J Mater Sci (2012) 47:1171 1186
DOI 10.1007/s10853-011-5774-3
MATERIALS IN NEW ZEALAND
A critical review of all-cellulose composites
" " "
Tim Huber Jörg Müssig Owen Curnow
" "
Shusheng Pang Simon Bickerton
Mark P. Staiger
Received: 9 February 2011 / Accepted: 7 July 2011 / Published online: 21 July 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Cellulose is a fascinating biopolymer of almost different processing routes have been applied to the man-
inexhaustible quantity. While being a lightweight material, ufacture of ACCs using a broad range of different solvent
it shows outstanding values of strength and stiffness when systems and raw materials. This article aims to provide a
present in its native form. Unsurprisingly, cellulose fibre comprehensive review of the background chemistry and
has been rigorously investigated as a reinforcing compo- various cellulosic sources investigated, various synthesis
nent in biocomposites. In recent years, however, a new routes, phase transformations of the cellulose, and
class of monocomponent composites based on cellulosic mechanical, viscoelastic and optical properties of ACCs.
materials, so-called all-cellulose composites (ACCs) have The current difficulties and challenges of ACCs are clearly
emerged. These new materials promise to overcome the outlined, pointing the way forward for further exploration
critical problem of fibre matrix adhesion in biocomposites of this interesting subcategory of biocomposites.
by using chemically similar or identical cellulosic materials
for both matrix and reinforcement. A number of papers
scattered throughout the polymer, composites and biomo- Cellulose the natural choice for composite materials
lecular science literature have been published describing
Introduction
non-derivatized and derivatized ACCs. Exceptional
mechanical properties of ACCs have been reported that
easily exceed those of traditional biocomposites. Several Cellulose is one of the most abundant biopolymers on earth
with *1.5 9 1012 tons of cellulose produced each year.
Thus, it presents an enormous amount of a renewable and
T. Huber M. P. Staiger (&)
biodegradable resource for raw materials [1, 2]. Cellulose
Department of Mechanical Engineering, University
fibres are widely recognised for their applicability in eco-
of Canterbury, Private Bag 4800, Christchurch, New Zealand
friendly composite materials, although unlocking their full
e-mail: [e-mail]
potential remains a challenge for load-bearing engineering
J. Müssig
applications.
Department for Biomimetics, University of Applied Sciences
Bremen, Neustadtswall 30, 28199 Bremen, Germany
Chemistry and phases of cellulose
O. Curnow
Department of Chemistry, University of Canterbury, Private Bag
The molecular composition of cellulose, isolated from
4800, Christchurch, New Zealand
plant cell walls, was first discovered and determined by
Anselme Payen (1795 1871). Next to plants, some algae,
S. Pang
fungi and bacteria species are also produce cellulose [1].
Department of Chemical and Process Engineering, University
of Canterbury, Private Bag 4800, Christchurch, New Zealand
Cellulose is a linear polymer composed from aldehyde
sugars, so-called D-anhydroglucopyranose units (C6H11O5/
S. Bickerton
IUPAC nomenclature: (3R,4S,5S,6R)-6-(hydroxymethyl)
Department of Mechanical Engineering, University of Auckland,
oxane-2,3,4,5-tetrol), often simply referred to as glucose
Tamaki Campus, Auckland, New Zealand
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1172 J Mater Sci (2012) 47:1171 1186
lower value of 75 GPa. Ishikawa et al. [8] published a
similar ranking for those polymorphs sourced from ramie
fibre. This variation in tensile properties is due to differ-
ences in the molecular structure of the different allo-
morphs. Deformation mechanisms involve complex
stretching and re-organisation of the hydrogen bonds,
which differs greatly for crystalline and amorphous phases.
The mechanical properties of cellulose compete well
Fig. 1 Molecular structure of a cellobiose unit
with other engineering materials such as aluminium
(70 GPa) or glass fibres (76 GPa) [14]. As a result of the
units, assembled into groups of two as   cellobiose  units. low density of 1.58 1.59 g/cm3 Wegst et al. ranked the
Figure 1 shows the typical molecular structure of a cellobi- specific stiffness of native cellulose of 67 GPa cm3 g-1 as
ose unit. A single glucose unit is a hexose that takes on one of
among the highest of all natural materials [5, 15, 16].
two forms (a or b), depending on the position of the hydroxyl Cellulose I is the strongest allomorph with a theoretical
groups. Individual cellulose chains are highly hydrophilic
ultimate tensile strength of about 13 17 GPa. Cellulose II
due to the large numbers of hydroxyl groups present. Native
and amorphous cellulose are less strong with tensile
cellulose or cellulose I is the most crystalline type of which strengths of *9 and 0.8 Ä… 0.1 GPa, respectively [17]. The
there are two forms: Ia and Ib. While the cellulose Ia crystal
high tensile strength and low density of the native cellulose
has a triclinic unit cell, the cellulose Ib crystal has a mono- crystal results in the highest specific tensile strength of any
clinic unit cell. Both, cellulose Ia and Ib are present in native
known natural polymers for cellulose I (667 MPa cm3 g-1)
cellulose structures but their ratio depends on the source of
[15].
cellulose. Other allomorphs of cellulose are possible out of
which the most common are cellulose II, III, and IV. Cel- The biocomposite development
lulose II can be formed by mercerisation or regeneration of
cellulose I [1, 3 5]. Cellulose III can be formed from either,
Early reports on the use of natural fibres in composites date
cellulose I or cellulose II by a treatment with liquid ammonia, back to the early 1970 and 1980 [18, 19], since then
resulting in either cellulose III1 or cellulose III2. Cellulose
modern advances in the development of cellulose fibre-
IV1 and Cellulose IV2 can be prepared by the corresponding
reinforced polymer composites have been the subject of
form of cellulose III by heating in glycerol [6]. several hundred studies. Due to the independence of cel-
Single-molecule cellulose chains interconnect via
lulosic fibres of crude oil and their vast availability, an
hydrogen bonds to form cellulose microfibrils that exhibit
improved CO2-balance compared with composites made
crystalline, paracrystalline and amorphous regions [7]. from industrially made fibres and fillers and good
Those microfibrils are present in the secondary cell wall of
mechanical properties, cellulose-containing composites
all plants, usually embedded in a matrix consisting of
have generated much interest amongst various industries,
hemicelluloses and lignin. The degree of polymerisation
especially the automotive industry [20 28].
