Green preparation of cellulose nanocrystal and its application - ACS

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Green preparation of cellulose nanocrystal and its application Xingya Kang, Shigenori Kuga, Chao Wang, Yang Zhao, Min Wu, and Yong Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02363 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Sustainable Chemistry & Engineering

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Green preparation of cellulose nanocrystal and its

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application

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Xingya Kang,†,‡ Shigenori Kuga,† Chao Wang,† Yang Zhao,†Min Wu*,†and Yong

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Huang*,†

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Mailing address:

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†

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Zhongguancun East Road, Haidian District, Beijing 100190, People’s Republic of

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China.

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‡

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29

School of Packaging and Printing Engineering, Henan University of Animal

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Husbandry and Economy, 2 Longzihu North Road, Zhengdong New District,

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Zhengzhou 450046, People’s Republic of China.

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Email address of corresponding author:

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*Min Wu. E-mail: [email protected].

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*Yong Huang. E-mail: [email protected]

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KEYWORDS: Cellulose nanocrystal (CNC), Ball milling, Centrifugation, Polyvinyl

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alcohol (PVA)

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ABSTRACT: A green method was used to prepare cellulose nanocrystal (CNC) with

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no use of harsh chemicals or organic solvents. This method was simple via ball

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milling cellulose with water followed by centrifugation. The diameter and length of

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CNC were 3-10 nm and 120-400 nm separately, with a aspect ratio of 20-60. The

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yield was about 20%. The thermal stability of CNC was the same as that of raw

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cellulose owing to lacking of chemical modification. So it was superior to CNC

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prepared by acid hydrolysis. Then polyvinyl alcohol (PVA)/CNC composite film was

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made by solution casting. The results showed that the tensile strength of composite

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film was 79% increased from neat PVA film when adding 5% CNC to PVA matrix. In

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addition, the composite films possessed good transparency and thermal stability.

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INTRODUCTION

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Cellulose nanocrystal (CNC) (nanowhisker) is rod-like particle with typical diameter

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of 1-50 nm and length of several hundred nanometers.1 Due to its unique

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physicochemical properties such as large specific surface area, high strength and low

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density, CNC has been an object of intense research. The traditional preparation is

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hydrolysis by strong mineral acids, which attack amorphous regions of cellulose,

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leaving crystalline region as CNC. In the post-processing, the excess acid must be

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removed by centrifugation and/or dialysis, which are laborious processes.

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On the other hand, ball milling is an effective method to produce nanocellulose.

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While dry ball milling of cellulose is known to cause its decrystallization,2-4 addition

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of liquid to the system prevents or retards the decrystallization, possibly due to


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shock-damping or specific interaction effects with cellulose. The influence depends

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heavily on the nature of the liquid added, and such effects have been reported in some

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instances.5-9

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As for water, its presence in ball milling is known to prevent decrystallization,

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leading to mere defibrillation of cellulose; therefore Zhang et al10 attempted

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preparation of cellulose nanofiber (CNF) by ball milling purified wood pulp with

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water aided by weak alkali pretreatment. Their results showed certain effectiveness,

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and the average diameter of the CNF was 100 nm. Abe et al11 bead-milled bleached

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wood pulp with 8 or 16 % NaOH solution and obtained CNF of 12-20 nm wide.

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However, the products of these two studies were not dispersed to the elementary

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fibril-level of 3-5 nm wide. While these studies used purified wood pulp as starting

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material, which were of high molecular weight and unfavorable for dispersion, Peyre

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et al12 showed that TEMPO oxidation of microgranular (microcrystalline) cellulose

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could give 5-9 nm wide CNCs by mild stirring.

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Thereupon we here examined ball milling of commercial microcrystalline

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cellulose without any pretreatment or use of harsh chemicals for preparing CNC.

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Because this procedure cannot give complete disintegration down to elementary-fibril

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level, the product was fractionated by centrifugation into the supernatant and

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precipitate. The yield of the two fractions and their effectiveness as reinforcement

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element for polyvinyl alcohol film was examined for mechanical, optical and thermal



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properties.

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EXPERIMENTAL SECTION

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Materials. Microcrystalline cellulose (MC, DP=262, Sigma Aldrich), CF11 (DP=200,

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Whatman), bleached kraft pulp cellulose (DP=1162, Kinleith), polyvinyl alcohol

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(degree of polymerization was 1700, degree of deacetylation was 99%, Aladdin,

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analytical grade). All the materials were used directly.

