Enzymatic-Assisted Modification of ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Enzymatic-Assisted Modification of Thermomechanical Pulp Fibers To Improve the Interfacial Adhesion with Poly(lactic acid) for 3D Printing Daniel Filgueira,*,† Solveig Holmen,‡ Johnny K. Melbø,§ Diego Moldes,† Andreas T. Echtermeyer,‡ and Gary Chinga-Carrasco*,§ †

Department of Chemical Engineering, Edificio Isaac Newton, Lagoas-Marcosende s/n, University of Vigo, 36310 Vigo, Spain Department of Mechanical and Industrial Engineering, NTNU, Richard Birkelandsvei 2B, 7491 Trondheim, Norway § RISE PFI, Høgskoleringen 6b, 7491 Trondheim, Norway ‡

S Supporting Information *

ABSTRACT: The present study is about the enzymatic modification of thermomechanical pulp (TMP) fibers for reduction of water uptake and their use in bio-based filaments for 3D printing. Additionally, TMP was used as a fiber reinforcing material and poly(lactic acid) (PLA) as the polymer matrix. The hydrophilic TMP fibers were treated via laccase-assisted grafting of octyl gallate (OG) or lauryl gallate (LG) onto the fiber surface. The modified TMP fibers showed remarkable hydrophobic properties, as demonstrated by water contact angle measurements. Filaments reinforced with OG-treated fibers exhibited the lowest water absorption and the best interfacial adhesion with the PLA matrix. Such higher chemical compatibility between the OG-treated fibers and the PLA enabled better stress transfer from the matrix to the fibers during mechanical testing, which led to the manufacture of strong filaments for 3D printing. All of the manufactured filaments were 3D-printable, although the filaments containing OG-treated fibers yielded the best results. Hence, laccase-mediated grafting of OG onto TMP fibers is a sustainable and environmentally friendly pathway for the manufacture of fully bio-based filaments for 3D printing. KEYWORDS: Laccase, Grafting, Octyl gallate, TMP, PLA, Biocomposites, 3D printing

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flour as well as different types of wood fiber composites with PLA and polypropylene (PP) matrixes. Thermomechanical pulp (TMP)−PLA biocomposites showed the best fiber dispersion and mechanical properties. Faludi et al.8 examined the interface of biocomposites consisting of PLA and six different lignocellulosic fibers. The bond between PLA and natural fibers was found to be strong. Peltola et al.9 also examined the influence of fiber refining on the morphology and properties using long pine TMP fibers with a PLA matrix. They found that the mechanical properties were better with TMP fiber, high volume fractions of fibers, and large fiber aspect ratios. These characteristics are also well-known from traditional short fiber composites made from inorganic fibers and petroleum-based matrix materials. Therefore, the combination of PLA as a thermoplastic matrix reinforced with TMP fibers would lead to the manufacture of a fully bio-based composite with promising mechanical properties.

INTRODUCTION

Biocomposites. A biocomposite is a material consisting of at least one phase that is biologically derived.1 Usually the dispersed phase is a biofiber, and the matrix phase may be composed of a bioplastic that originates from “green” sources.2,3 When both the matrix and the dispersed phase are derived from biological sources, the biocomposite is also classified as fully bio-based. Thermoplastics such as poly(lactic acid) (PLA), starch, and lignin are the main matrixes used in fully bio-based composites, while wood, flax, and hemp fibers are the main dispersed phases.2,4 The properties of a given biocomposite depend on various aspects, including the structure and composition. Wood fibers have varying diameter and length, which influence the corresponding strength of the biocomposites. The higher the aspect ratio, the greater is the reinforcing effect. Additionally, a higher fraction of fibers will enhance the mechanical properties, e.g., mechanical strength, stiffness, and toughness.5,6 On the basis of the mechanical advantages of wood pulp fibers compared with, e.g., wood flour, various wood pulp fibers have been tested for biocomposite manufacturing. Peltola et al.7 examined the reinforcing effect of wood fibers contra wood © 2017 American Chemical Society

Received: July 13, 2017 Revised: September 1, 2017 Published: September 7, 2017 9338

