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Nanofibers

These have two dimensions in the nano range, and those of most interest for polymer applications are nanocellulose and the nanocarbon materials, carbon nanofibers and nanotubes. Asbestos and halloysite are examples of inorganic nanofibers.

Nanocellulose

Cellulose is a highly crystalline, high molecular weight, linear polymer and is present in many plants in the form of microfibrils. There are a number of different crystal forms and plant (native) cellulose is a mixture of two of the polymorphs, cellulose Ia and cellulose Ip. Cellulose provides much of the strength and stiffness of the plant. Plant fibers themselves are cellular biocomposites designed by nature and are essentially an amorphous matrix made principally of hemicellulose, lignin, waxes, and trace elements, reinforced by the cellulose microfibrils. These plant fibers are themselves cemented together by an intracellular ligneous material. The cellulose microfibrils typically have a diameter of about 2-20 nm and are made up of 30-100 cellulose molecules in an extended chain conformation.

The reinforcing ability of cellulose fibrils is ascribed to their crystalline nature and the extended chain conformation of the cellulose molecules. This particular conformation results from strong intermolecular hydrogen bonding caused by the high density of hydroxyl groups present in the cellulose molecule. The specific strength is significantly higher than steel.

The extraction of cellulose nanocrystals from cellulosic fibers is similar to that described for starch nanocrystals and usually involves an acid-induced destructuring process, involving hydrolysis of gylcosidic bonds. The hydrolysis step is followed by procedures such as centrifugation, dialysis, and ultrasonication. Various acids can be used for the hydrolysis, including sulfuric, hydrochloric, phosphoric, and nitric acids. Based on papermaking technology, hydrolysis with sulfuric acid has been most investigated and appears to be the most effective method. If hydrochloric acid is used, then the ability of the resulting nanocrystals to disperse in solvents is limited and the suspension is unstable, with a tendency to flocculate. On the other hand, if sulfuric acid is used for the hydrolysis, it reacts with the surface hydroxyl groups via esterification, with the formation of anionic sulfate ester groups (OSO3~). The presence of these negatively charged groups provides an electrostatic layer around the nanocrystals and promotes their dispersion and stability in water. However, as for starch, the sulfate ester groups also compromise the thermal stability of the nanocrystals. Neutralization of the sulfate ester groups by sodium hydroxide (NaOH) can be used to eliminate this problem.

The nomenclature used to describe acid-hydrolyzed crystalline cellulose nanoparticles has been uncontrolled until recently, with terms such as cellulose whiskers or nanowhiskers and nanocrystalline cellulose being employed. TAPPI (Technical Association of the Pulp and Paper Industry) has now proposed a standardized terminology - cellulose nanocrystal (Standard Terms and Their Definition for Cellulose Nanomaterial WI 3021).

The nanoparticles occur as high aspect ratio rod-like nanocrystals. Their actual dimensions depend on the plant origin and hydrolysis conditions. The length distribution is quite broad, due to the diffusion-controlled hydrolysis process. The aspect ratio varies with the plant from about 10-70.

Like starch, the use of cellulose nanocrystals in polymer composites is currently limited by cost and processing issues and the “green” and sustainable badge is compromised by the chemical processing needed to extract the crystals and by the amount of co-product which needs to be utilized.

The processing problems are those common to many solution processed fine particles; the need to isolate them in a redispersible form for most polymer uses. Most progress has been made with direct transfer into a polymer from an aqueous dispersion of the cellulose nanocrystals. This can be achieved by mixing an aqueous dispersion of the cellulose nanocrystals with either water soluble polymers or polymer aqueous dispersions (lattices). Correctly implemented, this method preserves the original dispersion of the nanoparticles, and has been used extensively in the academic work. For commercial purposes, natural rubber latex or styrene butadiene rubber latex are of most interest. Solution casting techniques have also been used with some success in the laboratory, but are not suited to industrial applications. Producing a dry powder suitable for melt processing or adding to bulk rubbers on a commercial scale is far more difficult and still has not been achieved. Problems include thermal degradation and poor dispersion. The dried particles agglomerate due to hydrogen bonds between nanoparticles. Functionalizing the surface of the cellulose nanocrystals before drying can help with the agglomeration and dispersion issues.

Commercial development of nanocellulose is at a very early stage. A large pilot plant was opened in 2013 at the University of Maine (USA). Another pilot plant came on stream in Mumbai, India, in 2014 (at the Central Institute for Research on Cotton Technology, CIRCOT).

More details can be found in the reviews by Miller and Hobbie (2013) and Mariano et al. (2014). Cellulose fillers in general are also discussed in ? Chaps. 16, “Fillers from Organic Sources,” and ? 22, “Sustainable and Recycled Particulate Fillers.”

 
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