Handbook of Composites from Renewable Materials, Nanocomposites: Science and Fundamentals

Preface

The concept of green chemistry and sustainable development policy impose on industry and technology to switch raw material base from the petroleum to renewable resources. Remarkable attention has been paid to the environmental friendly, green and sustainable materials for a number of applications during the past few years. Indeed, the rapidly diminishing global petroleum resources, along with awareness of global environmental problems, have promoted the way to switch toward renewable resources-based materials. In this regards, bio-based renewable materials can form the basis for variety of eco-efficient, sustainable products that can capture and compete markets presently dominated by products based solely on petroleum-based raw materials. The nature provides a wide range of the raw materials that can be converted into a polymeric matrix/adhesive/reinforcement applicable in composites formulation. Different kinds of polymers (renewable/nonrenewable) and polymer composite materials have been emerging rapidly as the prospective substitute to the ceramic or metal materials, due to their advantages over conventional materials. In brief, polymers are macromolecular groups collectively recognized as polymers due to the presence of repeating blocks of covalently linked atomic arrangement in the formation of these molecules. The repetitive atomic arrangements forming the macromolecules by forming covalent links are the building block or constituent monomers. As the covalent bond formation between monomer units is the essence of polymer formation, polymers are organic or carbon compounds of either biological or synthetic origin. The phenomenon or process of polymerization enables to create diverse forms of macromolecules with varied structural and functional properties and applications. On the other hand, composite materials, or composites, are one of the main improvements in material technology in recent years. In the materials science field, a composite is a multiphase material consisting of two or more physically distinct components, a matrix (or a continuous phase) and at least one dispersed (filler or reinforcement) phase. The dispersed phase, responsible for enhancing one or more properties of matrix, can be categorized according to particle dimensions that comprise platelet, ellipsoids, spheres, and fibers. These particles can be inorganic or organic origin and possess rigid or flexible properties.

The most important resources for renewable raw materials originate from nature such as wood, starch, proteins, and oils from plants. Therefore, renewable raw materials lead to the benefit of processing in industries owing to the short period of replenishment cycle resulting in the continuous flow production. Moreover, the production cost can be reduced by using natural raw materials instead of chemical raw materials. The waste and residues from agriculture and industry have also been used as an alternative renewable resources for producing energy and raw materials such as chemicals, cellulose, carbon, and silica. For polymer composites applications, an intensifying focus has been directed toward the use of renewable materials. Bio-based polymers are one of the most attractive candidates in renewable raw materials for use as organic reinforcing fillers such as flex, hemp, pine needles, coir, jute, kenaf, sisal, rice husk, ramie, palm, and banana fibers which exhibited excellence enhancement in mechanical and thermal properties. For green polymer composites composed of inorganic reinforcing fillers, renewable resources-based polymers have been used as matrix materials.

Significant research efforts all around the globe are continuing to explore and improve the properties of renewable polymers-based materials. Researchers are collectively focusing their efforts to use the inherent advantages of renewable polymers for miscellaneous applications. To ensure a sustainable future, the use of bio-based materials containing a high content of derivatives from renewable biomass is the best solution.

This volume of the book series Handbook of Composites from Renewable Materials is solely focused on the Nanocomposites: Science and Fundamentals Some of the important topics include but not limited to preparation, characterization, and applications of nanomaterials from renewable resources; hydrogels and its nanocomposites from renewable resources: preparation of chitin-based nanocomposite materials through gelation with ionic liquid; starch-based bionanocomposites; biorenewable nanofiber and nanocrystal; investigation of wear characteristics of dental composite reinforced with rice husk-derived nanosilica filler particles; performance of regenerated cellulose/vermiculite nanocomposites fabricated via ionic liquid; preparation, structure, properties, and interactions of the PVA/cellulose composites; green composites with cellulose nanoreinforcements; biomass composites from bamboo-based micro/nanofibers; synthesis and medicinal properties of polycarbonates and resins from renewable sources; nanostructured polymer composites with modified carbon nanotubes; organic–inorganic nanocomposites derived from polysaccharides; natural polymer-based nanocomposites; cellulose whisker-based green polymer composites; poly (lactic acid) nanocomposites reinforced with different additives; nanocrystalline cellulose; halloysite-based bionanocomposites; nanostructurated composites based on biodegradable polymers and silver nanoparticles; starch-based biomaterials and nanocomposites; green nanocomposites based on PLA and natural organic fillers; and chitin and chitosan-based nanocomposites.

Several critical issues and suggestions for future work are comprehensively discussed in this volume with the hope that the book will provide a deep insight into the state of art of "Nanocomposites" of the renewable materials. We would like to thank the Publisher and Martin Scrivener for the invaluable help in the organisation of the editing process. Finally, we would like to thank our parents for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

University of Cranfield, U.K.

Manju Kumari Thakur, M.Sc., M.Phil., Ph.D.

Himachal Pradesh University, Shimla, India

Michael R. Kessler, Ph.D., P.E.

Washington State University, U.S.A.

Chapter 1

Preparation, Characterization, and Applications of Nanomaterials (Cellulose, Lignin, and Silica) from Renewable (Lignocellulosic) Resources

K.G. Satyanarayana1*, Anupama Rangan2, V.S. Prasad3 and Washington Luiz Esteves Magalhães4

1Poornaprajna Institute of Scientific Research (PPISR), Bangaluru, Karnataka, India

2Department of Pharmaceutical Chemistry, Vivekananda College of Pharmacy, Bangalore, India

3Chemical Sciences & Technology Division, National Institute for Interdisciplinary Science & Technology (NIIST-CSIR), Thiruvananthapuram, Kerala, India

4Department of Technology of Forestry Products, Embrapa Forestry, Colombo PR, Brazil

*Corresponding author: [email protected], [email protected]

Abstract

Safer ecological/environmental requirements have necessitated the use of renewable bioresources to address the issues of sustainability of the resources. In this perspective, biomass is attractive due to its abundance, renewability, and low cost. However, there are some limitations for industrial uptake of materials derived from biomass for structural and other applications. As the demand for developing functional materials increases, macro- to nanosize reduction of materials provides an alternative for varied applications presenting advantages in behavior and functionality. This has triggered development and use of nanomaterials along with the need to find new sources to produce them. While most of nanofibers from lignocellulosic materials refer to nanocellulose (NC), there have also been attempts to obtain nanolignin and nanosilica from wood and similar materials. Surface modification and functionalization of NC from various sources including natural fibers can lead to various nanomorphologies which have potential for application in storage and delivery of drugs and cosmetics. Lignin is the second most abundant natural renewable biopolymer. Recent advances in bioengineering and biotechnology have brought lignin into the limelight as a value-added product in spite of this being mostly regarded as an undesired by-product. Silica with high purity and amorphous nature has many industrial applications. With the progress of nanotechnology and increase in demand, several silica-processing industries have started producing nanosilica particles. Accordingly, this chapter presents preparation methods of cellulose, lignin and silica in nanoform, their characterization, and applications.

