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J Polym Res (2017) 24:191
https://doi.org/10.1007/s10965-017-1343-z
ORIGINAL PAPER
Butanol-mediated oven-drying of nanocellulose with enhanced
dehydration rate and aqueous re-dispersion
Zahid Hanif 1 & Hyeonyeol Jeon 1 & Thang Hong Tran 1,2 & Jonggeon Jegal 1 & Seul-A. Park 1 &
Seon-Mi Kim 1 & Jeyoung Park 1,2 & Sung Yeon Hwang 1,2 & Dongyeop X. Oh 1,2
Received: 8 August 2017 / Accepted: 27 September 2017
# Springer Science+Business Media B.V. 2017
Abstract The application potential of nanocellulose has been
previously hindered by the costly and slow drying methods
that this material requires, including freeze/supercritical drying process. The main issue for nanocellulose commercialization is how effectively and rapidly its high water contents (90–
99%) can be removed, all of which raise its transportation and
processing costs. Oven-drying is the fastest, most economical,
and most scalable method for dehydrating nanocellulose, but
causes strong interfibrillar aggregation and leads to poor aqueous re-dispersion. Here, we report that the problems of
nanocellulose oven-drying are comprehensively overcome
by adding tert-butanol (t-BuOH) to the nanocellulose solution
at >90%. In a lab-scale comparison, the t-BuOH-mediated
oven-drying of aqueous nanocellulose showed lower drying
times by a factor of 2–12 compared to water only oven-drying
and freeze drying of the same material. The dispersibility of
this dried nanocellulose is as high as the never-dried material
Zahid Hanif and Hyeonyeol Jeon contributed equally to this work.
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10965-017-1343-z) contains supplementary
material, which is available to authorized users.
* Jeyoung Park
jypark@krict.re.kr
* Sung Yeon Hwang
crew75@krict.re.kr
* Dongyeop X. Oh
dongyeop@krict.re.kr
1
Research Center for Bio-based Chemistry, Korea Research Institute
of Chemical Technology (KRICT), Ulsan 44429, Republic of Korea
2
Advanced Materials and Chemical Engineering, University of
Science and Technology (UST), Daejeon 34113, Republic of Korea
in terms of particle size, light transmittance, and sedimentation. t-BuOH reduces interfibrillar shrinkage due to the lower
surface tension of t-BuOH compared to water, and a remaining t-BuOH/water mixture decreases interfibrillar adhesion
and contact.
Keywords Nanocellulose . Oven-drying . Co-solvent .
Drying rate . tert-Butanol . Re-dispersion
Introduction
Cellulose is the most abundant biopolymer and consists of
rigid crystalline nanofibers, which can be exfoliated by chemically or mechanically breaking interfibrillar hydrogen bonds
to be a sort of nanomaterials, called nanocellulose [1–11]. The
unprecedented features of nanocellulose such as biocompatibility, high aspect ratio (10–200), high specific surface area,
low oven expansion (~2.7 ppm/K), and remarkable elastic
modulus (80–120 GPa) have received considerable interests
of materials research in recent years [6–11].
The general form of nanocellulose is an aqueous suspension
with the concentration of 0.1–10 wt% [1–11]. There are three
types of nanocellulose − cellulose nanocrystals (CNC), cellulose nanofilbrils (CNF), and 2, 2, 6, 6-tetramethylpiperidine
(TEMPO)-CNF (Fig. 1a). CNC is prepared through the controlled acid hydrolysis of bulk cellulose, and it has the width of
5–20 nm and the length of 100–300 nm. The presence of
anionic sulfate on CNC stabilizes its nano-dispersion in aqueous media. CNF is prepared only through the mechanical homogenization of wet cellulose pulp, and it has the width of 5–
50 nm and the length of several μm. However, CNF aqueous
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J Polym Res (2017) 24:191
Page 2 of 11
Fig. 1 Drying rate (min/mL) of CNC, CNF, and TEMPO-CNF samples
with varying t-BuOH contents at (a) 80, (b) 100, and (c) 120 °C. As a
standard drying rate measurement, a 10 mL sample in a 20 mL vial was
dried under an oven of desired temperature, and then the value was
determined by total drying time (min) for the sample of 10 mL
suspensions are opaque due to unfibrillated or partially fibrillated agglomeration. TEMPO-mediated oxidation reaction
transforms the primary hydroxyl groups of cellulose on the
surface of CNF into anionic charged carboxylate groups,
which repel each other to facilitate the nanodispersion of
CNF. This process gives transparent solution, and the resultant
material is here called TEMPO-CNF.
