All-cellulose composites from unbleached hardwood kraft pulp reinforced with nanofibrillated cellulose

In the present work, nanofibrillated cellulose (NFC) from bleached eucalyptus pulp was prepared, characterized and used as reinforcement in an unbleached eucalyptus fiber matrix. First, the NFC was fabricated through TEMPO-mediated oxidation and characterized for the degree of polymerization, water retention value, cationic demand and carboxyl content. Intrinsic mechanical properties were also calculated by applying the rule of mixtures, which determines the coupling (f _c) and efficiency factor (η _e) of cellulose nanofibrils within the matrix. The results showed that the average intrinsic tensile strength and Young’s modulus of NFC are estimated to be 6,919 MPa and 161 GPa, respectively. After characterization, the NFC was used as reinforcement in the preparation of biocomposites in the form of paper handsheets, which were physically and mechanically analyzed. The presence of NFC induced an increase in the density of biocomposites and significant enhancement of the mechanical properties as well as an important reduction in porosity. Finally, f _c and η _e were determined from the mean intrinsic properties.     

Evaluation of new organosolv dissolving pulps. Part II: Structure and NMMO processability of the pulps

The supermolecular structure of dissolving pulps produced from hardwood by the organosolv processes Acetosolv, Formacell, and Milox was characterized by physical methods (TEM, WAXS, SAXS, NMR) and compared with conventional Sulfite and standard commercial dissolving pulps. The suitability of the pulps for the NMMO technology was tested by spinning fibres and blowing films, whose structural and mechanical properties have also been determined. With TEM it was shown that the TCF-bleached organosolv pulps have only the primary (Formacell), the primary and S1 (Milox), or mainly the S1 (Acetosolv) layers exposed to the surface, whereas Sulfite pulping exposes the S2 cell wall layer. Especially for Milox and Acetosolv Eucalyptus wood pulps, a reduced degree of crystallinity was found, both with WAXS and NMR. The SAXS results indicate a lower pore intersection length for the new pulps as compared to conventional pulps. Unbleached organosolv pulps show a lower crystallinity, very low pore intersection lengths, and an average crystallite shape different from their bleached counterparts. The dissolution behaviour in NMMO and the processability of the bleached organosolv pulps was satisfactory so far. Fibres and films could be produced with structural and mechanical properties comparable with conventional Sulfite and standard commercial dissolving pulp products. However, unbleached organosolv pulps did not meet the requirements of the NMMO process.     

Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research.     

Agriculture crop residues as a source for the production of nanofibrillated cellulose with low energy demand

Nanofibrillated cellulose (NFC) from three agricultural crop (rice straw, corn and rapeseed stalk) residues was isolated with high-yield production using either high pressure homogenisation or a high speed blender. The fibres were extracted from the neat biomass via an NaClO₂/acetic acid and alkali pulping process. TEMPO-mediated oxidation pretreatment at pH 7 and 10 was accomplished to facilitate the release of the cellulose microfibrils. The fibrillation yield, transparency degree and morphological characteristics of the ensuing NFC were analysed using the centrifugation method, transmittance measurement and SEM observation. The energy consumption during the disintegration process was also accessed. It was shown that the mode of lignin removal and the fibre pretreatment notably affected the nanofibrillation efficiency and energy demand. A successful production of NFC with yield exceeding 90 %, using a simple Waring blender, was achieved when the NaClO₂/acetic acid delignification followed by a TEMPO-NaBr–NaClO oxidation at pH 10 was adopted.     

Effect of residual lignin and heteropolysaccharides in nanofibrillar cellulose and nanopaper from wood fibers

Unbleached (UN), oxygen-delignified and fully-bleached (FB) birch fibers with a residual lignin content of ca. 3, 2 and <1 %, respectively, were used to produce nanofibrillated cellulose (NFC) and nanopaper by using an overpressure device. The tensile index, elongation and elastic modulus of nanopaper were compared and the effect of residual cell wall components accessed. Under similar manufacturing conditions, UN NFC produced nanopaper with a density of 0.99 g/cm³, higher than that from FB NFC (0.7 g/cm³). This translated in much lower air permeability in the case of UN nanopaper (1 and 11 mL/min for UN and FB samples, respectively). Fundamentally, these observations are ascribed to the finer fibrils produced during microfluidization of UN fibers compared to those from lower yield counterparts (AFM roughness of 8 and 17 nm and surface areas of 124 and 98 m²/g for NFC from UN and FB fibers, respectively). As a result, values of stress at break and energy absorption of nanopaper from high yield fibers are distinctively higher than those from fully bleached NFC. Interactions of water with the surface and bulk material were affected by the chemical composition and structure of the nanofibrils. While UN nanopaper presented higher water contact angles their sorption capacity (and rate of water absorption) was much higher than those measured for nanopaper from FB NFC. These and other observations provided in this contribution are proposed to be related to the mechanoradical scavenging capacity of lignin in high shear microfluidization and the presence of residual heteropolysaccharides.     

