Caracterização de aditivos poliméricos melhoradores de índice de viscosidade e estudo reológico de lubrificantes automotivos

Abstract

Viscosity Index Improvers (VIIs) are among the most important additives used for automotive lubricant oil formulation. In this study, eight commercial VIIs based on poly(styrene-co-isoprene) hydrogenated - viscosity modifiers serie A - and poly(alkyl methacrylate) viscosity modifiers serie B, have been characterized by Thermogravimetric Analysis (TG), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). It was observed that the VIIs, based on poly(styrene-co-isoprene) hydrogenated, have higher thermal stability than the VIIs based on poly(alkyl methacrylate), but both are suitable for use in automotive engines. However, it should be noted that the commercial MIVs series B were supplied in diluted oil. This fact could have influenced the degradation temperature of the samples. The main absorption bands of the VIIs analyzed were identified by FTIR analysis. The influence of VIIs on the characteristics of lubricating oils was evaluated through kinematic viscosity tests, calculation of viscosity index and study of the rheological behavior. Light and medium parafinic base oil were additivated with two types of VIIs, from serie A, A2 and A5 types, at different concentrations. It was observed that the parafinic oils are good solvents for both additives. The kinematic viscosity of additivated parafinic base oils increased with increasing concentration of VII. A5 showed the greatest thickening effect due to its polymer molecular structure, specially the number of styrene blocks. For all the studied concentrations, there was a significantly increase on the viscosity index (VI) of the samples in comparison with the pure oils. MIV A5 showed better performance than MIV A2, allowing the use of a smaller amount of additive to achieve the same IV. Light and medium parafinic base oil additivated with A2 and A5, in different concentrations, showed, predominantly, viscous behavior, and good stability to deformation. It was observed that additivated parafinic base oils exhibited non-Newtonian behavior. This behavior was better described by the Power Law of Ostwald de Waele. From Creep / Recovery tests, it was verified that an increase in additive concentration, for a given time value, caused a decrease in the value of compliance. This fact was attributed to the presence of polymer, that even at low concentrations, provides increased resistance to deformation to an applied stress. It was observed the dependence of complex viscosity (*) on temperature for additivated parafinic base oils. The complex viscosity decreased with increasing temperature, particularly in samples with high concentration of additive. However, it remained, for all polymer concentrations studied, in the temperature range considered, in amounts exceeding those of pure oils, attesting the effectiveness of the additives serie A as MIVs.