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.
The
present study aims at supplying a more in-depth picture of
the generation of environmentally persistent free radicals (EPFRs)
from phenol (PhOH) on ZnO/SiO2 systems, by exploring the
properties of ZnO nanoparticles (NPs) with different intrinsic defectivity
grown on highly porous silica with a spherical (ZnO/SiO2_S) and wormlike morphology (ZnO/SiO2_W). In detail, besides
an extensive structural, morphological, and surface investigation,
the occurrence of inequivalent defect centers in the samples was tracked
by photoluminescence (PL) experiments, which unveiled, for ZnO/SiO2_W, intense blue emissions possibly involving radiative recombination
from Zni excited levels to the valence band or to VZn levels. Electron spin resonance (ESR) spectra corroborated
these results and revealed a remarkably different behavior of the
samples in the EPFR formation model reaction. In fact, upon PhOH contact,
the ESR spectrum of ZnO/SiO2_S showed the exclusive presence
of a weak isotropic signal ascribable to a PhenO• EPFR. Instead, for ZnO/SiO2_W, intense features associated
with oxygen species in proximity of VO
+, VZn
–, and (VZn
–)2
– centers dominate the spectra, while
a minor contribution of the PhenO• radical can be
discovered only by signal simulation. These outcomes definitively
envisage a role of the intrinsic defectivity of ZnO NPs on the final
yield and stability of EPFR generation, with VO
+ and VZn
– defects possibly involved
in dissociative adsorption or oxidation processes at the oxide surface.
Although this work focuses on ZnO, it is expected to foster a critical
re-examination and integration of important results on other metal
oxide/silica systems already reported in the literature, offering
the chance to better evaluate the dependence of EPFR generation on
the oxide defects chemistry.
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