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Holzforschung, Vol. 60, pp. 53–58, 2006 • Copyright by Walter de Gruyter • Berlin • New York. DOI 10.1515/HF.2006.010
Article in press - uncorrected proof
Reinforcing potential of wood pulp-derived microfibres in a
PVA matrix
Ayan Chakraborty
1
, Mohin Sain
2,
* and Mark
Kortschot
1
1
Department of Chemical Engineering and Applied
Chemistry, University of Toronto, Toronto, Ontario,
Canada
2
Faculty of Forestry, University of Toronto, Toronto,
Ontario, Canada
*Corresponding author.
Professor Mohini Sain, Faculty of Forestry, University of
Toronto, 33 Willcocks Street, Toronto, Ontario M5S 3B3,
Canada
Tel.: q1-416-946-3191
Fax: q1-416-978-3834
E-mail: m.sain@utoronto.ca
Abstract
In this study, the reinforcing potential of cellulose ‘‘micro-
fibres’’ obtained from bleached softwood kraft pulp was
demonstrated in a matrix of polyvinyl alcohol (PVA).
Microfibres are defined as fibres of cellulose of 0.1–1
mm
in diameter, with a corresponding minimum length of
5–50
mm. Films cast with these microfibres in PVA
showed a doubling of tensile strength and a 2.5-fold
increase in stiffness with 5% microfibre loading. The
theoretical stiffness of a microfibre was calculated as 69
GPa. The study also demonstrated that the strength of
the composite was greater at 5% microfibre loading
compared to 10% loading. Comparative studies with
microcrystalline cellulose showed that the minimum
aspect ratio of the reinforcing agent is more critical
than its crystallinity in providing reinforcement in the
composite.
Keywords: aspect ratio; cellulose crystallinity; microfibre;
reinforcement.
Introduction
Preparation of biocomposites using cellulose microfibrils
is expanding rapidly in composite science. The produc-
tion of such cellulose microfibrils from different non-
wood sources has been reported over the past decade.
For example, Dufresne et al. (1997) produced microfibrils
by chemically separating the pectin from cellulose chains
and subsequent pressurisation in a laboratory homoge-
nizer. Production of microfibrils by similar methods from
sugar beet has been reported by Dinand et al. (1999).
Similarly, cellulose microfibril suspensions have been
produced from potato tuber cells by alkali hydrolysis, and
the preparation method of a starch-cellulose microfibril
composite has been reported (Dufresne et al. 2000).
Composite films developed via solution casting in poly-
mer matrices such as plasticised starch showed
improved mechanical performance of the composites
compared to the pure polymer, and consequently dem-
onstrated the reinforcing potential of these microfibrils.
The generation of cellulose microfibrils from wood
sources has been relatively restricted. Nevertheless, the
studies of Turbak et al. (1983) and Herrick et al. (1983)
on the generation of ‘‘microfibrillated cellulose’’ (MFC)
from softwood sulfite pulp via high-pressure homogenis-
ing action deserve special mention. The process led to
opening up and unravelling of the fibres to produce a
mesh of smaller fibrils and microfibrils. It did not, how-
ever, lead to the isolation of microfibrils as individual enti-
ties separate from the cell wall. Such MFCs from kraft
pulp were also obtained by Nakagaito and Yano (2004,
2005) by refining the pulp slurry and subsequently pass-
ing it through a high-pressure homogenizer. Incorpora-
tion of these MFCs in a phenol-formaldehyde resin led
to a composite of superior mechanical properties. Micro-
fibrils from wood pulp fibres were obtained by Taniguchi
(1996) in a supergrinder, in which the fibres were broken
apart to diameters in the range of 100 nm and less.
This paper discusses the reinforcing potential of
‘‘microfibres’’ generated from bleached softwood kraft
pulp, as reported by Chakraborty et al. (2005). As
opposed to the microfibrils or ‘‘microfibrillated cellulose’’
discussed above, the microfibres are discrete fibres free
at both ends, and have a distinct aspect ratio (length/
diameter). It is well known in composite science that the
fibres must have a minimum aspect ratio to effectively
transfer stress from the fibres to the matrix, and thereby
impart their full reinforcing potential in composites (Hull
1996; Piggott 2002). The basis of this requirement is
derived from the shear lag theory first enunciated by Cox
(1952). The minimum aspect ratio for which the peak
(central) stress in the fibres closely approaches the max-
imum possible (at which the fibre strain at the centre
approximates the value being imposed on the composite)
is known as the stress transfer aspect ratio, denoted by
s
t
(Hull 1996). If the fibre aspect ratio is below this value,
the matrix strain exceeds that of the fibre throughout the
whole length of the fibre. Therefore, a minimum aspect
ratio of s
t
is required to ensure that the fibre strain
approaches the matrix strain and the fibres transfer the
maximum possible stress to the matrix. Accordingly, a
microfibre is defined in this case as a fibre consisting of
continuous cellulose chains with negligible lignin and
hemicellulose content, and having a diameter between
0.1 and 1
mm, with a minimum aspect ratio of 50, i.e.,
with a corresponding length of 5–50
mm. In the longitu-
dinal direction, it consists of alternating crystalline and
amorphous zones of cellulose. Laterally, however, the
chains are linked together by a combination of hydrogen
bonding, amorphous cellulose molecules, and traces of
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54 A. Chakraborty et al.
