A critical review of all cellulose composites

M A T E R I A L S I N N E W Z E A L A N D

A critical review of all-cellulose composites

Tim Huber

•

Jo¨rg Mu¨ssig

•

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

inexhaustible quantity. While being a lightweight material,
it shows outstanding values of strength and stiffness when
present in its native form. Unsurprisingly, cellulose fibre
has been rigorously investigated as a reinforcing compo-
nent in biocomposites. In recent years, however, a new
class of monocomponent composites based on cellulosic
materials, so-called all-cellulose composites (ACCs) have
emerged. These new materials promise to overcome the
critical problem of fibre–matrix adhesion in 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-
lecular science literature have been published describing
non-derivatized

and

derivatized

ACCs.

Exceptional

mechanical properties of ACCs have been reported that
easily exceed those of traditional biocomposites. Several

different processing routes have been applied to the man-
ufacture of ACCs using a broad range of different solvent
systems and raw materials. This article aims to provide a
comprehensive review of the background chemistry and
various cellulosic sources investigated, various synthesis
routes, phase transformations of the cellulose, and
mechanical, viscoelastic and optical properties of ACCs.
The current difficulties and challenges of ACCs are clearly
outlined, pointing the way forward for further exploration
of this interesting subcategory of biocomposites.

Cellulose—the natural choice for composite materials

Introduction

Cellulose is one of the most abundant biopolymers on earth
with *1.5 9 10

tons of cellulose produced each year.

Thus, it presents an enormous amount of a renewable and
biodegradable resource for raw materials [

]. Cellulose

fibres are widely recognised for their applicability in eco-
friendly composite materials, although unlocking their full
potential remains a challenge for load-bearing engineering
applications.

Chemistry and phases of cellulose

The molecular composition of cellulose, isolated from
plant cell walls, was first discovered and determined by
Anselme Payen (1795–1871). Next to plants, some algae,
fungi and bacteria species are also produce cellulose [

Cellulose is a linear polymer composed from aldehyde

sugars, so-called

-anhydroglucopyranose units (C

IUPAC nomenclature: (3R,4S,5S,6R)-6-(hydroxymethyl)
oxane-2,3,4,5-tetrol), often simply referred to as glucose

T. Huber

M. P. Staiger (

Department of Mechanical Engineering, University
of Canterbury, Private Bag 4800, Christchurch, New Zealand
e-mail: mark.staiger@canterbury.ac.nz

J. Mu¨ssig
Department for Biomimetics, University of Applied Sciences
Bremen, Neustadtswall 30, 28199 Bremen, Germany

O. Curnow
Department of Chemistry, University of Canterbury, Private Bag
4800, Christchurch, New Zealand

S. Pang
Department of Chemical and Process Engineering, University
of Canterbury, Private Bag 4800, Christchurch, New Zealand

S. Bickerton
Department of Mechanical Engineering, University of Auckland,
Tamaki Campus, Auckland, New Zealand

123

J Mater Sci (2012) 47:1171–1186

DOI 10.1007/s10853-011-5774-3

units, assembled into groups of two as ‘‘cellobiose’’ units.
Figure

shows the typical molecular structure of a cellobi-

ose unit. A single glucose unit is a hexose that takes on one of
two forms (a or b), depending on the position of the hydroxyl
groups. Individual cellulose chains are highly hydrophilic
due to the large numbers of hydroxyl groups present. Native
cellulose or cellulose I is the most crystalline type of which
there are two forms: I

and I

. While the cellulose I

crystal

has a triclinic unit cell, the cellulose I

crystal has a mono-

clinic unit cell. Both, cellulose I

and I

are present in native

cellulose structures but their ratio depends on the source of
cellulose. Other allomorphs of cellulose are possible out of
which the most common are cellulose II, III, and IV. Cel-
lulose II can be formed by mercerisation or regeneration of
cellulose I [

–

]. Cellulose III can be formed from either,

cellulose I or cellulose II by a treatment with liquid ammonia,
resulting in either cellulose III

or cellulose III

. Cellulose

and Cellulose IV

can be prepared by the corresponding

form of cellulose III by heating in glycerol [

Single-molecule

cellulose

chains

interconnect

via

hydrogen bonds to form cellulose microfibrils that exhibit
crystalline, paracrystalline and amorphous regions [

Those microfibrils are present in the secondary cell wall of
all plants, usually embedded in a matrix consisting of
hemicelluloses and lignin. The degree of polymerisation
(DP) of cellulose varies widely depending on the source,
ranging from 300 in wood fibres up to 10,000 for plant
fibres and bacterial cellulose. Cellulose content, DP and the
lateral arrangement of the microfibrils determine the tensile
properties of a plant fibre. Cellulose microfibrils can be
classified as nanomaterials given the lateral dimensions of
a microfibril is in the range of 5–50 nm [

–

Mechanical properties of cellulose

An average Young’s modulus of 10.3 GPa was calculated
for amorphous cellulose using a force-field model [

Using X-ray diffraction, Sakurada et al. [

] determined

the Young’s modulus of elementary cellulose fibril of
bleached ramie fibre to be 134 GPa. Using a similar set-up,
Nishino et al. [

] measured the elastic modulus of several

cellulose polymorphs. Cellulose I was found to have a
modulus of 138 GPa, whereas cellulose IV exhibited a

lower value of 75 GPa. Ishikawa et al. [

] 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

with other engineering materials such as aluminium
(70 GPa) or glass fibres (76 GPa) [

]. As a result of the

low density of 1.58–1.59 g/cm

Wegst et al. ranked the

specific stiffness of native cellulose of 67 GPa cm

-1

among the highest of all natural materials [

Cellulose I is the strongest allomorph with a theoretical

ultimate tensile strength of about 13–17 GPa. Cellulose II
and amorphous cellulose are less strong with tensile
strengths of *9 and 0.8 ± 0.1 GPa, respectively [

]. The

high tensile strength and low density of the native cellulose
crystal results in the highest specific tensile strength of any
known natural polymers for cellulose I (667 MPa cm

-1

)

[

The biocomposite development

Early reports on the use of natural fibres in composites date
back to the early 1970 and 1980 [

], since then

modern advances in the development of cellulose fibre-
reinforced polymer composites have been the subject of
several hundred studies. Due to the independence of cel-
lulosic fibres of crude oil and their vast availability, an
improved CO

-balance compared with composites made

from industrially made fibres and fillers and good
mechanical properties, cellulose-containing composites
have generated much interest amongst various industries,
especially the automotive industry [

