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Wood cellulose biocomposites with fibrous structures at micro- and nanoscale
Composites Science and Technology 71 (2011) 382–387
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Wood cellulose biocomposites with fibrous structures at micro- and nanoscale Houssine Sehaqui a, Maël Allais b, Qi Zhou a,c, Lars A. Berglund a,d,⇑ a
Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-10044 Stockholm, Sweden Ecole Nationale Supérieure d’Arts et Métiers, Esplanade des Arts et Métiers, 33405 Talence, France c School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, SE-10691 Stockholm, Sweden d Wallenberg Wood Science Center, Royal Institute of Technology, SE-10044 Stockholm, Sweden b
a r t i c l e
i n f o
Article history: Received 16 September 2010 Received in revised form 30 November 2010 Accepted 3 December 2010 Available online 9 December 2010 Keywords: Nanofibrillated cellulose (NFC) A. Fibers A. Nano composites B. Mechanical properties Scanning electron microscopy (SEM)
a b s t r a c t High-strength composites from wood fiber and nanofibrillated cellulose (NFC) were prepared in a semiautomatic sheet former. The composites were characterized by tensile tests, dynamic mechanical thermal analysis, field-emission scanning electron microscopy, and porosity measurements. The tensile strength increased from 98 MPa to 160 MPa and the work to fracture was more than doubled with the addition of 10% NFC to wood fibers. A hierarchical structure was obtained in the composites in the form of a micro-scale wood fiber network and an additional NFC nanofiber network linking wood fibers and also occupying some of the micro-scale porosity. Deformation mechanisms are discussed as well as possible applications of this biocomposites concept. Ó 2010 Published by Elsevier Ltd.
1. Introduction Biocomposites based on plant fibers are interesting for many branches of industry. The low cost of many types of plant fiber feedstock has historically been an important motivation. Recently, also environmental concerns have increased the interest in materials from renewable resources. Currently used biocomposites include thermoplastics combined with plant fibers. Woodthermoplastic composites may serve as an example, where typically polyethylene (PE) or polypropylene (PP) for melt-processing is combined with wood fibers, for instance in the form of saw dust [1–3]. This type of biocomposite has low material cost but shows low strength and brittleness [4]. Another more traditional type of biocomposites is paperboard and fiberboard materials [2], which are essentially porous fibrous networks where interfiber bonding may be ensured through polymer binders. They are also used in large-volume applications where high production rate and low cost are more important than materials performance. Despite the industrial focus on large-volume products of low mechanical performance (modulus typically below 5 GPa, strength below 70 MPa) [4], cellulose has remarkable potential as a highstrength constituent in biocomposites. Wood fibers (30 lm diameter, 3 mm length), which have been chemically processed ⇑ Corresponding author at: Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-10044 Stockholm, Sweden. Tel.: +46 8 7908118; fax: +46 8 7906166. E-mail address: [email protected] (L.A. Berglund). 0266-3538/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2010.12.007
to high cellulose content, can be mechanically disintegrated [5– 8] to form nanofibers (diameter 5–20 nm, length 2–10 lm). Note that the somewhat misleading term microfibrillated cellulose (MFC) introduced in Ref. [5] and frequently used also today, is replaced by nanofibrillated cellulose (NFC) in the present study. The motivation is that due to improved preparation procedures, the lateral fibril dimension in studies after 2004 [6] is really nanoscale, and this was not the case in Refs. [5,9]. The different terminologies related to cellulose units are summarized in Table 1. Materials based on cellulose nanofibers show interesting characteristics. For instance, nanopaper made from wood cellulose nanofibers shows high work of fracture and can have a modulus of 13.2 GPa, a tensile strength above 200 MPa, and a strain to failure of 10% [10], much superior to microstructured paper and paperboard materials. Tailored cellulose nanocomposites with nanostructured matrix distribution can even show a tensile strength above 300 MPa [11] for the in-plane random fibril orientation case. Nanofibers can also be used in high-porosity cellulose aerogels and foams [12,13], optically transparent biocomposites [14], as oxygen barriers [15], to reinforce the cell walls of polymer foams [16], and as templates for magnetic nanoparticle hybrids [17]. Cellulose nanocomposites as a concept was first reported by Favier et al., who discussed the effect of the cellulose nanofiber network on modulus in a thermoplastic composite [18]. The potential of the cellulose nanofiber network as strength and work to fracture enhancing reinforcement of polymers is apparent from cellulose nanopaper performance [10] and from performance of biocomposites with very soft matrices [19]. Mechanical beating
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H. Sehaqui et al. / Composites Science and Technology 71 (2011) 382–387 Table 1 Terminologies related to cellulose units. Term
Diameter
Cellulose microfibrils
3 nm thick in plants and more than 20 nm in certain algae and tunicates [32,33] 15–30 nm up to about 5 lm due to aggregation [34]
Microcrystalline cellulose (MCC) Cellulose nanocrystals (CNC) Cellulose whiskers Microfibrillated cellulose (MFC) Nanofibrillated cellulose (NFC)
From 3 nm to 50 nm depending on the source [35,36]
10–20 nm from tunicate mantle [18] 25–100 nm from wood pulp fibers [5]
2. Experimental 2.1. Materials NFC was prepared according to the method reported by Henriksson et al. [27]. It was obtained in the form of a 2 wt.% suspension in water. The wood fibers used were from the same source as the NFC, a never-dried bleached sulphite softwood pulp from spruce, kindly supplied by Nordic Paper Seffle AB (Säffle, Sweden). Its hemicelluloses and lignin contents were 13.8 wt.% and 0.7 wt.%, respectively.
