High Solid Content Production of Nanofibrillar Cellulose via

Research Article pubs.acs.org/journal/ascecg

High Solid Content Production of Nanofibrillar Cellulose via Continuous Extrusion Rim Baati,† Albert Magnin,‡ and Sami Boufi*,† †

Faculty of Science−LMSE, University of Sfax, BP 802-3018 Sfax, Tunisia Laboratoire Rhéologie et Procédés, University Grenoble Alpes, LRP, CNRS, LRP, F-38000 Grenoble, France

‡

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S Supporting Information *

ABSTRACT: Cellulose nanofibrils (CNFs) with a high solid content of 10% were produced from oxidized holocellulose pulps by continuous extrusion using a twin-screw minicompounder in replacement of the high pressure homogenization or grinding, conventionally used to produce CNFs. A strong gel with high yield in nanofibrillated material exceeding 80% was obtained after 30 min of recirculation through the extruder at room temperature. The effect of the chemical composition of the pulp and the carboxyl content on the fibrillation yield and the properties of the ensuing CNFs were investigated. CNF material with a lateral dimension lower than 5 nm and a length within the micrometer scale was produced through this simple approach, and no evolution in crystallinity in terms of crystalline indexes was observed after prolonged extrusion up to 30 min. Dynamical rheology showed that the CNF suspension exhibited an elastic gel-like behavior over the whole concentration domains, ranging from 1 to 10% w/w. The described preparation method is easy to implement, without any clogging risk during the disintegration process and can be scaled up for a high capacity and cost-effective production of CNFs at high consistency. KEYWORDS: Nanofibrillar cellulose, Extrusion, Rheology, Modulus

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INTRODUCTION During the last two decades, the production of nanosized cellulose nanofibrils commonly known as CNFs or NFC (nanofibrillar cellulose) has emerged as one of the most promising nanomaterials derived from renewable natural resources and has aroused considerable academic and industrial interest.1 The nanoscale dimensions, biodegradable character, high potential bonding, high aspect ratio, light weight, and sustainability are the driving forces toward the use of CNF in a wide range of applications in innovative nanostructured materials, highlighted in a number of recent reviews.2,3 Furthermore, according to the current knowledge, nanocellulose is classified as a nontoxic material4 without any adverse effects on the environment.5 These benefits facilitate the use of nanocellulose and eliminate safety concerns, commonly encountered for mineral and carbon nanofillers. CNFs combine various properties of cellulose, such as hydrophilicity, broad chemical-modification capacity, and crystallinity, which contribute to expanding the potential use of this nanomaterial. CNFs are composed of nanosized thin flexible fibrils composed of both crystalline and amorphous domains with a width in the range of 5−50 nm and a length within the micrometer scale. They are produced from cellulose fibers after an intensive mechanical shearing action to delaminate the cell wall and break down the hydrogen-bonding network holding together the cellulose microfibrils. This can be done via high© 2017 American Chemical Society

pressure homogenization (HPH), microfluidization, ultrasonication,6 and high speed-disintegration.7 Another promising strategy to produce CNFs is grinding8,9 where the cellulose suspension with consistency between 1 and 2.5% is grinded through adjustable stones with an upper static disk and a lower stone rotating at 1400−1500 rpm. However, even though grinding is easier to run than high pressure homogenizer or microfluidizer, the energy consumption is still high (between 10 and 30 kWh/kg)10 and the consistency of the CNF remains quite low. Moreover, the ensuing CNFs seem to be less homogeneous in terms of fibril diameters and bundles of fibrils can still be observed.11 Although all of the above-reported processes were effective in breaking down cellulose to nanoscale and despite the commercial availability of the microfluidizer, grinder or the HPH at different scales, the widespread use of CNFs is still limited and below expectations. Three main obstacles are facing the large-scale production of CNFs. The first one is the high energy consumption involved in the production of CNFs when a high pressure homogenizer or a microfluidizer are used. Even though the chemical pretreatments contributed to notably reducing the energy input to less than 20 kWh/kg of dry CNF, this is still too high. The second obstacle is the low consistency of the CNFs produced via these disintegration modes. Received: November 4, 2016 Revised: January 18, 2017 Published: January 19, 2017 2350