(DP) of cellulose varies widely depending on the source,
The most commonly used natural fibres for composite
ranging from 300 in wood fibres up to 10,000 for plant
applications are wood, jute, flax, sisal and hemp, although
fibres and bacterial cellulose. Cellulose content, DP and the many others are also suitable for biocomposites [29 33].
lateral arrangement of the microfibrils determine the tensile
The high specific tensile strength and stiffness of natural
properties of a plant fibre. Cellulose microfibrils can be
fibres makes them a lightweight alternative to traditional
classified as nanomaterials given the lateral dimensions of reinforcements such as glass fibres or other fillers. Natural
a microfibril is in the range of 5 50 nm [1, 8 10].
fibres are also less hazardous to handle and require less
energy during processing compared with glass or carbon
Mechanical properties of cellulose fibres. The fibres themselves also sequestrate carbon
dioxide during growing and are biodegradable [34]. How-
An average Young s modulus of 10.3 GPa was calculated
ever, due to different growing conditions, natural fibres
for amorphous cellulose using a force-field model [11]. usually show a large scatter of properties compared to
Using X-ray diffraction, Sakurada et al. [12] determined
industrially made glass fibres. A strong quality manage-
the Young s modulus of elementary cellulose fibril of
ment during fibre harvesting and processing and extreme
bleached ramie fibre to be 134 GPa. Using a similar set-up,
care while determining fibre properties are necessary to
Nishino et al. [13] measured the elastic modulus of several
produce reliable and reproducible results [35, 36]. Several
cellulose polymorphs. Cellulose I was found to have a
traditional processing methods for thermosetting and ther-
modulus of 138 GPa, whereas cellulose IV exhibited a moplastic polymers have been modified to allow the use of
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J Mater Sci (2012) 47:1171 1186 1173
natural fibres. New processing routes have also been Other approaches to produce all-cellulosic composites
developed that allow more rapid fabrication of biocom- involve chemical treatments, such as oxypropylation or
posite components at production rates demanded by benzylation, to generate cellulose derivatives that form the
industry [37]. matrix phase and are covered in the second part of this
A promising development in biocomposites research has review.
been the transition from petroleum-based polymer (e.g.
polyethylene, polypropylene) to naturally derived bio-
polymer (e.g. polylactides, palm oil-based resins, starch) Non-derivatized all-cellulose composites
matrices to produce composites that aim to be completely
biodegradable and CO2 neutral [32]. Introduction
While composites based on cellulose reinforcement and
petroleum-based polymers are usually referred to as eco- or The commonly accepted definition of a composite is a
biocomposites, composites based entirely on naturally material that consists of two or more distinct materials to
derived fibres and biopolymers have been named green improve the stiffness, strength and/or toughness over the
(bio) composites [32, 34]. individual constituents. However, in a monocomponent or
Bio- and green composites are finding applications in a single polymer composite, reinforcing and matrix phases
wide range of applications from structural to biomedical are based on the same material. Theoretically, this would
[32, 38 42]. Nevertheless, the inherent chemical incom- lead to an interfaceless composite where boundaries
patibility between a hydrophobic polymer matrix and between reinforcement and matrix are indistinct in the
hydrophilic cellulose [43, 44] causes interfacial bonding presence of ideal chemical bonding. Therefore, the need for
between the cellulosic and biopolymer components to be energy intensive fibre treatments or coupling agents for
often weak, particularly in the case of thermoplastic bio- improving interfacial bonding could be drastically reduced
polymers [45]. This leads to an inefficient stress transfer or even completely eliminated.
under load and thus low mechanical strength and stiffness While the reinforcement and matrix of monocomponent
[20, 43, 46, 47]. The chemical compatibility can be composites are necessarily of the same chemical compo-
improved by a chemical treatment of the fibre or matrix. sition, physical morphology and/or structural phases of the
Silane, alkaline, acetylation, chemical grafting, and corona two components may differ in reality. The performance of
discharge treatments provide widely varying degrees of a monocomponent or single-polymer composite is best
improvement [29, 45, 48 53]. Interfacial bonding can also illustrated with an example of the concept as put forward
be increased by using nanosized forms of cellulose such as by Capiati and Porter [63]. In this work, high-density
bacterial cellulose [54, 55], microfibrillated cellulose [56, polyethylene composites were produced with a gradient of
57] and cellulose whiskers [58 61] that provide an changing morphology between the reinforcing fibres and
increased surface area per volume. While significant the matrix material resulting in an improved interfacial
improvements in mechanical properties can be obtained, shear strength in the range between glass fibre-reinforced
the above methods also add cost and complexity to the polyester and epoxy resins. In addition to the enhanced
formulation of biocomposites. bonding at the reinforcement matrix interface, monocom-
Cellulose reinforced cellulosic structures such as vul- ponent composites can also provide a more straightforward
canized cellulosic fibres have been reported decades ago path for recycling as the fibre and matrix do not require
and found applications, for example, as vulcanized paper separating (e.g. all-polypropylene composites [64 67]).
[62]. However, growing environmental awareness and A recent summary of different single polymer composites
increasing interest in sustainable material concepts have is presented by Matabola et al. [68].
lead to the development of bio- and green composites for The concept of an all-cellulose composite was first
structural composite applications. The newly developed discussed by Nishino et al. [14]. ACCs can be considered
all-cellulose composites (ACCs) described in this review bio-derived monocomponent composites; although strictly
represent an approach to formulating green composites that speaking, the same source of cellulosic materials would
aim to eliminate the chemical incompatibilities between need to be used for the reinforcing and matrix phases.
reinforcement and matrix phases by utilising cellulose for While the ease of recycling is an important advantage for
both components. ACCs show the potential to be the next thermoplastic-based monocomponent composites, the main
step in the development of more sustainable composites. driver for the development of ACCs is to improve chemical
The processing, characterisation, properties and applica- bonding at the reinforcement matrix interface. During the
tions of this promising class of high strength biocomposite processing of ACCs, it is quite possible to have two or
materials is presented in detail in the first section of this more different allomorphs present. Cellulose molecules
review. strongly interact through hydrogen bonding, although the
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interaction between different cellulose allomorphs has not dinitrogen tetroxide/dimethylformamide (N2O4/DMF),
been quantified. Thus, the details of chemical bonding at N-methylmorpholine-N-oxide (NMMO), mineral acids,
allomorph boundaries in ACCs remain elusive. The mor- sodium hydroxide (NaOH), dimethylsulfoxide/tetra-
phological characteristics of the interface have only been butylammonium fluoride (DMSO/TBAF), dimethylimida-
observed qualitatively using for example scanning [69] or zolone/lithium chloride, and various molten salt hydrates
transmission electron microscopy [70]. and ionic liquids (ILs). Of these, LiCl/DMAc, NMMO,
NaOH and the ionic liquid 1-butyl-3-methylimidazolium
Preparation and synthesis of ACCs chloride (BmimCl) have been used mostly in the process-
ing of ACCs (Table 1). However, limited dissolution
Processing routes capacity, slow dissolution rates, toxicity and non-recyclability
are the reasons that prevent some of these solvents from
There are two distinct strategies in the literature for the being used in large industrial scales. It has been observed
preparation of ACCs (Fig. 2). The first of these methods that some ILs offer high cellulose dissolution rates. The
(2-step method) involves firstly dissolving a portion of low vapour pressure of ILs also makes them easy to reuse
cellulose in a solvent which is then regenerated in the and safer to handle, and has led to the term   green sol-
presence of undissolved cellulose. An example of this vents  [72 76]. In the following, we review in further
method was first given by Nishino et al. [69] in which Kraft detail properties of cellulose solvents used for the pro-
fibre was fully dissolved and then regenerated in the duction of ACCs.