Keywords: Nanomaterials, biomass, cellulose, lignin, silica, processing, structures and properties, applications

1.1 Introduction

Nano-based manufactured goods and nanotechnology has been gaining increased attention in the recent times. Indeed, nanotechnology is significantly affecting design and use of many products and processes across varied fields of scientific research and industrial applications. Greater expectations have been put forth on various aspects of nano-related things (science of nanomaterials, nanotechnology including nano-manufacturing, etc.) not only in the academic community, but also among investors, governments, and industrial sectors (Serrano et al., 2009; Tuuadaij & Nuntiya, 2008; Lin et al., 2011a,b). Reasons for this are obvious in that nano-related materials and processes exhibit unique characteristics. Nanomaterials exhibit enhanced properties and performance, while the technology provides approaches to fabricate new structures at atomic scale (Thakur et al., 2012a,b, 2014a,b). Although there are some limitations for industrial uptake of materials derived from biomass for structural and other applications, the demand for development of functional materials is increasing probably due to reduction in the size of these materials below the normal micro level. This offers advantage in behavior and functionality exhibited by the nanosized materials based on biomass. One of the applications of nanotechnology in the area of biomass has been the development of nanocellulose (NC) in virtue of its super functionalities, such as its extremely large, active surface area, and low cost (Hubbe et al., 2008; Yano et al., 2005). Thus, new world of novel materials and devices has arrived showing greater application potential than that was possible hitherto with normal materials and processes. In fact, these nano-related materials have already attained industrial and economic reality. Worldwide annual sources of naturally occurring nanoparticles is estimated to be the lowest from biomass with about 1.8 million tons, compared to 16.8 million tons from mineral aerosol and 3.6 million tons from marine salts (Gaffet, 2011). This large measure of nanoparticles highlights their possible applications in a variety of fields aiming at manufacturing or modifying available material resources for a variety of technological uses (Senff et al., 2010).

While this is the status of emerging materials and technology, many countries are projected to face sustainability issues in the coming years. The diminishing natural sources coupled with the increasing demand for clean and safer energy alternatives have necessitated the development of novel approaches using biodegradable renewable resources. The idea of shifting to renewable resources to produce fuel and value added products from lignocellulose is being explored extensively. This is because lignocellulose is the most abundant renewable biomass on earth and is mainly composed of cellulose, hemicelluloses and lignin. Cellulose and hemicellulose fractions are polymers of sugars and are potential sources of fermentable sugars. Lignin can be used for the production of chemicals, low end products such as adhesives as well as for generation of heat and power applications (Harmsen et al., 2010). The overall objective for research in this field is to prepare the required biomaterials from agro-industrial lignocellulosic wastes like sugar bagasse, wood residues, agricultural residues etc. There is a dire need for efficient technologies to be introduced using these lignocellulosic wastes as they are abundant, inexpensive and offer a distinctive resource for large-scale and cost-effective technologies, besides meeting the industrial demands for renewable resource.

At a more fundamental level, lignocellulosic biomass is made up of nanometer-size constitutive building blocks that provide mechanical strength besides serving multiple functions. In nature there are many examples of such efficient and optimized systems that are based on nanotechnology (Avila-Olias et al., 2013). Indeed, some of the inherent properties of the naturally occurring biomaterial composites such as bone, teeth or the shells of marine animals can be directly correlated to the nanometer dimensions of their building blocks (Sarikaya et al., 2003). Thus, it is logical to explore the use and application of nanotechnology-based methods as a promising approach for efficient utilization of the lignocellulosic resources. However, the complex structure of lignocellulose where the carbohydrates such as cellulose and hemicelluloses are extensively cross-linked with lignin acts as a barrier for efficient degradation of the lignocellulosic wastes to obtain value added products. The prospect of obtaining nanounits by systematic breakdown of the larger biopolymer is still an emerging and challenging field that holds great promise of revolutionizing utilization of lignocellulosic materials. In this regard, NC has been extensively studied and it has been used for diverse applications (Charreau et al., 2013).

With the above background of safer ecological/environmental requirements leading to the demand for the use of renewable bioresources, nanosized cellulose, lignin, and silica can be tapped to address sustainability issues as they offer distinct advantages over other materials. This chapter presents an overview of preparation methods of cellulose, lignin, and silica in nanoform, their characterization, and applications. In addition, market aspects and perspectives for these nanomaterials based on lignocellulosic biomass are also mentioned.

1.1.1 Cellulose and Nanocellulose

Cellulose is the most abundant renewable organic polymer produced in the world. There are two reports giving different amounts of cellulose available with one giving at 1.5 × 10¹² tons of the total annual biomass production (Klemm et al., 2005), while the second gives an annual production of more than 7.5 × 10¹⁰ tons (Habibi, 2014; Habibi et al., 2009). Sources of cellulose include mainly plants, animals, bacteria, and other organisms being others. Higher production of cellulose comes from pulp mill industry, which produces it from solid wood through chemical digestion. Cellulose is thus not only widely available from renewable sources but its production has low carbon footprint and it is biodegradable. As an inexhaustible sustainable biopolymer, cellulose has remarkable chemical and physical properties that can be utilized for wide range of applications. One example of use of cellulose is as a raw material for various applications including the production of nanocellulose. It may be noted that the source of the cellulose, being renewable and biodegradable, is produced from solid wood through chemical digestion, acidic or alkaline process thus making its production with low carbon footprint. Thus, cellulose is widely available and a low cost material. Hence, cellulose is a fascinating material and almost inexhaustible sustainable polymer, possessing remarkable chemical and physical properties for possible applications in a wide range of materials. Furthermore, cellulose has been exploited as a raw material in chemical industry as paper, cellophane films, nitrocellulose explosives, textiles, dietary fibers, etc. for about the last two centuries. Cellulose fibers are being used in plastic-based composites for more refined applications as well.

Cellulose exists in nature as assemblies of individual fibers rather than as isolated molecules. This is due to the process of spinning of individual cellulose molecules in a hierarchical order during biosynthesis (Somerville et al., 2006). The biopolymerization, spinning, and crystallization occur in plasma membrane complex that has a diameter of 30 nm and by specific enzymatic terminal complexes (TCs) which act as biological spinnerets. Celluloses from different sources may exhibit different packing as well as different morphologies as dictated by the biosynthetic conditions. Usually, almost 36 glucan chain assemblies are brought together through van der Waals′ forces and both intra- and intermolecular hydrogen bonds lead to larger units known as elementary fibrils or protofibrils. These pack into larger units or microfibrils which in turn are assembled into cellulose fibers.

Cellulose constitutes the structural element not only in plants, but also a prominent constituent of bacteria, fungi, algae, amoeba, and even sea animals. In general, cellulose is a fibrous, tough, water-insoluble substance that plays an essential role in maintaining the structure of plant cell walls which was first discovered and isolated by Paine (Habibi et al., 2009). The multiple physical and chemical aspects of cellulose have been extensively studied. Chemically, cellulose is a high molecular weight homopolysaccharide of β-1,4-anhydro-D-glucopyranose units. Regardless of its source, every unit is cork screwed 180° with respect to its neighbors, and the repeat segments frequently taken to be a dimer of glucose, known as cellobiose (Figure 1.1a and b). Each cellulose chain possesses a directional chemical asymmetry with respect to its molecular axis: having chemically reducing functionality, a hemiacetal unit and a pendant nonreducing hydroxyl end group. The degree of polymerization in the case of cellulose, which is mainly localized in the primary cell walls, is up to 20,000, though shorter chains can occur depending on its origin (Habibi et al., 2009). Approximately, it varies from 10,000 to 15,000 to glucopyranose units from wood to cotton, respectively.

