**2. Early basestocks**

The earliest lubricants used in turbine engines were highly refined mineral oils. Mineral oils, however, lack the high temperature stability to withstand the high temperatures found in different points of the engine. Severe oxidation of the light mineral oils results in significant increases in the viscosity of the oil. The problems of severe oxidation and degradation highlighted the need for a new class of oils, which lead to the development of a class of synthetic aviation turbine oils [3].

The search for a new class of synthetic aviation lubricants led to the examination of organic esters as basestocks. Over 3500 different esters were examined for lubricant use between 1937 and 1944. A number of candidates had some of the required properties, including thermal stability, pour point, or boiling point, but none met all of the requirements. It should be possible based on the number of di-acids and alcohols available to achieve the desired viscosity and other properties with a pure compound; however, to achieve the wide liquid range required for Air Force needs, the blending of multiple esters was thought to be preferable to a single compound. The U.S. pour point of −40°C and high temperature requirement for a 3 cS fluid could be met by the reaction of simple mono-alcohols with dibasic carboxylic acids. The earliest commercial lubricant based on organic esters involved branched alcohols such as 2-ethylhexyl alcohol and adipic acid, azelic acid, and sebacic acid (**Figure 1**).

**Figure 1.** Structures of common components of lubricant esters.

**•** High operating cost associated with added weight, making backup and redundant systems highly undesirable. This requirement leads to small volumes of lubricant and results in high

**•** Wide range of temperatures, pressures, and speeds that the lubricant is exposed to under

The combination of these requirements eliminates the possibility of an ideal natural lubricant or synthetic lubricant meeting all of the requirements. As speeds and temperatures increased, automotive lubricants initially used in aircraft were replaced by specialty lubricants for aircraft engines. As the propulsion systems changed from conventional engines to turbine engines, lubricants based on mineral oils were inadequate and a new class of lubricants was needed [2].

In the years after World War II, turbine engines were developed and began to dominate both military and commercial aircraft propulsion. Turbine engines require the lubricant to be stable and fluid over a wide range of temperatures. Ground starting temperature may be as low as -54°C and temperature as high as 300°C may be observed at times in the operating engine. Over time, a number of different lubricants were developed with very different chemistries and compatibilities. Many different locations had lubricants that were developed to suit the local conditions and needs. With a number of incompatible fluids, the global nature of aircraft use, and the grave consequences of mistakes, a need for standardization became apparent. To address this problem, a system of lubricant specifications has been developed for turbine

The earliest lubricants used in turbine engines were highly refined mineral oils. Mineral oils, however, lack the high temperature stability to withstand the high temperatures found in different points of the engine. Severe oxidation of the light mineral oils results in significant increases in the viscosity of the oil. The problems of severe oxidation and degradation highlighted the need for a new class of oils, which lead to the development of a class of synthetic

The search for a new class of synthetic aviation lubricants led to the examination of organic esters as basestocks. Over 3500 different esters were examined for lubricant use between 1937 and 1944. A number of candidates had some of the required properties, including thermal stability, pour point, or boiling point, but none met all of the requirements. It should be possible based on the number of di-acids and alcohols available to achieve the desired viscosity and other properties with a pure compound; however, to achieve the wide liquid range required for Air Force needs, the blending of multiple esters was thought to be preferable to a single compound. The U.S. pour point of −40°C and high temperature requirement for a 3 cS fluid could be met by the reaction of simple mono-alcohols with dibasic carboxylic acids. The earliest commercial lubricant based on organic esters involved branched alcohols such as 2-ethylhexyl

alcohol and adipic acid, azelic acid, and sebacic acid (**Figure 1**).

heat dissipation requirements and high operating temperatures.

normal operating and storage conditions.

36 Recent Progress in Some Aircraft Technologies

engine lubricants.

**2. Early basestocks**

aviation turbine oils [3].

Higher viscosity requirements of the British could be met using longer-chain acids and alcohols, but the low temperature requirements could not be met by these combinations. The solution was to use a slightly more viscous blend of the 3cS fluid with a viscosity improver, typically a polyglycol.

Two fundamentally different and incompatible lubricants were both unacceptable and major potential hazards. The use of the wrong lubricant was considered unacceptable even for emergency use. In the 1960s, an intermediate viscosity oil was developed with a viscosity of 5.5 cS at 99°C that could replace both of the fluids, especially for future engines. This fluid also was adopted by the U.S. Navy for use in its aircraft. The advent of supersonic aircraft led to a need for even higher oil temperatures and improved stability.

Hydrolysis is a characteristic reaction of esters in the presence of water. Hydrolysis can be avoided through the use of longer-chain and branched alcohols. The primary route for the degradation of the esters used in the lubricants described above is through an elimination reaction initiated by the loss of the hydrogen atom on the β carbon to the carboxylate (**Figure 2**).

