9.2 Definition and Classification of Plastics

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tapioca roots, etc. Polylactide (PLA) has characteristics comparable to PE, PP,

or PS. It is likely to be a substitute for low-density polyethylene and high-density

polyethylene (LDPE and HDPE), PS, and PET [8]. It can be produced when corn or

other carbohydrates are converted chemically into dextrose. Dextrose is fermented

into lactic acid and then polycondensed into monomers of lactic acid. There are

different stereo-chemical compositions of PLA, namely, L-, D, and L,D-lactide.

They have melting temperatures of 170–180 and 55^∘C for the optical pure L- and

D-lactide and the amorphous L,D-lactide, respectively [5]. It has use in the food

industry to package sensitive food items. This is also used in medical implants, drug

distribution system, tissue engineering etc. due to its capability to be integrated into

human and animal bodies.

Starch-Based Polymers These polymers are complex mixtures of starch and

biodegradable plastics. Some examples include polybutylene succinate, polylactic

acid, polybutylene adipate terephthalate (PBAT), polycaprolactone (PCL), and poly-

hydroxyalkanoates. Thermoplastic starch (TPS) is formed by the action of thermal

and mechanical energy [5]. Low water vapor barrier with reduced mechanical

properties, brittleness, and bad process ability are some of the drawbacks [5]. On the

other hand, applications include bottle manufacturing, food packaging, disposable

table ware, cutlery, and coffee machine capsules.

Polyethylene Succinate (PES) Polyethylene succinate (PES) is a thermoplastic

polyester which is made by co-polymerization of succinic anhydride and ethylene

oxide. An additional type of production is ethylene glycol and succinic acid poly-

condensation [9]. A bacterial strain called Pseudomonas sp. AKS2 is documented

to degrade this polymer in an efficient manner. In contrast to diversity of PCL

degrading microorganisms, the distribution of microbes that degrades PES is

restricted. Another PES-degrading thermophilic strain named Bacillus sp. TT96

has been isolated from the soil. Moreover, the genera Bacillus and Paenibacillus

phylogenetically have many mesophilic microbes that are isolated with the intrinsic

ability to degrade PES [10]. Plastics industries use PES to manufacture films for

livestock, in the form of paper coating material, and for shopping bags.

Polycaprolactone (PCL) PCL is a fossil-based biodegradable, bio-compatible, and

non-toxic polymer. It is partially crystalline and has a low melting point and

a glass transition temperature of 60 and –60 ^∘C, respectively [5]. It is made by

𝜀-caprolactone with ring-opening polymerization [5]. Microbial lipases and

esterases can degrade this. The causative bacteria for PCL biodegradation can

widely be found in the atmosphere. Aspergillus sp. ST-01, a fungal strain, has been

found to degrade PCL into a wide variety of produces such as butyric, caproic,

succinic, and valeric acids [11]. It is also an industrial polymer. It has a wide range

of applications in hot-melting glue and laminating bags. It is also found in model

making, prototyping, and molds making for reproduction. Due to its low melting

point and high biodegradability, pure PCL is mostly used in clinical applications.