5–30 nm from wood pulp fibers [6,8,37]
of wood fibers [20] results in a fibrillated wood fiber surface and considerably improves strength of corresponding paper sheets [21]. Beating requires large amounts of energy, and causes extensive cell wall damage. From an application point-of-view, it is interesting to consider the potential of biocomposites based on networks from wood fibers currently produced in large scale at pulp mills. Recently, compression molding of pure wood fibers (cellulose content > 95%) prepared by the sulphite process was reported [22]. A wet cake of wood fibers was subjected to elevated temperature and highpressure, so that a high-density (1330 kg/m3) molded all-cellulose biocomposite was obtained. Advantages of this concept include water-based processing (no solvents), molding of complex geometrical shapes, material from widely available renewable resource, biodegradability, recyclability, no odor problems (high cellulose content), and good mechanical performance. The Young’s modulus was as high as 13 GPa, and the strain-to-failure was 1.7–10.5% depending on interfiber bonding characteristics. The tensile strength is more moderate, 31–76 MPa. In contrast, the present study investigates the potential of pure wood fibers combined with NFC in a high-density type of fiberboard biocomposite. Corresponding ‘‘hardboard’’ biocomposites are based on non-collapsed lignin and hemicellulose-containing fibers [2]. A formaldehyde-containing ‘‘binder’’ adhesive is often used, which imparts environmental problems and often ruins recycling possibilities. Replacement of reactive binders with NFC is therefore interesting. The present processing route is not reactive hot pressing, but rather a lower temperature preparation route akin to paper processing, which also allows the use of nanofibers and nanoparticles [23]. In a previous study, Eriksen et al. added cellulose nanofibers to low density paper structures based on lignin-containing thermomechanical wood pulp fibers in order to improve the mechanical performance [24]. Also, Juntaro et al. used bacterial cellulose (BC) nanofibers to coat lignin-containing sisal fibers used in sisal-poly(L-lactic acid) (PLLA) biocomposites [25]. The transverse strength of the sisal-PLLA biocomposites was improved since the BC nanofibers improved the interfacial adhesion between the sisal plant fibers and the PLLA. In the present study, we use pure cellulose nanofibers (20 nm 5 lm) and also high cellulose content wood fibers (30 lm 3 mm). The use of fibrous constituents at two very different length scales is interesting, since the smaller scale NFC nanofibers may favorably influence the larger scale failure events. Lakes has analyzed mechanical property advantages of hierarchically structured porous materials [26]. In the present study, the strength-enhancing potential of cellulose nanofibers in cellulose fiberboard type of biocomposites is investigated. Cellulose biocomposites with fibrous structures at two length scales is a new concept and may be combined with a polymer to further modify properties and facilitate thermoforming (heating–shaping–cooling).
2.2. Preparation of wood fiber/NFC composites The never-dried softwood pulp had a water content of ca. 70 wt.%. It was first diluted to 2 wt.% by addition of water and dispersed for 10 min at 3000 rpm using a disintegrator (PTI disintegrator model number 95568) according to the SCAN-C 18:65 procedure, and then beaten in a PFI-mill (HAM-JERN, Norway, 4000 revolutions) following the SCAN-C 24:96 method. These mechanical treatments aimed at making the surface of the fibers more accessible for subsequent adsorption of NFC. The 2 wt.% NFC suspension was diluted to 0.2 wt.% by addition of water and mixed for 10 min with an Ultra Turrax mixer (IKA, D125 Basic). Different amounts of the 0.2 wt.% NFC suspension namely 20, 50, 70 and 100 g were added to 1 liter of the 0.2 wt.% wood fiber slurry in order to obtain the following compositions of NFC to the wood fiber, i.e. 2%, 5%, 7%, and 10%, respectively. The obtained slurry was thoroughly mixed with magnetic stirring for 3 days. The wood fiber/NFC composites were prepared by vacuum filtrating the mixed slurry on top of a 0.65 lm filter membrane (DuraporeÒ DAWP29325) in a semi-automatic sheet former (Rapid Köthen model RK3A-KWT) followed by drying the wet cake between two filter papers at a temperature of 93 °C and a pressure of 70 mbars for 12 min. The composite sheets thus obtained had a diameter of 200 mm and a thickness of 67–73 lm. In order to compare with our previous method [10], nanopaper sheets consisting of only NFC were also prepared using the same procedure. Typically, 1 liter of the 0.2 wt.% NFC suspension was first degassed using a vacuum pump prior to filtration and drying in the sheet former. The thickness of the nanopaper sheets was ca. 50 lm. All samples were conditioned at 50% relative humidity and 23 °C before testing. 2.3. Density and porosity The mercury displacement method [28] was used to determine the density of the pulp fiber/NFC composites. The sample was weighed both in the air and when it was submerged in mercury. The volume of the sample was calculated from its buoyancy (Eq. (1)), this allowed us to calculate its density. The porosity of the sample was calculated from its density (Eq. (2)) by assuming the density for the cellulose to be 1500 kg/m3.
V sample ¼
mair ðsampleÞ mHg ðsampleÞ qHg qair
qsample Porosity ð%Þ ¼ 100 1
qcellulose
ð1Þ
ð2Þ
2.4. Mechanical properties Tensile tests for the wood fiber/NFC composites and NFC nanopaper were performed on a universal material testing machine Instron 5566 equipped with a 500 N load cell. Specimen strips of 50 mm in length, 15 mm in width, and a thickness in the range
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of 67–73 lm were tested at 5 mm/min strain rate under a controlled relative humidity of 50% at 23 °C. Specimens (5–7) were tested per sample. Young’s modulus was determined from the initial linear part of the stress–strain curves, and the work to fracture was determined from the area under the stress–strain curve. The specific strength and modulus were calculated by dividing the tensile strength and Young’s modulus by the density of the material.
2.6. Dynamic mechanical thermal analysis (DMTA) DMTA measurements for wood fiber/NFC composites were performed using a DMA Q800, TA Instruments in tensile mode. The specimen was a rectangular strip with a length of 10 mm and a width of 4 mm. Temperature scans were performed at a heating rate of 3 °C/min and a frequency of 10 Hz. The scan was carried out from 50 °C to 300 °C in air.
2.5. Field-emission scanning electron microscopy (FE-SEM) 3. Results and discussion Sheet surfaces and fracture surface of the materials were observed by field-emission scanning electron microscopy using a Hitachi S-4800 with a cold field-emission electron source. The samples were coated with graphite and gold–palladium using Agar HR sputter coaters (total thickness ca. 5 nm). Secondary electron detector was used for capturing images at 0.7–1 kV.
