Water redispersible cellulose nanofibrils adsorbed

Cellulose (2014) 21:4349–4358
DOI 10.1007/s10570-014-0452-7
ORIGINAL PAPER
Water redispersible cellulose nanofibrils adsorbed
with carboxymethyl cellulose
Núria Butchosa • Qi Zhou
Received: 16 August 2014 / Accepted: 16 September 2014 / Published online: 25 September 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Cellulose nanofibrils (CNFs) are difficult
to redisperse in water after they have been completely
dried due to the irreversible agglomeration of cellulose
during drying. Here, we have developed a simple
process to prepare water-redispersible dried CNFs by
the adsorption of small amounts of carboxymethyl
cellulose (CMC) and oven drying. The adsorption of
CMC onto CNFs in water suspensions at 22 and
121 °C was studied, and the adsorbed amount of CMC
was measured via conductimetric titration. The waterredispersibility of dried CNFs adsorbed with different
amounts of CMC was characterized by sedimentation
test. Above a critical threshold of CMC adsorption, i.e.
2.3 wt%, the oven dried CNF–CMC sample was fully
redispersible in water. The morphology, rheological,
and mechanical properties of water-redispersed CNF–
CMC samples were investigated by field emission
scanning electron microscopy, viscosity measurement, and tensile test, respectively. The waterN. Butchosa
Department of Fibre and Polymer Technology, Royal
Institute of Technology, 100 44 Stockholm, Sweden
Q. Zhou (&)
Wallenberg Wood Science Center, Royal Institute of
Technology, 100 44 Stockholm, Sweden
e-mail: qi@kth.se
Q. Zhou
School of Biotechnology, AlbaNova University Centre,
Royal Institute of Technology, 106 91 Stockholm,
Sweden
redispersed CNFs preserved the original properties
of never dried CNFs. This new method will facilitate
the production, transportation and storage, and largescale industrial applications of CNFs.
Keywords Cellulose nanofibrils Carboxymethyl
cellulose Adsorption Redispersion
Introduction
Cellulose nanofibrils (CNFs), three orders of magnitude smaller than the micrometer scale intact plant
fiber cells, have unique properties such as large
specific surface area and high mechanical strength
and toughness. The extraction process of CNFs from
renewable resources like wood has been extensively
investigated in the past decade (Pei et al. 2013).
Typical processes include enzymatic (Henriksson
et al. 2007; Pääkkö et al. 2007) and chemical (Pei
et al. 2013; Saito et al. 2006) pretreatments on wood
pulp followed by mechanical disintegration. Stora
Enso has built a pre-commercial plant at Imatra,
Finland for the production of CNFs. Other potential
major producers of CNFs are UPM, Borregaard, and
Daicel Corp. CNFs have been widely utilized as
promising nanoscale building blocks in the fabrication
of new materials such as nanopaper (Henriksson et al.
2008; Sehaqui et al. 2010a), hydrogel, aerogel (Jin
et al. 2011; Sehaqui and Zhou 2011), and foam
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(Pääkkö et al. 2008; Sehaqui et al. 2010b), and as
mechanical reinforcing nanoparticles in polymer
composites (Hietala et al. 2013). CNF-based materials
have demonstrated their suitability in a large range of
applications such as substrates for solar cells (Fang
et al. 2014), display substrates and organic lightemitting diode materials (Nogi and Yano 2008),
transparent barrier coatings (Fukuzumi et al. 2009),
polymer or metal-ion batteries (Hu et al. 2010;
Nyström et al. 2009; Zhu et al. 2013), magnetic
nanocomposites (Olsson et al. 2010), etc. However,
CNFs are generally produced as aqueous suspensions
with low solid content and very high viscosity due to
the inherent hydrophilic nature of cellulose. Concentrated water suspensions of CNFs are difficult to
prepare due to the irreversible agglomeration of
cellulose during drying. This agglomeration process
is known as hornification or co-crystallization (Newman 2004), and it is caused by strong hydrogen
bonding between cellulose chains when they become
in contact. To avoid hornification, CNFs must be kept
in water suspension. This is a major drawback for
CNFs in transportation, storage, and large-scale
industrial applications in sectors such as composites
industries.
