This study investigates the performance of phenol–formaldehyde adhesives containing Eucalyptus lignin as an extender in their formulation. A commercial phenol–formaldehyde resin was used, and five different types of lignin were tested: (1) kraft lignin precipitated with carbon dioxide, (2) kraft lignin precipitated with sulfuric acid, (3) soda lignin precipitated with hydrochloric acid, (4) soda lignin precipitated with sulfuric acid, and (5) a second soda lignin where the wood underwent a phosphoric acid extraction process prior to alkaline extraction. The lignins were used both unmodified and activated through three different processes: hydroxymethylation, phenolysis in an acidic medium, and alkaline phenolysis. Adhesives were formulated with substitution percentages of the base resin ranging from 10% to 60%, in addition to a reference adhesive that contained no lignin. Wooden test specimens were manufactured to determine the tensile shear strength. Results indicate that best performance is achieved when lignins are activated through hydroxymethylation and when soda lignin is used. Under optimal conditions, it is possible to replace at least 45% of the base resin with activated Eucalyptus soda lignin, which represents a reduction of at least 30% in the cost of the final adhesive. This substitution results in a 46% increase in adhesive strength compared to the base adhesive (without lignin). These findings suggest that the valorization Eucalyptus soda lignin could have significant economic and environmental benefits.

This study examines the impact of precipitation conditions, specifically pH and temperature, on the production of Eucalyptus kraft lignin from black liquor, focusing on the resulting chemical properties. Additionally, the process of hydroxymethylation of lignin during precipitation in a single step was investigated. Lignins were characterized using wet chemical analyses, which included the determination of phenolic hydroxyl group content, carboxylic group content, syringyl-to-guaiacyl ratio, molecular weight distribution, and glass transition temperature. Additionally, ATR-FTIR spectra were recorded, and prediction models between these spectra and lignin descriptive parameters were established: phenolic OH content, carboxyl group content, S/G ratio, glass transition temperature, and molecular weight distribution. The results demonstrate that the properties of the precipitated lignin are influenced by both pH and temperature during precipitation, with the precipitation yield exhibiting the most pronounced variation. The study also concludes that lignin activation during precipitation is achievable, with the most favourable outcomes observed at pH 9 and 80 °C.