Cellulose Nanofibrils Suspension Research Articles

Effects of mechanical fibrillation time by disk grinding on the properties of cellulose nanofibrils

Cellulose nanofibrils (CNF) were successfully produced from a bleach kraft eucalyptus pulp by a supermasscolloider. Effects of grinding time on structure and properties of CNF and the corresponding CNF films were investigated. Grinding time was important to increase the optical transparency of CNF suspensions. The degree of polymerization (DP) and crystallinity index (CrI) of CNF decreased linearly with the increase in CNF suspension transparency. This suggests optical transparency of a CNF suspension can be used to characterize the degree of fibrillation. Specific tensile strength and Young’s modulus of the CNF films made of CNF suspension with only 0.5 h grinding were increased approximately 30% and 200%, respectively, compared with conventional handsheets prepared by valley beating to 300 Canadian Standard Freeness (CSF). Energy input was only 1.38 kWh/kg for 0.5 h grinding. Grinding beyond 0.5 h produced negligible improvement in specific tensile and specific modulus. Opacity of CNF films decreased rapidly during the first 1.5 h of fibrillation and then plateaued.

Current Progress in Rheology of Cellulose Nanofibril Suspensions

Cellulose nanofibrils (CNFs) are produced and commonly used in the form of aqueous suspensions or gels. A number of studies have focused lately on rheological properties of CNF suspensions, which gives insight into properties of such materials and can reflect their behavior during handling. This Review summarizes the recent progress in rheological studies on CNF aqueous suspensions using rotational rheometry. Here, we discuss linear viscoelastic properties, i.e., frequency-dependent storage and loss moduli; shear flow behavior, i.e., apparent viscosity and shear stress as a function of shear rate; local flow characteristics, etc. In this Review, we point out that the rheological behavior of at least two types of CNF suspensions should be distinguished: (i) ones produced using mechanical fibrillation with or without enzymatic pretreatment (no surface chemical modification), which possess highly flocculated structure, and (ii) ones produced involving chemical modification pretreatments, e.g., carboxylation, carboxymethylation, quaternization, or sulfonation, which possess better colloidal stability and do not evidently flocculate.

In-situ modification of cellulose nanofibrils by organosilanes during spray drying

Cellulose nanofibrils (CNFs) have shown their potential as the sustainable choice for reinforcements in thermoplastic composites. Studies on surface properties are important because they directly relate to the compatibility between CNFs and polymer matrices. In this study, CNF suspensions were treated with two different organosilane functionalities before being spray-dried (SD). Three concentration levels of organosilane solution were applied based on the mass of the CNFs in suspension: 1wt. %, 3wt. % and 5wt. %. Morphological and surface properties of the CNFs were measured to understand changes induced by the organosilane modification. Scanning electron microscopy analysis shows some aggregation of larger particles as well as many small twisted rectangular particles. Particle size distribution analysis (PSD) indicates certain differences among treated and untreated CNFs. Inverse gas chromatography (IGC) reveals that the organosilane treatment reduces the dispersion component of surface energy of dried CNFs and changed the acid-base characteristic of CNFs by the attachment of different functional end-groups.

Structure of cellulose -silica hybrid aerogel at sub-micron scale, studied by synchrotron X-ray tomographic microscopy

Synchrotron X-ray tomographic microscopy was used to study the three-dimensional structure and compatibility of a hybrid aerogel composite material synthesized by impregnation of a porous cellulosic scaffold with silica sol and subsequent gelation of the sol. The scaffolds were synthesized from freeze-drying of a water-based nanofibrillated cellulose suspension, in its native state and in the presence of methyltrimethoxysilane as a modifying agent. To retrieve the maximum information from the acquired tomography datasets, images were reconstructed using the Gridrec algorithm without phase processing and then starting from pre-processed data, the phase information was obtained using the Paganin's phase-retrieval algorithm. Differences in the three-dimensional morphology of the scaffolds and silica aerogel were analyzed and the sub-micron topological characteristics of the cellulosic network which influences the physical properties of the hybrid, e.g. porosity, degree of anisotropy and thickness of the agglomerated fibers were studied and discussed in the context of physical properties. This study illustrates the sensitivity and efficiency of synchrotron X-ray tomography for qualitative and quantitative studies of complex hybrid nano- and microscale structures to predict their macroscopic behavior. In combination with suitable structural numerical models, these results can be used to gain additional insights into thermal transport and mechanical properties.

Micro-mechanics of electrostatically stabilized suspensions of cellulose nanofibrils under steady state shear flow.

