Cellulose (2015) 22:1729–1741 DOI 10.1007/s10570-015-0582-6
ORIGINAL PAPER
Preparation of cellulose nanofiber from softwood pulp by ball milling Liyuan Zhang • Takuya Tsuzuki • Xungai Wang
Received: 29 October 2014 / Accepted: 17 February 2015 / Published online: 27 February 2015 Ó Springer Science+Business Media Dordrecht 2015
Abstract This paper reports the possibility of producing cellulose nanofiber from softwood pulp using a simple ball milling technique under ambient pressure and at room temperature. The effects of milling conditions including the ball-to-cellulose mass ratio, milling time, ball size and alkaline pretreatment were investigated. It was found that milling-ball size should be carefully selected for producing fibrous morphologies instead of particulates. Milling time and ball-tocellulose mass ratio were also found important to control the fiber morphology. Alkali pre-treatment helped in weakening hydrogen bonds between cellulose fibrils and removing small particles, but with the risks of damaging the fibrous morphology. In a typical run, cellulose nanofiber with an average diameter of 100 nm was obtained using soft mechanical milling conditions using cerium-doped zirconia balls of 0.4–0.6 mm in diameter within 1.5 h without alkaline pretreatment. L. Zhang T. Tsuzuki (&) X. Wang Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia e-mail:
[email protected] L. Zhang Department of Chemical Engineering, Monash University, Melbourne, VIC 3800, Australia T. Tsuzuki Research School of Engineering, College of Engineering and Computer Science, Australian National University, Canberra, ACT 0200, Australia
Keywords Cellulose nanofiber Ball milling Softwood pulp Single factor analysis
Introduction Cellulose nanofiber has been recognized as a new family of sustainable nanomaterials (Azizi Samir et al. 2005). It is a long filament with a typical lateral dimension of less than 100 nm and a longitudinal dimension of several microns. It has found wide applications in areas such as tissue engineering (Bodin et al. 2007), wound dressing (Brown Jr et al. 2006; Czaja et al. 2006), filtration media (Wertz and Schneiders 2009), electronics devices (Nogi and Yano 2008), carbon nanofiber production (Jazaeri et al. 2011), and hydrophobic coating (Arbatan et al. 2012). In order to realize wide-spread applications of cellulose nanofiber, it is critical to develop a rapid and economical production method. In the last couple of decades, various methods to manufacture cellulose nanofiber from regenerated cellulose and natural raw materials have been developed. These methods include electrospinning (Frey 2008; Kim et al. 2006), biosynthesis (Brown 1886; Recouvreux et al. 2011), and mechanical isolation. Some review papers have been published to compare these methods (Abdul Khalil et al. 2012; Lavoine et al. 2012; Siro´ and Plackett 2010). Conventional needlebased electrospinning techniques for producing
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cellulose nanofiber suffer from the use of toxic solvents, very low productivity (spinning rate: 10 ml/h), the use of high voltage (Ye 2007) and poor crystallinity. Biosynthesis by bacteria produces nanofiber with narrow diameter distributions, high aspect ratios and high crystallinity, but has limitations such as the requirement for strict and costly production conditions, poor reproducibility between the bacteria of different generations, and the complex post-treatment purification procedures. Abdul Khalil et al. (2014) reviewed the mechanical production methods of cellulose nanofiber. Among the mechanical methods, high pressure homogenization has been widely applied to isolate cellulose nanofiber from plants (Nakagaito and Yano 2004). In this method, raw cellulosic fibrils are first soaked in water and then subjected to a high-shear homogenizer at high pressure (up to 1500 bar) (Zimmermann et al. 2010). This method can produce fibers with a diameter range from 5 nm to 1 lm. However, the energy consumption is high. It has another drawback where fiber agglomeration could block the small slit in the homogenizer that leads to the immature termination of the production process. In order to shorten the process time and to reduce the energy consumption, pretreatments have often been applied in the mechanical methods. The pre-treatments include cryo-crushing (Bhatnagar and Sain 2005; Nishiyama et al. 2002; Sain