presence of ramie fibres. A second route (1-step method) NMMO belongs to the family of cyclic, aliphatic, ter-
involves partial dissolution of the surface of cellulosic tiary amine oxides, where the nitrogen carries the cyclic
fibres then regenerated in situ to form a matrix around the and aliphatic groups, and oxygen [81] (compare Fig. 3).
undissolved portion. An example of this method was first The highly polar N O group is responsible for the high
given by Gindl et al. [71] in which they partially dissolved hydrophilicity of NMMO and its complete miscibility in
cellulose I, resulting in volume fractions of up to 90% of water, as it readily forms hydrogen bonds. NMMO is a
the original fibre and 10% of newly regenerated cellulose powerful cellulose solvent due to the high polarity and
matrix. This method has also been described as   surface weakness of the N O bond [81].
selective dissolution  [97]. In these processing routes, the NMMO is used industrially in the Lyocell process for
dissolution step is followed by solvent removal and cellu- producing regenerated cellulose fibres. The main steps of
lose regeneration using water or other coagulants, after this process are the preparation of the slurry by dissolution
which the composites usually have to be dried. of cellulose (usually pulp or cotton) in a mixture of water,
NMMO, stabilizers and additives. The cellulose solubility
Cellulose dissolution depends on the mixing ratio of cellulose, water and
NMMO. A more detailed description of the dissolution
Known non-derivatising solvents for cellulose include process and the influencing factors can be found in the
lithium chloride/N,N-dimethylacetamide (LiCl/DMAc), review of Fink et al. [82]. The dissolution is followed by an
extrusion of the viscous dope at elevated temperatures
Table 1 Cellulose solvents used for the production of all-cellulose
composites and the year their functionality was reported
Solvent Year Reference
NMMO 1969 Johnson [77]
LiCl/DMAc 1981 McCormick [78]
NaOH urea 1995 Isogai and Antalla [79]
Ionic liquids 1934 Graenacher [80]
Fig. 3 Structural formula of the
NMMO molecule
Fig. 2 Schematic of two-step (a) and one-step (b) all-cellulose
composite preparation. The scheme of the one-step process is adapted
from Nishino and Arimoto [95]
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(generally 90 120 °C) through an air gap. The fibres are the dissolution, while NaOH can only partially dissolve
then coagulated into a precipitation bath, washed and dried. celluloses of low DP [102 105]. The alkaline solvent is
Around 99% of the NMMO can be recovered from the cooled to subzero temperatures. Once the subzero tem-
precipitation and washing baths [83]. perature is attained, the mixture is often stirred under
Lyocell fibres primarily consist of monoclinic cellulose thawing, resulting in cellulose dissolution [102, 106]. The
type-II crystallites [82, 84]. Crystallites with a length, solution can be centrifuged to separate the undissolved
width and thickness of 12 14, 8 10, and 3 4 nm, respec- portion from the truly dissolved cellulose [107, 108]. The
tively, accumulate into strand-shaped bundles with lengths dissolved cellulose is then transformed into a gel by a
of 150 550 nm, partly assembled into aggregates of thermal path, precipitated in an acidic medium or coagu-
30 60 nm in diameter [84]. The strength and stiffness lated. Depending on the process, different microstructures
of regenerated fibres formed from NMMO in the presence can be obtained [109]. Coagulation was used to produce
of additives can be up to 1.3 and 55 GPa, respectively [85]. cellulose membranes with varying pore geometries and
Another solvent for the preparation of ACCs is DMAc mechanical properties according to the coagulant type,
mixed with LiCl (see Fig. 4). Cellulose needs to undergo a concentration and coagulation time [110 112]. NaOH
so-called   activation procedure  during which the fibre is urea (NaOH/(NH2)2CO) and NaOH thiourea (NaOH/
penetrated with a polar medium [86]. Without activation, it (NH2)2CS) can also be used to process the cellulose into
can take several months for the dissolution to proceed textile fibres with mechanical properties close to com-
regardless of the crystallinity of the cellulose. Amorphous mercially available rayon fibre [102, 103].
cellulose obtained by ball-milling also proves difficult to Graenacher [80] was the first to discover an IL solvent
dissolve in the absence of the activation procedure [87]. system for cellulose, but this was thought to be of little
Interestingly, the activation step before the actual dissolu- practical value at the time. Much more recently the use of ILs
tion does not affect cellulose crystallinity [88]. as a solvent for cellulose has been reported by Swatloski et al.
There are two different ways to prepare the mixture: (i) the [73]. Ionic liquids are molten salts with melting points
LiCl/DMAc solution is prepared first and then the cellulose is below 100 °C. There is a wide range of possible cations
added, or (ii) cellulose and DMAc are mixed together fol- (e.g. alkylimidazolium ([R1R2IM]?), tetraalkylammonium
lowed by addition of LiCl [89]. Stirring is also critically ([NR4]?) and tetraalkylphosphonium ([PR4]?) and anions
important for the dissolution to proceed due to the hetero- (e.g. hexafluorophosphate ([PF6]-), nitrate ([NO3]-) or
geneous fibre-solvent mixture. It has been reported that the chloride, bromide and iodide salts [113]. Only some of them
solubility of cellulose increases with LiCl content [90]. are able to dissolve cellulose, but the number of possible ion
Many studies on ACCs report the use of LiCl/DMAc combinations is said to be as high as one trillion which leaves
[14, 69, 71, 91 99], which may be due to its ability to much scope for the development of new types of cellulose
completely dissolve high molecular weight cellulose [88]. solvents [114]. Their ability to dissolve cellulose originates
In those studies, a concentration of 8 wt% LiCl was used from their high effective polarity, due to their ionic character.
for the dissolution of cellulose. The most successful ILs in cellulose dissolution reported so
A more eco-friendly non-derivatizing solvent for cellu- far, are hydrophilic and consist of the cations methylimi-
lose is based on NaOH or aqueous NaOH solutions with dazoloium and methylpyridinium cores with allyl-, ethyl- or
additions of urea and/or thiourea used at sub ambient butyl side chains with chloride, acetate or formate anions
temperatures or other additives such as poly(ethylene gly- [115, 116]. Recently, Pinkert et al. [117] provided a detailed
col) (PEG) or zinc oxide [100, 101]. NaOH-urea-thiourea review of cellulose dissolution by ionic liquids. ILs combine
dissolution is a simple, safe process requiring minimal all of the desirable characteristics of the previous solvents
energy input. The addition of urea ((NH2)2CO) and/or including low volatility, low cost due to ease of recycling,
thiourea ((NH2)2CS) to aqueous NaOH greatly enhances capacity for rapid and complete dissolution of a broad range
of cellulose sources and with no requirement for pre-treat-
ment or activation. However, some ILs have proven to
Fig. 4 Structural formula of the
LiCl/DMAc molecule
be toxic and an environmental hazard, contradicting their
image as   green  solvents [118, 119].
