Open access peer-reviewed chapter

Preparation, Properties, and Advanced Functional Applications of Nanocellulose

Written By

Kaimeng Xu, Yu Chen, Guanben Du and Siqun Wang

Submitted: 31 May 2022 Reviewed: 13 June 2022 Published: 07 July 2022

DOI: 10.5772/intechopen.105807

From the Edited Volume

Wood Industry - Past, Present and Future Outlook

Edited by Guanben Du and Xiaojian Zhou

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Abstract

Cellulose is the most abundant natural polymer on earth, which widely exists in various biomasses such as wood, bamboo, and other forestry and agricultural crops. Nanocelluloses isolated by various physical, chemical, and mechanical ways, as the second-generation cellulose products, facilitate the special microstructural formation such as rigid nano crystals and flexible nano fibrils, which display the high specific surface area, the excellent comprehensive mechanical strengths and thermal stability, as well as easy tailorability. Nanocellulose has been considered as a most important renewable, biodegradable, high-added-value bioresource for the development of novel functional bio-products in the future of wood industry and its industrial cross fields, including construction, polymer adhesives, composite fabrication and reinforcement, green energy storage and adsorption system. This chapter aims to introduce the important preparation and isolation methods, the basic and special properties, and several novel advanced functional applications of nanocellulose.

Keywords

  • nanocellulose
  • nanocellulose crystals
  • nanocellulose fibrils
  • preparation
  • properties
  • advanced functional materials

1. Introduction

A green renewable era for novel biomass materials application is approaching due to the continuous shortage of petroleum and environmental pollution caused by non-biodegradable synthetic polymers. Biomass with inherent advantages of renewability, biodegradation, low cost, and zero carbon dioxide emissions has attracted great interest in academic and industrial fields to effectively mitigate environmental pollution, global warming, and energy crisis. Cellulose (CE), extracted from various biomass sources including wood, bamboo, cotton, flax, hemp, crop straws, bagasse, leaf, fruit, and other microorganisms [1], is the most abundant and widely distributed natural biopolymer in the world with an estimated annual production of 7.5 × 1010 tons [2]. The chains of cellulose consist of the repeating β-(1–4)-linked-D-glucose units, which assemble into microfibrils via the interactions of hydrogen bonding and van der Waals forces [3]. Cellulose has been widely used in the paper, textile, and chemical industries for several centuries.

The rapid development of nanotechnology and wood industry has ushered in a new global nano era in different fields. Nanocelluloses isolated by various physical, chemical, and mechanical methods as well as their combinations facilitate the formation of nanocrystals and nanofibrils. Compared with conventional celluloses, nanocelluloses with outstanding properties including high specific surface area, excellent tensile strength and Young’s modulus, thermal stability, easy adaptability and processing, high barrier, and interesting optical properties [4], have drawn considerable attention in various cross fields of wood industry, such as construction, composite fabrication, wood adhesives, gas barrier materials, filtration systems, sensors systems, energy storage, and other environmental-friendly products. Nanocellulose has been considered as a critical renewable high-added-value bioresources for the development of novel functional bio-products in future wood industry. According to the Global Industry Analysts, the market for nanocellulose-based materials exceeded a billion dollars by 2020.

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2. Preparation and properties of nanocellulose

2.1 Structure of cellulose

Cellulose (C6H10O5)n is a long polymer chain with ringed glucose molecules and a ribbon-like conformation [5]. The unique properties of cellulose such as hydrophilicity, insolubility in aqueous solvents, infusibility, and ease of functionalization are attributed to the intramolecular and intermolecular hydrogen bond as well as polymer chain length [6], as shown in Figure 1.

Figure 1.

Schematic representation of the chemical structure and intra-, intermolecular hydrogen bonds in cellulose [2, 6].

Cellulose molecules are usually assembled into elementary nanofibrils (protofibrils). The protofibrils are further assembled via hydrogen bonding into microfibrils corresponding to dimensions varying from 2 to 20 nm [5]. These inter- and intra-hydrogen bonding networks enhance the durability and the axial rigidity of cellulose fibrils. There are two major domains of native cellulose corresponding to crystalline and amorphous regions. The crystallinity ranges from 40 to 70% depending on the biomass source and the extraction method with enhanced resistance to chemical, mechanical, and enzymatic effects [2]. Furthermore, cellulose can form different crystal types via molecular orientations, intramolecular and intermolecular interactions, and van der Waals forces, and they can transform into each other by different forms of isolation and treatment [7, 8]. The amorphous region exhibits a lower density and easily reacts with other chemical groups [9].

2.2 Types of nanocellulose and their properties

The term “nano” originates from the Greek word “nanos” or Latin word “nanus,” which generally means “dwarf.” “Nano” represents a tiny scale of one in a billion. Generally, cellulose measuring nanometers in size in at least one dimension is considered nanocellulose. The basic properties of nanocellulose are similar to common cellulose including weak water solubility and ease of chemical modification despite various micro-morphologies under different physical, chemical, and biological treatments. However, they show outstanding mechanical properties, excellent thermal stability, large specific surface area, unique rheological and optical properties. The common nanocellulose can be categorized in two major types, cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) depending on their preparation methods, micro-morphology, and characteristics. The specific structures, properties, and yields of nanocellulose are closely related to the source of cellulose and isolation conditions [10].

