Novel Insight into the Separation and Composite Utilization of

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Novel insight into the separation and composite utilization of sclerenchyma fiber bundles of willow bark Jinze Dou, Jouni Paltakari, Leena-Sisko Johansson, and Tapani Vuorinen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04001 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Sustainable Chemistry & Engineering

Novel insight into the separation and composite utilization of sclerenchyma fiber bundles of willow bark Authorship. Jinze Dou*, Jouni Paltakari, Leena-Sisko Johansson, and Tapani Vuorinen*

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Vuorimiehentie 1, P.O. Box 16300, 00076 Aalto, Finland. *Corresponding author: Jinze Dou Tel: +358 504088797. E-mail:[email protected] Tapani Vuorinen Tel: +358 505160048. Fax: +358 9470 24259. E-mail:[email protected] Present address: Vuorimiehentie 1, Espoo, Finland

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ABSTRACT. The bark from fast-growing willow crops contains long and strong sclerenchyma fiber bundles, which could potentially replace the pulp and annual plant fibers currently used to reinforce green polymer composites. Here we successfully demonstrate the isolation of fiber bundles of willow bark with a simple alkali treatment under much milder conditions than what pulp fiber separation requires. The fiber bundles separated had hydrophobic surfaces, which made them compatible with polymers, in this case with polylactic acid, without using any additives. The most hydrophobic fiber bundles of willow bark provided the strongest and toughest composites, superior to the corresponding isotropic composites of pulp and flax fibers. Integration of the fiber bundle isolation with a prior recovery of hot water extractable aromatics from the bark and further processing of the debarked willow stems into bioethanol and lignin, for example, could make the full valorization of the willow biomass feasible. Keywords. Bark, Fiber bundle, Renewable resources, Sustainable chemistry, Willow.


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INTRODUCTION Wood and annual plants have been the dominant source for pulp, paper and board production from the past to the present. The biological origin of these materials make them renewable, recyclable and biodegradable which is required in today’s green society.1 Along with the rapid development of pulping technologies for isolating cellulose fibers from any lignocellulose to make paper, composites, textiles, etc., the recent trend has been away from native forests as the major fiber source towards use of fast-growing planted trees (e.g. eucalyptus, willow and poplar)2 and agricultural residues (e.g. rapeseed straw and pineapple leaves)3,4. Especially, deforestation in most of the world has decreased the supply of wood from natural forests for pulp production.5 Continuous bast fibers, formed by microscopic lignin-bound fibrous cells, provide the mechanical support to the soft outer regions of certain plants. Due to their high aspect ratio, stiffness, biodegradability and low cost, the bast fibers from non-wood plants, such as flax, hemp and ramie, have raised interest in increasing the strength and elasticity of polymers. On the other hand, their low impact strength, low durability and irregular fiber length, may limit the use of bast fibers as polymer reinforcement.6 Independent of their origin, pulp and bast fibers are typically polar on account of their hydroxyl groups that easily sorb moisture in contrast to thermoplastic polymers.7-9 Therefore, achieving the compatibility between the reinforcement and the thermoplastic matrix has been a major challenge. Grafting short-chain molecules onto fiber surfaces or using coupling agents, have been applied to improve the accessibility of polymer molecules by reducing the agglomeration tendency of filler particles.10-12 In addition, the utilization of sizing agents has been seen as another practical approach for modifying the natural fiber’s surface to improve accessibility.13 Bast fibers, or continuous fiber bundles (FB), of relatively long and thick-walled sclerenchyma fibers are characteristic of willow inner bark (WIB), which is additionally rich in aromatic



