3.4. Fiber Tensile Testing Once the geometry of a fiber had been characterized, the fiber was tested to failure on a Linear ExtensometerOnce the (LEX820)geometry tensileof a fiber frame had [been51]. Usingcharacterize the equipmentd, the fiber noted, was tested test methods to failure were on a derived Linear fromExtensometer ASTM C1557 (LEX820) [53]. Duringtensile tensileframe [51]. testing, Using the th break-forcee equipment detection noted, was test setmethods to 2 gmf were for flaxderived and 5from gmf ASTM for hemp C1557 and [53]. kenaf, During as flax tensile was noted testing, to failthe morebreak-force progressively, detection whereas was set hemp to 2 gmf and for kenaf flax were and more5 gmf rapid. for hemp This and load kenaf, data was as flax then was used noted with to the fail appropriate more progressively, cross-sectional whereas area tohemp calculate and kenaf axial stresswere more and determine rapid. This mechanical load data was properties, then used specifically with the elastic appropriate modulus cross-sectional and tensile strength. area to calculate Broken samplesaxial stress were and kept determine for further mechanical analysis suchproperties, as confocal specifically microscopy. elastic Fibersmodulus were and generally tensile testedstrength. in groupsBroken ofsamples 20, with were total kept valid for tests further counts analysis identified such inas Tableconfocal2. When microscopy. an invalid Fibers test were occurred generally (e.g., griptested or in adhesive groups of failures), 20, with the total fiber valid was tests discarded counts andidentified replaced in Table unless 2. otherwiseWhen an invalid noted. test occurred (e.g., grip or adhesive failures), the fiber was discarded and replaced unless otherwise noted. 2.3.5. Confocal Microscopy and Image Processing 2.3.5. Confocal Microscopy and Image Processing Following the methods identified by Hilda et al., confocal microscopy was performed in order to determineFollowing a fiber’sthe methods lignin contentidentified and by distribution Hilda et al., [ 46confocal]. Excess microscopy fiber length was samples performed were in wetted order withto determine 8 µL of water a fiber’s in order lignin to content activate and the distributi autofluorescenton [46]. lignin.Excess Afiber Leica length TCS SPEsamples spectral were confocal wetted fluorescencewith 8 µL of microscopewater in order using to theactivate LAS Xthe software autofluorescent package waslignin. used A Leica to image TCS the SPE fibers’ spectral morphology. confocal Excitationfluorescence of themicroscope autofluorescent using componentsthe LAS X was software performed package using was an argon used laser to emittingimage the at 488fibers’ nm withmorphology. an intensity Excitation of 61.7 of and the detection autofluorescent range of components 525–610 nm, was with performed the gain set using to 992. an Theseargon laserlaser settingsemitting result at 488 in nm the with excitation an intensity of lignin, of 61.7 which and then detection emits within range theof 525–610 above green nm, with light the spectrum gain set and to can992. beThese detected. laser settings Single images result ofin thethe fiberexcitation along of its lignin, length which and at then each emits breakpoint within were the above taken atgreen the mid-planelight spectrum using and a frame can be averaging detected. value Single of images 6 to eliminate of the fiber nonspecific along its signal, length and and then at each exported breakpoint from thewere software taken at in the .tiff mid-planeformat. using a frame averaging value of 6 to eliminate nonspecific signal, and then ImageJexported was from utilized the software to post-process in .tiff format. the imaged fibers [54]. The image was imported as a .tiff file, adjustedImageJ to 32-bit, was utilized and set to to post-process a minimum the threshold imaged of fibe 9,rs with [54]. no The maximum image was threshold. imported This as a adjusted .tiff file, theadjusted image to from 32-bit, the and black set and to a green minimum image threshold seen on the of left9, with of Figure no maximum3 to the black threshold. and white This image adjusted on thethe right.image The from rectangle the black tool and was green then image used toseen outline on th thee left fiber of areaFigure in small3 to the segments. black and Figure white3 (right) image alsoon the shows right. how The the rectangle rectangles tool were was overlaidthen used on to a typicaloutline fiber the fiber after area adjusting in small the typesegments. and threshold Figure 3 of(right) the image. also shows Themeasure how thefeature rectangles then were counted overlaid the pixels on a oftypical highlighted fiber after (lignin) adjusting to nonhighlighted the type and (cellulosic)threshold of pixels the image. for each The rectangle measure and feature calculated then thecounted percent the area pixels of lignin.of highlighted Excel was (lignin) utilized to tononhighlighted combine the lignin(cellulosic) and cellulosic pixels for pixel each countrectangl ofe each and individualcalculated rectangularthe percent area portion of lignin. into a Excel total, andwas thenutilized calculate to combine an overall the lignin percent and area cellulosic of lignin. pixel count of each individual rectangular portion into a total, and then calculate an overall percent area of lignin. 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Figure 3. ConfocalConfocal microscopy microscopy image image of of a a representative representative fi ﬁberber (left) that is processed using ImageJ to determine lignin content (right).

