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Enzymatic-assisted modification of TMP fibres for improving the interfacial adhesion with PLA for 3D printing Daniel Filgueira, Solveig Holmen, Johnny K. Melbø, Diego Moldes, Andreas Echtermeyer, and Gary Chinga-Carrasco ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02351 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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ACS Sustainable Chemistry & Engineering

ACS Sustainable Chemistry & engineering. Accepted version. GCH. Enzymatic-assisted modification of TMP fibres for improving the interfacial adhesion with PLA for 3D printing

Daniel Filgueiraa*, Solveig Holmenb, Johnny K. Melbøc, Diego Moldesa, Andreas T. Echtermeyerb, Gary Chinga-Carrascoc*

a

Department of Chemical Engineering, Edificio Isaac Newton, Lagoas-Marcosende s/n, University of Vigo,36310, Vigo, Spain b

Department of Mechanical and Industrial Engineering, NTNU, Richard Birkelandsvei 2B, 7491 Trondheim, Norway c

RISE PFI, Høgskoleringen 6b, 7491 Trondheim, Norway.

Corresponding author: [email protected] [email protected]

ABSTRACT The present study is about the enzymatic modification of Thermomechanical Pulp (TMP) fibres for reduction of water uptake and their use in bio-based filaments for 3D printing. Additionally, TMP was used as fibre reinforcing material and polylactic acid (PLA) as polymer matrix. The hydrophilic TMP fibres were treated via laccase-assisted grafting of Octyl Gallate (OG) or Lauryl Gallate (LG) onto the fibre surface. The modified TMP fibres showed remarkable hydrophobic properties, as shown by water contact angle measurement. Filaments reinforced with OG-treated fibres exhibited the lowest water absorption and the best interfacial adhesion with the PLA matrix. Such higher chemical compatibility between the OG-treated fibres and the PLA enabled a better stress transfer from the matrix to the fibres during the mechanical testing, which led to the manufacturing of strong filaments for 3D printing. All the manufactured filaments were 3D printable, although the filaments containing OG-treated fibres yielded the best results. Hence, laccase-mediated grafting of OG onto TMP fibres is a sustainable and environmentally-friendly pathway to manufacture fully bio-based filaments for 3D printing.

KEYWORDS: laccase, grafting, octyl gallate, TMP, PLA, biocomposites, 3D printing

1 ACS Paragon Plus Environment


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ACS Sustainable Chemistry & engineering. Accepted version. GCH.

INTRODUCTION Biocomposites A biocomposite is a material consisting of at least one phase that is biologically derived.1 Usually the dispersed phase is a biofibre, and the matrix phase may be composed by a bioplastic that originates from ”green” sources.2,3 When both the matrix and the dispersed phase derive from biological sources, the biocomposite is also classified as fully bio-based. Thermoplastics such as polylactic acid (PLA), starch or lignin are the main matrixes used in fully bio-based composites while wood, flax and hemp fibres are the main dispersed phase.2,4 The properties of a given biocomposite depend on various aspects, including the structure and composition. Wood fibres have varying diameter and length, which influences the corresponding strength of the biocomposites. The higher the aspect ratio, the more the reinforcing effect is. Additionally, a higher fraction of fibres will enhance the mechanical properties, e.g. mechanical strength, stiffness and toughness.5,6 Based on the mechanical advantages of wood pulp fibres compared to e.g. wood flour, various wood pulp fibres have been tested for biocomposite manufacturing. Peltola et al. examined the reinforcing effect of wood fibres contra wood flour, and examined different types of wood fibre composites with PLA and polypropylene (PP) matrix.7 Thermomechanical pulp (TMP)-PLA biocomposites showed the best fibre dispersion and mechanical properties. Faludi et al. examined the interface of biocomposites consisting of PLA and six different lignocellulosic fibres.8 The bond between PLA and natural fibres was found to be strong. Peltola et al. also examined the influence of fibre refining on morphology and properties, using long pine TMP fibres with PLA matrix.9 They found that mechanical properties were better with TMP fibre, high volume fractions of fibres and large fibre aspect ratios. These characteristics are also well known from traditional short fibre composites made from inorganic fibres and petroleum based matrix materials. Therefore, the combination of PLA as thermoplastic matrix reinforced with TMP fibres would lead to the manufacturing of a fully bio-based composite with promising mechanical properties.