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lignin and hemicellulose not removed in the pulping
processes.
In addition to using microfibrils as the reinforcing
agent, there is an increasing trend at present to isolate
pure cellulose crystallites and to use them in preparing
composites with superior mechanical performance. Such
highly crystalline cellulose whiskers have been isolated
from a variety of sources, such as the animal cellulose
tunicin (Favier et al. 1995), chitin (Morin and Dufresne
2002), bacterial cellulose (Grunert and Winter 2002), and
microcrystalline cellulose (Oksman and Mathew, submit-
ted). Reinforcing these high-crystallinity cellulose chains
in a variety of polymers by solution casting demonstrated
improved mechanical properties compared to the base
polymers. However, this increase in crystallinity is often
achieved at the cost of reducing the aspect ratio of the
fibres. Apart from investigating the reinforcing potential
of the microfibres, this paper also investigates the relative
importance of aspect ratio and crystallinity in imparting
superior mechanical properties to composites.
Experimental methods
Microfibres (MFs) were generated in the laboratory from
bleached kraft pulp (BKP) from northern black spruce wood by
a combination of high shear and high impact. The pulp was
obtained from Kimberly-Clark Forest Products Inc., Terrace Bay,
Ontario, Canada. The details of the microfibre generation pro-
cess were described by Chakraborty et al. (2005). In short, BKP
was passed through a PFI mill run for 125 000 revolutions, fol-
lowed by crushing under liquid nitrogen. The microfibres were
isolated by sieving through a 60-mesh screen, and characterised
using different microscopic techniques. As elaborated in the
paper, these fibres had diameters of 1
mm and less, and aspect
ratios in excess of 50. The MF suspension was maintained at a
consistency of 0.1%.
Results obtained with MF were compared to corresponding
results obtained using the source BKP and microcrystalline cel-
lulose (MCC) obtained from Aldrich Chemical Company, Mil-
waukee, USA.
Characterisation of microfibres and MCC
Both the MF sample and the MCC were characterised using
scanning electron microscopy (SEM). The MF sample was also
characterised using atomic force microscopy (AFM).
SEM A Hitachi S2500 SEM was used. A drop of the MF sus-
pension was placed on the SEM stub and allowed to dry before
analysis. On the other hand, MCC samples were placed directly
on the stub in the dry state. Both samples were gold-coated
and a voltage of 10 kV was used during SEM imaging.
AFM A droplet of the 0.1% MF suspension was placed on a
freshly cleaved mica surface and allowed to dry at 808C over-
night. The instrument was run in tapping mode using a Nanos-
cope IIIa Multimode AFM from Veeco, and commercial Si
cantilevers provided by MicroMasch.
Average degree of polymerisation and chain length
ASTM D 1795–96 was used to estimate the average degree of
polymerisation of BKP, MF and MCC. This test gives the value
of intrinsic viscosity, which may be converted to the average
degree of polymerisation (DP) and polymer chain length, as
described below.
A 14-ml aliquot of 0.5 M cupriethylene diamine (CED) solution
was transferred in a 300-size Ostwald viscometer placed in a
water bath at 258C. After 5 min, when the solution had reached
the bath temperature, suction was applied, and the solution was
drawn past the upper mark of the viscometer. The suction was
then released, and the time t
0
required by the meniscus to pass
from the upper mark to the mark below the lower bulb was
recorded.
BKP, MF and MCC were each dissolved in CED at concentra-
tions of 0.4, 0.2 and 0.1 g dl
y
1
. For each of these nine samples,
14 ml of the solution was passed through the viscometer, and
the time t for the solution to pass between the two marks in the
viscometer was noted.