3. Results Results and and Discussion Discussion InvestigatingInvestigating the causes causes of of high high variation variation in in natural natural fibers fibers consisted consisted of of several several hypothesis-driven hypothesis-driven experiments. To To begin, begin, various various cross-sectional cross-sectional area area approximations approximations were were hypothesized hypothesized to affect to aff theect standardthe standard deviation deviation with withrespect respect to the to mean the mean (variation). (variation). A cross-sectional A cross-sectional area approximation area approximation which minimizedwhich minimized variation variation was sought. was sought. Next, the Next, presence the presence of physical of physical defects defects such as such kinks, as kinks, fraying, fraying, and inconsistentand inconsistent thicknesses thicknesses believed believed to cause to cause variation variation in innatural natural fiber fiber properties properties were were investigated. investigated. Additionally, variationsvariations inin color color in in unbleached unbleached fibers fibers were were studied studied to to relate relate color color to theto the strength strength of the of thefiber. fiber. Finally, Finally, confocal confocal microscopy microscopy was utilized was utiliz to determineed to determine if lignin contentif lignin consistently content consistently indicates a indicatesfiber’s material a fiber’s properties. material properties. 3.1. Cross-Sectional Approximation 3.1. Cross-Sectional Approximation Using diametral and tensile data of the fibers tested, elastic modulus and tensile strength values Using diametral and tensile data of the fibers tested, elastic modulus and tensile strength values were calculated for each of the cross-sectional area shapes considered along with their standard were calculated for each of the cross-sectional area shapes considered along with their standard deviation. Results for both modulus and tensile strength for hemp, flax, and kenaf fibers are shown in deviation. Results for both modulus and tensile strength for hemp, flax, and kenaf fibers are shown Table2 for each cross-sectional area approximation. in Table 2 for each cross-sectional area approximation.

Table 2. Mean (standard deviation) of elastic modulus and tensile strength for each fiber type based on Table 2. Mean (standard deviation) of elastic modulus and tensile strength for each fiber type based all tests performed. on all tests performed. Minor Circle Major Circle Rectangular Elliptical Minor Circle Major Circle Rectangular Elliptical Approximation Approximation Approximation Approximation ApproximationElastic Tensile ElasticApproximationTensile ElasticApproximationTensile ElasticApproximationTensile Fiber Number of Modulus Strength Modulus Strength Modulus Strength Modulus Strength Type NumberValid Tests Elastic Tensile Elastic Tensile Elastic Tensile Elastic Tensile Fiber Type of Valid Modulus(GPa) Strength(MPa) Modulus(GPa) Strength(MPa) Modulus(GPa) Strength(MPa) Modulus(GPa) Strength(MPa) HempTests 296 (GPa) 8 (6) (MPa) 139 (119) (GPa) 61 (109) 937(MPa) (975) 15(GPa) (7) 249(MPa) (159) 21(GPa) (9) 337(MPa) (207) Flax 454 11 (11) 212 (212) 86 (71) 1255 (1086) 26 (15) 391 (287) 33 (19) 523 (381) HempKenaf 296 99 8 6(6) (5) 139 64 (119) (78) 61 30 (109) (29) 937 284 (330)(975) 11 15 (10) (7) 108249 (123) (159) 14 21 (13) (9) 138 337 (157) (207) Flax 454 11 (11) 212 (212) 86 (71) 1255 (1086) 26 (15) 391 (287) 33 (19) 523 (381) Kenaf 99 6 (5) 64 (78) 30 (29) 284 (330) 11 (10) 108 (123) 14 (13) 138 (157) Approximating the area as a circle using the minimum and maximum diameter of the fiber led toApproximating the largest and the smallest area as a material circle using property the minimum values, respectively. and maximum Rectangular diameter of and the ellipticalfiber led toapproximations the largest and resulted smallest in mid-range material modulus property and values, tensile strengths.respectively. These Rectangular results reflect and the elliptical expected approximationseffects on calculated resulted stress. in Emid-rangeffects on the modulus standard an deviationd tensile asstrengths. a percent These of mean results (variation) reflect didthe expectednot mirror effects the changes on calculated in material stress. property Effects values.on the Variationstandard indeviation both maximum as a percent and minimumof mean (variation)diameter approximations did not mirror the approached changes in 100%, material while property rectangular values. and Variation elliptical in approximations both maximum were and minimumlower. The lowestdiameter approximation approximations across allapproached fiber types was100%, the ellipticalwhile rectangular approximation. and In particular,elliptical approximationshemp had the lowest were variationlower. The of the lowest fiber typesapproximation tested. The across remaining all fiber high types variability was indicatedthe elliptical the approximation.need for further In investigation; particular, hemp however, had the ellipticallowest variation cross-sectional of the fiber area types was hereafter tested. The established remaining as high variability indicated the need for further investigation; however, the elliptical cross-sectional area was hereafter established as the default for calculating mechanical properties. This finding Sustainability 2019, 11, 2655 8 of 15

