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Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender
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Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender Sami Boufi ∗ , Achraf Chaker University of Sfax, Faculty of Science of Sfax, LMSE, BP 1171, 3000 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 5 December 2015 Accepted 16 May 2016 Available online xxx Keywords: Nanofibrillated cellulose Agricultural residue Corn stalk Cellulose Nanocomposites
a b s t r a c t Agricultural crop residues are an abundant and cheap source of cellulose fibers suitable for uses in composite, textile, pulp and paper manufacture. Besides, field crop fibers might be an attractive source for the production of value-added nanosized cellulose fibrils with a broad potential use. In this study, nanofibrillated cellulose (NFC) from corn stalk crop residues were produced with high yields using a simple high speed blender (HSB). It was shown that a full conversion of cellulose fibers into NFC was successfully achieved by disintegration during 30 min of fibers produced via a NaClO2 /acetic acid delignification mode and submitted to a TEMPO-mediated oxidation pre-treatment. The fibrillation yield, transparency degree, colloidal properties and morphological characteristics of the ensuing NFC were analyzed and compared according to the delignification mode and the carboxyl content. The reinforcing potential of the NFC produced with different carboxyl contents was investigated. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The last two decades have witnessed an increasing interest in the production of nanofibrillated cellulose (NFC) from a wide range of vegetal resources due to the broad possibility of use of this natural nanomaterial in the field of innovative materials (Kalia et al., 2014; Klemm et al., 2011). NFC are fibrils with width in the nanometer scale, typically between 5–50 nm depending on the extent of cell delamination and fibers pre-treatment (Khalil et al., 2012) and length in the range of 1–5 m. These bio-nanomaterials combine the advantageous properties of cellulose, namely the broad range of chemical modifications, non-toxic character (Vartiainen et al., 2011), biodegradability, renewability, and sustainability with the specific attribute of nanosized materials that expand the spectrum of potential applications. To produce NFC, the hierarchical structure of macroscopic fibers has to be broken up, by delaminating cellulosic fibers under an intense high mechanical shearing action. Since the advent of NFC in the 1983 by Turbak et al., (1983) high pressure homogenization (HPH) and micro-fluidization have been the main methods used to produce NFC. However, albeit the promising potential usefulness of NFCs in a multitude of applications, their scale-up production is still limited and below expectations. One major obstacle to this
∗ Corresponding author. E-mail address: Sami.Boufi@fss.rnu.tn (S. Boufi).
development is the high energy consumption involved during the mechanical disintegration of the fibers into nanofibers (Naderi et al., 2015). Another problem is the necessity to call on costly industrial equipment such as the high-pressure homogenizer or the microfluidizer. In both of these disintegration modes, a high pressure is requested to force the diluted suspension of fibers through a tight flow path in the case of the microfluidizer or a narrow nozzle (between 100–200 m) for the high-pressure homogenizer. This high pressure accounts for the huge energy consumption during the cell wall delamination process. It follows that any mechanical disintegration method operating under atmospheric pressure is likely to contribute to lower the energy input during the disintegration process. The fibers pre-treatment prior to the disintegration process is another approach currently adopted to decrease the energy demand. Chemical pre-treatment has emerged as one of the most efficient and popular pre-treatment strategies to facilitate the break-up of the fibers network by generating ionic or ionisable groups within the internal structure of the fibers. This can be achieved via a TEMPO-mediated oxidation (Saito et al., 2006), (Liimatainen et al., 2012), carboxymethylation (Siró et al., 2011) or via sulfonation with sodium bisulfate (Liimatainen et al., 2013) or quaternization (Chaker and Boufi, 2015, Saini et al., 2016). In general, a critical content in ionic groups is needed to effectively facilitate the release of the cellulose microfibrils and break down the cell wall of the fibers (Besbes et al., 2011). Another merit of the pre-treatment is the reduction of the risk of clogging during the homogenization process, namely when HPH and microfluidization
http://dx.doi.org/10.1016/j.indcrop.2016.05.030 0926-6690/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Boufi, S., Chaker, A., Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.05.030
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were used. In addition to the fibers pre-treatment approach, the delignification mode was recently shown to meaningfully affect the fibrillation efficiency of the bleached cellulose fibers. Leaving the highest content of hemicelluloses in the fibers after the pulping process contributed to make easier the conversion of the fibers into nanosised cellulose fibrils (Chaker et al., 2013, 2014). The combination of an enzymatic pre-treatment and a mechanical refining has also been extensively used as an eco-friendly approach to facilitate the disintegration of cellulose into nanofibrils and, thus, reduce energy consumption (Pääkkö et al., 2007; Qing et al., 2016). However, enzymatic treatments are known to affect fiber morphology. Enzymatic pre-treatments resulted in CNFs with shorter fibril lengths and higher crystallinity index. The ensuing nanofibrils appeared stiff and had rod-like shape looking quite similar to cellulose nanocrystals prepared through acid hydrolysis. Field crop residues and/or agricultural by-products such as cereal straw, cornstalks, corn straw, rice husk, bagasse and cereal straw are annually renewable, available in abundance throughout the world and currently of limited values (Reddy and Yang, 2004, 2005). The use of the cereal straw and other agricultural residues or by-products as a source of fibers or cellulosic based materials alleviate the shortage of wood resources and can have the potential to start a natural fiber industry in countries where there are little or no wood resources left, without adversely affecting soil fertility. However, agricultural cropresidues tend to have a high ash from 3% and higher making the pulping process more difficult than woody fibers. Corn straw is an annually renewable biomass, available in abundant volumes throughout the world that is often left behind after corn grain is harvest. In terms of chemical composition and ultrastructure, corn stalk can serve as a cheap source for NFC. Although several papers have dealt with the production of NFC from different agricultural products, most of them have used a high energy demanding disintegration process such as high pressure homogenization or microfluidization (Alemdar and Sain 2008; Hassan et al., 2012; Alila et al., 2013). This conventional approach led to a high-energy NFC cost production, making the use of agricultural by-products less attractive and unforeseeable as a resource for the production of NFC. As a result, an important challenge to be tackled is how to develop less energy consumption processes that are likely to make the production of NFC easier. In the present work, we pursue our investigation regarding the production of NFC from agricultural residue using a conventional high-speed blender (Boufi and Gandini 2015). One of the main advantages of the use of a highspeed blender lies in its simplicity and the possibility to scale-up the process for high volume production without any risk of clogging. By using this conventional high speed blender, our main objective is to highlight how the carboxyl content of fibers and the disintegration time affected the yield of fibrillation and the morphology of the ensuing nanofibrils,
2. Experimental section 2.1. Samples Corn stalk residue was obtained from local sources. The stem of the plants had a length of 0.7–1 m and diameter of 2–3 cm. After further drying at 80 ◦ C for 2 h, the stalk was ground to a coarse powder with about 0.5–2 mm length and crude fibers were Soxhlet extracted for 12 h, using a solvent mixture composed of toluene/ethanol (60/40 v/v). After drying at room temperature, the fibers were kept under mechanical stirring in hot water (70 ◦ C) for 1 h to remove pectin and sand. The fibers were recovered by filtration through Whatman 200 m filter and then submitted to the pulping process to remove lignin.