Several approaches have been attempted to achieve
water-redispersibility for CNFs and cellulose nanocrystals (CNCs) after drying. Dong and Gray (1997)
first studied redispersibility of dried CNC film samples made from suspensions with different counterions. They discovered that salt-form samples were
water-redispersible and turned into homogeneous
suspensions, while CNCs with H? as counterion did
not redisperse in water even with strong ultrasound
treatment. Further study by Beck et al. (2012)
revealed that the exchange of counterions of CNCs
from H? to Na? significantly decreases the intermolecular hydrogen bonding generated from cellulose
backbones, thus facilitates their redispersibility in
water even when fully dried. In a similar fashion,
Missoum et al. (2012) successfully prepared waterredispersible CNFs by freeze drying method after the
addition of sodium chloride. In order to avoid
intermolecular hydrogen bonds between fibrils and
thus to obtain water-redispersible CNFs, both covalent and non-covalent surface modification methods
have been exploited. Covalent surface carboxymetylation was found essential to obtain water-redispersible dry CNF powders (Eyholzer et al. 2010). The
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Cellulose (2014) 21:4349–4358
irreversible non-covalent attachment of carboxymethylcellulose (CMC) onto different wood pulp fibers
was first studied by Laine et al. (2000). The adsorbed
CMC improved the mechanical properties of handsheets prepared from wood pulp and aided paper
recycling (Duker et al. 2008). CMC adsorbed onto
cotton fibers was used to increase the charge density
and adsorption capacity for surfactant (Fras-Zemljic
et al. 2006; Zemljic et al. 2008). Therefore, CMC has
been successfully used as an anchoring polymer for
surface modification of cellulosic materials including
CNFs. Further CMC-assisted functionalizations such
as sequential ‘‘click’’ reactions, conjugation of biomolecules, and PEGylation have been demonstrated
(Filpponen et al. 2012). CMC has also been utilized to
improve wet-strength of nanofibrillated cellulose/
CMC composites after ionic crosslinking with glycidyl trimethyl ammonium chloride (Pahimanolis et al.
2013). Redispersibile microcrystalline cellulose
(MCC) products containing CMC as redispersing aid
(Avicel RC-591 and CL-611) have been commercially available for decades. These products typically
contains 8–18 wt% of CMC. Coating cellulose microfibrils with anionic polymeric additives such as CMC
and polyacrylate have been found to form a watersoluble interfacial film around the cellulose microfibrils while drying, which allows the recovering of
rheological properties after redispersion of dried
samples (Lowys et al. 2001). However, a weight
content of 30 % CMC was required in order to recover
the rheological behavior of the initial CNF
suspension.
In order to prepare water-redispersible CNFs, we
screened a series of hydrocolloids including cationic and
anionic xyloglucan derivatives, water-soluble chitosan
(50 % degree of acetylation), hydroxyethylcellulose,
and CMC with similar molecular weight as the
redispersing aids, and only CMC showed dispersing
and stabilizing effect. In this work, the adsorption
isotherm of CMC onto CNFs in water suspension at 22
and 121 °C was studied for the first time. The waterredispersibility of dried CNF samples adsorbed with
CMC was investigated in order to discover the minimum amount of CMC required for redispersion. The
morphology, viscosity, and mechanical properties of
water-redispersed dried CNF samples, which were
adsorbed with different amounts of CMC before drying,
were investigated and compared to the never-dried
CNFs.
Cellulose (2014) 21:4349–4358
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Denmark. Subsequently, the pretreated pulp was
passed 8 times through the microfluidizer (M-110EH,
Microfluidics Ind., Newton, MA) at room temperature
(21 °C), including first three passes through 400 and
200 lm chambers at a pressure of 900 bar and five last
passes through 200 and 100 lm chambers at a pressure
of 1,600 bar. A 2 wt% CNFs dispersion in water was
thus obtained. Figure 1 shows the STEM image and
width distribution profile of the CNFs. The nanofibril
length is several micrometers and nanofibril ends are
not apparent. The width of the cellulose nanofibrils was
in the range from 5 to 22 nm as measured from the
image by using imageJ (NIH, USA).
