Chemical modification of sugarcane bagasse and sisal fibers using hydroxymethylated lignin: Influence on impact strength and water absorption of phenolic composites

Chemical modification of fiber surfaces can increase wettability of composites reinforced by vegetal fibers and, consequently, the dispersion of the fiber in the matrix and mechanical properties can be improved. Although there are some studies about agents for chemical modifications of vegetal fiber surfaces, there are few data and discussion about the usage of lignin. In the present work, chemical modifications of sugarcane bagasse and sisal fibers using lignin (previously hydroxymethylated) were carried out under different reaction times (15, 30, and 60 min). The composition (holocellulose, hemicelluloses, cellulose, and lignin contents) of the treated and untreated fibers was evaluated. Phenolic composites were prepared using unmodified and modified fibers via compression molding process under temperature. Izod impact, water absorption tests, and scanning electron microscopy were performed to evaluate composite properties. The resin and lignin were characterized by size exclusion chromatography. Results showed that there was a tendency of reducing water absorption for composites prepared from modified fibers. Impact strengths of composites reinforced with sugarcane bagasse with modified fibers were similar to the ones with unmodified fibers (around 20 J/m). However, impact strengths for composites reinforced with modified sisal fibers (around 104 J/m for 15 min of reaction time) were higher than the ones with unmodified fibers (around 95 J/m). Therefore, the usage of lignin as a modifier agent of vegetal fiber surfaces to increase fiber–matrix adhesion for phenolic composites is a strategic alternative for improving products through simple, eco-friendly, and low-cost procedures.