Relationship between Viscosity Coefficients and Volumetric Properties: Measurements and Modeling for Pentaerythritol Esters

A rolling ball viscometer has been used in this work to determine the temperature and pressure dependencies of the dynamic viscosity of two pentaerythritol ester-based lubricants: pentaerythritol tetrapentanoate (PEC5) and pentaerythritol tetraheptanoate (PEC7). The viscosity measurements were performed along the six isotherms from 303.15 K to 353.15 K and at pressures up to a maximum of 60 MPa with an experimental uncertainty of ±3%. It has been observed that the dynamic viscosity increases as the size and branching of the chains of the pentaerythritol esters each increase. Based on the viscosity experimental data, the pressure and temperature viscosity coefficients, together with the viscosity index, have been determined for these fluids. The experimental viscosity values, together with literature data of other pentaerythritol esters, 2,3-dimethylpentane, and squalane were analyzed with several models, based on freevolume or hard-sphere concepts. Furthermore, we have analyzed one of the expressions proposed by Schmelzer et al. [J. Chem. Phys. 2005, 122, 074511/1-074511/11] that relates the pressure-viscosity coefficient with the thermal expansion coefficient, the isothermal compressibility, and the temperature-viscosity coefficient. The Schmelzer et al. relation is valid for 2,3-dimethylpentane but not for the other compounds. A more-general relation is proposed, which can explain the cases where the Schmelzer et al. equation fails.