Adsorption of CMC onto CNF
Fig. 1 STEM image of CNFs and corresponding histogram
showing the width distribution
Experimental
Materials
Carboxymethylcellulose (CMC, sodium salt, Mw =
2.5 9 105, DS = 0.90) was purchased from SigmaAldrich. A commercial never-dried softwood sulphite
pulp (Nordic Paper, Sweden) was used as a starting
material for CNFs production. The wood pulp contains
approximately 14 % of hemicelluloses, \1 % of
lignin, and almost 86 % cellulose with a degree of
polymerization (DP) of 1,200. CNFs were prepared
from the wood pulp according to the method reported
previously (Pääkkö et al. 2007; Henriksson et al. 2007;
Sehaqui et al. 2010a). In brief, the wood pulp was first
dispersed in water and subjected to a pretreatment step
involving mechanical beating and enzymatic degradation. The enzyme used was an endoglucanase,
FiberCare R, manufactured by Novozymes A/S,
CMC was dissolved in deionized water (1 wt%) and
stirred overnight. A series of CNF/CMC mixtures
were prepared by the addition of different aliquots of
CMC aqueous solution (1 wt%) to 40 g of 2 wt%
CNFs water suspension. Additional water was added
to each mixture to obtain a total weight of 80 g, i.e. a
constant CNFs concentration of 1 wt%. The CNF/
CMC suspensions were mixed at 12,000 rpm using an
Ultra-Turrax mixer (IKA, D125 Basic) for 2 min
before storing at room temperature (RT) overnight. To
study the effect of high temperature (HT) on the
adsorption, the suspensions were autoclaved at 121 °C
for 25 min after mixing and cooled down at room
temperature overnight. Subsequently, CNFs adsorbed
with CMC were precipitated by centrifugation (Avanti
J-26 XP, Beckman Coulter, USA) at 25,000 rpm for
20 min with unbound CMC remaining in the supernatant. The precipitated CNFs were redispersed in
water followed by centrifugation. This washing step
was repeated 3 times in order to completely remove
the unbound CMC. The modified CNFs samples were
coded as either CNF–CMC–HT-x or CNF–CMC–RTx, where the initial amount of CMC in suspension was
x mg per 1 g of CNFs at either high temperature
(121 °C) or room temperature (22 °C), respectively.
Drying and redispersion
To obtain dry cellulose, the pure CNFs and CNFs
adsorbed with CMC samples were simply dried from
their water suspension in an oven at 80 °C until
constant weight was reached. The dry samples were
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easily swelled and redispersed in water typically with
a solid concentration of 1 wt% by stirring with a
magnetic stirrer overnight followed by mixing with an
Ultra-Turrax mixer for 15 min.
Conductimetric titration
The amount of CMC adsorbed onto CNFs was
calculated from the carboxyl content as measured by
the electric conductivity titration method. In brief,
100 mL of CNFs suspension with a solid content of
0.1 wt% was mixed with 0.1 M HCl to set the pH value
in the range between 2.5 and 3. The suspension was
titrated with 0.01 M NaOH at a rate of 0.2 mL/min up
to pH 11, as measured by a pH station (FiveEasy,
Mettler-Toledo). The conductivity of the suspensions
was monitored with a conductimetric station (SevenCompact, Mettler-Toledo). The titration curve showed
typical presence of strong and weak acid groups. The
amount of strong acid corresponded with the added
HCl, and that of weak acid corresponded with the
carboxyl content (Araki et al. 2001; Perez et al. 2003).
The amount of weak acid (0.099 mmol/g) from the
pure CNFs was deduced to obtain the carboxyl content
of the adsorbed CMC. The results of three independent
titrations were averaged for each sample.
Cellulose (2014) 21:4349–4358
Viscosity
The viscosity measurements were performed on a con/
plate rheometer (RVDV-III, Brookfield) using a cone
CP-40 with an angle of 0.8°. The shear viscosity of the
water-redispersed CNFs and CNF-CMC suspensions
(0.5 wt%) was monitored by decreasing the shear rate
from 1,875 to 15 s-1 at 25 °C.
Mechanical properties
Cellulose nanopapers were prepared from the waterredispersed samples of CNFs adsorbed with CMC
according to a method developed previously (Henriksson et al. 2008; Sehaqui et al. 2010a). Tensile tests
of the nanopapers were performed using an Instron
universal material testing machine equipped with a
load cell of 500 N at 50 % relative humidity and
23 °C. Samples were cut into rectangular strips of
5 mm width. The gauge length was 50 mm and the
strain rate was 5 mm min-1. The results for each
material were based on at least five specimens. The
modulus was obtained from the slope at low strain and
the tensile strength was determined as the stress at
specimen breakage.
Results and discussion
Electron microscopy
Adsorption of CMC on CNFs
Field-emission scanning electron microscopy (FESEM) was conducted with a Hitachi S-4,800 SEM
working at low acceleration voltage (1 kV) and short
working distance (8 mm). A droplet of redispersed
water suspension of CNFs and CNFs adsorbed with
CMC with a solid content of 0.1 wt% was dried under
vacuum on a specimen mount covered with carbon
conductive tape. The vacuum dried samples were
coated with a thin layer of platinum–palladium with a
sputter coater (Cressington 208HR).
