Sulfonated cellulose nanofibrils obtained from wood pulp through regioselective oxidative bisulfite pre-treatment

The consecutive pre-treatment of cellulose with periodate and bisulfite was used as a new potential method to promote nanofibrillation of hardwood pulp and to obtain nanofibrils with sulfonated functionality. Nanofibrils having typical widths of 10–60 nm were obtained from sulfonated celluloses having low anionic charge densities (0.18–0.51 mmol/g) by direct high-pressure homogenization without the use of any mechanical pre-treatments. The aqueous nanofibrils existed as highly viscous and transparent gels and possessed cellulose I crystalline structures with crystallinity indexes of approximately 40 %. A transparent film was obtained from sulfonated nanofibrils having tensile strength of 164 ± 4 MPa and Young’s modulus of 13.5 ± 0.4 MPa. Oxidative sulfonation was shown to be a potential green method to promote nanofibrillation of cellulose, as it avoids the production of halogenated wastes, because the periodate used can be efficiently regenerated and recycled as shown in the preliminary experiments.     

Highly ductile fibres and sheets by core-shell structuring of the cellulose nanofibrils

A greater ductility of cellulosic materials is important if they are to be used in increasingly advanced applications. This study explores the potential for using chemical core-shell structuring on the nanofibril level to alter the mechanical properties of cellulose fibres and sheets made thereof. The structuring was achieved by a selective oxidation of the cellulose C2–C3 bonds with sodium periodate, followed by a reduction of the aldehydes formed with sodium borohydride, i.e. locally transforming cellulose to dialcohol cellulose. The resulting fibres were morphologically characterised and the sheets made of these modified fibres were mechanically tested. These analyses showed a minor decrease in the degree of polymerisation, a significantly reduced cellulose crystal width and a greater ductility. At 27 % conversion of the available C2–C3 bonds, sheets could be strained 11 %, having a stress at break of about 90 MPa, and consequently a remarkable tensile energy absorption at rupture of about 9 kJ/kg, i.e. 3–4 times higher than a strong conventional paper. Zero-span tensile measurements indicated that the treatment increased the ductility not only of sheets but also of individual fibres. This suggests that the amorphous and molecularly more mobile dialcohol cellulose is located as a shell surrounding the crystalline core of the cellulose fibrils, and that, at deformations beyond the yield point, this facilitates plastic deformation both within and between individual fibres.     

Cellulose nanofibrils derived from pulp as a high performance separator for lithium-ion batteries

Cellulose nanofibrils (CNFs) from hardwood bleached kraft pulp (HwBKP) are produced via enzymatic, chemical, and mechanical treatment. A nanoporous structured CNF-based separator is produced, and the electrochemical performance, morphology, and thermal stability analyses are performed in comparison to the commercial polyethylene separator. The results obtained show that the electrolyte-philic CNF separator has capacity retention of 88.6% over 200 cycles and very good ionic conductivity and wettability results due to its high hydrophilic nature. At 140°C, the CNF separator was resilient to heat and remained intact. The CNF separator reflects high thermal resistance and good electrolyte uptake properties that are among the mandatory requirements of a separator hence, a promising contender for use in lithium-ion batteries.     

Homo- and heteroflocculation of papermaking fines and fillers

The effects of cationic polyethylenimine (PEI) on the colloidal stability of anionic fines (microcrystalline cellulose or thermomechanical fines), fillers (clay) and their mixtures in deionized and tap water were investigated, using a photometric dispersion analyzer. Measurements confirmed that PEI flocculates all used materials by charge neutralization. As expected, higher additions of PEI lead to electrostatic stabilization of microcellulose and clay suspensions, but it was not possible to stabilize the suspension of fines using high additions of PEI. This is ascribed to the mechanical entanglements of fibrillar fines. In tap water, much more PEI is needed to reach optimum flocculation conditions than in deionized water. Heteroflocculation between PEI-coated clay and fines takes place with a rate which, for high fines concentration and a constant clay concentration, is independent of fines concentration. A theoretical model for the heteroflocculation of fines with PEI-coated clay has been developed, which explains the observed trends. In essence, clay particles can act as bridging agents for fines flocculation.     