**Figure 2.** Mechanism for β-elimination.

The β-elimination results in the decomposition of the ester to alkenes, carboxylic acids. Although the mechanism involves a base, it is simply an acceptor for the proton and could be a variety of species present in the lubricant. Because β-elimination requires a hydrogen atom on the β carbon atom, the elimination of hydrogen atoms at that position resulted in a series of more thermally stable lubricants.

#### **3. Current basestocks**

As turbine engine technology developed and the desire for supersonic flight became the norm for the military, higher bearing load and bearing temperatures were needed. These develop‐ ments required more stable lubricant basestocks. Because β-elimination reactions were one mode of degradation, alcohols that lacked hydrogen atoms at the β position were desired. Some common alcohols that fit the bill are the neopentyl polyols. These highly hindered alcohols react with acids of various chain lengths to form esters under acid catalyzed condi‐ tions, providing the lubricant esters in good yields. Current lubricants are based on the neopentyl polyols such as neopentyl glycol, pentaerythiol, and dipentaerythritol (**Figure 3**).

**Figure 3.** Structure of some esters commonly used in aircraft lubricants.

The esters used in the current aircraft lubricants use a mixture of carboxylic acids with between 5 and 9 carbon atoms, typically and may involve either linear acids or branched acids. Many common lubricants are based on a mixture of valeric acid, iso-valeric acid, heptanoic acid, nonanoic acid, and 3,5,5-trimethylhexanoic acid. The lubricants are prepared either from the acid-catalyzed esterification of the polyol with a mixture of the desired acids or it may involve the trans-esterification of a fatty acid methyl ester (FAME) with the polyol. The reaction can occur in a random manner, or can be controlled to occur stepwise, allowing more sterically hindered acids to react first and then complete the esterification with the less hindered acid [5]. The latter method allows esters to be prepared from biologically derived oils, such as vegetable oil [6]. The composition of the mixture of acids can be used to adjust the physical properties, including the viscosity and viscosity index of the resulting basestock [7]. The fact that a mixture is used also results in a wider liquid range for the resulting lubricant.

The β-elimination results in the decomposition of the ester to alkenes, carboxylic acids. Although the mechanism involves a base, it is simply an acceptor for the proton and could be a variety of species present in the lubricant. Because β-elimination requires a hydrogen atom on the β carbon atom, the elimination of hydrogen atoms at that position resulted in a series

As turbine engine technology developed and the desire for supersonic flight became the norm for the military, higher bearing load and bearing temperatures were needed. These develop‐ ments required more stable lubricant basestocks. Because β-elimination reactions were one mode of degradation, alcohols that lacked hydrogen atoms at the β position were desired. Some common alcohols that fit the bill are the neopentyl polyols. These highly hindered alcohols react with acids of various chain lengths to form esters under acid catalyzed condi‐ tions, providing the lubricant esters in good yields. Current lubricants are based on the neopentyl polyols such as neopentyl glycol, pentaerythiol, and dipentaerythritol (**Figure 3**).

of more thermally stable lubricants.

38 Recent Progress in Some Aircraft Technologies

**Figure 3.** Structure of some esters commonly used in aircraft lubricants.

**3. Current basestocks**

In general, esters based on the neopentyl polyols have excellent thermal stability and can perform well as lubricants. If, however, lower-quality reactants are used to prepare the ester, a substantially more reactive lubricant is obtained. Certainly, impurities in the alcohols that contain β-hydrogen atoms would significantly alter the high temperature stability. Other possible impurities include significant quantities of excess acid, which would increase the corrosivity of the oil; water, which has a significant solubility in the oil, can result in hydrolysis; and higher molecular weight polyols, such as dipentaerythritol or tripentaerythritol, can have a negative effect on the low temperature properties of the mixture.

The two grades of MIL-PRF-7808 that are available differ substantially in composition as is needed to obtain the difference in viscosity. The grade 3 lubricant because of the low temper‐ ature requirement has a higher content of low molecular weight acids and neopentylglycol as the dominant polyol. The total ion chromatogram from the gas chromatography-mass spectrometry (GC-MS) evaluation of a 3cS MIL-PRF-7808K grade 3 lubricant is shown in **Figure 4**.

**Figure 4.** A portion of the total ion chromatogram of a MIL-PRF-7808K grade 3 lubricant.

In an effort to obtain superior high temperature properties to the MIL-PRF-7808K grade 3 and still retain better low temperature properties than is available with MIL-PRF-23699, MIL-PRF-7808K grade 4 was developed. The grade 4 oil used a different mixture of acids and alcohols to achieve a good compromise in properties. The total ion chromatogram from the GC-MS evaluation of a 4cS MIL-PRF-7808K grade 4 lubricant is shown in **Figure 5**.

