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펄프·종이기술 51(2) 201988
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
Sung-Jun Hwang and Hyoung-Jin Kim†
Received March 14, 2019; Received in revised form April 11, 2019; Accepted April 19, 2019
ABSTRACT
Cellulose nanofiber (CNF) has a wide range of applications due to its advantageous prop-
erties, including renewability, biodegradability, high mechanical strength, dimensional
stability, thermal stability, and high resistance to water. Consequently, much research is
focused on its development and improvement. CNF is obtained most widely from both
softwood and hardwood, but it can be also be sourced from non-wood based materials
and micro organisms. However, improved living standard and economic growth combine
to raise the price of wood annually, coinciding with increased production of wood
products such as paper, tissue, wrapper, etc. Thus the use of non-wood based materials
as an alternative to wood pulp is increasing in a variety of industries all over the world.
In this study, we analyzed CNF manufactured from paper mulberry bast fiber in order to
confirm its applicability to various industries. After pre-treatment of the paper mulberry
bast pulp, a wet disk-mill and high pressure homogenizer (HPH) were employed se-
quently to manufacture CNF, which was then characterized by scanning electron micros-
copy (SEM), thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), and
X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR), and the
degree of polymerization (DP), tensile strength and elongation were measured. We
conclude that it is feasible to manufacture high quality CNF from paper mulberry bast fi-
ber.
Keywords: HwBKP, paper mulberry bast fiber, CNF, WDM, HPH
• Kookmin University Industry-Academic Cooperation Foundation, Kookmin University, Seoul, 136-702, Republic of Korea
† Corresponding Author: E-mail: [email protected]
Journal of Korea TAPPIVol. 51. No. 2, 2019, 88-99pISSN (Print): 0253-3200Printed in Korea http://dx.doi.org/10.7584/JKTAPPI.2019.04.51.2.88
1. Introduction
Various industries utilize wood as a raw material,
but for many that wood must originate from forest
more than 50 years old. However, in South Korea,
only 10% of forests are more than 30 years old
and, furthermore, due to environmental changes
resulting from global warming and desertification
there are severe shortages of wood.1)
Many advanced countries, especially those in
J. of Korea TAPPI Vol.51 No.2 Mar.-Apr. 2019 89
Sung-Jun Hwang·Hyoung-Jin Kim
Europe, the United States and Japan, have actively
been conducting advanced research on the manu-
facture of bio-composites, and they are now in the
commercialization phase using non-wood based
materials.2,3) In South Korea, however, research on
cellulose nanofiber using a non-wood based mate-
rial is less advanced.4)
A non-wood based material to replace wood
would be an excellent alternative for utilization in
a variety of industries. A promising candidate is
bast from the paper mulberry (Broussonetia papy-
rifera), which grows fast in South Korea whose
climate, soil, and precipitation levels are all suit-
able, and it does not require the use of pesticides.5)
Also, paper mulberry bast has high tensile and tear
strength characteristics, and has consequently
been used in products such as wallpaper, sound-
proofing material, and sanitary ware. The medical
product industries have also exploited its noise in-
sulation, antibacterial and fragility properties.6-8)
However, in South Korea, paper mulberry bast is
largely imported from China and Southeast Asia
due to the high labor costs and low production in
South Korea.9) Furthermore, in South Korea it is
produced by traditional manual processing so the
quality is not sufficiently uniform for commercial-
ization.10)
Recently, Company ‘A’ has introduced automated
paper mulberry bast pulp production in South
Korea, and mass production is now possible while
maintaining the characteristics of the fiber itself,
thus the manufacturing cost can be reduced and a
consistent quality maintained.11)
In the knowledge that research into cellulose
nanofiber (CNF) in South Korea has been held back
through lack of raw materials, this study aimed to
confirm the applicability to various industries of
CNF originating in South Korea from the new au-
tomated facility. For comparison (the control) we
selected hardwood bleached kraft pulp (HwBKP),
the most commonly used raw material in CNF
research in South Korea. After chemical composi-
tion analysis, the paper mulberry bast fiber was
pre-treated and then, due to its fiber strength and
length, it was processed through a wet disk-mill.
