16 Rubber & Plastics News • February 10, 2020 www.rubbernews.com Technical Role of zinc oxide in sulfur crosslinking By Gina Butuc, Arne Janssen, Kees The physical appearance of this product van Leerdam, Auke Talma is of a viscous liquid. Nouryon Functional Chemicals Executive summary The EPDM is produced by Arlanxeo Sulfur crosslinking was discovered by Charles Goodyear and Thomas Han- and commercialized under the trade and Anke Blume cock more than 150 years ago and led to the development of a new material name of Keltan 2450. This elastomer has University of Twente science application—rubber. Since the first discovery of ways of vulcanizing a low Mooney viscosity ML (1+4) @ 125C Either naturally derived or syntheti- rubber for improved dimensional stability, mechanical properties and chemical of 28, and a relatively high ethylene con- cally produced elastomers need to be resistance, sulfur continued to be analyzed to elucidate its role in the crosslink- tent of 48 percent, while the ethylene subject to further chemical processing ing process. norbornene content is of 4.1 percent. to achieve the dimensional stability re- Although the discovery of sulfur crosslinking was in itself a game changer, it Crosslinking agents are as follows: quired by rubber applications—and this was determined that vulcanization was too slow for commercial purposes and • Dicumyl peroxide (40 wt.%) on clay process is known as vulcanization. Sul- as such methods to expedite the crosslinking reaction were studied. Zinc oxide produced by Nouryon and commercial- fur crosslinking or vulcanization is the in combination with stearic acid were discovered as the best ways for improved ized under the trade name of Perkadox most common way of achieving the me- sulfur reactivity in the vulcanization process. Zinc ions combine with stearic BC-40K-pd, in powder form; chanical strength required in rubber acid and cyclic tetrasulfide (acting as a sulfur accelerator) to form an active • ZnO technical grade by Spectrum applications.1-4 complex which catalyzes the vulcanization process. Chemicals; and It is seldom that elemental sulfur is Because the mechanism of reaction is complex, analyzing the structure at the •Stearic acid technical grade by Spec- used as the sole crosslinking agent. Usu- nano level could yield an insight into the process. This paper is focusing on trum Chemicals. ally there are other sulfur compounds transmission electron microscopy (TEM) and energy-dispersive X-ray spectros- Compound mixing equipment was copy (EDX) for an in-depth analysis of the process, with an emphasis on ZnO performed in a Brabender internal mix- TECHNICAL NOTEBOOK crystallography/surface chemistry and its influence on sulfur crosslink process. er with a capacity of 0.5L and intermesh Edited by John Dick rotors. playing an important role in rubber In this research, the role played by the Mechanical properties were measured called accelerators that are helping in compounds, particularly in the sulfur ZnO as crosslinking facilitator was in- using a tensile tester by Instron, model the vulcanization process. Although the related vulcanization. Interestingly, in vestigated in a novel polymeric material 4500 series with a load cell of 5kN. process of vulcanization has been known the early years of rubber vulcanization, blend with increased resistance to Stress strain tests were performed ac- for more than 100 years, the mechanism ZnO only was used as an inorganic rein- non-polar hydrocarbon solvents. The cording to ASTM D1708 at a crosshead of reactions is still yet to be fully under- forcing filler. It was not until the 1920s novelty polymer-oligomer mixture for speed of 50 cm/min. stood. The main reason for this knowl- that it was proven that sulfur vulcaniza- rubber applications is based primarily The fluid immersion test was per- edge gap is the complexity of rubber tion process can be expedited using ZnO on EPDM. formed according to ASTM D471 at compounds, which often contain more in combination with stearic acid.10,11 The curative system for the blend is room temperature and atmospheric than five ingredients involved in the The role of ZnO and stearic acid was dual organic peroxide and sulfur, and pressure. The hydrocarbon fluid used vulcanization itself, therefore the full not fully understood and it is only in re- the focus of this investigation is on the for immersion is IRM 903, which com- mechanism of crosslinking is not fully cent years that scientists started study- role ZnO plays in the polysulfides cure. prises 45 percent normal paraffin, 50 understood. ing the chemical process by using both Dual cure is getting an increased atten- percent iso paraffin, and 5 percent aro- The mechanism of sulfur crosslinking small-angle neutron scattering and X-ray tion recently because it has the advan- matic hydrocarbon. has been elucidated in the case of ele- techniques, suggesting that an interme- tage