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154 Lubricant Additives: Chemistry and Applications

low free-amine content and better engine test performance than dispersants made from conven-

tional polyamines.

Imide and ester dispersants are made by reacting polyamines and polyhydric alcohols with

alkenylsuccinic anhydrides. The reaction typically requires a reaction temperature between 130

and 200°C to remove the resulting water and complete the reaction [44]. As mentioned earlier,

imide dispersants are made by the use of polyalkylene polyamines, such as diethylenetriamine

and triethylenetetramine. Many polyhydric alcohols can be used to make ester dispersants.

These include trimethylolpropane, tris(hydroxymethyl)aminoethane, and pentaerythritol. When

one uses tris(hydroxymethyl)aminoethane as the alcohol, one can obtain an ester dispersant

with basicity. The reactions to make succinimide and succinate dispersants are depicted in

Figure 5.11.

The alkylphenol-derived dispersants are made by reacting an alkylphenol, such as polyisobutyl-

phenol, with formaldehyde and a polyamine [58,69]. The result is the formation of 2-aminomethyl-

4-polyisobutylphenol. The reaction of ammonia or an amine, formaldehyde, and a compound with

active hydrogen(s), such as a phenol, is called the Mannich reaction [70,71]. Hence, such disper-

sants are called Mannich dispersants. For making phosphonate dispersants, the common method

is to react the free acid with an ole n epoxide, such as propylene oxide or butylene oxide, or an

amine [2,72,73]. These reactions are shown in Figure 5.12. Salts derived from the direct reaction

of amine and metal bases with ole n-phosphorus pentasul de adduct are also known [74,75]. It

is important to note that structures in  gures are idealized structures. The actual structures will

depend on the substrate (alkylphenol and alkenylsuccinic anhydride)-to-reactant (formaldehyde

and polyamines) ratio.

Because of the polyfunctionality of the succinic anhydride group and of the amines and

polyhydric alcohols, various dispersants can be made by altering the anhydride-to-amine or

anhydride-to-alcohol ratios. These dispersants differ not only in their molecular weight but

also in their properties. Polyfunctionality of the two reactants leads to dispersants, which have

molecular weights that are three to seven times higher than expected if the two reactants were

monofunctional.

OH OH

Polyisobutylene

Polyisobutylene + P

Phenol

Polyisobutylphenol

Polyisobutenyl

Pol

isobuten

lthio

hos

honic and

isobuten

hos

honic acids

or polyisobutenyl

Acid

Adduct

Phosphorus

pentasulfide

FIGURE 5.7 Synthesis of alkylphenols and alkenylphosphonic acids.

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Dispersants 155

The methods to make DVMs are shown in Figures 5.13 through 5.15. These are synthesized by

Grafting or reacting of a dispersancy-imparting monomer on an already-formed polymer,

as in the case of EPRs and SDRs [76–84].

Including such a monomer during the polymerization process, as in the case of polyacry-

lates and PMAs [85].

Introducing a reactive functional group in the polymer that can be reacted with a reagent to

impart dispersancy, as in the case of styrene–maleic anhydride copolymers [40,86–93].

Although most of the examples in Figures 5.13 through 5.15 pertain to the introduction of the basic

nitrogen-containing moieties, neutral DVMs are also known in the literature. These are made by

using nonbasic reactants, such as N-vinylpyrrolidinone, alcohols, or polyether-derived methacry-

late ester [79,94,95]. Recently, dispersant viscosity–improving additives with built-in oxidation

•

Diamines

Ethylenediamine

Triamines

Tetramines

Triethylenetetramine

Aminoethylaminoethylpiperazine

N,N-dimethylaminopropylamine

Primary amino group (1°)

Secondary amino group (2°)

Tertiary amino group (3°)

1°

3° 2°

2°

3°

2°

3°

2°

1°

3°

Diethylenetriamine Aminoethylpiperazine

Bis(aminoethyl) piperazine

Alcohols

Pentaerythritol

Trimethylolpropane

Tris

(

drox

meth

ane

Tris(hydroxymethyl)aminoethane

FIGURE 5.8 Amines and alcohols used to synthesize dispersants.

