Cellulosic Nanofibers: A Renewable Nanomaterial for Polymer Nanocomposites

The depletion of resources, the occupancy of landfills, and the economical and environmental burden spurred due to nonbio- degradable plastics have forced researchers to explore alternative biodegradable and renewable sources of polymers. The nanofib- rillated cellulose obtained from plants, bacteria, and tunicates is a low-density biodegradable polymer with excellent mechanical properties. However, the high energy consumption and cost in the mechanical separation of nanofibers are the major stumbling blocks in their wider acceptance on the industrial scale. Research on plant- based cellulose biofibers is a much-sought area for obtaining low- cost, sustainable, and biodegradable polymer nanocomposites. This chapter provides an insight into this area and unfolds in two major parts. Firstly, it investigates the identified cellulose-rich sources and the techniques to extract the cellulose nanofibers from them. In

Nanotechnology in Textiles: Advances and Developments in Polymer Nanocomposites Edited by Mangala Joshi

Copyright © 2020 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-81-5 (Hardcover), 978-1-003-05581-5 (eBook) www.jennystanford.com the latter part, it mainly focuses on the role of cellulose nanofibers in the preparation of their respective nanocomposites. Further we also discuss the various applications of cellulose nanofiber- based nanocomposites as enthralling tissue engineering scaffolds, packaging materials, sensors, optics, filtration media, and many more.

Introduction

Every year, over 300 million tons of plastic is produced from petroleum sources for applications in the field of construction, transport, packaging, medical devices, electrical and electronic applications, etc. [1]. Though it was invented to benefit humans, till date only 9% of the world's plastic has been recycled, 12% has been incinerated, and the rest will take more than 400 years to degrade in nature [2, 3]. The drastic accumulation of plastic and the scarcity of land to dump it in has led to its disposal in the oceans. By 2050, it is believed that there will be more plastic in the oceans than fishes. Moreover, the ingestion of plastics is lethally affecting aquatic life and wildlife and ultimately human life. Hence, the scientific community is continuously working on exploring more sustainable, natural, and environment-friendly polymers. These may be obtained from cellulose, starch, lactic acids, etc. [4]. It is estimated that every year nearly Ю10 to 1011 tons of cellulose is produced from plant sources, out of which only 6*109 tons is utilized, mainly in the paper, chemical, and textile industries [5]. Because of their ubiquitous existence, abundance, and free availability, cellulose- based nanofibers need to be explored as the ultimate biodegradable and sustainable source of polymers and the various fields they can conquer.

Cellulose: Chemical Constituents, Structural Aspects, and Properties

Cellulose is the most abundant hydrophilic, semicrystalline polymer obtainable from numerous natural sources, including all green plants, some marine animals (tunicates), algae, fungi, invertebrates,

An insight into the plant cell wall, where (a), (b), and (c) represent the arrangement of cellulose at the microlevel, nanolevel, and molecular level, respectively [16]

Figure 15.1 An insight into the plant cell wall, where (a), (b), and (c) represent the arrangement of cellulose at the microlevel, nanolevel, and molecular level, respectively [16].

and bacteria (Acetobacter and Agrobacteriumi) [6-9]. Cellulose is a glucon polymer containing unbranched chains of anhydroglucose rings joined through 1-4 p glycosidic linkage containing hydroxyl groups (Fig 15.1c) [9-11]. Cellulose nanofibers are lightweight and low-density fibers that offer specific characteristics such as large surface areas, high aspect ratios, and a highly porous meshwork [12].

Plant- and bacteria-based cellulose nanofibers are being widely studied as a reinforcement in nanocomposites, tissue engineering scaffolds, filtration media, high-tech transparent films, and optical devices [13].

The following two sections provide an overview of plant- and bacteria-based cellulose nanofibers in terms of chemical components and structural aspects.

Cellulosic nanofibers (plant based)

Cellulose is the most commonly occurring organic polymer on earth since it is the main constituent of the plant cell wall. The plant cell wall is 90% carbohydrates, and its main constituents are cellulose, hemicellulose, and lignin [14]. Cellulose nanofibers are found to be in a compact form inside the microfibrillated bundles

Type of fiber

Cellulose

wt%

Lignin

wt%

Hemicellulose

wt%

Pectin

wt%

Wax

wt%

Microfibrillar angle (deg.)

