%DVW)LEHUktappi.kr/xml/19644/19644.pdf · 2019. 12. 5. · j. of korea tappi vol.51 no.2 mar.-apr. 2019 4voh +vo)xbohh)zpvoh +jo,jn &vspqf uif6ojufe4ubuftboe+bqbo ibwfbdujwfmz cffodpoevdujohbewbodfesftfbsdipouifnbov

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


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



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


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


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



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


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.



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


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


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


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℃



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


coatings, membrane filters, reinforcing fillers,

packaging, etc.

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