**2. Composition of turbine engine lubricants**

Turbine engine lubricants have changed dramatically over the years in response to the increasing stresses applied to the lubricant. In particular higher shear stress, higher operating temperatures and lower storage temperatures have made changes in both basestocks and additive packages necessary. Natural petroleum based oils could not meet the temperature demands which made the selection of synthetic materials, modified with a number of additives necessary for this application [3]. In order to meet the demands for modern aircraft, lubricants based on synthetic esters were developed and have been refined many times, both in terms of the basestocks and the additive packages to meet the current specifications.

### **2.1 Basestock composition**

The composition of lubricant basestocks for turbine engines is somewhat variable as long as they can meet the performance requirements set forth in the standards SAE5780 for commercial aircraft and either MIL-PRF 23699 [4] or MIL-PRF 7808 [5] for military aircraft. One of the requirements is to be compatible with all of the previously approved lubricants in a given specification to avoid the inevitable mixing. Esters have been used since the 1940 as synthetic basestocks that have desirable thermal properties, however no single ester meets all requirements. Modern lubricant basestocks use a mixture of a number of esters in order to tailor the properties of the lubricant to the desired properties. These specifications have resulted in the use of certain common ester basestocks. Ester basestocks for turbine engines are all ester based using polyols and common carboxylic acids. Some of the common alcohols used are shown n **Figure 1**.

The polyols shown have been selected because they are highly hindered and also lack hydrogen atoms in the β position. Previous studies have shown that increases thermal and hydrolytic stability results when there is no hydrogen atom present on the β carbon atom. The carboxylic acids used to make the esters are a combination of linear and branched acids with a blend being frequently used to arrive at the desire viscosity. Normally 5 cs (SAE5780 and MIL-PRF 23699) basestocks

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**Figure 2.**

**Figure 1.**

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

structures of various additives are shown in **Figure 3**.

*Common polyols used to make ester-based lubricant basestocks.*

*Some of the acids used in the preparation of synthetic lubricants.*

use pentaerythritol and dipentaerythritol for the alcohols and C5-C10 linear and branched acids. Lower viscosity lubricants (Mil-PRF 7808) are of the based on neopentyl glycol and trimethylolopropane as the alcohol and C5-C12 linear carboxylic acids. The incorporation of branched acids in lubricants has a significant effect on the thermal stability and physical properties of the lubricant. Some of the different

Lubricants with ester basestocks require a series of additives in order to lubricate under the conditions observed in turbine engines. Typical additive packages include antioxidants, typically an aromatic amine, an anti-wear additive, typically a phosphate ester and possibly an antifoaming additive and a viscosity index modifier. The

Most additives degrade as a part of their mechanism of action, which means that their concentration is constantly decreasing. Many of them also degrade though other mechanisms as well. In general, when the additives have degraded beyond a certain point, either they must be replenished or the lubricant must be changes.

*DOI: http://dx.doi.org/10.5772/intechopen.82398*

acids are shown in **Figure 2**.

**2.2 Common additives**

*Turbine Engine Lubricant and Additive Degradation Mechanisms DOI: http://dx.doi.org/10.5772/intechopen.82398*

use pentaerythritol and dipentaerythritol for the alcohols and C5-C10 linear and branched acids. Lower viscosity lubricants (Mil-PRF 7808) are of the based on neopentyl glycol and trimethylolopropane as the alcohol and C5-C12 linear carboxylic acids. The incorporation of branched acids in lubricants has a significant effect on the thermal stability and physical properties of the lubricant. Some of the different acids are shown in **Figure 2**.