To further characterize the size distribution of the
never dried CNFs, 7 ll of very dilute sample
suspension (0.005 wt%) was deposited on a carbon
coated cooper grid and stained with 2 wt% uranyl
acetate. After drying at ambient conditions, the
sample was observed using a Hitachi S-4800 scanning electron microscope equipped with transmitted
electron detector (STEM) to capture transmitted
electron images.
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The adsorbed amount of CMC onto CNFs as a
function of added amount of CMC in water suspension
of CNFs at room temperature 22 °C (RT) and high
temperature 121 °C (HT) is plotted in Fig. 2. Following the classification of adsorption isotherms by
Brunauer et al. (1940), the adsorption curve at RT
shows a typical shape of type III isotherm, which is
characteristic for unfavorable adsorption. Interestingly, the adsorption at HT follows a type II or BET
isotherm, which is characteristic for multilayer
adsorption on a non-porous material (Brunauer et al.
1938; Ebadi et al. 2009). The adsorption of anionic
CMC onto slightly negatively charged CNFs is
unfavorable due to charge–charge repulsion. The
irreversible adsorption relies on the attachment of
unsubstituted cellulose segments in CMC on cellulose
chains exposed on the surface of CNFs by hydrogen
bonding, similar to the adsorption of a nonionic
Cellulose (2014) 21:4349–4358
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irreversible adsorption of CMC onto CNFs in water
suspension at different temperatures. The adsorbed
amount of CMC could be possibly further increased by
tuning pH or by the addition of cations in the CNF
water suspensions (Duker et al. 2007; Laine et al.
2000; Liu et al. 2011).
Redispersion of dried CNF–CMC samples
Fig. 2 Adsorbed amount of CMC onto CNFs as a function of
the initial amount of CMC in CNFs water suspensions at room
temperature 22 °C (RT) and high temperature 121 °C (HT)
polymer, methylcellulose, onto wood pulp (Ishimaru
and Lindström 1984). As shown in Fig. 2, such
irreversible attachment was significantly strengthened
by increasing temperature. At HT, almost all the CMC
added in the suspension was adsorbed onto CNFs
when the initial amount of CMC was lower than
20 mg/g CNFs. When the initial amount of CMC was
gradually increased from 20 to 100 mg/g CNFs, the
final amount of bound CMC slowly increased from 17
to 23 mg/g CNFs. Further increasing the initial
amount to 200 mg/g CNFs did not increase the
adsorption, indicating that the surface of CNFs was
saturated with negative charges from bound CMC. To
achieve an adsorption level of 23 mg CMC per gram
of CNFs, the initial amount of CMC was only 100 mg
at HT as compared to 300 mg at RT. After the
saturation point achieved at HT, i.e. when the initial
amount of CMC was higher than 200 mg/g CNFs, the
adsorption became unfavorable, similar to the adsorption at RT in which the amount of bound CMC
gradually increased with increasing initial amount of
CMC. The amount of bound CMC was in the range of
23–59 mg/g CNFs by increasing the initial amount of
CMC from 200 to 400 mg/g CNFs at HT. Thus, the
adsorbed amount of CMC on CNFs is in the range of
0.09–0.22 mg/m2, assuming an average diameter of
10 nm for the CNFs. This result is similar to the
surface coverage of 0.33–0.34 mg CMC/m2 on regenerated cellulose films as studied by surface plasmon
resonance at different electrolyte conditions (Liu et al.
2011). To our knowledge, we are the first to report the
The stability of water-redispersed CNFs and CNF–
CMC suspensions was qualitatively assessed by
sedimentation test. As shown in Fig. 3A, the oven
dried CNFs (b) and the four CNF–CMC samples, in
which the initial contents of CMC were 20 (c, CNF–
CMC–HT-20) and 50 (d, CNF–CMC–HT-50) mg/g of
CNFs at 121 °C and 100 (f, CNF–CMC–RT-100) and
200 (g, CNF–CMC–RT-200) mg/g of CNFs at 22 °C,
precipitated from water suspensions within 3 and 24 h.
