**4.3. Preparation of green composites: mechanical properties and water vapor barrier**

From a tensile test, basic mechanical properties of a TPS can be obtained such as tensile strength (the maximum tensile stress a TPS can withstand before it breaks), percentage of elongation at breakage (E) (flexibility), percentage of elongation at yield (EY), and modulus of elasticity (EM) (stiffness) [67, 68].

It has been reported that the mechanical properties of a starch film is affected by the glass transition temperature, degree of crystallinity of the films, amylose content, plastifier type, and content and the storage conditions.

Studies of the barrier properties of starch films are important in order to estimate the shelf life of a food product. This barrier property depends on the starch source, and on the quantity and type of plastifier used, among the most important. Gas barrier properties for a TPS film include water vapor permeability (WVP), oxygen permeability [14], and aroma permeability. WVP is used to describe the ability of the film to control water vapor transportation between a food system and its surrounding. TPS films are not considered good water vapor barriers [68].

The use of CNF has shown to be a viable option for the improvement of mechanical and bar‐ rier properties of TPS films. **Table 3** presents a number of studies on the use of starch from different botanical sources to obtain TPS films reinforced with CNF from different materials. However, there are in fact very few reports relating to the use of CNF as reinforcement ma‐ terials in TPS films. A number of studies have researched the use of undervalued residues as source material for the procurement of cellulose fibers or CNF such as cassava bagasse, bar‐ ley husk, and sugarcane bagasse, as shown in **Table 3**.


The use of CNF has shown to be a viable option for the improvement of mechanical and bar‐ rier properties of TPS films. **Table 3** presents a number of studies on the use of starch from different botanical sources to obtain TPS films reinforced with CNF from different materials. However, there are in fact very few reports relating to the use of CNF as reinforcement ma‐ terials in TPS films. A number of studies have researched the use of undervalued residues as source material for the procurement of cellulose fibers or CNF such as cassava bagasse, bar‐

> **Preparation of TPS reinforced with CNF or cellulose fibers**

A mixture containing starch, glycerol or a mixture of glycerol with sorbitol, stearic acid and different quantities of CNF from cassava bagasse. The films were prepared by compression molding at 140°C

The films were prepared by *casting*. A suspension containing 3% starch in distilled water, 0.30 g of glycerol/g dry starch, 0.01 g of guar gum/g dry starch, 10 and 20 g of cellulose fiber/100 g dry starch.The solution was heated at 90°C for 10 min and poured onto acrylic

A mixture of starch, sorbitol, stearic acid and CNF (5, 10, 15, and 20 g/100 g dry starch). The TPS films with CNF were manufactured using a twin screw extruder. Pieces of the extruded materials were compression molded into thin films with a thickness of 0.3 mm

plaques

**Most important results** 

The addition of 10% and 20% of CNF significantly reduced the elastic module of the TPS films

The addition of cellulose fibers in the films increased the TS and decreased elongation. The WVP of the starch film with 20% of cellulose fibers was lower than that of the film without

fibers

An increase in TS of TPS films was observed with the addition of CNF

**Reference**

[49]

[72]

[74]

ley husk, and sugarcane bagasse, as shown in **Table 3**.

94 Composites from Renewable and Sustainable Materials

**fiber** 

**Preparation of CNF or isolation of cellulose**

Acidic hydrolysis with sulfuric acid (H2SO4) at 60°C for 40 min. Excess acid was removed by centrifugation. Dialysis of the suspension and ultrasonic treatment

the barley husk. Removal of lignin and hemicellulose by alkaline treatment at 80°C for 4 h. Bleaching to remove residual lignin in sodium acetate buffer and a solution of sodium chlorite at 95°C

for 4 h

fibrillation

the wood flour was treated with acetic acid and sodium chlorite between 70 and 75°C for 58 h.The CNF were obtained from delignified wood flour through mechanical

**Potato** Wood flour To obtain the cellulose,

**Type of starch**  **Type of fiber** 

bagasse

**Barley grain**Barley husk Removal of lipids from

**Cassava** Cassava



**Type of starch** 

**Potato tuber**  **Type of fiber** 

Potato tuber **Preparation of CNF or isolation of cellulose**

Potato pulp was treated with NaOH (2%) at 80°C for 2.5 h. The cellulose was submitted to a bleaching process with a solution of sodium chlorite (NaClO2). The resultant cellulose was washed with distilled water and lyophilized.