–

The most commonly used natural fibres for composite

applications are wood, jute, flax, sisal and hemp, although
many others are also suitable for biocomposites [

–

The high specific tensile strength and stiffness of natural
fibres makes them a lightweight alternative to traditional
reinforcements such as glass fibres or other fillers. Natural
fibres are also less hazardous to handle and require less
energy during processing compared with glass or carbon
fibres. The fibres themselves also sequestrate carbon
dioxide during growing and are biodegradable [

]. How-

ever, due to different growing conditions, natural fibres
usually show a large scatter of properties compared to
industrially made glass fibres. A strong quality manage-
ment during fibre harvesting and processing and extreme
care while determining fibre properties are necessary to
produce reliable and reproducible results [

]. Several

traditional processing methods for thermosetting and ther-
moplastic polymers have been modified to allow the use of

Fig. 1

Molecular structure of a cellobiose unit

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J Mater Sci (2012) 47:1171–1186

123

natural fibres. New processing routes have also been
developed that allow more rapid fabrication of biocom-
posite components at production rates demanded by
industry [

A promising development in biocomposites research has

been the transition from petroleum-based polymer (e.g.
polyethylene, polypropylene) to naturally derived bio-
polymer (e.g. polylactides, palm oil-based resins, starch)
matrices to produce composites that aim to be completely
biodegradable and CO

neutral [

While composites based on cellulose reinforcement and

petroleum-based polymers are usually referred to as eco- or
biocomposites, composites based entirely on naturally
derived fibres and biopolymers have been named green
(bio) composites [

Bio- and green composites are finding applications in a

wide range of applications from structural to biomedical
[

–

]. Nevertheless, the inherent chemical incom-

patibility between a hydrophobic polymer matrix and
hydrophilic cellulose [

] causes interfacial bonding

between the cellulosic and biopolymer components to be
often weak, particularly in the case of thermoplastic bio-
polymers [

]. This leads to an inefficient stress transfer

under load and thus low mechanical strength and stiffness
[

]. The chemical compatibility can be

improved by a chemical treatment of the fibre or matrix.
Silane, alkaline, acetylation, chemical grafting, and corona
discharge treatments provide widely varying degrees of
improvement [

–

]. Interfacial bonding can also

be increased by using nanosized forms of cellulose such as
bacterial cellulose [

], microfibrillated cellulose [

] and cellulose whiskers [

–

] that provide an

increased surface area per volume. While significant
improvements in mechanical properties can be obtained,
the above methods also add cost and complexity to the
formulation of biocomposites.

Cellulose reinforced cellulosic structures such as vul-

canized cellulosic fibres have been reported decades ago
and found applications, for example, as vulcanized paper
[

]. However, growing environmental awareness and

increasing interest in sustainable material concepts have
lead to the development of bio- and green composites for
structural composite applications. The newly developed
all-cellulose composites (ACCs) described in this review
represent an approach to formulating green composites that
aim to eliminate the chemical incompatibilities between
reinforcement and matrix phases by utilising cellulose for
both components. ACCs show the potential to be the next
step in the development of more sustainable composites.
The processing, characterisation, properties and applica-
tions of this promising class of high strength biocomposite
materials is presented in detail in the first section of this
review.

Other approaches to produce all-cellulosic composites

involve chemical treatments, such as oxypropylation or
benzylation, to generate cellulose derivatives that form the
matrix phase and are covered in the second part of this
review.

Non-derivatized all-cellulose composites

Introduction

The commonly accepted definition of a composite is a
material that consists of two or more distinct materials to
improve the stiffness, strength and/or toughness over the
individual constituents. However, in a monocomponent or
single polymer composite, reinforcing and matrix phases
are based on the same material. Theoretically, this would
lead to an interfaceless composite where boundaries
between reinforcement and matrix are indistinct in the
presence of ideal chemical bonding. Therefore, the need for
energy intensive fibre treatments or coupling agents for
improving interfacial bonding could be drastically reduced
or even completely eliminated.

While the reinforcement and matrix of monocomponent

composites are necessarily of the same chemical compo-
sition, physical morphology and/or structural phases of the
two components may differ in reality. The performance of
a monocomponent or single-polymer composite is best
illustrated with an example of the concept as put forward
by Capiati and Porter [

]. In this work, high-density

polyethylene composites were produced with a gradient of
changing morphology between the reinforcing fibres and
the matrix material resulting in an improved interfacial
shear strength in the range between glass fibre-reinforced
polyester and epoxy resins. In addition to the enhanced
bonding at the reinforcement–matrix interface, monocom-
ponent composites can also provide a more straightforward
path for recycling as the fibre and matrix do not require
separating (e.g. all-polypropylene composites [

–

]).

A recent summary of different single polymer composites
is presented by Matabola et al. [

The concept of an all-cellulose composite was first

discussed by Nishino et al. [

]. ACCs can be considered

bio-derived monocomponent composites; although strictly
speaking, the same source of cellulosic materials would
need to be used for the reinforcing and matrix phases.
While the ease of recycling is an important advantage for
thermoplastic-based monocomponent composites, the main
driver for the development of ACCs is to improve chemical
bonding at the reinforcement–matrix interface. During the
processing of ACCs, it is quite possible to have two or
more different allomorphs present. Cellulose molecules
strongly interact through hydrogen bonding, although the

J Mater Sci (2012) 47:1171–1186

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123

interaction between different cellulose allomorphs has not
been quantified. Thus, the details of chemical bonding at
allomorph boundaries in ACCs remain elusive. The mor-
phological characteristics of the interface have only been
observed qualitatively using for example scanning [

] or

transmission electron microscopy [

Preparation and synthesis of ACCs

Processing routes

There are two distinct strategies in the literature for the
preparation of ACCs (Fig.

). The first of these methods

(2-step method) involves firstly dissolving a portion of
cellulose in a solvent which is then regenerated in the
presence of undissolved cellulose. An example of this
method was first given by Nishino et al. [

] in which Kraft

fibre was fully dissolved and then regenerated in the
presence of ramie fibres. A second route (1-step method)
involves partial dissolution of the surface of cellulosic
fibres then regenerated in situ to form a matrix around the
undissolved portion. An example of this method was first
given by Gindl et al. [

] in which they partially dissolved

cellulose I, resulting in volume fractions of up to 90% of
the original fibre and 10% of newly regenerated cellulose
matrix. This method has also been described as ‘‘surface
selective dissolution’’ [

]. In these processing routes, the

dissolution step is followed by solvent removal and cellu-
lose regeneration using water or other coagulants, after
which the composites usually have to be dried.

