Production of cellulose nanofibrils: A review of recent advances

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.

/pdf/current-characterization-methods-for-cellulose-nanomaterials-1mwbef779r.pdf

Current characterization methods for cellulose nanomaterials

Chemical modification of cellulose nanocrystals is an increasingly popular topic in the literature. This review analyses the type of cellulose nanocrystal modification reactions that have been published in the literature thus far and looks at the steps that have been taken towards analysing the products of the nanocrystal modifications. The main categories of reactions carried out on cellulose nanocrystals are oxidations, esterifications, amidations, carbamations and etherifications. More recently nucleophilic substitutions have been used to introduce more complex functionality to cellulose nanocrystals. Multi-step modifications are also considered. This review emphasizes quantification of modification at the nanocrystal surface in terms of degree of substitution and the validity of conclusions drawn from different analysis techniques in this area. The mechanisms of the modification reactions are presented and considered with respect to the effect on the outcome of the reactions. While great strides have been made in the quality of analytical data published in the field of cellulose nanocrystal modification, there is still vast scope for improvement, both in data quality and the quality of analysis of data. Given the difficulty of surface analysis, cross-checking of results from different analysis techniques is fundamental for the development of reliable cellulose nanocrystal modification techniques.

https://pubs.rsc.org/en/content/articlepdf/2014/nr/c4nr01756k

Surface modification of cellulose nanocrystals.

Cellulose nanocrystals are unique nanomaterials derived from the most abundant and almost inexhaustible natural polymer, cellulose. These nanomaterials have received significant interest due to their mechanical, optical, chemical, and rheological properties. Cellulose nanocrystals primarily obtained from naturally occurring cellulose fibers are biodegradable and renewable in nature and hence they serve as a sustainable and environmentally friendly material for most applications. These nanocrystals are basically hydrophilic in nature; however, they can be surface functionalized to meet various challenging requirements, such as the development of high-performance nanocomposites, using hydrophobic polymer matrices. Considering the ever-increasing interdisciplinary research being carried out on cellulose nanocrystals, this review aims to collate the knowledge available about the sources, chemical structure, and physical and chemical isolation procedures, as well as describes the mechanical, optical, and rheological properties, of cellulose nanocrystals. Innovative applications in diverse fields such as biomedical engineering, material sciences, electronics, catalysis, etc, wherein these cellulose nanocrystals can be used, are highlighted.

/pdf/cellulose-nanocrystals-synthesis-functional-properties-and-4ej01axfo4.pdf

Cellulose nanocrystals: synthesis, functional properties, and applications.

Nanocellulose as a natural source for groundbreaking applications in materials science: Today’s state

Cellulose nanofibers have a bright future ahead as components of nano-engineered materials, as they are an abundant, renewable and sustainable resource with outstanding mechanical properties. However, before considering real-world applications, an efficient and energetically friendly production process needs to be developed that overcomes the extensive energy consumption of shear-based existing processes. This paper analyses how the charge content influences the mechanical energy that is needed to disintegrate a cellulose fiber. The introduction of charge groups (carboxylate) is achieved through periodate oxidation followed by chlorite oxidation reactions, carried out to different extents. Modified samples are then subjected to different levels of controlled mechanical energy and the yields of three different fractions, separated by size, are obtained. The process produces highly functionalized cellulose nanofibers based almost exclusively on chemical reactions, thus avoiding the use of intensive mechanical energy in the process and consequently reducing drastically the energy consumption.

Energy requirements for the disintegration of cellulose fibers into cellulose nanofibers

Electrosterically stabilized nanocrystalline cellulose (ENCC) was modified in three ways: (1) the hydroxyl groups on C2 and C3 of glucose repeat units of ENCC were converted to aldehyde groups by periodate oxidation to various extents; (2) the carboxyl groups in the sodium form on ENCC were converted to the acid form by treating them with an acid-type ion-exchange resin; and (3) ENCC was cross-linked in two different ways by employing adipic dihydrazide as a cross-linker and water-soluble 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide as a carboxyl-activating agent. Films were prepared from these modified ENCC suspensions by vacuum filtration. The effects of these three modifications on the properties of films were investigated by a variety of techniques, including UV–visible spectroscopy, a tensile test, thermogravimetric analysis (TGA), the water vapor transmission rate (WVTR), and contact angle (CA) studies. On the basis of the results from UV spectra, the transmittance of these films was as high as ...