The cellulose microfibrils were obtained by submitting the cellulose to a homogenization process with distilled water at

500 bars and 90–95°C

crystalline region of the cellulose was carried out by acid hydrolysis (H2SO4, 64%) for 30 min at 45°C. The CNF obtained was washed and neutralized by dialysis. Finally, the

**Corn** – Extraction of the

**Preparation of TPS reinforced with CNF or cellulose fibers** 

(4% w/v) in 100 ml of deionized water. 1.5% of acetic acid and 2.5% of glycerol were added. The mixture was gelatinized at 105°C for a period of 15–20 min. The mixture was placed on glass trays at 50°C, for 6 h

A suspension of cellulose microfibrils (3.3%) was mixed with a gelatinized solution of starch (3.1%). Glycerol was used as plasticizer. The mixture was homogenized and air bubbles were eliminated at reduced pressure. The suspension was poured into a

Teflon mold.

The nanocomposites were obtained by *casting*. A solution was prepared containing 3.58 g of normal corn starch, 1.93 g glycerol, 35 g of distilled water, and different quantities of waxy corn starch nanocrystals (0, 50, and 100%) and CNF (0, 50, and 100%). The solution was gelatinized at 90°C. The mixture **Most important results** 

as filler improves the

properties of the film

mechanical

(TS and E)

The cellulose microfibrils reinforce the starch matrix in the film (greater tension module in comparison with the film without cellulose microfibrils)

The addition of waxy starch nanocrystals and CNF increased the TS and reduced WVP of TPS films. Moreover, with the addition of the CNF, deformation values decreased and

**Reference**

[73]

[76]

**fiber** 

96 Composites from Renewable and Sustainable Materials


**Table 3.** Research work on the use of starch from different botanical sources to obtain TPS films reinforced with CNF.

**Table 3** presents information regarding the isolation of cellulose fibers, CNF preparation, the preparation of TPS films reinforced with CNF or with cellulose fibers, and indicates the most significant results.

In general, one can observe that the isolation of cellulose fiber consists in exposing the plant material to high temperatures by means of an alkaline treatment, with the purpose of elimi‐ nating lignin and hemicellulose. In addition, the material is exposed to a bleaching process at high temperatures with a solution of sodium chlorite.

For the procurement of CNF, the cellulose fibers are treated with hydrolysis between 45 and 60°C. After hydrolysis, the CNF are recovered by centrifugation, dialysis, and subsequent treatment with ultrasonic bath.

Diverse studies are available which report the use of the casting technique for the production of biodegradable starch films. The most commonly used plasticizer is glycerol. To prepare the solution used to form the films, the starch is mixed with glycerol, water, and different quantities of CNF (between 2.5 and 50% with respect to the starch) as reinforcing agents. In some cases, guar gum is used to avoid sedimentation of the fibers. The solution for film formation is heated in order to achieve gelatinization of the starch for a specific period of time. After gelatinization, the solution is poured into Petri dishes for the formation of the films, and conditioned for evaluation. Very few studies have been published reporting on the use of injection molding and extrusion to obtain TPS films reinforced with CNF.

The mechanical properties and water barrier properties of TPS films reinforced with CNF have been reported in a number of publications. In general, CNF facilitates an increase in tensile strength, a decrease in deformation values, an increase in Young's module, and a decrease in WVP of TPS films. The chemical structure of cellulose and starch is similar. When they are mixed to produce a filmogenic solution, interactions among the OH groups of both polymers are produced by hydrogen bridges, producing a rigid network that increases the TS [4, 5, 69]. The addition of CNF favors high values of TS for TPS films. This may be due to the fact that a greater contact surface is produced between CNF and the starch chains [70].

The nanometric size of CNF allows a low WVP value of the starch films, which favors the generation of a network of hydrogen bridges between the starch chains and the CNF, causing the water molecule to follow a path with many "curves and bends" and thus reducing its diffusion through the starch films [71]. In addition, the cellulose is less hydrophilic than starch, due to its higher crystallinity and compact microfibrillar arrangement, making it more hydrophobic [69].

**Table 3** shows research works on the use of starch from different botanical sources, such as cassava [4, 49], barley grain [72], potato [73, 74], corn [75, 76], chayote [48], tamarind seeds [77], and wheat [78], to obtain TPS films reinforced with CNF.
