Preparation and characterization of high yield cellulose nanocrystals (CNC) derived from ball mill pretreatment and maleic acid hydrolysis

Journal Pre-proof Preparation and Characterization of High Yield Cellulose Nanocrystals (CNC) Derived from Ball Mill Pretreatment and Maleic Acid Hydrolysis Frederikus Tunjung Seta (Data curation) (Methodology) (Software) (Visualization) (Writing - original draft), Xingye An (Conceptualization) (Methodology) (Software) (Supervision) (Validation) (Writing - review and editing), Liqin Liu (Software) (Data curation), Hao Zhang (Software) (Data curation), Jian Yang (Software) (Data curation), Wei Zhang (Resources) (Software) (Data curation), Shuangxi Nie (Software) (Data curation) (Writing review and editing), Shuangquan Yao (Software) (Data curation), Haibing Cao (Software) (Data curation), Qingliang Xu (Software) (Data curation), Yifan Bu (Software) (Data curation), Hongbin Liu (Project administration) (Supervision)

PII:

S0144-8617(20)30116-8

DOI:

https://doi.org/10.1016/j.carbpol.2020.115942

Reference:

CARP 115942

To appear in:

Carbohydrate Polymers

Received Date:

14 December 2019

Revised Date:

31 January 2020

Accepted Date:

31 January 2020

Please cite this article as: Seta FT, An X, Liu L, Zhang H, Yang J, Zhang W, Nie S, Yao S, Cao H, Xu Q, Bu Y, Liu H, Preparation and Characterization of High Yield Cellulose Nanocrystals (CNC) Derived from Ball Mill Pretreatment and Maleic Acid Hydrolysis, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115942

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Preparation and Characterization of High Yield Cellulose Nanocrystals (CNC) Derived from Ball Mill Pretreatment and Maleic Acid Hydrolysis Frederikus Tunjung Setaa,c, Xingye Ana*, Liqin Liua, Hao Zhanga, Jian Yanga, Wei Zhanga, Shuangxi Nieb, Shuangquan Yaob, Haibing Caod, Qingliang Xud, Yifan Bua, Hongbin Liua* a

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, No.

b

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29, 13th Street, TEDA, Tianjin 300457, P. R. China

Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light

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Industry and Food Engineering, Guangxi University, Nanning 530004, P. R. China c

Center for Pulp and Paper, Ministry of Industry of Indonesia, Jalan Raya Dayeuhkolot No.132,

d

-p

Bandung 40258, Indonesia

Zhejiang JingXing Paper Joint Stock Co., Ltd., No. 1, Jingxing Industry Zone, Jingxing First

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Road, Caoqiao Street, Pinghu, Zhejiang Province, 314214, P. R. China

(Hongbin Liu)

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Graphical Abstract

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*Corresponding Authors’ E-mail: [email protected] (Xingye An); [email protected]

A facile and green approach to prepare cellulose nanocrystals (CNC) with high yield and colloidal stability from bamboo fibers by maleic acid hydrolysis is presented. The aim to pretreat bamboo fibers with ball mill before maleic acid hydrolysis is to destroy and open up the solid structure of bamboo fibers, to facilitate the following accessibility of maleic acid molecules into

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bamboo fiber structure, and to help expose more free hydroxyl groups from C2, C3, and C6 of cellulose chains that can be esterified by maleic acid molecules to form cellulose maleate. Ball mill pretreatment would guarantee more maleic acid molecules into the destroyed bamboo fiber structure and induce sufficient hydrolysis of amorphous regions of fibers, thus releasing more satisfactory CNC particles, i.e., a higher CNC yield; in addition, more maleic acid molecules would be esterified with hydroxyl groups during hydrolysis to impart rich carboxyl groups to

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CNC, i.e., a higher colloidal stability, compared with the control CNC sample without ball mill

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Highlights:

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pretreatment.

Ball mill process can open up the firm structure of bamboo fibers

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Ball mill-treated bamboo fibers would expose more accessible hydroxyl groups



More acid molecules can be penetrated into the structure of destroyed fibers



Maleic anhydride can be more easily reacted with –OH to generate –COOH

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The method induces a higher CNC yield and colloidal stability than the control

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

ABSTRACT The target of the study is to improve the yield and the colloidal stability of cellulose nanocrystals (CNC) that is obtained through maleic acid hydrolysis. Herein, a facile/ green approach 2

to prepare CNC with high yield and colloidal stability from bamboo fibers is presented. Ball mill pretreatment can break down and open up the structure of bamboo fibers, thus exposing more hydroxyl groups on the surface of pulp fibers and increasing the access of acid molecules into pulp fibers. The maleic acid molecules can easily hydrolyze cellulose, thus releasing more crystalline parts; maleic acid anhydride can react with hydroxyl groups to generate more –COOH groups on CNC. The yield of resultant CNC was 10.55-24.50%, which was much higher than 2.80% of the control. The study put forward a facile approach to prepare CNC with high yield

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and colloidal stability, and paves a possible way for industrialization of CNC production.

Keywords: Cellulose nano-crystals (CNC); Ball mill pretreatment; Maleic acid hydrolysis; Yield;

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Colloidal stability

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1. Introduction

Cellulose is one of the most important bio-based materials that have attracted massive

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usability in recent decades because of their excellent low toxicity, biocompatibility, and biodegradability for numerous utilizations (Bian, Chen, Dai, & Zhu, 2017). Cellulose can be

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harvested from various sources, such as bamboo. As one of the most desirable non-wood raw material for papermaking process, bamboo can be a kind of potential bioresource to alleviate the current shortage of wood-based pulp due to its combination of advantages, such as being rich in

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cellulose, fast growth and regrowth, easy propagation, and large-area planting in many countries (Nayak & Mishra, 2016; Huang, Fei, Wei, & Zhao, 2016; Lu et al., 2018; Bystriakova & Kapos, 2006).

