Ultra-Strong Green Plastics from Marine-Sourced Alginate Dendritic Colloids and Chitin Nanocrystals with a “Cement–Mortar” Structure

The escalating environmental burden of petrochemical plastics necessitates biodegradable alternatives with balanced strength, flexibility, and water resistance. Here, sodium alginate (SA) and chitin nanocrystals (ChNCs), both derived from marine resources, were integrated to fabricate reinforced films. ChNCs were sustainably prepared via a microwave-assisted ferric chloride hexahydrate (FeCl 3 ·6H 2 O) hydrolysis route, with surface charge density tuned by controlled deacetylation to optimise hydrogen bonding and electrostatic interactions with SA. In parallel, dendritic colloidal SA was generated by ultra-high-shear homogenization in an environmentally benign manner, serving as flexible nodes in a “cement–mortar” architecture with rigid ChNCs to achieve synergistic reinforcement. Compared with neat SA films, the composite containing ∼36% deacetylated ChNCs exhibited increases of 196.03% in tensile strength, 150.80% in elongation at break, and 44.41% in Young’s modulus (308.46 MPa, 20.29%, 8845.21 MPa). Rheological and isothermal titration calorimetry analyzes confirmed the essential role of electrostatic balance and colloidal networks. The films maintained transparency and biodegradability while displaying enhanced thermal stability, water resistance, and barrier performance. This scalable strategy offers marine resource-based packaging films with tunable interfaces, addressing the long-standing strength–ductility trade-off in polysaccharides and holding promise for eco-plastics.