Tannin-based non-isocyanate polyurethane wood adhesive rich in complicated cross-linked networks with ultra-strong bonding property and excellent water resistance by introducing poly-urea structure

Developing of bio-based wood adhesives is an inevitable common topic in the development at top speed of wood industry. Herein, a bio-based tannin wood adhesive, named as tannin-based non-isocyanate polyurethane-urea (TNIPU-U), was designed which showing a ultra-strong adhesion and water resistance performance by two-step approach. A plausible mechanism was proposed, which implied that urea acted as the crosslinker to connect with derived intermediates containing free amino groups rooted from the reaction of tannin, dimethyl carbonate (DMC), and HMDA, to introduce urea linkages in traditional TNIPU. Furthermore, addition of urea to act as a trapping agent to capture excessive free HMDA in TNIPU adhesive, promoting additional polyurea formation, leading to avoidance of its emission during the hot-pressing process. This will convert the imperfection of TNIPU to a superiority by a eco-friendly strategy. The complicated cross-linked networks will be obtained that are rich in physical entanglement, covalent bonds, and hydrogen bonds. Compared with the TNIPU, the shear strength of TNIPU-U adhesive, after soaked in hot water and boiling water for 3 h, exhibited increase by 17.88 % and 34 %, respectively, indicating the potential for use under harsher conditions. Moreover, the TNIPU-U adhesive exhibits an excellent bonding performance (1.93 MPa) that greatly exceeds the China National Standard (GB/T 9846–2015, ≥ 0.7 MPa) requirement for exterior-grade plywood type I. It is comparable to commercial phenol-formaldehyde resin on the aspect of water tolerance. It can be applied to particleboard preparation and bamboo bonding as well, showing a well-adopted feature. This study provides an efficient, low-cost, and sustainable strategy for high performance bio-based wood adhesives.