Towards semi-structural cellulose nanocomposites â the need for

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Towards semi-structural cellulose nanocomposites – the need for scalable processing and interface tailoring Farhan Ansari, and Lars A. Berglund Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00142 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Towards semi-structural cellulose nanocomposites – the need for scalable processing and interface tailoring Farhan Ansari and Lars A Berglund* Fiber and Polymer Technology and Wallenberg Wood Science Centre, KTH Royal Institute of Technology, SE-10044, Sweden

Abstract: Cellulose nanocomposites can be considered for semi-structural load-bearing applications where modulus and strength requirements exceed 10 GPa and 100 MPa, respectively. Such properties are higher than for most neat polymers, but typical for molded short glass fiber composites. The research challenge for polymer matrix biocomposites is to develop processing concepts which allows high cellulose nanofibril (CNF) content, nanostructural control in the form of well-dispersed CNF, the use of suitable polymer matrices as well as molecular scale interface tailoring to address moisture effects. From a practical point of view, the processing concept needs to be scalable, so that large scale industrial processing is feasible. The vast majority of cellulose nanocomposite studies elaborate on materials with low nanocellulose content. An important reason is the challenge to prevent CNF agglomeration at high CNF content. Research activities are therefore needed on concepts with potential for rapid processing

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with controlled nanostructure, including well-dispersed fibrils at high CNF content so that favorable properties are obtained.

This perspective discusses processing strategies, agglomeration problems, opportunities and effects from interface tailoring. Specifically, preformed CNF mats can be used to design nanostructured biocomposites with high CNF content. Since very few composite materials combine functional and structural properties, CNF materials are an exception in this sense. The suggested processing concept could include functional components (inorganic clays, carbon nanotubes, magnetic nanoparticles, among others). In functional three-phase systems, CNF networks are combined with functional components (nanoparticles or fibril coatings) together with a ductile polymer matrix. Such materials can have functional properties (optical, magnetic, electric etc) in combination with mechanical performance, and the comparably low cost of nanocellulose may facilitate the use of large nanocomposite structures in industrial applications.

Keywords: biocomposite, nanocellulose, interphase, epoxy, unsaturated polyester, moisture, mechanical properties, thermoset


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1. Introduction Recently, the interest in materials from renewable resources, such as plants, has increased. These materials were, for obvious reason, dominant in prehistoric and historic times, at least until the 1500’s. Metal use then increased, as well as the use of concrete, and during the 20th century, synthetic polymers reached a strong position. Replacement of synthetic polymers with more eco-friendly materials from renewable resources is desirable, but also challenging in terms of requirements for physical performance, moisture durability and scalable processing methods. It is almost ironic that the performance of one of the most recent material developments, plantbased nanobiocomposites, relies on cellulose, one of the oldest material constituents on earth. Red algae have cellulose nanofibrils in the cell wall, and fossils date from 1.6 billion years ago.1 However, cellulose has also been found in the cell walls of the much older cyanobacteria2 and its function to mechanically stabilize cell walls of organisms has been critical to the development of life on earth. The cell wall of wood contains roughly 40% of cellulose in the form of “microfibrils”, which are long, fibrous nanofibrils of about 4 nm in diameter. The mechanical properties of wood largely rely on the high crystal modulus (≈ 136 GPa) and strength (≈ 3 GPa) of these cellulose nanofibrils (CNF),3,

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which has been an active research field during the past decade.5-11

Recently, so called microfibrillated cellulose,12 a coarser form of nanocellulose, has become available on large industrial scale. Due to the specific challenges with small nanofibril components, both applied and more basic research progress is needed to support development of nanocellulosic materials. CNFs are long slender fibrils, obtained by mechanical disintegration of the wood cell wall.13-15 The structure is shown in Figure 1 (a). Another form of nanocellulose is the rod-like cellulose


