Nanopaper Properties and Adhesive Performance of Microﬁbrillated Cellulose from Different (Ligno-)Cellulosic Raw Materials

Article Nanopaper Properties and Adhesive Performance of Microﬁbrillated Cellulose from Different (Ligno-)Cellulosic Raw Materials

Stefan Pinkl 1,*, Stefan Veigel 2,Jérôme Colson 2 and Wolfgang Gindl-Altmutter 1,2

1 Competence Centre for Wood Composites and Wood Chemistry, Wood K Plus, Linz 4040, Austria; [email protected] 2 Department of Materials Science and Process Engineering, BOKU—University of Natural Resources and Life Sciences, Vienna 1180, Austria; [email protected] (S.V.); [email protected] (J.C.) * Correspondence: [email protected]; Tel.: +43-147654-89150

Received: 10 July 2017; Accepted: 27 July 2017; Published: 31 July 2017

Abstract: The self-adhesive potential of nanocellulose from aqueous cellulosic suspensions is of interest with regard to a potential replacement of synthetic adhesives. In order to evaluate the performance of microfibrillated cellulose from different (ligno-)cellulosic raw materials for this purpose, softwood and hardwood powder were fibrillated and compared to sugar beet pulp as a representative non-wood cellulose resource, and conventional microfibrillated cellulose produced from bleached pulp. An alkali pre-treatment of woody and sugar beet raw materials enhanced the degree of fibrillation achieved, same as TEMPO-mediated oxidation of microfibrillated cellulose. Nanopapers produced from fibrillated material showed highly variable density and mechanical performance, demonstrating that properties may be tuned by the choice of raw material. While nanopaper strength was highest for TEMPO-oxidated microfibrillated cellulose, fibrillated untreated sugar beet pulp showed the best adhesive performance. Different microscopic methods (AFM, SEM, light microscopy) examined the interface between wood and fibrillated material, showing particular distinctions to commercial adhesives. It is proposed that fibrillated material suspensions, which achieve bond strength up to 60% of commercial urea-formaldehyde adhesive, may provide a viable solution to bio-based adhesives in certain applications where wet-strength is not an issue.

Keywords: nanocellulose; sugar beet; wood; nanopaper; bio-adhesive

1. Introduction Adhesive bonding is a key processing step in the production of modern engineered materials. Recently, there is a growing interest in bio-based solutions for adhesive bonding as an alternative to well-established high-performance fossil-based adhesives. One approach consists of the synthesis and use of partially or fully bio-based adhesives [1]. Lignin is a widely available resource in this context and intense efforts are being undertaken towards the partial replacement of phenol in the synthesis of phenol-formaldehyde based resins [2]. Even so, the most important hindrance with regard to broad lignin utilization in adhesive synthesis is still its lack of reactivity compared to phenol [1]. Tannins as a bio-based adhesive component extracted from the bark or xylem of trees are polyhydroxyphenols which start auto condensation when hexamine is added. Particleboards with internal bond strength of 0.9 MPa are already reported [1,3–5]. A different approach of substituting fossil-based adhesives may be found in the use of intrinsic bonding forces for binderless bio-based materials, such as those realised in paper. Numerous examples for such approaches may be found in literature. Binderless wood ﬁberboards are achieved with the aid of self-bonding forces of lignin in well-established wet-processing technology, where decomposition

Polymers 2017, 9, 326; doi:10.3390/polym9080326 www.mdpi.com/journal/polymers Polymers 2017, 9, 326 2 of 12 products of hemicellulose, hydrogen bonding and molecular entanglement contribute to the formation of bonds [6–11]. Similarly, the thermal behavior of lignin at higher temperatures was proposed to be responsible for the bonding of high-density coconut husk boards [12]. Steam explosion is a useful pre-treatment in this context, as shown for softwood fiber [13,14], and particleboard from oil palm frond [15]. In addition to the activation of native lignin, technical lignin may be an additive providing adhesive bonding, as shown for sheets of steam-exploded Miscanthus sinensis. Resulting fiber boards show flexural strengths higher than 60 MPa and internal bond strengths of 3.5 MPa [16]. Carbohydrates are also an interesting additive enabling the manufacture of binderless boards. The addition of 20% glucose to oil palm biomass improved the internal bond strength of particleboards from 0.6 to 1.7 MPa [17]. Hemicellulose as a branched polymer with a lower molecular weight compared to cellulose also offers possibilities to develop self-adhesive forces. Hemicelluloses can be easily hydrolyzed into their constituting monosaccharides (pyranoses and hexoses) with acids and mild temperatures (100–130 ◦C). Hydrolysis of pyranose leads to furfural while hexoses degrade to 5-hydroxymethyl-2-furfuraldehyde, whereby both molecules are able to polymerize again [18,19]. In wood bonding, these molecules are able to react with other wood components e.g., lignin. From bagasse, lignocellulosic composites can be produced by in situ formation of self-adhesive forces [20]. The Thermodyn process from 1940 is also based on these self-adhesive forces [21]. Through their high specific surface area and hydrophilic surface, cellulose nanomaterials are also capable of providing adhesion. It is well known that the addition of microfibrillated cellulose to the papermaking process benefits paper strength [22,23]. Recently, several studies demonstrated the usefulness of wet microfibrillated cellulose for providing adhesive bonding in paper laminates [24] and in various wood-based composite board products [25]. A similar mechanism is at work in the Zelfo process, where partial fibrillation of plant material allows the production of novel, fully bio-based and binderless structural materials [26]. In the present study we will apply the concept of using fibrillated cellulosic material as a binder to a broad variety of fibrillated (ligno-)cellulosic materials. The hypothesis that fibrillated material can build-up substantial adhesive forces will be examined in the present paper together with the performance of nanopapers produced from these materials.