DOI: 10.1021/acssuschemeng.7b02351 ACS Sustainable Chem. Eng. 2017, 5, 9338−9346

Research Article

ACS Sustainable Chemistry & Engineering Fiber Modification. Apart from the structure of the fiber, the interfacial bonding between the fiber and the polymer is vital to acquire good reinforcement from the dispersed phase and a strong biocomposite. Because of poor compatibility between the fiber and matrix, methods to improve the adhesion between the phases (also called adhesion promoters) are often required. Chemical alteration of the fiber is an example of this.4 Adhesion promoters not only contribute to a better-reinforced biocomposite but also can contribute to a reduction in water absorption and an improvement of processability.10 One limitation of wood pulp fibers with respect to their application in biocomposites is the hydrophilic and hygroscopic nature of the lignocellulosic fibers, i.e., they absorb water. Water absorption may lead to weakening and reduce the dimensional stability of biocomposites. This may be considered a drawback in certain applications where durability is a requirement. However, water absorption due to the fiber content may also be an advantage in biocomposites that are required to be biodegradable. Water absorption may lead to increased biological activity and thus potentially accelerate biodegradation. Several physical and chemical methods for the modification of lignocellulosic fibers have been reported.11 Most of them involve the use of oil-derived reagents and/or high consumption of energy and time. An alternative and sustainable process is the laccase-assisted modification of lignocellulosic fibers. Laccase is an enzyme that can carry out one-electron oxidation of phenolic substrates. Therefore, laccase may activate wood fibers by means of oxidation of phenolic moieties in lignin.12 Laccase can expand their oxidation capability by means of the so-called laccase mediator system (LMS). A mediator forms a radical when it reacts with the enzyme. Additionally, this radical can undergo oxidation reactions that cannot be carried out by the enzyme. This radicalized mediator can access locations that are not accessible to the enzyme because of its larger size. In addition, laccase is able to graft some hydrophobic compounds onto lignocellulosic surfaces.13 This strategy has been used to improve the interfacial adhesion of jute fibers to polypropylene and epoxy matrixes.14−17 Laccase has also been used for other applications regarding wood fibers, e.g., biobleaching, removal of lipophilic extractives, and production of particle boards from TMP fibers.18−21 3D Printing. Additive manufacturing (AM), also known as 3D printing, is based on a layer-by-layer production process that facilitates the production of objects in three dimensions. There is a broad spectrum of technologies within AM, including fused deposition modeling (FDM), selective laser melting (SLM), selective laser sintering (SLS), stereolithography (SLA), and electron beam melting (EBM), among others. Additionally, AM technology has been broadly applied in areas ranging from the medical sector to the aerospace and automotive industries, with applications depending on the technology and the material used. With different AM technologies, there are various materials that can be used, e.g., plastic, metal, ceramic, glass, and composites.22 The most frequently applied process when it comes to thermoplastics is FDM. During the FDM process, the 3D printing unit presses the thermoplastic through a nozzle and prints a given part layer-by-layer using a bottom-to-top approach. Every layer in the structure is built up by threads, and the printing pattern can be raster, contours, or a combination of these two.23 The application of FDM processes for 3D printing of biocomposites containing wood pulp fibers

has several challenges that should be considered in this respect. First, wood pulp fibers have a low thermal degradation temperature, which should not be exceeded in the FDM process. This restricts the use of FDM to certain polymers with low enough processing temperatures.24 Second, 3D printing generally uses a nozzle of minor size (0.4 mm), which can cause problems when printing filaments containing wood pulp fibers. Finally, poor fiber distribution in the filaments may lead to blockage in the minor nozzle. Strong and homogeneous biocomposites with low water uptake would offer a new grade of filaments for 3D printing by FDM. Hence, in the present study TMP fibers were modified via laccase-assisted grafting of octyl gallate (OG) and lauryl gallate (LG). Before the enzymatic hydrophobization of the fibers, lipophilic extractives were removed from the TMP fibers by treatment with laccase and a low-molecular-weight phenolic compound. Hence, for the first time a full enzymatic treatment to remove and graft target compounds onto the surface of TMP fibers was performed. The modified TMP fibers were used as a fiber reinforcing material for the manufacture of PLA-based biocomposite filaments. The mechanical properties and water absorption of the filaments were measured. The extruded filaments were used for 3D printing by FDM technology. The tensile strength of 3D-printed dog bone samples and the quality of the 3D-printed objects were assessed.

