**3. Lubricant basestock degradation mechanisms**

Conventional lubricants are petroleum based and consist of hydrocarbons including a huge number of isomers. The primary degradation mechanism for hydrocarbons is oxidation, which leads to the formation of alcohols and carboxylic acids. Synthetic lubricants typically by oxidation to carboxylic acids, aldehydes and ketones under extreme conditions, and degrade by hydrolysis, due to the presence of water and in some cases by transesterification with phosphate ester additives. In addition to the degradation of the basestock due to oxygen, the role of bearing surfaces where extremely high temperatures and pressures; along with the presence of metals and surface treatments such as metal carbides most be considered. In addition, lubricant esters can act synergistically with certain additives [6] and can react differently in the present of metals and or metal carbides.

### **3.1 Hydrolysis**

The hydrolysis of the ester basestock is the reaction of the basestock with water to form an alcohol and a carboxylic acid. This reaction is catalyzed by acids or bases, which are frequently present within the lubricant and does require water. The water can come from various sources, including contamination of the lubricant and the exposure of the lubricant to the environment. Water is soluble in typical ester basestocks to a level of about 500 ppm, meaning that water is readily available in the lubrication systems for turbine engines. The mechanism for the hydrolysis of esters is shown in **Figure 3**.

**79**

**Figure 4.**

*Mechanism for the acid catalyzed hydrolysis of esters.*

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

Hydrolysis of esters can occur through either an acid or base catalyzed mechanism, with significant differences in the mechanism. The acid catalyzed mechanism [7] begins with the protonation of the carbonyl oxygen atom, followed by a water molecule attacking the carbonyl carbon atom of the ester. The carbonyl carbon normally has a partial positive charge which is increased by the protonation of the oxygen atom yielding the hemiacetal shown in **Figure 4**. One of the water can be transferred to the alcohol oxygen atom and then the alcohol is lost completing the

The base catalyzed mechanism [8] involves a water molecule attacking the carbonyl carbon atom, followed by transfer of a proton to the carbonyl oxygen atom. The base the assist with the transfer of the proton from the carbonyl oxygen atom the oxygen atom of the alcohol as the alcohol leaves forming the carboxylic acid. The two hydrolysis mechanisms require that water be able to attack the carbonyl

Ester based lubricants are all subject to high temperature oxidation which has the most detrimental effect on their properties. Early work examined changes in the bulk composition of ester based lubricants showing the formation of a wide range of acids. The lighter carboxylic acids were attributed to oxidation of the acid chains. Other products were attributed to oxidation of the alcohol [10]. Later work proposed an explanation for oxidation that is based on a radical chain mechanism. Oxidation occurs through a complex radical chain mechanism which is common to a wide range of organic materials. The initial stages of the oxidation involve the formation of an alkyl peroxy radical by reaction with oxygen. The reaction is propagated by the attack of an alkyl peroxy radical on a methylene group of the ester. The α position of the acid, has been shown to be significantly more reactive than other methylene groups in the carboxylic acid [11]. This reaction is significantly hindered in the polyol esters, especially when branched chain acids with branches at C-2 are included. A more recent study, using isotope labelling techniques has shown that the initial site of oxidation is at C-1 of the alcohol, cleaving the carbon–oxygen

group of the ester. The use of hindered alcohols such as the various neopentyl alcohols (**Figure 1**) reduces the ability of the water to approach the carbonyl carbon atom. The use of branched chain acids further reduces the ability of water to attack the carbonyl, resulting in an increase in the hydrolytic stability of the ester [9].

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

hydrolysis.

**3.2 Oxidation**

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

Hydrolysis of esters can occur through either an acid or base catalyzed mechanism, with significant differences in the mechanism. The acid catalyzed mechanism [7] begins with the protonation of the carbonyl oxygen atom, followed by a water molecule attacking the carbonyl carbon atom of the ester. The carbonyl carbon normally has a partial positive charge which is increased by the protonation of the oxygen atom yielding the hemiacetal shown in **Figure 4**. One of the water can be transferred to the alcohol oxygen atom and then the alcohol is lost completing the hydrolysis.

The base catalyzed mechanism [8] involves a water molecule attacking the carbonyl carbon atom, followed by transfer of a proton to the carbonyl oxygen atom. The base the assist with the transfer of the proton from the carbonyl oxygen atom the oxygen atom of the alcohol as the alcohol leaves forming the carboxylic acid.

The two hydrolysis mechanisms require that water be able to attack the carbonyl group of the ester. The use of hindered alcohols such as the various neopentyl alcohols (**Figure 1**) reduces the ability of the water to approach the carbonyl carbon atom. The use of branched chain acids further reduces the ability of water to attack the carbonyl, resulting in an increase in the hydrolytic stability of the ester [9].

#### **3.2 Oxidation**

*Aerospace Engineering*

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

Conventional lubricants are petroleum based and consist of hydrocarbons including a huge number of isomers. The primary degradation mechanism for hydrocarbons is oxidation, which leads to the formation of alcohols and carboxylic acids. Synthetic lubricants typically by oxidation to carboxylic acids, aldehydes and ketones under extreme conditions, and degrade by hydrolysis, due to the presence of water and in some cases by transesterification with phosphate ester additives. In addition to the degradation of the basestock due to oxygen, the role of bearing surfaces where extremely high temperatures and pressures; along with the presence of metals and surface treatments such as metal carbides most be considered. In addition, lubricant esters can act synergistically with certain additives [6] and can

The hydrolysis of the ester basestock is the reaction of the basestock with water to form an alcohol and a carboxylic acid. This reaction is catalyzed by acids or bases, which are frequently present within the lubricant and does require water. The water can come from various sources, including contamination of the lubricant and the exposure of the lubricant to the environment. Water is soluble in typical ester basestocks to a level of about 500 ppm, meaning that water is readily available in the lubrication systems for turbine engines. The mechanism for the hydrolysis of

the additive packages at acceptable levels.

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

**3. Lubricant basestock degradation mechanisms**

react differently in the present of metals and or metal carbides.

**78**

**3.1 Hydrolysis**

**Figure 3.**

esters is shown in **Figure 3**.

Ester based lubricants are all subject to high temperature oxidation which has the most detrimental effect on their properties. Early work examined changes in the bulk composition of ester based lubricants showing the formation of a wide range of acids. The lighter carboxylic acids were attributed to oxidation of the acid chains. Other products were attributed to oxidation of the alcohol [10]. Later work proposed an explanation for oxidation that is based on a radical chain mechanism.

Oxidation occurs through a complex radical chain mechanism which is common to a wide range of organic materials. The initial stages of the oxidation involve the formation of an alkyl peroxy radical by reaction with oxygen. The reaction is propagated by the attack of an alkyl peroxy radical on a methylene group of the ester. The α position of the acid, has been shown to be significantly more reactive than other methylene groups in the carboxylic acid [11]. This reaction is significantly hindered in the polyol esters, especially when branched chain acids with branches at C-2 are included. A more recent study, using isotope labelling techniques has shown that the initial site of oxidation is at C-1 of the alcohol, cleaving the carbon–oxygen

**Figure 4.** *Mechanism for the acid catalyzed hydrolysis of esters.*

bond between the first carbon of the alcohol and the ester oxygen, followed by further oxidation at that carbon to form the organic acid [12]. After the initial attack, the reaction can progress to form anhydrides which continue to react to form aldehydes, acids and eventually high molecular weight compounds which can form sludge in the engine. The mechanism of the initial stages of the oxidation of the esters is shown in **Figure 5**.
