Ultra-Tough Vanillin-Based Polyester Hot-Melt Adhesive through Multiple Oxygenated Groups

To limit the use of petrochemical resources in the context of the energy crisis, it is critical to produce sustainable thermoplastic adhesives. However, it is still difficult for biomass-based hot-melt adhesives to attain superior adhesion strength and recyclability. An intriguing option for sustainable polyester hot-melt adhesives is provided by vanillin, an aromatic lignin derivative with a structure of many oxygen-containing functional groups. Charged negative oxygen-containing functional groups can easily interact with positively charged substrates such as metals, and copolyesters containing vanillin-derived monomers can be used as hot-melt adhesives to achieve a favorable balance between substrate adhesion and cohesion of the polyester glue layer. Here, we designed and modified the lignin-derived aromatic chemical vanillin for copolymerization with 1,4-butanediol (BDO), dimethyl terephthalate (DMT), and diethylene glycol (DEG) to obtain partial biobased copolyesters. The produced copolyesters exhibited excellent adhesion to diverse metal substrates because the monomer generated from vanillin contained a range of oxygen-containing groups including methoxy, hydroxyl, carboxyl groups, and ether bonds. With single-lap shear strengths of 10.6, 14.4, and 15.1 MPa for aluminum, brass, and iron, respectively, the copolyester with 30% vanillin-derived monomer content demonstrated outstanding adhesion. The resultant copolyesters can be recycled by dissolving them in a particular solvent at room temperature and exhibiting good resistance to heat, acids, and alkalis. Additionally, we evaluated the potential interaction of naturally oxygenated biomass derivatives with metal atoms using density functional theory (DFT), assessed the rational material design, and explained how exceptional adhesion properties are achieved.