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extractives.14,15 In addition to WIB, the bark (WB) of fast growing willow crops contain a thin layer of outer bark. The Kraft cooking process is known as the most cost-efficient delignification method for producing paper-grade pulp in a huge industrial scale.16 In reality, the same economy of scale could not be applied for deconstructing WB, which forms only ca. 10 % of the fast growing willow crops. On the other hand, separation of the bast fibers by the traditional retting treatment is by far too slow to be industrially feasible.17 Therefore, we have undertaken an in-depth study on using a green, simple and low cost technology to separate FB from WB for composite application. Combining the fiber separation with recovery of the aromatic extracts of WB by a prior hot water extraction treatment and conversion of the debarked biomass into sugars, their fermentation products and lignin might make the overall fractionation approach feasible. MATERIALS AND METHODS Materials. All reagents were used as received unless described otherwise. The screener used for collecting the accepted fiber bundles was purchased from IKEA. One-year-old ‘Klara’ hybrid willow stems were harvested from a plantation of Pajupojat Oy-WillowPartners in Southern Finland (Kouvola, Finland) on 18 May, 2017. The bark and inner bark of the stems were peeled manually with a scalpel and stored at 20°C for further experiment. Hackled flax fibers were obtained from Metropolia University of Applied Sciences (Helsinki, Finland). According to SEM imaging, the width of the fibers varied in the range of 10-40 µm. Hydrogen peroxide (30%), magnesium sulfate (MgSO4), diethylenetriaminepentaacetic acid (DTPA), pure cellulose (Whatman) and sodium hydroxide pellets (NaOH) were purchased from VWR. Arabinose, rhamnose, galactose, xylan, dimethyl sulfoxide-d6 and pyridine-d5 were supplied from Sigma-Aldrich. Polylactic acid (PLA) grade Ingeo 3251D was obtained from NatureWorks. Methods


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Fiber bundle fraction and composite making. As depicted in Figure 1, tap water together with willow bark (WB) or willow innerbark (WIB) (liquid-to-bark ratio 20:1) were added in a temperaturecontrolled reactor for collecting the hot water extracts (HWE) after 20 mins incubation at 80°C. The alkaline treatments (AT) were experimented in a silicon oil-bath heated iron container (Haatotuote, model 43427) while the alkaline hydrogen peroxide treatments (AHPT) were carried out in plastic bags, which were placed in a preheated water bath (the detailed cooking recipes are seen in Tables S1 and S2, SI). MgSO4 was added to prevent catalytic decomposition of H2O2. Kraft pulp of hot water extracted WB (control) was prepared using an air-heated rotating autoclave (Table S6, SI). The spent liquor and fiber bundles (FB) were separated using a nylon filtration bag. The filtered FB was further washed using distilled water and then mixed with a mechanical mixer at 1000 rpm for 10 mins to liberate FB from their flocs. The diluted spent liquor was further dialyzed (> 6-8 kDa) and centrifuged before freeze-drying to obtain the high molecular weight (HMW) fractions for NMR analysis. The solid residue was screened using an IKEA filter (pore size of 2 mm) to separate FBs from impurities (i.e. outer bark, not well-cooked materials, parenchyma cells, fines etc). The FB sheet was formed with a Lorentzen&Wettre (L&W) laboratory hand sheet mold (type FI101) from diluted FB suspensions. The couched sheets were then wet-pressed with L&W SE040 press (2 min, 490 kPa) and finally dried with L&W drum dryer (type FI119) at drum surface temperature of 65°C. The sheets were cut into a customized size (14 cm x 4.4 cm). The grammages of the FB sheets were: ATWB 147.6 g/m2, ATWIB 162.9 g/m2, AHPTWB 124 g/m2, AHPTWIB 100 g/m2, Kraft HWEWB 138.9 g/m2. Before fabrication of the composite structures, the PLA powders and the FB sheets were stored at room temperature to reach equilibrium condition. The composite structures were fabricated by placing the customized FB sheet in the middle of two evenly distributed and pre-weighed polylactic



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acid (PLA) layers. Finally, the composite structure was assembled by heating the layered structure at 205°C under a pressure of 9.8 bar in a hot press (Fabriks Märke, Karlstad) for 2 minutes.