Influence of cellulose sources on dissolution
Various cellulosic materials including wood pulps, ramie,
sisal and regenerated cellulose fibres, microcrystalline
cellulose powder, bacterial cellulose and filter paper have
been used to produce ACCs. The time required for
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cellulose dissolution depends strongly on the fibre struc- Longer dissolution times lead to a reduction of cellulose I
ture, especially the degree of orientation and crystallinity in crystallites resulting in a change of crystallinity after
the outer part of the fibre. Thus, the dissolution conditions regeneration of the cellulose. Nishino et al. [69, 95] and
need to be tailored to different sources of cellulose in Soykeabkaew et al. [97] reported that after dissolution with
forming ACCs. Soykeabkaew et al. compared immersion LiCl/DMAc the regenerated phase was non-crystalline or
times necessary to form a matrix phase of highly orientated amorphous based on WAXS. On the contrary, Duchemin
cellulose structures present in Bocell fibres to less ordered et al. [120] obtained results from WAXS and NMR that
cellulose configurations in the outer regions of Lyocell suggest that exposure of crystalline cellulose to LiCl/
fibres. They reported that the Bocell fibres needed more DMAc results in peeling away of thin layers from the
than 1 h immersion time to form a matrix phase while for original crystallites which retain some molecular ordering.
the Lyocell fibres less than 20 min were sufficient. Longer After solvent removal, these thin layers can form a para-
immersion times also lead to a reduction in fibre diameter crystalline phase that is distinct from typical amorphous
as the fibre surface dissolution increases [97, 98]. cellulose and closer in structure to cellulose I. They also
suggested that the presence of a paracrystalline   matrix  is
Cellulose regeneration one of the underlying reasons for the high mechanical
properties of ACCs. Zhao et al. [115] observed that dis-
The steps involved in the regeneration of the dissolved solution of cellulose I using the IL BmimCl results in a
cellulose are (i) removal of the solvent by a coagulant matrix phase consisting of cellulose II. Gindl and Keckes
(water, alcohol or acetone are commonly used) and then [71] also identify the regenerated phase of dissolved cel-
(ii) removal of the coagulant through evaporative drying. lulose I as cellulose II.
Cellulose regeneration is an important step in processing
ACCs as it controls the precipitation of the final cellulose Mechanical properties
phases. Duchemin et al. suggested that the regeneration
rate controls the phase composition, which in turn will The mechanical properties of anisotropic composites
dictate the physical properties of the ACC. Cellulose strongly depend on the dissolution and regeneration con-
phases of higher crystallinity are observed as the rate of ditions. The longitudinal tensile strength decreases with
regeneration is decreased. This is thought to be due to the increasing dissolution time due to a decreasing cross-
dissolved cellulose chains having greater time to order sectional area of the load-bearing cellulose fibrils and
themselves into a lower energy configuration. Thus, the therefore a reduction in fibre volume fraction. The trans-
rate of application of the coagulant for removal of the verse tensile strength follows an opposite trend as the
solvent and then subsequent drying rate can be manipulated matrix phase increased and the interface becomes more
to given varying properties in the final ACC [70]. homogenous. Soykeabkaew et al. [97] reported that
Contact of the coagulant with the cellulose will lead to assumption based on their experiments with unidirectional
swelling especially if water is used. Distortion of the ramie fibre composites. Immersion times of 12 h lead to a
sample due to warping is even more apparent in thicker decrease in longitudinal tensile strength of about 60%
samples as a diffusion gradient of coagulant from the compared to immersion times of 6 h. Simultaneously, the
surface to the interior of the solution results in differential transversal tensile strength is increased by about 33%.
shrinkage and subsequent delamination and void forma- However, there are very few publications stating trans-
tion. Furthermore, the dissolved and undissolved portions versal strength and stiffness of unidirectional composites to
of cellulose will swell by different amounts which upon verify this assumption. Extending the dissolution time can
regeneration again can cause differential shrinkage during lead to over-dissolution of the fibres, resulting in a rapid
regeneration, leading to the formation of voids at the fibre decrease in tensile properties for either isotropic or aniso-
matrix interface [70]. tropic ACCs [69, 99]. The application of the solvent can
also affect the composition of the reinforcing fibres. Gindl
Phase characterisation et al. [93] observed that the hardness of the fibres changes
when treated with LiCl/DMAc, presumably due to partial
Identification and characterisation of cellulose phases dissolution of cellulose within the cell walls.
