Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Table 4. NYCOBASE 9600X/NYCOPERF AO 337, fully
formulated fluid, Pan evaporation test, 230°C, Thermogravimetric analysis, 250°C, Micro- coking test, 230-280°C
The TGA graph clearly shows 3 phases that outline the properties that we are looking for in such a base fluid: • A slow evaporation phase
• A rapid decomposition phase (pyrolysis) • A polymerization phase, leaving very little residue
Efficiently thickening the selected base fluid One fundamental aspect of this study is to ascertain that the selected base fluid can be properly thickened to produce mechanically stable greases. Inorganic thickeners were preferred for this project, as they are easy to handle, cost-effective and suitable for extreme temperatures. The selected base fluid was therefore thickened with bentonite clay and silica, and the resulting greases were fully formulated with antioxidants, Phosphorus and Sulfur additives, and metal deactivator (Table 5).
As expected from infusible thickeners, dropping points are very high. They are in fact more of an oil separation phenomenon. Oil separations are of a very satisfactory level, even though slightly on the high side.
Evaporation rates are partly due to Sulfur additive that contributes to roughly 2% mass loss, but still show very satisfactory levels.
Thermo-gravimetric analysis at 230°C and 250°C, under oxygen flow, was used to assess the thermo-oxidative stability. Such measurements may be used to provide an indication of relative grease life in operation (2) – but the severity of the test (high temperature, small quantity of grease, pure oxygen flow) must be taken into account.
In addition:
• Commercially available 345 mm2/s PFPE based, PTFE thickened grease and
• Laboratory PAO 40 based greases, thickened with bentonite clays, and treated with same additive systems (NYCOPERF AO 337/aminic antioxidants)
Table 5. Consistency and mechanical stability of clay and silica thickened greases •
NYCOPERF AO 337 contains 30% active material, it was therefore compared with a 3% treat rate of aminic anti-oxidants. It was possible to achieve a remarkably stable NLGI 1 grease, either in P100,000 worked penetration or in the Roller test. Bentonite clay, however, must be carefully selected to match the polarity of the medium, as some bentonite thickeners did result in mechanically unstable greases.
Hydrophilic fumed silica, as expected, resulted in greases that were not as stable mechanically. However, these greases were evaluated all the same to understand the impact of thickening agents on thermal stability.
were compared with ester based greases. Results lead to the following conclusions:
Profile of curves is similar to those of base fluids, and shows the 3 distinct phases observed earlier (evaporation, decomposition, deposit formation). A notion of induction time is clearly visible (Figure 5).
• It is confirmed NYCOPERF AO 337 provides better thermal stability than traditional aminic anti-oxidant system (Figure 5).
• Silica seems to be more neutral at high temperatures, compared to bentonite clay, thus showing slightly better results (Figure 5). This is attributable to the partially organic nature of bentonite clay contributing to thermal instability.
• Commercially available PFPE/PTFE grease shows remarkable inertia at 250°C (Figure 5), as expected.
Table 6. High temperature properties
No.95 page 4
Evaluation of grease performance at high temperatures As a first step usual properties were tested on the obtained greases: dropping point, oil separation and evaporation rate at 200°C (Table 6).
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LUBE MAGAZINE NO.123 OCTOBER 2014
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