Wood fiber/NFC composites were prepared according to a filtration procedure developed for nanopaper [23]. The advantage of this method is that it prevents nanofibers from going through the filter, thus wood fiber/NFC composites can be prepared conveniently. It was desirable to have NFC homogeneously distributed
Fig. 1. Measured density (a) and estimated porosity (b) of the wood fiber/NFC composites and 0% NFC biocomposite reference.
Fig. 2. Surface texture of the 0% NFC reference (a and b) and 10% wood fiber/NFC composite (c and d). Scale bars are 1 mm (a and c) and 100 lm (b and d).
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in the material and long mixing times were therefore used. Compared with the use of wood fibers only, the filtration time of the pulp fiber/NFC slurry is increased considerably by the use of the fine membrane and by addition of NFC, and is more than 5 min. 3.1. Structure In order to analyze the mechanical behavior of the present porous, fibrous biocomposite, the density needs to be measured carefully so that porosity and solid wood fiber/NFC fraction can be estimated. Density measurements were performed using the mercury displacement technique described in Section 2. The results are presented in Fig. 1. The densities of the wood fiber/NFC biocomposites were in the range of 937–970 kg/m3 whereas the density of the 0% NFC reference was 830 kg/m3. The addition of 2% NFC increased density considerably, but higher amounts of NFC did not induce
further systematic density increase. Addition of 2% NFC induces a more compact wood fiber network structure. Regarding the location of NFC, one may consider two possibilities. One is that NFC coats the wood fibers, as in the case of sisal and BC reported by Juntaro et al. [25]. Another possibility is that NFC forms porous membranes or foams in the pores of the larger scale wood fiber network. The structure of the biocomposites was studied by FE-SEM. Fig. 2 shows the FE-SEM micrographs for the surfaces of wood fiber/NFC composites. There is a clear difference in surface texture of the wood fiber/NFC composites compared to the wood fiber reference. The surface was much smoother when NFC was added, which is interesting in a technical context such as printing. The pores apparent in Fig. 2a and b for the 0% NFC reference, were not as apparent after the addition of NFC, see Fig. 2c and d. Nanoporous NFC fibrillar structures fill the micropores formed by the wood fiber network, although cellulosic NFC also adsorbs to wood fiber cellulose surfaces. Fig. 3 illustrates an NFC-rich region in the composite with 10% NFC. It provides additional information on the material structure. Significant nanoscale porosity is apparent. Furthermore, since the width of a collapsed wood fiber is in the 30–40 lm range, Fig. 3 emphasizes the large difference in scale between the two types of reinforcement structures (wood fibers and NFC). It also becomes clear that dense regions of NFC, such as the one in Fig. 3, contribute substantial load-carrying ability to the material. 3.2. Stress–strain behavior
Fig. 3. High magnification FE-SEM image (scale bar 1 lm) of the 10% wood fiber/ NFC composite (10% refers to NFC to wood fiber solid content).
Fig. 4. Tensile stress–strain curves of the wood fiber/NFC composites. Percentages of NFC are expressed next to the curve, 100% NFC refers to the nanopaper sample.
The uniaxial stress–strain curves in tension are presented in Fig. 4. Tensile properties are also summarized in Table 2. The wood fiber/NFC composites have higher modulus, higher strength and larger strain-to-failure as compared with the biocomposite from wood fibers only (0% NFC). The 0% NFC biocomposite has a strength of 97.5 MPa whereas the 2% wood fiber/NFC composite has a strength as high as 141 MPa. The strength was further increased to 160 MPa at 10% NFC addition. The strain-to-failure ec was 4.2% when 10% NFC was added as compared to ec = 2.7% for 0% NFC. Due to the addition of NFC, the Young’s modulus increased by ca. 25% for all the composites. There was no strong effect on modulus as the NFC content was increased above 2%. It means further NFC addition above 2% did not markedly improve interfiber stress transfer mechanisms in the elastic region. The combination of strength, stiffness and ductility improvements, lead to increased work to fracture by a factor of 2 when only 2% of NFC was added. Fiberboard data are typically 5 GPa or lower for Young’s modulus and below 56 MPa for bending strength (tempered hardboard, about 1000 kg/m3 density) and below 33 MPa for tensile strength [4]. Although strength data may be difficult to compare, the significantly higher modulus for the present materials indicate a much more favorable structure. The deformation mechanisms for the biocomposites in Fig. 4 may be summarized as follows. In the early linear region the fibrous network deformation is elastic and no damage is induced.
