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.
