Lubricant Additives

104 Lubricant Additives: Chemistry and Applications

been used in water-based formulations with good pump wear characteristics [155,156]. One of the
most recent applications has been in refrigeration compressor oils (e.g., for automobiles) that are
compatible with the more ecologically acceptable refrigerants. The reason for their selection in this
application has probably been their good hydrolytic stability in view of the need for a long fl uid life
[157,158]. Other automotive industry applications for these products include use as friction modi-
fi ers, for example, in automatic transmission fl uids [159], or possibly as detergents in engine oils
[160–162] to keep insoluble combustion and oil oxidation products dispersed in the oil.
An alternative method for incorporating phosphorus into dispersant is exemplifi ed in Ref. 160.
This method involves reacting P 2 S 5 with a sulfurized hydrocarbon, such as sulfurized polyisobutyl-
ene, at high temperatures to form a thiophosphorus acid (see reaction 3.23, Figure 3.22). This inter-
mediate is then reacted with propylene oxide to form the hydroxypropyl esters of the phosphorus
acid (see reaction 3.24, Figure 3.22).
Aminoethane phosphonate copolymers have also been claimed to provide dispersancy, corrosion
protection, and pour point depression [163]. Among other applications mentioned in the literature
for these products or their salts in lubricating oils are the extrusion, cold rolling, and cold forging of
aluminum [164], offering improved rust inhibition [165] and antioxidant performance [166].

3.5.7 A SUMMARY OF THE PROPOSED MECHANISM FOR ANTIWEAR AND

EXTREME-PRESSURE ACTIVITY OF PHOSPHORUS-BASED ADDITIVES

In attempting to produce an explanation for the activity of phosphorus-containing additives, it is
not easy, as explained earlier, to compare the results of the preceding investigations because condi-
tions vary from one investigation to another. No report evaluates all the different types of additives
with the same (high) level of purity under identical test conditions. However, it is possible to draw
together some of the more consistent “threads” running through the many papers. One parameter
highlighted in past reports (and confi rmed by recent observations) is that the presence of oxygen
on the metal surface appears to be important for the activity of neutral aryl phosphates. This could
perhaps be one of the major reasons why TCP is sometimes found to be inactive. The composition
of the fi lm formed on the surface is not yet completely defi ned, but current work points toward the
formation of a self-regenerating polyphosphate layer in which amorphous carbon may be providing
the lubrication benefi ts. The mechanism of formation of the polyphosphate layer and the role, for
example, of moisture is not yet clear but appears to be a stepwise process as follows:

• The adsorption of the material onto the surface (occurring through the –P=O and –P–OH
  bonds in the molecule).


• Either the hydrolysis of a –P–OR bond to form –P–OH (probably arising from water on the
  surface but may also occur in solution) with the formation of acid phosphates/phosphites

PIB + P 2 S 5 PIB P OH

S

OH

P OH

O

PIB

S

OH

PIB P O

S

OH

H 2 O

+^2 (CH^3 CH CH^2 ) (OCH^2 CH

CH 3

OH) 2

where PIB = polyisobutylene

(3.23)

(3.24)

FIGURE 3.22 An example of the preparation of a phosphorus-based detergent. (From Colyer, C.C., Gergel,
W.C ., Chemistry and Technology of Lubricants, VCH Publishers, New York, 1992. With permission.)