Methods of Production of Nanocellulose and Nanofibrillated Cellulose
Since nanocellulose is a material of great interest nowadays and is intensely studied, a number of techniques of extraction were introduced. All the methods lead to different types of nanocellulosic materials, depending on the origin of cellulose pulp and its pretreatment (Abdul Khalil et al. 2012). The chemical, biological, and mechanical treatments are implemented in different combinations. We describe briefly some of the methods below.
Chemical and Biological Treatments
In order to obtain CNC, acid hydrolysis of the cellulose pulp in concentrated sulfuric acid is commonly applied (Table 4.1). However, hydrochloric, phosphoric, and hydrobromic acids are also used to extract CNC. The acid contributes not only to the isolation of CNC but also to the dispersing of them due to the esterification (Phanthong et al. 2018). After neutralization of the suspension, it may be subjected to mechanical treatment to improve the separation of nanocrystals. In most cases, sonication is implemented for this purpose (Kettunen 2013).
Enzymatic hydrolysis is a frequently used biological technique to obtain CNF (Table 4.1). Wood pulp is slowly degraded in mild conditions with one or more enzymes, for example, endoglucanases, which attack only amorphous regions of fibre; after that it may be subjected to homogenization or another mechanical treatment. Enzymatic hydrolysis is usually used in a combination with other methods in order to accelerate the process.
Cellulose nanofibrils could be prepared by oxidation processes, for example, by using TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl radical) or its derivatives as a catalyst and hypochlorite as an oxidant in different media. The method may consist of one or several steps in which lignocellulose is treated by oxidizing agents to remove both lignin and hemicelluloses and to obtain oxycellulose simultaneously (Duran et al. 2012). TEMPO-oxidized cellulose nanofibres mostly have uniform width (3^1 nm) with a high aspect ratio (Phanthong et al. 2018).
Carboxymethylation is one more chemical pretreatment for obtaining CNF. The purpose is to introduce anionic charges on the fibril surface to increase the electrostatic repulsion between the fibrils, and thus facilitate the fibrillation. Etherification is carried out with monochloroacetic acid and NaOH in an alcohol media. After carboxymethylation. the fibres are homogenized using one of the mechanical methods (Kettunen 2013).
The mechanical high shear force techniques are used to cleavage the cellulose fibres in longitudinal axis to extract CNF. The main disadvantage of these methods is the high level of power consumption. High pressure homogenization is one of the conventional methods when impact and shear forces in fluid are generated to cleavage cellulose microfibrils into nanometre size in diameter (Phanthong et al. 2018).
The treatment of lignocellulosic biomass with short-time high pressure steam, followed by decompression (explosion) leads to fiberization or ‘mulching’ of fibres. As a result, CNF is obtained. The pressure increases and drops resulting in a substantial break down of the lignocellulosic structure, hydrolysis of the hemicellu- loses fraction, depolymerization of the lignin components, and defibrillization (Abdul Khalil et al. 2012).
In a number of studies, a combination of chemical and mechanical methods is applied. One of the examples was described in the paper Phanthong et al. (2018). High-intensity ultrasonication process implements the hydrodynamic forces of the ultrasound. After the removal of lignin and hemicelluloses by chemical methods, the samples are rinsed in water until the residues get neutralized, and then the ultrasonic treatment is carried out. The mechanical oscillating power is produced resulting in the formation, expansion, and implosion of microscopic gas bubbles when the liquid molecules absorb ultrasonic energy.
One of the popular methods, especially before the 2000s, is CNF production by ball milling. When the centrifugal force creates the shear forces between balls and the surface of a dish, the cellulose fibrils are split into the smaller size in diameter reaching nanometre sizes (Phanthong et al. 2018).
Direct Acid Hydrolysis of Waste Paper
In recent years, the viability of the direct production of nanocellulose from recycled paper was proved by numerous reports. The first approach to extract nanocellulose is acid hydrolysis. This method requires such pretreatment as mercerization and bleaching. The aim of the alkali treatments is to ensure the hydrolysis of hemicelluloses and removal of undesirable amorphous type polymer components, while the bleaching treatment is primarily aimed at removing lignin (Cherian et al. 2010; Li et al. 2009; Ndazi et al. 2007; Danial et al. 2015).