**4. Cellulose**

Cellulose is the most abundant renewable biomass material in nature and is also a major component of plant cell walls. It has been widely used in the form of wood and plant fibers as an energy source, for building materials, and for clothing. Cellulose can be used as feedstocks for producing biofuels, bio‐based chemicals, and high value‐added bio‐based materials. In the past, much attention has been given to the conversion of cellulose‐to‐cellulose‐based compo‐ sites due to its properties relating to mechanical strength, biocompatibility, biodegradation, and bioactivity, as well as its potential applications which include biomedical, antibacterial, water pretreatment, and in the field of functional materials for photocatalysis [38].

The basic building block of cellulose is β‐d‐glucose (C6H12O6) (**Figure 3**). To form the "cello‐ biose" molecule, one water molecule is eliminated for every two glucose molecules. Then, from the condensation of various cellobiose molecules, the cellulose molecule is formed. The cellulose chains are found very close to each other due to their hydrophilic character [39].

**Figure 3.** Segment of a cellulose molecule.

The elemental composition of cellulose was discovered in 1842 by the French chemist Anselme Payen [40] and cellulose consists of: 44.0–45.0% carbon (C), 6.0–6.5% hydrogen (H), and 48.5– 50.0% oxygen (O).

**Table 2** presents a list of the chemical composition of several materials containing cellulose [41].

In plants, cellulose can be found in the form of microfibers in the primary, secondary, and tertiary cell walls, organized either unidirectional or as a woven mesh. Cellulose microfibers are 5 nm thick strands of glucopyranose molecular chains, whose Young's module is 134 GPa, with a density of 1.5 g/cm3 , and a tensile strength (TS) estimated at approximately 2 GPa. Cellulose microfibers have a diameter of 20–200 Å, while the length can reach several dozen microns. These characteristics are responsible for the interesting mechanical properties of the


microfibers, which are extracted from the biomass by means of a chemical treatment followed by a mechanical treatment; the aim being to obtain a homogenous suspension of individual microfibers [42, 43].

**Table 2.** Chemical composition of materials containing cellulose [41].

In this process, the disintegration of the cellulose was achieved by the generation of high shear forces. As a consequence of the above, the microfibers detach from the surrounding material and remain united to other microfibers, thereby forming a network of fibers with nanometric dimensions. The cellulose obtained from this procedure is denominated cellulose nanofibers. These CNF are packages of fibers on a nanometric scale. It has been determined that, within the plant, these CNF are capable of supporting the pressure exerted by the water contained in the plant cells and thus the interest in using cellulose in this form for nanocomposites, based on the concept of introducing nanometric loads (fillings) in a polymeric matrix [44].

#### **4.1. Extraction of cellulose nanofillers**

**4. Cellulose**

90 Composites from Renewable and Sustainable Materials

**Figure 3.** Segment of a cellulose molecule.

with a density of 1.5 g/cm3

50.0% oxygen (O).

Cellulose is the most abundant renewable biomass material in nature and is also a major component of plant cell walls. It has been widely used in the form of wood and plant fibers as an energy source, for building materials, and for clothing. Cellulose can be used as feedstocks for producing biofuels, bio‐based chemicals, and high value‐added bio‐based materials. In the past, much attention has been given to the conversion of cellulose‐to‐cellulose‐based compo‐ sites due to its properties relating to mechanical strength, biocompatibility, biodegradation, and bioactivity, as well as its potential applications which include biomedical, antibacterial,

The basic building block of cellulose is β‐d‐glucose (C6H12O6) (**Figure 3**). To form the "cello‐ biose" molecule, one water molecule is eliminated for every two glucose molecules. Then, from the condensation of various cellobiose molecules, the cellulose molecule is formed. The cellulose chains are found very close to each other due to their hydrophilic character [39].

