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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.

3. 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 ful 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 modi 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 ole 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 NO

is more prevalent in diesel

•

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Dispersants 145

engines and gasoline engines that are subjected to severe service, such as long-distance driving for

extended periods. The NO

formation initiates when the temperature reaches 137°C [10,11]. Zinc

dialkyldithiophosphates are commonly used as oxidation inhibitors in engine oils [12,13]. All these

acids are neutralized by basic detergents to form inorganic metal salts and metal carboxylates. These

compounds are of low hydrocarbon solubility and are likely to fall out of solution.

The aldehydes and ketones undergo aldol-type condensation in the presence of bases or acids to

form oligomeric or polymeric compounds. These can further oxidize to highly oxygenated hydrocar-

bons, commonly referred to as oxygenates. The oxygenates are usually of sticky consistency, and the

term resin is often used to describe them [14]. Resin is either the basic component in or the precur-

sor to all types of deposits. Common types of deposits include varnish, lacquer, carbon, and sludge

[15,16]. Varnish, lacquer, and carbon occur when resin separates on hot surfaces and dehydrates or

polymerizes to make tenacious  lms. The quantity and the nature of deposits depend on the proxim-

ity of the engine parts to the combustion chamber. The parts closer to the combustion chamber, such

as exhaust valve head and stem that experience approximate temperatures of 630–730°C [17,18], will

develop carbon deposits. The same is true of the combustion chamber wall, piston crown, top land,

and top groove, which are exposed to approximate temperatures of 200–300°C. Carbon deposits are

more common in diesel engines than in gasoline engines and result from the burning of the liquid

lubricating oil and the high-boiling fractions of the fuel that adhere to hot surfaces [19].

As we move away from these regions to the low-temperature regions, such as the piston skirt,

the deposits are not heavy and form only a thin  lm. For diesel engine pistons, this type of deposit

is referred to as lacquer; for gasoline engine pistons, this type of deposit is called varnish. The

difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely

fuel-derived. In addition, the two differ in their solubility characteristics. That is, lacquer is water-

soluble and varnish is acetone-soluble [15]. Lacquer usually occurs on piston skirts, on cylinder

walls, and in the combustion chamber, whereas varnish occurs on valve lifters, piston rings, piston

skirts, valve covers, and positive crankcase ventilation (PCV) valves.

The coolest parts of the engine, such as rocker arm covers, oil screen, and oil pan, that are

exposed to temperatures of ≤200°C experience sludge deposits. Sludge can be watery or hard in

consistency, depending on the severity of service. If the service is extremely mild and of short

duration, as in the case of stop-and-go gasoline engine operation, the sludge is likely to be watery

or mayonnaiselike [15]. This type of sludge is called low-temperature sludge, which occurs when

the ambient temperature is <95°C. The high-temperature sludge is more common in diesel engines

and gasoline engines with long, continuous operation. This type of sludge occurs when the ambient

temperature is >120°C and is hard in consistency. In the former case, the engine does not get hot

enough to expel combustion water, which stays mixed with oil, imparting sludge, a mayonnaiselike

appearance. In the latter case, however, the ambient temperature is high enough to expel water,

thereby resulting in hard sludge. Sludge is common in areas that experience low oil  ow, such as

crankcase bottoms and rocker boxes.

Another component of the combustion ef uent that must be considered is soot. Soot not only

contributes toward some types of deposits such as carbon and sludge, but it also leads to a viscosity

increase. These factors can cause poor lubricant circulation and lubricating  lm formation, both of

which will result in wear and catastrophic failure. Soot is particulate in nature and results from the

incomplete combustion of the fuel and of the lubricating oil from the crankcase that might enter the

combustion chamber by traveling past the piston rings [20]. Fuel-derived soot is a chronic problem

in the case of diesel engines because diesel fuel contains high-boiling components that do not burn

easily. In addition, diesel engine combustion is largely heterogeneous, with poor air–fuel mixing,

hence poor combustion [20]. Soot is made of hydrocarbon fragments with some of the hydrogen

atoms removed. The particles are charged and hence have the tendency to form aggregates. When

aggregates occur on surfaces, such as those of the combustion chamber, soot deposits result. These

deposits are soft and  aky in texture. If these occur in oil, lubricant experiences an increase in vis-

cosity. A soot-related viscosity increase usually requires the presence of polar materials in oil that

