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We propose a new idea for facile and green synthesis of urea-based highly branched polymers (HBPs) using polyamines and urea via deamination. More importantly, our strategy is to directly use the synthesized HBPs to construct plywood. The HBPs would be further condensed and solidified during the hot pressing process, resulting in the aldehyde-free wood adhesive with a high bonding performance. In this work, water-soluble HBPs were synthesized by one-pot and one-step strategy via polycondensations between urea (U) and tris(2-aminoethyl)amine (TAEA), with no solvents and catalysts used, as well as purification was also avoided. Here, the TAEA was selected as a model compound of polyamine. Urea was used in this synthesis because of it is one of the cheapest chemicals. Structure characterizations show that the polymers contain both terminal amine and urea groups and their relative contents varied with respect to different monomer ratios. For the first time, the performances of the HBPs water solutions directly used as wood adhesive resins were investigated. The resins showed excellent water resistance, as the bonding strengths were still greater than 1.8 MPa after 7 h boiling water soaking, indicating that the cured resins that consist of polyurea and biuret linkages are quite stable toward hydrolysis. In a wide range of monomer ratio of 1.5−2.5, the bonding performance of the resins did not vary significantly. This study extended the potential applications of HBPs to the wood industry and provides a possible and potential solution to the formaldehyde emission problem of the wood-based products. The advantages of HBP resins over traditional wood adhesives in the synthesis and applications make large-scale application of such resins practical and attractive.

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Abstract:To explain the reason why using phenol-formaldehyde (PF) resin improves the water resistance of soy-based adhesive, the performance of soy-based adhesive cross-linked with hydroxymethyl phenol (HPF) and the reaction between HPF and a common dipeptide N-(2)-L-alanyl-L-glutamine (AG) being used as a model compound were studied in this paper. The DSC and DMA results indicated the reaction between HPF and soy-based adhesive. The soy-based adhesive cross-linked with HPF cured at a lower temperature than the adhesive without HPF. The former showed better mechanical performance and heat resistance than the latter. The ESI-MS, FT-IR and 13C-NMR results proved the reaction between HPF and AG. Because of the existence of branched ether groups in the 13 C-NMR results of HPF/AG, the reaction between HPF and AG might mainly happened between hydroxymethyl groups and amino groups under a basic condition.

Abstract:To prepare a low-formaldehyde soy-based adhesive with good water resistance, phenol-formaldehyde modified with glutaraldehyde (PFG) with lower free phenol and free formaldehyde contents was used to cross-link the soy-based adhesive. The results showed that the mechanical properties and water resistance of plywood prepared with soy-based adhesive with PFG was better than that of plywood with the same amount of phenol-formaldehyde (PF). The reaction between phenol and glutaraldehyde was proved by 13 C-NMR. Under the optimized preparation conditions for plywood, that is to say, press temperature 160 • C, press time 4 min and resin loading 320 g·m −2 , type I plywood could be prepared with 9% PFG as a cross-linker of soy-based adhesive. The Differential Scanning Calorimetry (DSC) result confirmed the cross-linking reaction between soy-based adhesive and PFG or PF. The activation energy of soy-based adhesive with cross-linker PFG was higher than that with PF resin.