**4.2. Green composites of TPS and cellulose fibers**

*4.1.1. Extraction by chemical hydrolysis*

92 Composites from Renewable and Sustainable Materials

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].

microcrystalline cellulose (MCC) by Battista in 1975 [52].

for hydrolysis.

nm [59].

*4.1.2. Extraction by mechanical force*

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

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

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

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 Human beings are known to be highly dependent on synthetic polymers derived from petroleum for the elaboration of diverse packaging and utensils, giving rise to environmental problems [45]. However, reports have been published of the combination of natural fibers, which are completely biodegradable, for the development of green composites. These mate‐ rials are environmentally friendly and their use can reduce contamination on a global level [45].

Nanocomposites contain materials with a nanometric dimension ranging between 1 and 100 nm. These materials surpass conventional composite materials due to their superior thermal, mechanical, and barrier properties [62, 63]. Biodegradable polymers, in particular, may require improvement in terms of brittleness, low thermal stability, and poor barrier properties [63].

The physical and mechanical properties of the polymeric material reinforced with cellulose are strongly dependent on their structure, relaxation, and morphological processes, as well as a good dispersion of the fiber in order to achieve the minimization of holes. Indeed, a good dispersion of fibers in the polymer matrix has been reported as something very difficult to achieve [64].

The processing of TPS reinforced with cellulose fibers processing is similar to most conven‐ tional synthetic thermoplastic processing [65]. Most thermoplastic operations involve heating and forming into desired shapes, and then cooling. Processing techniques used on thermo‐ plastics can also be used in the TPS reinforced with cellulose fibers. These include extrusion, injection molding, internal mixing, compression molding, and others [66].
