Accurate viscosity data of liquids over wide ranges of temperature and pressure are needed for the design, operation and optimization of equipments in lubricant industry. The viscosity of a lubricant has a marked effect on wear due to its relation with the film thickness and it is a key requirement for systems operating under hydrodynamic or elastohydrodynamic lubrication (EHL). The effectiveness of oil in the rolling element bearings and gears, the energy losses due to viscous dissipation, the improvement of the fatigue life and reductions in friction and wear are entirely dependent on its viscosity [1]. Thus, the energy requirement of an appliance compressor can be reduced by decreasing the lubricant viscosity. However, if the lubricant viscosity is reduced too strongly, it is no longer high enough to ensure that a full fluid film is entrained; metal to metal contact occurs, so some wear is unavoidable [2]. For this reason it is interesting to known the temperature and pressure dependencies of the lubricant viscosity.

Dynamic viscosity of three mixtures of pentaerythritol ester lubricants (PEs) has been measured using a rolling-ball viscometer up to 60 MPa, from 303.15 to 353.15 K with an experimental uncertainty of 3% [3]. The first one is a multicomponent mixture of several PEs named in the present work as PEC5-C9 lubricant, the second one is a binary mixture of pentaerythritol tetra(2-ethylhexanoate), PEB8, and pentaerythritol tetraheptanoate, PEC7, with a PEB8 mole fraction of 0.6670 and the third one is another binary mixture of PEB8 and pentaerythritol tetrapentanoate, PEC5, with a PEB8 mole fraction of 0.6911. The two binary mixtures, PEB8 + PEC7 and PEB8 + PEC5, have been prepared with close nominal viscosity at 313 K and 0.1 MPa than the PEC5-C9 lubricant (~33 cSt).

The viscosities of these binary mixtures have been compared with the predicted values obtained by using several viscosity models (Grunberg-Nissan and Katti-Chaudhri mixing laws, self-referencing model, hard-sphere theory and free-volume model). All methods predict dynamic viscosity values for the two binary mixtures that agree with the experimental data within average mean deviation of 10% over the entire temperature and pressure ranges. The best predictions were found with the free-volume model, for which the average mean deviation for both mixtures is lower than 4%. Parameters values for self-referencing model were determined from experimental viscosity data of several pure PEs. These parameters permit to estimate viscosity values of PE lubricant of unknown composition, when a viscosity value at any temperature and pressure is available. This model predicts the viscosities of PEC5-C9 lubricant with an average deviation of 4%.

References:

[1] Godfrey, D., Herguth, W.R., “Physical and chemical properties of industrial mineral oils affecting lubrication. Part I”, Lub. Eng. 51, 397 - 400, 1995. [2] Boyde, S.,“Green lubricants. Environmental benefits and impacts of lubrication”, Green Chem., 4, 293 - 307, 2002. [3] Lugo, L., Canet, X., Comuñas, M.J.P., Pensado, A.S., Fernández, J., “Dynamic Viscosity under Pressure for mixtures of Pentaerythritol Ester Lubricants with 32 Viscosity Grade: Measurements and Modeling”, Ind. Eng. Chem. Res., 46, 1826 - 1835, 2007.