Figure 1. Experimental flow of fiber bundle separation and composite application. Processes with dashed lines were not carried out in this study. WB (Willow bark); WIB (Willow inner bark); HWEWB (Hot water extracted willow bark); HWEWIB (Hot water extracted willow inner bark). Ash determination. The carbohydrate and lignin contents of WB, WIB and FB made of them were characterized followed the reference standard (NREL/TP-510-42618). The sugar determination was performed using a HPAEC-PAD (Dionex ICS-3000, CarboPac PA20 column, Sunnyvale, CA, USA).18 Ash samples were digested with a mixture of nitric acid and hydrogen peroxide in a microwave oven (Milestone, Ethos). K and Na contents were determined with F-AAS (Varian 240), Ca and Mg contents were analyzed with ICP-OES (Perkin-Elmer, 7100 DV) after dilutions with MQwater.19 Morphology of fiber bundles and composites breaking fracture. Scanning electron microscopy (SEM) imaging was performed using a Zeiss Sigma VP. The WB fragments and composite fractures were sputter-coated with gold to ensure electric conductivity. The images were taken at 3-4 kV operating voltage.


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Chemical analysis. The solution-state NMR experiments were performed in DMSO-d6-pyridined5 solvent at 27°C on a Bruker Avance III spectrometer operating at 400.13 and 100.61 MHz for 1H and 13C, respectively. NMR data was processed using Topspin 3.0 software. Chemical shifts (δ) are denoted in ppm and coupling constants (J values) are given in Hz. The central DMSO solvent peak was used as internal reference (δ 13C 39.5, δ 1H 2.49 ppm). Phase-sensitive 2D 1H–13C HSQC spectra were acquired (spectral widths of 12 ppm for 1H and 220 ppm for 13C) using a relaxation delay of 2 s, 1K data points, 256 t1 increments, and 64 transients. An adiabatic version of the HSQC experiment was used (hsqcetgpsisp.2 pulse sequence from the Bruker Library). The spectral images were colored using Adobe Illustrator CS6 and chemical structures were redrawed using ChemDraw Pro 14.0. The solid-state 13C CP/MAS spectra were acquired on a Bruker AVANCE III spectrometer operating at 100.61 MHz, the freeze-dried samples were packed up using a 4.0 mm ZrO2 rotor. The following parameters were adopted: a relaxation delay of 5 s, a spectral width of 306 ppm, 40K transients of 2K data points, a contact time of 1 ms, and a spinning rate of 8 kHz. Surface lignin content. Surface lignin content of FB was evaluated with X-ray photoelectron spectroscopy (XPS), using an AXIS Ultra electron spectrometer by Kratos Analytical. For the experiment, the material was first acetone extracted20. FB were secured onto the XPS sample holder with UHV compatible carbon tape, together with an in-situ reference sample of pure cellulose filter paper (Whatman)21. Each sample batch was pre-evacuated overnight. XPS spectra were acquired from several locations (less than 1 mm analysis area) using low power monochromated X-ray irradiation at 100W and under naturalization. Both wide energy range elemental spectra (80 eV PE, 1 eV step) and high-resolution regional spectra (20 eV PE, 0.1 eV step) of C 1s and O 1s were recorded. As the samples were quite rough, some locations were charging; those data points were excluded from further analysis. After the measurements, atomic concentrations were calculated from the wide data and the high-resolution carbon C 1s signal was split into Gaussian components using



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CasaXPS software. Surface lignin content was calculated from the non-cellulosic component, with the help of the in-situ reference data.22 Tensile testing. Density of the composites was computed by dividing sample basis weight (TAPPI standard T410) by the thickness (TAPPI Method T411) and size (15 mm wide and 130 mm length) from the compression molded pieces. Tensile tests (ISO 527-2) were carried out using an MTS 400/M Vertical Tensile Tester (2 kN load cell, 12 mm min-1 cross-head speed and 100 mm clamp span). The tensile strength, tensile index, breaking strain, and elastic modulus were computed from the stressstrain curves. RESULTS AND DISCUSSION Prior to FB separation the valuable aromatic hot water extracts, consisting mostly of picein, triandrin and (+)-catechin, were removed from willow bark and inner bark in ca. 20% yield. Picein and (+)catechin can be used as antioxidants in the food industry while triandrin has been proposed for medical treatment of rheumatic conditions.15 Hot water extracted (HWE) WB and willow inner bark (WIB) were subjected to alkali treatments (AT) and alkaline hydrogen peroxide treatments (AHPT) at 80-120oC to enable separation of FB (Figure 1). The amount of NaOH required to accomplish the FB separation depended on temperature and the origin of the sample (WB or WIB) but was unaffected by the addition of H2O2 (Tables S1-S2). The latter observation was unexpected, as alkaline peroxide is known to oxidize certain lignin structures. At 100oC, 13 and 17 % dosages of NaOH were required to isolate FB of WIB and WB, respectively, to yield mostly (90-100 %) of individual FB (Table S2). Samples prepared with these alkali dosages were selected for more detailed chemical analyses and composite reinforcement testing. Under these conditions, the overall yield of AT and AHPT ranged from 30-35 % (HWEWB) to ≥ 40 % (HWEWIB). The screened yield of FB (accept) varied from ≤ 20 % (HWEWB) to 32-34 % (HWEWIB). FB are not present in the outer bark, which explains the lower yields from WB over WIB. In addition to ca. 100 µm wide individual and associated FB in the