and Bhatnagar 2008), alkali pretreatment (Abe et al. 2007, 2009; Iwamoto et al. 2007), and TEMPO-mediated oxidation (Fukuzumi et al. 2008; Saito et al. 2007). However, alkali pretreatment reduces the strength of cellulose nanofiber. TEMPO-mediated oxidation leads to a significant decrease in the thermal degradation point (Fukuzumi et al. 2008) and reduces the mechanical strength of the cellulose nanofiber. Enzyme pretreatment reduces the pass-number to go through a homogenizer, but the pretreatment itself was a complicated process (Henriksson et al. 2007, 2008). Therefore, alternative methods to efficiently produce cellulose nanofiber are required. Ball milling is a top-down technique to form micro to nano scale materials, by inducing heavy cyclic deformation in materials. Ball milling is nowadays widely used for the preparation of nanoparticles because of its simple operation, use of relatively inexpensive equipment and its applicability to essentially all classes of materials (Koch 1997). However,
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one of the problems is the decrystallization of materials during ball milling. In addition, the production of nanofiber using ball milling without forming particulates is not a straightforward task. In spite of these challenges, our preliminary work demonstrated that ball milling can produce highly crystalline cellulose nanofiber from softwood pulp (Zhang et al. 2010; Tsuzuki et al. 2010). However, the factors that may influence the quality and the yield of cellulose nanofiber by ball milling were not studied in detail. In this paper, the single factor effects of milling conditions on the production of cellulose nanofiber are investigated. The factors include the mass ratio between balls and cellulose, milling time, and ball diameters. The effect of alkali pre-treatment was also studied. In the past, chemical pre-treatment was shown to effectively reduce the number of stone-grinding processes required to obtain cellulose nanofiber (Abe et al. 2007). However, its effect on ball milling has not been studied. Since cellulose could be damaged easily by acid (Bondeson et al. 2006; Correˆa et al. 2010; Eichhorn 2011; Hubbe et al. 2008; Moran et al. 2008) or strong alkaline solutions, in this study, only a mild alkaline (sodium carbonate) solution was used. Postproduction-treatments such as dimensional homogenization were also studied as necessary parts of cellulose nanofiber production process.
Materials and methods Materials Dry sheets of commercial Northern Bleached Softwood Kraft (NIST reference material 8495, manufactured by Grande Prairie pulp mill, Canada) were used as a raw material. The pulping and bleaching processes have removed most of the lignin and hemicellulose, leaving predominantly cellulose in the raw material. Zirconia balls ZirconoxÒ with diameters less than 1 mm were used as grinding media, including yttriumdoped-zirconia balls with a diameter of 0.3 mm and a density of 1200 kg/m3, as well as cerium-dopedzirconia balls with a diameter of 0.4–0.6 and 0.8–1.0 mm and a density of 6200 kg/m3 (Klausen Pty Ltd). Sodium carbonate (Sigma-Aldrich) was used to prepare the alkali solutions of different concentrations (1, 2, and 4 %). Nylon meshes with openings of 250 and 133 lm and a bench-top Eppendorf
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centrifuge were used to remove the residual large cellulose fibrils after milling and to concentrate cellulose nanofiber suspension.
samples were washed by diluting with deionized water and then centrifuging. The washing was repeated till the pH value of the supernatant became 7.
Soft mechanical milling
Morphology characterization
Before milling, the dry softwood pulp sheets were made into water-swollen fibrils, by soaking 5 cm 9 5 cm pieces of the dry pulp in water with a solid content of 10 wt% overnight at room temperature. The swollen pulp fibrils were first disintegrated by a kitchen blender for 5–10 min and then diluted to 1 wt%. The 25 grams of the diluted fibril slurry were put into a plastic specimen container of 70 ml capacity. Zirconia balls were added into the container before it was subjected to ball milling in a Spex 8000M Mixer/ Mill (SPEX CertiPrep Group). The swollen fibrils and the balls were mixed during the milling as illustrated in Fig. 1. Different milling conditions were used to study the effects of these single factors.
