Soy protein/sodium lignosulfonate biocomposite film for agricultural crop cultivation

The widespread use of petroleum-based plastic mulch films in agriculture has led to severe environmental pollution, which has driven increasing interest in the development of biodegradable plastics derived from biomass resources. Herein, eco-friendly composites/films were fabricated using soy protein isolate (SPI) and sodium lignosulfonate (LS) as the primary biomass components through a simple casting method. The SPI/LS composites exhibited impressive mechanical properties, including high tensile strength (15.4 MPa), fracture strain (290 %), toughness (33 MJ‧m^-3), Young's modulus (100 MPa), and excellent fatigue resistance (maintaining 5 MPa strength after 10 cycles of repeated stretching at 150 % strain). These properties were closely linked to multiple molecular interactions within the composites. Fourier transform infrared spectroscopy and X-ray diffraction analysis revealed that the protein's secondary structure transitioned to a more ordered state, with enhanced hydrogen bonding between SPI and LS. Furthermore, the SPI/LS films demonstrated excellent UV-shielding, moisture retention capabilities, and enhanced crop seed germination rates. Plant seedling tests indicated that after biodegradation in soil, the films released nitrogen-containing ions such as NH₄⁺ vital for healthy root growth. Therefore, the eco-friendly and biodegradable SPI/LS composites/films hold great potential as mulching films for agricultural crop cultivation, improving crop yields while reducing the need for fertilizers.