Precise Depolymerization of Poly(3-hydoxybutyrate) by Pyrolysis

Abstract

Poly(3-hydroxybutyrate) (PHB) is a well known microbial and biodegradable polymer, which is accumulated and stored by prokaryotic microorganisms at levels up to 90% of their cellular dry weight (Steinbuchel & Valentin, 1995). PHB has been attracting much interest from researchers not only as an environmentally compatible thermoplastic, but also as a polymeric material obtainable from renewable resources and having a high melting temperature of around 180 °C (Marchessault et al., 1981). A major problem of PHB when used as a thermoplastic is its thermal instability during meltprocessing. Therefore, intense interest has been shown in the thermal degradation of PHB and other related poly(hydroxyalkanoate)s (PHAs). Recently, it has been demonstrated that PHB is a chemically recyclable material with end products such as crotonic acid, linear oligomers having a crotonate end group (Morikawa & Marchessault, 1981), and a cyclic trimer (Melchiors et al., 1996). Melchiors et al. found that cyclic oligomers were obtained via back-biting reactions in a toluene solution with catalysts such as dibutyltin dimethoxide. However, the present review concentrates upon the thermal degradation of PHB in melt. If plastic materials originating from renewable resources can be efficiently recycled through precise control of their thermal degradation, an ideal recycling system could be constructed for plastic products, in which the resources and production energy of the materials are minimized.