2. Materials and Methods

2.1. Preparation of Fibrillated Material Dried sugar beet pulp obtained from a sugar factory in Tulln (Agrana Zucker GmbH, Tulln an der Donau, Austria) was milled in a cutting mill (SM1, Retsch GmbH, Haan, Germany), first to a particle size <2 mm and in a second run <0.2 mm. One variant was fibrillated without further pre-treatment, whereas a second variant was extracted in batches of 20 g with 200 mL NaOH (0.5 M) for 2 h at 80 ◦C while stirring. The supernatant was removed and the pulp was washed and centrifuged before extracting a second time with 100 mL NaOH (0.5 M) for 15 min at 20 ◦C while stirring. The washed residue was bleached with 10 g NaClO2 (80% Techn., Carl Roth GmbH & Co. KG, Essen, Germany) in an acetate buffer (pH = 4.9, prepared from 5 g acetic acid (100% p.a., Carl Roth GmbH & Co. KG), 5 g sodium acetate (≥98.5% pure, Carl Roth GmbH & Co. KG) and 25 mL deionized water) for 2 h at 70 ◦C while stirring. Then the suspension was centrifuged and washed until the chlorine smell disappeared [27]. For reference purposes, a broad variety of cellulosic sources was used for the production of fibrillated material. Fibrillated wood was prepared by disintegrating beech- and spruce wood boards into chips and subsequent milling as described above. While one variant of wood particles was further fibrillated without chemical pre-treatment, a second variant was extracted in batches of 10 g with 250 mL 5% (w/v) NaOH (≥99% p.a., Carl Roth GmbH & Co. KG) for 2 h at 20 ◦C while stirring. Thereafter the supernatant was decanted and wood particles were washed with deionized water to pH = 7. Conventional microfibrillated cellulose was produced from lignin-free never-dried beech Polymers 2017, 9, 326 3 of 12 sulphite pulp provided by Lenzing AG, Wien, Austria. While one variant of pulp was fibrillated as received, a second variant was treated by oxidation using the well-known TEMPO/NaClO/NaClO2 system [28,29] at 60 ◦C for 2 h. All the variants described above were diluted to a solid content of 0.5% in water and dispersed using an Ultra Turrax device (IKA®-Werke GmbH & CO. KG, Staufen, Germany). Mechanical fibrillation was carried out by applying 20 passes through a laboratory homogenizer APV1000 (SPX FLOW Inc., Delavan, WI, USA) at a pressure of 70–80 MPa. An overview of sample codes used together with the most relevant treatments is given in Table1.

Table 1. Sample overview with corresponding codes and most relevant chemical treatment.

CODE Chemical treatment prior to ﬁbrillation sugar beet spruce no treatment beech MFC

NaOH sugar beet NaOH extraction + NaClO2 bleaching NaOH spruce NaOH extraction NaOH beech

TEMPO MFC oxidation by TEMPO/NaClO/NaClO2 system

2.2. Film Casting and Tensile Test After homogenization, the suspensions (solid content 0.5%) were ﬁlled in petri dishes (1000 mL) and stored under a fume hood at room temperature for 48 h to evaporate water. Test specimens were cut with a razor blade punch in dog bone shape, with 25 mm test length. Outer parts were strengthened with an edging tape to prevent breaks in the clamping area. After one week storage at 20 ◦C and 65% RH, specimens were tested with a static materials testing machine (Z020, Zwick GmbH & Co. KG, Ulm, Germany). All samples were tested to failure with a constant test speed of 1 mm·min−1. Tested ﬁlms were used for further microscopic studies.

2.3. Microscopy Fibrillated suspensions were further diluted to 0.005% (w/w) with deionized water and dropped on conventional mica platelets glued on atomic force microscope (AFM) specimen discs. The AFM (Dimension Icon, Bruker, Bremen, Germany) was used in tapping mode with an Otespa tip (resonant frequency 300 kHz, spring constant 42 N·m−1). Overview pictures (20 µm × 20 µm, amplitude error picture) and zoom pictures (2 µm × 2 µm, Z sensor picture) were consulted for analysis. On zoom pictures, the height of the 10 smallest fibrils was measured to determine an average smallest fibril diameter. Therefore the height profile extraction function of an AFM visualization and analysis software (Gwyddion 2.44, Brno, Czech Republic) was applied. Scanning electron microscope (SEM) (Quanta FEG 250, FEI, Brno-Cernovice,ˇ Czech Republic) was used for pictures of fractured film surfaces after the tensile tests, operated with a 5 kV electron beam. Before scanning, surfaces were coated with gold vapour using a sputter coater (Scancoat Six, HHV Ltd., Crawley, UK) adjusting 1 mbar, 20 mA and 3–4 kV for 240 s.

2.4. Determination of Adhesive Bond Strength by Lap-Shear Testing Homogenized suspensions were adjusted to a solid content of 3% (w/w) by careful drying in a drying chamber (Memmert UFB 500, Schwabach, Germany) and further used to glue spruce slats following the European standard EN 302-1:2013 [30]. Slats (length 500 mm × width 120 mm, annual ring alignment 30–90◦) conditioned at 20 ◦C and 65% RH were planed to a final thickness of 5 mm shortly before gluing. Aliquots of suspensions corresponding to 30 g m−2 solid matter Polymers 2017, 9, 326 4 of 12 werePolymers spread 2017, 9 with, 326 a spatula. At a specific pressure of 0.5 MPa and 50 ◦C the slats remained4 of 13 in a laboratory press (Langzauner GmbH, Lambrechten, Austria) for 72 h. 10 test specimens (Langzauner GmbH, Lambrechten, Austria) for 72 h. 10 test specimens (length 150 mm × width 20 × (lengthmm) were 150 mm cut afterwidth a further 20 mm) storage were at cut above after climate a further conditions storage until at above constant climate weight. conditions Testing untilof constantlongitudinal weight. tensile Testing shear of longitudinal strength was tensile performed shear strength in tensile was performedtest mode inwith tensile 0.15 test mm·min mode with−1 −1 0.15deformation mm·min rate.deformation rate. InIn a a second second experiment,experiment, the the applied applied amount amount of fib ofrillated fibrillated material material was increased was increased to 60 and to 120 60 andg −2 120m g−2 msolidsolid matter. matter. Therefore, Therefore, the solid the content solid content of the suspensions of the suspensions was increased was increased to 6.7% and to 6.7% 11.7% and 11.7%which which is more is moreor less or the less maximum the maximum applicable applicable amount without amount squeezing without squeezing out of the slats. out of Pressing the slats. Pressingparameters parameters were adjusted were adjusted to 120 °C to and 120 1 ◦hC pressing and 1 h time pressing in order time to inaccount order for to accountincreased for moisture. increased moisture.Mean values Mean valueswere calculated were calculated for each for group each group and andcompared compared by one-way by one-way analysis analysis of ofvariance variance (Post-hoc-test(Post-hoc-test Tukey Tukey b, b, Levene’s Levene’s test test for for equalityequality ofof variances, Kolmogorov-S Kolmogorov-Smirnovmirnov test test for for normal normal distribution,distribution,p ≤p ≤0.05). 0.05).

