Biobased and Biodegradable Poly(HydroxyButanoicAcid) Copolymers: A Review of Properties and Applications

摘要

A remarkable discovery, dating back to 1926, documents the presence of thermoplastic polyesters within bacterial cells (1, 2). The basic polyester that Lemoigne isolated and characterized was poly (3-hydroxybutanoic acid) or P(3HB). This discovery is of particular significance because it preceded the recognition of polymeric/macromolecular structures first by Herman Staudinger and later by Wallace Carothers. It was later determined that the P(3HB) granules within the bacterial cells serve as an intracellular food and energy source and are produced in response to a nutrient limitation in their immediate vicinity so as to prevent starvation during times of scarcity (3, 4). P(3HB) is an ideal carbon storage medium because it is inert to water, chemicals and osmosis, and can be readily converted to acetic acid by a series of enzymatic reactions (5). Bacterial polyesters became commercially significant when ICI (Imperial Chemical Industries) started producing a PHB copolymer under the tradename "Biopol". Other corporations, most notably W. R. Grace Company, also invested considerable effort looking into the possibility of producing such polymers on a commercial scale. These initial efforts were abandoned possibly due to the high investment required for commercial-scale fermentation and product recovery processes (6). More recent discoveries in genetic engineering led to the creation of a new company, Metabolix (7-10). In 2006, Metabolix formed a 50-50 joint venture with Archer Daniels Midland (ADM) to commercialize the production of PHB copolymers under the tradename Mirel~(TM). These polymers are made by microbial fermentation of sugars such as corn sugar or cane sugar or vegetable oils. Because P(3HB) is stored by bacteria for eventual breakdown and consumption, these polyesters are biodegradable in a variety of environments wherein the macromolecule is hydrolyzed enzymatically to monomeric form. Mirel~(TM) polymers and their products are known to biodegrade in soil, home compost, and industrial compost sites; they also biodegrade in fresh water and sea water environments. Because of their biobased contant (the use of sustainable materials for their production) and their ability to biodegrade, PHB copolymers are being considered for a variety of thermoplastics applications. In this presentation, we will begin with a brief historical perspective of these bacterial polyesters. Subsequently, the melt rheology and solid-state properties of these PHB copolymers will be discussed as a function of molecular weight and copolymer composition. Finally, the degradation behavior of these polymers will be outlined.