Effects of boric acid on the morphology and transparency of solution-cast ethylene–vinyl alcohol films for packaging applications

Ethylene–vinyl alcohol (EVOH) is widely used in packaging applications owing to its excellent oxygen barrier properties, mechanical strength, and transparency. However, the inherent hydrophilicity of this polymer results in moisture absorption, which leads to performance degradation in humid environments. Furthermore, the whitening of EVOH films often occurs during solution casting, which limits their applicability. To address these drawbacks, EVOH films were chemically modified by crosslinking with boric acid (BA). The introduction of BA controlled the crystallinity and promoted polymer chain rearrangement, thereby reducing the degree of structural heterogeneity and enhancing the barrier, mechanical, and optical performances. Additionally, EVOH/BA films were fabricated with different BA concentrations (0, 1, 3, and 5 wt%). In the absence of BA and at a low BA concentration of 1 wt%, the film exhibited significant whitening owing to mesophase accumulation and the formation of porous structures. Upon increasing the BA content, the degree of film whitening progressively diminished, with a fully transparent film being obtained at a BA concentration of 3 wt%. The enhanced crosslinking density at this concentration also contributed to an improved barrier performance, thermal stability, and mechanical strength. However, a slight decline in performance was observed at a higher BA concentration of 5 wt% owing to excessive crosslinking. Based on these results, a 3 wt% BA concentration was considered optimal. This work represents a simple yet effective strategy for improving the structural integrities and functional performances of solution-cast EVOH films, offering practical potential for advanced packaging applications.