Alkali Treated Kraft Lignin as a Component in Flakeboard Resins

Summary Southern pine kraft lignin was reacted with NaOH (15 and 20% based on dry lignin) at 170, 200, and 250°C for 30 and 60 min. Sweetgum flake boards bonded with phenolic resins containing 50% hydroxymethylated lignin prepared from some of the alkali treated lignins wcre compared with boards bonded with a neat PFresin. Results indicate that boards bonded with lignin-containing resins have a satisfactory bending strength. Boards bonded with resins prepared by reacting lignins with phenol and formaldehyde have better internal bond strength and durability than those bonded with lignin-extended resins. However, there is no evident advantage in using alkali treated lignin in the manufacture of phenol-ligninformaldehyde resins for flakeboard application. NMR spectral analyses indicated that the phenolic hydroxyl content of alkali treated lignins increases with increasing severity of reaction condition. The increase in the phenolic hydroxyl content, however, is accompanied by a decrease in free C5 positions in the guaiacyl moieties due to the formation of GC5-C5G and GCS-CH2-C5G bonds during the reaction. Because of condensation of C5 positions, the degree of hydroxymcthylation of alkali treated lignins with formaldehyde also decreases with increasing severity of the reaction.