The isolation of cellulose into nanocellulose generally entails two major stages: pretreatment of lignocellulosic feedstocks to obtain pure cellulose and transformation of cellulose to nanocellulose. Specifically, the hemicelluloses and lignin as well as extracts including fat, sugar, rosin, tannins, resin, terpenoids, terpene, flavonoids, waxes, and fatty acids in various biomass feedstocks are eliminated by different pretreatments [11]. The pure cellulose is then isolated by top-down methods such as physical, chemical, and mechanical techniques and their combinations. In addition, bacterial cellulose (BC) and cellulose nanofibers obtained by electrospinning (ECNF), which are generated by bottom-up methods, are also considered as nanocellulose. BC is self-assembled from low-molecular-weight sugars via bacterial metabolism, whereas ECNF are formed by electrospinning using different solvents of cellulose [12].

2.2.1 Cellulose nanocrystals

CNCs are also known as cellulose nanowhiskers or nanocrystalline celluloses. CNCs are rigid with a high modulus of elasticity (about 140 GPa) owing to their inherent crystallinity and orientation of hydrogen bonding, which is a rod-like or needle-like microstructure. The diameter and length of CNCs are less than 100 nm and around 100–400 nm, respectively. The degree of crystallinity usually changes from 54 to 84% [2]. Acid hydrolysis is a facile process used primarily. The crystallinity and size of CNCs are determined by the types of cellulose, the methods of isolation, and the corresponding parameters (hydrolysis time and temperature). The short rod-like CNCs with high degree of crystallinity originate mainly in the woody biomass and are obtained by acid hydrolysis. However, the CNCs with higher aspect ratios are typically obtained from bacteria and tunicates [13], as seen in Figure 2.

Figure 2.

Transmission electron microscope (TEM) images of cellulose nanocrystals derived from (a) wood, (b) bamboo, (c) straw, and (d) tunicate [14, 15, 16, 17].

2.2.2 Cellulose nanofibrils

Cellulose nanofibrils (CNFs) with the same name, such as cellulose nanofibers, nanofibrillar cellulose, microfibrillated cellulose, and cellulose microfibrils, are long and flexible nanofibers, entangled into a mesh, which can be isolated mechanically (Figure 3). Their diameter and length range from 2 to 50 nm and 500 nm to several microns, respectively [10]. CNFs exhibit a stable configuration mainly due to their cellulosic molecular chains connected by hydrogen bonds. Typical mechanical ways including homogenization, refining, grinding, microfluidization, cryocrushing, and ultrasonication are used to defibrillate the cellulose, thus producing CNFs [12]. Some chemical pretreatments can be introduced to synergistically break the cellulose hydrogen bonds, resulting in enhanced accessibility of hydroxyl groups, which change the crystallinity and promote the reactivity of fibers. The inherent mechanical properties of CNFs are excellent corresponding to the ultimate strength of 2–6 GPa and an elastic modulus of 138 GPa in the crystalline region [18, 19]. The longer length with the higher aspect ratio and surface area as well as additional hydroxyl groups, which are easily modified on the surface of CNFs compared with CNCs. CNFs also exhibit outstanding thermal stability, optical transmittance, and gas barrier properties.

Figure 3.

Atomic force microscopy (AFM) images of cellulose nanofibrils from hardwood kraft pulp [12].

2.2.3 Electrospun cellulose nanofibers

Electrospinning is a novel, facile, and efficient way to fabricate continuous submicron or nanocellulose fiber. They have great potential for various applications due to their unique interwoven porous structure, low carbon footprint, and green synthesis. Electrospinning can be also used for the orientation and assembly of nanofibers to decrease the Gibbs free energy. During the electrospinning process, the cellulose solution is in a high voltage electric field. The droplets overcome the surface tension and then jet in a filament to the collection device through the air during solvent evaporation. The electrospinning process is influenced by the source of cellulose, concentration, viscosity, and surface tension of cellulose solution as well as electric field intensity, the receiving distance, and air temperature and humidity. In recent decades, the ECNFs and their derivatives have attracted great interest. The selection and development of the appropriate solvent system are the most important prerequisite and guarantee for the preparation of high-quality cellulose nanofibers by electrospinning.

2.2.4 Bacterial cellulose nanofibers

Bacterial cellulose (BC) is synthesized by specific microorganisms in the sugar source. Its chemical structure contains linear glucan molecules connected via hydrogen bonds, similar to the plant cellulose, and is free of lignin, hemicelluloses, and pectin. BC is assembled by twisting ribbons with cross-sectional area of 210–420 nm2 and length of 1–9 μm. The degree of polymerization and crystallinity of BC are about 3000–9000 and 80–90%, respectively [20, 21]. Approximately 200,000 glucose molecules are formed via polymerization by a single Acetobacter xylinum cell per second. The cellulose synthase or terminal complexes existing in surface pores enter the media [22], which typically form ribbon-like bundles. Some pore-like sites are arranged in rows along the cell axis and juxtaposed with the cellulosic ribbon outside the cell [23]. A discrete lipopolysaccharide layer is formed as the extruding sites for precellulosic polymer in several chain groups assemble into subfibrils and microfibrils. A fibrillar ribbon with a width of 50–80 nm finally was formed by tightly aggregation [24].