Moisture content wt%

Bast

Jute

61-71.5

12-13

13.6-20.4

0.2

0.5

8

12.6

Flax

71

2.2

18.6-20.6

2.3

1.7

10

10

Hemp

70.2-74.4

3.7-5.7

17.9-22.4

0.9

0.8

6.2

10.8

Ramie

68.6-76.2

0.6-0.7

13.1-16.7

1.9

0.3

7.5

8

Kenaf

31-39

15-19

21.5

Leaf

Sisal

67-78

8-11

10-14.2

10

2

20

11

PALF

70-82

5-12

14

11.8

Henequen

77.6

13.1

4-8

Seed

Cotton

82.7

5.7

0.6

Fruit

Coir

36-43

41-45

0.15-0.25

3-4

41-45

8

Source: [1] clasped together by polysaccharides, glycoproteins, and lignin, as shown in Fig. 15.1b. Cellulose forms the skeleton of the cell wall, while hemicellulose serves as a connector for the cellulosic and noncellulosic polymers [14]. Lignin lends structural support and as it is less hydrophilic than cellulose and hemicellulose, it prevents water absorption and rotting of the cell wall due to a microbial attack [15].

On the basis of the arrangement or order of cellulose chains, the cell wall is distinguished into two layers, the primary cell wall and the secondary cell wall (Fig 15.1a) [16,17]. The primary cell wall has less ordered cellulose chains and consists of cellulose microfibrils (9%-25%), hemicellulose (25%-50%), and pectins (10%-35%) [14]. The secondary cell wall, obtained from the primary cell wall, is characterized by highly ordered and condensed cellulose microfibrils aligned parallel to the fiber axis [17]. The secondary cell wall is composed of cellulose (40%-80%), hemicellulose (10%- 40%), and lignin (5%-25%) [14]. The diameter of the cellulose nanofiber varies from 5 to 50 nm and it is several micrometers long [18].

Although the chemical contents of all natural fibers are the same, the chemical composition and the degree of polymerization vary as per the origin of the plant, the part of the plant, the age and process of extraction of fibers, etc. [19]. Table 15.1 provides the chemical composition and structural parameters of some known natural fibers. Cotton fibers have the maximum cellulose content, while coir has the minimum.

As discussed earlier, cellulose is contained in both crystalline and amorphous regions, which helps it to inherit excellent mechanical properties, such as a high Young’s modulus, high tensile strength, and a low thermal expansion coefficient [13, 20]. Table 15.2 denotes the moduli of engineering materials compared to cellulose.

From Table 15.2, it can be observed that crystalline cellulose is twice as stiff as aluminum and glass while five times as stiff as steel. Further, since the diameters of cellulose nanofibers are less than one-tenth of the visible light wavelength, they are free from light scattering and can rotate the plane of polarized light [21], which makes them optically transparent.

Table 15.2 Moduli of engineering materials compared to cellulose

Material

Modulus

(GPa)

Density (mg/m 3)

Specific modulus (GPa mg/m 3)

Aluminum

69

2.7

26

Steel

200

7.8

26

Glass

69

2.5

28

Crystalline cellulose

138

1.5

92

Source: [2]

Bacterial cellulosic nanofibers (cultured)

This is a highly pure and crystalline form of cellulose obtained from the extracellular secretion of Acetobacter xylinum bacteria in well-monitored culture conditions [22]. An ultrafine 3D meshwork of entangled cellulose nanoribbons is formed typically 75-150 nm wide and several micrometers long [23]. A bacterial cellulosic nanofiber (BCNF) has highly ordered cellulose chains compared to the cellulose chains in plant cell walls, which endows it with excellent physical properties, such as a high Young’s modulus (138 GPa), high tensile strength (2 GPa), and a lower thermal coefficient (0.1*10_6 1/K) [24]. The BCNF has excellent bioaffinity, which is exploited primarily in biomedical applications such as tissue engineering and scaffold, artificial skin, artificial blood vessels and nerves, and wound healing mats [23, 25]. However, both plant- based and bacteria-based cellulose need proper processing to get nanofibers of the desired size and for final applications.

Recent Separation Techniques and Processing of CNFs and BCNFs

The extraction of nanofibers from plant and bacterial sources and their processing for desired application is the most crucial step. Plant-based cellulosic nanofibers (CNFs) are embedded with a variety of other chemical components, like lignin, pectin, hemicellulose, and polysaccharides, which themselves are utilized as food and fuel sources and therefore need to be carefully extracted. On the other hand, bacteria-based CNFs have the chances of getting contaminated in the early stages of synthesis, and hence the culture needs to be monitored carefully. The following section discusses the various steps involved in getting pure nanofibers or nanocrystals from a microfibrillar bundle of cellulose.