present in ACCs has been mainly carried out with wide- The inherent properties of the reinforcement will also
angle X-ray scattering (WAXS) [69, 71, 92, 95, 97, 115] affect the properties and processing of ACCs. Many dif-
and to a lesser extent solid state nuclear magnetic reso- ferent combinations of fibre, matrix, and solvent systems
nance (NMR) [120]. have been studied in the literature, giving a large range of
The treatment of cellulose with LiCl/DMAc leads to a properties for ACCs. Table 2 lists the tensile properties of
decrease in crystallinity depending on the immersion time. ACCs made using different materials and solvents,
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Table 2 Overview over isotropic (ISO) and unidirectional (UD) all-cellulose composites produced by one and two step processes and after additional fibre or composite treatment
No. Cellulose source Cellulose source Solvent Fibre volume Tensile Tensile Young s Strain to Strain to Reference
for matrix for reinforcement fraction in % strength strength modulus failure failure
: (MPa) ? (MPa) : (GPa) : (%) ? (%)
Two step process
1 Cotton linter pulps Cellulose whiskers (ISO) NaOH/urea 10 124 5.1 Qi et al. [125]
2 Cotton linter pulps Cellulose whiskers (ISO) NaOH/urea 20 117 5.9 Qi et al. [125]
3 Cellulose powder Hemp fibre (ISO) NMMO 40 28.9 1.8 20.8 Quajai and Shanks [122]
4 Wood pulp Ramie fibre (UD) LiCl/DMAc 80 480 12 4 5 Nishino et al. [69]
5 Ramie fibre Ramie fibre (UD) LiCl/DMAc 85 410 25 4.8 Qin et al. [99]
6 Filter paper Rice husks (ISO) Ionic liquid (BmimCl) 40 57.5 1.74 5.67 Zhao et al., 2009, [115]
7 Filter paper Rice husks (ISO) Ionic liquid (BmimCl) 60 56 2.92 2.76 Zhao et al., 2009, [115]
No. Cellulose source Reinforcement Solvent Fibre volume Tensile Tensile Young s Strain Strain to Reference
for matrix and type fraction in % strength strength modulus to failure failure
reinforcement : (MPa) ? (MPa) : (GPa) : (%) ? (%)
One-step process
8 MCC (ISO) LiCl/DMAc 242.8 13.1 8.6 Gindl and Keckes [71]
9 Beech pulp (ISO) LiCl/DMAc 80 154 12.2 0.023 Gindl et al. [93]
10 Filter paper (ISO) LiCl/DMAc 16 211 8.2 3.8 Nishino and Arimoto [95]
11 Ramie fibre (UD) LiCl/DMAc 85 480 29 26 3.7 4.5 Soykeabkaew et al. [97]
12 LDR-Lyocell fibre (UD) LiCl/DMAc 72 250 9 24 Soykeabkaew et al. [98]
13 HDR-Lyocell fibre (UD) LiCl/DMAc 73 350 12 10 Soykeabkaew et al. [98]
14 Bocell fibre (UD) LiCl/DMAc 88 910 23 8.2 Soykeabkaew et al. [98]
15 MCC (ISO) LiCl/DMAc 58.7 3.2 2.5 Duchemin et al. [70]
16 MCC (ISO) LiCl/DMAc 105.7 6.9 3.3 Duchemin et al. [70]
17 BC (ISO) LiCl/DMAc 411 18 4.3 Soykeabkaew et al. [96]
18 MCC (ISO) Ionic liquid (BmimCl) 91.8 5.75 3.76 Duchemin et al. [126]
19 Filter paper (ISO) Ionic liquid (BmimCl) 124 10.8 2 Duchemin et al. [126]
No. Cellulose Cellulose Solvent Additional processing step Fibre Tensile Tensile Young s Strain to Strain to Reference
source source for volume strength strength modulus failure failure
for matrix reinforcement fraction (%) :(MPa) ? (MPa) : (GPa) : (%) ? (%)
All-cellulose composites prepared with further fibre or composite processing
20 Wood pulp Ramie fibre (UD) LiCl/DMAc Immersing in water, acetone, DMAc 80 400 17 25 3 21 Nishino et al. [69]
21 MCC (ISO) LiCl/DMAc Wet drawing to align cellulose fibrils 428 95 33.5 2.3 Gindl and Keckes [92]
22 Ramie fibre Ramie fibre (UD) LiCl/DMAc Mercerization 85 540 25 4.8 Qin et al. [99]
Included are the types of cellulose source, reinforcement, solvent and fibre fraction used. Both tensile properties parallel (:) and transverse (?) to the fibre direction are given where available
MCC microcrystalline cellulose, BC bacterial cellulose, LDR low draw ratio, HDR high draw ratio
J Mater Sci (2012) 47:1171 1186
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1178 J Mater Sci (2012) 47:1171 1186
The strain to failure of ACCs (eACC) is largely dominated
f
by the type of reinforcement used. For example, ACCs
reinforced with low strain ramie fibre (ef = 1.2 3.8%,
[121]), show lower values of eACC (3.7 4.8%) when com-
f
pared with reinforcement made from high strain Lyocell
fibre (ef = 9.4 27.9%, [98] which gives higher eACC
f
(10 24%) (Table 2). ACCs based on hemp fibre reported by
Quajai et al. [122] achieved values of 20% for eACC. This is
f
unexpected as ef of hemp fibres is only 1.6% [121]. In fact,
this last example emphasises the difficulty in comparing
different ACCs in the literature due to differences in pro-
cessing that can greatly influence the final properties.
Fig. 5 Ranges of tensile strengths and Young s moduli of isotropic
Other chemical or mechanical processing steps have
ACCs (dashed) compared with traditional isotropic biocomposites
been used to influence ACC properties (Table 2). The
(dotted). The numbers and letters of the references are found in
positive effect of a treatment with an alkali solution aso
Tables 2 and 3, respectively
known as   mercerization  on lignocellulosic fibres,
namely an improvement of tensile properties and absorp-
tion characteristics, is well known [123, 124]. When
applied to the ACCs, a swelling of the fibres occurs that
fills in voids and cracks significantly improving the inter-
face and, therefore, the tensile properties of the composite.
SEM pictures of an untreated (a) and mercerized (b) com-
posite can be seen in Fig. 7 [99].
Furthermore, Gindl et al. reported that wet drawing of
the composites after regeneration of the dissolved cellulose
could change the orientation of the cellulose crystals within
the composite towards a unidirectional direction according
to the direction of the applied load. The wet state changes
Fig. 6 Ranges of tensile strengths and Young s moduli of unidirec-
the molecule mobility as the water adsorption weakens the
tional ACCs (dashed) compared with traditional unidirectional
biocomposites (dotted). The numbers and letters of the references inter- and intramolecular hydrogen bonds. By drawing the
are found in Tables 2 and 3, respectively
composites, the crystal orientation changes linearly with
the draw ratio, the ratio between specimen length after and
demonstrating the large variability possible with formula- before stretching, whereas the overall crystallinity of the
tion and processing. Thus, judging and comparing the
composite stays unaffected. Drying the samples afterwards
influence of various parameters and properties between
causes the molecule chains to keep their positions resulting
different studies of ACCs is difficult. However, it is of
in anisotropy of the specimens with an improved tensile
interest to compare the family of ACCs with other bio- strength in longitudinal direction [91, 92].
composites. In general, the tensile strength of ACCs is
significantly higher compared with the more traditional
Viscoelastic properties
isotropic and unidirectional biocomposites (compare
Figs. 5, 6; Table 3).
