Deep eutectic solvent processing of seagrass biomass from beach wrack for sustainable polypropylene composites in large format additive manufacturing

Seagrass-rich beach wrack collected from the Bay of Cádiz (Spain) was valorized as a reinforcement for polypropylene (PP) composites manufactured by large-format additive manufacturing by fused granular fabrication (LFAM-FGF). Two delignification approaches were investigated with NaOH (SG-NaOH) and a deep eutectic solvent (DES) based on lactic acid and choline chloride (SG-DES). While SG-NaOH achieved greater lignin removal, SG-DES yielded a cellulose-rich material with higher crystallinity and a distinctive nanofibrillar morphology. When incorporated into PP at 15 wt%, untreated seagrass (SG-UT) and SG-NaOH resulted in a marked reduction in tensile strength and ductility. In contrast, composites reinforced with SG-DES exhibited a significant increase in Young's modulus while maintaining tensile strength comparable to neat PP and preserving high elongation at break. Thus, despite its lower delignification efficiency, the DES treatment produced the most effective reinforcement among the investigated formulations, highlighting that extensive lignin removal is not necessarily required to achieve high composite performance. In addition, all seagrass-filled formulations showed improved dimensional stability during LFAM-FGF printing, effectively suppressing the warping commonly observed in neat PP, enabling the fabrication of large-scale demonstrator parts which maintained their structural integrity after more than one year of outdoor exposure.