as well as a lower solvency available in the market. Blends of naphthenic oils with paraffinic Group II and Group III products were presented then as alternatives to fill in the gap left by Group I products. But base oil blends are not only useful in order to match the properties of paraffinic Group I oils. This article describes how by blending different types of base oils, one can not only match the viscosity and/or solvency of Group I base oils, but one can also improve certain properties in lubricating greases, potentially reducing the need for some additives.
Selecting the right base oils The final properties of base oils are affected by a number of factors. These factors include the crude oil type, which will determine the type of base oil obtained (naphthenic or paraffinic); the refining method (solvent extraction or hydrotreatment), as well as its severity.[7] In paraffinic base oils, the refining method and its severity determines to which API group they belong; i.e. Group I are solvent refined, while Group II are hydrotreated, and Group III are severely hydrotreated. The same refining technology is used for naphthenic oils, resulting in oils with different characteristics depending on the refining method and severity.
In this article, three different sets of blends are studied, all of them based on naphthenic base oils obtained by different refining methods. It can be observed in Table 1, that the hydrotreated naphthenic oil (A 1) has a lower pour point than a Group II paraffinic oil having the same viscosity at 40˚C (B 1). In the same table, it can be seen how the naphthenic/paraffinic blend, despite having a higher viscosity at 40˚C, maintains a low pour point, at the same level as the solvent refined naphthenic oil with the same viscosity at 40˚C (D 1).
Table 1 shows also how the solvency of these oils, expressed as their aniline point, varies. The naphthenic base oil (A 1), presents the highest solvency power (lowest aniline point), while the paraffinic Group II oil (B 1) has the lowest solvency power (highest aniline point). The naphthenic/paraffinic blend has similar aniline point than the solvent refined naphthenic base oil (D 1).
Table 3. Residue based heavy naphthenic oil blended with paraffinic Group I oil.
Grease production and characterization NLGI grade 2 Lithium greases were produced using an open kettle pilot plant (10Kg capacity). Lithium hydroxide and 12-hydroxystearate were used as thickener. The oil mixtures were used both for cooking and cooling the greases. No additives were included in any of the formulations.
Table 1. Hydrotreated and solvent refined naphthenic base oils, a paraffinic Group II oil, plus a blend between a hydrotreated naphthenic and B 1.
In Table 4, it can be appreciated how the penetration values for all of the greases correspond to NLGI grade 2, and that the greases are mechanically stable (the difference between 100000 and 60 strokes penetration is quite low for all of them). The dropping point is also around the same for all the samples, and is typical for lithium greases with this consistency.
Table 2. Solvent refined naphthenic oil blended with different paraffinic oils.
The main difference between the greases is their soap content, which is known to depend on both the oil viscosity and its solvency power. It can be appreciate in Table 4, how the grease based on oil C 1 has a lower thickener content than both grease B 1 and A 1, in part due to the higher viscosity of the oil. It is clear by comparing the results between C 1 and D 1, than some synergistic effect occurs while blending naphthenic and paraffinic base oils, because despite having the similar viscosity and aniline point than D 1, the grease based on C 1 presents lower thickener content than the one based on D 1. The grease having the highest thickener content was grease B 1, which was based only on Group II paraffinic base oil. The rest of the greases Continued on page 8
LUBE MAGAZINE NO.126 APRIL 2015 7
Similar results can be observed in Table 2 where a solvent refined naphthenic oil is blended with paraffinic oils from the different groups (Group I, II and III) to the same viscosity at 40˚C. The fact that the naphthenic oil is produced from an oil having a high sulphur content, by solvent extraction, results in an oil and blends with relatively high sulphur content. This sulphur is not reactive and the Cu corrosion tests (ASTM D130) for all samples was 1. Anyhow, the sulphur seems to confer special properties to this oil, and the blends based on it, as it is going to be explained later on.
It can also be seen in Table 2, that the properties of the blends; such as aniline point, VI, and pour point; are not only affected by the relative amount of naphthenic oil in them, but also by the type of paraffinic oil that it is blended with. This is reflected, for example, in the aniline point of blends A 2 and C2. It can be seen in the table how A 2 has a lower aniline point that C 2, despite having a lower amount of naphthenic oil. But one has to remember than paraffinic Group I base oils have much lower aniline points than the paraffinic Group III ones with the same viscosity.
In Table 3, a heavy naphthenic oil based on a heavy residue is blended with paraffinic Group I oil (SN 500) to different viscosities. The higher the viscosity, the higher the content of naphthenic oils, which gets reflected in the aniline point and the viscosity index of the blends.
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