Chemistry of EPDM vulcanisation
Martin van Duin, LANXESS Elastomers
Terpolymerisation of ethylene, propylene and a diene monomer results in EPDM polymers, which i) have a low glass transition temperature and are amorphous (or very low crystalline) at application temperatures and ii) can be crosslinked, and, thus, can be applied as an elastic rubber. Because of its low level of unsaturation in side groups, EPDM has an excellent resistance against oxygen, ozone, heat and irradiation. The apolar character of EPDM and the very high entanglement density explain the very high acceptance of plasticiser oil and, as a consequence, of carbon black. Combined with the very low density, EPDM compounds are among the most cost-effective rubber products. The apolar character also results in a poor resistance against oil and apolar solvents though. EPDM is the largest, non-tire, synthetic rubber (1100 kton/year) with main applications in outdoor and elevated temperatures, such as automotive sealing systems, window gaskets, roof sheeting, hoses, tires and tubes, mechanical goods, wire & cable and thermoplastic vulcanisates. Crosslinking of EPDM is typically performed with accelerated sulfur (~80%), sometimes with peroxide and, for special applications like thermoplastic vulcanisates, with activated resol systems. The current views on the chemistry of these 3 EPDM vulcanisation systems will be presented, supported by results from experimental studies using state-of-the-art analytical techniques. Recent developments by LANXESS, such as high-VNB-EPDM for enhanced peroxide curing efficiency and the use of zeolites to improve the cure characteristics of resol curing will be highlighted.
Crosslinking of fluoroelastomers and the influence on final properties
Matthias Lueckmann*, Wolfgang Steinhoff, DuPont de Nemours (Germany) GmbH
Crosslinking of rubber materials generates chemical bonds between chain segments and forms a stable three dimensional structure. The properties of uncured rubber are changing from more viscous behaviour with high remaining deformations to high elastic deformation behaviour after crosslinking that minimizes the irreversible flow. As specialty rubbers like FKM and AEM are used in demanding environments, the crosslinking process is important to provide materials that can withstand e.g. high temperatures. At current, fluoroelastomers are cured with crosslinking systems based on bisphenol or a combination of peroxide with coagent. Crosslinking mechanisms will be presented as well as efficient designs to control rate and state of cure. In addition, the appropriate final properties of the elastomers will be discussed. Regarding the crosslinking with bisphenol, the influence of polymer structure and metal oxides will be shown.
Vulcanization of silicone rubber, different chemistry and the effects on process parameters and applications
Dr. Konrad Alfons Wierer*, Michael Hirth, Wacker Chemie AG
The silicone rubbers are classified according to the different vulcanization ( RTV1, RTV2, HTV, LSR). Based on the chemistry of reactions the vulcanization (condensation, radical und addition curing) is described including main components and byproducts. The different reactivity and the resulting set up for the process parameters are explained. The type of vulcanization influences also the properties of the cured end product and their acceptance for different end uses like industrial, food contact and medical applications.
Vulcanization with sulfur/accelerators or organic peroxides?
Leo Nijhof, AkzoNobel Functional Chemicals B.V.
The vulcanization of rubbers with sulfur/accelerators or organic peroxides is frequently subject of discussion when formulating rubber compounds. In this presentation the pre and contras of both systems are compared. Differences in rubber properties are given. It will be shown that with the selection of the correct cure system most important technical requirements can be fulfilled. Additionally information will be provided how to improve peroxide based cure systems.