Cellulose dissolution

Known non-derivatising solvents for cellulose include
lithium

chloride/N,N-dimethylacetamide

(LiCl/DMAc),

dinitrogen

tetroxide/dimethylformamide

/DMF),

N-methylmorpholine-N-oxide (NMMO), mineral acids,
sodium

hydroxide

(NaOH),

dimethylsulfoxide/tetra-

butylammonium fluoride (DMSO/TBAF), dimethylimida-
zolone/lithium chloride, and various molten salt hydrates
and ionic liquids (ILs). Of these, LiCl/DMAc, NMMO,
NaOH and the ionic liquid 1-butyl-3-methylimidazolium
chloride (BmimCl) have been used mostly in the process-
ing of ACCs (Table

). However, limited dissolution

capacity, slow dissolution rates, toxicity and non-recyclability
are the reasons that prevent some of these solvents from
being used in large industrial scales. It has been observed
that some ILs offer high cellulose dissolution rates. The
low vapour pressure of ILs also makes them easy to reuse
and safer to handle, and has led to the term ‘‘green sol-
vents’’ [

–

]. In the following, we review in further

detail properties of cellulose solvents used for the pro-
duction of ACCs.

NMMO belongs to the family of cyclic, aliphatic, ter-

tiary amine oxides, where the nitrogen carries the cyclic
and aliphatic groups, and oxygen [

] (compare Fig.

The highly polar N–O group is responsible for the high
hydrophilicity of NMMO and its complete miscibility in
water, as it readily forms hydrogen bonds. NMMO is a
powerful cellulose solvent due to the high polarity and
weakness of the N–O bond [

NMMO is used industrially in the Lyocell process for

producing regenerated cellulose fibres. The main steps of
this process are the preparation of the slurry by dissolution
of cellulose (usually pulp or cotton) in a mixture of water,
NMMO, stabilizers and additives. The cellulose solubility
depends on the mixing ratio of cellulose, water and
NMMO. A more detailed description of the dissolution
process and the influencing factors can be found in the
review of Fink et al. [

]. The dissolution is followed by an

extrusion of the viscous dope at elevated temperatures

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 [

]

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 [

]

LiCl/DMAc

1981

McCormick [

]

NaOH–urea

1995

Isogai and Antalla [

]

Ionic liquids

1934

Graenacher [

]

Fig. 3

Structural formula of the

NMMO molecule

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J Mater Sci (2012) 47:1171–1186

123

(generally 90–120

°C) through an air gap. The fibres are

then coagulated into a precipitation bath, washed and dried.
Around 99% of the NMMO can be recovered from the
precipitation and washing baths [

Lyocell fibres primarily consist of monoclinic cellulose

type-II crystallites [

]. Crystallites with a length,

width and thickness of 12–14, 8–10, and 3–4 nm, respec-
tively, accumulate into strand-shaped bundles with lengths
of 150–550 nm, partly assembled into aggregates of
30–60 nm in diameter [

]. The strength and stiffness

of regenerated fibres formed from NMMO in the presence
of additives can be up to 1.3 and 55 GPa, respectively [

Another solvent for the preparation of ACCs is DMAc

mixed with LiCl (see Fig.

). Cellulose needs to undergo a

so-called ‘‘activation procedure’’ during which the fibre is
penetrated with a polar medium [

]. Without activation, it

can take several months for the dissolution to proceed
regardless of the crystallinity of the cellulose. Amorphous
cellulose obtained by ball-milling also proves difficult to
dissolve in the absence of the activation procedure [

Interestingly, the activation step before the actual dissolu-
tion does not affect cellulose crystallinity [

There are two different ways to prepare the mixture: (i) the

LiCl/DMAc solution is prepared first and then the cellulose is
added, or (ii) cellulose and DMAc are mixed together fol-
lowed by addition of LiCl [

]. Stirring is also critically

important for the dissolution to proceed due to the hetero-
geneous fibre-solvent mixture. It has been reported that the
solubility of cellulose increases with LiCl content [

Many studies on ACCs report the use of LiCl/DMAc

[

–

], which may be due to its ability to

completely dissolve high molecular weight cellulose [

In those studies, a concentration of 8 wt% LiCl was used
for the dissolution of cellulose.

A more eco-friendly non-derivatizing solvent for cellu-

lose is based on NaOH or aqueous NaOH solutions with
additions of urea and/or thiourea used at sub ambient
temperatures or other additives such as poly(ethylene gly-
col) (PEG) or zinc oxide [

100

101

]. NaOH-urea-thiourea

dissolution is a simple, safe process requiring minimal
energy input. The addition of urea ((NH

)

CO) and/or

thiourea ((NH

)

CS) to aqueous NaOH greatly enhances

the dissolution, while NaOH can only partially dissolve
celluloses of low DP [

102

–

105

]. The alkaline solvent is

cooled to subzero temperatures. Once the subzero tem-
perature is attained, the mixture is often stirred under
thawing, resulting in cellulose dissolution [

102

106

]. The

solution can be centrifuged to separate the undissolved
portion from the truly dissolved cellulose [

107

108

]. The

dissolved cellulose is then transformed into a gel by a
thermal path, precipitated in an acidic medium or coagu-
lated. Depending on the process, different microstructures
can be obtained [

109

]. Coagulation was used to produce

cellulose membranes with varying pore geometries and
mechanical properties according to the coagulant type,
concentration and coagulation time [

110

–

112

]. NaOH–

urea (NaOH/(NH

)

CO) and NaOH–thiourea (NaOH/

(NH

)

CS) can also be used to process the cellulose into

textile fibres with mechanical properties close to com-
mercially available rayon fibre [

102

103

Graenacher [

] was the first to discover an IL solvent

system for cellulose, but this was thought to be of little
practical value at the time. Much more recently the use of ILs
as a solvent for cellulose has been reported by Swatloski et al.
[

]. Ionic liquids are molten salts with melting points

below 100

°C. There is a wide range of possible cations

(e.g. alkylimidazolium ([R

IM]

), tetraalkylammonium

([NR

]

) and tetraalkylphosphonium ([PR

]

) and anions

(e.g. hexafluorophosphate ([PF

]

), nitrate ([NO

]

) or

chloride, bromide and iodide salts [

113

]. Only some of them

are able to dissolve cellulose, but the number of possible ion
combinations is said to be as high as one trillion which leaves
much scope for the development of new types of cellulose
solvents [

114

]. Their ability to dissolve cellulose originates

from their high effective polarity, due to their ionic character.
The most successful ILs in cellulose dissolution reported so
far, are hydrophilic and consist of the cations methylimi-
dazoloium and methylpyridinium cores with allyl-, ethyl- or
butyl side chains with chloride, acetate or formate anions
[