Figure 1.1 (a) Structure of cellulose. (b) Intramolecular network in a cellulose structure. (Reproduced from Habibi et al., 2010 with the kind permission of American Chemical Society.)

The possibility of formation of hydrogen bonds between the hydroxyl groups of cellulose plays a major role in the formation of fibrillar and semicrystalline order, which determines the important physical properties of these materials (Figure 1.1b) (Fleming et al., 2000).

The presence of a large number of functional groups such as hydroxyls, acetals, and hemiacetals in the cellulose structure, allow significant surface modification to graft a myriad of functional (macro) molecules using various synthetic strategies. This extends their use for a wide range of high-end applications, mostly to improve the processability and performances of the material.

1.1.1.1 Types of Nanocellulose

The lignocellulosic material is naturally nanostructured and cellulose is presented as ‘nanofibers′ in nature. These nanofibers are formed by semicrystallized cellulose molecules through hydrogen bonds and are then bundled as microfibrils glued together by lignin, hemicellulose, extractives, inorganic, and water. There are some strategies to remove the cellulose from the lignocellulosic material keeping its nanosized dimensions. For instance, pulping followed by mechanical fibrillation or controlled acid hydrolysis is common among researchers and industries (Lahtinen et al., 2014; Stelte & Sanadi, 2009).

In present times, these nanosized materials with one dimension in the nanometer range having different nomenclatures (‘crystallites′, ‘nanocrystals′, ‘whiskers′, ‘nanofibrils′, and ‘nanofibers′) have been attracting scientists toward their isolation, characterization, and possible applications (Klemm et al., 2011). These are called ‘nanocellulose′, ‘nanolignin′, and ‘nanosilica′ derived from cellulose, lignin, and silica, respectively. With advances in nanosciences, elongated fibrillar or defect-free rod-like crystalline or spherical particles having nanoscale range at least in one dimension have been prepared using mechanical shearing or controlled acid hydrolysis. However, it is interesting to note that the use of cellulosic fibers dates back thousands of years as lumber, textile, or cordage.

The focus on ‘nano′ technology involving these cellulosic substrates has generated tremendous attention because of their exceptional physical and chemical properties. Several reviews have been published in recent years underlining the importance of these emerging renewable building blocks (Dufresne, 2013; Eichhorn, 2011; Giri & Adhikari, 2013; Habibi, 2014; Habibi et al., 2009, 2010; Jonoobi et al., 2015; Jorfi & Foster, 2015; Klemm et al., 2011; Lee K.Y et al., 2014; Ng et al., 2015; Park, 2010; Ray & Sain, 2015; Rebouillat & Pla, 2013; Salas et al., 2014; Siro & Plackett, 2010; Siqueira et al., 2010a; Tkacheva et al., 2013). In fact, cellulose in nanoforms has revolutionized the whole range of composites, which are biodegradable with extreme properties. They are ecofriendly systems, which are in greater demand in advanced functional material industries.

Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. The nanodimensions of the structural elements result in a high surface area. This leads to powerful interaction of cellulosic moieties with surrounding species, such as water, organic and polymeric compounds, nanoparticles, and living cells. Most importantly, these novel NCs open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose.

Most of the chemical modifications carried out on NCs are similar to that done on cellulose fibers. Generally, these do not affect the crystalline structure and preserve the original morphology. This is of great importance especially in producing high strength composites.

Nanofibrils having limited number of defects or amorphous regions are formed if those biological spinnerets are not disturbed during biosynthesis (Habibi, 2014; Vincent, 2002; Williamson et al., 2002).

In a unique manner, these NCs combine important cellulose properties such as hydrophilicity, broad chemical modification capacity, and versatile semicrystalline fiber morphologies with the specific features of nanoscale materials: features mainly caused by the very large surface area of these materials. Based on their dimensions, functions, and preparation methods, which in turn depend mainly on the cellulosic source and on the processing conditions, NCs may be classified in three main subcategories, which are shown in Table 1.1.

Table 1.1 Various types of NCs from lignocellulosic materials.

Adapted from Klemm et al. (2011) with the kind permission of the Publishers-Wiley.

Based on these morphological features, cellulose fibers can be dissociated transversely at the amorphous regions present along their axis leading to nanometric and highly crystalline defect-free rod-like fragments, referred here after as ‘cellulose nanocrystals′ (CNCs). Cellulose fibers can be laterally disintegrated by mechanical shearing into their substructural nanoscale units (nanofibrils) resulting in nanofibrillated cellulose (NFC). The latter nanosized biomaterial biosynthesized through microorganisms is known as bacterial nanocellulose (BNC). The term ‘nanocellulose′ commonly refers to all these types of nanometric cellulosic substrates including CNCs, NFCs, and BNC. There is still a considerable discrepancy in the terminology and definition of these products and that is dictated by the large variability of starting raw materials and processing methodologies, leading to some ambiguity.

Microfibrillated cellulose (MFC) is a new kind of functional nanomaterials. Due to its advantages of biocompatibility, biodegradability, excellent mechanical, special optical and high barrier properties, it has extensive application prospects as nanopaper, aerogel, nanocomposite materials, in medicine, etc.

CNCs, crystallites, whiskers, rod-like cellulose microcrystals as well as spherical particles are synthesized by various methods (Mueller et al., 2015).

1.1.2 Lignin and Nanolignin

Lignin as the second most abundant biopolymer has been mostly regarded as an undesired by-product that is usually incinerated for energy production. It is a major byproduct from industries involved in paper making, ethanol production from biomass, etc. (Thakur et al., 2014). It is still a challenge to find applications for lignin in the high end product line. Intelligent design systems based on nanotechnology can optimize and maximize the usage of lignin across many fields (Ross & Mazza, 2010). In the recent years, lignin is being recognized as a raw material with a high recovery potential with low costs and a negligible environmental pollution hazard (Matsushita et al., 2006; Popa et al., 2011). In fact, due to its attractive properties, such as high abundance, low density, environmental friendliness, and antioxidant, antimicrobial, and biodegradable nature, neutrality to CO2 along with reinforcing capability, lignin has gained significance in the development of novel polymer composite materials (Thakur et al., 2014).

The structural heterogeneity in lignin, which varies with the source of origin, growth and harvest conditions, drying procedures make it difficult to obtain lignin in high physical and chemical purity (Ross & Mazza, 2010). Indeed, this limitation can be addressed by novel methodologies that aim at producing nanolignin efficiently and consistently. There are many patents that are filed covering mostly the production of the nanolignin, which indicate the vast potential of nanolignin (Zhiming et al., 2013; Zhiqiang et al., 2011).