Table 2 Mechanical properties of the wood fiber/NFC biocomposites. % NFC in the composites (%)
Tensile strength (MPa)
Specific strength (MPa g1 cm3)
Strain-tofailure (%)
Young’s modulus (GPa)
Specific modulus (GPa g1 cm3)
Work to fracture (MJ/m3)
0 2 5 7 10 100
97.5 ± 15 141 ± 13 141 ± 8 146 ± 14 160 ± 9 235 ± 21
117 ± 28 149 ± 29 151 ± 20 155 ± 24 165 ± 16 218 ± 27
2.7 ± 0.5 3.4 ± 0.5 3.6 ± 0.3 3.9 ± 0.2 4.2 ± 0.5 5.2 ± 1.2
8.1 ± 1.2 10.5 ± 1 10.2 ± 0.5 10.0 ± 0.5 10.1 ± 0.3 13.5 ± 0.3
9.7 ± 2.3 11.2 ± 2.2 10.9 ± 1.4 10.6 ± 1.2 10.4 ± 0.8 12.5 ± 0.7
1.73 ± 0.5 3.35 ± 0.6 3.52 ± 0.3 3.80 ± 0.4 4.42 ± 0.7 7.26 ± 2.6
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The yielding ‘‘knee’’ is likely to be associated with wood fiber–fiber debonding [9,10]. This is delayed to higher strain and stress as NFC is added since fiber–fiber adhesion or toughness is improved. The presence of small scale NFC structures improves the mechanical integrity of the larger scale wood fiber network structure in the ‘‘plastic’’ region at higher strains. One may speculate that the interfiber load transfer is improved at higher strains by surviving NFC membrane/foam links. After the yielding knee, a strain-hardening region follows with similar strain-hardening slope with and without NFC. In this region we expect fiber slippage to dominate [10,29]. Ultimate strength is improved considerably by addition of NFC, and the reason is most likely that wood fiber damage is delayed to higher strains due to improved fiber–fiber stress transfer in the strain-hardening region. Perhaps one of the most remarkable effects is the improved mechanical behavior with addition of only 2% of NFC. Part of the explanation is in the reduced porosity, see Fig. 1, which is improving Young’s modulus, see Table 2. However, the dramatic increase in strength cannot be explained by reduced porosity only. Also, NFC addition introduces new stress transfer mechanisms and non-critical but energy-absorbing failure events at a smaller length scale. As a consequence, strength and strainto-failure is dramatically increased, see Table 2, especially for the composition with 10% added NFC. Note that specific strength of the 10% NFC composition is very favorable at 165 MPa g1 cm3, and compares reasonably well with the present nanopaper at 218 MPa g1 cm3. The interpretation of failure mechanisms is supported by fractography information in Fig. 5. In Fig. 5a, the material consists of
wood fibers only and the fracture surface is highly irregular and dominated by pulled out wood fibers. The characteristic pull-out length is in the order of several hundred micrometers. The fibers are flat and ribbon-like since they are collapsed from the mechanical beating. In contrast, the composite containing 10% NFC in Fig. 5b presents a much more flat fracture surface. The NFC improves interfiber bonding so that the characteristic pull-out length becomes much shorter than 100 lm. The improved stress transfer correlates with increased tensile strength. 3.3. Thermomechanical properties In order to learn about dynamic thermal mechanical properties of the wood fiber/NFC composites, three materials were studied, namely the reference biocomposite (0% NFC), the 5% NFC composite and the 100% NFC nanopaper. Data for storage modulus E0 and tan d are presented in Fig. 6. The level of the E0 data at room temperature are similar to the static tensile moduli. It is interesting to note the high modulus values at low temperatures. Under conditions of low molecular mobility, cellulose composites can be very stiff. Moduli are also well preserved until temperatures as high as 200 °C. The 5% NFC composite has higher E0 than the 0% NFC composite and lower E0 than NFC nanopaper. Tan d curves show that all biocomposites have three different relaxations. The primary one at around 210 °C corresponds to the glass transition temperature of the disordered cellulose regions [30]. The transition at 70 °C does not correspond to any wellestablished transition in cellulose. Both the wood fibers and the
Fig. 5. Fracture surfaces from tensile tests of the 0% NFC reference (a), and 10% NFC/wood fiber composite (b). The scale bars are 50 lm.
Fig. 6. Storage modulus E0 (a) and tan delta (b) as a function of temperature for the 0% NFC, 5% NFC and 100% NFC materials. 0% NFC means 100% wood fibers, 5% NFC means 95% solid content by weight of wood fibers and 100% NFC is a nanopaper sample.
H. Sehaqui et al. / Composites Science and Technology 71 (2011) 382–387
NFC contain about 14% of hemicellulose. It is likely that the 70 °C transition corresponds to the glass transition of moisture plasticized hemicelluloses. The transition at the lowest temperature may correspond to the beta-transition in cellulose containing some moisture, a transition associated with localized main-chain relaxations [31]. NFC means 95% solid content by weight of wood fibers and 100% NFC is a nanopaper sample. 4. Conclusions Wood cellulose biocomposites with fiber network structures at both micro- and nanoscale were successfully processed by filtering and drying of a fibrous water suspension. The porosity was around 35%, and the structure consisted of collapsed wood cellulose fibers with nanofibers adsorbed to fibers but also filling the micro-scale pore-space. The Young’s modulus was typically around 10 GPa, and the fiber/fibril orientation distribution was random-in-the-plane. The high cellulose content (around 85 wt.%) ensures recyclability, degradability and decreases the risk for odor problems. The presence of nanofibers increased the tensile strength and work to fracture of the composite considerably. The strength increased from 98 MPa (wood fiber reference) to 160 MPa (10% NFC, 90% wood fibers by weight), and the work to fracture was more than doubled (from 1.7 MJ/m3 to 4.4 MJ/m3). The reason for this strong improvement is the presence of fiber network structures at two length scales (micro and nano). The NFC fibril network improves load transfer between wood fibers as damage starts to develop. This delays the development of large scale damage sites to higher stress and strain. The concept of biocomposites with fibrous networks at two length scales is of interest for the development of new types of binderless fiberboard composites with improved mechanical performance. Acknowledgement The Swedish Center for Biomimetic Fiber Engineering (Biomime, www.biomime.org) is acknowledged for financing H. Sehaqui and Dr. Q. Zhou. References [1] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Prog Polym Sci 1999;24:221–74. [2] Stark N, Cai Z, Carll C. Wood-based composite materials. In: Bergman R et al. editors. Wood handbook. Forest Products Laboratory, Madison, WI; 2010. p. 1– 28 [chapter 11]. [3] Berglund LA, Rowell R. Wood composites. In: Rowell RM, editor. Wood chemistry and wood composites. Taylor & Francis; 2005. p. 279–302. [4] Cai Z, Ross R. Mechanical properties of wood-based composite materials. In: Bergman R et al., editors. Wood handbook. Forest Products Laboratory, Madison, WI; 2010. p. 1–12 [chapter 12]. [5] Turbak AF, Snyder FW, Sandberg KR. Microfibrillated cellulose, a new cellulose product: properties, uses, and commercial potential. J Appl Polym Sci Appl Polym Symp 1983;37:815–27. [6] Nakagaito AN, Yano H. The effect of morphological changes from pulp fiber towards nano-scale fibrillated cellulose on the mechanical properties of highstrength plant fiber based composites. Appl Phys A Mater Sci Process 2004;78:547–52. [7] Henriksson M, Henriksson G, Berglund LA, Lindstrom T. An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. Eur Polym J 2007;43:3434–41. [8] Saito T, Kimura S, Nishiyama Y, Isogai A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007;8:2485–91. [9] Morseburg K, Chinga-Carrasco G. Assessing the combined benefits of clay and nanofibrillated cellulose in layered TMP-based sheets. Cellulose 2009;16: 795–806.