From an industrial perspective, the main engineering
issue of nanocellulose is the drying process [12, 13]. The
excessive water contents of aqueous nanocellulose solutions (>90 wt%) decreases the efficiency of transportation
and processing. In addition, in order to compound or mix
nanocellulose with (generally hydrophobic) synthetic
polymers or organic materials, this drying process is required and must not damage the nanofiber structure.
Freeze-drying and spray-drying are currently-used drying
processes for nanocellulose. Freeze-drying, spray-drying,
and supercritical-drying facilitate the re-dispersion of the
resultant solid in an aqueous solution by preventing interfibril aggregation. However, freeze drying generally takes
3–5 days to fully dry the nanocellulose [12, 14]. Spraydrying and supercritical drying also require high installation and maintenance costs [12, 15].
Oven-drying is the fastest and most economical drying
method available, and holds good production scalability potential in terms of industrial use. However, the oven-drying of
nanocellulose produces highly aggregated lumps that
are poorly dispersible in aqueous media. Oven-dried
nanocellulose has a constricted structure due to the high surface tension of water [16], and the hydroxyl groups of ovendried nanocellulose form irreversible interfibrillar hydrogen
bonds [12]. Some studies reported that tert-butanol
(t-BuOH) can exfoliate condensed cellulose structure within
free-drying or extraction [17–19]. However, it effects on the
oven-drying of nanocellulose has been poorly reported.
As such, this paper suggests a strategy to improve the
re-dispersibility of oven-dried CNC, TEMPO-CNF, and
CNF in aqueous media, and to accelerate the rate of
oven-drying with the addition of t-BuOH. Within ovendrying, the addition of t-BuOH decreases contraction and
prevents interfibrillar contact and adhesion. If this process
is used in industry, t-BuOH can be readily collected and
recycled. In this study, the effects of concentration, ovendrying temperature, drying rate, and water/t-BuOH ratio on
the morphology and aqueous re-dispersion of the
dehydrated products were investigated. In addition, this
work would be valuable in the biofuel research field
[20–27]. The disintegrated cellulose structure produced
by this method can improve the enzymatic or catalytic
conversion of dried cellulosic biomass to biofuel e.g. glucose and hydroxymethylfurfural (HMF) because the breaking of the interfibrillar hydrogen bond network allows enzyme or catalyst to infiltrate [20–27].
Experimental
Materials
Never-dried nanocellulose (CNC, CNF, and TEMPOCNF) were purchased from the University of Maine (ME,
USA), and materials information is as follows. CNC is an
aqueous gel with a concentration of ~10 wt%, and features
fibers 5–20 nm wide and 150–200 nm in length. CNF is an
J Polym Res (2017) 24:191
aqueous gel with a concentration of ~3 wt%, with fibers
~50 nm wide and several μm in length. TEMPO-CNF is an
aqueous solution with a concentration of ~1 wt%, having
fibers 5–20 nm wide and several μm in length. t-BuOH
(anhydrous, ≥ 99.5%) was purchased from Sigma
Aldrich. Deionized water was used for all drying experiments. All chemicals were used without further purification. University of Maine does not provide the methods to
prepare the three types of nanocellulose in detail. General
methods for the three different nanocellulose are as follows. First, CNC is obtained by hydrolyzing cellulose pulp
in 64 wt% sulfuric acid (aq) for 25 min at 45 °C [2, 13].
Then, the acidic product is vigorously washed with water.
Second, TEMPO-CNF is obtained by oxidizing cellulose
pulp (1 g) in water (100 ml) with TEMPO (0.1 mmol),
sodium bromide (1 mmol), and sodium hypochlorite
(5 mmol) [3, 7, 9]. Then, the oxidized product is vigorously washed with water. Third, CNF is obtained by mechanically homogenizing wet cellulose pulp that is never-dried
during the purification process from wood to pulp [1, 6].