Films Prepared from Electrosterically Stabilized Nanocrystalline Cellulose

A novel biopolymer-based aerogel was developed by freeze-drying a hydrogel, synthesized from cross-linking bifunctional hairy nanocrystalline cellulose and carboxymethylated chitosan through a Schiff base reaction. The nanocelluloses, bearing aldehyde and carboxylic acid groups, facilitated the cross-linking with chitosan through imine bond formation while providing negatively charged functional groups, and chitosan was modified to accommodate carboxylic acid. The potential of this bioaerogel in environmental remediation was examined in a model system comprising methylene blue, a cationic dye. Electrostatic complexation between acidic groups on the anionic aerogel with the dye resulted in time-dependent dye adsorption, with long-time equilibrium dye concentration fitting well to the Langmuir isotherm, yielding a maximum adsorption capacity of ∼785 mg g–1 and equilibrium constant K ∼ 0.0089 at room temperature. Dynamics of adsorption was modeled by numerically solving the unsteady-state diffusion–adsorptio...

Reusable Green Aerogels from Cross-Linked Hairy Nanocrystalline Cellulose and Modified Chitosan for Dye Removal.

Softwood cellulose pulp was oxidized by a two-step oxidation process with sodium periodate followed by sodium chlorite at pH 5.0. The oxidized product was first separated into two fractions by centrifugation, and the supernatant was further separated in two fractions by addition of ethanol and centrifugation. Different levels of oxidation were performed on cellulose, and the mass ratio and carboxyl content of each fraction were determined. The first precipitate, which amount decreases with increasing oxidation level, consists of short fiber fragments (microfibrils) with length of 0.6–1.8 μm and width around 120 nm, which for sufficiently high oxidation levels, could readily be made into cellulose nanofibrils by stirring. The second precipitate (after alcohol addition) has a very high crystalline index of 91 % and contains rod-like particles with length of 120–200 nm and diameter around 13 nm, reminiscent of nanocrystalline cellulose. The supernatant contains water-soluble dicarboxylated cellulose, as proven by liquid C-13 NMR.

Highly charged nanocrystalline cellulose and dicarboxylated cellulose from periodate and chlorite oxidized cellulose fibers

We produced novel nanocellulose particles made from cellulose fibers by periodate oxidation. For partial oxidation [degree of substitution (DS) <2], three products were generated after the periodate oxidized fibers were heat treated: fibrous cellulose, rod-like dialdehyde cellulose (DAC) nanofibers which we refer to as sterically stabilized nanocrystalline cellulose (SNCC), and dissolved DAC which is a copolymer of cellulose and DAC which we refer to as dialdehyde modified cellulose (DAMC). The products were separated by centrifugation and cosolvent addition. SNCC has similar dimension (100–200 nm in length and 5–8 nm in width) as conventional nanocrystalline cellulose (NCC) made by sulfuric acid hydrolysis. Several techniques were applied to characterize SNCC and its properties are compared to NCC. DAMC was found to be soluble in hot water or a few solvents (such as dimethyl formamide and dimethyl acetamide) at elevated temperature, but was insoluble in most common solvents at room temperature. The molecular weight of DAC (DS = 2) produced under various conditions (heating time and temperature) was determined by gel permeation chromatography. It was shown that the molecular weight decreased from 85.1 to 4.1 kDa with heating time and residence time when cooled down to room temperature.

Han Yang

Papers

Energy requirements for the disintegration of cellulose fibers into cellulose nanofibers

Films Prepared from Electrosterically Stabilized Nanocrystalline Cellulose

Reusable Green Aerogels from Cross-Linked Hairy Nanocrystalline Cellulose and Modified Chitosan for Dye Removal.

Highly charged nanocrystalline cellulose and dicarboxylated cellulose from periodate and chlorite oxidized cellulose fibers

Preparation and characterization of sterically stabilized nanocrystalline cellulose obtained by periodate oxidation of cellulose fibers