Switchable ionic liquids enable efficient nanofibrillation of wood pulp

Use of switchable ionic liquid (SIL) pulp offers an efficient and greener technology to produce nanofibers via ultrafine grinding. In this study, we demonstrate that SIL pulp opens up a mechanically efficient route to the nanofibrillation of wood pulp, thus providing both a low cost and chemically benign route to the production of cellulose nanofibers. The degree of fibrillation during the process was evaluated by viscosity and optical microscopy of SIL treated, bleached SIL treated and a reference pulp. Furthermore, films were prepared from the fibrillated material for characterization and tensile testing. It was observed that substantially improved mechanical properties were attained as a result of the grinding process, thus signifying nanofibrillation. Both SIL treated and bleached SIL treated pulps were fibrillated into nanofibers with fiber diameters below 15 nm thus forming networks of hydrophilic nature with an intact crystalline structure. Notably, it was found that the SIL pulp could be fibrillated more efficiently than traditional pulp since nanofibers could be produced with more than 30% less energy when compared to the reference pulp. Additionally, bleaching reduced the energy demand by further 16%. The study demonstrated that this switchable ionic liquid treatment has considerable potential in the commercial production of nanofibers due to the increased efficiency in fibrillation.     

Study on the compatibility of unbleached and bleached bamboo-fiber with LLDPE matrix

This article presents the comparative study on the compatibility of unbleached and bleached bamboo-fibers (UBF and BBF) with LLDPE matrix. Thermal characterization of composites was conducted by differential scanning calorimetry and thermogravimetric analysis. The results suggested that BBF are more compatible with matrix than the unbleached ones. The X-ray diffraction pattern shows no change in the intensity peak position of matrix in the presence of bamboo fibers. Morphological studies were done by scanning electron microscopy to get an idea of the compatibility of the fibers with the matrix. This study gives us the idea of using the bleached fiber for the preparation of the composites.     

Carboxymethylated nanofibrillated cellulose: rheological studies

The rheological properties of carboxymethylated nanofibrillated cellulose (NFC), investigated with controlled shear rate- and oscillatory measurements, are reported for the first time. It was shown that the rheological properties of the studied system are similar to those reported for other NFC systems. The carboxymethylated NFC systems showed among other things high elasticity and a shear thinning behaviour when subjected to increasing shear rates. Further, the shear viscosity and storage modulus of the system displayed power-law relations with respect to the dry content of the NFC suspension. The exponential values, 2 and 2.4 respectively, were found to be in good agreement with both theoretical predictions and published experimental work. Furthermore, it was found that the pulp consistency at which NFC is produced affects the properties of the system. The rheological studies imply that there exists a critical pulp concentration below which the efficiency of the delamination process diminishes; the same adverse effect is also observed when the critical concentration is significantly exceeded due to a lower energy input during delamination.     

Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose

Bionanocomposites of hydroxypropyl cellulose (HPC) and nanofibrillated cellulose (NFC) were prepared by solution casting. The various NFC were in form of powders and were prepared from refined, bleached beech pulp (RBP) by mechanical disintegration, optionally combined with a pre- or post mechanical carboxymethylation. Dynamic mechanical analysis (DMA) and tensile tests were performed to compare the reinforcing effects of the NFC powders to those of their never-dried analogues. For unmodified NFC powders an inferior reinforcing potential in HPC was observed that was ascribed to severe hornification and reagglomeration of NFC. In contrast, the composites with carboxymethylated NFC showed similar behaviors, regardless of the NFC suspensions being dried or not prior to composite preparation. SEM characterization confirmed a homogeneous dispersion of dried, carboxymethylated NFC within the HPC matrix. These results clearly demonstrate that drying of carboxymethylated NFC to a powder does not decrease its reinforcing potential in (bio)nanocomposites.     

