Borax as a Cross-Linking Agent in Pectin Films: Effects of Borax Concentration and Temperature on Film Properties

The present study evaluates the effect of borax as a cross-linker at different concentrations (0%, 1.32%, 2.61%, and 3.87%w/w) on the physicochemical properties of the films comprised of pectin, glycerol, and borax. The film-forming solutions were made by two distinct preparation methods: i. a heated system, at 85 °C, without stirring (thermal method, TM) and ii. a nonheated high-speed stirring system, 30,000 rpm (agitation method, AM). Films produced by both methods were compared to assess their structural, thermal, mechanical, and antimicrobial properties. Steady-state and real-time infrared spectroscopy (FTIR) indicated the formation of B-O-C moieties, with cross-linking efficiency favored under both higher pH and temperature. SEM images and mechanical analyses revealed that AM gave rise to markedly coherent and continuous films, whereas TM-derived films exhibited surface defects and breakpoints, particularly at higher borax concentrations. Thermal analyses (TGA/DTA) indicated that borax enhances the thermal stability of the films regardless of the preparation method. Increasing borax content, however, generally reduced tensile strength and elongation at break, suggesting a competitive balance between cross-linking and plasticization. The films did not display significant antibacterial effects, likely due to partial solubilization during diffusion-based assays. Overall, the results demonstrate that temperature and borax concentration in the film-forming solution play a key role toward pectin film properties.