Search

This research is directed to the characterization of the thermodynamic properties and viscosity of lubricant base oils modified with polymeric additives. Several groups of mineral and synthetic base oils, including Ultra S4, Ultra S8, and poly alpha olefin PAO 4 have been studied. Among the various types of additives explored were viscosity index modifiers, polyisobutylene polymers (PIBs), and dispersants. The viscosity index modifiers are studied in terms of different polymer architectures, molecular weights, presence or absence of functional groups, and their concentrations. The dispersants are studied in terms of concentration, molecular weight, and presence of capping groups. Density data, as the basic thermodynamic data, are generated using a high-pressure variable-volume view-cell over a pressure range from 10 to 40 MPa and a range of temperatures from 298 to 398 K. The density data are then correlated with the Sanchez-Lacombe equation of state, from which key thermodynamic properties, namely isothermal compressibility, isobaric expansivity, and internal pressure are derived. These properties offer a rational approach to better understand molecular packing in lubricants under high pressure and temperature conditions which has direct impact on film formation. Viscosity determinations are carried out using a custom-designed high-pressure rotational viscometer. Data were generated in the pressure range from 10 to 40 MPa, but at temperatures ranging from 298 to 373 K as a function of shear rate up to 1270 s-1. Viscosity data were then correlated with density which provides interpretations in terms of free-volume and density scaling models. The molecular parameters produced from these correlations support the interpretation of molecular packing under high pressure and temperature conditions. The results of this study included several key findings. With regards to density, the addition of viscosity index modifiers to Ultra S4 base oil caused the density to increase, except for the addition of functionalized olefin copolymers (OCPs) which caused the density to decrease. This was true with both high and low molecular weight additives. In the case of Ultra S8 base oil, the addition of OCPs generally decreased the density, while the addition of polymethacrylates (PMAs) caused the density to increase. In terms of compressibility and expansivity, the addition of high molecular weight viscosity index modifiers to Ultra S4 base oil generally decreased both these properties. However, the compressibility increased with the addition of 5 wt % functionalized PMA and 2 wt % star styrene butadiene (SSB). Furthermore, there was less of a decrease in compressibility with the addition of functionalized additives. With the addition of low molecular weight viscosity index modifiers to Ultra S4 base oil, little change was observed in compressibility, and the expansivity decreased to a lesser degree than with the addition of high molecular weight viscosity index modifiers. Viscosity index modifiers did not alter the compressibility of Ultra S8 base oil. Compared to Ultra S4, expansivity in Ultra S8 decreased to a lesser extent. The internal pressure was observed to be lowered to a greater degree in either Ultra S4 or Ultra S8 base oil with the addition of additives with more rigid internal structures (PMA and SSB). The decrease occurred to a greater degree with the addition of the higher molecular weight versions of additives studied and/or with the incorporation of functional groups to the additives. Although density changes were often greater with the addition of additives to the Ultra S8 base oil, all other derived thermodynamic properties, including internal pressure, changed to a greater degree with the addition of additives to the lower molecular weight Ultra S4 base oil. The viscosity generally increased to varying degrees with the addition of different additives to either base oil. The addition of functionality and higher molecular weight additives led to more consistent viscosity increases at higher temperatures. At the highest viscosity isotherm tested, 373 K, the addition of viscosity index modifiers resulted in similar viscosity values in either base oil, even though the viscosity of Ultra S4 at 373 K is much lower than the viscosity of Ultra S8 at this temperature. However, at 298 K, the viscosity index modifiers increased the viscosity of the Ultra S8 base oil to much higher values than the viscosity of the Ultra S4 base oil. Model based correlations of viscosity showed that with addition of high molecular weight viscosity index modifiers to Ultra S4 base oil, the parameters that are linked to free-volume overlap and the density dependence were more sensitive to the addition of OCPs than with the addition of PMAs and SSBs. These changes were reflected in larger free-volume overlap parameters and larger density exponent values. However, with a low molecular weight addition, the resulting parameters changed more with the addition of PMAs than OCPs. Overall, the addition of polymers with more rigid architecture led to more similar changes in correlative parameters across molecular weights from that of the original base oil, while for the OCP addition, the molecular weight had more of an influence on the degree of change. With addition of viscosity index modifiers to the Ultra S8 base oil, the architecture of the additive had more of an influence on the viscosity correlation parameters as the addition of PMA led to more noticeable changes in the parameters (resulting in lower free-volume overlap parameters, and a lower density exponent) than the addition of OCP, irrespective of the molecular weight or functionality. In either base oil, the addition of PMA led to lower free-volume overlap parameters and density exponent values than the addition of OCP. In this study it was observed that the addition of functionality, or polar groups to viscosity index modifiers, led to more desirable thermodynamic and rheological property changes to the lubrication base oil. This change was more definitive with the addition of polymers with more rigid architecture, such as PMAs and SSBs in contrast to the OCPs. The study on the addition of PIBs and capped or uncapped dispersants showed little variation in the resulting density and viscosity values when added to Ultra S4 base oil. However, the compressibility in these systems generally increased while the expansivity decreased except with the addition of PIBs. The internal pressure decreased to similar levels for all additive additions, except for the lowest molecular weight PIB, in which there was little change. The study on the addition of PIBs to different base oils showed that low molecular weight PIBs had the potential to disrupt the packing of a more uniform PAO 4 base oil and change the thermodynamic properties and correlation parameters to a greater degree than with the addition of higher molecular weight PIBs. This resulted in higher compressibility and internal pressure values with the addition of low molecular weight PIB compared to the higher molecular weight PIBs. However, there was little variation in viscosity with any of the PIB additions, except for the highest molecular weight PIB.

