Lubricant Additives

8 Lubricant Additives: Chemistry and Applications

antioxidants in some petroleum base oils, and many patents have been issued on such compositions
[50,51]. Table 1.1 summarizes the patenting activities of the past three decades on the stabilization
of various lubricants with organophosphites. For optimum antioxidant performance, phosphites are
customarily blended with aminic or HP antioxidants that can lead to synergistic effect. For better
hydrolytic stability, tri-substituted phosphites with sterically hindered structures such as tris-(2,4-
di-tert-butylphenyl) phosphite and those based on pentaerythritol as described in the U.S. Patent
5,124,057 [52] are preferred. The aluminum, calcium, or barium salts of alkyl phosphoric acids are
another type of phosphorus compound that displays antioxidant properties [53,54].

1.5 SULFUR–PHOSPHORUS COMPOUNDS

The identifi cation of sulfur and phosphorus compounds as powerful antioxidants for protection
of hydrocarbons has led to the development of oil-soluble antioxidants, having both elements
in one molecule. Numerous patents have been issued on such compositions, and a considerable
number have been used commercially [60–67]. In fact, antioxidants containing both sulfur and
phosphorus are usually more effective and effi cient in a wider variety of base stocks than those
containing only phosphorus or sulfur. Many commercial oils have employed one kind or other of
these sulfur–phosphorus-type additives.
One widely used class of sulfur–phosphorus additive is the metal dialkyldithiophosphates,
which are typically prepared by the reaction of phosphorus pentasulfi de with alcohols to form
dithio-phosphoric acids, followed by neutralization of the acids with an appropriate metal com-
pound. Many types of alcohols such as the aliphatic, cyclic [62], and phenolic derivatives have
been used, and those of relatively high molecular weight (such as lauryl, octyl, cyclohexyl, methyl
cyclohexyl alcohols, and amyl [65] or butyl phenols) are preferred to give suffi cient thermal stability
to the fi nal products while rendering suffi cient solubility in oils. For the second-step reaction, zinc,

TABLE 1.1

Applications of Organophosphites as Antioxidants for Lubricants

Applications Phosphites Supplementary Antioxidants References

Compressor oils Trinonylphenyl phosphite, tributyl

phosphite, tridecylphosphite,

triphenylphosphite,

trioctylphosphite,

dilaurylphosphite

Secondary aminic and hindered

phenolic

55

Automotive and industrial

lubricants

Triaryl phosphites, trialkyl

phosphites, alkyl aryl phosphites,

acid dialkyl phosphites

Secondary aminic and hindered

phenolic

56

Automotive and industrial

lubricants

Triphenyl phosphite, diisodecyl

pentaerythritol diphosphite,

tri-isodecyl phosphite, dilauryl

phosphite

Secondary aminic and hindered

phenolic

57

Hydraulic fl uids, steam

turbine oils, compressor oils,

and heat-transfer oil

Steric hindered tributyl phosphite,

bis(butylphenyl pentaerythritol)

diphosphite

(3,5-Di-t-butyl)4-hydroxybenzyl

isocyanurate

52

Steam turbine oils, gas turbine

oils

Triphenyl phosphite, trialkyl-

substituted phenyl phosphite

Alkylated diphenylamine,

phenyl-naphthylamine

58

Hydraulic fl uids, Automatic

transmission fl uids

Trialkyl phosphites Secondary aminic and hindered

phenolic including bis-phenol

59