PRETREATMENT WITH ALKALINE AND REFINING FOR
PRODUCING NANOFIBER DURING TEMPO OXIDATION
PROCESS
Author: Yun Qian, Lingling Shen, Guolin Tong* (*Corresponding author: gtong@njfu.edu.cn)
Affiliation: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University,
210037, China.
ABSTRACT: Alkaline pretreatment and PFI refining were used to loose the fiber's microstructure and helped
the TEMPO-mediated oxidation process. 5 hrs oxidation produced 1.18 mmol/g carboxyl content for bleached
softwood pulp with pretreatment, higher than fibers without pretreatment. High-speed dispersion and
ultrasonication were used as post-treatments, and both methods produced nanofibers. Nanofibers obtained by
rotator were 1-2 μm long and 20 nm wide, while nanofibers produced with ultrasonic treatment were several
millimeters long and 50-80 nm wide.
Keywords: Pretreatment; Post-treatment; TEMPO oxidation; Nanofibers.
INTRODUCTION
Nanofibers can be obtained by mechanical, chemical and biological methods. TEMPO-mediated oxidation is
a good way to produce nanofibers. However, this method take a long time to disperse microfibers into water [1].
Previous researches show ultrasonic treatment could facilitate the preparation of high carboxyl content
nanocrystals from natural fibers by TEMPO oxidation [2-4]. But, the energy needed to prepare nanocrystals from
ultrasonic-assisted TEMPO oxidation is high. To reduce the energy consumed in the preparation, two different
ultrasonic systems, probe ultrasonic generator and ultrasonic cleaning bath, were used. It is found that probe
ultrasonic generator was more efficient because probe ultrasonic generator avoid the energy consumed by
ultrasonic transmission medium [5]. Yet, the energy consumption is still high, and most of the energy is consumed
to break the compacted microstructure of fibers for oxidation.
NaOH can penetrate through cell walls to make fibers swell up, which make the chemical agents of TEMPO
oxidation more easily to access the hydroxyl groups. Besides, some hydrogen bonds are broken, which decrease
the difficulty of separating nanocrystals from fibers [6]. Mechanical pretreatments are used to loose the fiber's
microstructure. Refining can break the P wall and expose fibrils from S wall [7]. As more fibers are fibrillated, more
surface area will be created for reaction. In this research, those pretreatments were combined to loose fiber’s
microstructure, and help oxidation. Then, high-speed dispersion and/or ultrasonication were used as
post-treatment.
EXPERIMENTAL
Materials
Fully bleached softwood kraft pulp (from Howe Sound Pulp & Paper Corporation, Canada), NaClO (50 g/L
available chlorine, Shanghai Jiuyi Chemical Co. Ltd., China), TEMPO (Changzhou JiaNa Chemical Co. Ltd.,
China), NaBr (Sinopharm Chemical Regent Co. Ltd., China), and other chemicals were all used as received
without further purification.
Methods
NaOH pretreatment and PFI refining
NaOH solutions, which concentrations were range from 0% to 12%, were used to swell fibers. 30 g dry fibers
were added to NaOH solution and stored for designed hours before refining 25 min by PFI refiner. Beating degree
of pulp was used to measure the effect of fibrillation. The optimum conditions were selected to prepare fibers with
least cost and time. SEM observation also applied to determine the changes during chemical and mechanical
pretreatment.
TEMPO oxidation and preparation of nanofibers
2 g dry fully bleached softwood pulp fibers, 0.32 g NaBr and 0.032 g TEMPO were dispersed in about 150
mL water. Then, 20 mmol NaClO solution were added in the mixture, and the pH was adjusted to 10 by 2 M HCl.
The whole mixture was adjusted to 200 mL by adding water. A pH meter and 2 M NaOH was used to maintain the
pH at 10. The reaction was operated at room temperature for maximum 5 hours. After oxidation, the mixture was
separated and washed by filtration immediately. Oxidized fibers were stored in refrigerator at 4 oC for further
analysis. Fibers without any pretreatment were also oxidized in the same conditions as a control. Redox titration
was used during reaction to represent the oxidation trend.
Conductimetric titration method was used to determine the carboxyl content of oxidized fibers. Nanofibers
were produced by post-treatments. Oxidized fibers were dispersed water with concentration of 0.2%, and treated
with a high-speed rotator from a vacuum homogeneous emulsifying machine. Another 2 g oxidized fiber sample
was added to water with the concentration of 1.5%, and was treated by probe ultrasonic generator. Nanofibers
were observed by TEM.