It is of critical importance to understand the chemical nature of lignin to explore its application in the nanoform. Unlike cellulose, lignin has a complex chemical structure that is formed by phenyl propane units, which are derived from precursors like p-hydroxy phenyl alcohol, guaiacyl alcohol and syringyl alcohol. Chemically, lignin is a highly branched poly phenolic polyether. The chemical structural diversity of lignin stems due to the varied amounts of the three-hydroxyl cinnamyl alcohols that are linked together with different degrees of methoxylation/ethoxylation. The basic structure and polymerization pattern in lignin is shown in Figure 1.2 (Crestini et al., 2010). It may be noted that the second structure shown below emphasizes the nonuniformity in the structure of lignin, which is in contrast to that of cellulose. It shows the variations in the interactions of the monomeric units of lignin displaying structural heterogeneity and complexity in lignin.

Figure 1.2 Chemical structure of lignin. (Reproduced from Crestini et al., 2010 with the kind permission from Elsevier.)

1.1.3 Silica and Nanosilica

Silicon is the second most abundant element in earth′s crust and soil. Silica is the main constituent of more than 95% of all earth′s rocks (Von Sapei, 2007). Thus, it belongs to silicate compounds and is the common name for materials composed of silicon oxide (SiO2). This occurs both in crystalline and amorphous forms. The former form has limited direct applications due to its low reactivity although it is more abundant in the earth′s crust (Wang et al., 2011). Some plants, algae, and animals can accumulate silicon in their tissues, in general, as nanostructures. The silica originated from plants, algae or animals is called ‘biogenic silica′ (BSi). In general, the plants that accumulate more silica are those from the monocot compared to nonmonocotyledonous species. Silicon accumulation occurs more often in primitive land plants. Plants and animals accumulate silicon through uptake of orthosilicic acid [Si(OH)4] present in the water or soil below a concentration range of 0.1–0.6 mM and below pH 9 (Currie & Perry, 2007; Hodson et al., 2005; Lopez et al., 2005; Narayanan & Sakthivel, 2011).

The silica uptake by rice plants influences the productivity and quality of grains and health of plants more than lignin content besides its importance for the digestibility of the rice straw (Van Soest, 2006). Grasses also take up silica. Moreover, its digestibility decreases, while abrasiveness of the grasses increases with increasing silica content. Additionally, silica acts as defense for insect folivores affecting growth rate of the insect and digestion efficiency (Massey et al., 2006). In fact, many plants that accumulate silicon, substitute lignin by silica with advantage of energy consumption and keep silica and lignin in close contact.

Until now, just the silicatein in enzyme with dual activities (silica polymerase and silica esterase) for Si transportation and silica deposition detected in diatoms were isolated for material applications (Hodson et al., 2005; Müller et al., 2008; Müller et al., 2013).

Preparation of inorganic materials requires pure form of silica with minimum impurities, but of amorphous nature (Chandrasekhar et al., 2002). This suggests that amorphous silica will have many industrial applications. These include abrasives, advanced materials (SiC, Si3N4), cements, ceramic pigments, glasses, refractory, microelectronics, mortars, and zeolites (Carneiro et al., 2015). Besides, the importance of micro silica, increasing attention has also been given to their nanosized particles in several scientific areas and in industrial segments leading to industrial scale production (Carneiro et al., 2015; http://www.tsi.com/uploadedFiles/Site…/Nano-A4_5001286A_WEB.pdf). Applications of nanosilica in the societally important areas relate to human health (biomedical and biotechnological applications), food and agriculture, communication, energy, environment, etc. To meet the demands of the above-mentioned large areas of applications of nanosilica, it is being produced from different sources of silica, mostly from silica precursors such as silicon alkoxides, using several methods such as chemical processes involving vapor phase reactions, sol–gel, and other techniques (Carneiro et al., 2015). Considering the demerits of these processes (energy intensive due to the use of high temperature and pressure, environmentally not friendly, etc.), researchers have looked at resources which are cheaper, easily available, renewable and environmentally friendly besides having great potential for sustainability. One such resource is biomass (lignocellulosic materials) in view of their capability to accumulate high amounts of silicon. Besides this, hierarchically structured plants can yield nanosilica, being referred to as ‘biogenic silica′, which is one of the hierarchical porous structures (Carneiro et al., 2015). Various parts of plants such as stem/barks/fibers of banana, kenaf, pineapple leaf, sisal, Phormium tenax (harakeke), mengakuang leaf, mulberry bark as well as agro-industrial residues such as sugarcane pulp, and husks of rice and coffee (Carneiro et al., 2015) have been used as raw materials for preparing nanomaterials including nanosilica. Among these, rice plant is a heavy Si accumulator. In rice plants, at least three enzymes of the group nodulin-26 intrinsic protein III subgroup of aquaporins (Lsi1, Lsi2, and Lsi6) are responsible for the silicic acid uptake in the root and transportation to the xylem (Yamaji et al., 2008; Currie & Perry, 2007). However, there are evidences for a passive uptake through transpiration process, besides the most common active uptake of silicon from water-soluble orthosilicic acid (Ding et al., 2008).

Other plant source of recent studies to produce nanosilica is the Equisetum species (Equisetum hyemale and Equisetum arvensis). These are among the higher terrestrial plants that accumulate silicon as hydrate amorphous silica (‘BSi′). This form of silica can reach up to 25 % by dry weight. This is in the form of protrusions on the stems (Carneiro et al., 2015). Figure 1.3 shows photographs of these species. Studies indicate that the carbohydrates act as templates for nucleation, growth, and deposition of silica in such plants. In addition, since the content of lignin is not high, silica may substitute lignin for reinforcement. In fact, there is lack of lignin deposition on the outer tissue layer of the E. hyemale (Sapei et al., 2007; Gierlinger et al., 2008).

Figure 1.3 Macrophotographs of Horsetail plants (Equisetum spp.): (a) E. hyemale and (b) E. arvensis (Courtesy: Google images).

More details about the concentration of silica in these species are reported elsewhere (Carneiro et al., 2015). Unlike other plant materials mentioned above, leaves or stems or roots of Equisetum species are not considered separately for producing nanosilica because of the following reasons: (i) economic non viability to industrially exploit them, (ii) small ratio of leaves to stem as observed by the morphology of these types of plants, and (iii) no possibility to remove the roots in view of their functions, which involve avoiding excessive nutrient and exporting carbon from the soil in order to maintain the fertility of the soil after many plantation cycles.

1.2 Preparation of Nanomaterials

1.2.1 Nanocellulose from Lignocellulosic Materials

It may be noted that until now, cellulose was not produced in laboratories by just a wet chemistry procedure. Instead, greater production of cellulose comes from wood pulping industry. Artificially, nanostructured cellulose is obtained from cellulose pulp by three main routes: hydrolysis, electrospinning, and mechanical fibrillation. Under the mechanical treatment, three main technologies, namely homogenization (by means of a Manton–Gaulin homogenizer for example), microfluidization and microgrinding, are widely used (Habibi et al., 2009). In fact, novel methods used for the preparation of NC range from top-down methods involving enzymatic/chemical/physical methodologies for their isolation from wood and forest/agricultural residues to the bottom-up production of cellulose nanofibrils from glucose by bacteria.