Cellulose nanocrystals (CNC) can be isolated from many biomass resources, such as wood (Abushammala, Krossing, & Laborie, 2015; Du et al., 2017; Isogai & Zhou, 2019), bagasse

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(Mandal & Chakrabarty, 2011; Sofla, Brown, Tsuzuki, & Rainey, 2016), cotton (ElazzouziHafraoui et al., 2008), waste office paper (Yeganeh, Behrooz, & Rahimi, 2017), kenaf (Sulaiman, Chan, Chia, Zakaria, & Jaafar, 2015), algae (Sinha et al., 2015), and bamboo (Yu et al., 2012; Brito, Pereira, Putaux, & Jean, 2012; Hong, Chen, & Xue, 2016). The physicochemical properties of the CNC are regarded to be determined by its source, for example, CNC from hardwood (Moon, Martini, Nairn, Simonsen, & Youngblood, 2011) will have 140-150 nm on length, 4-5 nm in width, 43-65 % on crystallinity; from softwood will have 100-150 nm on 3

length, 10-20 nm in width, 78-82 % on crystallinity; and cotton 70-300 nm on length, 5-11 nm in width, 74-91 % on crystallinity (Li, Wang, & Liu, 2011). CNC has attracted more and more attention in recent years in various applications, such as food industry, paper-making area, catalyst area, etc. (An, Long, & Ni, 2017; Yang, 2017), thanks to its excellent inherent advantages, such as large specific area, perfect mechanical properties, etc. (George & Sabapathi, 2015). The basic method to extract CNC from pulp fibers is to remove the amorphous regions of

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cellulose thus releasing the crystalline parts, i.e. CNC particles. Several methods have been developed to isolate CNC, such as acid hydrolysis (for example sulfuric acid (H2SO4) (Trachea, Hussin, Haafiz, & Thakur 2017; Mahmud et al., 2019; Pirich et al., 2019; Yang, 2017) and

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hydrochloric acid (HCl) (Cheng et al., 2017; Trachea et al., 2017), mechanical method (Amin, Annamalai, Morrow, & Martin, 2015; Zhang, Tsuzuki, & Wang, 2015; Zhao, Kuga, Jiang, Wu,

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& Huang, 2016), enzymatic hydrolysis (Chen, Deng, Shen, & Jiang, 2012; Xua et al., 2013), ionic liquid treatment (Tan, Hamid, & Lai, 2015), subcritical water hydrolysis (Cantero,

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Bermejo, & Cocero, 2015; Novo, Bras, Garcia, Belgacem, & Curvelo, 2015; Meillisa, Woo, & Chun, 2015), oxidation method (Sun, Hou, Liu, & Ni, 2015), and the combined processes (Lu, et

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al., 2016; Lu et al., 2013; Rohaizu & Wanrosli, 2017; Tang et al., 2015). The acid hydrolysis is the most commonly used to produce CNC (Moon et al., 2011; Song, Ji, Wang, Wei, & Yu, 2018) due to its high efficiency for CNC preparation. The main function of acid catalysis during CNC

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preparation is to remove the amorphous structure of cellulose and produce nano-crystals by its ability to release hydronium ions for hydrolytic cleavage of glycosidic bonds in cellulose molecular chains within amorphous regions along the cellulose fibrils (Ng et al., 2015). However, the main shortcomings of the CNC resulted by acid hydrolysis are those: 1) it has low thermal stability and is difficult to be functionalized due to the presence of sulfate groups

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(for sulfuric acid-induced CNC); 2) it has tendency to be aggregated due to the bare surface charge density (for hydrochloric acid-induced CNC) (Yu et al., 2013). Furthermore, the potential excessive degradation of cellulose, large amounts of effluent released from the neutralization stage process and corrosion hazards of high concentration of strong acid to the equipment and environment (Chen, Zhu, Baez, Kitin, & Elder, 2016) are the other drawbacks of this method. Solid acid, such as maleic acid, can be utilized to hydrolyze cellulose to produce CNC (Bian et al., 2017; Espinosa, Kuhnt, Foster, & Weder, 2013; Lu et al., 2016; Yeganeh et al., 2017), 4

considering the advantages of maleic acid: 1) highly safe storage; 2) low transportation cost; 3) environmentally friendliness and harmless to equipment; 4) maleic acid solution has a higher boiling point (it can achieve effective acid hydrolysis at temperatures around 100 °C without boiling); 5) possible surface modification with –COOH during maleic acid hydrolysis. Some studies have been published to utilize maleic acid as the catalyst to isolate CNC from cellulose resource, for example, Wang, Chen, Zhu, & Yang (2017), Chen, Zhu, Baez, Kitin, & Elder (2016), Bian et al. (2017) have successfully produced CNC by using maleic acid as the catalyst,

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but all of these experiments showed low CNC yield (< 10%), considering the weak acid nature of maleic acid. Also, the low surface charge density (-COOH) of CNC induced by maleic acid hydrolysis would cause serious colloidal aggregation during the storage of CNC colloids.

