Strength and barrier properties of MFC films

Kristin Syverud Æ Per Stenius

Received: 11 March 2008 / Accepted: 14 July 2008 / Published online: 19 August 2008

� Springer Science+Business Media B.V. 2008

Abstract The preparation of microfibrillar cellulose

(MFC) films by filtration on a polyamide filter cloth, in a

dynamic sheet former and as a surface layer on base paper

is described. Experimental evidence of the high tensile

strength, density and elongation of films formed by MFC

isgiven.Typically,aMFCfilmwithbasisweight35 g/m2

had tensile index 146 ± 18 Nm/g and elongation

8.6 ± 1.6%. The E modulus (17.5 ± 1.0 GPa) of a film

composed of randomly oriented fibrils was comparable to

values for cellulose fibres with a fibril angle of 50�. The

strength of the films formed in the dynamic sheet former

wascomparable to thestrengthof theMFCfilmsprepared

by filtration. The use of MFC as surface layer (0–8% of

total basis weight) on base paper increased the strength of

the paper sheets significantly and reduced their air

permeability dramatically. FEG-SEM images indicated

that the MFC layer reduced sheet porosity, i.e. the dense

structure formed by the fibrils resulted in superior barrier

properties. Oxygen transmission rates (OTR) as low as

17 ml m-2 day-1 were obtained for films prepared from

pure MFC. This result fulfils the requirements for oxygen

transmission rate in modified atmosphere packaging.

Keywords MFC � Strength � Barrier � MFC films �Microfibrillar cellulose � Nano cellulose � Paper

Introduction

The production of microfibrillar cellulose (MFC

hereafter) from wood fibres was demonstrated by

Turbak et al. (1983) in the early 1980s. The smaller

dimensions and the large surface area of MFC

compared to fibres open up for new, perhaps unfore-

seen possibilities to utilise cellulose-based materials.

However, so far profitable production of MFC has

been restricted by high energy consumption, difficul-

ties with up-scaling and runnability problems. During

the last years research within this area has increased

rapidly and breakthroughs in the production methods

are expected (see e.g. Paakko et al. 2007; Abe et al.

2007).

It has become relatively easy to produce MFC in

laboratory scale, which offers the possibility to

perform more extensive research with MFC. Several

recent publications demonstrate how MFC can be

utilized for various purposes, e.g. in nanocomposites

(Nakagaito and Yano 2005; Bruce et al. 2005;

Malainine et al. 2005; Lopez-Rubio et al. 2007), as

dispersion stabilizers (Oza and Frank 1986; Ougiya

et al. 1997; Khopade and Jain 1990; Andresen and

Stenius 2007), and as antimicrobial films (Andresen

et al. 2007). However, little has been reported about

K. Syverud (&)

Paper and Fibre Research Institute (PFI), Høgskoleringen

6b, NO-7491 Trondheim, Norway

e-mail: kristin.syverud@pfi.no

P. Stenius

Ugelstad Laboratory, Department of Chemical

Engineering, NTNU, Høgskoleringen 6b, NO-7491

Trondheim, Norway

123

Cellulose (2009) 16:75–85

DOI 10.1007/s10570-008-9244-2

the use of MFC in paper applications or the properties

of pure MFC films.

The decomposition of wood fibres into micro- or

nanofibrillar cellulose gives a material which retains

many of the advantageous properties of cellulose

fibres, such as the ability to form hydrogen bonds,

resulting in a strong network. Taniguchi (1998)

reports that the tensile strength of a MFC film from

wood pulp is approximately 2.5 times higher than the

tensile strength of a printing paper. The type of

printing paper or the basis weight of films and paper

are not specified. Production of MFC films is also

described by Dufresne et al. (1998). They use MFC

produced from sugar beet pulps and study the

influence of pectins on mechanical properties. Hen-

riksson and Berglund (2007) report an E modulus of

14 GPa for pure MFC films and E = 16.6 GPa for

composite films of MFC and melamine formaldehyde

(MF) containing 9% MF.

Petrochemically based polymers predominate in

packaging of foods due to their ease of processing,

excellent water barrier properties and low cost.

However, there is an increasing interest in replacing

synthetic polymers with more sustainable materials.