A mass-colloidal mill, high-shear mill, cryocrushing, and a refining equipment produces cellulose mechanically defibrillated nanofibrils after some passages in the mill with or without addition of enzymes, oxidizers, or others chemicals. However, the main challenge is the high-energy consumption in the case of mechanical fibrillation, which may make it not economically feasible process for industrial production of NC. Besides, the strong polarity of MFC restricts its good dispersion in nonpolar matrices and limits its applications in nanocomposites production. This has been overcome by pretreatments before mechanical isolation with a view to reduce the high-energy consumption (Zhou et al., 2014).

On the other hand, enzymatic or acidic controlled hydrolysis modifies part of the cellulose chain into sugars (or other soluble products). The remaining constitute undissolved cellulose nanorods (or also called nanowhiskers) having typical dimensions of 5 nm in diameter and 200 nm in length (dimensions depends mostly on cellulose source) (Dufresne 2012; Einchhorn et al., 2013; Filson et al., 2009; Williamson 2002).

Electro spinning method involves pulling a yarn with nanometer diameter through the application of high voltage (~10–30 kV) to a drop of dissolved cellulose. It may be noted that (i) use of nonsolvent (nonsolvent is very common in the literature, in this case it may be water, which does not dissolve cellulose and thus cellulose is regenerated) is necessary to regenerate cellulose and (ii) dissolution of the solvent. The process can be varied to obtain cellulose nanospheres instead of long fibers. The latter process is called ‘electrospraying′ (Dufresne 2012; Gu et al., 2014, Habibi 2009; Magalhães et al., 2009, 2011). The latter process refers to electro-spinning wherein a sub-micrometric yarn is pulled while the former process- electro spraying is used to produce sub-micrometric drops.

Details of all the above processes with some specific examples are given in the following paragraphs.

1.2.1.1 Mechanical Shearing and Grinding

This process consists of three steps. In the first step, the material (e.g., lignocellulosic fibers) is subjected to an alkali pulping process (Karimi et al., 2014). Then, pulped fibers are bleached. This is followed by mechanical shearing to separate the pulped and the bleached fibers into their constituent nanoscale cellulosic fibers. This process has been recently successfully applied to kenaf bast fibers by treatment procedures. The authors have studied the influence of each type of treatment procedures on various properties of fibers. These include chemical composition, morphology, functional groups, crystallinity, and thermal behavior of fiber hierarchy at different stages of purification. Many experimental techniques such as Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermal properties by thermogravimetry analysis (TGA), and scanning and transmission electron microscopes (SEM and TEM) were used to characterize the obtained NC.

Figure 1.4 shows macrophotograph of a microfludizer normally used in the preparation of NC (Kamel, 2007).

Figure 1.4 Homogenizer (microfluidizer) used for the production of MFC. (Reproduced from Kamel, 2007 with the kind permission of Publishers of eXPRESS Polymer Letters.)

This has a refiner in the shape of disk in which the dilute fiber suspension to be treated is forced through a gap between the rotor and stator disks. These have surfaces fitted with bars and grooves, against which the fibers are subjected to repeated cyclic stresses. Due to this mechanical treatment, irreversible changes in the fibers occur. Thus, the bonding potential in these fibers increases through modification of their morphology and size.

The next step is the homogenization process. This involves subjecting the refined dilute slurries of cellulose fibers to high pressure by feeding them to valve assembly loaded with a spring high pressure. In view of opening and closing of the valve in rapid succession, the fibers are subjected to a large pressure drop along with shearing and impact forces. A high degree of microfibrillation of the cellulose fibers taking place due to the combination of these forces leads to MFC. The paper industry normally uses the above refining process.

1.2.1.2 Steam Explosion/High-Pressure Homogenization

One of the effective methods for the preparation of nanofibers from biomass is reported to be steam explosion (Cherian et al., 2008). The process consists of subjecting the cellulosic biomass to steam (for a short duration), which is produced at high pressure in a batch reactor and then compressing it rapidly followed by discharging it to atmospheric pressure. During the above process, the material (lignocellulosic fibers, for example) in dry condition is first saturated using steam at high temperature and pressure. Then, the pressure is released suddenly. This would result in flash evaporation of water leading to rupture of the biomass due to the thermomechanical force resulted during the process. Then, obtained nanofibrils would be completely separated from the bundles into individual fibrils. A few researchers have used this process. Figure 1.5 shows a schematic of this process.

Figure 1.5 Process diagram for a typical aqueous/steam explosion system. (Reproduced from Rebouillat and Pla, 2013 with the kind permission of the Authors/Publishers.)

This process has been successfully used to isolate or to obtain nanofibrils from pineapple leaf fibers (Cherian et al., 2010) and Helicteres isora plant fibers (Chirayil et al., 2014a). Thus obtained nanofibrils have been characterized for their dimensions, morphology and crystallinity using XRD, SEM, atomic force microscope (AFM) and TEM techniques. The authors observed that obtained nanofibrils showed network like structure with a length of 300 nm, width of 20 nm and an aspect ratio of 15.

It is also reported by some researchers that after the sudden disintegration of the starting material into a fibrous dispersed solid, it is subjected to the following processes sequentially: (1) extraction with water at 80 °C for 1 h, using a fiber to water ratio of 1:10; (ii) filtration and washing with water; (iii) extraction with 20 wt% NaOH at 80 °C for 1 h using a fiber to liquor ratio of 1:10; and (iv) bleaching with a mixture of H2O2 and NaOH in a stirred tank reactor at 65 °C for 2 h. Finally, bleached fibers are diluted to 1% consistency followed by neutralization with sodium metabisulfite to decompose the residual H2O2, and stabilize the brightness (Rebouillat and Pla, 2013).

In fact, one can also use this method as pretreatment to reduce the content of noncellulosic compounds that cement the fiber aggregates, whereby the reinforcement ability of the obtained individual nanofibrils could be increased in composites.

High-pressure homogenization (HPH) process, one of the efficient methods, has been used to prepare NC from lignocellulosic materials. The principle of this process is similar to those of explosion of steam and ammonia explosion processes (Chen et al., 2010). The process consists of the following steps: First, subject the materials to a high pressure followed by its rapid reduction. Material is crushed under sudden reduced pressure with focused turbulent eddies and strong shearing forces (Ye & Harte, 2014; Zhang et al., 2012) resulting in nanosize materials. It may be noted that the valve of the homogenizer could be clogged by the lignocellulosic raw material used due to insolubility of natural cellulose in water and most of the organic solvents. This suggests some pretreatment of the material (by steam explosion or microfluidizer processing or other methods) is necessary to get better results in this processing method.

Other researchers who have used steam explosion process to prepare NC include Abraham et al. (2013), Cherian et al. (2010), Deepa et al. (2011), and Kaushik et al. (2011). Some details of these are given below.