2.3 Isolation methods of nanocellulose

See Figure 4.

Figure 4.

Preparation method of nanocellulose.

2.3.1 Isolation methods of CNF

Mechanical isolation of CNF is usually performed after chemical pretreatment of cellulose. An appropriate pretreatment changes the crystallinity of cellulose and increases its reactivity. Cellulosic nanofibers are usually prepared under high-speed shear force and friction to dissociate the raw material into microfilament bundles with nanometer width and micron length. The main processing equipment utilizes high-pressure homogenization (HPH), microfluidization, grinding, ball milling, and cryocrushing. The first three techniques are the most common methods of mechanical isolation, as shown in Figure 5.

Figure 5.

The most common mechanical method for preparing CNF: (a) homogenization, (b) microfluidization, (c) grinding [25].

2.3.1.1 High-pressure homogenization

HPH is the most widely utilized method for preparing CNFs. It was first used to prepare CNFs from wood pulp in 1983 [26]. HPH is performed by sending the fiber slurry suspension into the container through a small nozzle, which generates high shear in the suspension under high speed, high pressure, and fluid impact, thereby reducing the fiber size to the nanometer scale [27]. HPH is a highly efficient isolation technique for refining cellulose fiber sheets owing to its simplicity and lack of organic solvents. Many researchers have attempted to use various feedstocks in HPH. Sugar beet was successfully isolated at 30 MPa for 10–15 cycles by Leitner et al. [28]. Habibi et al. [29] used bleached cellulose residues to extract about 2–5 nm wide CNFs through HPH of 15 times at 50 MPa. Clogging is one of the most significant limitations of HPH. Due to uneven particle distribution, small orifice size, and high homogenization pressure, homogenization often leads to pipe clogging and pump wear. In addition, the homogenization time is prolonged and the energy consumption is increased. To overcome the disadvantages of clogging and wearing, the size is usually reduced prior to HPH. Therefore, a series of experiments were carried out using kenaf bast fibers to produce nanofibers [30]. Kenaf bast fibers were pretreated by refining and low-temperature crushing to obtain 10–90 nm wide nanofibers. CNFs with a width of 20–25 nm and 15–80 nm were obtained by pretreatment of kenaf core and stem by grinding. Zimmermann et al. [31] used milling pretreatment of wheat straw and wood fiber prior to homogenization of 150 MPa to obtain the nanofibrous cellulose.

2.3.1.2 Microfluidization

Microfluidization is also a common method used to produce CNFs from pretreated cellulose based on a principle similar to HPH. Microfluidization is mainly conducted in a homogeneous cavity under a pressurizing mechanism, with usually “N” and “Y” types inside the homogeneous chamber. The microfluidizer combines the advantages of microfluidization and ultrahigh pressure (>300 MPa). Cellulose is passed through an N-shaped or Y-shaped channel at Mach speed in a pressurized chamber. It is simultaneously subjected to shear force, high-frequency oscillation, cavitation, and shocking, resulting in the breakdown of intermolecular hydrogen bonds of cellulose and the fibrillation [32].

2.3.1.3 Grinding

Grinding is a facile and low energy consumption method. It facilitates the possibility of industrial manufacture of CNFs. Generally, slurry is passed through static and rotating grinding stones in the instrument. The process of fibrillation in the shredder disrupts the hydrogen bonds and destroys the cell wall structures using shear forces to turn cellulose raw materials into nanoscale fibers [33]. Compared with HPH, traditional disc grinding has a lower efficiency due to the difficulty in tuning the gap of the two disks after gradually reducing the cellulose size. Meanwhile, the improper adjustment of the two grinding disks at a high speed of rotation results in collision and friction with the metal or inorganic fragments. In addition, the cellulose is easily embedded in the grooves of grinding discs during the grinding process, resulting in uneven scale. New grinding methods such as supermasscolloider and planetary ball milling have been shown to overcome the disadvantage of low efficiency. Furthermore, due to the presence of holes inside the chamber, it is easy to incorporate cellulose during the grinding process. The grinding procedure, however, is difficult to clog, and a large quantity of raw materials can be treated simultaneously. However, heat is easily generated by high-speed rotating disc grinding and cannot be dissipated rapidly (Figure 6).

Figure 6.

Schematic diagram of the preparation of CNF by ball milling [34].

