Construction of a Fully Biobased Plastic from a Starch Tung Oil Acid Ester

Renewable and biodegradable starch-based materials offer attractive alternatives to petroleum-based plastics, yet their processing and performance require optimization. This study employed starch (ST) and tung oil (TO) to synthesize a starch tung oil acid ester (ST-TO) via esterification in a CO2-switchable solvent, followed by thermal cross-linking of conjugated double bonds. By systematically controlling the degree of substitution (DS, from 1.42 to 2.40), the hydroxy groups were progressively replaced, which weakened hydrogen bonding and reduced the glass transition temperature to as low as 88.99 °C, thereby imparting thermal processability. The resulting cross-linked bioplastics exhibited tunable mechanical properties, with optimal performance at DS of 1.82 (SP3): tensile strength of 16.1 ± 3.3 MPa, pencil hardness of 9H, water contact angle exceeding 90°, and gel content up to 98.98%. SP3 maintained dimensional stability in acids, bases, and organic solvents over 30 days, withstood thermal exposure up to 250 °C without deformation, and achieved approximately 90% mass loss in soil within 90 days. This work demonstrates that precise control over substitution and cross-linking yields starch-based plastics with integrated thermal processability, robust mechanical performance, exceptional chemical resistance, and natural biodegradability, offering a viable pathway toward sustainable alternatives to conventional petroleum-based materials.