115

116

]. Recently, Pinkert et al. [

117

] provided a detailed

review of cellulose dissolution by ionic liquids. ILs combine
all of the desirable characteristics of the previous solvents
including low volatility, low cost due to ease of recycling,
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
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

Fig. 4

Structural formula of the

LiCl/DMAc molecule

J Mater Sci (2012) 47:1171–1186

1175

123

cellulose dissolution depends strongly on the fibre struc-
ture, especially the degree of orientation and crystallinity in
the outer part of the fibre. Thus, the dissolution conditions
need to be tailored to different sources of cellulose in
forming ACCs. Soykeabkaew et al. compared immersion
times necessary to form a matrix phase of highly orientated
cellulose structures present in Bocell fibres to less ordered
cellulose configurations in the outer regions of Lyocell
fibres. They reported that the Bocell fibres needed more
than 1 h immersion time to form a matrix phase while for
the Lyocell fibres less than 20 min were sufficient. Longer
immersion times also lead to a reduction in fibre diameter
as the fibre surface dissolution increases [

Cellulose regeneration

The steps involved in the regeneration of the dissolved
cellulose are (i) removal of the solvent by a coagulant
(water, alcohol or acetone are commonly used) and then
(ii) removal of the coagulant through evaporative drying.
Cellulose regeneration is an important step in processing
ACCs as it controls the precipitation of the final cellulose
phases. Duchemin et al. suggested that the regeneration
rate controls the phase composition, which in turn will
dictate the physical properties of the ACC. Cellulose
phases of higher crystallinity are observed as the rate of
regeneration is decreased. This is thought to be due to the
dissolved cellulose chains having greater time to order
themselves into a lower energy configuration. Thus, the
rate of application of the coagulant for removal of the
solvent and then subsequent drying rate can be manipulated
to given varying properties in the final ACC [

Contact of the coagulant with the cellulose will lead to

swelling especially if water is used. Distortion of the
sample due to warping is even more apparent in thicker
samples as a diffusion gradient of coagulant from the
surface to the interior of the solution results in differential
shrinkage and subsequent delamination and void forma-
tion. Furthermore, the dissolved and undissolved portions
of cellulose will swell by different amounts which upon
regeneration again can cause differential shrinkage during
regeneration, leading to the formation of voids at the fibre–
matrix interface [

Phase characterisation

Identification and characterisation of cellulose phases
present in ACCs has been mainly carried out with wide-
angle X-ray scattering (WAXS) [

115

]

and to a lesser extent solid state nuclear magnetic reso-
nance (NMR) [

120

The treatment of cellulose with LiCl/DMAc leads to a

decrease in crystallinity depending on the immersion time.

Longer dissolution times lead to a reduction of cellulose I
crystallites resulting in a change of crystallinity after
regeneration of the cellulose. Nishino et al. [

] and

Soykeabkaew et al. [

] reported that after dissolution with

LiCl/DMAc the regenerated phase was non-crystalline or
amorphous based on WAXS. On the contrary, Duchemin
et al. [

120

] obtained results from WAXS and NMR that

suggest that exposure of crystalline cellulose to LiCl/
DMAc results in peeling away of thin layers from the
original crystallites which retain some molecular ordering.
After solvent removal, these thin layers can form a para-
crystalline phase that is distinct from typical amorphous
cellulose and closer in structure to cellulose I. They also
suggested that the presence of a paracrystalline ‘‘matrix’’ is
one of the underlying reasons for the high mechanical
properties of ACCs. Zhao et al. [

115

] observed that dis-

solution of cellulose I using the IL BmimCl results in a
matrix phase consisting of cellulose II. Gindl and Keckes
[

] also identify the regenerated phase of dissolved cel-

lulose I as cellulose II.

Mechanical properties

The mechanical properties of anisotropic composites
strongly depend on the dissolution and regeneration con-
ditions. The longitudinal tensile strength decreases with
increasing dissolution time due to a decreasing cross-
sectional area of the load-bearing cellulose fibrils and
therefore a reduction in fibre volume fraction. The trans-
verse tensile strength follows an opposite trend as the
matrix phase increased and the interface becomes more
homogenous. Soykeabkaew et al. [

] reported that

assumption based on their experiments with unidirectional
ramie fibre composites. Immersion times of 12 h lead to a
decrease in longitudinal tensile strength of about 60%
compared to immersion times of 6 h. Simultaneously, the
transversal tensile strength is increased by about 33%.
However, there are very few publications stating trans-
versal strength and stiffness of unidirectional composites to
verify this assumption. Extending the dissolution time can
lead to over-dissolution of the fibres, resulting in a rapid
decrease in tensile properties for either isotropic or aniso-
tropic ACCs [

]. The application of the solvent can

also affect the composition of the reinforcing fibres. Gindl
et al. [

] observed that the hardness of the fibres changes

when treated with LiCl/DMAc, presumably due to partial
dissolution of cellulose within the cell walls.

The inherent properties of the reinforcement will also

affect the properties and processing of ACCs. Many dif-
ferent combinations of fibre, matrix, and solvent systems
have been studied in the literature, giving a large range of
properties for ACCs. Table

lists the tensile properties of

ACCs made using different materials and solvents,

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123

Table

Overview

over

isotropic

(ISO)

and

unidirectional

(UD)

all-cellulose

composites

produced

one

and

two

step

processes

and

after

additional

fibre

composite

treatment

No.

Cellulose

source

for

matrix

Cellulose

source

for

reinforcement

Solvent

Fibre

volume

fraction

Tensile

strength

(MPa)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Strain

failure

(%)

Strain

failure

(%)

Reference

Two

step

process

Cotton

linter

pulps

Cellulose

whiskers

(ISO)

NaOH/urea

124

5.1

al.

[

125

]

Cotton

linter

pulps

Cellulose

whiskers

(ISO)

NaOH/urea

117

5.9

al.

[

125

]

Cellulose

powder

Hemp

fibre

(ISO)

NMMO

28.9

1.8

20.8

Quajai

and

Shanks

[

122

]

Wood

pulp

Ramie

fibre

(UD)

LiCl/DMAc

480

Nishino

al.

[

]

Ramie

fibre

Ramie

fibre

(UD)

LiCl/DMAc

410

4.8

Qin

al.

[

]

Filter

paper

Rice

husks

(ISO)

Ionic

liquid

(BmimCl)

57.5

1.74

5.67

Zhao

al.,

2009,

[

115

]

Filter

paper

Rice

husks

(ISO)

Ionic

liquid

(BmimCl)

2.92

2.76

Zhao

al.,

2009,

[

115

]

No.