DOI: 10.1021/acssuschemeng.6b02673 ACS Sustainable Chem. Eng. 2017, 5, 2350−2359

Research Article

ACS Sustainable Chemistry & Engineering

Delignification Processes. Soda Pulping Procedure. The wood chips were added to water (solid content of 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 to remove the residual lignin. The bleaching treatment was carried out at 70 °C for 1 h at pH 4.8. The solution is composed of equal parts of aqueous chlorite (1.7 wt % NaClO2 in water) and an acetate buffer. NaClO2/Acetic Acid Pulping Procedure (Holocellulose Fibers). The NaClO2/acetic acid (AA) pulping process was carried out as follows: 5 g of dry Soxhlet extracted biomass were added to water and mixed to form a suspension at a solid content 4 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 removal of any liquor. Fresh charges of sodium chlorite and acetic acid were added to the reaction every 1.5 h for up to 6 h. Chemical Composition. The determination of the basic chemical composition was conducted following TAPPI standard protocols. (TAPPI T 257 cm-02). 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). TEMPO-Mediated Oxidation. The TEMPO-mediated oxidation was carried out at pH 10 following the method reported in the literature.19 In brief, bleached cellulose fibers (2 g) were suspended in 100 mL water. TEMPO (30 mg) and NaBr (250 mg) were added to the suspension. Then 50 mL of a commercial NaClO solution (4 wt %) was added dropwise to the cellulose suspension at a temperature around 5 °C and 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. Carboxyl Content. The carboxyl content of the oxidized cellulose was determined using a conductometric titration, as described elsewhere.19 Transmittance Measurements. NFC suspensions (0.1% loading) were introduced into quartz cuvettes and the transmittance was measured between 400 and 800 nm using a Shimadzu UV−vis spectrophotometer. The spectrum of a cuvette filled with water was used as a blank. Twin Screw Extrusion of Pulp. Samples were processed in a laboratory scale corotating conical twin-screw mini extruder (TSE) DSM-Xplore 15 cm3 microextruder, comprised of a clamshell barrel with a conical twin-screw extruder, which can be operated in batch and continuous modes ensured by a recirculation channel and a control valve built into the barrel (Figure 1). Pulps with a solid content of 10% were fed into the barrel and continuously extruded at a constant screwspeed of 240 rpm via recirculation for 10−40 min. Three CNF samples with different carboxyl contents between 300 to 900 μmol/g were produced via TSE. We denote these samples as CNF-X, whereby X corresponds to the carboxyl content as determined by conductometric titration. Yield of Nanofibrillated Cellulose. Centrifugation of a diluted CNF suspension was shown to be an efficient means to separate the unfibrillated materials19 from those partially fibrillated. 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 nonfibrillated 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 1

