A Thermal-Triggered Phase Transition Method for Developing Strong and Water-Resistant Collagen-Based Wood Adhesives from Leather Waste

Developing sustainable biomass-based wood adhesives to replace fossil-derived resins is crucial for low-VOC, formaldehyde-free bonding solutions. Proteinaceous materials offer great potential due to their unique structural properties and abundant active groups. However, balancing hydrophilic adhesion with hydrophobic protection remains challenging. In this study, collagen from chrome shavings was modified sequentially with epichlorohydrin (ECH) and long-chain silane to form composite emulsions consisting of a protein network encapsulating hydrophobic regions. The ECH-modified adhesive achieved a dry strength of 2.23 MPa but failed under wet conditions (0 MPa). Silane modification significantly enhanced its water resistance, yielding dry/wet strengths of 2.65/1.81 MPa, meeting the Class II plywood standard of GB/T 9846-2015. Mechanism studies revealed that ECH cross-linking amino/carboxyl groups form a stable three-dimensional network, while silane grafts onto hydroxyl groups to form Si–C and C–O–Si bonds, improving stability and imparting hydrophobicity. This dual-modification strategy enables a thermal-triggered phase transition from ‘hydrophobic core’ to ‘surface hydrophobic protection’. This approach not only resolves the inherent conflict between adhesion and hydrophobicity in collagen-based materials but also promotes the sustainable transformation of the leather industry by using chrome shavings.