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Microfibrillated cellulose and new nanocomposite materials:a review
Istvan Siro • David Plackett
Received: 11 September 2009 / Accepted: 29 January 2010 / Published online: 21 February 2010
� Springer Science+Business Media B.V. 2010
Abstract Due to their abundance, high strength and
stiffness, low weight and biodegradability, nano-scale
cellulose fiber materials (e.g., microfibrillated cellu-
lose and bacterial cellulose) serve as promising
candidates for bio-nanocomposite production. Such
new high-value materials are the subject of contin-
uing research and are commercially interesting in
terms of new products from the pulp and paper
industry and the agricultural sector. Cellulose nanof-
ibers can be extracted from various plant sources and,
although the mechanical separation of plant fibers
into smaller elementary constituents has typically
required high energy input, chemical and/or enzy-
matic fiber pre-treatments have been developed to
overcome this problem. A challenge associated with
using nanocellulose in composites is the lack of
compatibility with hydrophobic polymers and various
chemical modification methods have been explored
in order to address this hurdle. This review summa-
rizes progress in nanocellulose preparation with a
particular focus on microfibrillated cellulose and also
discusses recent developments in bio-nanocomposite
fabrication based on nanocellulose.
Keywords Microfibrillated cellulose �Nanocellulose � Nanofibrils � Nanocomposites �Bacterial cellulose
Introduction
Cellulose is one of the most abundant biopolymers on
earth, occurring in wood, cotton, hemp and other
plant-based materials and serving as the dominant
reinforcing phase in plant structures. Cellulose is also
synthesized by algae, tunicates, and some bacteria
(Klemm et al. 2006; Henriksson and Berglund 2007;
Iwamoto et al. 2007). Despite its relative chemical
simplicity, the physical and morphological structure
of native cellulose in higher plants is complex and
heterogeneous. Furthermore, cellulose molecules are
intimately associated with other polysaccharides and
lignin in plant cell walls, resulting in even more
complex morphologies (Clowes et al. 1968).
The production of nano-scale cellulose fibers and
their application in composite materials has gained
increasing attention due to their high strength and
stiffness combined with low weight, biodegradability
and renewability. Application of cellulose nanofibers
in polymer reinforcement is a relatively new research
field. Although there is growing publication activity
as indicated by Fig. 1, the number of reports is still
modest when compared to publications dealing with
organic micro- and macrofillers or inorganic
I. Siro (&) � D. Plackett
Risø National Laboratory for Sustainable Energy,
Technical University of Denmark (DTU), P.O. Box 49,
4000 Roskilde, Denmark
e-mail: isir@risoe.dtu.dk
123
Cellulose (2010) 17:459–494
DOI 10.1007/s10570-010-9405-y
nanofillers (e.g., nanoclays). The limited application
of cellulose nanofibers to date may be partly because
the separation of plant fibers into smaller elementary
constituents has typically been a challenging process
to perfect with high energy demands.
In this review we describe various approaches to
the preparation of nanocellulosic materials from plant
sources. The focus is on the extraction and investi-
gation of microfibrillated cellulose (MFC) in partic-
ular; however, to put this topic in context, cellulose
whiskers and bacterial cellulose are also briefly
discussed in particular sections of the text and
applications of nanocellulosics in films or in other
nanocomposite materials are included. Although it
has been suggested that cellulose nanofibers could be
used as a rheology modifier in foods, paints, cosmet-
ics and pharmaceutical products (Turbak et al. 1983),
discussion of these applications is beyond the scope
of this paper. A notable feature of cellulose nanof-
ibers is their hydrophilicity, which makes them
suitable for combination with hydrophilic polymers
but generally incompatible with hydrophobic matri-
ces. Various chemical modification methods have
been explored to open up possibilities for combining
cellulose with hydrophobic polymers and these are
discussed here. Overall, this review aims to summa-
rize current knowledge on the development and
application of cellulosic nanofibers with a particular
focus on MFC and polymer reinforcement.
It is worth noting here that anomalies exist in the
literature regarding the nomenclature applied to
cellulosics. The term ‘‘microfibril’’ is generally used
to describe the 2–10 nm thick fibrous cellulose
structures with the length of several tens of microns
formed during cellulose biosynthesis in higher plants
(Krassig 1993). Depending on their origin, the
microfibril diameters may vary. In wood, for exam-
ple, the lateral dimension for microfibrils is around 3–
5 nm (Ohad and Mejzler 1965). However, cellulose
microfibrils also form intertwined aggregates with
widths of 20–25 nm in the parenchyma cell wall
(Clowes et al. 1968). ‘‘Nanofibril’’ and ‘‘nanofiber’’
are also used as synonyms for ‘‘microfibril’’.
The term ‘‘microfibrillated cellulose’’ (MFC),
production of which is described in this review,
should not be confused with the term ‘‘microfibril’’.
Although the thickness of MFC nano-elements could,
in principle, be as small as 3–10 nm, it is typically in
the range of 20–40 nm since MFC usually consists of
aggregates of cellulose microfibrils (Svagan et al.
2007). Various terms are used to describe MFC in the
literature, including: microfibril (Andresen and Ste-
nius 2007; Andresen et al. 2007; Cheng et al. 2007;
Aulin et al. 2008), microfibril aggregates (Henriksson
and Berglund 2007; Henriksson et al. 2007; Iwamoto
et al. 2007), microfibrillar cellulose (Stenstad et al.
2008; Werner et al. 2008; Syverud and Stenius 2009),
nanofibril (Ahola et al. 2008b; Henriksson et al.
2008), nanofiber (Bhatnagar and Sain 2005; Abe et al.
2007; Stenstad et al. 2008), nanofibrillar cellulose
(Ahola et al. 2008a, b; Paakko et al. 2008; Stenstad
et al. 2008; Syverud and Stenius 2009) or fibril
Fig. 1 Illustration of the
annual number of scientific
publications and patents
since 1983, when the term
‘‘microfibrillated
cellulose’’ was first
introduced. Data analysis
completed using SciFinder
Scholar search system on 07
September, 2009 using the
search term
‘‘microfibrillated cellulose’’
and its synonyms such as
‘‘nanofibrillar cellulose’’,
‘‘cellulose nanofibers’’ and
‘‘cellulose nanofibrils’’
460 Cellulose (2010) 17:459–494
123
aggregates (Hult et al. 2001, 2002; Virtanen et al.
2008). In this review we have used the terms
presented by the original authors of a particular work
but elsewhere have applied the general term ‘‘nano-
fiber’’ to describe the individual elements of nano-
cellulosics such as MFC.
When subjected to acid hydrolysis, cellulose
microfibrils undergo transverse cleavage along the
amorphous regions and the use of sonication results
in a rod-like material with a relatively low aspect
ratio referred to as ‘‘cellulose whiskers’’ (Ranby
1952). The typical diameter of these whiskers is
around 2–20 nm, but there is a wide length distribu-
tion from 100 to 600 nm and in excess of 1 lm in
some cases (Hubbe et al. 2008). Due to the near
perfect crystalline arrangement of cellulose whiskers,
this form of nanocellulose has a high modulus and
therefore significant potential as a reinforcing mate-
rial (Eichhorn et al. 2001). Synonyms for cellulose
whiskers include ‘‘nanowhiskers’’ (de Rodriguez
et al. 2006; Oksman et al. 2006; Petersson and
Oksman 2006; Petersson et al. 2006, 2007; John and
Thomas 2008), ‘‘nanorods’’ (Dujardin et al. 2003;
Samir et al. 2005), and ‘‘rod-like cellulose crystals’’
(Iwamoto et al. 2007). The properties of cellulose
whiskers and their application in nanocomposites
have been previously reviewed (Samir et al. 2005).
Strong hydrogen bonding between the individual
cellulose crystals (whiskers) promotes re-aggregation
during spray-drying procedures (Levis and Deasy
2001), which leads to another cellulose structure
called ‘‘microcrystalline cellulose’’ (MCC). The
length dimension of MCC is generally greater than
1 lm. MCC is a commercially available material
mainly used as a rheology control agent and as a
binder in the pharmaceutical industry (Janardhnan
and Sain 2006). Dimensional parameters for the
various forms of nanocellulose are summarized in
Table 1.
Sources of nanocellulose
Wood
The mechanical extraction of nanofibers from wood
dates back to the 1980s when Herrick et al. (1983)
and Turbak et al. (1983) produced MFC from wood
pulp using cyclic mechanical treatment in a high-
pressure homogenizer. The homogenization process
resulted in disintegration of the wood pulp and a
material in which the fibers were opened into their
sub-structural microfibrils (Andresen et al. 2006).
The resulting MFC gels consisted of strongly entan-
gled and disordered networks of cellulose nanofiber.
Bleached kraft pulp has often been used as the
starting material for research on MFC production
(Bhatnagar and Sain 2005; Iwamoto et al. 2005;
Janardhnan and Sain 2006; Saito and Isogai 2006,
2007; Saito et al. 2006, 2007, 2009).
Agricultural crops and by-products
Although wood is certainly the most important
industrial source of cellulosic fibers, competition
from different sectors such as the building products
and furniture industries and the pulp and paper
industry, as well as the combustion of wood for
energy, makes it challenging to supply all users with
the quantities of wood needed at reasonable cost. For
this reason fibers from crops such as flax, hemp, sisal,
and others, especially from by-products of these
different plants, are likely to become of increasing
interest. These non-wood plants generally contain
less lignin than wood and therefore bleaching
processes are less demanding. Other examples of
agricultural by-products which might be used to
derive nanocellulose include those obtained from the
cultivation of corn, wheat, rice, sorghum, barley,
sugar cane, pineapple, bananas and coconut crops.
Table 1 Nanocellulose dimensions (Samir et al. 2005; Tanem et al. 2006; Hubbe et al. 2008)
Cellulose structure Diameter (nm) Length (nm) Aspect ratio (L/d)
Microfibril 2–10 [10,000 [1,000
Microfibrillated cellulose (MFC) 10–40 [1,000 100–150
Cellulose whisker 2–20 100–600 10–100
Microcrystalline cellulose (MCC) [1,000 [1,000 *1
Cellulose (2010) 17:459–494 461
123
Today, these agricultural by-products are either
burned, used for low-value products such as animal
feed or used in biofuel production. Because of their
renewability crop residues can be valuable sources of
natural nanofibers (Reddy and Yang 2005). In
addition, when by-products, such as pulps after juice
extraction, are used as raw materials, fewer process-
ing steps to obtain cellulose are required (Bruce et al.
2005). It should also be noted that in agricultural
fibers the cellulose microfibrils are less tightly wound
in the primary cell wall than in the secondary wall in
wood, thus fibrillation to produce nanocellulose
should be less energy demanding (Dinand et al.
1996a).
In the literature there are reports of cellulose
microfibril extraction from diverse non-wood sources
including wheat straw and soy hulls (Alemdar and
Sain 2008a), sugar beet pulp (Dufresne et al. 1997;
Dinand et al. 1996a, 1996b, 1999; Gousse et al. 2004;
Habibi and Vignon 2008), potato pulp (Dufresne
et al. 2000), swede root (Bruce et al. 2005), bagasse
(Bhattacharya et al. 2008), sisal (Moran et al. 2008),
algae (Imai et al. 2003), stems of cacti (Malainine
et al. 2005; Habibi et al. 2009) and banana rachis
(Zuluaga et al. 2007).
Bacterial cellulose
In addition to its plant origins, cellulose fibers are
also secreted extracellularly by certain bacteria
belonging to the genera Acetobacter, Agrobacterium,
Alcaligenes, Pseudomonas, Rhizobium, or Sarcina
(El-Saied et al. 2004). The most efficient producer of
bacterial cellulose (BC) is Acetobacter xylinum (or
Gluconacetobacter xylinus), a gram-negative strain
of acetic-acid-producing bacteria (Brown 2004;
Klemm et al. 2006). There are important structural
differences between BC and, for example, wood
cellulose. BC is secreted as a ribbon-shaped fibril,
less than 100 nm wide, which is composed of much
finer 2–4 nm nanofibrils (Iguchi et al. 2000; Brown
and Laborie 2007). In contrast to the existing
methods for obtaining nanocellulose through
mechanical or chemo-mechanical processes, BC is
produced by bacteria through cellulose biosynthesis
and the building up of bundles of microfibrils
(Nakagaito et al. 2005). These microfibril bundles
have excellent intrinsic properties due to their high
crystallinity (up to 84–89%; Czaja et al. 2004),
including a reported elastic modulus of 78 GPa
(Guhados et al. 2005), which is higher than that
generally recorded for macro-scale natural fibers
(Mohanty et al. 2000) and is of the same order as the
elastic modulus of glass fibers (70 GPa; Saheb and
Jog 1999; Juntaro et al. 2007). Compared with
cellulose from plants, BC also possesses higher water
holding capacity, higher degree of polymerization (up
to 8,000), and a finer web-like network (Klemm et al.