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146 Lubricant Additives: Chemistry and Applications

have the ability to associate with soot. These can be additives or polar lubricant oxidation and deg-

radation products. Carbon deposits are lower in carbon content than soot and, in most cases, contain

oily material and ash. This makes knowledge of the ash-forming tendency of a lubricant important

to a formulator. This concern was addressed in Chapter 4.

When soot associates with resin, one gets either resin-coated soot particles or soot-coated resin

particles [16]. The  rst type of particles results when resin is in excess, and the second type results

when soot is in excess. The amount of soot in resin determines the color of the deposits: the higher

the soot, the darker the deposits. Sludge results when resin, soot, oil, and water mix [9].

Deposit formation in gasoline engines is initiated by NO

and oxidation-derived hydroperoxides

that react with hydrocarbons in the fuel and the lubricant to form organic nitrates and oxygenates

[14,21]. Being thermally unstable, these species decompose and polymerize to form deposits. The

deposits typically include resin, varnish, and low-temperature sludge. In diesel engines, however,

soot is an important component of the deposits, which include lacquer, carbon deposits, and high-

temperature sludge [16]. Typically, carbon deposits are of high metal content, which is mainly due

to the presence of detergent additives in the lubricant [22,23].

Detailed mechanism of deposit formation in engines is described elsewhere [24,25]. The mecha-

nism is based on the premise that both the lubricant and the fuel contribute toward deposit forma-

tion. The role of the blowby, NO

, and high-temperature oxidative and thermal degradation of the

lubricant, described earlier, are substantiated [24]. The importance of oxygenated precursors—their

decomposition, condensation, and polymerization to form deposits—is also supported. The deposit

precursors consist of approximately 15–50 carbon atoms and contain multiple hydroxy and carboxy

functional groups. Because of the polyfunctionality, these molecules have the ability to thermally

polymerize to high-molecular-weight products [14,16]. As mentioned earlier, soot associates with

polar oxidation products in oil to cause a viscosity increase. Viscosity increase can also occur in

gasoline engine oils that have little or no soot. This happens when the oxygen content of the precur-

sors is low and the resulting polymer is of low molecular weight and of good oil solubility [14]. This

phenomenon is commonly referred to as oil thickening [6]. Conversely, if the oxygen content of the

precursors is high, the polymerization results in the formation of high-molecular-weight products of

low lubricant solubility. Such products constitute resin, which is of low oil solubility and separates on

surfaces. If the surfaces are hot, subsequent dehydration and polymerization lead to the formation of

varnish, lacquer, and carbon deposits. It is important to note that deposits are a consequence of lubri-

cant oxidation that accelerates once the oxidation inhibitor package in the lubricant is exhausted.

Three other internal combustion engine problems—oil consumption, ring sticking, and cor-

rosion and wear—are also related to lubricant degradation. Oil consumption is a measure of how

much lubricant travels past piston rings into the combustion chamber and burns. A certain minimum

amount of the lubricant is necessary in the vicinity of the piston rings to lubricate cylinder walls and

cylinder liners and hence facilitate piston movement and minimize scuf ng. However, if too much

lubricant ends up in the combustion chamber, serious emission problems will result. Modern piston

designs, such as articulated pistons and pistons with low crevice volume, allow just enough lubricant

to minimize scuf ng, but without adversely contributing to emissions [26,27]. Other parameters

that affect oil consumption include the integrity of pistons and cylinders and the viscosity, volatility,