RESULTS AND DISCUSSION
NaOH Pretreatment and PFI Refining
Different NaOH concentrations and reaction time affected the fibrillation degree of fibers. Fig. 1 shows higher
concentration of NaOH shortened the time of beating pulp. When the time reached 25 min, different concentration
of NaOH had same result. Higher concentration of NaOH also made fiber yield lower, that’s because most soluble
components in pulp, such as hemicellulose, β- and γ-cellulose, were dissolved in water [6]. After NaOH
pretreatment, the residual fibers were mostly α-cellulose. From Fig. 2, it was found that the optimum conditions
were 10% NaOH pretreatment within 4 hrs, and then treated with 25 min PFI refining.
Fig.1 The relationship between beating degree of
pulp and the concentration of NaOH.
Fig.2 The curve of yield vs. the concentration of
NaOH, and the curve of yield vs. Reaction time.
TEMPO Oxidation of Fibers With and Without Pretreatment
To study the effects of pretreatment on TEMPO oxidation, fibers with and without NaOH pretreatment and
PFI refining were used. It was indicated from Fig. 3 and Fig. 4 that the NaOH pretreatment and PFI refining could
facilitate the oxidation, and produce more carboxyl content. That’s because pretreatment and refining created
more caves and pits in the fiber surface (as shown in Fig. 5), thus accelerated the oxidation. After 5 hrs reaction,
carboxyl content of fibers with pretreatment was 1.18 mmol/g, and higher than fibers without pretreatment.
As presented in Fig. 5, fibers were fibrillated, primary wall (P) and secondary wall (S1) were destroyed, and
some lumens were broken. So much more surface areas were created.
Fig.3 Residual available chlorine vs. oxidation time.
Fig.4 Carboxyl content vs. oxidation time.
(a)
(b)
(c)
(d)
Fig.5 SEM images of fibers. (a, b) Fibers without pretreatment; (c, d) Fibers with NaOH pretreatment and
refining.
Fig.6 Nanofibers obtained by high-speed rotator.
Fig.7 Nanofibers obtained by probe ultrasonic
generator.
Post-treatment of Oxidized Fibers
A high-speed rotator and a probe ultrasonic generator were used to prepare nanofibers from oxidized fibers.
After 1.5 hrs dispersion, nanofibers were obtained from 0.2% oxidized fiber suspension, and their TEM images
were shown in Fig. 6. But when using ultrasonic generator to prepare nanofibers, only 15-20 min was needed to
obtain nanofibers. The TEM images of nanofibers produced by ultrasonic generator were shown in Fig. 7.
It’s easily found that the nanofibers had different morphologies via two post-treatments. Nanofibers produced
by high-speed rotator were <20 nm wide and most of them were 1-2 μm long. However, nanofibers produced by
ultrasonication were much wider and longer than those in Fig. 6. Nanofibers were several micrometers long and
50-80 nm wide.
The phenomenon was supposed to be the different dispersion mechanism of high-speed rotator and
ultrasonication. High-speed rotator spitted or ripped nanofibers from cell wall, thus the nanofibers were slim and
thin. But, in ultrasonication system, nanofibers were produced by the destruction of amorphous region.
Ultrasonication destroyed amorphous region in cell wall, thus nanofibers were separated as their connection
regions were broken. It was possible that nanofibers were bundles of fibrils, and nanofibers produced by
ultrasonication contained more fibrils than those produced by high-speed rotator.
CONCLUSIONS
NaOH pretreatment and PFI refining created more surface area for TEMPO oxidation, and accelerated the
oxidation. The carboxyl content of fibers with pretreatment was 1.18 mmol/g after 5 hrs oxidation of bleached
softwood pulp.
Nanofibers were obtained by high-speed rotator after 90 min dispersion. Most nanofibers were 1-2 μm long,
and the width were less than 20 nm.
Nanofibers were prepared by probe ultrasonic generator after 15-20 min ultrasonic treatment. The nanofibers
were 50-80 nm wide, and the length was several micrometers.
ACKNOWLEDGMENTS
The authors are grateful for the support of “Twelve Five” Nation Science Support Program, China, Grant No.
2011BAC11B01, PAPD of Jiangsu Higher Education Institutions, and the Fund of Jiangsu Provincial Key Lab of
Pulp and Paper Science and Technology, Grant No. 2010-09.
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