2006; Wan et al. 2006; Barud et al. 2008). In
addition, BC is produced as a highly hydrated and
relatively pure cellulose membrane and therefore no
chemical treatments are needed to remove lignin and
hemicelluloses, as is the case for plant cellulose (Wan
et al. 2006; Barud et al. 2008).
Recent studies have highlighted the potential of
BC as a reinforcement in nanocomposites (Nakagaito
et al. 2005; Nogi et al. 2005, 2006b; Yano et al. 2005;
Maeda et al. 2006; Sanchavanakit et al. 2006; Ifuku
et al. 2007; Juntaro et al. 2007; Grande et al. 2008;
Juntaro et al. 2008). More detailed discussion of BC
preparation and use is presented elsewhere (Iguchi
et al. 2000; Brown 2004; El-Saied et al. 2004; Shoda
and Sugano 2005; Wan et al. 2006).
Preparation and characterization
of microfibrillated cellulose
The production of MFC by fibrillation of cellulose
fibers into nano-scale elements requires intensive
mechanical treatment. However, depending upon the
raw material and the degree of processing, chemical
treatments may be applied prior to mechanical fibril-
lation. These chemical processes are aimed to produce
purified cellulose, such as bleached cellulose pulp,
which can then be further processed. There are also
examples with reduced energy demand in which the
isolation of cellulose microfibrils involves enzymatic
pre-treatment followed by mechanical treatments
(Henriksson et al. 2007; Paakko et al. 2007). Depend-
ing upon the raw materials and fibrillation techniques,
the cellulose degree of polymerization, morphology
and nanofiber aspect ratio may vary (Svagan et al.
2008). Examples of cellulose nanofiber preparation
methods, including MFC, are shown in Table 2.
462 Cellulose (2010) 17:459–494
123
Ta
ble
2E
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ple
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ho
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oto
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llat
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ar
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eral
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gan
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ain
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tin
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tory
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bri
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)
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se,
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llag
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98)
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pfi
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sev
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.(2
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emic
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edia
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gar
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and
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no
n(2
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Cellulose (2010) 17:459–494 463
123
Mechanical treatments
Refining and high-pressure homogenization
Methods for producing MFC were first reported by
Herrick et al. (1983) and Turbak et al. (1983). The
principle of their methods was to pass dilute cellu-
losic wood pulp-water suspensions through a
mechanical homogenizer, in which a large pressure
drop facilitated microfibrillation. MFC is now a
commercial product available from various compa-
nies and other organizations (e.g. Daicel, Japan;
Rettenmaier, Germany or Innventia AB, Sweden).
The manufacture of MFC is now generally per-
formed by a mechanical treatment consisting of
refining and high pressure homogenizing process steps
(Nakagaito and Yano 2005; Paakko et al. 2007;
Stenstad et al. 2008). Using a disk refiner, the dilute
fiber suspension is forced through a gap between rotor
and stator disks. These disks have surfaces fitted with
bars and grooves against which the fibers are subjected
to repeated cyclic stresses. This mechanical treatment
brings about irreversible changes in the fibers, increas-
ing their bonding potential by modification of their
morphology and size (Nakagaito and Yano 2004).
However, mechanical refining methods tend either to
damage the microfibril structure by reducing molar
mass and degree of crystallinity or fail to sufficiently
disintegrate the pulp fiber (Henriksson et al. 2007). The
refining process is carried out prior to homogenization
due to the fact that refining results in external
fibrillation of fibers by gradually peeling off the
external cell wall layers (P and S1 layers) and exposing
the S2 layer. Internal fibrillation loosens the fiber wall
which prepares the pulp fibers for subsequent homog-
enization treatment (Nakagaito and Yano 2004, 2005).
During homogenization dilute slurries of refined
cellulose fibers are pumped at high pressure and fed
through a spring-loaded valve assembly. As this valve
opens and closes in rapid succession, the fibers are
subjected to a large pressure drop with shearing and
impact forces. This combination of forces promotes a
high degree of microfibrillation of the cellulose fibers,
resulting in the production of MFC (Nakagaito and
Yano 2004). Zimmermann et al. (2004) and Lopez-
Rubio et al. (2007) reported the mechanical fibrillation
process using a microfluidizer in the homogenization
step. Such mechanical dispersion of pulp fibers leads to
fibril structures with diameters between 20 and 100 nmTa
ble
2co
nti
nu
ed
Met
ho
dR
awm
ater
ial
Pro
ced
ure
Nan
ofi
ber
dim
ensi
on
Ref
eren
ces
En
zym
atic
pre
-
trea
tmen
t
Ble
ach
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lpE
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mat
icp
re-t
reat
men
tb
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ng
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OS
1(i
sola
ted
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min
fect
edE
lmtr
ees)
,fo
llo
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hsh
ear
refi
nin
g,
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ocr
ush
ing
and
dis
per
sio
nin
wat
erb
ya
dis
inte
gra
tor
10
–2
50
nm
inw
idth
,th
e
maj
ori
tyis
aro
un
d2
5–
75
nm
Jan
ard
hn
anan
dS
ain
(20
06
)
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ach
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lfite
soft
wo
od
cell
ulo
sep
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Refi
nin
gto
incr
ease
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acce
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fth
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esu
bse
qu
ent
mo
no
com
po
nen
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do
glu
can
ase
trea
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zym
atic
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dre
fin
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e;h
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-pre
ssu
reh
om
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eniz
ing
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nm
ind
iam
eter
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pez
-Ru
bio
etal
.(2
00
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kk
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al.
(20
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agan
etal
.(2
00
7)
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ftw
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lfite
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lp;
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tin
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ith
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e(N
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ure
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gen
izat
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iam
eter
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rik
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erg
lun
d
(20
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rik
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464 Cellulose (2010) 17:459–494
123
and estimated lengths of several tens of micrometers.
When a cellulosic pulp fiber suspension is homoge-
nized the procedure is often repeated several times in
order to increase the degree of fibrillation. For
example, Leitner et al. (2007) ran a suspension of
sugar beet pulp cellulose through a high-pressure
laboratory homogeniser operated at 300 bar for 10–15
cycles. However, with increasing homogenization
cycles, the energy demand increases and can be as
high as 30,000 kWh/t (Nakagaito and Yano 2004;
Lindstrom 2007). Iwamoto et al. (2005) reported that
after 14 cycles, further homogenizing up to 30 cycles
did not improve fibrillation. This observation was
supported by Malainine et al. (2005) who achieved the
desired fibrillation by applying 15 passes through a
laboratory homogenizer operated at 500 bar. Dufresne
et al. (2000) also used the same operating conditions to
produce MFC from potato pulp.
Cryocrushing
Alemdar and Sain (2008a) extracted MFC from wheat
straw and soy hulls via mechanical treatment involving
cryocrushing followed by disintegration and fibrilla-
tion. These authors found that almost 60% of the
nanofibers had a diameter within a range of 30–40 nm
and lengths of several thousand nanometers. Cryoc-
rushing is an alternative method for producing nanof-
ibers in which fibers are frozen using liquid nitrogen
and high shear forces are then applied (Chakraborty
et al. 2005). When high impact forces are applied to the
frozen fibers, ice crystals exert pressure on the cell
walls, causing them to rupture and thereby liberating
microfibrils (Wang and Sain 2007a). The cryocrushed
fibers may then be dispersed uniformly into water
suspension using a disintegrator (Janardhnan and Sain
2006) before high pressure fibrillation. Bhatnagar and
Sain (2005) obtained nanofibers with an estimated
diameter of 5–80 nm by applying cryocrushing of
chemically treated flax, hemp, and rutabaga fibers.
Cryocrushing combined with a high-pressure fibrilla-
tion process was used also by Wang and Sain (2007a, b)
for the isolation of nanofibers with diameters in the
range 50–100 nm from soybean stock.
Grinding
Modified commercial grinders with specially designed
disks have been used by some researchers in order to
fibrillate cellulose fibers. In such equipment the
cellulose slurry is passed between a static grind stone
and a rotating grind stone revolving at *1,500 rpm.
The fibrillation mechanism of the grinder treatment can
be explained as follows: the cell wall structure
consisting of nanofibers in a multi-layered structure
and hydrogen bonds is broken down by the shearing
forces generated by the grinding stones and then nano-
sized fibers are individualized from the pulp. As an
example, Taniguchi and Okamura (1998) obtained
microfibrillated fibers having diameters in the range
20–90 nm by a unique super-grinding procedure.
When homogenized cellulosic pulp was subjected to
a grinder treatment by Iwamoto et al. (2005, 2007), the
fibril bundles were further fibrillated and 10 repetitions
of the grinder treatment resulted in uniform nanofibers
50–100 nm wide. During the grinding process, the
shearing force generated by the grinding stones could
degrade the pulp fibers, which might affect the
reinforcing potential of MFC and therefore the phys-
ical properties of composites based on the fibrillated
pulp fibers (Iwamoto et al. 2007).
As a result of the complicated multilayered structure
of plant fibers and interfibrillar hydrogen bonds, a
common feature of all disintegration methods is that
the material obtained consists of aggregated nanofibers
with a wide distribution in width (Abe et al. 2007).
However, Abe et al. (2007) also reported an efficient
extraction of wood cellulose nanofibers as they exist in
the cell wall, with a uniform width of 15 nm, by a very
simple mechanical treatment. This result was achieved
by keeping the material in the water-swollen state after
the removal of lignin and hemicellulose, thus avoiding
the generation of strong hydrogen bonding between the
cellulose bundles, which often takes place during
drying processes (Hult et al. 2001).
Pre-treatments
A major obstacle that needs to be overcome is the
high energy consumption connected to the mechan-
ical disintegration of the fibers into MFC, which often
involves several passes through the disintegration
device. Values around 20,000–30,000 kWh/tonne are
not uncommon. Even higher values reaching
70,000 kWh/tonne have also been reported (Eriksen
et al. 2008). By combining the mechanical treatment
with certain pre-treatments (e.g., chemical or
enzyme) it is possible to decrease the energy
Cellulose (2010) 17:459–494 465
123
consumption significantly to the level of 1,000 kWh/
tonne (Ankerfors and Lindstrom 2007).
Alkaline pre-treatment
Before mechanical processing, a number of researchers
have applied alkaline treatment of fibers in order to
distrupt the lignin structure and help to separate the
structural linkages between lignin and carbohy-
drates (Dufresne et al. 1997; Wang and Sain 2007a,
b, c; Wang et al. 2007). Purification by mild alkali
treatment results in the solubilization of lignin and
remaining pectins and hemicelluloses. Alkali extrac-
tion needs to be carefully controlled to avoid
undesirable cellulose degradation and to ensure that
hydrolysis occurs only at the fiber surface (Bhatnagar
and Sain 2005; Wang and Sain 2007a) so that intact
nanofibers can be extracted.
Oxidation pre-treatment
Saito et al. (2006) introduced an oxidation pre-
treatment of cellulose, applying 2,2,6,6-tetramethylpi-
peridine-1-oxyl (TEMPO) radicals before mechanical
treatment in a Waring-blender. TEMPO-mediated
oxidation is a promising method for surface modifi-
cation of native celluloses, by which carboxylate and
aldehyde functional groups can be introduced into
solid native celluloses under aqueous and mild
conditions (Saito and Isogai 2005, 2006, 2007; Saito
et al. 2005, 2007, 2009; Habibi and Vignon 2008;
Lasseuguette et al. 2008). In the case of such oxi-
dations the nature of the products obtained is highly
dependent on the starting materials. When regener-
ated and mercerized celluloses are used, water-
soluble b-1,4-linked polyglucuronic acid sodium salt
(cellouronic acid) with a homogeneous chemical
structure can be obtained quantitatively as the
oxidized product (Isogai and Kato 1998; Tahiri and
Vignon 2000). On the other hand, when native
celluloses are used, the initial fibrous morphology is
mostly maintained, even after the TEMPO-mediated
oxidation under harsh conditions (Saito and Isogai
2005). In this case, the oxidation occurred only at the
surface of the microfibrils, which became negatively
charged. This negative charge resulted in repulsion of
the nanofibers, thus easing fibrillation.
As can be seen in Fig. 2. NaBr and NaClO are
generally used as additional catalyst and primary
oxidant respectively at pH 9–11. In order to avoid
undesirable side reactions under alkaline conditions,
such as significant depolymerization or discoloration
of the oxidized cellulose due to the presence of
aldehyde group residuals, Saito et al. (2009) applied a
TEMPO/NaClO/NaClO2 system under neutral or
slightly acidic conditions. These authors demon-
strated that the new oxidation system allowed almost
complete maintenance of the original DP, uniform
nanofiber distribution (&5 nm in width) and a
material free of aldehyde groups. Films prepared
from TEMPO-oxidized cellulose gels had high
transparency, high toughness and low density.
