Synthesis of lignin-formaldehyde resin as wood adhesive and usage of trimethyl stearyl amine-montmorillonite as filler to improve plywood performances under dry/wet conditions

Lignin (L) is a biopolymer obtained from non-toxic renewable sources and composed of phenylpropane units bearing phenolic hydroxyl groups on benzene rings. In this study, a new method was developed for the production of lignin formaldehyde (LF) resin, which can also be defined as green wood adhesive, by complete replacement of phenol (P), which is used in PF synthesis and produced from petroleum fossil fuels, with L. Reactive lignin molecules (Lr) were prepared by Elbs persulfate oxidation using constant potassium persulfate (KPS) content (0.1 mol/L) in three different concentrations of NaOH (1.24, 2.80, and 5.50 mol/L). The samples of Lr were synthesized with two different concentrations of L (1.60 and 6.80 mol/L) in two different reaction times (2 h and 5 h). As to the results of acid-base titration/¹H NMR and gravimetry, the reactive lignin molecules prepared using 6.80 mol/L of L in 5.50 mol/L of aqueous NaOH solution for 2 h were chosen for the preparation of LF resin due to their higher phenolic hydroxyl (3.25/3.45 mmol/g) and solids (38.9%) contents. The molecular weights/molecular weight distributions and hydrogen bonds between phenolic/alcoholic hydroxyl groups were analyzed using GPC and FTIR/¹³C and ¹H NMR, respectively. The results for dry/wet bonding strengths (6.15/2.2 MPa) of plywood panels prepared by adding 10 wt% of trimethyl stearylamine-montmorillonite (TMSA-MMT) into LF resin were comparable with PF/Wheat Flour (WF) as conventional wood adhesive (8.10/3.2 MPa). Also, the free formaldehyde contents of LF and LF/TMSA-MMT (0.25 and 0.06%, respectively) were lower than those of PF and PF/WF (0.70 and 0.12%) and comparable to the values of the Chinese National Standard (GB/T 14732 − 2006) limitation (≤ 0.3%).