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nanocrystals (CNC) produced by acid hydrolysis of cellulose. CNC will not be discussed in detail, since CNF is likely to be more feasible for large-scale industrial applications in loadbearing biocomposites. One reason is that the lower aspect ratio and the acid hydrolysis preparation route mean that the reinforcement efficiency will be lower than for CNF (lower aspect ratio), and the cost will be higher (lower yield, and acid handling challenges). Polymer matrix biocomposites with plant fibers are widely used.5, 16 The reasons include high and tunable mechanical properties, improved moisture stability, biobased origin of the fiber phase, facile melt processing and cost advantages. Typical mechanical properties of current polymer matrix biocomposites are Young’s modulus (E) = 5-7 GPa, and tensile strength (σ*) = 40-70 MPa, with brittleness and low strain to failure (≈ 2%), see Figure 1b.17-19 Replacing plant fibers with wood CNF is of interest, and this possibility is considered in the present perspective. In the context of engineering applications, CNF films, laminates or inorganic CNF hybrids without a continuous polymer matrix are likely to suffer from intrinsic moisture sensitivity. The use of a continuous polymer matrix phase is crucial, since this polymer transfers stresses between fibrils, reduces moisture sorption, allows for geometrical shaping processes and provides opportunity for property tailoring. In the mid-1990s, Favier et al. prepared composites using very high aspect ratio (length/diameter) CNC (from tunicate) as reinforcement for a polymer matrix with low glass transition temperature (Tg). Dynamic mechanical analysis showed strong reinforcement effects above the polymer Tg, which was attributed to a ‘percolating’ network of inter-connected CNC rods.20 Thereafter, several studies have investigated nanocellulose as potential reinforcement for different thermoset and thermoplastic polymers.9, 21 An overwhelming majority of these studies show improved modulus and strength with respect to the neat polymer. The modulus and


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strength are usually highest with a few weight % of nanocellulose, and thereafter decrease with further addition of nanocellulose,22-25 presumably due to agglomeration of the nanocellulose reinforcement. The nanocellulose content at which the modulus/strength begin to decrease varies over a wide range (between 0.1 and 25 weight %),22-25 depending on the specific nanocellulose/polymer/solvent system. However, the often observed decrease in modulus with increasing volume fraction (Vf) is somewhat unexpected since this phenomenon is largely unknown in glass and carbon fiber composites. Perhaps the most likely explanation is very strong nano-reinforcement aggregation. The role of a low Vf ‘percolating’ network in providing mechanical stability above the polymer Tg was highlighted in early studies on nanocellulose composites.20 However, semi-structural applications require that materials are used well below their Tg, and possess high modulus and strength. This could be achieved by incorporating high contents of nanocellulose in composites with high modulus polymer matrices. One may note that water soluble thermoplastic polymers can be efficiently reinforced with high volume fractions of nanocellulose, leading to substantial improvements in strength and stiffness.26-31 The main reason is that such polymers can be conveniently processed in a hydro-colloidal CNF suspension, and therefore aggregation effects are not as severe. However, such composites are likely to have limited practical application due to their high moisture sensitivity.

1.1 Preliminary Analysis The engineering science challenge is to develop nanocellulose biocomposites as semistructural load-bearing materials, for use in a society committed to sustainable development. Needs include higher mechanical performance and better moisture stability than for current plant fiber composites. Preliminary analysis shows that properties of biocomposites with high volume fraction (Vf) of CNF can indeed compete with molded, industrially used glass fiber and plant


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fiber/thermoset composites. The comparison is based on random-in-the-plane fiber orientation distribution.17,

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CNF/thermoset biocomposites show higher strength and comparable

modulus with glass fiber composites (Figure 1b) for semi-structural applications (automotive, marine and general industry). Note that currently used “Plant fiber/thermoset” composites using the ≈ 30 micrometer diameter fibers, have substantially lower strength than CNF nanocomposites. Another important advantage becomes apparent in a plot of strength vs failure strain, where CNF/thermoset composites not only show higher strength but also preserve matrix strain to failure and show better ductility than existing biocomposites and glass fiber composites, see Figure 1(b).

1.2 Nanostructure and processing: challenges However, mixing and processing of CNF in polymer biocomposites is not straightforward due to the nanoscale size of this reinforcement. Interestingly, despite high initial hopes, large-scale structural nanocomposites for load-bearing structures appear not to have been industrially realized with carbon nanotubes (CNT), clay or related nanoscale reinforcements.40 Micron scale aggregation and the use of low nanoparticle content have been suggested as the primary reasons.40,

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Several materials science and engineering problems remain to be solved, but

cellulose has obvious advantages in terms of cost and eco-friendly attributes42 compared with many nanoscale reinforcements. Any materials and processing concept aiming for semistructural biocomposites based on nanocellulose needs to address the following challenges: • CNF agglomerates: If CNFs agglomerate and form concentrated microscale particles, the advantages of nanoscale fibrils are lost. As the material is subjected to mechanical load, microscale agglomerates serve as defects and will initiate failure at low strain, so that ultimate strength is compromised. Agglomeration can also cause heterogeneous packing, accompanied by