The pulping procedure for the crude fibers was carried out as follows: 2.1.1. Delignification processes NaClO2 /acetic acid pulping mode The NaClO2 /AA pulping process was carried out as follows: Five grams of dry Soxhlet extracted biomass were added to water and mixed to form suspensions at a solid content 10 wt%. Then, 0.5 g of sodium chlorite (NaClO2 ) and 0.5 mL of acetic acid per gram of dry biomass were added, and the suspension was kept under mechanical stirring at a temperature of 70 ◦ C for 6 h without the removal of any liquor. Fresh charges of sodium chlorite (0.25 g/g fibers) and acetic acid (0.25 g/g fibers) were added to the reaction every 1.5 h for up to 6 h. The pulp yield is calculated through Eq. (1):
Pulp Yield% =
mw (1 − MC/100 md
× 100
(1)
where, mw , md and MC are the weight of the wet biomass recovered, the weight of the dry sample used and MC the moisture content of the recovered solids, respectively. The ensuing pulp recovered after washing with water was white and no bleaching treatment was implemented. 2.2. NaOH pulping mode The extracted biomass was added to water (solid content 10 wt%) and then pulped with a 5 wt% NaOH solution for 2 h at 70–80 ◦ C under mechanical stirring. This treatment was repeated three times until the fibers were well individualized. The ensuing fibers were subsequently filtered and rinsed with distilled water and twice bleached with NaClO2 at 70 ◦ C and pH 4–5 to remove the residual lignin. 2.3. Chemical composition The determination of the basic chemical composition was conducted following TAPPI standard protocols. (TAPPI T 257 cm02). Samples were first submitted to Soxhlet extraction with ethanol/toluene and water. Then, the chemical contents were determined using the following methods, Ash (Tappi T 211 om-93) extractive (Tappi T264 om-07), Klason lignin (Tappi T222 om-88), and hemicelluloses (Tappi T249-cm-85). 2.4. TEMPO-mediated oxidation The TEMPO-mediated oxidation was carried out at pH 10, using NaClO as oxidizing agent and TEMPO as a catalyst. Briefly, cellulose fibers (2 g) were suspended in 200 mL water. TEMPO (30 mg) and NaBr (250 mg) were added to the suspension. Then 50 mL of a commercial NaClO solution (12◦ ) was added dropwise to the cellulose suspension at a temperature around 5 ◦ C which was kept constant throughout the oxidation reaction. The pH was maintained around 10 by the continuous addition of a 0.1 M aqueous solution of NaOH. The oxidation was stopped by adding ethanol (20 mL) and the pH was adjusted to 7 by adding 0.1 M HCl. 2.5. Carboxyl content The carboxyl content of the oxidized cellulose was determined using conductimetric titration, as described elsewhere (Besbes et al., 2011). 2.6. Fibrillation process using a conventional high speed blender (HSB): the fibers in a water suspension at a concentration of 2 wt% were disintegrated
Please cite this article in press as: Boufi, S., Chaker, A., Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.05.030
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during 15–30 min in a “one-step operation” using a domestic high speed blender (MOULINEX 400 W) with a constant running speed of 11 000 rpm, in which the blades rotate in a recessed section at the bottom of a container of 1 L capacity. 2.7. Yield in nanofibrillated cellulose The yield is defined as the ratio of the amount of NFC in the supernatant to the initial amount of NFC before ultra-sonication and centrifugation. This was shown to be an efficient means to separate the unfibrillated materials from those partially fibrillated (Besbes et al., 2011). The protocol was carried out as follows: a dilute suspension with about 0.1 wt% solid content (Sc) was centrifuged at 4500 rpm for 20 min to separate the nanofibrillated material (in supernatant fraction) from the non-fibrillated or partially fibrillated ones which settle down. Then, the sediment fraction was dried to a constant weight at 90 ◦ C. The yield was calculated from Eq. (2)
Yeild% =
1−
weight of dried sediment weight of diluted sample × %Sc
× 100
3
Table 1 Chemical composition (wt %) of the corn stalk biomass and prior and after lignin extraction via NaClO2 /acetic acid pulping process. Constituents
Corn stalk biomass
Pulp composition
Cellulose Hemicellulose Lignin Extractible Ash
38 32 19 4 7
69 31 – – –
CrI Pulp yield Fibres length (m) Fibres width (m)
51 – – –
63 65 680 21
*As% of dry matter.
the maximum intensity (FWHM), in radians, and is the diffraction angle
(2) 2.11. Energy consumption
The results represented the average values of three replications. 2.8. AFM observation The morphology of the nanofibers was studied by atomic force microscopy (AFM) (FlexAFM from Nanosurf) using a tapping mode. The samples were prepared by depositing a drop of diluted NFC suspension (with a solid content about 0.01 wt%) on the surface of a silicon wafer and leaving it to dry for 1 h.
The instrument used for the energy consumption measurement was a power analyzer from SCOMEC DIRIS A20 which is an integrating tool that measures precisely the total amount of electrical energy, the voltage and the current intensity supplied to the electrical equipment in a given period of time.
Intensity
A ZEISS SUPRA40 FE-SEM was used to obtain images by capturing secondary electrons emitted from the surface of a NFC sample, prepared from a drop of the NFC suspension (with a solid content about 0.02 wt%) deposited on the surface of a silicon wafer and coated with a thin carbon layer, applied by ion sputtering with a thickness limited to 2–3 nm). To ensure a good image resolution without any damage for the samples during the analysis, the acceleration voltage was maintained at a relatively low range (2–5 kV).
SiO2
A
2.9. Field-emission scanning electron microscopy (FE-SEM)
Corn stalk
NFC
Corn pulp
2.10. Determination of the crystalline index 5
CrI% =
I
002 − Iam I002
× 100
(3)
K ˇ. cos
(4)
Where K is a dimensionless shape factor and usually taken to be 0.9, ´˚ is the X-ray wavelength,  is the line broadening at half (1.54 A)
15
20
25
30
35
40
B (c) (b) (a) 1710 cm-1
where I002 is the maximum intensity of the (002) diffraction peak, taken at 2 between 22◦ and 23◦ for cellulose I, and Iam is the intensity of the amorphous diffraction peak taken at 2 between 18◦ and 19◦ for cellulose I. Scherrer’s equation was used to calculate the crystallite size, T (nm), perpendicular to the (200) plane for cellulose I crystals: T=
10
2θ (°)
Transmittance
The crystallinity was evaluated from an X-ray diffraction (XRD) pattern obtained using a BRUKER AXS diffractometer (Madison, WI) with a Cu-K␣ radiation, generated at 30 kV and an incident current of 100 mA. The (2) angular region from 5◦ to 40◦ was scanned by steps of 0.05◦ using a step time of 10 s. The crystalline index (CrI) was calculated by equation 3 using the diffraction intensities of the crystalline structure and that of the amorphous fraction, according to the method of Segal (Segal et al., 1959):
1510 cm-1
1360 cm-1 1308 cm-1 -1
895 cm
Nea t corn stalk fibers-NaClO2 Fibe rs-Oxydized
1800
1700
1600
1500
1400 1300 1200 1100 -1 Wave number (cm )
1000
900
800
Fig. 1. (A), XRD patterns for neat corn stalk, corn pulp and NFC, and (B) FTIR spectra of: native cornstalk, NaClO2 deliginified, NaOH deliginified, and oxydized fibers from corn stalk.
Please cite this article in press as: Boufi, S., Chaker, A., Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.05.030
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Neat
320
485
900
500-15%H
15% Hemicellulose
100
Fibrillation yield (%)
90 80 70 60 50
The NFC gel with 2% solid content was mixed with the latex in order to obtain nanocomposite films with weight fraction of cellulose ranging from 0% to 15%. After stirring for 1 h, the mixture was cast in a Teflon mould and stored at 40 ◦ C until water evaporation was completed. A transparent to translucent film, depending on the NFC content, was obtained with a thickness in the range of 300–400 m. 2.11.2. Tensile tests The non-linear mechanical behaviour of the films was analyzed using an Instron testing machine in tensile mode, with a load cell of 100 N working at a strain rate of 10 mm/min at 25 ◦ C. The specimens were obtained using a cutting device.
