Kraft lignin as fiber treatment for natural fiber-reinforced composites

Pine kraft lignin was sedimented from an aqueous NaOH solution (pH 13) onto flax fibers by submerging flax fiber mats in the alkaline solution for 5 min. This treatment has been used to improve the interfacial interactions between the hydrophilic flax and a hydrophobic soy-oil-based liquid molding resin. The flax fiber mat weight increase as a function of the lignin content in the alkaline treatment solution was determined and visualized using electron microscopy. Lignin sedimentation was found to decrease the contact angle between the resin and the treated flax fibers. Tensile and flexural properties of the composite samples showed an increase in mechanical properties with increasing lignin sedimentation, with a positive deviation for low lignin sedimentation. These results are in line with previous published data with hemp fiber composites [Thielemans et al. (2002), J. Appi. Polym. Sci., 83, 323]. Improved properties are attributed to the improved wettability, while the low lignin content deviation is due to small lignin deposits protruding from the surface and thus improving mechanical interlocking. Features on the fracture surfaces, visualized by electron microscopy, are consistent with these findings. The soy-oil-based resin behaves similar to existing petroleum-based unsaturated polyester and vinylester resins and its use is not inherent to the success of the lignin treatment.