Enzymatic pre-treatment
Enzymatic pre-treatments enable the manufacture of
MFC with significantly reduced energy consumption
(Paakko et al. 2007). In nature, cellulose is not
degraded by a single enzyme but a set of cellulases
are involved. These can be classified as A- and B-
type cellulases, termed cellobiohydrolases, which are
able to attack highly crystalline cellulose and C- and
D-type cellulases or endoglucanases which generally
require some disorder in the structure in order to
degrade cellulose (Henriksson et al. 1999, 2005).
Cellobiohydrolases and endoglucanases show strong
synergistic effects (Berghem and Pettersson 1973;
Henriksson et al. 2007). During preparation of MFC,
isolated cellulases have been applied which modify
rather than degrade the cellulose (Henriksson and
Henriksson 2004). Janardhnan and Sain (2006)
prepared MFC by treating bleached kraft pulp with
OS1, a fungus isolated from trees infected with Dutch
elm disease. A significant shift towards lower fiber
diameters occurred as a result of this enzyme
treatment. Fungus OS1 had only mild activity against
cellulose, which is of interest as this minimizes the
loss of cellulose during MFC preparation. Henriksson
et al. (2007) and Paakko et al. (2007) found that
endoglucanase pre-treatment facilitates disintegration
of cellulosic wood fiber pulp into MFC nanofibers.
Moreover, the MFC produced from enzymatically
pre-treated cellulosic wood fibers showed a more
favorable structure than nanofibers produced by
subjecting pulp fiber to strong acid hydrolysis.
Pretreated fibers subjected to the lowest enzyme
concentration (0.02%) were successfully disinte-
grated while molecular weight and fiber length were
466 Cellulose (2010) 17:459–494
123
well preserved (Henriksson et al. 2007). Lopez-Rubio
et al. (2007) and Svagan et al. (2007) also combined
mechanical and enzymatic treatments. The cell wall
delamination was carried out by treating the pulp in
four separate steps: a refining step using an Escher–
Wyss refiner in order to increase the accessibility of
the cell wall to the subsequent enzyme treatment, an
enzymatic treatment step using monocomponent
endoglucanase, a second refining stage, and finally
a step in which the pulp slurry was passed through a
high-pressure microfluidizer.
Nanofiber morphology in MFC
The ultrastructure of cellulose derived from various
sources has been extensively studied. Techniques
such as transmission electron microscopy (TEM),
scanning electron microscopy (SEM), field-emission
scanning electron microscopy (FE-SEM), atomic
force microscopy (AFM), wide-angle X-ray scatter-
ing (WAXS) and solid state 13C cross-polarization
magic angle spinning (CPMAS) NMR spectroscopy
have been used to characterize MFC morphology.
These methods have typically been applied to inves-
tigation of dried MFC morphology.
Although a combination of microscopic techniques
with image analysis can provide information on MFC
nanofiber widths, it is more difficult to determine
nanofiber lengths because of entanglements and
difficulties in identifying both ends of individual
nanofibers. It is often reported that MFC suspensions
are not homogeneous and that they consist of
cellulose nanofibers and nanofiber bundles. In addi-
tion, suspensions may contain a certain amount of
larger fiber fragments and unfibrillated fibers (And-
resen et al. 2006; Andresen and Stenius 2007). The
evolution of larger nanofiber agglomerates is due to
the high density of hydroxyl groups on the surface of
the microfibrils, which can strongly interact and lead
to agglomeration (Zimmermann et al. 2004). The
network of single nanofibers with superimposed
fibers can be seen in the TEM micrograph of an
MFC gel (Fig. 3).
Nanofiber diameter distribution is strongly influ-
enced by the features of the manufacturing process
and the nature of the cellulose source. In particular
Fig. 2 Scheme of TEMPO-
mediated oxidation of
cellulose (reprinted from
Saito et al. 2006 with
permission from Elsevier)
Cellulose (2010) 17:459–494 467
123
the presence of non-cellulosic polysaccharides such
as hemicellulose and pectin can be a factor. Various
authors (Dinand et al. 1999; Hult et al. 2000, 2001,
2003; Zhang and Tong 2007; Iwamoto et al. 2008;
Virtanen et al. 2008; Zhang et al. 2008) have reported
that higher hemicellulose content correlates with a
smaller nanofiber aggregation size, which suggests
that hemicellulose limits the association between
cellulose nanofibers. Iwamoto et al. (2008) reported
that hemicelluloses in MFC pulp facilitated nanofi-
brillation and improved the physical properties of
nanocomposites. In contrast, Alemdar and Sain
(2008a) assumed that the removal of hemicellulose
and lignin is associated with higher tensile strength
due to increased relative crystallinity of nanofibers,
although no tensile data were reported. These authors
found that nanofiber crystallinity determined by X-
ray diffractometry (XRD) was increased by 35% for
wheat straw nanofibers and 16% for soy hull nanof-
ibers when non-cellulosic compounds were removed.
Pectins are also present in cellulose pulps and they
strongly influence the mechanical behaviour of
cellulose. For example, Dufresne et al. (1997)
reported that the removal of pectins from sugar beet
pulp decreased the tensile modulus of films made of
the pulp in dry atmosphere. Indeed, pectins are
thought to act as a binder between the cellulose
microfibrils and to improve the mechanism of load
transfer when samples are subjected to mechanical
stress. This binding mechanism increases the cohe-
sion within the material due to hydrogen bonding
and/or covalent interactions between pectins, hemi-
celluloses, and cellulose microfibrils.
The influence of cellulose pulp chemistry on its
microstructure has been investigated by Ahola et al.
(2008b). Using AFM, these authors compared the
microstructure of two types of MFC prepared at
Innventia AB (formerly STFI-Packforsk, Stockholm,
Sweden). MFC generation 1 was manufactured from
a sulfite pulp of high hemicellulose content while
MFC generation 2 was produced from a dissolving
pulp with lower hemicellulose content. As a result of
the chemistry involved in producing generation 2
(carboxymethylation), it differs significantly from
generation 1 in having charged groups on the
nanofiber surfaces (Wagberg et al. 2008). As
observed by Ahola et al. (2008b) MFC generation 1
formed a more apparent network structure while the
charged nanofibers (MFC generation 2) formed
smoother and denser films. These observations have
been confirmed recently in a study by Aulin et al.
(2009) in which different nanoscale cellulose model
films were characterized.
The same two generations of MFC were investi-
gated by Siro and Plackett (2008). AFM revealed a
generally finer and more homogeneous size distribu-
tion of nanofibers in MFC generation 2 films relative
to MFC generation 1; however, this technique also
showed the presence of some large fiber aggregates in
MFC generation 2. It was also reported that these
structures could be disintegrated and gel transparency
improved by further homogenization steps. However,
it has to be emphasized that the role of nanofiber
aggregation in influencing macroscale properties of
pulp (e.g., strength) is not yet well understood.
Besides microscopic techniques, the extent of
fibrillation can be assessed indirectly by other
measurements. Degree of cellulose polymerization
(DP) is reported to correlate strongly with the aspect
ratio of the nanofibers; longer fibrils are associated
with higher cellulose DP. The DP for the raw material
(sulfite pulp) is often reported to be around 1,200–
1,400, while mechanical isolation of nanofibers may
result in about 30–50% decrease in DP (Henriksson
et al. 2007). Depending on the nature of the starting
material, the DP of MFC may be even lower. For
Fig. 3 TEM image of MFC gel showing cellulose nanofibers
and nanofiber bundles. Scale bar corresponds to 0.2 lm
468 Cellulose (2010) 17:459–494
123
example, Iwamoto et al. (2007) reported that the DP
of MFC decreased from about 770 to 525 after nine
passes through a grinder. High cellulose DP is
desirable for MFC nanofibers since this is correlated
with increased nanofiber tensile strength (Henriksson
et al. 2007). Although the strength of microfibrils has
not been clarified yet, it can be estimated to be at least
2 GPa based on the experimental results for tensile
strength of 1.7 GPa (Page and Abbot 1983) obtained
for Kraft pulp, which mainly consists of cellulose
microfibrils in which 70–80% of the microfibrils are
distributed parallel to the fiber direction (Yano and
Nakahara 2004).
Water retention, as well as viscosity, both related to
the exposed surface area of cellulose, may also serve as
an approximate estimate of fibrillation. Generally, the
viscosity of the pulp fiber suspension increases
dramatically during MFC preparation (Bruce et al.
2005; Iwamoto et al. 2005; Henriksson et al. 2007).
Preparation and properties of microfibrillated
cellulose films
MFC gels can be converted to films by dilution and
dispersion in water and then either cast (Dufresne
et al. 1997; Andresen et al. 2006, 2007; Saito et al.
2006) or vacuum filtered (Iwamoto et al. 2005, 2007;
Nakagaito and Yano 2005, 2008a, b; Henriksson and
Berglund 2007; Henriksson et al. 2008; Seydibeyoglu
and Oksman 2008). When the water is removed from
the MFC gel, a cellulose nanofiber network is formed
with interfibrillar hydrogen bonding. Stiff and strong
films are formed and the fibrillar nature of MFC film
surfaces can be illustrated microscopically (Fig. 4).
When water is removed by freeze-drying, aqueous
MFC gels can be converted to flexible and deform-
able ‘‘sponge-like’’ aerogels, as demonstrated by
Paakko et al. (2008). Crosslinkers are not needed in
this process since hydrogen bonding and nanofiber
entanglement provide the necessary properties.
Mechanical properties
Mechanical properties of MFC films prepared from
different cellulose sources and by different proce-
dures are summarized in Table 3. Taniguchi and
Okamura (1998) succeeded in preparing MFC from
different sources such as wood pulp, cotton cellulose
and tunicin cellulose by a simple mechanical process.
The MFC suspensions were then transformed into
homogeneous, strong and translucent films with
thickness of 3–100 lm by solvent casting. The
tensile strength of wood pulp MFC and tunicin
MFC films was *2.5 times that of print-grade paper
and 2.7 times that of polyethylene (PE). However, the
measured tensile strength values were not specified.
Henriksson et al. (2007) reported that the mechan-
ical properties of MFC films were reduced when
immersed in water but much of the structure was
retained. The nanofibers in the film were not redis-
persible in water which is due to the strong interac-
tion between adjacent nanofibers after drying, most
likely dominated by hydrogen bonding. Despite
random in-plane MFC orientation, MFC films have
interesting mechanical properties. As discussed by
Fig. 4 FE-SEM images of the surface of MFC films prepared
from: A MFC 1 gel manufactured from a sulfite pulp containing a
high amount of hemicelluloses and B MFC 2 gel manufactured
from dissolving pulp with lower hemicellulose content. Scalebars correspond to 200 and 300 nm respectively (MFC provided
by Innventia AB, Stockholm, Sweden)
Cellulose (2010) 17:459–494 469
123
Berglund (2006), Young’s modulus may approach
20 GPa and strength can reach 240 MPa. However,
most literature indicates lower modulus and strength
values. Zimmerman et al. (Zimmermann et al. 2004,
2005a, b) reported that the tensile strength of pure
MFC films almost reached the strength of clear wood
(80–100 MPa) while a modulus of elasticity was
found to be *6 GPa, which was similar to that
reported by Leitner et al. (2007) for cellulose
nanofibril sheets prepared from sugar beet pulp chips
via solvent casting. These authors also performed
wide-angle X-ray scattering (WAXS) on the dried
cellulose nanofibril sheets and found homogeneous
azimuthal distribution of scattering intensity, con-
firming the random orientation of cellulose nanofi-
bers. Values of 104 MPa and 9.4 GPa were measured
for tensile strength and modulus of elasticity respec-
tively for sugar beet-derived MFC.
Bruce et al. (2005) reported modulus of elasticity
and tensile strength of 7 GPa and 100 MPa
respectively for cellulose sheets made from high-
pressure homogenized swede root pulp. On the other
hand, significantly lower tensile modulus values
(&2.5–3.2 GPa) were measured by Dufresne et al.
(1997) for MFC films prepared from sugar beet pulp.
These authors also noted that the cellulose microfi-
brils were stiffer in the presence of pectins, which is
one of the main compounds in this type of pulp (25–
30 wt%).
More recently, Henriksson et al. (2008) explored
the structure–mechanical property relationships for
pure MFC films prepared from nanofibrils of different
cellulose molar mass. The porosity of the films was
modified by introducing solvents other than water.
