Nanopaper from almond (Prunus dulcis) shell

Five pulping methods using different reagents were used for the delignification of almond shells: sodium hydroxide 7.5 % v/v for 24 h at 60 °C, potassium hydroxide 7.5 % v/v for 24 h at 60 °C, formic acid/water 90/10 v/v, organosolv with ethanol/water 60/40 v/v and sodium hydroxide 15 % v/v in an autoclave for 90 min at 120 °C. The resulting cellulose pulps were evaluated using TAPPI standard methods and X-ray diffraction (XRD) to determine the lignin content and crystallinity changes. After pulping, fibers were bleached with sodium chlorite and hydrogen peroxide to obtain pure cellulose. The resulting pulps were characterized by XRD and thermogravimetry to determine the cellulose purification rates and changes in crystallinity. Then, the different pulps were acetylated, hydrolyzed and homogenized to obtain cellulose nanofibers. Nanofiber sizes were assessed by atomic force microscopy and XRD to evaluate the effect of hydrolysis on nanofibers. Finally, nanopaper sheets were produced and the properties were compared to conventional micropaper. The different treatments influenced the amount of lignin eliminated, which had a direct relationship on the subsequent bleaching treatments to obtain pure cellulose. Hence, the different chemical methods influenced the crystallinity of the fibers which also influenced the yield of cellulose nanofibers and different nanopapers.     

Mesoporous structures in never-dried softwood cellulose fibers investigated by nitrogen adsorption

Nitrogen adsorption was used to characterize mesoporous structures in never-dried softwood cellulose fibers. Distinct inflections in desorption isotherms were observed over the relative vapor pressure (P/P₀) range of 0.5–0.42 for never-dried cellulose fibers and partially delignified softwood powders. The reduction in N₂ adsorption volume was attributed to cavitation of condensed N₂ present in mesopores formed via lignin removal from wood cell walls during delignification. The specific surface areas of significantly delignified softwood powders were ~150 m² g^?1, indicating that in wood cell walls 16 individual cellulose microfibrils, each 3–4 nm in width, form one cellulose fibril bundle surrounded with a thin layer of lignin and hemicelluloses. Analysis of N₂ adsorption isotherms indicates that mesopores in the softwood cellulose fibers and partially delignified softwood powders had peaks ranging from 4 to 20 nm in diameter.     

Multi-layer nanopaper based composites

Native cellulose nanofibrils (CNF) were prepared from bleached birch pulp without any chemical or enzymatic pretreatment. These CNF were modified by adsorption of a small amount of water-soluble polysaccharides and used to prepare nanopapers, which were processed into composites by lamination with an epoxy resin and subsequently cured. The results were compared to the properties of composites prepared using bacterial cellulose nanopapers, since bacterial cellulose constitutes highly pure and crystalline cellulose. It was found that both types of nanopapers significantly improved both the thermal stability and mechanical properties of the epoxy resin. As anticipated, addition of only 2 wt% of water-soluble polysaccharides efficiently hindered crack-propagation within the nanopaper and significantly improved the tensile strength and work of fracture compared to composites containing a conventional nanopaper reinforcement. The mechanical properties of the composites thus reflected the improvement of the nanopaper properties by the polysaccharides. Moreover, it was possible to predict the properties of the final composite from the mechanical performance of the nanopapers.     

Assessment of Bleached and Unbleached Nanofibers from Pistachio Shells for Nanopaper Making

Cellulose and lignocellulose nanofibrils were extracted from pistachio shells utilizing environmentally friendly pulping and totally chlorine-free bleaching. The extracted nanofibers were used to elaborate nanopaper, a continuous film made by gravimetric entanglement of the nanofibers and hot-pressed to enhance intramolecular bonding. The elaborated nanopapers were analyzed through their mechanical, optical, and surface properties to evaluate the influence of non-cellulosic macromolecules on the final properties of the nanopaper. Results have shown that the presence of lignin augmented the viscoelastic properties of the nanopapers by ≈25% compared with fully bleached nanopaper; moreover, the hydrophobicity of the lignocellulose nanopaper was achieved, as the surface free energy was diminished from 62.65 to 32.45 mNm⁻¹ with an almost non-polar component and a water contact angle of 93.52°. On the other hand, the presence of lignin had an apparent visual effect on the color of the nanopapers, with a ΔE of 51.33 and a ΔL of −44.91, meaning a substantial darkening of the film. However, in terms of ultraviolet transmittance, the presence of lignin resulted in a practically nonexistent transmission in the UV spectra, with low transmittance in the visible wavelengths. In general, the presence of lignin resulted in the enhancement of selected properties which are desirable for packaging materials, which makes pistachio shell nano-lignocellulose an attractive option for this field.     

