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Poor points for pour points? Well, not quite, but a Nynas study shows that pour points should be viewed with scepticism when picking a lubricant for cold climates.
A Nynas study of pour points has found that samples with naphthenic oils provided more stable friction characteristics than paraffinic oils (Gr II and Gr III). The reason was likely waxy residue left over from refining the paraffinic oils. "We found that up to 10 °C above the pour point of the paraffinic oils, wax started to precipitate and crystals grow," observes Mehdi Fathi-Najafi, Senior Technical Coordinator, Nynas. "This rapidly caused the viscosity to increase and made these fluids show unstable friction behaviour."
Out in the field, what problems does that crystallisation pose? "It may affect the fluid's ability to flow into the contact and can result in contacts being starved of lubricant, leading to friction and destructive wear," Fathi-Najafi says. Then there's the question of disturbances in flowability. The ASTM D97 defines the pour point as the lowest temperature when movement in a test specimen can be observed. But it only illustrates the when, not the why. When movement ceases, it may be due to increased viscosity and/or crystals forming, which solidifies the specimen, remarks Fathi-Najafi. While the former still behaves as a Newtonian fluid, the latter does not.
"For products containing wax, the pour point certainly needs to be viewed with scepticism," says Fathi-Najafi. The study found that paraffinic oils perform much better in low temperatures when blended with naphthenic oils. "Unless similar attention is paid to the nature of the fluid, the use of the pour point as a guide for choosing a lubricant may actually jeopardise the performance of a lubricated system," he says.
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