Preparation of nanocellulose solutions with t-BuOH
Co-solvents of water/t-BuOH were prepared in different volume ratios of 10:0, 9:1, 5:5, and 1:9. CNC, CNF, and
TEMPO-CNF dispersions were prepared with the concentration of 0.1 wt% in water only or co-solvent. Each 10 mL
sample was placed in a 20 mL glass vial and transferred into
a conventional lab oven at a fixed temperature. Never-dried
CNC, CNF, and TEMPO-CNF suspensions with cellulose
concentrations of 0.1 wt% were prepared as control groups.
Generally, CNC, CNF, and TEMPO-CNF are aqueous dispersion forms, and freeze-dried CNC, CNF, and TEMPO-CNF
are not readily dispersed in pure t-BuOH. It is practically
difficult to prepare nanocellulose dispersion in pure t-BuOH.
The drying effects of nanocellulose in 100% t-BuOH are
merely beyond the scope of the present study.
Page 3 of 11 191
Characterization of aqueous dispersions of oven-dried
nanocellulose
Particle size was determined by dynamic light scattering
(DLS, Malvern Zetasizer 3000 Malvern, Worcestershire,
UK). As DLS is a light scattering method, the particle size is
represented as the hydrodynamic diameter of an equivalent
sphere. Thus, it does not represent the actual physical dimensions of CNC rods, but for comparison purposes such data are
considered valid. CNF and TEMPO-CNF have dimensions of
at least several micrometers. Thus, DLS cannot characterize
nanoscale dispersions of CNF and TEMPO-CNF. All measurements were conducted in triplicate at 25 °C. UV-Vis transmittance data is one of the well-known tool for analyzing
TEMPO-CNF dispersions [16]. Remarkable low transmittance value of CNF is caused by its unfibrillated or partly
fibrillated cellulose contents. UV-Vis transmittance spectra
were recorded using a Shimadzu UV-2600 (Kyoto, Japan) to
analyze TEMPO-CNF. Dispersion stability for CNF was qualitatively examined by cross polarizer images and precipitation
tests after 1 day. Scanning electron microscopy (SEM, Hitachi
S-4800, Japan) was utilized for the microstructural analysis of
dried nanocellulose samples. Chemical analyses were performed with a Fourier transform infrared (FT-IR) spectrometer (iS50-FT-IR, NICOLET, Wisconsin, USA) using KBr pellets at the range of 4000–500 cm−1 (wavenumbers). Inherent
densities of the samples were obtained using a helium pycnometer (Accupyc 1340, Micromeritics, GA, USA). The residual solvent content of oven-dried nanocellulose was investigated using a thermal gravimetric analysis (TGA, Pyris 1,
Perkin Elmer, USA). Analysis of individual oven-dried
nanocellulose samples weighing 3–5 mg was carried out at
a constant heating rate of 10 °C/min from room temperature
to 220 °C [28]. The residual solvent content is the weight loss
with the heating.
Results and discussion
Sample preparation
Oven-drying and re-dispersion
The oven-drying of nanocellulose suspensions in water and
water/t-BuOH co-solvents were carried out at 80, 100, and
120 °C for 8, 12, 16, 20, and 24 h. BFully-dried^ conditions
were defined as <0.1% weight change at room temperature
under 20–30% relative humidity for 1 h. The fully dried samples were re-dispersed in water with bath sonication for
20 min to obtain aqueous nanocellulose dispersions at
~0.1 wt%. Lab-scale freeze-drying was conducted using a
freeze dryer (TFD5505, Ilshin, Korea) at −50 °C. Before
freeze-drying, 10 mL samples were placed into 20 mL glass
vials and frozen at −70 °C for at least 6 h.
We formulated a hypothesis that the water/t-BuOH co-solvent
system in the oven-drying of nanocellulose has three advantages over pristine water. First, t-BuOH can accelerate this
drying rate due to its higher vapor pressure (4.1 KPa at
20 °C) and lower boiling point (82.2 °C) compared to water
(2.3 KPa at 20 °C, and 100 °C). Second, it can reduce the
shrinkage of nanocellulose suspensions during drying process
due to its low surface tension [16, 29, 30]. Third, after ovendrying, a remaining water/t-BuOH mixture can give more
increased interfibrillar distance than water only because
t-BuOH has higher molecular size than water; it can reduce
interfibrillar contacts and adhesion [31] (Scheme 1).
