Superstrong Adhesive of Isocyanate-Free Polyurea with a Branched Structure

Using, urea, one of the cheapest chemicals and convenient deamination polycondensation (solvent-free, catalyst-free, one-pot, one-step) provides an industrially relevant and environmentally benign synthesis of branched polyurea. Inspired by the structural analysis of traditional resin adhesives, we hypothesized that the higher the cross-linking degree of the adhesive after curing, the better the bonding performance. Improving the branching cross-linking degree of resin adhesives is the key to improve their bonding performance. In this work, in order to verify the relationship between the branching degree of the polymer adhesives and their bonding properties, branched polyureas with different branching degrees were designed and synthesized. Five polyamines of PA4N, PA5N, PA6N-1, PA6N-2, and PA7N were synthesized from ethylenediamine, diethylenetriamine, tris(2-aminoethyl)amine, triethylenetetramine, and tetraethylenepentamine, respectively. Polycondensation of urea with polyamine (PA4N, PA5N, PA6N-1, PA6N-2, and PA7N) by deamination achieved branched polyureas including PA4N–urea, PA5N–urea, PA6N-1–urea, PA6N-2–urea, and PA7N–urea via a solvent-free, catalyst-free, one-pot, and one-step approach. The polyureas were detailedly investigated as robust adhesives for wood bonding. The bonding performance of the branched polyureas, including PA4N–urea, PA5N–urea, PA6N-1–urea, PA6N-2–urea, and PA7N–urea, was represented by lap shear strengths of 1.52, 2.08, 2.23, 2.36, and 2.64 MPa for poplar wood after soaking the specimens in boiling water for 3.0 h, respectively, which indicated the superior lap shear strength and enhanced water resistance for use as a wood adhesive. As we expected, the results showed that the bonding strength of branched polyurea increased with an increase of the number of terminal functional groups. Besides, the adhesive performance on other substrates, including glass, aluminum, stainless steel, and polyvinyl chloride, was also studied, and the adhesion strengths to glass, aluminum, stainless steel, and polyvinyl chloride are 3.52, 3.29, 1.45, and 1.25 MPa, respectively.