Preparation and properties of holocellulose nanofibrils with different hemicellulose content

Park, C. W., Han, S. Y., Choi, S. K., and Lee, S. H. (2017). "Preparation and properties of holocellulose nanofibrils with different hemicellulose content," BioRes. 12(3), 6298-6308.

Abstract

The hemicellulose content in holocellulose was adjusted by an alkaline treatment. The effects of this treatment on the defibrillation efficiency of holocellulose nanofibrils (HCNFs) were investigated by wet disk-milling (WDM), along with their morphological and physical properties. In addition, the tensile properties of nanopaper sheets fabricated with these HCNFs were investigated. As the hemicellulose content decreased, the average diameter and the filtration time of the HCNFs decreased, whereas specific surface area and crystallinity index increased. An increase in the WDM time reduced the average diameter and crystallinity of the HCNFs and increased their filtration time and specific surface area. The tensile strength and elastic modulus of the nanopaper sheets increased with increased hemicellulose content and WMD time.

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Preparation and Properties of Holocellulose Nanofibrils with Different Hemicellulose Content

Chan-Woo Park,^a Song-Yi Han,^a Seon-Kang Choi,^b and Seung-Hwan Lee ^a,*

Keywords: Holocellulose nanofibril; Alkaline treatment; Wet disk-milling; Nanopaper; Defibrillation

Contact information: a: Department of Forest Biomaterials and Engineering, Kangwon National University, Chun-Cheon 200-701, Republic of Korea; b:Department of Agricultural Life Industry, Kangwon National University, Chun-Cheon 200-701, Republic of Korea;

* Corresponding author: lshyhk@kangwon.ac.kr

INTRODUCTION

Cellulose is the most abundant, renewable natural biopolymer on earth. It is generally obtained from woody biomass via photosynthesis – a process that produces approximately tens of billions of tons of cellulose annually (Cheng et al. 2007; Habibi et al. 2010). In the wood cell wall, cellulose molecules form microfibrils via intra- or intermolecular hydrogen bonds (Aulin et al. 2011). The cellulose microfibril has a diameter of 10 nm to 30 nm and can be isolated after chemical or biological pretreatments through mechanical defibrillation processes, such as wet disk-milling (WDM), high-pressure homogenizing, and ball milling (Habibi et al. 2010; Siró and Plackett 2010; Wang et al. 2012; Espinosa et al. 2013; Jang et al. 2013; Peng et al. 2013; Yu et al. 2013; Zhang et al. 2015). However, it is difficult to degrade the wood cell wall into its constituents because of its structural heterogeneity and complexity, endowing it with very strong recalcitrant characteristics against chemical or biological treatments (Himmel et al. 2007). Therefore, it is indispensable to pretreat lignocellulosic biomass before the defibrillation process to overcome its recalcitrance (Chang et al. 2012; Jang et al. 2013). The main role of the pretreatment is to effectively disturb the tightened structure of lignocellulosic biomass by removing hemicellulose and lignin (Lee et al. 2010; Siró and Plackett 2010). To this end, various pretreatment methods for the improvement of the defibrillation process have been reported, such as alkaline, enzyme, hot-compressed water (HCW), ozone, and steam explosion treatments, etc. However, it is not always necessary for these pretreatments to completely remove the hemicellulose and lignin (Iwamoto et al. 2008; Lee et al. 2010; Abraham et al. 2011; Jang et al. 2013). The partial removal of hemicellulose and lignin can improve defibrillation, and the resulting product has a different chemical composition, which will influence the properties of the fibrillated products. Jang et al. (2013) reported that the partial removal of lignin and hemicellulose by ozone and steam treatments improved the efficiency of defibrillation. Iwamoto et al. (2008) reported that the amount of hemicellulose in pulp affects the ease of defibrillation and the properties of the produced paper sheets. Galland et al. (2015) prepared holocellulose nanofibers (HCNFs) using the acid chlorite method for delignification, reporting that the content of hemicellulose in HCNF influenced the properties of the nanopaper sheets. The obtained HCNFs are unique in terms of their high molecular weight and cellulose-hemicellulose core-shell structure. This structure can also be designed by coating hemicellulose on a pure cellulose nanofiber surface, resulting in the improvement of the hygromechanical properties and moisture stability (Prakobna et al. 2015a,b).

