Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. Most importantly, these novel nanocelluloses open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose. The nanodimensions of the structural elements result in a high surface area and hence the powerful interaction of these celluloses with surrounding species, such as water, organic and polymeric compounds, nanoparticles, and living cells. This Review assembles the current knowledge on the isolation of microfibrillated cellulose from wood and its application in nanocomposites; the preparation of nanocrystalline cellulose and its use as a reinforcing agent; and the biofabrication of bacterial nanocellulose, as well as its evaluation as a biomaterial for medical implants.

Nanocelluloses: A New Family of Nature-Based Materials

Due to their abundance, high strength and stiffness, low weight and biodegradability, nano-scale cellulose fiber materials (e.g., microfibrillated cellulose and bacterial cellulose) serve as promising candidates for bio-nanocomposite production. Such new high-value materials are the subject of continuing research and are commercially interesting in terms of new products from the pulp and paper industry and the agricultural sector. Cellulose nanofibers can be extracted from various plant sources and, although the mechanical separation of plant fibers into smaller elementary constituents has typically required high energy input, chemical and/or enzymatic fiber pre-treatments have been developed to overcome this problem. A challenge associated with using nanocellulose in composites is the lack of compatibility with hydrophobic polymers and various chemical modification methods have been explored in order to address this hurdle. This review summarizes progress in nanocellulose preparation with a particular focus on microfibrillated cellulose and also discusses recent developments in bio-nanocomposite fabrication based on nanocellulose.

Microfibrillated cellulose and new nanocomposite materials: a review

Native wood celluloses can be converted to individual nanofibers 3–4 nm wide that are at least several microns in length, i.e. with aspect ratios >100, by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water. Preparation methods and fundamental characteristics of TEMPO-oxidized cellulose nanofibers (TOCN) are reviewed in this paper. Significant amounts of C6 carboxylate groups are selectively formed on each cellulose microfibril surface by TEMPO-mediated oxidation without any changes to the original crystallinity (∼74%) or crystal width of wood celluloses. Electrostatic repulsion and/or osmotic effects working between anionically-charged cellulose microfibrils, the ζ-potentials of which are approximately −75 mV in water, cause the formation of completely individualized TOCN dispersed in water by gentle mechanical disintegration treatment of TEMPO-oxidized wood cellulose fibers. Self-standing TOCN films are transparent and flexible, with high tensile strengths of 200–300 MPa and elastic moduli of 6–7 GPa. Moreover, TOCN-coated poly(lactic acid) films have extremely low oxygen permeability. The new cellulose-based nanofibers formed by size reduction process of native cellulose fibers by TEMPO-mediated oxidation have potential application as environmentally friendly and new bio-based nanomaterials in high-tech fields.

TEMPO-oxidized cellulose nanofibers

This paper provides an overview of recent progress made in the area of cellulose nanofibre-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibres (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterisation of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibres covered are those extracted from plants by acid hydrolysis (nanowhiskers), mechanical treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibril-based composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymerisation from the surface of cellulose nanowhiskers by atom transfer radical polymerisation and ring opening polymerisation, and methods to analyse the dispersion of nanowhiskers. The applications and new advances covered in this review are the use of cellulose nanofibres to reinforce adhesives, to make optically transparent paper for electronic displays, to create DNA-hybrid materials, to generate hierarchical composites and for use in foams, aerogels and starch nanocomposites and the use of all-cellulose nanocomposites for enhanced coupling between matrix and fibre. A comprehensive coverage of the literature is given and some suggestions on where the field is likely to advance in the future are discussed.

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Review: current international research into cellulose nanofibres and nanocomposites

A new type of nanocellulosic material has been prepared by high-pressure homogenization of carboxymethylated cellulose fibers followed by ultrasonication and centrifugation. This material had a cylindrical cross-section as shown by transmission electron microscopy with a diameter of 5−15 nm and a length of up to 1 μm. Calculations, using the Poisson−Boltzmann equation, showed that the surface potential was between 200 and 250 mV, depending on the pH, the salt concentration, and the size of the fibrils. They also showed that the carboxyl groups on the surface of the nanofibrils are not fully dissociated until the pH has reached pH = ∼10 in deionized water. Calculations of the interaction between the fibrils using the Derjaguin−Landau−Verwey−Overbeek theory and assuming a cylindrical geometry indicated that there is a large electrostatic repulsion between these fibrils, provided the carboxyl groups are dissociated. If the pH is too low and/or the salt concentration is too high, there will be a large attract...