Cellulose

source

for

matrix

and

reinforcement

Reinforcement

type

Solvent

Fibre

volume

fraction

Tensile

strength

(MPa)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Strain

failure

(%)

Strain

failure

(%)

Reference

One-step

process

MCC

(ISO)

LiCl/DMAc

242.8

13.1

8.6

Gindl

and

Keckes

[

]

Beech

pulp

(ISO)

LiCl/DMAc

154

12.2

0.023

Gindl

al.

[

]

Filter

paper

(ISO)

LiCl/DMAc

211

8.2

3.8

Nishino

and

Arimoto

[

]

Ramie

fibre

(UD)

LiCl/DMAc

480

3.7

4.5

Soykeabkaew

al.

[

]

LDR-Lyocell

fibre

(UD)

LiCl/DMAc

250

Soykeabkaew

al.

[

]

HDR-Lyocell

fibre

(UD)

LiCl/DMAc

350

Soykeabkaew

al.

[

]

Bocell

fibre

(UD)

LiCl/DMAc

910

8.2

Soykeabkaew

al.

[

]

MCC

(ISO)

LiCl/DMAc

58.7

3.2

2.5

Duchemin

al.

[

]

MCC

(ISO)

LiCl/DMAc

105.7

6.9

3.3

Duchemin

al.

[

]

(ISO)

LiCl/DMAc

411

4.3

Soykeabkaew

al.

[

]

MCC

(ISO)

Ionic

liquid

(BmimCl)

91.8

5.75

3.76

Duchemin

al.

[

126

]

Filter

paper

(ISO)

Ionic

liquid

(BmimCl)

124

10.8

Duchemin

al.

[

126

]

No.

Cellulose

source

for

matrix

Cellulose

source

for

reinforcement

Solvent

Additional

processing

step

Fibre

volume

fraction

(%)

Tensile

strength

(MPa)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Strain

failure

(%)

Strain

failure

(%)

Reference

All-cellulose

composites

prepared

with

further

fibre

composite

processing

Wood

pulp

Ramie

fibre

(UD)

LiCl/DMAc

Immersing

water,

acetone,

DMAc

400

Nishino

al.

[

]

MCC

(ISO)

LiCl/DMAc

Wet

drawing

align

cellulose

fibrils

428

33.5

2.3

Gindl

and

Keckes

[

]

Ramie

fibre

Ramie

fibre

(UD)

LiCl/DMAc

Mercerization

540

4.8

Qin

al.

[

]

Included

are

the

types

cellulose

source,

reinforcement,

solvent

and

fibre

fraction

used.

Both

tensile

properties

parallel

)

and

transverse

)

the

fibre

direction

are

given

where

available

MCC

microcrystalline

cellulose,

bacterial

cellulose,

LDR

low

draw

ratio,

HDR

high

draw

ratio

J Mater Sci (2012) 47:1171–1186

1177

123

demonstrating the large variability possible with formula-
tion and processing. Thus, judging and comparing the
influence of various parameters and properties between
different studies of ACCs is difficult. However, it is of
interest to compare the family of ACCs with other bio-
composites. In general, the tensile strength of ACCs is
significantly higher compared with the more traditional
isotropic

and

unidirectional

biocomposites

(compare

Figs.

; Table

Interestingly, a comparison of unidirectional ACCs with

traditional biocomposites does not reveal dramatic differ-
ences in the Young’s modulus. The underlying reasons
may be complex given the variable formulations but may
be due to either (i) the cellulose solvent decreasing the
modulus of the reinforcing fibres in ACCs or (ii) the
modulus of biocomposites being dominated by the modulus
of the fibres, with the fibre-matrix interfacial strength being
less important. In contrast, significant increases in modulus
are observed for isotropic ACCs compared with traditional
biocomposites in which it could be envisaged that the high
matrix properties of ACCs dominate this behaviour.

The strain to failure of ACCs (e

ACC

) is largely dominated

by the type of reinforcement used. For example, ACCs
reinforced with low strain ramie fibre (e

= 1.2–3.8%,

[

121

]), show lower values of e

ACC

(3.7–4.8%) when com-

pared with reinforcement made from high strain Lyocell
fibre (e

= 9.4–27.9%, [

] which gives higher e

ACC

(10–24%) (Table

). ACCs based on hemp fibre reported by

Quajai et al. [

122

] achieved values of 20% for e

ACC

. This is

unexpected as e

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.

Other chemical or mechanical processing steps have

been used to influence ACC properties (Table

). The

positive effect of a treatment with an alkali solution aso
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.

[

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
the molecule mobility as the water adsorption weakens the
inter- and intramolecular hydrogen bonds. By drawing the
composites, the crystal orientation changes linearly with
the draw ratio, the ratio between specimen length after and
before stretching, whereas the overall crystallinity of the
composite stays unaffected. Drying the samples afterwards
causes the molecule chains to keep their positions resulting
in anisotropy of the specimens with an improved tensile
strength in longitudinal direction [

Viscoelastic properties

The viscoelastic properties of ACCs are determined by the
viscoelastic properties of the cellulose chains and allo-
morphs. Amorphous cellulose exhibits a viscoelastic
behaviour involving different molecular motions depend-
ing on temperature [

135

136

]. Due to its high content of

hydroxyl groups, hydrogen bonds are formed with neigh-
bouring units of the same molecule, with neighbouring
chains and any water that is present. Intra- and intermo-
lecular hydrogen bonds are responsible for the thermal
stability of the cellulose molecule. Intramolecular hydro-
gen bonds increase the stiffness of the polymer while the
intermolecular hydrogen bonds and van der Waals inter-
actions are responsible for single chains arranging into a

Fig. 5

Ranges of tensile strengths and Young’s moduli of isotropic

ACCs (dashed) compared with traditional isotropic biocomposites
(dotted). The numbers and letters of the references are found in
Tables

and

, respectively

Fig. 6

Ranges of tensile strengths and Young’s moduli of unidirec-

tional ACCs (dashed) compared with traditional unidirectional
biocomposites (dotted). The numbers and letters of the references
are found in Tables

and

, respectively

1178

J Mater Sci (2012) 47:1171–1186

123

2D sheet [

137

]. The stable structure of the cellulose mol-

ecule is the result of a dense network of hydrogen bonds
and their different bonding patterns within a single cellu-
lose structure and their dependency on different tempera-
tures [