Nanofibrillated cellulose (NFC) reinforced polyvinyl alcohol (PVOH) nanocomposites: properties, solubility of carbon dioxide, and foaming

Polyvinyl alcohol (PVOH) and its nanofibrillated cellulose (NFC) reinforced nanocomposites were produced and foamed and its properties—such as the dynamic mechanical properties, crystallization behavior, and solubility of carbon dioxide (CO₂)—were evaluated. PVOH was mixed with an NFC fiber suspension in water followed by casting. Transmission electron microscopy (TEM) images, as well as the optical transparency of the films, revealed that the NFC fibers dispersed well in the resulting PVOH/NFC nanocomposites. Adding NFC increased the tensile modulus of the PVOH/NFC nanocomposites nearly threefold. Differential scanning calorimetry (DSC) analysis showed that the NFC served as a nucleating agent, promoting the early onset of crystallization. However, high NFC content also led to greater thermal degradation of the PVOH matrix. PVOH/NFC nanocomposites were sensitive to moisture content and dynamic mechanical analysis (DMA) tests showed that, at room temperature, the storage modulus increased with decreasing moisture content. The solubility of CO₂ in the PVOH/NFC nanocomposites depended on their moisture content and decreased with the addition of NFC. Moreover, the desorption diffusivity increased as more NFC was added. Finally, the foaming behavior of the PVOH/NFC nanocomposites was studied using CO₂ and/or water as the physical foaming agent(s) in a batch foaming process. Only samples with a high moisture content were able to foam with CO₂. Furthermore, the PVOH/NFC nanocomposites exhibited finer and more anisotropic cell morphologies than the neat PVOH films. In the absence of moisture, no foaming was observed in the CO₂-saturated neat PVOH or PVOH/NFC nanocomposite samples.     

Development of continuous process enabling nanofibrillation of pulp and melt compounding

Microfibrillated cellulose (MFC), a mechanically fibrillated pulp mostly consisting of nanofibrils, is a very attractive material because of its high elastic modulus and strength. Although much research has been done on composites of MFC and polypropylene (PP), it has been difficult to produce such composites at an industrial level because of the difficulties in using MFC in such composites are not only connected to the polarity (that can be improved with compatibilizers), but also with the challenge to make a homogeneous blend of the components, and also the low temperature stability of cellulose that could cause problems during processing. We developed a new processing method which enables continuous microfibrillation of pulp and its melt compounding with PP. Never-dried kraft pulp and powdered PP were used as raw materials to obtain MFC by kneading via a twin-screw extruder. Scanning electron microscopy showed nano to submicron wide fibers entangled in the powdered PP. MFC did not aggregate during the melt compounding process, during which the water content was evaporated. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to reinforce interfacial adhesion between the polar hydroxyl groups of MFC and non-polar PP. We investigated the effect of MAPP content on the mechanical properties of the composite, which were drastically improved by MAPP addition. Needle-leaf unbleached kraft pulp (NUKP)-derived MFC composites had better mechanical properties than needle-leaf bleached kraft pulp (NBKP)-derived MFC composites. Injection molded NUKP-derived MFC composites had good mechanical and thermal properties. The tensile modulus of 50 wt% MFC composite was two times, and the tensile strength 1.5 times higher than that of neat PP. The heat distortion temperature of 50 wt% MFC content composite under 1.82 MPa flexural load was increased by 53 °C, from 69 to 122 °C. This newly developed continuous process using powder resin has the potential for application at an industrial level.     

Sorption of spruce O-acetylated galactoglucomannans onto different pulp fibres

Sorption of spruce acetylated galactoglucomannans (GGM) onto different pulps, among which unbleached and peroxide-bleached mechanical pulps, and unbleached and bleached kraft (BK) pulps, was studied as a means of understanding the retention of acetylated GGMs in mechanical pulping and papermaking. The fibre surface coverage of lignin and carbohydrates was estimated by X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA). GGM sorption was clearly favoured on kraft pulps. Hardly any differences in sorption were, however, observed between unbleached and BK pulps, even if the surface coverage of lignin was lower on the bleached pulp. Neither thermomechanical pulp (TMP) nor chemithermomechanical pulp (CTMP) manufactured from spruce sorbed any acetylated GGMs. Peroxide bleaching of the pulp did not increase sorption. Only CTMP produced from aspen sorbed some GGMs. The anionic charge of neither chemical nor mechanical pulps influenced GGM sorption.     