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Furthermore, the firm structure of bamboo pulp fibers that will be applied in this study for CNC preparation would retard maleic acid molecules penetrating into the fiber structure for effective

Mechanical

treatment

(i.e.,

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cellulose hydrolysis.

microfluidization,

ultrasonication,

high-pressure

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homogenization, and ball milling) is another alternative method and has been widely used for the production of cellulose in nano-size particles. The mechanical method can be used either in

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combination or a single method. The advantages of ball mill treatment are green, simple, high efficiency, no introduction of other groups, etc., which has attracted numerous studies on it. Song et al. (2018) used ball mill treatment as a pretreatment method for oxalic acid hydrolysis. The

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resulted products have a relatively high yield of 59 % and the maximum degradation temperature of 332 ℃. Phanthong, Guan, Ma, Hao, & Abudula (2016) also used ball mill pretreatment method before sulfuric acid hydrolysis, and they found that the crystallinity and crystal size of ball-milled cellulose decreased with the increase of ball milling time, and the mild acid hydrolysis of the ball-milled cellulose resulted in the increasing of crystallinity and thermal

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stability of CNC at the high-temperature range but without any changes in chemical structure. Direct production of CNC by ball mill treatment has also been investigated. Amin et al. (2015) successfully prepared CNC with a high aspect ratio and high thermal stability via High Energy Ball Mill (HEBM) treatment. However, the single mechanical ball milling without other pre or post treatment would induce CNC products with low crystallinity and high energy consumption as well (Trachea, et al., 2017; Phanthong et al., 2016).

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In this work, ball mill pretreatment will be utilized before the maleic acid hydrolysis of bamboo pulp fibers to enhance the yield and colloidal stability of CNC. The hypotheses for the study are that: 1) ball mill pretreatment can break down and open up the structure of bamboo fibers, thus increasing the accessibility of maleic acid molecules into amorphous domains and exposing more hydroxyl groups on the surface of pulp fibers; 2) The maleic acid molecules in the following acid hydrolysis can easily react with β-1,4-glucosidic linkages in amorphous domains of fibers, releasing more crystalline parts (i.e. CNC) from pulp fibers (a higher CNC yield); and 3) in addition, the maleic acid anhydride can easily react with the exposed hydroxyl

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groups to generate more –COOH groups on the surface of CNC, thus improving the CNC colloidal stability. SEM, AFM, XRD, FTIR were carried out to characterize the morphology of

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bamboo fibers and CNC, the crystallinity and surface functional groups of the CNC, respectively. The yields, particle size distribution, zeta potential, and colloidal stability analyses

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of CNC were also investigated. 2. Materials and Methods

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2.1 Materials

Bleached bamboo pulp board (air-dried) from native one-year-old Moso bamboo was

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obtained from Yibin Paper Co. Ltd., in Sichuan Province, China. The moisture content of this pulp was 8.56%. The chemical composition of bamboo pulp in this experiment was shown in Table 1 and determined according to the standards provided by Technical Association of Pulp

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and Paper Industry (TAPPI). The lignin, α-cellulose, hemicellulose, and ash contents of the bamboo pulp were determined according to TAPPI standard methods T222 os-74, T203 cm-99, T223cm-01, and T413 om-93 respectively. The procedures could also be found in previous studies (Cordeiro, Belgacem, Torres, & Moura, 2004; Espino et al., 2014; Sheltami, Abdullah, Ahmad, Dufresne, & Kargarzadeh, 2012; Ververis, Georghiou, Christodoulakis, Santas, &

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Santas, 2004). The summarized chemical analyzed result can be seen in Table 1. Each of the measurement was calculated from the average of three parallel runs. Maleic acid was purchased from Nine-Dinn Chemistry (Shanghai) Co. Ltd. The purity of maleic acid is 99.0 %. Sodium hydroxide was purchased from Tianjin Yongda Chemical Reagent Co. Ltd. Deionized (DI) water was utilized in the experiment. All other chemicals were of analytical grade and used without further purification. Table 1. Chemical composition of bamboo pulp 6

Sample Bamboo bleached pulp

α-cellulose

Hemicellulose

Lignin

Ash

(%)

(%)

(%)

(%)

79.34 ± 2.1

15.10 ± 1.3

0.08 ± 0.003

1.1 ± 0.03

2.2 Ball Mill Pre-treatment The dried pulp samples were soaked in DI water overnight and disintegrated using IKA T18 Ultra-turrax homogenizer for about 10 minutes. The bamboo pulps were separated and diluted

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into distilled water for desired concentration (wt %) and the total suspension weight is 300 g. The prepared pulp suspensions were pretreated in the ball mill (CSN0.3 from Chile (Shanghai)

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Mechanical Technology Co., Ltd.), in which the small balls are made from zirconia with 0.5 mm in diameter and total ball weight is 350 g. The ball mill processes were carried out by varying the

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milling time, speed, and pulp consistency, which have been listed in Table 2. The Centrifugation process (20 minutes at 10,000 rpm) was carried out after ball mill pre-treatment followed by

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freeze-drying process to obtain the dried treated fiber samples. The abbreviations of pulp fiber samples after ball mill pretreatment and the conditions of ball mill pre-treatment are summarized in Table 2.

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Table 2. The Abbreviations of Bamboo Fiber Samples after Ball Mill Pretreatment and the Conditions of Ball Mill Pre-treatment

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Condition

Sample Name

Speed

Time

(wt %)

(rpm)

(h)

BM1

1

1,500

1

BM2

1

1,500

2

BM3

1

1,500

3

BM4

1

1,500

4

BM5

1

1,500

5

BM6

1

1,750

2

BM7

1

2,000

2

BM8

0.5

1,500

2

BM9

1.5

1,500

2

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Consistency

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Note: BM stands for Ball Milling 2.3 Maleic Acid Hydrolysis Process Maleic acid hydrolysis of bamboo pulp fibers was conducted using liquor to pulp weight ratio of 100:1 in a maleic acid solution of 75 wt % concentration at 110 °C for 3 h. The suspensions were constantly mixed using a mechanical stirrer during the hydrolysis process. Silicone bath DF-101S from Kenu was used in the hydrolysis process as the heater source. The reactions were quenched by adding 150 mL of DI water to the suspensions at the end of each

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reaction. The suspension was washed and centrifuged at 10,000 rpm for 20 min to discard the supernatant and remove the excess acid. The precipitate was then mixed with 30% sodium hydroxide solution and distilled water to a total concentration of 1 wt % and dialyzed in the

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distilled water for a week or until the pH of the dialysis water no longer changed. The dialyzed sample was subjected to an ultrasonication at 60% power for 4 minutes (4 seconds on and 8

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seconds off).