Two of CNFs were isolated by mechanical treat-
ment via wet disk-mill (WDM) and high-pressure
homogenizer (HPH) sequently. The resulting CNF
was then characterized by field emission scanning
electron microscopy (FE-SEM), thermogravimetric
analysis (TGA), derivative thermogravimetry
(DTG), X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FT-IR), degree of polymer-
ization (DP), tensile strength and elongation.
2. Materials and Methods
2.1 MaterialsPaper mulberry bast pulp was purchased from the
Forest Cooperative Federation in Icsan, Korea. The
pulp was derived from branches of trees between
1 and 3 years old that were pulped using the auto-
mated process at Company ‘A’ in Korea. The pulp-
ing process conditions are shown in Table 1. HwBKP
was purchased from Company ‘B’ in Korea as a
control pulp.
2.2 Methods2.2.1 Chemical composition
The composition of the extractives was analyzed
by the alcohol-benzene extraction method accord-
ing to KS M 7039 standard. The klason lignin
Table 1. Pulping process conditions of paper mulberry bast fiber
Condition Control
Cutting length, (cm) 5-10 cm
Chemical dosage, (%) NaOH 20%
Liquor to solid ratio 1:10
Cooking temperature, (℃) 105℃
Cooking time, (min) 90 min
펄프·종이기술 51(2) 201990
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
content was analyzed according to the KS M 7045
standard, and the holocellulose content was ana-
lyzed according to the method. The ash content
was analyzed according to KS M ISO 1762 stan-
dard.
2.2.2 Pre-treatment process of paper mulberry bast for the manufacture of CNF
In a WDM, due to their length and strength, the
fibers in paper mulberry bast pulp, become tangled
together, and do not receive frictional force
between the two millstones (GA6-120SD type).
Furthermore, when pulp is injected into a HPH
without pre-treatment, the pipelines become
blocked and the fibers are not subject to the shear
force. Therefore, paper mulberry bast pulp requires
pre-treatment before it is passed to either a WDM
or HPH.
Pulping chemicals remaining on the paper
mulberry bast pulp were removed by a washing
process. Reject fibers were filtered out using a
Somerville screen (3.0 mm hole type). Fibers pass-
ing through the screen were then injected into a
standard pulp disintegrator (Pulp disintegrator,
L&W, Sweden), and the pulp was dried for 24 h in
an oven dryer adjusted at 105℃. The dried pulp
was cut by hand, then subjected to a beating pro-
cess using a valley beater (LB-20, Metrotec.,
Spain) for 1 h before processing in the WDM.
2.2.3 Manufacture of CNF suspensions
HwBKP and paper mulberry bast pulp were sus-
pended at 1.0 wt.% concentration. The suspensions
were then passed through a WDM (Wet disk-mill,
MKCA6-2, Masuko Co. Ltd., Japan). The suspen-
sion rotation speed was set at 1,800 rpm, and the
clearance between upper and lower disks was re-
duced to 200-230 μm from the zero point. The
pulp was passed through the WDM up to 20 times.
A high concentration slurry of manufactured mi-
crofibrillated cellulose with WDM was adjusted to
0.2 wt% to pass through the high pressure homog-
enizer (MN400BF, Picomax, Germany), whose
pressure was adjusted to 20,000 psi. The two fiber
types were passed up to 20 times through the HPH.
2.2.4 Preparation of CNF samples for analysis
2.2.4.1 Preparation for FE-SEM
The manufactured CNF solutions were diluted to
0.001 wt.%, and then sonicated (Sonic Dismembra-
tor Model 100, Fisher Scientific Inc., USA) for
1 min. The suspensions were vacuum-filtrated on
a polytetrafluoroethylene (PTFE) membrane filter
with a pore size of 0.2 μm (ADVANTEC, Japan).
The filtrated products, remaining on the PTFE fil-
ter, were immersed in tert-butyl alcohol for 30
min. This procedure was repeated 30 times to
completely exchange water with tert-butyl alcohol,
and after which the sample was freeze-dried using
a freeze dryer (FDB-5520, Operaon Co. Ltd.,
Korea) at -55℃ for 3 h to prevent the aggregation
of the CNF. The freeze-dried samples were coated
with osmium tetroxide using an osmium plasma
coater (HPC-1 SW, Vacuum Device Inc., Japan).
