Lubricant Additives

158 Lubricant Additives: Chemistry and Applications

in some cases good thermal and oxidative stability, their use is advantageous. They usually replace
additives, called viscosity modifi ers, in the package. Since they impart some dispersancy because
of their structure, the amount of polymeric dispersant in engine oil formulations is somewhat
decreased [79,98].
Both the connecting group and the polar moiety are important to the dispersancy of the disper-
sant molecule. They must be considered together since both contribute toward polarity. In Mannich
dispersants, the phenol functional group, and in imide and ester dispersants, succinimide, succinate,
and phosphonate functional groups are also polar, the same as the amine and the alcohol-derived
portion of the molecule. The polarity is a consequence of the electronegativity difference between
carbon, oxygen, nitrogen, and phosphorus atoms. The greater the electronegativity difference, the
stronger the polarity. This implies that groups that contain phosphorus–oxygen bonds are more
polar than those containing carbon–oxygen bonds, carbon–nitrogen bonds, and carbon–phosphorus
bonds. The electronegativity difference for such bonds is 1.4, 1.0, 0.5, and 0.4, respectively [99].
However, since dispersants have many bonds with various combinations of atoms, the overall
polarity in a dispersant and its ability to associate with polar materials are not easy to predict.

FIGURE 5.12 Synthesis of Mannich and phosphonate dispersants.

++

OH

R

CH 2 O H 2 N

N

N

H

Polyisobutylphenol

Polyalkylenepolyamine

OH

H 2

R

C

H

N

NH

N

NN

HN

or

H

CH 2

OH

C

N

H

H 2

R

NH

N

Polyaminomethylpolyisobutylphenols

S

PIB P OH

OH

+

O

Propylene

oxide

Polyisobutenylthiophosphonic acid

PIB

S CH 3

CH 3

OCH 2 CH

OCH 2 CH

P OH

OH

Bis-hydroxypropyl

polyisobutenylthiophosphonate