2.3.1.4 Cryocrushing

The cryocrushing technique is used to obtain CNFs at low temperature via mechanical fibrillation. Cryocrushing of fibers is generally preceded after chemical pretreatment. The celluloses are rapidly frozen under liquid nitrogen, followed by treatment with high shear force, resulting in longitudinal decomposition and formation of CNFs [12]. The width of the CNFs extracted by cryocrushing is between 5 and 80 nm, and the length is several thousands of nanometers [35]. This technique is rarely used in commercial applications due to its limited ability to produce CNFs.

2.3.1.5 Oxidation

Oxidation via 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is used for surface modification of native cellulose, which results in the oxidation of hydroxyl groups into carboxyl groups in mild aqueous systems. This method preserves the fibrous morphology of native cellulose, and the oxidation reaction only occurs on the cellulose surface. The surface negative charges lead to mutual repulsion between fibers, which facilitates the disintegration of fine fiber bundles and nanofibrillation. The most common oxidation method used is TEMPO/NaBr/NaClO in the aqueous system at pH = 9–11, TEMPO and NaBr play a catalytic role, and NaClO oxidizes the fiber surface [36]. Saito et al. used TEMPO for carboxylation of cellulose surface and successfully obtained fibers with a width of 3–4 nm and a length of several micrometers with a mechanical method involving pure nanofibrils [37]. To prevent side reactions, such as the degradation or discoloration of oxidized cellulose due to aldehyde groups, Hirota et al. utilized a TEMPO/NaClO/NaClO2 oxidation system in neutral or slightly acidic conditions at pH 4.8–6.8 and a reaction temperature of 60°C, the carboxyl content can reach 1.87 mmol g-1 [38]. Further, the TEMPO/NaClO/NaClO2 oxidation system can maintain the original polymerization of cellulose with uniform diameter and basically no aldehyde groups. Although the TEMPO oxidation method introduces a large number of carboxyl groups, which may reduce the degree of cellulose molecule polymerization, the crystal structure and crystallinity of cellulose are basically unchanged. The nanofibrils are uniformly dispersed in water, and the aspect ratio of the fibers is larger possibly due to many functional groups such as carboxyl groups on the surface.

2.3.2 Isolation methods of CNC

Acid hydrolysis is used as a facile process for the isolation of cellulose nanocrystals. This process involves acid-induced decomposition with the diffusion of acid molecules into cellulose microfibrils. The preparation of CNC via acid hydrolysis is mainly based on the difference in hydrolysis kinetics between the amorphous and the crystalline regions. It disrupts the glycosidic bonds in the cellulose molecular chain along the amorphous domains of the cellulose fibers, resulting in fracture of the hierarchical structure of cellulose bundles into CNCs. Acid hydrolysis decreases the degree of polymerization of CNCs. When the cellulose is partially hydrolyzed, a large amount of water is added for dilution, and the residual acid and impurities are removed by centrifugal dialysis.

2.3.2.1 Sulfuric acid hydrolysis

Historically, sulfonated cellulose nanocrystals (SCNCs) were successfully prepared for the first time in 1947 via sulfuric acid hydrolysis by Nickerson [39]. The concentration, temperature, and time of sulfuric acid hydrolysis play an important role in particle size, morphology, and physicochemical properties of CNCs [40]. Therefore, the reaction parameters and cellulose raw materials should be considered. Nagarajan et al. [41] reported incomplete hydrolysis when sulfuric acid concentration is less than 63 wt%. In this process, particles with low crystallinity are produced and small amounts of amorphous and aromatic polymers are dissolved. The productivity is enhanced when the sulfuric acid concentration ranges between 63% and 64% and the temperature is between 45°C and 60°C for 30–120 min [40, 42]. Similarly, when the concentration of sulfuric acid is greater than 65 wt%, there is a possibility of swelling of the crystalline region [43]. Sulfuric acid hydrolysis occurs between sulfuric acid and sodium hydroxyl groups on the surface of the crystal sulfate of electric charge. When the sulfate group is introduced into the surface of nano crystal, its dispersibility in water is improved, but its thermal stability is reduced.

2.3.2.2 Hydrochloric acid hydrolysis

Hydrochloric acid hydrolysis is another available method for preparing CNCs. The efficiency of hydrochloric acid hydrolysis of CNCs is enhanced at an acid concentration of 2.5–6.0 N and a temperature of 60–105°C for 2–4 h [44, 45]. Compared with sulfuric acid hydrolysis, CNCs hydrolyzed by hydrochloric acid carry no charged groups on the surface, so the dispersibility of the product in water is limited, resulting in easy aggregation and flocculation. The rheological properties of the two acid-hydrolyzed cellulose nanocrystals are different. The viscosity of nanocrystal suspensions obtained by hydrolysis of sulfuric acid is not correlated with time. However, the nanocrystal suspension hydrolyzed by hydrochloric acid exhibits thixotropy when the mass fraction is above 0.5 wt%, and anti-thixotropy when the mass fraction is below 0.3 wt% [45]. The nanocrystals obtained by blending sulfuric acid and hydrochloric acid have the same size as the nanocrystals hydrolyzed by sulfuric acid. However, the surface charge of the nanocrystals can be adjusted by changing the ratio of the two components.