Dynamic rheology behavior of electron beam-irradiated cellulose pulp/NMMO solution

The rheological behavior of irradiated cellulose pulp solution by electron beam was investigated. Storage modulus G′, loss modulus G″, the dependence of complex viscosity η* and frequency ω of cellulose solutions were measured by DSR-200 Rheometer (Rheometrics co., USA). The molecular weight of irradiated cellulose was measured via the intrinsic viscosity measurement using an Ubbelohde capillary viscometer. The crystalline structure was studied by FTIR Spectroscopy. The results congruously showed that the molecular weight of pulp cellulose decrease and the molecular weight distribution of cellulose become narrow with increase in the irradiation dose. Moreover, the crystalline structure of the cellulose was destroyed, the force of the snarl between the cellulose molecules weakens and the accessibility of pulp spinning is improved. The study supplies some useful data for spinnability of irradiated cellulose and technical data to the filature industry.     

Swelling of mechanical pulp fines

The objective of the study was to determine the swelling of different types of mechanical pulp fines. The physical and chemical characteristics of the fines were also examined. It was found that the degree of swelling correlates with the proportion of fibrillar material, that is fibrillar content of the fines. The fines with the lowest fibrillar content had a swelling comparable to mechanical pulp fibres (0.69g/g), whereas the fines with a high fibrillar content had a swelling comparable to neverdried kraft pulp fibres (1.41g/g). Hemicellulose content and charge could not explain the differences in swelling of different types of the mechanical pulp fines. While the lignin content appears to be an important factor in the degree of swelling of mechanical pulp fines, the results suggest that structural differences between the particles are also important. The bulk elastic modulus was determined by measuring the change in swelling for a known change in osmotic pressure. All the mechanical pulp fines had a high bulk elastic modulus compared to kraft fines. However, fibrillar fines had a lower bulk elastic modulus than flakelike fines. Mechanical pulp fines, both fibrillar and flakelike varieties, did not hornify appreciably. The swelling of both the fines and the fibre fractions increased slightly with the specific energy consumption in the refining.     

Characterization of the supermolecular structure of cellulose in wood pulp fibres

The hierarchic organization of cellulose fibrils (microfibrils) was investigated in holocellulose, sulphite pulp and kraft pulp using TEM, XRD, ED and FTIR. There were remarkable differences in both the fibril structure and fibril aggregation between the samples. TEM observations revealed more intimately associated fibrils in the kraft pulp compared to the sulphite pulp and the holocellulose, results in agreement with previous CP/MAS ¹³C-NMR data [Hult E.-L. et al. (2002) Holzforschung 56: 231–234]. Furthermore, the cellulose crystallinity was higher in the kraft pulp sample. With respect to the cellulose I and I allomorphs, these samples were controversial when different analytical techniques were applied. Due to the small fibril size and the low degree of order of cellulose in these samples, the concept of crystalline triclinic and monoclinic components as determined by diffraction analysis may not be adequate. Instead the fibril can be regarded to have different degrees of lateral order (including paracrystalline ordering) that can be reoriented to I type conformation and packing upon pulping.     

Effect of microfibrillated cellulose and fines on the drainage of kraft pulp suspension and paper strength

Different types of microfibrillated cellulose (MFC) and fines suspensions were produced, characterized, and then added to a papermaking pulp suspension. High and medium molar mass cationic polyelectrolytes were used as fixatives. The drainage behavior of the pulp suspensions with additives were evaluated against the strength properties of hand sheets made thereof. The effects of salt concentration, pH, fixative type, dosage and type of fibrillar material on drainage were examined. All the MFC and fines samples produced had clearly different properties due to their dissimilar production methods, and they also introduced specific responses on the measured drainage and paper strength. Generally, the addition of MFC decreased the drainage rate of pulp suspension and increased the strength of paper. However, it was shown that by optimum selection of materials and process conditions an enhancement of the strength properties could be achieved without simultaneously deteriorating the drainage.     

