Introduction of primary chemical bonding in lignin-based PP composites for mechanical reinforcement via reactive extrusion

Lignin is a by-product of paper and pulp production resulting from extraction of cellulose, the world's most abundant natural polymer. In this study, we manufactured polypropylene (PP)/PP-grafted maleic anhydride (PP-g-MAH)/glass fiber (GF)/lignin composites via reactive extrusion process for value-addition of lignin, often discarded as wood waste. To enhance adhesion in composites at interphases, diisocyanate was added in-situ during the extrusion process, leading to chemical primary bonding. Structure-property relationship of PP/PP-g-MAH/GF/lignin composites revealed two different reinforcement mechanisms that were supported by FT-IR, TGA, hardness, tensile, flexural, and impact tests, SEM, and density. Up to 7.5 wt% of isocyanate loading, linear urethane bonding was formed; further addition of isocyanate resulted in evolution of chemical crosslinking via allophanate bonding.