Environmental and Agricultural Microbiology. Группа авторов

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4.5). Cargill Dow LLC developed an alternative pathway of melt polycondensation and the use of a tin catalyst to obtain commodity PLA applicable for packaging industries. This pathway involves the pre-polymer formation of aqueous lactic acid and converting into lactide stereoisomer through intramolecular cyclization. In the next step, lactide ring-opening produces high Mw PLA [21] using stannous octoate Sn(Oct)2 as a catalyst [22].

      4.3.3 Biodegradability of Polylactic Acid

      PLA attains researchers’ keen attention among all the biodegradable plastics due to its complete application in the agriculture and packaging industry, as PLA films are mechanically stable and biodegradable. Biodegradation of PLA occurs in the following two pathways: (a) Fragmentation of the polymer leading chains occurs via hydrolysis in the presence of acidic or basic conditions. Moisture plays a vital role in lowering Mn values below 40000, (b) followed by bio-assimilation of disintegrated oligomers by environmental microorganisms to carbon dioxide, water, etc. Biodegradability depends on the chemical structure as well as the polymer sources. Proteinase K was a widely used microorganism for PLA bio-degradation [23]. However, they are able to degrade oligomer products or lactic acids but not PLA itself. Degradability can be controlled in various synthetic ways, and a few popular methods are discussed as follows.

      4.3.4 Copolymerization Method

      The degree of crystallization is directly connected to the rate of bio-degradability and amorphous copolymers between L-lactide and glycolic acid monomers. The prepared poly(lactide-co-glycolide) shows faster degradation than PLA, and increasing glycolide contribution rate further increases [24]. Grafting copolymerization between L-lactide into chitosan (high content) using a tin catalyst increases the thermal stability and degradability rate [25].

      4.3.5 Blending Method

      Biodegradability rate can be increased in the blending method in between PLA and lactic acid (0%–5% of lactic acid content), and the changes were observed in chemical or physical properties. Lactate is an available substrate for different bacterial species to facilitate PLA degradation by providing carbon and energy source [26].

      4.3.6 Nanocomposite Formation

      PLA nanocomposites with montmorillonites (nanoclay) can enhance the degradation rate because hydroxyl groups belonging to the silicate layers facilitate the hydrolysis process. Nanoclays’ effect on PLA biodegradability is enhanced by their excellent dispersion over the polymer surface and depending upon their chemical structures and affinity toward bacterium [27].

      4.3.7 Summary

      Along with conventional uses, exciting PLA or its stereocomplex PLA applications are possible due to its favorable mechanical properties, tunable degradation rates, and high biocompatibility. These specific properties are possible with its copolymer such as Poly(lactic-co-glycolic acid) (PLGA) and which can be further utilized in periodontal regenerative medicine.

      Definition

      PHAs are the family of bio-polyesters and are among well-known biodegradable plastics and well recognized as entirely biosynthetic and biodegradable with almost zero toxic waste be recycled into organic waste [30–32]. PHAs act as microbial reserve compounds for energy [33] and carbon [34] and hold a great potential to replace the petroleum-based compounds in the plastic market are termed as “green plastics” [35]. They show a wide range of properties that can be accessed biosynthetically by selected prokaryotes, and this opens the potential market for substituting petroleum-based products such as elastomers, thermoplastics by PHAs.

      Rising concern for greenhouse gas emissions facilitates bio-based materials and promotes the PHA market in the future. PHAs commercial use increased from an estimated value of 10,000 metric tons (MT) to 34,000 MT in 2018, with a CAGR of 27.7% [36]. PHA biopolymer is expensive as compared to PP and PE. This high price is due to the high purity of substrates such as glucose, its production in various batches, and a large amount of solvents [2]. With the increasing availability of renewable raw material and increasing demand to use biodegradable polymers for bio-medical use and food, applications are beneficial to the PHA market, and its market is expected to US$93.5 million by 2021 from US$73.6 million in 2016.

      4.4.1 Biosynthesis of Polyhydroxyalkanoates

      In addition to PHA’s biosynthesis, an alternative and convenient synthetic approach to obtain PHAs is via the ring-opening polymerization (ROP) of β-lactones, including anionic, “coordination-insertion,” organo-catalyzed, enzymatic, and cationic processes.

      4.4.2 Application of PHAs

      PHAs can be used in various applications [37], such as

       - Packaging films for foods, containers, and bags.

       - Precursors for different chiral compounds.

       - Act as a probe for drug delivery, herbicides, and insecticides.

       - Disposable products like utensils, diapers, cups, etc.

       - Medical applications such as surgical pins,

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