Improving the properties of urea-lignin-glyoxal resin as a wood adhesive by small addition of epoxy

The aim of this work was to improve the properties of plywood panels bonded with urea-lignin-glyoxal (ULG) resin by small addition of epoxy resin. For this reason, various contents (1, 3 and 5% on resin solids) of an epoxy resin were added to the prepared ULG resin. The structural, thermal and physicochemical properties of the resins prepared as well as the water absorption and wet and dry shear strength of the plywood panels bonded with them were measured according to standard methods. The results showed that the addition of an epoxy resin significantly accelerated the gel time and increased the ULG resin viscosity, density and solids content. The cross linking structure in the cured ULG resin with the epoxy resin was confirmed by FTIR analysis. The curing temperature of the ULG resin also decreases by addition of the epoxy resin as shown by differential scanning calorimetry (DSC). As a consequence, the plywood panels bonded with ULG resins containing any amount of the epoxy resin presented a better dimensional stability than those bonded with the control resin. The dry and wet tensile shear strength of plywood panels bonded with ULG resins indicate that they could be significantly improved by increasing the percentage of the epoxy resin added.