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16 Rubber & Plastics News • February 10, 2020 www.rubbernews.com Technical Role of in 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 ML (1+4) @ 125 C 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 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. produced by Nouryon and commercial- fur crosslinking or vulcanization is the in combination with stearic 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 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 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 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 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 -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 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 , struments X-MaxN 150 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 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. 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 . 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 , from Gronin- tember 1996 as a member of the product on surface analysis from the Eindhoven gen State University and a post-doctorate development 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 Anke Blume studied chemistry at A.G. She worked there in different posi- of 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 , German Institute for Rubber Technol- and silane for rubber applications. In Oc- at Nouryon Func- radical polymerization, thermoset and ogy, which she finished November tober 2013, she reduced her working time tional Chemicals. sulfur chemistry. He is the inventor of 1995. at Evonik and started at the University She has more Perkalink 900, an antireversion agent for After nine months of post-doctoral of Twente as chair of Elastomer Technol- than 20 years of elastomer compounds. Talma holds a doc- study at the DIK, Blume started in Sep- ogy and Engineering. experience in the polymer field, ei- ther polymer syn- thesis or applica- Butuc tion development and polymer testing, ranging from bench chemist to business management. Butuc is the inventor or co-inventor of six U.S. and international patents and the author of four technical arti- cles and two book chapters. She holds a master’s degree in organic chemis- try from the University of Bucharest and a bachelor’s in physical chemistry from the same university. Arne Janssen currently is work- ing for Zeiss Microscopy as product manager for electron microscopy. Janssen holds a doctorate in geolog- ical and sciences/geosciences from Westfalische Wilhelms—Uni- SEPTEMBER 15-17, 2020 | AKRON, OH versitat Muenster. Kees van Leerdam is an analytical scientist, specializing in microscopy and surface analysis. Van Leerdam THE LARGEST SHOW IN NORTH AMERICA has worked in different science-re- lated roles in the for almost 30 years, serving many different markets such as catalysis, fibers, pharmacy, , paper, coatings and . Van Leer- BOOTHS AVAILABLE FOR 2020 SHOW source and Super-X energy dispersive sili- con drift detectors (SDD’s). The instru- Network with the Generate Leads Raise your ment was mainly operated in STEM mode at an accelerating voltage of 200 keV with Best in the Industry a beam current of 500 pA. Imaging was and New Business Company Pro le commonly done using a high angle annu- lar dark field (HAADF) detector. EDX spectrum images (SI) were ac- quired and processed in the Thermo Scientific Velox Software. Typical acqui- sition conditions were image sizes of 1,024 by 1,024 pixels at a dwell time of 25 μs/pixel. Electron transparent films for the STEM-EDX investigations were pre- pared with a Leica UC7 Ultramicrotome under cryogenic conditions (-120ºC). The thin sections were collected dry on holey filmed Cu grids.

Results and discussion This paper is focused around the role of ZnO as crosslinking facilitator for sulfur cure. The curative system used is a dual system organic peroxide and sul- fur generated in situ by the CTS in a blend of EPDM and CTS (Fig. 1). The Interested in Exhibiting? dual cure system is used for improved scorch time during curing, improved mechanical properties and better com- Looking to Register? pression set. While the peroxide cure mechanism is understood, the sulfur cure is more complex in this system and Have a Paper to Present? it is facilitated by the presence of ZnO and stearic acid. Polysulfides are known to have a high resistance against swelling in hydrocar- bon solvents.18,19 CTS was chosen for this NOW ACCEPTING HERZLICH experiment because of its sulfur donor AWARD SUBMISSIONS properties in such a manner that it has the right scorch time (other types of poly- See Crosslinking, page 18 visit itec-tireshow.com 18 Rubber & Plastics News • February 10, 2020 www.rubbernews.com Technical

but the mechanism of the reaction is not shaping on a two-roll mill and cured at compound (EPDM-CTS-CB) versus sili-