The barrier properties of plastics are low in compar-

ison to traditional packaging materials, such as glass

and hermetics. Hence, polymer films with low

permeability to low molecular gases, such as O2

and CO2, have been developed. Such polymers are

used in connection with modified atmosphere pack-

aging (MAP), where a controlled atmosphere within

the packaging is desired (Ackermann et al. 1997).

Large efforts have been spent to improve the barrier

properties of commonly used plastics such as poly-

ethylene (PE). The crystallinity of the polymers is of

large importance for the barrier properties of the

polymers (Lagaron et al. 2004).

The MFC fibrils have dimensions in the nano-

scale, ranging upwards from less than 10 nm in

diameter. It is well known that it is difficult for other

molecules to penetrate the crystalline parts of cellu-

lose fibrils. These properties, in combination with the

ability of the dried fibrils to form a dense network

held together by strong inter-fibrillar bonds, suggest

that due to the barrier and strength properties of the

films, MFC may be an interesting alternative to e.g.

plastics

In the present paper focus is set on the strength and

barrier properties of MFC films prepared from

microfibrils from wood cellulose fibres. The use of

pure MFC and MFC as a layer on base paper in order

to increase barrier properties are demonstrated.

Experimental

Raw materials

Microfibrillar cellulose (MFC) was prepared by

disintegration and homogenization of fully bleached

spruce sulphite pulp from Borregaard ChemCell.

Pulp fibres were cut to an average length of approx.

1 mm and diluted to 1% consistency before homog-

eniziation in a Gaulin M12 homogenizer with a

pressure drop of 600 bar at each pass. Details of the

procedure are described elsewhere (Andresen et al.

2006).

The substrate (base) paper was prepared from fully

bleached softwood kraft pulp from Sodra Cell, Tofte,

Norway.

Preparation of test specimens

Three series of samples were prepared; 1) films of

MFC having no orientation, 2) films of oriented MFC

prepared in a dynamic sheet former and 3) layers of

MFC on substrate base paper, prepared by spraying of

MFC suspensions on the paper, using the dynamic

sheet former.

(1) MFC films were prepared by from an approx-

imately 0.1% MFC suspension poured into a

cylindrical mould. The bottom of the cylinder was a

layered structure consisting of polyamide filter cloth,

coarseness 235 mesh (top), a filter paper (middle) and

a supporting Cu wire (bottom). The water was

removed by free suction through the polyamide film

into the filter paper and drainage through the bottom

as well as evaporation from the top. The mould was

6 cm in diameter, allowing for preparation of test

pieces with dimensions 1.5 9 5 cm. The film thick-

nesses were 20–33 lm, corresponding to basis

weights between 15–30 g/m2 The films were dried

by evaporation at room temperature. They could be

easily removed from the polyamide filter, without

visible remnants of the MFC in the cloth.

(2) Films of oriented MFC were prepared using a

dynamic sheet former (FiberTech, Sweden) and two

wires with coarseness 125 and 250 mesh. The

76 Cellulose (2009) 16:75–85

123

headbox consistency was 0.06%. Using the 125 mesh

wire the wire loss was 20%. The basis weights of the

two types of films thus produced were 24 and

16 g/m2. They were pressed (1 bar), using blotting

paper on top of the film during pressing. The films

were dried on a drum drier at 50 �C. The wire was not

removed until after drying.

(3) MFC as a surface layer on paper. Oriented

sheets were prepared using a dynamic sheet former

(FiberTech, Sweden). The base paper was made from

unbeaten softwood pulp. MFC was deposited on the

top of the wet base paper in the dynamic sheet

former. The top layer and the base paper were thus

combined wet in wet. The extent to which this

resulted in mixing of the MFC with fibres was not

investigated, but it has later been shown for films

prepared in the same way (Eriksen et al. 2008) that

the mixing is negligible. The total basis weight of the

sheets was in all cases 90 g/m2. The basis weights of

the top layers were varied from 2 to 8 g/m2, i.e. the

basis weight of the base paper varied between 88 and

82 g/m2. A reference sheet without any coating was

also prepared.

