Dispersants 147
lubricant oxidation and degradation, such as carboxylic and sulfonic acids. Fuel sulfur–derived
piston ring wear and cylinder wear are serious problems in large, slow-speed marine diesel engines
that use a high-sulfur fuel. Corrosive wear is controlled by the use of lubricants with a base reserve,
that is, those containing a large quantity of basic detergents. This was discussed in Chapter 4. Abrasive
wear results from the presence of the particulate matter, such as large soot particles, in the lubricant.
Dispersants are crucial to the control of soot-related wear.
5.3 DEPOSIT CONTROL BY DISPERSANTS
Fuel and lubricant oxidation and degradation products, such as soot, resin, varnish, lacquer, and
carbon, are of low lubricant (hydrocarbon) solubility, with a propensity to separate on surfaces.
The separation tendency of these materials is a consequence of their particle size. Small particles
are more likely to stay in oil than large particles. Therefore, resin and soot particles, which are
the two essential components of all deposit-forming species, must grow in size through agglom-
eration before separation. Growth occurs either because of dipolar interactions, as is the case in
resin molecules, or because of adsorbed polar impurities such as water and oxygen, as is the case
in soot particles. Alternatively, soot particles are caught in the sticky resin. Dispersants interfere
in agglomeration by associating with individual resin and soot particles. The particles with asso-
ciated dispersant molecules are unable to coalesce because of either steric factors or electrostatic
factors [28]. Dispersants consist of a polar group, usually oxygen- or nitrogen-based, and a large
nonpolar group. The polar group associates with the polar particles, and the nonpolar group keeps
such particles suspended in the bulk lubricant [16]. Neutral detergents, or soaps, operate by an
analogous mechanism.
5.4 DESIRABLE DISPERSANT PROPERTIES
Dispersing soot, deposit precursors, and deposits is clearly the primary function of a dispersant.
Dispersants, in addition, need other properties to perform effectively. These include thermal and
oxidative stability and good low-temperature properties. If a dispersant has poor thermal stabil-
ity, it will break down, thereby losing its ability to associate with and suspend potentially harmful
products. Poor oxidative stability translates into the dispersant molecule contributing itself toward
deposit formation. Good low-temperature properties of a lubricant are desired for many reasons:
ease of cold cranking, good lubricant circulation, and fuel economy. Base oil suppliers have devel-
oped a number of ways to achieve these properties. The methods they use include isomerization of
the base stock hydrocarbons through hydrocracking and the use of special synthetic oils as addi-
tives. Since dispersant is one of the major components of the engine oil formulations, its presence
can adversely affect these properties, which must be preserved.
5.5 DISPERSANT STRUCTURE
A dispersant molecule consists of three distinct structural features: a hydrocarbon group, a polar
group, and a connecting group or a link (see Figure 5.1). The hydrocarbon group is polymeric
FIGURE 5.1 Graphic representation of a dispersant molecule.
Nitrogen- or oxygen-
derived functionalityPolar moietyConnecting group
Hydrocarbon group