Characterization of two maritime pine tannins as wood adhesives

Maritime pine tannins from the two suppliers BIOLANDES and DRT were used in combination with paraformaldehyde (5%), hexamine (6%) and glyoxal (9%) as hardener in the preparation of adhesives for wood-based panels. Both tannins showed similar structures according to the matrix-assisted laser desorption/ionization-time of flight analysis. A high reactivity has been checked by gel time measurements and the viscosity increase. Already the BIOLANDES and the DRT tannin alone registered an internal bond (IB) of 0.4MPa without any hardener. The BIOLANDES tannin with paraformaldehyde could fulfil the European standard EN 312. In general, DRT tannin obtained the best mechanical results. However, the pH and the moisture content play a fundamental role for the final IB. Thus, DRT tannin with glyoxal as hardener at high pH and moisture content gave an IB of 0.51MPa. The mechanical resistance differences between BIOLANDES and DRT tannin were confirmed by the C-13 NMR spectra. DRT tannin presents a lower polymerization de than BIOLANDES, and more reactive sites are available.