Formulating and designing energy efficient polymer additives and base oil molecules is one way to reduce energy losses in mechanical systems. However, the first step to designing novel polymers and base oils with improved performance under a wider range of operating conditions is to decipher the effect of their structure and chemistry on the bulk properties of lubricants. Therefore, this research aimed to investigate the factors and mechanisms that influence the key bulk properties (density, viscosity, traction coefficient) of liquid lubricants at the molecular level using molecular dynamics (MD) simulations, machine learning (ML), and quantitative-structure-property-relationship (QSPR) modeling. In the first study, the prospect of improving the mechanical efficiency (ME) of hydraulic systems by formulating fluids with viscosity modifiers (VMs) was tested in a pump dynamometer. Lower viscosity fluids provided better ME but decreasing the viscosity of base oil by adding VM did not have the same effect. Simulations showed that viscosity was directly correlated to the elongation of the polymers under shear, which, together with calculations of the key shear rate range in a pump, suggested ways of designing VMs to achieve a specific viscosity profile that maximizes ME. The second study presented a model for predicting the critical shear rate, intending to identify a fluid that shear thins in the key shear rate range in a pump. The model was applied to predict the properties of fluids formulated with VMs and validated by comparison to viscosities obtained from experimental measurements and MD simulations across many decades of shear rates. Results demonstrated that polymer molecular weight plays an important role in determining the critical shear rate, whereas polymer concentration primarily affects the Newtonian viscosity. The simulations showed the molecular origins of shear thinning and critical shear rate. In the third study, MD simulations were used to identify the QSPR of polymer-enhanced lubricants having commercial grades chemistries. The molecular origins of differences in the viscosity index, thickening efficiency, and traction coefficient between the fluids were investigated by calculating multiple structural properties of the polymers in the simulations. In the fourth study, a python package called PyL3dMD was developed, which enables users to compute nearly 2000 dynamic molecular descriptors by post-processing MD simulation trajectories. This was then used in our fifth study to relate 3D conformations of 305 complex hydrocarbons to their temperature-dependent density and viscosity and, as a result, developed advanced ML-based QSPR models. The models predicted density and dynamic viscosity with the coefficient of determination values of 99.6% and 97.7%, respectively, for all data sets, including a test data set of 45 molecules. Overall, this dissertation demonstrated the viability of various techniques in understanding molecular interactions and facilitating novel and innovative designs of polymeric additives and base oils for various applications with improved performance.