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MATERIALS AND METHODS

Materials. PLA pellets (Ingeo Biopolymer 4043D) were purchased from Nature Works. Spruce TMP fibers were kindly provided by Norske Skog Saugbrugs (Halden, Norway). Laccase from Myceliophthora termophila (NS51003) was provided by Novozymes (Bagsværd, Denmark). The activity of the enzyme was calculated by the 2,2′azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) oxidation assay. One unit of activity was defined as the amount of enzyme that oxidized 1 μmol of ABTS/min at 25 °C and pH 7 (0.1 M phosphate buffer). All of the other chemical reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA) at reagent grade and used without further purification. Modification of TMP Fibers. Spruce TMP fibers (18 g of ovendried pulp (odp)) were suspended in 1.8 L of phosphate buffer (0.1 M, pH 7) at 50 °C. After 30 min under constant agitation, 27 mL of laccase enzyme (175 units/g of odp) and vanillin (V) or guaiacol (G) (10 mM) were added to the solution. Thirty minutes later, 80 mL of an acetone solution containing octyl gallate (OG) or lauryl gallate (LG) (0.15 M) was added to the system. The reaction was conducted for 2 h at 50 °C under permanent agitation. After the enzymatic treatment, the fibers were dried at room temperature for 24 h and washed with 1.8 L of distilled water/acetone solution (60:40 v/v) during 1 h at 50 °C. Finally, the fibers were extensively washed with distilled water and dried at 50 °C for 12 h. Treatments without addition of laccase and/or vanillin or guaiacol were carried out as control tests. Formation of TMP Sheets and Measurement of Water Contact Angle. Sheets were prepared from enzymatically modified TMP fibers. The fibers (1 g of odp) were suspended in distilled water (0.4% consistency) and filtered through a cellulose filter (ø = 50 mm, pore ø = 0.45 μm) in a Büchner funnel under vacuum. The resultant sheets were dried at 50 °C during 4 h. Sheets with unmodified TMP fibers were prepared as a reference. The hydrophobicity of the TMP sheets was evaluated by measurement of the water contact angle (WCA). A drop of distilled water was deposited on the surface of the TMP sheet and the contact angle was measured with a MobileDrop GH11 goniometer (KRÜ SS). Contact angle measurements were carried out with intervals of 15 s during 2 min. The drop shape was analyzed with the DSA2 software (KRÜ SS). 9339


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ACS Sustainable Chemistry & Engineering Fiber Morphology. The TMP fibers were stained with bromine and assessed as described by Reme et al.25 Bromine reacts with the lignin in the fiber structure. TMP fibers before and after grinding were assessed by scanning electron microscopy (SEM) on a Hitachi SU3500 scanning electron microscope in secondary electron imaging (SEI) and backscatter electron imaging (BEI) modes to assess the fiber morphology and the fiber surface areas rich in lignin. The fibers were additionally embedded in epoxy resin and prepared for cross-sectional analysis as described by Reme et al.25 Additionally, fibers before and after grinding were assessed with a fiber tester device (L&W FiberTester Plus, code 912. Software: version 4.0−3.0). In order to assess the fiber width and length distributions 40 mL suspensions (0.1% solid content) were run through the fiber tester in standard mode. Extrusion of Biocomposite Filaments. PLA filaments were reinforced with enzymatically modified TMP fibers. Control filaments with unmodified TMP fibers and without fibers were also prepared. An overview of the series that were prepared is given in Table 1.

modeled using a three-dimensional computer-aided design (CAD) program. The software used for the 3D printing was Cura 15.04. Tensile Testing. The tensile strength properties of the extruded biocomposite filaments and the printed dog bones (PLA, P10OGT, and P20OGT) were determined with a Zwick Roell Proline analyzer and a 2.5 kN load cell, combined with the software TestExpertII. Four test specimens of each filament sample were cut to 70 ± 1 mm. The test speed was set to 20 mm/min and the initial position to 340 mm. The grip distance was 50 mm. The filament samples after tensile testing were attached to a pinch and further coated with gold, using an Agar automatic sputter coater. The fracture surfaces were visualized with SEM in SEI mode. The acceleration voltage was 5 kV, and the working distance was 5−17 mm.

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RESULTS AND DISCUSSION Fiber Morphology. Greater fiber length has in earlier studies been found to yield better mechanical properties, but on the other hand, they cause difficulties in dispersion and may yield higher amounts of fiber agglomerates in the biocomposite.7,9,26,27 In this study, the fibers were milled to 30 mesh to ease the processing during filament manufacturing, increase fiber dispersion, and avoid clogging of the 3D printing head. The reduction in fiber length due to the grinding can be observed in Figure 1. The average fiber lengths of TMP and

Table 1. Composition of Biocomposite Filaments for 3D Printing series

PLA (%)

TMP fiber (%)

TMP fiber modification

PLA P10T P20T P10LGT P20LGT P10OGT P20OGT

100 90 80 90 80 90 80

− 10 20 10 20 10 20

− − − LG LG OG OG

In order to obtain a homogeneous blend, PLA pellets and TMP fibers were ground in a Thomas Wiley Mini-Mill cutting mill to 10 and 30 mesh, respectively. The milled PLA and the fibers were oven-dried at 105 °C for 1 h, and the blending was performed at two different TMP fiber loads, 10% and 20%. The blend was extruded in a Noztek Xcalibur extruder, which has three different heating chambers for the total control of the extrusion temperature. The temperatures were set in decreasing order (175, 170, and 165 °C) to obtain filaments with low porosity and a diameter of approximately 2 mm. The speed of the screw extruder and the fan were set at 12 mm/s and 65%, respectively. All of the filaments were spooled with a Filabot spooler at the output of the extruder. Because of some fiber clots in the PLA filaments, especially with fiber loads of 20%, a second extrusion was carried out. The filaments obtained after the first extrusion were cut into small pellets (10 mm length), which were extruded again under the same conditions as for the first extrusion. Water Uptake. The hydrophobicity of the filaments was assessed by measuring their water absorption. Four samples (ø = 2.2 ± 0.1 mm; length = 70 ± 2 mm) of each filament were submerged in 50 mL of distilled water in a Falcon tube for 32 days. Prior to initiation of the water absorption test, the samples were completely dried at 105 °C for 4 h, and the dried weight (W0) was measured. The samples were weighed every 24 h (Wi). The water uptake of the filaments (in wt %) was calculated using the following equation:

water uptake =

Wi − W0 × 100 W0

Figure 1. SEM images of TMP fibers (left) before and (right) after milling.

ground TMP were 1.5 and 0.4 mm, respectively. The corresponding diameters were 33 and 38 μm, respectively. The increase in diameter is probably due to opening of the fiber walls due to the grinding process. Additionally, the fines fraction increased from 43% to 70% after grinding. The TMP fibers contain lignin, which is exemplified as brighter areas in Figure 2. The lignin-containing areas correspond to part of the middle lamella of the fiber walls. SEM in BEI mode produces images where the contrast is determined by the average atomic weight of the local areas in the specimen.25 Bromine reacts with lignin and increases the average atomic weight of the lignin-rich areas, which thus appear brighter when observed in SEM BEI mode.

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3D Printing. PLA filaments (ø = 2.2 ± 0.1 mm) reinforced with TMP fibers were used to print 3D model figures (ø = 15 mm) and dog bones (length = 80 mm, width = height = 3 mm) on an Ultimaker Original 3D printer. Only dog bones of the series PLA, P10OGT, and P20OGT were tested because of limited filament amounts, and these series appeared to be the most interesting to assess in this study. The 3D printer was equipped with a nozzle having a diameter of 0.4 mm. The print speed and temperature were set at 15 mm/s and 210 °C, respectively. The design of a model figure and dog bones were

Figure 2. TMP fibers stained with bromine: (left) surface images acquired in SEM BEI mode; (right) cross-sectional images. The white arrows indicate bright areas rich in lignin. 9340


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ACS Sustainable Chemistry & Engineering Water Contact Angle. The WCA was measured in order to assess the hydrophobicity of the TMP sheets. The higher the WCA, the higher is the grafting of the hydrophobic compounds on TMP fibers. Unmodified TMP fibers showed highly hydrophilic behavior, since the average time for the complete absorption of a droplet of water was below 20 s (Figure 3). Such behavior was expected

molecular size (60−100 kDa). However, such steric hindrance could be solved with the laccase mediator system, since smallsized mediators may penetrate the fiber wall structure. Two different low-molecular-weight phenolic compounds were assessed as mediators: guaiacol (G) and vanillin (V). The used mediators have low redox potentials (0.454 and 0.620 mV, respectively). This parameter is extremely important since lowredox-potential laccases like that from Myceliophthora thermophila cannot oxidize high-redox-potential mediators.34 The grafting of LG onto TMP fibers seems to be slightly improved since the WCA was also improved by means of the laccase mediator system reaction using vanillin (denoted as “L+V +LG”). Nevertheless, when guaiacol was used as the mediator (denoted as “L+G+LG”), the grafting of LG and the hydrophobicity of the TMP fibers was remarkably enhanced. In fact, the WCA of the TMP sheets remained practically constant along the WCA test. Guaiacol and vanillin are phenolic compounds with a methoxy group at the ortho position (Figure 4). In addition,

Figure 3. Water contact angles of the sheets prepared with TMP fibers that were treated with laccase (L), vanillin (V), guaiacol (G), octyl gallate (OG), and/or lauryl gallate (LG). TMP = untreated TMP fibers.

since spruce TMP fibers have a carbohydrate content of approximately 70%.28 On the other hand, hydrophobization of TMP fibers was performed in preliminary experiments by means of the enzymatic treatment with LG (denoted as “L+LG (not preactivated)”). However, the surface hydrophobicity was very low since the WCA was insignificant after 30 s of testing. Therefore, a different strategy was used in order to enhance the hydrophobicity of the TMP fibers. Laccase was added to the fiber dispersion 30 min before addition of the LG (denoted as “L+LG (preactivated)”). It is known that the maximum number of phenoxy radicals in laccase-treated TMP fibers is obtained after 30 min of reaction.29 Therefore, the objective was to preactivate the fibers by producing as many phenoxy radicals as possible prior to addition of the hydrophobic compound. The WCA of the resultant sheets indicated that the grafting of LG onto TMP fibers was remarkably improved with respect to non-preactivated fibers. Nevertheless, the fibers did not show the expected results in comparison with previous studies.13,30 It is worthy of notice that the enzymatic activation of mechanical pulps is extremely influenced by the chemistry and physical accessibility of the fiber’s surface lignin.29 In addition to lignin, the extractives in the fiber surface are increased during the mechanical pulping.31 The agglomeration of such extractives on the fiber surface may form films that could limit the enzymatic oxidation of lignin.32 In addition, because of their low redox potential, laccases cannot directly oxidize rigid structures like lipophilic extractives.33 The combination of laccase with some synthetic or natural low-molecular-weight phenolic compounds (mediators) is an eco-friendly approach for the direct removal of lipophilic extractives from the fiber surface.19,34 The mediators work as an electron carrier between laccase and the lipophilic extractives, enabling their oxidation and improving their removal.35 At the same time, the penetration of laccase within the fibers is limited by its large