2.2.4.2 Preparation for FT-IR, XRD
The CNF solutions manufactured via HPH were
put into a tube, and homogenized with tert-butyl
alcohol. The homogenized slurries were suspended
at 1.0 wt.% concentration and centrifuged with
10,000 rpm. After removing the supernatant, the
sediment was put into a tube and freeze-dried
using a freeze dryer (FDB-5520) at -55℃ for 3 h.
The resulting CNF samples were labeled and used
for FT-IR and XRD (Table 2).
2.2.4.3 Preparation for TGA, DP, tensile strength and
elongation
CNF suspensions of 0.2 wt.% (220 mL) were pre-
pared and sonicated for 1 min, then vacuum-
filtered on a silicone-coated filter (Phase Separator
Paper, 123 mm). The products were compressed
J. of Korea TAPPI Vol.51 No.2 Mar.-Apr. 2019 91
Sung-Jun Hwang·Hyoung-Jin Kim
with a press at a pressure of 1 kgf for 10 min and
the compressed samples were dried and pressed
again using a drum dryer (Gon, 400r, GIST Co.
Ltd.).
Sheets of CNF with a diameter of 72±3 mm were
manufactured, according to the processing condi-
tions and methods shown in Table 3.
2.2.5 Field emission scanning electron microscope (FE-SEM)
CNF morphology was examined using FE-SEM
(Hitachi S-4800, Hitachi Ltd., Japan) with an ac-
celerating voltage of 5 kV. Before examination,
samples were coated with osmium tetroxide using
an osmium plasma coater (HPC-1 SW) for 10 sec-
onds to eliminate the electron charging effects. The
diameter of at least 500 individual fibers was
measured for each sample by Image J software
(Image J 1.45, National Institute of Health (NIH),
USA).
2.2.6 FT-IR
Changes in functional groups of the samples at
each step were determined by FT-IR (Frontier,
PerkinElmer Inc., UK) according to the ATR
method in the frequency range of 4,000-400 cm-1,
at a resolution of 4 cm-1.
2.2.7 XRD
XRD patterns of the sample were obtained using
a HRXRD (Panalytical, X’pert-proMPD, Nether-
lands) with Cu-Kα radiation: 10-60° (2θ). The op-
erating voltage and current were 40 kV and 25 mA,
respectively. The crystallinity index of CNF was
Table 2. Manufacturing conditions of CNF samples by different mechanical treatments
Material Method Treatment conditions Sample name
Paper mulberry bast fiber
Valley beater 1 h P-Blank
WDM10 passes P-W10
20 passes P-W20
HPH10 passes P-H10
20 passes P-H20
HwBKP
Pulper only 20 min H-Blank
WDM10 passes H-W10
20 passes H-W20
Table 3. Manufacturing conditions of CNF sheet by different mechanical treatment
Material Method Treatment condition Sample name
Paper mulberry bast fiber
Valley beater 1 h P-Blank sheet
WDM 20 passes P-W20 sheet
HPH
5 passes P-H5 sheet
10 passes P-H10 sheet
20 passes P-H20 sheet
HwBKP
Pulper only 20 min H-Blank sheet
WDM 20 passes P-W20 sheet
HPH
5 passes H-H5 sheet
10 passes H-H10 sheet
20 passes H-H20 sheet
펄프·종이기술 51(2) 201992
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
calculated according to Segal et al. (1959),12) and
the average crystallite size was calculated accord-
ing to Monshi et al. (2012).13)
2.2.8 TGA
The thermal stability of each sample was deter-
mined using a thermogravimetric analyzer (SDT
Q600, TA Inc., UK). A sample (20 mg) was heated
from 20℃ to 600℃ at a rate of 10℃/min under a
nitrogen atmosphere.