In this study, we characterized and modeled the rheology of TEMPO-oxidized cellulose nanofibril (NFC) aqueous suspensions with electrostatically stabilized and unflocculated nanofibrous structures. These colloidal suspensions of slender and wavy nanofibers exhibited a yield stress and a shear thinning behavior at low and high shear rates, respectively. Both the shear yield stress and the consistency of these suspensions were power-law functions of the NFC volume fraction. We developed an original multiscale model for the prediction of the rheology of these suspensions. At the nanoscale, the suspensions were described as concentrated systems where NFCs interacted with the Newtonian suspending fluid through Brownian motion and long range fluid-NFC hydrodynamic interactions, as well as with each other through short range hydrodynamic and repulsive colloidal interaction forces. These forces were estimated using both the experimental results and 3D networks of NFCs that were numerically generated to mimic the nanostructures of NFC suspensions under shear flow. They were in good agreement with theoretical and measured forces for model colloidal systems. The model showed the primary role played by short range hydrodynamic and colloidal interactions on the rheology of NFC suspensions. At low shear rates, the origin of the yield stress of NFC suspensions was attributed to the combined contribution of repulsive colloidal interactions and the topology of the entangled NFC networks in the suspensions. At high shear rates, both concurrent colloidal and short (in some cases long) range hydrodynamic interactions could be at the origin of the shear thinning behavior of NFC suspensions.

Rheology of cellulose nanofibrils in the presence of surfactants.

Cellulose nanofibrils (CNFs) present unique opportunities for rheology modification in complex fluids. Here we systematically consider the effect of ionic and non-ionic surfactants on the rheology of dilute CNF suspensions. Neat suspensions are transparent yield-stress fluids which display strong shear thinning and power-law dependence of modulus on concentration, G' ∼ c(2.1). Surfactant addition below a critical mass concentration cc produces an increase in the gel modulus with retention of optical clarity. Larger than critical concentrations induce significant fibril aggregation leading to the loss of suspension stability and optical clarity, and to aggregate sedimentation. The critical concentration was the lowest for a cationic surfactant (DTAB), cc ≈ 0.08%, while suspension stability was retained for non-ionic surfactants (Pluronic F68, TX100) at concentrations up to 8%. The anionic surfactant SDS led to a loss of stability at cc ≈ 1.6% whereas suspension stability was not compromised by anionic SLES up to 8%. Dynamic light scattering data are consistent with a scenario in which gel formation is driven by micelle-nanofibril bridging mediated by associative interactions of ethoxylated surfactant headgroups with the cellulose fibrils. This may explain the strong difference between the properties of SDS and SLES-modified suspensions. These results have implications for the use of CNFs as a rheology modifier in surfactant-containing systems.

Effect of xylan in hardwood pulp on the reaction rate of TEMPO-mediated oxidation and the rheology of the final nanofibrillated cellulose gel

alkali-washed nanofibrillated cellulose (NFC) samples, obtained from hardwood kraft pulp, with different amounts of retained xylan were prepared to study the influence of xylan on the water-retention properties of NFC suspensions. In this study, NFC was produced using an oxoammonium-catalyzed oxidation reaction that converts the cellulosic substrate to a more highly oxidized material via the action of the nitroxide radical species 2,2,6,6-tetramethylpiperidine-1-oxyl. Reduction of the xylan content in NFC was achieved by cold alkali extraction of kraft pulp. The pulps were then oxidized to a set charge under constant chemical conditions, and the reaction time was determined. The xylan content of the feed pulp was found to have a large negative influence on the oxidation rate of the pulp, as the oxidation time shortened when xylan was removed, from 220 min (for 25.2 % xylan content) to 28 min (for 7.3 % xylan content). Following fibrillation by homogenization, the swelling of the NFC was determined by a two-point solute exclusion method. The distribution of hemicellulose over the fibril surface was observed by atomic force microscopy. Xylan was found to be distributed unevenly over the surface, and its presence increased the water immobilized within flocs of NFC, i.e., so-called network swelling. The swelling of the NFC had a large impact on its rheology and dewatering. Comparison of the morphological and swelling properties of the suspensions with their rheological and dynamic dewatering behavior showed that reducing the xylan content in NFC results in a weaker gel structure of the nanocellulose suspension. The results indicate that most of the water is held by the swollen structure by means of xylan particles trapped within the hemicellulose layer covering the fibril surface. Samples with high xylan content had high shear modulus and viscosity and were difficult to dewater.