The elemental composition of cellulose was discovered in 1842 by the French chemist Anselme Payen [40] and cellulose consists of: 44.0–45.0% carbon (C), 6.0–6.5% hydrogen (H), and 48.5–

**Table 2** presents a list of the chemical composition of several materials containing cellulose [41]. In plants, cellulose can be found in the form of microfibers in the primary, secondary, and tertiary cell walls, organized either unidirectional or as a woven mesh. Cellulose microfibers are 5 nm thick strands of glucopyranose molecular chains, whose Young's module is 134 GPa,

Cellulose microfibers have a diameter of 20–200 Å, while the length can reach several dozen microns. These characteristics are responsible for the interesting mechanical properties of the

, and a tensile strength (TS) estimated at approximately 2 GPa.

water pretreatment, and in the field of functional materials for photocatalysis [38].

Several processes have been used to extract highly purified CNF from cellulosic materials. All these methods lead to different types of magnesium stearate and stearic acid, depending on the cellulose raw material and its pretreatment, and more importantly, depending on the disintegration process itself [45]. CNF extraction can be divided into: chemical hydrolysis and extraction by mechanical force.

#### *4.1.1. Extraction by chemical hydrolysis*

A commonly used extraction methodology of CNF is acidic hydrolysis of the amorphous regions surrounding the embedded CNF and cleavage of the bundles, followed by filtration or centrifugation to exclude dissolved noncrystalline elements [46, 47]. The methodology is beneficial in that it can be performed on very small quantities of cellulose, it requires only the simplest laboratory equipment, and the CNF can be obtained without any induced imperfec‐ tions caused by mechanical processing. The conditions typically involve the use of aqueous solutions of sulfuric acid, stirred at 45–60°C at atmospheric pressure until a homogeneous beige solution is obtained [48, 49]. After acid hydrolysis, the suspension is diluted with water in order to stop the chemical reaction. This suspension is subjected to centrifugation to obtain the cellulose and eliminate excess acids. The resulting cellulose is washed with water using centrifugation and is finally dialyzed using membranes to reach a neutral pH [50].

This procedure results in CNF having anionic groups on the surfaces (leading to electrostatic stabilization of the nanocrystals in suspension) with the ability to form chiral nematic liquid crystalline phases in concentrated solutions [51]. The form of cellulose obtained was denoted microcrystalline cellulose (MCC) by Battista in 1975 [52].

Exaggerated hydrolysis can typically be noted as the solutions turn dark or black in color as the degradation of the CNF occurs. This phenomenon was reported by Roman et al. in 2004 [53], who assigned the crystal degradation to potential induced thermal degradation related to the sulfate groups, introduced as a functional surface on the CNF when sulfuric acid is used for hydrolysis.

#### *4.1.2. Extraction by mechanical force*

The mechanical methods to extract CNF from wood pulp and parenchyma cells typically involve a high‐pressure homogenizer treatment [42, 54], a microfluidizer [55, 56], a high‐ pressure refiner, a super‐grinder treatment [57], or ultrasonication [58]. The form of cellulose obtained was denoted microfibrillated cellulose (MFC) by Herrick et al. and Turbak et al. in 1983 [42, 43]. These processing methodologies have in common the fact that they rely on the application of high shear forces on cellulose fiber suspensions in order to mechanically liberate the CNF from the original plant cell wall structure. In a high‐pressure homogenizer this is achieved by allowing a cellulose suspension to pass under high pressure through a thin slit where it is subjected to high shear forces. The shear forces serve to disintegrate the microfibrils or microfibril bundles in the plant cell wall, resulting in CNF with diameters of about 5–100 nm [59].

High‐intensity ultrasonication consists of a combination of chemical pretreatment and high‐ intensity ultrasonication. To obtain the cellulose fiber, first, the lignin is eliminated from the samples by immersion for 1 h at 75°C in a solution of sodium chlorite, previously acidified [60, 61]. After the bleaching, the samples are treated with potassium hydroxide (3%) at boiling point for 2 h. After which, they are subjected once again to a treatment with potassium hydroxide at a different concentration (6%), the aim being to eliminate hemicellulose, residual starch, and pectin. After the application of the chemical treatment, the samples are washed with distilled water and the resulting cellulose fibers are immersed in distilled water. A total of 120 ml of this solution containing purified cellulose fibers is placed in an ultrasound generator of 20–25 kHz in frequency equipped with a cylindrical titanium alloy probe tip of 1.5 cm in diameter. The subsequent ultrasonication is conducted for 30 min to isolate the CNF.
