Natural rubber (NR) nanocomposites reinforced with carbon nanotubes (CNTs) and zinc oxide–decorated CNTs (CNT/ZnOx) were prepared, with bis(triethoxysilylpropyl) tetrasulfide (TESPT) as coupling agent. The influence of ZnO decoration and TESPT on curing, mechanical, dynamic mechanical, thermal, and electrical properties was investigated. Rheometric analysis showed TESPT accelerated curing, reducing cure time (tc90) and enhancing the torque difference (ΔM) by more than 50% compared to gum NR. Tensile strength improved from 14.94 MPa (gum NR) to 19.26 MPa in NR–CNT3/ZnO3–TESPT, while Young’s modulus nearly doubled. DMA revealed Tg upshifts from −55.72°C (gum NR) to −44.29°C, with tanδmax reduced from 1.98 to 1.65, indicating restricted chain mobility and stronger filler–rubber interactions. Bound rubber and swelling confirmed increased chemical linkages, with crosslink density highest in TESPT-modified CNT/ZnO composites. Morphology showed finer dispersion and reduced pull-out in TESPT systems. Electrical conductivity improved by nearly five orders of magnitude with CNT3/ZnO3, and further with TESPT, while the dielectric constant also increased due to enhanced interfacial polarization. Collectively, the results demonstrate ZnO decoration on CNTs and TESPT coupling act synergistically to produce nanocomposites with superior mechanical strength, thermal stability, and electrical performance through optimized dispersion, interfacial bonding, and network continuity.
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