Enhancement of Mechanical Properties and Solvent Resistance of Natural Rubber/Ethylene–Propylene–Diene Monomer Blend Reinforced with Carbon Nanotubes Functionalized by Ionic Liquids and Silane-Containing Fillers

Abstract

This work examines the reinforcing efficiency of carbon nanotubes (CNTs) and their surface-functionalized counterparts in natural rubber/ethylene–propylene–diene monomer (NR–EPDM) composites. CNTs were functionalized using ionic liquids—1-hexadecyl-3-methylimidazolium bromide (HDMIM-Br) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI)—as well as silane coupling agents, namely 3-aminopropyltriethoxysilane (APTES) and bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT). Composites containing 0–8 phr CNTs were fabricated via melt mixing and systematically evaluated in terms of curing behavior, mechanical performance, swelling resistance, and morphological characteristics. Rheometric analysis revealed that all CNT-reinforced systems exhibited increased minimum torque, maximum torque, torque difference, and cure rate index, along with reduced scorch and optimum cure times, indicating accelerated vulcanization kinetics. The incorporation of pristine CNTs resulted in enhancements of 25% in tensile strength, 26% in modulus at 100% elongation, and 22% in tear strength, with optimum performance observed at 5 phr. However, further increases in CNT loading led to performance deterioration due to filler agglomeration. Surface-functionalized CNTs displayed markedly superior reinforcing efficiency, attributable to improved dispersion and stronger filler–matrix interfacial interactions. In particular, HDMIM-Br- and BMI-modified CNTs increased tensile strength by 32 and 39% and modulus at 100% elongation by 35 and 44%, respectively. Even greater improvements were achieved with silane-functionalized CNTs, where APTES- and TESPT-modified systems enhanced tensile strength by 44 and 52% and tear strength by 32 and 37%, respectively. In addition, all functionalized CNT-based composites exhibited higher crosslink density, hardness, abrasion resistance, and swelling resistance, together with reduced compression set and only minimal reductions in rebound resilience. Overall, the findings demonstrate the synergistic effectiveness of ionic liquid and silane functionalization in improving CNT dispersion, strengthening filler–matrix interactions, and significantly enhancing the multifunctional performance of NR–EPDM composites.