Dynamic Viscosity under Pressure for Mixtures of Pentaerythritol Ester Lubricants with 32 Viscosity Grade: Measurements and Modeling L. Lugo,* X. Canet, † M. J. P. Comun ˜ as, A. S. Pensado, and J. Ferna ´ ndez Laboratorio de Propiedades Termofı ´sicas, Dpto. de Fı ´sica Aplicada, Facultade de Fı ´sica, UniVersidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain, and Laboratoire de Thermodynamique, Faculte ´ Polytechnique de Mons, Belgium The dynamic viscosity under pressure of three mixtures of pentaerythritol ester lubricants (PEs) has been measured using a rolling-ball viscometer for several temperatures with an experimental uncertainty of 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, xPEB8 + (1 - x)PEC7 and xPEB8 + (1 - x)PEC5, have been prepared with the same viscosity grade as the PEC5-C9 lubricant (VG32). A total of 1176 experimental measurements of the rolling time have been performed at pressures up to 60 MPa for the determination of 196 dynamic viscosity data points. 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 an 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%. Parameter values for the self-referencing model were determined from experimental viscosity data of several pure PEs. These parameters permit the estimation of 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%. Introduction People are paying increasingly close attention to the environ- ment with the development and progress of society. Following the Kyoto conference on climate change, energy efficiency is becoming an important performance characteristic for lubrication and all refrigeration and air-conditioning systems. The appropri- ate selection of a lubricant can have a significant impact on the overall efficiency of operation of domestic appliances and other refrigeration and air conditioning systems. In the lubrication field, including automotive and marine engine oils, compressor oils, hydraulic fluids, gear oils, and grease formulations, greater attention is being placed on the use of synthetic fluids. It is imperative in the present situation to study green lubricants. It is estimated that ∼10% of global lubricating oil production is fully synthetic products. 1 Synthetic fluids differ from mineral oil in that they have generally better defined chemical structures, but also a wider range of chemical functionalities. Although, in recent years, remarkable progress in green chemical technol- ogy has been made, some problems remain to be solved, such as the compatibility among base oils, thickeners, and additives, as well as the development of novel additives and methods to assess the biodegradability of green lubricants. 2 Synthetic lubricants 3,4 are manufactured from a number of differing chemical bases. Several classes of compounds have been developed to provide base stocks for commercial synthetic fluids such as polyalphaolefins, PAOs, polyalkylene glycols, PAGs and esters, especially polyol esters, POEs. Different studies point out that these kinds of synthetic lubricants can be really called green lubricants. 5-8 Among these green lubricants, POEs seem to be the lubricants of choice for use with the natural refrigerant CO 2 or with non- chlorinated refrigerants, such as HFCs, for reasons of miscibility, low toxicity, and excellent biodegradability, and also because of their inherently good lubricity. 9,10 Polyol esters are made by reacting a multifunctional alcohol with a monofunctional acid. Neopentyl glycol and pentaerythritol esters are sometimes known as neopentyl polyols because their structures are based on the hydrocarbon neopentane. 11 Polyol esters are used in a wide variety of applications, namely, refrigeration compressors, aviation, greases, air compressors, metal working, fire resistant and biodegradable hydraulic fluids, and chain oils. Polyol ester base oils combine both excellent performance, including high- temperature applications, and biodegradability. 5,12-14 Generally, linear polyol esters tend to be used when high degrees of biodegradability are required. 11 POEs are less toxic and tend to be more effective lubricants than mineral oils, and they can be obtained using a significant proportion of raw materials derived, or potentially derivable, from renewable resources, 5,15,16 e.g., through the hydrolysis of fats and oils to produce the constituent fatty acids as raw materials for chemical synthesis. A wide variety of natural sources, including solid fats and low-grade or waste materials such as tallow from rendering of animal carcasses or tall oil from wood pulp processing, can be converted through controlled chemical processing into pure fatty acids of consistent quality. Saturated short-chain fatty acids are used to make high-stability polyol esters that are used in high- performance synthetic car engine oils, jet engine lubricants, and compressor oils. Other benefits include extended life, reduced maintenance and downtime, lower energy consumption, and reduced smoke and disposal. Pentaerythritol esters (PEs) are a family of polyol ester synthetic lubricants manufactured by * To whom correspondence should be addressed. Tel.: 34981563100, ext. 14046. Fax: 34981520676. E-mail: falugo@usc.es. † Faculte ´ Polytechnique de Mons. 1826 Ind. Eng. Chem. Res. 2007, 46, 1826-1835 10.1021/ie061187r CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007