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Preparation of CNC. Three kinds of cellulose material with different DP were used:

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microcrystalline cellulose, CF11 and bleached kraft pulp. 1.1 g of cellulose and 20.9 g

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of deionized water were added to a 45 mL zirconia pot containing seven zirconia balls

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(10 mm). Ball milling was carried out at 300 rpm for 0.5 to 16 h by a Fritsch P7

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planetary mill. After ball milling, cellulose slurry was centrifuged at 4000 rpm for 5

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min to give supernatant and precipitation. Both fractions were freeze-dried and

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weighed. Samples were marked as “S/P-milling time”. “S” means supernatant and “P”

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means precipitation. For example, “S-6” means the supernatant after 6-hour milling.

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Preparation of PVA-CNC composite films. In this section, CNC was referred to S-6

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from MC. 5 g of PVA was dissolved in 95 g of deionized water at 90°C oil bath under

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stirring. Different amounts of CNC was dispersed in the PVA solution at room

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temperature. Weight percent of CNC in composite films was 1, 3, 5, 7, 10%,

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respectively. The mixed solution was sonicated for 20 min and cast in the glass petri


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dish to dry in ambient air for 1 day. PVA/P-6 film was also prepared for comparison.

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P-6 was also prepared from MC.

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Characterization. CNC was dispersed in water to make 0.01wt% suspension and

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dropped on carbon film for TEM test (JEOL 2100) at 200 kV, or on freshly cleaved

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mica for AFM characterization (Bruker Multimode 8) in scanasyst mode.

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The freeze-dried sample was ground with KBr for FTIR (Varian 3100) in

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transmission mode. The FTIR spectra of PVA/CNC composite films were measured in

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reflection mode.

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XRD was measured by an X’Pert PRO X-ray diffractometer (Bruker AXS GmbH).

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The degree of crystallinity was calculated by eq 1, where I002 is the intensity of the

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crystalline region of cellulose (2θ = 22.2°) and Iam is the intensity of amorphous refion

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(2θ = 18.6°).

（（%）） =

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× 100

(1)

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Thermogravimetry of cellulose samples and PVA/CNC composite film was done by

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TGA (Q 50) in nitrogen atmosphere from room temperature to 600 oC with heating

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rate of 10 oC/min。

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The composite films were cut into 5 mm wide and 30 mm long strip for tensile test

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by a universal testing machine (MTS Sintech-1). The gauge of the strips was 20 mm

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and the strain rate was 25 mm/min. The samples were conditioned in measuring



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environment for 48 h before testing. Measurement was done for more than five test

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pieces.

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The surface and cross-section of PVA/CNC films were examined by SEM (JEOL-4800). The cross-section of samples were fractured in liquid nitrogen.

Light transmittance of PVA/CNC films were measured using UV−vis-NIR

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spectrometer (Varian Cary 5000) from 200 to 800 nm.

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RESULTS AND DISCUSSION

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The morphology and size of CNC. Figure 1 showed TEM images of supernatant and

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precipitate of microcrystalline cellulose ball milled with water for 6 h. The

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supernatant consisted of rod- or needle-like particles, mostly 10 nm wide and several

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hundred nanometers long (Figure 1a). The results were similar for CF11 as shown in

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Figure 2a. Though the length was that of typical CNC, the width was much larger

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than that of elementary fibrils, indicating that ball milling without surface-modifying

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agent was not enough for ultimate dispersion. Still, dispersion of these particle in

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supernatant was very stable at least for one week, showing flow birefringence (Figure

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3).

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The precipitate fractions of MC and CF11 consisted of incompletely dispersed long

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fibers (Figure 1b and 2b). Though their width was not much larger than that of

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supernatant fractions, their longer length would cause extensive entanglement and

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aggregation, which would lead to sedimentation on centrifugation. This factor was


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likely to be the cause of centrifugal separation of the tow fractions.

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On the other hand, pulp cellulose ball milled with water for 6 h was totally

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sedimented by centrifugation, the supernatant also consisted of long fiber aggregates

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(Figure 4). This behavior confirmed the role of cellulose DP in preparation of CNC

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via this new method.