The viscoelastic properties of ACCs are determined by the
Interestingly, a comparison of unidirectional ACCs with
viscoelastic properties of the cellulose chains and allo-
traditional biocomposites does not reveal dramatic differ- morphs. Amorphous cellulose exhibits a viscoelastic
ences in the Young s modulus. The underlying reasons
behaviour involving different molecular motions depend-
may be complex given the variable formulations but may
ing on temperature [135, 136]. Due to its high content of
be due to either (i) the cellulose solvent decreasing the
hydroxyl groups, hydrogen bonds are formed with neigh-
modulus of the reinforcing fibres in ACCs or (ii) the
bouring units of the same molecule, with neighbouring
modulus of biocomposites being dominated by the modulus
chains and any water that is present. Intra- and intermo-
of the fibres, with the fibre-matrix interfacial strength being
lecular hydrogen bonds are responsible for the thermal
less important. In contrast, significant increases in modulus
stability of the cellulose molecule. Intramolecular hydro-
are observed for isotropic ACCs compared with traditional
gen bonds increase the stiffness of the polymer while the
biocomposites in which it could be envisaged that the high
intermolecular hydrogen bonds and van der Waals inter-
matrix properties of ACCs dominate this behaviour.
actions are responsible for single chains arranging into a
123
J Mater Sci (2012) 47:1171 1186 1179
Table 3 Overview over the tensile properties of different isotropic and unidirectional biocomposites
Matrix Fibre type Fibre fraction Tensile strength Young s Strain to Reference
in (vol.% or (MPa) modulus failure (%)
wt%) (GPa)
Isotropic composites
PBAT Flax 30 (vol.%) 32 4.1 2 Bodros et al. [25] (Fig. 5a)
PBS Flax 30 (vol.%) 49 3.8 2.5 Bodros et al. [25] (Fig. 5b)
PHB Flax 30 (vol.%) 40 4.5 1.8 Bodros et al. [25] (Fig. 5c)
PLA MCC 5 (wt%) 31.9 1.5 [100 Petersson and Oksman [127] (Fig. 5d)
PLA Cellulose whiskers 5 (wt%) 47 2.1 5.4 Mathew et al. [128] (Fig. 5e)
PLA Cellulose microfibres 5 (wt%) 59 2.3 3.3 Mathew et al. [128] (Fig. 5f)
PLA Cordenka 40 (vol.%) 57.97 4.85  Bax and Müssig [129] (Fig. 5g)
PLA Flax 40 (vol.%) 54.15 6.31  Bax and Müssig [129] (Fig. 5h)
PCL Starch nano-crystals 50 (wt%) 15.5 0.384 5.0 Habibi and Dufresne [40] (Fig. 5i)
PCL Cellulose nano-crystals 50 (wt%) 18.7 0.442 8.6 Habibi and Dufresne [40] (Fig. 5j)
Polyester Banana 40 (vol.%) 68 1.87 6 Sreekuma et al. [130] (Fig. 5k)
PTP Hemp 21 (wt%) 63 7  Müssig et al. [42] (Fig. 5l)
Unidirectional composites
Epoxy Flax 50 (wt%) 119 30  Bos [29] (Fig. 6m)
PP Flax 55 (vol.%) 320.7 28.2  Madsen and Liholt [131] (Fig. 6n)
PP Jute 21.2 (vol.%) 141 11  Khondker et al. [132] (Fig. 6o)
Epoxy Flax 49 (vol.%) 284 26  van de Weyenberg et al. [133] (Fig. 6p)
PLA Kenaf 70 (vol.%) 230 24  Ochi [134] (Fig. 6q)
Starch Flax 60 (wt%) 78 9.3  Romhany et al. [44] (Fig. 6r)
Epoxy Sisal 46 (vol.%) 211 19.7 1.9 Oksman et al. [33] (Fig. 6s)
Fig. 7 SEM pictures of ramie
fibre-reinforced cellulose
composite made with 4%
cellulose concentration solution
in an untreated (a) and
mercerized (b) state [99].
Reprinted from Ref. [99].
Copyright 2008, with
permission from Elsevier
2D sheet [137]. The stable structure of the cellulose mol- shown by the analyses of other polysaccharides such as
ecule is the result of a dense network of hydrogen bonds dextran, that the c-relaxation is mainly associated with a
and their different bonding patterns within a single cellu- rotation of the CH2OH groups of the cellulose molecule
lose structure and their dependency on different tempera- rather than the rotation of its OH groups [140]. The bcell-
tures [138]. relaxation is associated with cooperative but localised
At low temperatures, the molecular mobility is consid- motion of segments of the main chain of the cellulose
ered as localised at the molecular level, giving rise to molecule depending on the water content [139]. The tem-
secondary relaxations [139]. Two secondary relaxations of perature range for the bcell-relaxation was reported as
amorphous cellulose are reported, occasionally referred to -83.15 to 43.15 °C [140, 141]. At higher temperatures,
as ccell and bcell [140, 141]. The c-relaxation is reported to cellulose exhibits three further transitions, designated a1, a2
appear at a constant temperature of -123 °C. It has been and a3 by Manabe et al. [142] and measured for regenerated
123
1180 J Mater Sci (2012) 47:1171 1186
cellulose. Thea3 transition is very closely related to the water the LiCl/DMAc solvent system show variations in optical
content, and therefore not associated with the inherent transparency as a function of the dissolution time, with
molecular motion but with the cooperative motion of cellu- increasing time resulting in greater transparency (Fig. 8).
lose chain segments and absorbed water molecules. The a2 As the dimensions of the used cellulosic materials in other
transition can be separated into two distinct relaxations (a2,1 studies exceed the nanometer range, the transparency of
and a2,2) that are caused by micro-Brownian motions of those ACCs has been attributed to the good adhesion
amorphous chain segments [142 144]. The a1 transition is between reinforcement and matrix phase and closure of
associated with motions in the non-crystalline regions as the internal pores in the cell wall [95, 97].
crystalline segments are constrained. However, as the a1
transition also coincides with the onset chemical decompo-
sition of the molecule due to the high temperature, it is Derivatized all-cellulose composites
possible that released chain segments from the crystalline
regions also contribute to the relaxation [145]. Introduction
The viscoelastic properties of ACCs have been investi-
gated using dynamic mechanical analysis (DMA) [13, 69, The field of derivatized cellulose, of which much is beyond
95, 97, 98]. The storage modulus (E0) of the composites the scope of this article, has been reviewed elsewhere [147
formed with LiCl/DMAc as solvent system are reported to 152]. One of the first processes using cellulose derivatives
be lie between 1010 and 1011 Pa between temperatures of was the viscose process, discovered by British chemists
-150 and 0 °C. E0 only gradually decreases with increas- Charles Cross, Edward Bevan and Clayton Beadle in 1891
ing temperature as a consequence of strong hydrogen [153]. The expression   Viscose  originates from the highly
bonding in cellulose I [95, 97, 98]. At temperatures above   viscous cellulose  solution obtained during the dissolution
250 °C, the cellulose composites start do degrade, resulting process, that was later contracted to   Viscose  . A major
in severe drop in E0. The decrease in E0 at higher temper- drawback of this process is the contamination of the waste-
atures than 250 °C of those composites goes in hand with water by carbon disulfide and other polluting sulphur
the reported a2 transition of regenerated cellulose. by-products during cellulose derivatization. This is one of
the main reasons that many improvements have been made to
Optical properties the process and the resulting products such as rayon fibre in
the last century to improve fibre quality and reduce the
Optically transparent cellulose-containing composites environmental impact [153]. The so-called man-made cel-
based on bacterial cellulose, epoxy and acrylic resins have lulose fibres have shown a steady rate of production in the
been produced; according to Nogi et al. [146], the optical range of 2,500 to 3,000 kt/year in the last decades with the
transparency is due to the dimensions of the bacterial cel- main application being textile fibres [154].
lulose ribbons lying in the nanometre range. However, Three of the most recent approaches for the production
ACCs have also been observed to be optically transparent of thermoplastic   all-plant fibre composites  , self-
under certain conditions as first reported by Gindl et al. reinforced cellulose composites and cellulosic nanocom-
[71] and attributed to the lateral dimension of the used posites via benzylation, oxypropylation and esterification,
microcrystalline crystals of 1 3.5 nm. ACC films based on respectively, are presented.