2.3.2.3 Phosphoric acid hydrolysis

Phosphoric acid hydrolysis is a mild method. The CNCs prepared via phosphoric acid hydrolysis were thermally stable than those obtained by sulfuric acid [46]. Nevertheless, the colloidal stability of the suspension obtained after hydrolysis is not comparable to that of sulfuric acid hydrolysis. Acid concentration of 70–75 wt%, duration between 80 and 120 min, and temperatures from 100 to 120°C resulted in higher efficiency of phosphoric acid hydrolysis [47]. In addition, the phosphoric-acid-hydrolyzed CNCs exhibited better dispersibility in polar solvents than sulfuric-acid-hydrolyzed CNCs, and it can also be used as flame retardants and bio-bone scaffold materials [48].

2.3.2.4 Organic acid hydrolysis

Organic acids are recyclable, milder, and environmentally friendlier than other classical inorganic acids. Xie et al. [49] reported that 91 ± 2% of oxalic acid was recycled when sulfuric acid/oxalic acid mixture was used to prepare CNCs. Further, these organic acids are less corrosive to the processing equipment. Therefore, organic acid hydrolysis will be increasingly utilized in industrial manufacture of CNCs. Li et al. [50] reported a method of CNC synthesis from formic acid under mild conditions. Initially, the amorphous domains of cellulose are destroyed by formic acid (FA), releasing CNCs. Further oxidation using TEMPO increased the charge density on the CNC surface. The CNCs hydrolyzed by formic acid and modified by TEMPO exhibit higher crystallinity and surface charge density. The hydrolytic efficiency was further improved by Du et al. using FeCl3 addition to the formic acid hydrolysis system [51]. The results indicated that the particle size of the synthesized CNCs was decreased with increasing ferric chloride dosage in the formic acid hydrolysis.

2.3.2.5 Solid acid hydrolysis

Although most reports of the preparation of nanocellulose crystals entail inorganic acid hydrolysis, the method has disadvantages such as easy corrosion of equipment, environmental pollution, difficult to control the degree of hydrolysis, and low yield. The strong acid cation exchange resin in the solid acid catalyst can be used to replace the homogeneous acid catalysts such as sulfuric acid and hydrochloric acid. The preparation of CNCs via solid acid hydrolysis is environmentally friendly and increases the recoverable yield. Tang et al. [52] used solid acid hydrolysis to prepare CNC from MCC by mixing the strong acid cation exchange resin with water in a ratio of 1:12 with continuous stirring. The crystallinity of the CNCs is higher than that of the CNCs prepared by sulfuric acid. However, the reaction time is long and the reaction efficiency is low due to the limited contact area between solid acid and cellulose. Therefore, it is still in the stage of laboratory research.

2.3.2.6 Enzymatic hydrolysis

Enzymatic hydrolysis of CNCs is a cheaper alternative to acid hydrolysis, which eliminates harsh chemicals and requires less energy consumption for fibrillation. Cellulose can be selectively degraded by the enzyme in the amorphous or crystalline regions. Cellulase is a multicomponent enzyme system with synergistic actions. It comprises endoglucanases (EGs), cellobiohydrolases (CBHs), and β-glucosidase (GBs) depending on the different catalytic reactions. EGs randomly cleave the amorphous regions within the cellulose polysaccharide chain, producing oligosaccharides of different lengths and new chain ends [53]. CBH acts on the ends of cellulosic polysaccharide chains to release glucose or cellobiose. GB hydrolyzes cellobiose to yield two molecules of glucose [12]. Under the action of cellulase, not only the amorphous region is cleaved by EG but also the crystalline region is destroyed by CBHs. Therefore, the degradation of CBHs to the cellulose crystal region should be avoided during enzymatic hydrolysis. The three components of cellulase are separated from each other by pretreatment. Amorphous regions of cellulose are maintained for EG hydrolysis, while additional crystalline regions are preserved during enzymatic hydrolysis.

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3. Advanced functional applications of nanocellulose

3.1 Modifier of wood adhesives

Wood adhesive is the one of the most important components in the manufacture of wood-based composites such as medium-density fiberboards (MDF), particle boards (PB), and plywoods, which directly determines the comprehensive properties of panels. Urea-formaldehyde (UF), phenolic-formaldehyde (PF), and melamine-urea-formaldehyde (MUF) adhesives are widely used in the wood industry. Several studies reported chemical modification of wood adhesive but few described modifications of nanocellulose [54]. Surface morphology, chemistry, and adhesive properties affect the interfacial bonding and sufficient cross-linking reaction between wood and adhesives during the curing process. Nanocellulose with an inherent advantage of high stiffness and specific surface area at an appropriate amount improves the viscosity and stiffness of the wood adhesives. Nanocellulose can also fill the rough surface and hole in wood-unit surface, resulting in decreased porosity [55]. Nanocelluloses generated via various methods of isolation and modification exhibit differential effects on the bonding between wood and adhesive. The entangled microfibrillated cellulose (MFC) was prepared by a strong mechanical shearing resulted in high viscosity of adhesive [56]. TEMPO-CNF with long fibrillated morphology and negatively charged surface improves the CNF dispersion in the polar adhesive. Surface wettability of wood adhesive can be altered by various levels of energy dissipation with the addition of the modified nanocellulose. Aminopropyltriethoxysilane modification effectively reduced the surface energy of CNC, leading to a remarkable increase in the contact angle of CNC and urea formaldehyde resin [57].