40 30 20 10
3. Results and discussion
0 10
20
25
30
Desintegration time (min) Fig. 2. Evolution of the fibrillation yield vs the disintegration time at different carboxyl content, using a high speed blender.
2.11.1. Nanocomposites processing Commercial acrylic latex obtained by the copolymerization of styrene (S) and butyl acrylate (BuA) was used as a matrix. The size of the polymer particles was around 150 nm and the solid content 50 wt%. The glass–rubber transition temperature (Tg) of this poly(Sco-BuA) copolymer was about −10 ◦ C.
3.1. Pulp characterization The chemical composition of corn stalk is given in Table 1. The main constituents are polysaccharides (70 wt%), with about 38% cellulose, and 32% of hemicelluloses, respectively. The content of lignin is lower than 20%, making its whole extraction from the fibers easier. This composition is in agreement with literature data (Reddy and Yang, 2005). The low cellulose content in agricultural crop, compared to woody plant, is explained by the low exigency in terms of weight that the plants have to support during their growth. Due to the high content of hemicelluloses in corn stalk, we have adopted the NaClO2/acetic acid extraction process that selectively removed lignin with less or no impact on hemicelluloses. Even
Fig. 3. Schematic illustration of the aspect of the cellulose nanofibrils according to their hemicelluloses content: (A) hemicelluloses content 30%, (B) Hemicellulose content (<20%).
Please cite this article in press as: Boufi, S., Chaker, A., Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.05.030
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Fig. 4. AFM height images of NFC prepared via high speed blender with carboxyl content 300 and 900 mol/g, and via high pressure homogenization and the corresponding height profile analysis (the arrow marks the points the points used for the measurements of the height profile.
though hemicelluloses are amorphous polysaccharide, we infer that leaving them within the fibers is likely to be beneficial for the properties of NFC. The chemical composition and the physical properties of the cellulose fibers after the lignin extraction are given in Table 1. The yield in fibers after the pulping process was about 65%, which is quite high if compared to conventional soda pulping methods. The presence of high residual lignin in the fibers after the lignin extraction accounted for the high pulping yield. No change in the native crystalline structure was noted after the pulping process, with cellulose I polymorph as shown in the XRD (Fig. 1A). The XRD diffraction patterns of the corn stalk and the pulp after lignin extraction were given in the supplementary material. The FTIR spectra of the original corn stalk and the ensuing fibers after lignin extraction are given in Fig. 1. The main results revealed the vanishing of the peak at 1520 cm−1 assigned to C C stretching of the aromatic ring, confirmed the total removal of lignin after the delignification process. The band at 1730 and 1245 cm−1 of
the acetyl and uronic ester groups of the hemicelluloses remained unchanged after the extraction with NaClO2 /AA, in good agreement with the high content of hemicellulose in this sample. The characteristic bands vibration of the glucosic ring appeared in the 1100–1000 cm−1 region and the small sharp band at 895 cm−1 is characteristic of -glycosidic linkage between the sugar units. The small variations of the absorption spectra bands at 1368 and 1316 cm−1 for native and treated fibers indicated that the cellulose underwent a minor change during the process of delignification. After the TEMPO-mediated oxidation (spectrum c), an intensification in the C O band at 1720 cm−1 is observed as a result of the generation of the carboxyl groups. 3.2. Nanofibrillation behaviour of the fibers The fibrillation behaviour of the untreated and the oxidized fibers at three levels of carboxyl content was investigated by following the fibrillation yield and the optical transparency of the
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Fig. 5. FE-SEM images of NFC from oxidized corn pulp with carboxyl content 480 mol/g at different time of disintegration via high speed blender (A) after 10 min, (B) after 20 min and (C) after 30 min.
NFC suspension at regular interval of disintegration in a high speed blender. The first remark to highlight is the successful production of NFC using a conventional high-speed blender as a mechanical disintegration device instead of the high-pressure homogenizer. However, the fibrillation yield is strongly dependent on the carboxyl content of the fibers. After 10 min of disintegration, the fibrillation yield grew from 50% to about 85% as the carboxyl content increased from about 300 mol/g to 900 mol/g, and further disintegra-
tion during 20 min brought the fibrillation yield to about 75 and 100%, respectively. At a carboxyl content of about 900 mol g−1 , a quasi-transparent thick gel with full fibrillation was obtained after blending during 30 min. However, when neat pulp was used, the fibrillation yield did not exceed 30% even after high speed blending prolonged for 40 min. Presumably, the carboxyl content of the fibers (about 125 mol/g) is not enough to bring about enough electrostatic repulsion within the cell wall and overcome the interaction between neighbouring microfibrils through hydrogen bonding. Independently of their chemical structure, the content of the ionic groups generated during the chemical pre-treatment was shown to be determinant on both the energy reduction during the disintegration as well as the fibrillation efficiency. Although the mechanism of action of the ionic groups in the fibrillation process is still a matter of debate, it is assumed that the introduction of the ionic groups facilitated the break-down of the cell wall of the fibers through different effects: (a) electrostatic repulsion among the appended charged groups; (b) increasing level of osmotic pressure brought by the difference in the ionic concentration in counter-ions of the carboxylic groups between the inside and the outside of the fibber’s wall; (c) the ionic groups enhanced the hydration and swelling of the fibers, making them more flexible; and finally, (d) the chemical pre-treatment inevitably led to a chain scission within the amorphous zones that facilitated the release of the elementary microfibrils. Another key parameter that contributed to make the production of NFC via a high-speed blender possible is the chemical composition of the fiber after lignin removal, and more specifically, the residual content of hemicellulose left in the fibers. The high content of hemicelluloses is expected to further facilitate the fibrillation process by preventing fibrils aggregation during lignin removal as highlighted in our previous work13 . Hemicelluloses are believed to be intimately bounded to the cellulose fibrils through hydrogen and covalent bonding and to fill the space as an amorphous constituent between cellulose fibrils. It follows that the presence of hemicelluloses as a protective layer surrounding the microfibrils will prevent the fibrils from coming close together to aggregate and collapse under the effect of interfibrillar hydrogen bonding interaction. Further evidence of the key role of hemicelluloses content could be highlighted by comparing the fibrillation yield of pulp from corn stalk with hemicelluloses content of 15%, obtained through NaOH pulping process (see inset in Fig. 2). When pre-treated by TEMPOmediated oxidation at a carboxyl content of about 500 mol/g, the fibrillation yield did not exceed 38% after disintegration for more than 30 min. A schematic illustration depicting the nanoscale organization of the cellulose fibrils according to the hemicelluloses content is given in Fig. 3. It follows that when the high fraction of hemicelluloses is left in the fiber after the delignification process, the breakup of the cell wall during the mechanical disintegration is easier to implement, and the released fibrils are less aggregated with lateral size close to that corresponding to the elementary crystallites (about 3–4 nm). The strong correlation between hemicelluloses content and fibrils aggregation was shown also to affect other properties such as fibers recyclability, fibers strength, fibers porosity, fibers accessibility and fibers stiffness. By studying the effect of the hemicelluloses content on the ultrastructure of the fibers after lignin removal, a correlation between the ultrastructure of the fibres and the hemicellulose content was proposed (Duchesne et al., 2001). The pulp with high hemicellulose content had a porous surface structure with high relief, whereas fibres with low hemicellulose content, the fibril aggregates formed a much more compact surface structure. Wan et al. (2010) noted the same trend, where a decrease in the hemicelluloses content was shown to partially promote irreversible microfibril aggregation resulting in more compact fibrillar structures with increase in the fibril aggregate size (Wan et al.,
