Valorisation of Ulva spp. green seaweed and its residues from pressurized hot water extraction: A comparative study of casting and melt-compounding techniques for film production

This study investigates the valorisation of Ulva spp. biomass after Autohydrolysis (AH) extraction performed at 120-200 °C as feedstock for biopolymer films, providing the first comparative assessment of casting and melt-compounding techniques. Compositional analysis confirmed that the biomass contained carbohydrates (27–36%, with 4-24% of cellulose), proteins (11–26%) and ashes (10–42%). The seaweed biomass also contained phenolic compounds (12–64 mg Gallic Acid Equivalents (GAE)/g sample), which exhibited antioxidant properties (5–11 mg Trolox Equivalent Antioxidant Capacity (TEAC)/g sample). Lower molecular weights were observed with increasing AH temperature, indicating partial depolymerization of the macromolecules. Films were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD), together with mechanical and water-barrier properties. The casting method produced continuous, slightly transparent films with Internal Transmittance (Ti) values of 1–40%, higher stiffness (Young’s Modulus, E: 399.83–1852.85 MPa), lower flexibility (Elongation at Break, ɛb: 1.34–12.28%), low Water Vapor Permeability (WVP: 1.53–2.28 × 10 -10 g·m·Pa -1 ·s -1 ·m -2 ) and moderate hydrophilicity (contact angle: 57.95°–65.60°). Melt-compounding films were opaque, more homogeneous films with lower stiffness but, in some cases, greater extensibility (ɛb: 1.31–13.33%), higher WVP (7.31–9.12 × 10 -10 g·m·Pa -1 ·s -1 ·m -2 ) and increased hydrophilicity (contact angle: 23–44°). Thermal–mechanical processing also influenced molecular organization and the distribution of hydrophilic groups. Overall, this study demonstrates, for the first time, that AH residues derived from Ulva spp. can be transformed into bio-based films with tunable structural, mechanical, and barrier properties. • Green seaweed Ulva spp. was valorized for film production • Films were produced using both casting and melt-compounding techniques • Autohydrolysis temperature influenced films mechanical and barrier properties • Ulva-based films as a potential alternative to conventional plastics