In this study, the hemicellulose content in HCNF was adjusted by an alkali treatment, and its effect on the defibrillation efficiency, morphological, and crystalline properties of HCNF was investigated. Furthermore, the tensile properties of HCNF nanopaper sheets were also investigated.

EXPERIMENTAL

Materials

A Korean white pine (Pinus koraiensis Sieb. et Zucc.) sample was obtained from Kangwon National University’s Experimental Forest, and it was cutter-milled to 0.45 mm. Sodium chlorite (NaClO₂), acetic acid (CH₃COOH), and sodium hydroxide (NaOH) were purchased from Daejung Chemical & Materials Co. (Siheung, Republic of Korea). Other chemicals were purchased from commercial sources and used without further purification.

Delignification and alkaline treatment

Delignification was performed by following the Wise method (Wise et al. 1946). First, wood powder (20 g) and distilled water (1,200 mL) were poured into a round flask (2-L), and they were kept in a water bath at 80 °C for 20 min under constant stirring at 150 rpm. The delignification reaction was initiated by adding sodium chlorite (8 g) and acetic acid (1600 µL) into the suspension, which was continuously stirred for 1 h. The same amount of sodium chlorite and acetic acid were added every hour, and the process was repeated 7 times. The residue, holocellulose, was then vacuum-filtrated and washed with distilled water several times until the pH was neutralized.

The hemicellulose content in holocellulose was adjusted via an alkaline treatment. First, 30 g of holocellulose were poured into 1%, 3%, and 5% sodium hydroxide solutions (750 mL), and the reaction was performed at 25 °C for 24 h under constant stirring at 150 rpm. Next, the reactant was further reacted at 80 °C for 2 h to extract the hemicellulose, according to Abe et al. (2007). At the end of the reaction, a 10% acetic acid solution (750 mL) was added into the reactant for neutralization. Then, the insoluble residue was vacuum filtrated and washed with distilled water several times. The content of hemicellulose was calculated by subtracting the weight of the pure cellulose obtained by using a 17.5% NaOH treatment on the holocellulose. The sample codes and composition of cellulose and hemicellulose are summarized in Table 1.

Table 1. Sample Codes and Chemical Composition of Holocellulose

Preparation of HCNF

The holocellulose was suspended to a 1.0 wt.% concentration (1500 mL), and then subjected to WDM (Supermasscolloider, MKCA6-2, Masuko Sangyo Co. Ltd., Kawaguchi, Japan). The rotational speed was set at 1,800 rpm, and the clearance between the upper and lower disks was reduced to between 100 µm and 150 µm from the zero point, at which distance the disks would begin to rub. The operation was repeated until the fifth pass.

Methods

Morphology

The HCNF suspensions were diluted to 0.001 wt.% and then sonicated using an ultrasonicator (VCX130PB, Sonics & Materials Inc., Newtown, USA) for 1 min. The suspensions were vacuum-filtrated on a polytetrafluoroethylene (PTFE) membrane filter with a pore size of 0.2 μm (ADVANTEC^®, Toyo Roshi Kaisha Ltd., Tokyo, Japan). The filtrated products were stacked on the PTFE filter and immersed in tert-butyl alcohol for 30 min. This procedure was repeated thrice to completely exchange the water with tert-butyl alcohol. The HCNFs were freeze-dried using a freeze dryer (FDB-5502, Operon Co. Ltd., Gimpo, Korea) at -55 °C for 3 h to prevent the aggregation of HCNFs. The freeze-dried samples were coated with carbon using a vacuum evaporator (JEE-400, JEOL Ltd., Tokyo, Japan) with an electric current of 30 mA. The morphologies of the HCNFs were observed with a scanning electron microscope (SEM, S-4800, Hitachi Ltd., Tokyo, Japan) in the Central Laboratory of Kangwon National University. The diameter of the individual fibers was measured at least 800 times for each sample by an open-sourced Image J program (1.45 ver. for Windows, National Institute of Mental Health [NIMH] of the National Institute of Health [NIH], Bethesda, USA).