Key role of the hemicellulose content and the cell morphology on the nanofibrillation effectiveness of cellulose pulps

The effect of the hemicellulose content and that of the fibre morphology on the nanofibrillation behaviour of delignified cellulose pulps were studied. For this purpose, pulps from two non-woody plants, alfa (Stipa tenacissima) and sunflower (Helianthus annuus), were delignified using NaClO₂/acetic acid and the NaOH pulping processes to obtain fibres with different hemicellulose contents. The ensuing fibres were characterized by chemical analysis, SEM, FTIRS and X-ray diffraction. The fibres were then disintegrated into nanofibrillated cellulose (NFC) using either a high pressure homogenizer or a domestic blender. The degree of fibrillation and the morphology of the nanofibrillated fractions were evaluated by centrifugation and Field-emission scanning electron microscopy. Pulps containing the highest hemicellulose content showed higher yields of the nanofibrillated fraction and a better aptitude for the individualization of the microfibrils. Furthermore, it was shown that fibres from sunflowers exhibiting a thinner cell wall were easier to fibrillate and could be disintegrated into NFC by just using a simple domestic-blender once deliginification process was carried out using the NaClO₂/acetic acid method. Eucalyptus fibres were also used to further confirm the key role of hemicelluloses in the nanofibrillation process of woody plants.     

Comparison of hydrothermal,hydrotropic and organosolv pretreatment for improving the enzymatic digestibility of bamboo

Pretreatment is the crucial step to disrupt the recalcitrant structure of lignocellulosic biomass for improving the enzymatic hydrolysis efficiency. Typically, hydrothermal, organosolv and hydrotropic pretreatments are environmentally benign and effective methods. In this work, effects of hydrothermal, organosolv and hydrotropic pretreatments on improving enzymatic hydrolysis of bamboo were comprehensively compared. Hydrotropic pretreatment was more effective in removal lignin and xylose from bamboo fiber cell wall. However, the surface coverage by lignin and extractives were dramatically displaced during organosolv pretreatment as investigation by X-ray photoelectron spectroscopy. After pretreatments, the crystallinity of cellulose in pretreated substrates has a significant reduction, and pores were exposed on fiber surface. The residual content of acetyl and phenolic groups in hydrotropic pretreated substrates is lower than organosolv pretreated substrates. In order to deeply assess the delignification of pretreatments, the isolated lignins obtaining from pretreatments process were characterized by Fourier transform infrared spectroscopy also. It was revealed that hydrotropic lignin contained more phenolic hydroxyl group and syringyl units than organosolv lignin. Compared to hydrothermal and organosolv pretreatment, cellulase adsorption capacity of pretreated substrates was notably improved by hydrotropic pretreatment, which indicating the better enzyme accessibility of cellulose. Eventually, the maximum glucose yield was obtained from hydrotropic pretreated substrates.     

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.     

Accelerated weathering of unbleached and bleached Kraft wood fibre reinforced polypropylene composites

In this paper, combined moisture/ultraviolet (UV) weathering performance of unbleached and bleached Kraft wood fibre reinforced polypropylene (PP) composites was studied. Composites containing 40 wt% fibre with 3 wt% of a maleated polypropylene (MAPP) coupling agent were fabricated using extrusion followed by injection moulding. Composite mechanical properties were evaluated, before and after accelerated weathering for 1000 h, by tensile and impact testing. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were also carried out to assess the changes occurring during accelerated weathering. Bleached fibre composites initially showed higher tensile and impact strengths, as well as higher thermal stability and greater crystallinity. During accelerated weathering, both unbleached and bleached fibre composites reduced tensile strength (TS) and Young's modulus (YM), with the extent of the reduction found to be similar for both unbleached and bleached fibre composites. Evidence supported that the reduction of TS and YM was due to PP chain scission, degradation of lignin and reduced fibre-matrix interfacial bonding.     