The single factor effects on the preparation of cellulose nanofiber by ball milling were evaluated based on these morphologies. Scanning electron microscopy (SEM) images of the obtained samples were taken to characterize sample morphologies using a Zeiss Supra 55VP SEM. Samples No. 001–014 were diluted to a concentration of 0.1 g/100 ml, and dried on an aluminium stab in an oven at 60 °C overnight. Before imaging, all the dried samples were coated with gold (total thickness ca. 2 nm) by a sputter coater. The secondary electron detector was used for capturing images at the electron-acceleration voltage of 2 kV. Images were acquired at magnifications in the range from 5009 to 10,0009 in determining the distribution of fiber diameter. The images were analyzed for cellulose nanofiber dimension using the method proposed by Batchelor (Zhang et al. 2012). All the fibers that could be recognized in each SEM image were measured. This was done manually by drawing a line across the fiber in the image and then measuring the length of the line using the Image-pro Plus image analysis software. Each image was manually marked to ensure that an individual fiber in the image was only counted once. The measured fiber diameters were sorted into bins of 5 nm width, with the values of the bin center being used for all calculations. The counts in a bin were then normalized by image area to give the frequency description, counts/m2. When the measurement range for fiber diameters overlapped between images, the highest frequency of counts/m2 measured at any of the magnifications was selected as the count frequency for that particular bin, which then allowed for a single distribution of fiber diameter to be estimated across the whole range of fiber diameters measured.
Single factor effects study The single factors of ball-to-cellulose mass ratio, milling time, ball size and concentrations of sodium carbonate in alkali-pretreatment were studied. The ball-to-cellulose mass ratio of 40:1, 60:1, 80:1 and 200:1 were studied as shown in Table 1. Milling times of 30, 60, 90 and 150 min were trialed as shown in Table 2. The ball diameters of 0.3, 0.4–0.6 and 0.8–1.0 mm were used as shown in Table 3. Alkalipretreatment with sodium carbonate solutions with concentrations of 0.8, 1.6 and 3.0 % were studied as shown in Table 4. After milling, the alkali-pretreated
Crystallinity study
Fig. 1 Schematic diagram of cellulose nanofiber production by ball milling
The crystal structure was studied by X-ray diffraction (XRD) measurements. A Pan-Analytical X’Pert PRO MRD was used with Cu Ka radiation at a step width of 0.02°. The operation voltage and current were set at 40 kV and 30 mA, respectively. Oven-dried fiber
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Table 1 Runs of different ratio of balls to cellulose Sample ID
Mass of cellulose (g)
Solid content (%)
Mass ratio (balls:cellulose)
Diameter of balls (mm)
Mass of balls (g)
Milling time (min)
No. 001
0.25
1.00
40:1
0.4–0.6
10
60
No. 002
0.25
1.00
60:1
0.4–0.6
15
60
No. 003
0.25
1.00
80:1
0.4–0.6
20
60
No. 004
0.25
1.00
200:1
0.4–0.6
50
60
Milling time (min)
Table 2 Runs with different milling time Sample ID
Mass of cellulose (g)
Solid content (%)
Mass ratio (balls:cellulose)
Diameter of balls (mm)
Mass of balls (g)
No. 005
0.25
1.00
40:1
0.3
10
No. 006
0.25
1.00
40:1
0.3
10
90
No. 007
0.25
1.00
40:1
0.3
10
150
30
Table 3 Runs with balls of different diameters Sample ID
Mass of cellulose (g)
Solid content (%)
Mass ratio (balls:cellulose)
Diameter of balls (mm)
Mass of balls (g)
Milling time (min)
No. 008
0.25
1.00
40:1
0.3
10
60
No. 001
0.25
1.00
40:1
0.4–0.6
10
60
No. 009
0.25
1.00
40:1
0.8–1.0
10
60
Table 4 Runs of sodium carbonate solvents with different concentrations Sample ID
Solvent
Conc. of solvent (%)
Mass of cellulose (g)
Solid content (%)
Mass ratio (balls:cellulose)
Mass of balls (g)
Milling time (min)
No. 001
Sodium carbonate
0
0.25
1.00
40:1
10
60
No. 010
Sodium carbonate
0.8
0.25
1.00
40:1
10
60
No. 011
Sodium carbonate
1.6
0.25
1.00
40:1
10
60
No. 012
Sodium carbonate
3.0
0.25
1.00
40:1
10
60
The diameter of balls used in this table was 0.4–0.6 mm
sheeting was used for the measurements. The XRD samples were mounted on a zero diffraction plate made of single crystal silicon. Crystallite sizes, Dcr, were calculated using the Scherrer equation for the diffraction peak at 22.5°:
maximum (FWHM) and h is the Bragg angle. The degree of crystallinity was estimated using the Segal method that is frequently used for cellulose (Segal et al. 1959):
Kk Dcr ¼ b cos h
Icr ¼ ð1Þ
where K is the Scherrer constant (=0.94), k is the wavelength (=0.1542 nm), b is the full width at half
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Imax Imin 100 Imax
ð2Þ
where Imax is the maximum intensity of the peak at 22.5° and Imin is the peak intensity of the amorphous fraction represented by the diffraction intensity at 19°.
