Combined treatments of fiber surface etching/silane-coupling for enhanced mechanical strength of aramid fiber-reinforced rubber blends

This study sought to improve the interfacial bonding force between aramid fiber (AF) and a rubber matrix. We pretreated the aramid fiber with anhydrous calcium chloride (CaCl2) and sodium hydroxide (NaOH) solutions. We then performed a separate modification using two different methods, first, grafting the aramid fiber with silane coupling agent γ-Methacryloxypropyl trimethoxy silane (KH570) and secondly in situ formation of silica on the surface of aramid fiber. In addition, mechanical blending was adopted to prepare natural rubber/cis-1,4-polybutadiene rubber/aramid fiber (NR/BR/AF) composites. Finally, aramid fibers were characterized via Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electronic microscopy (SEM), then the mechanical properties of the corresponding composites analyzed. Results revealed that the synergistic action of anhydrous CaCl2 and NaOH solutions caused an increase in surface roughness and the contact area of the aramid fiber. The Sol-gel method allowed successful coating of the secondary modified aramid fiber surface with a layer of in situ-generated silica. Consequently, this improved surface reactivity of the fiber and the rubber matrix, thus promoting their combination. Moreover, we successfully grafted KH570 onto the surface of the aramid fiber through an oxygen-containing group on the surface of the pretreated aramid fiber. Tensile properties of AF composites exhibited a 31.9% increase after secondary modification with KH570 whereas their DIN abrasion performance increased by 16.2%. Notably, a significant improvement in interfacial adhesion was recorded in modified AF prepared NR/BR composites, which further improved the mechanical properties of NR/BR composites.