Modifying the Structure of Lignin with the Purpose of Increasing its Use in Phenol-Formaldehyde Adhesives

Background and Objectives: One of the potential applications of lignin, due to its structural similarity to phenol, is in the production of phenol-formaldehyde (PF) wood adhesives. The main challenge in this area is lignin's lower chemical activity compared to phenol, which has made it impossible to use large quantities of lignin in preparing phenol-formaldehyde adhesives. In the previous researches, different methods have been used to modify the lignin structure, but none of these methods have enabled the use of large amounts of lignin in this adhesive. In the present study, a two-stage method was used to enhance lignin properties. At first lignin with deep eutectic solvents of the amino acid-based lysine type (lys-AADES) has been modified and then methylolation of AADES- modified lignin by the non-toxic aldehyde entitled glyoxal was employed. The modified lignin was substituted for phenol in phenol-formaldehyde adhesive in large quantities and the properties of them was measured according to standard methods.

.Materials and Methods: the chemical modification of lignin was carried out in two stages: first, the lignin network structure was separated with a lysine-based amino acid deep eutectic liquid (Lys-AADES) and then methylolation reaction was performed by reaction of lignin with a non-toxic aldehyde entitled glyoxal. The thermal and structural properties of lignin were measured before and after chemical modification. Then, the 70, 80, and 90% modified lignin as substitute of phenol was used in phenol-formaldehyde adhesives to prepare lignin-phenol-formaldehyde (LPF) resin, and the resulting adhesive was used to make plywood. Finally, the various properties of the prepared adhesive together with the physical and mechanical properties of the produced plywood were investigated according to standard methods.

Results: The results of FTIR analysis confirmed the excellent cleavages of the ether bonds in lignin network and the introduction of the amino group (NH2) into the lignin structure by AADES treatment. Also the introduction of reactive methylol groups (CH2OH) to lignin structure after the methylolation process has been confirmed by FTIR analysis. The results of differential scanning calorimetry (DSC) analysis also showed that the glass transition temperature (Tg) of virgin lignin decreased from 91°C to 79°C after AADES treatment, and then it more decreased to 71°C after glyoxalation.The physicochemical properties of the prepared adhesive showed that LPF adhesives containing modified lignin had faster gelation time and higher viscosity compared to those adhesives containing virgin lignin. Also the plywood containing LPF adhesive with modified lignin showed greater shear strength (dry and wet), higher dimensional stability, and lower formaldehyde emission compared to LPF adhesive containing virgin lignin.

Conclusion: It should be noted that, based on the results of this research, modifying of lignin with lysine-based amino acid deep eutectic solvent and then glyoxalation of it has enabled the use large quantities of lignin in the structure of PF adhesives. The properties of the produced LPF adhesive and the plywood produced from it are comparable to those of conventional PF adhesives. The results of this research can help open new windows for developing the application of lignin in the adhesive and wood industries.