From these experiments, the relative viscosity (h
rel
) for each
sample at each of the three concentrations was obtained from
the ratios of outflow times as follows:
h st/t . (1)
rel 0
There are a number of different equations used to correlate
the viscosity of a non-Newtonian fluid with concentration. One
of these is Martin’s equation (Utracki and Simha 1963; Bird et
al. 1977), which is used in ASTM D 1795–96 to relate the vis-
cosity of the different cellulose solutions in CED with their
concentrations:
lnw(h –1)/cxslnwhxqkwhxc, (2)
rel
where ks0.30 is a constant, and c is the cellulose concentration
(g dl
y
1
) of the solution. A plot of lnw(h
rel
y1)/cx versus c was
extrapolated to the point cs0. The intercept gives the value of
intrinsic viscosity whx in dl g
y
1
. To obtain the average degree of
polymerisation of the cellulose fibres, the value of intrinsic vis-
cosity was multiplied by 190 (as mentioned in ASTM D 1795).
The length of each cellobiose unit is 1.03 nm. A cellobiose mol-
ecule, in turn, is composed of two glucose units. Therefore, the
average cellulose chain length for BKP, MF and MCC was cal-
culated by multiplying the average degree of polymerisation by
1.03/2.
Crystallinity measurement
X-Ray diffraction studies were performed to evaluate the crys-
tallinity of BKP, MF and MCC. A D8 Advance Bruker AXS dif-
fractometer was used, with a Cu point focus source, a graphite
monochromator, and a 2D-area detector GADDS system. Sam-
ples were analysed using transmission mode.
Casting composite films with PVA
Four compositions were prepared for making films: pure PVA,
and PVA composites with BKP, MF and MCC, each at a con-
centration of 5% in the respective composite.
PVA film A 3-g sample of Elvanol PVA obtained from DuPont
Polymer Products, Wilmington, USA, was mixed with 150 ml of
water in a beaker. This beaker was placed on a hot plate, and
the temperature of the water was maintained at 808C. The con-
tents of the beaker were stirred for 30 min until the PVA was
completely dissolved in water. The solution was then gently
poured into a Petri dish, taking care not to entrap any bubbles
in the solution. The dish was kept for 24 h in an oven maintained
at 658C. At the end of the 24-h period, all the water had evap-
orated, leaving behind a thin film of PVA. This film was removed
from the dish and prepared for evaluating mechanical properties,
as described later.
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Reinforcement with microfibres in PVA 55
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Figure 1 Typical SEM images of (a) microfibres and (b) micro-
crystalline cellulose.
Figure 2 (a) SEM image of individual microfibres with high
sample dilution; and (b) AFM image of microfibres.
PVA-BKP composite film Air-dried BKP was placed in2lof
water in a fibre disintegrator, which was rotated for 30 000 rev-
olutions. The resulting fibres were subsequently made into a
suspension of 0.1% consistency. A 150-ml sample of this sus-
pension containing 0.15 g of fibres was poured into a beaker
and 2.85 g of PVA was added. The beaker was kept on a hot
plate so that the solution temperature in the beaker was main-
tained at 808C. From this point onwards, the film casting process
was similar to that used for PVA. The resulting film produced
was a PVA-BKP composite with 5% BKP.
PVA-MF composite film A 150-ml sample of the previously
prepared 0.1% MF suspension in water was poured into a beak-
er. This sample contained 0.15 g of MF. Then 2.85 g of PVA was
added to the suspension. From this point onwards, the method
for preparing the PVA-MF composite with 5% MF was similar to
that for the PVA-BKP composite.
PVA-MCC composite film A 2.85-g sample of PVA was
placed into 150 ml water in a beaker. The PVA was dissolved
using a method similar to that for preparing the pure PVA film.
However, in this case, 0.15 g of MCC in powder form was added
to the water after the PVA was completely dissolved. The PVA
solution containing MCC was stirred for another 5 min before
pouring the suspension in a Petri dish and putting it in an oven,
as for pure PVA.
In addition to the composite films prepared with 5% MF load-
ing, PVA-MF composite films were also prepared with 10%,
15% and 20% MF content. These films were prepared in the
same way as for the composite with 5% MF, except that 1.35 g,
0.85 g and 0.6 g of PVA were added to 150 ml of the MF sus-
pension for films with 10%, 15% and 20% MF content,
respectively.
Evaluation of tensile properties
The mechanical strength and modulus of the films of pure PVA
and its composites were analysed with a model 3397-36 Sin-
tech-1 instrument in tensile mode with a load cell of 50 lb using
the ASTM D 638 procedure. The specimens were cut in a dumb-
bell shape with an ASTM D 638 type V die. The tensile tests
were performed at a crosshead speed of 2.5 mm min
y
1
. At least
five measurements were taken for each sample.
