Mimosa tannin based NIPU wood adhesive with significant substitution of hexamethylenediamine using polyethyleneimine

Tannin-based non-isocyanate polyurethane (NIPU) adhesive exhibits an excellent performance for wood bonding, but contains excess toxic hexamethylenediamine (HMDA), which is emitted to the environment during the preparation of wood panels. Herein, branched polyethylenimine (PEI)/tannin mixtures were utilized to share this adhesive in one integral formulation via various additions at the expense of HMDA. The physical properties, including pH, non-volatile content, viscosity, tannin- and bio-sourced-contents were investigated and compared to a traditional NIPU. The results have shown that the viscosities of the modified adhesives were increased significantly, with improvements at about 2–5% of bio-sourced content and around 4–8% of tannin content being achieved. Moreover, the HMDA content was decreased from 31 % for control T-NIPU-0-0 adhesive to 19 % for the modified adhesives. Possible undergoing reactions of the tannin-NIPU based adhesives series was investigated by Fourier transform infrared spectroscopy (FT-IR) and ¹³ C nuclear magnetic resonance (NMR) analyses. The main tannin-based NIPU structure was maintained even after tannin/PEI mixtures were added. Thermomechanical analysis (TMA) results showed two typical curing stages for either the control adhesive (T-NIPU-0-0) or the modified ones, but different curing performance was observed for modified adhesives, particularly at the first stage. The thermal stability of the modified adhesives was enhanced due to the inserted tannin/PEI mixture, which was proven by thermogravimetric analysis (TGA). The mechanical properties indicated that formulations with suitable weight ratios of tannin/PEI mixtures, such as T-NIPU-4-1 and T-NIPU-5-1 adhesives, could hold a similar performance to the control T-NIPU-0-0 adhesive, which satisfied the European Norm EN 312–2010 for 12- mm boards of type P2. Moreover, a small addition of glycerol diglycidyl ether (GDE) into the T-NIPU-4-1 adhesive endowed an acceptable bonding performance while undergoing different preparation conditions. Therefore, the modified adhesives with the branched PEI/tannin adduct inherited outstanding bonding performance compared to classical tannin-NIPU adhesives along with a noteworthy decreased release of harmful substances such as HMDA, which is in line with the requirements for a cleaner production process.