Abstract
ZnO is a worldwide used activator for a rubber vulcanization process, which promotes fast curing kinetics and high cross-linking densities of rubber nanocomposites (NCs). However, its extended use together with leaching phenomena occurring during the production and life cycle of rubber products, especially tires, entails potential environmental risks, as ecotoxicity toward aquatic organisms. Pushed by this issue, a novel activator was developed, which introduces highly dispersed and active zinc species in the vulcanization process, reducing the amount of employed ZnO and keeping high the curing efficiency. The activator is constituted by Zn(II) single sites, anchored on the surface of SiO2 nanoparticles (NPs) through the coordination with functionalizing amino silane groups. It behaves as a double-function material, acting at the same time as a rubber reinforcing filler and a curing activator. The higher availability and reactivity of the single-site Zn(II) centers toward curative agents impart faster kinetics and higher efficiency to the vulcanization process of silica/isoprene NCs, compared to conventionally used ZnO activators. Moreover, the NCs show a high cross-linking degree and improved dynamic mechanical properties, despite the remarkably lower amount of zinc employed than that normally used for rubber composites in tires. Finally, the structural stability of Zn(II) single sites during the curing reactions and in the final materials may represent a turning point toward the elimination of zinc leaching phenomena.
Highlights
Rubber is a material used in many widespread applications,[1−4] thanks to its unique properties, as low hardness, high elasticity, and high elongation at break
Its mechanical properties are commonly enhanced by adding carbon rbeliancfko5r−ci9ngtonatnhoefilrluerbsb, eer.gm., aStirOix2, silicates, POSS, which promote and the formation of a percolative filler network inside the rubber nanocomposites (NCs).[10−12] Among curing methods, vulcanization is a consolidated key technology in the rubber industry to cure unsaturated rubber chains through sulfur cross-links at high temperature and formation of poly- to monosulfide bridges between them.[13,14]
In the field of the rubber vulcanization process, employed for tire production, one main environmental concern is related to the use of ZnO, a fundamental curing activator that increases the sulfur vulcanization efficiency, by reducing reaction time, saving energy and costs of the whole process.[22−25] ZnO is generally employed in conjunction with accelerators and coactivators, in a complex sequence of multistep reactions, which lead to sulfur cross-link formation,[26−29] through the following reaction mechanism: (i) ZnO reacts with SA to create highly reactive Zn(II)-SA adducts; (ii) Zn(II)-SA reacts with an accelerator and Received: April 26, 2021
Summary
Rubber is a material used in many widespread applications (e.g., tires, tubes, footwear and gloves, glues, etc.),[1−4] thanks to its unique properties, as low hardness, high elasticity, and high elongation at break. Article sulfur to form Zn complexes containing polysulfidic ligands, the active sulfurating agents; (iii) active sulfurating agents react with the polymer forming polysulfidic pendant cross-link intermediates; (iv) polysulfidic bridges form between the chains and progressively shorten through decomposition and rearrangement reactions promoted by Zn(II) centers, giving rise to highly cross-linked products According to this mechanism, ZnO and especially Zn(II) centers have demonstrated to play a main role on both the kinetics of the curing reaction and the properties of vulcanized rubber NCs.[23,30−32] due to the highly hydrophilic character of ZnO, opposed to rubber hydrophobicity, high oxide amounts are used in tire formulations in order to overcome its tendency to agglomerate and to guarantee a uniform crosslink formation inside the rubber matrix
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