Efeito de íons metálicos sobre a viscosidade intrínseca e sobre a degradação térmica da borracha natural da maniçoba (Manihot glaziovii)

Abstract

The examination of the effect of metallic ions over the intrinsic viscosity and thermic degradation of maniçoba's (Manihot glaziovii) natural rubber (NR) has been the main aim of this work. The inclusion of the metallic ions to the NR was carried out by mixing-up aqueous solutions of the metallic chlorides with chloroformic solution of the NR. The metal slender films of NR were prepared by evaporation of the solvent over a teflon thin layer, Intrinsic viscosity of the toluenic solutions prepared from the films were determined at 30°C. It was observed that the metallic ions cause four different effects over the intrinsic viscosity of the natural rubber: 1) Invariability with the concentration: K+ ion; 2) lncreasing with the concentration: Ca2+ and Sr2+ ions; 3) Decreasing with the increasing of the concentration: Zn2+, Mg2+ and Cu2+ ions; 4) Initial decreasing followed bv increasing with the concentration: AI3+ ion. The analysis of the different behaviors of intrinsic viscosity [η] and ionic rays allows us to stablish: a) Univalent ion can not promote alteration of [η], in other words, it doesn't interact with the NR significantly; b) Big divalent ions cause increasing of [η], probably due to the formation of crossed links; c) Small divalent ions cause decreasing of [η] probably due to the intrachain links; d) Small trivalent ions shows the two different behaviors mentioned on b and c. At short concentration, it promotes decreasing of [η] by the effect of intrachain interaction. With the increasing of concentration, the effect of the multivalent charge predominates, with the formation of cross linking. Films of NR doped with K+, Ca2+ and Cu2+ were exposed to a thermic treatment in the presence of atmospheric air during time intervals ranging from 15 to 120 minutes, at constant temperature of 90°C. From the determination of [η] of these films we found that the data obtained for the film dopped with K+ and Ca2+ ions were not conclusive to prouve the effect of these ions on the polymer thermal degradation. On the other hand, Cu2+ acted like an oxidant asent of the degradation.