A series of experiments was conducted to optimize 1) molar ratio of formaldehyde/phenol, 2) sodium hydroxide catalyst, and 3) reactant concentration of phenolic copolymer to lignin in terms of providing the best performance of lignin-phenolic resin systems for structural flakeboard panels. At 25/75 (w/w) mixture of phenolic/lignin, a methylolated lignin blended with phenolic resin synthesized with a formaldehyde/phenol ratio of 3 consistently resulted in higher bond strength, lower thickness swell, and smaller linear expansion of structural flakeboard. Gluebond performance improved as the NaOH/phenol ratio increased from 0.2 to 0.7. Further increases in sodium hydroxide adversely affected gluebond performance. Panel strength properties decreased with increasing resin solids content. Adhesives formulated with 75% of methylolated lignins as substi tutes for phenolic resins can be used to make structural flakeboards with acceptable properties.Incorporation of lignin into phenol-formaldehyde resins has been one of the primary areas of research in lignin utilization. When lignin is used directly as a replacement for phenol, the maximum acceptable level of replacement is about 20% (1). However, a higher degree of replacement can be achieved using modified lignin (2-7). Of all lignin modification treatments, those that increase chemical reactivity or reactive sites, such as phenolation or methylolation, seem to be the most promising.
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