Tuning Polyphenols for Thermosetting Wood Adhesives: Class 'A1' Non-Isocyanate Polyurethanes from Condensed Tannins

This study aimed to develop nonisocyanate polyurethane (NIPU) wood adhesives from Quebracho condensed tannins (CT). Following the carbonation of CTs with dimethyl carbonate, condensation with multifunctional amines delivered two distinct NIPU formulations: TANIPU-DA when hexamethylene diamine (DA) was used and TANIPU-DA-TD when successively using DA and hexamethylenetetramine (TD). For both TANIPUs, urethane formation was confirmed with Fourier-transform infrared (FTIR), viscosity measurements, and size exclusion chromatography (SEC). Gelation and vitrification were evidenced with DMA in the 120-150 °C and the 160-220 °C ranges, respectively. TANIPU-DA-TD wood-bonded assemblies reached a lap shear strength exceeding 10 N/mm², qualifying TANIPU-DA-TD as a class 'A1' wood adhesive according to European standards. TANIPU-DA and TANIPU-DA-TD achieved relatively high wet strengths of 2.5 ± 1.3 and 3.8 ± 1.2 N/mm², respectively. This study demonstrates that tannin-based NIPUs can meet the dry bonding strength requirements for wood adhesives, suggesting a viable, more environmentally friendly alternative to traditional formaldehyde- and isocyanate-based wood adhesives.