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contain naphthenic oil, and it can be seen that among the ones based on the solvent refined naphthenic oils (A 2, B 2, and C 2), it is the grease having the lowest naphthenic content (grease B 2) the one presenting the highest thickener content. For greases based on higher viscosity oil blends (A 3, B 3 and C 3) the thickener content is around 5%.
dewaxing process is not as effective as for Group II and III oils, so it is possible that the wax content for that sample is higher than for B 2, and C 2. Evidently the presence of naphthenic oils in most of the samples, improves the flowability of the greases at low temperatures. This improvement is evident in some of the blends which show similar, if not lower flow pressures than pure naphthenic greases, like in the case of the samples C1 and C 2. These two samples present also lower thickener content, and a lower thickener content is known to contribute to a better grease flowability.[8]
Increasing the temperature range of the grease The benefits of the naphthenic/paraffinic blends are evident at low temperatures, but what happens to the greases at high temperatures? Do the greases maintain their structure and reduce the friction between metal surfaces in an effective way?
A Mini Traction Machine (MTM2) was used in order to measure the lubricating properties of the greases in a mixed rolling/ sliding contact. The measurements for the different greases were obtained at a temperature of 110˚C. The applied load was 20N, resulting in a contact pressure of 0.82GPa. Both the ball and the disk were made of steel (AISI 52100), the ball having a diameter of 19.05mm and the disk was 4.5mm thick with a diameter of 46mm. A PTFE grease scoop that allows the grease to be fed to the contact, while testing on the MTM was used during the tests.
Table 4. Characteristics of the different greases produced. *The number between parentheses is the difference between the 100000 and 60 strokes penetration.
Figure 2. Stribeck curve of the greases at 110˚C. Figure 1. Flow pressure of some of the grease samples at -35˚C.
The low temperature properties of the greases can be seen in Figure 1. This figure shows the results of the flow pressure measurements for some of the grease samples at -35 ˚C, according to the standard method DIN 51805. It can be appreciated on the Figure how all the samples containing naphthenic oil in their composition present lower flow pressure values than the grease based only on paraffinic base oils (sample B 1). This sample is followed by sample A 2, as the sample having the second highest flow pressure, probably due to the fact that it contains paraffinic Group I oil. In Group I refining, the
It can be seen in Figure 2 how at these conditions, the pure naphthenic greases (Grease A 1 and D 1) present higher friction coefficients, than the paraffinic based grease (Grease B 1) at all speeds. In the same figure, it can also be observed how the grease based on the naphthenic/paraffinic blend (Grease C 1), has similar friction coefficients than Grease B 1 at speeds up to 100mm/s. At higher speeds Grease C 1 presents higher friction coefficients than Grease B 1, bur lower than greases A 1 and D 1. So it can be concluded than by blending a paraffinic Group II oil with a naphthenic one, like in the case of Grease C 1, one can obtain a grease that shows good flowability at -35˚C, as well as low friction coefficients at 110˚C. In other words, a grease with a naphthenic character at low temperatures and a paraffinic one at high temperatures.
Better oxidation stability It has already been mentioned, that the solvent refined naphthenic oil used to make blends A 2, B 2 and C 2, has a high sulphur level. It is known that some sulphur components present in the oil can act as natural anti-oxidants, effectively protecting the oil from oxidation.[7] The rotary “bomb” test (ASTM D
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LUBE MAGAZINE NO.126 APRIL 2015
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