Typically, CNFs are produced at a solid content between 0.5 up to 2.5 wt % at maximum, which led to a large dilution effect when CNFs are used as an additive over 5% and contributed to increase the transport cost. The third issue is the high cost of the high-pressure homogenizer or the microfluidizer and their frequent clogging during the disintegration process. In this sense, the production of CNFs at a high consistency would be an attractive alternative to reduce the energy consumption and avoid the excessive dilution effect when CNFs are used as an additive. To the best to our knowledge, only two publications have tackled the high consistency production of CNFs. The first one was reported by Ho et al.12 in 2015 who used a twinscrew extruder for the disintegration of fibers at 28% consistency. In this approach, the fibrillated cellulose was produced by multiple passes of never-dried refined NBKP (needle-leaf bleached kraft pulp) through a twin-screw extruder. The fibrillated material was obtained as a humid powder with varying solid contents from 32.75 up to 45% depending on the number of passes. Although, a successful fibrillation of the fibers was observed after several passes through the extruder, the ensuing fibrils were quite large with a size exceeding 100 nm. The diluted suspension of the fibrillated material was fairly opaque showing a significant sedimentation upon standing for several hours. Moreover, the excessive heat generated during the extrusion process led to a severe degradation of cellulose, namely after 10 passes. This result in a huge drop in the degree of polymerization, crystallinity degree, and strength of the nanopaper prepared from the extruded CNFs. The second work was recently reported by Liimatainen et al.13 focusing on the production of microfibrillated celluloses using a high consistency (7.5−15%) using a PFI mill. The fibers were first chemically oxidized by periodate chlorite to increase the carboxyl content to about 1.7 mmol/g and, then, disintegrated at a solid content between 7.5 to 15% using a laboratory-scale PFI mill. An opaque gel was obtained after 5000 milling revolutions, and partly transparent viscous gels were obtained after 30 000 milling evolutions. The lateral dimension of the ensuing CNF was relatively large ranging from 10 to 100 nm and the length was up to the micron-scale. However, microsized large cell wall fragments were observed in addition to the nanosized fraction material. Nanopapers with an ultimate tensile strength and a Young’s modulus of 61−115 MPa and 8− 11 GPa, respectively, were prepared from the as prepared CNFs gel. The objective of the present work was to explore the possibility of the continuous production of CNFs at a high solid content of 10% using a twin-screw extrusion for the disintegration process. For this purpose, TEMPO-mediated holocellulose oxidized fibers were used, and the effect of the carboxyl content on the extent of fibrillation and the rheology of the ensuing CNFs gel was investigated. The successful production of CNFs with a yield exceeding 80% via twin-screw extrusion was demonstrated when oxidized holocellulose fibers with a minimum carboxyl content of 300 μmol/g were used.

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EXPERIMENTAL SECTION

Materials. Wood chips (approximately 1 cm × 0.5 cm × 0.2 cm) were prepared from Eucalyptus Grandis wood (about 7 years old) kindly provided by a local fiber board mill. They were air-dried to a final humidity of 8−10% and then stored for later use. Before lignin extraction, wood ships were ground by means of a cutting mill (Cutting Mill SM 100 from Retsch) to pass a screen of 2 mm aperture. The pulping procedure was carried out as follows:

⎛ ⎞ weight of dried sediment yield % = ⎜1 − ⎟ × 100 (weight of diluted sample × % Sc) ⎝ ⎠ (1) 2351


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The chemical composition of fibers, their crystalline index, and carboxyl content were given in Table 1. Table 1. Chemical Composition (wt %) of the Wood and Bleached Fibers Produced via NaClO2/Acetic Acid and NaOH Pulping Process constituents cellulose hemicellulose lignin extractible ash CrI pulp yield

Figure 1. Twin-screw mini extruder used for the disintegration of fibers.

(2)

where Ww and Wd are the weight of wet and dry samples. Field-Emission Scanning Electron Microscopy (FE-SEM). A Weiss SEM was used to obtain images by capturing secondary electrons emitted from the surface of a CNF sample prepared from a drop of the CNF suspension (with a solid content of about 0.05 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 to 3 nm. To ensure a good image resolution without any damage to the samples during the analysis, the acceleration voltage was maintained at a relatively low range (2−5 kV). Determination of the Crystalline Index. The crystallinity was evaluated from an X-ray diffraction (XRD) pattern obtained using a BRUKER AXS diffractometer (Madison, WI) with a Cu Kα radiation, generated at 30 kV and an incident current of 100 mA. The (2θ) 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 eq 3 using the diffraction intensities of the crystalline structure and that of the amorphous fraction, according to the method of Segal et al.:16

⎡I − I ⎤ am CrI % = ⎢ 002 ⎥ × 100 ⎣ I002 ⎦

fiber composition (NaClO2 pulping)

fiber composition (NaOH pulping)

