Submitted to: Journal of Applied Polymer Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/1999
Publication Date: N/A
Interpretive Summary: There is a growing public awareness and concern over environmental pollution caused by careless disposal of synthetic plastic materials into marine waters around the world. An international marine pollution treaty has been ratified that sets restrictions on plastic disposal from private vessels at sea. Biodegradable plastics prepared from renewable farm biopolymers and a microbial polyester (PHBV) have received considerable interest as natural alternatives to conventional synthetic plastics derived from petroleum. The environmental performance of bioplastics made from blends of cornstarch and PHBV was assessed in tropical coastal waters. Their biodegradation was monitored for one year. Starch in the composite bioplastics degraded rapidly, while PHBV degraded relatively slowly. To study the underlying degradation of starch and PHBV, a mathematical model was developed from which degradation patterns and lifetimes of the individual biopolymers could be predicted. The model for biodegradation of a bioplastic composed of 30% starch and 70% PHBV revealed that the degradation of PHBV was delayed and lagged 46 days until more than 65% of the starch was consumed. Computed degradation profiles also predicted that the starch in the composite would be completely biodegraded in 174 days while residual PHBV would persist in the marine environment for 1107 days. The model was validated by later experimental findings. This study is of interest to scientists, environmentalists, and plastic manufacturers.
Technical Abstract: Plastic prepared from formulations of cornstarch and poly(beta- hydroxybutyrate-co-beta-hydroxyvalerate) (PHBV) biodegraded in tropical coastal waters. Biodegradation was monitored for one year. Starch- PHBV bioplastic appeared to lose weight in two overlapping phases until both biopolymers were entirely consumed. To examine the underlying degradation of starch and PHBV from biphasic weight-loss profiles, a semi-empirical mathematical model was developed from which degradation profiles and lifetimes of the individual biopolymers could be predicted. The model predicted that starch and PHBV in the bioplastic had half-lives of 19 days and 158 days, respectively. Computed profiles also predicted that the starch in the composite would be completely degraded in 174 days while residual PHBV would persist in the marine environment for 1107 days. The model further revealed that, for a 30% starch:70% PHBV composite, PHBV degradation was delayed 46 days until more than 65% of the starch was consumed. This suggested that PHBV degradation was metabolically repressed by glucose derived from starch. Glucose repression of microbial PHBV degradation was substantiated in 91 of 100 environmental isolates. The validity of the elaborated model was proven when its revelations and predictions were later confirmed by chemical analysis of residual bioplastic samples.