Lube-Tech
direct performance comparison with the saturated variant too, the estolide is convincing with a higher stability. Since the molecular structure of the estolide somewhat rules out stabilisation of the double bonds due to effects like mesomerism, the better values can be presumed to be due primarily to a steric effect.
The oxidation at the double bond proceeds initially via the formation of peroxyl (ROO.) radicals (Stolz, et al., 2019). However, these are adjacent to the space-consuming estolide bond. The double bonds of an unsaturated capping acid might, in this case, have a negative impact (Cermak, et al., 2003).
Resistance to hydrolysis In recent years, esters have increasingly been used in lubricating preparations in which resistance to hydrolysis is deemed a crucial selection criterion. A good example of this is their use in the production of lubricating grease.
With the known ester compounds, the ester may be split into its fatty acid and alcohol components during in-situ saponification in the presence of water. The outcome of this is incomplete formation of the lubricating grease’s structure and thus a significant reduction in the quality of the end product is noticeable. As illustrated in Figure 4, the alkaline saponification is irreversible.
For applications in the lubricants industry too, the presence of water can be troublesome and lead to hydrolysis of the Base Oils. Constituents such as acids and bases or metals such as copper, iron or nickel act catalytically and promote this unwanted effect.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.131 page 3
of an estolide. It is evident that the ester bond of a short-chain saturated complexester is far more prone to hydrolysis than that of an unsaturated ester. But the estolide exhibits the best values here too with persuasively good resistance to hydrolysis.
Figure 5: Results of resistance to hydrolysis according to ASTM D2619
The good resistance to hydrolysis of an unsaturated complex ester has already been reported previously (Stolz, et al., 2019). Unlike complex esters though, the molecular structure of the estolide has a significant number of secondary ester bonds that are far more resistant to hydrolysis due to direct branching on the ester group (Leslie R. Rudnick, 1999). This theory is also supported by other papers written on the subject (Cermak, et al., 2001).
Wear properties Additives are an indispensable part of the high performance lubricants that are required in the industry today. Only by adding these synthetic agents can the end product be given the desired properties tailored precisely to the specific purpose of use.
Figure 4: Saponification reaction
Figure 5 contrasts the resistance to hydrolysis of a saturated and an unsaturated complex ester and that
28 LUBE MAGAZINE NO.160 DECEMBER 2020
To minimise wear, extreme pressure (EP) additives are added to the lubricant and these release chemical compounds at the high temperatures that arise. The released substances react with the metal surface under the conditions that prevail to form layers which
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