42 26.4 28 3 0.6

69 31 0

86 14 0

79 64

80 48

All of the pretreated holocellulose fibers produced through the NaClO2/AA pulping mode were successfully disintegrated by twin-extrusion giving a thick gel after continuous extrusion for 10 min. The gel consistency and transparency evolved according to the carboxyl content and changed with the increase of the extrusion time. The nonoxidized fibers did not turn to a gel even after more than 30 min of continuous extrusion. The change in the fibrillation yield and in the transparency degree at 700 nm according to the carboxyl content and the recirculation time through the extruder are given in Figure 2. A steady raise in the fibrillation yield is observed with increasing time of extrusion, passing from 53% after 10 min to 81% after 40 min of extrusion. The transparency degree followed also the same trend, starting at 61% for 10 min to 75% after 40 min of extrusion. From the aspect of the gel in terms of the viscosity, the fibrillation degree, and the transparency degree, we could conclude that after only 10 min of extrusion, more than 50% of the fibers were converted to a nanofibrillar cellulose. Further increase in the extrusion time led to a more efficient fibrillation of the material and increased the consistency, the yield in nanosized fibrils and the transparency of the gel. It is worth noting that no meaningful increase in the temperature of the gel was noted during the extrusion, even after 40 min. The fibers prepared through NaOH pulping either pretreated via TEMPO-mediated oxidation or untreated could not be transformed into a gel and no disintegration of fibers were observed by extrusion, even for fibers with a carboxyl content of 900 μmol/g. We have noted that fibers were accumulated at the bottom of the screw and seem to be compressed without being able to get through the recirculating channel. This results in the clogging of the entry of the channel with no possible circulation through the twin screws of the compounder. Two possible reasons might explain the unsuccessful fibrillation of NaOH pulped fibers. The first one is the lack of enough energy transferred to fibers to break down the strong hydrogen bonding holding the elementary fibrils together within the cell wall. The second one would be the insufficient hydration of NaOH pulped fibers making them hard to drag through the conical mixing screws. Even though the oxidation treatment is known to facilitate the breakdown of the cell wall, this treatment was shown to be ineffective in promoting the breakdown of the cell wall via TSE for NaOH pulped fibers. The enhancement of the hydration and the swelling of the fibers by the oxidation treatment could be seen from the evolution of WRV, which reflects the capability of fibers to

The results represented the average values of three replications. Water Retention Capacity. The water retention value (WRV) was evaluated by centrifugation according to the method of Okubayashi et al.15 In brief, about 0.3−0.5 g of fiber was soaked into distilled water for 24 h at ambient temperature. After centrifugation at 4000 g for 10 min, the weight of the fiber was measured and the water retention value (WRV) was calculated from eq 2.

WRV(g/g) = (Ww − Wd)/Wd

wood composition

(3)

where I002 is the maximum intensity of the (002) diffraction peak, taken at 2θ between 22° and 23° for cellulose I, and Iam is the intensity of the amorphous diffraction peak taken at 2θ between 18° and 19° for cellulose I. AFM Observation. The morphology of the CNF was studied by atomic force microscopy (AFM; Flex AFM 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.

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RESULTS AND DISCUSSION Fibrillation Process with TSE. The extrusion of the pulp was carried out on four samples differing by their carboxyl content and the mode of lignin extraction. The first lignin extraction mode was a conventional NaOH pulping, and the second was conducted with NaClO2/acetic acid, originally known as the Wise method,17 which is recognized to remove lignin while preserving the highest amount of hemicellulose.18 2352


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Figure 2. Evolution of the (A) fibrillation yield, (B) transmittance of the CNF-300, 500, and 900 gel at 700 nm, and (C) aspect of the CNF gel at different extrusion times and carboxyl content for cellulose fibers from NaClO2/AA pulping oxidized at different levels.

