The main goal of this article is to provide an overview of recent research in the area of cellulose nanomaterial production from different sources. Due to their abundance, renewability, high strength and stiffness, eco-friendliness and low weight, numerous studies have been reported on the isolation of cellulose nanomaterials from different cellulosic sources and their use in high-performance applications. This report covers an introduction to the definition of nanocellulose as well as the methods used for isolation of nanomaterials (including nanocrystals and nanofibers, CNCs and CNFs, respectively) from various sources. The web-like network structure (CNFs) can be extracted from natural sources using mechanical processes, which include high-pressure homogenization, grinding and refining treatments. Also, rod-like CNCs can be isolated from sources such as wood, plant fibers, agricultural and industrial bioresidues, tunicates and bacterial cellulose using an acid hydrolysis process. Following this, the article focuses on the characterization methods, material properties and structures. Encyclopedic characteristics of CNFs and CNCs obtained from different source materials and/or studies are also included. The current report is a comprehensive review of the literature regarding nanocellulose isolation and demonstrates the potential of cellulose nanomaterials for a wide range of high-tech applications.

Different preparation methods and properties of nanostructured cellulose from various natural resources and residues: a review

A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from molecular cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mechanical reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are critical for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphology, surface chemistry, surface charge, purity, crystallinity, rheological properties, mechanical properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.

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Current characterization methods for cellulose nanomaterials

In this review we introduce recent advances in the development of cellulose nanomaterials and the construction of high order structures by applying some principles of colloid and interface science. These efforts take advantage of natural assemblies in the form of fibers that nature constructs by a biogenetic bottom-up process that results in hierarchical systems encompassing a wide range of characteristic sizes. Following the reverse process, a top-down deconstruction, cellulose materials can be cleaved from fiber cell walls. The resulting nanocelluloses, mainly cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC, i.e., defect-free, rod-like crystalline residues after acid hydrolysis of fibers), have been the subject of recent interest. This originates from the appealing intrinsic properties of nanocelluloses: nanoscale dimensions, high surface area, morphology, low density, chirality and thermo-mechanical performance. Directing their assembly into multiphase structures is a quest that can yield useful outcomes in many revolutionary applications. As such, we discuss the use of non-specific forces to create thin films of nanocellulose at the air–solid interface for applications in nano-coatings, sensors, etc. Assemblies at the liquid–liquid and air–liquid interfaces will be highlighted as means to produce Pickering emulsions, foams and aerogels. Finally, the prospects of a wide range of hybrid materials and other systems that can be manufactured via self and directed assembly will be introduced in light of the unique properties of nanocelluloses.

Nanocellulose properties and applications in colloids and interfaces

The fibrillation of pulp fiber was attempted by two methods, a high-pressure homogenizer treatment and a grinder treatment. The grinder treatment resulted in the successful fibrillation of wood pulp fibers into nanofibers. The nanofibers demonstrate promising characteristics as reinforcement material for optically transparent composites. Due to the size effect, the nanofiber-reinforced composite retains the transparency of the matrix resin even at high fiber content such as 70 wt %. Since the nanofiber is an aggregate of semi-crystalline extended cellulose chains, its addition also contributes to a significant improvement in the thermal expansion properties of plastics while maintaining its ease of bending. Cellulose nanofibers have tremendous potential as a future resource since they are produced in a sustainable manner by plants, one of the most abundant organic resources on earth.

Optically transparent composites reinforced with plant fiber-based nanofibers(ABSTRACTS (MASTER THESIS FOR GRADUATE SCHOOL OF AGRICULTURE))

https://pubs.rsc.org/en/Content/ArticlePDF/2015/GC/C4GC02398F

Comprehensive elucidation of the effect of residual lignin on the physical, barrier, mechanical and surface properties of nanocellulose films

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.