Sustainability 2019, 11, x FOR PEER REVIEW 8 of 15 the default for calculating mechanical properties. This finding reflects conclusions made by Virk et al., holdingreflects conclusions elliptical approximation made by Virk et. as al, the holding best estimation elliptical approximation given the impracticalities as the best estimation of convex-hull given estimationthe impracticalities [48]. of convex-hull estimation [48]. To betterbetter understand understand the the remaining remaining causes causes of the of variation the variation of the tensileof the strengthtensile strength values observed values forobserved each fiber,for each microscopy fiber, microscopy of post-failure of post-failure fibers was fibers performed was performed to identify to identify the breakage the breakage types. Twotypes. representative Two representative fibers arefibers shown are shown in Figure in Figure4, with 4, respective with respective tensile tensile strengths strengths of 153 of MPa 153 MPa and 41.8and MPa.41.8 MPa. Imaging Imaging of the of stronger the stronger fiber reveals fiber reveals what is what actually is actually a fiber bundlea fiber breakingbundle breaking into multiple into strandsmultiple (left). strands Alternatively, (left). Alternatively, imaging ofimaging the lower of tensilethe lower strength tensile fiber strength shows fiber a clear shows breakage a clear of whatbreakage appeared of what to beappeared a single to strand be a (right).single strand While normalization(right). While innormalization calculating stress in calculating should account stress forshould load account variations, for load it is apparentvariations, that it is there apparent are additional that there eff areects additional impacting effects fiber strength.impacting It fiber was positedstrength. that It was the high posited numerical that the variability high numerical in the obtained variability data in was the the obtained result of data physical was dithefferences result of in thephysical individual differences defects in and the fiber individual morphology defects of and each fiber fiber. morphology Thus, a larger of bundleeach fiber. will achieveThus, a higherlarger strengthsbundle will as loadachieve redistribution higher strengths may occur as load through redistribution the surrounding may fibers.occur Inthrough this vein, the moresurrounding detailed explorationsfibers. In this into vein, the emoreffects ofdetailed physical explorations defects and fiberinto morphologythe effects of were physical conducted. defects and fiber morphology were conducted.

Figure 4. White-light microscopy (at l0l0× (left) and 20× 20 (rig(right)ht) magnification) magnification) of of two two broken flax flax fibers fibers × × representative of thethe failurefailure variationsvariations notednoted inin allall fiberfiber typestypes tested.tested.

