Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
compared to Group III and Group IV base fluids. This unique combination of properties makes esters a candidate base stock technology for future ultra low viscosity engine oils.
GpI/GpII (ISO100) (solid) Figure 3. Noack volatility vs CCS @ -35O Oxidation Stability
Oxidation stability is a highly desired feature for high temperature or long-life lubricants and is strongly influenced by the basic chemical composition of the base fluid, impurities and additive technology.
Thermal cracking and oxidation proceed via a free radical reaction and so if maximum oxidation resistance is required from an ester it would be designed so that it is based on a polyol alcohol reacted with fully saturated acids, or diacids or trimellitic acid reacted with fully saturated alcohols. The choice of alcohols or acids is in part dictated by the other properties that are required from the ester so some compromise in terms of thermal or oxidation stability may be required to ensure other key properties aren’t compromised.
Table 5 shows the dry TOST (ASTM D943) oxidation stability properties for three different types of ester based environmentally acceptable hydraulic fluids (ISO46). Vegetable esters are the least oxidatively stable and therefore are limited to low temperature or short-life lubrication. Unsaturated oleochemical esters (for example oleate esters) have improved oxidation stability properties compared to vegetable esters but fully saturated oleochemical esters have the greatest resistance to oxidation and are the technology of choice for higher temperature or long-life lubricants.
Trimellitate (ISO 320) (liquid) Poloyl ester (ISO 320) (liquid) Figure 4. Deposit forming tendencies of different base stocks
Group I / Group II blends and PAO 40 form hard resinous deposits as they begin to thermally breakdown, whereas specially designed oxidation resistant esters are less prone to polymerisation and remain liquid for much longer than hydrocarbon base fluids.
Renewability and Biodegradability Some applications require the use of products with high renewability content and in those situations natural esters and synthetic oleochemical esters are the products of choice. Natural esters would be classified as being 100% renewable and synthetic oleochemical esters would typically have a renewability content in the range 70 – 100%.
Biodegradability and renewability when combined together are the two properties which allow esters to be completely differentiated from all other base stock types. Whilst some progress is being made on developing new low viscosity hydrocarbon base fluids with high renewability content, they cannot compare to esters which can range in viscosity up to and including ISO 1000.
Table 5. Dry TOST test for esters
Deposits and deposit control are also dependent upon thermal and oxidative stability characteristics of the base fluid and also on the solvency properties of the base fluid and hence its ability to dissolve deposits.
36 LUBE MAGAZINE NO.129 OCTOBER 2015
Biodegradability and renewability need not be mutually inclusive, it is perfectly feasible to make esters from petrochemical feedstocks (0% renewable) which are biodegradable and to make esters from renewable or sustainable based feedstocks (with or without the use of some petrochemcial feedstocks) which are very resistant to biodegradation. Building in resistance to biodegradation can be very beneficial, especially in environments where biodegradation leads to a severe deterioration in lubricant performance, for example, in metalworking fluids.
C for a selection of base stocks
No.100 page 5
Figure 4 demonstrates the appearance and form of several types of base stocks when subjected to high thermal stress. In this example a thin film of lubricant is placed into an aluminium pan, which is then placed in a forced draft oven at 260O of 8 hours.
C for a period
PAO 40 (solid)
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