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accept, fragments of parenchyma and outer bark were present in the reject fractions (Figure 2, Figures S1-S4).

Figure 2. Examples on willow bark fragments after alkaline treatment imaged by Scanning Electron Microscopy (SEM): a) individual fiber bundle; b) fiber bundle association; c) parenchyma cells mixed with fibers; d) inner (right) and outer (left) layers of outer bark. 13

C CP/MAS NMR spectroscopy provided an overall view on the chemical composition of the FB

accept fractions (Figure 3, Figures S5-S8). All NMR peaks were assigned based on the previous works reported earlier.23,24 The aromatic carbon signal pattern at 130-160 ppm showed that the residual lignin was of the guaiacyl-syringyl type and that the average residual lignin content was ca. 10 % or similar to the average Klason lignin content of FB (Figure 4b, Table S3). For comparison, HWEWB and HWEWIB contained, respectively, 32 and 24 % (Klason) lignin. Cellulose and hemicelluloses were the main components of FB with their characteristic signals at 60-110 ppm. The aliphatic carbon at 25-40 ppm originated possibly from suberin that is present in the outer bark.25 The signal at 58 ppm arised mainly from methoxyl groups of lignin and, in part, similar groups in hemicelluloses and suberin.



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Figure 3. 13C CP/MAS NMR spectra of fiber bundle fractions obtained through alkaline treatment of hot water extracted willow bark for 60 min at 100oC. Treatments: a) WB + 17 % NaOH (ATWB); b) WIB + 13 % NaOH (ATWIB); c) WB + 17 % NaOH + 4 % H2O2 (AHPTWB); d) WIB + 13 % NaOH + 4 % H2O2 (AHPTWIB). The monosaccharide composition of the acid hydrolysates of FB (Figure 4a) showed that the main polysaccharides of FB were cellulose and xylan. The content of the neutral monosaccharides that are characteristic of pectin (arabinose, rhamnose, galactose) was low in FB in comparison with the high content of these sugars in the original and hot water extracted WB and WIB. The degradation of the pectin in the alkaline conditions applied26 was confirmed by the fact that arabinan was the main component of the high molecular weight (HMW) fraction of the spent liquor from the alkaline treatments, separated by dialysis (> 6-8 kDa) and analyzed by 2D HSQC NMR spectroscopy (Figure S9). The typical 1H–13C correlations of syringyl or guaiacyl units were absent in the spectra27-30, suggesting that lignin was solubilized in lower molecular weight fragments. Statistically the quality of FB separation depended less on delignification than removal of pectin. Thus, the mild alkaline conditions applied were able to liberate FB from the parenchyma cells but not the sclerenchyma fibers from each other. The inefficiency of H2O2 to promote delignification and FB separation was unexpected. The presence of some manganese in the bark (Table S4) obviously catalyzed the decomposition of H2O2 and depolymerized cellulose to some extent (Table S5) even though MgSO4 was added as an inhibitor.