Hydrophobic cellulose nanopaper through a mild esterification procedure

Films of cellulose nanofibrils (CNF) (referred to as nanopaper) present a great potential in many applications due to the abundance, low environmental impact, excellent oxygen barrier properties and good mechanical performance of CNF. However, the strong hygroscopic character of the natural nanofibers limits their use in environments with high relative humidity. In this paper, we introduce a simple route for the esterification and processing of CNF with the aim of reducing their hydrophilicity, and producing hydrophobic cellulose nanopaper with reduced moisture sensitivity. The preparation steps of hydrophobic nanopapers involve vacuum filtration, solvent exchange from water to acetone, and reaction with anhydride molecules bearing different hydrophobic alkyl chains by hot pressing. Porous films having a surface area between 38 and 47 g/m² and pore sizes in the 3–200 nm range are obtained. This method preserves the crystalline structure of native cellulose, and successfully introduces hydrophobic moieties on CNF surface as confirmed by FTIR, XPS and elemental analysis. As a result, modified nanopapers have a reduced moisture uptake, both higher surface water contact angle and wet tensile properties as compared with reference non-modified nanopaper, thus illustrating the benefit of the modification for the use of cellulose nanopaper in humid environments.     

Controlled pinewood fractionation with supercritical ethanol: A prerequisite toward pinewood conversion into chemicals and biofuels

The objective of this work was to investigate the ability of supercritical (SC) ethanol conditions to attack preferentially the lignin fraction against the carbohydrate fraction and their effects on the product distribution among gases, light products, bio-oils, and chars. In this study, the conversion of each pinewood component was determined by the analysis of solid residues to quantify cellulose, hemicellulose, lignin, and char contents. It is shown that, by tuning the temperature, hemicellulose and lignin are already transformed in subcritical ethanol conditions, lignin being more reactive than hemicellulose. In contrast, native wood cellulose is recalcitrant to liquefaction in SC ethanol near the critical point (T_c = 241 °C and P_c = 61 bar), but 20% of native wood cellulose is converted in SC ethanol at 280 °C. Besides, the severity of the conditions, in terms of temperature and treatment time, does not significantly influence the yields of gases, light products, and bio-oils but strongly enhances char formation. Interestingly, the increase in SC ethanol density does not change the conversion of biomass components but has a marked effect on bio-oil yield and prevents char formation. The optimum fractionation conditions to convert the lignin component, while keeping unattacked the cellulose fraction with a minimum formation of char, are dense SC ethanol, at 250 °C for 1 h, in batch conditions. However, although lignin is more reactive than hemicellulose under these conditions, these fractions are converted, in a parallel way, to around 50% and 60%, respectively.     

Residual lignin inhibits thermal degradation of cellulosic fiber sheets

The market for cellulosic fiber based food packaging applications is growing together with the importance of improving the thermal durability of these fibers. To shed light on this, we investigated the role of residual lignin in pulp on the thermal stability of refined pulp sheets. The unbleached, oxygen delignified, and fully bleached pulp sheets were studied after four separate refining degrees. Comparison by Gurley air resistance, Bendtsen porosity, and the oxygen transmission rate tests showed that lignin containing sheets had better air and oxygen barrier properties than fully bleached sheets. Sheet density and light scattering coefficient measurements further confirmed that the lignin containing pulps underwent more intense fibrillation upon refining that changed the barrier properties of the sheets. Thermal treatments (at 225 °C, 20 and 60 min, in water vapor atmospheres of 1 and 75 v/v %) were applied to determine the thermal durability of the sheets. The results revealed that the residual lignin in pulps improved the thermal stability of the pulp sheets in the hot humid conditions. This effect was systematically studied by tensile strength, brightness, and light absorption coefficient measurements. The intrinsic viscosity results support the findings and suggest that lignin is able to hinder the thermal degradation of pulp polysaccharides. In spite of the fact that lignin is known to enhance the thermal yellowing of paper, no significant discoloration of the pulp sheets containing residual lignin was observed in the hot humid conditions (75 v/v %). Our results support the idea of lignin strengthening the thermal durability of paper.     