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Fig. 6. (A) Visible transmittance spectra of the NFC suspension for corn stalks produce via high speed blender (and HPH for sake of comparison) at different carboxyl content. (B) Visual aspect of the NFC gel at a solid content of 1 wt% according to their carboxyl content (value in yellow) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
2010). The phenomenon of fibril aggregation during drying of pulp is also on the origin of the harder disintegration of hornified fibers into NFC compared to never dried pulp as recently highlighted by (Kekäläinen et al., 2014). The morphology of the dispersed nanofibrils was evaluated by AFM imaging of extremely diluted NFC suspension deposited on a wafer substrate and dried at room temperature. Examples of images are shown in Fig. 4. All of the NFC samples with different carboxyl contents were composed of nanosized thin fibrils forming entangled network. The fibrils look highly flexible as attested by the formation of bended zone and their length ranging from 200 nm up to several m. The width of the fibril was estimated from the height profile and was found to be between 3 and 5 nm for the individual non-aggregated fibrils. The larger width distribution around 5 nm was found for the NFC produced from fibers with the lowest carboxyl content of 320 and over 500 mol g−1 , the width level off around 3 nm, which is close to the average thickness of cellulose crystallites (around 3.5 nm) calculated on the basis of the Scherrer’s equation. This means that the NFC produced through high-speed blender was composed by individual elementary cellulose fibrils. Further confirmation of the nanoscale dimension of the NFC from HSB was given by FE-SEM observation (Fig. 5) after 10, 20 and 30 min disintegration in a HSB. FE-SEM observation gives larger view than AFM and provides additional indication about the evolution of the fibrils morphologies in terms of width and length distribution during the disintegration process. During the beginning of the mechanical disintegration, the fibers were progressively broken down and fragments of partially fibrillated material with width in the range of 100–200 nm can be seen, in addition with nanosized fibrils 10–20 nm in width. With increasing time of disintegration, the content of nanosized fibrils increased at the expense of the larger fibrils, which is an indication that the cell walls of fibers were progressively broken down, releasing the elementary fibrils building up the fiber. We should note that, prior to the FESEM observation, the micro-sized fragment of fibers were removed by centrifugation. This treatment was necessary to make the observation of the nanosized fibrils possible. The transparency degree of the ensuing NFC was shown to be greatly dependent on both the disintegration time and the carboxyl content. As shown in Fig. 6, at a carboxyl content of 320 mol g−1 , the NFC gel is opaque and turned to a translucid gel over a carboxyl content of 500 mol g−1 . At a carboxyl content of 900 mol g−1 , a highly transparent thick gel is obtained After 30 min of disintegra-
tion. The evolution in the optical transparency is further highlighted by the transmittance measurement of a diluted suspension of the NFC. The transmittance level for NFC-320 did not exceed 70% at a wavelength of 800 nm and grow to over 77% and 85% for a carboxyl content of 480 and 900 mol g−1 , respectively. The evolution in the optical transparency is explained by the change in the fibrillation yield and the dispersability of the cellulose nanofibrils in water. The enhancement in the transmittance with increasing content of ionic groups is indicative of a decrease in the amount of non-fibrillated and partially fibrillated fractions responsible for a light-scattering phenomenon. In addition, the increase in the carboxyl content is expected to enhance the colloidal stability of the cellulose nanofibrils through electrostatic stabilization brought by ionized carboxylic groups on the surface of the nanofibrils. This can be seen from the increase in the absolute value of the zeta-potential of the NFC as the carboxyl content is going up (Table 2). The value of zeta-potential lower than −30 mV for NFC with a carboxyl content over 500 accounted for the good dispersion stability of the NFC suspension in water and good transparency, even at a solid content over 1 wt.% The energy consumed during the disintegration process for the NFC with different carboxyl content was also measured using a power analyser device and the data were collected in Table 2. The energy consumption after disintegration during 30 min was about 11 KWh/Kg which is much lower as those reported for high pressure homogenization (Chaker et al., 2014; Klemm et al., 2011). 3.3. Reinforcing potential of NFC The reinforcing potential of the NFC produced from corn stalk via high speed blender was investigated by preparing nanocomposite films using solvent-casting and analysing their mechanical properties by tension testing tensile. A ductile water-borne acrylic matrix with a Tg around −10 ◦ C was chosen as a matrix in order to reach the limit strength without premature breaking of the sample due to excessive rigidity. To compare the reinforcing potential of the different NFC, the relative modulus and tensile strength for the nanocomposite films vs the content of NFC was plotted (Fig. 7). This parameter represents the increment in the tensile modulus and strength for the nanocomposite with respect to the neat matrix, and gives better indication about the reinforcing potential of the nanofiller. For all
Please cite this article in press as: Boufi, S., Chaker, A., Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.05.030
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Table 2 Physical and colloidal properties of NFC prepared via HSB at different carboxyl content. Fibers characteristic
Neat fibers
NFC-320 (blender)
NFC-480 (blender)
NFC-900 (blender)
Fibrillation yield Viscosity at 1 s−1 (Pa.s) Fibrils width (nm) Zeta-potential (mV) Energy consumption (KWh/Kg dry NFC)
40 4
80 13.3 3.5 −25 10.2a
87 24.8 2.7 −32 11a
100 31.4 2.5 −40 11.2a
a
−15 –
After 30 min disintegration in a HSB.
A
the fibrils is likely to reduce the density of bonded area between the fibrils within the entangled network generated by the cellulose nanofibrils (Boufi, 2014). Indeed, it is well reported that the unusual strong reinforcing effect brought by nanosized cellulose fibrils is ascribed to the set-up of an interconnected and entangled network held-up by strong hydrogen bonding (Favier et al., 1995). These entanglements play an important role in the force transferring from matrix to fibrils and from fibrils to fibrils.
4. Conclusion
B
The potential use of corn stalk as a raw material for the production of nanofibrillated cellulose with low energy demand was demonstrated in the present study. The NFC was produced via disintegration using a conventional high-speed blender. The yield in nanofibrillated material was shown to depend on the hemicellulose and the carboxyl content. A full conversion of fibers from corn stalk into NFC by disintegration during 30 min in a high-speed blender would be possible if the hemicelluloses content of the fibers was over 30% and the carboxyl content exceeded 500 mol g−1 . AFM observation confirmed the nanoscale of the cellulose fibrils produced through HSB with width around 3 nm, which presumably correspond to the ultimate elementary fibril. The NFC produced showed a high reinforcing potential when incorporated into a waterborne polymer matrix. At a content of 10 wt.%, the increment in the modulus and in the tensile strength was about 125 and 10 times that of the neat matrix. The possibility of producing NFC from corn stalk and, in general, from any agricultural crop residue using a high speed blender is expected to open the way towards less exigent technology and energetically cost-effective production of nanofibrillar cellulose from crop residues.
References Fig. 7. increment in (A) the tensile modulus, and (B) the tensile strength of anocomposite films vs NFC content prepared via high speed blender.
the NFC produced, the inclusion of the NFC into the ductile acrylic matrix led to a strong enhancement of both of the tensile modulus and tensile strength, which is in agreement with the well-known reinforcing capacity of NFC. For instance, at 10 wt% NFC, the increment in the tensile strength is 9.5, 10.2 and 13 fold higher than that of the neat matrix in presence of NFC-300, NFC-480, NFC-900 and NFC-500-H. When comparing the reinforcing potential of NFC with three carboxyl degree, it can be seen that NFC-320 brought stronger stiffness enhancement than NFC-900 albeit the lower fibrillation yields of the former. A possible reason for this apparent discrepancy is the difference in fibrils length, and accordingly, in the aspect ratio between the different NFCs. Indeed, as revealed by the AFM observation, although NFC-300 exhibited larger width than NFC-900, the fibrils look qualitatively longer than those from NFC-900, and encompasses lower fraction with short fibrils. The lower length of
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Further reading Boufi, S., Kaddami, H., Dufresne, A., 2014. Mechanical performance and trans-parency of nanocellulose reinforced polymer nanocomposites. Macromol. Mater. Eng. 299, 560-568.
