**2.2 Common additives**

*Aerospace Engineering*

concern [2].

is an important diagnostic of lubricant health.

lubricant degradation will be examined.

**2.1 Basestock composition**

common alcohols used are shown n **Figure 1**.

**2. Composition of turbine engine lubricants**

and the additive packages to meet the current specifications.

result in the failure of the engine. In turbine engine applications, additive depletion

In addition to lubricant degradation being important to engine health there are significant implications to human health. On the vast majority of commercial aircraft, the air used to pressurize the cabin is drawn from the engine just after the compressor section. Lubricant degradation products have been shown to pass from the engine into the cabin on seal failure with severe health effects. Of perhaps greater significance is the normal low level leakage of lubricants and degradation products into the cabin under normal flight conditions. It is known that all seals leak some and some of the leaked material can be transmitted into the passenger cabin as both vapors and nano-droplets. The chronic toxicity of these materials is of great

In this chapter, the composition of typical turbine engine lubricants will be presented in Section 2. The decomposition mechanisms of the basestock are presented in Section 3, followed by the additive degradation mechanism in Section 4. Finally, in Section 5 synergistic and antisynergistic interactions of lubricants and additives are examined. Changes in bearing systems and the incorporation of ionic liquids and nanoparticles will be included and finally in Section 6, some of the consequences of

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

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

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

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.

#### **Figure 1.**

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

**Figure 2.**

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

**Figure 3.** *Structures of some lubricant additives used for turbine engines.*

Fortunately, most turbine engines lose some lubricant under normal operating conditions and the oil lost is replenished on a regular basis. These procedures maintain the additive packages at acceptable levels.