Fibre modification Apart from the structure of the fibre, the interface bonding between the fibre and the polymer is vital to acquire a good reinforcement from the dispersed phase and a strong biocomposite. Due to poor compatibility between fibre and matrix, methods of improving adhesion between phases are often required, also called adhesion promoters. Chemical alteration to the fibre is an example of this.4 Adhesion promoters do not only contribute to a more reinforced biocomposite, but can also contribute to a reduction in water absorption and an improvement of processability.10 One limitation of wood pulp fibres with respect to their application in biocomposites is the hydrophilic and hygroscopic nature of the lignocellulosic fibres, i.e. they absorb water. Water absorption may lead to weaken 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 fibre 2 ACS Paragon Plus Environment

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ACS Sustainable Chemistry & engineering. Accepted version. GCH. 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. There are several physical and chemical methods that have been reported for the modification of lignocellulosic fibres.11 Most of them involve the use of oil-derived reagents and/or a high consumption of energy and time. An alternative and sustainable process is the laccase-assisted modification of lignocellulosic fibres. Laccase is an enzyme capable to carry out one electron oxidation of phenolic substrates. Therefore, laccase may activate wood fibres 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 reacting with the enzyme. Additionally, this radical can undergo oxidation reactions that cannot be carried out by the enzyme. This radicalized mediator can access locations, which are not accessible for the enzymes due to 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 fibres with polypropylene and epoxy matrixes.14,15,16,17 Laccase have also been used for other applications regarding wood fibres, e.g. biobleaching, removing of lipophilic extractives or particleboards production from TMP fibres.18,19,20,21

3D printing Additive manufacturing (AM), also known as 3D printing, is based on a layer-by-layer production process, facilitating the production of objects in three dimensions. There is a broad spectre of technologies within AM, amongst others fused deposition modelling (FDM), selective laser melting (SLM), selective laser sintering (SLS), stereolithography (SLA) and electron beam melting (EBM). Additionally, AM technology has a broad application, from medical sector to aerospace and automotive industry, with applications depending on technology, and not to mention material used. With the 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, on a bottom-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 fibres has several challenges that should be considered in this respect. Firstly, wood pulp fibres 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 Secondly, 3D printing generally uses a nozzle of minor size (0.4 mm), which can cause problems when printing filaments containing wood pulp fibres. Finally, poor fibre 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 fibres were modified, via laccase-assisted grafting of Octyl Gallate (OG) and Lauryl Gallate (LG). Before the enzymatic hydrophobization of the fibres, lipophilic extractives were removed from the TMP fibres by means of a 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 fibres was performed. The modified TMP fibres were used as fibre reinforcing material for the manufacturing of PLA-based biocomposite filaments. 3 ACS Paragon Plus Environment


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ACS Sustainable Chemistry & engineering. Accepted version. GCH. 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 dogbone samples and the quality of the 3D printed objects was assessed. MATERIALS AND METHODS Materials PLA pellets (Ingeo™ Biopolymer 4043D) were purchased from Nature Works. Spruce TMP fibres 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’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) oxidation assay. One unit of activity was defined as the amount of enzyme that oxidized 1 μmol of ABTS per minute at 25 ⁰C and pH 7 (0.1 M phosphate buffer). All the other chemical reagents were purchased from Sigma-Aldrich (St. Louis, USA) at reagent grade and used without further purification.

TMP fibres modification Spruce TMP fibres (18 g of oven dried pulp (odp)) were suspended in 1.8 L of phosphate buffer (0.1 M, pH 7) at 50 ⁰C. After 30 minutes under constant agitation, 27 mL of laccase enzyme (175 U / g odp) and vanillin (V) or guaiacol (G) (10 mM) were added to the solution. 30 minutes later, 80 mL of an acetone solution containing Octyl Gallate (OG) or Lauryl Gallate (LG) (0.15 M) were added to the system. The reaction was conducted during 2 hours at 50 ⁰C under permanent agitation. After the enzymatic treatment, the fibres were dried at room temperature during 24 h and washed with 1.8 L of distilled water: acetone solution (60:40 %, v/v) during 1 h at 50 ⁰C. Finally the fibres were extensively washed with distilled water and dried at 50 ⁰C for 12 h. Treatments without adding laccase and/or the vanillin or guaiacol were carried out as control tests.