Figure 4. Chemical structures of guaiacol and vanillin.

vanillin possesses an aldehyde group at the para position. The presence of an electron-accepting group (i.e., the aldehyde group) at the para position could destabilize the phenoxy radicals, reducing their oxidation capacity.28 Hence, it seems that guaiacol has a high capacity to oxidize and remove lipophilic extractives from the TMP fiber surface, since the initial WCA of the TMP sheets treated with only laccase and guaiacol (“L+G”) was significantly lower than the WCA on the untreated TMP fibers (Figure 3). Moreover, TMP fibers were Soxhlet extracted with hexane in order to remove the lipophilic extractives. After the extraction process, the fibers were enzymatically treated with LG without addition of guaiacol, and the resultant sheets showed remarkable hydrophobic behavior (data not shown). Thus, the guaiacol-mediated reaction seems to enable the removal of lipophilic extractives from the fiber surface, which allows the activation of phenolic lignin moieties for the grafting of the hydrophobic compounds. For the enzymatic hydrophobization of the fibers, gallate compounds (OG and LG) are especially interesting. These phenolic compounds are more or less hydrophobic depending on the length of their aliphatic chain. In this way, the hydrophobicity of the enzymatically treated fibers may be tailored depending on the grafted gallate.36 Therefore, LG and OG were tested for the hydrophobization of TMP fibers. The WCA of the TMP sheets made from OG-treated fibers was slightly lower than the WCA yielded by the LG-treated fibers. OG possesses a shorter aliphatic chain than LG (Figure 5). Hence, the WCA on the modified TMP sheets was in accordance with the length of the aliphatic chain of the grafted compound. 9341


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Figure 5. Scheme of the laccase-mediated grafting of LG or OG onto lignocellulosic fiber surfaces. The potential activated groups in lignin, LG, and OG are highlighted.

treated fibers and the PLA matrix. This is potentially due to the eight-carbon chain of OG, which could favor entanglements and interdiffusion of the modified fibers with the PLA matrix, as discussed in the next sections. The water absorption of the filaments was high during the first 100 h of the test and remained practically constant for the rest of the time (Figure 6). However, after 400 h, the filaments

Thus, the laccase-assisted hydrophobization of the TMP fibers was performed using guaiacol as the mediator. TMP fibers treated with OG or LG showed similar hydrophobic characteristics. However, the surfaces of the hydrophobized fibers were chemically different, since OG and LG have aliphatic chains with different lengths (Figure 5). Such a difference could have an important effect on the interfacial adhesion between the hydrophobized fibers and the PLA matrix. Water Uptake. Reinforcement of the polymer matrix with wood fibers potentially improves the mechanical properties and enhances the biodegradability of the biocomposites. At the same time, the hydrophilicity of lignocellulosic fibers may induce problems of thickness swelling and poor dimensional stability. Hence, water absorption is a key parameter to determine in biocomposites. Laccase-assisted hydrophobization of the TMP fibers remarkably reduced the water absorption of the filaments. Filaments reinforced with OG-treated fibers, both P10OGT and P20OGT, evidenced a clear reduction of water (50%) with respect to the filaments filled with unmodified TMP fibers. Filaments reinforced with LG-treated fibers (P20LGT) exhibited also a significant lower water absorption (40%) than P20T filaments. Although LG is more hydrophobic than OG, the filaments reinforced with OG-treated fibers showed less water absorption than those filled with LG-treated TMP fibers. There are three main mechanisms of water absorption in biocomposites: diffusion, capillarity, and transport of water molecules.37 Capillarity depends on the interface of the matrix− fiber system. Hence, the interfacial adhesion between the fibers and the PLA had an important impact on the filaments’ water absorption. It is worthy of notice that the LG- and OG-treated fibers showed similar hydrophobicities as measured by the water contact angle. Thus, it is expected that the lower water absorption shown by the P10OGT and P20OGT filaments is directly related to better interfacial adhesion between the OG-

Figure 6. Water absorption (wt %) by the filaments during 32 days of water immersion.