SEM revealed a fine web-like and highly fibrous
network structure on the surface of MFC films. The
typical lateral dimensions of nanofibers were found to
be 10–40 nm, suggesting that they consist of cellu-
lose microfibril aggregates rather than smaller indi-
vidual microfibrils. Despite a relatively high porosity
Table 3 Mechanical properties of MFC films. Test conditions are reported in the original references
Raw material Preparation
procedure
Max. Stress
(MPa)
Modulus of
elasticity (GPa)
Strain at
break (%)
References
Sugar beet pulp Casting ND 2.5–3.2 ND Dufresne et al. (1997)
Softwood sulfite pulp Casting 80–100 *6a *1 mma Zimmermann et al. (2004,
2005a, b)
Swede root pulp Filtering 100 7 ND Bruce et al. (2005)
Softwood dissolving pulp Vacuum filtering 104 14.0 2.6 Henriksson and Berglund
(2007)
Sugar beet pulp chips Casting 104 9.3 3.2 Leitner et al. (2007)
Bleached sulfite softwood
cellulose pulp
Casting 180 13.0 2.1 Svagan et al. (2007)
Softwood dissolving pulp;
bleached sulfite softwood
Vacuum filtering 129–214 10.4–13.7 3.3–10.1 Henriksson et al. (2008)
Wood powder,
holocellulose pulp
Vacuum filtering 213–240 12.8–15.1 3.2–4.4 Iwamoto et al. (2008)
Commercial MFC (Daicel
Chemical Industries)
Vacuum filtering 140–160 8.5–10.5a 5–11 Nakagaito and Yano
(2008a, b)
Never-dried softwood and
hardwood bleached kraft
pulp
Vacuum filtering 222–233 6.2–6.9 7.0–7.6 Fukuzumi et al. (2009)
Commercial MFC (Daicel
Chemical Industries)
ND 67–68a ND 2.7–2.8a Iwatake et al. (2008)
Bleached spruce sulfite pulp Vacuum filtering 104–154 15.7–17.5 5.3–8.6 Syverud and Stenius (2009)
Hardwood bleached kraft
pulp
Vacuum filtering 222–312 6.2–6.5 7.0–11.5 Saito et al. (2009)
a Values estimated from charts presented in the original reference
470 Cellulose (2010) 17:459–494
123
(up to 28%) for the water-based MFC films, the
Young’s modulus (13.2 ± 0.6 GPa) and tensile
strength (214 ± 6.8 MPa) were remarkably high.
These values decreased significantly with increased
porosity. Assuming that MFC film is a random
network of ideal straight and infinite fibers, Syverud
and Stenius (2009) suggested that the maximum
theoretical E-modulus might be one-third that of the
individual fibers (i.e., &27 GPa). In reality, how-
ever, since MFC networks deviate from the ideal,
significantly lower values are reported (Table 3).
Optical properties
Reinforcing elements with diameters less than one-
tenth of visible light wavelengths are not expected to
cause light scattering (Yano et al. 2005). MFC is
typically in this size range and therefore, unless
significant nanofiber agglomeration occurs, highly
transparent MFC films should be expected. Indeed,
Fukuzumi et al. (2009) reported 78% and 90% light
transmittance at 600 nm for 20 lm-thick TEMPO-
oxidized MFC films prepared from hardwood and
softwood cellulose respectively.
In the case of carboxymethylated MFC with low
hemicellulose content, nanofibers tend to form large
fiber fragments and aggregates of micron size, which
can compromise film transparency. Siro and Plackett
(2008), however, improved transparency of such
films by subjecting the initial MFC gel to as many
as three additional homogenization steps before film
preparation, which resulted in the disintegration of
larger fiber aggregates. As a consequence, light
transmittance at 600 nm for 20 lm-thick films was
improved from 61 to 82%.
Nogi et al. (2009) have recently evaluated the
influence of film surface roughness on film transpar-
ency. These researchers found that surface light
scattering significantly reduced the light transmit-
tance of nanocellulose films. When film surfaces
were polished or impregnated with an optically
transparent polymer layer (e.g., using an acrylic
resin) the total light transmittance could be increased
up to 89.7% (Nogi and Yano 2009).
Barrier properties
Generally speaking, it is difficult for diffusing mole-
cules to penetrate the crystalline parts of cellulose
fibrils (Syverud and Stenius 2009). Due to relatively
high crystallinity (Lu et al. 2008b, Aulin et al. 2009), in
combination with the ability of the nanofibers to form a
dense network held together by strong inter-fibrillar
bonds, it has been hypothesized that MFC might act as
a barrier material (Syverud and Stenius 2009).
Although the number of reported oxygen permeability
values is limited, reports attribute high oxygen barrier
properties to MFC films. As an example, Syverud and
Stenius (2009) reported an oxygen transmission value
of 17.75 ± 0.75 ml m-2 day-1 for 21 lm-thick MFC
films measured at 23 �C and 0% RH using a Mocon
Coulox oxygen sensor ((Modern Controls, Inc., Min-
neapolis, USA).
Fukuzumi et al. (2009) reported more than a 700-
fold decrease in oxygen permeability of polylactide
(PLA) film when an MFC layer was added to the PLA
surface. It is hypothesized that MFC, being highly
hydrophilic, tends to absorb a significant amount of
moisture. Water absorption and swelling of MFC is a
complex phenomenon, which is thought to be influ-
enced both by the molecular structure of cellulose
and the mesostructure of the films (Aulin et al. 2009).
To the authors’ knowledge only one study has been
published so far presenting water uptake of neat MFC
films (Aulin et al. 2009); however, no results were
presented regarding the water vapor permeability of
such films. It has been reported that MFC addition
can decrease the moisture uptake of potato starch
films and amylopectin films (Dufresne et al. 2000;
Svagan et al. 2009).
The influence of MFC film density and porosity on
film permeability remains relatively unexplored. Some
authors have reported significant porosity in MFC films
(Henriksson and Berglund 2007; Svagan et al. 2007;
Henriksson et al. 2008), which seems to be in
contradiction with high oxygen barrier properties. A
possible explanation could be that MFC films contain
closed pores in the core of the cross section and it might
be inferred that good oxygen barrier properties occur as
a result of close nanofiber ordering and packing as well
as the effect of cellulose crystallinity.
Microfibrillated cellulose nanocomposites
Nanocomposites in general are two-phase materials
in which one of the phases has at least one dimension
in the nanometer range (1–100 nm). The advantages
Cellulose (2010) 17:459–494 471
123
of nanocomposite materials when compared with
conventional composites are their superior thermal,
mechanical and barrier properties at low reinforce-
ment levels (e.g., B5 wt%), as well as their better
recyclability, transparency and low weight (Oksman
et al. 2006; Sorrentino et al. 2007).
Biodegradable polymers, in particular, may require
improvement in terms of brittleness, low thermal
stability and poor barrier properties (Sorrentino et al.
2007). A number of researchers have therefore
explored the concept of fully bio-derived nanocom-
posites as a route to development of bioplastics or
bioresins with better properties (Plackett et al. 2003;
Oksman et al. 2006; Petersson et al. 2007). Biopoly-
mer-based nanocomposites have also been the subject
of recent reviews (Pandey et al. 2005; Akbari et al.
2007; Rhim 2007; Rhim and Ng 2007).
Due to their abundance, biodegradability and
relatively low price, there is a significant history of
using cellulose fibers from plants as reinforcement in
composite materials (Bledzki and Gassan 1999;
Saheb and Jog 1999; Eichhorn et al. 2001; Wambua
et al. 2003; John and Anandjiwala 2008; John and
Thomas 2008); however, the application of nanoscale
cellulose fibers for this purpose is a relatively new
research area. The properties of fiber-reinforced
composites depend on many factors, including fiber
size, fiber/matrix adhesion, volume fraction of fiber,
fiber aspect ratio, fiber orientation, and stress/transfer
efficiency through the interface (Dufresne et al.
2003). Fiber content in composites is a critical factor
since fiber agglomeration through hydrogen bonding
tends to occur at higher filler loadings (Tserki et al.
2006; Sanchez-Garcıa et al. 2008).
As illustrated in Fig. 5, nanocomposites based on
nanocellulosic materials such as microfibrillated
cellulose or bacterial cellulose have been prepared
with petroleum-derived non-biodegradable polymers
such as polyethylene (PE) or polypropylene (PP) and
also with biodegradable polymers such as PLA,
polyvinyl alcohol (PVOH), starch, polycaprolactone
(PCL) and polyhydroxybutyrate (PHB). Chemical
modification of cellulose has been explored as a route
for improving filler dispersion in hydrophobic poly-
mers. MFC-based nanocomposites are discussed in
the following section with particular emphasis on the
reinforcing potential of such nanocellulose materials.
However, some BC-based nanocomposites are also
presented briefly.
MFC nanocomposites with hydrophilic polymers
Thermoset resins
As noted before, due to the hydrophilic nature of MFC,
many studies have focused on nanocomposites based
on hydrophilic matrices. For example, Nakagaito and
Yano (2004, 2005) impregnated microfibrillated Kraft
pulp with a phenol–formaldehyde resin and then
compressed the resulting material under high pressure
to produce high-strength cellulose nanocomposites.
This study was also designed to clarify how the degree
of fibrillation of pulp fibers affects the mechanical
properties of cellulose composites. It was found that
fibrillation that only influences the fiber surfaces is not
effective in improving composite strength but that a
complete fibrillation of the bulk of the fibers is
required. In the study by Nakagaito and Yano (2004)
this was achieved by 16–30 passes through a refiner,
followed by high pressure homogenization. The
resulting nanocomposites had Young’s modulus and
bending strength values of 19 GPa and 370 MPa
respectively, as determined by a three-point bending
test (Nakagaito and Yano 2005). More recently, the
same authors used aqueous sodium hydroxide-treated
MFC and phenolic resin to produce nanocomposites
(Nakagaito and Yano 2008a). The tensile properties of
MFC-resin nanocomposites were compared with those
of wood pulp-resin composites and it was shown that
the tensile strength of the MFC composites was
significantly higher than that of the pulp composites,
regardless of the treatment or resin content. In contrast,
the Young’s modulus values were practically the same.
These authors thus confirmed the advantage of nano-
structured composites over microstructured compos-
ites in terms of obtaining high strength and ductility.
Tensile tests showed that strong alkali-treated MFC
nanocomposites with resin content around 20 wt% had
strain at fracture values twice as high as those of
untreated MFC nanocomposites based on the same
resin.
Henriksson et al. (2007) produced nanocomposite
films of MFC and melamine formaldehyde (MF)
resin as a potential material for use in loudspeaker
membranes, where a high Young‘s modulus and low
density are required. The MF-MFC nanocomposites
showed average Young’s modulus as high as
16.6 GPa and average tensile strength as high as
142 MPa, while density was higher than for a
472 Cellulose (2010) 17:459–494
123
conventional paper prepared from pulp fibers. The
combination of properties attainable with MFC films
and composites, including high mechanical damping,
demonstrates that these materials have the potential
to be used as loudspeaker membranes.
Bruce et al. (2005) prepared composites based on
swede root MFC and different resins including four
types of acrylic and two types of epoxy resins. All the
composites were significantly stiffer and stronger
than the unmodified resins. The main merit of the
study was that it demonstrated the potential for
fabricating nanocomposites with good mechanical
properties from vegetable pulp in combination with a
range of resins.
Aside from good mechanical properties, high
composite transparency can be important for some
applications (e.g., in the optoelectronics industry).
Iwamoto et al. (2005, 2007) reported that because of
the size of nanofibers MFC-reinforced acrylic resin
retains the transparency of the matrix resin even at
fiber contents as high as 70 wt%. Bacterial cellulose
with nanofiber widths of &10 nm also has potential
as a reinforcing material for transparent composites.
As an example, Nogi et al. (2005, 2006a, b) obtained
transparent composites by reinforcing various acrylic
resins with BC at loadings up to 70 wt%.
By reducing the average fiber size (d & 15 nm),
Abe et al. (2007) fabricated an MFC-acrylic resin
nanocomposite with transmittance higher than that of
BC nanocomposite in the visible wavelength range
and at the same thickness and filler content. This
finding indicated that nanofibers obtained from wood
in this study were more uniform and thinner than BC
nanofibers.
Another remarkable and potentially useful feature
of cellulose nanofibers is their low thermal expansion
coefficient (CTE), which can be as low as
0.1 ppm K-1 and comparable with that of quartz
glass (Nishino et al. 2004). This low CTE combined
with high strength and modulus could make cellulose
nanofiber a potential reinforcing material in roll-to-
roll technologies (e.g., for fabricating flexible dis-
plays, solar cells, electronic paper, panel sensors and
actuators). As an example, Nogi and Yano (2008)
prepared a foldable and ductile transparent nanocom-
posite film by combining low-Young‘s modulus
transparent acrylic resin with 5 wt% of low-CTE
and high-Young‘s modulus bacterial cellulose. The
same researchers reported that transparent cellulose
nanofiber sheets prepared from MFC and coated with
acrylic resin have low CTEs of 8.5–14.9 ppm K-1
and a modulus of 7.2–13 GPa (Nogi et al. 2009; Nogi
and Yano 2009).