Flocculation behavior of cellulose nanofibrils under different salt conditions and its impact on network strength and dewatering ability

Flocculation of cellulose nanofibrils (CNF) was induced by charge neutralization with different types and concentrations of salt, and its effects on the network strength and dewatering ability of the CNF suspension were investigated. Aggregation of the CNF suspension was evaluated by measurement of light transmission using Turbiscan equipment. This procedure enabled us to characterize the aggregation and sedimentation behavior of nanofibrils under different salinity conditions. Aggregation and sedimentation of CNF occurred on salt addition because the degree of compression of the electrical double layer was changed by adsorption of cations onto the CNF, depending on the type and concentration of salt. The changes in network strength of the CNF suspension due to flocculation were evaluated using a conventional oscillatory rheometer. The viscosity, storage modulus, and yield stress of the CNF suspension increased with an increase in the ionic strength of the suspension. Microrheology measurements using the dynamic light scattering technique were also adopted to characterize the viscoelastic properties of the CNF suspension, revealing that CNF suspension containing a high concentration of salt showed more solid-like behavior. In addition, the aggregation degree of the CNF affected the dewatering ability of the CNF suspension. Bivalent cations were more effective for increasing the network strength and dewatering for a small amount of added salt compared with monovalent cations.

Preparation of cross-linked cellulose nanofibril aerogel with water absorbency and shape recovery

Cellulose nanofibril (CNF) aerogels are promising materials for various applications because of their highly porous and ultralight characteristics. The fiber network of CNF aerogels held together by hydrogen bonding and mechanical entanglement of adjacent fibers is easily destroyed when it is exposed to water. In this study, cross-linked CNF aerogels were prepared using maleic acid and sodium hypophosphite as cross-linking agents. In the first step of treatment, CNF dispersed in water was reacted with maleic acid to form an ester linkage. Sodium hypophosphite was then added to the maleic acid-functionalized CNF suspension and the suspension was rapidly frozen using liquid nitrogen. CNF aerogel was obtained after freeze drying. The cross-linking of the cellulose was formed by the reaction between the carbon–carbon double bond of maleic acid-functionalized CNF and hypophosphite. The cross-linked CNF aerogel exhibited good network stability in water and springiness after compression.

Effect of the oxidation treatment on the production of cellulose nanofiber suspensions from Posidonia oceanica: The rheological aspect

Different grades of cellulose nanofibrils (CNF) were prepared from Posidonia oceanica balls and leaves (POB and POL). Pretreatment using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation was performed to facilitate the fibrillation during ultrafine friction grinding process. The ensuing CNF batches were compared in terms of morphology and degree of fibrillation. The rheological properties of the produced CNF suspensions were also analyzed for varying doses of sodium hypochlorite used during the TEMPO- mediated oxidation procedure. The stronger fibrous network structures were formed when increasing the oxidant concentration, which was confirmed by the increase of the storage moduli value. P. oceanica balls were found to undergo stronger fibrillation and, consequently, to form stronger networks, compared to P. oceanica leaves, when using equivalent concentration of the oxidizing agent.

Controlled Silylation of Nanofibrillated Cellulose in Water: Reinforcement of a Model Polydimethylsiloxane Network.

A comparative approach for the surface silylation of nanofibrillated cellulose (NFC) in water is proposed through an environmentally friendly sol-gel route based on alkoxysilanes. NFC suspensions were freeze-dried under controlled conditions in the presence of methyltrimethoxysilane used as a model alkoxysilane. Two different protocols that involve different pH values (0.4 and 4) and post-treatment procedures were investigated and compared. Protocol 1 led to a network of nanofibrils in which polysiloxane particles were dispersed, and protocol 2 produced a scaffold of cellulosic fibrils coated by a polysiloxane layer bonded firmly to the cellulosic substrate. Different from protocol 1, protocol 2 imparted the cellulosic material with hydrophobic properties and improved its thermal stability. Moreover, if 1 wt % of fibrils treated by protocol 2 were incorporated into a model polydimethylsiloxane network, substantial improvements of the static and dynamic mechanical properties of the composite were noted.

Concentration effect of TEMPO-oxidized nanofibrillated cellulose aqueous suspensions on the flow instabilities and small-angle X-ray scattering structural characterization

The rheological behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized nanofibrillated cellulose (NFC) suspensions at different concentrations were examined using Couette with smooth and serrated surfaces and vane-in-cup geometries. A slippage against the walls of rheometer rotor or/and stator (wall-slip) as well as flow localization within a specific volume of the sample (shear banding) took place. These phenomena were visualized using a technique based on introduction of titanium dioxide pigment into the specific volume of NFC suspension and monitoring its deformation. The flow instabilities appeared more pronounced with an increase of the NFC concentration. It was shown that the use of serrated geometry is necessary to compete with the wall-slip but is not sufficient to prevent it completely. Serrated Couette was found to be the most appropriate geometry (among the tested) to measure the rheological properties of NFC suspensions. Small-angle X-ray scattering was used to provide a supplementary information about the structural organization of NFC suspensions. The statistical average diameter of 4.8 nm was determined from the scattering data for the NFC swollen in water.