with a fiber content of 20% (P20T, P20OGT, and P20LGT) exhibited a secondary mechanism of water absorption compared with the filaments with a fiber content of 10%. Such a secondary effect of water absorption may be related to microcracks that may appear in the PLA matrix as a result of fiber swelling. This effect was not observed in the filaments with a fiber content of 10%. Hence, it seems that the higher the fiber content in the filaments is, the higher is the amount of microcracks formed as a result of fiber swelling. Nevertheless, such secondary mechanism of water absorption was less 9342


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ACS Sustainable Chemistry & Engineering pronounced in P20OGT filaments than in P20LGT filaments. Thus, better interfacial adhesion between the matrix and the fibers can reduce the impact of the microcracks potentially formed due to fibers swelling. Tensile Strength and Fracture Surface Assessment. Good stress transfer from the matrix to the fibers is required to achieve the full potential of the mechanical properties of the TMP-fiber-reinforced biocomposites. Such a characteristic depends on the bonding between the matrix and the fibers. The porosity and the number of protruding fibers in the fracture surface will indicate how strong the interfacial adhesion is between the fibers and the matrix. In addition, good fiber dispersion within the matrix will lead to a homogeneous distribution of the load during mechanical testing. The tensile strengths of the filaments manufactured in this study show remarkable differences. The OG-modified fibers yielded the highest tensile strength among the biocomposite filaments. This is considered a confirmation of the fiber modification, which led to a better interfacial adhesion with the PLA matrix and thus stronger filaments (Figure 7). However, the tensile

Figure 8. Maximum force of the filaments.

Figure 7. Tensile strengths of the filaments.

strength of most of the biocomposite filaments was unexpectedly lower than that of the neat PLA filaments. This was most probably caused by a porous structure and thus a thicker filament. For the same mass, a porous filament leads to a larger cross-sectional area and thus to an underestimation of the tensile strength. On the other hand, the maximum force, which is not influenced by the varying cross-sectional area, is a representative measure of the load-bearing capacity of the filaments. When considering the maximum force of the filaments, the P10OGT and P20OGT filaments showed considerably higher values than the neat PLA filaments (Figure 8). However, the P10T, P20T, P10LGT, and P20LGT showed lower maximum force values than the neat PLA. This is probably due to weaker interfacial adhesion between the fibers and the PLA. The varying results may also be due to poor homogeneity and dispersion of the fibers in the PLA matrix, which can cause fiber agglomerates that may have initiated the failure and crack propagation.38 The fracture surface of the filaments was analyzed by SEM (Figure 9). The P20T filaments exhibited a heterogeneous distribution of big and small pores in the fracture surface. Poor chemical compatibility between the hydrophobic PLA and the hydrophilic TMP fibers led to fiber agglomerations and heterogeneous dispersion of the fibers in the polymer matrix. The gaps between the fibers and the matrix confirmed the poor bonding between the two phases. In addition, fibers protruding from the matrix were also observed.

Figure 9. SEM images of the fracture surfaces. The left panels show the fracture areas of (top to bottom) PLA, P20T, P20LGT, and P20OGT filaments. The right panels present images acquired at higher magnification, showing the fibers (white arrows) in the PLA matrix and the pores (black arrows).

Regarding the enzymatically hydrophobized fibers, the fracture surface of P20LGT filaments had a large number of pores. However, the size of the pores was smaller than that observed in P20T filaments. In addition, a homogeneous distribution of the pores along the surface fracture was noted, which indicates improved chemical compatibility between the PLA matrix and the LG-treated TMP fibers. Nonetheless, there was still a significant amount of fibers pulled out from the PLA matrix. The results were significantly improved with OG9343


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ACS Sustainable Chemistry & Engineering treated TMP fibers. P20OGT filaments exhibited a smooth fracture surface. Fiber aggregation disappeared, and just micropores were observed. Enzymatic hydrophobization of the fibers with OG improved the interface adhesion with the PLA matrix. Moreover, fiber breakage was observed. Hence, improved stress transfer from the matrix to the fibers was achieved. The improved fiber−matrix adhesion led to a significant reduction of water absorption of the filaments (Figure 6) and a corresponding increase in tensile strength (Figure 7). 3D Printing. Some 3D printed objects are displayed in Figure 10. All of the filaments manufactured in the study were

Figure 11. Tensile strengths of 3D-printed dog bones (PLA, P10OGT, and P20OGT). The inset shows the P10OGT filament and the corresponding 3D printed PLA (white) and P10OGT (yellow/brown) dog bones.

