Biodegradable Polymers, Blends and Composites

Book Preface

In developing countries, environmental contamination caused by polymeric materials has reached high levels. Fossil fuel-derived polymers are not biodegradable, and their resistance to microbial degradation causes them to be stored in the environment. Furthermore, oil prices have recently risen dramatically. Evidence like this has fueled research in biodegradable polymers (Mochizuki & Hirami, 1997). These polymers are improved using techniques such as blending and composite forming, resulting in new blends with different properties such as high efficiency, strength, and good process-ability (Hamad et al., 2014; Nair & Laurencin, 2007; Liu & Zhang, 2011; Ramesh & Rajeshkumar, 2018). Biobased degradable plastics and polymers were first devel-oped during the 1980s. Biodegradable polymers can be used in a variety of forms, from nondegradable to naturally degradable. These naturally degradable polymers can be obtained from renewable sources, while synthetic polymers are made from nonrenewable petroleum-based sources (Jo et al., 1992). Polylactic acid (PLA), poly-butylene succinate (PBS), polybutylene succinate adipate (PBSA), polybutylene adipate co-terephthalate (PBAT), polycaprolactone (PCL), and thermoplastic starch (TPS) have all sparked interest in biodegradable polymers (Srithep et al., 2011). Biodegradable poly-mers have low viscosity, poor resilience, low modulus and thermal sensitivity, and high cost. To overcome these drawbacks, various methodologies such as copolymerization, compositing, and mixing are commonly used. Blending is the most popular among them, owing to its lower cost compared to the preparation of copolymers (Bhatia et al., 2007; Raghavan & Emekalam, 2001; Ramesh, Kumar et al., 2020). In Fig. 1.1, an overview of these polymers is provided based on their origin and production methods (Mochizuki & Hirami, 1997).

Biodegradable polymers
Biodegradation occurs as a result of the reaction of enzymes and/or chemical oxidation in living organisms. This biodegradation happens in two phases. The first phase is the fragmentation of polymers into lower molecular mass organisms by abiotic reactions such as oxidation, photodegradation, or hydrolysis, or biotic reactions such as micro-organism degradation. In the second phase the bioassimilation and mineralization of polymer particles takes place by microorganisms (Jo et al., 1992). For researchers and scientists, reducing the widespread use of petrochemical thermoplastics (polypro-pylene, polystyrene, polyethylene, polyvinyl chloride, and so on) and thermosets (epoxy, polyester, vinyl ester, phenols, and so on) has become a problem. Researchers and scientists are interested in developing biopolymers to replace petrochemical poly-mers in this context. The main difference between petrochemical polymers and bio-polymers is their structure. Biopolymers have well-defined primary, secondary, and tertiary structures, whereas petrochemical polymers have repeated units called mono-mers. Starch is a naturally occurring biopolymer, while PLA and polyhydroxybutyrate (PHB) are two more widely used biopolymers.

Polyhydroxybutyrate
This is a biopolymer with a number of characteristics, including being insoluble in water, and having low moisture absorption and strong ultraviolet resistance. PHB, on the other hand, has lower thermal stability since its glass transition temperature is about 2C. This also has a poor tolerance for acids and bases. It is suitable for medical use because it is biodegradable. PHB is the most common polymer in the polyhydroxyalka-noates (PHA) family, and it refers to the short-chain length PHA with monomers contain-ing a number of carbon atoms. A biosynthetic method for the production of PHB has been developed due to the fermentation of sugar by the bacterium Alcaligenes eutrophus.The PHB homopolymer is highly crystalline in nature, exceedingly brittle, and moderately hydrophobic, much like other PHA homopolymers. As a result, the PHA homopoly-mers have in vivo degradation with respect to time (Mochizuki & Hirami, 1997).

Poly-lactic acid

This is a biodegradable polymer made from potato, sugarcane bagasse, maize, and other agricultural fermentation wastes. PLA is a lactic acid cyclic dimer made from D-or L-lactic acid polycondensation or lactide ring opening polymerization. The nat-ural isomer is L-lactide, while the synthetic mixture is D-lactide (Ayala et al., 2009).

PLA is a hydrophobic nature polymer due to the availability of eCH3 groups. It is much more resistant to hydrolysis than polyglycolide due to the steric shielding effect of methyl groups. The average glass transition temperature for commercial PLA is 64C, the elongation at break is around 31%, and the tensile load carrying capacity is about 32 MPa (Ray et al., 2005). It has excellent mechanical properties and is easy to process. However, it has some disadvantages, such as low impact strength, higher water absorption, and high brittleness. It has a wide variety of uses, including pharmaceuticals, packaging, textiles, and household items. PLA can be processed to improve chain mobility and allow the crystallization process. This process is done with oligomeric acid and citrate ester with low-molecular-weight polyethylene glycol (Ray & Bousmina, 2005). The rate of PLA degradation varies depending on the crys-tallinity index. PLA has a low degradation value when compared to polyglycolide, and so certain copolymers have been examined as bioresorbable orthopedic materials (Ramesh, Rajeshkumar, & Balaji, 2021). The biodegradation nature of PLA also can be improved by a grafting process. During this process L-lactide on chitosan was used to perform ring opening polymerization with a catalyst. The transition tem-perature and thermal stability of grafted copolymers rise as the grafting percentage rises. As the lactide content rises, the graft polymer’s degradation decreases (Zhao et al., 2012).

Polyglycolide

Polyglycolide is the simple linear aliphatic polyester made by ring opening poly-merization of a cyclic lactone and glycolide. It is crystalline in nature, with a crys-tallinity index of around 50%, and hence is insoluble in several organic solvents. Polyglycolide has a melting temperature of 220e225C and the range of the glass transition temperature is 35e40C(Xie et al., 2013). The polyglycolide has good mechanical strength. However, due to its poor solubility and high rate of acid-producing degradation, its biomedical applications are limited. As a result, caprolactone, lactide, or trimethylene carbonate glycolide copolymers have been developed for medical instruments (Nair & Laurencin, 2007; Ramesh & Rajeshkumar, 2021).

Natural rubber
Natural rubber has been used to prepare biocomposites by reinforcing high-strength natural or synthetic fibers due to their high strength ratio, high water resistance, and high durability. Latex is a product that is typically obtained from a rubber tree. Latex is a milky, sticky colloid that is collected using tapping processes. During tapping, in-cisions in the bark are made, and the fluid is collected in vessels. The latex is extracted and poured into dry rubber preparation coagulation tanks or ammoniation-sealed airtight containers. Ammoniation keeps the latex in a colloidal state for a long time. It is normally coagulated under formic acid cleaning control or refined into latex concentrate for the manufacture of dipped products. Natural rubber is used mainly in industries, including transportation, pharmaceutical, agricultural, and aerospace (Mochizuki & Hirami, 1997; Jo et al., 1992).

Starch
Starch, a polymeric carbohydrate, is one of the most widely used biopolymers pro-vided by green plants. It contains a significant number of glucose-bound glycosidic bonds. It is found in a variety of crops, including wheat, corn, and rice. It is a white color powder that is odorless, tasteless, and soluble in water and ethanol. It can be con-verted to sugar and used to make ethanol for whisky, beer, and biofuels by malting and fermenting it (Jo et al., 1992).