Using carboxyl groups to improve the compatibility of XNBR/lignin composites

Abstract The paper and pulp industry produces lignin as a byproduct, which could be a bio‐based reinforcing filler for rubber. Carboxylated nitrile rubber (XNBR) contains carboxyl groups that form ionic bonds with zinc oxide, potentially increasing compatibility with lignin, compared to usual nonpolar rubbers. This study employed the traditional mixing method, two‐roll mill, to incorporate hardwood Kraft lignin without chemical or physical modification as a reinforcing filler in commercial XNBR. A mixture design of experiments was used to explore the effect on rubber/lignin interaction of the carboxyl group content (from 1% to 7% in blends of XNBR) and the amount of lignin (from 0 to 40 phr). Adding 40 phr of lignin increased stress at 100% in XNBR 7% from 1.7 to 6.3 MPa. In XNBR 1%, the increase was from 1.2 to 1.9 MPa. Lignin showed better interaction and dispersion with XNBR 7%, determined from response surface of G ′ at high deformations and SEM, respectively. Loss of thermal transition in DMA indicates interaction through ionic groups. These results show that the presence of carboxyl groups enhances the rubber/lignin interaction. This research open possibilities of compatibilization of lignin, offering guidance for future studies and technologies involving lignin in technical applications. Highlights Lignin dispersion increased as the carboxyl group content increased to 7% (w/w). Stress at 100% elongation increased 370% with 40 phr of lignin and 7% carboxyl. Rubber/lignin interaction as per G ′ increased with carboxyl groups. Loss of thermal transition suggests lignin/carboxy/zinc oxide interaction. Lignin can be used as a reinforcing filler in nitrile carboxylated rubber.