with values of PLA made by traditional manufacturing methods (approximately 60 MPa). This was most probably due to the relatively high porosity of the dog bones caused by the 3D printing setup applied in this study. More work should be done to investigate the cause of the reduced strength and to change the 3D printing process to reduce the porosity. However, the main purpose of this study was to modify the surface of TMP fibers in order to improve the adhesion between the fibers and PLA. The applied modification contributes to a reduction in water uptake (Figure 6) and to an improvement in the tensile strength of filaments (Figure 8) and printed objects (Figure 11). Final Remarks. Laccase-assisted grafting of LG provided hydrophobic properties to the TMP fibers and reduced the water absorption of the fiber-reinforced filaments. However, the interfacial adhesion between LG-treated fibers and PLA was not optimal as observed by SEM. Such low chemical compatibility between the LG-treated fibers and PLA had a considerable effect on the poor tensile strength and maximum force observed during the mechanical tests. On the other hand, the results of the present study evidenced that laccase-mediated grafting of OG onto TMP fibers increased the water contact angle of the fibers and improved their interfacial adhesion with PLA. This improvement in the adhesion of the fiber−matrix system had a remarkable impact on the maximum force needed to break the filaments and in the tensile strength of the 3Dprinted dog bones. In addition, filaments reinforced with OGtreated fibers showed good performance during the 3D printing. Therefore, the manufacture of fiber-reinforced biocomposites with enhanced mechanical properties and relatively low water absorption was achieved. The hydrophobization of TMP fibers was performed under soft conditions of temperature with a short time of reaction and a minor volume of organic solvent. Hence, the consumption of energy and chemical reagents for the fiber modification were relatively low. Moreover, in a biorefinery context, previous recovery of lipophilic extractives from the TMP fibers would allow easier enzymatic modification (reduction of reaction time, no mediators would be needed) and would become a source of added-value chemicals. Thus, the development of an efficient, sustainable, and environmentally friendly large-scale process for natural fiber functionalization is feasible. However, one limitation for upscaling the laccase-mediated grafting of OG onto lignocellulosic fibers could be the costs associated with laccase. Nevertheless, laccases are already integrated into existing industrial processes such as textile dyeing.39 In

Figure 10. 3D-printed biocomposites containing 10% and 20% TMP fibers are depicted in the left and middle panels, respectively. The right panels show some local details from the areas marked with dashed rectangles in the middle panels.

3D-printable. However, there were slight differences between the six biocomposite series and the PLA series used as a control. The objects printed with the P10T, P20T, P10LGT, and P20LGT filaments revealed some poor delineation of the object edges. The print quality seemed to be improved when printing with the P10OGT and P20OGT filaments, which provides additional evidence of the suitability of the OGmodified fibers for the manufacture of biocomposite filaments for 3D printing. The series PLA, P10OGT, and P20OGT were also used to 3D-print dog bones. The tensile strength of the dog bones was assessed. The results seem to validate the good performance of the OG-modified fibers in reinforcing the filaments and the 3Dprinted dog bones (Figure 11). Unfortunately, the tensile strengths of all of the 3D-printed materials were low compared 9344