2.2.9 DP
To a sample (0.25 g) of the manufactured CNF
sheet was added 25 mL of distilled water and
25 mL of cupriethylenediamine (CED) sequentially,
and the mixture stirred until the samples were
dissolved in the CED solution. A sample of the CED
solution was placed into a capillary viscometer (SI
Analytics GMbH, Germany) to measure the cellu-
lose viscosity by determining the limiting viscosity
number in the CED solution (standard KS M ISO
5351). Then, after substituting values in the fol-
lowing formula,14) the calculated result values were
multiplied by the molecular weight of cellulose in
order to convert from viscosity to DP.
V K DP= ( )
where:
V=cellulose viscosity, mPa·s (cP)
K, ɑ=Mark-Houwink-Sakurada constant,
(K=0.98×10-2, ɑ=0.905)
DP=cellulose DP
2.2.10 Tensile strength and elongation
Samples of manufactured CNF sheets were cut to
the dimensions 5×0.7-0.8×50 mm (width×thick-
ness×length) and dried for 24 h in an oven dryer
at 23±1℃ and 50±2% relative humidity according
to standard KS M ISO 187. The tensile and elonga-
tion were measured using a testing machine (GB/
H50K, Tinius Oslen, USA), at a cross-head speed
of 10 mm/min, with a specimen span length of
30 mm.
3. Results and Discussion
3.1 Chemical compositionTable 4 summarizes the chemical composition of
the paper mulberry bast and HwBKP fibers. The
holocellulose, lignin, extractives and ash content of
paper mulberry bast fiber were measured at
93.23%, 1.76%, 2.84%, and 4.09% respectively.
Even though the paper mulberry bast fiber under-
went a mass automated pulping process, the char-
acteristics of the original non-wood based material
(high cellulose and low lignin content) are main-
tained. In addition, the lignin, extractives and ash
levels of HwBKP were higher than those of the
paper mulberry bast fiber; this is due to the
HwBKP bleaching process which involves the use
of H2O2.
3.2 FE-SEMFE-SEM images of the paper mulberry bast and
HwBKP fibers are shown in Figs. 1-2, respectively.
The micrographs show that the diameter of the
Table 4. Chemical composition of paper mulberry bast and HwBKP fibers
Component Paper mulberry bast fiber HwBKP
Holocellulose, (%) 93.2 97.3
Lignin (klason), (%) 1.8 0.3
Extractives (alcohol-benzene), (%) 2.8 0.2
Ash, (%) 4.1 0.5
J. of Korea TAPPI Vol.51 No.2 Mar.-Apr. 2019 93
Sung-Jun Hwang·Hyoung-Jin Kim
paper mulberry bast fibers lies within the range
150-500 nm after 10 passes through the WDM,
while after 20 passes the diameter decreases to
100-300 nm.
The diameter of the HwBKP fibers lies within the
range 120-300 nm after 10 passes through the
WDM, while after 20 passes the diameter decreases
to 120-200 nm. The WDM treatment reduced both
the diameter and length of the cellulose fibers
through frictional force between two millstones,
the size of the cellulose fibers remained below 1 μm.
The average diameters of the paper mulberry bast
fibers after 5, 10, and 20 treatments by HPH were
74, 58, and 50 nm, caused by the shear forces
arising from the high temperature and pressure
during HPH. In addition, the average diameters of
the HwBKP fibers after 5, 10, and 20 HPH treat-
ment were 74, 62, and 45 nm, respectively. It is
believed that 5 HPH treatments was sufficient to
produce CNF with a diameter of less than 100 nm,
however, after 5 passes the tangled fibers were
dispersed instead of reduced in size.15)
3.3 FT-IRFT-IR spectra of the cellulose fibers are shown in
Fig. 3. The O-H stretching intramolecular hydro-
gen bonds for cellulose I was shown in the spectral
bands at 3,175-3,490 cm-1, and C-H stretching was
shown in the spectral bands at 2,850-2,970 cm-1.