138

At low temperatures, the molecular mobility is consid-

ered as localised at the molecular level, giving rise to
secondary relaxations [

139

]. Two secondary relaxations of

amorphous cellulose are reported, occasionally referred to
as c

cell

and b

cell

[

140

141

]. The c-relaxation is reported to

appear at a constant temperature of -123

°C. It has been

shown by the analyses of other polysaccharides such as
dextran, that the c-relaxation is mainly associated with a
rotation of the CH

OH groups of the cellulose molecule

rather than the rotation of its OH groups [

140

]. The b

cell

relaxation is associated with cooperative but localised
motion of segments of the main chain of the cellulose
molecule depending on the water content [

139

]. The tem-

perature range for the b

cell

-relaxation was reported as

-83.15 to 43.15

°C [

140

141

]. At higher temperatures,

cellulose exhibits three further transitions, designated a

, a

and a

by Manabe et al. [

142

] and measured for regenerated

Table 3

Overview over the tensile properties of different isotropic and unidirectional biocomposites

Matrix

Fibre type

Fibre fraction
in (vol.% or
wt%)

Tensile strength
(MPa)

Young’s
modulus
(GPa)

Strain to
failure (%)

Reference

Isotropic composites

PBAT

Flax

30 (vol.%)

4.1

Bodros et al. [

] (Fig.

PBS

Flax

30 (vol.%)

3.8

2.5

Bodros et al. [

] (Fig.

PHB

Flax

30 (vol.%)

4.5

1.8

Bodros et al. [

] (Fig.

PLA

MCC

5 (wt%)

31.9

1.5

[100

Petersson and Oksman [

127

] (Fig.

PLA

Cellulose whiskers

5 (wt%)

2.1

5.4

Mathew et al. [

128

] (Fig.

PLA

Cellulose microfibres

5 (wt%)

2.3

3.3

Mathew et al. [

128

] (Fig.

PLA

Cordenka

40 (vol.%)

57.97

4.85

–

Bax and Mu¨ssig [

129

] (Fig.

PLA

Flax

40 (vol.%)

54.15

6.31

–

Bax and Mu¨ssig [

129

] (Fig.

PCL

Starch nano-crystals

50 (wt%)

15.5

0.384

5.0

Habibi and Dufresne [

] (Fig.

PCL

Cellulose nano-crystals

50 (wt%)

18.7

0.442

8.6

Habibi and Dufresne [

] (Fig.

Polyester

Banana

40 (vol.%)

1.87

Sreekuma et al. [

130

] (Fig.

PTP

Hemp

21 (wt%)

–

Mu¨ssig et al. [

] (Fig.

Unidirectional composites

Epoxy

Flax

50 (wt%)

119

–

Bos [

] (Fig.

Flax

55 (vol.%)

320.7

28.2

–

Madsen and Liholt [

131

] (Fig.

Jute

21.2 (vol.%)

141

–

Khondker et al. [

132

] (Fig.

Epoxy

Flax

49 (vol.%)

284

–

van de Weyenberg et al. [

133

] (Fig.

PLA

Kenaf

70 (vol.%)

230

–

Ochi [

134

] (Fig.

Starch

Flax

60 (wt%)

9.3

–

Romhany et al. [

] (Fig.

Epoxy

Sisal

46 (vol.%)

211

19.7

1.9

Oksman et al. [

] (Fig.

Fig. 7

SEM pictures of ramie

fibre-reinforced cellulose
composite made with 4%
cellulose concentration solution
in an untreated (a) and
mercerized (b) state [

Reprinted from Ref. [

J Mater Sci (2012) 47:1171–1186

1179

123

cellulose. The a

transition is very closely related to the water

content, and therefore not associated with the inherent
molecular motion but with the cooperative motion of cellu-
lose chain segments and absorbed water molecules. The a

transition can be separated into two distinct relaxations (a

2,1

and a

2,2

) that are caused by micro-Brownian motions of

amorphous chain segments [

142

–

144

]. The a

transition is

associated with motions in the non-crystalline regions as the
crystalline segments are constrained. However, as the a

transition also coincides with the onset chemical decompo-
sition of the molecule due to the high temperature, it is
possible that released chain segments from the crystalline
regions also contribute to the relaxation [

145

The viscoelastic properties of ACCs have been investi-

gated using dynamic mechanical analysis (DMA) [

]. The storage modulus (E

) of the composites

formed with LiCl/DMAc as solvent system are reported to
be lie between 10

and 10

Pa between temperatures of

-150 and 0

°C. E

only gradually decreases with increas-

ing temperature as a consequence of strong hydrogen
bonding in cellulose I [

]. At temperatures above

250

°C, the cellulose composites start do degrade, resulting

in severe drop in E

. The decrease in E

at higher temper-

atures than 250

°C of those composites goes in hand with

the reported a

transition of regenerated cellulose.

Optical properties

Optically

transparent

cellulose-containing

composites

based on bacterial cellulose, epoxy and acrylic resins have
been produced; according to Nogi et al. [

146

], the optical

transparency is due to the dimensions of the bacterial cel-
lulose ribbons lying in the nanometre range. However,
ACCs have also been observed to be optically transparent
under certain conditions as first reported by Gindl et al.
[

] and attributed to the lateral dimension of the used

microcrystalline crystals of 1–3.5 nm. ACC films based on

the LiCl/DMAc solvent system show variations in optical
transparency as a function of the dissolution time, with
increasing time resulting in greater transparency (Fig.

As the dimensions of the used cellulosic materials in other
studies exceed the nanometer range, the transparency of
those ACCs has been attributed to the good adhesion
between reinforcement and matrix phase and closure of
internal pores in the cell wall [

Derivatized all-cellulose composites

Introduction

The field of derivatized cellulose, of which much is beyond
the scope of this article, has been reviewed elsewhere [

147

–

152

]. One of the first processes using cellulose derivatives

was the viscose process, discovered by British chemists
Charles Cross, Edward Bevan and Clayton Beadle in 1891
[

153

]. The expression ‘‘Viscose’’ originates from the highly

‘‘viscous cellulose’’ solution obtained during the dissolution
process, that was later contracted to ‘‘Viscose’’. A major
drawback of this process is the contamination of the waste-
water by carbon disulfide and other polluting sulphur
by-products during cellulose derivatization. This is one of
the main reasons that many improvements have been made to
the process and the resulting products such as rayon fibre in
the last century to improve fibre quality and reduce the
environmental impact [

153

]. The so-called man-made cel-

lulose fibres have shown a steady rate of production in the
range of 2,500 to 3,000 kt/year in the last decades with the
main application being textile fibres [