and 23.1° corresponding to (101), (101)̅ , (002) planes, respectively, indicating the preservation of the native crystalline structure of cellulose I after the oxidation treatment and the extrusion process. The crystallinity did not undergo any evolution after a 30 min extrusion. As shown in Figure S1, the CrI remained roughly unchanged around 0.8 for all samples even for CNF with a carboxyl content of 900 μmol/g, which has the highest fibrillation yield exceeding 80%. This means that, contrary to the high pressure homogenization which has a negative effect on the crystallinity,19 the disintegration with TSE did not impart the crystallinity of the CNF. This effect is likely to be the result of the huge difference in the shear rate applied during the high pressure homogenization and TSE. In the former, shear rate as high as 106 s−1 is often attained20 as the suspension is forced to pass across the narrow channel or nozzle under high pressure (between 50 to 100 MPa). On the other hand, the shearing rate in a twin-screw extruder is much lower and should not exceed 300−3000 s−1. The mixing action in twin screw extruders is based on an elongational flow, where intensive shear stress and elongation stress are transferred to fibers as the material is forced to drag through the small gap between screws (typically of the order of a few microns). Referring the literature data,21,22 the average shear rate might be modeled as a function of screw speed and parameters by eq 4:

retain water inside the cell wall. As shown in Table 2, the oxidation treatment strongly increased the WRV, especially Table 2. WRV of Untreated and TEMPO-Oxidized Fibers at Different pH Values untreated fibers (NaClO2 pulping) untreated fibers (NaOH pulping) oxidized fibers (NaClO2 pulping) oxidized fibers (NaOH pulping)

pH = 4

pH = 7

pH = 9

310 247 603 487

341 305 724 544

402 338 784 605

over pH 7, where a high fraction of the carboxylic groups were fully ionized. Moreover, it could be seen that NaClO2/AA pulped fibers exhibited (neat and oxidized) higher WRV than NaOH pulped fibers, and their oxidation treatment contributed to increase the WRV much more than NaOH pulped fibers. The presence of higher amounts of residual hemicelluloses in NaClO2/AA pulped fibers (see Table 1) might explain their higher WRV compared to NaOH pulped fibers. The presence of high amounts of residual hemicelluloses in these fibers reduces the magnitude of the hydrogen bonds interaction among the microfibrils network and favors the diffusion of water to the space between the fibrils. In NaOH pulped fibers, we infer that the removal of a high fraction of hemicelluloses led to the development of irreversible hydrogen bonds among the microfibrils, making them more difficult to fibrillate via TSE. The beneficial role of hemicelluloses in facilating the disintegration of fibers into CNF was also confirmed in our previous work.14 Wide-angle X-ray diffraction patterns of the different oxidized fibers and the ensuing CNF after 30 min of extrusion are shown in Figure S1 (Supporting Information). For all samples, WAXD revealed the presence of the main characteristic peaks of cellulose I (JCPDS. No. 03-0226) at 2θ values of 15.2°, 16.7°,

γ ̇ = KN α

(4)

where γ̇ is the average shear rate (s−1), K (sα−1/revα) is a constant depending on the geometry of the extruder screws, α is a constant to account for material conveying in the extruder, and N (rev/s) is the screw speed. Based on the literature data, most of the γ̇ and K values fell in the range of 20−40 and 0.8− 1.2, respectively. Accordingly, for a screw speed of 240 rpm, the shear rate should be around 300 s−1, which is much lower than 2353


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Figure 3. (A) AFM height images of CNFs with different carboxyl contents, produced after 30 min of extrusion. (B) Cross-section profile analysis (the arrow marks the points used for the measurements of the height). (C) Height distribution for the whole image using Gyddion software.

CNFs as the oxidation degree of the fibers is increasing. The profile of CNF-300 showed a main population of fibrils centered on 2 nm and a second population with a large distribution extending up to 6 nm height. As the fibers oxidation degree increased to 500 μmol/g, the fibrils population distribution was less polydisperse and fibrils with height extending to 6 nm vanished, indicating a decrease in the width of CNFs. For CNF-900, the height distribution became mainly centered on 2 nm, and cellulose fibrils look more uniform in width. However, fibrils length was adversely affected by the oxidation level. This became mainly visible for CNF-900, where short fibrils with a length less than 500 nm became visible. This phenomenon is well documented in the literature and was explained by the cleavage of the cellulose backbone during the oxidation process as a result of the β-elimination and through a radical scission.24 SEM observation was also used to investigate the change in the fiber morphology during the TSE (Figure 4). Prior to the TSE, fibers were flat with a diameter about 12 μm. No visible damage of the fibers was visible after the oxidation treatment, even at a carboxyl content of 900 μmol/g. After 10 min of extrusion (Figure 4B), the cell wall of fibers was delaminated and the structure of the fiber was opened revealing macrofibrils with about a 100 nm width, partially connected to each others. We can also see that a fraction of macrofibrils was broken down into fine elementary fibrils forming an interconnected network