**Figure 5.** A portion of the total ion chromatogram of a MIL-PRF-7808K grade 4 lubricant.

This lubricant contains primarily the pentaerythritol esters of valeric acid, heptanoic acid, octanoic acid, and 3,3-dimetylheptanoic acid. Also observed between 20 and 23 min retention time are four peaks that correspond to isomers of the tri-cresyl phosphate additive. The large number of different esters helps with the wide liquid range desired, whereas the higher molecular weights lead to an increase in viscosity from 3 to 4 cS.

A further increase in molecular weight is desirable for MIL-PRF-23699 lubricants, which have a still better thermal stability and viscosity. The change in molecular weight can be seen in the distribution of esters in **Figure 6**.

**Figure 6.** Total ion chromatogram of an MIL-PRF-23699 HTS lubricant showing the retention times where the esters elute.

The analysis of the acid composition of this lubricant indicates that at least six different acids are present in the mixture. The ability of multiple manufacturers to use somewhat different acid and alcohol blends yet achieve the same properties and have a compatible lubricant is thought to be a huge advantage.

Lubricant degradation at high temperatures is a significant problem. Degradation can result in an increase in acid number, an increase in viscosity, or an increase in the reactivity of the lubricant. For hindered esters where the β-elimination (discussed earlier) is blocked, there are several possible reactions, including hydrolysis of the ester, trans-esterification of the ester with another ester, and oxidation of the ester.

alcohols to achieve a good compromise in properties. The total ion chromatogram from the

This lubricant contains primarily the pentaerythritol esters of valeric acid, heptanoic acid, octanoic acid, and 3,3-dimetylheptanoic acid. Also observed between 20 and 23 min retention time are four peaks that correspond to isomers of the tri-cresyl phosphate additive. The large number of different esters helps with the wide liquid range desired, whereas the higher

A further increase in molecular weight is desirable for MIL-PRF-23699 lubricants, which have a still better thermal stability and viscosity. The change in molecular weight can be seen in the

**Figure 6.** Total ion chromatogram of an MIL-PRF-23699 HTS lubricant showing the retention times where the esters

The analysis of the acid composition of this lubricant indicates that at least six different acids are present in the mixture. The ability of multiple manufacturers to use somewhat different acid and alcohol blends yet achieve the same properties and have a compatible lubricant is

Lubricant degradation at high temperatures is a significant problem. Degradation can result in an increase in acid number, an increase in viscosity, or an increase in the reactivity of the lubricant. For hindered esters where the β-elimination (discussed earlier) is blocked, there are

GC-MS evaluation of a 4cS MIL-PRF-7808K grade 4 lubricant is shown in **Figure 5**.

**Figure 5.** A portion of the total ion chromatogram of a MIL-PRF-7808K grade 4 lubricant.

molecular weights lead to an increase in viscosity from 3 to 4 cS.

distribution of esters in **Figure 6**.

40 Recent Progress in Some Aircraft Technologies

thought to be a huge advantage.

elute.

The hydrolysis of esters is a well-known reaction that involves as the initial step the attack of either a water molecule of a hydroxide ion at the carbonyl carbon. The use of highly hindered esters reduces the rate of acid- and base-catalyzed hydrolysis by blocking easy access to the carbonyl carbon of the ester. The rate of hydrolysis can be reduced if water is excluded from the oil; however, it is soluble to a concentration of approximately 0.5% in these lubricants. Hydrolysis typically results in the presence of acids (increased acid number) and partial esters, where the polyol has one or more hydroxyl groups present.

The process of trans-esterification can be best seen as changes in the acids attached to a given molecule. This process has long been known because, if a lubricant is prepared containing two initial esters with the same alcohol and different acids [i.e. PE(*n*C5)4 and PE(*n*C7)4], after a period of time, the acids are scrambled and PE(*n*C5)3(*n*C7), etc., are found in the mixture. This process does not normally result in a serious change in the properties of the lubricant and is actually thought to be beneficial by increasing the number of different components. However, if transesterification occurs with a phosphate ester from an additive, changes of reactivity are possible (**Figure 7**).

**Figure 7.** Reaction of a lubricant ester with phosphate esters by trans-esterification.

Possibly the most detrimental reaction to the properties of polyolester-based lubricants is hightemperature oxidation. The initial stages of the oxidation involve the attack of an alkyl peroxy radical on a methylene group of the ester. The position α to the carbonyl has been shown to be significantly more reactive than other methylene groups in the molecule [8]. After the initial attack, the reaction can progress to form anhydrides, which continue to react to form alde‐ hydes, acids, and eventually high molecular weight compounds that can form a sludge in the engine (**Figure 8**) [9].

**Figure 8.** Thermal oxidation of PE ester at high temperature to give either anhydrides or an aldehyde and an acid.

The production of either an anhydride or an aldehyde is undesirable because they are reactive and have a tendency to polymerize, creating high molecular weight species. These reactions are eliminated in the presence of either BHT or an amine antioxidant until those concentrations are depleted.
