| PHB and its copolymers with polyhydroxyvalerate (PHV) are melt-processable semi-crystalline thermoplastics made by biological fermentation from renewable carbohydrate feedstocks. They have been described as "the first example of a true thermoplastic from biotechnology" and are also biodegradeable. Although quite stable under everyday conditions they degrade slowly in the body and when composted or in landfill sites. [The HB monomer unit is a normal constituent of human blood.] Their chemical resistance is somewhat limited as they are attacked by acids and alkalis and dissolve in chlorinated solvents. Rather remarkably, they are optically active polymers with a chiral site in each molecular repeat unit, all of which are in the D- (or R) configuration. PHB homopolymer is a stiff and rather brittle polymer of high crystallinity, whose mechanical properties are not unlike those of polystyrene, though it is less brittle and more temperature resistant. Hence, copolymers are preferred for general purposes. It is believed that the most likely area for the application of homopolymer is in the medical/biological fields. Information provided by Goodfellow. |
