Cellulose (2009) 16:75–85
DOI 10.1007/s10570-008-9244-2
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
Ó 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
is given. Typically, a MFC film with basis weight 35 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
was comparable to the strength of the MFC films prepared
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.
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
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 unforeseen possibilities to utilise cellulose-based materials.
However, so far profitable production of MFC has
been restricted by high energy consumption, difficulties 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. Pääkkö 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; López-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
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76
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. Henriksson 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 comparison 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 packaging (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 polyethylene (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 nanoscale, 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 cellulose 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
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Cellulose (2009) 16:75–85
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 homogeniziation 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 Södra 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 approximately 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 thicknesses 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
Cellulose (2009) 16:75–85
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 according 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.
77
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 fieldemission 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.
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Cellulose (2009) 16:75–85
Table 1 Strength and permeability of MFC films prepared by free drying
Sample
code
Basis weight
(g/m2)
Thickness
(lm)
A
17 ± 1
21 ± 1
Density
(kg/m3)
811 ± 47
Tensile index Tensile strength
(Nm/g)
(MPa)
Elongation
(%)
E modulus
(GPa)
Air permeability
(nm Pa-1 s-1)
129 ± 16
5.3 ± 1.0
15.7 ± 1.3
13 ± 2
104
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 (%)
MD
CD
MD
Air permeability
(nm Pa-1 s-1)
Total
MFC
CD
G
88.9
0
150 ± 5
593
35 ± 1
18 ± 1
2.0 ± 0.9
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
–
(6.5 ± 0.9) 104
MD = Machine direction, CD = Cross direction
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 elongation 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 continually as the basis weight increased. The E moduli of
123
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.
Cellulose (2009) 16:75–85
79
180
12
160
10
120
Elongation, %
Tensile index, Nm/g
140
100
80
60
8
6
4
40
20
2
0
0
5
10
15
20
25
30
35
40
Basis weight, g/m2
0
0
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
5
10
15
20
25
30
35
40
Grammage, g/m2
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
180
20
160
18
140
E-modulus, GPa
Tensile index, Nm/g
16
120
100
14
12
10
8
6
4
80
2
0
0
60
0
0,2
0,4
0,6
0,8
1
1,2
Density, g/cm3
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
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
5
10
15
20
25
30
35
40
Grammage, g/m2
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)
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.
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Cellulose (2009) 16:75–85
25
100000
20
10000
Airperm. (nm/Pa s)
Tensile stiffness (MNm/kg)
80
15
10
5
100
10
0
0
5
10
15
20
Basis weight
25
30
35
40
(g/m2)
50
45
40
35
30
25
20
15
10
0
2
4
6
8
10
MFC layer (g/m2)
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
Discussion
Strength properties
Tensile indices
As noted above, the strengths of the MFC films
(tensile index 129–146 Nm/g) are much higher than
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1
0
10
20
30
40
MFC basis weight (g/m2)
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)
Tensile index (kNm/kg)
1000
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)
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 transversal) (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 underestimated 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
Cellulose (2009) 16:75–85
81
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
Polyethylene HD
–
–
25
25
–
–
7800a
2600a
21
–
3b
Cellophane
Two measurements of oxygen permeability were made for each MFC sample; both results are given in the table
a
Parry (1993)
b
Kjellgren and Engström (2006)
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 elongation 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 experimental 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
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82
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 strengthincreasing 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.
Fig. 8 MFC film in two magnifications
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Cellulose (2009) 16:75–85
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
Cellulose (2009) 16:75–85
83
Fig. 9 (Upper left) Base paper. (Upper right and lower images) Base paper coated with 8 g/m2 MFC in three magnifications
with thickness of 20.8 lm are reported to have
OTR & 3 ml m-2 day-1 (Kjellgren and Engström
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 permeability 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 importance in spite of the additional surface charge.
The E modulus of a film composed of randomly oriented fibrils with thickness in the same
range as the thickness of a single spruce fibre, is
123
84
comparable to values for cellulose fibres with
fibril angle 50°.
The dense structure formed by the fibrils gives
superior barrier properties. MFC films with thicknesses 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 permeability 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 Södra
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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