Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/1998
Publication Date: N/A
Interpretive Summary: Conventional plastic is produced from imported petroleum and contributes to the nation's increasing trade deficit. Furthermore, there are environmental concerns regarding its disposal, particularly in marine waters where an international accord (MARPOL Treaty) bans dumping plastic at sea. A potential solution to both problems is degradable plastics derived from renewable farm resources. The environmental performance of bioplastics made of cornstarch or a bacterial polyester (PHBV) or blends thereof was assessed in tropical coastal water. Starch degraded rapidly (2% weight loss/day), while PHBV was lost relatively slowly (0.1% weight loss/day). A predictive model based on blends of 30% starch-70% PHBV, which produces a bioplastic of reasonable consumer quality, was developed. The model was validated by experimental findings and showed that the starch component in the plastic blend would persist in the environment (tropical coastal water) for about 150 days and the PHBV component would persist for about 1000 days. This study is of interest to scientists, environmentalists, and plastic producers.
Technical Abstract: Extruded bioplastics were prepared from cornstarch or poly(hydroxybutyrate- co-valerate) (PHBV) or blends thereof. Blended formulations incorporated 30% or 50% starch in the presence or absence of polyethylene oxide (PEO), which enhances adherence of starch granules to PHBV. Degradation of these formulations was monitored for a year at four stations in southwest Puerto Rican coastal water. Two stations were within a mangrove stand. Two were sited offshore, of which one station was on the shoulder of a reef and one was at an open location with a sandy bottom. Microbial enumerations of the four stations revealed considerable flux in populations over the course of the year. However, in general, overall population densities were an order of magnitude less at the offshore sandy bottom station and, as determined by clearing zones on minimal media plates, starch-degraders were approximately 50-fold more prevalent than PHBV-degraders at all stations. Accordingly, the degradation of bioplastic, as evidenced by weight loss an deterioration in tensile properties, correlated with the amount of starch present (neat starch > 50% starch > 30% starch > neat PHBV). Incorporation of PEO into blends slightly retarded or had little effect on the rate of degradation. The rate of starch loss from neat samples was about 2%/day, while neat PHBV was lost at a rate of about 0.1%/day. Biphasic weight loss was observed for starch-PHBV blends at all stations. A predictive mathematical model for individual polymer loss from a 30% starch-70% PHBV formulation was developed and experimentally validated. The model showed that PHBV degradation was delayed 50 days until over 80% of the starch was consumed and predicted that starch and PHBV in the blend have half-lives of 19 and 158 days, respectively.