Mechanical Performance Under Compressive Load of Carboxylated Nitrile Rubber and Kraft Lignin Composites

ABSTRACT Lignin is a promising biobased material that enhances the sustainability of rubber composites as a reinforcing filler. This study investigates 10 formulations of carboxylated nitrile rubber (XNBR) filled with Kraft lignin, focusing on their compressive mechanical properties, which are related to sealing applications. Key assessments include stress–strain behavior under compression, stress relaxation, and compression set. Characterization techniques like micro‐computed tomography (micro‐CT) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR) were also employed. Micro‐CT imaging indicates that XNBR with 7% carboxylic acid exhibits significantly better lignin dispersion than formulations with just 1% carboxylic acid. The compressive properties of the composites depend on the rubber's carboxylic acid content; for instance, adding lignin to rubber with 1% carboxylic acid had minimal impact on stress at 50%, while it nearly doubled in rubber with 7%. Higher lignin content leads to increased hysteresis, stress relaxation, and compression sets, particularly as carboxylic acid content rises. ATR‐FTIR analysis suggests that ionic bonds in carboxylated rubbers play a crucial role in their interaction with lignin, enhancing mechanical performance.