144 Lubricant Additives: Chemistry and Applications
is the case in imide/amide dispersants. The basicity of the imide/amide dispersants is due
to the presence of the amine functionality. Amines are weak bases and therefore possess
minimal acid-neutralizing ability. Conversely, detergents, especially basic detergents, con-
tain reserve metal bases as metal hydroxides and metal carbonates. These are strong bases,
with the ability to neutralize combustion and oxidation-derived inorganic acids, such as
sulfuric and nitric acids, and oxidation-derived organic acids.- Dispersants are much higher in molecular weight, approximately 4–15 times higher, than
the organic portion (soap) of the detergent. Because of this, dispersants are more effective
in fulfi lling the suspending and cleaning functions than detergents.
As mentioned in Chapter 4, dispersants, detergents, and oxidation inhibitors make up the general
class of additives called stabilizers and deposit control agents. The goal of oxidation inhibitors is
to minimize the formation of deposit precursors, such as hydroperoxides and radicals [3,4]. This is
because these species are reactive, and they attack the hydrocarbon base oil and additives, which
make up the lubricant, to form sludge, resin, varnish, and hard deposits. The goal of the dispersant
and the soap portion of the detergent is to keep these entities suspended in the bulk lubricant. This
not only results in deposit control but also minimizes particulate-related abrasive wear and viscosity
increase. When the lubricant in the equipment is changed, the deposit precursors and the deposit-
forming species are removed with the used oil.
The dispersants suspend deposit precursors in oil in various ways. These comprise the
following:
Including the undesirable polar species into micelles.
Associating with colloidal particles, thereby preventing them from agglomerating and fall-
ing out of solution.
Suspending aggregates in the bulk lubricant, if they are formed.
Modifying soot particles so as to prevent their aggregation. The aggregation will lead to oil
thickening, a typical problem in heavy-duty diesel engine oils [5,6].
Lowering the surface/interfacial energy of the polar species to prevent their adherence to
metal surfaces.5.2 NATURE OF DEPOSITS AND MODE OF THEIR FORMATION
A number of undesirable materials result from the oxidative degradation of various components of
the lubricant. These are base oil, additives, and the polymeric viscosity modifi er, if present. In engine
oils, the starting point for the degradation is fuel combustion, which gives rise to hydroperoxides and
free radicals [7]. The compounds in the fuel that are most likely to form peroxides, hydroperoxides,
and radicals include highly branched aliphatics, unstaurates such as olefi ns, and aromatics such as
alkylbenzenes. All these are present in both gasoline and diesel fuels. American Society for Testing
and Materials (ASTM) test methods D 4420 and D 5186 are used to determine the aromatic content
of gasoline and diesel fuels, respectively [8]. The fuel degradation products (peroxides, hydroperox-
ides, and radicals) go past the piston rings into the lubricant as blowby and, because they are highly
energetic, attack largely the hydrocarbon lubricant. Again, the highly branched aliphatic, unsatu-
rated, and aromatic structures are among those that are highly susceptible. ASTM Standard D
5292 is commonly used to determine the aromatic content of the base oil [8]. The reaction between
the contents of the blowby and these compounds results in the formation of the lubricant-derived
peroxides and hydroperoxides that either oxidatively or thermally decompose to form aldehydes,
ketones, and carboxylic acids [3,4,9]. Acids can also result from the high-temperature reaction of
nitrogen and oxygen, both of which are present in the air–fuel mixture; the oxidation of the fuel
sulfur; and the oxidation, hydrolysis, or thermal decomposition of additives such as zinc dialkyl-
dithiophosphates. The reaction between nitrogen and oxygen to form NOx is more prevalent in diesel