3. Results3. Results and and Discussion Discussion

3.1.3.1. Microfibrillated Microfibrillated Cellulose Cellulose from from Different DifferentRaw Raw MaterialsMaterials TheThe morphology morphology of microfibrillatedof microfibrillated cellulose cellulose from from different different raw raw materials materials was was studied studied by means by ofmeans transmitted of transmitted light microscopy light microscopy in order toin obtainorder to an obtain optical an impression optical impression of the degree of the of degree fibrillation of achieved.fibrillation In addition,achieved. atomicIn addition, force microscopyatomic forc wase microscopy used to obtain was used quantitative to obtain measurements quantitative of themeasurements smallest fibril of diameters the smallest achieved. fibril diameters At first achiev glance,ed. light At first microscopy glance, ligh showst microscopy very clear shows differences very betweenclear differences the different between raw materials the differe usednt raw (Figure materials1). used (Figure 1).

FigureFigure 1. Light1. Light microscopic microscopic images images of of different different ﬁbril fibril suspensions: suspensions: (a )(a spruce;) spruce; (b )(b beech;) beech; (c) ( sugarc) sugar beet; (d)beet; MFC; (d ()e MFC;) NaOH (e) NaOH spruce; spruce; (f) NaOH (f) NaOH beech; beech; (g) NaOH (g) NaOH sugar sugar beet; beet; (h) TEMPO (h) TEMPO MFC. MFC.

WithWith all all samples, samples, the the beneficial beneficial effecteffect ofof alkali pre-treatment or or TEMPO TEMPO oxidation, oxidation, respectively, respectively, onon the the degree degree of fibrillationof fibrillation achieved achieved is evident. is eviden Event. Even so, theso, woodthe wood powders powders spruce spruce and beechand beech appear appear to be more recalcitrant towards fibrillation even after alkali pre-treatment, as numerous to be more recalcitrant towards fibrillation even after alkali pre-treatment, as numerous larger fiber larger fiber fragments are prevalent (Figure 1e,f). This effect is more pronounced for NaOH spruce fragments are prevalent (Figure1e,f). This effect is more pronounced for NaOH spruce compared to compared to NaOH beech. Compared to wood powders, sugar beet pulp provided small fibrils as NaOH beech. Compared to wood powders, sugar beet pulp provided small fibrils as far as this can far as this can be deduced from light microscopy (Figure 1c). While MFC (Figure 1d) still showed a be deduced from light microscopy (Figure1c). While MFC (Figure1d) still showed a high amount of high amount of fibrils discernible by light microscopy, this was no longer the case for NaOH sugar fibrilsbeet discernibleand TEMPO by MFC, light respectively, microscopy, indica this wasting novery longer fine fibrils the case (Figure for NaOH 1g,h). sugar beet and TEMPO MFC, respectively,In general, it indicatingis difficult veryto capture fine fibrils the entire (Figure structural1g,h). variability of (partly) fibrillated cellulose withIn general,one microscopic it is difficult method to capturealone. Therefore, the entire AFM structural was used variability complementary of (partly) to fibrillatedlight microscopy cellulose within oneorder microscopic to cover the method smallest alone. fibrillary Therefore, structures AFM found was used in each complementary variant. It was to assumed light microscopy that the in ordersmallest to cover fibril the fraction smallest is of fibrillary particular structures relevance found to the in performance each variant. of It MFC was assumedproducts, that because the smallest even

Polymers 2017, 9, 326 5 of 13 Polymers 2017, 9, 326 5 of 12 though its mass fraction may be small, its contribution to the overall accessible specific surface area Polymers 2017, 9, 326 5 of 13 is significant.fibril fraction In is accordance of particular with relevance light microscopy to the performance (Figure of 1), MFC AFM products, (Figure because 2) also even reveals though distinct its differencesmassthough fraction itsin massfibril may fraction befineness small, may its between be contribution small, itsthe contribu toindividu the overalltional to accessiblevariantsthe overall studied specific accessible surfacesolely specific areabased surface is significant. on area visual inspection.Inis accordance significant. Quantification with In accordance light microscopyof fibril with height light (Figure microscopydirectly1), AFM from (Figure(Figure AFM2 1),) images also AFM reveals (Figure shown distinct 2) in also Figure differences reveals 3 confirms distinct in fibril this visualfinenessdifferences impression. between in Withfibril the individual regardfineness to between variants the average studiedthe individuvalues solely ofal based fibrilvariants onheight visualstudied shown, inspection. solely it shouldbased Quantification onbe notedvisual that of thesefibrilinspection. values height represent directly Quantification from only AFM the of fibril lower images height tail shown directlyof the in Figuredi fromstribution 3AFM confirms images of fibril this shown visual diameters in impression. Figure and 3 confirms do With not regard thisgive an indicationtovisual the average ofimpression. characteristic values With of fibril overallregard height tofibril the shown, averagediameters. it values should of be fibril noted height that shown, these values it should represent be noted only that the lowerthese tail values of the represent distribution only ofthe fibril lower diameters tail of the and distribution do not give of anfibril indication diameters of and characteristic do not give overall an indication of characteristic overall fibril diameters. fibril diameters.

FigureFigureFigure 2. 2.Atomic 2.Atomic Atomic force force force microscopicmicroscopic microscopic (AFM) (AFM)(AFM) images images ofof of th theee smallestsmallest smallest fibril ﬁbrilfibril fraction fractionfraction found foundfound in in differentin different different fibrilﬁbril fibrilsuspensions: suspensions: suspensions: (a) ( aspruce;(a)) spruce; spruce; (b ()(b bbeech;)) beech;beech; ( c ()c )sugar sugar beet;beet; beet; ( (dd)) ) MFC;MFC; MFC; ( e( )e )NaOH NaOH spruce; spruce;spruce; (f) ( fNaOH(f)) NaOH NaOH beech; beech; beech; (g) (NaOHg)(g NaOH) NaOH sugar sugar sugar beet; beet; beet; (h ()h (TEMPOh)) TEMPO TEMPO MFC. MFC. MFC.