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Easy production of cellulose nanofibrils from corn stalk by a conventional high speed blender Sami Boufi ∗ , Achraf Chaker University of Sfax, Faculty of Science of Sfax, LMSE, BP 1171, 3000 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 5 December 2015 Accepted 16 May 2016 Available online xxx Keywords: Nanofibrillated cellulose Agricultural residue Corn stalk Cellulose Nanocomposites
a b s t r a c t Agricultural crop residues are an abundant and cheap source of cellulose fibers suitable for uses in composite, textile, pulp and paper manufacture. Besides, field crop fibers might be an attractive source for the production of value-added nanosized cellulose fibrils with a broad potential use. In this study, nanofibrillated cellulose (NFC) from corn stalk crop residues were produced with high yields using a simple high speed blender (HSB). It was shown that a full conversion of cellulose fibers into NFC was successfully achieved by disintegration during 30 min of fibers produced via a NaClO2 /acetic acid delignification mode and submitted to a TEMPO-mediated oxidation pre-treatment. The fibrillation yield, transparency degree, colloidal properties and morphological characteristics of the ensuing NFC were analyzed and compared according to the delignification mode and the carboxyl content. The reinforcing potential of the NFC produced with different carboxyl contents was investigated. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The last two decades have witnessed an increasing interest in the production of nanofibrillated cellulose (NFC) from a wide range of vegetal resources due to the broad possibility of use of this natural nanomaterial in the field of innovative materials (Kalia et al., 2014; Klemm et al., 2011). NFC are fibrils with width in the nanometer scale, typically between 5–50 nm depending on the extent of cell delamination and fibers pre-treatment (Khalil et al., 2012) and length in the range of 1–5 m. These bio-nanomaterials combine the advantageous properties of cellulose, namely the broad range of chemical modifications, non-toxic character (Vartiainen et al., 2011), biodegradability, renewability, and sustainability with the specific attribute of nanosized materials that expand the spectrum of potential applications. To produce NFC, the hierarchical structure of macroscopic fibers has to be broken up, by delaminating cellulosic fibers under an intense high mechanical shearing action. Since the advent of NFC in the 1983 by Turbak et al., (1983) high pressure homogenization (HPH) and micro-fluidization have been the main methods used to produce NFC. However, albeit the promising potential usefulness of NFCs in a multitude of applications, their scale-up production is still limited and below expectations. One major obstacle to this
∗ Corresponding author. E-mail address: Sami.Boufi@fss.rnu.tn (S. Boufi).
development is the high energy consumption involved during the mechanical disintegration of the fibers into nanofibers (Naderi et al., 2015). Another problem is the necessity to call on costly industrial equipment such as the high-pressure homogenizer or the microfluidizer. In both of these disintegration modes, a high pressure is requested to force the diluted suspension of fibers through a tight flow path in the case of the microfluidizer or a narrow nozzle (between 100–200 m) for the high-pressure homogenizer. This high pressure accounts for the huge energy consumption during the cell wall delamination process. It follows that any mechanical disintegration method operating under atmospheric pressure is likely to contribute to lower the energy input during the disintegration process. The fibers pre-treatment prior to the disintegration process is another approach currently adopted to decrease the energy demand. Chemical pre-treatment has emerged as one of the most efficient and popular pre-treatment strategies to facilitate the break-up of the fibers network by generating ionic or ionisable groups within the internal structure of the fibers. This can be achieved via a TEMPO-mediated oxidation (Saito et al., 2006), (Liimatainen et al., 2012), carboxymethylation (Siró et al., 2011) or via sulfonation with sodium bisulfate (Liimatainen et al., 2013) or quaternization (Chaker and Boufi, 2015, Saini et al., 2016). In general, a critical content in ionic groups is needed to effectively facilitate the release of the cellulose microfibrils and break down the cell wall of the fibers (Besbes et al., 2011). Another merit of the pre-treatment is the reduction of the risk of clogging during the homogenization process, namely when HPH and microfluidization
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were used. In addition to the fibers pre-treatment approach, the delignification mode was recently shown to meaningfully affect the fibrillation efficiency of the bleached cellulose fibers. Leaving the highest content of hemicelluloses in the fibers after the pulping process contributed to make easier the conversion of the fibers into nanosised cellulose fibrils (Chaker et al., 2013, 2014). The combination of an enzymatic pre-treatment and a mechanical refining has also been extensively used as an eco-friendly approach to facilitate the disintegration of cellulose into nanofibrils and, thus, reduce energy consumption (Pääkkö et al., 2007; Qing et al., 2016). However, enzymatic treatments are known to affect fiber morphology. Enzymatic pre-treatments resulted in CNFs with shorter fibril lengths and higher crystallinity index. The ensuing nanofibrils appeared stiff and had rod-like shape looking quite similar to cellulose nanocrystals prepared through acid hydrolysis. Field crop residues and/or agricultural by-products such as cereal straw, cornstalks, corn straw, rice husk, bagasse and cereal straw are annually renewable, available in abundance throughout the world and currently of limited values (Reddy and Yang, 2004, 2005). The use of the cereal straw and other agricultural residues or by-products as a source of fibers or cellulosic based materials alleviate the shortage of wood resources and can have the potential to start a natural fiber industry in countries where there are little or no wood resources left, without adversely affecting soil fertility. However, agricultural cropresidues tend to have a high ash from 3% and higher making the pulping process more difficult than woody fibers. Corn straw is an annually renewable biomass, available in abundant volumes throughout the world that is often left behind after corn grain is harvest. In terms of chemical composition and ultrastructure, corn stalk can serve as a cheap source for NFC. Although several papers have dealt with the production of NFC from different agricultural products, most of them have used a high energy demanding disintegration process such as high pressure homogenization or microfluidization (Alemdar and Sain 2008; Hassan et al., 2012; Alila et al., 2013). This conventional approach led to a high-energy NFC cost production, making the use of agricultural by-products less attractive and unforeseeable as a resource for the production of NFC. As a result, an important challenge to be tackled is how to develop less energy consumption processes that are likely to make the production of NFC easier. In the present work, we pursue our investigation regarding the production of NFC from agricultural residue using a conventional high-speed blender (Boufi and Gandini 2015). One of the main advantages of the use of a highspeed blender lies in its simplicity and the possibility to scale-up the process for high volume production without any risk of clogging. By using this conventional high speed blender, our main objective is to highlight how the carboxyl content of fibers and the disintegration time affected the yield of fibrillation and the morphology of the ensuing nanofibrils,
2. Experimental section 2.1. Samples Corn stalk residue was obtained from local sources. The stem of the plants had a length of 0.7–1 m and diameter of 2–3 cm. After further drying at 80 ◦ C for 2 h, the stalk was ground to a coarse powder with about 0.5–2 mm length and crude fibers were Soxhlet extracted for 12 h, using a solvent mixture composed of toluene/ethanol (60/40 v/v). After drying at room temperature, the fibers were kept under mechanical stirring in hot water (70 ◦ C) for 1 h to remove pectin and sand. The fibers were recovered by filtration through Whatman 200 m filter and then submitted to the pulping process to remove lignin.
