Curing behavior and viscoelastic properties of pine and wattle tannin-based adhesives studied by dynamic mechanical thermal analysis and FT-IR-ATR spectroscopy

This study investigated the curing behavior and viscoelastic properties of two types of tannin-based adhesives, wattle and pine, with three hardeners: paraformaldehyde, hexamethylenetetramine and TN (tris(hydroxylmethyl)nitromethane), by FT-IR-ATR spectroscopy and dynamic mechanical thermal analysis (DMTA). Using FT-IR-ATR spectroscopy, the chemical structures of tannin-based adhesives were determined and the degrees of curing as a function of temperature and time were compared with the conversion degrees of the hydroxyl groups. Paraformaldehyde was shown to be more reactive with wattle tannin than the other hardeners, while hexamethylenetetramine was the most reactive with the pine tannin. As the quantity of hardener was increased, the conversion degree also increased. The storage modulus (E′), loss modulus (E″) and loss factor (tan δ) of each adhesive system were obtained by DMTA. With increasing temperature, as the tannin-based adhesives hardened, the storage modulus (E′) increased in all adhesive systems. The curing behavior results obtained by DMTA showed a similar tendency as seen from the FT-IR-ATR spectroscopy results. The curing behavior of the tannin-based adhesives was successfully determined using FT-IR-ATR spectroscopy in combination with DMTA.