Poly(styrene-co-butyl acrylate) latex
In a study by Malainine et al. (2005) nanocomposite
materials were prepared by solution casting from an
Fig. 5 Classification of
nanocomposites based on
nanocellulosic materials
(e.g. microfibrillated
cellulose, bacterial
cellulose) reported in the
literature
Cellulose (2010) 17:459–494 473
123
aqueous suspension of cellulose microfibrils obtained
from Opuntia ficus-indica cladodes (i.e., modified
stems) and a latex of poly(styrene-co-butyl acrylate).
These researchers observed a significant reinforcing
effect of cellulose microfibrils on the matrix. The
tensile modulus increased from 0.6 MPa to 34.5 MPa
when adding 10 wt% filler. A tensile strength of
14.5 MPa was obtained for poly(styrene-co-butyl
acrylate) containing 10 wt% MFC. The elongation
at break decreased as a function of the MFC content
from 362 to 115%. It was also proved that the thermal
stability of the composite was significantly enhanced,
while swelling behaviour of the polymeric matrix was
found to decrease significantly with MFC addition,
even at low filler loadings.
Samir et al. (2004) compared the reinforcing effect
of cellulose nanofibers (MFC) and nanowhiskers in the
same type of latex as used by Malainine et al. (2005).
Cellulose microfibrils were extracted from sugar beet
and also further processed to whiskers via progressive
acid hydrolysis. Composites were prepared with 6 wt%
filler content by solvent casting. It was found that both
the tensile modulus and the tensile strength of the
composites increased when adding the cellulosic filler,
but the reinforcing effect depended on the morphology
of the filler. Incorporation of cellulose nanofibrils
resulted in significantly higher tensile modulus
(114 MPa) and tensile strength (6.3 MPa) when com-
pared to nanowhisker addition (tensile modulus and
strength of 31 and 1.5 MPa respectively). In compar-
ison, the relevant data for pure matrix were 0.2 and
0.18 MPa for modulus and strength respectively.
More recently, Dalmas et al. (2007) dispersed
cellulose nanofibrils obtained from sugar beet pulp in
the same poly(styrene-co-butyl acrylate) latex. Two
methods were used for composite processing. The
mixture was either cast or freeze dried and then hot
pressed. In the first case, formation of a rigid nanofibril
network linked by strong hydrogen bonds took place
leading to high mechanical reinforcement and thermo-
mechanical stability. On the other hand, when cellulose
nanofibrils were freeze-dried the creation of strong
contacts between nanofibrils was prevented and there-
fore only a limited reinforcement was observed.
Ethylene vinyl alcohol copolymers
Fernandez et al. (2008) prepared solvent-cast films
from ethylene vinyl alcohol copolymers (EVOH29
and EVOH44) with 2 wt% nanofiber (MFC) content
using isopropanol : water (70 : 30) as a solvent.
Polarized light optical microscopy and AFM con-
firmed that the MFC microfibrils were well dispersed
across the sample thickness, although some MFC
agglomeration also took place. MFC microfibrils
could not be discerned from the matrix using SEM
suggesting that good dispersion and interfacial adhe-
sion of cellulose MFC microfibrils in the EVOH
matrix had been achieved. However, MFC tends to
further diminish the relatively low water barrier
performance of EVOH and therefore low dose
gamma irradiation (up to 30 kGy) was applied in
order to counteract this detrimental effect. A decrease
in polar groups due to irradiation was confirmed,
which could in principle lead to a decrease in the
hydrophilicity of the polymer.
Polyurethanes
Polyurethanes are relatively polar polymers and can
interact with the polar groups of cellulose, leading
to good interfacial adhesion, which is essential for
enhanced mechanical properties (Aksoy et al. 2007).
PU-MFC composite materials were prepared
recently using a film stacking method in which the
PU films and non-woven cellulose fibril mats were
stacked and compression moulded (Seydibeyoglu
and Oksman 2008). The thermal stability and
mechanical properties of the pure PU were
improved by MFC reinforcement. Nanocomposites
with 16.5 wt% fibril content had tensile strength and
E-modulus values nearly five times and thirty times
higher respectively than the corresponding values
for the matrix polymer.
Shape memory polymers (SMPs) are capable of
fixing a transient shape and of recovering their
original dimensions by the application of an external
stimulus (Tobushi et al. 1996a, b). Auad et al.
(2008) reported that MFC was utilized to reinforce
shape memory polyurethanes. The resulting nano-
composites showed higher tensile modulus and
strength than unfilled films (53% modulus increase
at 1 wt% nanocellulose) with higher elongation at
break. Meanwhile the nanocomposites retained
shape memory properties equivalent to those of the
neat polyurethane with recoveries of the order of
95%.
474 Cellulose (2010) 17:459–494
123
Poly(vinyl alcohol)
Poly(vinyl alcohol) or PVOH is water-soluble, has
excellent chemical resistance and is biocompatible
and biodegradable. There is therefore interest in a
wide range of practical applications for this polymer
(Ding et al. 2004). In particular, PVOH is an ideal
candidate for biomedical applications including tissue
reconstruction and replacement, cell entrapment and
drug delivery, soft contact lens materials and wound
covering bandages for burn victims (Hassan and
Peppas 2000; Paradossi et al. 2003). Although PVOH
hydrogels can have mechanical properties similar to
some soft biological tissues, in order to mimic other
tissues and, in particular, for medical device appli-
cations, improvement of PVOH durability is required.
One approach is to create a PVOH-based nanocom-
posite that possesses the required properties. As
examples, Wan et al. (2006) and Millon and Wan
(2006) tested BC as a potential reinforcing material in
PVOH for medical device applications. These authors
developed a PVOH-BC nanocomposite with mechan-
ical properties tunable over a broad range, thus
making it appropriate for replacing different tissues.
A number of applications using MFC for reinforcing
PVOH have been reported. For example, Zimmer-
mann et al. (2004) dispersed MFC into PVOH and
generated fibril-reinforced PVOH nanocomposites
(fibril content 20 wt%) with up to three times higher
E-modulus and up to five times higher tensile strength
when compared to the reference polymer.
A blend containing 10% cellulose nanofibers
obtained from various sources, such as flax bast
fibers, hemp fibers, kraft pulp or rutabaga and 90%
poly(vinyl alcohol) was used for making nanofiber-
reinforced composite material by a solution casting
procedure (Bhatnagar and Sain 2005). Both tensile
strength and Young’s modulus were improved com-
pared to neat PVOH film, with a pronounced four- to
five-fold increase in Young’s modulus observed.
Similarly, Wang and Sain (2007a, b) reported that
soybean stock-based nanofiber-reinforced PVOH
films (up to 10% nanofiber content) demonstrated a
two-fold increase in tensile strength when compared
with films without filler. When higher loading of
MFC was used, the relative enhancement of mechan-
ical properties was even more remarkable. Bruce
et al. (2005) reported approximately five times higher
tensile strength for PVOH containing 50 wt% MFC
when compared to the neat polymer. Such improve-
ment in mechanical properties can be explained by
strong interfacial bonding between the cellulose
nanofibers and PVOH.
Leitner et al. (2007) prepared PVOH nanocom-
posites with a range of nanocellulose contents (0–90
wt%). At a cellulose content of 50 wt%, the modulus
of elasticity of PVOH increased by a factor of 20 and
tensile strength increased by a factor of 3.5. Both
parameters increased further at cellulose contents of
70 and 90 wt% respectively. It was also reported that
tensile strength increased linearly as a function of
filler content, which suggests that cellulose content
was the major determinant of strength in these
composites.
Lu et al. (2008b) observed no further improve-
ment of mechanical properties when MFC was
applied above 10 wt%. In their recent study MFC
suspension was added at 1, 5, 10, and 15 wt%
loadings to PVOH matrix. A steady increase in film
modulus and strength was observed until a plateau
was reached at 10 wt% MFC. Dynamic mechanical
analysis (DMA) showed an increase of storage
tensile modulus in the glassy state with increasing
MFC content. The thermal stability of the PVOH
composite films was slightly increased with the
addition of MFC.
Starch
Starch from a variety of crops such as corn, wheat,
rice and potatoes is a source of biodegradable plastics
which are readily available at low cost when com-
pared with most synthetic plastics (Ma et al. 2008a).
Starch is comprised of amylose, a linear polymer with
molecular weight between 103 and 106 and amylo-
pectin, a branched polymer with a-(1–6)-linked
branch points (Dufresne and Vignon 1998). In the
glassy state starch tends to be brittle and is very
sensitive to moisture. In order to extrude or mold an
object from starch, it is often converted into a
thermoplastic starch (TPS; Dufresne et al. 2000). TPS
is obtained after disruption and plasticization of
native starch by applying thermomechanical energy
in a continuous extrusion process (Averous 2004).
Water and glycols are commonly used plasticizers,
although urea (Huang et al. 2006; Ma et al. 2008a, b)
and formamide (Ma and Yu 2004a, b; Wang et al.
2008) have also been explored.
Cellulose (2010) 17:459–494 475
123
Both native starch and TPS can suffer from poor
mechanical properties and high water uptake com-
pared to conventional polymers, moreover these
properties may change after processing (Averous
2004; Ma et al. 2008a). Two main approaches have
been applied to overcome these problems—blending
or chemical modification. Starch can be blended with
biodegradable polymers such as PHB (Lai et al. 2006;
Thire et al. 2006), PLA (Jang et al. 2007; Hao et al.
2008; Ning et al. 2008), PCL (Wang et al. 2005; Rosa
et al. 2006; Kim et al. 2007; Sarazin et al. 2008) and
chitosan (Xu et al. 2005; Durango et al. 2006;
Nakamatsu et al. 2006). Fabrication of composites
based on organic or inorganic reinforcement is
another possible solution to improve the performance
of starch films. Application of fillers such as clays
(Huang et al. 2004; Yoon and Deng 2006; Lee et al.
2007a, b; Cyras et al. 2008), natural fibers (Romhany
et al. 2003; Alvarez et al. 2004; Ma et al. 2005;
Duanmu et al. 2007) cellulose whiskers (Cao et al.
2008; Mathew et al. 2008), or microcrystalline
cellulose (Kumar and Singh 2008; Ma et al. 2008a;
Kadokawa et al. 2009) are reported. MFC and BC
have also been reported as promising candidates for
starch reinforcement (Grande et al. 2008; Mondragon
et al. 2008; Sreekala et al. 2008) and some of these
applications are discussed here.
Dufresne and Vignon (1998, 2000) aimed to
improve the thermomechanical properties and to
reduce the water sensitivity of potato starch-based
nanocomposites, while preserving the biodegradabil-
ity of the material through addition of MFC. The
cellulose filler and glycerol plasticizer content were
varied between 0–50 wt% and 0–30 wt% respec-
tively. MFC significantly reinforced the starch
matrix, regardless of the plasticizer content, and the
increase in tensile modulus as a function of filler
content was almost linear. The tensile modulus was
found to be *7 GPa at 50 wt% cellulose content
compared to *2 GPa for unreinforced samples (0%
MFC). However, it was noted that when the samples
were conditioned at high relative humidity (75%
RH), the reinforcing effect of the cellulose filler was
strongly diminished. Since starch is more hydrophilic
than cellulose, in moist conditions it absorbs most of
the water and is then plasticized. The cellulosic
network is surrounded by a soft phase and the
interactions between the filler and the matrix are
strongly reduced. Besides improving mechanical
properties of starch, addition of MFC to the matrix
resulted in a decrease of both water uptake at
equilibrium and the water diffusion coefficient.
A different approach to achieve starch-nanocellu-
lose composites has been presented by Grande et al.
(2008). In their study, starch was added to the culture
medium of cellulose-producing bacteria (Acetobacter
sp.) in order to introduce the granules into the
forming network of cellulose. The application of such
a bottom-up technique allowed the preservation of
the natural ordered structure of cellulose nanofibers.
The BC-starch mats were hot pressed to obtain
nanocomposite sheets. Atomic force microscopy and
environmental scanning electron microscopy (ESEM)
revealed that starch acted as a matrix which filled the
voids in the BC network. The gelatinized starch
formed a homogenous layer on BC fibers and a
typical brittle fracture surface of the composites was
observed. Using MFC, a moulded product with a
bending strength of 250 MPa was obtained by Yano
and Nakahara (2004) without the use of binders.