Results and discussion
SEM images
The images of MFs obtained by SEM imaging have been
analysed in detail by Chakraborty et al. (2005). A typical
SEM image of MFs is shown here in Figure 1a. An SEM
of image of MCC is presented in Figure 1b. As outlined
by the same authors, at high sample dilution, the MFs
are fibres with discrete aspect ratios, instead of being an
entangled mass. This is illustrated in Figure 2a for a
0.03% MF suspension.
AFM images
An AFM image of the MF samples at 0.1% consistency
is shown in Figure 2b. This image confirmed that the
fibres in the isolated MF suspension were less than 1
mm
in diameter. Even though some bundles of larger diam-
eter were visible, a closer look at the image shows that
these bundles were most likely a result of physical entan-
glement of several individual fibrillated MFs with dia-
meters much less than 1
mm.
The aspect ratios of the MFs and MCCs were calcu-
lated by analysing several SEM images. The correspond-
ing values for MF spanned from approximately 50 to
several hundreds, whereas for MCC the range was
between 1 (essentially spherical particles) and 5. For soft-
wood BKP, an average fibre length of 2 mm and diame-
ters between 5 and 20
mm were obtained, giving an
aspect ratio of 100–400.
Degree of polymerisation, chain length and
crystallinity
The average degree of polymerisation and chain length
of BKP, MF and MCC are listed in Table 1. The corre-
sponding aspect ratio and crystallinity of these materials
are also presented in the same table. As the MFs are
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56 A. Chakraborty et al.
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Table 1 Average degree of polymerisation (DP), average chain
length, aspect ratio, and crystallinity.
DP Chain Aspect Crystallinity
length ratio (%)
BKP 2230 1150 100–400 45
MF 1000 520 )50 54
MCC 225 110 1–5 64
BKP, bleached kraft pulp; MF, microfibres; MCC, microcrystalline
cellulose.
Figure 3 Tensile properties of pure PVA and its composites
with bleached kraft pulp (BKP), microfibres (MF) and microcrys-
talline cellulose (MCC): (a) peak stress; and (b) stiffness.
generated from BKP, there is a preferential breakage
along the amorphous zones. Therefore, as expected, the
decrease in chain length of the MFs was accompanied
by an increase in crystallinity compared to BKP. MCC, on
the other hand, is prepared chemically by removing
amorphous zones in cellulose using acid hydrolysis.
Therefore, its decrease in chain length was also accom-
panied by a substantial increase in crystallinity.
Tensile properties
The tensile strength and modulus of pure PVA and its
composites with BKP, MF and MCC are shown in Figure
3. As this figure illustrates, the composite with MF had
the highest strength, followed by MCC and BKP. The
strength of the PVA-MF composite was approximately
double that of pure PVA. The stiffness of the PVA-MF
composite was also the highest, and approximately
2.5-fold that of pure PVA. This was a significant improve-
ment in mechanical properties, which demonstrates the
immense potential of MFs as a biodegradable reinforcing
agent.
It is obvious from Figure 3 that MFs are better rein-
forcing agents compared to both MCC and BKP. A pos-
itive effect in the case of MFs compared to BKP is
expected, because MFs have much lower amorphous
content than BKP. However, MCC has even lower amor-
phous content than MF, which implies that MCC should
have a stronger reinforcing potential than MF. But in
effect, this is not the case, as Figure 3 illustrates. The
reason for and implications of the highest reinforcing
potential of MFs compared to both BKP and MCC are
explained later.
Noting the stiffness of the pure PVA matrix and the
corresponding value for the MF-PVA composite at 5%
reinforcement, as shown in Figure 3b, the theoretical
stiffness of the MFs may be calculated by assuming a
simple rule of mixtures: the modulus of a composite is
equal to the sum of the modulus of the respective phases
(matrix or fibre) multiplied by the volume fraction of that
phase in the composite. For example, if E
m
, E
f
and E
c
denote the elastic modulus of the matrix, the MFs and
the composite, respectively, and v
f
denotes the volume
fraction of MFs in the composite, then the rule of
mixtures would give:
E svEq(1–v )E . (3)
cff fm
The value of v
f
was calculated knowing that the mass
fraction of MFs in the composite was 5%, and that the
density of cellulose and the starch polymer was 1500 and
1300 kg m
y
3
, respectively. This gave a value of 4.4% for
v
f
. Noting the values of E
c
for the MF-starch composite
(5.25 GPa) and the value of E
m
for pure PVA (2.3 GPa)
from Figure 3, Eq. (3) yielded a value of approximately
69 GPa for the stiffness E
f
of the MFs. Similar calcula-
tions for BKP and MCC gave much lower stiffness values
of 39 and 23 GPa, respectively.
