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

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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

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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

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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

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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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