Sugar-painting inspired branched ureido polymers as high-performance formaldehyde-free wood adhesive

Developing multifunctional adhesives with strong adhesion, excellent water-resistance, sustainability, low cost, and simple production approach is still a great challenge. Here, inspired by sugar painting, this work reports the synthesis of thermoset branched ureido polymers (3 N-Un) containing a heterocyclic structure and many ureido groups. The synthesized branched polyureas using low-cost raw materials showed self-healing properties, compact microstructures, and robust cohesive forces due to their strong multiple H-bonds. The 3 N-Un used as a formaldehyde-free wood adhesive exhibits remarkable bonding and thermal properties after curing as well as excellent adhesion to various materials and tissues. Interestingly, using NaIO4 solution to oxidize the surface of poplar veneer can make it functionalized with aldehyde and carboxyl groups and then, covalently cross-linked with ureido polymer adhesives, thus improving the bonding strength and decreasing the hot-pressing temperature (from 200 to 120 °C). The dry lap shear strength and wet lap shear strength (soaking in hot water at 63 °C for 3 h) of the plywood made with 3 N-U3 adhesive and poplar veneers (surface treated by NaIO4 solution) are 2.33 MPa and 1.98 MPa, respectively, which are much higher than the requirements of Type II Plywoods in China National Standard (GB/T 9846-2015). This investigation provides a novel method for the synthesis of environmentally friendly adhesives and the covalent cross-linking of the wood interface with adhesives in wood-based panel industries.