Specific surface area (SSA)

The HCNF suspensions (30 mL) were centrifuged (VS-21SMTi, Vision Scientific Co. Ltd., Bucheon, Republic of Korea) at 20,000 rpm for 15min, and the supernatant was drained. Then, tert-butyl alcohol was poured into the centrifuge tube, and it was mixed with the centrifuged HCNFs using a vortex mixer (IKA^® MS2 mini shaker, IKA Werke, Germany). This solvent exchange was repeated 15 times to completely exchange the water with tert-butyl alcohol and prevent the aggregation of HCNFs. The solvent exchanged HCNFs were freeze-dried with powder at -55 °C for 12 h. The specific surface area of the HCNFs was measured using a sorption analyzer (BELSORP-Max, BEL Japan Inc., Toyonaka, Japan) by N₂adsorption at 77 K.

Crystallinity index and average crystallite size

The x-ray diffraction profiles of the HCNFs were detected using a diffractometer (RINT TTR III, Rigaku Co. Ltd., Tokyo, Japan) with Cu-Kα radiation (K = 0.15418 nm). The samples were scanned in 2θ ranges varying from 0° to 40° at a 2°/min scanning rate. The crystallinity index and average crystallite size of the HCNFs were calculated from the x-ray diffraction profiles following Eq. 1 (Segal et al. 1959) and Eq. 2 (Monshi et al. 2012), respectively,

(1)

where I₂₀₀ is the maximum intensity of the peak from the (200) lattice plane (2θ = 22.5°), and I_amor is the peak intensity of the amorphous phases (2θ = 18.5°). Equation 2 is listed as,

(2)

where K is the Scherrer constant (0.89), λ is the wavelength of the X-ray (0.15418 nm), is the width of the half-height of the peak angle at the (200) plane in radians, and is the Bragg angle corresponding to the (200) plane.

Nanopaper preparation and tensile testing

First, 0.2 wt.% HCNF suspensions (220 mL) were prepared, sonicated for 1 min, and vacuum-filtrated on a silicone-coated filter (Whatman® No. 2200 125, GE Healthcare Ltd., Buckinghamshire, UK). At this stage, the filtration time was measured as a criterion of the degree of fibrillation. The filtrated HCNFs were sandwiched between the silicone-coated filters and pressed at 105 °C with a pressure of 5 MPa for 2 min using a hot press.

For the tensile test, 7 specimens with dimensions of 5.0 mm × 0.7 – 0.8 mm × 50.0 mm (width × thickness × length) were cut from the nanopaper and kept in a thermo-hygrostat at 25 °C and 50% relative humidity to minimize the effect of the relative humidity on the tensile properties of the nanopaper sheets. The tensile test was conducted using a testing machine (H50K, Hounsfield Test Equipment Ltd., Redhill, UK), at a cross-head speed of 10 mm/min, with a specimen span length of 30 mm.

RESULTS AND DISCUSSION

Morphological Properties

As shown in Table 1 of the Experimental section, the content of hemicellulose in holocellulose was successfully adjusted by changing the concentration of the NaOH solution, in which a range of hemicellulose of 9.2 wt.% to 22.6 wt.% was obtained. The effects of the hemicellulose content and the number of WDM passes on the morphological properties of the HCNFs were investigated. Figure 1 shows the SEM images of various HCNFs. All of the samples showed a fine and uniform morphology, and a decreased diameter was observed as the hemicellulose content decreased, and the number of WDM passes increased.

Table 2 and Fig. 2 summarize the average diameter and distribution of HCNFs with different hemicellulose content and number of WDM passes, respectively. The average diameter of the HCNFs decreased as the hemicellulose content decreased, as shown in Table 2. Holocellulose has a core-shell structure, i.e., hemicellulose covered cellulose, thus the diameter of HCNF may be subject to the amount of hemicellulose. The average diameter decreased for all of the samples as the number of WDM passes increased.