The resultant cellulose slurry after maleic acid hydrolysis and following purification process

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was further centrifuged at 2,000 rpm, 3,000 rpm for 5 minutes, and 10,000 rpm for 10 minutes to separate the small particles from the cellulose slurry to obtain the supernatant (containing CNC

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particles) and precipitation phases (bigger cellulose parts, non-CNC particles). The dried nanocrystal products were finally obtained by freeze-drying the supernatant phase. Herein, the prepared CNC samples derived from bamboo pulp fibers without ball mill pretreatment were

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noted as CNCB, while CNC samples from ball mill pretreated bamboo pulp fibers were named as CNCBMx, in which x is referred to the sample number of ball mill pretreatment shown in Table 2.

2.4 CNC Yield Measurement

CNC Yields were measured by the gravimetric analysis method. The weight of the final

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dried CNC sample was designated as M2 and the dry weight of the initially dried bamboo pulp was determined as M1. The final yield was calculated from the average of three parallel runs of measurements for error analysis. The yield of CNC sample was given as follows: 𝑦𝑖𝑒𝑙𝑑(%) =

M2 × 100% 𝑀1

2.5 Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) Analyses

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For the SEM observation of bamboo pulp fibers, pre-weighed samples were put on a desiccator for overnight at a room temperature. The samples then mounted on a conductive carbon tape and coated with gold before the scanning process. The SEM pictures were captured using Oxford instrument with 10 kV beam. Atomic force microscopy (AFM) observations of CNC samples were performed on Multimode Bruker equipment with tapping mode. The sample used for AFM imaging was produced by placing a few drops of CNC with 0.005% solution (wt %) onto a 1 x 1 cm square of

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freshly cleaved mica, then air-dried overnight. 2.6 Particle Size, Zeta Potential, Surface Charge, and Carboxyl Group Content Measurement

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All the CNC samples were diluted into 0.5 wt % and dispersed under ultrasonic treatment (Scientz SB-5200DTD) for 20 min before particle size and zeta potential measurement with DI

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water as the dispersant, at room temperature (25 oC). Dilute aqueous CNC suspensions were used to measure the particle size by monitoring the scattered light intensity at an angle 90o. The Zeta

converting the mobility into potential value.

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potential was calculated by measuring the electrophoretic mobility of the suspension and

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Charge densities of the CNC samples were measured using a Mütek PCD 04 Particle Charge Detector based on the colloidal titration method. In the colloidal titration, CNC colloids were titrated with standard cationic polyelectrolyte [poly (diallyl dimethyl ammonium chloride)

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(poly-DADMAC)] (concentration = 0.001 N).

To measure the carboxyl group content of CNC, 10 g of CNC suspension containing 0.1 wt % of CNC was mixed with 10 mL of 1 mol/mL NaOH solution and stirred for 30 min at room temperature. The mixture then titrated with 1 mol/mL HCl solution. The carboxylic acid content was calculated using the following equation.

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𝐶𝑂𝑂𝐻 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 =

(10 · 𝐶(𝑁𝑎𝑂𝐻)) − (𝑉 · 𝐶(𝐻𝐶𝑙)) (mmol/g CNC) 𝑊

Where C was the concentration of NaOH (mol/L) and HCl (mol/L), V was the volume of

HCl required to reach the end of the titration (mL), and w was the weight of the dry sample (g). The final results were calculated from the average of three parallel runs of measurements for error analysis. 2.7. Thermogravimetry Analyses (TGA) 9

Thermal stability of bamboo pulp fibers and CNC samples were tested by Thermogravimetric analysis (TGA) analysis, using a Discovery SDT650 TGA with temperatures from ambient temperature to 600 °C under a nitrogen steam with a flow rate 20 ml/min and a heating rate of 10 °C/min. Approximately 5 mg of sample was used for the TGA test. 2.8 Fourier Transform Infrared Spectroscopy (FTIR) Analyses Each sample was weighed 1 mg approximately, followed by milling together with 100–200 mg of dried potassium bromide (KBr) powder in an agate mortar. The mixture powder then

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pressed by a hydraulic press at a pressure of 10 MPa for 1 minute until a homogeneous thin film was obtained. The as-prepared thin film was recorded its spectra on an FTIR-650 (Tianjin Gangdong Science and Technology Development Co. Ltd.). The spectra were recorded in the

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range of 4000 to 650 cm-1. 2.9 Crystallinity Analyses

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To determine the crystallinity index (CrI) of bamboo pulp fibers and CNC samples, a common method developed by Segal and coworkers (Park, Baker, Himmel, Parilla, & Johnson,

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2010; Seagal, Creely, Martin, & Conrad, 1959) was applied. The proposed method was for empirical measurements to allow the rapid comparison of cellulose samples. CrI was calculated

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from the ratio of the height of the 002 peaks (I002) and the height of the minimum (IAM). Where I002 is the maximum intensity of the (002) lattice diffraction peak and IAM is the intensity scattered by the amorphous part of the sample. The diffraction peak for plane (002) is located at

