Structure–Property Relationships in Multifunctional Starch/Agar Biocomposite Films Reinforced with Alkaline–Peroxide Treated Hemp Fibers for Sustainable Applications

As plastic pollution worsens globally, developing biocomposite films has become essential for a cleaner and more sustainable future. In this study, nine biocomposite films were prepared by solution casting using a fixed starch amount (3 g) with varied agar proportions (0%, 50%, and 75% of starch weight) and alkaline hydrogen peroxide-treated hemp fibers (0%, 15%, and 30% of the total starch–agar mass). Films were evaluated for color parameters (total color difference, yellowness index, and whiteness index), moisture content, water absorption, water solubility, and biodegradability (over three months), and analyzed by differential scanning calorimetry, Fourier-transform infrared spectroscopy, and scanning electron microscopy. Results showed increasing hemp fiber content increased total color difference and yellowness index, while agar improved surface uniformity and whiteness. Adding agar significantly (p < 0.05) reduced moisture content, while water absorption increased with higher fiber loading. Water solubility was greatest in films containing starch, 50% agar and hemp. After three months in soil, starch-based and starch/hemp fiber films completely decomposed, whereas starch/agar and starch/agar/hemp fiber films degraded partially (about 52–75%), indicating that agar’s higher density limited microbial activity. The analysis confirmed strong hydrogen bonding between polysaccharides and hemp fibers, reducing crystallinity and affecting thermal stability. Scanning electron microscopy images showed good interfacial adhesion at 15% hemp loading but some agglomeration at 30%. Overall, films composed of starch with 50% agar and hemp fibers provided the most balanced overall performance among the tested formulations in terms of structural integrity, moisture resistance, and biodegradability, making them promising candidates for sustainable applications. • Systematic variation of agar (0–75%) and hemp fiber (0–30 wt%) revealed strong compositional effects on film color, moisture, and solubility. • FTIR and DSC analyses confirmed enhanced hydrogen bonding and altered thermal stability due to fiber–polysaccharide interactions. • Soil burial tests showed complete biodegradation of starch-only and SH films, while agar-rich films degraded more slowly (≈52–75%). • Optimized formulations (particularly SAH15) achieved balanced mechanical integrity, moisture resistance, and biodegradability for sustainable packaging and eco-friendly applications.