Polyamide-formaldehyde resin as a low-toxic adhesive for wood bonding

Formaldehyde-based resins are widely used in the field of wood adhesives. However, high formaldehyde emission is still a serious problem. Therefore, a low-toxic wood adhesive was successfully synthesized using branched polyamide and formaldehyde via hydroxymethylation in this work. The polyamide was firstly synthesized by condensation between diethylenetriamine (DETA) and urea (U), with no solvents and catalysts used, which also avoided purification. Then the polyamide directly reacted with formaldehyde with different F/U molar ratios to obtain polyamide-formaldehyde resins. The characterization results show that the polyamide contains terminal amine and urea groups, and the formaldehyde was basically reacted completely. The test results of adhesive performance show dry bonding strength of polyamide-formaldehyde resin (F/U = 2) reached 1.72 MPa and wet bonding strength was still greater than the standard requirements (≥0.7 MPa), but the formaldehyde emission was only 0.2 mg/L, which was well under the E0 standard (≤0.5 mg/L), indicating that adhesive resins introduced with stable polyurea linkages and branched cross-linked structure have great performance. Low-cost reactants, a simple preparation process, and great adhesive performance will increase the potential application of the resins in the field of the wood industry.