understood. 177 C. The fillers used for these applica- ca-filled compound (EPDM-CTS-SiO2) Crosslinking EPDM has a fully saturated backbone tions were selected to have a comparable at smaller scale of 20 µm appears to and unsaturation is added in the small surface area for ease of comparison, have a more homogeneous CB-filled Continued from page 17 side chains as crosslinking sites. Its which is 60 (m2/g) for the silica particles compound than the silica-filled com- , such as polysulfides containing structure is indicated in Fig. 2. and 83 (m2/g) for the carbon black. pound. The homogeneity of the two ma- end groups, exhibit high scorch).18 Because rubber compounds are hav- Testing at the macro structure level, terials is similar, but the contrast given CTS as a blend with EPDM with the ing a complex composition, the formula- Shore A hardness has an expected lower by silica and CTS makes it appear as resulting EPDM-CTS blend targeted to tions presented in this paper are scaled value for the non-filled compound (45) as slightly less homogeneous. SEM pictures have an improved resistance against back to basic contributors to the elasto- compared to the filled rubbers (54 for of CB and silica-filled compounds, re- swelling in hydrocarbon solvents. The mer network to facilitate the investiga- silica-filled material and 56 for CB-filled spectively, are shown in Fig. 6. CTS is undergoing a ring opening reac- tion on sulfur crosslinking and the role material). Mechanical properties are Further research is focused on im- tion from which it is releasing sulfur played by ZnO and stearic acid. presented in Fig. 3. The unfilled com- proved of the two phases species in the form of mono and disul- Presented in this paper, there are pound shows as expected lower moduli, EPDM and CTS using a compatibilizer. fide, which can further participate in three compounds having the composi- tensile strength and elongation at break. At even higher magnifications (nm crosslinking reactions. The ring opening tions listed in Table 1 that were mixed Interestingly, the compound contain- scale) by using STEM/EDX the role of reaction should be facilitated by ZnO, using an internal mixer followed by ing carbon black has much improved ZnO can be analyzed in depth in gener- mechanical properties when compared ating the sulfur in-situ. Fig. 7a identi- Fig. 4: Hydrocarbon absorption of EPDM-based compounds. against the compound containing silica. fies a small agglomerate of ZnO in the This could be explained by carbon EPDM compound. Using EDX imaging black’s affinity to the EPDM elastomer, it also can be identified the presence of therefore presenting a good reinforcing two other Zn species, the first one is ZnS behavior. By comparison, silica—being a present as small (3 nm) nanoparticles in polar material—has a greater affinity abundance, particularly surrounding the toward the CTS part of the network, ZnO agglomerate. which is first less abundant, and second The presence of ZnS is identified by is the weaker of the two main phases superimposing the two images identify- because of its low molecular weight. ing Zn presence and S presence. The Also, the silica filler used in this experi- second Zn compound is a faint, feather- ment does not contain a coupling agent, like structure, which is considered to be which is known to enhance the reinforc- zinc stearate. Although the ZnO and ing silica properties. Future studies of ZnS are found in a close neighborhood of compounds containing silica fillers are each other, with ZnS being more scat- incorporating coupling agents, which tered and surrounding the ZnO clusters, have the potential to improve the filler the zinc stearate compound is generally reinforcement behavior. found at a bit larger distance from ZnO, A fluid immersion test was performed suggesting a higher mobility. according to ASTM D471. Materials were It seems to be that both ZnO and ZnS immersed in the test fluid and removed being inorganic materials do not solubi- periodically for measuring of fluid absorp- lize in EPDM, but zinc stearate has a tion/adsorption until equilibrium. Fluid relatively higher solubility in the poly- absorption measurements were performed mer matrix. Zinc stearate appears at a Fig. 5: SEM images of unfilled and CB-filled compounds. by weighing of the test specimens. The distance further away from the ZnO test results are presented in Fig. 4. cluster supporting the idea of a EPDM peroxide cured was used with- higher solubility of this chemical com- out CTS as a reference material, also pound in EPDM. containing five phr of ZnO and stearic CB particles also are playing a sup- acid, respectively. It is known that port role, facilitating the sulfur generat- EPDM performs poorly when exposed to ing reaction, as seeing in Fig. 7b-c, hydrocarbon solvent.20 Therefore, it is no where sulfur as clusters of ZnS are seen surprise that the reference material is on the surface of CB particles. Fig. 7d is the worst performer in the fluid expo- a close look at clusters of ZnO and ZnS. sure