When 2 wt% oxidized tapioca starch was added, the
yield strain doubled and the bending strength reached
310 MPa.
Recently, nanocomposites from wheat straw
nanofibers and thermoplastic starch from modified
potato starch were prepared by the solution casting
method (Alemdar and Sain 2008b). Thermal and
mechanical performance of the composites was
compared with the pure thermoplastic starch using
thermogravimetric analysis (TGA), dynamic mechan-
ical analysis (DMA) and tensile testing. The tensile
strength and modulus were significantly enhanced in
the nanocomposite films, which could be explained
by the uniform dispersion of nanofibers in the
polymer matrix. The modulus of the TPS increased
from 111 to 271 MPa with maximum (10 wt%)
nanofiber filling. In addition, the glass transition (Tg)
of the nanocomposites was shifted to higher temper-
atures with respect to the pure TPS.
Mondragon et al. (2008) applied glyceryl monos-
tearate (GMS) as surfactant in TPS-MFC nanocom-
posites prepared by solution casting. As expected,
cellulose nanofibers derived from husks and corncobs
increased the Young‘s modulus and tensile strength
of TPS films due to the strong interactions between
the starch matrix and the high aspect ratio nanofibers.
It was also noticed that mechanical properties could
be further improved by the application of GMS
476 Cellulose (2010) 17:459–494
123
surfactant. This was attributed to the formation of
amylose-GMS complexes, which could increase the
V-type crystallinity and interact with the hydroxyl
groups of cellulose. Although the relative increase of
Young’s modulus was higher than that reported by
Alemdar and Sain (Alemdar and Sain 2008b), the
absolute values were significantly lower in the study
by Mondragon et al. (2008) (see Table 4). This might
be explained by the difference both in TPS and in
MFC sources.
Besides solution casting, the dispersion of nanof-
ibers in TPS was also performed via melt mixing by
Chakraborty et al. (2007). MFC suspension was
poured into molten thermoplastic starch to obtain
fiber loadings up to 2 wt% and then composites were
compression molded into thin films. A maximum of
20% increase in tensile strength and a 100% increase
in stiffness due to cellulose reinforcement was
reported.
Takagi and Asano (2008) investigated the effect of
processing conditions on the mechanical properties
and internal microstructures of composites consisting
of a dispersion-type biodegradable resin made from
esterified starch and cellulose nanofibers. All samples
with nanofiber loading of 70 wt% were prepared by
hot pressing at 140 �C and pressures of 10–50 MPa.
It was found that the density of the composites
increased with increased molding pressure. Moreover
both extra stirring and vacuum drying of the disper-
sion before molding resulted in the removal of voids.
Density was used as an indicator for the mechanical
strength of the composites. Although similar densities
were measured for vacuum-treated and extra-stirred
samples, the latter showed significantly higher flex-
ural strength, which was explained by differences
between their internal microstructure and fiber
dispersion.
Amylopectin
To the authors’ knowledge, unlike amylopectin, there
are no reports to date on amylose-nanocellulose
composites, which could be explained by the fact that
amylopectin may require more improvement in terms
of mechanical properties. Indeed, studies have shown
that under dry or moderate humidities amylose films
are mechanically stronger and exhibit better gas
barrier properties than amylopectin films (Forssell
et al. 2002; Myllarinen et al. 2002a, b). Research has
therefore focused on amylopectin-nanocellulose
composites.
Amylopectin films with enhanced properties
through addition of MFC have been reported by
Lopez-Rubio et al. (2007). In this research, gelati-
nized films of amylopectin, glycerol plasticizer and
MFC were cast at 50% relative humidity and the
properties were investigated after 10 days of storage.
Amylopectin films were brittle and impossible to
handle unless glycerol was added at 38 wt% or above.
However, when MFC was added it was possible to
produce stiff and strong films that could be handled
without the addition of glycerol. This effect was
attributed to film reinforcement by the cellulose
nanofibers combined with an indirect plasticizing
effect as a result of water present in the MFC.
Svagan et al. (2007) combined MFC with a
viscous amylopectin-glycerol blend and cast homo-
geneous films with higher (10–70%) MFC content.
The films with 70 wt% MFC showed high tensile
strength (160 MPa), high tensile modulus (6.2 GPa)
and very high work of fracture (9.4 MJ m-3).
Microscopy revealed a layered nanocomposite struc-
ture and good MFC dispersion, which would explain
the good mechanical properties observed.
Besides improving the mechanical properties of
amylopectin films, reducing the moisture sensitivity
of such films has also been a target. Svagan et al.
(2009) demonstrated that moisture diffusivity was
significantly reduced by adding MFC (up to 70 wt%)
to an amylopectin matrix, while maximum moisture
uptake decreased moderately. These authors cited
three possible reasons for the observed reduction in
moisture diffusivity: (1) cellulose characteristics and
geometrical impedance, (2) swelling constraints due
to a high-modulus/hydrogen bonded fiber network
and (3) strong molecular interactions between cellu-
lose nanofibers and with the amylopectin matrix
(Svagan et al. 2009).
In recent years, nanocomposite foams have also
received attention. Novel biomimetic nanocomposite
foams were prepared by Svagan et al. (2008) based
on MFC in an amylopectin matrix and the use of a
lyophilization process. The cellulose nanofibril con-
tent ranged from 10 to 70 wt% (dry weight basis).
The resulting nanocomposite foams showed micro-
cellular structure with cell dimensions in the range
20–70 lm. Compared to unreinforced amylopectin
foam, a significant improvement in modulus and
Cellulose (2010) 17:459–494 477
123
Ta
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mix
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nea
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.(2
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(0.2
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.(2
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.(2
00
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478 Cellulose (2010) 17:459–494
123
yield strength was observed. This improvement is due
to improved mechanical properties of the nanofibril-
reinforced foam cell wall and the microcellular
structure. Such improvements are not possible with
micrometer-scale cellulose fibers because of their
large dimensions relative to foam cell wall
thicknesses.
Poly(ethylene oxide)
Poly(ethylene oxide) or PEO is a highly biocompat-
ible, biodegradable, hydrophilic and flexible polymer
which has found many different applications (e.g.,
drug delivery, mucoadhesive, dispersant, surfactant,
hydrogel, electrolyte solvent in lithium polymer cell,
flocculation and rheology control agents (Brown and
Laborie 2007; Kaczmarek et al. 2007). The possibil-
ity for hydrogen bonding between the PEO ether
oxygen and the cellulose hydroxyl groups suggests
that PEO may be efficient at altering the crystalliza-
tion of cellulose, yielding a range of fiber and
nanocomposite morphologies. By tailoring the com-
position and morphology of BC-PEO nanocompos-
ites, it is further hypothesized that physical, thermal
and mechanical properties might be fine-tuned
(Brown and Laborie 2007). It was demonstrated that
by adding PEO to the growth medium of Acetobacter
xylinum, finely dispersed BC-PEO nanocomposites
could be produced in a wide range of compositions
and morphologies. Due to BC incorporation the
decomposition temperature of PEO increased by
approximately 15 �C and the tensile storage modulus
of PEO improved significantly, especially above
50 �C. The reinforcing effect was proportional to
the cellulose content (Brown and Laborie 2007). To
the authors’ knowledge there have been no studies on
MFC-PEO systems so far.
Chitosan
Partial deacetylation of chitin results in the produc-
tion of chitosan, a polysaccharide composed of
glucosamine and N-acetyl glucosamine (Crini and
Badot 2008). After purification and derivatization,
chitosan can be dissolved in water at suitable pH and
solution-cast to films (Chenite et al. 2001). Hosokawa
et al. (1990, 1991) reported a novel and biodegrad-
able composite film derived from chitosan and MFCTa
ble
4co
nti
nu
ed
Mat
rix
Pro
ced
ure
Fib
erco
nte
nt
(%)
Max
.S
tres
s
(MP
a)
Mo
du
lus
of
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tici
ty
(GP
a)
Str
ain
atb
reak
(%)
Ref
eren
ces
Th
erm
op
last
icst
arch
So
luti
on
cast
ing
0–
10
ND
(0.1
1)/
0.1
5–
0.2
7N
DA
lem
dar
and
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n(2
00
8b
)
0–
20
(2.5
/4.5
)/5
–1
4d
(0.0
2)/
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25
–0
.14
d(3
8/5
5)/
70
–3
2d
Mo
nd
rag
on
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.(2
00
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tin
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luti
on
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ing
0–
10
(1.8
)/4
.1–
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(20
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.(2
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aV
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ated
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sen
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inth
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rig
inal
refe
ren
ceb
Dep
end
ing
on
mat
rix
nat
ure
and
fib
erco
nte
nt
cD
epen
din
go
np
last
iciz
erco
nte
nt
(0–
30
wt%
)an
dco
nd
itio
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%)
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rfac
tan
tw
asap
pli
edo
rn
ot
Cellulose (2010) 17:459–494 479
123
that has a high oxygen-gas barrier capacity and is
hydrophilic but insoluble in water. Composite films
were prepared in the ranges between 0 and 40 wt%
chitosan in MFC. The composite films had a max-
imum wet strength (60 MPa) at 10–20% chitosan on
cellulose, while the dry tensile strength was found to
be more than 100 MPa at the same chitosan loading.
In this case the strength of cellulose nanofiber films
was increased by the addition of chitosan.
There are examples of research in which a chitosan
matrix was strengthened by the addition of cellulose
nanofibers (Gallstedt and Hedenqvist 2006; Nordqvist
et al. 2007). It is known that acetic-acid-protonated
chitosan (chitosan A) becomes very flexible when
exposed to water (Chen et al. 1994), which opens up
the possibility of forming a rigid chitosan film into
various shapes in its wet state at room temperature.
However, due to contact with water, the film creases
and adjacent surfaces adhere to each other, making it
very difficult to retain a flat film. Furthermore, the
water-plasticized film has very low strength, which
prevents shaping involving large strains (Nordqvist
et al. 2007). Recently a new route to enhance the wet
properties of chitosan-acetic-acid-salt films by adding
MFC at 5 wt% loading has been presented by
Nordqvist et al. (2007). FE-SEM revealed that
cellulose fibrils and fibril bundles were evenly
distributed in the chitosan matrix and formed a
percolated/interconnected network. Due to the size of
the filler (&56 ± 20 nm in width) the transparency
of the composite was not compromised. It was also
observed that MFC fibrils restricted the deformation
of the film associated with exposure to water. The
reinforcing effect was also apparent and when 5 wt%
MFC was added the wet strength increased from 12 to
25% of the dry strength and the wet modulus
increased from 0.4 to 3% of the dry modulus.
Although MFC improved the mechanical properties
of the dry films, its role was more important in the
presence of a ‘‘weak’’ wet chitosan matrix.
Ciechanska (2004) presented a manufacturing
technique for BC/chitosan nanocomposite materials
suitable for medical applications. In particular, the
modification of the BC occured during microbiolog-
ical synthesis by introducing selected bioactive
polysaccharides, such as various chitosan forms and
their derivatives, into the culture medium. As a result
several advantageous features of a composite were
achieved, including good mechanical properties in
the wet state and improved moisture-holding
capacity.
Modification of microfibrillated cellulose
Due to the hydrophilic nature of cellulose, MFC
cannot be uniformly dispersed in most non-polar
polymer media. Consequently, MFC modification is
of interest in order to improve compatibility with a
wider variety of matrices. Although many methods
have been proposed for cellulose surface modification
(John and Anandjiwala 2008; John et al. 2008),
including corona or plasma discharges (Bataille et al.
1989), surface derivatization (Hafren et al. 2006),
graft copolymerization (Gruber and Granzow 1996)
or application of surfactant (Heux et al. 2000; Bonini
et al. 2002), reports on surface modification of
nanocellulosic fibers, in particular MFC, are limited
in number. Some approaches aiming to hydrophobize
nanocellulosic materials are briefly discussed in the
following.
Acetylation
In research by Kim et al. (2002), BC was partially
acetylated to modify its physical properties while
preserving the microfibrillar morphology. In this
case, the degree of acetyl substitution had a crucial
influence on material properties. Ifuku et al. (2007)
found that acetylation improved the transparency and
reduced the hygroscopicity of BC/acrylic resin com-
posite materials. However, the composites had an
optimum degree of substitution (DS) and excessive
acetylation reduced their properties. Acetylation has
also been reported to improve the thermal degrada-
tion resistance of BC fibers (Nogi et al. 2006a). No
studies reporting acetylation of MFC surfaces have
been published so far.
Silylation
Gousse et al. (2004) utilized isopropyl dimethylchlo-
rosilane for surface silylation of cellulose microfibrils
resulting from the homogenization of parenchymal
cell walls. These authors claimed that microfibrils
retained their morphology under mild silylation
conditions and could be dispersed in a non-flocculat-
ing manner into organic solvents. Andresen et al.