Abraham et al. have used raw banana fibers to produce cellulose nanofibers (CNFs) for preparing natural rubber (NR) latex-based nanocomposites films using cross-linking agents (Abraham et al., 2013). These nanocomposites films were characterized for physicomechanical properties. Cherian et al. (2010) have isolated NC from pineapple leaf fibers by steam explosion. Nanocomposites prepared using these NC in NR showed significant improvement of Young′s modulus and tensile strength compared to the matrix (NR) particularly at higher loading of nanocellulose fibers (NCFs).

Similarly, Deepa et al. (2011) have prepared cellulose fibers using banana fibers by steam explosion and characterized them for their structure, morphology and thermal properties. They observed high-percentage yield along with aspect ratio of resulting nanofibers by this method of preparation compared to conventional methods with increasing cellulose content in the fibers from 64% to 95%. The authors have attributed the later to the combined effect to the following: (i) removal of noncellulosic constituents such as hemicelluloses and lignin during steam explosion, (ii) bleaching and acid treatments or by the alkali and acid treatments. They have also observed reduction in the fiber diameter during steam explosion followed by acid treatments.

Kaushik et al. (2011) prepared uniformly dispersed NC fibrils from wheat straw by a combined process of alkali steam explosion and high shear homogenization. Uniform distribution was attributed to shearing of the fiber agglomerates by high shear produced during the process. These were characterized using AFM, TEM, and SEM techniques. The authors observed that these fibrils were of 10–50 nm dia when prepared. Their size decreased with alkali treatment to just 10–15 nm range, but exhibited better thermal stability compared to the original material.

Chirayil et al. in a recent review have reported about the preparation of NC from different lignocellulosic fibers by this technique (Steam explosion). They have mentioned that these nanocellulosic fillers at very low volume fraction improved tensile strength and modulus of polymers in a more efficient manner than that has been reported for conventional micro- or macrocomposite materials (Chirayil et al., 2014b).

High pressure homogenization (HPH) pretreatment methods have been used for enhancing enzymatic digestibility of four different kinds of lignocellulosic biomass (Jin et al., 2015). It is reported that the principle of this process (HPH) is similar to that of steam explosion and ammonia explosion (Chen et al., 2010). In this process, the raw material is initially subjected to high force at very high pressure. Then, pressure is reduced instantly at a faster rate. The blasting effect causes a combination of huge pressure drop with highly focused turbulent eddies leading to strong shearing forces to crush the raw materials (Ye & Harte, 2014; Zhang et al., 2012). This process does not involve any chemical treatments and thus has a promising potential for broad applications. For example, HPH pretreatment was used to disintegrate sewage sludge for improving sludge biodegradation (Lan et al., 2013; Zhang et al., 2012). However, this method was seldom tested for lignocellulosic biomass pretreatment. Chen et al. (2010) reported the HPH combined with alkaline treatment on sugarcane bagasse, which exhibited a significant increase of enzymatic digestibility of 95.5%.

HPH has also been used for isolating the NC from sugarcane bagasse fibers (Li et al., 2012). Cellulose nanofibrils from microcrystalline cellulose (MCC) have also been prepared using a high-pressure homogenizer (Lee et al., 2009). In this case, size of most cellulose fibrils was in the range from 28 to 100 nm.

On the other hand, a mechanical pretreatment has also been used to prepare homogeneous bundles of cellulose fibril suspensions (CFB) before subjecting the biomass to high shearing in the homogenizer (Zimmermann et al., 2010). It was observed that the size of the cellulose fibers could be minimized to overcome the problem of clogging of the homogenizer by the fibers.

HPH method has also been successfully used in the case of bamboo (Wang HK et al., 2015), grass clipping, corn straw, catalpa sawdust and pine sawdust (Jin et al., 2015) and cotton (Wang YH et al., 2015). Some details of these are given here. NCFs of bamboo were obtained using both bamboo pulp sheets mostly containing mainly fibers and bamboo processing residues containing mostly parenchymal cells (Wang HK et al., 2015). Obtained NCFs were characterized for chemical composition, tensile properties, and morphology. In addition to the relative ease of the procedure and the low-energy required in this process, the authors have opined that NCFs could be obtained at relatively low costs as the bamboo fibers normally contain a high ratio of parenchyma cells. Thus bamboo processing residues produced in a number of industrial bamboo applications represented a very promising source of raw material for the production of NCFs.

Cellulose nanofibrils have been reported to be obtained from pineapple leaf fibers for the first time using steam explosion process in combination with acid treatment (Cherian et al., 2010). It was found that the above method was effective in both depolymerization and defibrillation of pineapple fibers leading to their nanofibrils. The authors opined that obtained nanofibrils could be promising versatile material for use in a wide range of biomedical and biotechnological applications.

Schematic diagram depicting the combined chemical and mechanical treatments to obtain nanofibers from lignocellulosic materials is shown in Figure 1.6.

Figure 1.6 Process diagram for a typical aqueous/steam explosion system. (Reproduced from Rebouillat and Pla, 2013 with the kind permission of the Authors/Publishers.)

NC was obtained using cotton fibers using HPH method in conjunction with the use of ionic liquids (1-butyl-3-methylimidazolium chloride ([Bmim] Cl) by Wang YH et al. (2015). Obtained product exhibited narrow particle size distribution having diameter in the range of 10–20 nm.

From the foregoing, it becomes evident that this method (HPH) is simple, highly efficient and does not require any organic solvent as reported elsewhere (Keeratiurai & Corredig, 2009). In view of these, it is considered to be one of the efficient methods for refining biomass.

1.2.1.3 Chemical Methods (Acid Hydrolysis, Alkaline Treatment and Bleaching)

According to Jiang et al. (2013a,b), acid hydrolysis process helps in breaking the disordered and amorphous parts of cellulose releasing single and well-defined elementary cellulose nanofibrils. Properties and morphology of the obtained material depends on various factors such as source of raw materials (nature of acid including its concentration, ratio of the acid-to-cellulosic fibers) used and processing conditions (temperature and time of reaction) (Eichhorn 2011; Habibi et al.; 2010; Klemm et al., 2011).

General procedure followed in this method is as follows: First, known weight of the plant fibers from which NC is to be obtained are cut into small, but uniform size (approximately 5–10 cm). These are then subjected to steam explosion with 2 % NaOH solution as pretreatment in an autoclave, with suitable temperature and pressure conditions are maintained for about 1 h. Fibers are taken out of autoclave after releasing the pressure and washed in distilled water till the solution becomes neutral. By this treatment, noncellulosic components such as hemicelluloses, pectins and lignin of the fibers are removed. The insoluble residue was further delignified using acidified sodium chlorite solution at a lower temperature (approximately, 70 °C) for about 1 h. The process should be repeated till white bleached product (pulp) is obtained. This pulp is then neutralized using NaOH solution followed by thorough washing using distilled water. The product upon drying in a vacuum oven is subjected to acid hydrolysis using oxalic acid for about 3 h in an autoclave maintained at a given pressure. Finally, pressure is released immediately and the product is taken out from the autoclave, filtered and rinsed in distilled water till pH of 7 is reached. The obtained product is then suspended in water with mechanical stirring. Then, the yield of the NC obtained from the source fiber is calculated using the equation:

The above method has been successfully used for the preparation of NC from banana rachis, sisal, kapok, pineapple leaf and coir (Deepa et al., 2015), pomelo fruit peels (Yongvanich, N, 2015), Carmagnola hemp fibers (Luzia et al., 2014), coconut husk fibers (CHF) (Nascimento et al., 2014), corn/maize straw (Zea mays) (Rehman et al., 2014), oil palm empty fruit bunch (Salehudin et al., 2014), cotton (Gossypium hirsutum) linters (Moraisa et al., 2013), citrus waste (Mariño et al., 2015), an agro-waste, lotus leaf stalks (LLS) (Chen YD et al., 2015).

