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On the development and applications of cellulosic nanofibrillar and nanocrystalline materials.

Nanoscience, where physics, chemistry, biology and materials science converge, deals with the manipulation and characterization of matter at molecular to micron scale. Nanotechnology is the emerging engineering discipline that applies nanoscience to create products. Because of their size, nanomaterials have the ability to impart novel and/or significantly improved physical (strength, stiffness, abrasion, thermal), chemical (catalytic, ion-exchange, membranes), biological (anti-microbial, compatibility) and electronic (optical, electrical, magnetic) properties. While chemistry and physics of simple atoms and molecules are fairly well understood, predictable and no longer considered overly complex, serious attempts to bridge across the length scales from nano to macro remain a major challenge, and will occupy researchers and scientists for years ahead.

This review paper highlights the potential significant benefits emanating from fibre engineering and selective design of lignocellulosics using evolutionary developmental processes with the objective of producing high value-added products of superior end-use performance for existing and new markets. The development and application of nanotechnologies to increase the potential utilization of renewable biomaterials so as to reduce supply chain costs (better with less material) and add functionality will improve the competitiveness of forestry materials and consolidate their utilization in "smart" packaging, E-paper, advanced engineering and structural composite materials, and cosmetics.

Les nanosciences, ou convergent la physique, la chimie, la biologie et la science des materiaux, traite de la manipulation et de la caracterisation de matieres a des echelles allant de l'echelle moleculaire au micron. La nanotechnologie est une discipline emergente de l'ingenierie et recourt aux nanosciences pour creer des produits. En raison de leur taille, les nanomateriaux ont la capacite de conferer des proprietes nouvelles ou largement ameliorees que ce soit en terme physique (force, rigidite, abrasion, thermique), chimique (catalytique, echange d'ions, membranes), biologiques (anti-microbiens, compatibilite) et electronique (optique, electrique, magnetique). Bien que la chimie et la physique des atomes et des molecules simples sont assez bien comprises, sont predictibles et ne sont plus considerees comme tres complexes, les tentatives serieuses pour passer au echelles de longueur allant du nano au macro demeurent un defiimportant, et cela occupera les chercheurs et les scientifiques pour plusieurs annees.

Dans cette article de revue, on souligne les avantages potentiels significatifs venant de l'ingenierie des fibres et de la conception selective de lignocelluloses a l'aide de procedes de developpement evolutifs, pour fabriquer des produits a forte valeur ajoutee ayant une performance d'usage final superieur pour des marches existants et nouveaux. Le fait de developper et d'appliquer les nanotechnologies pour accroitre l'utilisation potentielle des biomateriaux renouvelables dans le but de reduire les couts de la chaine d'approvisionnement (faire mieux avec moins de materiaux) et d'ajouter des proprietes fonctionnelles, ameliorera la competitivite des materiaux forestiers et consoliderera leur utilization dans les emballages << intelligents >>, le papier electronique, les materiaux d'ingenierie avancee et a base de composites structuraux ainsi que les cosmetiques.

Keywords: lignocellulosics, nanomaterials, nanofibrillar whiskers, nanocrystalline suspensions, nanocomposites


The term nanotechnology embraces a broad range of science and technology working at a length scale of approximately 1 to 100 nanometres to manipulate atoms and molecules in order to create useful structures (Drexler, 1981). The many successes that are currently attributed to nanotechnology have in fact been the result of years of research into conventional fields like materials or colloids sciences, and some of today's nanotechnology products are even mundane, when compared to (unchallenged) popular perceptions: stain-resistant trousers, better sun cream and tennis rackets reinforced with carbon nanotubes.