3.2. Observed Physical Defects The observed importance of physical differences differences in tested fibersfibers suggested the need for further examination of their impact on mechanical properties. It was hypothesized that variation of material properties couldcould be be limited limited by by accounting accounting for for physical physic dialff erencesdifferences in the in form the form of defects. of defects. Data were Data sorted were basedsorted onbased observed on observed characteristics characteristics made before, made during,before, andduring, after and each after fiber each was fiber tested. was To tested. minimize To variability, hemp fibers were utilized; thus, the baseline values used were 21 9 GPa for modulus and minimize variability, hemp fibers were utilized; thus, the baseline values used± were 21 ± 9 GPa for 337 207 MPa for strength, as shown in Table2. modulus± and 337 ± 207 MPa for strength, as shown in Table 2. Using the observations taken during fiber fiber selection (Table 11),), fibersfibers werewere firstfirst sortedsorted toto removeremove transient datadata duedue toto gripgrip failure failure or or unexplained unexplained loss loss of of load load carrying carrying capability; capability; however, however, there there was was no changeno change in the in the baseline baseline mean mean or standard or standard deviation. deviati Next,on. Next, only only specimens specimens of consistent of consistent thickness thickness along thealong length the length of the fiberof the were fiber considered, were considered, and while and there while was there no significantwas no significant change in change the modulus in the values,modulus the values, mean the of the mean strength of the increased strength increased to 390 MPa towith 390 MPa a negligible with a negligible drop in the drop standard in the deviationstandard (207deviation MPa to(207 206 MPa MPa). to Following 206 MPa). the Following results from the the results analyzed from hemp the analyz data, aed similar hemp investigation data, a similar of a subsetinvestigation of the flaxof a fibers subset tested of the was flax performed fibers test ased these was groupsperformed had theas these largest groups number had of the defect-free largest tests.number Separate of defect-free groups, whichtests. Separate varied in groups, both type whic andh processingvaried in both methods, type wereand processing tested and methods, analyzed. Inwere these tested datasets, and analyzed. variation In of boththese modulusdatasets, andvariat tensileion of strength both modulus before andand aftertensile filtering strength for before kinks, inconsistencies,and after filtering fraying, for kinks, and transient inconsistencies, data due fray to griping, failureand transient or unexplained data due loss to ofgrip load failure carrying or capabilityunexplained is shownloss of inload Figure carrying5. capability is shown in Figure 5. Sustainability 2019, 11, x FOR PEER REVIEW 9 of 15 Sustainability 2019, 11, 2655 9 of 15

Effect of Filtering for Observed Defects on Standard 90 Deviation (Flax Fibers) 80 Strength (Filtered) 70 Modulus (Filtered) 60 Strength (Unfiltered) Modulus (Unfiltered) 50