The two CNF–CMC samples with the initial content of
CMC of 100 mg/g at 121 °C (e, CNF–CMC–HT-100)
and 300 mg/g at 22 °C (h, CNF–CMC–RT-300) were
stable for the whole studied period of time, i.e. 24 h, as
compared to the never dried CNFs (a) sample. This
indicates that the water-resdispersibility of dried CNFs
relies on the adsorbed amount of CMC. When the
surface of the CNFs was saturated with negative
charges introduced by the adsorbed CMC, the
agglomeration of CNFs induced by the intermolecular
hydrogen bonding between cellulose upon drying was
minimized, thus stable suspensions were obtained
when fully dried CNF-CMC samples were redispersed
in water. The required amount of adsorbed CMC to
achieve a stable suspension after redispersion was
23 mg CMC/g CNFs, i.e. a total surface carboxyl
content of 0.185 mmol/g on CNFs. This value corresponds to a degree of substitution (DS) of 0.03, which
is lower than the DS (0.08–0.22) of water-redispersible nanofibrillated cellulose prepared by carboxymethylation and mechanical disintegration (Eyholzer
et al. 2010). This indicates that the adsorption of CMC
onto CNFs is rather homogeneous while covalent
carboxymethylation is a heterogeneous reaction. Figure 3B shows the photograph of dried CNF–CMC–
HT-100 sample, which was prepared by the addition of
100 mg CMC/g CNFs in suspension at 121 °C followed by careful washing of unbound CMC and oven
drying. 100 mL of 1 wt% suspension was placed in a
glass beaker in the oven at 80 °C with ventilation and
dried overnight until constant weight was reached. The
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Cellulose (2014) 21:4349–4358
Morphology, viscosity and mechanical properties
of redispersed CNF–CMC samples
Fig. 3 A Sedimentation test for 0.2 wt% water suspensions of
never dried CNFs (a) and redispersed oven dried CNFs (b),
CNF–CMC–HT-20 (c), CNF–CMC–HT-50 (d), CNF–CMC–
HT-100 (e), CNF–CMC–RT-100 (f), CNF–CMC–RT-200 (g),
and CNF–CMC–RT-300 (h). B Photographs of oven dried and
C water-redispersed CNF–CMC–HT-100 sample with a solid
content of 5 wt%
CNF–CMC sample was removed from the glass wall
of the beaker and broken into pieces ready for
redispersion. The dried sample contained only
23 mg CMC/g CNFs (i.e. 2.3 % by weight of CNFs).
Interestingly, this sample could be redispersed in
water even at a solid content as high as 5 wt%, and a
homogeneous and stable hydrogel was prepared
(Fig. 3C). Such procedure to prepare CNF suspensions with high solid content is much easier than the
conventional method using high speed centrifugation
or filtration to concentrate the typical 2 wt% suspension of never dried CNFs (Pääkkö et al. 2007; Sehaqui
et al. 2010b).
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Figure 4 shows FE-SEM images of vacuum dried
CNFs water suspensions. The individualized nanofibers from never dried CNFs suspension formed a weblike structure (Fig. 4A). Due to the agglomeration of
CNFs during drying, both redispersed CNFs (Fig. 4B)
and CNF–CMC–HT-10 (Fig. 4C) showed large CNFs
aggregates of more than 25 lm. With increasing initial
added amount of CMC, the water-redispersed CNF–
CMC–HT-20 sample (Fig. 4D) showed a web-like
structure with fewer large CNFs aggregates, while the
water-redispersed
CNF–CMC–HT-100
sample
(Fig. 4E) showed a web-like structure of individualized nanofibers without any aggregates, similar to that
for never dried CNFs. This further indicates that
negative charges on the surface of CNFs are essential
for a complete redispersion in water after drying. The
addition of 100 mg CMC/g CNFs at HT to achieve an
adsorption level of 23 mg/g is necessary to prepare
water-redispersible dry CNF–CMC sample, as also
indicated by the sedimentation test.
The viscosities as a function of shear rate for
suspensions of never dried CNFs and water-redispersed dry CNFs and CNF–CMC samples with a
solid content of 0.5 wt% were investigated. As shown
in Fig. 5, all samples showed a non-Newtonian
behavior, with a large decrease of viscosity with
increasing shear rate, i.e. shear thinning. This pseudoplastic behavior is typical for water suspensions of
cellulose nanocrystals (Araki et al. 1998) and CNFs
(Pääkkö et al. 2007). It is caused by the breakage of
entanglements between cellulose particles and their
alignment when shear rate increases. The viscosity of
water-redispersed dry CNFs at shear rate of 15 s-1
was 4.2 9 10-2 Pa s, almost 3 times lower than that
of never dried CNFs (1.1 9 10-1 Pa s). The viscosity
of the water-redispersed CNF–CMC suspensions
increased with the adsorbed amount of CMC on
CNFs. For the CNF–CMC–HT-100 sample with
2.3 wt% CMC adsorbed on CNFs, the viscosity was
1.0 9 10-1 Pa s at shear rate of 15 s-1, similar to that
of never dried CNFs suspension. This is a remarkable
improvement since a weight content of 30 % CMC
was required in order to recover the rheological
behavior of the initial CNFs suspension as reported
previously (Lowys et al. 2001).