Fig. 1. SEM images of HCNFs with different hemicellulose content and number of WDM passes

As shown in Fig. 2, the distribution of the diameters became narrow as the content of hemicellulose decreased in both cases of 1 and 3 WDM passes. All HCNFs showed a diameter of less than 55 nm. The diameter distribution peak shifted to a smaller size with an increased number of WDM passes. Iwamoto et al. (2008) reported that holocellulose prepared by delignification using the Wise method, and pure cellulose prepared using a 6% potassium hydrate solution after delignification from Sitka spruce, were fibrillated uniformly via WDM into diameters of 10 nm to 20 nm. Nobuta et al. (2016) removed lignin and hemicellulose from kenaf bast fiber using the Wise method and an alkaline treatment using a 4% KOH solution, respectively, and prepared HCNF and pure cellulose nanofibrils (PCNF) by WDM. The diameters of HCNF and PCNF were reported to be 18 nm and 17 nm, respectively. Their results were similar to those obtained in this study.

Fig. 2. Diameter distribution of HCNFs with different hemicellulose content and number of WDM passes; Note: the class intervals of the diameter distribution were 1 nm

Table 2. Average Diameter of HCNFs with Different Number of WDM Passes

Specific Surface Area and Filtration Time

Table 3 shows the effect of the hemicellulose content and number of WDM passes on the specific surface area and filtration time of HCNFs. The specific surface area showed an increasing tendency as the hemicellulose content and number of WDM passes decreased and increased, respectively. This result corresponded well with the tendency of the HCNF diameter to decrease with decreased hemicellulose content and increased number of WDM passes. Moreover, these results supported that the removal of hemicellulose improved the defibrillation efficiency. The filtration time of the HCNF suspensions was shortened as the hemicellulose content decreased, despite their increased specific surface area. This may have been due to the strong hydrophilicity of hemicellulose. Kim et al. (2017) investigated filtration time of HCNF and BKP-NF (bleached kraft pulp-based nanofibril) suspensions, and the filtration time of HCNF suspensions was noticeably longer than that of the BKP-NF suspension. Iwamoto et al. (2008) reported that a KOH treatment to remove hemicellulose in holocellulose reduced the water retention value of HCNFs due to the reduction of hydrophilic amorphous hemicellulose substances. Pejic et al. (2008) reported that the water retention value of hemp fibers decreased due to the removal of hemicellulose. Similar to the specific surface area, the filtration time increased with an increased number of WDM passes.

Table 3. Specific Surface Area and Filtration Time of HCNFs with Different Hemicellulose Content Obtained by Different Number of WDM Passes

Crystallinity Index and Average Crystallite Size

Figure 3 shows the effects of hemicellulose content and the number of WDM passes on the X-ray diffraction pattern. All of the samples showed a typical cellulose I pattern, but the peak at 22.5° (2θ) became larger with decreased hemicellulose content. Table 4 summarizes the crystallinity index and average crystallite size of the various HCNFs. As the hemicellulose content decreased, the crystallinity index increased when 1 and 3 WDM passes were performed. Wan et al. (2010) reported that the crystallinity index of eucalyptus kraft pulps increased as the amount of hemicellulose decreased. The increase of the crystallinity index may have been due to the removal of amorphous hemicellulose. The crystallinity index was also influenced by the number of WDM passes. With an increased number of WDM passes, the crystallinity index of all HCNFs decreased. It is thought that the crystalline structure of cellulose collapses by shear and impact forces during the WDM process. Iwamoto et al. (2007) also reported a decreasing tendency of crystallinity with an increasing number of WDM passes. Yu and Wu (2011) investigated the effect of ball-milling on the crystallinity of microcrystalline cellulose and reported that crystalline structures can be destroyed by severe ball-milling treatments. In all of the samples, the average crystallite size fell within the range of 3.2 nm to 3.6 nm. With decreased hemicellulose content, the average crystallite size was slightly increased. In contrast, a decreasing tendency of the average crystallite size was found with an increased number of WDM passes. Wan et al. (2010) also reported that the average width of cellulose crystallite increased from 4.3 nm to 4.6 nm as the hemicellulose content decreased from 27.62% to 9.09%.