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Dimensional homogenization To remove the remaining large fibers from nanofibres, the aqueous suspension of the milled sample was screened firstly by diluting the milled sample with deionized water and then passing the fiber suspension through mesh filters with 250 and 133 lm openings. The sample suspension that passed through the filters was collected and centrifuged at 4,500 rpm for 10 min. The supernatant was discarded after centrifuging. The milky nanofiber suspension at the bottom of the centrifuge tubes was collected and diluted again with deionized water. The above procedure was repeated twice to complete the dimensional homogenization (i.e., diameter-purification) process. Samples before and after the process were dried in an oven at 60 °C overnight and then observed using a SEM. From the SEM images, the median diameter, maximum diameter, and the arithmetic average diameter before and after process were obtained statistically.
Results and discussions Homogeneity of cellulose nanofiber The colors of the samples before and after ball milling were both white and no visible difference was detected. Figure 2a shows the morphology of cellulose fibrils before ball milling. Figure 2b–d show the morphologies of sample No. 001 from low to high magnifications. Figure 2d illustrates that cellulose nanofiber was obtained after ball milling. In Fig. 2b, c, nanofiber network was not observed as the magnifications were much lower than Fig. 2d. However, Fig. 2b, c showed a high degree of homogeneity of cellulose nanofiber obtained using the ball milling technique. Single factor effects on cellulose nanofiber production Mass ratio between balls and cellulose Figure 3a–d show the SEM images of the samples milled using different ball-to-cellulose mass ratio of 40:1, 60:1, 80:1 and 200:1 under the conditions shown
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in Table 1. It is evident that the ball-to-cellulose mass ratio significantly influenced the production of cellulose nanofiber. At the ratio of 40:1, nanofiber was obtained with a small amount of nanoparticles. At the ratio of 60:1, nanofiber was obtained with a large amount of nanoparticles. When the ratio was increased to 80:1, nanofiber was shortened and continuous gellike objects were formed along with many nanoparticles. With a higher ratio of 200:1, no fibrous morphology was found. Figure 3a, b demonstrated the possibility of obtaining cellulose nanofiber while preventing particle formation, by controlling the mass ratio between balls and cellulose. In wet conditions, cellulose fibers were fully swollen with water and hydrogen bonding between nanofibers in the pulp was weakened. The mechanical agitation induced by the contact of the milling balls to the fibrils helps defibrillate the bundle of nanofibers. The fibrous morphology was completely damaged by the large ball-to-cellulose mass ratio of 200:1 (Fig. 3d). A higher ball-to-cellulose ratio results in a higher number of ball-to-fiber collision events, leading to the damage of the fibrous morphology. The results suggest that, when the ball diameter of 0.4–0.6 mm was used, the ball-to-cellulose mass ratio should be less than 80:1 to prevent the damage of fibrous morphology. To reduce the amount of the particles formed during the ball milling, the ball-tocellulose mass ratio should be less than 60:1. Figure 4 shows the X-ray diffraction patterns of the raw pulp and Samples No. 001, 003 and 004. The Miller indices were assigned to the diffraction peaks according to the recent theoretical calculation (French 2014). The raw pulp showed the cellulose-Ia crystal structure having preferred orientation along the fiber axis, typical of plant fibers. After milling, a new peak appeared at around 29°. This peak position is similar to that of the cellulose-Ia with random orientation of the crystallites, but a lower angle than that of cellulose-Ib crystal structure (French 2014). Figure 5 shows the crystallinity and crystallite size as a function of ball mass in Table 1. As the ball mass (i.e. ball-to-cellulose ratio) increased, the crystallinity decreased gradually. However, even using the high ball-to-cellulose ratio of 200:1 (Sample No. 004), the crystallinity was over 70 %. The crystallite size showed almost no change over the studied range of ball-to-cellulose ratio.
