Roles of Dinuclear Bridging Bidentate Zinc/Stearate Complexes in Sulfur Cross-Linking of Isoprene Rubber

Abstract

The roles of the intermediate [Zn 2 (μ-O 2 CC 17 H 35 ) 2 ] 2+ •4X (X; a hydroxyl group, water, and/or a rubber segment) in the sulfur cross-linking of isoprene rubber are clarified for the first time using in situ time-resolved zinc K-edge X-ray absorption fine structure spectroscopy and in situ time-resolved infrared spectroscopy along with density functional theory calculations. The combined experimental and computational investigation suggests that N-(1,3benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS) is most easily hydrolyzed on the dinuclear bridging bidentate zinc/ stearate intermediate, when a water molecule coordinates to the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the benzothiazole group in CBS. The newly produced intermediate with coordinated 1,3-benzothiazole-2-thiolate and cyclohexylamine (CHA) is also found to most readily induce a sulfur insertion among possible candidates to generate subsequent intermediates, when CHA is removed from the intermediate and a water molecule coordinates to the zinc cation which is coordinated by the nitrogen atom of benzothiazole group. The novel dinuclear bridging bidentate zinc/stearate complexes apparently accelerate the sulfur cross-linking of isoprene rubber. Despite the long history of rubber science and technology, these intermediates have been mysterious. The present work will clarify the vulcanization mechanism and will advance the rubber chemistry for a new paradigm of vulcanization technique in the 21 st century.

Effect of fatty acids on the accelerated sulfur vulcanization of rubber by active zinc/carboxylate complexes

“…2 and S2. † These changes in their FT-IR spectra were similar to those of ZnO(0.5)-StH(2) and IR-ZnSt 2 (4.5) in our previous papers, 13,14 where the formation of a specic structural complex was reported. The identied structure was [Zn 2 (m-O 2 CC 17 H 35 ) 2 ] 2+ $4X, where X was a hydroxyl group, water and/or rubber segment (intermediate I), as shown in Fig.…”

“…These results agree well with the absorption bands observed in IR-ZnO(0.5)-StH(2) at 144 C in our previous study. 13,14 In addition, Dn of the COO À antisymmetric and symmetric stretching of these two bands was 173 cm À1 for all samples. Therefore, it was conrmed that the generated zinc/carboxylate complexes in all samples had the bridge bidentate-type coordination.…”

“…In the common rubber compounding containing StH and ZnO, it is well-known that StH reacts with ZnO to form ZnSt 2 as an essential cure activator for the vulcanization reaction. In our previous study, the generation of ZnSt 2 was conrmed by the presence of COO À antisymmetric and symmetric stretching bands at 1537 and 1398 cm À1 , respectively, for the spectra of IR-ZnO(0.5)-StH(2) at 35 C. 13,14 This observation corresponds well with the bands detected in IR-ZnO(0.5)-StH(2.0) and IR-ZnSt 2 (4.5) at 35 C in this study. Thus, the bands of COO À antisymmetric and symmetric stretching that are present in other samples suggest that the reaction between ZnO and each fatty acid also partially occurred during the rubber mixing process.…”

“…This observation suggests that each sample can form a tetrahedral coordination structure of dinuclear-type bridging bidentate of zinc/carboxylate complex at high temperature, similarly with our previous results observed in isoprene rubber mixed with ZnSt 2 or ZnO/StH, as reported. 13,14 Therefore, the XAFS and FT-IR results suggested that the aliphatic chain length of fatty acids does not affect the generation of the dinuclear bridging bidentate zinc/carboxylate complex intermediate in the rubber matrix at high temperature, however, it inuences the reactivity of the fatty acid with ZnO.…”

“…In 2019, we nally revealed the vulcanization mechanism via the intermediate I, 14 where the hydrolysis reaction of N-cyclohexyl benzothiazole sulfenamide (CBS), the insertion reaction of sulfur and sulfur cross-linking reaction were found to be accelerated. Furthermore, the visualization of this speculated two-phase morphology was conrmed by using the atomic force microscopy nanomechanical mapping with the twodimensional mapping of reliability index.…”