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J Polym Res (2017) 24:191
Scheme 1 Expected mechanism
of t-BuOH-mediated drying of
nanocellulose: weak and open
structure gives re-dispersion of
dried nanocellulose
To demonstrate this behavior, CNC, CNF, and TEMPO-CNF
suspensions of ~0.1 wt% in water only and co-solvents were
oven-dried at 80, 100, and 120 °C for 8, 12, 16, 20, and 24 h.
nanocellulose by a factor of 2–12 compared to water-only
oven-drying and freeze drying.
Re-dispersibility of t-BuOH-mediated oven-dried CNC
Improved oven-drying rate of nanocellulose with t-BuOH
The drying rates of co-solvents in the presence of
nanocellulose were evaluated at different temperatures as
shown in Scheme 1 and Fig. 1. As a standard oven-drying
method, 10 mL samples in 20 mL vials were dried in an
oven. Naturally, the drying rate gradually increased as the
temperature. As expected, the drying rate gradually increased as the increasing t-BuOH contents due to the higher
vapor pressure and lower boiling point of t-BuOH (compared to water). At 80 °C, the t-BuOH addition increased
the drying rate of nanocellulose, suggesting that t-BuOH
addition can reduce the required oven drying temperature
for nanocellulose to below the boiling point of water. In a
lab-scale comparison (10 mL liquid in a 20 mL vial), this
oven-drying method shows a notable improvement in drying
rate compared to freeze-drying, an already-established method for dehydrating nanocellulose. Laboratory freeze-drying
generally takes 100–120 h to fully dry 10 mL of these three
different nanocellulose suspensions. However, oven-drying
at 80 °C generally required 2.5–8 h to fully dry these same
three nanocellulose suspensions (10 mL). Overall, the
t-BuOH mediated oven-drying lowered the drying time of
CNC suspensions (10 mL, 0.1 wt%) with different t-BuOH
contents were oven-dried at different temperatures (Fig. 2). At
80 °C (below the boiling point of water), the CNC suspensions with t-BuOH contents of 0–10 vol% were not fully dried
after 16 h due to high water contents.
The fully oven-dried CNC samples were re-dispersed in
water to the concentrations of 0.1 wt%. Figure 2 shows that
the aqueous re-dispersibility of the oven-dried CNC samples
which were evaluated by a zeta particle size analyzer. Neverdried CNC samples at 0.1 wt% concentration were used as a
control group. At all studied temperatures, the average particle
size of oven-dried CNC steadily decreased as the increasing
t-BuOH concentration to 70–80 nm, comparable to that of the
control sample. This implies that the t-BuOH-mediated ovendried CNC samples were exfoliated in water as much as the
never-dried CNC samples. Generally, the water-only ovendried CNC had an average particle size that increased with
increasing temperature and drying time, likely due to the formation of hydrogen bonds and cross-links between individual
nanocrystals [32]. Prolonged drying times and higher temperatures remove surface-bound water and degrade the porous
structures into which water molecules can move [13]. It was
difficult to investigate the aggregation of CNC samples using
J Polym Res (2017) 24:191
Page 5 of 11 191
CNF showed >98% transmittance at 400–800 nm (Figure S3
and S4). At 80 °C, the TEMPO-CNF suspensions prepared
with 0–10 vol% of t-BuOH were not fully dried after 16 h due
to the high water contents. At this temperature, the light transmittance of the dried samples typically decreased with increasing drying time and decreasing t-BuOH content, dropping
below 90% at 400–500 nm due to the removal of surfacebound water molecules (Fig. 3 and S4). However, the dried
samples prepared with >50% t-BuOH exhibited >96% transmittance at this same wavelength range and temperature
(Fig. 3 and S4). This suggests that t-BuOH-mediated ovendrying improved the aqueous re-dispersibility of the fullydried TEMPO-CNF. At drying temperatures >100 °C, the
light transmittance of the samples did not show a distinct
dependence on the t-BuOH contents and exact temperature;
the samples prepared at >100 °C showed remarkably low light
transmittance regardless of the t-BuOH content. Hightemperature thermal treatment induced yellowing and
interfibrillar cross-linking of TEMPO-CNF [32], because
TEMPO-CNF has reactive aldehyde and carboxylate group
[33]. It is worth noting that oven-drying at 120 °C in the
presence of water/t-BuOH (1:9) gave the lowest value of
transmittance (<80%) among all samples (Fig. 3 and S4).
