Alpha (Vitrea) Transition in Vulcanized Natural Rubber/Styrene Butadiene Rubber Blends Prepared by Mechanical and Solution Mixing

Abstract

The preparation method of an elastomeric blend can influence the mechanical properties of the vulcanized compound. In this research elastomeric blends composed by natural rubber and styrene butadiene rubber were mixed using two different methods: by mixing in a roll mill and by dissolution of both elastomers in toluene, mixing of both solutions with the curatives and the evaporation of the solvent. Samples with different Natural Rubber/Styrene Butadiene Rubber relation were prepared by both methods and vulcanized at 433K with a system based on sulphur and accelerator (N-t-butyl-2-benzothiazole sulfenamide) up to the time of optimum cure. The blend composition and the preparation methods have a strong influence in the mechanical dynamic properties. Scanning Electron Microscopy observations indicate that, in the blends prepared by the dissolution method, the samples show better miscibility of the constitutive phases than those prepared by the roll milling method. The temperature dependence of the internal friction was studied for each sample using a subresonant forced pendulum at 1 Hz between 190K and 250K. Depending on the blend composition, one or two glass transition temperatures (Tg) associated to the α-relaxation were measured. In the last case each Tg corresponds to each elastomeric phase of the compound. The loss tangent data for each compound was analyzed using a mixture law of two phases in the frame of the Rouse theory. The adjustment of the data to the proposed model was very good for both preparation method and the whole composition range of the compounds. Then it was possible to obtain the Tg, the main relaxation time and the activation energy values of each compound and, in some samples, the respective values for each elastomeric phase.