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156 Lubricant Additives: Chemistry and Applications

FIGURE 5.9 Manufacture of polyamines.

C=CH

+ CI

CICH

CI CICH

CICH

NHCH

Ethylene

Ethylenedichloride

Chloroethylamine

Ethylenediamine

Diethylenetriamine

Aminoethylpiperazine

FIGURE 5.10 Amine–anhydride reaction products. (Based on Harrison, J.J., Ruhe, R., Jr., William, R., U.S.

Patent 5,625,004, April 29, 1997.)

(a) Primary amine

(b) Secondary amine

Polyisobutenyl

+ RNH

+ R

Polyisobutenyl

Imide

Amide

Salt

O NHR

− +

inhibiting and antiwear moieties have been reported in the patent literature [77,96,97]. Dispersant

polymers containing oxidation-inhibiting moieties are commercially available from Texaco Chemi-

cal Company now part of Ethyl Petroleum Additives Company. As the examples show, grafting

usually allows the introduction of the connecting group in the dispersant polymers at the same time

as the polar moiety.

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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  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 af 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 af 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 ef ciency and

FIGURE 5.11 Synthesis of imide and ester dispersants.

PIB

Polyisobutenylsuccinimide

Polyalkylenepolyamine

Polyisobutenylsuccinic

anhydride

Polyisobutenylsuccinic

anhydride

Polyisobutenylsuccinate ester

Polyhydric

alcohol

PIB = Polyisobutenyl

R′

HOH

PIB

R′

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158 Lubricant Additives: Chemistry and Applications

in some cases good thermal and oxidative stability, their use is advantageous. They usually replace

additives, called viscosity modi ers, in the package. Since they impart some dispersancy because

of their structure, the amount of polymeric dispersant in engine oil formulations is somewhat

decreased [79,98].

Both the connecting group and the polar moiety are important to the dispersancy of the disper-

sant molecule. They must be considered together since both contribute toward polarity. In Mannich

dispersants, the phenol functional group, and in imide and ester dispersants, succinimide, succinate,

and phosphonate functional groups are also polar, the same as the amine and the alcohol-derived

portion of the molecule. The polarity is a consequence of the electronegativity difference between

carbon, oxygen, nitrogen, and phosphorus atoms. The greater the electronegativity difference, the

stronger the polarity. This implies that groups that contain phosphorus–oxygen bonds are more

polar than those containing carbon–oxygen bonds, carbon–nitrogen bonds, and carbon–phosphorus

bonds. The electronegativity difference for such bonds is 1.4, 1.0, 0.5, and 0.4, respectively [99].

However, since dispersants have many bonds with various combinations of atoms, the overall

polarity in a dispersant and its ability to associate with polar materials are not easy to predict.

FIGURE 5.12 Synthesis of Mannich and phosphonate dispersants.

Polyisobutylphenol

Polyalkylenepolyamine

Polyaminomethylpolyisobutylphenols

PIB

Propylene

oxide

Polyisobutenylthiophosphonic acid

PIB

OCH

Bis-hydroxypropyl

polyisobutenylthiophosphonate

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Dispersants 159

4-Vinylpyridine

Ethylene − propylene copolymer

Radical

initiator

n −1

Dis

ersant olefin co

mer

(

DOCP

)

FIGURE 5.13 Dispersant viscosity modi er synthesis through grafting.

COOR

n−x

C C OCH

C COOR

Dimethylaminoethyl

methacrylate

Radical

initiator

Alkyl methacrylate

OCH

Pol

acr

late-T

e Dis

ersant Viscosit

Modifier

COOR

FIGURE 5.14 Dispersant viscosity modi er synthesis through copolymerization.