that of the HPH for which the shear rate is in the order of 105− 106 s−1. The morphology of the CNFs prepared via extrusion with different carboxyl contents was analyzed by AFM observations of samples deposited on wafer substrate and dried at room temperature. For all samples, observation revealed individual fibrils with a quite uniform width distribution and a length being dependent on the oxidation extent (Figure 3). The diameter of CNF was estimated from the height profile to avoid tip artifacts effect. Indeed, as the lateral section of CNF is smaller than the tip radius (around 10 nm), tip convolution effect might occur and the objects appear broader than they are in reality. For this reason, the height profile gives better indication about the topography and the width of the object. Indication about the fibril size was provided from the height profile of the fibrils and was found to be in the range of 2−5 nm for the individual nonaggregated fibrils, which is indicative of the efficient fibrillation and individualization of CNFs through the TSE process. These values should not be considered as the exact width of the CNF given the risk of distortion effect when tiny hydrated or soft objects with size lower than 5−3 nm were considered. The vertical dimensions might appear smaller than expected and the height of the object is underestimated due to different effects such as tip compression, dehydration and the tip geometry itself.23 Further insight in the height distribution profile (Figure 3C) revealed a narrowing in the thickness of 2354


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Figure 4. FE-SEM observation of (A) oxidized fibers with carboxyl content 300 and 500 μmol/g, extruded fibers after 15 min extrusion with (B) carboxyl content 300, (C) carboxyl content 500, and of (D) CNF after 30 min extrusion (CNF-500).

G″ to a higher magnitude, indicating the formation of a stiffer gel. The gel-like structure of CNFs suspension is a typical behavior of CNFs suspension in water as its solid content exceeded a critical threshold (between 0.2 and 0.5% depending on the pretreatment and the fibrillation degree). This was explained by the formation of entangled network acting as physical cross-linking sites held by fibril−fibril hydrogen bonding. The flexible property of cellulose fibrils resulting from the presence of alternating amorphous and crystalline domains is at the origin of the formation of the entangled network in water. Though the literature data of the rheological behavior of CNFs at a solid content exceeding 3% are scarce, the values of G′ at 3 and 7% are within the range of those reported by Päak̈ kö et al.25 For example, for CNFs prepared via the high pressure homogenization of an enzymatic pretreated fibers, G′ values at 3 and 5.9% CNF were around 5 × 104 and 1.5 × 105 MPa at a frequency of 1 rad/s, respectively. Presently, at 3 and 5%, G′ reached about 3.9 × 104 and 8 × 104 MPa for CNF-500 and 8 × 104 and 18 × 104 Pa for CNF-900, after 30 min extrusion at 240 rpm. The plateau levels of the modulus, G′ and G″, at 3.3 rad/s were plotted versus the dry content, for the three CNF suspension with different carboxyl content (Figure 6). For all the CNF suspensions, a power-law fitting of G′ with the dry content was observed (G′ = KCα), which is in agreement with the literature data.26,27 The exponent value α was found around