TMP sheets formation and measurement of water contact angle Sheets were prepared from enzymatically-modified TMP fibres. The fibres (1 g odp) were suspended in distilled water (0.4% of 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 ⁰C during 4 hours. Sheets with unmodified TMP fibres were prepared as reference. The hydrophobicity of the TMP sheets was evaluated by measurement of water contact angle (WCA). A drop of distilled water was deposited on the surface of the TMP sheets and the contact angles were measured with a goniometer MobileDrop GH11 (KRÜSS). Contact angle measurements were carried out with intervals of 15 seconds during 2 minutes. The drop shape was analyzed with the DSA2 software (KRÜSS).


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ACS Sustainable Chemistry & engineering. Accepted version. GCH. Fibre morphology The TMP fibres were stained with Bromine and assessed as described by Reme et al.25 Bromine reacts with the lignin in the fibre structure. TMP fibres before and after grinding were assessed with a scanning electron microscope (SEM) (Hitachi SU3500 Scanning Electron Microscope) in secondary electron imaging (SEI) and backscatter electron imaging (BEI) modes to assess the fibre morphology and the fibre surface areas rich in lignin. The fibres were additionally embedded in epoxy resin and prepared for crosssectional analysis, as described by Reme et al.25 Additionally, fibres before and after grinding were assessed with a Fibre Tester device (L&W FiberTester Plus, Code 912. Software: Version 4.0-3.0). In order to assess the fibre width and length distribution 40 ml suspensions (0.1% solid content) were run through the Fibre Tester in standard mode.

Extrusion of biocomposite filaments PLA filaments were reinforced with enzymatically modified TMP fibres. Control filaments with unmodified TMP fibres and without fibres were also prepared. An overview of the series that were prepared is given in Table 1. Table 1 Composition of biocomposite filaments for 3D printing. Series PLA P10T P20T P10LGT P20LGT P10OGT P20OGT

PLA (%) 100 90 80 90 80 90 80

TMP fibre (%) 10 20 10 20 10 20

TMP fibre modification LG LG OG OG

In order to obtain a homogeneous blend, PLA pellets and TMP fibres were ground in a Thomas Wiley Mini-Mill Cutting mill to mesh 10 and 30, respectively. The milled PLA and the fibres were oven dried (105 ⁰C during 1 hour) and the blending was performed at two different TMP fibre loads, 10% and 20 %. The blend was extruded in a Noztek Xcalibur, which has three different heating chambers for the total control of the extrusion temperature. The temperatures were set in decreasing order (175 ⁰C, 170 ⁰C and 165 ⁰C) to obtain filaments with low porosity and a diameter of approx. 2 mm. The speed of the screw extruder and the fan were set at 12 mm / s and 65 %, respectively. All the filaments were spooled with a Filabot spooler at the output of the extruder. Due to some fibre clots in the PLA filaments, especially with fibre 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 of the first extrusion.

Water uptake



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ACS Sustainable Chemistry & engineering. Accepted version. GCH. 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 during 32 days. Prior to initiate the water absorption test, the samples were completely dried (105 ⁰C during 4 hours) and the dried weight (W0) was measured. The samples were weighed every 24 hours (Wi). The water uptake of the filaments was measured using the following equation, Water uptake (weight %) = =

× 100

(1)

3D printing PLA filaments (ø = 2.2 ± 0.1 mm) reinforced with TMP fibres were used for printing 3D model figures (Ø = 15 mm) and dogbones (Length = 80 mm, width and height = 3 mm), in an Ultimaker Original 3D printer. Only dogbones of the series PLA, P10OGT and P20OGT were tested due to limited filament amount and these series appeared to be the most interesting to assess in this study. The 3D printer was equipped with a nozzle diameter of 0.4 mm. The print speed and temperature were set at 15 mm / s and 210 ⁰C, respectively. The design of a model figure and dogbones were modelled in a three dimensional computer aided program (CAD). 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 dogbones (PLA, P10OGT and P20OGT) were determined with a Zwick Roell Proline and a load cell of 2.5 kN, 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 initial position 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 working distance was 5-17 mm.

