Improving physical and mechanical properties of new lignin- urea-glyoxal resin by nanoclay

To eliminate toxic formaldehyde from wood based panels, glyoxal, a low volatility and nontoxic aldehyde, was used to react with urea and lignin to prepare a glyoxalated lignin- urea-glyoxal (LUG) wood adhesive resin. Moreover, another objective of this research work was to improve the physical and mechanical properties of the new LUG resins by nanoclay addition. For the preparation of LUG resin, glyoxalated lignin (15 mass%) was added instead of second urea to the urea-glyoxal resin synthesis under acid conditions. The LUG resin so prepared was mixed with 1, 2 and 3 mass% nanoclay by mechanically stirring for 5 min at room temperature. Then, the physicochemical and structural properties of the prepared resins as well as the water absorption and the mechanical properties of the plywood panels bonded with it were measured according to standard methods. The physicochemical test results indicated that the gel time of the LUG resin was markedly slower than that of the UF resin. Plywood panels prepared with the LUG resin also presented lower water absorption as well as weaker shear strength than those prepared with the UF resin. Addition of nanoclay changed the physicochemical properties of the resins as the gelation time of the LUG resin was shorter when adding sodium montmorillonite (NaMMT). Higher shear strength values and lower water absorption were achieved by continuously increasing nanoclay proportion from 1 to 3 mass%. Furthermore, addition of nanoclay had more influence on panels bending strength than their flexural modulus. X-ray diffraction (XRD) analysis also indicated that NaMMT exfoliated completely when mixed with LUG resin.