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a diffraction angle of around 2θ = 22.0o and the intensity scattered by the amorphous part was measured as the lowest intensity at a diffraction angle of around 2θ = 18.0o. The crystallinity index (CrI) was determined by using Segal's method as stated below: 𝐶𝑟𝐼 (%) =

𝐼002 − 𝐼AM × 100 % 𝐼002

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3. Results and Discussions

3.1 Schematic of CNC Preparation via Ball Mill Pretreatment and Following Maleic Acid Hydrolysis Process

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Figure 1. Schematic of CNC preparation via combining ball mill pretreatment and followed by

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maleic acid hydrolysis for high CNC yield and colloidal stability

Figure 1 illustrates the schematic of CNC preparation via maleic acid hydrolysis process

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facilitated by ball mill pretreatment, in which, the bamboo pulp fibers are pretreated in ball mill equipment at modified conditions, therefore, the straight and intact bamboo pulp fibers became

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shorter and crinkle after going through small balls and suffering high shear force treatment between balls. The inherent firm structure of bamboo fibers can be then destroyed and opened up, thus more hydroxyl groups will expose on the surface of bamboo fibers. Furthermore, maleic

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acid molecules can be more accessible into the damaged/loose fiber structure during the following maleic acid hydrolysis.

As shown in Figure 1, we proposed a reaction mechanism for the hydrolysis of ball-milled bamboo pulp by maleic acid. There are three functions of the maleic acid during the hydrolysis process: 1) maleic acid molecules react with β-1,4-glucosidic linkages/hydrogen bonds between

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anhydroglucose units in cellulose chains and release cellulose with small and uniform particle size; 2) maleic acid can hydrolyze the disordered regions of ball-milled bamboo pulp to release the crystalline region of the cellulose; and 3) maleic acid can catalyze the esterification process of hydroxyl groups on the exposed cellulose chains with rich maleic anhydride anionic groups, thus resulting in good colloidal stability of the final CNC in aqueous solution. The hydrolysis mechanism of hemicellulose was similar to cellulose. Because of the amorphous and heterogeneous structure and lower degree of polymerization of hemicellulose 11

compared with cellulose, maleic acid molecules will break down hemicellulose easier and faster to form shorter chained oligomers, thus most of hydrolyzed hemicellulose will be removed during the washing-centrifugation process and very small amount of hemicellulose will still be remained and counted as a CNC yield (Hilpmann et al., 2016; Rosli, Ahmad, & Abdullah, 2013). Herein, it can be hypothesized that the maleic acid hydrolysis can be facilitated by the ball mill pretreatment for CNC preparation with higher CNC yield and colloidal stability. The strong shear and friction forces from the balls also weaken the inter hydrogen bonds between fibers (Lin,

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Guo, Xu, & Wu, 2019; Piras, Fern´andez-Prieto, & Borggraeve, 2019). In this case, maleic acid hydrolysis can be more prone to proceed and more beneficial to CNC preparation. The hypotheses of this study are based on:

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1) Ball mill mechanical treatment can open up/ loosen the firm structure of bamboo fibers, and expose more accessible hydroxyl groups on it.

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2) More maleic acid molecules can be penetrated into the structure of bamboo fibers, and react with exposed hydroxyl groups to generate more carboxyl groups on CNC surface.

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3) Maleic acid molecules can be more easily to react with β-1,4-glucosidic linkages/ hydrogen bonds between anhydroglucose units in cellulose chains and release more CNC

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particles with small and uniform particle size.

4) CNC derived from the combined method has a higher yield and colloidal stability compared with the control sample without ball mill pretreatment.

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3.2 Analyses of Yields of CNC Samples

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Figure 2. The yields of CNC samples derived from different conditions

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The yields of CNC samples isolated from different conditions are shown in Figure 2. It can be clearly noted that the yield of CNCB with desirable particle size (has been separated and

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classified by the given centrifugation process) that is derived from maleic acid hydrolysis of bamboo pulp fibers without ball mill pretreatment is only about 2.8±0.36%, which is comparably low due to the weak acid property and the poor accessibility of acid molecules into fiber structure (Bian et al., 2017; Chen et al., 2016; Wang et al., 2017). It can also be found from Figure 2 that CNC yields increased from 2.8±0.36 to 24.50±2.09 %

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when the milling time increased to 5 h at 1 wt% pulp consistency and 1,500 rpm, which indicated that ball mill pretreatment can effectively facilitate the following maleic acid hydrolysis and enhance the yield of CNC and that the ball milling duration played a vital role in the process. It may be ascribed to the fact that the high shears force of ball mill pretreatment can break up the firm structure of bamboo pulp fibers (Figure 3), thus increasing the accessibility of maleic acid molecules. Lin et al. (2019) also applied ball mill pretreatment to open up the

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structure of microcrystalline cellulose to make carboxymethyl cellulose, and found that ball mill pretreatment can help improve the properties of final product and increase the yield as well. In addition, it can also be concluded that the ball milling speed also facilitated the following acid hydrolysis for higher CNC yield, which indicated that high ball milling speed would induce high shear force, thus helping open up the firm fiber structure and improving the accessibility of maleic acid molecules into bamboo fiber structure. Interestingly, the higher consistency of bamboo pulp (from 0.5 to 1.5 wt %) during ball milling pretreatment would lead to a lower CNC

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yield (from 18.02±0.75 to 11.72±0.39 %), which may be due to the reason that small zirconia balls would have a more sufficient contact with bamboo pulp fibers when it is in lower pulp concentration. The excessive and non-uniform high shear force would be induced between

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zirconia balls and bamboo pulp fibers when it is in high pulp consistency, thus resulting in low milling efficiency of ball mill pretreatment. It can also be evidenced from the literature, in which

substrate (Ogi, Zulhijah, Iwaki, & Okuyama, 2017).