RESULTS AND DISCUSSION Fibre morphology Greater fibre length has in earlier studies been found to yield higher mechanical properties, but on the other hand cause difficulties in dispersion, and may yield higher amount of fibre agglomerates in the biocomposite.7,9,26,27 In this study, the fibres were milled to mesh 30 to ease the processing during filament manufacturing, increase fibre dispersion and avoid clogging of the 3D printing head. The reduction of fibre length due to the grinding is observed in Figure 1. The average fibre length of TMP and ground TMP was 1.5 and 0.4 mm, respectively. The corresponding diameters were 33 and 38 µm, respectively. The increase in diameter is probably due to an opening of the fibre walls due to the grinding process. Additionally, the fines fraction increased from 43 to 70% after grinding. The TMP fibres contain lignin, which is exemplified as brighter areas in Figure 2. The lignin-containing areas correspond to part of the middle lamella of the fibre walls. SEM in BEI mode produces images 6 ACS Paragon Plus Environment

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ACS Sustainable Chemistry & engineering. Accepted version. GCH. 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 number of the lignin-rich areas, which thus appear brighter when observed in SEM BEI mode.

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

Figure 2. TMP fibres stained with bromine. Left) Surface images acquired in SEM BEI mode. Right) Crosssectional images. The white arrows indicate bright areas rich in lignin.

Water contact angle The WCA was measured in order to assess the hydrophobicity of the TMP sheets. The higher the WCA the higher the grafting of the hydrophobic compounds on TMP fibres. Unmodified TMP fibres showed a highly hydrophilic behaviour, since the average time for the complete absorption of a droplet of water was below 20 seconds (Figure 3). Such behaviour was expected since spruce TMP fibres have a carbohydrate content of approx. 70 %.28 On the other hand, hydrophobization of TMP fibres was performed in preliminary experiments by means of the enzymatic treatment with LG (L+LG (not preactivated)). However, surface hydrophobicity was very low since the WCA was insignificant after 30 seconds of test. 7 ACS Paragon Plus Environment


ACS Sustainable Chemistry & engineering. Accepted version. GCH. Therefore, a different strategy was performed in order to enhance the hydrophobicity of the TMP fibres. Laccase was added to the fibre dispersion 30 min before adding the LG (L+LG (preactivated)). It is known that the maximum amount of phenoxy radicals in laccase-treated TMP fibres is obtained after 30 min of reaction.29 Therefore, the objective was the preactivation of the fibres by producing as much as possible phenoxy radicals prior to adding the hydrophobic compound. The WCA of the resultant sheets indicated that the grafting of LG onto TMP fibres was remarkably improved with respect to non-preactivated fibres. Nevertheless, the fibres did not show the expected results in comparison with previous studies.13,30

TMP L+LG (no preactivated) L+V+LG L+G+OG

L+G L+LG (preactivated) L+G+LG

100

Water contact angle (⁰)

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80 60 40 20 0 0

30

60

90

120

150

Time (s) Figure 3. Water contact angle of the sheets prepared with TMP fibres which were treated with: L = laccase; V = vanillin; G = guaiacol; OG = octyl gallate; LG = lauryl gallate. TMP = untreated TMP fibres.

It is worth to notice that the enzymatic activation of mechanical pulps is extremely influenced by the chemistry and physical accessibility of the fibre´s surface lignin.29 In addition to lignin, the extractives in the fibres surface are increased during the mechanical pulping.31 The agglomeration of such extractives on the fibre surface may form films which could limit the enzymatic oxidation of lignin.32 In addition, due to their low redox-potential, laccases cannot oxidize directly 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 fibres 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 fibres is limited by its large molecular size (60 - 100KDa). However, such steric hindrance could be solved with the laccase-mediator system, since small size mediators may penetrate the fibre wall structure.


