Structure and properties of formaldehyde-free faba bean starch composite foams influenced by additives and lignin-glyoxal co-polycondensation

• Compression-molded air-classified faba bean starch biofoams are fabricated • Reinforced biofoams show anisotropic strength: radial > tangential (∼375–500%) • Lignin-glyoxal crosslinking enhances crystallinity and thermal stability (>300°C) • Functional performance shows high impact resistance (49.7 J/m) and recovery (93.9%) Foams fabricated from polymeric sources are increasingly being adopted as sustainable alternatives to petroleum-based foams but often suffer from poor mechanical strength and high water sensitivity. Crosslinking can reinforce the polymer network, enhancing structural integrity and resilience. However, many conventional crosslinkers are formaldehyde-based, raising environmental and health concerns. These limitations highlight an urgent need for novel crosslinking strategies. In this study, compression-molded biofoams were fabricated using air-classified faba bean starch (FBS), with a lignin–glyoxal (LiG) resin incorporated as a formaldehyde-free crosslinker, alongside additives such as glycerol and a chemical blowing agent. Results showed that the LiG resin reinforced the starch network, leading to progressive improvements in properties with increasing LiG content. Compressive strength reached 749.47 kPa, while apparent density (0.60–1.25 g/cm³), impact resistance (25.30–49.65 J/m), and recovery after compression (81.59–93.89%) steadily increased with higher LiG loading. Thermal stability was also improved, with decomposition temperatures exceeding 300°C, consistent with the formation of a denser crosslinked network. Morphological analysis revealed a predominantly closed cell structure, and X-ray diffraction indicated increased crystallinity, with three distinct peaks observed. This study demonstrates a simple, effective, and formaldehyde-free strategy for producing high-performance starch-based biofoams with enhanced mechanical and thermal properties, underscoring their potential as sustainable materials for packaging and cushioning applications.