The shape, size and surface properties of obtained NC in the above studies have been found to depend on the source and hydrolysis conditions. For example, a comparative study of the fundamental properties of raw material used, bleached cellulose and NC revealed an average diameter in the range of 10–25 nm, high crystallinity, high thermal stability and a great potential to be used with acid coupling agents due to a predominantly basic surface (Deepa et al., 2015). Other results reported are NC from Citrus waste that had 55% crystallinity, an average diameter of 10 nm and a length of 458 nm (Mariño et al., 2015); NC extracted from oil palm empty fruit bunch has diameter between 50 and 90 nm (Salehudin et al., 2014); individual NFC from lotus leaf stalks had a width of 20±5 nm and length on a micron scale (Chen YD et al., 2015); NC from cotton (Gossypium hirsutum) linters were of 177 nm long and 12 nm wide, with an aspect ratio of 19 (Moraisa et al., 2013); NC whiskers from unripe coconut husk fibers (CHF) showed an average length of 172±88 nm and a diameter of 8±3 nm, (aspect ratio of 22±8) (Nascimento et al., 2014). It is interesting to note that extraction of NC from raw cotton linter did not require pulping (Moraisa et al., 2013).

It is reported that the yield of NC obtained using different methods of hydrolysis remains approximately at about 19%, the same level, as seen in the case of CNC from Carmagnola hemp fibers (Luzia et al., 2014). Further, both the chemical or mechanical methods mentioned above either produce low yields or severely degrade the cellulose, besides being neither environment friendly nor energy efficient (Wang et al., 2009). Similarly, value of depolarization ratio of the nanocrystals extracted from corn/maize straw (Zea mays) by an environmental friendly multistep procedure involving alkaline treatment and a totally chlorine-free bleaching is reported to be the same as that of cotton whiskers (Rehman et al., 2014). The authors opined that this ratio does not depend on the cellulose source. The maize whiskers are arranged laterally in bundles with average thickness around five times that of the crystallite.

It may be noted that all the above-mentioned processes based on the chemical degradation of cellulose polymer, have significant disadvantages. For example, mass loss of cellulose occurs due to the degradation of cellulose to glucose or as water-soluble oligomers. In addition, washing and purification of cellulose becomes more complicated due to the use of strong mineral acids, besides creating complex environmental impact. In view of these limitations, industrial use of these methods on a relatively large scale may be disadvantageous (Rebouillat & Pla, 2013).

1.2.1.4 Ultrasonication

With a view to overcome the problems being encountered in the case of chemical and mechanical methods to isolate fibrils from several cellulose resources, a novel method based on high-intensity ultra-sonification (HIUS) has been developed by Wang et al., (2009). The process uses ultrasound of typical frequency of 20–50 kHz. This is termed as ‘sonification′. The principle involved in this process is generation of high frequency by electronically, which is transmitted to the lignocellulosic material through a metal probe (normally of titanium alloy) oscillating with high frequency (Dufresne, 2012). When this is kept in the container having the lignocellulosic material, localized high pressure builds up in the material due to high frequency of the probe. The cells in the material would break open as result of HIUS treatment, which produces very strong mechanical oscillating power. Then separation of cellulose fibrils from its biomass takes place by the action of hydrodynamic forces of the ultrasound. Obtained nanofibers would be of varying sizes due to the cavitations and impaction resulted from the generated high pressure. These nanofibers are then cooled to room temperature, when it is possible for the fibers to get collected at the bottom of the container. It may be noted that there are six factors that may affect the efficiency of fibrillation. These include power used to generate the sound, temperature, time, concentration of suspension used, size of fibers, and distance between the fiber and the ultrasonic probe (Wang & Cheng, 2009).

Schematic diagram of the process is shown below for the separation of CNFs from wood in Figure 1.7.

Figure 1.7 Schematic of the procedure for the separation of CNFs from polar wood. (Reproduced from Chen et al., 2011b with the kind permission of Elsevier.)

Some of the lignocellulosic fibers used to prepare nanofibers include: bamboo, cotton, hemp and ramie fibers (Zhao et al., 2007). Size of nanofibers obtained from the above materials is reported to be of uniform and in the range of 25–120 nm dia. The authors have observed that the degree of fibrillation of the cellulose fibers treated by HIUS was significantly increased. It was also that observed increasing frequency of the ultrasonification resulted in faster disaggregation process of the fibers.

1.2.1.5 Other Methods

It is well known that isolation of MFC/NFC or substructural fibrils, having lengths in the micron scale and widths ranging from 10 to a few hundred nanometers consist of disintegrating the cellulose fibers along their long axis depending on the nature of the plant cell walls. Accordingly, a combination of various methods discussed in the above sections has been attempted. These include several simple mechanical methods, sometimes in combination with enzymatic or chemical pretreatments. The aqueous suspensions of these nanofibers formed exhibit gel-like characteristics in water with pseudo plastic and thixotropic properties even at low solid content.

For example, coconut (Cocos nucifera L.) palm petioles fall off naturally and if not properly processed, may lead to an environmental hazard. Therefore, these petioles were used to prepare CNFs. For this purpose, chemical pretreatments followed by different mechanical processing, including grinding (G), grinding followed by ultrasonication (GU), and grinding followed by homogenization (GH) have been used (Zhao, Y. 2015). FTIR spectra analysis showed that the chemical treatment removed most of hemicelluloses and lignin from the palm petioles, leaving only the cellulose. SEM observations of obtained NCFs showed that diameter of these obtained after GU method was in the range of 50–100 nm with an aspect ratio over 1000.

Very recently, Gamelasa et al. (2015) have used one chemical treatment, with two different mechanical methods to prepare CNFs using eucalyptus pulp fibers. First, the pulp was bleached by chemical treatment using NaOCl in combination of NaBr TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) as catalysts for oxidation. This is followed by two distinct mechanical treatments using a homogenizer. The initial treatment was 5 passes at 300 bars and the second was 15 passes at 600 bars. The later consisted of 5 passes at 300 bars and 10 passes at 600 bars. These resulted in two types of NCFs. These were stored in a freeze drier till their characterization.