The pulping procedure for the crude fibers was carried out as follows: 2.1.1. Delignification processes NaClO2 /acetic acid pulping mode The NaClO2 /AA pulping process was carried out as follows: Five grams of dry Soxhlet extracted biomass were added to water and mixed to form suspensions at a solid content 10 wt%. Then, 0.5 g of sodium chlorite (NaClO2 ) and 0.5 mL of acetic acid per gram of dry biomass were added, and the suspension was kept under mechanical stirring at a temperature of 70 ◦ C for 6 h without the removal of any liquor. Fresh charges of sodium chlorite (0.25 g/g fibers) and acetic acid (0.25 g/g fibers) were added to the reaction every 1.5 h for up to 6 h. The pulp yield is calculated through Eq. (1):
Pulp Yield% =
mw (1 − MC/100 md
× 100
(1)
where, mw , md and MC are the weight of the wet biomass recovered, the weight of the dry sample used and MC the moisture content of the recovered solids, respectively. The ensuing pulp recovered after washing with water was white and no bleaching treatment was implemented. 2.2. NaOH pulping mode The extracted biomass was added to water (solid content 10 wt%) and then pulped with a 5 wt% NaOH solution for 2 h at 70–80 ◦ C under mechanical stirring. This treatment was repeated three times until the fibers were well individualized. The ensuing fibers were subsequently filtered and rinsed with distilled water and twice bleached with NaClO2 at 70 ◦ C and pH 4–5 to remove the residual lignin. 2.3. Chemical composition The determination of the basic chemical composition was conducted following TAPPI standard protocols. (TAPPI T 257 cm02). Samples were first submitted to Soxhlet extraction with ethanol/toluene and water. Then, the chemical contents were determined using the following methods, Ash (Tappi T 211 om-93) extractive (Tappi T264 om-07), Klason lignin (Tappi T222 om-88), and hemicelluloses (Tappi T249-cm-85). 2.4. TEMPO-mediated oxidation The TEMPO-mediated oxidation was carried out at pH 10, using NaClO as oxidizing agent and TEMPO as a catalyst. Briefly, cellulose fibers (2 g) were suspended in 200 mL water. TEMPO (30 mg) and NaBr (250 mg) were added to the suspension. Then 50 mL of a commercial NaClO solution (12◦ ) was added dropwise to the cellulose suspension at a temperature around 5 ◦ C which was kept constant throughout the oxidation reaction. The pH was maintained around 10 by the continuous addition of a 0.1 M aqueous solution of NaOH. The oxidation was stopped by adding ethanol (20 mL) and the pH was adjusted to 7 by adding 0.1 M HCl. 2.5. Carboxyl content The carboxyl content of the oxidized cellulose was determined using conductimetric titration, as described elsewhere (Besbes et al., 2011). 2.6. Fibrillation process using a conventional high speed blender (HSB): the fibers in a water suspension at a concentration of 2 wt% were disintegrated
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during 15–30 min in a “one-step operation” using a domestic high speed blender (MOULINEX 400 W) with a constant running speed of 11 000 rpm, in which the blades rotate in a recessed section at the bottom of a container of 1 L capacity. 2.7. Yield in nanofibrillated cellulose The yield is defined as the ratio of the amount of NFC in the supernatant to the initial amount of NFC before ultra-sonication and centrifugation. This was shown to be an efficient means to separate the unfibrillated materials from those partially fibrillated (Besbes et al., 2011). The protocol was carried out as follows: a dilute suspension with about 0.1 wt% solid content (Sc) was centrifuged at 4500 rpm for 20 min to separate the nanofibrillated material (in supernatant fraction) from the non-fibrillated or partially fibrillated ones which settle down. Then, the sediment fraction was dried to a constant weight at 90 ◦ C. The yield was calculated from Eq. (2)
Yeild% =
1−
weight of dried sediment weight of diluted sample × %Sc
× 100
3
Table 1 Chemical composition (wt %) of the corn stalk biomass and prior and after lignin extraction via NaClO2 /acetic acid pulping process. Constituents
Corn stalk biomass
Pulp composition
Cellulose Hemicellulose Lignin Extractible Ash
38 32 19 4 7
69 31 – – –
CrI Pulp yield Fibres length (m) Fibres width (m)
51 – – –
63 65 680 21
*As% of dry matter.
the maximum intensity (FWHM), in radians, and is the diffraction angle
(2) 2.11. Energy consumption
The results represented the average values of three replications. 2.8. AFM observation The morphology of the nanofibers was studied by atomic force microscopy (AFM) (FlexAFM from Nanosurf) using a tapping mode. The samples were prepared by depositing a drop of diluted NFC suspension (with a solid content about 0.01 wt%) on the surface of a silicon wafer and leaving it to dry for 1 h.
The instrument used for the energy consumption measurement was a power analyzer from SCOMEC DIRIS A20 which is an integrating tool that measures precisely the total amount of electrical energy, the voltage and the current intensity supplied to the electrical equipment in a given period of time.
Intensity
A ZEISS SUPRA40 FE-SEM was used to obtain images by capturing secondary electrons emitted from the surface of a NFC sample, prepared from a drop of the NFC suspension (with a solid content about 0.02 wt%) deposited on the surface of a silicon wafer and coated with a thin carbon layer, applied by ion sputtering with a thickness limited to 2–3 nm). To ensure a good image resolution without any damage for the samples during the analysis, the acceleration voltage was maintained at a relatively low range (2–5 kV).
SiO2
A
2.9. Field-emission scanning electron microscopy (FE-SEM)
Corn stalk
NFC
Corn pulp
2.10. Determination of the crystalline index 5
CrI% =
I
002 − Iam I002
× 100
(3)
K ˇ. cos
(4)
Where K is a dimensionless shape factor and usually taken to be 0.9, ´˚ is the X-ray wavelength,  is the line broadening at half (1.54 A)
15
20
25
30
35
40
B (c) (b) (a) 1710 cm-1
where I002 is the maximum intensity of the (002) diffraction peak, taken at 2 between 22◦ and 23◦ for cellulose I, and Iam is the intensity of the amorphous diffraction peak taken at 2 between 18◦ and 19◦ for cellulose I. Scherrer’s equation was used to calculate the crystallite size, T (nm), perpendicular to the (200) plane for cellulose I crystals: T=
10
2θ (°)
Transmittance
The crystallinity was evaluated from an X-ray diffraction (XRD) pattern obtained using a BRUKER AXS diffractometer (Madison, WI) with a Cu-K␣ radiation, generated at 30 kV and an incident current of 100 mA. The (2) angular region from 5◦ to 40◦ was scanned by steps of 0.05◦ using a step time of 10 s. The crystalline index (CrI) was calculated by equation 3 using the diffraction intensities of the crystalline structure and that of the amorphous fraction, according to the method of Segal (Segal et al., 1959):
1510 cm-1
1360 cm-1 1308 cm-1 -1
895 cm
Nea t corn stalk fibers-NaClO2 Fibe rs-Oxydized
1800
1700
1600
1500
1400 1300 1200 1100 -1 Wave number (cm )
1000
900
800
Fig. 1. (A), XRD patterns for neat corn stalk, corn pulp and NFC, and (B) FTIR spectra of: native cornstalk, NaClO2 deliginified, NaOH deliginified, and oxydized fibers from corn stalk.
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Neat
320
485
900
500-15%H
15% Hemicellulose
100
Fibrillation yield (%)
90 80 70 60 50
The NFC gel with 2% solid content was mixed with the latex in order to obtain nanocomposite films with weight fraction of cellulose ranging from 0% to 15%. After stirring for 1 h, the mixture was cast in a Teflon mould and stored at 40 ◦ C until water evaporation was completed. A transparent to translucent film, depending on the NFC content, was obtained with a thickness in the range of 300–400 m. 2.11.2. Tensile tests The non-linear mechanical behaviour of the films was analyzed using an Instron testing machine in tensile mode, with a load cell of 100 N working at a strain rate of 10 mm/min at 25 ◦ C. The specimens were obtained using a cutting device.