Research Article

ACS Sustainable Chemistry & Engineering

polylactic acid as a replacement of glass fiber reinforced polypropylene, macro and micro-mechanics of the Young’s modulus. Composites, Part B 2017, 125, 203−210. (7) Peltola, H.; Päak̈ könen, E.; Jetsu, P.; Heinemann, S. Wood based PLA and PP composites: Effect of fibre type and matrix polymer on fibre morphology, dispersion and composite properties. Composites, Part A 2014, 61, 13−22. (8) Faludi, G.; Dora, G.; Imre, B.; Renner, K.; Móczó, J.; Pukánszky, B. PLA/lignocellulosic fiber composites: Particle characteristics, interfacial adhesion, and failure mechanism. J. Appl. Polym. Sci. 2014, 131 (4), 39902. (9) Peltola, H.; Laatikainen, E.; Jetsu, P. Effects of physical treatment of wood fibres on fibre morphology and biocomposite properties. Plast., Rubber Compos. 2011, 40 (2), 86−92. (10) La Mantia, F. P.; Morreale, M. Green composites: A brief review. Composites, Part A 2011, 42 (6), 579−588. (11) George, J.; Sreekala, M. S.; Thomas, S. A review on interface modification and characterization of natural fiber reinforced plastic composites. Polym. Eng. Sci. 2001, 41 (9), 1471−1485. (12) Kudanga, T.; Nyanhongo, G. S.; Guebitz, G. M.; Burton, S. Potential applications of laccase-mediated coupling and grafting reactions: A review. Enzyme Microb. Technol. 2011, 48 (3), 195−208. (13) Garcia-Ubasart, J.; Esteban, A.; Vila, C.; Roncero, M. B.; Colom, J. F.; Vidal, T. Enzymatic treatments of pulp using laccase and hydrophobic compounds. Bioresour. Technol. 2011, 102 (3), 2799− 2803. (14) Dong, A.; Yu, Y.; Yuan, J.; Wang, Q.; Fan, X. Hydrophobic modification of jute fiber used for composite reinforcement via laccasemediated grafting. Appl. Surf. Sci. 2014, 301, 418−427. (15) Ni, X.; Dong, A.; Fan, X.; Wang, Q.; Yu, Y.; Cavaco-Paulo, A. Jute/polypropylene composites: Effect of enzymatic modification on thermo-mechanical and dynamic mechanical properties. Fibers Polym. 2015, 16 (10), 2276−2283. (16) Ni, X.; Dong, A.; Fan, X.; Wang, Q.; Yu, Y.; Cavaco-Paulo, A. Enzyme-mediated surface modification of jute and its influence on the properties of jute/epoxy composites. Polym. Compos. 2017, 38, 1327− 1334. (17) Dong, A.; Wu, H.; Fan, X.; Wang, Q.; Yu, Y.; Cavaco-Paulo, A. Enzymatic hydrophobization of jute fabrics and its effect on the mechanical and interfacial properties of jute/PP composites. eXPRESS Polym. Lett. 2016, 10 (5), 420−429. (18) Singh, G.; Kaur, K.; Puri, S.; Sharma, P. Critical factors affecting laccase-mediated biobleaching of pulp in paper industry. Appl. Microbiol. Biotechnol. 2015, 99 (1), 155−164. (19) Gutiérrez, A.; Rencoret, J.; Ibarra, D.; Molina, S.; Camarero, S.; Romero, J.; Del Río, J. C.; Martínez, Á . T. Removal of lipophilic extractives from paper pulp by laccase and lignin-derived phenols as natural mediators. Environ. Sci. Technol. 2007, 41 (11), 4124−4129. (20) Felby, C.; Pedersen, L. S.; Nielsen, B. R. Enhanced auto adhesion of wood fibers using phenol oxidases. Holzforschung 1997, 51 (3), 281−286. (21) Euring, M.; Rühl, M.; Ritter, N.; Kües, U.; Kharazipour, A. Laccase mediator systems for eco-friendly production of mediumdensity fiberboard (MDF) on a pilot scale: Physicochemical analysis of the reaction mechanism. Biotechnol. J. 2011, 6 (10), 1253−1261. (22) Ford, S.; Despeisse, M. Additive manufacturing and sustainability: an exploratory study of the advantages and challenges. J. Cleaner Prod. 2016, 137, 1573−1587. (23) Zein, I.; Hutmacher, D. W.; Tan, K. C.; Teoh, S. H. Fused deposition modeling of novel scaffold architectures for tissue engineering applications. Biomaterials 2002, 23 (4), 1169−1185. (24) Schwarzkopf, M. J.; Burnard, M. D. Wood-Plastic Composites  Performance and Environmental Impacts. environmental impacts of traditional and innovative forest-based products 2016, 19−43. (25) Reme, P. A.; Johnsen, P. O.; Helle, T. Assessment of fibre transverse dimensions using SEM and image analysis. J. Pulp Pap. Sci. 2002, 28 (4), 122−128. (26) Migneault, S.; Koubaa, A.; Erchiqui, F.; Chaala, A.; Englund, K.; Krause, C.; Wolcott, M. Effect of fiber length on processing and

addition, laccase immobilization would allow reutilization of the enzyme, which would significantly decrease the production costs associated with laccase.40 Finally, the stable bonding of OG on the lignocellulosic fiber surface is expected to provide new properties (antifungal, antibacterial, antioxidant) in addition to hydrophobicity.36,41 Therefore, the grafting of OG, besides improving the interfacial adhesion between fibers and the PLA matrix, could potentially provide biological activity to the biocomposites. Such properties could extend the uses of filaments with OG-modified wood pulp fibers for, e.g., antibacterial 3D-printed devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02351. Parameters measured in the tensile strength test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: gary.chinga.carrasco@rise-pfi.no. *E-mail: damfi[email protected]. ORCID

Diego Moldes: 0000-0001-6745-4320 Gary Chinga-Carrasco: 0000-0002-6183-2017 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was funded by the ValBio-3D Project (Grant ELAC2015/T03-0715 Valorization of residual biomass for advanced 3D materials; Research Council of Norway, Grant 271054) and the FiberComp Project (High performance wood fiber composite materials; Research Council of Norway, Grant 245270). The authors also thank François Bru for assistance in some laboratory testing. The COST action FP1405 is acknowledged for funding an STSM of D.F. to RISE PFI, where these activities were performed. Funding from Xunta de ́ Galicia (Project 09TMT012E Novos tratamentos biocataliticos para a mellora da durabilidade da madeira. Valorización de extractivos da madeira e de lignina kraft) is also appreciated.

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REFERENCES

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