154

Three of the most recent approaches for the production

thermoplastic

‘‘all-plant

fibre

composites’’,

self-

reinforced cellulose composites and cellulosic nanocom-
posites via benzylation, oxypropylation and esterification,
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-OH

þ NaOH ! Wood-O

þ H

Wood-ONa

þ ClCH

-C

! Wood-O CH

þ NaCl

ð1Þ

Fig. 8

Optical transparency of ACCs as a function of dissolution

time in DMAc/LiCl. Reprinted from Ref. [

permission from Elsevier

1180

J Mater Sci (2012) 47:1171–1186

123

where, Wood-OH represents the hydroxyl groups mainly
present in cellulose [

157

158

]. The matrix material is

swollen in NaOH and afterwards transferred to the benzyl
chloride. The solution is stirred for several hours at tem-
peratures above 100

°C. It is then washed to remove

inorganic salts, benzyl chloride and its by-products to yield
a liquid matrix phase consisting of the used cellulosic
material. Due to the complexity of wood and its macro-
molecules, the extent of benzylation is measured indirectly
by observing the weight gain as the modification proceeds.
Benzylated fibres show the formation of a thermoplastic
region surrounding the fibre core [

157

–

160

]. The com-

posites can then be simply formed by hot-pressing [

157

–

160

]. Fibre volume fractions of up to 40% have been

achieved using this method. Lu et al. reported that the rate
of benzylation is hardly affected by the wood source, but
depends strongly on the amounts of NaOH and benzyl
chloride (C

Cl) as well as the processing tempera-

tures and reaction times [

]. Increasing the

degree of benzylation of cellulosic material leads to a
decrease in crystallization due to disruption of the hydro-
gen bonds within the cellulose molecules, also reducing the
final mechanical properties. This is explained by the larger
benzyl molecule introducing a larger free volume and
causing a change in the supramolecular structure of the
molecule [

157

–

159

It has been shown that it is also possible to produce ‘‘all-

plant fibre composites’’ by an oxypropylation treatment of
cellulosic fibres as first reported by Gandini et al. in 2005.
This is done by using a Brønsted base to activate the
hydroxyl groups of the cellulose followed by an anionic
polymerisation of the PO in a ‘‘grafting from’’ process. The
fibres are first immersed in a solution of ethanol and
potassium hydroxide for several hours. After the alcohol
has evaporated, the fibres are then mixed with propylene
oxide (PO) under nitrogen atmosphere in an autoclave at
temperatures of 130–150

°C. The PO homopolymer cre-

ated by chain-transfer reactions can be removed by a
Soxhlet extraction using hexane. This effectively grafts a
thermoplastic polymer matrix onto the outer surfaces of the
fibres, with the amount measured by the weight gain [

162

–

164

]. In this reaction, cellulose I is converted to an oxy-

propylated amorphous derivative. de Menezes et al. [

163

]

report that the amorphous regions of the cellulosic mate-
rials are more prone to modification than crystalline
regions, while the degree of modification is dependent on
the amount of PO used. With increasing amounts of PO,
the crystalline regions will also take part in the reaction
although increased degradation of the cellulose structure
and reduction in mechanical properties can occur. As for
benzylated cellulose, oxypropylated cellulose can be hot-
pressed to form a composite film, with fibre fractions
ranging from 10 to 40 vol.% [

–

Matsumura et al. produced cellulose nanocomposites by

partial esterification of wood pulp by a treatment with a
p-toluene sulphonic/hexanoic anhydride system using a
cyclohexane based reaction medium that caused no swell-
ing of the cellulose fibres. To produce the composites, pulp
fibres were exposed to a hexanoylation reaction to receive
heterogeneously hexanoylated pulp fibres. The hexanoy-
lation was assumed to start from both the surface of indi-
vidual microfibrils and unordered regions within the fibre
followed by hexanoylation of the microfibril core. Those
fibres were mixed with water or methanol and filtered
afterwards to receive a uniform fibre mat in disc shape. The
discs were compression moulded at 155–170

°C and at

room temperature. The so produced thermoplastic com-
posites of unmodified cellulose I and esterified cellulose
were semitransparent [

165

166

The degree of benzylation is decisive for the compos-

ites’ properties. A higher presence of benzyl groups is
reported to increase the viscosity of the material. This can
lead to insufficient wetting of the fibres and result in a weak
interface that reduces the flexural and tensile strength at a
fibre volume share of 40% as the fibre are not completely
surrounded by the matrix phase [

158

160

]. An overview of

the tensile properties of benzylated and oxypropylated
ACCs is given in Table

. The thermomechanical and

viscoelastic properties of benzylated ACCs have been
analysed with thermomechanical analysis (TMA) [

158

159

]. Benzylated ACCs exhibit thermoplastic behaviour

due to the benzylated cellulosic material with a softening
temperature in the range of about 90 to 120

°C depending

on the source of cellulose and degree of benzylation. Lu
et al. [

159

] suggested that, as the plastification does not

change the chemical backbone of the cellulose molecule
chains, that softening should not be attributed to the a

transition of the cellulose but could rather be caused by
inter-molecular slips.

Analogous, for oxypropylated cellulose, the amount of

PO used is a determining factor for the tensile properties,
as the use of higher amounts leads to a decrease in strength
and stiffness but an increase in elongation to break. Those
changes are likely to be caused by an increase of the
thermoplastic amorphous phase [

162

For the esterified composites, the degree of substitution

(DS) during the hexanoylation reaction is the decisive
factor for the mechanical performance. Similar to oxy-
propylated cellulose, elongation increased with DS, while
strength and stiffness decrease at highest tested DS of 2.0
[

166

Comparing the reported tensile properties of non-deriv-

atized ACCs with those of benzylated, oxypropylated and
esterified ACCs shows that the biggest differences seem to
be in tensile strength. The strongest non-derivatized ACCs
are about five to ten times stronger than their derivatized

J Mater Sci (2012) 47:1171–1186

1181

123

counterparts, while the Young’s moduli and strain at failure
are in a similar range. Based on the reported values of filter
paper in Tables

and

one can assume that the non-

derivatized formation of the composite using LiCl/DMAc
leads to better tensile properties than the oxypropylation,
but without a comparative study the influence of processing
on composite properties are not evaluable. Therefore,
the biggest difference seems to be the thermoformability of
the derivatized ACCs.

Future challenges and applications for all-cellulose
composites

In spite of the promising properties of ACCs, there still
remains much research required to understand the funda-
mentals of these materials and to find suitable industrial
applications.