Nanotechnology will inevitably impact, for instance, papermaking--the technology of converting lignocellulosics into paper and board products--through the incorporation of novel chemicals and finishing equipment. Potential improvements in productivity, product performance, uniformity and quality control could result from the application of (i) nano-enabled retention and drainage systems, (ii) nanofiltration systems to filter out various contaminants in papermaking effluents, (iii) nano-enabled roll covers for high-stress calendering applications, as in supercalendering, (iv) nanocoatings to create new functionality for paper and to enable dry coating processes, (v) nano-metal oxides to improve the rigidity and toughness of boilers and other high-pressure, high-temperature equipment, and (vi) nano-scale sensors for detecting temperature and moisture profiles, web uniformity, etc. The science underlying these technologies is sophisticated and the products would indicate big improvements on what has gone before. However, they do not really represent a decisive break from the past since the developmental processes enabled by nanotechnology primarily represent an incremental change. One such important and fast-evolving process of potential significant impact on paper manufacture is dry coating. (1) This process uses nanoparticles that bind with fibres and with each other without requiring binding agents, thus eliminating the need for fillers and wet-pressing in papermaking. By providing a well-covered and smoother surface for printing, the palpable benefits will stem from potential improvements in brightness, gloss, printability, ink fixing and densities of the finished product. The ensuing profitability will be a function of the margin of costs incurred by the new technology and the price tag associated with the resulting product attributes.

In order to reinvigorate the forestry sector and pulp manufacture for the 21st century, we must venture into evolutionary developmental processes aiming to re-engineer and custom design the industry's primary raw material, the lignocellulosic fibres, and profit from their unique properties. Lignocellulosic fibres are built from nanofibrils comprising a number of chain molecules in close alignment and lie parallel to one another (see Figure 1) with cross-sectional dimensions in the nanometre range (Frey-Wyssling and Muhlethaler, 1965). These nanofibrils comprise crystallites linked by amorphous areas (Figure 2a). A cellulose crystallite is essentially an aggregation of cellulose molecular chains of relatively low flexibility which tend to aggregate in parallel bundles with the desirable consequence of their axial physical properties approaching those of perfect crystals (Battista, 1975). The size, shape and orderliness of the crystallite regions play a predominant role as far as such fibre properties as tensile strength, density, rigidity, swelling and heat-sensitivity are concerned. The less ordered areas and the ratio of such amorphous areas to the crystalline regions are more influential in controlling extensibility.


Products with specific end-use requirements have long been developed through a judicious selection of fibre properties, whereby fibres are processed for many different requirements, for example, super absorbent hygiene products, ultra-soft tissue products or ultra-lightweight coated paper. The competitiveness of forestry materials rests on associating product development with the concept of fibre engineering and selective design, by using new technical tools to manipulate and re-structure fibres (and their constituents) at a scale as small as scientifically possible in order to add functionality. For instance, isolation of the crystallites (Figure 2b) can yield individual reinforcing elements, the cellulose nanofibrils, of excellent physical attributes approaching those of perfect crystals. Table 1 compares some key attributes relevant to the reinforcement efficiency of cellulose nanofibrils and Kraft softwood fibres. The tremendous surface area, very close spacing, extremely high stiffness and strength, and high aspect (length-to-diameter) ratio of cellulose nanofibrils provide them with a strong potential as high-performance reinforcement in advanced materials.

Forestry-based products have long capitalized on the ability of cellulosic materials to form fibres, and new forestry-based, functional materials have the potential to compete with other established ones, e.g. plastics, as well as metals and metal alloys, not only on performance, as indicated by the scientific evidence, but also on merits of recyclability, biodiversity, biodegradability, sustainability and being a renewable resource--see Table 1 and Appendix I.