The C-O stretching vibration for the acetyl, uronic
ester linkages in hemicellulose, and carboxylic
group of ferulic, p-coumeric acids in lignin were
shown in the spectral bands at about 1,737 cm-1. In
addition, a spectral band was observed in the re-
gion 1,645 cm-1 due to O-H bending from absorbed
water. A peak at 1,428 cm-1 is due to CH2 scissor-
ing motion in cellulose. Furthermore, peaks at
1,370 cm-1 (C-H bending), 1,335 cm-1 (OH plane
bending), 1,317 cm-1 (CH2 wagging), 1,055 cm-1
(C-O-C pyranose ring stretching) were observed,
Fig. 1. FE-SEM Images (×3,000) of paper mulberry bast fibers: (a) WDM 10 passes, (b) WDM 20 passes, (c) HPH 10 passes, (d) HPH 20 passes.
Fig. 2. FE-SEM Images (×3,000) of HwBKP fibers:(a) WDM 10 passes, (b) WDM 20 passes, (c) HPH 5 passes, (d) HPH 10 passes.
펄프·종이기술 51(2) 201994
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
and the spectral bands observed at 1,032 cm-1 and
898 cm-1 are typical of cellulose.16,17) Although, the
pulps were treated by mechanical treatment, the
functional groups of the fibers remained un-
changed.
3.4 XRDThe XRD patterns of the HwBKP and paper mul-
berry bast fiber treated with WDM and HPH are
shown in Fig. 4. The two samples exhibited a sharp
high peak at 2θ=22.7° and a weaker diffraction
peak at 2θ=15°, both of which are attributable to
cellulose I.18) The calculated crystallinity index (CrI)
and the average crystallite size (ACS) of the
HwBKP and paper mulberry bast fibers after me-
chanical treatment are shown in Table 5. It can be
seen that the CrI and ACS of two types of fiber de-
creased after WDM and HPH treatment. This was
probably due to the breakdown of cellulosic hydro-
gen bonds by the impact of the shear force and
pressure from WDM and HPH. However, as the
number of passes through WDM increased, the
ACS of samples increased, this is probably due to
the agglomeration of cellulose by WDM treatment.
On the contrary, as the number of passes through
HPH increased, the ACS of samples treated with
HPH decreased, it is probably due to the dispersed
effect of fibers’ bundles by HPH treatment.19)
Fig. 3. FT-IR spectra of nanofibers after WDM and HPH treatment of (a) paper mulberry bast fiber and (b) HwBKP.
J. of Korea TAPPI Vol.51 No.2 Mar.-Apr. 2019 95
Sung-Jun Hwang·Hyoung-Jin Kim
3.5 TGATGA and derivative thermogravimetry (DTG)
were performed to study the thermal properties of
the paper mulberry bast and HwBKP fibers arising
from WDM and HPH treatment. The moisture was
removed in the temperature range 60-110℃, the
hemicellulose was substantially decomposed in the
temperature range 220-300℃, and the cellulose
and lignin were substantially decomposed in the
temperature range 290-400℃ (Fig. 5). Above 400 ℃,
black carbonaceous residue, known as char, re-
mained.16,20) Table 6 summarizes that the data for
maximum decomposition temperature, the tem-
perature at a weight loss of 10%, 50%, and the
weight loss ratio at 400℃. As the number of passes
through WDM and HPH increased, the maximum
decomposition temperature and the temperature at
a weight loss of 10% and 50% for two fiber types
decreased. Furthermore the decomposition tem-
perature of paper mulberry bast fibers was higher
than that for HwBKP fibers. In addition, the DSC
curve showed two large peaks centered at 310℃
and 430℃ due to exothermic reactions from cellu-
lose.21)
3.6 DPThe DP for the paper mulberry bast was 2 or
3 times higher than that of HwBKP, and remained
higher for all treatments (Fig. 6). The paper mul-
berry bast fibers maintained a DP as high as natu-
ral non-wood material, despite the nanoscale size
of the fibers. The DP for both fiber types reduced
Table 5. XRD analysis parameters for crystallinity index (CrI) and average crystallite size (ACS)
Material Method Treatment condition CrI ACS
Paper mulberry bast fiber
Beater 1 h 73% 12.18 nm
WDM10 passes 63% 4.24 nm
20 passes 58% 4.32 nm
HPH10 passes 56% 3.84 nm
20 passes 53% 3.58 nm
HwBKP
Pulper only 20 min 71% 4.82 nm
WDM10 passes 66% 3.85 nm
20 passes 65% 4.00 nm
Fig. 4. XRD diffractograms of nanofiber by passing WDM and HPH of (a) paper mul-berry bast fiber and (b) HwBKP.