The hydrophilicity of cellulose will require additional

processing to avoid swelling and degradation in long-term
applications. Even while processing, hydrophilicity of the
cellulosic raw materials can cause a significant amount of
swelling in the ACCs and a resulting amount of shrinkage
after drying. It would be interesting to see how the swelling
and shrinkage correlate with the amounts and types of
cellulose used and the applied processing steps. Unfortu-
nately, this subject has not been addressed by the
researchers so far, as predictable and controllable shrinkage
will be essential for industrial scale production of those
composites.

Although several research papers have been published

on cellulose dissolution with ILs for different applications,
only few research groups have focused on using ILs to
process composite materials. The same is true for ACCs
processed with NMMO and NAOH/urea. Until now, the
effect of the different solvents on the mechanical properties
of single cellulosic fibres or the resulting ACCs has not

been investigated in detail. This will be necessary to judge
the effectiveness of the solvent and therefore allow a
classification of the different processing methods, involved
costs and resulting material properties. The costs of the
solvent could be minimised if it can be recovered and
recycled, as it is the case for NMMO and ILs, assuming the
recycling process is not too cost-intensive. On the other
hand, in case of a non-recyclable and possibly hazardous
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
also thinkable that different solvents will only work with
cellulose sources of low DP and crystallinity whereas
others might be the ideal choice for high-class raw
materials.

Furthermore, more research is necessary to know how

the different solvent systems interact with the other com-
ponents such as hemicelluloses and lignin of financially
attractive cellulose sources like natural fibres and wood in
contrast to highly purified cellulosic materials such as
microcrystalline cellulose or filter paper.

The interfacial properties between the cellulosic matrix

and reinforcement has not been analysed with the typical
tests developed for glass- or carbon fibre-reinforced com-
posites such as pull-out or single-fibre fragmentation tests.
Hence, no values of the interfacial shear strength of those
composites have been published. This is somewhat sur-
prising, as the improved interface seems to be the driving
factor behind the development of ACCs. So far, the quality
of the fibre-matrix-interface has only been judged using
SEM pictures. A specific analysis of the interfacial prop-
erties of ACCs is necessary to verify the proposed positive
effect of a monocomponent composite and verify a possi-
ble connection to the improved tensile strength. Under-
standing the formation, structure and quality of the
interface will be essential for the production of predictable
composite materials.

Table 4

Overview of isotropic (ISO) and unidirectional (UD) ACCs based on cellulose derivatives produced by benzylation, oxypropylation

and esterification

Cellulose
source

Reinforcement

Treatment

Fibre volume
fraction (%)

Tensile strength
(MPa)

Young’s
modulus (GPa)

Strain at
failure (%)

Reference

Wood sawdust

Sisal fibre (UD)

Benzylation

4.8

Lu et al. [

158

]

Wood sawdust

Sisal fibre (UD)

Benzylation

4.2

Lu et al. [

158

]

Wood sawdust

Sisal fibre (ISO)

Benzylation

2.35

2.1

Zhang et al. [

160

]

Sisal fibre

Benzylation

–

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

–

0.8

Matsumura et al. [

166

]

Wood pulp

ISO

Esterification

–

1.3

Matsumura et al. [

166

]

The cellulose sources, reinforcements, fibre volume fractions and tensile properties are shown

1182

J Mater Sci (2012) 47:1171–1186

123

It has already been mentioned that so far the structure of

those composites is limited to a simple two-dimensional
geometry. Compositions that are more complex would
allow competition with other fibre-reinforced polymers.

As well as structural applications, it could also be pos-

sible to use ACCs for biomedical applications. Materials
made from bacterial cellulose [

167

] and other cellulosic

materials have successfully been tested for such applica-
tions [

168

–

170

]. The good mechanical properties of the

ACCs, therefore might allow an application as substitution
of bone or cartilage material [

Another application could be so-called ‘‘smart materi-

als’’ in which regenerated cellulose [

171

] and cellophane

[

172

] are used as electro-active paper. As those can be used

for a broad variety of applications such as sensors, elec-
trical displays and micro robots [

173

], the improved

mechanical properties of the ACCs might provide an
attractive alternative to mere cellulose paper.

The ACCs produced so far have been produced in lab-

oratory scale experiments and little is known on their
processability with mass manufacturing methods. As so far
all those composites had a film or sheet like structure, blow
moulding seems to be one applicable method as it is suc-
cessfully applied for NMMO-solutions [

174

]. Irrespective

of the manufacturing method, the production costs of the
ACCs will depend on several other aspects. The quality of
the cellulose source will play a major role in the calcula-
tion. Highly purified cellulose or other necessary pretreat-
ments such as excessive drying of the raw materials will
increase the costs of the composite whereas the use of low
cost raw materials, such as waste products of, for example,
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
existing materials. The example of the viscose process
shows, that using biological raw materials and biodegra-
dation of the end product count little when the process-
ing involve highly hazardous materials [

175

]. The eco-

friendliness of the ACCs will therefore strongly depend on
the involved materials and processing methods and the
corresponding energy needs. It will be necessary to do
environmental audits on the composites and their produc-
tion as soon as they are ready for industrial fabrication to
judge how ‘‘green’’ those materials really are. In addition
to that degradation studies on those composites might
provide important information as well.

Conclusions

All-cellulose composites are an interesting new develop-
ment in the area of green and biocomposites. Their superior
tensile properties compared to other biocomposites and the

suggested improved interfacial properties are especially
remarkable. The broad variety of possible cellulose
resources, cellulose solvents and the different processing
possibilities promise a wide range of materials for various
applications. However, a manufacturing process has to be
found that can be easily controlled and can be used for
different cellulose sources. It would be beneficial if such a
process could be at least partially automated and adapted to
already existing polymer processing methods. That way,
the transformation from laboratory to industrial scale will
become a lot easier. However, before that can be done a lot
of work is still required to characterise and fully understand
those composites. Many aspects, for example the different
phases of cellulose within the composites and their relation
to mechanical and thermal properties need closer investi-
gations. Furthermore, it has to be determined how the raw
materials, cellulose solvents and processing methods
influence the material properties of the composites.
Impressive as the mechanical properties of the ACCs are, it
will be necessary to identify the single aspects that might
be responsible for those properties. A closer inspection of
the interface between reinforcing material and matrix in the
ACCs is necessary to explain the benefits of monocom-
ponent composites.

A real classification of this new class of materials is

surely premature at this stage, but nonetheless it can be said
that, based on the results reported so far, ACCs could play
an important role in the area of biocomposites in the future.

Acknowledgement

The authors acknowledge the financial support

of the New Zealand Foundation for Research, Science, and
Technology.

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