is a polyhydroxyalkanoate
Polyhydroxyalkanoates or PHAs are linear polyesters produced in nature by bacterial fermentation of sugar or lipids. They are produced by the bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to give materials with extremely different properties...
(PHA), a polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
belonging to the polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
s class that was first isolated and characterized in 1925 by French
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
A microbiologist is a scientist who works in the field of microbiology. Microbiologists study organisms called microbes. Microbes can take the form of bacteria, viruses, fungi, and protists...
Maurice Lemoigne. PHB is produced by microorganisms (like Ralstonia eutrophus
or Bacillus megaterium
Bacillus megaterium is a rod-shaped, Gram-positive, endospore forming, species of bacteria used as a soil inoculant in agriculture and horticulture.Bacterium is arranged into the streptobacillus form....
) apparently in response to conditions of physiological stress. The polymer is primarily a product of carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
assimilation (from glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
) and is employed by micro-organisms as a form of energy storage molecule to be metabolized when other common energy sources are not available. Microbial biosynthesis of PHB starts with the condensation
A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...
of two molecules of acetyl-CoA
Coenzyme A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All sequenced genomes encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it as a substrate...
to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.
The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but many other polymers of this class are produced by a variety of organisms: these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers.
Currently, plastics and synthetic polymers are mainly produced from hydrocarbons, which do not decompose and they are
stored, burnt or recycled at end of life. However, it is possible to produce compostable plastic using PHB.
Fermentation can also be used to produce the alcohol needed to produce plastic, although this renewable source of energy is expensive and is not used commercially.
Imperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...
had developed the material to pilot plant
A pilot plant is a small chemical processing system which is operated to generate information about the behavior of the system for use in design of larger facilities....
stage in the 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those of polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
In 1996 Monsanto (who sold PHB as a copolymer with PHV under the trade name Biopol) bought all patents for making the polymer from ICI/Zeneca. However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria. But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.
In June 2005, a US company, Metabolix, received the Presidential Green Chemistry Challenge Award (small business category) for their development and commercialisation of a cost-effective method for manufacturing PHAs in general, including PHB.
Another group of researchers at Micromidas Inc.
have begun to produce PHB from the bacteria in municipal waste water. This approach shows promise for the future of human waste disposal and biodegradable plastic production.
Biopol is currently used in the medical industry for internal suture. It is non toxic and biodegradable, so it does not have to be removed after recovery.
Properties of PHB
- Water insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available biodegradable plastic
Biodegradable plastics are plastics that will decompose in natural aerobic and anaerobic environments. Biodegradation of plastics can be achieved by enabling microorganisms in the environment to metabolize the molecular structure of plastic films to produce an inert humus-like material that is...
s, which are either water soluble or moisture sensitive.
- Good oxygen permeability.
- Good ultra-violet resistance but poor resistance to acids and bases.
- Soluble in chloroform and other chlorinated hydrocarbons.
- Biocompatible and hence is suitable for medical applications.
- Melting point 175°C., and glass transition temperature 2°C.
- Tensile strength 40 MPa
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
, close to that of polypropylene.
- Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments.
- Less 'sticky' when melted, making it a potentially good material for clothing in the future