Low curing temperature tannin-based non-isocyanate polyurethane (NIPU) wood adhesives: Preparation and properties evaluation

Bio-sourced glycerol diglycidyl ether (GDE) has been used in appropriate proportions as an enhancer to decrease the curing temperature, and improve the bonding performance, while reducing the emission of harmful substances, such as hexamethylenediamine, in the preparation process of tannin-based NIPU adhesives. The route of suggested enhancement was investigated by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and Fourier transform infrared spectroscopy (FT-IR). The epoxy groups of GDE easily cross-link with molecules containing amino groups, promoting the reactions proceeding and resulting in a lower curing temperature. In parallel, the esterification reaction between the C3 hydroxyl groups of the flavonoid tannin heterocyclic ring and the epoxy groups of GDE occurred as well. The reduced curing temperature, improved fracture surface morphology, and enhanced thermal stability were demonstrated by thermomechanical analysis (TMA), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA), respectively. The bonding performance was significantly enhanced, satisfying the standard requirements for interior grade plywood (≥0.7 MPa), even though under a relatively lower curing temperature compared to unmodified tannin NIPU adhesives. Therefore, this modification strategy enhanced the bonding properties, reduced the energy consumption during preparation, and decreased release of harmful substances, which is in line with the requirements for a cleaner production process.