10 Standard Deviation (% Percent Mean) (% Percent Deviation Standard 0 5 6 7 8 9 10 11 12 13 14 15 Flax Fiber Group Number Figure 5. Comparison of strength and modulus standard deviations as a percent of the mean resulting Figure 5. Comparison of strength and modulus standard deviations as a percent of the mean resulting fromfrom observed observed defect defect filtering filtering for for several several groupsgroups ofof flaxflax fibersfibers as noted in Table 22.. Within the subset of flax fiber data, groups 5–9 and 11–14 consisted of 40 fibers selected by eye and Within the subset of flax fiber data, groups 5–9 and 11–14 consisted of 40 fibers selected by eye observed for the previously specified defects. Variation of modulus improved in groups 7 and 9 after and observed for the previously specified defects. Variation of modulus improved in groups 7 and 9 filtering,after filtering, with an with average an average improvement improvement of 6%. of Tensile 6%. Tensile strength strength variation variation dropped dropped in groups in groups 8 and 8 9, withand an 9, averagewith an improvementaverage improvement of 10%. Although of 10%. Alth variationough variation was reduced, was bothreduced, the rangeboth the and range magnitude and ofmagnitude variation in of modulus variation and in tensilemodulus strength and tensile remained strength high, atremained 20–74% andhigh, 29–66%, at 20–74% respectively. and 29–66%, It was theorizedrespectively. that observationsIt was theorized by eyethat were observations not sufficient by eye to observewere not all sufficient defects, to and observe as a result, all defects, unobserved and defectsas a result, were unobserved still accounting defects for were a significant still accoun portionting for of thea significant variation. portion of the variation. WhiteWhite light light microscopy microscopy waswas utilizedutilized asas aa more detaileddetailed method of of defect defect identification identification to to select select fibersfibers without without any any observed observed physicalphysical defects.defects. Groups Groups 10 10 and and 15 15 in in Figure Figure 5 consisted of of 10 10 fibers fibers selectedselected and and tested, tested, intending intending to lookto look more more closely closely at the at impactsthe impacts of these of these defects defects that resulted that resulted in groups in 5–9groups and 11–14,5–9 and respectively. 11–14, respectively. Observations Observations were still were made still by made eye forby filteringeye for filtering purposes, purposes, and because and defectsbecause were defects already were accountedalready accounted for in the for preselection, in the preselection, variation variation did not did change not change after after filtering filtering (see group(see group 15 of Figure15 of Figure5). However, 5). However, several several data points data inpoints group in 10group were 10 unusable were unusable due to gripdue failures,to grip andfailures, a resulting and a increaseresulting in increase variation in variation due to the due limited to the dataset limited size dataset is noted size is as noted these as fibers these were fibers not easilywere replaced.not easily replaced. CombiningCombining the the results results of of fibers fibers filtered filtered only only by eyeby observationseye observations and comparingand comparing them them to the to results the ofresults fibers of preselected fibers preselected with white with light white microscopy light microscopy shows thatshows more that accurately more accurately assessing assessing fiber defects fiber decreasesdefects decreases variation. variation. Fibers undergoing Fibers undergoing white light white microscopy light microscopy preselection preselection had variation had variation in tensile in strengthtensile strength of 49%, whileof 49%, those while only those filtered onlyby filtered eye observations by eye observations had variation had variation of 74%. Forof 74%. modulus, For variationmodulus, reduced variation from reduced 74% tofrom 36% 74% between to 36% eye be observationtween eye observation filtered and filtered white lightand white microscopy light preselectedmicroscopy fibers, preselected respectively. fibers, Overall,respectively. accounting Overall, for accounting material defectsfor material reduced defects the magnitudereduced the of standardmagnitude deviations of standard with deviations respect to with the meanrespect value to the of mean both modulusvalue of both and modulus strength. and strength.

3.3.3.3. Stem Stem Diameter Diameter and and FiberFiber ColorColor StemStem diameter diameter (stems (stems< 1<1 cm, cm, 1–2 1–2 cm cm stems, stems, and and 2–3 2–3 cm cm stems) stems) of hempof hemp fibers fibers grown grown together together were tested.were tested. After testing After testing at least at 60 least fibers 60 from fibers each from of each these of groups these groups and using and the using elliptical the elliptical cross-sectional cross- areasectional approximation, area approximation, the modulus the valuesmodulus and values tensile and strengths tensile strengths were found were to found decrease to decrease as diameter as Sustainability 2019, 11, 2655 10 of 15 Sustainability 2019, 11, x FOR PEER REVIEW 10 of 15 diameterincreased, increased, matching matching the diameter the diameter trends noted trends previously noted previously [22,38]. While[22,38]. this While was this not was a new not finding, a new finding,it confirmed it confirmed the fiber the testing fiber methodstesting methods utilized utilized herein. herein. AnalysisAnalysis was was performed performed based based on on the the observed observed color color of of each each fiber fiber (i.e., (i.e., light, light, spotted, spotted, and and dark), dark), whichwhich may may be be associated associated with with degree degree of of retting retting [4 [444].]. Unbleached Unbleached hemp hemp from from the the 296 296 records records grouped grouped togethertogether in in Table Table 22 werewere separatedseparated andand sortedsorted accordingaccording toto thesethese threethree categorizations.categorizations. Mean and standardstandard deviation ofof each each group group was was determined. determined. Little Little difference difference from thefrom overall the overall mean and mean standard and standarddeviation deviation in Table2 in above Table was 2 above noted was for thenoted light for and the spotted light and groups, spotted though groups, the though dark group the dark was groupnoted was to have noted reduced to have standard reduced deviations. standard Thesedeviations. were furtherThese were reduced further when reduced the same when defect the groups same noted above were deselected, resulting in 22 6 GPa and 296 135 MPa for modulus and strength, defect groups noted above were deselected, resu± lting in 22 ± 6 ±GPa and 296 ± 135 MPa for modulus andrespectively. strength, Itrespectively. is notable that It is 12% notable of the that records 12% analyzedof the records consisted analyzed of the consisted darker fiber of the color, darker indicating fiber color,that darker indicating fibers that are darker less common. fibers are To less better commo understandn. To better these understand implications these as well implications as those from as well the assources those offrom variation the sources already of variation observed, already the composition observed, of the the composition fiber was taken of the into fiber consideration was taken into with considerationthe use of confocal with the microscopy. use of confocal microscopy.