Pretreatments

The complex hierarchical structure of a plant cell wall offers resistance toward depolymerization. The main aim of pretreatment is to shatter the lignocellulosic complex and store the contents for further valorization while separating the nanocellulosic contents [26, 27]. The various chemical and enzymatic treatments are meant to obtain and solubilize the nanocellulosic contents so that they can be sent for further mechanical separation, the techniques which are highly expensive when implemented single-handedly to obtain the nanofibers.

Enzymatic treatment

It is the most environment-friendly method and employs several enzymes to degrade lignin and hemicellulose as well as selectively hydrolyze a few organic components of the cellulosic microfibril [28]. A set of cellulases containing cellobiohydralases (types A and B) efficiently attack highly crystalline cellulose while endoglu- canases (types C and D) are found to attack disordered cellulose [29, 30].

Chemical treatment

Chemical treatment is used to separate the cellulose from the polymer matrix of lignin, hemicellulose, and various polysaccharides and to hydrolyze it and obtain nanocellulosic contents [31]. Ideally, the chemicals should not disrupt the structure of the cellulose and effectively solubilize and recover hemicellulose and lignin contents without forming toxic by-products.

The following chemical methods are used either alone or together to get nanocellulosic contents in cost-effective ways.

• Acid hydrolysis: The process is carried out with either strong or dilute concentrations of sulfuric acid/hydro- chloric acid/phosphoric acid/nitric acid. The main action of acid hydrolysis is the hydrolytic cleavage of the glycosidic bond between two unhydroglucose units, dissolving the amorphous region of the cellulose [32, 33]. The reaction time, temperature, and concentration of acids affect the efficiency of the hydrolysis [33]. The end product of hydrolysis will be a stable colloidal solution containing high-aspect-ratio crystals of pure cellulose [9].

Treatment with concentrated acids causes corrosion in reactors and leads to complete degradation of lignin and hemicellulose, with the direct hydrolysis of cellulose to glucose. On the other hand, dilute acid causes minimal toxicity and removes hemicellulose, with minimal degradation of lignin and cellulose [31].

  • Alkaline hydrolysis: The alkalis preferred for hydrolysis are NaOH, KOH, Ca(OH)2, hydrazine, and ammonium hydroxide [34, 35]. The mechanism of action of the base is to attack the ester linkages present between hemicellulose, cellulose, lignin, and other carbohydrates and form alkali salts of carboxylic acids [36]. It is mainly responsible for the dissolution of all cross-linkers present inside and outside of the cellulose. The pretreatment of the lignocellulose material with dilute NaOH has shown a decrease in the crystallinity and the degree of polymerization with an increase in the porosity and surface area of cellulose contents [37].
  • Organosolv: Volatile organic solvents, such as methanol, ethanol, acetone, ethylene glycol, and ethyl acetate, are preferred either in their pure forms or in their aqueous forms for an organosolv pretreatment [38]. Organic solvents are generally used along with an inorganic acid catalyst under heating conditions to dissolve lignin and hemicellulose. The OH- ions from the organic solvent hydrolyze the acid ester bonds and ether linkages present in-between lignin- hemicellulose compounds and break down the aromatics and polysaccharides of the lignocellulosic material [31].

In terms of benefits, this process utilizes volatile, water- soluble, and less expensive alcohols, which can be easily recovered by simple distillation. However, the solvent- treated material needs extensive washing to recover the solvent, which leads to more energy consumption for the entire procedure. Due to this reason the organosolv process is not recommended, though it helps in complete fractionation and effective recovery of lignin for specialty chemical synthesis [38].

  • Ionic liquids: Ionic liquids are thermally stable cationic salts of imidazolium, which has the ability to dissolve cellulose, lignin, and hemicellulose without disrupting the cellulose chain’s structure [39]. Ionic liquids can be used in mild conditions, and their chemistry can be tuned to create different dissolving capacities as per the targeted compounds. They primarily attack fi glycosidic bonds and cause hydrolysis of cellulose [31]. Ionic liquids are reusable at room temperature. However, these are expensive for large-scale operations.
  • Oxidative delignification: This treatment utilizes mainly organic peroxides, ozone, oxygen, or air as an oxidative agent to catalyze delignification [40]. The mechanism of action of these oxidative agents is to attack and oxidize the aromatic ring of lignin and react with part of hemicellulose to form carboxylic acid compounds [31].

Generally, oxidative treatments are followed after alkaline pretreatments. The combination of these two processes will induce the bleaching of cellulose and complete degradation of lignin and hemicellulose in the presence of an alkali. After the pretreatment, the cellulose microfibrils are ready to undergo a major size reduction in a mechanical separator.

 
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