Composition–Property Relationships of Bioplastic Films Valorizing Green, Brown, and Red Macroalgal Polysaccharides, Biodiesel-Derived Bio-Glycerol and Vinegar: A Circular and Blue Economy Approach

Persistent petroleum-based packaging pollutes marine ecosystems, emits greenhouse gases and threatens human health, whereas starch-based alternatives compete with food crops. In this study, polysaccharides green-extracted from three seaweeds, Ulva fasciata starch (UFS), Sargassum latifolium alginate (SLA), and Pterocladia capillacea agar (PCA), were cast into bioplastic films using only water, biodiesel-derived bio-glycerol and vinegar. Six formulations (three neat polymers and three 1:1 binary blends) were prepared at constant plasticizer and cross-linker levels and characterized for thickness, density, solubility in neutral, acidic and alkaline media, soil-burial mass loss, and spectroscopic, crystallographic, morphological, thermal and mechanical properties. All formulations formed continuous, flexible, and self-supporting films. The neat-alginate film dissolved completely in water within 30 min, whereas its blends with starch or agar retained over 99% of their mass, showing that an insoluble partner polysaccharide can stabilize alginate without calcium crosslinking. The alginate/agar film combined a tensile strength of 1.19 ± 0.35 MPa with an elongation at break of 85.96 ± 2.5% and remained below 1.4% solubility in all media, while neat agar was the most extensible (102.87 ± 3.0%). Soil burial removed 91.8–100% of the initial mass within four weeks, at composition-dependent rates. These films are promising candidates for short-life dry-food packaging in circular and blue bioeconomies.