3.4. Fiber Composition 3.4. Fiber Composition Given the observation that defect-free, dark-colored hemp fibers have reduced mechanical property Given the observation that defect-free, dark-colored hemp fibers have reduced mechanical variation, fiber composition was investigated using fluorescence confocal microscopy. Using methods property variation, fiber composition was investigated using fluorescence confocal microscopy. Usingsimilar methods to those usedsimilar in [46to ],those the fibers’ used components in [46], the were fibers’ revealed components using autofluorescence were revealed at using green autofluorescenceemission wavelengths. at green Theemission dark wavelengths. region of the The fiber dark is representativeregion of the fiber of theis representative cellulose, while of the the cellulose,bright-green while portions the bright-green are the lignin portions components are the throughoutlignin components the fiber. throughout The pectin the wall fiber. is not The shown pectin in wallconfocal is not microscopy, shown in confocal allowing microscopy, the clear view allowing of the internalthe clear components view of the ininternal the fibers. components Using confocal in the fibers.microscopy, Using theconfocal compositions microscopy, of a varietythe compositions of previously of a broken variety fibers of previously were analyzed. broken Infibers particular, were analyzed.the percent In area particular, of lignin the of apercent given fiber area was of lign foundin of and a comparedgiven fiber with was tensile found strength and compared (Figure6 )with and tensileelastic strength modulus (Figure (Figure 6)7). and elastic modulus (Figure 7). Comparison of Tensile Strength with Lignin Content 1200 Flax (Unbleached) Flax (Bleached) Hemp 1000 Kenaf

800

600 R² = 0.0656

400 Tensile Strength (MPa) Strength Tensile R² = 0.0706 200

R² = 0.3283 0 0% 10% 20% 30% 40% 50% 60% 70% Area Lignin Content

Figure 6. Comparison of change in the area of lignin content to tensile strength of various ﬂax, hemp, Figure 6. Comparison of change in the area of lignin content to tensile strength of various flax, hemp, and kenaf ﬁbers with weak trends shown (no trend shown for kenaf due to limited data points). and kenaf fibers with weak trends shown (no trend shown for kenaf due to limited data points). Sustainability 2019, 11, x FOR PEER REVIEW 11 of 15

Characterization of several fibers of each type and corresponding material property data provided the opportunity to establish trends between lignin content and tensile strength of the different bast fibers. The standard error of regression values (R2) of each of the fiber types were very low, with the exception of kenaf, which only had two data points. These results were not significant enoughSustainability to establish2019, 11, 2655 an identifiable trend relating the lignin content and tensile properties of 11these of 15 natural fibers. Comparison of Elastic Modulus with Lignin Content 8

