The efficiency of sulfur vulcanization reaction in rubber industry is generally improved thanks to the combined use of accelerators (as sulphenamides), activators (inorganic oxides), and co-activators (fatty acids). The interaction among these species is responsible for the formation of intermediate metal complexes, which are able to increase the reactivity of sulfur towards the polymer and to promote the chemical cross-links between the rubber chains. The high number of species and reactions that are involved contemporarily in the process hinders the complete understanding of its mechanism despite the long history of vulcanization. In this process, ZnO is considered to be the most efficient and major employed activator and zinc-based complexes that formed during the first steps of the reaction are recognized to play a main role in determining both the kinetic and the nature of the cross-linked products. However, the low affinity of ZnO towards the rubber entails its high consumption (3–5 parts per hundred, phr) to achieve a good distribution in the matrix, leading to a possible zinc leaching in the environment during the life cycle of rubber products (i.e., tires). Thanks to the recent recognition of ZnO ecotoxicity, especially towards the aquatic environment, these aspects gain a critical importance in view of the urgent need to reduce or possibly substitute the ZnO employed in rubber vulcanization. In this review, the reactivity of ZnO as curing activator and its role in the vulcanization mechanism are highlighted and deeply discussed. A complete overview of the recent strategies that have been proposed in the literature to improve the vulcanization efficiency by reducing the amount of zinc that is used in the process is also reported.

Silica–styrene butadiene rubber (SBR) nanocomposites were prepared by using shape-controlled spherical and rod-like silica nanoparticles (NPs) with different aspect ratios (AR = 1–5), obtained by a sol–gel route assisted by a structure directing agent. The nanocomposites were used as models to study the influence of the particle shape on the formation of nanoscale immobilized rubber at the silica-rubber interface and its effect on the dynamic-mechanical behavior. TEM and AFM tapping mode analyses of nanocomposites demonstrated that the silica particles are surrounded by a rubber layer immobilized at the particle surface. The spherical filler showed small contact zones between neighboring particles in contact with thin rubber layers, while anisotropic particles (AR > 2) formed domains of rods preferentially aligned along the main axis. A detailed analysis of the polymer chain mobility by different time domain nuclear magnetic resonance (TD-NMR) techniques evidenced a population of rigid rubber chains surrounding particles, whose amount increases with the particle anisotropy, even in the absence of significant differences in terms of chemical crosslinking. Dynamic measurements demonstrate that rod-like particles induce stronger reinforcement of rubber, increasing with the AR. This was related to the self-alignment of the anisotropic silica particles in domains able to immobilize rubber.

The present study reports on a simple preparation strategy of a hybrid catalyst, TiO₂/HKUST-1, containing TiO₂ anatase nanoparticles (NPs) with tailored morphology and photocatalytic activity coupled with a porous metal-organic framework (MOF), namely HKUST-1, as an advanced material for the CO₂ photocatalytic reduction. In detail, TiO₂/HKUST-1 catalyst was prepared via an easy slow-diffusion method combined with a step-by-step self-assembly at room temperature. The growth of crystalline HKUST-1 onto titania surface was achieved by functionalizing TiO₂ nanocrystals, with phosphoesanoic acid (PHA), namely TiO₂-PHA, which provides an intimate contact between MOF and TiO₂. The presence of a crystalline and porous shell of HKUST-1 on the TiO₂ surfaces was assessed by a combination of analytical and spectroscopic techniques. TiO₂/HKUST-1 nanocomposite showed a significant efficiency in reducing CO₂ to CH₄ under solar light irradiation, much higher than those of the single components. The role of MOF to improve the photoreduction process under visible light was evidenced and attributed either to the relevant amount of CO₂ captured into the HKUST-1 porous architecture or to the hybrid structure of the material, which affords enhanced visible light absorption and allows an effective electron injection from TiO₂-PHA to HKUST-1, responsible for the photochemical reduction of CO₂.

Silica–natural rubber composites were prepared by in situ sol–gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler–filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler–rubber interactions. Both filler–rubber and strong filler–filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler–filler interaction, and it is maximum when also the filler–rubber interaction occurs. Strong silica–rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.