The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes

Method for producing paper by the addition of a retention - dry strength agent system. The system consists of: cationic starch with a substitution degree of at least 0.005 in an amount of at least 1%; anionic high-polymer with a molecular weight greater than 106 and in amount of 0.003 - 0.5%, and cationic synthetic polymer in an amount of 0.005 - 0.5%.

Method of making paper with high filler content

This contribution reviews the field of cellulose surface modification for the application of lignocellulosic fibres in composite materials. There are varying needs to modify cellulosic surfaces, depending on the particular composite to be produced and the particular process to be used for the purpose. There is generally a need to enhance the adhesion between the fibres and the matrix material and there is the need to restrict water sorption into the reinforcing fibres and this clearly requires two different approaches. Finally, there is the need to find various aids (dispersants etc) for the processing. Providers of cellulose composites today apply a pragmatic approach, often applying techniques used in the fields of other composite materials, neglecting the particular interactions to be targeted in the manufacturing process using lignocellulosics, in a more or less successful way. History repeats itself; technology comes before the science. Cellulose modification has a long technical history, particularly for various types of wooden materials and wood composites. There are, however, few approaches, which lend themselves to be directly applied to the cellulose composites in focus today. There are also few industrial operations apart from common sizing (to make cellulose surfaces hydrophobic) and wet strengthening of paper, where the cellulose surface chemistry is deliberately altered. Said technologies are both mature and fairly advanced from a technical point of view and with a considerable history as well. More recently, during the last decade, there has however, been a renewed and active research in the field of cellulose composites, particularly for thermoplastic composites and a number of different approaches for the surface modification of lignocellulosics have been tried out. There are a number of mechanisms by which the coupling between lignocellulosic fibres and various matrix materials may be increased and excellent reviews are available. Numerous coupling agents have been tried out, such as maleated PP (polypropylene), various organosilanes, triazines etc. Needless to say, many of these coupling agents were developed for glass fibre composites and progress has generally been slow, although coupling agents such as maleated PP are extensively used in a rapidly expanding industry of cellulose reinforced PP in USA. Some of these approaches will be reviewed. The pulp and paper scientific community has long identified the need to alter the surface chemistry of fibres, primarily to reinforce the fibre-fibre bond strength, but also from a host of other purposes to reinforce the competitiveness of bio-composite materials. Hence, in our laboratories, the bipolar activator (Laine et al 2000, 2002) and polyelectrolyte multilayering (Wagberg et al 2002) technologies have been developed and these technologies have the promise of providing a generic platform for topochemical surface modification of lignocellulosic surfaces. Secondly, there are the various biotechnical approaches (selective enzymes, target molecules), which undoubtedly also have the promise of creating appropriate surface modification platforms. Whether these approaches may contribute to the future developments in composites, outside the pulp and paper sector is not known, but will be addressed in our future research efforts at the newly started, Biofibre Materials Centre (BiMaC) at KTH.

An overview of some possibilities to modify fibre surfaces for tailoring composite interfaces

The regular transmittance of light or similar radiation through a flowing suspension of particles fluctuates because of the random occurrence of particles in the beam.In the work presented here, a theory for this fluctuating behaviour with the emphasison dispersions of mm-length slender cylindrical particles having circular crosssections is given. The particles in question are wood pulp fibres, which as a first approximation are considered to have a cylinder shape. Four possible measurementprinciples are described theoretically and experimentally. The four principles are for the measurement of concentration, length distribution characterized as lengthclasses, mean length, and mean width. The usefulness in industrial process monitoring of two of these principles is exemplified with pulp measurements. In order to estimate model errors, numerical simulations were used. Although other techniques such as image analysis may compete, the technique presented here is attractive because of the simplicity of the measurement device used.

The Development of an Integrated Retention Control System

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The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes

Method of making paper with high filler content

An overview of some possibilities to modify fibre surfaces for tailoring composite interfaces

The Development of an Integrated Retention Control System