**4. Lubricant additive degradation**

Lubricant additives are in many ways designed to degrade as they serve their purpose in the formulated lubricant. As the lubricant is lost in service primarily due to leakage, new lubricant is added which act to replenish the additives used. Lubricant loss is typically estimated at as much as one quart per hour depending on the engine [17]. It is possible to use the amount of remaining additives to determine the need for engine service or lubricant replacement. One example of an instrument for the analysis of remaining antioxidant as an engine diagnostic is RULER [18].

#### **4.1 Phosphate esters**

Phosphate esters are normally required as an extreme pressure or anti-wear additive. The phosphate esters react with the metal surface to form a lubricious polymeric coating. The coating protects the bearing under conditions of start-up, inadequate flow or extreme shear, where the coating wears away, but is continuously reformed from unreacted phosphate ester in the lubricant. The mechanism of action of the additive causes its degradation over time [19].

### *4.1.1 Hydrolysis*

Hydrolysis of phosphate esters is the reaction of the triester with water to form a diester and an aromatic alcohol. The diester can further react under the same conditions to form the monoester and eventually phosphoric acid. Two classes of mechanisms have been proposed for the hydrolysis in aqueous solution, dissociative mechanisms that proceed through a PO3 <sup>−</sup> anion and associative mechanisms then proceed through a penta coordinate phosphorous intermediate [20]. The likely mechanism in the non-polar lubricant medium where the attacking species is a water molecule is most likely through the associative mechanism, which does not require formation of a PO3 <sup>−</sup> ion. The mechanism of the reaction is based on the addition of water to the phosphorus atom, followed by loss of a proton and elimination of the alcohol (phenol) [21]. The mechanism is shown schematically in **Figure 7**.

**Figure 7.** *Mechanism for the hydrolysis of phosphate esters in polyol ester-based lubricants.*

Hydrolysis is an important degradation mechanism because it forms a range of phosphate partial esters, some of which do not form a lubricous coating on the bearing and contribute to the acids contained in the lubricant.