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entrapment of air voids, which is negative for optical and barrier properties. Effects from agglomeration are further aggravated due to the hydrophilic character of nanocellulose. The agglomerated regions have a high concentration of cellulose-cellulose interfaces which are rich in hydroxyl groups and act as sorption sites for moisture.43, 44 This will reduce mechanical and physical properties even further. • Moisture stability: Many industrial applications of cellulosic materials requires moisture stability. This is to realize the full mechanical property potential and to limit dimensional changes, and therefore the adsorption of moisture needs to be minimized. Since moisture sorption is dominated by surface hydration of cellulose fibrils, the interface between cellulose and the polymer matrix is critical, as will be discussed. To address agglomeration and interface problems, several surface modification strategies (based on covalent bonds, electrostatic interaction or polymer physical sorption) are suggested.4549

These strategies capitalize on the reactive CNF surface hydroxyl groups (or their ionic

derivatives), so that the surface becomes relatively hydrophobic after the modification. Different aspects of nanocellulose surface modification have been detailed in extensive reviews.6, 8, 50 In biocomposites studies, the strength at a given volume fraction of modified nanocellulose maybe higher than for composites with native nanocellulose,51, 52 and the maximum strength is typically obtained at a higher volume fraction (Vf). This indicates that the dispersion is relatively better for surface-modified nanocellulose, but agglomeration problems can still not be avoided above a certain nanocellulose content.22, 25, 52 The intrinsic nanocellulose moisture stability also increases as shown by reduced moisture sorption53 and increased tensile stiffness of wet nanocellulose network.54 Strategies based on ionic crosslinking55 and polymer sorption28, 43, 56, 57 have also been reported to affect the dispersion and moisture stability.


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• Processing concepts need to be scalable: The control of nanostructure during processing, especially with high nanocellulose content, is a limiting factor for engineering biocomposites. Solvent casting is well suited for fundamental studies, but solvent handling and slow drying are major obstacles which hinder industrial processing based on this concept. Melt processing using thermoplastic polymers is directly scalable and is investigated by Yano and coworkers.58-60 This approach does not usually provide sufficient mechanical properties for semi-structural applications since so far the CNF content is low due to high melt viscosity arising from physical CNF entanglement effects. The concept proposed in the present perspective instead utilizes thermoset precursors and shaping methods such as compression molding. The strategy to mix CNF with liquid thermoset precursors, and cast or mold a composite from the liquid mixture, has so far met with limited success. Typical data are 3-5 GPa in modulus and ≈ 50 MPa in strength.9 One reason for this is that most studies focus on low volume fraction (Vf) of CNF, and if higher Vf is attempted, agglomeration takes place and the mechanical properties are reduced, see Figure 2. This is not only negative from an application perspective, but also the real potential of polymer matrix CNF nanocomposites becomes unclear. Moreover, relative contributions from different factors such as agglomeration, intrinsic CNF properties etc cannot be separated. Important research questions are related to the molecular interactions at the CNFpolymer interface, modifications to the bulk thermoset polymer properties due to the presence of CNF as well as the constrained geometrical nanostructural conditions, and the relative distributions of the polymer matrix and CNFs in a high Vf nanocomposite. The illustrated decrease in properties at high Vf in Figure 2 is, unfortunately, widely supported by experimental data in the literature. The class of biocomposites considered in Figure 2 are short fiber


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composites (high fiber aspect ratio) with in-plane random fiber distribution, and predictive micromechanics models for stiffness properties are available in standard texts.61 A major change in processing strategy, instead of incremental improvements, is probably required in order to realize the potential of nanocellulose biocomposites. A prepreg-based approach using thermoset polymers is of interest. This approach is based on the preparation of a porous nanopaper structure of high specific surface area. Porous cellulose nanopaper is in essence a nanoporous CNF mat, analogous to a random-in-the-plane glass or carbon fiber mat, which can be impregnated by thermoset polymer precursors. This “prepreg” can be further processed, shaped and cured to form a CNF/thermoset polymer nanocomposite material.