and sealing characteristics of the lubricant. Pistons with stuck rings and out-of-square grooves and

cylinders with increased wear will result in a poor seal between the crankcase and the combustion

chamber [15]. As a consequence, a larger amount of blowby will enter the crankcase and increase the

rate of lubricant breakdown. This will complicate the situation further. Ring sticking occurs when

sticky deposits form in the grooves behind the piston rings. This is a serious problem because it not

only results in a poor seal but also leads to poor heat transfer from the cylinder to the wall. If not con-

trolled, this will result in nonuniform thermal expansion of the pistons, loss of compression, and ulti-

mately the failure of the engine [15]. The wear of pistons and the cylinders is undesired for the same

reasons. Wear of engine parts is either corrosive or abrasive. Corrosive wear arises from the attack

of fuel sulfur-derived products, such as sulfur oxides or sulfuric acid, or the acidic by-products of

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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 functionality

Polar moiety

Connecting group

Hydrocarbon group

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148 Lubricant Additives: Chemistry and Applications

in nature, and depending on its molecular weight, dispersants can be classi ed into polymeric

dispersants and dispersant polymers. Polymeric dispersants are of lower molecular weight than

dispersant polymers. The molecular weight of polymeric dispersants ranges between 3,000 and

7,000 as compared to dispersant polymers, which have a molecular weight of 25,000 and higher.

Although various ole ns, such as polyisobutylene, polypropylene, polyalphaole ns, and mixtures

thereof, can be used to make polymeric dispersants, the polyisobutylene-derived dispersants are the

most common. The number average molecular weight (Mn) of polyisobutylene ranges between 500

and 3000, with an Mn of 1000–2000 being typical [29]. In addition to Mn, other polyisobutylene

parameters, such as molecular weight distribution and the length and degree of branching, are also

important in determining the overall effectiveness of a dispersant.

Substances obtained through a polymerization reaction, especially those made by using an acid

catalyst or a free-radical initiator, often contain molecules of different sizes. Molecular weight

distribution, or polydispersity index, is commonly used to assess the heterogeneity in molecular

size. Polydispersity index is the ratio of weight average molecular weight (Mw) and Mn, or Mw/Mn

[30–32]. These molecular weights are determined by subjecting the polymer to gel permeation

chromatography (GPC). The method separates molecules based on size [33]. The larger molecules

come out  rst, followed by the next size. When the molecules are of the same size, Mw/Mn equals

1 and the polymer is called a monodisperse polymer. The polymers with an index >1 are called

polydisperse polymers. For most applications, monodispersity is desired. Polyisobutylene, derived

from acid-catalyzed polymerization reaction, typically has a polydispersity index between 2 and 3.

This will impact many of the dispersant properties described below.

Dispersant polymers, also called dispersant viscosity modi ers (DVMs) and dispersant viscos-

ity index improvers (DVIIs), are derived from hydrocarbon polymers of molecular weights between

25,000 and 500,000. Polymer substrates used to make DVMs include high-molecular-weight ole n

copolymers (OCPs), such as ethylene–propylene copolymers (EPRs), ethylene–propylene–diene

copolymers (EPDMs), polymethacrylates (PMAs), styrene–diene rubbers (SDRs) of both linear and

star con gurations, and styrene–ester polymers (SEs).

The polar group is usually nitrogen- or oxygen-derived. Nitrogen-based groups are derived

from amines and are usually basic in character. Oxygen-based groups are alcohol-derived and are

neutral. The amines commonly used to synthesize dispersants are polyalkylene polyamines such

as diethylenetriamine and triethylenetetramine. In the case of DVMs or dispersant polymers, the

polar group is introduced by direct grafting, copolymerization, or by introducing a reactable func-

tionality. The compounds used for this purpose include monomers such as 2- or 4-vinylpyridine,