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GUAIACOL

VANILLIN

Figure. 4. Chemical structure of guaiacol and vanillin

Two different low molecular weight phenolic compounds were assessed as mediators, i.e. Guaiacol (G) and Vanillin (V). The used mediators have low redox-potentials, 0.454 mV and 0.620 mV, respectively. This parameter is extremely important since low redox-potential laccases like M. Thermophila cannot oxidize high redox-potential mediators.34 The grafting of LG onto TMP fibres seems to be slightly improved since the WCA was also improved by means of laccase-mediator system reaction using vanillin (L+V+LG). Nevertheless, when guaiacol was used as mediator, the grafting of LG and the hydrophobicity of the TMP fibres was remarkably enhanced (L+G+LG). 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 orto position (Figure 4). In addition, vanillin possesses an aldehyde group at the para position. The presence of electron acceptor groups, i.e. aldehyde groups, at the para position could destabilize the phenoxy radicals, reducing their oxidation capacity.28 Hence, it seems that guaiacol have a high capacity to oxidize and remove lipophilic extractives from the TMP fibres surface, since the initial WCA of the TMP sheets treated only with laccase and guaiacol (L+G) was significantly lower than the WCA on the untreated TMP fibres (Figure 3). Moreover, TMP fibres were Soxhlet extracted with hexane in order to remove the lipophilic extractives. After the extraction process, the fibres were enzymatically treated with LG without adding guaiacol, and the resultant sheets showed a remarkable hydrophobic behaviour (data not shown). Thus, the guaiacolmediated reaction seems to enable the removal of lipophilic extractives from the fibres surface, which allows the activation of phenolic lignin moieties for the grafting of the hydrophobic compounds.



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

For the enzymatic hydrophobization of the fibres, gallate compounds (OG and LG) are especially interesting. These phenolic compounds are more or less hydrophobic depending on the length of their aliphatic chain. By this way, the hydrophobicity of the enzymatically-treated fibres may be tailored depending on the grafted gallate.36 Therefore, LG and OG were tested for the hydrophobization of TMP pulp. The WCA of the TMP sheets made from OG-treated fibres was slightly lower than the WCA yielded by the LG-treated fibres. 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. Thus, the laccase-assisted hydrophobization of the TMP fibres was performed using guaiacol as mediator. Both TMP fibres treated with OG or LG showed similar hydrophobic characteristics. However, the surface of the hydrophobized fibres was chemically different, since OG and LG have aliphatic chains with a different length (Figure 5). Such difference could have an important effect on the interfacial adhesion between the hydrophobized fibres and the PLA matrix.

Water uptake The reinforcement of polymer matrix with wood fibres potentially increases the mechanical properties and enhances the biodegradability of the biocomposites. At the same time, the hydrophilicity of lignocellulosic fibres may induce problems of thickness swelling and poor dimensional stability. Hence, water absorption is a key parameter to determine in biocomposites.


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ACS Sustainable Chemistry & engineering. Accepted version. GCH. Laccase-assisted hydrophobization of the TMP fibres reduced remarkably the water absorption of the filaments. Filaments reinforced with OG-treated fibres, both P10OGT and P20OGT, evidenced a clear reduction of water (50%) with respect to the filaments filled with unmodified TMP fibres. Filaments reinforced with LG-treated fibres (P20LGT) exhibited also a significant lower water absorption (40 %) than P20T filaments. Although LG is more hydrophobic than OG, the filaments reinforced with OGtreated fibres showed a minor water absorption than those filled with LG-treated TMP fibres. 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-fibre system. Hence, the interfacial adhesion between the fibres and the PLA had an important impact in the filaments water absorption. It is worth to notice that LG- and OG-treated fibres showed a similar hydrophobicity measured as water contact angle. Thus, it is expected that the lowest water absorption shown by the P10OGT and P20OGT filaments is directly related to a better interfacial adhesion between OG-treated fibres and PLA matrix. This is potentially due to the eight carbons chain of OG which could favour entanglements and interdiffusion of the modified fibres with the PLA matrix, as commented in next sections. The water absorption of the filaments was high during the first 100 hours of the test and remained practically constant the rest of the time (Figure 6). However, after 400 hours, the filaments with a fibre content of 20 % (P20T, P20OGT and P20LGT) exhibited a secondary mechanism of water absorption compared to the filaments with a fibre content of 10 %. Such secondary effect of water absorption may be related to microcracks that may appear in the PLA matrix, as a result of fibre swelling. This effect was not observed in the filaments with a fibre content of 10 %. Hence, it seems that the higher the fibre content in the filaments, the higher the amount of microcracks formed due to fibres swelling. Nevertheless, such secondary mechanism of water absorption was less pronounced in P20OGT filaments than in P20LGT filaments. Thus, a better interfacial adhesion between the matrix and the fibres could reduce the impact of the microcracks formed due to fibres swelling.