Figure 3. Box and whisker plot of the height of the smallest fibrils found in AFM topography images shown in Figure 2 (10 fibrils each were measured). FigureFigure 3. Box 3. Box and and whisker whisker plot plot of of the the height height of of the the sm smallestallest fibrils ﬁbrils found inin AFMAFM topography topography images images 3.2. Nanopapers Prepared from Different Fibrillated Materials shownshown in Figure in Figure 2 2(10 (10 fibrils ﬁbrils each each were were measured). measured). The tensile strength and modulus of elasticity of dry films of fibrillated cellulosic materials 3.2. Nanopapersprovide valuable Prepared insights from Differentinto the differFibrillatedent performance Materials of fibrillated materials [31–33], which is The tensile strength and modulus of elasticity of dry films of fibrillated cellulosic materials provide valuable insights into the different performance of fibrillated materials [31–33], which is

Polymers 2017, 9, 326 6 of 12

3.2.Polymers Nanopapers 2017, 9, 326 Prepared from Different Fibrillated Materials 6 of 13

why Thenanopapers tensile strength were produced and modulus and tested of elasticity prior to ofadhesion dry films testing. of fibrillated Overall, cellulosica great diversity materials of providefilm thickness, valuable ranging insights between into the 0.02 different mm and performance 0.15 mm ofand, fibrillated concurrently, materials density [31–33 ranging], which from is why 0.3 nanopapersg·cm−3 up to were 1.4 g·cm produced−3 was and obtained tested prior(Figure to adhesion4a). The lowest testing. film Overall, densities a great were diversity observed of film for −3 thickness,fibrillated ranginguntreated between wood 0.02 powders, mm and showing 0.15 mm that and, these concurrently, materials density were rangingparticularly from recalcitrant 0.3 g·cm −3 uptowards to 1.4 proper g·cm fibrillation,was obtained whereas (Figure fibrillated4a). The lowest sugar filmbeet densitiesyielded high-density were observed nanopaper for fibrillated even untreatedwithout pre-treatment. wood powders, Improved showing that fibrillation these materials after werealkali particularly pre-treatment, recalcitrant which towards was already proper fibrillation,presumed based whereas on fibrillatedthe optical sugar impression beet yielded given high-densityby light microscopy nanopaper (Figure even 1), without is confirmed pre-treatment. by the Improvedconsistent fibrillationincrease in afternanopaper alkali pre-treatment,density observed which for wasthese already variants. presumed As for MFC, based high on the density optical is impressionobserved, which given is by furthermore light microscopy improved (Figure by1 TEMPO), is confirmed oxidation by thepre-treatment consistent increase to the highest in nanopaper density densityvalues among observed all variants for these studied. variants. As for MFC, high density is observed, which is furthermore improved by TEMPO oxidation pre-treatment to the highest density values among all variants studied.

Figure 4. Box and whisker plots of (a) density; (b) tensile strength; (c) modulus of elasticity; and (Figured) elongation 4. Box and at break whisker for nanopapers plots of (a) produceddensity; (b from) tensile different strength; raw ( materials.c) modulus of elasticity; and (d) elongation at break for nanopapers produced from different raw materials. The mechanical performance (strength and modulus of elasticity, Figure4b,c) of nanopapers in The mechanical performance (strength and modulus of elasticity, Figure 4b,c) of nanopapers in tensile testing shows trends similar to film density, with the notable exception of untreated fibrillated tensile testing shows trends similar to film density, with the notable exception of untreated sugar beet. Even though high-density nanopaper with an average density of 1.25 g·cm−3 was obtained, fibrillated sugar beet. Even though high-density nanopaper with an average density of 1.25 g·cm−3 the mechanical performance of this material is modest and very similar to untreated fibrillated wood was obtained, the mechanical performance of this material is modest and very similar to untreated powder. Overall, this group of three untreated fibrillated materials shows the lowest mechanical fibrillated wood powder. Overall, this group of three untreated fibrillated materials shows the properties compared to all other variants. While this poor performance may be explained by a low lowest mechanical properties compared to all other variants. While this poor performance may be degree of fibrillation together with a low film density in the case of untreated fibrillated spruce explained by a low degree of fibrillation together with a low film density in the case of untreated and beech, untreated sugar beet presumably performs poorly due to the presence of a substantial fibrillated spruce and beech, untreated sugar beet presumably performs poorly due to the presence percentage of non-cellulosic constituents. Roughly, sugar beet pulp only consists of 20–25% weight of a substantial percentage of non-cellulosic constituents. Roughly, sugar beet pulp only consists of cellulose [34,35] compared to 50% in wood. 20–25% weight cellulose [34,35] compared to 50% in wood. Alkali pre-treatment, which results in increased film density, is also beneficial to the mechanical performance of sugar beet, spruce, and beech. Finally, TEMPO MFC, which is known to provide