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Wood cellulose biocomposites with fibrous structures at micro- and nanoscale Houssine Sehaqui a, Maël Allais b, Qi Zhou a,c, Lars A. Berglund a,d,⇑ a
Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-10044 Stockholm, Sweden Ecole Nationale Supérieure d’Arts et Métiers, Esplanade des Arts et Métiers, 33405 Talence, France c School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, SE-10691 Stockholm, Sweden d Wallenberg Wood Science Center, Royal Institute of Technology, SE-10044 Stockholm, Sweden b
a r t i c l e
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Article history: Received 16 September 2010 Received in revised form 30 November 2010 Accepted 3 December 2010 Available online 9 December 2010 Keywords: Nanofibrillated cellulose (NFC) A. Fibers A. Nano composites B. Mechanical properties Scanning electron microscopy (SEM)
a b s t r a c t High-strength composites from wood fiber and nanofibrillated cellulose (NFC) were prepared in a semiautomatic sheet former. The composites were characterized by tensile tests, dynamic mechanical thermal analysis, field-emission scanning electron microscopy, and porosity measurements. The tensile strength increased from 98 MPa to 160 MPa and the work to fracture was more than doubled with the addition of 10% NFC to wood fibers. A hierarchical structure was obtained in the composites in the form of a micro-scale wood fiber network and an additional NFC nanofiber network linking wood fibers and also occupying some of the micro-scale porosity. Deformation mechanisms are discussed as well as possible applications of this biocomposites concept. Ó 2010 Published by Elsevier Ltd.
1. Introduction Biocomposites based on plant fibers are interesting for many branches of industry. The low cost of many types of plant fiber feedstock has historically been an important motivation. Recently, also environmental concerns have increased the interest in materials from renewable resources. Currently used biocomposites include thermoplastics combined with plant fibers. Woodthermoplastic composites may serve as an example, where typically polyethylene (PE) or polypropylene (PP) for melt-processing is combined with wood fibers, for instance in the form of saw dust [1–3]. This type of biocomposite has low material cost but shows low strength and brittleness [4]. Another more traditional type of biocomposites is paperboard and fiberboard materials [2], which are essentially porous fibrous networks where interfiber bonding may be ensured through polymer binders. They are also used in large-volume applications where high production rate and low cost are more important than materials performance. Despite the industrial focus on large-volume products of low mechanical performance (modulus typically below 5 GPa, strength below 70 MPa) [4], cellulose has remarkable potential as a highstrength constituent in biocomposites. Wood fibers (30 lm diameter, 3 mm length), which have been chemically processed ⇑ Corresponding author at: Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-10044 Stockholm, Sweden. Tel.: +46 8 7908118; fax: +46 8 7906166. E-mail address: [email protected] (L.A. Berglund). 0266-3538/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2010.12.007
to high cellulose content, can be mechanically disintegrated [5– 8] to form nanofibers (diameter 5–20 nm, length 2–10 lm). Note that the somewhat misleading term microfibrillated cellulose (MFC) introduced in Ref. [5] and frequently used also today, is replaced by nanofibrillated cellulose (NFC) in the present study. The motivation is that due to improved preparation procedures, the lateral fibril dimension in studies after 2004 [6] is really nanoscale, and this was not the case in Refs. [5,9]. The different terminologies related to cellulose units are summarized in Table 1. Materials based on cellulose nanofibers show interesting characteristics. For instance, nanopaper made from wood cellulose nanofibers shows high work of fracture and can have a modulus of 13.2 GPa, a tensile strength above 200 MPa, and a strain to failure of 10% [10], much superior to microstructured paper and paperboard materials. Tailored cellulose nanocomposites with nanostructured matrix distribution can even show a tensile strength above 300 MPa [11] for the in-plane random fibril orientation case. Nanofibers can also be used in high-porosity cellulose aerogels and foams [12,13], optically transparent biocomposites [14], as oxygen barriers [15], to reinforce the cell walls of polymer foams [16], and as templates for magnetic nanoparticle hybrids [17]. Cellulose nanocomposites as a concept was first reported by Favier et al., who discussed the effect of the cellulose nanofiber network on modulus in a thermoplastic composite [18]. The potential of the cellulose nanofiber network as strength and work to fracture enhancing reinforcement of polymers is apparent from cellulose nanopaper performance [10] and from performance of biocomposites with very soft matrices [19]. Mechanical beating
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H. Sehaqui et al. / Composites Science and Technology 71 (2011) 382–387 Table 1 Terminologies related to cellulose units. Term
Diameter
Cellulose microfibrils
3 nm thick in plants and more than 20 nm in certain algae and tunicates [32,33] 15–30 nm up to about 5 lm due to aggregation [34]
Microcrystalline cellulose (MCC) Cellulose nanocrystals (CNC) Cellulose whiskers Microfibrillated cellulose (MFC) Nanofibrillated cellulose (NFC)
From 3 nm to 50 nm depending on the source [35,36]
10–20 nm from tunicate mantle [18] 25–100 nm from wood pulp fibers [5]
2. Experimental 2.1. Materials NFC was prepared according to the method reported by Henriksson et al. [27]. It was obtained in the form of a 2 wt.% suspension in water. The wood fibers used were from the same source as the NFC, a never-dried bleached sulphite softwood pulp from spruce, kindly supplied by Nordic Paper Seffle AB (Säffle, Sweden). Its hemicelluloses and lignin contents were 13.8 wt.% and 0.7 wt.%, respectively.