On the morphology of cellulose nanofibrils obtained by TEMPO-mediated oxidation and mechanical treatment

The morphological properties of cellulose nanofibrils obtained from eucalyptus pulp fibres were assessed. Two samples were produced with the same chemical treatment (NaClO/NaBr/TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation), but distinct mechanical treatment intensities during homogenization. It was shown that the nanofibrils production yield increases with the mechanical energy. The effect of mechanical treatment on the yield was confirmed by laser profilometry of air-dried nanocellulose films. However, no significant differences were detected regarding the nanofibrils width as measured by atomic force microscopy (AFM) of air-dried films. On the other hand, differences in size were found either by laser diffraction spectroscopy or by dynamic light scattering (DLS) of the cellulose nanofibrils suspensions as a consequence of the differences in the length distribution of both samples. The nanofibrils length of the more nanofibrillated sample was calculated based on the width measured by AFM and the hydrodynamic diameter obtained by DLS. A length value of ca. 600nm was estimated. The DLS hydrodynamic diameter, as an equivalent spherical diameter, was used to estimate the nanofibrils length assuming a cylinder with the same volume and with the diameter (width) assessed by AFM. A simple method is thus proposed to evaluate the cellulose nanofibrils length combining microscopy and light scattering methods.

Repeated homogenization, a route for decreasing the energy consumption in the manufacturing process of carboxymethylated nanofibrillated cellulose?

Energy-efficient manufacturing is a requirement for the large-scale production of nanofibrillated cellulose systems. Today, most strides involve the development of new physicochemical pre-treatment processes or invention of new or further-development of existing mechanical delamination processes; these approaches are often costly. In this communication, it was shown that significant reduction in energy consumption (~30–50 %) of the nanofibrillated cellulose (NFC) manufacturing process is possible by the wise employment of existing machinery and processes—without deterioration of the attractive properties of the NFC product. The suggested approach has further the additional advantage of increasing the yield of nanofibrillation, robustness of the manufacturing process, and increasing the out-put of the process. These notions can lead to significant energy savings in the NFC production. The novel manufacturing protocol involved repeated homogenization of the pulp suspension at low (~400 bar) applied homogenization pressures (as compared to those currently applied, ~1700 bar). It was hypothesized that this manufacturing protocol leads to a more homogeneous and effective shearing of the fibres, than is achievable by one time homogenization of the pulp slurry at high applied pressures. The investigations further showed that there exists an apparent yield of fibrillation (of about 40 %), above which the rheological properties of the NFC systems and the mechanical properties of the resulting NFC films remain unaffected. This observation [which is in accordance to published observations by e.g. Fall (2013)] can be employed to reduce the specific energy consumption in the NFC manufacturing process. Finally, the investigations also showed that the route that is used for dilution of concentrated NFC systems can significantly alter the properties of resulting NFC-films. It was postulated that a protocol based on the severe shearing of the NFC suspension (prior to NFC-film formation), e.g. by high pressure homogenization, can lead to a more homogeneous and better fibrillated system, which in turn might lead to the overestimation of the actual properties of the studied NFC system.

Self-assembled optically transparent cellulose nanofibril films: effect of nanofibril morphology and drying procedure

Cellulose nanofibril (CNF) films currently provide great opportunity in many applications with advantages of excellent mechanical strength, high light transmittance, and good barrier properties. However, processes for preparing CNFs are typically tedious and vary, along with their properties. Here, five preparation methods using various combinations of filtration, freeze-drying, and casting are applied to produce CNF films, and their major properties are compared. Three different types of CNFs having a range of fiber diameter and aspect ratio were examined using each of these five preparation methods. Because of limited hydrogen bonds and nanofibril arrangement, the freeze-dried CNF films displayed reduced mechanical strength and light transmittance compared to the other methods, although freeze-drying was relatively fast. Some effects of film production methods on measured crystallinity were also observed with freeze-dried samples having lower crystallinity than films similarly produced by filtration and drying. Free-standing CNF films produced by casting at room temperature required long times and mold growth was sometimes observed, but cast films made from 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized CNFs had the highest light transmittance of any samples. Filtration of CNF suspensions followed by air- or oven-drying produced films with minimal defects, high mechanical strength, and good light transmittance with relatively little effort. Therefore, this filtration procedure is recommended for producing CNF films.