Nanocrystalline Cellulose Suspensions

Extraction is central to further developing and processing cellulose nanocrystals into functional, high value-added materials, and as such several techniques are available (Revol et al., 1997; Cavaille et al., 2000a; Hanna et al., 2001). A reliable recipe for their production with uniform size, shape and charge distribution is necessary; however, it is challenging. The success of any approach would be measured by two essential factors: (1) The feasibility of cost-effective commercial scale-up, and (2) the manner in which yield of the cellulose nanocrystals may be maximized. Typically, two primary steps comprise the process of extracting cellulose nanofibrils from wood (Ranby, 1951): (1) a controlled chemical pretreatment to destroy the molecular bonds whereby crystalline nanofibrils are hinged together in a network structure; and (2) appropriate use of mechanical energy to disperse substantial quantities of unhinged nanofibrils in the aqueous phase. Since the early introduction of these steps, the process has evolved and been applied to various wood and non-wood species (Revol et al., 1992, 1994, 1997; Sassi and Chanzy, 1995; Guo and Gray, 1994; Dong et al., 1998; Dinand et al., 1999; Cavaille et al., 2000a, b; Heux et al., 2000; Araki et al., 2001; Hanna et al., 2001).

Revol et al. (1997) have pioneered the development of novel self-organizing systems of nanocrystals of cellulose, and expended tremendous efforts in studying the multi-faceted physio-chemical properties of this material. Employing acid hydrolysis, typically using concentrated mineral acids, such as sulphuric or hydrochloric, individual crystallites are prepared from wood pulp (Revol et al., 1992) or cotton (Revol et al., 1994; Dong et al., 1998)--each having a different extent of cellulose purity: cotton-seed fluff is composed of [less than or equal to] 94% cellulose, and wood contains [less than or equal to] 55%. Revol et al. (1992) outline the preparation procedure, which begins with initial acid action to remove the polysaccharide material closely bonded to the microfibrillar surface, resulting in an overall decrease in amorphous material. Subsequent hydrolysis breaks down those portions of the long glucose chains in accessible, non-crystalline regions. A levelling-off degree of polymerization is achieved: this corresponds to the residual highly crystalline regions of the original cellulose fibre. When this level is reached, hydrolysis is terminated by rapid dilution of the acid. A combination of centrifugation and extensive dialysis is then employed to fully remove the acid, and a brief sonication completes the process to disperse the individual particles of cellulose and yield an aqueous suspension, as shown in Figure 3. The cellulose rods that remain after this treatment are almost entirely crystalline and as such are termed crystallites.


The precise physical dimensions of the crystallites depend on several factors, including the source of the cellulose, the exact hydrolysis conditions, and ionic strength (Fleming et al., 2001). Besides, complications in size heterogeneity are inevitable owing to the diffusion-controlled nature of the acid hydrolysis (Fleming et al., 2000). Typical figures for crystallites derived from different species vary: (3 - 5) x 180 [+ or -] 75 nm for bleached softwood Kraft pulp (Araki et al., 1998), 7 x (100 - 300) nm for cotton (Dong et al., 1998), and 20 x (100 - 2000) nm for Valonia (Imai et al., 1998). The high axial (length to width) ratio of the rods is important for the determination of anisotropic phase formation (Revol et al., 1992; Dong et al., 1998).

When aqueous suspensions of cellulose nanocrystals are allowed to dry, the chiral nematic orientational order present in the suspension remains in the solid (Revol et al., 1997, 1998). (2) Some of the possible applications of chiral nematic cellulose crystallites include optically variable iridescent pigments, particularly as iridescent, nontoxic and moisture responsive films and pigments in cosmetics and security papers--since the optical properties cannot be reproduced by printing or photocopying.

The Case for Cellulose Nanofibrils as High-Performance Reinforcement

A primary interest in nanocomposites (3) largely stems from the automotive and aerospace industries' perpetual search for ways to make lighter, stronger parts that will help reduce vehicle/aircraft weight. The materials that are made from a variety of polymers and contain relatively low loadings (under 6% by weight) of nanoscale mineral particles, for example clay, of which montmorillonite is the most common type used for nanocomposite formation. Other types of clay can also be used, depending on the precise properties required from the product. These clays include hectorites (magnesiosilicates), which contain very small platelets, and synthetic clays (e.g. hydrotalcite), which can be produced in very pure form and can carry a positive charge on the platelets, in contrast to the negative charge found in montmorillonites. One useful example, polyolefin nanocomposites, have been successfully manufactured using commercially available surface-modified montmorillonite (Nanomer[TM]). These nanocomposites are notable for their improved mechanical properties, such as tensile strength and modulus, and for their dimensional stability (Dong et al., 2004; Guan et al., 2005).