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More accurate width determination was possible by AFM. The CNC made from

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MC had width of 3-12 nm and lengths 120-400 nm, with the aspect ratio of 20-60

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(Figure 5a). Figure 5b was the AFM image of the thin nanofiber in the precipitate,

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showing width of 3-8 nm. Though entangled by the long lengths, the fibers were well

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dispersed into near-elementary fibril levels. Figure 6 showed supernatant and

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precipitate fractions from CF11. They also had widths of 2-10 nm.

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The yield of CNC. Figure 7 showed the yield of CNC from MC and CF11 obtained

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by different ball milling time. The yield of CNC increased rapidly in the first 4 h, then

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nearly levelled off after 8 h. Conventional CNC preparation by mineral acids was

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known to give low yield, less than 30%.13 Therefore, together with eliminating

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necessity of harsh treatment and laborious purification, use of ball milling for CNC

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preparation can be a practical method.

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Crystallinity of CNC. The crystallinity of CNC affects its properties in application.

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Figure 8 showed the XRD patterns of supernatant CNC and precipitate samples ball

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milled with water for 0.5 h and 6 h. All the patterns were that of cellulose I, and there



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was little apparent difference between the samples, including the original cellulose,

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but the degree of crystallinity were reduced by ball milling as shown in Table 1. This

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obviously reflects mechanical damage by ball milling, probably accompanying

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decrease in degree of polymerization. On the other hand, there was no additional

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sharp peaks from inorganic contamination which often encountered in dry ball

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milling.

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It is known that acid treatment for CNC preparation can cause enhancement of

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crystallinity due to removal of the amorphous regions of cellulose14. In contrast, such

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effect was not observed in the case of ball milling. Also, the difference between the

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supernatant and precipitate fractions was negligibly small, indicating the ball milling

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action gave similar effects to both fractions except for the fiber length and aggregating

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tendency.

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The surface groups of CNC. The FTIR spectra of both fractions of ball milled

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cellulose were almost the same with each other, but slightly different from that of

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original cellulose (Figure 9). The main difference was in i) sharpness of OH stretching

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band at 3000-3700 cm-1, indicating enhanced homogeneity in hydrogen bonding

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patterns; ii) weakening of CH stretching band at 2800-3000 cm-1; iii) weakening of

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multiple peaks at 1250-1500 cm-1. Interpretation of these changes needs further

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studies.

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Thermogravimetric analysis of CNC. Figure 10 showed the thermogravimetric


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curves of S-6 and P-6 obtained from the two kinds of starting cellulose. Because no

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new functional groups were introduced by ball milling, the thermal decomposition

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behavior was nearly the same as the original cellulose. Only small decrease in the

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decomposition temperature, 5-10 K, was noted (Table 2). This was in sharp contrast to

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the CNC prepared by sulfuric acid treatment, which started to decompose at about

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220°C due to presence of sulfate ester of surface hydroxyls.15-16 Here was another

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advantage of ball mill-derived CNC over acid hydrolysis-derived CNC in practical

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application.

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PVA/CNC composite films. The supernatant and precipitate fractions from

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microcrystalline cellulose ball milled with water for 6 h were used for fabrication of

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nanocomposite with PVA via water-based mixing. Figure11a showed the effect of

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CNC addition on the mechanical properties of the films. The tensile strength

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increased at first, and then decreased. The maximum of 85 MPa was attained at 5%

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CNC, which was 79% increase from neat PVA film. The decrease at higher CNC

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addition was likely to result from inhomogeneous dispersion of CNC in the film. The

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elongation at break decreased linearly with increase of CNC content, giving 31%

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decrease from the neat PVA. Thus addition of CNC made the film stronger and stiffer.

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Addition of the precipitate fraction to PVA was also effective, but the extent of

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improvement was lower than that of CNC; the maximum strength was obtained at 3%

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loading, with 40% increase in tensile strength (Figure 11b). This was because of the

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aggregates of precipitate.



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SEM of fractured cross section of the composite film (Figure 12) showed gradual

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roughening with increased loading of CNC, suggesting influence of aggregation of

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CNC particles. Still, the images showed no indication of phase separation or fibril

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plucking, indicating good adhesion between CNC and PVA matrix. This adhesion is

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likely to be based on hydrogen bonding and van der Waals forces.