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Figure 4. (a) Carbohydrate composition and (b) overall chemical composition of willow bark and fiber bundle fractions made of it through 60 min alkaline treatments at 100oC. Sample codes: WB: willow bark; WIB: willow inner bark; HWEWB: hot water extracted WB; HWEWIB: hot water extracted WIB; ATWB: WB + 17 % NaOH; ATWIB: WIB + 13 % NaOH; AHPTWB: WB + 17 % NaOH + 4 % H2O2; AHPTWIB: WIB + 13 % NaOH + 4 % H2O2. Sugars: arabinose (Ara), rhamnose (Rha), galactose (Gal), glucose (Glc), xylose (Xyl), mannose (Man).

Hot water extraction reduced the acetone extract content of WB (20 %) and WIB (8 %) to the level of 3-4 % (Figure 4b). The alkaline treatments lowered the content further to below 1 % (Table S2). High-resolution X-ray photoelectron spectroscopy (XPS) of acetone-extracted FB provided data on O/C ratio and C 1s region of the spectrum (Figure 5a and Table S5). In principle, both information



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could be applied for quantification of lignin on the surface (top 10 nm)31. However, the O/C ratio and C-C carbon content did not correlate unambiguously (Figure 5b).20 Thus, surface lignin coverage was evaluated by Gaussian component fitting into from high-resolution C 1s spectra (Figure 5a).22,32,33 The lowest binding energy component CC at 285 eV was used as the marker for lignin. All the extracted FB samples had elevated levels of this non-cellulosic component, and it was highest for the ATWB, yielding a nominal lignin surface coverage of 40%, 10% higher than the other selected samples (30%). As the 13C CP/MAS NMR spectra showed (Figure 3), the samples studied here are more complex than just cellulose and lignin. Although the values presented may not denote the real surface lignin content, they at least qualitatively demonstrate the abundancy of hydrophobic components, such as lignin and suberin, on FB surfaces (see Figure 3 and Figures S10-S11).

Figure 5. (a) Normalized XPS spectra of extracted willow bark fiber bundles and a pure cellulose reference (Whatman). (b) Correlation plot of the O/C atomic concentration ratio with the percentage of aliphatic C1 carbon for willow bark fiber bundle samples. Also shown are theoretical points for a milled wood lignin and pure cellulose, determined by XPS. For abbreviations, see Figure 3.


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Inspired by their high hydrophobicity (surface C-C carbon content), FB webs were heat pressed with polylactic acid (PLA) to make FB/PLA composites. Flax fibers were applied as a reference, because they form a major part of global bast fiber production and because they have been commonly used as the reinforcement of polymers such as PLA.34,35 FB with the highest C-C carbon surface coverage (ATWB) provided the best reinforcement at a constant FB/PLA mass ratio of 25:75 (Figure 6, Table S6). Only flax fibers organized parallel with the direction of tension outperformed the isotropically oriented ATWB FB. FB/PLA mass ratio of 20:80 yielded the maximum increase in elastic modulus and tensile strength. Higher FB content obviously decreased the fiber-matrix contact through aggregation of FB as visualized in Figure S12 (see Figures S13-23 for detailed fracture images).

Figure 6. Reinforcement of PLA with willow bark fiber bundles (for abbreviations, see Figure 3), isotropical (FLAX_ISO), parallel to load (FLAX_MD) and antiparallel to load (FLAX_CD) flax fibers and kraft pulp of hot water extracted willow bark (Kraft HWEWB). The most hydrophobic FB (ATWB) formed also the toughest sheets, measured by the tensile energy absorption (TEA), with PLA outperforming even the flax fibers independent of their orientation



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(Figure S24). FB share in the range of 15-20 % maximized TEA and the bending stiffness of the material. In all the aspects the hydrophobic FB were superior to kraft pulp made of hot water extracted willow bark, which demonstrates in this case the advantage of using FB instead of delignifying them further to individual pulp fibers that are currently used in large-scale composite applications. Moreover, kraft pulping requires a higher chemical dosage, use of sodium sulfide, a much higher temperature and a longer reaction time (Table S6, SI). Figure 7 compares the tensile fracture surfaces of the composite sheets (25/75 mixing ratio between the fibers and PLA) imaged by SEM. The use of the most hydrophobic FB (ATWB) resulted in few FB pullouts and uneven fracture surfaces, which indicates good dispersion, compatibility and adhesion of FB with PLA (Figure 7a-d). The orientation of the fibers, exemplified with flax in Figure 7e-g, is another factor influencing the mechanical properties of the composite.34 Flax fibers organized parallel with the tension provided the composite the maximal strength and toughness and the tension subjected to the fibers is well visualized in Figure 7g. Therefore, good dispersion of hydrophobic FB and their favorable alignment should result in optimal mechanical properties.35 However, the experimental setup didn’t allow to demonstrate the effect of unisotropical alignment of WB FB in this study.