PLA

P10T

P20T

P10OGT

P20OGT

P20LGT

7

Water absorption (%)

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6 5 4 3 2 1 0 0

200

400

600

800

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




Tensile strength and fracture surface assessment A good stress transfer from the matrix to the fibres is required to achieve the full potential of the mechanical properties of the TMP fibre-reinforced biocomposites. Such characteristic depends on the bonding between the matrix and the fibres. The porosity and the amount of protruding fibres in the fracture surface will indicate how strong the interfacial adhesion is between the fibres and the matrix. In addition, good fibre dispersion within the matrix will lead to a homogeneous distribution of the load during mechanical testing. The tensile strength of the filaments manufactured in this study shows remarkable differences. The OG-modified fibres yielded the highest tensile strength among the biocomposite filaments. This is considered a confirmation of the fibre modification, which led to a better interfacial adhesion with the PLA matrix and thus stronger filaments (Figure 7). However, the tensile strength of most of the biocomposite filaments was unexpectedly lower than the neat PLA filaments. This is most probably caused by a porous structure and thus a thicker filament. Considering 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 (kN), 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 show considerable higher values than the neat PLA filaments (Figure 8). However, the P10T, P20T, P10LGT and P20LGT have lower maximum force than the neat PLA. This is probably due to a lower interfacial adhesion between the fibres and the PLA. The varying results may also be due to a poor homogeneity and distribution of the fibres in the PLA matrix. A poor homogeneity and dispersion of fibres cause fibre agglomerates, which may have initiated the failure and crack propagation.38

60 50

Tensile Strength (MPa)

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40 30 20 10 0 PLA

P10T

P20T

P10LGT

P20LGT

P10OGT

P20OGT

Figure 7. Tensile strength of the filaments.


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300

Maximum Force (N)

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250 200 150 100 50 0 PLA

P10T

P20T

P10LGT

P20LGT

P10OGT

P20OGT

Figure 8. Maximum force (N) of the filaments.

The fracture surface of the filaments was analyzed by SEM (Figure 9). The P20F filaments exhibited a heterogeneous distribution of big and small pores in the fracture surface. A poor chemical compatibility between the hydrophobic PLA and the hydrophilic TMP fibres leads to fibre agglomerations and heterogeneous dispersion of the fibres in the polymeric matrix. The gaps between fibres and matrix confirmed a poor bonding between the two phases. In addition, fibres protruding from the matrix were also observed. Regarding the enzymatically hydrophobized fibres, the fracture surface of P20LGT filaments had a great amount of pores. However, the size of the pores was smaller than the observed in P20T filaments. In addition, a homogeneous distribution of the pores along the surface fracture was noted, which indicates an improved chemical compatibility between the PLA matrix and the LG-treated TMP fibres. Nonetheless, there was still an important amount of fibres pull-out from the PLA matrix. The results were significantly improved with OG-treated TMP fibres. P20OGF filaments exhibited a smooth fracture surface. Fibres aggregation disappeared and just micropores were observed. Enzymatic hydrophobization of the fibres with OG improved the interface adhesion with PLA matrix. Moreover, fibre breakage was observed. Hence, an improved stress transfer from the matrix to the fibres was achieved. The improved fibre-matrix adhesion leads to a significant reduction of water absorption of the filaments (Figure 6) and a corresponding increase in tensile strength (Figure 7).



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PLA

P20T

P20LGT

P20OGT

Figure 9. SEM images of the fracture surface. The left panel shows the fracture area of a PLA filament, P20T, P20LGT and P20OGT filaments. The right panel exemplifies images acquired with higher magnification and show the fibres (white arrows) in the PLA matrix and the pores (black arrows).