Conversion of cellulose materials into nanostructured ceramics by biomineralization

Synthesis of hierarchically ordered silica materials having ordered wood cellular structures has been demonstrated through in-situ mineralization of wood by means of surfactant-directed mineralization in solutions of different pH. At low pH, silicic acid penetrates the buried interfaces of the wood cellular structure without clogging the pores to subsequently “molecularly paint” the interfaces thereby forming a positive replica following calcinations. At high pH, the hydrolyzed silica rapidly condenses to fill the open cells and pits within the structure resulting in a negative replica of the structure. Surfactant-templated mineralization in acid solutions leads to the formation of micelles that hexagonally pack at the wood interfaces preserving structural integrity while integrating hexagonally ordered nanoporosity into the structure of the cell walls following thermal treatment in air. The carbothermal reduction of mineralized wood with silica at high temperature produces biomorphic silicon carbide (SiC) materials, which are typical aggregations of β-SiC nanoparticles. To understand the roles of each component (lignin, crystalline cellulose, amorphous cellulose) comprising the natural biotemplates in the transformation to SiC rods, three different cellulose precursors including unbleached and bleached pulp, and cellulose nanocrystals have been utilized. Lignin in unbleached pulp blocked homogeneous penetration of silica into the pores between cellulose fibers resulting in non-uniform SiC fibers containing thick silica layers. Bleached pulp produced uniform SiC rods with camelback structures (80 nm in diameter; ∼50 μm in length), indicating that more silica infiltrates into the amorphous constituent of cellulose to form chunky rather than straight rod structures. The cellulose nanocrystal (CNXL) material produced clean and uniform SiC nanowires (70 nm in diameter; >100 μm in length) without the camelback structure.     

High strength modified nanofibrillated cellulose-polyvinyl alcohol films

In this study surface-modified nanofibrillated cellulose (NFC) was used at low levels (0.5 to1.5 wt%) as a reinforcement in a polyvinyl alcohol (PVA) matrix. The modified-NFC–PVA composite films prepared using the solution casting technique showed improved mechanical performance. Birch pulp cellulose was initially modified by allylation using a solvent-free, dry modification method followed by subsequent epoxidation of the allyl groups and finally grinding the pulp to yield epoxy-NFC. In order to obtain optimal mechanical performance, epoxy-NFC with different degrees of substitution was evaluated in the reinforcement of PVA. The addition of 1 wt% epoxy-NFC (degree of substitution, DS 0.07) enhanced the modulus, strength, and strain of pure PVA film by 307, 139 and 23 %, respectively, thus producing the best performing film. The results demonstrate the favourable effect of chemically functionalized NFC on the mechanical properties of polyvinyl alcohol compared to unmodified NFC as reinforcement. In order to improve industrial and economic feasibility, the manufacture of the composite was also done in situ by grinding cellulose directly in PVA to produce the new biocomposite in a one-step process.     

Comparison of nano- and microfibrillated cellulose films

Nanocellulose is an interesting building block for functional materials and has gained considerable interest due to its mechanical robustness, large surface area and biodegradability. It can be formed into various structures such as solids, films and gels such as hydrogels and aerogels and combined with polymers or other materials to form composites. Mechanical, optical and barrier properties of nanofibrillated cellulose (NFC) and microfibrillated cellulose (MFC) films were studied in order to understand their potential for packaging and functional printing applications. Impact of raw material choice and nanocellulose production process on these properties was evaluated. MFC and NFC were produced following two different routes. NFC was produced using a chemical pretreatment followed by a high pressure homogenization, whereas MFC was produced using a mechanical treatment only. TEMPO-mediated oxidation followed by one step of high pressure (2,000 bar) homogenization seems to produce a similar type of NFC from both hardwood and softwood. NFC films showed superior mechanical and optical properties compared with MFC films; however, MFC films demonstrated better barrier properties against oxygen and water vapor. Both the MFC and NFC films were excellent barriers against mineral oil used in ordinary printing inks and dichlorobenzene, a common solvent used in functional printing inks. Barrier properties against vegetable oil were also found to be exceptionally good for both the NFC and MFC films.     

Extension of Donnan theory to predict calcium ion exchange on phenolic hydroxyl sites of unbleached kraft fibers

The Donnan theory is extended to formulate a model for determination of the distribution coefficient of calcium in slurries of fully bleached and unbleached kraft fibers at different pH, taking into account the presence of both carboxylic and phenolic groups in the fibers. The intrinsic dissociation constants of the carboxylic acid groups and phenolic hydroxyl, which are the key inputs of the extended model, were determined by potentiometric titration. The extension improves the model prediction significantly, particularly when the presence of phenolic lignin in fibers becomes significant. However, when the pH exceeds 10, the model underestimates the distribution coefficient, suggesting that other factors may influence the fiber charge. The structural changes of the fiber wall at high pH and the presence of hydroxyl groups on the cellulose, which ionize at high pH, may be major factors.