Biodegradable Plastics: Natural versus "Synthetic"
Authors: Vipin Chandra Kaliaa,b* , Subhasree Raya,b
aMicrobial Biotechnology and Genomics, CSIR - Institute of Genomics and Integrative Biology (IGIB), Delhi University Campus, Mall Road, Delhi-110007.
bAcademy of Scientific & Innovative Research (AcSIR), 2, Rafi Marg, Anusandhan Bhawan, New Delhi- 110001.


Introduction

Plastics are one of the most widely used synthetic chemical products. They are important components of our daily life. A major issue related to the plastics is their non-biodegradable nature, which makes their disposal a cause of worry for Environmentalists and Health Managers. An alternative is to look for biodegradable plastics. There are “synthetic” bioplastics, which are prepared by mixing chemicals and biological compounds, but the degradation is limited by the contribution made by the biomatter. In contrast, microbes under certain stressed environmental conditions can metabolize carbon compounds in to polyhydroxyalkanoates (PHAs), which are completely degradable natural bioplastics.

The potential alternatives to non-biodegradable plastics

“Synthetic” Biodegradable Plastics

  • Polylactic acid (PLA): PLA is a biodegradable aliphatic polyester, which can be produced from cane sugar or corn starch, tapioca starch. Here, bacterial fermentation results in the production of lactic acid, which is then polymerized into PLA. It gets naturally degraded in soil although the susceptibility to get degraded is lower compared to other aliphatic polyesters such as poly(ε-caprolactone (PCL).
  • Polybutylene succinate (PBS): It involves esterification of succinic acid (produced by microbial fermentation of renewable sources) with 1,4-butanediol resulting in the formation of oligomers of PBS.
  • Polytrimethylene terephthalate (PTT): The process for producing PTT involves condensation polymerization of 1,3-propanediol (produced by bacterial conversion of glycerol rich biodiesel industry effluent), and terephthalic acid or dimethyl-terephthalate.
  • Starch and Cellulose based Bioplastics: Biomass from vegetable oil, corn, pea starch, banana peels, and tapioca are added to chemicals.
Natural Biodegradable Plastics

· PolyHydroxyAlkanoates (PHAs)

Microbes have the unique ability to withstand stress conditions by diverting their metabolic routes. Microbes under normal conditions metabolize organic carbon (C) compounds through Tri-carboxylic acid cycle to generate energy. However, in the presence of excess of C and limitations of minerals such as N, K, O, Mg, etc., TCA cycle is cut short and terminates in the production of poly-3-hydroxybutyrate (PHB). PHB has physico-chemical properties quite similar to petroleum based polypropylene. There are other possibilities of incorporating different fatty acids and supplements leading to the production of biopolymers with different compositions including co-polymers. These variations are grouped as Polyhydroxyalkanoates (PHAs). PHBs are brittle in nature, whereas PHAs are more ductile and less elastic than plastics. PHAs have wider applications, the best being their use as implants in Medical Industry.

The limits of degradation

The term “synthetic” Biodegradable Plastics is misleading as the limit of biodegradation is regulated by the quantity of biological matter. Biological component may constitute only up to 30% of the final product. In contrast, PHAs can be termed as true Bioplastics, as these are synthesized by biological routes from renewable materials. Their complete degradation is also executed through enzymes: PHA depolymerase.

The Truth

PHAs are the true bioplastics.

References:

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About Author / Additional Info:
Researchers in Microbial Biotechnology and Genomics at CSIR-IGIB, Delhi.