Toyota Central R&D Laboratory has pioneered the development of nanocomposites for more than a decade, when they found a way to disperse montmorillonite clay (MMT) into nylon-6 (Roukes, 2001). The nylon-clay hybrid was subsequently used to injection-mould a timing-belt cover. Volvo has also conducted research on the potential for using thermoplastic olefin (TPO) nanocomposites for automotive body panels. Automotive test panels moulded from a TPO with 5% MMT had comparable flexural stiffness and higher impact resistance than panels made from a TPO filled with 20% talc. Moreover, MMT-based resins exhibit 50 - 75% reduced peak heat-release rate, and form a carbonaceous-silicate char on the surface that insulates the underlying material (Seeman and Belcher, 2002). The superior barrier properties of nanocomposites further make them attractive for packaging and medical applications. International Paper's Packaging Development Center has recently revealed their new technology for gas and moisture barrier for beverage packaging. The technology employs a nanoclay composite coating; similar nanocomposite coatings have also been used by International Paper in inkjet and digital printing paper, which have been marketed under the brand Jet Print Photo paper. (4)

Cellulose nanocrystals have been investigated as fillers in a number of matrix systems, including siloxanes, poly(caprolactone), glycerol-plasticized starch, styrene-butyl acrylate latex, cellulose acetate butyrate and epoxies. The motivation for using cellulose nanocrystals was sparked by the same interest in other nanocomposite systems utilizing montmorrilonite clays and carbon nanotubes, namely: superior properties and very high aspect ratio. At relatively low nanocrystal loadings the composite reaches a percolation threshold. This is the filler level at which the filler particles begin to contact each other and form a three-dimensional network. The elasticity modulus builds up very rapidly from this point to extremely high values, whereas the shear modulus increases by more than three orders of magnitude at nanocrystal loadings of only 6% (Favier et al., 1995a; Grunert and Winter, 2002; Samir et al., 2004a, b). Cellulose nanocrystals, in this respect, function no differently than nanoclays or carbon nanotubes--except that they are readily available from renewable, recyclable natural resources.

Rapidly biodegradable hydrophobic material (RBHM) film coatings made of cellulose-latex nanocomposites (Loelovich and Figovsky, 2001) have recently been commercialized for use in packaging to improve the dimensional stability, hygroexpansivity and toughness of sack paper, with potential application in food packaging. (5) Furthermore, cellulose nanofibrils with dimensions of ~5 x 200 nanometres have been extracted, incorporated into an aqueous solution of a thermoplastic, and processed by different means including freeze-drying/moulding and casting/evaporation (Favier et al., 1995b; Helbert et al., 1996). Cellulose nanofibrils were shown to provide high reinforcing effects, where the elastic modulus of elastomers, for example, was increased by more than 1000 times at 15% cellulose nanofibril volume fraction (Figure 4) (Hajji et al., 1996, Dufresne et al., 1997a, b).


Drawing upon studies of biological systems where optimal properties are not always provided by the strongest and stiffest interfaces, surface modification may be used as an important tool to effect cross-linking between cellulose nanocrystals. For instance, bones and muscles are connected via tendons, which are quite flexible at small strains, with an increasing modulus of elasticity as the strain increases. The key here is energy absorption by the interphase. With the cellulose nanocrystal systems discussed here, one should be able to design and build nanosystems with various types of bonding. The concept is thus to build a material composed of an aligned network of rigid, strong "rods" held together by polymer "springs." The specific interactions between cellulose nanofirbils in polymer composites involve the development of strong hydrogen bonds between cellulose nanofibrils, which can be simulated in the context of percolation theories (Favier et al., 1995a, b; Dufresne et al., 1997a, b). Interaction and bonding of cellulose nanofibrils induce mechanical percolation of nanofibrils, which differs from geometrical percolation. Cellulose nanofibrils thus act as those exposed on the surface of cellulose fibres (by fibrillation) in paper sheets during water evaporation. Surface modification/treatment is primarily responsible for the stabilization of the cellulose nanocrystalline aqueous suspension, and as such, entanglement between flexible fibrils may play a crucial part in the mechanical behaviour of the nanocomposite, mainly in the non-linear range. (6)


