Pulsed Field Gradient NMR Spectroscopy: Applications in Determining the Pressure Viscosity Coefficient and Low-Temperature Flow Properties of Lubricant Base Oils

Abstract

The pulsed field gradient (PFG) NMR technique was employed to measure the self-diffusion coefficients (D) of hydrocarbon lubricant base fluids under ambient pressure conditions. This parameter was then related to various rheological properties, including the kinematic viscosity and the pressure viscosity coefficient (PVC). The results demonstrate that D varies inversely with the fluid viscosity and PVC. This relationship was then used to predict PVC from D for a series of compositionally diverse base oils, providing an R 2 value of 0.86. PFG NMR spectroscopy was also used to study the molecular dynamics of the liquid-solid transitions of the oils at low temperatures ranging from +40 to -40 °C. The self-diffusion coefficient decreased linearly with decreasing temperature to 0 °C and then "leveled off" at lower temperatures. This observation can be explained, using spatially heterogeneous dynamics and other mechanistic models, by the onset of wax crystallization.

Correlation of structure and properties of groups I to III base oils

“…However, larger decrease in the values of α at higher temperatures corresponds to thinner film formation at that temperature. The behaviour is similar to VI characteristics depicted for base fluids, and groups III to V base oils exhibit low values of α compared with groups I and II base oils …”

“…Densities at different temperatures were determined to calculate the dynamic viscosities at 30°C, 40°C, 60°C, 80°C and 100°C, respectively. These values were used to estimate pressure–viscosity coefficients . A high shear rate (10 5 –10 4 s −1 ) viscometer was used to determine the CCS viscosities at low temperatures as per ASTM D‐5293 for observable cranking speed in cold conditions.…”

The detection and identification of selected lube base oil using terahertz time-domain spectroscopy

“…As shown in Fig. 3 [16]. In the liquid phase, various types of vibration occur, where the characteristic frequencies are described as intra-molecular and intra-molecular vibrations.…”

“…The chemical composition of LBO represents a complex mixture of hydrocarbons ranging in carbon number from 20 to 40 depending on the viscosity grade. Base oil typically contain normal paraffins, branched or isoparaffins, cyclic paraffins (up to 5-or 6-membered rings), aromatic ring structures (mono-, di-, tri-, and polynuclear), and very small quantities of heterocyclic (sulfer, nitrogen, and oxygen) compounds [16]. For different viscosity index of LBO products were evaluated in this study, the collected THz-TDS spectra contained the necessary analytical information to identify the viscosity index, such as spectral variations related to different chain lengths of normal linear hydrocarbons.…”

“…Because the chemical composition of base oil directly determines its physical and chemical performance characteristics, absorption coefficient and refractive index have been developed that utilize compositional data to predict properties such as oxidation lifetime, viscosity, viscosity index, and engine test performance. Such structure-property correlations can be used to design better lubricants and to optimize refining processes [16].…”