2. Processing concept for nanostructural control The suggested processing strategy has practical resemblance to conventional papermaking (Figure 3). It is based on impregnation of preformed CNF network, and can be used to prepare composites with high nanocellulose content. The starting material/precursor is a CNF network, obtained by filtration of a dilute hydro-colloidal CNF suspension.62, 63 The filtered CNF/water mat is self-supporting and easy to handle. For epoxies or unsaturated polyester resins, it is subjected to solvent exchange by placement in a bath containing the solvent. The choice of solvent is primarily guided by the solubility of the monomers, but typical organic solvents such as acetone are common, and also used in existing industrial thermoset-based processes. The CNF/solvent template is then impregnated in a solution of resin monomers or prepolymers, followed by drying and curing. Impregnation by liquid thermoset precursors for epoxy, polyester or vinylester resins is in general not possible, due to the nanoscale porosity of the “nanopaper” and lack of sufficient resin-cellulose compatibility.


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This strategy results in fairly homogenous impregnation, which is facilitated by the rapid diffusion of epoxy or unsaturated polyester precursors into the CNF network. Low viscosity of the impregnating resin liquid solution, together with the porosity (pore size ≈ 10-100 nm)

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the CNF mat ensure a fast process where homogeneous impregnation can be achieved without vacuum. Resin content in the final composite can be tuned by variation of the concentration of the solution. Thus the same starting material (CNF fibre mat) is used to prepare composites with different fibre content. Since this fiber mat is formed by controlled filtration from water, the composite nanostructure is not strongly dependent on CNF/solvent interactions (unlike in solvent cast systems). Composites with very high CNF content and retained nanostructure can thus be produced.37-39, 65, 66 The starting fibre mat is prepared from a hydro-colloidal suspension where CNF is welldispersed in water, and the CNF network is formed during filtration. It is critical to start with a well-dispersed nanofiber suspension, since degree of dispersion has a strong influence on the composite nanostructure. The rate of water removal during filtration can be increased by applying pressure.67 Once the fibre mat is prepared, CNF agglomeration can still occur during solvent exchange, evaporation68 and resin impregnation. A sharp change in the chemical and thermodynamic characteristics of the solvent used, may cause the network structure to collapse. To avoid this, step-wise solvent exchange has advantages,69 where the CNF/water template is first placed in a mixture of solvent/water, and eventually in pure solvent.

2.1 Potential The proposed methodology is related to conventional papermaking, and has potential as a continuous process (Figure 3). The processing strategy is compatible with well-established molding processes for conventional micron-scale fibre/thermoset composites. Another technical


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prospect is that prepregs can easily be laminated after drying, and molded to obtain complex shapes of desired thickness.70, 71 Additionally, a simplified approach could involve the use of a dried porous CNF mat as the starting material for impregnation (structure shown in Figure 3).64, 72

This may minimize costs for transportation of CNF, since CNF is typically isolated and stored

as a gel of ≈ 2-10% in water. The drying method is important and determines the nanostructure of CNF nanopaper. It needs to preserve the high specific surface area of the mat, which is possible by supercritical carbon dioxide evaporation. Even sophisticated drying methods may cause some fibril agglomeration, but the concept is still worthwhile since there are handling advantages associated with dried porous mats. The concept can be used to prepare samples large enough for qualified studies of nanostructural effects on mechanical and hygro-thermal performance of engineering biocomposites, and is considered for industrial processing of polymer matrix biocomposites of controlled nanostructure.

3. Effects of nanostructure There are several studies in the literature where the prepreg approach has been used. In the work by Yano and co-workers, CNF biocomposites prepared by “prepreg-based” processing showed high optical transparency and in-plane thermal conductivity with acrylics, phenol formaldehyde or epoxy.66,

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They reported favourable physical properties even though the

fiber mat was dried after filtration and then impregnated. No ductility data are presented, possibly because the polymers are brittle. Henriksson et al. used solvent exchange (instead of drying), and reported high mechanical properties with melamine formaldehyde and hyperbranched polyesters.37, 65 The processing was later extended to unsaturated polyester and epoxy composites, with tailored interfaces for reduced moisture sensitivity.38, 39, 75 Impregnation with thermoplastic polymers has also been studied,76, 77 but effects from CNF network pore size


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and polymer molecular weight are likely to be critical for impregnation with long polymer chains.