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160 Lubricant Additives: Chemistry and Applications

Because some of the materials with which the dispersant associates are acidic, such as carboxylic

acids derived from lubricant oxidation, the presence of an amine nitrogen is an advantage because

of its basic character. Therefore, in certain gasoline engine tests, nitrogen dispersants are superior to

ester dispersants. Ester dispersants are usually superior in diesel engine tests because of their higher

thermo-oxidative stability. Mannich dispersants are good low-temperature dispersants; hence, they

are typically used in gasoline engine oils.

As mentioned earlier, commercial polyisobutylenes have a molecular weight distribution.

This will lead to dispersant structures of varying size, hence molecular weight. An optimum ratio

between the molecular weight of the hydrocarbon chain and that of the polar functionality (polar/

nonpolar ratio) is a prerequisite for good dispersancy. If a dispersant composition has an exces-

sive amount of components with short hydrocarbon chains, that is, of low molecular weight, its

associating ability increases, but its oil solubility suffers. This is likely to deteriorate its disper-

sancy, especially after associating with polar impurities. Such structures in dispersants are, there-

fore, undesired. Their formation can be minimized by using polyole ns of low polydispersity

index, controlling the formation of low-molecular-weight components, removing such components

through distillation [100], or postreacting with another reagent, preferably of the hydrocarbon type.

Polyole ns of low polydispersity index (≤2) are available from BP and Exxon Chemical Company.

Controlling the formation of low-molecular-weight components is exempli ed by the use of boron

tri uoride catalyst for making alkylphenols instead of aluminum chloride, which tends to fragment

polyisobutylene. Removing the lower-molecular-weight components, although not easy, is possible

at the precursor stage, which is before reacting with the alcohol or the amine. A number of reagents

can be used for the postreaction [101]. Hydrocarbon posttreatment agents include polyepoxides

[102], polycarboxylic acid [103], alkylbenzenesulfonic acids [104], and alkenylnitriles [105].

Whenever postreacted dispersants are used in engine oils, improved dispersancy, viscosity index

credit, improved  uorocarbon elastomer compatibility, hydrolytic stability, and shear stability are

often claimed.

5.7.2 THERMAL AND OXIDATIVE STABILITY

All the three components of the dispersant structure determine its thermal and oxidative stability,

the same as dispersancy. The hydrocarbon group can oxidize in the same manner as the lubricant

hydrocarbons to form oxidation products that can contribute toward deposit-forming species [4,9].

(This is described in Section 5.2.) Although the rate of oxidation is quite slow for largely paraf nic

hydrocarbon groups, such as polyisobutyl group, it is quite high for those that contain multiple bonds,

such as polyisobutenyl, and the benzylic groups. The benzylic functional group is present in styrene

FIGURE 5.15 Dispersant viscosity modi er synthesis through chemical reaction.

Styrene−maleic

anhydride polymer

ROH

R′

Styrene ester−based

dispersant viscosity

modifier

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Dispersants 161

butadiene and styrene ester–derived dispersant polymers. Purely paraf nic hydrocarbon groups that

contain tertiary hydrogen atoms, such as EPRs, oxidize at a faster rate than those that contain only

primary and secondary hydrogen atoms. Styrene isoprene–derived materials contain both benzylic

and tertiary hydrogen atoms. This implies that highly branched alkyl groups, such as polyisobutyl

and polyisobutenyl, have a higher susceptibility toward oxidation than linear or unbranched alkyl

groups. Dispersant polymers with built-in oxidation-inhibiting moieties are known in the literature

[77,78,96]. The polar moiety in an amine-derived dispersant is also likely to oxidize at a faster rate

than the oxygen-derived moiety because of the facile formation of the amine oxide functional group on

oxidation. Such groups are known to thermally undergo β-elimination [40], called the cope reaction, to

form an ole n. This can oxidize at a faster rate as well as lead to deposit-forming polymeric products.

