Wood Adhesive from Lignin Biomass: Effects of Crosslinker and Nano-Reinforcement

Abstract

Lignin accounts for about 30% of the dry mass of wood and provides rigidity and stability to woody plants. In pulping operations, lignin is removed to liberate the paper making fiber. Lignosulfonate is a byproduct of sulfite pulping. An estimated one million tons of lignosulfonate is produced annually. This byproduct is easily recoverable but underutilized. Due to the function of lignin as a natural binder of adjacent cells, lignin biomass has been investigated for wood adhesives. This study aims to improve the competitiveness of lignosulfonate-based adhesives by incorporating bio-based additives. The specific objective is to examine the effects of citric acid as a crosslinker and cellulose nanofiber as reinforcement. To achieve the research objective, adhesives were prepared in various mass ratios of lignosulfonate, citric acid, and cellulose nanofiber. The adhesives were applied to wood surfaces and hot pressed to form single lap joints. Lap joint specimens were pulled apart to test for the shear bond strength of the adhesives. Results showed that the addition of citric acid increased the shear bond strength by 29% and the addition of cellulose nanofiber increased the shear bond strength by 27%. In wet conditions, lignosulfonate and citric acid did not perform nearly as well as phenol formaldehyde, but by incorporating 25% phenol formaldehyde to lignosulfonate and citric acid made the adhesive directly comparable to neat phenol formaldehyde. This suggests potential for lignosulfonate to significantly add bio-based content and reduce the non-renewable carbon in petroleum-based wood adhesives.