5–30 nm from wood pulp fibers [6,8,37]
of wood fibers [20] results in a fibrillated wood fiber surface and considerably improves strength of corresponding paper sheets [21]. Beating requires large amounts of energy, and causes extensive cell wall damage. From an application point-of-view, it is interesting to consider the potential of biocomposites based on networks from wood fibers currently produced in large scale at pulp mills. Recently, compression molding of pure wood fibers (cellulose content > 95%) prepared by the sulphite process was reported [22]. A wet cake of wood fibers was subjected to elevated temperature and highpressure, so that a high-density (1330 kg/m3) molded all-cellulose biocomposite was obtained. Advantages of this concept include water-based processing (no solvents), molding of complex geometrical shapes, material from widely available renewable resource, biodegradability, recyclability, no odor problems (high cellulose content), and good mechanical performance. The Young’s modulus was as high as 13 GPa, and the strain-to-failure was 1.7–10.5% depending on interfiber bonding characteristics. The tensile strength is more moderate, 31–76 MPa. In contrast, the present study investigates the potential of pure wood fibers combined with NFC in a high-density type of fiberboard biocomposite. Corresponding ‘‘hardboard’’ biocomposites are based on non-collapsed lignin and hemicellulose-containing fibers [2]. A formaldehyde-containing ‘‘binder’’ adhesive is often used, which imparts environmental problems and often ruins recycling possibilities. Replacement of reactive binders with NFC is therefore interesting. The present processing route is not reactive hot pressing, but rather a lower temperature preparation route akin to paper processing, which also allows the use of nanofibers and nanoparticles [23]. In a previous study, Eriksen et al. added cellulose nanofibers to low density paper structures based on lignin-containing thermomechanical wood pulp fibers in order to improve the mechanical performance [24]. Also, Juntaro et al. used bacterial cellulose (BC) nanofibers to coat lignin-containing sisal fibers used in sisal-poly(L-lactic acid) (PLLA) biocomposites [25]. The transverse strength of the sisal-PLLA biocomposites was improved since the BC nanofibers improved the interfacial adhesion between the sisal plant fibers and the PLLA. In the present study, we use pure cellulose nanofibers (20 nm 5 lm) and also high cellulose content wood fibers (30 lm 3 mm). The use of fibrous constituents at two very different length scales is interesting, since the smaller scale NFC nanofibers may favorably influence the larger scale failure events. Lakes has analyzed mechanical property advantages of hierarchically structured porous materials [26]. In the present study, the strength-enhancing potential of cellulose nanofibers in cellulose fiberboard type of biocomposites is investigated. Cellulose biocomposites with fibrous structures at two length scales is a new concept and may be combined with a polymer to further modify properties and facilitate thermoforming (heating–shaping–cooling).
2.2. Preparation of wood fiber/NFC composites The never-dried softwood pulp had a water content of ca. 70 wt.%. It was first diluted to 2 wt.% by addition of water and dispersed for 10 min at 3000 rpm using a disintegrator (PTI disintegrator model number 95568) according to the SCAN-C 18:65 procedure, and then beaten in a PFI-mill (HAM-JERN, Norway, 4000 revolutions) following the SCAN-C 24:96 method. These mechanical treatments aimed at making the surface of the fibers more accessible for subsequent adsorption of NFC. The 2 wt.% NFC suspension was diluted to 0.2 wt.% by addition of water and mixed for 10 min with an Ultra Turrax mixer (IKA, D125 Basic). Different amounts of the 0.2 wt.% NFC suspension namely 20, 50, 70 and 100 g were added to 1 liter of the 0.2 wt.% wood fiber slurry in order to obtain the following compositions of NFC to the wood fiber, i.e. 2%, 5%, 7%, and 10%, respectively. The obtained slurry was thoroughly mixed with magnetic stirring for 3 days. The wood fiber/NFC composites were prepared by vacuum filtrating the mixed slurry on top of a 0.65 lm filter membrane (DuraporeÒ DAWP29325) in a semi-automatic sheet former (Rapid Köthen model RK3A-KWT) followed by drying the wet cake between two filter papers at a temperature of 93 °C and a pressure of 70 mbars for 12 min. The composite sheets thus obtained had a diameter of 200 mm and a thickness of 67–73 lm. In order to compare with our previous method [10], nanopaper sheets consisting of only NFC were also prepared using the same procedure. Typically, 1 liter of the 0.2 wt.% NFC suspension was first degassed using a vacuum pump prior to filtration and drying in the sheet former. The thickness of the nanopaper sheets was ca. 50 lm. All samples were conditioned at 50% relative humidity and 23 °C before testing. 2.3. Density and porosity The mercury displacement method [28] was used to determine the density of the pulp fiber/NFC composites. The sample was weighed both in the air and when it was submerged in mercury. The volume of the sample was calculated from its buoyancy (Eq. (1)), this allowed us to calculate its density. The porosity of the sample was calculated from its density (Eq. (2)) by assuming the density for the cellulose to be 1500 kg/m3.
V sample ¼
mair ðsampleÞ mHg ðsampleÞ qHg qair
qsample Porosity ð%Þ ¼ 100 1
qcellulose
ð1Þ
ð2Þ
2.4. Mechanical properties Tensile tests for the wood fiber/NFC composites and NFC nanopaper were performed on a universal material testing machine Instron 5566 equipped with a 500 N load cell. Specimen strips of 50 mm in length, 15 mm in width, and a thickness in the range
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of 67–73 lm were tested at 5 mm/min strain rate under a controlled relative humidity of 50% at 23 °C. Specimens (5–7) were tested per sample. Young’s modulus was determined from the initial linear part of the stress–strain curves, and the work to fracture was determined from the area under the stress–strain curve. The specific strength and modulus were calculated by dividing the tensile strength and Young’s modulus by the density of the material.
2.6. Dynamic mechanical thermal analysis (DMTA) DMTA measurements for wood fiber/NFC composites were performed using a DMA Q800, TA Instruments in tensile mode. The specimen was a rectangular strip with a length of 10 mm and a width of 4 mm. Temperature scans were performed at a heating rate of 3 °C/min and a frequency of 10 Hz. The scan was carried out from 50 °C to 300 °C in air.
2.5. Field-emission scanning electron microscopy (FE-SEM) 3. Results and discussion Sheet surfaces and fracture surface of the materials were observed by field-emission scanning electron microscopy using a Hitachi S-4800 with a cold field-emission electron source. The samples were coated with graphite and gold–palladium using Agar HR sputter coaters (total thickness ca. 5 nm). Secondary electron detector was used for capturing images at 0.7–1 kV.