3.1 Mechanical properties In contrast to many other nanocomposite systems, CNF-thermoset biocomposites from prepreg show improved modulus and strength with increasing CNF content, even up to 60-80% of CNF (Figure 4a).36-39 This indicates that the “prepreg” processing approach allows for high nanocellulose content without significant CNF agglomeration. This is in contrast to numerous studies on solvent cast CNF composites, and can be attributed to the use of a preformed nanostructured CNF mat as the starting material. Moreover, the composites can also have high ductility (strain to failure > 6%) due to the synergy between the CNF network and the ductile thermoset matrix.38, 39 An important distinction must be made between the networks discussed here and low Vf ‘percolating network’ widely discussed in literature. Percolation of CNF is typically achieved at a very low nanocellulose content, and implies the first formation of a continuous network of physically connected nanocellulose throughout the composite. The current network structure consists of high Vf (15-80 vol%) of physically entangled, “nonwoven” fibrils where fibrils are also connected through strong secondary interactions.63 The mechanical property data are an achievement in the context of semi-structural nanocomposites. A closer analysis of modulus using micromechanics models to back-calculate the CNF reinforcement efficiency, shows that the effective contribution from CNF decreases as the CNF content increases.38,

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This means that further optimization of the processing stage

using specific resin/solvent systems may lead to further improvements in properties. Moreover, if a CNF network with oriented fibrils78-80 is used for impregnation, the modulus and strength of the composites will be much higher than for the currently reported composites with random-in-


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plane orientation of CNF. Ductility will decrease for more highly oriented biocomposites. The in-plane random orientation of CNF fibrils is a practically interesting case, for instance as molded biocomposite components for transport applications. In terms of property predictions, conventional micromechanics models for short fiber composites have not been very successful, probably because of the strong effects from the CNF state of dispersion. It is apparent that the stress-strain behavior of the predried neat CNF network is important for the tensile behaviour of corresponding polymer matrix composites.9

3.2 Interfacial tailoring and moisture durability During prepreg based processing, resin curing occurs in the presence of hydroxyl groups at the cellulosic CNF surface. Due to the high specific surface area of the CNF mat, a large number of hydroxyls are available to interact with the resin monomers, either via covalent linkages or by secondary interactions (Figure 4b, bottom panel). In the specific case of epoxy, covalent interactions reduce the moisture sensitivity of the composites when the CNF is uniformly dispersed in the matrix.38,

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CNF agglomeration (at higher CNF content) is detrimental to

moisture stability due to higher fraction of moisture sensitive CNF/CNF bonds. With unsaturated polyester matrices, the CNF-polymer interactions are not covalent but through secondary bonds and the mechanical performance is compromised at high humidity.39 Thus, a highly nanostructured materials must account for homogeneous CNF dispersion as well as controlled interfacial characteristics. The mechanical properties at low moisture content (ambient conditions, relative humidity (RH) 50%) are relatively independent of interface interactions as nanocomposites based on epoxy and unsaturated polyester (UP) matrices are compared.39 Possibly, the high intrinsic mechanical performance of the CNF mat is helpful for the UP composite. In addition, the high


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specific interface area may allow for strong secondary interactions between the UP matrix and the CNF, so that mechanical performance is high at ambient conditions. Since the high specific area of the CNF is largely preserved after filtration from water, the prepreg concept can also be used as an independent surface modification strategy to modify the individual CNF surface. In this case, the reactive units are impregnated into the preformed CNF mat, reactions carried out and any free molecules are washed out after the reaction.53, 54, 81-84 The modification reactions are performed on a preformed CNF network formed by filtration from water. This has advantages since the need for dispersing CNF in less polar organic solvents is avoided (which would be required if the CNF is pre modified to increase hydrophobicity and then dispersed for filtration/casting), resulting in strong CNF network with minimum CNF aggregation.68 The impregnation, reaction and washing steps are carried out by placing the template in a solvent. This allows for conservative use of chemicals and energy, since energy intensive washing steps at low concentration (such as centrifugation) can be avoided.

Cellulose-Epoxy reactions The cellulose-epoxy reactions are highly relevant since epoxies are widely used (composites, adhesives, coatings). The cellulose hydroxyl-epoxy reactions are amine catalysed and proceed at rapid rate, as was established by investigating model mono-epoxide molecules.81 Such reactions from a solid substrate in a solvent-free environment are interesting, and possibly due to the high specific surface area of the fibrils. The reaction scheme could further be used as an independent surface modification strategy where epoxide molecules with desired functional groups (such as aromatic, allyl) can be attached to the nanocellulose surface. This may lead to moisture-stable CNF substrates81 or improve nanocellulose compatibility with hydrophobic polymers.