N-vinylpyrrolidinone, and N,N-dialkylaminoalkyl acrylate and unsaturated anhydrides and acids

such as maleic anhydride, acrylic acid, and glyoxylic acid. The details of these reactions are

described in Section 5.6, which deals with the dispersant synthesis. Amine-derived dispersants are

called nitrogen or amine dispersants, and those that are alcohol-derived are called oxygen or ester

dispersants [28]. Oxygen-derived phosphonate ester dispersants were popular at one time, but their

use in engine oils is now restrained because of the phosphorus limit. Phosphorus limit pertains

to its tendency to poison noble metal catalysts used in catalytic converters. Formulators prefer to

take advantage of the phosphorus limit by using zinc dialkyldithiophosphates, which are excellent

oxidation inhibitors and antiwear agents. In the case of amine dispersants, it is customary to leave

some of the amino groups unreacted to impart basicity to the dispersant. The reasons for this are

described in Section 5.7.

5.6 DISPERSANT SYNTHESIS

Since it is not easy to attach the polar group directly to the hydrocarbon group, except in the case

of ole ns that are used to make DVMs, the need for a connecting group or a link arises. Although

many such groups can be used, the two common ones are phenol and succinic anhydride. Ole n, such

as polyisobutylene, is reacted either with phenol to form an alkylphenol or with maleic anhydride

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Dispersants 149

to form an alkenylsuccinic anhydride. The polar functionality is then introduced by reacting these

substrates with appropriate reagents.

5.6.1 THE HYDROCARBON GROUP

Polyisobutylene is the most common source of the hydrocarbon group in polymeric dispersants. It is

manufactured through acid-catalyzed polymerization of isobutylene [34,35]. Figure 5.2 depicts the

mechanism of its formation. In Figure 5.2, polyisobutylene is shown as a terminal ole n, whereas

in reality it is a mixture of various isomers. Those that predominate include geminally disubstituted

(vinylidene), trisubstituted, and tetrasubstituted ole ns. Figure 5.3 shows their structure and the pos-

sible mechanism of their formation. Polyisobutylenes of structures I and II result from the loss of a

proton from carbon 1 and carbon 3 of the intermediate of structure V. Polyisobutylenes of structures

III and IV result from the rearrangement of the initially formed carbocation, as shown in Figure 5.3.

The reactivity of these ole ns toward phenol and maleic anhydride varies. In general, the more sub-

stituted the ole n, the lower the reactivity, which is a consequence of the steric factors. Similarly,

the larger the size of the polyisobutyl pendant group, that is, the higher the molecular weight, the

lower the reactivity. This is due to the dilution effect, which results from low ole n-to-hydrocarbon

ratio. As mentioned earlier, polyisobutylene is the most commonly used ole n. One of the reasons

for its preference is its extensive branching. This makes the derived dispersants to possess excellent

oil solubility, in both nonassociated and associated forms. However, if the hydrocarbon chain in the

dispersant is too small, its lubricant solubility greatly suffers. Because of this, the low-molecular-

weight components in polyisobutylene are not desired. This is despite their higher reactivity. These

must be removed, which is carried out through distillation. Alternatively, one can minimize the

formation of these components by decreasing the amount of the catalyst during polymerization and

by lowering the polymerization reaction temperature.

A new class of dispersants derived from ethylene/α-OCP with an Mn of 300–20,000 has also

been reported, primarily by the Exxon scientists [36,37]. Such dispersants are claimed to have supe-

rior low- and high-temperature viscometrics than those of the polyisobutylene-derived materials.

As mentioned earlier, dispersant polymers are derived from EPRs, styrene–butadiene copoly-

mers, polyacrylates, PMAs, and styrene esters. The ethylene–propylene rubbers are synthesized

by Ziegler–Natta catalysis [38]. The styrene–butadiene rubbers are synthesized through anionic

polymerization [38]. Polyacrylates and PMAs are synthesized through polymerization of the

monomers using free-radical initiators [38]. Styrene esters are made by reacting styrene–maleic

Isobutylene

R = polyisobutyl

− H

FIGURE 5.2 Acid-catalyzed polymerization of isobutylene.

