1
Nanocellulose: A Cutting Edge Biopolymer - An Overview

Bidisha Saha and Mainak Mukhopadhyay*

Department of Bioscience, JIS University, Kolkata, West Bengal, India

Abstract

As the ever-increasing need for plastics and other polymeric materials, producing them ethically is an essential part of the business. Making it a sustainable and environmentally friendly industry is therefore critical. Filler for either a synthetic matrix or a natural starch matrix, cellulose presents a great chance to lessen the impact of non-biodegradable elements. Due to their excellent mechanical properties, biodegradability, biocompatibility, high specific surface area, and rich hydroxyl groups for extensive chemical modification, nanocelluloses have recently become increasingly popular as naturally derived, bio-based nanometer-sized reinforcement in a wide variety of technological areas. However, for its intrinsic hydrophilicity and difficulties in dispersion inside a hydrophobic matrix, the extraction of nanocellulose from cellulosic biomass and its dispersion in the matrix remain major hurdles. The literature provides a summary of recent advancements in nanocellulose research, including methods of extraction, surface modification, and polymeric composite applications. This chapter demonstrates how the source of the cellulosic materials and the processing factors affect the morphologies and performances of nanocellulose. Although nanocellulose is derived from organisms or plants that initially appear to be fragile, it can be used as a reinforcement material or the primary component to create high-value, cutting-edge products like high-performance nanocomposites, multifunctional hydrogels, conductive filaments, medical dressings, and energy storage materials. The development of high-performance nanocellulose-reinforced polymer composites is presented, along with research priorities and directions.

Keywords: Nanocellulose, natural biopolymer, nanocellulose-based nanocomposites, reinforced polymer composites, polymer, functionalization

1.1 Introduction

The use of biomasses for the preparation of polymer-based composites is expanding quickly in academia as well as industry due to the overuse and exhaustion of petroleum-based feedstocks as well as significant damage to the environment. Cellulose is being used for decades as a renewable energy source since it is the biomass resource with the highest availability. It is the main carbon source for the growing renewable sector that is creating functional composites [1-3]. It can be derived from numerous different sources including plants, fungi, and marine species, and because of its sustainability, non-toxicity, and biodegradable properties, it has a broad spectrum of applications in the creation of paper, coatings, cosmetics, and pharmaceutical industries. Cellulose is composed of glucose units connected by 1,4-glycosidic linkages. Many freely reactive hydroxyl groups are also present on the exterior of cellulose at the C2, C3, and C6 locations, which results in hydrogen bonding between the linear chains [4-6]. Cellulose is a perfect bio-filler for synthetic or natural polymers due to its excellent physicochemical, mechanical, and chemical characteristics. The inherent hydrophilicity, poor solubility, and infusible process ability of cellulose limit its employment in high-value-added domains, despite the fact that it is a good bio-filler with many beneficial features [7, 8].

In recent years, it has become increasingly difficult to obtain cellulose's nanoscale structured components for use in the production of numerous valuable bio-compatible nanocomposite and commercialized cellulose compounds. In general, there are three different varieties of this cellulose-based nanostructured material, often known as nanocellulose: cellulose nanofibrils (CNFs), cellulose nanocrystals (CNCs), and bacterial nanocellulose (BNC) [9, 10]. This classification is based on the raw material, production process, and fiber morphology. However, there are still some significant challenges to the use of nanocellulose. Although there are numerous physical, chemical, and biosynthetic methods for producing nanocellulose, these processes remain dependent on intensive chemical processing, specialized equipment, or unorthodox raw materials. An economically feasible, efficient, and sustainable processing technology for nanocellulose is needed to decrease manufacturing expenses and prices. Polymeric composites with excellent biocompatibility and low toxicity can benefit from using nanocellulose as nanofiller for reinforcement. Though it has a propensity to clump easily because of its hydrophilicity which frequently leads to poor dispersion in the matrix and decreased performance of composites. Additionally, the keratinization that may happen as a result of nanocellulose drying could make it difficult to obtain dry nanoparticles or result in product flaws [11].

The goal of this chapter is to summarize the recent advances in the study of nanocellulose, provide a brief overview to the processes involved in its extraction and modification, and discuss its uses in polymeric composites, hydrogels, and filaments, as well as nanocellulose's potential uses in biomedicine.

1.2 Nanocellulose: A Brief Overview

1.2.1 Structure and Source of Cellulose

A crystalline structure of cellulose known as primary fibrils is created when the newly produced cellobiose units are connected to one another. To create macro-fibrils, or cellulosic fibres, these latter are grouped together to make micro-fibrils. The unique characteristics of cellulose, such as chirality, ease of chemical functionalization, hydrophilicity, insolubility in most aqueous solvents, and infusibility, are due to its intra- and intermolecular chemical groups. Anhydroglucose units (AGUs), which make up the majority of cellulose, are repeated in the 4C1-chain structure with each monomer unit corkscrewed at an angle of 180 degrees with respect to its neighbours [12]. Evidently, cellulose is produced from wood and cotton, with respective polymerization degrees of 10,000 AGUs and 15,000 units, respectively. The degree of polymerization and the polymeric chain size imparts significant result on cellulose's characteristics. Structured (crystalline) and unstructured (amorphous) regions both combine to form natural cellulose fiber. The crystallinity of the material can range from 40 to 70%, depending on the selection of extraction method. Compared to the crystalline sections, the amorphous regions are less packed and more likely to have interactions with adjacent molecular groups [13-15]. Depending on the molecular orientations, van der Waals, intra- and intermolecular interactions, isolation technique, and treatment procedure, cellulose exists in a variety of polymorphs, including cellulose I, II, III (I), III (II), IV (I), and V. These polymorphs can be interconverted by thermal or chemical treatments [16, 17]. Table 1.1 lists some of the sources of cellulose, including waste paper, wood, grass, herbaceous plants, agricultural crops, animal sources, algae, and bacterial sources. Figure 1.1 illustrates the evolution of cellulose from natural sources to fundamental molecules. Natural resource availability, origin and maturity, pretreatment and processing techniques, and reaction conditions can all affect the qualities of the cellulose that is produced [21-23]. In general, to remove non-cellulosic elements from bleaching, delignification, lignocellulosic sources, and the removal of extractive components (flavonoids, free sugars, fat, resin, tendon, terpenoids, tannins, terpenes, and waxes) are necessary [24, 25]. These pre-treatments can performed by a variety of chemical, physical, biological, and combined approaches [26-29]. They enable lignocellulose's intractability to be overcome as well as its compact structure. More than 40% of the total processing cost goes towards pretreatments [30]. These pretreatments enable access to the cellulose fraction, disrupt the links between cellulose and non-cellulosic substances (lignin and hemicellulose), and permit the separation of pure and microcrystalline cellulose. Some pretreatments may have a negative impact on the process due to the creation of potentially hazardous wastes, ineffective separation, cellulose degradation and loss, as well as the high overall costs of the procedure. Numerous studies are still being done in order to improve the processes' efficacy and effectiveness, lower their costs, and lessen their negative effects on the environment while also better understanding the phenomena that can arise during pretreatments. In order to get pure cellulose from animal cellulose, Trache et al. (2017) [18] claim that pretreatments are frequently required. Bacterial cellulose does not include extractives, hemicellulose, or lignin, in contrast to biomass-based cellulose; hence, pretreatments are not necessary. However, the price of producing it on an industrial scale is still quite high [31].

Table 1.1 Several sources for cellulose fibre synthesis [18-20].