Ethanol production from pretreated olive tree wood and sunflower stalks by an SSF process

Olive tree wood and sunflower stalks are agricultural residues largely available at low cost in Mediterranean countries. As renewable lignocellulosic materials, their bioconversion may allow both obtaining a value-added product, for fuel ethanol, and facilitating their elimination. In this work, the ethanol production from olive tree wood and sunflower stalks by a simultaneous saccharification and fermentation (SSF) process is studied. As a pretreatment, steam explosion at different temperatures was applied. The water insoluble fractions of steam-pretreated sunflower stalks and steamed, delignified olive tree wood were used as substrates at 10% w/v concentration for an SSF process by a cellulolytic commercial complex and Saccharomyces cerevisiae. After 72-h fermentation, ethanol concentrations up to 30 g/L were obtained in delignified steam-pretreated olive tree wood at 230°C and 5 min. Sunflower stalks pretretated at 220°C and 5 min gave maximum ethanol concentrations of 21 g/L in SSF experiments.     

Fabrication of polyacrylonitrile/lignin-based carbon nanofibers for high-power lithium ion battery anodes

Low-cost carbon nanofibers are fabricated from lignin, the second most abundant raw material in wood after cellulose and polyacrylonitrile mixture as a carbon precursor by electrospinning, followed by suitable heat treatments. As the lignin content in the precursor increases, the carbon nanofibers become thinner, as seen from scanning electron microscopy images. However, their carbon structure and electrochemical performance are found to be very similar, even though surface functional groups on carbon nanofibers are slightly different from each other. For example, in the initial charge (lithium insertion) and discharge (lithium deinsertion) process, the reversible specific capacities of the various carbon nanofibers come from different precursor ratios of lignin and polyacrylonitrile are similar. Even at a fast (7 min) charge and discharge condition, the carbon nanofibers prepared from the lignin-containing precursors show a discharge capacity of 150 mAh g^?1. The lignin-based carbon nanofibers thus show promise for use in high-power lithium ion battery anodes with low price.     

From paper to nanopaper: evolution of mechanical and physical properties

In the present work the evolution of physical and mechanical properties of papers and nanopapers is studied. Handsheets made of eucalyptus fibres reinforced with 0, 25, 50, 75 and 100 wt% of nanofibrillated cellulose (NFC) content were fabricated using a Rapid Köthen-like equipment. The obtained papers and nanopapers were physical- and mechanically-characterized. The results showed a significant increase in density and a reduction of porosity in the samples during their transition from paper to nanopaper; besides, nanopapers were more transparent and smoother than normal papers. These physical changes where more evident with increasing amounts of NFC. Regarding mechanical properties, nanopapers with a 100 wt% content of NFC improved their strength and rigidity in 228 and 317 %, respectively, in comparison with normal papers. The evolution of strength and rigidity from paper to nanopaper was linear in relation to the amount of NFC, which means that the ultimate tensile strength was mainly dependant on nanofibril failure.     

Preparation of tough cellulose II nanofibers with high thermal stability from wood

Well-dispersed cellulose II nanofibers with high purity of 92 % and uniform width of 15–40 nm were isolated from wood and compared to cellulose I nanofibers. First, ground wood powder was purified by series of chemical treatments. The resulting purified pulp was treated with 17.5 wt% sodium hydroxide (NaOH) solution to mercerize the cellulose. The mercerized pulp was further mechanically nanofibrillated to isolate the nanofibers. X-ray diffraction patterns revealed that the purified pulp had been transformed into the cellulose II crystal structure after treatment with 17.5 wt% NaOH, and the cellulose II polymorph was retained after nanofibrillation. The cellulose II nanofiber sheet exhibited a decrease in Young’s modulus (8.6 GPa) and an increase in fracture strain (13.6 %) compared to the values for a cellulose I nanofiber sheet (11.8 GPa and 7.5 %, respectively), which translated into improved toughness. The cellulose II nanofiber sheet also showed a very low thermal expansion coefficient of 15.9 ppm/K in the range of 20–150 °C. Thermogravimetric analysis indicated that the cellulose II nanofiber sheet had better thermal stability than the cellulose I nanofiber sheet, which was likely due to the stronger hydrogen bonds in cellulose II crystal structure, as well as the higher purity of the cellulose II nanofibers.     