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Interphase effects Another effect from high specific fibril-matrix interface area is that polymer matrix characteristics are substantially modified. High Vf of well dispersed CNF fibrils means that the distance between two fibrils is at the order of 10 nm or smaller,37 and a large volume of the thermoset matrix is in close vicinity to the fibril surface. The ‘interphase’ region dominates the matrix properties, which are very different from the bulk matrix state. This can manifest itself as a strong increase in the glass transition temperature37-39 since the mobility of polymer chains in the interphase region is lowered due to the proximity to the CNF fibrils. In reactive systems, the increase in thermoset Tg can be extremely high, since effective cross-link density is increased due to chemical bonds between the thermoset and the fibril surface.37, 38 Low amounts of CNF in thermoplastic matrices can also improve the matrix properties beyond bulk polymer data, for instance by increased physical ageing rate,52 or increased crystallization rate.85, 86 Such changes in polymer matrix structure compared with the bulk polymer reference are often the cause of strong improvements in polymer properties due to addition of a small amount of nanoparticles.

3.3 Functional CNF composites With growing interest in multifunctional materials for electronic device applications, nanoparticles such as carbon nanotube (CNT), clay, graphene are of interest. Functional composites with these nanoparticles can be designed by extending the current processing strategy to either precipitate the functional nanoparticle on CNF or add them to a CNF/water suspension and filter.87 In ternary composites containing CNF as an additional component, CNF network formation aids in homogenous nanoparticle dispersion, and also provides additional mechanical support in the final composite. For instance, highly magnetic CNF/epoxy/cobalt-ferrite nanoparticle composites with up to 70 wt% of the ferrite nanoparticle were prepared using the precipitation approach.87 Incorporation of as little as 20 wt% of CNF led to sufficient structural


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support during processing, and resulted in functional and ductile thermoset nanocomposites that could be shaped by compression molding.87 The CNF + thermoset polymer, in these systems, acts as a high performance “matrix” where the mechanical load bearing properties of the CNF is synergistically combined with the functional properties of added nanoparticles. This approach could further expand the range of CNF based ‘smart’ materials,88-94 while making them suitable for semi-structural applications.

4. Conclusions and future perspective Research interest in CNF nanocellulose biocomposites is driven not only by environmental concerns, but also the high mechanical performance potential of these materials, and the possibility to add functional properties. Recent research demonstrates the critical importance of avoiding nanocellulose agglomeration and the need for CNF-polymer interface tailoring. For semi-structural biocomposites of high mechanical performance, materials design must include high cellulose content with controlled nanostructure, so that high ductility and ultimate strength of the nanocellulose network can be translated into biocomposite properties. A prepreg-based processing approach is a promising route towards realizing this goal. The adaptability of the process allows for convenient control of the nanocellulose volume fraction, and high volume fraction of well dispersed nanocellulose is attainable. The in-situ curing of thermosets offers possibilities for interface tailoring by chemical reactions with cellulose hydroxyls (i.e. amine-catalyzed epoxide reactions with hydroxyls). Excellent mechanical (modulus ≈10 GPa, strength ≈150 MPa, strain to failure ≈6%) and hygro-thermal properties have been realized, due to synergistic contributions from random-in-plane fiber networks and ductile polymer matrices. Furthermore, composites combining mechanical and functional properties


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(magnetic, conductive, optical) can be obtained by adding a third component, and this requires only minor adaptations to the process. Further scientific work may investigate different aspects to fully explore the promised potential. The design of complex multi-phase materials with several functional nanoparticles could be an interesting route, as would be the choice of other matrices (including in-situ polymerization of thermoplastics). High specific surface area of the nanocellulose network creates a large amount of sites for cellulose interaction with the polymer matrix. Interactions with cellulose fibrils and the nanoscale distribution of polymer matrix domains, lead to constraint effects on the dynamics of the polymer matrix. Such effects can possibly be used in order to improve polymer matrix properties compared with data for the polymer in bulk state. The processing approach is scalable since it is related to industrially established methods already used in preparation of fiber composite prepregs and adhesive films based on thermoset precursors. The filtration steps to obtain the CNF nanopaper “mat” are related to conventional papermaking, and CNF nanopaper is already close to market introduction. Consequently, large structures with thick and complex shapes may be obtained by laminating and/or molding the prepregs. Several practical challenges need to be addressed before truly nanostructured polymer matrix biocomposites will be available at large scale. Although the use of organic solvents is feasible (consistent with existing thermosets), selection of the specific solvent system needs to be addressed as well as the modification of existing processing technologies. An alternative to solvent-borne systems is to use thermoset precursor or monomer liquids which are highly compatible with cellulose, showing instantaneous wetting.