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Influence of CNC addition on transparency was not significant, as shown in

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Figure 13. Transmittance was 91.9% at 550 nm for neat PVA, and linearly decreased

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to 85.1% for 10% CNC composite film. This behavior was understandable in view of

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the closeness of refractive indexes of the two materials, about 1.58 of cellulose vs.

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1.49-1.52 of PVA.

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Figure 14 showed thermal properties of PVA/CNC composite films. There were

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three thermal decomposition stages for neat PVA. The first stage was below 150°C,

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evaporation of adsorbed water; the second stage was 250-350°C, decomposition of

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PVA with weight loss of ~75%. The last stage was above 400°C, which was the

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decomposition of residual char. The thermal decomposition temperature of pure PVA

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film was 240°C, and those of composite films were about 255°C, as shown in Figure

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14a. Therefore, it could be considered the thermal stability of PVA/CNC composite

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films improved by addition of CNC. Differential TGA (Figure 14b) showed the

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influence of CNC addition clearly as a shoulder at ca. 350°C, which agreed with the

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main decomposition range of pure cellulose (data not shown). This behavior indicated


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that decomposition of the two components took place nearly independently.

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CONCLUSIONS

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Cellulose nanocrystal was prepared by ball milling cellulose with water followed by

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centrifugation. This method was simple and green with no use of harsh chemicals or

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organic solvents, offering a new route of cellulose nanocrystal preparation. The

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diameter of CNC was 3-10 nm and the length was 120-400 nm, so the aspect ratio

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was 20-60. Though complete dispersion into 3-5 nm width was not possible,

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supernatant fraction after centrifugation, with typical yield of 20%, making maximum

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surface area and aspect ratio beneficial for composite additive. The thermal stability

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of CNC was higher than CNC prepared by acid hydrolysis due to lack of chemical

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modification. The PVA/CNC composite film made by solution casting showed

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significant enhancement of mechanical strengths, while showing no detrimental effect

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on transparency. Furthermore, the thermal stability of composite films were improved

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to some extent. Cellulose is abundantly reproducible and biodegradable; the

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physicochemical procedure here does not involve harsh/harmful chemicals or drastic

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conditions. Therefore the approach is expected to be useful for providing an

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environment-friendly and sustainable new materials.

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Figure 1. TEM of S-6 (a) and P-6 (b) of microcrystalline cellulose.

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Figure 2. TFM image of S-6 (a) and P-6 (b) obtained from CF 11.

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Figure 3. The flow birefringence of CNC


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Figure 4. TFM image of S-6 obtained from pulp cellulose.

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Figure 5. AFM images of S-6 (a) and P-6 (b) obtained from MC.

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Figure 6. AFM images of S-6 (a) and P-6 (b) obtained from CF11.

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Figure 7. The yield of CNC of different ball milling time obtained from MC and

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CF11.


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Figure 8. XRD spectra of CNC and precipitation of different ball milling time

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obtained from MC (a) and CF 11 (b).

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Figure 9. FTIR spectra of S-6 and P-6 obtained from MC (a) and CF 11 (b).

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Figure 10. TGA of S-6 and P-6 obtained from MC and CF11.

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Figure 11. The mechanical properties of PVA/CNC (a) and PVA/P-6 (b) composite

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films.


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Figure 12. The morphology of cross-section of PVA/CNC films.

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Figure 13. The optical transparency of PVA/CNC films. (a) Photograph of PVA/CNC

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films; (b). Light transmittance of PVA/CNC films.



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Figure 14. Thermal degradation of PVA/CNC films under nitrogen. (a) TGA; (b)

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DTGA.

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Table 1. The crystalline degree of CNC and precipitation of different ball milling

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time.

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Table 2. Thermal decomposition temperatures of S-6 and P-6 obtained from MC and

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CF11.

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AUTHOR INFORMATION

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Corresponding Author

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*Min Wu. E-mail: [email protected]. Tel: +86-10-82543500.

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*Yong Huang. E-mail: [email protected]. Tel: +86-10-82543478.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

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This study was supported by the National Natural Science Foundation of China

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(No.51373191,

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Professorships.

51472253),

and

Chinese

Academy

of

Sciences

Visiting

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REFERENCES

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Cellulose nanocrystal and the flow birefringence phenomenon.


Green preparation of cellulose nanocrystal and its application - ACS

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