Oriented sheets coated with surface modified MFC

were prepared using the same procedure. The MFC

was in this case modified by grafting of amine (bis-

(3-aminopropyl)amine) on MFC surfaces modified

with isocyanate, giving positively charged MFC, or

of succinic anhydride, resulting in a higher negative

charge of the fibres by introduction of acid groups.

Detailed descriptions of the modification procedures

are given by Stenstad et al. (2008).

The sheets were pressed and dried according to a

standardised procedure (ISO 5269-1:1998). The wire

side of the paper was in contact with a drying plate

while the top side with the MFC layer was covered

with blotting paper in the same way as in the

preparation of oriented MFC sheets.

Characterization

The test specimens were analysed by measuring

tensile strength and elongation (ISO 1924-2:1994),

grammage (ISO 536:1995), thickness and density

(ISO 534:1988). Six replicates were used for these

measurements. Air permeability was assessed accord-

ing to ISO 5636-5:2003 using three replicates. The

tensile strength and elongation were measured with a

Zwick material tester (T1-FRxxMOD.A1K, serial no.

119249, model 2005). Conditions in the test room

were: relative humidity 50%, temperature 23 �C. The

E-modulus was calculated from the slope of a linear

regression of the steepest part of the stress-strain

curve of the MFC-films.

The oxygen permeability was measured according

to the ASTM standard D3985 (23 �C, 0% RH on the

top side, 50% RH on the bottom side). The MFC

films were mounted in a cell where 100% O2 was

flushed on the top side and 100% N2 on the bottom

side. The amount of O2 transferred through the films

was assessed by a Mocon Coulox oxygen sensor in

the N2 gas flow. Two replicates were measured for

each sample.

Surface characteristics were assessed by field-

emission scanning electron microscopy, FEG-SEM.

Platinum-coated samples were investigated with a

Zeiss Gemini Supra 55VP FEG-SEM instrument. The

images were acquired with a lateral secondary

electron (SE) detector at 600009 magnification.

The Inlens SE-detector capability of the FEG-SEM

was used to obtain high resolution images at

1700009 magnification.

Results

Strength properties

The strength properties and permeability of the

random MFC films, the oriented MFC films and the

sheets with layers of MFC are shown in Tables 1–3

respectively.

In order to facilitate comparison of results for the

different types of sheets, selected results are also

plotted in Figs. 1–7.

The tensile indices of MFC films, oriented MFC

sheets and the paper sheets coated with MFC layers

are compared in Fig. 1. The tensile indices of the

MFC films and sheets were considerably higher than

typical tensile indices for paper, which are in the

range of 10–100 kNm/kg, depending on the type of

paper (Fellers and Norman 1996). The difference was

not just a result of differences in densities, as seen

Fig. 2, which shows the tensile indices of MFC films

and sheets as a function of their densities. Although

the densities of the oriented MFC films were much

lower than those of the films of randomly dispersed

MFC, the tensile indices were not much different.

Cellulose (2009) 16:75–85 77

123

The thickness of the MFC films ranged from 21 to

33 lm. The diameter of a typical Norwegian spruce

fibre (Picea abies) in wood is 30 ± 10 lm (Fengel

and Grosser 1976). Thus, the thickness of the MFC

films was in the same range as the thickness of a

single fibre.

The elongation of the MFC films ranged from 5.3

to 8.6% (Fig. 3), increasing with basis weight. The

low elongation of the oriented sheet prepared on a

125 mesh wire was probably due to the removal of

fine material from the suspension (20%). The elon-

gation of the paper sheets, whether coated with MFC

or not, was lower than that of the MFC films, but

increased when the thickness of the coating

increased.

Figure 4 shows the E-modulus of the MFC films

and oriented sheets. The E values increased contin-

ually as the basis weight increased. The E moduli of

the oriented sheets were much lower than those of the

films. This may be due to the use of thickness values

in the calculations. If the densities are underestimated

this will also give low values of the E modulus.

Figure 5 shows the tensile stiffness of the same

samples. In this case the data are normalized by the

amount of material. The tensile stiffnesses are in the

same range for all MFC films.