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3.3 Analyses of AFM Images of CNC Samples

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showed that ball mill treatment can effectively cause the morphology changes of a majority of

Figure 3. AFM images of (a): CNC samples hydrolyzed from bamboo pulp fibers without ball mill pretreatment (CNCB, 0.005 wt%); (b) (c) and (d): CNC samples (0.005 wt%) hydrolyzed

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from bamboo pulp fibers with different ball mill pretreatment conditions (CNCBM1, CNCBM5 and CNCBM8). AFM images of CNC samples are shown in Figure 3. It indicated that CNC derived from bamboo pulp fibers without ball mill pretreatment (CNCB, Figure 3(a)) showed a more aggregated CNC dispersion and vague CNC outline, compared with that of (c) CNCBM5 and (d) CNCBM8 samples. In addition, particle size of CNCB sample was in a wide distribution, which means maleic acid hydrolysis without ball mill pretreatment just produced uneven CNC particle

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size. It can be hypothesized that once the ball mill pretreatment was utilized before maleic acid hydrolysis, the treated bamboo pulp with opened up fiber would let more maleic acid molecules access into its broken and loose structure and isolate more CNC particles with carboxyl groups,

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compared the AFM images of Figure 3(b) to Figure 3(a). It was also indicated that CNCBM5 had a desirable CNC particle size with a width of ca. 15 nm and length of ca. 150 nm, and

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uniform distribution, which demonstrated that ball milling pretreatment can effectively facilitate the following maleic acid hydrolysis for CNC preparation with high yield and colloidal stability.

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If we reduce the mill duration from 5 to 2 h and the pulp concentration from 1 to 0.5 wt % (as shown in Table 2), CNC colloid with high yield and uniform particle size/distribution can also be

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obtained (CNCBM8) compared with those of CNCBM5. Similar conclusions can also be identified from the yields and photographs analyses of CNC samples. 3.4 Analyses of SEM Images of Bamboo Pulp Fibers

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The morphological structure changes of bamboo pulp fibers after ball mill pre-treatment are shown in Figure 4. It was illustrated that bamboo pulp fibers without ball mill pre-treatment have a smooth surface and stiff structure (Figure 4(a) (b)), thus would impede the accessibility of maleic acid molecules into fiber structure, which would lead to an insufficient hydrolysis and less CNC releasing (Figure 2 and Figure 5). Hence, ball mill mechanical treatment can

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effectively destroy/ open up the firm structure of bamboo fibers (Figure 4(c)-(f)), thus increasing the accessibility of maleic acid molecules and exposing more hydroxyl groups on the surface of bamboo fibers. It has been reported that the morphology changes on the fibers and the decrease of fiber size after ball milling can facilitate the fiber swelling in aqueous solvent (Nuruddin, Hosur, Uddin, Baah, & Jeelani, 2016). In addition, bamboo fibers were crushed and milled into smaller parts, with a decrease in length and width, indicating that the milling pretreatment can

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also break the cellulose crystallites along the direction perpendicular to the crystallite axis

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(Zheng, Fu, Li, & Wu, 2018).

Figure 4. SEM images of (a) pristine bamboo pulp fibers; (b) enlarged image from (a); (c) ball

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mill treated fibers (BM2, 1 wt %, 1500 rpm, 2 h); (d) ball mill treated fibers (BM6, 1 wt %, 1750 rpm, 2 h); (e) ball mill treated fibers (BM7, 1 wt %, 2000 rpm, 2 h); and (f) ball mill treated fibers (BM9, 1.5 wt %, 1500 rpm, 2 h) It can also be concluded that the ball milling duration, speed and pulp consistency play a vital role in the morphology in bamboo structure, as shown in Figure 4(c)-(f) and Figure S1. After 2 h of ball milling, bamboo fibers began to have a distinct morphology change compared to the control. The longer milling duration and higher speed of ball mill pretreatment would 16

produce a higher mechanical shear force and induce a decreased particle size and reduced crystallinity of bamboo fibers, which can be supported by the previous reports (Lin et al., 2019; Piras et al., 2019). Interestingly, after 5 h of ball milling (Figure S1(f)), fiber size was decreased into micro scale and the re-aggregation phenomenon of tiny fibers happened. The results were possibly caused by the activation/ interactions of fiber surfaces generated by milling process (Ogi et al., 2017; Zheng et al., 2018). It was found from Figure 4(c) and (f) that high fiber concentration would increase the friction and shear force between fibers, thus improving the ball

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milling pretreatment and further destroying the fiber structure (Gotor, Achimovicova, Real, & Balaz, 2013).

3.5 Analyses of Particle Size, Zeta Potential, Surface Charge, and Carboxyl Content Group

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of CNC Samples

The CNC colloidal stability was determined by Zeta potential, particle charge density

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measurement, and sedimentation test. The average length and Zeta potential of CNC samples derived from various ball mill pretreatment and maleic acid hydrolysis are listed in Figure 5(a).

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CNC sample without ball milling process (CNCB) showed the highest negative value (-8.66 ± 0.27 mV), while CNCBM5 sample had a lowest Zeta potential of -36.58 ± 0.77 mV after 5 h of

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ball mill pretreatment. Herrera et al. (2018) also reported that cellulose nanocrystals would aggregate together if its zeta potential within the range of -30 to 30 mV. In addition, the lengths of CNC particles shown in Figure 5(a) were well consistent with those of Figure 3. Hence, it can

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be seen that ball mill pretreatment can effectively improve the Zeta potential of CNC colloids, along with the decreased particle size of CNC samples, compared to the results of CNCB, CNCBM1, CNCBM2, and CNCBM3 etc. samples, indicating that ball milling duration have a positive effect on the Zeta potential and particle size of CNC samples. It should be noted that CNCBM5 sample has a highest yield and lowest zeta potential as

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well as desirable and uniform particle size, compared with other CNC samples. Besides milling duration, milling speed and pulp concentration also has a vital effect on the final properties of CNC. If the pulp concentration for milling process was increased from 1 to 1.5 wt %, the yield of CNCBM9 decreased accordingly (Figure 2), along with the increased particle size and decreased zeta potential, which may be ascribed to the fact that: high pulp concentration can induce high shear force and inhomogeneous milling treatment between pulp fibers and zirconia beads.

