Submitted to: Starch
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 24, 1997
Publication Date: N/A
Interpretive Summary: Concerns over the landfill burdens of plastics, as well as legislation to eliminate the dumping of plastics at sea (MARPOL Treaty), have increased awareness of the need for biodegradable plastics. Starch- based plastics use an inexpensive and renewable resource, yet such materials can be extremely brittle unless other materials, called plasticizers, are included to increase flexibility. Under certain conditions, common plasticizers such as urea can produce noxious products. Amino acids were tested and shown to be effective plasticizers with the added benefit of providing nitrogen required for biodegradation. These amino acids plasticizers produce nontoxic, environmentally friendly blends for single-use, disposable or compostable starch-based plastics. The research benefits the fledgling bioplastics industry, as well as the general American populous, by potentially replacing imported petroleum-derived plastic materials with renewable, home-grown commodities.
Amino acids of differing hydrophobicity were tested as plasticizers in starch-glycerol (4:1 weight ratio) blends and were compared to the more conventional plasticizers, urea, sucrose and ammonium chloride. In mechanical tests (tensile strength and percent elongation) on extruded ribbons containing up to 20 weight percent plasticizer, glycine and isoleucine were found to be extremely poor plasticizers. However, lysine behaved similarly to sucrose. Proline compared favorably to urea. Moreover, at concentrations of 23 and 29 weight percent, proline was superior to urea in its ability to increase the percent elongation of the starch-glycerol mixture. The glass transition temperature (Tg) for the standard starch-glycerol samples was 40 deg C, as determined by differential scanning calorimetry. Added lysine, sucrose, proline and urea effected the Tg similarly. At 5 weight percent, all Tg's increased, while at 20 weight percent they dropped to room temperature or below. Isoleucine, at low or high concentration, yielded a Tg of 60 deg C, consistent with very brittle material. Biodegradation experiments were conducted on selected formulations by monitoring CO2 evolution after inoculation with Aspergillus niger. Little CO2 accumulated from the standard starch-glycerol mixture or sucrose plasticized blends, likely due to a lack of combined nitrogen. In contrast, proline and urea-plasticized blends were rapidly metabolized.