Dewatering of cellulose nanofibrils using ultrasound

Although cellulose nanomaterials have promising properties and performance in a wide application space, one hinderance to their wide scale industrial application has been associated with their economics of dewatering and drying and the ability to redisperse them back into suspension without introducing agglomerates or lose of yield. The present work investigates the dewatering of aqueous suspensions of cellulose nanofibrils (CNFs) using ultrasound as a potentially low-cost, non-thermal, and scalable alternative to traditional heat-based drying methods such as spray drying. Specifically, we use vibrating mesh transducers to develop a direct-contact mode ultrasonic dewatering platform to remove water from CNF suspensions in a continuous manner. We demonstrate that the degree of dewatering is modulated by the number of transducers, their spatial configuration, and the flow rate of the CNF suspension. Water removal of up to 72 wt.% is achieved, corresponding to a final CNF concentration of 11 wt.% in 30 min using a two-transducer configuration. To evaluate the redispersibility of the dewatered CNF material, we use a microscopic analysis to quantify the morphology of the redispersed CNF suspension. By developing a custom software pipeline to automate image analysis, we compare the histograms of the dimensions of the redispersed dewatered fibrils with the original CNF samples and observe no significant difference, suggesting that no agglomeration is induced due to ultrasonic dewatering. We also perform SEM analysis to evaluate the nanoscale morphology of these fibrils showing a width range of 20 nm–4 um. We estimate that this ultrasound dewatering technique is also energy-efficient, consuming up to 36% less energy than the enthalpy of evaporation per kilogram of water. Together with the inexpensive cost of transducers (<?$1), the potential for scaling up in parallel flow configurations, and excellent redispersion of the dewatered CNFs, our work offers a proof-of-concept of a sustainable CNF dewatering system, that addresses the shortcomings of existing techniques.

     

Interactions between inorganic nanoparticles and cellulose nanofibrils

Nanofibrillated cellulose (NFC) is increasingly utilized in materials and biomedical applications consequently increasing interest in the modification of its surface properties. Besides modification using polyelectrolytes and polysaccharides, NFC can be combined with solid particles enabling formation of fibril network loaded with particles. Use of particles enabling easy functionalization could be beneficial for the development of hybrid structures, and lead to preparation of nanocomposites and functional materials. In order to explore interactions related to preparation of such structures, the interactions between nanosized precipitated calcium carbonate (nanoPCC) and nanoclay particles and NFC were examined by observing adsorption of the particles on NFC substrate using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) imaging. By a treatment with carboxymethylated cellulose (CMC), the anionicity of the NFC substrate could be increased, providing an additional tool to affect the interplay between NFC and the inorganic particles. For slightly cationic nanoPCC particles an increase in the anionicity of the NFC by the CMC treatment increased the affinity, while the opposite was true for anionic nanoclay. Additionally, for interactions between nanoclay and NFC, dispersion stability was an important factor. QCM-D was successfully used to examine the adsorption characteristics of nanoparticles although the technique is commonly used to study the adsorption of thin polymer layers. Distinct adsorption characteristics were observed depending on the nanoparticle used; nanoclay particles deposited as a thin layer, whereas nanoPCC particles formed clusters.     

Interaction between microfibrillar cellulose fines and fibers: influence on pulp qualities and paper sheet properties

Due to the high potential of cellulose nanoparticles in composite materials and for both fundamental and technological considerations, we investigated the interaction between microfibrillar cellulose and fibers. The contribution to the paper properties of fines added to a pulp suspension was determined. The impact of various proportions of fines added to a softwood kraft pulp on the paper strength and how they affected porosity and density was evaluated. The respective effects of dried fines (dead fines), originating from paper or board degradation, and the newly formed secondary fines (fresh fines) generated during refining were examined. The nature of the bonding between the fines and the fibers versus physical retention was characterized in the pulp suspension. For the first time the respective parts in the interaction of hydrogen bonds and mechanical associations were demonstrated and quantified. The amount of H-bonded fresh fines exceeded that of dead fines by more than 30 %. The results revealed that, for both types, the amount of H-bonded fines reached a threshold, independently of the proportion of fines added to the fibers. Addition of fines significantly affected the porosity of papers, fresh fines decreasing porosity more readily than dead fines. All the results are convergent to indicate that fresh fines penetrate more evenly and more deeply into the fiber network and induce better bonding that produces a closure of the fiber mat structure. They also demonstrate that incorporating an optimal proportion of fresh cellulose fines in fiber networks can bring significant improvement to the final composite material.     

