Lubricant Additives

Dispersants 157

5.7 DISPERSANT PROPERTIES

A dispersant consists of a hydrocarbon chain, a connecting group, and a polar functionality. Although
each structural feature imparts unique properties to a dispersant, the dispersant’s overall perfor-
mance depends on all the three. The overall performance is assessed in terms of its dispersancy,
thermal and oxidative stability, viscosity characteristics, and seal performance. These criteria
primarily relate to engine oils, where dispersants fi nd major use.

5.7.1 DISPERSANCY

As mentioned, dispersancy pertains to a dispersant’s ability to suspend by-products of combustion,
such as soot, and lubricant degradation, such as resin, varnish, lacquer, and carbon deposits. The
overall performance of a dispersant depends on all the three of its structural features: the hydrocarbon
chain, the connecting group, and the polar moiety. The molecular weight of the hydrocarbon group
in a dispersant determines its ability to associate with undesirable polar species and suspend them
in the bulk lubricant. For dispersants that have the same connecting group and the polar moiety, the
lower the molecular weight, the higher the ability to associate with polar materials and the lower the
ability to suspend them. Because of the trade-off between the two properties, the hydrocarbon chain
must have the correct size and branching.
The size affects a dispersant’s affi nity toward polar materials, and branching affects its solu-
bility, both before association and after association with the species, a dispersant is designed to
suspend in oil. Experience has demonstrated that hydrocarbon groups containing 70–200 carbon
atoms and extensive branching, as in the case of polyisobutylenes, are extremely suitable to design
dispersants with good dispersancy. The hydrocarbon chains of larger size, even if the branching is
similar, lead to dispersants with low affi nity toward polar materials.
That is why dispersant polymers possess lower dispersancy than polymeric dispersants. How-
ever, since dispersant polymers have additional attributes, such as good thickening effi ciency and

FIGURE 5.11 Synthesis of imide and ester dispersants.

O

PIB

O

O

O

PIB

O

O

N

PIB

O

C

C

H 2

H 2

CH 2

H 2 C

O

+

+

H 2 N

H

N

N NH

N

Polyisobutenylsuccinimide

Polyalkylenepolyamine

Polyisobutenylsuccinic

anhydride

Polyisobutenylsuccinic

anhydride

Polyisobutenylsuccinate ester

Polyhydric

alcohol

PIB = Polyisobutenyl

R′

R

HOH 2 C C CH 2 OH O

O

O C

O

R

O

O

PIB CH 2

CH 2

CH 2

CH 2

C

R

R′

R′