Ionic liquid- modified lignin as a bio- coupling agent for natural fiber- recycled polypropylene composites

Abstract In this study, the influence of ionic liquid-treated lignin as a bio coupling agent on the various properties of bagasse fiber-recycled polypropylene composites was investigated. For this purpose, extracted lignin from Bagasse Soda black liquor was modified with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) ionic liquid and then different ratios of unmodified and modified lignin (0, 1, 3 and 5 wt%) were added to the bagasse fiber/recycled polypropylene mixture. The composites were then manufactured by injection molding after mixing mechanically of the bagasse fiber, virgin or modified lignin and recycled polypropylene. Structural and thermal properties of lignin after modification by ionic liquid along with physical (water absorption and thickness swelling) and mechanical (flexural modulus, flexural strength, tensile strength and impact strength) properties of the manufactured composites were measured according to ASTM standard methods. The FTIR analysis indicated lignin rearrangements showing that some bonds changed or formed after its modification with IL. The DSC analysis showed that the glass transition temperature (Tg) of lignin decreased from 92 °C to 80 °C by increasing the weight ratio of ionic liquid to lignin from 20 to 30 wt%. The panel testing results indicated that all physical and mechanical properties of the composites were continuously improved by increasing the lignin content from 1 to 5 wt%. Thus, the composites with IL-treated lignin presented higher dimensional stability and mechanical strength compared to those prepared with unmodified lignin. Scanning electron microscopy (SEM) showed that treatment with ionic liquid improves the uniform dispersion of lignin in the polymer matrix.