Effects of Pinus pinaster bark extracts content on the cure properties of tannin-modified adhesives and on bonding of exterior grade MDF

Phenol-urea-formaldehyde-tannin (PUFT) adhesives with different degrees of phenol substitution by Pinus pinaster bark tannins were thermally characterized by dynamic mechanical thermal analysis (DMTA) and by differential scanning calorimetry (DSC). They were employed to prepare exterior grade Medium Density Fiberboards (MDF) according to European Standards. DMTA and DSC experiments showed, first, that the tannin-modified adhesives hardened in the low temperature range (30-110degreesC) and, second, that increasing the tannin content of the adhesives reduced the curing temperature, obtaining at least the same mechanical strength (stiffness) and higher curing enthalpies (DeltaH) than the commercial phenolic resin. Although only the MDF boards made using the lowest viscosity tannin-modified adhesive (PUFT-10), with a 44% phenol replacement by tannin, met the outdoor requirements, all the other tannin-modified adhesives boards met the interior grade specifications. Among the board properties evaluated, the low value of thickness swelling after 24-h water immersion of MDF boards prepared using the PUFT-10 (6.6%) is particularly noticeable, which means an improvement of almost 50% compared to that of the commercial PF (12.6%).