Influence of biological origin on the tensile properties of cellulose nanopapers

Cellulose - Cellulose nanopapers provide diverse, strong and lightweight templates prepared entirely from sustainable raw materials, cellulose nanofibers (CNFs). Yet the strength of CNFs has not...     

Enhancement of the Enzymatic Digestibility and Ethanol Production from Sugarcane Bagasse by Moderate Temperature-Dilute Ammonia Treatment

In this study, sugarcane bagasse was pretreated with ammonium hydroxide, and the effectiveness of the pretreatment on enzyme hydrolysis and ethanol production was examined. Bagasse was soaked in ammonium hydroxide and water at a ratio of 1:0.5:8 for 0–4 days at 70 °C. Approximately, 14–45 % lignin, 2–6 % cellulose, and 13–22 % hemicellulose were removed during a 0.5- to 4-day ammonia soaking period. The highest glucan conversion of sugarcane bagasse soaked in dilute ammonia at moderate temperature by cellulase was accomplished at 78 % with 75 % of the theoretical ethanol yield. Under the same conditions, untreated bagasse resulted in a cellulose digestibility of 29 and 27 % of the theoretical ethanol yield. The increased enzymatic digestibility and ethanol yields after dilute ammonia pretreatment was related to a combined effect of the removal of lignin and increase in the surface area of fibers.     

Preparation of tough hydrogels based on β-chitin nanofibers via NaOH treatment

Cellulose and chitin exist in nature as highly crystalline nanofibers. Previously, we reported preparing unique hydrogels from cellulose nanofibers by a simple NaOH treatment without use of any specific solvents or cross-linking agents. In the present study, a similar gel preparation was applied to β-chitin nanofibers extracted from purified squid pen powder. The crystal structure of chitin nanofibers was transformed from β-chitin to α-chitin by NaOH(aq) treatment above 30 wt%. The crystal conversion involving the interdigitation among adjacent nanofibers caused the formation of stable hydrogels with a α-chitin nanofiber network. The use of ethanol voided the dissolution during neutralization and enabled preparation of a higher crystalline hydrogel with high mechanical strength. It achieved a Young’s modulus of 16.6 MPa, a tensile strength of 7 MPa and a strain at break of 52.2 %, on average. Finally, we note that the shrinkage of the cellulose I and β-chitin nanofibers in aqueous NaOH solutions was caused by the release of tensile residual stress due to the intracrystalline swelling in NaOH solutions.     

Cellulosic nanoparticles from alfa fibers (<Emphasis Type="Italic">Stipa tenacissima)</Emphasis>: extraction procedures and reinforcement potential in polymer nanocomposites

The microstructure and chemical composition of alfa (Stipa tenacissima) were investigated. The polysaccharide and lignin contents were around 70 and 20 wt%, respectively. From the bleached and delignified fibers, two types of nanosized cellulosic particles were extracted, namely cellulose nanocrystals and microfibrillated cellulose (MFC). The former correspond typically to the elementary crystallite units of the cellulose fibers, with a rod-like morphology and an aspect ratio of about 20. The latter, mechanically disintegrated from oxidized bleached fibres, presents an entangled fibrillar structure with widths in the range 5-20 nm. The reinforcing potential of the ensuing nanoparticles was investigated by casting a mixture of acrylic latex and aqueous dispersion of cellulose nanoparticles. Thermo-mechanical analysis revealed a huge enhancement of the stiffness above the glass transition of the matrix. Significant differences in the mechanical reinforcing capability of the nanoparticles were reported.     

