Lubricant Additives

160 Lubricant Additives: Chemistry and Applications

Because some of the materials with which the dispersant associates are acidic, such as carboxylic
acids derived from lubricant oxidation, the presence of an amine nitrogen is an advantage because
of its basic character. Therefore, in certain gasoline engine tests, nitrogen dispersants are superior to
ester dispersants. Ester dispersants are usually superior in diesel engine tests because of their higher
thermo-oxidative stability. Mannich dispersants are good low-temperature dispersants; hence, they
are typically used in gasoline engine oils.
As mentioned earlier, commercial polyisobutylenes have a molecular weight distribution.
This will lead to dispersant structures of varying size, hence molecular weight. An optimum ratio
between the molecular weight of the hydrocarbon chain and that of the polar functionality (polar/
nonpolar ratio) is a prerequisite for good dispersancy. If a dispersant composition has an exces-
sive amount of components with short hydrocarbon chains, that is, of low molecular weight, its
associating ability increases, but its oil solubility suffers. This is likely to deteriorate its disper-
sancy, especially after associating with polar impurities. Such structures in dispersants are, there-
fore, undesired. Their formation can be minimized by using polyolefi ns of low polydispersity
index, controlling the formation of low-molecular-weight components, removing such components
through distillation [100], or postreacting with another reagent, preferably of the hydrocarbon type.
Polyolefi ns of low polydispersity index (≤2) are available from BP and Exxon Chemical Company.
Controlling the formation of low-molecular-weight components is exemplifi ed by the use of boron
trifl uoride catalyst for making alkylphenols instead of aluminum chloride, which tends to fragment
polyisobutylene. Removing the lower-molecular-weight components, although not easy, is possible
at the precursor stage, which is before reacting with the alcohol or the amine. A number of reagents
can be used for the postreaction [101]. Hydrocarbon posttreatment agents include polyepoxides
[102], polyca rboxylic acid [103], alkylbenzenesulfonic acids [104], and alkenylnitriles [105].
Whenever postreacted dispersants are used in engine oils, improved dispersancy, viscosity index
credit, improved fl uorocarbon elastomer compatibility, hydrolytic stability, and shear stability are
often claimed.

5.7.2 THERMAL AND OXIDATIVE STABILITY

All the three components of the dispersant structure determine its thermal and oxidative stability,
the same as dispersancy. The hydrocarbon group can oxidize in the same manner as the lubricant
hydrocarbons to form oxidation products that can contribute toward deposit-forming species [4,9].
(This is described in Section 5.2.) Although the rate of oxidation is quite slow for largely paraffi nic
hydrocarbon groups, such as polyisobutyl group, it is quite high for those that contain multiple bonds,
such as polyisobutenyl, and the benzylic groups. The benzylic functional group is present in styrene

FIGURE 5.15 Dispersant viscosity modifi er synthesis through chemical reaction.

CC

HHH

C C

C

O O O

CH 2

Styrene−maleic

anhydride polymer

n n

H 2 N

ROH

R

H

C C C

H H

O CCO

CH 2

R′

R R′

N

N

NH OR

Styrene ester−based

dispersant viscosity

modifier