Lubricant Additives

Antioxidants 21

1.10.2.1 Metal Catalysis

1.10.2.1.1 Init iat ion Step

MRHMHR

(n 1)→ n  i

(1.13)

MOM O

n

2

(n 1)

2

→ 


(1.14)

1.10.2.1.2 Propagation Step

M ROOH M H ROO

(n1)→ n i

(1.15)

M ROOH M HO RO

n(→ n1)
i

(1.16)

1.10.3 HIGH-TEMPERATURE LUBRICANT DEGRADATION

The preceding discussion provides the basis for the autoxidation stage of lubricant degradation
under both low and high-temperature conditions. The end result of low-temperature oxidation is the
formation of peroxides, alcohols, aldehydes, ketones, and water [185,186]. Under high-temperature
oxidation conditions (>120°C), breakdown of peroxides including hydroperoxides becomes pre-
dominant, and the resulting carbonyl compounds (e.g., reactions 1.8 and 1.9) will fi rst be oxidized
to carboxylic acids as shown in Figure 1.6. As an immediate result, the oil acidity will increase. As
oxidation proceeds, acid or base-catalyzed Aldol reactions take place. The reaction mechanism is
illustrated in Figure 1.7 [187]. Initially, α,β-unsaturated aldehydes or ketones are formed, and fur-
ther reaction of these species leads to high-molecular-weight products. These products contribute to
oil viscosity increase and eventually can combine with each other to form oil-insoluble polymeric
products that manifest as sludge in a bulk oil oxidation environment or as varnish deposits on hot
metal surface. Oil viscosity increase and deposit formation have been identifi ed to be the principal
oil-related factors to engine damages [188].

1.10.4 EFFECT OF BASE STOCK COMPOSITION ON OXIDATIVE STABILITY

Mineral base stocks used to formulate lubricants are hydrocarbons that are originated from crude
oils and essentially contain mixtures of n-paraffi ns along with isoparaffi ns, cycloparaffi ns (also
called naphthenes), and aromatics having about 15 or more carbon atoms [189]. In addition, small
amounts of sulfur-, nitrogen-, and oxygen-containing species may be present depending on the
refi nery techniques employed. In the American Petroleum Institute (API) base oil classifi cation
system, mineral oils largely fall into the groups I, II, III, and V, with some distinctions shown in
Table 1.3 in terms of saturates, sulfur contents, and viscosity index. Group I base oils still dominate
the base oil market, accounting for more than 50% of global capacity. Groups II and III base stocks

O

CR

O

CR

O

R C OO

O

C OOH

O

C OH + O 2

−ROOH

ROO O 2 RH

−R

2 R

H

• 
  
  
  
  • • •
  
  
  
  

2 R

FIGURE 1.6 High-temperature (>120°C) lubricant degradation leading to the formation of carboxylic acids.