@article{32af1053e3ec4c07bb119fde064cda14,
title = "Elucidation of the role of ZnO in sulfur cure in novel EPDM-CTS blends",
abstract = "A new concept is introduced in this study, suggesting the role played by the ZnO crystal in sulfur crosslinking polymers, particularly, rubber blends of EPDM-CTS. The study is conducted in a polymer blend of ethylene propylene diene terpolymer (EPDM) and cyclic tetrasulfide (CTS). CTS is a reactive low molecular weight polymer, which can act as a sulfur donor. It is proposed that the sulfur crosslinking occurs at the surface of the ZnO crystals. A reaction site is created on the surface of the ZnO crystal by the reaction with stearic acid, which creates a “template” or active site on the surface as a catalytic site for the CTS reaction to take place. Various experiments are performed to substantiate the newly proposed role the crystalline ZnO structure plays in influencing the initiation of the sulfur crosslinking. The investigation is conducted in rubber compounds in the absence of fillers and other additives and additionally in chemical model studies. The analyses conducted in rubber compounds are performed on fully or partially cured rubber to study the evolution of the chemical process involved in the CTS reaction and the curing of the rubber blends. Step-cured compound allows analyses of the transformation of the ZnO crystal size during the sulfur crosslinking process, and the ability to gauge the step transformation that the ZnO crystal is undergoing during the crosslinking process. Additional chemical model studies are used in the study of the influence of the ZnO crystal on sulfur crosslinking, to confirm the reaction path and by-products of reaction. Lastly, quantum mechanical (QM) and molecular mechanics (MM) calculations are applied in support of the suggested mechanism of reaction.",
keywords = "UT-Gold-D, Cyclic tetrasulfide, Mechanism of crosslinking, Mechanism of reaction, Modelling studies, Rubber blend, S(T)EM-EDX, Sulfur crosslinking, XRD, ZnO crystal, Crystalline template",
author = "Butuc, {Steluta Gina} and {van Leerdam}, Kees and Brenda Rossenaar and Jose Swart and Frank Geurts and Bianca Bossinga-Geurts and Paul Verwer and Auke Talma and Anke Blume",
note = "Funding Information: The uncured compound was analyzed as a reference. Polymeric domains can be distinguished in the STEM-HAADF image of Fig. 3. The continuous polymeric phase is EPDM, while the discontinuous and darker grey colored phase is CTS. This is supported by the S and O element maps, which show that CTS domains have a higher S and O content. Also, the clay particles used as a carrier for the peroxide formulation are clearly visible in Fig. 3, as can be judged from their characteristic claylike shape in combination with the identification of the elements Al and O.All experimental data, either in-rubber or solid-state experiments supports the claim of the ZnO crystal acting as a base for the crosslinking reaction involving CTS. With these experiments the concept of the ZnO surface reactivity introduced by Nieuwenhuizen and Heideman [5,6,16], can be explained in greater detail. It proves that the crystalline structure of the ZnO plays a more important role than a simple adsorption-desorption process. Furthermore, the reactivity of the ZnO crystal is proven to be different depending on the facet of the crystal.A new concept is introduced in this paper, the concept of ZnO forming a “template” support for the ring opening reaction of CTS (Fig. 18). A reactive site is created on the surface of the ZnO crystal by the reaction with stearic acid. Two sulfur atoms and the allylic group of the polymer are fitted in the “template” where a six-member ring intermediate is formed. Upon the CTS ring opening reaction, additional by-products of reaction are ZnS and H2O. Based on all results from the described experiments and analyses, a “template” as an active site is proposed for the ring opening reaction in the presence of an allylic hydrocarbon site. Publisher Copyright: {\textcopyright} 2022",
year = "2023",
month = jan,
doi = "10.1016/j.polymertesting.2022.107843",
language = "English",
volume = "117",
journal = "Polymer testing",
issn = "0142-9418",
publisher = "Elsevier Ltd",
}