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

Masuko refi ning of unbleached kraft birch pulp has been noted to result in more thorough fi brillation than the refi ning of its bleached counterpart. This result is observed through different fractionation behavior of once refi ned pulps and is further supported by different relative changes in pulp viscosity. The formation of mechanoradicals during refi ning is observed with electron paramagnetic resonance spectroscopy, and the role of lignin and hemicelluloses [quantifi ed using kappa number determination and UV resonance Raman (UVRR) spectroscopy] in the progress of refi ning is discussed. Lignin, a known antioxidant, is capable of stabilizing radicals, which could potentially counteract recombination reactions between highly reactive cellulose radicals. On the other hand, lignin ’ s ability to promote fi brillation could also lie in its amorphous nature and not solely in its antioxidant characteristics. Furthermore, bleaching removes not only lignin but also hemicelluloses, which affects both the charge density and the structure of the fi ber material, and this is another likely contributor to the easier fi brillation of lignin-containing pulps.

Mechanoradical formation and its effects on birch kraft pulp during the preparation of nanofibrillated cellulose with Masuko refining

This review outlines the present state and recent progress in the area of lignin-containing cellulose nanofibrils (LCNFs), an emerging family of green cellulose nanomaterials. Different types of LCNF raw materials are described, with main focus on wood-based raw materials, and the properties of the resulting LCNFs are compared. Common problems faced in industrial utilization of CNFs are discussed in the light of potential improvements from LCNFs, covering areas such as chemical and energy consumption, dewatering and redispersibility. Out of the potential applications, barrier films, emulsions and nanocomposites are considered.

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On the potential of lignin-containing cellulose nanofibrils (LCNFs): a review on properties and applications

In this paper, porous nanofibrillated cellulose (NFC) films were utilized to produce water-resistant, porous cellulose films. Film porosities of ~ 50% were achieved through solvent exchange from water to acetone, and the resulting films were hydrophobized with an epoxy modifier in non-swelling conditions in acetone, yielding films that were non-wettable by water but permeable to water vapor. The mass transport mechanisms of gaseous and liquid water were studied by water vapor transfer rate (WVTR), water vapor uptake and water contact angle measurements to unfold how these properties were achieved. Surface hydrophobization was found to decrease the moisture uptake but it did not prevent it completely. The WVTR values were in effect similar for the initial and hydrophobized films, even if the water contact angles were higher in the latter. We anticipate that the porous and hydrophobic NFC films presented in this paper may find applications in sportswear, medical, or personal hygiene products.

Water vapor mass transport across nanofibrillated cellulose films: effect of surface hydrophobization

The potential of neutral sulphite pulps from softwood with different yields (c. 58–84%) and high residual lignin contents (c. 10–25 wt%) was investigated as a raw material for lignin-containing cellulose nanofibrils (LCNFs) by following their fibrillation during grinding. It was found that the lower yield (58–65%) pulps needed two grinding cycles to produce fibrillated fibers with water retention values (WRV) as high as 400 g/g (at the energy consumption level of 1400 kWh/t). In contrast, the high yield (77–84%) pulps fibrillated more slowly, requiring five grinding cycles to reach comparable WRV values. Apparently, higher crosslinking degrees of lignin in the high yield pulps are hampering the fibrillation, although the high hemicellulose contents (21–24 wt%) and the high charge densities (200–350 µmol/g, originating from carboxylic and sulphonic acid groups) of the pulps were expected to enhance the fibrillation. Nevertheless, regardless of the different fibrillation behaviour, most of the pulps formed c. 10–15% of nanosized material below 30 nm and significant amounts of fibrils with size under 100 nm based on the centrifugation method and FE-SEM images. As the pulps were of moderate to high yield and fibrillated easily without any chemical or enzymatic pretreatments, they show promise for cost-efficient production of LCNFs. The nanopapers prepared from the fibrillated pulps showed tensile strengths (73–125 MPa) comparable with the nanopapers from high yield mechanical pulps, whereas the water contact angles (41°–58°) were closer the those of chemical pulps.

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Iina Solala

Papers

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

Mechanoradical formation and its effects on birch kraft pulp during the preparation of nanofibrillated cellulose with Masuko refining

On the potential of lignin-containing cellulose nanofibrils (LCNFs): a review on properties and applications

Water vapor mass transport across nanofibrillated cellulose films: effect of surface hydrophobization

Fibrillation and characterization of lignin-containing neutral sulphite (NS) pulps rich in hemicelluloses and anionic charge