Polymers 2017, 9, 326 7 of 12

Alkali pre-treatment, which results in increased film density, is also beneficial to the mechanical performance of sugar beet, spruce, and beech. Finally, TEMPO MFC, which is known to provide Polymers 2017, 9, 326 7 of 13 nanopapers of superior performance [36–38], clearly shows the highest tensile strength. Interestingly, thenanopapers modulus ofof elasticity superior of TEMPOperformance MFC [36–38], nanopaper clearly is matched shows bythe alkali-treated highest tensile fibrillated strength. beech (NaOHInterestingly, beech). the Overall, modulus the of strength elasticity and of stiffnessTEMPO ofMFC nanopapers nanopaper produced is matched in by the alkali-treated present study comparefibrillated well beech with (NaOH the values beech). found Over inall, literature the strength [31]. Thisand alsostiffness holds of truenanopapers for the elongation produced in at the break (Figurepresent4d). study For this compare parameter, well with highest the valuesvalues indicatingfound in literature a certain mobility[31]. Thisof also fibril-fibril holds true bonds for the were observedelongation for NaOHat break sugar (Figure beet 4d). and For MFC this nanopapers.parameter, highest values indicating a certain mobility of fibril-fibrilScanning bonds electron weremicroscopy observed forof NaOH the fracture sugar beet surfaces and MFC of nanopapers. nanopapers tested in tension helps understandingScanning someelectron of themicroscopy differences of the observed fracture during surfaces mechanical of nanopapers characterisation. tested in tension Clearly, helps the woodunderstanding powders directly some of subjected the differences to fibrillation observed without during anymechanical pre-treatment characterisation. other than Clearly, swelling the in waterwood show powders insufficient directly fibrillation subjected to (Figure fibrillation5a,b). without Even though, any pre-treatment occasionally, other fine than individual swelling fibrils in arewater observed, show insufficient large micron-scale fibrillation fibrous (Figure cell 5a,b). wall Even fragments though, dominate occasionally, the appearancefine individual of fibrils fracture surfaces.are observed, Furthermore, large micron-scale the structure fibrous appears cell ratherwall fragments loose and dominate porous, correspondingthe appearance wellof fracture with the lowsurfaces. density Furthermore, measured for the these structure variants. appears Compared rather toloose fibrillated and porous, wood corresponding powder, fibrillated well with sugar the beet low density measured for these variants. Compared to fibrillated wood powder, fibrillated sugar pulp (Figure5c) exhibits a more compact and consolidated surface, and only few individual fibrils beet pulp (Figure 5c) exhibits a more compact and consolidated surface, and only few individual may be discerned. Contrary to that, the MFC fracture surface (Figure5d) is very rough, with many fibrils may be discerned. Contrary to that, the MFC fracture surface (Figure 5d) is very rough, with fine fibrils protruding from the material. As to the effect of alkali pre-treatment before fibrillation, many fine fibrils protruding from the material. As to the effect of alkali pre-treatment before only marginal changes are observed for NaOH spruce (Figure5e), whereas NaOH beech (Figure5f) fibrillation, only marginal changes are observed for NaOH spruce (Figure 5e), whereas NaOH beech shows(Figure much 5f) shows finer fibrils much comparedfiner fibrils tocompared untreated to beech.untreated Similarly, beech. Similarly, also the fracturealso the fracture surface ofsurface NaOH sugarof NaOH beet (Figuresugar beet5g) (Figure shows 5g) a much shows more a much fibrillary more fibrillary appearance appearance similar similar to MFC, to MFC, compared compared to the untreatedto the untreated variant. variant. The highly The fibrillaryhighly fibrillary appearance appear ofance fracture of fracture surfaces surfaces for these for these two variantstwo variants agrees wellagrees with well the hypothesiswith the hypothesis of inter-fibrillar of inter-fibril bond mobilitylar bond being mobility responsible being responsible for their high for elongationtheir high at breakelongation (Figure 4atd). break On the(Figure contrary, 4d). TEMPOOn the contrary, MFC (Figure TEMPO5h) differsMFC (Figure from all 5h) other differs variants from inall that other the fracturevariants surface in that is densethe fracture and smooth surface overall, is dense with and many smooth very overall, small fibrils with discernible,many very correspondingsmall fibrils withdiscernible, limited elongation corresponding at break. with limited elongation at break.

FigureFigure 5. Representative5. Representative SEM SEM images images of fractureof fracture surfaces surfaces of nanopapers of nanopapers after after tensile tensile testing: testing: (a) spruce; (a) (b)spruce; beech; (b ()c )beech; sugar (c) beet; sugar ( dbeet;) MFC; (d) MFC; (e) NaOH (e) NaOH spruce; spruce; (f )(f NaOH) NaOH beech;beech; ( (gg)) NaOH NaOH sugar sugar beet; beet; (h)(h TEMPO) TEMPO MFC. MFC.

3.3. Adhesive Strength by Lap-Shear Testing 3.3. Adhesive Strength by Lap-Shear Testing The adhesive strength obtained for different fibrillated materials shown in Figure 6 varied The adhesive strength obtained for different ﬁbrillated materials shown in Figure6 varied roughly roughly between average values of 1.5 up to 3.0 MPa. Among the tested variants, sugar beet and between average values of 1.5 up to 3.0 MPa. Among the tested variants, sugar beet and NaOH NaOH beech show best performance. While NaOH beech also performed well in terms of nanopaper beechstrength show and best modulus, performance. the good Whileperformance NaOH of beechuntreated also fibrillated performed sugar well beet in does terms not ofcorrespond nanopaper to nanopaper performance. Typically, comparable set-ups using spruce wood as a substrate for widely-used commercial wood adhesives, shear strength values in the order of 7 MPa are achieved

Polymers 2017, 9, 326 8 of 12

Polymersstrength 2017, and9, 326 modulus, the good performance of untreated ﬁbrillated sugar beet does not correspond8 of 13 to nanopaper performance. Typically, comparable set-ups using spruce wood as a substrate for [39].widely-used Thus, even commercial though substantial wood adhesives, self-adhesive shear strength (nanopapers) values in the and order adhesive of 7 MPa strength are achieved (lap-shear [39]. specimens)Thus, even is though developed, substantial absolute self-adhesive values (nanopapers)reach only andapprox. adhesive half strengththe magnitude (lap-shear of specimens) standard industrialis developed, adhesives. absolute values reach only approx. half the magnitude of standard industrial adhesives.

FigureFigure 6. Box 6. Box and and whisker whisker plot plot of of the the adhesive adhesive strength strength expressed expressed by shear strengthstrength of of bonded bonded wood wood specimensspecimens for for fibrillated ﬁbrillated material material from from different different raw raw materials. materials.

A discussionA discussion of of the the systems systems observed observed inin viewview of of adhesion adhesion theories theories may may help help in understanding in understanding the thedifferences differences observed. observed. According According to theto the state state of the of artthe in art wood in wood adhesion, adhesion, polar interactionspolar interactions with wood with wood–OH –OH groups groups play aplay dominant a dominant role in role wood in adhesion, wood adhesion, while interlocking while interlocking betweenthe between cured adhesivethe cured adhesiveand the and porous the woodporous structure wood structure may also providemay also aminor provide contribution a minor contribution to overall bond to strengthoverall [bond40]. strengthAs shown [40]. inAs Figure shown7, penetrationin Figure 7, ofpenetration adhesive into of adhesive the porous into wood the porous structure, wood which structure, is common which for is commonindustrial for woodindustrial adhesives, wood isadhesives, not present is whennot present using fibrillatedwhen using (ligno-)cellulose. fibrillated (ligno-)cellulose. On the contrary, On thea contrary, very sharp a interfacevery sharp between interface compressed between fibrillated compressed material fibrillated between material the bonded between wood the pieces bonded is wooddiscernible pieces is for discernible the adhesive for bondthe adhesive lines found bond in thelines present found study. in the Thus,present the st potentialudy. Thus, contribution the potential of contributionthis mechanism of this is mechanism lost. Furthermore, is lost. penetration Furthermore, of adhesive penetration into woodof adhesive pores is into an indicatorwood pores of good is an indicatorwetting, of which good maximises wetting, which the contact maximises surface betweenthe contact the roughsurface wood between substrate the andrough the wood adhesive substrate used. Lack of penetration into pores is thus clearly a shortcoming of fibrillated (ligno-)cellulose compared to and the adhesive used. Lack of penetration into pores is thus clearly a shortcoming of fibrillated conventional industrial adhesives. (ligno-)cellulose compared to conventional industrial adhesives. With regard to polar interactions, fibrillated (ligno-)cellulose disposes of a high adhesion potential to woody substrate [41], which is facilitated by the intrinsic high specific surface area of this material. With regard to AFM characterisation results shown in Figure3, it was proposed that the size of the smallest fibrils may be an indicator for differences in the overall specific surface area of the variants studied. A simple correlation analysis of this parameter and nanopaper strength on the one hand, and adhesive strength on the other hand, is shown in Figure8. While there is no correlation between nanopaper strength and fibril diameter of smallest fibrils, a clear trend of decreasing adhesion strength with increasing diameter of smallest fibrils is apparent. The smallest fibril diameters in ascending order were observed for TEMPO MFC, sugar beet, and NaOH beech (Figure3). Interestingly, TEMPO MFC, which performs best in terms of nanopaper strength and modulus, shows reduced adhesion strength compared to sugar beet and NaOH beech,

