**2. Biodegradable polymers: classifications and properties**

Biodegradable plastics can be derived from either synthetic or natural resources and are commonly referred to as "biopolymers" [5]. They are defined as polymers


**Table 1.**

*Classification of biodegradable polymers [7, 19–22].*

*Design for Sustainability with Biodegradable Composites DOI: http://dx.doi.org/10.5772/intechopen.88425*

that naturally degrade and assimilate in the environment into water (H2O) and carbon dioxide (CO2) by means of microorganisms [23]. In regard to the biopolyesters, their hydrolysable ester bonds are what make them biodegradable, while for the natural polymers, the process is usually through hydrolysis [7]. Moreover, the means of fabrication are categorized into three main classes: (1) chemical polymerization of monomers originating from biological processes such as in the case of polylactic acid (PLA), (2) chemosynthesis of the polymers in microorganisms such as polyhydroxyalkanoate (PHA), and (3) modification of natural polymers, i.e., starch [5]. **Table 1** summarizes the different typologies used to categorize the different types along with their source and methods of production. Also, characteristic examples of each type are indicated, and their main features highlighted.

#### **2.1 Bio-polyesters: synthetic polymers**

Among the most representative of the synthetic polymers are the aliphatic biopolyesters (listed in **Table 1**), PLA, PHA, and polycaprolactone (PCL) [18]. Polycondensation of bifunctional monomers and ring-opening polymerization processes are commonly used to yield high molecular weight polymers [7]. A comprehensive overview of the different chemical synthesis methods used to synthesize them is reviewed in [23]. The prime interest in this class of materials is due to the fact that they exhibit mechanical properties equivalent to petroleum-based polymers such polyethylene (PE) and polypropylene (PP) [19]. A summary of the reported mechanical and physical properties of the bio-polyesters and natural biopolymers in comparison to the polyolefin low-density polyethylene (LDPE) is provided in **Table 2**.

#### *2.1.1 Polylactic acid*

Polylactic acid is a high molecular weight, crystalline thermoplastic obtained from the ring polymerization of lactide [20]. It was first synthesized in 1931 by a DuPont scientist and was derived from agricultural products such as corn [22]. The typical glass transition temperature (Tg) falls in between 40 and 70°C, while the melting temperature (Tm) is between 130 and 180°C, as referred in **Table 2**. Additionally, it exhibits high strength, where the average tensile strength is 50 MPa compared to 14 MPa of polyolefins such as LDPE.

**Table 3** demonstrates the essential differences between the different biopolymers with regard to their cost, mechanical properties, hydrophilicity, and biodegradation rate. PLA is a hydrophobic polymer due to the methane side group present along the chain's backbone. Thus, it is more resistant to hydrolysis than PHAs, and hence, their biodegradation rate is relatively slow [7]. Moreover, the hydrolytic degradation process needs to be catalyzed at high temperatures, normally in the


**Table 2.**

*Physical and mechanical properties of biodegradable polymers compared to non-biodegradable LDPE.*

range of 60°C. This means that it is not compostable at home, for instance, and requires a specific compostable environment [22]. Despite its high strength, PLA is limited due to its brittleness thermal instability [20]. Additionally, PLA polymers are derived from nonrenewable resources that make their sustainability a questionable matter. While other semisynthetic variants are fabricated; however, their use would not be favorable due to being partially degradable [7].

produce. Owing to their attractive attributes, this category provides a promising sustainable candidate for green product design over their synthetic equivalents.

Natural polymers form during the ecological growth cycle of living organisms [7]. They are mainly derived from biomass fragmentation processes, where polysaccharides are classified as the most representative family of natural polymers.

The main polysaccharides explored in various applications are starch and cellulose-based derivatives [7]. Owing to being abundant, low in cost, and biodegradable, starch-based polymers are among the most extensively studied biodegradable polymer and are considered one of the most favorable candidates for sustainable materials [2, 6, 7, 14–16, 19, 20, 25, 28, 29]. Starches are hydrophilic carbohydrate materials that are regenerated by photosynthesis from plants such as

Starch is primarily composed of two glucose homopolymers: (1) linear amylase and (2) highly branched amylopectin [1, 16]. Different sources yield different proportions of the homopolymers in the range of 10–25% amylose and 75–90% amylopectin [6]. This leads to variable properties, where high amylose content in starch leads to an improvement in mechanical properties such strength and elongation [7, 20]. Additionally, the hydroxyl side groups present in the polymeric chain aid in the rapid biodegradation of the biopolymer [6]. The polymer is considered as highly sustainable, where it is worth noting that during the natural assimilation process, starch is hydrolyzed into glucose that is further metabolized into CO2 and H2O. Afterwards, an ecological equilibrium is created, whereas aforementioned the starch is regenerated by the natural photosynthesis process of plants as they absorb the processed CO2 [6, 27]. Nevertheless, it is important to note that native starch by itself cannot be processed and it must undergo a modification process to improve its

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

molding leads to a more amorphous structure and ductile properties [16].

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

**2.2 Agropolymers: natural polymers**

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

*Design for Sustainability with Biodegradable Composites*

*2.2.1 Polysaccharides: starch polymers*

wheat, corn, rice, and potato [6, 28].

processability.

**43**
