Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHA; Polyhydroxyfatty ) or polyhydroxy fatty acids ( PHF) are naturally occurring water-insoluble and linear polyesters produced by many bacteria as reserve materials for carbon and energy. In nature, they are produced by fermentation of sugars or fats. These biopolymers are biodegradable, and are used for the production of bioplastics.

Biosynthesis

The biosynthesis of PHA is ( lack of the macro elements phosphorus, nitrogen deficiency in trace elements or lack of oxygen, for example ) is triggered at the same time over-supply of carbon sources usually caused by certain deficiency conditions. The polyesters are deposited in the form of highly refractive granules as storage substances in the cells. Depending on the micro-organism and cultivation conditions homo-or copolyesters are produced with different hydroxy alkanoic acids.

As a key enzyme in the biosynthesis PHA synthases apply. These use coenzyme A thioesters of (R ) -hydroxy fatty acids as substrates. The two classes of PHA synthases differ in the specific use of hydroxy short or medium chain length. This results in two types of PHA yield:

• Poly ( HAKKL ) and scl - PHA (short chain - length PHAs) from hydroxy short chain length of three to five carbon atoms can be synthesized by many bacteria, including Cupriavidus necator and Alcaligenes latus (PHB).
• Poly ( HAMKL ), mcl- PHAs also (middle chain length - PHAs ) from hydroxy fatty acids with medium chain length with six to 14 carbon atoms, for example, be built from Pseudomonas putida.

A few bacteria, including Aeromonas hydrophila and Thiococcus pfennigii, synthesize copolyester consisting of above two types of hydroxy fatty acids or at least possess enzymes that body in the position.

Another great, too technically possible, synthesis can be carried out with the help of soil organisms. For lack of nitrogen and phosphorus they produce three kilograms of sugar one kilograms PHA. The polymers can be extracted from the cell material in granules of the bacteria.

The simplest and most common form of PHA is produced by fermentation of poly (R-3 -hydroxybutyrate) ( polyhydroxy butyric acid, PHB or poly ( 3HB ) ). This consists of 1,000 to 30,000 Hydroxyfettsäureeinheiten. In addition to 3 -hydroxybutyric acid around 150 other hydroxy fatty acids as PHA blocks are known. After completion of the biosynthesis the bacteria consist of 80 weight percent of the polyester.

Structure of poly -3- hydroxyvalerate ( PHV )

Structure of poly-4- hydroxybutyrate ( P4HB )

Properties

PHA polymers are thermoplastic processable on conventional equipment and are deformable and more or less elastic depending on the composition. Depending on the chemical composition ( homo-or copolyesters contained hydroxy fatty acids ) to distinguish their properties. They are UV - stable, stand in contrast to other bioplastics such as polylactic acid polymers of partially temperatures up to 180 ° C and exhibit a low permeation of water. The crystalline fraction may range from a few to 70%. Improve processability, impact resistance and flexibility. Valeriate by higher shares in the material PHA polymers are soluble in halogenated hydrocarbons such as chloroform or dichloromethane.

PHB is similar in its material properties, the polypropylene (PP ) has good resistance to moisture and has aroma barrier properties. Just out polyhydroxybutyric synthesized PHB is relatively brittle and stiff, PHB copolymers emthalten other fatty acids such as beta - hydroxy valeric acid, can also be elastic.

Production and processing

In the industrial PHA production of polyester is extracted by optimizing the fermentation conditions microbially from sugar or glucose from the bacteria and purified. The British chemical company Imperial Chemical Industries (ICI ), developed in the 1980s, a fermentation produced copolyester of 3 -hydroxybutyric acid and 3- hydroxy valeric acid, which was sold under the name " Biopol.RTM " and developed in the USA by the company Monsanto and Metabolix on. As a raw material for the fermentation can be used carbohydrates such as glucose and sucrose, as well as vegetable oil or glycerin from biodiesel production. Industrial research is working on methods by which transgenic plants are developed, expressing the PHA synthesis pathways of bacteria and thus produce in their tissues PHA as energy storage. Are processed PHA especially in injection molding, extrusion and extrusion blow molding into films and hollow bodies.

Use

Plastics from PHA find as biodegradable elastomers and thermoplastics using, for example, for packaging material, particularly for food. They are also used in the medical field, for example, as the body absorbable materials such as sutures, implants and sustained release pharmaceutical preparations. Commercial significance are particularly copolymers, which allow a better property setting. The market share of PHA bioplastics at all in Western Europe in 2008 was less than five percent. For the future, a strong market growth is expected.