Polymers 2017, 9, 326 9 of 12 respectively. The alkaline treatment of beech wood deacetylates hemicellulose and increases the amount of OH-groups making them more water soluble [42]. This alteration of chemical composition could be a reason for higher adhesive strength because of a better penetration into wood in comparison Polymersto TEMPO 2017, 9 MFC, 326 [43]. It is proposed that particularly in the case of sugar beet, the high content9 of 13 of non-cellulosic carbohydrates may help to explain the good adhesion performance of sugar beet, which shows the best performance of all variants studied. In order to investigate whether further improvement was possible by simply adding more material, the spreading quantity of sugar beet suspension was increased from 30 up to 120 g·m−2 (Figure9). Overall, there is a clear trend of increasing adhesive strength with increasing spreading quantity and strength levels up to more than 60% of the strength level of commercial urea formaldehyde adhesive or similar strengths as starch Polymersbased wood 2017, 9 adhesives, 326 [44] are achieved with this fully bio-based variant. 9 of 13

Figure 7. Incident light microscopic image of a typical wood adhesive bond line using the industrial wood adhesive urea-formaldehyde compared to a typical bond line using fibrillated material produced in the present study (inset). Arrows denote the approximate thickness of the bond line.

With regard to polar interactions, fibrillated (ligno-)cellulose disposes of a high adhesion potential to woody substrate [41], which is facilitated by the intrinsic high specific surface area of this material. With regard to AFM characterisation results shown in Figure 3, it was proposed that the size of the smallest fibrils may be an indicator for differences in the overall specific surface area Figure 7. Incident light microscopic image of a typical wood adhesive bond line using the industrial of the variants studied. A simple correlation analysis of this parameter and nanopaper strength on wood adhesiveadhesive urea-formaldehyde urea-formaldehyde compared compared to a to typical a typical bond linebond using line ﬁbrillated using fibrillated material producedmaterial the oneproducedin the hand, present inand the study adhesive present (inset). study strength Arrows (inset). on denote Arrows the other the de approximatenote hand, the is approximate shown thickness in Figure ofthic thekness bond 8. of line. the bond line.

Figure 8. Scatter plots of (a) the correlation between fibril diameter and lap shear strength of bonded Figure 8. Scatter plots of (a) the correlation between fibril diameter and lap shear strength of bonded wood specimens with fibrillated material from different raw materials as well as (b) the non-correlating wood specimens with fibrillated material from different raw materials as well as (b) the comparison of fibril diameter and tensile strength of nanopapers produced from different raw materials. non-correlating comparison of fibril diameter and tensile strength of nanopapers produced from different raw materials.

While there is no correlation between nanopaper strength and fibril diameter of smallest fibrils, a clear trend of decreasing adhesion strength with increasing diameter of smallest fibrils is apparent. The smallest fibril diameters in ascending order were observed for TEMPO MFC, sugar beet, and Figure 8. Scatter plots of (a) the correlation between fibril diameter and lap shear strength of bonded NaOH beech (Figure 3). Interestingly, TEMPO MFC, which performs best in terms of nanopaper wood specimens with fibrillated material from different raw materials as well as (b) the non-correlating comparison of fibril diameter and tensile strength of nanopapers produced from different raw materials.

Polymers 2017, 9, 326 10 of 13 strength and modulus, shows reduced adhesion strength compared to sugar beet and NaOH beech, respectively. The alkaline treatment of beech wood deacetylates hemicellulose and increases the amount of OH-groups making them more water soluble [42]. This alteration of chemical composition could be a reason for higher adhesive strength because of a better penetration into wood in comparison to TEMPO MFC [43]. It is proposed that particularly in the case of sugar beet, the high content of non-cellulosic carbohydrates may help to explain the good adhesion performance of sugar beet, which shows the best performance of all variants studied. In order to investigate whether further improvement was possible by simply adding more material, the spreading quantity of sugar beet suspension was increased from 30 up to 120 g·m−2 (Figure 9). Overall, there is a clear trend of increasing adhesive strength with increasing spreading quantity and strength levels up to more than 60% of the strength level of commercial urea formaldehyde adhesive or similar strengths as starch based wood adhesives [44] are achieved with this fully bio-based Polymers 2017, 9, 326 10 of 12 variant.

Figure 9. Box and whisker plot of the adhesive strength expressed by shear strength of wood specimens Figure 9. Box and whisker plot of the adhesive strength expressed by shear strength of wood bonded with ﬁbrillated sugar beet pulp using increasing spreading quantities. specimens bonded with fibrillated sugar beet pulp using increasing spreading quantities.