test performed by an increase of Similar features are seen in the case 131 percent of its original weight. By of silica-filled compound in Fig. 8, the comparison, a compound containing 25 same three types of Zn species are pres- percent CTS is showing a reduction of ent. Once again ZnO and ZnS are clus- fluid absorption of approximately 28 tered together while zinc stearate gets percent. The EPDM-CTS compounds diffused within the EPDM network. Fig. 6: SEM image of CB and silica-filled compounds. also containing silica and carbon black, One noticeable difference, at least quali- respectively, showed an even higher re- tatively, is in the relative amount of ZnS sistance to swell in hydrocarbon solvent, present at the surface of silica, which is but this superior result is a filler effect. significantly lower than CB. Looking at the micro level and analyz- These imaging evidences are support- ing the structure of the compounds by ing the hypothesis the sulfur generating SEM (Fig. 5), the homogeneity of the reaction being catalyzed by the ZnO compounds can be compared. In the non- crystals in the presence of stearic acid. filled compound (EPDM-CTS), the con- Additional supporting STEM images tinuous is EPDM while the dis- of EPDM-CTS compound filled with sili- creet phase is the CTS. It is expected ca also is indicating the presence of all and proved in this experiment that the three Zn species, ZnO, ZnS and zinc dispersion is greatly improved when stearate. Once again ZnO and ZnS are filler is added, as indicated in Fig. 5 for within close proximity, but zinc stearate the carbon black-filled compound is diffusing away from the surface of (EPDM-CTS-CB). ZnO and into the EPDM matrix as indi- A side by side comparison of CB-filled cated in Fig. 9.

Fig. 7: STEM/EDX images of CB-filled compound. Summary This research project takes a different approach to investigate the sulfur cross- linking by ways of imaging of ZnO. With the use of STEM/EDX, it is demonstrated that the ring opening sulfur generating reaction occur at the surface of ZnO pres- ent in the compound and the reaction is facilitated by the presence of stearic acid. Resulting free sulfur is captured by the ZnO, which turns into ZnS. It also is concluded that zinc stearate is less effi- cient in sulfur crosslinking because of its solubility in the polymeric matrix. See Crosslinking, page 19 www.rubbernews.com Rubber & Plastics News • February 10, 2020 19 Akron rubber workers: It’s time to tell your stories By Sue Walton the oral history project but wants more, so it has sched- Crain’s Akron Business uled recording sessions from March through May AKRON—The city of Akron is asking rubber workers across Summit County. and their families to share their voices to help preserve “This is our opportunity to celebrate the people who the industry’s legacy. made Akron the rubber capital of the world,” Love said In September, the city announced it would commemo- in the release. “We want to help visitors and residents rate its heritage as the Rubber City by installing a 12-foot learn about Akron’s unique history, how it shapes our bronze statue of a rubber worker in a new roundabout at identity and how it is driving our future.” Main and Mill streets. The design of the work, by sculptor Collected stories will be archived with the Ak- Alan Cottrill and to be installed later this year, was in- ron-Summit County Public Library and be available spired by the image on the cover of the book “Wheels of online, with excerpts featured as part of the interactive Fortune” by David Giffels and Steve Love, both formerly kiosk downtown, Akron Stories said. writers for the Akron Beacon Journal newspaper. Akron resident Miriam Ray is founder of the Akron Sto- But the statue isn’t the only thing that looks to pre- ries initiative, according to its website, and has worked to- serve Akron’s rich rubber industry history. As part of ward making the storytelling project a reality since 2016. This rendering shows the bronze sculpture of a rub- the project, an interactive kiosk, with audio and visual “From the very beginning, I asked people for their sto- ber worker planned for a new roundabout at Main and storytelling featuring former rubber workers, will be ries and quickly realized they were as important as the Mill streets in downtown Akron. installed in the northeast corner of the intersection. rubber worker statue itself,” Ray said in a statement. “We Akron Stories, a group led by artist and business are three generations removed from these workers and it of their working mothers. … Many (workers) were ac- owner Mac Love, is leading the storytelling project, is so important that future generations get to know them.” tively involved in the union, standing up for their rights which is being funded, along with the kiosk, by the sale Added Josy Jones, the project’s director of storytell- and shaping a future for their children.” of commemorative bricks. About 130 bricks already