Poly(3-hydroxybutyrate-''co''-3-hydroxyvalerate), commonly known as PHBV, is a polyhydroxyalkanoate-type polymer. It is

biodegradable Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...

, nontoxic, biocompatible plastic produced naturally by

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and a good alternative for many non-biodegradable synthetic polymers. It is a

thermoplastic A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains as ...

linear aliphatic

polyester Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natura ...

. It is obtained by the copolymerization of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. PHBV is used in speciality packaging, orthopedic devices and in controlled release of drugs. PHBV undergoes bacterial degradation in the environment.

History

PHBV was first manufactured in 1983 by

Imperial Chemical Industries Imperial Chemical Industries (ICI) was a British Chemical industry, chemical company. It was, for much of its history, the largest manufacturer in Britain. Its headquarters were at Millbank in London. ICI was listed on the London Stock Exchange ...

(ICI). It is commercialized under the trade name Biopol. ICI (

Zeneca Zeneca (officially Zeneca Group PLC) was a British multinational pharmaceutical company headquartered in London, United Kingdom. It was formed in June 1993 by the demerger of the pharmaceuticals and agrochemicals businesses of Imperial Chemi ...

) sold it to

Monsanto The Monsanto Company () was an American agrochemical and agricultural biotechnology corporation founded in 1901 and headquartered in Creve Coeur, Missouri. Monsanto's best-known product is Roundup, a glyphosate-based herbicide, developed ...

in 1996. This was then obtained by Metabolix in 2001. Biomer L is the trade name of PHBV from Biomer.

Synthesis

PHBV is synthesized by bacteria as storage compounds under growth limiting conditions. It can be produced from glucose and propionate by the recombinant ''

Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...

'' strains. Many other bacteria like '' Paracoccus denitrificans'' and ''

Ralstonia eutropha ''Cupriavidus necator'' is a Gram-negative soil bacterium of the class Betaproteobacteria. Taxonomy ''Cupriavidus necator'' has gone through a series of name changes. In the first half of the 20th century, many micro-organisms were isolated ...

'' are also capable of producing it. It can also be synthesized from genetically engineered plants. PHBV is a

copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...

of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. PHBV may also be synthesized from butyrolactone and valerolactone in the presence of oligomeric aluminoxane as

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

Structure

The monomers, 3-hydroxybutanoic acid and 3-hydroxypentanoic acid, are joined by ester bonds; the back bone of the polymer is made up of carbon and oxygen atoms. The property of the PHBV depends upon the ratio of these two

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

s in it. 3-hydroxybutanoic acid provides stiffness while 3-hydroxypentanoic acid promotes flexibility. Thus PHBV can be made to resemble either

polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer Propene, propylene. Polypropylene belongs to the group of polyolefin ...

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

by changing the ratio of monomers. Increase in the ratio of 3-hydroxybutanoic acid to 3-hydroxypentanoic acid results in an increase in melting point, water permeability,

glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...

(T_g) and tensile strength. However impact resistance is reduced.

Properties

PHBV is a thermoplastic polymer. It is brittle, has low elongation at break and low impact resistance.

Uses

PHBV find its application in controlled release of drugs, medical implants and repairs, specialty packaging, orthopedic devices and manufacturing bottles for consumer goods. It is also biodegradable which can be used as an alternative to non biodegradable plastics

Degradation

When disposed, PHBV degrades into carbon dioxide and water. PHBV undergo bacterial degradation. PHBV, just like fats to human, is an energy source to microorganisms. Enzymes produced by them degrade it and are consumed. PHBV has a low

thermal stability In thermodynamics, thermal stability describes the stability of a water body and its resistance to mixing.Schmidt, W. 1928. Über Temperatur und Stabilitätsverhältnisse von Seen. Geogr. Ann 10: 145 - 177. It is the amount of work needed to tra ...

and the

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

occurs at the ester bond by β elimination reaction.

Hydrolytic Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

degradation occurs only slowly making it usable in medical applications.

Drawbacks

PHBV, being biodegradable, biocompatible and renewable, is a good alternative for synthetic nonbiodegradable polymers made from petroleum. But it has the following drawbacks, * Expensive * Low thermal stability * Brittle * Primitive mechanical properties * Processing difficulty

References

{{Reflist Biodegradable plastics Polyesters Thermoplastics Copolymers Bioplastics Biochemistry