Benzylated, oxypropylated and esterified celluloses
In the last decade, alternative approaches to form all-
cellulosic structures using cellulose derivatives have been
undertaken we term these materials derivatized ACCs.
The first approach involves the production of all-plant or
all-wood fibre composites by a benzylation treatment of the
cellulose source [155, 156]. This is based on a Williamson
synthesis reaction involving nucleophilic substitution of an
oxide or a phenoxide ion for a halide ion
Wood-OHþNaOH!Wood-O NaþþH2O
Fig. 8 Optical transparency of ACCs as a function of dissolution
Wood-ONaþClCH2-C6H6!Wood-O CH2 ð1Þ
time in DMAc/LiCl. Reprinted from Ref. [97]. Copyright 2008, with
C6H6þNaCl
permission from Elsevier
123
J Mater Sci (2012) 47:1171 1186 1181
where, Wood-OH represents the hydroxyl groups mainly
Matsumura et al. produced cellulose nanocomposites by
present in cellulose [157, 158]. The matrix material is
partial esterification of wood pulp by a treatment with a
swollen in NaOH and afterwards transferred to the benzyl
p-toluene sulphonic/hexanoic anhydride system using a
chloride. The solution is stirred for several hours at tem- cyclohexane based reaction medium that caused no swell-
peratures above 100 °C. It is then washed to remove
ing of the cellulose fibres. To produce the composites, pulp
inorganic salts, benzyl chloride and its by-products to yield
fibres were exposed to a hexanoylation reaction to receive
a liquid matrix phase consisting of the used cellulosic
heterogeneously hexanoylated pulp fibres. The hexanoy-
material. Due to the complexity of wood and its macro- lation was assumed to start from both the surface of indi-
molecules, the extent of benzylation is measured indirectly
vidual microfibrils and unordered regions within the fibre
by observing the weight gain as the modification proceeds.
followed by hexanoylation of the microfibril core. Those
Benzylated fibres show the formation of a thermoplastic
fibres were mixed with water or methanol and filtered
region surrounding the fibre core [157 160]. The com- afterwards to receive a uniform fibre mat in disc shape. The
posites can then be simply formed by hot-pressing [157 discs were compression moulded at 155 170 °C and at
160]. Fibre volume fractions of up to 40% have been
room temperature. The so produced thermoplastic com-
achieved using this method. Lu et al. reported that the rate
posites of unmodified cellulose I and esterified cellulose
of benzylation is hardly affected by the wood source, but
were semitransparent [165, 166].
depends strongly on the amounts of NaOH and benzyl
The degree of benzylation is decisive for the compos-
chloride (C6H5CH2Cl) as well as the processing tempera- ites properties. A higher presence of benzyl groups is
tures and reaction times [157 159, 161]. Increasing the
reported to increase the viscosity of the material. This can
degree of benzylation of cellulosic material leads to a
lead to insufficient wetting of the fibres and result in a weak
decrease in crystallization due to disruption of the hydro- interface that reduces the flexural and tensile strength at a
gen bonds within the cellulose molecules, also reducing the
fibre volume share of 40% as the fibre are not completely
final mechanical properties. This is explained by the larger
surrounded by the matrix phase [158, 160]. An overview of
benzyl molecule introducing a larger free volume and
the tensile properties of benzylated and oxypropylated
causing a change in the supramolecular structure of the
ACCs is given in Table 4. The thermomechanical and
molecule [157 159].
viscoelastic properties of benzylated ACCs have been
It has been shown that it is also possible to produce   all- analysed with thermomechanical analysis (TMA) [158,
plant fibre composites  by an oxypropylation treatment of
159]. Benzylated ACCs exhibit thermoplastic behaviour
cellulosic fibres as first reported by Gandini et al. in 2005.
due to the benzylated cellulosic material with a softening
This is done by using a Brłnsted base to activate the
temperature in the range of about 90 to 120 °C depending
hydroxyl groups of the cellulose followed by an anionic
on the source of cellulose and degree of benzylation. Lu
polymerisation of the PO in a   grafting from  process. The
et al. [159] suggested that, as the plastification does not
fibres are first immersed in a solution of ethanol and
change the chemical backbone of the cellulose molecule
potassium hydroxide for several hours. After the alcohol
chains, that softening should not be attributed to the a1
has evaporated, the fibres are then mixed with propylene
transition of the cellulose but could rather be caused by
oxide (PO) under nitrogen atmosphere in an autoclave at
inter-molecular slips.
temperatures of 130 150 °C. The PO homopolymer cre- Analogous, for oxypropylated cellulose, the amount of
ated by chain-transfer reactions can be removed by a
PO used is a determining factor for the tensile properties,
Soxhlet extraction using hexane. This effectively grafts a
as the use of higher amounts leads to a decrease in strength
thermoplastic polymer matrix onto the outer surfaces of the
and stiffness but an increase in elongation to break. Those
fibres, with the amount measured by the weight gain [162 changes are likely to be caused by an increase of the
164]. In this reaction, cellulose I is converted to an oxy- thermoplastic amorphous phase [162].
propylated amorphous derivative. de Menezes et al. [163]
For the esterified composites, the degree of substitution
report that the amorphous regions of the cellulosic mate- (DS) during the hexanoylation reaction is the decisive
rials are more prone to modification than crystalline
factor for the mechanical performance. Similar to oxy-
regions, while the degree of modification is dependent on
propylated cellulose, elongation increased with DS, while
the amount of PO used. With increasing amounts of PO,
strength and stiffness decrease at highest tested DS of 2.0
the crystalline regions will also take part in the reaction
[166].
although increased degradation of the cellulose structure
Comparing the reported tensile properties of non-deriv-
and reduction in mechanical properties can occur. As for
atized ACCs with those of benzylated, oxypropylated and
benzylated cellulose, oxypropylated cellulose can be hot- esterified ACCs shows that the biggest differences seem to
pressed to form a composite film, with fibre fractions
be in tensile strength. The strongest non-derivatized ACCs
ranging from 10 to 40 vol.% [157 160, 162 164].
are about five to ten times stronger than their derivatized
123
1182 J Mater Sci (2012) 47:1171 1186
counterparts, while the Young s moduli and strain at failure been investigated in detail. This will be necessary to judge
are in a similar range. Based on the reported values of filter the effectiveness of the solvent and therefore allow a
paper in Tables 2 and 4 one can assume that the non- classification of the different processing methods, involved
derivatized formation of the composite using LiCl/DMAc costs and resulting material properties. The costs of the
leads to better tensile properties than the oxypropylation, solvent could be minimised if it can be recovered and
but without a comparative study the influence of processing recycled, as it is the case for NMMO and ILs, assuming the
on composite properties are not evaluable. Therefore, recycling process is not too cost-intensive. On the other
the biggest difference seems to be the thermoformability of hand, in case of a non-recyclable and possibly hazardous
the derivatized ACCs. solvent, the costs of its disposal must be taken into account.