4.4. Conclusions Conclusions TheThe results results presented presented above demonstrate thatthat fibrillatedfibrillated materialmaterial derived derived from from various various sources sources of of(ligno-)cellulosic (ligno-)cellulosic raw raw materials materials delivers delivers nanopapers nanopapers with with highly highly variable variable properties. properties. Overall, Overall, density densityof the nanopapers of the nanopapers formed formed correlates correlates well with well their with mechanical their mechanical performance, performance, with highest with densitieshighest densitiesoften also often showing also showing highest strengthhighest strength and stiffness, and stiffness, as is well as known. is well Sugarknown. beet, Sugar which beet, contains which containscomparatively comparatively less cellulose, less cellulose, is an exception is an exception to this trend.to this trend. Regarding Regarding adhesive adhesive performance performance of the offibrillated the fibrillated materials, materials, fibrillated fibrillated sugar beetsugar and beet fibrillated and fibrillated NaOH pre-treatedNaOH pre-treated beech wood beech perform wood performbest, with best, values with up values to 60% up ofto the60% strength of the strength of standard of standard industrial industrial adhesives. adhesives. It is concluded It is concluded that the thatadhesion the adhesion potential potential of fibrillated of fibrillated (ligno-)cellulose (ligno-)cellulose is very promising is very promising in dry condition, in dry butcondition, is also limitedbut is alsoby lacklimited of penetration by lack of penetration into the porous into woodthe porous structure. wood structure. Acknowledgments: Thanks to the Austrian Research Promotion Agency (FFG) supporting this project in Acknowledgments:collaboration with theThanks Fritz to Egger the Austrian GmbH & Research Co., OG Promotio as industrialn Agency partner, (FFG) especially supporting Martin this Steinwender project in collaborationposthumous. with Thanks the to Fritz Agrana Egger Research GmbH & Innovation& Co., OG Center as industrial GmbH for partner, supporting espe withcially raw Martin material, Steinwender especially posthumous.Walter Hein. Thanks to Agrana Research & Innovation Center GmbH for supporting with raw material, especiallyAuthor Contributions: Walter Hein. Stefan Pinkl, Stefan Veigel, and Wolfgang Gindl-Altmutter conceived and designed the experiments; Stefan Pinkl, Stefan Veigel, and Jerome Colson performed the experiments; Stefan Pinkl and Wolfgang Gindl-Altmutter analyzed the data; Stefan Pinkl and Wolfgang Gindl-Altmutter wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

1. Pizzi, A. Recent developments in eco-efﬁcient bio-based adhesives for wood bonding: Opportunities and issues. J. Adhes. Sci. Technol. 2006, 20, 829–846. [CrossRef] 2. Kalami, S.; Arefmanesh, M.; Master, E.; Nejad, M. Replacing 100% of phenol in phenolic adhesive formulations with lignin. J. Appl. Polym. Sci. 2017, 134, 30. [CrossRef] 3. Pizzi, A. Chemistry and Technology of Cold- and Thermosetting Tannin-Based Exterior Wood Adhesives. Ph.D. Thesis, University of the Orange Free State, Bloemfontein, South Africa, 1978. 4. Pizzi, A. Research vs. Industrial practice with tannin-based adhesives. In Adhesives from Renewable Resources; American Chemical Society: Washington, DC, USA, 1989; Volume 385, pp. 254–267. Polymers 2017, 9, 326 11 of 12

5. Luckeneder, P.; Gavino, J.; Kuchernig, R.; Petutschnigg, A.; Tondi, G. Sustainable Phenolic Fractions as Basis for Furfuryl Alcohol-Based Co-Polymers and Their Use as Wood Adhesives. Polymers 2016, 8, 15. [CrossRef] 6. Felby, C.; Hassingboe, J.; Lund, M. Pilot-scale production of fiberboards made by laccase oxidized wood fibers: Board properties and evidence for cross-linking of lignin. Enzyme Microb. Technol. 2002, 31, 736–741. [CrossRef] 7. Felby, C.; Pedersen, L.S.; Nielsen, B.R. Enhanced auto adhesion of wood fibers using phenol oxidases. Holzforschung 1997, 51, 281–286. [CrossRef] 8. Felby, C.; Thygesen, L.G.; Sanadi, A.; Barsberg, S. Native lignin for bonding of fiber boards—Evaluation of bonding mechanisms in boards made from laccase-treated fibers of beech (Fagus sylvatica). Ind. Crop. Prod. 2004, 20, 181–189. [CrossRef] 9. Widsten, P.; Kandelbauer, A. Laccase applications in the forest products industry: A review. Enzyme Microb. Technol. 2008, 42, 293–307. [CrossRef] 10. Widsten, P.; Laine, J.E.; Tuominen, S.; Qvintus-Leino, P. Effect of high defibration temperature on the properties of medium-density fiberboard (mdf) made from laccase-treated hardwood fibers. J. Adhes. Sci. Technol. 2003, 17, 67–78. [CrossRef] 11. Widsten, P.; Tuominen, S.; Qvintus-Leino, P.; Laine, J.E. The influence of high defibration temperature on the properties of medium-density fiberboard (mdf) made from laccase-treated softwood fibers. Wood Sci. Technol. 2004, 38, 521–528. [CrossRef] 12. Van Dam, J.E.G.; van den Oever, M.J.A.; Teunissen, W.; Keijsers, E.R.P.; Peralta, A.G. Process for production of high density/high performance binderless boards from whole coconut husk—Part 1: Lignin as intrinsic thermosetting binder resin. Ind. Crop. Prod. 2004, 19, 207–216. [CrossRef] 13. Angles, M.N.; Ferrando, F.; Farriol, X.; Salvado, J. Suitability of steam exploded residual softwood for the production of binderless panels. Effect of the pre-treatment severity and lignin addition. Biomass Bioenergy 2001, 21, 211–224. [CrossRef] 14. Angles, M.N.; Reguant, J.; Montane, D.; Ferrando, F.; Farriol, X.; Salvado, J. Binderless composites from pretreated residual softwood. J. Appl. Polym. Sci. 1999, 73, 2485–2491. [CrossRef] 15. Suzuki, S.; Shintani, H.; Park, S.Y.; Saito, K.; Laemsak, N.; Okuma, M.; Iiyama, K. Preparation of binderless boards from steam exploded pulps of oil palm (elaeis guineensis jaxq.) fronds and structural characteristics of lignin and wall polysaccharides in steam exploded pulps to be discussed for self-bindings. Holzforschung 1998, 52, 417–426. [CrossRef] 16. Velasquez, J.A.; Ferrando, F.; Salvado, J. Effects of kraft lignin addition in the production of binderless fiberboard from steam exploded miscanthus sinensis. Ind. Crop. Prod. 2003, 18, 17–23. [CrossRef] 17. Lamaming, J.; Sulaiman, O.; Sugimoto, T.; Hashim, R.; Said, N.; Sato, M. Influence of chemical components of oil palm on properties of binderless particleboard. Bioresources 2013, 8, 3358–3371. [CrossRef] 18. Ellis, S.; Paszner, L. Activated self-bonding of wood and agricultural residues. Holzforschung 1994, 48, 82–90. [CrossRef] 19. Zhang, D.H.; Zhang, A.J.; Xue, L.X. A review of preparation of binderless fiberboards and its self-bonding mechanism. Wood Sci. Technol. 2015, 49, 661–679. [CrossRef] 20. Mobarak, F.; Fahmy, Y.; Augustin, H. Binderless lignocellulose composite from bagasse and mechanism of self-bonding. Holzforschung 1982, 36, 131–135. [CrossRef] 21. Jost, J.; Runkel, R. Verfahren und Vorrichtung zur Herstellung von Formlingen aus Holz, Holzabfaellen oder verholzten Pflanzenteilen unter Druck bei hoeheren Temperaturen. German Patent DE000000841055B, 2 November 1948. 22. Osong, S.H.; Norgren, S.; Engstrand, P. Processing of wood-based microfibrillated cellulose and nanofibrillated cellulose, and applications relating to papermaking: A review. Cellulose 2015, 23, 93–123. [CrossRef] 23. Boufi, S.; González, I.; Delgado-Aguilar, M.; Tarrès, Q.; Pèlach, M.À.; Mutjé, P. Nanofibrillated cellulose as an additive in papermaking process: A review. Carbohydr. Polym. 2016, 154, 151–166. [CrossRef][PubMed] 24. Yousefi Shivyari, N.; Tajvidi, M.; Bousfield, D.W.; Gardner, D.J. Production and characterization of laminates of paper and cellulose nanofibrils. ACS Appl. Mater. Interfaces 2016, 8, 25520–25528. [CrossRef][PubMed] 25. Tajvidi, M.; Gardner, D.J.; Bousfield, D.W. Cellulose nanomaterials as binders: Laminate and particulate systemscellulose nanomaterials as binders: Laminate and particulate systems. J. Renew. Mater. 2016, 4, 365–376. [CrossRef] Polymers 2017, 9, 326 12 of 12