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Industrial production of biocomposites of high nanocellulose content requires low cost for the nanocellulose component. In Japan, focus of massive industrial development appears to be smalldiameter fibrils, often with modified chemical functionality compared with native cellulose. The objective is advanced applications, and cost estimates can be in the range of 100 US$ per kg, or even more. In north America and Europe, several companies have lower cost products, often termed “microfibrillated cellulose” (MFC), although fibril diameters can be around 100 nm. One important application area, already developed, is MFC as a dry strength additive for paper board. This indicates low material cost. Recent market analyses have assumed a material cost in the range 4-11 US$ per kg.95 Energy requirements for nanocellulose of MFC production have been discussed in the literature, and values of around 2000 kWh per ton, or even below, have been reported.96, 97 Japanese efforts on thermoplastic cellulose nanocomposites from melt-processing98 involves active participation from many industrial companies. In combination with arguments from the present study, this suggests that the ratio between performance and cost for nanocellulose biocomposites is certainly sufficiently high to merit further development work. Furthermore, cellulose has high potential as a more eco-friendly reinforcement phase for polymers than glass fibers.42 Corresponding Author *Lars A Berglund ([email protected])

Acknowledgements The authors thank the Knut and Alice Wallenberg (KAW) foundation for funding through the Wallenberg Wood Science Center (WWSC).

References ACS Paragon Plus Environment

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60. Semba, T.; Ito, A.; Kitagawa, K.; Nakatani, T.; Yano, H.; Sato, A., Thermoplastic composites of polyamide-12 reinforced by cellulose nanofibers with cationic surface modification. J. Appl. Polym. Sci. 2014, 131. 61. Hull, D.; Clyne, T. W., An Introduction to Composite Materials. Cambridge University Press: 1996. 62. Houssine, S.; Liu, A.; Zhou, Q.; Berglund, L., Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures. Biomacromolecules 2010, 11, 2195 - 2198. 63. Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindstrom, T.; Nishino, T., Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 2008, 9, 1579-85. 64. Sehaqui, H.; Zhou, Q.; Ikkala, O.; Berglund, L. A., Strong and Tough Cellulose Nanopaper with High Specific Surface Area and Porosity. Biomacromolecules 2011, 12, 3638-44. 65. Henriksson, M.; Berglund, L. A., Structure and Properties of Cellulose Nanocomposite Films Containing Melamine Formaldehyde. J. Appl. Polym. Sci. 2007, 106, 2817-2824. 66. Nakagaito, A. N.; Yano, H., Novel High-Strength Biocomposites Based on Microfibrillated Cellulose Having Nano-Order-Unit Web-Like Network Structure. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 155-159. 67. Österberg, M.; Vartiainen, J.; Lucenius, J.; Hippi, U.; Seppälä, J.; Serimaa, R.; Laine, J., A Fast Method to Produce Strong NFC Films as a Platform for Barrier and Functional Materials. ACS Appl. Mater. Interfaces 2013, 5, 4640-4647. 68. Johansson, L.-S.; Tammelin, T.; Campbell, J. M.; Setala, H.; Osterberg, M., Experimental evidence on medium driven cellulose surface adaptation demonstrated using nanofibrillated cellulose. Soft Matter 2011, 7, 10917-10924. 69. Ishii, D.; Tatsumi, D.; Matsumoto, T., Effect of Solvent Exchange on the Solid Structure and Dissolution Behavior of Cellulose. Biomacromolecules 2003, 4, 1238-1243. 70. Galland, S. Compression-moulded and multifunctional cellulose network materials. KTH Royal Institute of Technology, Stockholm, 2013. 71. Mautner, A.; Lucenius, J.; Österberg, M.; Bismarck, A., Multi-layer nanopaper based composites. Cellulose 2017, 24, 1759-1773. 72. Nissilä, T.; Karhula, S. S.; Saarakkala, S.; Oksman, K., Cellulose nanofiber aerogels impregnated with bio-based epoxy using vacuum infusion: Structure, orientation and mechanical properties. Compos. Sci. Technol. 2018, 155, 64-71. 73. Iwamoto, S.; Nakagaito, A. N.; Yano, H.; Nogi, M., Optically Transparent Composites Reinforced with Plant Fiber-Based Nanofibers. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 1109-1112. 74. Shimazaki, Y.; Miyazaki, Y.; Takezawa, Y.; Nogi, M.; Abe, K.; Ifuku, S.; Yano, H., Excellent Thermal Conductivity of Transparent Cellulose Nanofiber/Epoxy Resin Nanocomposites. Biomacromolecules 2007, 8, 2976-2978. 75. Ansari, F.; Sjöstedt, A.; Larsson, P. T.; Berglund, L. A.; Wågberg, L., Hierarchical wood cellulose fiber/epoxy biocomposites – Materials design of fiber porosity and nanostructure. Composites, Part A 2015, 74, 60-68. 76. Jonoobi, M.; Aitomäki, Y.; Mathew, A. P.; Oksman, K., Thermoplastic polymer impregnation of cellulose nanofibre networks: Morphology, mechanical and optical properties. Composites, Part A 2014, 58, 30-35. 77. Capadona, J. R.; Van Den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C., A Versatile Approach for the Processing of Polymer Nanocomposites with Self-Assembled Nanofibre Templates. Nat. nanotechnol. 2007, 2, 765-9. 78. Sehaqui, H.; Ezekiel Mushi, N.; 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, 1043-9.