Another important factor is the amount of dissolvable
cellulose and resulting necessary amount of solvent. It is
Future challenges and applications for all-cellulose also thinkable that different solvents will only work with
composites cellulose sources of low DP and crystallinity whereas
others might be the ideal choice for high-class raw
In spite of the promising properties of ACCs, there still materials.
remains much research required to understand the funda- Furthermore, more research is necessary to know how
mentals of these materials and to find suitable industrial the different solvent systems interact with the other com-
applications. ponents such as hemicelluloses and lignin of financially
The hydrophilicity of cellulose will require additional attractive cellulose sources like natural fibres and wood in
processing to avoid swelling and degradation in long-term contrast to highly purified cellulosic materials such as
applications. Even while processing, hydrophilicity of the microcrystalline cellulose or filter paper.
cellulosic raw materials can cause a significant amount of The interfacial properties between the cellulosic matrix
swelling in the ACCs and a resulting amount of shrinkage and reinforcement has not been analysed with the typical
after drying. It would be interesting to see how the swelling tests developed for glass- or carbon fibre-reinforced com-
and shrinkage correlate with the amounts and types of posites such as pull-out or single-fibre fragmentation tests.
cellulose used and the applied processing steps. Unfortu- Hence, no values of the interfacial shear strength of those
nately, this subject has not been addressed by the composites have been published. This is somewhat sur-
researchers so far, as predictable and controllable shrinkage prising, as the improved interface seems to be the driving
will be essential for industrial scale production of those factor behind the development of ACCs. So far, the quality
composites. of the fibre-matrix-interface has only been judged using
Although several research papers have been published SEM pictures. A specific analysis of the interfacial prop-
on cellulose dissolution with ILs for different applications, erties of ACCs is necessary to verify the proposed positive
only few research groups have focused on using ILs to effect of a monocomponent composite and verify a possi-
process composite materials. The same is true for ACCs ble connection to the improved tensile strength. Under-
processed with NMMO and NAOH/urea. Until now, the standing the formation, structure and quality of the
effect of the different solvents on the mechanical properties interface will be essential for the production of predictable
of single cellulosic fibres or the resulting ACCs has not composite materials.
Table 4 Overview of isotropic (ISO) and unidirectional (UD) ACCs based on cellulose derivatives produced by benzylation, oxypropylation
and esterification
Cellulose Reinforcement Treatment Fibre volume Tensile strength Young s Strain at Reference
source fraction (%) (MPa) modulus (GPa) failure (%)
Wood sawdust Sisal fibre (UD) Benzylation 30 90 15 4.8 Lu et al. [158]
Wood sawdust Sisal fibre (UD) Benzylation 40 68 20 4.2 Lu et al. [158]
Wood sawdust Sisal fibre (ISO) Benzylation 15 32 2.35 2.1 Zhang et al. [160]
Sisal fibre UD Benzylation  43 3 Lu et al. [159]
Filter paper ISO Oxypropylation  18.7 1.18 2.7 de Menezes et al. [162]
Filter paper ISO Oxypropylation  25.7 1.31 4.91 de Menezes et al. [162]
Wood pulp ISO Esterification  25 0.8 6 Matsumura et al. [166]
Wood pulp ISO Esterification  20 1.3 5 Matsumura et al. [166]
The cellulose sources, reinforcements, fibre volume fractions and tensile properties are shown
123
J Mater Sci (2012) 47:1171 1186 1183
It has already been mentioned that so far the structure of suggested improved interfacial properties are especially
those composites is limited to a simple two-dimensional remarkable. The broad variety of possible cellulose
geometry. Compositions that are more complex would resources, cellulose solvents and the different processing
allow competition with other fibre-reinforced polymers. possibilities promise a wide range of materials for various
As well as structural applications, it could also be pos- applications. However, a manufacturing process has to be
sible to use ACCs for biomedical applications. Materials found that can be easily controlled and can be used for
made from bacterial cellulose [39, 167] and other cellulosic different cellulose sources. It would be beneficial if such a
materials have successfully been tested for such applica- process could be at least partially automated and adapted to
tions [41, 168 170]. The good mechanical properties of the already existing polymer processing methods. That way,
ACCs, therefore might allow an application as substitution the transformation from laboratory to industrial scale will
of bone or cartilage material [54]. become a lot easier. However, before that can be done a lot
Another application could be so-called   smart materi- of work is still required to characterise and fully understand
als  in which regenerated cellulose [171] and cellophane those composites. Many aspects, for example the different
[172] are used as electro-active paper. As those can be used phases of cellulose within the composites and their relation
for a broad variety of applications such as sensors, elec- to mechanical and thermal properties need closer investi-
trical displays and micro robots [173], the improved gations. Furthermore, it has to be determined how the raw
mechanical properties of the ACCs might provide an materials, cellulose solvents and processing methods
attractive alternative to mere cellulose paper. influence the material properties of the composites.
The ACCs produced so far have been produced in lab- Impressive as the mechanical properties of the ACCs are, it
oratory scale experiments and little is known on their will be necessary to identify the single aspects that might
processability with mass manufacturing methods. As so far be responsible for those properties. A closer inspection of
all those composites had a film or sheet like structure, blow the interface between reinforcing material and matrix in the
moulding seems to be one applicable method as it is suc- ACCs is necessary to explain the benefits of monocom-
cessfully applied for NMMO-solutions [174]. Irrespective ponent composites.
of the manufacturing method, the production costs of the A real classification of this new class of materials is
ACCs will depend on several other aspects. The quality of surely premature at this stage, but nonetheless it can be said
the cellulose source will play a major role in the calcula- that, based on the results reported so far, ACCs could play
tion. Highly purified cellulose or other necessary pretreat- an important role in the area of biocomposites in the future.
ments such as excessive drying of the raw materials will
Acknowledgement The authors acknowledge the financial support
increase the costs of the composite whereas the use of low
of the New Zealand Foundation for Research, Science, and
cost raw materials, such as waste products of, for example,
Technology.
the wood or textile industry will reduce the expenses.
A further aspect that must be mentioned is whether
ACCs can really be considered a   greener  alternative to
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