High-Value Upcycling of Waste Polystyrene Foam into Wood Adhesives via a Catalyst-Free Cross-Linking Strategy

The rapid accumulation of waste polystyrene (PS) foam has emerged as a critical environmental challenge due to its persistent nature and limited recyclability. Herein, we demonstrate a mild and catalyst-free strategy that enables the conversion of waste PS foam into a value-added wood adhesive (WPS–WA). Ethyl acetate serves as a mild solvent that dissolves waste PS, rosin, and epoxy resin E51, forming a homogeneous, single-phase system. Subsequently, the carboxyl groups of rosin react with the epoxy groups of E51 epoxy resin through epoxy ring-opening esterification, leading to the formation of a cross-linked network. During simultaneous cross-linking and solvent evaporation, the linear PS chains become physically entrapped and entangled within the rosin-epoxy cross-linked network. The synergistic integration of these two structural components endows the resulting adhesive with excellent bonding performance. Notably, the WPS–WA2:1:2:10 formulation achieved a wood butt-joint tensile strength of 3.45 ± 0.05 MPa, comparable to that of commercial wood adhesives, while maintaining its bonding performance after short-term simulated sunlight irradiation and low-temperature storage at −40 °C for 36 h. The conversion of waste PS foam into a formaldehyde-free wood adhesive through a mild and sustainable process offers a promising strategy for plastic waste valorization and the development of value-added adhesives.