Qinglin Wu

During the past decade, nanocellulose has attracted considerable attention because of its unique physical and chemical properties and the growing interest in the bioconversion of renewable lignocellulosic biomass. Nanocellulose refers to cellulose broken down to the level of molecules and atoms. Substantial academic and industrial interests have been directed toward the potential applications of nanocellulose for various fields, including high performance composites, electronics, biomedical materials and energy.

What is nanocellulose?
Cellulose, one of the world’s most abundant, natural and renewable biopolymer resources, is widely present in various forms of biomass, such as trees, plants, tunicates (marine invertebrate animals) and bacteria. In plant cell walls, approximately 36 individual cellulose molecule chains connect with each other through hydrogen bonding to form larger units known as elementary fibrils, which are packed into larger microfibrils. These microfibrils have disordered (amorphous) regions and highly ordered (crystalline) regions (Figure 1). In the crystalline regions, cellulose chains are closely packed together by a strong and highly intricate network, while the amorphous domains are regularly distributed along the microfibrils. When the amorphous regions of cellulose microfibrils are selectively hydrolyzed, these microfibrils break down into shorter crystalline parts, which are generally referred to as nanocellulose (Figure 1). Nanocellulose is further divided into short rod-shaped cellulose nanocrystals (CNCs) and longfiber- shaped cellulose nanofibers (CNFs).

Nanocellulose has nanoscale dimensions and excellent mechanical properties. Its tensile strength is stronger than steel and many other materials (Figure 2). Because of an abundance of hydroxyl groups on its surface, nanocellulose can be modified to accomplish many functions, including polymer grafting, which could successfully facilitate the incorporation and dispersion of nanocellulose into different polymer matrices.

Nanocellulose production
The production of nanocellulose is generally carried out in two steps. The first step consists of pretreatment of the raw material like wood chips to obtain purified cellulosic fibers, an essential pulping process for paper making. The pretreatments can be chemical, thermal, mechanical, biological or a combination of the processes. The second step consists of transformation of the purified cellu losic fibers into microfibrils (CNFs) or short crystals (CNCs).

The main processes typically used are mechanical treatment, acid hydrolysis, and enzymatic hydrolysis. The treatments can be applied separately or combined. For example, enzymatic or acid hydrolysis pretreatments followed by mechanical treatments are normally carried out to enable quick disintegration of cellulosic fibers. The pretreatments allow opening the fiber structure to facilitate access to the cellulose microstructure for effective processing. Thus, a sufficiently selective pretreatment helps increase the access to the microsites and still maintain a desired degree of polymerization in the cellulose chain.

Nanocellulose products
As a green building block, numerous products can be derived from nanocellulose. Some of the emerging products include additives for paints, varnishes, coatings, adhesives, drilling and other fluids, polymers (thermosets and thermoplastics), hydrogels, cosmetics and pharmaceuticals. Products that can be derived from nanocomposites include optical devices, nanofilters, highstrength yet lightweight body armor, fuel-efficient yet super-durable vehicles, medical devices, bendable battery systems, flexible electronic displays and biofuels. Figure 3 shows some potential markets for CNCs. Among the products that CNCs will play an important role in developing are fluid loss control agents in the next generation of smart drilling fluids.

Nanocellulose film and membranes are being considered as good candidates to replace traditional polymer and glass substrate in energy devices. A new scientific term, green electronics, has been created with the goal of helping to produce eco-friendly electronics. Figure 4 shows a new generation of a flexible Li-Ion battery based on nanocellulose technology. The new architectural design provides unprecedented advances in the electrochemical performance. It also provides shape flexibility and safety tolerance far beyond those achievable with conventional battery technologies for development of next-generation energy storage systems.
 
Eco toxicological aspects
Nanomaterials such as nanocellulose have unusual physical and mechanical properties, which are not found in the bulk materials. However, because of their small size, there could be a range of potential health hazards, known as nanotoxicity. For nanocellulose materials, an effort has been made to investigate their safety in terms of both the environment and living beings, as well as possible modifications of production techniques that can reduce any toxicity present. For example, using a 3-D in vitro model of the human epithelial airway barrier, it was observed that cellulose nanofibers isolated from cotton elicited a significantly lower cytotoxicity and pro-inflammatory response than multiwalled carbon nanotubes and asbestos fibers. Thus, nanocellulose is generally considered as nontoxic as table salt.

Standardization
As government, academic and industrial research and development organizations endeavor to understand and develop nanocellulose-based products for multiple industrial sectors, there is a great need for international standards for cellulose nanomaterials. A committee within the International Nanotechnology Division (INSCC) of TAPPI, an international paper and pulp technical organization, has been formed to help guide and coordinate standards development worldwide.

Future perspectives
Nanocellulose with its lightweight, high-strength and transparent properties is of great interest for many applications in a wide variety of areas. Nanocellulose is quickly becoming available commercially worldwide. The manufacturing emphasis is placed on rapid scaleability. Research efforts focus on new applications and ensuring safety of the new materials in the environment, the workplace and the products.

Nanocellulose will compete with petrochemical products, and their industry has nearly a century of development experience. Partnerships among the forest products industry, the petrochemical industry and the manufacturing industry are the key to introduce “green” nanocellulose environmentally sustainable to a large consumer market with competitive cost and performance.

Qinglin Wu is the Roy O. Martin Sr. Professor in Composites and Engineered Wood Products in the School of Renewable Natural Resources.

(This article was published in the Winter 2015 issue of Louisiana Agriculture.)