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150 Lubricant Additives: Chemistry and Applications

anhydride copolymer or styrene–maleic anhydride–alkyl acrylate terpolymer with alcohols, usually

in the presence of a protic acid, such as sulfuric or methanesulfonic acid, catalyst. Since complete

esteri cation of the anhydride is hard to achieve, the neutralization of the residual carboxylic acid

anhydride is carried out by alternative means [38–40].

FIGURE 5.3 Polyisobutylene structures and the mode of their formation.

Terminal olefin (vinylidene)

IX VIII III

Trisubstituted olefin

Tetrasubstituted olefin

VI VII

Trisubstituted olefin

Tetrasubstituted olefin

Carbocation intermediate

Trisubstituted olefin

R = polyisobutyl

II III

3 CH

to C

hydride

transfer

to C

methide

transfer

to C

methide

transfer

to C

hydride

transfer

proton

loss

Not shown

by arrow

proton

loss

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Dispersants 151

5.6.2 THE CONNECTING GROUP

As mentioned in Section 5.5, succinimide, phenol, and phosphonate are the common connecting

groups used to make dispersants. Of these, succinimide and phenol are the most prevalent [2].

Succinimide group results when a cyclic carboxylic acid anhydride is reacted with a primary amino

group. Alkenylsuccinic anhydride is the precursor for introducing the succinimide connecting

group in dispersants. Alkenylsuccinic anhydride is synthesized by reacting an ole n, such as

polyisobutylene, with maleic anhydride [2]. This is shown in Figure 5.4.

The reaction is carried out either thermally [29,41,42] or in the presence of chlorine [43]. The

thermal process involves heating the two reactants together usually >200°C [29,41,42], whereas the

chlorine-mediated reaction with a mixture is carried out by introducing chlorine to react contain-

ing polyisobutylene and maleic anhydride [43–48]. Depending on the manner in which chlorine is

added, the procedure is either one-step or two-step [44]. If chlorine is  rst reacted with polyisobu-

tylene before adding maleic anhydride, the procedure is considered two-step. If chlorine is added

to a mixture of polyisobutylene and maleic anhydride, it is a one-step procedure. The one-step

procedure is generally preferred.

The chlorine-mediated process has several advantages, which include having a low reaction

temperature, having a faster reaction rate, and working well with internalized or highly substi-

tuted ole ns. The low reaction temperature minimizes the chances of thermal breakdown of poly-

isobutylene and saves energy. The major drawback of the chlorine process is that the resulting

dispersants contain residual chlorine as organic chlorides. Their presence in the environment is

becoming a concern because they can lead to the formation of carcinogenic dioxins. A number

of strategies are reported in the literature to decrease the chlorine content in dispersants [49–54].

The thermal process does not suffer from the presence of chlorine, although it is less energy-ef -

cient and requires the use of predominantly a terminal ole n, that is, the polyisobutylene of high

vinylidene content.

The mechanism by which the two processes proceed is also different [46,47,50–52]. The ther-

mal process is postulated to occur through an ene reaction. The chlorine-mediated reaction is postu-

lated to proceed through a Diels–Alder reaction. The mechanism of the diene formation is shown in

Figure 5.5. Chlorine  rst reacts with polyisobutylene 1 to form allylic chloride II. By the loss of the

chloride radical, this yields the intermediate III, which through C

to C

methyl radical transfer is

converted into the intermediate IV. A C

to C

hydrogen shift in the intermediate results in the for-

mation of the radical V. This radical can lose hydrogen either from C

to yield the diene VI or from

to result in the diene VII. The resulting dienes then react with maleic anhydride through a 4 + 2

addition reaction, commonly called a Diels–Alder reaction [55], to form alkenyltetrahydrophthalic

anhydrides [50,52]. These reactions are shown in Figure 5.6.