Stress concentration at the bonding interface of wood and brittle adhesives (UF, PF, and MUF) is high corresponding to the density of cross-linked methylene and the degree of crystallinity. The reaction of hydroxyl groups and methylol groups in nanocellulose and UF promotes the ductility of adhesives. The crystalline regions improve the hydrolytic stability, resulting in reduced release of formaldehyde. The formaldehyde emission can be reduced by 13% by adding the modified CNCs at optimum levels (1 wt%) [57]. The major challenge for the adhesive modification by nanocellulose is the suspension of nanocellulose during the synthesis [58]. The higher the content of nanocellulose suspension incorporated, the higher the content of water in the adhesive, which reduces the solid content, resulting in slow curing process. The UF resin can be synthesized in various reaction conditions including weak acid, alkaline, and strong acids. Therefore, positively charged or uncharged nanocellulose under strong acid environment is essential due to the easy agglomeration of TEMPO-CNF at pKa = 3.50 via hydrogen bonding. The addition of nanocellulose or other cross-linkers such as poly(vinyl alcohol) effectively promotes the adhesive properties [59]. This provides a novel way to produce eco-friendly wood adhesives and reduce the use of petroleum-based polymers in wood adhesives in the future.

3.2 Novel energy storage application

Nanocellulose for novel energy storage application has received considerable attention due to its inherent structure and properties. Lithium-ion batteries (LIBs) with excellent properties including high energy density, non-memory effect, long-life cycles and low self-discharge rates, are key contributors to green energy storage [60]. Carbonaceous materials pyrolyzed by various precursors are the most utilized anodes industrially in lithium-ion batteries, resulting in low cost and various allotropic forms and morphologies [61]. Most anodes derived from natural cellulose are based on modified or synthetic porous carbon using chemical solvents or metals.

Nevertheless, directly pyrolyzed natural cellulose is an alternative candidate with acceptable electrochemical performance. The CNF carbon as anode in LIBs exhibited higher capacity but a weaker rate compared with CNC carbon. The disordered carbons used as LIB anodes were obtained by pyrolysis of microcrystalline cellulose in the temperature range of 950–1100°C [62]. Natural bacterial cellulose-based carbon treated by freeze-drying and carbonization showed good properties because of low charge transfer resistance and high specific surface area. To effectively promote the electronic conductivity of carbonaceous materials, nitrogen (N) doping in carbonaceous materials with similar atomic radii leads to desirable lattice mismatch, improves interfacial characteristics of the electrolyte and electrode, which facilitate electrical conductivity [63]. Unlike the traditional petroleum-based precursors from melamine or polyacrylonitrile for synthesizing N-doped carbons, novel chitosan (CS) contains 40% carbon and 8% nitrogen [64]. The N-doped porous carbon anodes derived from chitosan for the supercapacitance were successfully prepared by fine-tuning the hydrothermal carbonization parameters [65].

The natural nitrogen-doped porous carbon anode in LIBs was prepared by CS/CNC biocomposites using a facile procedure [60]. The N-doping content, the interfacial compatibility, and the pyrolysis temperature have a synergistic effect on the electrochemical performance of anodes. The outstanding cycling stability and coulombic efficiency of anodes are found at a pyrolyzed temperature of 1200°C. The addition of CNC has a positive effect on the rate retention and cycling performance. The CS/CNC anodes at the ratio of 10 wt% have high porosity, C/N ratio, and the multiscale pore distribution, resulting in an average specific capacity of 333 mAh g−1 at 100 mA g−1, retention capacity of 251 mAh g−1 at 2000 mA g−1, and almost constant capacity of 327 mAh g−1 after cycling. The CE/CS composite nanofibrous mats were obtained by single-nozzle and coaxial-nozzle electrospinning and then pyrolyzed at 900°C. The porous structure of carbon nanofibrous mats releases partial mechanical stress generated by the insertion and extraction of lithium ions. Their rough surface was conductive to the formation of solid electrolyte film, reducing the co-intercalation of solvent molecules and improving the cycling stability. The conventional carbon nanofibrous mats with a CE/CS ratio of 5:5 showed optimum electrochemical performance including a good rate performance (399 mAh g−1 at 30 mA g−1), a high specific capacity (327 mAh g−1 at 100 mA g−1), and an excellent cycling stability after 300 cycles. The carbon nanofibrous mats with the core-shell structure (CE as core and CS as shell) and the same ratio of CE/CS had higher pyridinic-N and pyrrolic-N content but a lower degree of graphitization and specific surface area in comparison with the common one. The N was more uniformly doped into the carbon nanofibrous mats via single-nozzle electrospinning compared with the coaxial technique, leading to a promotion for comprehensive electrochemical performance [66].

