Sepiolite Nanofibers Decorated with Zn Single Sites—A Sustainable Filler for Mechanically Robust, Self-Healing and Recyclable Ionomeric Elastomer Composites

In light of the persistent challenges associated with the recycling of certain polymers, particularly elastomers, the imperative to align them with the circular economy paradigm has become fundamental, given their crucial role in diverse applications. One potential pathway to achieve this objective is to imbue the materials with self-healing properties.<br/>Inspired by the energy dissipation and the spontaneous regeneration of sacrificial bonds in nature, several research efforts have been devoted to the incorporation of labile bonds into synthetic and natural rubbers for imparting both mechanical strength and self-repairing ability. However, significant issues arise considering the discrepancy between outstanding mechanical properties usually required for rubber-based products, which entails rigid and stable covalently bonded molecular network, and self-healing, which instead implies polymer chains mobility.<br/>In this context, ionomers represent promising materials combining the processability of thermoplastics with the elasticity of rubber. The ionic interactions occurring in ionomers usually entail electrostatic interactions between anions, such as carboxylates and sulfonates, and cations from Group 1A, Group 2A, or from transitional metals. These ionic interactions produce thermoreversible physical cross-links (i.e. aggregates or ionic clusters), which exert a remarkable influence on the mechanical and physical properties of the polymer and underpins the self- healing behaviour.<br/>Specifically, the possibility of generating reversible ionic dynamic interactions in rubbery matrices by ZnO introduction has been recently reported in the literature as a suitable and versatile strategy. It was demonstrated that, at a molecular scale, the Zn²⁺ cations of the oxide can form ionic interaction with the carboxylic groups of carboxylated nitrile rubber (XNBR) capable of thermoreversible association. However, the extended use of ZnO together with leaching phenomena occurring during the production and life cycle of rubber products, especially tires, raise potential environmental risks, as ecotoxicity toward aquatic organisms.<br/>Moreover, despite all the endeavors, achieving good repairability and maintaining satisfactory mechanical performances in crosslinked elastomeric networks still remain major challenges.<br/>Pushed by all these issues, the present work proposes the preparation and the exploitation of a novel multifunctional filler based on sepiolite nanofibers (Sep) decorated at the surface with Zn(II) single sites, able to enhance the reinforcement and to deliver thermally activated self-healing properties to carboxylated nitrile rubber (XNBR) composites, simultaneously.<br/>In detail, naturally available sepiolite was functionalized by an amino-substituted silane (3-aminopropyl-triethoxysilane, APTES), which provided the coordination through the amino groups of single Zn(II) centers onto the oxide surface. The structural, surface, and morphological features of the functionalized Sep fibers were extensively inspected by spectroscopic, morphological and thermogravimetric analyses.<br/>The Sep@APTES-Zn filler was then included in XNBR to produce composites in which the formation of thermally reversible ionic crosslinks between the Zn²⁺ cations and the carboxylate pendant groups of the polymer offers a route to rubber recovery after damage. Indeed, XNBR/Al₂O₃@APTES-Zn composites display excellent mechanical properties, remarkable self-repairability and pronounced recyclability, maintaining comparable tensile strength and elongation at break even after reprocessing. Finally, the structural stability of Zn(II) single sites during the curing, the processing and in the final materials provides a turning point toward the elimination of zinc leaching phenomena.<br/>These results may open new avenues to design and fabricate high-performance and sustainable rubber products with extended lifetime, with paramount fallouts for automotive and rubber components industries.