Bonding properties of polyurethan enhanced by branched polyamine and wood interfacial activation

Developing formaldehyde-free and excellent water resistance has constantly been at the center of attention of wood adhesives. In addition to enhancing the adhesive itself, the modification of wood interface can also improve its bonding strength and water resistance. In this study, aqueous solutions of NaIO4 and NaClO2 have been sprayed onto the surface of poplar veneer evenly. An activated wood interface equipped with -CHO and -COOH was prepared. Traditional polyurethane adhesives are primarily two-component, with excessive manufacturing cost, complicated production and use procedures. In this study, a single component high branched polyurethane adhesive (GI) was synthesized via a A2 +B3 oligomerization method by using glycerol and isophorone diisocyanate (IPDI). Using branched polyamine (PA4 N) as a modifier, GI was modified to GI-PA4 N, which was used as an adhesive and then hot pressed with the activated wood to make three-layer plywood. The bonding interface was equipped with urethane bonds, urea-based chemical bonds, amide bonds, Schiff bonds and hydrogen bond, which considerably enhanced the stability and water resistance of the bonding interface. FT-IR and XPS confirmed the existence of -CHO and -COOH at the interface of the activated wood, and characterized the synthesis mechanism and bonding mechanism of GI-PA4 N. The results indicated that when the mass ratio of polyurethane to polyamine was 4:1, the overall performance of the adhesive was the best. The dry bonding strength increased from 1.92 MPa to 3.63 MPa, and the wet bonding strength increased from 1.77 MPa to 3.13 MPa, much higher than the minimum requirement of GB/T 17657–2013 (0.7 MPa). A formaldehyde-free, high-performance plywood with excellent water resistance was successfully prepared using wood interfacial activation and branched polyamine modification.