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Figure 5(b) shows the carboxyl group contents and surface charge densities of several typical CNC samples, i.e., CNCB, CNCBM2, CNCBM5, and CNCBM7. It can be clearly noted that ball milling process can significantly increase the surface charge density of CNC samples, and that carboxyl groups from esterification reaction induced by maleic acid hydrolysis mainly contributed to the charge density (Hoeng, Denneulin, Neuman, & Bras, 2015). Higher charge density would lead to a higher CNC colloidal stability, as shown in Figure 5(c) and (d). The photograph of CNCBM4 (B, 0.1 wt %) and CNCBM7 (C, 0.1 wt %) colloid samples showed

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perfect high colloidal stability after standing 5 days, while the photograph of CNCB sample (A, 0.1 wt %) displayed aggregation phenomenon, which was ascribed to the low surface charge

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density.

Figure 5. (a) The particle size and zeta potential results of CNC samples; (b) the carboxyl

group content and surface charge density of CNCB, CNCBM2, CNCBM5 and CNCBM7; (c) the photograph of CNCB (A, 0.1 wt %), CNCBM4 (B, 0.1 wt %) and CNCBM7 (C, 0.1 wt %) colloid samples and (d) the photograph of corresponding CNC colloid samples after standing 5 days. 18

3.6 TGA Analyses of Bamboo Fibers and CNC Samples

(b)

o Temperature (%) ( C) Weight

80

(1) (2) (3) (4)

(4) (3)

Bamboo Pulp CNC Bamboo BM7 CNC BM7

60

40

(2)

20

0.0

-0.5

(%/℃ Weight Deriv. ) Deriv. Weight (%/oC)

(a) 100

(4) -1.0

(2) -1.5

(3)

(1) (2) (3) (4)

-2.0

(1)

0

-2.5

0

100

200

300

400

500

600

200

Temperature (℃ ) Weight (%)

250

300

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(1)

Bamboo Pulp CNCB BM7 CNCBM7

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400

450

500

Temperature Temperature (o(℃ C) )

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Figure 6. (a) TGA and (b) DTG curves of (1) bamboo pulp fibers, (2) CNC sample derived from maleic acid hydrolysis without ball mill pretreatment (CNCB), (3) ball milled bamboo pulp

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fibers (BM7), and (4) CNC sample from the combined method (CNCBM7), respectively. The TGA spectra of raw bamboo pulp, ball-milled bamboo pulp and CNC samples are

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shown in Figure 6. It was noted that all samples illustrated same thermal degradation trend and had an initial amount of weight loss at the temperature below 100 °C with a mass loss about 10%

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because of the evaporation of absorbed water on the surfaces of sample (Santos et al., 2013; Tan et al., 2015). DTG curves showed that all samples have a one-step pyrolysis process, which was in good accordance with the work done by Yu and his coworkers (Yu et al., 2013). The weight

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loss of the pulp fiber was ascribed to the cellulose degradation of depolymerization, dehydration, or decomposition of glycosyl units followed by the formation of charred residue (Song et al., 2018).

Decomposition curves of raw bamboo pulp had a higher temperature range of 288-382 °C and weight loss of almost 90% than that of ball-milled bamboo (BM7) (272-367 °C, weight loss

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of 70%). While CNC samples from ball-milled bamboo pulp (CNCBM7) had a lower decomposition temperature range of 200-365 °C and a lower weight loss of 65% than that of CNCB sample (219-366 °C, weight loss of 80%). The lower onset decomposition temperature of BM and CNCBM7 sample might due to the breakdown of some crystalline regions (can be seen in XRD result) during the ball milling process (Nuruddin et al., 2016). In addition, it was found from Figure 6(b) that the derivative weight loss (%/°C) of CNCBM7 sample had a lower level but a higher amount of char residue, compared to other three samples, indicating that CNC 19

sample derived from ball milling process and maleic acid hydrolysis had a higher thermal stability, which is consistent with the previous literature (Phanthong et al., 2016; Sofla et al., 2016).

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3.7 FTIR Analyses of Bamboo Fibers and CNC Samples

Figure 7. FTIR spectra of (a) raw bamboo pulp, (b) ball milled bamboo pulp (BM3), (c) CNC

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from raw bamboo pulp (CNCB), and (d) CNC from ball milled bamboo pulp (CNCBM3). FTIR spectra of the pristine bamboo pulp, ball-milled bamboo pulp, and CNC samples (CNCB and CNCBM3) are shown in Figure 7. All spectra possessed the same absorption peaks at around 3400, 2900, 1420, 1330, 1045 cm−1, which are associated with the native cellulose type I (An, Wen, Cheng, Zhu, & Ni, 2016; Phanthong et al., 2016). It can be clearly seen that BM3

