Thermosetting environment‐friendly polyamide adhesives prepared with citric acid and hexanediamine

Abstract Thermosetting polyamide resins prepared by reacting citric acid with hexanediamine (CH) were synthesized and used for plywood bonding in this work, and the effects of reaction time, molar ratio of citric acid, and hexanediamine on the bonding performance have been studied. Fourier transform‐infrared (FT‐IR) and liquid chromatography‐mass spectrometry (LC–MS) were used to determine the structures of the resins. Differential scanning calorimeter (DSC), dynamic mechanical properties (DMA), and thermogravimetric analysis (TG) were used to analyze its thermal properties. The results show that amide bonds can be formed by the reaction of carboxyl and amino groups, which acted as reaction raw materials that can undergo further polycondensation to render possible the formation of a cured cross‐linked network, thus to ensure the bonding performance of the adhesives. Meanwhile, CH adhesives presented excellent bonding strength and water resistance above the standard requirements (≥0.7 MPa). In particular, the plywood prepared by CH resin with molar ratio of citric acid to hexanediamine of 1:1.5 and reaction time of 3 h exhibited the best performance, the dry shear strength being as high as 1.96 MPa, 24 h cold water soaking strength of 0.87 MPa, and 3 h hot water (63°C) soaking wet strength attaining 1.24 MPa.