펄프·종이기술 51(2) 201996
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
as the number of passes through WDM and HPH
increased.
3.7 Tensile strength and elongationThe highest tensile strength values for both paper
mulberry bast and HwBKP fibers were reached
after 5 passes through HPH (Fig. 7). It is believed
that the surface area of the fibers increased as the
content of OH- hydrophilic group of the fibers in-
creased, the latter creating a strong bond between
the fibers due to the frictional force applied by the
millstones during WDM, and the shear force from
Fig. 5. TGA and DTG analysis of paper mulberry thin sheet (a), (b) and Hw-BKP (c), (d).
Table 6. Thermal parameters for the thermograms of paper mulberry bast fibersand Hw-BKP fi-bers after processing with WDM and HPH
Material MethodTreatmentcondition
Maximumtemperature
10% loss temperature
50% loss temperature
Paper mulberry bast fiber
Beater 1 h 378℃ 312℃ 372℃
WDM 20 passes 361℃ 314℃ 365℃
HPH10 passes 357℃ 311℃ 362℃
20 passes 359℃ 312℃ 364℃
HwBKP
Pulper only 20 min 372℃ 309℃ 365℃
WDM 20 passes 352℃ 293℃ 353℃
HPH10 passes 347℃ 291℃ 349℃
20 passes 345℃ 290℃ 349℃
J. of Korea TAPPI Vol.51 No.2 Mar.-Apr. 2019 97
Sung-Jun Hwang·Hyoung-Jin Kim
the principle of expansion and contraction during
HPH.22) However, the tensile strength decreased
with more than 5 passes through HPH. It is con-
sidered that the excessively high temperature and
pressure of HPH damaged the fibers and reduced
the DP and crystallinity, thus temperature and
pressure should be reduced if more than 5 passes
through HPH are used. Under all conditions, ten-
sile strength of sheets manufactured from paper
mulberry bast fiber were higher than that for
HwBKP, probably due to the high density of the
manufactured sheets and the high polymerization,
and crystallinity of the paper mulberry bast fiber.
Elongation values for the two fiber types followed
the same trend for tensile strength (Fig. 7). In
particular, elongation decreased significantly after
5 passes through HPH; it is believed that brittle-
ness increased due to the fibers in the sheets being
strongly bonded while the diameter and length of
the fibers decreased.
4. Conclusions
Characterization of the fibers by SEM showed
that fiber diameter decreases with an increase in
the number of passes through an HPH; final cellu-
lose nanofiber size was 30-80 nm after 20 passes.
HPH also induced a reduction in the DP and crys-
tallinity. FT-IR showed no differences in the fibers
after mechanical treatment. A TGA curve of the
isolated cellulose nanofiber after high pressure ho-
mogenization showed that degradation has only a
minimal effect on the thermal decomposition of the
nanocellulose. Residual weight loss at 312℃ and at
over 400℃ was 50% and 27 %, respectively. XRD
results indicated that both crystallinity and crystal
size decreased with an increase in the number of
HPH treatments. The average decrease in crystal-
linity and crystal size after 20 passes was 53% and
3.6 nm, respectively. In addition, the tensile
strength and elongation of two species increased
with WDM and up to 5 HPH treatments, with both
parameters higher in paper mulberry bast fiber
than in HwBKP.
This research has confirmed that paper mulberry
bast fibers could provide the raw material for a
range of applications, including nanocomposites,
Fig. 7. Tensile strength and elongation of sheets of paper mulberry bast and HwBKP.
Fig. 6. DP of paper mulberry bast fiber and HwBKP.
펄프·종이기술 51(2) 201998
Studies on the Characteristics of CNF from Paper Mulberry Bast Fiber
coatings, membrane filters, reinforcing fillers,
packaging, etc.
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