Fig. 3. X-ray diffraction spectra of HCNFs with varying hemicellulose content obtained using a different number of WDM passes

Table 4. Crystallinity Index and Average Crystallite Size of HCNFs with Different Hemicellulose Content Obtained Using a Different Number of WDM Passes

Tensile Properties

Table 5 shows the effect of the hemicellulose content and number of WDM passes on the density, tensile strength, and elastic modulus of nanopaper sheets prepared with the different HCNFs. Even though the density was lower in the nanopaper sheets from HCNFs with higher hemicellulose content, the tensile strength and elastic modulus increased with increased hemicellulose content. In the authors’ previous study (Park et al. 2017), the HCNF was prepared by the Wise method and WDM from Korean white pine, and the pure cellulose nanofiber (PCNF) was prepared by 17.5% NaOH treatment and WDM from the obtained holocellulose. The tensile strength and elastic modulus of the non-alkaline treated HCNF nanopaper were 155 to 178 MPa and 3.9 to 4.4 GPa, respectively, which is higher than the tensile properties of the PCNF with tensile strength of 59 to 67 MPa and elastic modulus of 1.7 to 1.9 GPa. Galland et al. (2015) reported an improvement in the tensile strength and elastic modulus of nanopaper with higher hemicellulose content. They explained this phenomenon in terms of the better packing effect due to hemicellulose-coated cellulose in individual HCNFs. Iwamoto et al. (2008) also reported that hemicellulose provides a better adhesion between cellulose nanofibrils, contributing the enhanced tensile properties to the nanopaper sheets. Molin and Teder (2002) reported that pulp paper sheets with higher hemicellulose content showed a higher stiffness and tensile strength, compared to pulp paper sheets with a lower hemicellulose content. The explanation for this was that hemicellulose on the surface of the pulp fibers adhered to the fibers in the sheets, inhibiting their deformation. Furthermore, the tensile strength and elastic modulus also improved with an increased number of WDM passes. This may have been due to the enhancement of defibrillation, i.e., the decrease in the diameter of HCNF, which would increase hydrogen-bonding sites between HCNFs.

Table 5. Density, Tensile Strength, Elastic Modulus, and Specific Tensile Strength of Nanopaper Sheets Prepared from HCNFs with Different Hemicellulose Content and Number of WDM Passes

CONCLUSIONS

The effects of hemicellulose content on the defibrillation efficiency of holocellulose and the properties of HCNF prepared by WDM were investigated.
The defibrillation efficiency of holocellulose improved with decreased hemicellulose content and increased number of WDM passes, which produced smaller fibers. The decrease of the HCNF diameter resulted in an increase of specific surface area.
As the hemicellulose content decreased and number of WDM passes increased, the crystallinity index of the HCNFs increased.
The filtration time of the HCNF suspensions and the tensile properties of the nanopaper sheets increased with increased hemicellulose content. This may have been caused by the strong water holding capacity and adhesion effect between the nanofibrils of hemicellulose.

ACKNOWLEDGMENTS

This research was supported by a grant (Project No. S211316L010120) from the Forest Science & Technology Projects, Forest Service, Republic of Korea, and by the Basic Science Research Program through the National Research Foundation of Korea by the Ministry of Education (No. NRF-2015R1D1A1A01061522). The authors thank Dr. Sun-Young Lee and Dr. Sang-Jin Chun at the Division of Wood Processing, National Institute of Forest Science in the Seoul, Republic of Korea for their help with the tensile tests.

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Article submitted: March 22, 2017; Peer review completed: June 24, 2017; Revised version received and accepted: July 4, 2017; Published: July 17, 2017.

DOI: 10.15376/biores.12.3.6298-6308