ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS Effect of Metal Oxides as Activator for Sulphur Vulcanisation in Various Rubbers Zinc oxide  Sulphur vulcanisation  Acti- vators  Metal oxides  Time-concentra- tion development of reaction products Two widely different rubbers, viz. EPDM and s-SBR have been selected in the present study. The influence of the basi- city, the crystal structure and the ability to form complexes of various metal oxi- des were studied. It was observed that neither CdO, PbO, BaO, CaO, MgO and BeO are proper substitutes for ZnO as activator in thiuram-accelerated vulca- nisation of EPDM, nor do they show a synergistic effect with ZnO. In s-SBR compounds, however, it is demonstrated that CaO and MgO can function as acti- vator of cure for sulphur vulcanisation, retaining the curing and physical prop- erties of the rubber vulcanisates. Model Compound Vulcanisation has been used to elucidate the influence of the activators MgO and CaO on the vul- canisation mechanism. It can be concluded that depending on the exact requirements for a specific compound, certain metal oxides repre- sent an alternative route to reduce the zinc level and therefore to minimise the environmental impact. G. Heideman, J. W. M. Noordermeer, R. N. Datta, Enschede (The Nether- lands), B. Van Baarle, Eindhoven (The Netherlands) Corresponding author: J. W. M. Noordermeer University of Twente Faculty of Science and Technology Department of Rubber Technology P.O. Box 217 NL-7500 AE Enschede E-mail: J.W.M.Noordermeer@utwente.nl Einfluss von Metalloxiden auf die Aktivierung der Schwefelvul- kanisation von Kautschuken Zinkoxid  Schwefel Vernetzung  Ak- tivator  Metall Oxide  Zeit-Konzen- trationsverlauf von Reaktionsproduk- ten In dieser Studie wurden verschiedene Oxide in zwei unterschiedlichen Poly- meren, EPDM und s-SBR, getestet. Der Einfluss des Sa ¨ uregrades, der Kristall- struktur und der Fa ¨ higkeit Komplexe zo formen, wurden untersucht. Es zeigte sich, daß weder CdO, PbO, BaO, CaO, MgO noch BeO als Ersatz fu ¨ r ZnO in einem Thiuram-Vulkanisationssy- stem in EPDM in Frage kommen. Es konnte auch kein synergistischer Ef- fekt der Kombination der genannten Oxide mit ZnO festgestellt werden. Im Gegensatz dazu ko ¨ nnen CaO und MgO als Ersatz fu ¨ r ZnO eingesetzt werden, ohne daß das Vulkanisati- onsverhalten und die mechanischen Eigenschaften des Vulkanisates be- einflußt werden. Mit Hilfe von Modellcompound-Vul- kanisationen wurde der Einfluß von MgO und CaO auf den Mechanismus der Vulkanisationsreaktion unter- sucht. In dieser Untersuchung konnte ge- zeigt werden, daß abha ¨ ngig von den Produktanforderungen alternative Metalloxide geeignet sein ko ¨ nnen, die Zinkkonzentration in einer Gummi- mischung zu verringern und so die Umweltbelastung zu reduzieren. It is generally known that for efficient vul- canisation of rubbers by elemental sulphur or by sulphur donors the presence of a me- tal activator is necessary [1]. Zinc oxide is the most effective activator for sulphur vul- canisation. A great deal of attention has been paid to the problem of reducing the zinc content in rubber products. A completely zinc-free vulcanisation system based on sulphur keeps on being intri- guing. There have been a number of inve- stigations comparing different metal oxi- des as vulcanisation activators, mostly with tetramethylthiuram disulphide (TMTD) in NR, with variable results. It is concluded that a variety of metal oxides can accelerate cure, but the degree of ac- celeration varies with the specific metal ion used [2]. In a research by Chapman, it was found that of metal oxides other than ZnO, CdO on average appears to be best, follo- wed by lead- and mercury-oxide [3]. Lau- tenschlaeger et al. studied the effects of different metal oxides in accelerated sul- phur vulcanisation with 2-methyl-2-pente- ne, as a model olefin [4]. The results of a comparison of ZnO, CdO, CaO, and a com- bination of ZnO and aniline also indicated that CdO is the most effective oxide, resul- ting in a high yield in monosulphides and a relatively low production of byproducts. In an investigation of the crosslinking of a model olefin with sulphur, tetraethylthiu- ramdisulphide (TETD) and various metal oxides, cupric oxide was found to give hig- her yields than zinc oxide, and nickel oxide was also quite effective [5]. Some metal oxides behave synergistically with ZnO. Replacement of half of the ZnO by an equivalent amount of CdO, PbO, Bi 2 O 3 , CaO, HgO or CuO in an effici- ent vulcanisation (EV) system gave higher moduli. Other oxides gave moduli equal to or lower than with ZnO alone. CaO and MgO are apparently not so promising in conventional sulphur/sulphenamide cu- res, especially not in combination with ZnO. It has been reported that CaO and MgO interfere with the efficient activation of ZnO [4]. Where ZnO is commonly used in the vulca- nisation recipe, not much is known about the accelerating properties of other metal oxides, so, a comprehensive study was considered worthwhile. In the first part of this article the sulphur vulcanisation of EPDM and s-SBR rubber with several al- ternative metal oxides as activators is de- scribed. The objective of the second part is to gain additional insight into the me- chanistic details of sulphur vulcanisation with other metal oxides as activator in or- der to judiciously reduce zinc oxide levels in rubber compounding. Other metal oxides in comparison with ZnO It has been reported that the high activity of ZnO can be explained on the basis of the chemistry of complex formation. A prece- ding reaction with stearic acid forms the rubber hydrocarbon-soluble zinc stearate and liberates water before the onset of crosslinking [6]. Furthermore, ZnO is in many vulcanisation systems a precursor to zinc-derived accelerators [7]. It has fur- ther been suggested in many different stu- dies that Zn 2+ -ions form these active com- plexes with accelerators, which are more reactive than the free accelerator [8 – 10]. Complex formation of the zinc ion with dif- ferent accelerators is critical to get efficient curing. 30 KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 1-2/2005