Biodegradable Bioplastics: Polyhydroxyalkanoate Natural Polymers
Polyhydroxyalkanoates (PHA)s natural polymers have become one of the widely emerging bioplastics to be used in commercially applications, and are known as microbial polyesters or bacterial plastics. PHAs belong to a family of biopolyesters produces by various microorganisms to yield polyesters with varying side-chain lengths. In addition to their biodegradability and biocompatibility, another widely recognized characteristic to PHAs includes chirality. They exhibit highly stereoregular polymer backbones with R configuration.
Figure 1: General structure of Polyhydroxyalkanoates (PHA) Polymer Materials.
Poly-b-hydroxybutyrate (PHB) and poly(hydroxybutyrate-co-hydroxyvalerate) polymer materials are the most studied polymers due to their attractive properties. Depending on the type of microorganism, carbon source that the cells are grown on, fermentation conditions including the types of enzymes used. The final properties of the PHAs primarily depend on the monomer composition, and monomer with C3-C5 typically yields thermoplastics, while C6-C14 chain length monomers yield elastomeric polymers.
PHB natural polymer is a melt processable, hydrophobic, semi-crystalline polyester that resembles properties of conventional thermoplastics such as polypropylene and polyethylene. As PHB is almost 80% crystalline, it exhibits a high melting point. It has an excellent resistance to organic solvents, however its applications are limited due to its high stiffness and brittleness. Further, at processing temperatures, PHB tend to degrade rapidly minimizing its usefulness further.
poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB-co-HV)) is commercially available polyester (BioPolTM), and less stiff and brittle than PHB homopolymer, making it easier to process.
PHAs can be modified to get less crystalline and improved physical properties by blending them with various other polymers such as polysaccharides, polyvinyl alcohol, polycaprolactone and polyvinyl chloride.
Miscibility of PHB with various polymers
Polylactic acid (PLA) – immiscible
(PHBV) – miscible depending on the PHBV content
Polycaprolactone (PCL) – immiscible
Polyethylene glycol (PEG) – miscible
Polyvinyl alcohol (PVOH) – miscible depending on the PVOH content
Poly(ethylene oxide) – miscible