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Fig. 2 SEM images of the a fibers before milling and b–d after milling (Sample No. 001), at different magnifications (from low to high)
The insignificant change in crystal structure, crystallinity and crystallite size (Figs. 4, 5) is in stark contrast to the drastic change in morphology (Fig. 3). For this reason, further investigation of the effects of milling conditions was carried out mainly using SEM imaging techniques.
magnification. However, high magnification images showed that cellulose nanofiber networks were obtained among the large fibers. Longer milling times provide a higher number of ball-to-fiber collision events, leading to a higher chance of the breakdown of hydrogen bonding between cellulose fibrils to extract the nanofiber.
Milling time Ball diameter SEM images of the samples milled for different times under the conditions in Table 2 are shown in Figs. 6, 7 and 8. In Sample No. 005 that was milled for 30 min, a large amount of micro fibrils remained (Fig. 6). Thus cellulose nanofiber could not be formed by ball milling for such a short milling time. Figures 7 and 8 show the morphologies of Samples No. 006 and No. 007 at a low and a high magnification, respectively. Some large fibers were observed at a low
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Figures 9, 10 and 11 show SEM images of the Samples Nos. 008, 001 and 009 milled with balls of different diameters under the conditions described in Table 3. It can be seen that, using small balls of 0.3 mm in diameter, a large amount of micron-scale fibers still remained (Fig. 9a) and that cellulose nanofiber started forming from the edge of large fibers (Fig. 9b). After ball milling using larger balls of
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Fig. 3 SEM images of samples milled with different ball-to-cellulose ratios a 40:1(Sample No. 001); b 60:1 (Sample No. 002); c 80:1 (Sample No. 003); d 200:1 (Sample No. 004)
0.4–0.6 and 0.8–1.0 mm in diameter, large fibers were scarcely observed and the images showed a cloud-like appearance at low magnifications (Figs. 10a, 11a). At high magnifications, cellulose nanofiber network were clearly evident (Figs. 10b, 11b). These results suggest that ball size plays an important role in the cellulose nanofiber production. When small milling balls with a diameter of 0.3 mm were used, nanofiber could be formed but the amount of nanofiber produced was very low. Larger balls, such as balls with diameters of 0.4–0.6 and 0.8–1.0 mm, may have had sufficient kinetic energy to break the fibrils from micro to nano scale more efficiently than smaller milling media. However, with larger balls, the chance of damaging nanofibers to form particulates also increases.
Alkali-pretreatment Figure 12 shows the SEM images of the samples milled in sodium carbonate solutions of 0, 0.8, 1.6 and 3.0 wt% concentrations under the conditions described in Table 4. The images were taken after removing the residual sodium carbonate from the milled samples by washing. Figure 12a, c show that Samples No. 001 and 010 had fibrous morphologies co-existing with small amounts of nanoparticles. As shown in Fig. 12a–c, the amount of nanoparticles decreased as the concentration of sodium carbonate increased. Figure 12d shows an appearance of Sample No. 012, in which cellulose nanofiber was evident without particulates but with a large amount of short fiber-fragments. It
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Fig. 6 SEM image of Sample No. 005 (30 min)
Fig. 4 XRD patterns of a raw pulp, b Sample No. 001, c No. 003 and d No. 004
Due to the higher surface areas of cellulose nanoparticles than that of cellulose nanofiber, nanoparticles may have dissolved selectively in the sodium carbonate solution. However, too much alkali will further dissolve the nanofiber. Milling in an alkaline solution has a drawback as it will add another step in the posttreatment process to remove the residual alkali, which should be considered in practical cellulose nanofiber production. Dimensional homogenization
Fig. 5 XRD crystallite size and crystallinity of raw pulp and Samples No. 001, 003 and 004 (diamond crystallinity, square crystallite size)
indicates that cellulose fibers were damaged when the concentration of sodium carbonate increased to 3.0 wt%. From the results shown above, it is apparent that milling in sodium carbonate solutions influences cellulose nanofiber production. The reason might be that sodium carbonate helps to dissolve and remove the cellulose nanoparticles produced by ball milling.
