Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE Hydrolytic stability Figure 3. Radical oxidation.
The order of -CH3 > CH2 > CH applies for the stability of the individual chain links of saturated fatty acids (Leslie R. Rudnick, 1999). The -CH3 group is 15 times more stable than a -CH2 group. Consequently short-chain fatty acids have better stability. Branched-chain fatty acids should also show worse stability, which is, however, often hidden by steric effects. Of interest, however, is how the individual secondary carbon atoms of the aliphatic chain differ. This has been considered by P. Sniegoski in detail in the model of the neopentyl-hexanoates (see Figure 4) (Sniegoski, 1977).
Unlike mineral oils, esters have a polar group, the ester group. This causes a variety of positive properties, such as, for example, the affinity to metal surfaces, high viscosity indices and, last but not least, good biodegradability. At the same time, however, this functional group is susceptible to hydrolysis under certain process conditions.
The hydrolysis of the ester is running rather slowly. Lubricant esters, which have a content of a few hundred ppm water are stable under normal storage conditions, but they do not have an increase in the acid number. This only occurs under the influence of catalysts. In general, a distinction is made between an acid and a basic catalysed ester hydrolysis. Also, metals act catalytically on the hydrolysis. The reaction speed of the hydrolysis, as seems likely, is affected by the temperature.
Figure 4. Methylene groups of the neopentyl-hexanoats.
Whether the -C atom of the alcohol group had a different reactivity was also investigated. Some authors suggest a 4.5 fold higher affinity to the formation of the RO.2-radical (
V.N.Bakunin, 1992). Sniegoski could now show that the methyl group has a significantly higher stability than the methylene group. At the same time, the secondary carbon group shows a much lower tendency to the formation of radicals in the -position. This applies both to the acyl and alcohol component. The author thinks that the electron affinity of the carbonyl oxygen is responsible for this.
30 LUBE MAGAZINE NO.150 APRIL 2019
There are 8 different mechanisms for the ester hydrolysis (Smith, et al, 2007). These are divided into acidic and alkaline catalysis, the attack on the alkyl or acyl carbon and a mono- or bimolecular mechanism. The base catalysed ester decomposition progresses as shown in the reaction diagram in Figure 5. Unlike during acid catalysis no free acid is formed but the corresponding salt, which causes a not reversible reaction. The catalytic converter, the basic connection, enters the reaction as a stochiometric component and is consumed. This process plays a role primarily in greases.
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Figure 5. Saponification reaction.
Of greater significance is the acid-catalysed ester decomposition according to the AAC2-mechanism (A = acid, AC = acyl decomposition 2 = bimolecular mechanism).
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