(see inset of Figure 4). As the residence time in the extruder is increasing, the fraction of macroscopic fibers is decreasing at the expense of nanosized fibrils. Further confirmation of the nanoscale dimension of CNF was given by FE-SEM observation (Figure 4C) with a larger area view of CNFs, which clearly revealed the efficient fibrillation of the fibers into CNFs with size lower than 10 nm. Rheological Behavior of CNFs from Twin-Screw Extrusion. To investigate the influence of fibrils consistency on the rheological properties of the CNFs, the mother suspension of CNFs gel, with a consistency of 10%, was diluted at different solid content, thoroughly mixed and subjected to 10 s sonication, to fully homogenize the mixture. Then, the viscoelastic properties were investigated within linear and nonlinear domains at a solid content from 10 to 1%. The storage modulus (G′) and loss modulus (G″) as a function of the angular frequency (ω) of CNFs suspensions at various solid contents are shown in Figure 5a. Over the whole range of frequencies investigated, both G′ and G″ were nearly frequency-independent, i.e., G′ ≈ ω0; with G′ being much higher than G″ (G′ about ten times that of G″ at the same concentration), which is typical of solidlike behavior of CNFs suspensions. This behavior remained unchanged over the entire range of solid content from 10 to 1%. This means that over the whole solid content from 1 to 10%, the CNFs suspension produced via TSE was structured in a 3D interconnected network responsible for gel-like structure. The increase in the dry content of the CNFs gel led to a steady shift of the G′ and 2355


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Figure 5. (A) Storage modulus G′ (solid symbols). (B) Loss modulus G″ (open symbols) versus angular frequency ω for CNF-500. (C) Visual aspect of CNF gel at different solid contents (the original CNF gel was produced via TSE at 10% starting from NaClO2/AA oxidized fibers).

theory proposed by de Gennes28 and also lower than that reported by the literature for CNF prepared via high pressure homogenization18 (α = 2.5−3). The parameter K was dependent on the CNF origin and increased with the carboxyl content (K was equal to 416, 1233, and 2435 for CNF-300, CNF-500, and CNF-900, respectively). A linear dependence between K and the carboxyl content was found as shown in Figure 6 (see inset). According to Tatsumi et al.,29 the K value gave an indication about the network strength and should be proportional to the square of the aspect ratio of fibrils. Presently, the increase in the K value with the carboxyl content of CNF might be explained by the amplification of interaction between fibrils−fibrils through hydrogen bonding as the carboxyl content is getting higher. The cohesion of the entangled CNF network became stronger and the stiffness of the gel further increased. Nonlinear Viscoelastic Behavior. To further investigate the microstructure of the CNF gels at different dry content, the transition from linear to nonlinear viscoelastic behavior under strain was studied. The evolution of G′ as a function of the strain at a constant angular frequency for CNF-500 at different dry contents is shown in Figure 7. To better highlight the

Figure 6. Storage modulus G′ as a function of concentration at 25 °C and at an angular frequency of 3.3 rad/s.

1.4−1.6, regardless of the CNF origin. This value was lower than the theoretical value (α = 2.25) reported by the scaling 2356


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Figure 7. Strain amplitude dependence of the normalized elastic modulus G′/G′0.

Figure 8. Power consumption (instantaneous and total), torque, and temperature parameters during the TSE process.

deviation from the linear region, the values of the elastic modulus G′ are normalized by those in the linear region at low strain amplitude, where G0′ is constant. The storage modulus is constant up to a critical strain γc in the range of 2−3%, depending of the CNF origin, above which the storage modulus decreases with strain and the viscoelastic behavior becomes nonlinear. The drop in the storage modulus over γc led to the irreversible deformation of the gel, which is the result of the breakdown in the network generated by the entangled CNFs. Compared to a covalently cross-linked gel with typical critical strain in the range of 10−20%,30 the lower γc for CNFs is presumably due to the difference in the crosslinking mode; i.e., chemical cross-linking for the former and physical cross-linking for the later, which is a short-range interaction very sensitive to the deformation. γc seems to be little sensitive to dry contents up to 3%, and a decrease in γc was observed at 1% which is the consequence of the decrease in the density of CNF entanglement as water is added. At the same dry content, γc was also dependent on the carboxyl content, attaining about 1 and 3% for CNF-300 and CNF-900, respectively. Although the difference was not significant, the lower value for CNF-300 might be the consequence of the decrease in the strength of the CNF network arousing from the decrease in the carboxyl content. Energy Consumption. To get an indication about the amount of the energy consumed and the evolution of the temperature of the CNF gel during the extrusion process, the evolution of the power consumption (instantaneous and total), the torque and the temperature during extrusion process was collected from TSE and reported in Figure 8. It can be seen that the power consumption grew rapidly during the first 6 min to stabilize at constant level after 10 min. More than 75% of the whole energy was consumed during the first 3 min, suggesting that most of destructuration process occurred during the first stage of the extrusion process and then lower energy is consumed to further disintegrate the fibers into CNFs. Based on the data provided by the TSE, the energy consumption during the fibrillation with TSE might be estimated to about 4.1 kWh/kg. This value is lower than that required for high pressure homogenizer (between 30 and 50 kWh/kg)31 or even for the stone grinder known to consume lower energy (between 5 and 30 kWh/kg).32,33 In addition, it is wise to remind that TSE is the only route that is able to produce CNF with such a high solid content amounting 10%. However, it is worth noting that the successful production of CNF via TSE

