Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 65

into the molecule as, for example, in thiophosphates and chlorinated phosphates. These are, how-
ever, outside the scope of this discussion, but the performance of mixtures of compounds separately
containing phosphorus and sulfur or chlorine will be mentioned.
In addition to examining the impact of ashless phosphorus compounds on lubrication
performance, this chapter also looks at their performance as antioxidants, rust inhibitors, and metal
passivators. Additionally, their polar nature makes them good solvents and assists the solution of
other additives in nonpolar base stocks. The versatility displayed by phosphorus-containing addi-
tives is such that usage of these products continues to grow nearly half a century after their intro-
duction, and they fi nd application in the latest technological developments.

3.2 HISTORICAL BACKGROUND

Until the 1920s, additive-free mineral oils met the majority of industry’s lubrication requirements.
In the applications where their performance was unsatisfactory, an increase in viscosity or the sul-
fur content of the oils then available usually provided adequate lubrication. For very severe applica-
tions, the oil would be blended with animal or vegetable oils—for example, tallow or rapeseed oils
were used for steam engine cylinder lubrication. Fish oils were used in the early locomotive axle
boxes, while castor oil reduced friction in worm gear drives and fl owers of sulfur were added to
cutting oils. However, when hypoid gears were introduced, they quickly revealed the limited lubri-
cation of oils then available. This resulted in the development of additives such as sulfurized lard
oil and lead naphthenate. These were followed by sulfurized sperm oil, an additive that eventually
became widely used in both industrial and automotive applications.
The earliest type of an organic phosphorus chemical to fi nd use as a lubricating oil additive
is thought to have been a neutral triaryl phosphate, specifi cally tricresyl phosphate (TCP). This
material was originally synthesized in about 1854 [1] although trialkyl phosphates were synthe-
sized slightly earlier, in about 1849 [2]. Commercial production of TCP began in about 1919, when

Phosphate esters

Phosphites and phosphonates

Alkyl or aryl phosphates

Neutral phosphates Acid phosphates

Non-ethoxylated Ethoxyalkyloxy-

Amine salts Amine salts

Neutral amine salts

Alkyl or aryl phosphites/phosphonates

Neutral phosphites/phosphonates Acid phosphites/phosphonates

FIGURE 3.1 The main classes of phosphorus-containing additives for lubricating oils.