40 30 20 10
3. Results and discussion
0 10
20
25
30
Desintegration time (min) Fig. 2. Evolution of the fibrillation yield vs the disintegration time at different carboxyl content, using a high speed blender.
2.11.1. Nanocomposites processing Commercial acrylic latex obtained by the copolymerization of styrene (S) and butyl acrylate (BuA) was used as a matrix. The size of the polymer particles was around 150 nm and the solid content 50 wt%. The glass–rubber transition temperature (Tg) of this poly(Sco-BuA) copolymer was about −10 ◦ C.
3.1. Pulp characterization The chemical composition of corn stalk is given in Table 1. The main constituents are polysaccharides (70 wt%), with about 38% cellulose, and 32% of hemicelluloses, respectively. The content of lignin is lower than 20%, making its whole extraction from the fibers easier. This composition is in agreement with literature data (Reddy and Yang, 2005). The low cellulose content in agricultural crop, compared to woody plant, is explained by the low exigency in terms of weight that the plants have to support during their growth. Due to the high content of hemicelluloses in corn stalk, we have adopted the NaClO2/acetic acid extraction process that selectively removed lignin with less or no impact on hemicelluloses. Even
Fig. 3. Schematic illustration of the aspect of the cellulose nanofibrils according to their hemicelluloses content: (A) hemicelluloses content 30%, (B) Hemicellulose content (<20%).
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Fig. 4. AFM height images of NFC prepared via high speed blender with carboxyl content 300 and 900 mol/g, and via high pressure homogenization and the corresponding height profile analysis (the arrow marks the points the points used for the measurements of the height profile.
though hemicelluloses are amorphous polysaccharide, we infer that leaving them within the fibers is likely to be beneficial for the properties of NFC. The chemical composition and the physical properties of the cellulose fibers after the lignin extraction are given in Table 1. The yield in fibers after the pulping process was about 65%, which is quite high if compared to conventional soda pulping methods. The presence of high residual lignin in the fibers after the lignin extraction accounted for the high pulping yield. No change in the native crystalline structure was noted after the pulping process, with cellulose I polymorph as shown in the XRD (Fig. 1A). The XRD diffraction patterns of the corn stalk and the pulp after lignin extraction were given in the supplementary material. The FTIR spectra of the original corn stalk and the ensuing fibers after lignin extraction are given in Fig. 1. The main results revealed the vanishing of the peak at 1520 cm−1 assigned to C C stretching of the aromatic ring, confirmed the total removal of lignin after the delignification process. The band at 1730 and 1245 cm−1 of
the acetyl and uronic ester groups of the hemicelluloses remained unchanged after the extraction with NaClO2 /AA, in good agreement with the high content of hemicellulose in this sample. The characteristic bands vibration of the glucosic ring appeared in the 1100–1000 cm−1 region and the small sharp band at 895 cm−1 is characteristic of -glycosidic linkage between the sugar units. The small variations of the absorption spectra bands at 1368 and 1316 cm−1 for native and treated fibers indicated that the cellulose underwent a minor change during the process of delignification. After the TEMPO-mediated oxidation (spectrum c), an intensification in the C O band at 1720 cm−1 is observed as a result of the generation of the carboxyl groups. 3.2. Nanofibrillation behaviour of the fibers The fibrillation behaviour of the untreated and the oxidized fibers at three levels of carboxyl content was investigated by following the fibrillation yield and the optical transparency of the
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Fig. 5. FE-SEM images of NFC from oxidized corn pulp with carboxyl content 480 mol/g at different time of disintegration via high speed blender (A) after 10 min, (B) after 20 min and (C) after 30 min.
NFC suspension at regular interval of disintegration in a high speed blender. The first remark to highlight is the successful production of NFC using a conventional high-speed blender as a mechanical disintegration device instead of the high-pressure homogenizer. However, the fibrillation yield is strongly dependent on the carboxyl content of the fibers. After 10 min of disintegration, the fibrillation yield grew from 50% to about 85% as the carboxyl content increased from about 300 mol/g to 900 mol/g, and further disintegra-
tion during 20 min brought the fibrillation yield to about 75 and 100%, respectively. At a carboxyl content of about 900 mol g−1 , a quasi-transparent thick gel with full fibrillation was obtained after blending during 30 min. However, when neat pulp was used, the fibrillation yield did not exceed 30% even after high speed blending prolonged for 40 min. Presumably, the carboxyl content of the fibers (about 125 mol/g) is not enough to bring about enough electrostatic repulsion within the cell wall and overcome the interaction between neighbouring microfibrils through hydrogen bonding. Independently of their chemical structure, the content of the ionic groups generated during the chemical pre-treatment was shown to be determinant on both the energy reduction during the disintegration as well as the fibrillation efficiency. Although the mechanism of action of the ionic groups in the fibrillation process is still a matter of debate, it is assumed that the introduction of the ionic groups facilitated the break-down of the cell wall of the fibers through different effects: (a) electrostatic repulsion among the appended charged groups; (b) increasing level of osmotic pressure brought by the difference in the ionic concentration in counter-ions of the carboxylic groups between the inside and the outside of the fibber’s wall; (c) the ionic groups enhanced the hydration and swelling of the fibers, making them more flexible; and finally, (d) the chemical pre-treatment inevitably led to a chain scission within the amorphous zones that facilitated the release of the elementary microfibrils. Another key parameter that contributed to make the production of NFC via a high-speed blender possible is the chemical composition of the fiber after lignin removal, and more specifically, the residual content of hemicellulose left in the fibers. The high content of hemicelluloses is expected to further facilitate the fibrillation process by preventing fibrils aggregation during lignin removal as highlighted in our previous work13 . Hemicelluloses are believed to be intimately bounded to the cellulose fibrils through hydrogen and covalent bonding and to fill the space as an amorphous constituent between cellulose fibrils. It follows that the presence of hemicelluloses as a protective layer surrounding the microfibrils will prevent the fibrils from coming close together to aggregate and collapse under the effect of interfibrillar hydrogen bonding interaction. Further evidence of the key role of hemicelluloses content could be highlighted by comparing the fibrillation yield of pulp from corn stalk with hemicelluloses content of 15%, obtained through NaOH pulping process (see inset in Fig. 2). When pre-treated by TEMPOmediated oxidation at a carboxyl content of about 500 mol/g, the fibrillation yield did not exceed 38% after disintegration for more than 30 min. A schematic illustration depicting the nanoscale organization of the cellulose fibrils according to the hemicelluloses content is given in Fig. 3. It follows that when the high fraction of hemicelluloses is left in the fiber after the delignification process, the breakup of the cell wall during the mechanical disintegration is easier to implement, and the released fibrils are less aggregated with lateral size close to that corresponding to the elementary crystallites (about 3–4 nm). The strong correlation between hemicelluloses content and fibrils aggregation was shown also to affect other properties such as fibers recyclability, fibers strength, fibers porosity, fibers accessibility and fibers stiffness. By studying the effect of the hemicelluloses content on the ultrastructure of the fibers after lignin removal, a correlation between the ultrastructure of the fibres and the hemicellulose content was proposed (Duchesne et al., 2001). The pulp with high hemicellulose content had a porous surface structure with high relief, whereas fibres with low hemicellulose content, the fibril aggregates formed a much more compact surface structure. Wan et al. (2010) noted the same trend, where a decrease in the hemicelluloses content was shown to partially promote irreversible microfibril aggregation resulting in more compact fibrillar structures with increase in the fibril aggregate size (Wan et al.,