(2006) hydrophobized MFC via partial surface
480 Cellulose (2010) 17:459–494
123
silylation using the same silylation agent and reported
that when silylation conditions were too harsh, partial
solubilization of MFC and loss of nanostructure could
occur. Films prepared from the modified cellulose by
solution casting showed a very high water contact
angle (117–146�). It is probable that in addition to
decreased surface energy, higher surface roughness as
a result of modification could contribute to increased
hydrophobicity. It has also been reported that hydro-
phobized MFC could be used for the stabilization of
water-in-oil type emulsions (Andresen and Stenius
2007).
Application of coupling agents
Lu et al. (2008a) successfully modified MFC by
applying three different coupling agents, 3-amino-
propyltriethoxysilane, 3-glycidoxypropyltrimethoxy-
silane, and a titanate coupling agent (Lica 38), in
order to enhance the adhesion between microfibrils
and epoxy resin polymer matrix. The surface mod-
ification changed the character of MFC from hydro-
philic to hydrophobic while the crystalline structure
of the cellulose microfibrils remained intact. Among
the tested coupling agents, the titanate gave the most
hydrophobic surface, possibly due to the lower
polarity of the titanate modifier alkyl chain. Unlike
silane coupling, titanate coupling is thought to occur
via alcoholysis, surface chelation or coordination
exchange. When there are hydroxyl groups present on
the surface of the substrate, the monoalkoxy- and
neoalkoxy-type titanium-derived coupling agents
react with the hydroxyl groups to form a monomo-
lecular layer.
Grafting
Stenstad et al. (2008) reported three methods for
modification of MFC by heterogeneous reactions in
both water and organic solvents. Epoxy functionality
was introduced onto the MFC surface by oxidation
with cerium (IV) followed by grafting with glycidyl
methacrylate. Reactive epoxy groups could serve as a
starting point for further functionalization with
ligands which typically do not react with the surface
hydroxyls present in native MFC. A major advantage
of this technique is that the reaction is conducted in
aqueous media, thereby avoiding the use of organic
solvents and laborious solvent exchange procedures.
By this method, cellulose nanofibers with a surface
layer of moderate hydrophobicity can be prepared. In
the same research, a far more hydrophobic MFC
surface was obtained by grafting of hexamethylene
diisocyanate, followed by reaction with amines.
Succinic and maleic acid groups can be introduced
directly onto the MFC surface as a monolayer by a
reaction between the corresponding anhydrides and
the surface hydroxyl groups of the MFC (Stenstad
et al. 2008). Siqueira et al. (2009) applied N-
octadecyl isocyanate (C18H37NCO) as the grafting
agent in order to improve MFC compatibility with
polycaprolactone.
The potential use of chemically coated hemp
nanofibers as reinforcing agents for biocomposites
was explored by Wang and Sain (2007c). The cellulose
nanofibers were treated using five different chemicals:
ethylene acrylic acid, styrene maleic anhydride, gua-
nidine hydrochloride, and Kelcoloids� HVF and LVF
stabilizers (propylene glycol alginate). Bio-nanocom-
posites were prepared from PLA and PHB as matrices.
Nanofibers were only partially dispersed in the poly-
mers and therefore mechanical properties were lower
than those predicted by theoretical calculations.
Although the purpose of nanocellulose modifica-
tion is usually to enhance compatibility with non-polar
polymers, thus to improve mechanical properties,
chemical modification may also add extra functional-
ity to nanocellulosic materials. For example, posi-
tively charged amine-functionalized MFC is reported
to be antimicrobially active in biomedical applications
(Thomas et al. 2005). Andresen et al. (2007) also
added extra functionality to microfibrillated cellulose
film by covalently grafting the cellulose with octa-
decyldimethyl(3-trimethoxysilylpropyl) ammonium
chloride (ODDMAC). The surface-modified MFC
films showed antibacterial activity against both
Gram-positive and Gram-negative bacteria, even at
very low concentrations of antimicrobial agent on the
surface, killing more than 99% of E. coli and S. aureus
when the atomic concentration of ODDMAC nitrogen
on the film surface was 0.14% or higher.
MFC nanocomposites with hydrophobic polymers
Polyethylene and polypropylene
There have been a number of research studies aimed
at dispersing MFC in hydrophobic polymers such as
Cellulose (2010) 17:459–494 481
123
PE and PP with mixed results. Wang and Sain
(2007a, b) applied solid phase melt-mixing followed
by compression molding to produce MFC-reinforced
composites. In their study nanofiber was directly
incorporated (up to 5 wt%) into PE and PP matrix
using a Brabender mixer and an ethylene–acrylic
oligomer emulsion was used as a dispersant. This
work demonstrated that coating MFC fibers with the
dispersant improved the cellulose dispersion in these
polymers. As a consequence, the mechanical proper-
ties of PP and PE nanocomposites were slightly
improved when compared to pure matrices; however,
this enhancement was not considered significant.
Cheng et al. (2007) fabricated PP composites from
filtered mats of regenerated cellulose (Lyocell) fibril
aggregates and PP fibers using compression molding.
Commercial MFC was also used for composite
preparation. Fibrillation of Lyocell fibers was per-
formed by high intensity ultrasonication. SEM
revealed that the fibril aggregates had good distribu-
tion in the thickness of the composites. However,
some porosity in the PP matrix and some gaps
between fibers and PP matrix were observed, sug-
gesting a lack of good adhesion. On the other hand,
tensile test results showed that the fiber-reinforced
composites had higher tensile modulus and strength
than unreinforced PP (Table 4).
Poly(lactide)
Polylactide derived from 100% renewable resources
(e.g. corn, wood residues or other biomass) is of
interest not only because of the need to ultimately
replace many synthetic polymers but also because of
PLA’s useful physical and mechanical characteristics
(Plackett et al. 2006). PLA is one of the few commer-
cially available biopolymers which has similar prop-
erties to those of fossil fuel-based commodity plastics
(Mathew et al. 2005; Ren et al. 2007). The properties of
PLA are for example sometimes compared with those
of polystyrene. In addition to its advantages, PLA also
has some shortcomings which restrict its applications,
such as its inherent brittleness (Baiardo et al. 2003;
Huda et al. 2006, 2008), low thermal stability (Tsuji
and Fukui 2003) and relatively high price (Park and Im
2003). Another disadvantage of PLA is that the gas and
water vapour barrier properties are not sufficient for
some uses (Ljungberg and Wesslen 2002). A detailed
description of PLA synthesis and properties can be
found in the literature (Duda and Penczek 2003; Mehta
et al. 2005; Gupta and Kumar 2007; Singh and Ray
2007).
Several approaches have been proposed for
improving PLA properties, including blending (Lee
et al. 2004; Wu and Liao 2005; Omura et al. 2006;
Harada et al. 2007; Liao and Wu 2008) and
copolymerization (Bigg 2005; Tamura et al. 2006)
Preparation of nanocomposites has also been consid-
ered as a promising method for PLA property
improvement (Okubo et al. 2005; Petersson and
Oksman 2006; Petersson et al. 2007; Sanchez-Garcıa
et al. 2008).
In terms of combining MFC with PLA, Okubo
et al. (2005, 2009) reported an effective technique for
improving the mechanical properties of PLA-based
bamboo fiber composites by adding MFC up to 20
wt%. In this study the MFC, PLA and bamboo fiber
were mixed in water and the dispersion was vacuum
filtered. Composites were fabricated by hot pressing
the dried filtered sheets. Applying three-point bend-
ing and micro-drop tests proved that the mechanical
properties of the composites were significantly
enhanced even at low MFC loadings (B10 wt%).
It has to be emphasized that most of the studies
reported in the literature refer to either solution
casting or vacuum filtering followed by hot pressing
to produce MFC reinforced biocomposites and there
have been only a few trials involving more industri-
ally practical methods. In one such case, Chakraborty
et al. (2007) used a twin-screw Brabender mixer to
disperse cellulose microfibrils (MFC) in water solu-
tion in a PLA matrix and the resulting compound was
then hot pressed at 190 �C. A calcofluor dye was
added to the composite in order to impart fluores-
cence to cellulose and allow assessment of fiber
distribution by laser confocal microscopy. Micro-
scopic images showed uniform dispersion of MFC in
the PLA matrix, indicating the potential of this
preparation method.
In contrast, Mathew et al. (2006) reported a non-
uniform dispersion of cellulose fillers in PLA matrix
when nanocomposites of PLA with 5 wt% cellulose
nanowhiskers and MFC were prepared by twin-screw
extrusion. The reinforcements were fed into the
extrusion process by liquid pumping using water as
the pumping medium. However, this study revealed
that fast evaporation of the water led to reaggregation
of the fillers in the polymer matrix. The generation of
482 Cellulose (2010) 17:459–494
123
microscale agglomerates together with the non-uni-
form dispersion of whiskers and fibers in the matrix
was considered to be the explanation for the poor
mechanical properties obtained.
Fabrication of MFC/PLA nanocomposites based
on a papermaking-like process has been presented as
an industrially practical method (Nakagaito et al.
2009). In this procedure PLA microfibers were mixed
with MFC in an aqueous suspension followed by
dewatering through a sieve consisting of a metal wire
mesh. The obtained sheets were then dried at 105 �C
and eight layers were sandwiched and hot pressed.
The process was characterized by high yields (&88–
99%) and relatively fast dewatering times (13–
100 min) and seemed to deliver uniform nanofiber
dispersion, which resulted in enhanced reinforcement
with increasing MFC content.
Solvent exchange (i.e., shifting from water to a more
apolar organic solvent) has been suggested recently as
a possible way of preparing MFC/PLA nanocompos-
ites. In a study by Iwatake et al. (2008) MFC was
premixed with PLA using acetone and the mixture was
kneaded after the removal of the solvent to attain
uniform dispersion. Indeed, these researchers suc-
ceeded in fabricating a composite sheet with signifi-
cantly more uniform filler distribution compared to
sheets prepared via direct introduction of MFC to the
molten PLA matrix. As a consequence of uniform
distribution the Young’s modulus and tensile strength
of PLA increased by 40 and 25% respectively without a
reduction of yield strain at a fiber content of 10 wt%.
Furthermore, the storage modulus of the composites
was kept constant above the glass transition temper-
ature of the matrix polymer (Iwatake et al. 2008). More
recently, Suryanegara et al. (2009) applied the same
method but then exchanged acetone for dichlorometh-
ane and showed that the resulting MFC-PLA nano-
composites had improved storage modulus when
compared to neat PLA. While Iwatake et al. (2008)
used fully amorphous PLA, which has a low storage
modulus at high temperature, Suryanegara et al. (2009)
demonstrated that crystallization of PLA increases the
storage modulus as well as the strength and Young’s
modulus of PLA/MFC nanocomposites without sig-
nificant reduction in the strain at break. These authors
also noticed that MFC can act as a nucleating agent for
the crystallization of PLA.
Chemical modification of cellulose has also been
explored as a possible route for improving filler
dispersion in a PLA matrix. Wang and Sain (2007c)
used chemically treated cellulose nanofibers extracted
from hemp to prepare PLA and PHB nanocomposites.
Composites containing 5 wt% nanofibers were
prepared by melt blending the polymer with the fiber
followed by granulating and injection molding steps.
The major difficulty was to ensure a uniform
distribution of nanofibers in the matrix. Indeed,
TEM showed that fiber agglomerates were present
in the polymer matrix and nanofibers were only
partially dispersed. This observation was reflected in
the mechanical properties of the composites.
Although there were some improvements in the case
of nanocomposite materials compared to the pure
PLA, these were less significant than expected based
on theoretical calculations.
Juntaro et al. (2007, 2008) developed a technique for
modifying natural fiber (hemp and sisal) surfaces to
improve the interaction between the fibers and poly-
mers by attaching nano-scale BC to the fiber surfaces.
These modified natural fibers were then incorporated
into cellulose acetate butyrate (CAB) and poly-L-lactic
acid (PLLA) polymers. Unidirectional natural fiber-
reinforced composites were manufactured to investi-
gate the impact of the surface modification on the fiber
and the interface-dominated composite properties. It
was reported that both the tensile strength parallel to
and perpendicular to the BC-modified natural fibers
increased significantly, especially in the case of the
PLLA matrix. In the case of modified sisal-reinforced
PLLA, the parallel strength increased by 44% and the
off-axis composite strength by 68%. The explanation
for these improvements was that the presence of the
nanofibers enhanced the interfacial adhesion between
the primary fibers and the polymer. Water absorption
was also reduced by BC grafting onto sisal fibers. In
principle, this approach could be successfully applied
to any natural fibers (hemp, flax, jute), provided their
surfaces are sufficiently hydrophilic (Juntaro et al.
2008).