This can be explained in that the aldehyde and carboxylate
groups of TEMPO-CNF reacted with t-BuOH to form a hydrophobic nanocellulose surface [34].
Re-dispersibility of oven-dried CNF
Fig. 2 Average zeta particle size of aqueous re-dispersed CNC samples
oven-dried for different times at (a) 80, (b) 100, and (c) 120 °C. The water
only and water/t-BuOH (9:1) samples were not fully dried at 80 °C for
16 h (red box)
UV-Vis spectroscopy because the increased particles sizes in
oven-dried CNC were smaller than the 400–800 nm radiation
used in such studies. Thus, UV-Vis spectra shows >90% of
transmittance for all examined samples (Figure S1 and S2).
Re-dispersibility of t-BuOH-mediated oven-dried
TEMPO-CNF
The fully oven-dried TEMPO-CNF samples were redispersed in water to concentrations of 0.1 wt%. The redispersibility of the TEMPO-CNF was evaluated by UV-Vis
transmittance (Fig. 3). As a control, never-dried TEMPO-
The oven-dried CNF samples were re-dispersed in water to
concentrations of 0.1 wt%. The aqueous re-dispersibility was
qualitatively assessed for each CNF sample by observing precipitation after 1 day of re-dispersion (Fig. 4). DLS and UVVis are not suitable methods to evaluate the aqueous redispersibility of CNF because of the high opacity and particle
sizes. Except for the CNF re-dispersion sample prepared with
90% t-BuOH at 80 °C, all other CNF samples exhibited precipitation within 1 day (Fig. 4). This behavior indicates that
high t-BuOH contents (~90%) are necessary for high redispersion stability. After the water only oven-drying, redispersed CNC and TEMPO-CNF samples featured partial
aggregation but did not show precipitation (Figure S1 and
S3). However, after the water only oven-drying, re-dispersed
CNF featured high aggregation, and precipitated in aqueous
media due to a lack of surface repulsive charges on CNF [35].
Thus, compared to CNC and TEMPO-CNF, the aqueous redispersibility of oven-dried CNF was significantly enhanced
by the t-BuOH addition.
Morphology of oven-dried nanocellulose
In order to investigate the cause of the improved redispersibility in oven-dried nanocellulose, the morphologies
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J Polym Res (2017) 24:191
Fig. 3 UV-Vis transmittance
spectra of re-dispersed TEMPOCNF samples after oven-drying at
80, 100, and 120 °C with (a–c)
water only and (d–f) 9:1, (g–i)
5:5, (j–l) 1:9, and water/t-BuOH
co-solvents. The water only and
water/t-BuOH (9:1) samples were
not fully dried at 80 °C for 16 h
(red box)
of oven-dried CNC, TEMPO-CNF, and CNF samples dried at
80 °C for 24 h with different t-BuOH contents were characterized by SEM. The CNC, TEMPO-CNF, and CNF samples
that were oven-dried with t-BuOH contents of <10% showed
compact lamellar structures and smooth particle surfaces. As
the t-BuOH contents in the nanocellulose suspension increased, the inter-lamellar gaps and interfibrillar spacing of
the three types of oven-dried nanocellulose increased correspondingly. In the SEM images of the samples prepared with
90% t-BuOH (Fig. 5d, h, l), the nanofibril texture of the different oven-dried nanocellulose types can be observed. This
suggests that the interfibrillar distance of nanocellulose was
relatively conserved by t-BuOH addition within the context of
oven-drying. The comparatively more porous structure of
Fig. 4 Sedimentation of aqueous re-dispersed CNF samples oven-dried at 80, 100, and 120 °C with water only and water/t-BuOH co-solvents. The
water only and water/t-BuOH (9:1) samples were not fully dried at 80 °C for 16 h (red box)
J Polym Res (2017) 24:191
Page 7 of 11 191
Fig. 5 SEM images of (a–d) CNC, (e–h) TEMPO-CNF, and (i–l) CNF dehydrate samples oven-dried with water only and water/t-BuOH co-solvents
oven-dried nanocellulose with t-BuOH could allow water
molecules to penetrate into the dried nanocellulose [36–38].