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ACS Sustainable Chemistry & engineering. Accepted version. GCH. 3D Printing Some 3D printed objects are exemplified in Figure 10. All the filaments manufactured in the study were 3D printable. However, there were slight differences between the six biocomposite series and the PLA series used as control. The objects printed with the P10T, P20T, P10LGT and P20LGT filaments reveal some poor delineation of the object edges. The print quality seems to be improved when printing with the P10OGT and P20OGT filaments, which provides additional evidence of the suitability of the OGmodified fibres for manufacturing of biocomposite filaments for 3D printing. The series PLA, P10OGT and P20OGT were also used to 3D print dogbones. The tensile strength of the dogbones was assessed. The results seem to validate the good performance of the OG-modified fibres in reinforcing the filaments and the 3D printed dogbones (Figure 11). Unfortunately, the tensile strength of all 3D printed materials was low compared to values of PLA made by traditional manufacturing methods (approx. 60 MPa). This was most probably due to the relatively high porosity of the dogbones caused by the 3D printing set up 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 fibres in order to improve the adhesion between the fibres and PLA. The applied modification contributes to a reduction of water uptake (Figure 6) and to an improvement of the tensile strength of filaments (Figure 8) and printed objects (Figure 11).



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Figure 10. 3D printing. 3D printed biocomposites containing 10% and 20% TMP fibres are given in the left and middle panels, respectively. The right panel exemplifies some local details from the areas marked with dashed rectangles in the middle panel.


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25

Tensile strength (MPa)

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20 15 10 5 0 PLA

P10OGT

P20OGT

Figure 11. Tensile strength of 3D printed dogbones (PLA, P10OGT and P20OGT). The inset exemplifies the P10OGT filament and the corresponding 3D printed PLA (white) and P10OGT (yellow/brown) dogbones.

Final remarks Laccase-assisted grafting of LG provided hydrophobic properties to the TMP fibres and reduced the water absorption of the fibre-reinforced filaments. However, the interfacial adhesion between LGtreated fibres and PLA was not optimal as observed by SEM. Such low chemical compatibility between the LG-treated fibres and PLA had a considerable effect on the poor tensile strength and maximum force observed during the mechanical test. On the other hand, the results of the present study evidenced that laccase-mediated grafting of OG onto TMP fibres increased the water contact angle of the fibres and improved their interfacial adhesion with PLA. Such improvement in the adhesion of the fibre-matrix system had a remarkable impact on the maximum force needed to break the filaments and in the tensile strength of the 3D printed dogbones. In addition, filaments reinforced with OG-treated fibres showed a good performance during the 3D printing. Therefore, manufacturing of fibre-reinforced biocomposites with enhanced mechanical properties and relatively low water absorption was achieved. The hydrophobization of TMP fibres was performed in soft conditions of temperature, short time of reaction and with a minor volume of organic solvent. Hence, the consumption of energy and chemical reagents for the fibre modification were relatively low. Moreover, in a biorefinery context, a previous recovery of lipophilic extractives from TMP fibres would allow an 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 fibre functionalization is feasible. However, one limitation for upscaling the laccasemediated grafting of OG onto lignocellulosic fibres could be the costs associated with laccase. Nevertheless, laccases are already integrated in existing industrial processes like textile dyeing.39 In addition, laccase immobilization would allow the enzyme reutilization which would decrease significantly the production costs associated to laccase.40



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ACS Sustainable Chemistry & engineering. Accepted version. GCH. Finally, the stable bonding of OG on the lignocellulosic fibres 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 fibres with PLA matrix, could potentially provide biological activity to the biocomposites. Such properties could extend the uses of filaments with OG-modified wood pulp fibres for e.g. antibacterial 3D printed devices.

ASSOCIATED CONTENT Supporting information Parameters measured in the tensile strength test.

ACKNOWLEDGEMENTS 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 no. 271054) and the FiberComp project (High performance wood fiber composite materials, Research Council of Norway, Grant no. 245270). The authors also thank François Bru for assistance in some laboratory testing. The COST action FP1405 is acknowledged for funding a STSM of the first author to RISE PFI, where these activities were performed. Funding from Xunta de Galicia (Project 09TMT012E Novos tratamentos biocatalíticos para a mellora da durabilidade da madeira. Valorización de extractivos da madeira e de lignina kraft) are also appreciated.

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For Table Of Contents Use Only

Synopsis: The enzymatic modification to functionalize lignocellulosic fibres is a sustainable and ecofriendly process for the manufacture of strong filaments for 3D printing.


Enzymatic-Assisted Modification of ... - ACS Publications

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