Poly(caprolactone)
Poly(e- caprolactone) or PCL is an oil-derived biode-
gradable and semicrystalline polyester. PCL has good
water, oil, solvent, and chlorine resistance, a low
melting point, and low viscosity and is easily processed
using conventional melt blending technologies (Gross
and Kalra 2002; Nair and Laurencin 2007). PCL has
Cellulose (2010) 17:459–494 483
123
low tensile strength (approximately 23 MPa) but an
extremely high elongation at break ([700%; Gunatil-
lake et al. 2008). MFC could be a potential candidate
for reinforcing PCL; however, due to the hydrophobic
nature of PCL, good dispersion of MFC in the matrix
requires some modification of the cellulose. Grafting,
for example, has been reported as a promising route for
compatibility enhancement.
In a study by Lonnberg et al. (2008) MFC was
successfully grafted with different molecular weights
of PCL in order to improve compatibility with a PCL
matrix. The PCL-grafted MFC films were hot-pressed
together with a PCL film to produce a laminate and
the interfacial adhesion was tested using DMA. The
laminates consisting of PCL-grafted MFC films
showed a significant improvement of the interfacial
adhesion when compared with laminates incorporat-
ing ungrafted MFC films. More recently, Siqueira
et al. (2009) applied N-octadecyl isocyanate as a
grafting agent for modifying the surface of two types
of cellulose nanofillers. In this study the reinforcing
capacity of nanowhiskers and MFC in PCL matrix
was compared. Substantial differences were observed
depending on the nature of the nanocellulosic filler.
These differences were mainly attributed to the fact
that, unlike the rod-like whiskers, MFC is capable of
forming an entangled network. In particular, grafted
MFC-reinforced PCL composites possessed higher
modulus and lower elongation at break at a given
loading level compared to cellulose whisker-rein-
forced nanocomposites.
Polyelectrolyte multilayers
The fabrication of polyelectrolyte multilayers (PEMs)
via consecutive addition of oppositely charged poly-
electrolytes is considered a useful tool for assembly of
nanocomposite materials (Decher and Schlenoff
2003). Wagberg et al. (2008), for example, success-
fully prepared polyelectrolyte multilayers (PEMs) by
combining different types of polyelectrolytes and
carboxylated MFC. PEMs were made by layer-by-
layer deposition either by dipping or by a spray coating
method. In this study, silicon oxide surfaces were first
treated with cationic polyelectrolytes before the sur-
faces were exposed to MFC. The build-up of the layers
was monitored with ellipsometry and it was shown that
it is possible to form very well-defined layers by
combinations of MFC, different types of
polyelectrolytes and different solution ionic strengths
during the adsorption of the polyelectrolyte. It was also
found that the PEMs were basically compact films of
cellulose with some cationic polyelectrolyte mixed/
intercalated between the fibrils.
Utsel et al. (2008) utilized MFC as an anionic
component for building up a thermoresponsive PEM
using a layer-by-layer technique. The cationic com-
ponent was synthesized from N-isopropylacrylamide
(NIPAAm) and from (3-acrylamidopropyl)trimethy-
lammonium chloride (APTAC) by atom transfer
radical polymerization (ATRP), resulting in a new
kind of thermoresponsive polyelectrolyte. The use of
a quartz crystal microbalance with dissipation mon-
itoring (QCM-D) revealed that the viscoelastic prop-
erties of the polymers adsorbed on surfaces changed
linearly with temperature in the interval 24–40 �C,
both for a single layer and for a multilayer with MFC.
In a recent study (Salmi et al. 2009) nanofibrillar
cellulose (i.e., MFC) prepared from bleached sulfite
pulp was combined with cationic polyacrylamide (C-
PAM) in order to prepare PEMs.
Patent literature
In the following section existing patents on the
manufacturing of microfibrillated cellulose and its
use as reinforcement in composites are briefly
reviewed. Other applications of MFC such as absor-
bent sheets, filtering materials or rheology modifiers
for food and cosmetics are not discussed. Results
given in the examples are included here to the extent
that they are considered relevant for this review.
Preparation of microfibrillated cellulose
MFC preparation methods were first patented in the
1980s by Herrick (1984) and by Turbak et al. (1985).
Since then, numerous patents on preparation of MFC
from different cellulose sources have appeared.
A wet grinding of pretreated cellulose pulp in a
vibration mill was used to prepare microfibrillated
cellulose fibers by Ishikawa et al. (1994). The pre-
treatment consisted of treating the pulp with
enzymes, alkali or acid. In the patent of Takami
(1999), MFC was manufactured by dispersing cellu-
lose fibers, in particular bleached kraft pulp, in a
30:70 ethanol:H2O mixture and finally grinding the
484 Cellulose (2010) 17:459–494
123
dispersions. Suzuki and Hattori (2004) patented a
method in wich a cellulose pulp having 1–6% solid
content was repeatedly treated in a disk refiner in
order to generate MFC. The authors claimed that their
method allows the production of high quality MFC
with good stability and efficiency.
Vegetable pulps, especially sugar beet pulp, were
utilized in a French patent to produce MFC via
homogenization followed by passing the cellulose
suspension through a small diameter orifice (Dinand
et al. 1996b). Taniguchi (2003) utilized natural
cellulose fibers derived from different sources (such
as cotton, hemp, wood pulp, seaweed, cereals, sea
squirts, bacteria, etc.) for the production of MFC by
fibrillating raw materials between rotating twin disks
while adding shear stress in the vertical direction of
fiber long axes. MFC was also successfully prepared
from pseudostem, leaves and inflorescence axis of
plantain plants (Musa paradisiacal) and banana
plants (Musa sapientum) as reported in the patent of
Mathew et al. (2007). Sain and Bhatnagar (2008)
invented a method to obtain MFC from renewable
feedstocks such as natural fibers and root crops via
cryocrushing of the pre-treated pulp with liquid
nitrogen. The pulp was hydrolyzed at a moderate
temperature of 50–90 �C and then both acidic and
alkali extractions were performed. More recently,
stems and leaf flowers of water hyacinth (Eichornia
crassipes) were reported as possible sources for the
production of MFC in the patent of Mathew et al.
(2009).
Several patents report the application of enzyme
pretreatments in order to ease the disintegration of
cellulose. As an example, Lindstrom et al. (2007)
utilized one or more wood degrading enzymes at a
relatively low dosage and the pretreated pulp was
then subjected to high shear homogenization. Other
patents applying enzyme pretreatments include those
of Ishikawa et al. (1994), Shibuya and Hayashi
(2008), or Yano et al. (2008). In the latter patent
microfibrillation was completed through ultrasonic
homogenization. Ultrasonic homogenization was also
applied by Isogai et al. (2008) when TEMPO-
oxidized pulp was subjected to microfibrillation.
MFC nanocomposites
The use of MFC to strengthen paper is the subject of
several patents (Anonymous 1983a, b; Katsura 1988;
Matsuda et al. 2001). However improvement of
barrier properties of paper is also a main target. Koga
(2000) patented a method which is aimed at fabri-
cating a gas barrier and moisture-resistant paper
laminate containing one or more layers of MFC. The
laminate is claimed to be useful for packaging of
food, medical and electronic parts.
A number of patents cover the use of MFC as a
reinforcement in resins. For example, Hayashi and
Shimo (2006) reported the use of MFC in phenolic
resin for the preparation of car fenders. Kitagawa and
Yano (2008) mixed MFC with resin powders with an
average particle size of 1–1,000 lm in order to obtain
molded products with high strength. Horiuchi et al.
(2008) patented a low cost method for preparing a
phenolic resin containing a large amount of cellulose
nanofibers as reinforcing material. In this patent a roll
kneading machine and a Banbury mixer are utilised
to directly knead the resin/nanofiber mixture.
In other patents, MFC has been subjected to
surface treatment followed by composite fabrication.
Yano and Ifuku (2008) utilized 3-aminopropyl-
triethoxysilane for surface treatment of cellulose
fibers, which were immersed in ethoxylated polypro-
pylene glycol 700 dimethacrylate and irradiated with
UV light to obtain a test piece with high modulus.
Nakahara (2008) grafted MFC with either PVOH or
PLA, followed by dewatering and kneading with
PLA. The resulting materials were then compression
molded. The product possessed flexural modulus of
5.1–5.7 GPa, flexural strength of 100–105 MPa and
an impact strength of *26 J m-1. A molded article
with improved tensile properties was produced as
described in a patent of Yano and Nakagaito (2008).
In this case MFC was pretreated with an alkali
solution before blending with a resin. Sumi et al.
(2009) utilized graft polymerization of softwood
cellulose pulp with methyl methacrylate in aqueous
media and the pulp was then fibrillated and mixed
with poly(methyl methacrylate) resin to obtain a high
strength, highly transparent composite. Miyazaki
et al. (2008) fabricated a cellulose nanofiber-rein-
forced resin composite with uniform thermal con-
ductivity as a result of coating the fibers with
aluminium triisopropoxide.
In a Japanese patent from 1993, MFC-containing
biodegradable films and sheets were cast from
slurries composed of MFC, chitosan, diglycerol and
water. The resulting films were flexible and
Cellulose (2010) 17:459–494 485
123
translucent (Nishama et al. 1993). There are very few
patents on combinations of MFC and hydrophobic
polymers. Nakahara et al. (2008) suggested an
interesting method to overcome the problem of
MFC-hydrophobic polymer incompatibility. These
authors simultaneously cut conifer kraft pulp fibers
and PLA fibers into 1 mm length and dispersed them
in water. The mixture was then treated in a refiner
and a homogenizer to achieve the desired nanoscale
structure, followed by filtering, pulverizing and
finally injection molding. The resulting test piece
showed a flexural modulus of 5.5 GPa and a bending
strength of 60 MPa. Hashiba (2009) succeeded in
fabricating a thermoplastic resin composite by mix-
ing an aqueous dispersion of MFC with PLA resin in
an agitator. The mixture was then pulverized and
injection molded to give a test piece with high
flexural strength and modulus (110 MPa and 4 GPa,
respectively).
The patent examples cited above give an insight
into the many potential applications of MFC. In
parallel with increasing research activity in this field,
the number of publications and patented applications
should continue to rise.
Concluding remarks
Development of nanocomposites based on nanocell-
ulosic materials is a rather new but rapidly evolving
research area. Cellulose is abundant in nature,
biodegradable and relatively cheap, and is a promis-
ing nano-scale reinforcement material for polymers.
The combination of biodegradable cellulose and
biodegradable, renewable polymers is particularly
attractive from an environmental point of view.
Furthermore, application of nanocellulosic fillers
such as microfibrillated cellulose improves polymer
mechanical properties such as tensile strength and
modulus in a more efficient manner than is achieved
in conventional micro- or macro-composite materials.
Packaging is one area in which nanocellulose-
reinforced polymer films could be of interest and,
as shown, it is possible to produce such films with
high transparency and with improved oxygen barrier
properties. High oxygen barrier is often a requirement
for food and pharmaceutical packaging applications
and such improvement may be a key for capturing
new markets. Besides packaging, the electronic
device industry could also profit by using MFC in
the future. The low thermal expansion of nanocell-
ulosics combined with high strength, high modulus
and transparency make them a potential reinforcing
material in roll-to-roll technologies (e.g., for fabri-
cating flexible displays, solar cells, electronic paper,
panel sensors and actuators). The high number of
reactive hydroxyl groups on the surface of cellulose
also provides the possibility for fabricating a wide
range of functionalized MFC-based materials for
future advanced applications.
Although there have been many promising
achievements at laboratory or pilot scale, there are
several challenges to solve in order to be able to
produce cellulose-based nanocomposites at the indus-
trial scale. A major obstacle which needs to be
overcome for successful commercialization of MFC
is the high energy consumption connected to the
mechanical disintegration of the fibers into nanofi-
bers, often involving several passes through the
disintegration device. However, by combining the
mechanical treatment with certain pre-treatments
(e.g., chemical or enzyme treatments) researchers
have shown that it should be possible to decrease
energy consumption significantly.
In order to achieve improved mechanical proper-
ties in polymer nanocomposites, good filler-matrix
interaction is essential. Due to compatibility prob-
lems of nanocellulosic materials and hydrophobic
matrices, it can be anticipated that nanocomposites
based on hydrophilic matrix polymers will be easier
to commercialize. The improvement of compatibility
with apolar materials, on the other hand, requires
chemical modification of nanocelluloses. Although a
number of studies have been aimed at chemical
modification of nanocellulose (e.g., MFC), there is as
yet no industrially practical way to produce cellulose
nanocomposites based on hydrophobic biopolymers.
Consequently, more research targeting novel, envi-
ronmentally-friendly methods of modification, as
well as an understanding of the mechanism of
reactions occurring at the cellulose nanofiber polymer
matrix interface, is now required.
Acknowledgment The authors would like to express their
gratitude for the assistance of Vimal Katiyar (Risø-DTU) with
TEM imaging, as illustrated in Fig. 3. Funding provided by the
Danish Research Council for Technology and Production
Sciences to support the research of Istvan Siro is hereby
gratefully acknowledged.
486 Cellulose (2010) 17:459–494
123
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