was possible only when oxidized holocellulose fibers, and the conventional NaOH pulped fibers can not be disintegrated via the present approach. The main challenge for the future work is to adapt this disintegration mode to NaOH pulped fibers. This can be done via an appropriate physical pretreatment mode. This work is in progress and promises interesting results. Another advantage of the TSE mode is the low evolution of the temperature during the extrusion process. Starting from a temperature of about 22 °C, the temperature attained about 27 and 32 °C after 10 and 30 min extrusion, respectively. The low evolution of the temperature may be explained by the high shear thinning behavior of the CNFs gel (see Figure S2) and by the presence of water favoring heat dissipation by conduction. Referring to Figure S2, if we consider the shear rate being around 300 s−1 during the extrusion, then the viscosity of the mixture within the gap between screw flank and barrel should be lower than 2 Pa·s. This low viscosity avoided excessive heat generation by viscous friction and prevented any risk of thermal degradation of CNFs during the disintegration process.

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CONCLUSION A facile approach to produce CNFs at a high consistency (10 wt %) via continuous twin-screw extrusion is reported, using oxidized holocellulose fibers. The approach was shown to successfully breakdown the cell wall of fibers into nanofibrillar cellulose, when TEMPO-mediated oxidized holocellulose fibers were used. A carboxyl content of 300 μmol/g was shown to be enough to turn fibers into a thick gel after a 30 min extrusion. Increasing the carboxyl content up to 900 μmol/g greatly enhanced the yield in nanofibrillated material and meaningfully reduced the time to turn the fibers suspension into a gel during the extrusion. Based on AFM observations, the lateral size of the ensuing fibrils was found to be around 3 to 7 nm, depending on the carboxyl content and their length was within the micron scale. Increasing the carboxyl content led to the narrowing of the lateral sized distribution. However, a cutting effect of fibrils was observed as the carboxyl content attained 900 μmol/g. No evolution in the CrI of CNFs was noted after prolonged extrusion for all samples, irrespective of their carboxyl degree. This means that, contrary to the high pressure homogenization, which has a negative effect on the crystallinity, the disintegration with TSE did not impart the crystallinity of the CNF. The rheological behavior of the CNF gel investigated by dynamic measurement indicated that both G′ and G″ were 2357


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nearly frequency-independent, with G′ being much higher than G″ over the entire range of solid content from 10 to 1%. This confirmed the strong gel-like property of the CNF suspension fabricated by TSE. The present approach offers several advantages including easy implementation, high solid content compared to the HPH or grinder, cost-effectiveness, and possibility to scale-up production.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02673. X-ray diffractograms of neat fibers and of CNF with different carboxyl contents and viscosity vs shear rate for CNF gel at different solid contents (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 216 47274400. Fax: 216 74274437. E-mail address: sami. boufi@fss.rnu.tn. ORCID

Sami Boufi: 0000-0002-3153-0288 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the CNRS (France) and DGRST (Tunisia) for their financial support. The Laboratoire Rhéologie et Procédés is part of the Labex Tec 21 (Investissement d’Avenir, grant agreement no. ANR-11-LAB-0033) and of the Polynat Carnot Institut (Investissement d’Avenir, grant agreement no. ANR-11-CAR-030-01).

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REFERENCES

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High Solid Content Production of Nanofibrillar Cellulose via

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