The authors have observed that mechanical treatments increased the yield of nanofibrils, which was supported by laser profilometry of air-dried NC films. AFM of these films did not show any significant differences in their morphology. They observed size differences in the cellulose nanofibrils suspensions as determined by both laser diffraction spectroscopy and by dynamic light scattering (DLS). This has been attributed to the differences in the length distribution of both types of samples. Actual length of more number nonfibrallated fibers was calculated using the measured width of the nanofibrils using AFM, which was found to be ca. 600 nm. The hydrodynamic diameter as an equivalent spherical diameter was determined using DLS. Further based on their experimentally determined observations the authors have proposed a simple method to evaluate the cellulose nanofibrils length combining microscopy and light scattering methods.

Another popular method for the preparation of NC from a very variety of lignocellulosic materials is ‘electrospinning′ (Frey & Joo, 2005; Rebouillat & Pla, 2013). The use of an electrical charge to draw very fine (typically on the micro or nanoscale) fibers from a liquid is called ‘electro spinning′. This method has characteristics of electrospraying and dry spinning of fibers of a conventional solution.

A patented process presents the principle involved in this method (Frey & Joo, 2005), which is schematically shown in Figure 1.8 (Rebouillat & Pla, 2013). A solution of cellulose is first prepared, which is then subjected to extrusion or electrospun under high electric field. The charged stream of cellulosic matter would be ejected when the voltage is sufficiently high. This then followed with a rather complicated loop and 3D spiral deployment trajectory as can be seen in Figure 1.8. During this process, the solvent evaporates leaving behind randomly oriented nanofibers. These would be collected in a collector. In the patent, the solvents used are ethylene diamine with a salt selected from the potassium thiocyanate, potassium iodide and mixtures of salts. The authors have claimed that this process is quite simple and cost effective while the NC obtained is very pure.

Figure 1.8 Schematic showing the principle of electrospinning process. (Reproduced from Rebouillat and Pla, 2013 with the kind permission of the Publishers.)

Most of the other methods of preparing nanofibrillated cellulose (NFC), which are yet in early stages for large scale production include: high-speed blending (Uetani, 2010), cryocrushing (Chakraborty, 2005), and high-intensity ultrasonication (Chen, w et al., 2011b,c; Cheng, 2010; Wang, 2009). Using the above-mentioned technologies, many researchers have prepared the NFCs that were prepared using lignocellulosic materials from various sources. These sources are soft and hard woods (Abe et al., 2007; Chen W et al., 2011a–c; Qian et al., 2011; Rodionova et al., 2013; Spence et al., 2010), sugar beet pulp (Leitner et al., 2007), banana rachis (Elanthikkal et al., 2010; Zuluaga et al., 2009), Opuntia ficus-indica (cactus) (Habibi et al., 2009; Malainine et al., 2005), potato (Dufresne et al., 2000), wheat straw (Alemdar et al., 2008; Chen W et al., 2011), bamboo (Abe & Yano, 2010; Chen W et al., 2011a; Hrabalova et al., 2011), Luffa cylindrica (Siqueira et al., 2010b), and some seaweed (Nystro¨m et al., 2009; Thiripurasundari et al., 2012).

1.2.1.6 Functionalized Nanocellulose from Fibers

Another widely used method for the preparation of NC termed as ‘carboxyl-functionalized nanocellulose′ is tetramethylpiperidine-1-oxyl radical ‘2,2,6,6-Tetramethylpiperidin-1-yl oxyl or 2,2,6,6-tetramethylpiperidin-1-yl oxidanyl (TEMPO)′-mediated oxidation method. The process consists of oxidizing cellulose with sodium hypochlorite in the presence of a catalytic amount of sodium bromide and TEMPO under various conditions. A detailed review on this process has been reported (Isogai et al., 2011). In one study, two types of wood samples, one without any drying and the other once dried, were oxidized using TEMPO (Saito et al., 2007). This was mixed with water to get a slurry, which was then subjected to mechanical treatment. During this process, when carboxylate contents formed from the primary hydroxyl groups of the celluloses reached approximately 1.5 m mol/g, the oxidized cellulose/water slurries were mostly converted to transparent and highly viscous dispersions. This results in highly crystalline and individualized CNFs that are dispersed in water. This was confirmed by the TEM studies, which showed the size of these nanofibers to be 3–4 nm in width and a few microns in length. The authors observed that as long as carboxylate contents in the TEMPO-oxidized celluloses reached approx.1.5 m mol/g, there was no intrinsic difference between celluloses without drying and once dried for preparing the dispersion.

Milanovic et al. have reported preparation of NCFs from hemp fibers after treating them with TEMPO to get the oxidized hemp fibers. These treated fibers were finer, with a lower content of lignin and hemicelluloses, but with improved water uptake properties (Milanovic et al., 2012). The authors observed less carboxyl group in the resulting samples compared to those samples earlier obtained by Saito et al. (2007), in the oxidation of hard wood cellulose.

The above studies have found that TEMPO-mediated oxidation was very efficient for simultaneous removal of noncellulosic substances (reduction of lignin content up to 1.95%) and introduction of surface functional groups, i.e., aldehyde (0.415 m mol/g) and carboxyl groups (0.815 m mol/g).

Ultrasonication-combined TEMPO system (US-TEMPO) is reported to have given 25% more NC yield than that from the TEMPO-mediated oxidation (Mishra et al., 2011).

Cellulose nanowhiskers have been prepared from microcellulose crystals (MCC) of oil palm biomass by two different methods, viz., chemical and hydrolysis treatment (Haafiza et al., 2014). Effects of these on some of the physicochemical and thermal properties have been studied.

As an extension of the above-mentioned methods, attempts have been made by functionalization of NC for preparing green composites. This is given in a mini review that presents various methods of preparation of NC from plant materials besides discussing the chemical constitution and microfibrillar arrangement of NC in the cellulose bundles. The review also highlights their applications with particular reference to utilization of scaffolds for biomedical applications (Giri & Adhikari, 2013). According to the authors characteristics of obtained NC showed a wide variability depending on sources and isolation method used. Only dimensions did not show such dependence and remained at 100–300 nm as length and 5–50 nm as width.

Another recent review has reported not only on several processing techniques including functionalized method, which are highly effective in extracting nanocellulose crystals (NCC) from plant cell walls, but also current knowledge on the surface modification of NC (Ng et al., 2015). The review underlines the emergence of preparation of CNCs having controlled morphology, structure and properties, besides presenting functionality of products at each stage and their influences on the final reinforcing capability of NCC.

1.2.2 Nanolignin

Most of the studies involving nanolignin have been carried out using extracted lignin, or with industrial/commercially available lignin. Extracted lignins are usually from plant sources such as pine straw, wheat straw, alfalfa, and flax fiber via organosolv treatment procedures. Industrial lignin is usually obtained as a by-product from pulp and paper industry which processes hardwood and softwood. Thus although the nanolignin preparation methods discussed below do not start with lignocellulosic fibers as the raw material directly, the native source of lignin is still lignocellulosic biomass.

1.2.2.1 Precipitation Method

Frangville et al. (2012) have developed precipitation method for the fabrication of novel biodegradable nanoparticles from lignin.