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Fig. 6. (A) Visible transmittance spectra of the NFC suspension for corn stalks produce via high speed blender (and HPH for sake of comparison) at different carboxyl content. (B) Visual aspect of the NFC gel at a solid content of 1 wt% according to their carboxyl content (value in yellow) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
2010). The phenomenon of fibril aggregation during drying of pulp is also on the origin of the harder disintegration of hornified fibers into NFC compared to never dried pulp as recently highlighted by (Kekäläinen et al., 2014). The morphology of the dispersed nanofibrils was evaluated by AFM imaging of extremely diluted NFC suspension deposited on a wafer substrate and dried at room temperature. Examples of images are shown in Fig. 4. All of the NFC samples with different carboxyl contents were composed of nanosized thin fibrils forming entangled network. The fibrils look highly flexible as attested by the formation of bended zone and their length ranging from 200 nm up to several m. The width of the fibril was estimated from the height profile and was found to be between 3 and 5 nm for the individual non-aggregated fibrils. The larger width distribution around 5 nm was found for the NFC produced from fibers with the lowest carboxyl content of 320 and over 500 mol g−1 , the width level off around 3 nm, which is close to the average thickness of cellulose crystallites (around 3.5 nm) calculated on the basis of the Scherrer’s equation. This means that the NFC produced through high-speed blender was composed by individual elementary cellulose fibrils. Further confirmation of the nanoscale dimension of the NFC from HSB was given by FE-SEM observation (Fig. 5) after 10, 20 and 30 min disintegration in a HSB. FE-SEM observation gives larger view than AFM and provides additional indication about the evolution of the fibrils morphologies in terms of width and length distribution during the disintegration process. During the beginning of the mechanical disintegration, the fibers were progressively broken down and fragments of partially fibrillated material with width in the range of 100–200 nm can be seen, in addition with nanosized fibrils 10–20 nm in width. With increasing time of disintegration, the content of nanosized fibrils increased at the expense of the larger fibrils, which is an indication that the cell walls of fibers were progressively broken down, releasing the elementary fibrils building up the fiber. We should note that, prior to the FESEM observation, the micro-sized fragment of fibers were removed by centrifugation. This treatment was necessary to make the observation of the nanosized fibrils possible. The transparency degree of the ensuing NFC was shown to be greatly dependent on both the disintegration time and the carboxyl content. As shown in Fig. 6, at a carboxyl content of 320 mol g−1 , the NFC gel is opaque and turned to a translucid gel over a carboxyl content of 500 mol g−1 . At a carboxyl content of 900 mol g−1 , a highly transparent thick gel is obtained After 30 min of disintegra-
tion. The evolution in the optical transparency is further highlighted by the transmittance measurement of a diluted suspension of the NFC. The transmittance level for NFC-320 did not exceed 70% at a wavelength of 800 nm and grow to over 77% and 85% for a carboxyl content of 480 and 900 mol g−1 , respectively. The evolution in the optical transparency is explained by the change in the fibrillation yield and the dispersability of the cellulose nanofibrils in water. The enhancement in the transmittance with increasing content of ionic groups is indicative of a decrease in the amount of non-fibrillated and partially fibrillated fractions responsible for a light-scattering phenomenon. In addition, the increase in the carboxyl content is expected to enhance the colloidal stability of the cellulose nanofibrils through electrostatic stabilization brought by ionized carboxylic groups on the surface of the nanofibrils. This can be seen from the increase in the absolute value of the zeta-potential of the NFC as the carboxyl content is going up (Table 2). The value of zeta-potential lower than −30 mV for NFC with a carboxyl content over 500 accounted for the good dispersion stability of the NFC suspension in water and good transparency, even at a solid content over 1 wt.% The energy consumed during the disintegration process for the NFC with different carboxyl content was also measured using a power analyser device and the data were collected in Table 2. The energy consumption after disintegration during 30 min was about 11 KWh/Kg which is much lower as those reported for high pressure homogenization (Chaker et al., 2014; Klemm et al., 2011). 3.3. Reinforcing potential of NFC The reinforcing potential of the NFC produced from corn stalk via high speed blender was investigated by preparing nanocomposite films using solvent-casting and analysing their mechanical properties by tension testing tensile. A ductile water-borne acrylic matrix with a Tg around −10 ◦ C was chosen as a matrix in order to reach the limit strength without premature breaking of the sample due to excessive rigidity. To compare the reinforcing potential of the different NFC, the relative modulus and tensile strength for the nanocomposite films vs the content of NFC was plotted (Fig. 7). This parameter represents the increment in the tensile modulus and strength for the nanocomposite with respect to the neat matrix, and gives better indication about the reinforcing potential of the nanofiller. For all
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Table 2 Physical and colloidal properties of NFC prepared via HSB at different carboxyl content. Fibers characteristic
Neat fibers
NFC-320 (blender)
NFC-480 (blender)
NFC-900 (blender)
Fibrillation yield Viscosity at 1 s−1 (Pa.s) Fibrils width (nm) Zeta-potential (mV) Energy consumption (KWh/Kg dry NFC)
40 4
80 13.3 3.5 −25 10.2a
87 24.8 2.7 −32 11a
100 31.4 2.5 −40 11.2a
a
−15 –
After 30 min disintegration in a HSB.
A
the fibrils is likely to reduce the density of bonded area between the fibrils within the entangled network generated by the cellulose nanofibrils (Boufi, 2014). Indeed, it is well reported that the unusual strong reinforcing effect brought by nanosized cellulose fibrils is ascribed to the set-up of an interconnected and entangled network held-up by strong hydrogen bonding (Favier et al., 1995). These entanglements play an important role in the force transferring from matrix to fibrils and from fibrils to fibrils.
4. Conclusion
B
The potential use of corn stalk as a raw material for the production of nanofibrillated cellulose with low energy demand was demonstrated in the present study. The NFC was produced via disintegration using a conventional high-speed blender. The yield in nanofibrillated material was shown to depend on the hemicellulose and the carboxyl content. A full conversion of fibers from corn stalk into NFC by disintegration during 30 min in a high-speed blender would be possible if the hemicelluloses content of the fibers was over 30% and the carboxyl content exceeded 500 mol g−1 . AFM observation confirmed the nanoscale of the cellulose fibrils produced through HSB with width around 3 nm, which presumably correspond to the ultimate elementary fibril. The NFC produced showed a high reinforcing potential when incorporated into a waterborne polymer matrix. At a content of 10 wt.%, the increment in the modulus and in the tensile strength was about 125 and 10 times that of the neat matrix. The possibility of producing NFC from corn stalk and, in general, from any agricultural crop residue using a high speed blender is expected to open the way towards less exigent technology and energetically cost-effective production of nanofibrillar cellulose from crop residues.
References Fig. 7. increment in (A) the tensile modulus, and (B) the tensile strength of anocomposite films vs NFC content prepared via high speed blender.
the NFC produced, the inclusion of the NFC into the ductile acrylic matrix led to a strong enhancement of both of the tensile modulus and tensile strength, which is in agreement with the well-known reinforcing capacity of NFC. For instance, at 10 wt% NFC, the increment in the tensile strength is 9.5, 10.2 and 13 fold higher than that of the neat matrix in presence of NFC-300, NFC-480, NFC-900 and NFC-500-H. When comparing the reinforcing potential of NFC with three carboxyl degree, it can be seen that NFC-320 brought stronger stiffness enhancement than NFC-900 albeit the lower fibrillation yields of the former. A possible reason for this apparent discrepancy is the difference in fibrils length, and accordingly, in the aspect ratio between the different NFCs. Indeed, as revealed by the AFM observation, although NFC-300 exhibited larger width than NFC-900, the fibrils look qualitatively longer than those from NFC-900, and encompasses lower fraction with short fibrils. The lower length of
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Further reading Boufi, S., Kaddami, H., Dufresne, A., 2014. Mechanical performance and trans-parency of nanocellulose reinforced polymer nanocomposites. Macromol. Mater. Eng. 299, 560-568.
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