Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
widely used synthetic hydrocarbons for lubricant applications, and other types of synthetics including Group V polyisobutenes (PIB), which are oligomers of isobutylene, and esters, primarily dibasic esters or polyol esters. PAO remains the only synthetic base oil evaluated in this study.
Up until the ‘90s, most paraffinic oils were Group I oils. In response to increasing regulation and technical demands placed on automotive engine oils (e.g., fuel economy and emissions), the industry moved from solvent-refined Group I base oils to hydroprocessed Group II and III base oils. In contrast to Group I, naphthenics (Group V) can be produced via solvent extraction or hydroprocessing. Today’s state-of-the-art naphthenic refineries use severe hydroprocessing to produce high-quality “clean” or low PAH (polycyclic aromatic hydrocarbon) naphthenic base oils that have reduced sulfur content and lower PAH content while preserving valuable solvency characteristics.
Group I base oils contain a considerable amount of sulfur and saturates of less than 90%. While most commercially available Group II and Group III base oils have essentially no aromatic content, Group I and severely hydroprocessed naphthenic base oils have been processed to remove the PAHs while leaving other aromatics intact. The removal of the PAHs is necessary to produce a “clean” base oil that meets the Health, Safety, and Environmental (HSE) regulations of the applications for which they are used.
Solvency is affected most by the aromatic content and then by the naphthenic (i.e., saturated ring structures) content. As the aromatic content decreases, the solvency decreases, and as the naphthenic content increases, the solvency increases. When aromatics are hydroprocessed, they are converted to naphthenes. Therefore, to maintain solvency after reducing the amount of PAHs for HSE requirements, naphthenics are an outstanding way to increase the solvency of a lubricant formulation.
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Solvency is impacted by several somewhat related factors and can be assessed in terms of aniline point, viscosity index (VI), and viscosity-gravity constant (VGC). Aniline point, ASTM D611, characterizes solvency via a compatibility test between the oil and aniline, which is an aromatic amine. The aniline point is defined as the minimum equilibrium solution temperature for equal volumes of aniline and the oil sample. The more soluble the oil is in the aniline, the lower the temperature required for the oil and aniline to become miscible. Less soluble oils require higher temperatures. This is an example of the industry adage, “like dissolves like.” Aniline is a polar molecule, and polar base oils are more readily miscible with aniline at a lower temperature. The lower the aniline point, the higher the solvency of the base oil.
The viscosity index (VI), ASTM D2270, which is a dimensionless number, is used to characterize the variation of the kinematic viscosity of a petroleum product with temperature. The higher the VI number, the less change in viscosity due to temperature. The VI correlates with chemical structure, with aromatics having the lowest VI, then naphthenics, and paraffins having the highest VI; therefore, a higher VI indicates a lower solvency.
Viscosity-Gravity Constant (VGC), ASTM D2501, describes the general relation between specific gravity and Saybolt viscosity. The VGC is low for paraffinic (0.800) and high for aromatic (1.00) type oils. As the VGC increase, the solvency increases. VGC is often used in conjunction with aniline point since VGC is independent of molecular weight.
The thickener in a lubricating grease is the component that sets grease apart from fluid lubricants. Thickeners are molecules, polymers, or particles that are partially soluble in lubricating fluid; they arrange themselves in such a way that they impart a semi-solid consistency to the grease. Many different types of chemical compounds can be used to thicken grease.
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