In addition to cost and product differentiation, the forestry industry will need to consider pre-competitive technologies in order to healthily grow and secure a respectable return-on-investment. This review offers a sense of possible technological advances in the field of nanomaterials, with a vision of developing functional forestry-based materials that could find enhanced applications in a multitude of industries, for instance, packaging, cosmetics, advanced engineering materials, etc. Cellulosic materials have a distinct advantage over petrochemicals and other materials in that through "green chemistry" a completely environmentally-friendly system that possesses impressive strength-to-weight capabilities may be proposed. The extraction of cellulose nanofibrils is a reality, however, requiring alternate, more efficient methods for process optimization and commercialization. Nevertheless, careful attention need be placed on the scope and level of development insofar as concerns and fears raised by the public use of such novel materials.

Nanotechnology-based developments can provide incremental and evolutionary changes, as outlined in this review. Dry coating is perhaps one of the most promising technologies offering incremental improvements in mainly paper print quality. However, evolutionary nanotechnology processes to develop cellulose nanocrystals and/or nanofibrils into new high value-added products of superior performance hold the more significant promise of reinvigorating the forestry industry.


Chiral Nematic Behaviour in Nanocrystalline Cellulose

The phase-forming ability of a cellulose crystallite suspension depends on the mineral acid chosen for the initial hydrolysis (Revol et al., 1998). Use of either sulphuric or phosphoric acid yields a chiral nematic phase, but hydrochloric acid hydrolysis gives a viscous suspension that forms a birefringent, glassy phase after a "post-sulphation" treatment (Araki et al., 2000). The chiral nematic, or cholesteric, phase consists of stacked planes of molecules aligned along a director (n), with the orientation of each director rotated about the perpendicular axis from one plane to the next, as shown in Figure II.1a. The source of the chiral interaction is thought to arise from the packing of screwlike rods (see Figure II.1b), as postulated by Straley's statistical-mechanical theory (Straley, 1976), which was later experimentally confirmed using Small Angle Neutron Scattering (SANS) (Orts et al., 1998).

Chiral nematic liquid crystals whose pitch is of the order of the wavelength of visible light reflect circularly polarized light of the same handedness as the chiral nematic phase (Chandrasekhar, 1992). The wavelength of this selectively reflected light changes with viewing angle, leading to an iridescent appearance. It has long been known that cellulose derivatives can form iridescent liquid and solid phases. What is novel is that by simply casting films from suspensions (of low ionic strength) of cellulose crystallites, cellulose films with the optical properties of chiral nematic liquid crystals can also be prepared by allowing the suspension to dry in a relaxed state (Revol et al., 1998). By altering the salt contents of the suspension for a given source of cellulose and set of hydrolysis conditions, the films could be tailored to give different colours of reflected light. It should be noted that, by contrast to the aforementioned, evaporation under shear of a suspension of cellulose crystallites from the green alga Cladophora sp. gives a film with highly oriented uniaxial structure (Nishiyama et al., 1997).


Professor Derek Gray, McGill University, and Dr. Thomas Hu, Paprican, are gratefully acknowledged for critically commenting on this manuscript; as are the late Dr. Raj Seth and Dr. Paul Watson of Paprican.

Manuscript received June 14, 2006; revised manuscript received July 25, 2006; accepted for publication July 25, 2006.


