Dispersants 165
As mentioned earlier, soot-related viscosity increase and deposit-related factors are the primary
criteria for evaluating a dispersant’s performance. Moreover, as commented in Chapter 4, neutral
detergents (soaps) also help control deposits such as varnish, lacquer, sludge, and carbon. There-
fore, besides the control of soot-related viscosity increase, which is the sole domain of dispersants,
deposit control is the result of a joint performance of the detergent and the dispersant. However, in
this regard, the dispersant plays a more prominent role.
Besides engine oils, transmission fl uids are the primary users of dispersants. Certain parts of the
transmission see very high temperatures, which lead to extensive lubricant oxidation. The oxidation
products, such as sludge and varnish, appear on parts; for instance, clutch housing, clutch piston,
control valve body, and oil screen components. This can impair the functioning of these parts.
A turbohydramatic oxidation test (THOT) is used to determine a transmission fl uid’s oxidative
stability.
Polymeric dispersants are useful in controlling sludge buildup [129]. When friction modifi cation
of the transmission fl uid is the goal, either dispersants or their precursors, such as alkenylsuccinic
acids or anhydrides, are used in combination with metal sulfonates [130–134]. In many such formu-
lations, the borated dispersant and the borated detergent (metal sulfonate) are used.
Dispersants are used in gear oils to improve their properties also. Gear oils usually contain
thermally labile extreme-pressure additives. Their decomposition by-products are highly polar,
and dispersants are used to contain them to avoid corrosion and deposit formation [135,136].
Polymeric dispersants are used in hydraulic fl uids to overcome wet fi ltration (Association Française
de Normalisation [AFNOR]) problems, which is often required for HF-0-type fl uids [137]. Fil-
tration problems occur due to the interaction of water with metal sulfonate detergent and zinc
dialkyldithiophosphate that are used as additives in hydraulic fl uid formulations. Fouling is a
common problem in many processes, including refi nery processes. Fouling refers to the depo-
sition of various inorganic and organic materials, such as salt, dirt, and asphaltenes, on heat-
transfer surfaces and other processing equipment. This results in poor heat transfer, among other
problems. Antifoulants are chemicals used in refi nery operations to overcome fouling. Detergents
and dispersants are often used for this purpose [138–140].
TABLE 5.4
Current European Diesel Engine Tests
Engine Test Engine Type Evaluation Criteria
VW 1.6TC diesel intercooler Four-cylinder engine Piston deposits, varnish, and ring
sticking
VW D1 Four-cylinder direct-injection engine Piston deposits, viscosity increase, and
ring sticking
Peugeot XUD11ATE Four-cylinder indirect-injection engine Piston deposits and viscosity increase
Peugeot XUD11BTE Four-cylinder indirect-injection engine Piston deposits and viscosity increase
M-B OM 602A Five-cylinder indirect-injection engine Engine wear and cleanliness
M-B OM 364A/LA Four-cylinder direct-injection engine Bore polishing, piston deposits,
varnish, sludge, wear, and oil
consumption
M-B OM 441LA Six-cylinder direct-injection engine Piston deposits, bore polishing, wear,
oil consumption, valve train condition,
and turbo deposits
MAN 5305 Single-cylinder engine Piston deposits, bore polishing, and oil
consumption
Mack T-8 Multicylinder engine Soot-related oil thickening