Figure 7. SEM images of the tensile fractured surfaces from fiber/PLA composites (mass ratio 25:75): a) ATWB; b) ATWIB; c) AHPTWB; d) AHPTWIB; e) FLAX_CD; f) FLAX_ISO; g) FLAX_MD; h) 100% ATWB. For abbreviations, see Figures 2 and 5.


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Conclusion In summary, our research demonstrates that fiber bundles (FB) suitable for polymer composites can be isolated from the bark of fast growing willow crops (WB) with a simple alkali treatment. The FB separation could be integrated with separation of valuable hot water extracts15 from WB and conversion of the debarked wood e.g. into bioethanol, pyrolysis oil or biochar. For more efficient chemical and energy recovery, the fractionation could be integrated with existing biorefineries such as kraft pulp mills. Due to their hydrophobicity, FB from WB (Figure S25) exhibit better compatibility with polymers, such as PLA, than the currently applied reinforcing cellulosic fiber sources, e.g. flax and kraft pulp, do. The performance of FB of WB could be improved further through controlling their alignment in the composites. Thus, in comparison with normal kraft pulps, WB fiber bundles can be prepared under much milder conditions and no compatibilizers are needed in making the composites.

Acknowledgements The authors thank Xin Xia and Kai Kang for their skillful experimental assistance in the alkaline treatments and composite making, Dr. Alp Karakoc for his valuable input in reinforcement alignments for composite studying and Rita Hatakka for her skillful technical assistance in HPAEC analysis. SEM imaging made use of the premises of Aalto University Nanomicroscopy Center (Aalto-NMC). Markku Suutari from Paju Pojat Oy kindly supplied the willow stems. JD was funded by a three-year grant from the Finnish Cultural Foundation. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org/. Preliminary cooking parameters and selected conditions; The willow bark fragments after cooking imaged by Scanning Electron Microscopy (SEM); 13C CP/MAS NMR Spectrum of the fiber bundles



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from the selected conditions; Comparison of lignin contents of FB samples from the selected conditions between 13C CP/MAS NMR spectroscopy and standard method (NREL/ TP 510-42618); Aromatic/unsaturated and side-chain (δC/δH 48-140/2.5-8.0 ppm) regions in the 2D HSQC NMR spectra of the high molecular weight (HMW) fractions from selected conditions: ATWB; ATWIB; AHPTWB; AHPTWIB; Metal elements in Klara bark and selected fiber bundle conditions determined by ICP-AES; The O/C atomic concentration ratio and the percentage of aliphatic C1 carbon for willow bark fiber bundle samples, determined by XPS; 13C CP/MAS NMR spectra of the xylan from birch wood (a), HMW fraction of dissolved material (b) and fiber bundles (c) from ATWB; 13

C CP/MAS NMR spectra of HMW fractions from the selected conditions: a) ATWB; b) ATWIB;

c) AHPTWB; d) AHPTWIB; Properties of fiber-PLA composite sheets; SEM images of tensile fracture surfaces of PLA and its composite with fiber bundles; SEM images of a tensile fracture surfaces from the fiber bundle composites together with the original PLA; Scatterplot of tensile energy absorbed (a) and bending stiffness (b) as function of fiber proportion in fiber-PLA composites; Photographs of a) willow bark fiber bundles; b) fiber bundle sheet; c) fiber bundle composite with PLA.


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Synopsis. Highly unconventional approach for fiber bundle separation from willow bark by judiciously using a mild alkali treatment and further fabrication of composites of the fiber bundles with polylactic acid.


Novel Insight into the Separation and Composite Utilization of

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