have ing, in a statement: “We are hearing from people who Organizers want to hear from as many rubber work- been sold, according to a recent news release. They’re remember their family members coming home coated ers and their families as they can, and ask contributors available for purchase online. in black tire remnants. They remember the sounds and to schedule an interview at AkronStories.com or by The group already has collected dozens of stories for energy of Main Street and the brilliance and resilience calling 330-238-8588, the release said. Auto makers to stop monthly new sales reports By Urvaksh Karkaria The closely watched, publicly traded and troughs. We think this will just coaxing retailers to register as Automotive News startup chose not to advertise its month- smooth those.” sold when they are still in inventory. BMW For the longstanding auto industry in, month-out achievements as it Added Jominy: “There’s not a lot of good is under scrutiny by the Securities and tradition of reporting monthly new-vehi- launched, and Wall Street didn’t seem to news to go around. So how many times in Exchange Commission, accused of engag- cle sales, the dominoes are falling. mind its lack of transparency. Tesla’s a year do you want to give bad news?” ing in such tactics in the U.S. In fact, the game now is, more or less, over. stock rose to unprecedented heights, Shifting to quarterly reporting will be Audi, BMW, Nissan, Porsche and Volks- making the company worth more on pa- Skewed view? good for both dealers and manufactur- wagen have made back-to-back announce- per than GM and Ford combined. Measuring quarterly instead of month- ers, said Patrick Womack, general man- ments that they will stop reporting their “The auto industry is asking itself: ly also may paint a picture of a smoother ager of Laurel BMW of Westmont in model-by-model monthly sales results ‘Tesla doesn’t do monthly reporting and sales year. Year-over-year monthly com- suburban Chicago. It takes off some of across the U.S., switching instead to quar- they are being hugely rewarded, so why parisons can garner big headlines among the pressure to hit 30-day sales targets, terly reports. Toyota has joined the rebellion do we do it?’ ” Tyson Jominy, vice presi- consumers but are sometimes misleading said Womack, who also is chairman of by brands that represent the bulk of U.S. dent of the Power Information Network because of fluctuations in the number of the BMW National Dealer Forum. sales, although last week it held out the at J.D. Power, told Automotive News. selling days. “We won’t have to offer loser deals at possibility of at least making its monthly Auto makers apparently see an opportu- This year, an extra calendar weekend month end,” he said. “Little things disrupt results available on a less formal basis. nity to take a break from monthly pressures will boost sales volumes in February, the market month to month. So quarterly The stampede away from tradition fol- and find a truer picture of their market re- May, October and December over 2019 is probably the better and healthier way to lows similar decisions by U.S. auto makers. sults, especially now, at a time of falling tallies; March, June and November each look at sales performance.” First, it was the California upstart Tesla sales and shifting consumer preferences. will lose a weekend. And Labor Day But the impact will be felt by economists, refusing to go along with the industry es- Sales of some car models have plummeted weekend, typically the king of selling industry analysts and corporate strate- tablishment, providing only global quar- in the past two years, but only because weekends, moved to August for 2019 but gists, Jominy said. The monthly data has, terly results. In 2018, industry leader customers are driving off dealership lots in reverts to September this year. for decades, been “a fantastic barometer” of General Motors balked, saying it was done new crossovers instead of sedans. The reporting change also might lead consumer spending strength, he said. with the practice. And last year, GM’s two “We think it’s a good way for us to pro- to healthier industry behavior. “Economists look to the auto industry Detroit competitors, Ford Motor Co. and vide a clearer picture of the sales perfor- The competitive pressure of the monthly for signs of what’s truly going on with Fiat Chrysler, followed suit. mance over a longer of time,” a “sales race” has fostered dubious practices consumer demand,” Jominy said. Many observers lay the disruption at Nissan spokesman said. “There’s a lot of and creative accounting. Auto makers have Those economists will soon have to find the feet of Tesla. monthly variances in sales—there’s spikes been accused of padding their numbers by another way to read the tea leaves. Technical

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Fig. 8: STEM/EDX images of silica-filled compound. Fig. 9: Additional STEM/EDX images of silica-filled compound.