7 Flax (Unbleached) Flax (Bleached) Hemp 6 Kenaf

5 R² = 0.053 4

R² = 0.038 Elastic Modulus (GPa) Elastic 2

1 R² = 0.697

0 0% 10% 20% 30% 40% 50% 60% 70% Area Lignin Content

Figure 7. Comparison of change in the area of lignin content to elastic modulus of various flax, hemp, Figureand kenaf 7. Comparison fibers with of weak change trends in the shown area (no of lignin trend co shownntent forto elastic kenaf duemodulus to limited of various data points). flax, hemp, and kenaf fibers with weak trends shown (no trend shown for kenaf due to limited data points). Characterization of several fibers of each type and corresponding material property data provided the opportunityComparing the to establish percent area trends of betweenlignin in ligninthe same content fibers and with tensile their strength respective of theelastic diff erentmodulus bast yieldedfibers. Thesimilar standard results error to that of regressionof the tensile values streng (R2th.) of The each highest of the standard fiber types error were of veryregression low, with value, the withexception the exception of kenaf, of which kenaf, only belonged had two to data bleached points. flax These and resultswas 0.70. were This not reflects significant a weak enough trend to correlatingestablish an increasing identifiable lignin trend content relating to thean increase lignin content in modulus. and tensile The remaining properties fiber of these types natural showed fibers. no trend,Comparing indicated by the standard percent error area ofof ligninregression in the valu samees fibersof less withthan their0.1. Ov respectiveerall, the elastic estimations modulus of ligninyielded content similar did results not correlate to that of with the the tensile material strength. properties The highest of the standardnatural fibers. error of regression value, withNoting the exception the relationship of kenaf, to belongedfiber color, to color bleached and composition flax and was were 0.70. also This compared. reflects aFive weak fibers trend of eachcorrelating color noted increasing above ligninwere tested content and to averaged. an increase Using in modulus. a subgroup The of remaining the hemp fiberfiber typesdataset showed in the colorno trend, section indicated above, byit was standard noted errorthat the of regression dark fiber valueshad a lower of less percent than 0.1. lignin Overall, content the estimations(3.0%), while of thelignin light content fiber had did a not higher correlate percent with area the of material lignin (34.5%). properties The of lignin the natural content fibers. of the spotted fiber was notedNoting to be between the relationship the light toand fiber dark color, fibers color (29. and3%). composition These data wereappear also to compared.complement Five the fibersresults of notedeach colorabove, noted where above less werevariation tested was and noted averaged. to be as Usingsociated a subgroup with the ofdarker the hemp fiber color, fiber dataset and suggest in the thatcolor lignin section content above, may it was impact noted variation. that the dark fiber had a lower percent lignin content (3.0%), while the lightThus, fiber hadit was a higher deemed percent appropriate area of to lignin investigate (34.5%). the The variation lignin content of the oflignin the spottedand cellulosic fiber was content noted alongto be betweenthe length the and light through and dark the fibersthickness (29.3%). of each These fiber. data Looking appear at to Figure complement 8 below, the it results is seen noted that ligninabove, content where changes less variation along wasthe length noted toof bethe associated fiber. As a with result, the a darkerfiber which fiber has color, a low and percent suggest area that oflignin lignin content overall may may impact still have variation. cross sections along the length of dramatically higher lignin content. In practice,Thus, that it was means deemed a given appropriate estimate toof investigatethe lignin percentage the variation over of a the segment lignin andof the cellulosic fiber might content not accuratelyalong the reflect length a and low through point where the thickness breakage ofcould each occur. fiber. In Looking a similar at fashion, Figure8 the below, variation it is seen of lignin that lignin content changes along the length of the fiber. As a result, a fiber which has a low percent area of lignin overall may still have cross sections along the length of dramatically higher lignin content. In practice, that means a given estimate of the lignin percentage over a segment of the fiber might not accurately reflect a low point where breakage could occur. In a similar fashion, the variation of lignin Sustainability 2019, 11, x FOR PEER REVIEW 12 of 15

content throughout the thickness of the fiber may similarly affect the failure. Observing the failure point of the fibers, it was seen that the location of the break changed through the thickness of the fiber. This indicates that the sample did not have a clean fracture, but instead was jagged due to progressive fracture. Considering the lignin contents associated with the colors noted above, the reduced lignin content associated with the darker color appears to be related to the reduced variation, Sustainability 2019, 11, 2655 12 of 15 while the increased lignin of the light and spotted fibers relates to higher variation. Further investigation of these relationships, including validation of the lignin and cellulose approximations contentfor these throughout bast fibers, theis warranted. thickness of the fiber may similarly affect the failure. Observing the failure pointFurther of the fibers, refinement it was of seen the thatcellulose-to-lignin the location of theinvestigative break changed methods through in combination the thickness with of thephysical fiber. Thisdefect indicates control thatare thesuggested sample didas continuing not have a cleansteps fracture,toward understanding but instead was fiber jagged variability. due to progressive This will fracture.allow for Consideringeffectively studying the lignin the contents impacts associated of fiber type, with thegrowing colors location, noted above, and theretting reduced process lignin on contentvariation associated of mechanical with properties. the darker colorIn addition, appears these to be are related trackable to the characteristics reduced variation, that would while lend the increasedtoward automated, lignin of the in-line light and quality spotted methods fibers relates attainable to higher at the variation. throughputs Further needed investigation for economic of these relationships,feasibility. Thus, including as more validation is learned ofabout the these lignin and and other cellulose sources approximations of variation, natural for these fibers bast may fibers, be isfully warranted. realized as replacements for synthetic materials in engineered applications.

