Lubricant Additives

Antioxidants 5

8. Boron compounds


9. Other organometallic compounds

1.2 SULFUR COMPOUNDS

The initial concepts of using antioxidants to inhibit oil oxidation date back to the 1800s. One of the
earliest inventions described in the literature [1] is the heating of a mineral oil with elemental sulfur
to produce a nonoxidizing oil. However, the major drawback to this approach is the high corrosivity
of the sulfurized oil toward copper. Aliphatic sulfi de with a combined antioxidant and corrosion
inhibition characteristics was developed by sulfurizing sperm oil [2]. Additives with similar func-
tionalities could also be obtained from sulfurizing terpenes and polybutene [3–5]. Paraffi n wax has
also been employed to prepare sulfur compounds [6–9]. Theoretical structures of several sulfur
compounds are illustrated in Figure 1.1. Actual compounds can be chemically complex in nature.
Aromatic sulfi des represent another class of sulfur additives used as oxidation and corrosion
inhibitors. Examples of simple sulfi des are dibenzyl sulfi de and dixylyl disulfi de. More complex
compounds of a similar type are the alkyl phenol sulfi des [10–15]. Alkyl phenols, such as mono- or
di-butyl, -amyl, or -octyl phenol, have been reacted with sulfur mono- or dichloride to form either
mono- or disulfi des. As shown in Figure 1.1, the aromatic sulfi des such as benzyl sulfi de have the
sulfur attached to carbon atoms in the alkyl side groups, whereas the alkyl phenol sulfi des have the
sulfur attached to carbon atoms in the aromatic rings. In general, the alkyl phenol sulfi de chemistry
appears to have superior antioxidant properties in many types of lubricants. Mono- and dialkyl-
diphenyl sulfi des obtained by reacting diphenyl sulfi de with C 10 –C 18 alpha-olefi ns in the presence
of aluminum chloride have been demonstrated to be powerful antioxidants for high-temperature
lubricants especially those utilizing synthetic base stocks such as hydrogenated poly-alpha-olefi ns,
diesters, and polyol esters [15]. The hydroxyl groups of the alkyl phenol sulfi des may also be treated

CH 2

CH 2

C

CH 2

CH

C

CH 3

S

C

CH 2 CH 3

S

Sulfurized dipentene

Sulfurized ester

CH 3 (CH 2 )x CH CH (CH 2 )x

S

CH 3

CH 3 (CH 2 )x CH CH (CH 2 )x CH 3

Sulfurized olefin

CH 3 (CH 2 )x C

O

O (CH 2 )x CH 3

CH 3 (CH 2 )x CCH 2 (CH 2 )x CH 3

O

CH CH (CH 2 )x

S

CH CH (CH 2 )x

CH 2 S CH 2

Dibenzyl sulfide

OH

R

HO

(S)x R

Dialkylphenol sulfide

S

O

S

CH 2

FIGURE 1.1 Examples of sulfur-bearing antioxidants.