These anhydrides can be converted into phthalic anhydrides through dehydrogenation by

using sulfur [50–52]. These compounds can then be transformed into dispersants by reacting with

polyamines and polyhydric alcohols [51,52]. During the thermal reaction of polyisobutylene with

maleic anhydride, that is, the ene reaction, the vinylidene double bond moves down the chain to

the next carbon. Since thermal reaction requires a terminal ole n, further reaction of the new

ole n with another mole of maleic anhydride will not occur if the double bond internalizes, and

Polyisobutylene

Polyisobutenyl

Maleic anhydride

Polyisobutenylsuccinic

anhydride

Heat

or CI

FIGURE 5.4 Alkenylsuccinic anhydride formation.

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152 Lubricant Additives: Chemistry and Applications

the reaction will stop at this stage. This is shown in reaction 5.3 of Figure 5.6. If the new double

bond is external, the reaction with another molecule of maleic anhydride is possible [45]. This is

shown in reaction 5.4.

For dispersants, polyisobutylphenol is the alkylphenol of choice. It is synthesized by reacting

polyisobutylenes with phenol in the presence of an acid catalyst [56–58]. Lewis acid catalysts, such

as aluminum chloride and boron tri uoride, are often employed. Boron tri uoride is preferred over

aluminum chloride because the reaction can be carried out at low temperatures, which minimizes

acid-mediated breakdown of polyisobutylene [58]. This is desired because dispersants derived from

low-molecular-weight phenols are not very effective. Other catalysts, such as sulfuric acid, meth-

anesulfonic acid, and porous acid catalysts of Amberlyst

type, can also be used to make alkylphe-

nols [59,60]. Polyisobutylene also reacts with phosphorus pentasul de through an ene reaction, as

described in Chapter 4. The resulting adduct is hydrolyzed by the use of steam to alkenylphosphonic

and alkenylthiophosphonic acids [2,3]. The methods to synthesize alkylphenols and alkenylphos-

phonic acids are shown in Figure 5.7.

A new carboxylate moiety derived from glyoxylic acid to make dispersants has been reported in

the literature [61–65]. However, at present, no commercial products appear to be based on this

chemistry.

5.6.3 THE POLAR MOIETY

The two common polar moieties in dispersants are based on polyamines and polyhydric alco-

hols. The structures of common amines and alcohols used to make dispersants are shown in

Figure 5.8.

The polyamines are manufactured from ethylene through chlorination, followed by the

reaction with ammonia [66]. The reaction scheme is given in Figure 5.9. As shown, polyamines

+ CI

−CI

•

−H

•

−H

•

(From C

)

CI H

•

III

•

transfe

•

transfer

(From C

)

•

VII

•

FIGURE 5.5 Mechanism of chlorine-assisted diene formation.

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Dispersants 153

contain piperazines as a by-product. Examining the structures of various amines, one can see

that they contain primary, secondary, and tertiary amino groups. Each type of amino group has

different reactivity toward alkenylsuccinic anhydride. The primary amino group reacts with

the anhydride to form a cyclic imide, the secondary amino group reacts with the anhydride to

form an amide/carboxylic acid, and the tertiary amino group does not react with the anhydride

at all [67].

However, it can make a salt if a free carboxylic acid functionality is present in the molecule,

as is the case in amide/carboxylic acid. These reactions are shown in Figure 5.10. New high-

molecular-weight amines derived from phosphoric acid–catalyzed condensation of polyhydroxy

compounds, such as pentaerythritol, and polyalkylene polyamines, such as triethylenetetramine,

are known [68]. These amines are claimed to form high total base number (TBN) dispersants with

Diels−Alder reaction

Ene reaction

C CH

(5.1)

(5.2)

(5.3)

(5.4)

FIGURE 5.6 Mechanism of alkenylsuccinic anhydride formation.

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