Highly Branched Polyurea-Based Resins: Clean Synthesis and Wood Bonding Performance

Development of green and safe adhesives with high performance is important for sustainability of the wood industry. In this work, highly branched polyureas (HBPUs) were synthesized via polycondensations of urea (U) with triethylenetetramine by a one-pot and one-step strategy. Structural characterization studies by employing Fourier-transform infrared spectrometry, electrospray ionization–mass spectrometry, gel permeation chromatography determination, and thermogravimetric analysis were also performed. HBPU solutions were used as formaldehyde-free wood adhesives and exhibited good bonding performance. The effects of the synthetic conditions and hot-press on bonding performance were investigated and discussed. Best bonding performance was achieved with a U/TETA molar ratio of 3.0, a reaction time of 13 h and a hot-press temperature of 220 °C.HBPUs were further reacted with formaldehyde (F) in different low F/U molar ratios to produce HBPU–formaldehyde (HBPUF) resins. Differential scanning calorimetry analysis demonstrated that the HBPUF resins exhibited similar curing behavior to conventional urea-formaldehyde (UF) resin, but relatively lower cure temperature. When HBPUF resins were used as wood adhesives, they showed better bonding performance and a much lower formaldehyde emission level than UF resin. Facile synthesis and satisfactory performance of HBPU and HBPUF resins make their applications in the wood industry promising.