Finally, Fig. 6 shows the tensile index of the

coated base paper as a function of the basis weight of

the MFC layer. An increase in tensile index in both

MD and CD as the thickness of the MFC layer

increases was observed. The figure also shows results

for sheets coated with films of cationically and

anionically modified MFC. There were no differences

in strengths between the samples coated with pure

MFC and to those with increased charges, whether

positive or negative.

Table 1 Strength and permeability of MFC films prepared by free drying

Sample

code

Basis weight

(g/m2)

Thickness

(lm)

Density

(kg/m3)

Tensile index

(Nm/g)

Tensile strength

(MPa)

Elongation

(%)

E modulus

(GPa)

Air permeability

(nm Pa-1 s-1)

A 17 ± 1 21 ± 1 811 ± 47 129 ± 16 104 5.3 ± 1.0 15.7 ± 1.3 13 ± 2

B 23 ± 1 23 ± 1 878 ± 24 126 ± 23 126 5.4 ± 1.5 16.7 ± 0.7 9 ± 2

C 30 ± 1 30 ± 1 974 ± 42 136 ± 14 136 8.0 ± 0.8 16.5 ± 0.2 11

D 35 ± 3 33 ± 2 1069 ± 70 146 ± 18 154 8.6 ± 1.6 17.5 ± 1.0 10 ± 1

Table 2 Strength and elongation of oriented films of MF

Sample code Wire

mesh

Basis weight

(g/m2)

Thickness

(lm)

Density

(kg/m3)

E modulus

(GPa)

Tensile index

(Nm/g)

Elongation (%)

MD CD MD CD

E 250 16 ± 1 32 ± 3 &500 7.1 ± 0.8 135 ± 28 102 ± 9 6.0 ± 1.3 6.9 ± 1.3

F 125 24 ± 1 84 ± 8 &300 6.1 ± 0.2 117 ± 8 84 ± 3 3.6 ± 0.5 4.0 ± 0.2

MD = Machine direction, CD = Cross direction. Values of the E modulus are in the MD

Table 3 Strength and permeability of paper sheets with MFC layers

Sample code Grammage

(g/m2)

Thickness (lm) Density (kg/m3) Tensile index

(Nm/g)

Elongation (%) Air permeability

(nm Pa-1 s-1)

Total MFC MD CD MD CD

G 88.9 0 150 ± 5 593 35 ± 1 18 ± 1 2.0 ± 0.9 – (6.5 ± 0.9) � 104

H 89.0 2 167 ± 5 532 33 ± 1 20 ± 1 2.4 ± 0.2 1.4 ± 1.1 (3.3 ± 1.6) � 104

I 91.3 4 171 ± 9 535 37 ± 2 23 ± 1 2.5 ± 0.1 2.5 ± 0.2 (2.6 ± 2.0) � 103

J 89.6 8 163 ± 6 549 40 ± 2 23 ± 1 2.6 ± 0.1 2.8 ± 0.2 360 ± 30

MD = Machine direction, CD = Cross direction

78 Cellulose (2009) 16:75–85

123

Barrier properties

The air permeability for base paper (the reference),

MFC coated base paper and MFC films are given in

Tables 1, 3 and Fig. 7. The air permeability

decreased upon coating with MFC and was further

reduced dramatically as the thickness of the MFC

layer increased. A constant level of about 10 nm/Pa s

was obtained for the MFC films.

Table 4 shows the oxygen transmission rate for

two MFC films compared with literature values for

various synthetic polymers.

0

20

40

60

80

100

120

140

160

180

0 5 10 15 20 25 30 35 40

Basis weight, g/m2

Ten

sile

inde

x, N

m/g

Fig. 1 Tensile index as a function of grammage. j: MFC

films, u: oriented MFC film, MD direction, 250 mesh, d:

oriented MFC film, MD direction, 125 mesh, s: tensile index

of paper sheets coated with unmodified MFC, total grammage

90 ± 1 g/m2, as a function of the grammage of the MFC

coating, m: Reference paper sheet

60

80

100

120

140

160

180

0 0,2 0,4 0,6 0,8 1 1,2

Density, g/cm3

Ten

sile

ind

ex, N

m/g

Fig. 2 Tensile index as a function of sheet density. j: MFC

films, u: Oriented MFC film, MD direction, 250 mesh, d:

Oriented MFC film, MD direction, 125 mesh

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40

Grammage, g/m2

Elo

ng

atio

n, %

Fig. 3 Elongation as a function of MFC grammage. d: MFC

films; j: oriented MFC sheet, MD, 250 mesh; h: oriented MFC

sheet, MD, 125 mesh; s: paper sheets coated by 2–8 g/m2 MFC

film, total grammage 90 ± 1 g/m2, u: reference sheet

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40

Grammage, g/m2

E-m

od

ulu

s, G

Pa

Fig. 4 E-modulus as a function of grammage of MFC films

prepared by filtration (j); oriented MFC films, 250 mesh,

MD direction (u); oriented MFC films, 125 mesh, MD

direction (d)

Cellulose (2009) 16:75–85 79

123

Discussion

Strength properties

Tensile indices

As noted above, the strengths of the MFC films

(tensile index 129–146 Nm/g) are much higher than

those of paper. The tensile indices of paper usually

range from 10 to 100 kNm/kg (Fellers and Norman

1996). The tensile strengths of the MFC films (104–

154 MPa) are comparable to the tensile strength of

cellophane (125 MPa longitudinal, 75 MPa transver-

sal) (Fink et al. 2001), but the E-moduli of the MFC

films are much higher (15.7–17.5 GPa vs. 3.7–

5.4 GPa). This may be due to the higher stiffness of

the crystalline cellulose fibrils in the MFC films,

compared to the amorphous structure of the cellulose

in the cellophane films.

The densities of the films of randomly oriented

MFC range from 0.8 to 1,1 g/cm3, but the densities of

the oriented films, in particular the one prepared with

a 125 mesh wire appear to be very low without any

marked effect on the strength which remained

constant within experimental error. For the 125 mesh

sheet the wire loss was 20%. The material that was

lost had probably very small dimensions, leaving a

coarser material in the resulting films. This may

explain of the much lower densities of these sheets.

The densities of the oriented films may be underes-

timated if the surface roughness is large. Large

surface roughness, due to e.g. wire mark, will lead to

too high thickness values and thus underestimated

densities. In any case, based on the results, the finest

material does not seem to have played any important

role for the strength properties.

The thickness of the MFC films was in the same

range as the thickness of a single fibre. In view of

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40

Basis weight (g/m2)

Ten

sile

stif

fnes

s (M

Nm

/kg)

Fig. 5 Tensile stiffness as a function of basis weight of MFC

films (m), oriented sheet, 250 mesh (j) and oriented sheet, 125

mesh (s)

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10

MFC layer (g/m2)

Ten

sile

inde

x (k

Nm

/kg)

Fig. 6 Tensile index of oriented base paper coated with

modified MFC as a function of the basis weight of the MFC

layer. u: MD, reference, j: MD, unmodified MFC, m: MD,

MFC modified with amine (cationic), d: MD, MFC modified

with succinic anhydride (anionic), e: CD, reference, h: CD,

unmodified MFC, D: CD, MFC modified with amine (cationic).

s: CD, MFC modified with succinic anhydride (anionic). Note

that some points overlap

1

10

100

1000

10000

100000

0 10 20 30 40

MFC basis weight (g/m2)

Airp

erm

. (nm

/Pa

s)

Fig. 7 Air permeabilities of the base paper (reference) (9),

MFC coated base paper, total grammage 90 ± 1 g/m2 (m) and

MFC films (u)

80 Cellulose (2009) 16:75–85

123

this, the tensile index values must be considered

remarkably high.

The elongations of the MFC films were high,

ranging from 5.3 to 8.6%. The films were free dried,

which normally gives higher values for elongation

compared to restricted drying. However, the elonga-

tion values for the oriented MFC film prepared on

125 mesh wire under restricted conditions was 6.0%

in MD and 6.9 in CD. The oriented film produced

with the coarser wire (sample F) had lower values,

3.6 and 4.0% in MD and CD respectively. According

to Dodson and Herdman (1982) the elongation is a

linear function of the shrinkage and the value of

restricted drying (zero shrinkage) is independent of

the manufacture process and orientation of the sheets.

