Assessment of the Mixing Procedure and the Effect of Carboxylated Nitrile Rubber as a Compatibilizer for Kraft Lignin/Nitrile Rubber Composites

ABSTRACT The growing interest in sustainable rubber materials motivates the use of renewable resources for the development of advanced materials. Lignin, valued for its abundance, biodegradability, and mechanical strengths, emerges as a promising alternative filler in rubber composites. This study produced composites using nitrile rubber (NBR) and a 70:30 blend of NBR with carboxylated nitrile rubber (XNBR), each incorporating 40 parts per hundred of rubber (phr) of kraft lignin. Two methods were tested for incorporating lignin: direct addition of 40 phr of lignin powder to the rubbers or compounding using masterbatches of NBR/lignin and XNBR/lignin, each with 130 phr of lignin produced by co‐coagulation. Results revealed that NBR/XNBR/lignin composites show thinner, more aligned lignin particles than NBR/lignin composites, with the masterbatch technique further reducing particle size. All lignin‐filled rubbers exhibited an increased reaction rate constant of vulcanization, lower covalent crosslink density, and higher ionic crosslink density compared to unfilled rubber. Notably, composites made with lignin masterbatch exhibited a reduced Payne effect, as well as higher tensile and tear strengths, than those made using lignin powder. These findings suggest that incorporating lignin via a masterbatch and blending with XNBR offers a compelling strategy for enhancing the properties of NBR‐based rubber composites.