Amino Acids as Plasticizers for Starch‐based Plastics

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 hydrochloride 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 (T g ) for the standard starch‐glycerol samples was 40°C, as determined by differential scanning calorimetry. Added lysine hydrochloride, sucrose, proline and urea effected the T g similarly. At 5 weight percent all T g 's increased, while at 20 weight percent they dropped to room temperature or below. Isoleucine, at low or high concentration, yielded a T g of 60°C, consistent with very brittle material. Biodegradation experiments were conducted on selected formulations by monitoring CO 2 evolution after inoculation with Aspergillus niger . Little CO 2 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.