DEVELOPMENT AND CHARACTERIZATION OF SUSTAINABLE AND ECO-FRIENDLY LIGNIN-BASED ADHESIVES FOR WOOD BINDING APPLICATIONS

Abstract

Lignin, a bio-based material, is the second most abundant organic polymer on Earth, following cellulose. It presents significant potential as a renewable resource for developing innovative compounds and materials. Despite its vast availability, lignin's current applications are largely restricted to lower-value uses such as energy production, heating, and chemical recycling within the pulp and paper industry. These applications represent a small fraction of lignin's potential, as its broader utilization in higher-value products remains limited and underexplored. Bio-based phenol-formaldehyde resol resins have been successfully synthesized by replacing petroleum-based phenol with lignin. This process uses sodium hydroxide as a catalyst for condensation polymerization. When heated, these lignin-formaldehyde resols show solidification characteristics similar to conventional phenolic resol resins. They exhibit a primary exothermic peak around 120–135°C, which is typical of standard phenolic resins, and a secondary peak between 90–100°C, likely due to exothermic reactions between free formaldehyde and phenol or lignin, resulting in the formation of methyl phenols. Using demethylated lignin instead of lignin can improve the curing process and the thermal stability of the resins, as indicated by a higher decomposition temperature and increased carbon residue at elevated temperatures when using lignin instead of phenol in practice, it should not exceed 100 wt.%. Retrieved lignin should also improve resins’ thermal resistance.