In addition, the elastic modulus, and thus also the

elongation, depends on the basis weight up to a level

of approx. 45 g/m2 for paper (Alava and Niskanen

2006). Thus, only samples with equal basis weight

can be compared. Based on this, the elongations of

samples E and F can be compared to samples A and B

respectively, where sample E and F represent the

elongation at no shrinkage while A and B correspond

to films with free drying. The values are however

quite similar except for the MFC sheet produced with

the coarser wire. As noted above, the fibrils with the

smallest dimensions were probably lost during the

sheet forming with this wire. Thus, it seems that the

finest material played a role in the elongation.

The relative importance of factors determining the

elongation in a MFC film may be different from

paper. A paper sheet has large porosity; in the fibre

wall due to the lumen and between the fibres because

they form a network. Each fibril in MFC is solid

without any pores. Their ability to swell and shrink is

therefore limited.

The elongation of cellophane films is reported to

be 25–75% (Fink et al. 2001), which is considerably

higher than that of MFC films. However, these values

are not directly comparable due to the different ways

of preparation of the films.

E-modulus

Figure 4 shows that the elastic modulus of MFC films

increased continually as the basis weight increased,

never reaching a constant level. According to Alva and

Niskanen (2006) the value of E of paper is usually

constant from basis weight 45 g/m2 and upwards. The

theoretical E-modulus of cellulose fibres as a function

of fibril angle was calculated and compared to exper-

imental results by Page et al. (1977). The theoretical E

modulus was 80 GPa at zero fibril angle and decreased

to 17–18 GPa at a fibril angle of 50�. The experimental

values were a little lower. The MFC films are

composed of fibrils that are randomly oriented. The

obtained values, 15.7–17.5 GPa, are in the same range

as for fibres with low fibril angle. According to Cox

(1952) the E-modulus of a random network of ideal

straight and infinite fibres is one third of the E-modulus

of the individual fibres. The maximal theoretical value

is according to this 1/3 � 80 & 27 GPa. For fibril

Table 4 Permeabilities of MFC films and literature values for films of synthetic polymers and cellophane

Sample Grammage

(g/m2)

Thickness

(lm)

Air permeability

(nm/Pa s)

Oxygen permeability

in the material

(ml m-2 day-1)

MFC film A 17 ± 1 21 ± 1 13 ± 2 17.0, 18.5

MFC film C 29 ± 1 30 ± 1 11 ± 3 17.0, 17.0

Polyester, oriented – 25 – 50–130a

Polyester, oriented,

PVdC coated

– 25 – 9–15a

EVOH – 25 – 3–5a

Polyethylene LD – 25 – 7800a

Polyethylene HD – 25 – 2600a

Cellophane 21 – 3b

Two measurements of oxygen permeability were made for each MFC sample; both results are given in the tablea Parry (1993)b Kjellgren and Engstrom (2006)

Cellulose (2009) 16:75–85 81

123

networks that deviate from the ideal situation the

values will be lower. For paper the values are typically

5 GPa (Alava and Niskanen 2006).

Surface layer of MFC on base paper

Comparison of the tensile indexes of the base paper

with the MFC layer and MFC films (Fig. 1) shows

that there is a large difference between the coated

samples and the pure MFC samples. The strength-

increasing potential of the MFC layer seems not to be

fully utilized. An explanation for this may be seen in

Figs. 8 and 9. Figure 8 shows a pure MFC film in two

magnifications. A smooth, continuous film is

observed. Figure 9 shows the base paper without

and with the MFC coating. The fibres in the base

paper are clearly visible through the MFC layer. In

addition, the surface of the MFC layer has signs of

disorder and discontinuities. This will most probably

reduce the strength. By optimizing the technique for

manufacture of coated sheets, it may be possible to

obtain a larger strength increase and utilize the

potential in the MFC layer better.

The elongation of the sheets with a layer of MFC

was about 2.5%, which is lower than elongation of

the pure MFC films, 5.3–8.6% (see Table 3 and

Fig. 3). When the sheets are exposed to a tensile

force, the least extensible part of the sheet will break

first. The whole load must then be carried by the other

parts of the sheet. This will make the strength of a

layered sheet poorer than a linear combination of the

strength of the two layers.