26. Arevalo, R.; Peijs, T. Binderless all-cellulose fibreboard from microfibrillated lignocellulosic natural fibres. Compos. A Appl. Sci. Manuf. 2016, 83, 38–46. [CrossRef] 27. Leitner, J.; Hinterstoisser, B.; Wastyn, M.; Keckes, J.; Gindl, W. Sugar beet cellulose nanofibril-reinforced composites. Cellulose 2007, 14, 419–425. [CrossRef] 28. Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A. Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation using tempo catalyst under neutral conditions. Biomacromolecules 2009, 10, 1992–1996. [CrossRef][PubMed] 29. Saito, T.; Hirota, M.; Tamura, N.; Isogai, A. Oxidation of bleached wood pulp by tempo/naclo/naclo2 system: Effect of the oxidation conditions on carboxylate content and degree of polymerization. J. Wood Sci. 2010, 56, 227–232. [CrossRef] 30. CEN. Adhesives for Load-Bearing Timber Structures—Test Methods—Part 1: Determination of Longitudinal Tensile Shear Strength; EN 302-1:2013; UNMZ: Prague, Czech Republic, 2013. 31. Henriksson, M.; Berglund, L.A.; Isaksson, P.; Lindstrom, T.; Nishino, T. Cellulose nanopaper structures of high toughness. Biomacromolecules 2008, 9, 1579–1585. [CrossRef][PubMed] 32. Ferrer, A.; Quintana, E.; Filpponen, I.; Solala, I.; Vidal, T.; Rodriguez, A.; Laine, J.; Rojas, O.J. Effect of residual lignin and heteropolysaccharides in nanofibrillar cellulose and nanopaper from wood fibers. Cellulose 2012, 19, 2179–2193. [CrossRef] 33. Henniges, U.; Veigel, S.; Lems, E.M.; Bauer, A.; Keckes, J.; Pinkl, S.; Gindl-Altmutter, W. Microfibrillated cellulose and cellulose nanopaper from miscanthus biogas production residue. Cellulose 2014, 21, 1601–1610. [CrossRef] 34. Dinand, E.; Chanzy, H.; Vignon, M.R. Suspensions of cellulose microfibrils from sugar beet pulp. Food Hydrocoll. 1999, 13, 275–283. [CrossRef] 35. Hietala, M.; Sain, S.; Oksman, K. Highly redispersible sugar beet nanofibers as reinforcement in bionanocomposites. Cellulose 2017, 24, 2177–2189. [CrossRef] 36. Gindl-Altmutter, W.; Veigel, S.; Obersriebnig, M.; Tippelreither, C.; Keckes, J. High-modulus oriented cellulose nanopaper. In Functional Materials from Renewable Sources; Liebner, F., Rosenau, T., Eds.; American Chemical Society: Washington, DC, USA, 2012; Volume 1107, pp. 3–16. 37. Sehaqui, H.; Mushi, N.E.; Morimune, S.; Salajkova, M.; Nishino, T.; Berglund, L.A. Cellulose nanofiber orientation in nanopaper and nanocomposites by cold drawing. ACS Appl. Mater. Interfaces 2012, 4, 1043–1049. [CrossRef][PubMed] 38. Lindström, T. Aspects on nanofibrillated cellulose (nfc) processing, rheology and nfc-film properties. Curr. Opin. Colloid Interface Sci. 2017, 29, 68–75. [CrossRef] 39. Konnerth, J.; Gindl, W.; Harm, M.; Müller, U. Comparing dry bond strength of spruce and beech wood glued with different adhesives by means of scarf- and lap joint testing method. Holz als Roh- und Werkstoff 2006, 64, 269–271. [CrossRef] 40. Gardner, D.J.; Blumentritt, M.; Wang, L.; Yildirim, N. Adhesion theories in wood adhesive bonding. In Progress in Adhesion and Adhesives; John Wiley & Sons, Inc.: San Francisco, CA, USA, 2015; pp. 125–168. 41. Gardner, D.J.; Oporto, G.S.; Mills, R.; Samir, M. Adhesion and surface issues in cellulose and nanocellulose. J. Adhes. Sci. Technol. 2008, 22, 545–567. [CrossRef] 42. Gregorova, A.; Wimmer, R.; Hrabalova, M.; Koller, M.; Ters, T.; Mundigler, N. Effect of surface modification of beech wood flour on mechanical and thermal properties of poly (3-hydroxybutyrate)/wood flour composites. Holzforschung 2009, 63, 565–570. [CrossRef] 43. Tondi, G.; Wieland, S.; Wimmer, T.; Schnabel, T.; Petutschnigg, A. Starch-sugar synergy in wood adhesion science: Basic studies and particleboard production. Eur. J. Wood Wood Prod. 2012, 70, 271–278. [CrossRef] 44. Wang, Z.J.; Li, Z.F.; Gu, Z.B.; Hong, Y.; Cheng, L. Preparation, characterization and properties of starch-based wood adhesive. Carbohydr. Polym. 2012, 88, 699–706. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).