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97. Bharimalla, A. K.; Deshmukh, S. P.; Patil, P. G.; Vigneshwaran, N., Energy Efficient Manufacturing of Nanocellulose by Chemo- and Bio-Mechanical Processes: A Review. World J. Nano Sci. Eng. 2015, 05, 204-212. 98. Suzuki, K.; Homma, Y.; Igarashi, Y.; Okumura, H.; Yano, H., Effect of preparation process of microfibrillated cellulose-reinforced polypropylene upon dispersion and mechanical properties. Cellulose 2017, 24, 3789-3801.


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Figure 1. (a) Micrographs showing the long and flexible structure of cellulose nanofibrils (left) and surface image of a porous CNF network prepared by filtration (right). (b) Ashby plots comparing strength vs modulus (left) and strength vs strain to failure (right) for glass fiber composites (Chopped Strand Mat, CSM and Sheet Molding Compound, SMC),

32-35

plant

fiber/thermoset composites17-19 and high volume fraction CNF/thermoset composites36-39. CNF/thermoset composites are comparable to industrial glass fiber composites in terms of modulus, and have a significant advantage in terms of strength and strain to failure. Data is replotted from ref 17-19, 34-39


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Figure 2: Representative changes of in-plane random short fiber composite properties (strength and modulus) with increasing CNF volume fraction (Vf) as typically reported experimental data in literature (solid line) and as theoretically predicted (classical micromechanics) (dashed lines). Upper left figure in inset shows experimental data from ref.24 Agglomeration of nanoparticles has been suggested as the primary reason for poor mechanical properties at high Vf. The inset sketches depict CNF dispersion in the composites at low Vf (lower left), at high Vf when agglomerated (lower right) and when well dispersed (upper right). Processing pathways to preserve CNF dispersion and obtain good mechanical properties at high Vf is discussed in later sections.


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Figure 3: Sketch of filtration-based processing to obtain nanostructured biocomposites with high CNF volume fraction. The starting CNF/water suspension is filtered and subjected to solvent exchange and impregnation steps to obtain the prepreg. The dried composite prepregs can either be stored for later processing or stacked and molded into thicker and more complex shapes via compression molding. As a demonstration, 3-5 layers of such prepregs were successfully molded into a protective smartphone phone casing. Top left image shows the porous structure of a dried CNF mat, the scale bar is 200 nm.


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Figure 4: Mechanical properties of nanostructured thermoset biocomposites (with high CNF volume fraction) prepared by prepreg approach. (a) Increasing strength and modulus with increasing CNF content indicate that the CNF was well dispersed in the composites even at high Vf. The data is replotted from ref 33, 36-39 (b) Mechanical properties at RH 50% and 90% for three different CNF/thermoset systems. At similar CNF volume fraction, the interface was tailored so


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that the secondary interactions between CNF and unsaturated polyester (UP) were changed to covalent interactions between CNF and epoxy (EP), improving moisture stability (sketch at the bottom). For the favourable CNF/EP system, increasing the CNF content from 15 to 30 vol% increases CNF-CNF agglomeration so that there is an increase in weakly bonded and moisture sensitive CNF/CNF interfaces (sketch on the right). Bar chart data are replotted from ref 38, 39.


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For table of content use only

Towards semi-structural cellulose nanocomposites – the need for scalable processing and interface tailoring Farhan Ansari and Lars A Berglund*

ToC figure


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