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From the morphologies of the milled samples shown in the SEM images, only Samples No. 001, 006, 007, 009, and 011 consisted of a large percentage of nanofibers. Among these samples, sample 006 was selected to study the effectiveness of the post-milling dimensional homogenization process. Table 5 shows the statistical results of fiber-diameter analysis on Sample No. 006 before and after the dimensionalhomogenization process. Before the process, the fiber diameter of 95 % threshold (D [0.95]) was under 132 nm, which implies the good dimensional homogeneity of cellulose nanofiber produced using ball milling even before the dimensional-homogenization process. After the process, the maximum fiber diameter decreased from 793 to 139 nm. At the same time, D [0.95], median diameter, and arithmetic average diameter were slightly reduced. The results indicate that the diameter-purification process was effective in removing large fibers to obtain good dimensional homogeneity.
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Fig. 7 SEM images of Sample No. 006 (60 min) taken at a low magnification and b high magnification
Fig. 8 SEM images of sample No. 007 (90 min) taken from a lower magnification to c higher one
Conclusions This study demonstrated that ball milling is an effective one-step process to produce cellulose
nanofiber from micron-sized cellulosic fibrils under ambient pressure and at room temperature. The single factors influencing the production process were studied, including ball-to-pulp mass ratio, milling time,
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Fig. 9 SEM images of Sample No. 008 milled with small balls (diameter of 0.3 mm) taken at a low magnification and b high magnification
Fig. 10 SEM images of Sample No. 001 milled with middle-size balls (diameter of 0.4–0.6 mm) taken at a low magnification and b high magnification
ball size and the alkali-pretreatment. Since the crystal structure, crystallinity and crystallite size of milled fibers were found to be not sensitive to the differences in milling conditions, morphological analysis by electron microscopy was used to study the effects of milling conditions. It was found that careful selection of milling-ball size is important to produce fibers instead of powders. If the balls are too small, they do not have sufficient impact energy to de-fibrillate nanofibers. If the ball size is too large, they have too high impact energy to damage the fibers to form particles. Milling time and
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ball-to-pulp mass ratio were also found important to control the number of collision events between balls and fibers and in turn the production efficiency. If the total number of collision events is too small, the defibrillation will be slow. Too many collision events lead to the damage of nanofibers in a short milling time. Alkali pre-treatment helped defibrillation by weakening hydrogen bonding between nanofibers, but it also damaged the fibrous morphology when the alkali concentration is high. Alkali pre-treatment also introduces an additional post-treatment process, causing more resource consumption and environmental
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Fig. 11 SEM images of Sample No. 009 milled with large balls (diameter of 0.8–1.0 mm) taken at a low magnification and b high magnification
Fig. 12 SEM images of samples milled in alkaline solutions a No. 001; b No. 010; c No. 011; d No. 012
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Table 5 Diameter analysis of cellulose nanofiber Sample No. 006 Sample No. 006
Median D (nm)
D [0.95] (nm)
Max D (nm)
Arithmetic average D (nm)
Standard deviation (nm)
Sampling size (counts)
Before purification
54
132
793
68
53
123
After purification
47
127
139
57
31
281
D is diameter
pressure. By careful control of milling conditions, cellulose nanofiber with high dimensional homogeneity and average fiber diameters less than 100 nm was obtained by ball milling. Acknowledgments Liyuan Zhang thanks IDP Education Australia Ltd. for the IDP Student Mobility Scholarship, Dr. Warren Batchelor for providing the standard pulp sample as starting materials, Deakin Microscopy Center for SEM imaging, Chuntao Zhang for helping with the experiments, Ruoyang Chen, Monash University and Hui He, Donghua University for helping with the figures, and Dr. Xu Li from Bio21 Institute, University of Melbourne, Dr. Tina Arbantan from Dulux Austrlia, Ltd., and Chenfan Xia, Deakin University, for proof reading the manuscript.
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