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9 Biodegradation of Plastics by Microorganisms

9.2.2.2

Based on Structure and Thermal Properties

Thermoplastics Thermoplastic materials are those materials that may be cooled

and heated several times without altering their chemical or mechanical properties.

Polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene

(PS), and polytetrafluoroethylene (PTFE) are some examples of common plastic

polymers. Their molecular range varies from 20 000 to 500 000 amu. These polymers

can be used for a variety of possible applications, such as wire and lighting systems,

matrix for natural or synthetic fibers. Thermoplastics may melt before going to the

gaseous state and are soluble in certain solvents. Most thermoplastic materials give

high strength, resistance to shrinkage, and stress-free bendability. Thermoplastics

may be extracted, processed, and synthesized by chemical processing from plants

in vast quantities.

Thermosetting Plastics Thermosetting or heat convertible plastics are substances

which cannot be reshaped by heat or pressure once they are in their final state.

They differ from the thermoplastic in the way that they are more resistant to

temperature and breakage. Due to the permanent chemical structure and oddly

related arrangement, these plastics cannot be recyclable. Thermosetting plastics are

preferred for many applications in the building materials, due to their long-term

characteristics. Common examples include vulcanized rubber, fiberglass, polyester

resins, polyurethane (PU), melamine, epoxy resin, and bake lite.

9.2.2.3

Characteristics of Different Biodegradable Plastics

Polyhydroxy-Alkanoates (PHA) Polyhydroxy-alkanoates (PHA) polymers are thermo-

plastic. They are easily biodegradable, non-toxic, and produced naturally by bacterial

fermentation of lipids and sugars. Production of PHA is encouraged by the avail-

ability of carbohydrates during the bacterial growth phase. It can be preserved in

microorganisms and weigh as much as 80% of the organism’s dry weight. The rate

of microbial degradation and the nature of end products depend on the soil, envi-

ronmental conditions, and the nature of PHA. Microorganisms are able to degrade

and utilize it for their carbon and energy requirement under restricted energy and

carbon sources. Some representative bacteria found responsible for the biodegrada-

tion of this kind of plastics that include Bacillus, Nocardiopsis, and Cupriavidus [7].

Again, a variety of fungal genera (Mycobacterium and Micromycetes) are recognized

to incorporate PHA by using aerobic and anaerobic mechanisms. It is less sticky

when melted than conventional polymers. They are soluble in halogenated solvents

such as chloroform, dichloromethane, or dichloroethane, but insoluble in water. It

is resistant to hydrolytic degradation. Despite having good resistance to ultraviolet

(UV), it shows low resistance to acids and bases. PHA polymers are used in med-

ical sector, packaging, and pharmaceutical industries due to their biocompatibility

and biodegradability. Disposable medical equipment, food packaging materials, and

some paints are also widely used PHA products.

Polylactide (PLA) It is biodegradable in nature and thermoplastic aliphatic polyester.

It can be obtained from renewable sources such as corn starches, sugarcane,