Nanocomposites of natural rubber and polyaniline-modified cellulose nanofibrils

Cellulose nanofibrils (CNF) were isolated from cotton microfibrils (CM) by acid hydrolysis and coated with polyaniline (PANI) by in situ polymerization of aniline onto CNF in the presence of hydrochloride acid and ammonium peroxydisulfate to produce CNF/PANI. Nanocomposites of natural rubber (NR) reinforced with CNF and CNF/PANI were obtained by casting/evaporation method. TG analyses showed that coating CNF with PANI resulted in a material with better thermal stability since PANI acted as a protective barrier against cellulose degradation. Nanocomposites and natural rubber showed the same thermal profiles to 200 °C, partly due to the relatively lower amount of CNF/PANI added as compared to conventional composites. On the other hand, mechanical properties of natural rubber were significantly improved with nanofibrils incorporation, i.e., Young’s modulus and tensile strength were higher for NR/CNF than NR/CNF/PANI nanocomposites. The electrical conductivity of natural rubber increased five orders of magnitude for NR with the addition of 10 mass% CNF/PANI. A partial PANI dedoping might be responsible for the low electrical conductivity of the nanocomposites.     

Intrinsic viscosity of aqueous suspensions of cellulose nanofibrils

Cellulose nanofibrils (CNF) from wood fibers are of increasing interest to industry because they are from renewable sources and are biodegradable. Owing to their high aspect ratio, they produce viscous suspensions and stiff gels that are strengthened by interfibrillar hydrogen bonds. In this study, the viscosity of aqueous CNF suspensions, at dilute concentrations ( \(nL^{3}<1\) ), was measured at various pH values by addition of HCl, and at various ionic strengths by addition of NaCl and \(\hbox {CaCl}_{2}\) . The results show that the primary electroviscous effect significantly increases the intrinsic viscosity. The intrinsic viscosity under conditions where the surface charge of nanofibrils is fully screened is in good agreement with the predictions of classical theory for dispersions of rodlike particles at low shear rates. Increasing the ionic strength up to \(\kappa d\approx 1\) decreases the intrinsic viscosity; at \(\kappa d>1\) , the intrinsic viscosity increases because of fibril aggregation and increase of the effective volume fraction.     

Production of cellulose nanofibrils via an eco-friendly approach

Cellulose - Swelling of cellulose fibers facilitates the fibrillation process by mechanical treatment. In this study, potassium hydroxide (KOH)-urea (KUr) solution was investigated to swell fibers...     

Length-controlled cellulose nanofibrils produced using enzyme pretreatment and grinding

The length of cellulose nanofibrils (CNFs) is a significant parameter for various applications. The goal of this research was to employ a fabrication method to produce length-controlled CNFs; the chosen technique was enzy-grinding (enzyme pretreatment followed by mechanical grinding). Here, we presented the results of the optimization of the diameter and length, the characterization of the properties of CNFs and nanofilms prepared using these fibrils. The cellulose morphology, crystallinity index (CrI), chemical structure, and thermal stability were investigated as functions of the enzyme loading and hydrolysis time. The results showed that enzy-grinding could effectively reduce the diameter and length of cellulose fibrils. The average diameter was about 8.6 ± 3.6 nm, and the length could be controlled over the range from 0.76 ± 0.38 μm to ≥ 4 μm (i.e. aspect ratios from 43 to ≥ 328). After the grinding process, the CNFs maintained high thermal stability and no change in the chemical structure compared to the original pulp. The transmittance and mechanical properties of the CNF films were strongly dependent on the fibril length. The fabrication of length-controlled CNFs using the enzy-grinding process is meaningful and significant research which could be relevant to the optimization of such materials for various applications.     