The separator of LIBs has two major functions: (1) preventing the direct contact of two electrodes; (2) enabling lithium-ion transportation by the electrolyte reservoir. It plays a significant role in LIB performance, such as cycle life, safety, and power density. The CNF film as a separator for LIBs exhibits high mechanical strength, thermal stability, and electrolytic property. Further, the CNF separator effectively promotes electrolyte wettability, ionic conductivity, and dimensional stability compared with commercialized separators fabricated from polypropylene/polyethylene/polypropylene. The CNF separators derived from Cladophora showed a small pore size but high Young’s modulus and ionic conductivity, corresponding to 5.9 GPa and 0.4 mS cm−1, respectively [67]. The CNF separators displayed high thermal stability under 150°C and were electrochemically inert in the potential range of 0–5 V compared with Li+/Li. The LiFePO4/Li cell with a CNF separator exhibited an excellent cycling performance (99.5%) and capacity retention [68].

3.3 Reinforcement of polymers

3.3.1 Reinforcement of thermoplastic resins

The role of nanocellulose in reinforcing polylactic acid (PLA)-based composites has been studied in recent decades. Most studies report that the mechanical strength and elastic modulus are improved after the incorporation of appropriate levels of nanocellulose. The poor interfacial compatibility between the nanocellulose with hydrophilic groups and the thermoplastic resins with hydrophobic groups limits the addition of nanocellulose in PLA. The ideal addition level of nanocellulose required to reinforce polymers is only 0.5–2 wt%. The surface properties of nanocellulose are generally modified by chemical or physical methods, which improved their dispersion and compatibility in PLA matrix. The CNC-graft-PLA/PLA nanocomposites modified by dicumyl peroxide were fabricated via reactive extrusion with a high grafting efficiency of 66% [69], which effectively promoted the interfacial bonding between CNCs and the PLA matrix, as well as the thermal stability of composites. The tensile strength of the composites increased by 40%, and its Young’s modulus significantly rose by 490%.

Compared with the rigid CNCs, CNFs with flexible long chains exhibit possible entanglement with the resin matrix. A novel micro encapsulation-mixing and melt-compression technique to obtain CNF-reinforced PLA composites with uniform dispersion was reported by Wang and Drzal [70]. The PLA microparticles were obtained via solvent evaporation and mixed with CNFs generated by HPH. The dense PLA/CNFs composite sheets were prepared by membrane filtration and compression molding. This approach prompted the dispersion of the CNFs in the matrix even at high levels (32 wt%), which increased the modulus by 58% and the strength by 210%, respectively. The introduction of CNFs alters the crystallization of the PLA. Homogeneous and stable crystals can be obtained by increasing CNF loading, where CNFs act as the nucleating agents to accelerate crystallization of PLA particles and reduce the cold crystallization temperature [71]. An electrospinning technique incorporating CNCs with PLA improved the dispersion of CNCs in the composites [72]. A CNC loading of 2–3 wt% can effectively increase the elastic modulus by 17% and tensile strength by 14%, as well as contribute to stable ductility of the CNC/PLA composites.

3.3.2 Reinforcement of thermosetting resins

Phenol-formaldehyde (PF) resin is a typical and widely used thermosetting polymer. The mechanical properties of micro- and nanocellulose fibers as reinforcements for PF composites were compared [73]. The nanocomposite showed better mechanical properties than the micro-composites. The CNFs facilitate stress transfer in composites. The hydrophilicity of CNF enhanced the interfacial compatibility between CNF and PF. The CNFs immersed and dispersed in PF before polymerization, resulting in strong bonding in the outer layer of CNFs corresponding to strong fiber-matrix adhesion, which explains the higher strength of the CNF-reinforced PF composites compared with cellulose microfibril-reinforced PF composites. The commercial glass fibers with addition amount of 20 wt% promote the mechanical strengths of PF. However, the incorporation of natural CNF into the PF significantly elevated the tensile strength, flexural strength, and impact strength of CNF/PF composites by 142, 280, and 133%, respectively, at low CNF loading [73, 74]. The TEMPO-CNF/PF composites were prepared by a three-step procedure including mixing of CNFs and resin, ultrafiltration, and hot molding. The special processes of ultrafiltration and drying effectively adsorbed the moisture and then released the residual stress of the composite films. The obtained films were flexible with an average elongation at break of 13–17%. The neutral pH, the high polymerization of CNF, and the special composite method may contribute to the excellent mechanical strength of the TEMPO-CNF/PF composites. The TEMPO-CNF/PF composite flexible films were obtained via impregnation of TEMPO-CNF with aqueous PF resin, which yielded a tensile stress and toughness of 248 MPa and 26 MJ m-3, respectively [75].

3.4 Adsorption treatment of wastes

The traditional activated carbon and zeolites are two most widely used adsorbents for the wasted water and air treatment. Nanocellulose, as a potential bio-adsorbent with the advantages of sustainability, nontoxicity, biodegradation, tiny size, large specific surface area, and the numerous choices to surface functionalization, has attracted tremendous interests in environmental remediation including metal ions, anionic and cationic dyes, and air pollutants.