of imparting best properties derived For imaging a scanning electron mi- mental sulfur, both radical or ionic initi- diate structure of ZnO/stearic acid is from each type of curative.16 For EPDM, croscope, (SEM) Zeiss CrossBeam 540 ated process.5-6 However, a large number formed.12,13 this kind of dual cure system could lead was used, equipped with an Oxford In- of sulfur crosslinking applications in- Aging of elastomer compounds is to improved scorch time during curing, struments X-MaxN 150 silicon drift de- volve crosslinking through sulfur accel- caused mainly by free sulfur resulted improved mechanical properties such as tector (SDD) for EDX (using Aztec soft- erators, and for these reactions it is gen- from polysulfic bonds cleavage into di- higher elongation at break and increased ware), and Atlas 5, a correlative workflow erally understood that the mechanism is sulfide or even monosulfide bridges. Be- tear strength, as well as better heat software for large area imaging and specific to each type of accelerator.7,8 sides participating in the crosslinking stability of the resulting compound and slice-and-view experiments for tomogra- In recent years, special attention is process, ZnO plays an important role in improved compression set. phy. SEM-EDX Spectrum Images (SI) given to polymeric sulfur either as is or reducing the rubber hardening as an In this complex system, the role of ZnO and large area SEM maps were acquired in the form of sulfur organic copolymers undesired side effect of oxidative aging is unclear, but with the help of analytical at a primary electron beam energy of 6 acting as vulcanization agents because occurring in sulfur cured compounds, techniques such as scanning electron keV at a beam current of 300 pA. they present the advantage of improved particularly in SBR. In this type of sys- microscopy (SEM) and transmission Cross-sections of the rubber samples solubility of sulfur species in the poly- tem, the main role of ZnO is to activate electron microscopy (TEM), both coupled for SEM were prepared either by cutting meric matrix.9 the sulfur cure having ZnS as a by-prod- with energy dispersive X-ray spectrosco- with a razor blade perpendicular to the It already is known that zinc oxide is uct of the cure reaction.14,15 py (EDX), the role of ZnO and stearic acid surface or by using a Leica EM UC7 ul- in facilitating sulfur cure of a non-polar tramicrotome under cryogenic conditions. Fig. 1: CTS molecular structure. Fig. 2: EPDM molecular structure.16 elastomer, using a sulfur donor type sys- The cross-sections were attached with tem could be explained. Silver DAG (provided by agar scientific) onto Al based stubs. A Leica EM ACE600 Materials and experimental high vacuum sputter/coater was used to description coat the samples with 12 nm amorphous Cyclic tetrasulfide (CTS) is produced C to increase the electron conductivity. by Nouryon Functional Chemicals and Imaging at higher magnifications was commercialized under the trade name of performed on a Thermo Scientific Talos Thioplast CPS200; and is a low molecu- F200X S/TEM Transmission Electron lar weight oligomer, with a Mw ~5,000 Microscope. This microscope is equipped and a high sulfur content of 50 percent.17 with a high brightness X-FEG electron Table 1: EPDM-CTS elastomer composition. Fig. 3: Mechanical properties of EPDM-CTS compounds. www.rubbernews.com Rubber & Plastics News • February 10, 2020 17 Technical The authors dam holds a doctorate in applied physics torate in organic chemistry, from Gronin- tember 1996 as a member of the product on surface analysis from the Eindhoven gen State University and a post-doctorate development group silica in the Applied Gina Butuc is a doctoral candidate University of Technology, and a mas- from the University of Twente. Technology Department of Degussa at the University of Twente, faculty ter’s degree in chemistry on catalysis Anke Blume studied chemistry at A.G. She worked there in different posi- of Engineering Technology, Depart- from Utrecht University. the university of Hanover Germany tions, always related to the development ment of Elastomer Technology and Auke Talma has been working for from October 1988 until July 1993. In of silica and silane for use in rubber. Engineering, and a technical devel- AkzoNobel/Nouryon for the past 35 the last year of her study, she came in Degussa changed owners several times opment manager years, in various functions ranging the first contact with rubber during and now is known as Evonik Resource for crosslinking from scientist to research and develop- her master thesis. Consequently, she Efficiency GmbH. Since August 2011, peroxides and ment director for basic and applied re- carried out her doctoral thesis at the Blume has been an IP manager for silica polymer additives search of additives for the rubber industry, German Institute for Rubber Technol- and silane for rubber applications.