Types of nanocellulose

Cellulosic source, processing conditions, functions, dimensions, and pretreatment methods have great influence on the properties and type of nanocellulose. There are three main subcategories of nanocellulose microfibrillated cellulose, nanociystalline cellulose, and bacterial nanocellulose. 

Microfibrillated cellulose [MFC], discovered by Sandberg in the late 1970s, is produced from suspensions of wood-based cellulose, called pulp. The pulp is mainly obtained by using a mixture of sodium hydroxide and sodium sulfide (kraft pulp] or by using salts of sulfurous acid [sulfite pulp]. The former consists of almost pure cellulose fibers, while latter contains more by-products. MFC 

In water, microlibrils of cellulose exhibit gel-like characteristic with pseudoplastic and thixotropic properties (Fig. 21.10). The viscosity of MFC gels decreases with shearing, which is very important from the industrial processing point of view [24,25]. 

Production of MFC has one serious drawback It is an energyconsuming process. During the first attempts of MFC production, a gel-like MFC from a sulfite-pulp suspension with high hemicellulose content about 27,000 kWh per ton of MFC was needed. Today, a combination of enz5miatic treatment and refining of sulfite pulp enables reducing consumption of energy even to 1500 kWh/ton MFC [24]. 

A wide range of patents related to MFC and its both, rheological and colloidal, properties were published nevertheless, there are still not many large-scale commercial applications of MFC. The paper industry tries to use MFC in, e.g., greaseproof paper. Moreover, there are studies proving that MFC can be blended with rubber latex in order to produce rubber products with enhanced mechanical properties. MFC is also of interest to the cosmetic, pharmaceutical, and sanitary industries [24]. 

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As described before, one type of nanocellulose is formed directly as the result of biosynthesis of special bacteria. A very pure product with subsequently reported important properties is formed that necessitates challenging biosynthesis/biotechnological handling and the development of large-scale production. 

The second chapter by Dieter Klemm, Dieter Schumann, Hans-Peter Schmauder, and coworkers focuses on the recent knowledge of cellulosics characterized by a property-determining supramolecular nanofiber structure. Topics in this interdisciplinary contribution are the types of nanocelluloses and their use in technical membranes and composites as well as in the development of medical devices, in veterinary medicine, and in cosmetics. 

There are four types of nanocellulose depending on their fabrication method bacterial cellulose, electrospun cellulose, microfibrillated cellulose (MFC) and whiskers of cellulose (nanorods). Bacterial cellulose is a nanomaterial derived from plant material by bacterial action in the presence of oxygen. Various strains of Acetobacter species [82,83] can be used to produce cellulose, although strains of pseudomonas, Achrobacter, Alcaligene, Aerobacter and Azotobacter [84] can also be used. The bacterial cellulose... 

Kose and Kondo studied tbe size effects of cellulose nanofibres on the crystallization behaviour of PLA. They discovered that the smaller size of cellulose nanofibres on tbe nanoscale does not necessarily make a better nucleating agent for PLA. Table 9.1 summarizes the Avrami kinetic parameters for the isothermal crystallization of the PLA and PLA biocomposites with different types of nanocelluloses as compared to PLA composites with talc and nanoclay. With the addition of unmodified and silylated CNCs as nucleating agents, the t 2 value increases with increasing T similar to that of nanoclay and com starch, but opposite to that of talc. Comparing the... 

Nanocellulose and its derivatives can be processed into different forms. Bacterial celulose is one such important type of nanocellulose. It has been processed into nanofibers for different applications. Figure 1.8 shows the different applications of cellulose acetate nanofibers [51]. Chapters 2 and 4 discuss the different perspectives of bacterial cellulose-based materials and their different applications. In these chapters the authors discuss in detail a vast collection of BC nanocomposites prepared using different polymer matrices such as natural polymers and thermoplastic matrices. In addition to this,... 

During the last two decades micro/nanocellulose-reinforced composites have been the subject of intensive research and a number of review papers have appeared covering this work [14, 17, 19, 24, 53, 173,174], Nanocellulose either in CNC or NFC form will result in varying reinforcement of nanocomposites. Also, different types of nanocellulose can be used in various forms of reinforcement, including distributed reinforcements, planar reinforcements, or continuous networked structures. 

Both types of nanocelluloses may show different properties as a nanoreinforcement. In this case, Xu et at performed a comparative study between CNCs and CNFs regarding their morphology, crystalline structure, dispersion properties, and reinforcing effect in polyethylene oxide (PEO) polymer matrix. These are nanoscale cellulose fibers having different shape, size, and compositions. 

Relatively speaking, NFCs are considered more ductile as compared to CNCs and can be applied as a more useful template for further functionalization [46]. For the systematic comparative study, the Halpin-Kardos and Ouali models were applied to simulate the modulus of the nanocomposites and, as predicted, the experimental values showed good agreements. Therefore, on the basis of type of nanocellulose, systematic comparative study can help to develop the criteria for selecting the proper nanocellulose as a biobased nanoreinforcement material in polymer nanocomposites [45]. Therefore, these nanocellulose types such as CNCs and NFCs have attracted a great interest in the field of industrial and biomedical applications. Therefore, CNCs are often referred to as microcrystals, whiskers, nanocrystals, nanoparticles, and microcrystallites. Hereafter, for convenience, they will be called "cellulose nanocrystals" (CNCs). 

A short summary of types of nanocellulose, its origin, and other features is presented in Table. 21.7. 

Type of nanocellulose Selected ref. and synonyms Typical sources Formation Average size... 

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