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sample has a same FTIR spectra with raw bamboo pulp when compared with Figure (a) and (b), indicating that the ball milling process cannot change the chemical structure/ composition of cellulose fibers. The absorbance peaks observed in the 1,045-1,160 cm-1 range were attributed to C–O stretching and C-H rocking vibrations of the pyranose ring skeleton. The peaks observed in the range of 1,420 cm-1 and 1,330 cm-1 in all spectra were attributed to the symmetric bending of CH2 and the bending vibrations of the C-H and C-O groups of the aromatic rings in polysaccharides. The absorbance peaks in the 3,400 cm-1 and 1,640 cm-1 regions are attributed to 20

the stretching and bending vibrations, respectively, of the OH groups of cellulose. The peak at 2,900 cm-1 is corresponded to C-H stretching. It can be noted that the CNCB and CNCBM3 samples demonstrated two new peaks at around 1730 cm-1 and 1580 cm-1 as shown in Figure (c) and (d), which were regarded as ester carbonyl groups (C=O) and alkenes functional group (C=C), respectively, indicating that maleic acid hydrolysis process has successfully endowed ester carbonyl groups and alkenes functional groups on the surface of CNC (Chen et al., 2016). It was also observed that CNCBM3 has a

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stronger absorbance of carboxyl group than CNC sample without ball mill pre-treatment, indicating that ball mill pretreatment can facilitate the following maleic acid for higher -COOH content, which is responsible for the higher CNC colloidal stability shown in Figure 5.

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3.8 Crystallinity Analyses of Bamboo Pulp Fibers and CNC Samples

Figure 8. (1) XRD spectra of (a) raw materials; (b) ball milled pulp (BM5, 1 wt %, 1500 rpm, 5 h); (c) ball milled pulp (BM8, 0.5 wt %, 1500 rpm, 2 h); (2) XRD spectra of (a) CNC from raw

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materials (CNCB); (b) CNC from BM5 (CNCBM5); (c) CNC from BM8 (CNCBM8). The crystallinity of raw bamboo pulp, ball-milled bamboo pulp, and the resulted CNC was

measured and determined by X-ray diffraction (XRD), which is shown in Figure 8. The XRD patterns of Figure 1(a) bamboo pulp fiber, 1(b) ball milled pulp fiber and 1(c) contained three characteristic peaks at 2θ=14.8° (101), 16.4° (10Ī) and 22.5° (002), which are attributed to cellulose I, indicating that ball milling process cannot change the characteristic XRD peaks of cellulose. Comparing with the crystallinity of bamboo pulp, BM5 and BM8 inserted in Figure 21

8(1), the crystallinity of the all ball-milled bamboo pulp decreased with the increase of milling time and the rotation speed of ball mill, which can be ascribed to the fact that ball milling process can destroy/ decrease the crystalline structure of bamboo fibers (Feng, Han, & Owen, 2004). As shown in Figure 8(2), it is noted that maleic acid hydrolysis for CNC preparation can lead to improved crystallinity because of the removal of the amorphous regions of cellulose (Kang et al., 2018), and that CNC samples from ball milled bamboo pulp fibers (CNCBM5 and CNCBM8) can have a higher crystallinity, thanks to the high accessibility of maleic acid

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molecules into cellulose fiber and high hydrolysis efficiency facilitated by ball mill pretreatment. 4. Conclusions

In this study, ball mill pretreatment was utilized on the bamboo pulp fibers before maleic

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acid hydrolysis to prepare cellulose nanocrystals (CNC) with higher yield and colloidal stability. Here, ball mill pre-treatment can break down and open up the firm structure of bamboo pulp

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fibers, thus facilitating the following maleic acid hydrolysis. It was found that ball mill pretreatment combined with maleic acid hydrolysis can effectively isolate CNC products from

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bleached bamboo pulp with a higher yield (10.55%-24.50%) than that of the CNC sample without ball mill pretreatment (2.80±0.36 %). The ball mill pretreated cellulose fibers showed a

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smaller size which can promote the following acid hydrolysis to obtain the desirable rod-shape CNC particles with a length of 105.6-223.8 nm. The photographs, FTIR, charge density, zeta potential, and carboxyl group content analyses showed that ball mill pre-treatment can facilitate

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the following maleic acid hydrolysis of bamboo pulp fibers for CNC preparation. The study designs a facile and green approach to prepare CNC from bamboo fibers by maleic acid hydrolysis with high yield and colloidal stability of CNC and paves a possible way for

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industrialization of CNC production.

Author Contribution Statement

1. Frederikus Tunjung Seta: Data curation, Methodology, Software, Visualization, Roles/Writing - original draft

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2. Xingye An: Conceptualization, Methodology, Software, Supervision, Validation, Writing - review & editing 3. Liqin Liu: Software, Data curation 4. Hao Zhang: Software, Data curation 5. Jian Yang: Software, Data curation

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6. Wei Zhang: Resources, Software, Data curation

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7. Shuangxi Nie: Software, Data curation, Writing - review & editing

9. Haibing Cao: Software, Data curation

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10. Qingliang Xu: Software, Data curation

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8. Shuangquan Yao: Software, Data curation

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11. Yifan Bu: Software, Data curation

12. Hongbin Liu: Project administration, Supervision

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5. Acknowledgments

The authors would like to acknowledge the financial support from the National Key Research and Development Plan (Grant 2017YFB0307902), the Natural Science Foundation of Tianjin (Grant: 19JCQNJC05400), the State Scholarship Fund for Post-Doctoral Fellow (PDF) from China Scholarship Council (CSC), the National Natural Science Foundation of China

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(Grant: 31670589), the basic scientific research operation foundation of Tianjin University of Science and Technology (Grant: 000040186), the research fund of Tianjin Key Laboratory of Pulp & Paper, China (Grant: 201806), the research fund of Zhejiang JingXing Paper Joint Stock Co., Ltd (Grant: 21843), and the Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, P. R. China (Grant: KF201815-4).

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