Enhanced enzymatic hydrolysis of steam-exploded douglas fir wood by alkali-oxygen post-treatment

Good enzymatic hydrolysis of steam-exploded Douglas fir wood (SEDW) cannot be achieved owing to the very high lignin content (>40%) that remains associated with this substrate. Thus, in this study, we investigated the use of alkali-oxygen treatment as a posttreatment to delignify SEDW and also considered the enzymatic hydrolyzability of the delignified SEDW. The results showed that under optimized conditions of 15% NaOH, 5% consistency, 110°C, and 3h, approx84% of the lignin in SEDW could be removed. The resulting delignified SEDW had good hydrolyzability, and cellulose-to-glucose conversion yields of over 90 and 100% could be achieved within 48 h with 20 and 40 filter paper units/g of cellulose enzyme loadings, respectively. It was also indicated that severe conditions, such as high NaOH concentration and high temperature, should not be utilized in oxygen delignification of SEDW in order to avoid extensive condensation of lignin and significant degradation of cellulose.     

Selective delignification of wood by white-rot fungi

The white-rot fungi,Cerrena unicolor, Ganoderma applanatum, G. tsugae,Ischnoderma resinosum, andPerenniporia medullapanis, caused two distinct types of decay. Large areas of decayed wood were selectively delignified and a typical white-rot causing a simultaneous removal of all cell wall components was present. Preferential lignin degradation was intermittently dispersed throughout the decayed wood. Scanning and transmission electron microscopy were used to identify the micromorphological and ultrastructural changes that occurred in the cells during degradation. In delignified areas the compound middle lamella was extensively degraded without substantial alteration of the secondary wall. The S₂ layer of the secondary wall was least affected. The loss of middle lamellae resulted in extensive defibration of the cells. Sulfuric acid lignin determinations indicated that 95–98% of the lignin was removed. Wood sugar analyses using high pressure liquid chromatography demonstrated that hemicelluloses were removed in preference to cellulose when lignin was degraded. The results suggest that a highly diffusible ligninolytic system was responsible for the selective degradation of the wood. In simultaneously white-rotted wood, all cell wall layers were progressively removed from the cell lumen toward the middle lamella, causing erosion troughs or holes to form. Large voids filled with fungal mycelia resulted from a coalition of degraded areas. Chemical analyses of white-rotted wood indicated lignin, cellulose, and hemicellulose were removed in approximately the same amounts. Degradation was confined to areas around fungal hyphae.     

Obtaining nanofibers from curauá and sugarcane bagasse fibers using enzymatic hydrolysis followed by sonication

This paper is an initial study of the implementation of two new enzymes, an endoglucanase and a concoction of hemicellulases and pectinases to obtain cellulosic nanoparticles. In this study, curauá and sugarcane bagasse were dewaxed and bleached prior to enzymatic action for 72 h at 50 °C, and then followed by sonication. The concentration between these two enzymes was varied, and for the concentrations and time of enzymatic treatment used, subsequent sonication was necessary for cellulose nanoparticle release. It was easier to extract cellulose nanofibers from sugarcane bagasse which resulted in nanoparticles without damage of cellulose chains. On the other hand, curauá fibers needed a higher concentration of enzymes and the nanofibers obtained displayed a decrease of crystallinity suggesting that the cellulose structure was compromized. For both fibers, cellulose nanocrystals (single crystals) and larger diameter nanofibers were attained after the sonication.     

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.     

Highly Transparent,Flexible, and Mechanically Strong Nanopapers of Cellulose Nanofibers @Metal–Organic Frameworks

Freestanding nanopapers were fabricated by the assembly of metal–organic frameworks (MOFs) onto cellulose nanofibers (CNFs). The CNFs are wrapped by continuously nucleated MOF layers (CNF@MOF) by interfacial synthesis, with the charge density on the surface of the CNFs and the dosage of the surfactant polyvinylpyrrolidone (PVP) being carefully adjusted. The obtained CNF@MOF nanofibers with long-range, continuous, hybrid nanostructures were very different to the composites formed by aggregation of MOF nanoparticles on the substrates. Four typical MOFs (HKUST-1, Al-MIL-53, Zn-MOF-74, ZIF-CO₃-1) were successfully grown onto CNFs in aqueous solutions and further fabricated into freestanding nanopapers. Because of their unique nanostructures and morphologies, the corresponding flexible nanopapers exhibit hierarchical meso-micropores, high optical transparency, high thermal stability, and high mechanical strength. A proof-of-concept study shows that the CNF@MOF nanopapers can be used as efficient filters to separate volatile organic compounds (VOCs) from the air. This work provides a new path for structuring MOF materials that may boost their practical application.