Lube-Tech
products with solubility problems. Another mechanism deals with colloidal processes. During degradation various oxygenated and hydrolysed compounds are formed, which organise themselves into micelles, vesicles, lamelas, etc. Resultant colloidal structures may precipitate as solids or combine into a viscous bottom phase. Usually both polymerisation and colloidal processes take place during degradation. Both of them are affected by basestock and additives. Intuitively, such solidification should be less likely if lubricant is stable oxidatively. However, this is not always the case. When standard oxidative stability tests were run on transmission oils (courtesy M. Henneberg from
C.C.Jensen A/S), their induction periods were ranked quite clearly, see Figure 6.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.116 page 5
RPVOT results to actual filtration issues. Thin film degradation tests were run on the lubricants, initially exposing their 500 µm films to 589 hrs of heating at 150°C. Volatile losses were significant, but did not exceed the volume of remaining films. Coupons with degraded films were placed into beakers with fresh respective oils and soaked at 90°C for 1-2 hrs. It became evident that 6X3 dissolved cleanly from the steel surface, while the rest of oils had some residues, especially CG3, see Figure. 7.
Figure 7. Residues of degraded films after soaking in fresh hot transmission oils and rinsing. The grooves should be disregarded as attempts to run friction tests before soaking
Figure 6. Oxidation rates in RPVOT stability tests of transmission oils (left) with the illustration of RPVOT assembly (right)
Rotary Pressurised Vessel Oxidation Test (RPVOT) is widespread in lubricant industry as a means to compare oxidative stability. In this series the vessel was loaded with 50 g of oil, 55 g copper catalyst, 5 g water and pressurised to 6.2 bar with oxygen. First it was heated to 150°C, during which the pressure went up to approximately 12 bar, and then went down due to oxygen consumption for degradation product formation. Pressure drop of 1.75 bar was assumed as a benchmark of the induction period. The sample 6X3 had a much shorter induction period than others, despite representing a well-known brand name. However, the filtration company, which was servicing transmissions on these lubricants, could not correlate
34 LUBE MAGAZINE NO.145 JUNE 2018
Since the scale of this comparative study was limited [6], only several coupons were tested. A replicate of 6X3 was run as well, unfortunately not for 828 hrs, but for 663 hrs. Its degraded film dissolved in fresh 6X3 just as easily. In this study some attempts were made to evaluate friction changes due to degradation before soaking. Consequently, some scars are visible on several coupons in Figure 7. Residue solubility of TM1 and S2G was similar, despite substantial difference in their RPVOT data. However, it can be suspected that CG3 would solidify during degradation faster than 6X3, contrary to RPVOT data in Figure 6. It is also worth referring to long-term vaporisation data, see Figure 5, where 6X3 appeared more “forgiving” than CG3.
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