The lower surface tension of t-BuOH compared to water may
minimize the capillary force effects of water during ovendrying [12, 16, 29]. The utilization of t-BuOH in a cosolvent dispersion not only increased the drying rate but also
reduced the nanofibrillar aggregation in order to avoid molecular contact and adhesion.
To further study the water-accessible morphology, we measured the density of oven-dried CNC, TEMPO-CNF, and
CNF samples dried at 80 °C for 24 h with different t-BuOH
contents using a gas pycnometer using Helium (Table S1).
The water only oven-dried CNC, TEMPO-CNF, and CNF
samples had the density of 1.58, 1.65, and 1.70 g/cm3, respectively. The t-BuOH-mediated oven-dried CNC, TEMPOCNF, and CNF samples had the density of 1.53, 1.60, and
1.56 g/cm3, respectively. Thus, the t-BuOH oven-drying increased the porosity of oven-dried nanocellulose.
Initial concentration effect
The high initial concentration of nanocellulose can shorten the
interfibrillar distance during the drying [13]. To investigate the
effects of the initial nanocellulose concentration to the aqueous re-dispersibility, higher concentration nanocellulose suspensions (1 wt% for CNC, 0.5 wt% for TEMPO-CNF, and
0.5 wt% for CNF) with water/t-BuOH content (1:9) were oven-dried. The morphology of these oven-dried CNC,
TEMPO-CNF, and CNF dehydrate samples were observed
using SEM. They presented analogous porous structure compared to the samples prepared with nanocellulose content ≈
0.1 wt% (Fig. 6a). Then, they were respectively re-dispersed
in water to the concentrations of 0.1 wt%. The re-dispersed
suspensions were observed between cross polarizers (Fig. 6b).
The presence of birefringence is typically an indication of
individual nanocellulose dispersion [39].
The limitation of this research is that the effects of much
high initial nanocellulose concentration (>several wt%) on
t-BuOH-mediated oven-drying cannot be investigated because never-dried CNC, TEMPO-CNF, and CNF have the
maximum concentrations of ~10, 1–2, and 1–2 wt%, respectively [1–5]. Hopefully, if a way to exfoliate pulp into
nanocellulose in a t-BuOH solution is invented [40, 41], it
can be possible to study the effects of high nanocellulose
concentration on oven-drying.
Water/t-BuOH solvent remaining on nanocellulose
after oven-drying
The residual solvents after oven-drying was further investigated using FT-IR, with spectra of CNC, CNF, and
TEMPO-CNF oven-dried in water-only or water/t-BuOH
(1:9) solvents being collected. The water only oven-dried
CNC, CNF, and TEMPO-CNF displayed absorption bands
at 3200–3600, 2850–3000, 1410–1430, and 1050–
1150 cm−1, which are assigned to the stretching modes of
-OH, −CH, CH2, and O-C, respectively [42, 43]. The peaks
at 3300–3450 cm−1 (assigned to OH groups) have a different peak shape in the spectra of nanocellulose samples
oven-dried with t-BuOH compared to those from the water
only oven-dried samples, likely due to the presence of
t-BuOH on the nanocellulose surface. The peak around
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J Polym Res (2017) 24:191
Fig. 6 a SEM images of (top) CNC (1 wt%), (middle) TEMPO-CNF
(0.5 wt%), and (bottom) CNF (0.5 wt%) dehydrate samples oven-dried
with water only and water/t-BuOH (1:9). b Cross polarizer images of
aqueous re-dispersed samples of (a). c FT-IR spectra of (top) CNC,
(middle) TEMPO-CNF, and (bottom) CNF dehydrate samples with water
only and water/t-BuOH (1:9)
1640 cm−1 is originated from absorbed water on the hydroxyl group of nanocellulose. This peak in the spectra of
nanocellulose samples oven-dried with t-BuOH have a different peak shape compared to those from the water only
oven-dried samples. It also supports our inference.
Interestingly, t-BuOH remained on the cellulose surface
after oven-drying, although it has a lower boiling point than
water. The affinity of t-BuOH towards cellulose can be attributed to the combined contribution of 1) the hydrogen bonding
between hydroxyl groups and 2) the hydrophobic interactions
between the butyl group of t-BuOH and the inherent hydrophobic domains of nanocellulose [44]. According to molecular geometry, hydrophobic interactions can strongly contribute
to aqueous adhesion [45–47]. Probably thus, a t-BuOH/water
mixture remained on the cellulose surface in aqueous environment within the context of oven drying.
