Polyhydroxyalcanoates (PHA) are bacteria-synthesized, intracellularly accumulated polyesters from saturated and unsaturated hydroxyalcanoic acids. They exist either as homopolymers or as copolymers of the various hydroxyalcanoic acids. From the large number of monomers capable of building PHA, from the possible variations in the linking of the monomers to the polymer, from variations in the chain length of the polymers, the possibility of producing blends from different PHA and the possibilityof additionally introducing chemical or physical modifications into the polymer skeleton, one derives an almost inexhaustible multiplicity within this polymer family. This great variety fundamentally assures the potential of producing PHA with extremely differing properties, thereby opening up a plethora of fields of application. Nevertheless, it is assumed that out of this fundamental multiplicity, only about 5 to 10 different polymers would be of interest for industrial production.

Fabrication and properties
PHAs are thermoplastic, biodegradable, biocompatible, and nontoxic. In terms of fabricability, they scarcely differ from plastics based on petrochemical raw materials so that the installations customarily used in plastics technology are also suitable for fabrication of PHAs. Because of their biodegradability and biotechnical production from biogenic raw materials, PHAs are considered alternatives to non-degradable polymers based on fossil-derived raw materials.

Applications and ecomonmical aspects
Accordingly, they have the potential as mass-produced plastics of gaining a share of the market in the field of packaging and coatings. Numerous patents dealing with other fields of application have been granted. These include hygienic articles (e.g., diaper components), fibers, adhesives, components of toner and developer fluids, carriers of flavoring substances in foods, biodegradable fishing nets, etc. They also come under consideration as biological materials for medical applications (e.g. implant materials, suture materials, controlled release of active principles). Despite many years of research in the PHA field, until now no commercial breakthrough has been achieved. Industrial processes for fermentative production of PHB-based packaging polymers were halted after a short time due to the unfavorable economic parameters and the absence of comparative advantages of the polymers. Even if all cost reduction potentials are exhausted, conventional PHA from microbial fermentation would be more expensive than petrochemical bulk polymers by a factor of 3. Therefore, the cost goals for bulk (packaging) polymers could well only be attainable in the long term with PHA production in transgenic useful plants. However, commercialization of bulk PHA from transgenic plans could not be realized in less than ten years because of the research work still required.

Polyhydroxybutyrat (PHB)

The best studied polymer within the PHA family is polyhydroxybutyrate (PHB), which has a model character for research on PHA and for establishing and optimizing biotechnical PHA production processes. The properties of PHB (brittleness, stiffness), however, are not optimal for many applications; for them, rather, other PHA with more favorable properties come under consideration.

Some experts continue seeing a huge potential for PHB in the bioplastics market due to the strong variability of some PHB properties. In addition, numerous companies worldwide announce that they are getting into PHB production or expanding their present production. In this sense, besides some small and medium-sized manufacturers, the South American sugar industry is now producing PHB on an industrial scale, resulting in price expectations of 5 € per kg. PHB is bio-degradable in both aerobic and anaerobic environments, has a melting point above 130° C, forms clear films, and captivates by means of its mechanical properties.

PHB, mixed with other ingredients, is also used as a PHB blend. Special material properties can be achieved this way, for example by addition of cellulose acetates. The range of properties of PHB blends extends from adhesives to hard rubber. Starch, cork and inorganic materials are also possible additives in place of cellulose acetate. Mixing with cost-effective additives (cellulose acetate is a cheap waste product from cigarette filter production) also has a favourable effect on the production costs of PHP blends. According to many researchers, this will lead to a reduction of the production costs in the domain of petroleum-based plastic materials in the medium term, resulting in positive assessments of the future possibilities of this material.

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