Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Optimizing thermo-oxidative resistance of base fluid
Synthetic esters, which are known for their high performance at higher temperatures, were considered and tested as possible base fluids for this work. Some basic features were used as good indicators of the performance under extreme conditions, like flash point and evaporation rate for instance.
Thermogravimetry analysis proved to be a very interesting tool to evaluate behaviour of base fluids (and resulting greases) and better understand volatility phenomena.
We also used the ASTM D4636 test, designed for jet engine oils, which evaluates the resistance of a lubricant fluid to catalysed oxidation at elevated temperatures, in the presence of metal coupons and with air bubbling (204°C, 72 h). Viscosity and acid number changes are measured after the test. This test is a stringent oxidation and corrosion test.
Another useful test is the GFC-Lu-27-T-07 micro-coking test, which was initially designed by the automotive industry to evaluate the coking propensity and the detergency ability and of oils in contact with hot metal surfaces, where a gradient of temperature is applied.
As synthetic products, the structure of esters can be tailored and designed to maximize thermo-oxidative stability and minimize volatility, which is one of the reasons why they are used in jet engine lubricants, amongst other applications.
In particular, neopolyol esters, a specific class of esters, are known to show excellent resistance to oxidation, thermal degradation and coking, thanks to their structure (Figure 1). They all have in common the presence of a quaternary carbon in their carbon chain that makes them particularly robust under higher temperatures: the absence of hydrogen in β of alcohol oxygen does inhibit thermal decomposition resulting from elimination reactions.
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Table 1. NYCOBASE 9600X
Figure 2. NYCOBASE 9600X - Thermogravimetry (dynamic)
Table 2. NYCOBASE 9600X – General properties, Dynamic thermogravimetric analysis under oxygen (10°C/min)
Figure 1. Neopolyols, neopolyol ester (Pentaerythritol ester)
By carefully selecting acids that are reacted with neopolyols, it is possible to further optimize resistance to thermo-oxidation, and eventually obtain a high viscosity ester demonstrating very low volatility and maximized thermo-oxidative stability (Table 1, Figure 2 - NYCOBASE 9600X). Tables 2 and 3 show ASTM D4636 oxidation and corrosion test results and GFC-Lu-27-T-07 micro-coking test results, in comparison with PAO of similar viscosity (400 mm2/s at 40°C).
Table 3. NYCOBASE 9600X and PAO 40, Thermo-oxidative stability, coking propensity 30 LUBE MAGAZINE NO.123 OCTOBER 2014
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