In this study, the effect of the molecular weight (MW) of urea–formaldehyde (UF) resins on their cure kinetics, interphase, penetration into wood, and adhesion strength was evaluated for the first time, to understand their contribution to cohesion and adhesion in bonding wood. UF resins with two final formaldehyde-to-urea (F/U) molar ratios (1.0 and 1.2) were prepared as low-viscosity resin (LVR) and as high-viscosity resin (HVR) through viscosity control. Five LVR/HVR blending ratios (100:0, 75:25, 50:50, 25:75, and 0:100) were used to obtain UF resins with different MW distributions and average MWs and, hence, different viscosities for the two molar ratios. As the viscosity during the condensation phase increased, the MW increased while the gelation time and the low molecular weight fraction decreased. The resins with F/U molar ratio of 1.2 had higher MW and activation energy than those with F/U molar ratio of 1.0. Isoconversional analysis showed that the 1.0 F/U molar ratio resin went through a multiple-step process in their curing mechanism, unlike the 1.2 F/U molar ratio resin, whose cohesion during bond formation was likely affected by the higher F content. As the MW increased, the resins with 1.0 and 1.2 F/U molar ratios exhibited the highest maximum storage modulus (E′max), greatest depth of resin penetration, thinnest bond-line, and highest adhesive strength at apparent weight-averaged Mw of 2000–2400 g/mol for the 1.0 F/U molar ratio resins (according to mixing ratios LVR/HVR = 50:50 and 25:75) and 3500–4500 g/mol for the 1.2 F/U molar ratio resins (again according to mixing ratios LVR/HVR = 50:50 and 25:75). These results suggest that the MW of UF resins has a big impact on cure kinetics that contributes to their cohesion behavior, while it also affects E′max, the depth of resin penetration, and the bond-line thickness, which all contribute to their adhesion behavior.