The CNFs modified by carboxylate groups have been proved to be an effective adsorbent to several metal ions such as Pb2+, Cd2+, and Ni2+, even the radioactive UO22+ [76], which is attributed to the attraction of negative charges [77]. The maximum adsorption capacity (167 mg g−1) for UO22+ can be achieved with a three times higher than other popular adsorbents including montmorillonite, silica, and hydrogels. The CNFs modified by 3-aminopropyltriethoxysilane showed a positive effect on adsorbing Ni2+, Cu2+, and Cd2+ in solutions [78]. The adsorption performance depends on the pH range. The mercerized nanocellulose modified by succinic anhydride can quickly adsorb Zn2+, Cd2+, Cu2+, Ni2+, and Co2+ above 50% within 5 min. The maximum adsorption capacities correspond to 1.6, 2, 2, 0.8, and 1.3 mmol g−1 [79]. CNC is also proved to be available for adsorption of Cd2+, Ni2+, and Pb2+ but corresponding to a low adsorption capacity of 8.55, 9.42, and 9.7 mg g−1 in 25 mg L−1 aqueous solutions [80]. The adsorption capacities of Ag+, Cu2+, and Fe3+ by CNC are 56, 20, and 6.5 mg g−1. The modification of CNC using phosphate groups effectively improved the adsorption capacity for several metal ions, corresponding to 136, 117, and 115 mg g−1, respectively [81]. The modification effect of CNC by succinic anhydride on the adsorption of Pb2+ and Cd2+ was investigated. There was a about 10-fold adsorption capacity higher for modified CNC than the control one. The CNC adsorbent can regenerate using a saturated sodium chloride solution without any loss of adsorption capacity after two recycles [82].

CNF and CNC were also investigated in the adsorption of anionic and cationic dyes. CNF obtained from kenaf by a combination of acidified-chlorite pretreatment, which was proved to have a good adsorption capacity on methylene blue (MB). The parameters for maximum adsorption capacity (123 mg g−1) are at 20°C and pH of 9 within 1 min. CNF can be regenerated by an acidic medium and has a six adsorption-desorption available cycles [83]. CNCs can also remove the MB from aqueous solutions. The maximum adsorption equilibrium capacity fitted by Langmuir and Freundlich isotherm models was 118 mg dye g-1 when the temperature and pH were set at 25°C and 9. The TEMPO-oxidized CNC corresponds to a higher adsorption capacity (769 mg dye g-1) [84]. Sodium-periodate-oxidized CNC modified by ethylenediamine has an effective adsorption capacity for acid red, light yellow K-4G, and Congo red 4BS anionic dyes corresponding to the adsorption capacities of 135, 183, and 200 mg g−1, respectively [85].

In addition, the toxic hydrogen sulfide (H2S) adsorption performance was also studied by modified microfibrillated cellulose, such as aminopropyltriethoxysilane (APS) and hydroxycarbonated apatite (HAP) [86]. The H2S adsorption capacity corresponding to 103.95 and 13.38 mg g−1 for APS/MFC and HAP/MFC can be achieved when the initial concentration was 80 mg L−1. Compared with activated carbons, montmorillonites, silica-aluminas, and mixed zinc/cobalt hydroxides with the maximum adsorption capacities of 2–70, 0.5–12, 117–207, and 24–228 mg g−1 [79], APS/MFC is considered a promising bio-adsorbent.

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4. Conclusions

Nanocellulose shows a great potential and brilliant prospect to be a novel functional bio-product with many inherent advantages including excellent mechanical properties and thermal stability, high specific surface area, available tailorability, and other special properties in the future of wood industry and its industrial cross fields, such as wood construction, wood adhesives, fabrication and reinforcement of composites, green energy storage and adsorption system. The nanocellulose-based composites with the functional polymers, inorganics, and metals are also a promising direction for the developing green novel bio-products with the low or even zero carbon footprint in the future. However, the most important issue on nanocelluloses is its manufacture on a large scale due to the limited transition technology from lab-scale products to industrial production. Future works should focus on optimizing the industrial process and continuously developing new or combining methods to produce nanocellulose with the low energy consumption and high efficiency. Also, the life cycle assessment, regulation, and standardization of nanocellulose for safety and environmental properties will be necessary for next commercialization.

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Acknowledgments

This work is supported by the following grants and programs: 1. Yunnan Provincial Applied and Basic Research Grants (202201AT070058, 2019FB067); 2. National Natural Science Foundation of China (32060381); 3. the High Level Innovative One-Ten-Thousand Youth Talents of Yunnan Province (YNWR-QNBJ-2020-203); 4. 111 Project (D21027).

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Conflict of interest

There is no conflict of interest in this field.

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Written By

Kaimeng Xu, Yu Chen, Guanben Du and Siqun Wang

Submitted: 31 May 2022 Reviewed: 13 June 2022 Published: 07 July 2022