Chemically modified cellulose micro- and nanofibrils as paper-strength additives

Chemically modified cellulose micro- and nanofibrils were successfully used as paper strength additives. Three different kinds of cellulose nanofibrils (CNFs) were studied: carboxymethylated CNFs, periodate-oxidised carboxymethylated CNFs and dopamine-grafted carboxymethylated CNFs, all prepared from bleached chemical fibres of dissolving grade, and one microfibrillated cellulose from unbleached kraft fibres. In addition to mechanical characterization of the final paper sheets the fibril retention, sheet density and sheet morphology were also studied as a function of addition of the four different cellulose fibrils. In general, the cellulose fibrils, when used as additives, significantly increased the tensile strength, Young’s modulus and strain-at-break of the paper sheets. The effects of the different fibrils on these properties were compared and evaluated and used to analyse the underlying mechanisms behind the strengthening effect. The strength-enhancing effect was most pronounced for the periodate-oxidised CNFs when they were added together with polyvinyl amine (PVAm) or poly(dimethyldiallylammonium chloride) (pDADMAC). The addition of periodate-oxidised CNFs, with pDADMAC as retention aid, resulted in a 37% increase in tensile strength at a 2 wt% addition and an 89% increase at a 15 wt% addition (from 67 to 92 and 125 kNm/kg, respectively) compared to a reference with only pDADMAC. Wet-strong sheets with a wet tensile index of 30 kNm/kg were also obtained when periodate-oxidised CNFs and PVAm were combined. This significant increase in wet strength is suggested to be the result of a formation of cross-links between the aldehyde groups, introduced by the periodate oxidation, and hydroxyl groups on the lignocellulosic fibres and the primary amines of PVAm. Even though less significant, there was also an increase in wet tensile strength when pDADMAC was used together with periodate-oxidised fibrils which shows that the aldehyde groups are able to increase the wet strength without the presence of the primary amines of the PVAm. As an alternative method to strengthen the fibre network, carboxymethylated CNFs grafted with dopamine, by an ethyl dimethylaminopropyl carbodiimide coupling, were used as a strength additive. When used as an additive, these CNFs showed a strong propensity to form films on and around the fibres and significantly increased the mechanical properties of the sheets. Their addition resulted in an increase in the Young´s modulus by 41%, from 5.1 to 7.2 GPa, and an increase in the tensile strength index of 98% (from 53 to 105 kNm/kg) with 5 wt% retained dopamine-grafted CNFs.     

DFT study of metal cation-induced hydrogelation of cellulose nanofibrils

We report a density functional theory study of cation-induced bonding between carboxylated cellulose nanofibrils (CNFs). We describe a methodology of using cleaved cellulose crystal unit cells to develop simple surface and molecular models of charged CNFs. We compare bond lengths, binding energies, and displaced solvation volumes for interfibril models intercalated with alkali, alkaline earth, main group, or transition metal cations, surrounded by an implicit solvent. We characterize the type of bonding interactions that occur between metal cations, Mⁿ⁺ and carboxylated CNF surfaces by calculating the electronic density of states and Mayer bond orders. We find that Mⁿ⁺–O interactions for alkaline earth metal systems are predominantly electrostatic whereas transition metal cations form stronger, more covalent bonds with enhanced valence orbital overlap. Our results show that multivalent—as opposed to monovalent—ions can create CNF networks by effectively crosslinking multiple fibrils through surface carboxylate anions. Our computational results agree with empirical models of metal–carboxylate binding, while also providing a deeper understanding of the bonding mechanisms for different cations. Our findings help to explain trends in recent CNF hydrogelation experiments, and we also predict the existence of two new hydrogels—CNF-Mg²⁺ and CNF-Zr⁴⁺.     

Degradation of TEMPO-oxidized cellulose fibers and nanofibrils by crude cellulase

The biodegradation behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose fibers (TOCs) suspended in water and TEMPO-oxidized cellulose nanofibrils (TOCNs) dispersed in water by a commercial crude cellulase was studied. Products crude cellulase-treated for 0–7 days were separated into water/ethanol-insoluble and -soluble fractions. Weight recovery ratios and viscosity-average degrees of polymerization of the water/ethanol-insoluble fractions clearly decreased with crude cellulase-treatment time, showing that both TOCs and TOCNs have biodegradability. Water/ethanol-soluble fractions were subjected to size-exclusion chromatography (SEC) with photodiode array (PDA) detection to obtain SEC elution patterns detected by reflective index and UV spectra of each SEC pattern elution slice. SEC–PDA and ¹³C-NMR analyses showed that glucuronosyl unit-containing molecules present on microfibril surfaces in TOCs and TOCNs were primarily cleaved by hydrolyzing enzymes present as contaminants in the crude cellulase to form glucuronic acid as one of the major water-soluble degradation compounds. After the glucuronosyl units in TOCs and TOCNs were degraded and removed from microfibril surfaces by the hydrolyzing enzymes, cellulose chains newly exposed on the microfibril surfaces were rapidly hydrolyzed by cellulases predominantly present in the crude cellulase to form cellobiose. Both TOCs and TOCNs having sodium carboxyl groups are thus biodegradable, but TOCN having free carboxyl groups had clearly low biodegradability by the crude cellulase. Thus, biodegradation behavior may be controllable by controlling the structure of carboxyl group counter ions in TOCs and TOCNs.