The rule of mixtures assumes perfect adhesion
between the matrix and the reinforcing phase, which is
not achieved in practice. There is always some slip
between the matrix and the fibres. Eq. (3) yields the max-
imum value of E
c
when the values of E
f
and E
m
are known.
However, in this case, we determined experimentally the
value of E
c
for the three composite systems, and the
theoretical value of E
f
was back-calculated. Therefore, it
may be concluded that the values of E
f
calculated above
represent the minimum stiffness values for the different
forms of cellulose.
As presented in Figure 4, the introduction of 10%, 15%
and 20% MF into the system caused a progressive
decrease in strength compared to a 5% MF composite.
Since the same MF consistency was used during the
solution casting process for all fibre loadings, the effect
observed is not likely the result of agglomeration of the
MFs due to increased hydrogen bonding among them.
Therefore, the strength losses can be attributed to
increased physical entanglement of the MFs and their
failure to retain distinct aspect ratios at higher MF load-
ing. This demonstrates the limits up to which the intro-
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Reinforcement with microfibres in PVA 57
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Figure 4 Tensile stress of PVA and its composites at different
microfibre loadings.
duction of microfibres can increase the tensile strength
of composites.
Implications of the relative importance of
crystallinity and chain length in providing
reinforcement
The observations indicate that crystallinity alone is not
the deciding factor in providing reinforcement. As dis-
cussed earlier, a fibre must have a minimum aspect ratio,
referred to as the stress transfer aspect ratio s
t
, to realise
a maximum reinforcing potential. If the aspect ratio is less
than s
t
, there will not be sufficient stress transfer along
the fibre surface. In such a case, even a very strong fibre
will not be able to provide sufficient reinforcement, result-
ing in failure of the composite at relatively small stresses.
If the length of the same fibre is such that the aspect
ratio for a given diameter exceeds s
t
, the tensile strength
of the composite will be substantially increased.
Given the higher crystallinity of MCC in comparison to
MF, the relatively poor tensile strength of the PVA-MCC
composite can be explained by the fact that MCC does
not satisfy the requirement for a minimum length to reach
an aspect ratio value of s
t
that is needed for sufficient
reinforcement.
Our experimental results demonstrate again the impor-
tant implications of the known requirement for a mini-
mum aspect ratio. There is a trend to remove the
amorphous moiety from cellulose and to isolate pure
crystalline zones for composite materials. However, this
is not enough. The crystallite lengths should also be tak-
en into consideration. If they are smaller than those cor-
responding to an aspect ratio s
t
for the system used, the
mechanical properties of the composites will be inferior.
Accordingly, there is a limit up to which the amorphous
zones should be removed from cellulose chains for best
results.
Conclusion
High reinforcing potential of microfibres has been dem-
onstrated in PVA films. For example, 5% MF in PVA com-
posite films produced a doubling of tensile strength, and
2.5-fold increase in stiffness. However, the strength of
the composite was greater at 5% MF loading compared
to loadings of 10% and higher, demonstrating that there
are limits for strength increment. Using a simple rule of
mixtures, we obtained a minimum theoretical value of
69 GPa for the stiffness of a microfibre.
The tensile properties of composites with MFs were
better than with MCC, which has a much higher degree
of crystallinity. This can be explained by the lack of suf-
ficient aspect ratio of the MCC. Accordingly, in cellulose
nanocomposites the cellulose chain or fibre should have
a minimum length that corresponds to the stress transfer
aspect ratio s
t
for a given fibre diameter. With the present
trend in preparing cellulose nanocrystals and using them
as reinforcing agents, care should be taken to ensure that
a minimum fibre length is maintained, even if it is at the
cost of the degree of crystallinity.
Acknowledgements
The authors would like to acknowledge the help of BIOCAP/
NSERC strategic funds, the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Ontario Graduate
Scholarship (OGS) for providing funding for the project. The
authors also acknowledge the assistance of Prof. Kristiina Oks-
man, Dr. Bjørn Steiner Tanem and Ingvild Kvien at NTNU, Trond-
heim, Norway, for performing the AFM imaging presented in this
paper.
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Received May 9, 2005. Accepted October 4, 2005.
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