Abstract: The addition of a minute amount of long-chain polymer molecules to a turbulent flow can cause a significant reduction of skin-friction drag in both internal and external flows. The explanation of interaction between polymers and turbulence remains the most difficult area of polymer flow phenomena, although a considerable research has been carried out in this field. Time-resolved, two-dimensional particle Image velocimetry (PIV) and three-dimensional particle tracking velocimetry (PTV) based on the “Shake-the-box” method are employed to investigate turbulent structures of Newtonian and non-Newtonian polymeric channel flow. Characterization of polymer effects on the turbulent flow and detailed rheological measurements, including shear viscosity, extensional viscosity and relaxation time, are carried out. The Reynolds number of the current investigation is 20,000 based on channel height and properties of pure water. The experimental investigations of the polymer flow are carried out over various concentrations of polymer solution, including dilute polymer solutions and shear thinning polymer solutions. The maximum drag reduction, ‘Virk's asymptote’, is also obtained. In comparison to Newtonian fluid (water), the measurements showed a significant modification of the near-wall turbulence structure of the polymeric solutions: The viscous sublayer and buffer layer thickens; and the log-region is shifted upwards toward higher velocities while it remains essentially parallel to water in dilute polymer solutions. In the case of shear thinning polymers with drag reduction, the profiles in the log-layer are not only shifted upward, but also have a higher slope than that of water and dilute polymer solutions. With respect to the Reynolds stresses, a monotonic decrease is observed as a function of drag reduction. The Reynolds shear stress is close to zero in shear thinning flows, especially at maximum drag reduction. The power spectral density (PSD) of streamwise and wall normal velocity fluctuations in the buffer and log-layer showed a reduction over all frequencies. The reduction in the energy intensity increases as drag reduction increases and the significant reduction is observed at high frequencies, implying large reduction in the small eddies. The quadrant analysis indicated a significant attenuation of ejection and sweep events compared with water flow, consistent with a large reduction in production of turbulence kinetic energy in polymeric flows. Polymer dynamics in channel flow is investigated through the deformation tensor along the Lagrangian trajectories, which is used to identify the role of extensional viscosity within the flow. The results showed that the main contribution to the elongation deformation comes from streamwise strain rate fluctuations while a larger contribution is observed in the buffer layer. Resistance of molecules to strong elongational deformation results in a significant increase in extensional viscosity in the buffer layer. The PSD of the streamwise, wall-normal and shear strain rate fluctuations for polymeric flows from Lagrangian method are suppressed compared with water flow and the magnitude of reduction increases with increasing drag reduction. In addition, the reduction in energy intensity in streamwise and shear strain rate fluctuations are higher in the buffer layer while the reduction in energy intensity in wall-normal strain fluctuations is almost the same in the buffer and log layer.

Polymer additives exemplify many classes of compounds which possess a wide variety of chemical (i.e., phenols, amides, esters) and physical (i.e., volatility, solubility) properties. They are incorporated into polyolefins and other such polymeric materials for a number reasons: (a) to prevent degradation by ultraviolet light, heat, and oxygen; (b) to aid in the processing of the polymer; and (c) to modify the physical properties of the polymer. Since the purity and amount of additive can affect polymer properties, it is very important to characterize and quantify additives in polymer products. Traditional liquid solvent/polymer extraction methods, which involve dissolution/precipitation, are time-consuming, uneconomical, and the recoveries are significantly lower than 90%. In recent years, analysis with supercritical fluids (SFs) has emerged as an alternative analytical technique because SFs afford higher diffusivity and lower viscosity. In this research, an on-line Supercritical Fluid Extraction (SFE)/Supercritical Fluid Chromatography (SFC) system was assembled to provide efficient extraction and separation of polymer additives with quantitative results. The effects of various SFE/SFC parameters, such as trapping temperature, injection temperature, extraction pressure and temperature, dynamic extraction time, and fluid flow rate on extraction and separation efficiencies of different additive standards (i.e., BHT, BHEB, Isonox 129, Irganox 1076 and Irganox 1010) were investigated. Optimized conditions were employed to quantitatively extract additives from LDPE. Identification of additives was performed by comparing the retention time with each additive standard. Results obtained from on-line SFE/SFC were compared to results from off-line SFE/High Performance Liquid Chromatography (HPLC) and off-line Enhanced Solvent Extraction (ESE)/HPLC.

The extraction of fifteen (15) polymer additives with supercritical carbon dioxide which are used as antioxidants, uv stabilizers, process lubes, flame retardants and antistats from eight formulations of polystyrene is demonstrated and compared to traditional dissolution/precipitation extractions. The purpose of the study was twofold: 1) the development of a high performance liquid chromatography (HPLC) method(s) for the additives and 2) the determination of the viability of supercritical fluid extraction (SFE) for the additives from polystyrene. Separation of some of the additives was achieved using reversed phase liquid chromatography. Nine of the additives were assayed in this manner while, the remaining six additives could not be assayed using reversed phase liquid chromatography. In order to develop an extraction method for the additives, the effects of static extraction time, CO2 density, and temperature were first investigated. These preliminary extractions revealed that a static extraction period which afforded an opportunity for the polymer to swell combined with a high CO2 density and extraction temperature above the glass transition (Tg) yielded quantitative recoveries of the additives. Triplicate extractions of the various polystyrene formulations matched additive recoveries obtained by the traditional dissolution/precipitation method.