Figure 8.8. Fluorescence image of fiberfiber break point above (left)(left) and below (right) the mid-plane of the fiber,fiber, indicatingindicating variationvariation inin ligninlignin throughthrough thethe fiberfiber thickness.thickness.

4. ConclusionsFurther refinement of the cellulose-to-lignin investigative methods in combination with physical defect control are suggested as continuing steps toward understanding fiber variability. This will allow In order for bast fibers to reach their potential for engineered uses, particularly as sustainable for effectively studying the impacts of fiber type, growing location, and retting process on variation replacements of synthetic fibers in composite applications, performance characterization of the fibers of mechanical properties. In addition, these are trackable characteristics that would lend toward is needed to reduce variation in mechanical properties. Building upon previous findings, this work automated, in-line quality methods attainable at the throughputs needed for economic feasibility. Thus, investigates the root causes of four main sources of mechanical property variability of large-scale, asas more is learned about these and other sources of variation, natural fibers may be fully realized as processed fibers: cross-sectional area approximation, physical defects, color, and fiber composition. replacements for synthetic materials in engineered applications. In line with previous findings, mean elliptical approximation of cross-sectional area provides 4.mechanical Conclusions properties with the lowest standard deviation as a percentage of the mean [48]. Variation of fiber mechanical properties can be reduced by accounting for various physical defects, including In order for bast fibers to reach their potential for engineered uses, particularly as sustainable kinks, geometric inconsistencies, fraying, and transient data due to unexplained loss of load-carrying replacements of synthetic fibers in composite applications, performance characterization of the fibers capability. Further, hemp fibers of darker color were found to have reduced standard deviation when is needed to reduce variation in mechanical properties. Building upon previous findings, this work compared to lighter or spotted fibers, even though the color was not found to significantly affect the investigates the root causes of four main sources of mechanical property variability of large-scale, mean strength or modulus of the fiber. as-processed fibers: cross-sectional area approximation, physical defects, color, and fiber composition. Finally, comparing modulus with lignin content for bleached flax fibers, a slight increase in In line with previous findings, mean elliptical approximation of cross-sectional area provides modulus was noted as the percent area of lignin increased. Excepting this weak trend, no other trends mechanical properties with the lowest standard deviation as a percentage of the mean [48]. Variation could be established between the lignin content and either tensile strength or modulus in any of the of fiber mechanical properties can be reduced by accounting for various physical defects, including other natural fibers. kinks, geometric inconsistencies, fraying, and transient data due to unexplained loss of load-carrying capability. Further, hemp fibers of darker color were found to have reduced standard deviation when compared to lighter or spotted fibers, even though the color was not found to significantly affect the mean strength or modulus of the fiber. Finally, comparing modulus with lignin content for bleached flax fibers, a slight increase in modulus was noted as the percent area of lignin increased. Excepting this weak trend, no other trends could be established between the lignin content and either tensile strength or modulus in any of the other natural fibers. Sustainability 2019, 11, 2655 13 of 15

Author Contributions: Conceptualization, J.W.N. and J.M.S.W.; Formal analysis, B.F. and H.R.; Investigation, B.F., H.R., and L.J.B.; Methodology, B.F., H.R., J.W.N., J.M.S.W., and L.J.B.; Project administration, J.W.N.; Resources, J.W.N., J.M.S.W., and L.J.B.; Supervision, J.W.N.; Writing—original draft, B.F., H.R. and J.M.S.W.; Writing—review & editing, J.W.N., J.M.S.W., and L.J.B. Funding: This research was funded in part through the SUNY New Paltz AC2 Summer Research Program (2018). Acknowledgments: The authors would like to acknowledge the contributions of Sigmund Bereday, Mark Castellanos, and Matthew Pero, who participated in experimentation and preliminary analysis. In addition, the authors acknowledge the support from the AC2 Program and Nancy Campos (Director), which allowed for this work to be initiated. Finally, this work would not have been possible without the contributions of Trey Riddle, Natalie Hendrix, and the rest of Sunstrand LLC’s engineering team. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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