ARS | Publication request: Biodegradation of Starch/polylactic Acid/poly(hydroxyester-Ether) Composite Bars in Soil

Submitted to: Polymer Degradation and Stability
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 9, 2002
Publication Date: January 15, 2003
Citation: SHOGREN, R.L., DOANE, W.M., GARLOTTA, D.V., LAWTON JR, J.W., WILLETT, J.L. BIODEGRADATION OF STARCH/POLYLACTIC ACID/POLY(HYDROXYESTER-ETHER) COMPOSITE BARS IN SOIL. POLYMER DEGRADATION AND STABILITY. 2003. v. 79. p. 405-411.

Interpretive Summary: Polylactic acid (PLA) is a plastic material that is beginning to be produced commercially in large quantities from fermentation of corn to lactic acid and subsequent polymerization to PLA. PLA is biodegradable under some conditions such as in compost but relatively little is known about how PLA might degrade in soil, lakes, etc. or how the rate of PLA degradation might be modified for specific applications. In this study, it was found that molded PLA bars degraded very little after one year in soil in Midwestern U.S. Addition of cornstarch to the PLA bars did not significantly accelerate degradation but further addition of an epoxy resin resulted in extensive starch degradation and a porous PLA bar. These results are valuable to both companies developing PLA based plastics and the general public since they indicate that care needs to be taken in disposing of PLA plastics in environments such as soil where degradation is slow. They also point toward the need for developing PLA plastics having faster degradation and may inspire scientists in industrial and academic settings to do so.

Technical Abstract: Injection molded tensile bars composed of native corn starch (0-70%), poly(D,L-lactic acid)(95%L)(PLA, 13-100%) and poly(hydroxyester-ether)(PHEE, 0-27%) were buried in soil for one year in order to study the effects of starch and PHEE on rates of biodegradation. Rates of weight loss increased in the order pure PLA (approx. 0%/yr.) less than starch/PLA (0-15%/yr.) less than starch/PHEE/PLA (4-50%/yr.) and increasing starch and PHEE contents. Weight losses were due to starch only with the degradation proceeding from outside to inside along a narrow zone. Tensile strength did not change with time for pure PLA and, after an initial decline, did not change much for the other compositions. Some formulations containing PHEE and lower (40%) starch levels had higher tensile strengths after initial exposure to soil than those without PHEE.