The results showed that layer of anionic or cationic

MFC gave the same strength as unmodified MFC.

This indicates that the hydrogen bonds between the

fibrils give equally strong bonds between MFC fibrils

with increased surface charge.

Barrier properties

The very low air permeability of the films implies

that there were no connected pores through the whole

cross section of the films. It was therefore of interest

to measure the oxygen transmission rate (OTR).

When there are no pores allowing for gas flow

through a material, the gas permeability will depend

on the dissolution of oxygen and its rate of diffusion

in the particular material.

In Table 4 values of the OTR for two MFC films are

compared to literature values of various synthetic

polymers. The values for the MFC films are 17.0

and 17.8 ml m-2 day-1. The recommended OTR for

modified atmosphere packaging is below 10–20 ml

m-2 day-1 (Parry 1993). Indeed, films manufactured

from pure MFC without any additions fulfil this

requirement. Compared to films made from synthetic

polymers and with approximately the same thickness,

the MFC films studied here were competitive with the

best synthetic polymers with respect to oxygen

transmission rate (PVdC coated, oriented polyester

and EVOH). The explanation for the good barrier

properties is probably that low permeability of

cellulose generally is enhanced with the crystalline

structure of the fibrils. Commercial cellophane films

Fig. 8 MFC film in two magnifications

82 Cellulose (2009) 16:75–85

123

with thickness of 20.8 lm are reported to have

OTR & 3 ml m-2 day-1 (Kjellgren and Engstrom

2006). The role of the crystalline structure of plastics

is discussed by Lagaron et al. (2004) where it is

emphasized that high crystallinity improves barrier

properties. A composite material of high density

polyethylene and cellulose showed very good barrier

properties towards oxygen. This was explained by the

presence of impermeable cellulose crystals (Fendler

et al. 2007).

However, it should be noted that permeability

measurements may be influenced by the tendency of

cellulose surfaces to adsorb water molecules. For

example, it has been found that the oxygen perme-

ability of cellophane increases by a factor 20 when

RH increases from 0 to 50% (Krochta et al. 1994).

The effect of RH on permeability of MFC films

would warrant further investigation.

Conclusions

MFC produced from spruce sulphite fibres can be

used to prepare films with high strength properties

and to improve the strength properties of paper by

application of thin layers of MFC. The strength

properties of MFC films are comparable to or higher

than those of cellophane, but the E-modulus of MFC

films is higher. This may be explained by the high

degree of crystallinity of the fibrils in the MFC films.

No significant differences were observed between

native MFC and MFC with increased surface charge,

whether positive or negative. Thus, the inter-fibrillar

hydrogen bonds seem to be of predominating impor-

tance in spite of the additional surface charge.

The E modulus of a film composed of ran-

domly oriented fibrils with thickness in the same

range as the thickness of a single spruce fibre, is

Fig. 9 (Upper left) Base paper. (Upper right and lower images) Base paper coated with 8 g/m2 MFC in three magnifications

Cellulose (2009) 16:75–85 83

123

comparable to values for cellulose fibres with

fibril angle 50�.

The dense structure formed by the fibrils gives

superior barrier properties. MFC films with thick-

nesses of 21–30 lm fulfill the requirements for

oxygen transmission rate in modified atmosphere

packaging and are comparable to the best synthetic

polymers, like PVdC coated oriented polyester.

The strength of base paper increases significantly

upon coverage with a layer of less than 10% MFC,

whether pure or surface modified, and the air perme-

ability decreases dramatically. The reduced surface

porosity induced by fibrils explains the improved

barrier properties. The reduced surface porosity may

also be beneficial for printing properties.

The results indicate that MFC may contribute

to broadening the applicability of cellulose-based

packaging.

Acknowledgements The authors gratefully acknowledge Sodra

Cell, Borregaard Industries, Akzo Nobel and the NANOMAT

programme of the Research Council of Norway for funding. Olav

Solheim (PFI) is thanked for data processing and Øyvind

Gregersen (NTNU) and Øyvind Eriksen for valuable discussions.

Per Olav Johnsen is acknowledged for acquiring the images.

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