Effect of Mesogenic Organic Salts on Vulcanization and Physical Properties of Natural Rubber Compounds
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Effect of Mesogenic Organic Salts on Vulcanization and Physical Properties of Natural Rubber Compounds F. Avalos1, M. Tellez-Rosas1, M.E. Castañeda-Flores1 , F. J. Martínez-Casado2, J. A. RodríguezCheda3, M. Arroyo4, and M. A. López-Manchado4 1 Universidad Autónoma Coahuila, Facultad de Ciencias Químicas, Saltillo Coahuila, México, 2 Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-UGR, Granada, Spain, 3 Departamento de Química-Física, Fac. CC QQ, Univ. Complutense de Madrid. Madrid, Spain. 4 Instituto de Ciencia y Tecnología de Polímeros, CSIC, Madrid, Spain, ABSTRACT The effect of mesogenic organic salts as reinforcing fillers for natural rubber has been investigated. The influence of cation size (thallium and sodium) and organic chain length (thallium (I) pentanoate and dodecanoate) on the vulcanization parameters, physical and mechanical characteristics and rheological behavior has also been analyzed. In general, the maximum torque of the vulcanizates increases in the presence of the salts and is clearly manifested in a sensible increase in tensile modulus and strength of the composites. The reinforcing effect of these salts is noticeable in the natural rubber matrix. The thallium (I) salts are more effective reinforcements than the sodium salt, and the length of the organic chain hardly has any influence on the mechanical properties. The composites based on the thallium (I) dodecanoate salt show a very peculiar rheological behavior with a “plateau” in the G´ and G” vs temperature graphics which is related with solid phase I, existing between 83.5 ºC and 127 ºC, characterized as a plastic condis phase. This issue is especially interesting for the fabrication of devices such as sensors to control, for instance, the security (resistance of a material) as a function of temperature. INTRODUCTION Linear chain alkanoate salts have been used as plasticizers and reinforcing agents for ionic copolymers of ethylene and methacrylic acid due to their similar structure (carboxylic salt head groups and long methylene sequences in their hydrocarbon tails).[1] According to Duruz, Michels and Ubbelhode,[2,3] one of the most interesting aspects of organic salts is the ease of varying the charge and/or the shape of the anion and/or the cation just to move a desired physical property to a range of interest. It was shown that the morphology and properties of ionomermetal soap blends depend on the characteristics of the metal soap. Thus, in some cases, alkyl tails easily form a rotator phase which can slowly co-crystallize with ethylene sequences in the ionomers, leading to a sensible increase in stiffness. However, up to date the use of organic mesogenic salts as reinforcement for non-ionic elastomers has not been explored yet, in our knowledge. The organic salts in general, and the anionic ones in particular, have been considered for a long time of a great interest, that has increased recently because most of them are considered Metal-Organic Frameworks (MOFs)[4,5] Moreover, it is well known that stearic acid and zinc oxide
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