Structural characteristics of nanofibrillated cellulose mats: Effect of preparation conditions

Nanofibrillated cellulose (NFC) mats, which have porous or dense structures, were prepared in this study. The effects of the number of grinding passes, suspension solids content, and drying conditions on the structural changes of the NFC mats were investigated. Water removal in the NFC suspensions for forming the mats was carried out using pressurized dewatering equipment. The field-emission scanning electron microscope observation showed that the nanofibrils were preserved by solvent exchange drying, whereas the partial aggregation of nanofibrils occurred by freeze drying. Properties such as shrinkage, density, porosity, and specific surface areas of the NFC mats changed depending on the preparation conditions of the NFC mats. The NFC mats, which have low density and high porosity, could be prepared by solvent exchange drying and freeze drying. Porosity of the NFC mats varied from 76 % to 96 %. The specific surface area of the NFC mat increased to 175 m2/g with an increase in the number of grinding passes.

Heterogeneous flow kinematics of cellulose nanofibril suspensions under shear.

The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The in situ optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.

Evaluation of Dewatering of Cellulose Nanofibrils Suspension and Effect of Cationic Polyelectrolyte Addition on Dewatering

ABSTRACT Since cellulose nanofibrils (CNF) has large specific surface area and high water holding capacity, it is very difficult task to remove water from the CNF suspension. However, dewatering of CNF suspension is a prerequisite of following processes such as mat forming and drying for the application of CNF. In this study, we evaluated the drainage of cellulose fibers suspension under vacuum and pressure con-ditions depending on the number of grinding passes. Also, the effect of the addition of cationic poly-electrolyte on dewatering ability of CNF suspension was investigated. Regardless of dewatering con-dition, the total drained water amount as well as the drainage rate were decreased with an increase in the number of grinding passes. Pressure dewatering equipment enables us to prepare wet CNF mat with rela-tively higher grammage. The cationic polyelectrolytes improved the dewatering ability of CNF suspen-sion by controlling the zeta potential of CNF. The fast drainage was obtained when CNF suspension had around neutral zeta potential.

Synthesis of cellulose nanofibril bound silver nanoprism for surface enhanced Raman scattering.

Silver nanoprisms (AgNPs) were robustly synthesized using TEMPO-oxidized cellulose nanofibrils (CNFs) as a dual capping and shape-regulating agent for the first time. Reducing AgNO3 with NaBH4 in CNF suspensions produced smaller but more uniform Ag nanospheres (AgNSs) with increasing Ag(+)/CNF ratios. CNF bound AgNSs were facilely transformed to AgNPs by etching with H2O2, supporting the capping and shape-regulating capability of CNFs. AgNPs could also be synthesized directly in a one-shot reduction reaction with NaBH4 in the presence of both CNFs and H2O2. The AgNPs transformed from CNF bound AgNSs are similar to those synthesized directly, but more stable against H2O2. Successful synthesis of AgNPs with 80-320 nm truncated edges was confirmed by light blue solution color, sharp out-of-plane quadruple resonance peak at 334 nm and prominent in-plane dipole resonance peaks at 762-900 nm. The [111] lattice plane of AgNP was clearly evident by its predominant XRD peak at 38°, confirming the unique shape-regulating ability of the nearly fully surface carboxylated CNFs. The CNF surface bound AgNPs were easily fabricated into freestanding CNF/AgNPs films that showed excellent surface enhanced Raman scattering of Rhodamine 6G with analytical enhancement factor of 5 × 10(3) in contrast to none from the CNF/AgNSs film.

The vane method and kinetic modeling: shear rheology of nanofibrillated cellulose suspensions

We conduct rheological characterization of nanofibrillated cellulose (NFC) suspensions, a highly non-Newtonian complex fluid, at several concentrations. Special care is taken to cope with the prevalent problems of time scale issues, wall depletion and confinement effects. We do this by combining the wide-gap vane geometry, extremely long measurement times, and modeling. We take into account the wide-gap related stress heterogeneity by extending upon mainstream methods and apply a gap correction. Furthermore, we rationalize the experimental data through a simple viscous structural model. With these tools we find that, owing to the small size of the particles subjected to Brownian motion, the NFC suspensions exhibit a critical shear rate, where the flow curve experiences a turning point. This makes the steady state of these suspensions at low shear rates non-unique. To optimize various mixing and pumping applications, such history dependent tendency of NFC suspensions to shear band needs to be taken into account.