Wood fiber/NFC composites were prepared according to a filtration procedure developed for nanopaper [23]. The advantage of this method is that it prevents nanofibers from going through the filter, thus wood fiber/NFC composites can be prepared conveniently. It was desirable to have NFC homogeneously distributed
Fig. 1. Measured density (a) and estimated porosity (b) of the wood fiber/NFC composites and 0% NFC biocomposite reference.
Fig. 2. Surface texture of the 0% NFC reference (a and b) and 10% wood fiber/NFC composite (c and d). Scale bars are 1 mm (a and c) and 100 lm (b and d).
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in the material and long mixing times were therefore used. Compared with the use of wood fibers only, the filtration time of the pulp fiber/NFC slurry is increased considerably by the use of the fine membrane and by addition of NFC, and is more than 5 min. 3.1. Structure In order to analyze the mechanical behavior of the present porous, fibrous biocomposite, the density needs to be measured carefully so that porosity and solid wood fiber/NFC fraction can be estimated. Density measurements were performed using the mercury displacement technique described in Section 2. The results are presented in Fig. 1. The densities of the wood fiber/NFC biocomposites were in the range of 937–970 kg/m3 whereas the density of the 0% NFC reference was 830 kg/m3. The addition of 2% NFC increased density considerably, but higher amounts of NFC did not induce
further systematic density increase. Addition of 2% NFC induces a more compact wood fiber network structure. Regarding the location of NFC, one may consider two possibilities. One is that NFC coats the wood fibers, as in the case of sisal and BC reported by Juntaro et al. [25]. Another possibility is that NFC forms porous membranes or foams in the pores of the larger scale wood fiber network. The structure of the biocomposites was studied by FE-SEM. Fig. 2 shows the FE-SEM micrographs for the surfaces of wood fiber/NFC composites. There is a clear difference in surface texture of the wood fiber/NFC composites compared to the wood fiber reference. The surface was much smoother when NFC was added, which is interesting in a technical context such as printing. The pores apparent in Fig. 2a and b for the 0% NFC reference, were not as apparent after the addition of NFC, see Fig. 2c and d. Nanoporous NFC fibrillar structures fill the micropores formed by the wood fiber network, although cellulosic NFC also adsorbs to wood fiber cellulose surfaces. Fig. 3 illustrates an NFC-rich region in the composite with 10% NFC. It provides additional information on the material structure. Significant nanoscale porosity is apparent. Furthermore, since the width of a collapsed wood fiber is in the 30–40 lm range, Fig. 3 emphasizes the large difference in scale between the two types of reinforcement structures (wood fibers and NFC). It also becomes clear that dense regions of NFC, such as the one in Fig. 3, contribute substantial load-carrying ability to the material. 3.2. Stress–strain behavior
Fig. 3. High magnification FE-SEM image (scale bar 1 lm) of the 10% wood fiber/ NFC composite (10% refers to NFC to wood fiber solid content).
Fig. 4. Tensile stress–strain curves of the wood fiber/NFC composites. Percentages of NFC are expressed next to the curve, 100% NFC refers to the nanopaper sample.
The uniaxial stress–strain curves in tension are presented in Fig. 4. Tensile properties are also summarized in Table 2. The wood fiber/NFC composites have higher modulus, higher strength and larger strain-to-failure as compared with the biocomposite from wood fibers only (0% NFC). The 0% NFC biocomposite has a strength of 97.5 MPa whereas the 2% wood fiber/NFC composite has a strength as high as 141 MPa. The strength was further increased to 160 MPa at 10% NFC addition. The strain-to-failure ec was 4.2% when 10% NFC was added as compared to ec = 2.7% for 0% NFC. Due to the addition of NFC, the Young’s modulus increased by ca. 25% for all the composites. There was no strong effect on modulus as the NFC content was increased above 2%. It means further NFC addition above 2% did not markedly improve interfiber stress transfer mechanisms in the elastic region. The combination of strength, stiffness and ductility improvements, lead to increased work to fracture by a factor of 2 when only 2% of NFC was added. Fiberboard data are typically 5 GPa or lower for Young’s modulus and below 56 MPa for bending strength (tempered hardboard, about 1000 kg/m3 density) and below 33 MPa for tensile strength [4]. Although strength data may be difficult to compare, the significantly higher modulus for the present materials indicate a much more favorable structure. The deformation mechanisms for the biocomposites in Fig. 4 may be summarized as follows. In the early linear region the fibrous network deformation is elastic and no damage is induced.
