Combined effect of urea, glycerol, bentonite, and sodium tetraborate on the optical and mechanical properties of corn and cassava starch mulching films

Biodegradable films used as soil covers can enhance agricultural productivity by reducing water evaporation, limiting soil–crop contact, and suppressing weed growth. This study explores the combined effects of additives (glycerol, urea, borax, and bentonite) on the mechanical and optical properties of corn and cassava starch films produced through thermal gelatinization. To analyze the effect of the studied additives, Response Surface Methodology using the Box-Behnken Design was employed. Starch suspensions at 5% (w/v) were thermally gelatinized, and the additives were added at varying levels: glycerol as a plasticizer and urea as an additive capable of modifying intermolecular interactions (15 to 30% w/w), borax as a crosslinking agent (3 to 6% w/w), and bentonite as a reinforcing agent (0 to 10% w/w). Optical properties were evaluated by UV–Vis spectrophotometry and mechanical strength by puncture tests. Cassava starch films showed reduced UV and PAR transmission, mainly affected by urea, while bentonite had a smaller effect; with glycerol, only bentonite was significant. Urea improved UV and PAR barrier properties but increased rigidity, indicating it modifies starch interactions rather than acting as a plasticizer. Corn starch films showed good model fit with both plasticizers: with glycerol, maximum strength occurred at 5% bentonite, while with urea its effect was more pronounced. Despite the incorporation of the studied additives into corn and cassava starch matrices exerting distinct effects on film properties, several of the evaluated formulations significantly improved these properties, rendering the resulting films promising candidates for use as biodegradable mulching materials in organic agriculture.