CARIOCA BEAN STARCH (PHASEOLUS VULGARIS L.) THERMOPLASTIC FILMS MODIFIED WITH SODIUM TRIMETAPHOSPHATE SHOWING IMPROVED WATER RESISTANCE AND WETTABILITY PROPERTIES

The growing demand for sustainable alternatives to non-biodegradable fossil polymers has driven the use of biopolymers in various applications. Starch-based biodegradable films are promising due to their abundance, low cost, and biodegradability, but their high water sensitivity limits applications. Carioca bean starch, still scarcely explored, presents technological potential that can be enhanced by chemical modifications such as crosslinking with sodium trimetaphosphate. This study aimed to investigate the influence of sodium trimetaphosphate modification on the water resistance of carioca bean-based thermoplastic films. Starch modification was performed in alkaline medium (pH 11) at 45 °C under stirring for 120 min, followed by neutralization, washing, and drying. Native starch was used as reference. The chemical characteristics of native starch showed a high amylose content (around 30–35%), corresponding to an approximate amylose:amylopectin ratio of 1:2, which were modified (or not?) upon the chemical modification. Thermal analysis confirmed structural changes associated with the modification, evidenced by an increase in the degradation temperature of the material, which may be related to the chemical modification. When thermoplasticized by twin-screw extrusion and hot-pressed, both native and modified bean starch-based thermoplastic granules were successfully hot-pressed into transparent films, whose visual appearance was not significantly altered by crosslinking. After immersion into distilled water for 6 days, the sodium trimetaphosphate modified thermoplastic starch films exhibited high resistance to aqueous medium, showing only 18% mass loss, compared to the reference materials that lost 94% of their initial mass. Moreover, the modified thermoplastic starch films acquired hydrophobic surface features, contrary to the neat ones, as evidenced by the water contact angle increase from 68° up to 117°. In summary, sodium trimetaphosphate modification significantly reduced water solubility and wettability, improving the water resistance of carioca bean starch-based thermoplastic films and reinforcing their potential as biobased materials in line with current technological innovation trends.