High pressure thermophysical behaviour of reference and new lubricants. Mineral, synthetic and vegetable oils and ionic liquids

Resumen

European annual consumption of lubricating oils is about 5.2 million tons, which only a 2-4% is of vegetable origin, and 1.2 million end up polluting soil, water, air or burned illegally. The consumption in Spain is around 500,000 tonnes per year and it is estimated a loss of about 200,000 t. Moreover, we must keep in mind that 40% of the river pollution is due to lubricants and 1 kg of mineral oil can contaminate 1 million litters of water. Thus, the objective of the singular and strategic BIOVESIN project, that is the framework of this PhD Thesis, is the generation of environmentally friendly lubricants, from vegetable oils and biodegradable additives selected for its optimal performance for each application, exhibiting good performance / cost ratio. Moreover, ionic liquids (ILs) are candidates for being used as lubricants due to the combination of properties such as high thermal stability, non-flammability, a wide temperature liquid range, good temperature-viscosity behaviour, very low vapour pressure and good flame resistance. Besides, they have a high polarity, which allows them to establish strong films of effective adsorption and tribochemical reactions that contribute to their anti-wear ability. LUBIONIC and RENELUBIL projects aim at the characterization of ILs for their use in lubrication. In this PhD Thesis we focus on the thermophysical properties of both fundamental and industrial interest: the density, viscosity and rheological study at high pressures of different lubricants (ionic liquids provided by Merck and reference oils, vegetable bases and oils developed in the BIOVESIN project), the miscibility of the blends developed biolubricant + gasoline and solubility of gases (CO2 and O2) in biolubricants. Viscosity-temperature-pressure studies are essential to avoid the wear by "pitting" (one type of surface deterioration of the gears) resistance of various lubricant formulations, the anti-wear properties of the gearbox oil or the lubricant properties of hydraulic fluids. The knowledge of density as a function of temperature and pressure is necessary in the development of hydraulic fluids and lubricants in general, among others. In fact, hydraulic fluids are effective as power transmitters when they have a very low compressibility. Moreover, the potential of a lubricant for energy dissipation diminishes with its compressibility. In the lubrication field, the density is also necessary for calculating the temperature increase in the oil films, the Reynolds number and the heat transfer coefficient. The obtained density values will be used to perform a correlation as a function of pressure and temperature, using the equation of state of Tammann-Tait. From this equation the isothermal compressibility (and the isobaric expansion coefficient (p) will be obtained. Miscibility of the new lubricant formulations with gasoline is essential for their use in two-stroke engines, since an immiscible lubricant may cause serious problems in the engine. Gas solubility tests in these oils are also important as the oil is burned with gasoline in the combustion chamber of such engines. Therefore, other goal is to measure solubility of CO2 and O2 in reference and developed oils.