*2.2.2 Modification of natural starch: gelatinization to form thermoplastic starch (TPS)*

The modification process initiates by applying thermomechanical processing to the starch granules mixed with water at temperatures in the range of 90–180°C, which causes expansion and disruption of the granules as a means of transforming the semicrystalline structure into an amorphous thermoplastic starch [28]. This process is referred to as "gelatinization," and at this stage, the starch is difficult to process, and the addition of a plasticizer such as glycerol or other polyhydroxy compounds is needed to reduce the glass transition temperature (Tg) and improve its flow properties and mechanical properties as reported by Elsayed et al. [28], Ibrahim et al. [29], and Mehanny et al. [30], where optimum conditions were found at glycerin content of 30 wt.%. Additionally, Vroman and Tighzert reported—in their review on biodegradable polymer—an improvement in the flexibility and elongation properties at glycerol contents higher than 20% [7]. Thus, the resultant properties and loss of crystallinity is a function of the type of starch used and supplied water and heat during the gelatinization process [16, 28]. Concurrently, the processing technique plays a vital role in the crystallinity and mechanical properties of TPS, where the shear stresses applied during the extrusion process allows for an efficient transfer of the water into the molecules, and also the use of injection molding leads to a more amorphous structure and ductile properties [16].

Additionally, it is argued that the addition of plasticizers retards the retrograding process that takes place as the polymer recrystallizes and becomes brittle with time [31]. However, the pure thermoplastic starch still acquires properties similar to that of native starch such as poor mechanical properties and high hydrophilicity [2, 20]. This is due to the fact that the plasticizers themselves increase the hydrophilic nature of the polymer and results in higher water permeability [14, 31]. This leads to thermal instability and the loss of mechanical properties [2].

the nature of these composites will be discussed in Section 3 along with introducing

A biocomposite polymer is classified as a material which combines a biodegradable polymer as its matrix and a biodegradable filler as the reinforcement [18]. Such composites are also commonly known as "green composites," and as the focus in this chapter is sustainability, the use of natural fibers as fillers will be the only class investigated [34]. Several researchers have demonstrated a high compatibility between starch- and natural-based fibers such as cellulose derivatives [2, 14, 16, 18, 28, 29, 33]. Considerable improvement in the mechanical properties of starch-based composites coupled with a reduction in water permeability has been reported. Additionally, Reddy et al. has reported the use of nano-fillers specifically cellulosebased in the fabrication of green composites, where significant enhancement in properties is anticipated [35]. The classification of natural fibers is presented in Section 3.1, and an overview of their impact as reinforcements on the TPS matrix is

Fiber fillers are added as the source of reinforcement and load bearing component within the composite matrix. They are of either natural or synthetic origin such as plants and carbon, respectively. However, natural fibers offer several benefits over their synthetic counterparts, one of them being that they are essentially biodegradable which is considered as a merit for the environment [8]. Additionally, high specific properties such as strength and low density along with being renewable and low in cost have led to their emergence as excellent substitutes for the man-made competitors [4, 36]. The fiber strength comes from the strong inter- and intramolecular bonds that make the fiber stiff and rigid developing intertwined threadlike structures [31]. In addition to the strong bonds, the higher the crystallinity of the filler material, the less exposed areas of the matrix that would absorb water and moisture. Each type differs slightly in their characteristics; there are three major classes upon which this family of fibers are classified: (1) plant-based fibers, usually referred to as bast fibers and are extracted from the outer bark of plant stems, such as flax, jute, and hemp; (2) leaf fibers, which are hard and strong fibers obtained from leaf tissues such as in the case of sisal and pineapple; and finally (3) seed fibers such as cotton and coir [4, 16]. Other types are extracted from wood or grass [4]. **Table 4** illustrates the mechanical properties of the characteristic natural fibers commonly used for each category compared to carbon fibers. It can be observed that the plant-based flax fibers exhibit the highest strength with a maximum of 1500 MPa, while that for the remaining bast and leaf fibers is less than 1000 MPa. Nevertheless, they all show high specific strength and specific stiffness properties compared to carbon fibers, where the specific gravity of carbon is much

Furthermore, it is important to note that natural fibers have wax on its surfaces and other elements such as lignin and hemicellulose, which leads to difficulty in the adhesion of the matrix to the fibers. Therefore, to improve the poor linkage and adhesion problem, the fibers undergo a surface chemical treatment before synthesis with the matrix, which also aim to reduce the fiber permeability to water [16]. Most treatments work on removing the hydrogen bonds on the surface so as to make it

the different types and characteristics.

*DOI: http://dx.doi.org/10.5772/intechopen.88425*

provided in Section 3.2.

higher than the natural counterparts.

**45**

hydrophobic and to improve the surface roughness.

**3.1 Natural fibers**

**3. Biodegradable starch-based composites**

*Design for Sustainability with Biodegradable Composites*

#### **2.3 Modification of thermoplastic starch**

#### *2.3.1 Acetylation*

To improve the properties of TPS, several methods were devised. The first technique is known as acetylation, where starch acetate is fabricated through the chemical mixture with pyridine and acetic acid [7]. The resultant polymer has a high content of the linear amylose polymer that is less hydrophilic, thus overcoming the permeability issue exhibited by pure TPS [7].

#### *2.3.2 Grafting*

Another powerful technique devised is grafting or copolymerization. Examples include grafting synthetic bio-polyesters such as PCL and PLA to the starch by a chemical bond [6]. However, it is argued that the rate of biodegradability is sacrificed under these conditions as the chains *will not* assimilate in nature readily nor easily [7].

#### *2.3.3 Blending*

At first, scientists used to blend starch with polyolefin synthetic polymers to achieve desirable superior properties; however these systems are partially biodegradable and thus are regarded unacceptable from a sustainability point of view [32]. Thus, the use of only biodegradable synthetic polymers is restricted while using this technique. A thorough review is provided in [7]. The most common components to blend with starch are the aliphatic bio-polyesters such as PLA, PCL, and PHA [33]. The resultant material achieves improved properties and cost competitiveness. On the other hand, a major shortcoming is reported, where it is outlined that starch and many polymers are immiscible, which, thereafter, causes these blends to become weak and eventually deteriorate [6].

Thereupon, it could be inferred that a large range of properties could be tailored among these polymers for specific applications. However, each of them exhibits a variety of limitations which restricts their use and applicability across many fields, and from a sustainability viewpoint, starch-based polymers still provide the best alternative especially if their shortcomings are overcome. Their main advantageous features rely on being the lowest cost material compared to other biodegradable polymers, which are processed by existent processing techniques used for conventional polymers. Also they are both renewable and biodegradable,where several studies have reported an immense improvement in the mechanical and physical properties of modified TPS. Different techniques such as blending, grafting, and acetylation have been implemented to improve properties; however they affect the biodegradability of TPS which is its key successor that establishes it as the front runner in the race toward achieving sustainable alternative materials [7]. Subsequently, it has been reported in the literature that tailoring starch-based composites that are dependent on natural resources yields optimum results while still preserving the biodegradability nature of the polymer [6]. Given their favorable potential,

the nature of these composites will be discussed in Section 3 along with introducing the different types and characteristics.