Table 2 Mechanical properties of the wood fiber/NFC biocomposites. % NFC in the composites (%)
Tensile strength (MPa)
Specific strength (MPa g1 cm3)
Strain-tofailure (%)
Young’s modulus (GPa)
Specific modulus (GPa g1 cm3)
Work to fracture (MJ/m3)
0 2 5 7 10 100
97.5 ± 15 141 ± 13 141 ± 8 146 ± 14 160 ± 9 235 ± 21
117 ± 28 149 ± 29 151 ± 20 155 ± 24 165 ± 16 218 ± 27
2.7 ± 0.5 3.4 ± 0.5 3.6 ± 0.3 3.9 ± 0.2 4.2 ± 0.5 5.2 ± 1.2
8.1 ± 1.2 10.5 ± 1 10.2 ± 0.5 10.0 ± 0.5 10.1 ± 0.3 13.5 ± 0.3
9.7 ± 2.3 11.2 ± 2.2 10.9 ± 1.4 10.6 ± 1.2 10.4 ± 0.8 12.5 ± 0.7
1.73 ± 0.5 3.35 ± 0.6 3.52 ± 0.3 3.80 ± 0.4 4.42 ± 0.7 7.26 ± 2.6
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The yielding ‘‘knee’’ is likely to be associated with wood fiber–fiber debonding [9,10]. This is delayed to higher strain and stress as NFC is added since fiber–fiber adhesion or toughness is improved. The presence of small scale NFC structures improves the mechanical integrity of the larger scale wood fiber network structure in the ‘‘plastic’’ region at higher strains. One may speculate that the interfiber load transfer is improved at higher strains by surviving NFC membrane/foam links. After the yielding knee, a strain-hardening region follows with similar strain-hardening slope with and without NFC. In this region we expect fiber slippage to dominate [10,29]. Ultimate strength is improved considerably by addition of NFC, and the reason is most likely that wood fiber damage is delayed to higher strains due to improved fiber–fiber stress transfer in the strain-hardening region. Perhaps one of the most remarkable effects is the improved mechanical behavior with addition of only 2% of NFC. Part of the explanation is in the reduced porosity, see Fig. 1, which is improving Young’s modulus, see Table 2. However, the dramatic increase in strength cannot be explained by reduced porosity only. Also, NFC addition introduces new stress transfer mechanisms and non-critical but energy-absorbing failure events at a smaller length scale. As a consequence, strength and strainto-failure is dramatically increased, see Table 2, especially for the composition with 10% added NFC. Note that specific strength of the 10% NFC composition is very favorable at 165 MPa g1 cm3, and compares reasonably well with the present nanopaper at 218 MPa g1 cm3. The interpretation of failure mechanisms is supported by fractography information in Fig. 5. In Fig. 5a, the material consists of
wood fibers only and the fracture surface is highly irregular and dominated by pulled out wood fibers. The characteristic pull-out length is in the order of several hundred micrometers. The fibers are flat and ribbon-like since they are collapsed from the mechanical beating. In contrast, the composite containing 10% NFC in Fig. 5b presents a much more flat fracture surface. The NFC improves interfiber bonding so that the characteristic pull-out length becomes much shorter than 100 lm. The improved stress transfer correlates with increased tensile strength. 3.3. Thermomechanical properties In order to learn about dynamic thermal mechanical properties of the wood fiber/NFC composites, three materials were studied, namely the reference biocomposite (0% NFC), the 5% NFC composite and the 100% NFC nanopaper. Data for storage modulus E0 and tan d are presented in Fig. 6. The level of the E0 data at room temperature are similar to the static tensile moduli. It is interesting to note the high modulus values at low temperatures. Under conditions of low molecular mobility, cellulose composites can be very stiff. Moduli are also well preserved until temperatures as high as 200 °C. The 5% NFC composite has higher E0 than the 0% NFC composite and lower E0 than NFC nanopaper. Tan d curves show that all biocomposites have three different relaxations. The primary one at around 210 °C corresponds to the glass transition temperature of the disordered cellulose regions [30]. The transition at 70 °C does not correspond to any wellestablished transition in cellulose. Both the wood fibers and the
Fig. 5. Fracture surfaces from tensile tests of the 0% NFC reference (a), and 10% NFC/wood fiber composite (b). The scale bars are 50 lm.
Fig. 6. Storage modulus E0 (a) and tan delta (b) as a function of temperature for the 0% NFC, 5% NFC and 100% NFC materials. 0% NFC means 100% wood fibers, 5% NFC means 95% solid content by weight of wood fibers and 100% NFC is a nanopaper sample.
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NFC contain about 14% of hemicellulose. It is likely that the 70 °C transition corresponds to the glass transition of moisture plasticized hemicelluloses. The transition at the lowest temperature may correspond to the beta-transition in cellulose containing some moisture, a transition associated with localized main-chain relaxations [31]. NFC means 95% solid content by weight of wood fibers and 100% NFC is a nanopaper sample. 4. Conclusions Wood cellulose biocomposites with fiber network structures at both micro- and nanoscale were successfully processed by filtering and drying of a fibrous water suspension. The porosity was around 35%, and the structure consisted of collapsed wood cellulose fibers with nanofibers adsorbed to fibers but also filling the micro-scale pore-space. The Young’s modulus was typically around 10 GPa, and the fiber/fibril orientation distribution was random-in-the-plane. The high cellulose content (around 85 wt.%) ensures recyclability, degradability and decreases the risk for odor problems. The presence of nanofibers increased the tensile strength and work to fracture of the composite considerably. The strength increased from 98 MPa (wood fiber reference) to 160 MPa (10% NFC, 90% wood fibers by weight), and the work to fracture was more than doubled (from 1.7 MJ/m3 to 4.4 MJ/m3). The reason for this strong improvement is the presence of fiber network structures at two length scales (micro and nano). The NFC fibril network improves load transfer between wood fibers as damage starts to develop. This delays the development of large scale damage sites to higher stress and strain. The concept of biocomposites with fibrous networks at two length scales is of interest for the development of new types of binderless fiberboard composites with improved mechanical performance. Acknowledgement The Swedish Center for Biomimetic Fiber Engineering (Biomime, www.biomime.org) is acknowledged for financing H. Sehaqui and Dr. Q. Zhou. References [1] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Prog Polym Sci 1999;24:221–74. [2] Stark N, Cai Z, Carll C. Wood-based composite materials. In: Bergman R et al. editors. Wood handbook. Forest Products Laboratory, Madison, WI; 2010. p. 1– 28 [chapter 11]. [3] Berglund LA, Rowell R. Wood composites. In: Rowell RM, editor. Wood chemistry and wood composites. Taylor & Francis; 2005. p. 279–302. [4] Cai Z, Ross R. Mechanical properties of wood-based composite materials. In: Bergman R et al., editors. Wood handbook. Forest Products Laboratory, Madison, WI; 2010. p. 1–12 [chapter 12]. [5] Turbak AF, Snyder FW, Sandberg KR. Microfibrillated cellulose, a new cellulose product: properties, uses, and commercial potential. J Appl Polym Sci Appl Polym Symp 1983;37:815–27. [6] Nakagaito AN, Yano H. The effect of morphological changes from pulp fiber towards nano-scale fibrillated cellulose on the mechanical properties of highstrength plant fiber based composites. Appl Phys A Mater Sci Process 2004;78:547–52. [7] Henriksson M, Henriksson G, Berglund LA, Lindstrom T. An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. Eur Polym J 2007;43:3434–41. [8] Saito T, Kimura S, Nishiyama Y, Isogai A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007;8:2485–91. [9] Morseburg K, Chinga-Carrasco G. Assessing the combined benefits of clay and nanofibrillated cellulose in layered TMP-based sheets. Cellulose 2009;16: 795–806.
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