Cellulose (2014) 21:4349–4358
4355
Fig. 4 FE-SEM images of suspensions of never dried CNFs (A, A0 ) and water-redispersed CNFs (B), CNF–CMC–HT-10 (C), CNF–
CMC–HT-20 (D, D0 ), and CNF–CMC–HT-100 (E, E0 ) observed at a magnification of 92,000 and 920,000
Furthermore, water-redispersed CNFs and CNF–
CMC samples were used to prepare nanopapers.
Typical stress–strain behavior in uniaxial tension and
associated mechanical property data of the CNFs and
CNF–CMC nanopapers are presented in Fig. 6 and
Table 1, respectively. The tensile strength and strainto-break of the nanopaper prepared from waterredispersion of dried CNFs were significantly lower
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Cellulose (2014) 21:4349–4358
Table 1 Mechanical properties of nanopapers from never
dried CNFs and water-redispersed CNF and CNF–CMC
samples
Sample
Fig. 5 Viscosity versus shear rate for never dried CNFs
suspension, water-redispersed CNFs and CNF–CMC suspensions with a solid content 0.5 wt%
Modulus
(GPa)
Tensile
strength
(MPa)
Strain-tobreak (%)
CNFs never dried
11.4 ± 0.2
197.9 ± 6.1
7.4 ± 0.4
CNFs dried
CNF–CMC–HT-50
10.3 ± 0.6
10.0 ± 0.3
125.5 ± 8.3
189.8 ± 7.8
4.5 ± 0.5
9.2 ± 0.7
CNF–CMC–HT-100
10.5 ± 0.3
190.8 ± 12.9
7.9 ± 0.8
CNFs adsorbed with CMC were slightly higher than
that for never dried CNFs. This might be caused by the
higher charge on the surface of CNFs, which could
assist the sliding of the nanofibrils during plastic
deformation as shown for carboxylated CNFs prepared by TEMPO-mediated oxidation (Sehaqui et al.
2012).
Conclusions
Fig. 6 Typical stress–strain curves of nanopapers prepared
from never dried CNFs and water-redispersed CNFs and CNF–
CMC samples
than those of never dried CNFs while the modulus was
maintained. Cellulose nanofibrils were not individualized when redispersed in water from fully dried
CNFs sample, thus failed to form a continuous
nanofiber network structure in the nanopaper. As the
water redispersibility of CNFs increased with increasing amount of adsorbed CMC, the tensile strength of
the nanopapers also increased compared to the dried
CNFs without CMC. The CNF–CMC–HT-100 sample
with only 2.3 wt% adsorbed CMC had tensile strength
and modulus values rather similar to those for never
dried CNFs, indicating good redispersibility in water
after drying. The strain-to-break of nanopapers from
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We have shown that water-redispersible dried CNFs
can be simply prepared by the adsorption of 2.3 wt%
CMC onto CNFs before oven drying. The irreversible
adsorption of 2.3 wt% CMC was achieved with the
addition of only 100 mg CMC/g CNFs in water
suspension followed by incubation at 121 °C for
25 min. The rheological and mechanical properties of
CNFs were conserved after completely dried and
water-redispersed. Interestingly, water suspensions of
CNFs with concentration as high as 5 wt% can be
readily prepared by redispersion from dry sample. A
homogenous coverage of negative charges on the
surface of the CNFs induced by the adsorbed CMC is
essential to avoid the agglomeration of CNFs upon
drying, thus cellulose nanofibrils can be easily redispersed in water without aggregates observed. With
this new method, water-redispersible dried CNFs can
be prepared without sophisticated chemical modification or drying process. Hence, CNFs transportation
and storage can be optimized, leading to an easier
implantation of CNFs in traditional polymer sectors
such as packaging and composites industries.
Acknowledgments The authors thank the Swedish Research
Council Formas (CarboMat, 2009-1687) and the Wallenberg
Wood Science Center for supporting this work.
Cellulose (2014) 21:4349–4358
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