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(1) The Finnish equipment manufacturer Metso Paper is a leading contender of this technology. NTERA (formerly known as Nanomat, founded by academics from University College Dublin and EPFL, Lausanne) and its spin-off Nanova, are involved in developing nanomaterials for paper coatings. Imerys is launching a new nanoclay particle coating for high-end magazine and packaging applications.

(2) A more in-depth discussion of the chiral nematic nature of nanocrystalline cellulose is presented in Appendix II.

(3) The term composite, in materials science, refers to the synergetic combination of materials to produce a new one with enhanced properties and end-use performance. In this section, reference is made to filled and fibre-reinforced composites, where each represents different performance capabilities. The major contribution to advanced materials research in cellulose nanocrystals has to channel into the development of nanofibre/fibril-reinforced composite systems whose properties approach those of pure cellulose crystals.

(4) Nanotechnology in Paper & Packaging News, Vol. 1, Issue 5, February 23, 2005.

(5) This product is developed by the Israeli start-up, Rademate Ltd., and is being distributed through a large Brazilian packaging manufacturer in South America, with plans for licensing worldwide according to Nanotechnology in Paper & Packaging News, Vol. 1, Issue 4, February 9, 2005.

(6) The influence of such interactions on the mechanical properties of cellulose nanocomposites can be predicted following the method of Quali et al. (1991) in their adaptation of the percolation concept to the classical series-parallel model of Takayanagi et al. (1964).

Wadood Hamad *

Pulp and Paper Research Institute of Canada, 3800 Wesbrook Mall, Vancouver, BC, Canada V6S 2L9

* Author to whom correspondence may be addressed.

E-mail address:
Table 1. Typical properties of cellulose nanofibrils ([dagger])
versus Kraft softwood pulp fibres (Clark, 1985; Frey-Wyssling and
Muhlethaler, 1965; Dufresne et al., 1997a, b; Hajji et al., 1996;
Helbert et al., 1996)

PROPERTY Cellulose Nanofibril

Length, nanometre 500
Diameter, nanometre 5
Specific Surface, 1/nanometre 0.048 [V.sub.f] ([double dagger])
Fibre Spacing, nanometre 5 [V.sub.f.sup.-0.5]
Aspect Ratio 100
Tensile Strength, MPa 10 000
Elastic Modulus, GPa 150

PROPERTY Softwood Kraft Pulp

Length, nanometre 1 500 000
Diameter, nanometre 30 000
Specific Surface, 1/nanometre 0.000008 [V.sub.f]
Fibre Spacing, nanometre 30 000 [V.sub.f.sup.-0.5]
Aspect Ratio 50
Tensile Strength, MPa 700
Elastic Modulus, GPa 20

([dagger]) Extracted from hydrolyzed native cellulose of animal
(e.g., tunicin) or plant (e.g., wood pulp, sugar beat, etc.) origins

([double dagger]) [V.sub.f] is fibre volume fraction

Properties of cellulose nanofibrils relative to metallic
and polymeric materials

MATERIAL Tensile Elasticity
 Strength Modulus
 (MPa) (GPa)

Cellulose nanofibrils 10 000 150
302 Stainless steel (a) 1280 210
Aluminium alloys 380 and LM6 (b) 330 71
Zirconia (c) 240 150
Aluminium with 20% particulate SiC (d) 593 121
Low-density polyethylene (e) 9 0.25
Nylon 6/6 30% glass filled (d) 186 9
0/90/ [+ or -]45 carbon in epoxy (f) 503 65


(a) ASM Metals Handbook, 10th ed., Vol. 1, 1990.

(b) ASM Metals Handbook, 10th ed., Vol. 2, 1990.

(c) ASM Engineered Materials Reference Book, 1989.

(d) Engineers Guide to Composite Materials, 1987.

(e) Chapman and Hall Materials Selector, 1997.

(f) ASM Engineers Guide to Composite Materials, 1987.
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Author:Hamad, Wadood
Publication:Canadian Journal of Chemical Engineering
Geographic Code:1CANA
Date:Oct 1, 2006
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