Development and characterization of XNBR compounds filled with lignin

Abstract

Lignin, a bio-based and renewable by-product of the pulp, paper, and biofuel industries, is a promising alternative to carbon black as a reinforcing agent in rubber formulations. It offers biodegradability, antioxidative properties, and UV resistance. However, its high polarity leads to poor dispersion and weak interactions with non-polar rubbers. Carboxylated nitrile rubber (XNBR), a modified version of nitrile rubber widely used in industry, has the potential to interact more effectively with lignin due to its ability to form oth covalent and ionic cross-links. This study explores the use of Kraft lignin as a reinforcing filler for XNBR, focusing on its effects on vulcanization, mechanical, thermal, morphological, and nanomechanical properties. The research was divided into two parts. Part I determined the optimal carboxyl content in XNBR (1–7% by mass) to enhance lignin compatibility, using a mixture design approach. The study also evaluated its suitability as a sealing material. XNBR with 7% carboxyl content showed the best reinforcement effect, with tensile stress at 100% strain increasing from 1.7 MPa (without lignin) to 6.3 MPa (with 40 phr lignin). Micro-computed tomography revealed lower lignin agglomeration in XNBR 7%. Dynamic mechanical analysis (DMA) and attenuated total reflectance Fourier-ransform infrared spectroscopy (ATR-FTIR) indicated that ionic cross-linking plays a key role in improving XNBR-lignin interactions. However, high compression set at elevated temperatures remains a challenge for sealing applications. Part II further investigated unfilled XNBR 7% to assess the influence of covalent and ionic cross-links on macroscopic and nanomechanical properties. Atomic Force Microscopy (AFM) nanomechanical mapping revealed a phase-separated morphology in unfilled XNBR, forming the basis for further analysis of lignin-filled compounds. Equilibrium swelling and AFM measurements showed that lignin reduced covalent cross-link density, likely by interfering with vulcanization, leading to increased elongation but also higher compression set at high temperatures. Meanwhile, ATR-FTIR, AFM, and swelling tests confirmed that ionic cross-links increased in number but changed in nature, maintaining modulus while making the material more mechanically instable at high temperatures. While ensuring mechanical stability at high temperatures remains a challenge, this study demonstrates the potential of Kraft lignin as a sustainable reinforcing filler for XNBR, contributing to the development of more eco-friendly rubber materials.