Other alcohol solvent effect
We observed that isopropanol (C3) with similar boiling point
of 82 °C provided good aqueous re-dispersion of dried
nanocellulose, but ethanol (C2) and methanol (C1) with lower
boiling point gave precipitation probably because they did not
remain on the cellulose surface after drying. The aqueous
dispersion state of oven-dried CNF with methanol, ethanol,
and isopropanol were presented in supporting information
J Polym Res (2017) 24:191
(Figure S5). 1-BuOH (C4), 1-pentanol (C5) and 1-hexanol
(C6) mixtures were hardly dried in our mild experimental
conditions because 1-BuOH, 1-entanol, and 1-hexanol have
higher boiling point of 117, 137, and 158 °C respectively than
water. In a future, in-depth study for other solvents effects
including non-alcohols in terms of surface tension and boiling
point is strongly required.
Aggregation mechanism of aqueous nanocellulose
within oven-drying
To solve the poor aqueous re-dispersion issue of nanocellulose
after oven-drying, the oven-drying mechanism was addressed
in detail. The oven-drying of nanocellulose takes place in
three major steps (Fig. 7a); the bulk water evaporates first,
causing capillary convection to occur because the high surface
tension of water pulls nanocellulose together [13, 29, 30]. As a
result, the dimensions of the nanocellulose suspension decrease. At the second stage, the surface-bound water of the
nanocellulose begins to evaporate. At the final third stage, the
Page 9 of 11 191
nanofibrils come into contact, and the interfibrillar adhesion
occurs due to the absence of water molecules (Fig. 7b).
It is difficult to directly measure and compare the interfibrillar
adhesion energy of nanocellulose in the presence and absence of
water due to complex structures of nanocellulose. It can be
presumed from an interaction force between small molecules ∝
1/ε where ε is dielectric constant of medium, and ε of vacuum
and water are 1 and ~80, respectively [48, 49]. Thus, the fully
dried nanocellulose is poorly re-dispersed in water because the
absence of water gives strong molecular interaction.
Conclusion
We devised a scalable and fast oven-drying method for CNC,
CNF, and TEMPO-CNF nanocellulose, one which is capable
of improving the aqueous re-dispersion of dried nanocellulose.
The addition of t-BuOH greatly raised the drying rate of aqueous nanocellulose, up to ~3 times that of oven-drying in the
absence of t-BuOH and ~3–20 times that of freeze-drying. This
Fig. 7 a Three different types of nanocellulose (CNC, TEMPO-CNF, and CNF) with their representative surface functional groups and drying
processes. b Nanocellulose contact and adhesion after dehydration
191
t-BuOH-mediated oven-dried nanocellulose showed a relatively more porous structure that was permeable to water molecules. After oven-drying, t-BuOH bound on the nanocellulose
surface reduced interfibrillar adhesion and contact. The redispersed CNC and TEMPO-CNF after t-BuOH-mediated
oven-drying showed a similar level of aqueous dispersibility
as the never-dried samples. In case of CNF, only the sample
oven-dried with 90% t-BuOH at 80 °C has relatively stable redispersion in terms of precipitation. This strategy will contribute to overcoming the high drying costs that are a major obstacle to commercial nanocellulose production. In industry,
t-BuOH can be collected and recycled using a simple condenser. In the present study, the oven-drying experiments of
nanocellulose were conducted at constant temperature. Thus,
there is still much to be learned about the temperature gradient
effects to nanocellulose re-dispersion and morphology within
the oven drying.
Acknowledgements This work was financially supported by the
Technology Innovation Program (10070150) funded by the Ministry of
Trade, Industry and Energy (MOTIE, Korea), and partially supported by a
grant from Research Center for Chemical Biotechnology of Korea
Research Institute of Chemical Technology (SI 1709).
Author contributions All authors have given approval to the final
version of the manuscript. Z. H. and H. J. conducted the experiments.
H. J., J. J., S. Y. H., J. P., and D. X. O. wrote the manuscript. S. P. and S. K.
conducted supplementary experiments and literature survey.
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