Study on preparation of tannin-hydroxymethylated furfuryl alcohol adhesives

To improve the water resistance of tannin adhesives, bio-sourced furfuryl alcohol was used to react with trace formaldehyde to synthesize hydroxymethylated furfuryl alcohol as a modifier for tannin adhesives. The bonding performance of tannin-hydroxymethylated furfuryl alcohol(T-PFA) with different pH values, amount of formaldehyde addition and mass ratio of tannin to furfuryl alcohol were studied by preparing laboratory three-layer plywood and evaluating their shear strength. At the same time, resorcinol and 5-hydroxymethylfurfural (HMF) were used as model compounds to prepare resorcinol-HMF resins and then were analyzed by means of electrospray ionization mass spectrometry(ESI-MS), nuclear magnetic resonance carbon( ¹³ C NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy, which aimed to explain their reaction mechanisms more clearly. The results showed that resorcinol could react with hydroxymethylfurfural and produce a branched macromolecular structure, thus demonstrating the feasibility of preparing wood adhesives with hydroxymethyl furfuryl alcohol modified tannins. In addition, plywood test results indicated that a T-PFA adhesive shows a good bonding performance and water resistance with a shear strength as high as 0.80 MPa after soaking in boiling water for 3h, which satisfied the requirements of Chinese National Standards GB/T 9846-2015(≥0.7 MPa).