From a thermal stability perspective, the hydrocarbon group in the case of high-molecular-

weight dispersant polymers, such as those derived from OCPs, is more likely to break down (unzip)

than that derived from the low-molecular-weight polymers. Dispersants based on 1000–2000

molecular weight polyisobutylenes are relatively stable, except at very high temperatures that are

experienced in some engine parts, such as near the top of the piston [17,18]. Thermal breakdown of

the oxidized amine polar group is mentioned in the previous paragraph.

The chemical reactivity of certain dispersants toward water and other reactive chemicals pres-

ent in the lubricant formulation is an additional concern. The most likely reaction site is the con-

necting group. The common connecting groups are amide and imide in amine-derived dispersants

and ester in alcohol-derived dispersants. All three can hydrolyze in the presence of water [106], but

at different rates. Esters are easier to hydrolyze than amides and imides. The hydrolysis is facilitated

by the presence of bases and acids. Basic detergents are the source of the metal carbonate and metal

hydroxide bases, which at high temperatures catalyze the hydrolysis reaction. Additives, such as

zinc dialkyldithiophosphates, are a source of strong acids that result when these additives hydro-

lyze, thermally decompose, or oxidize. The fate of the ester-, amide-, and imide-type dispersant

polymers, such as those derived from polyacrylates, PMAs, and styrene ester substrates, is the same.

Some OCP-derived dispersant polymers, such as those obtained by grafting of monomers 2- or

4-vinylpyridine and 1-vinyl-2-pyrrolidinone [76,80], do not suffer from this problem since they do

not contain easily hydrolyzable groups. Reactivity toward other chemicals present in the formula-

tion is again prevalent in the case of ester-derived dispersants. Reaction with metal-containing addi-

tives, such as detergents and zinc dialkyldithiophosphates, can occur after hydrolysis to form metal

salts. This can destroy the polymeric structure of a dispersant and hence its effectiveness. Some

formulations contain amines or their salts as corrosion inhibitors or friction modi ers. Depending

on the molecular weight and the ambient temperature, these can displace the polyol or sometimes

the polyamine, thereby altering the dispersant structure, hence its properties.

5.7.3 VISCOSITY CHARACTERISTICS

The amount of dispersant in automotive engine oils typically ranges between 3 and 7% by weight

[79], making it the highest among additives. In addition, dispersant is the highest molecular-weight

component except the viscosity improver [107]. Both of these factors can alter some physical prop-

erties, such as viscosity, of the lubricant. A boost in the viscosity of a lubricant at high temperatures

is desired, but at low temperatures it is a disadvantage. At high temperatures, the lubricant loses

some of its viscosity [108], hence its  lm-forming ability, resulting in poor lubrication. Maintaining

good high-temperature viscosity of a lubricant is therefore imperative to minimize wear damage.

This is usually achieved by the use of polymeric viscosity modi ers [3,109]. Some dispersants,

especially those that are based on high-molecular-weight polyole ns and have been oversuccinated

partly ful ll this need [44]. Therefore, the amount of polymeric viscosity modi er necessary to

achieve speci c high-temperature viscosity is reduced. Unfortunately, dispersants that provide a

viscosity advantage lead to a viscosity increase at low temperatures as well. The low-temperature

viscosity requirements for engine oils have two components: cranking viscosity and pumping

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162 Lubricant Additives: Chemistry and Applications

viscosity [110]. Cranking viscosity is an indication of how easily the engine will turn over in

extremely cold weather conditions. Pumping viscosity is the ability of the lubricant to be pumped

to reach various parts of the engine. For cold weather operation, low to moderate cranking and

pumping viscosities are highly desirable. Although pumping viscosity and the pour point can be

lowered by the use of additives, called pour point depressants [3,13], lowering cranking viscosity

is not easy. In the case of base oils, this is usually achieved by blending carefully selected base

stocks. An ideal polymeric dispersant must provide high-temperature viscosity advantage with-

out adversely affecting the cold-cranking viscosity of the lubricant. Dispersant polymers have

the same requirement. Good high-temperature viscosity to cranking viscosity ratio in polymeric

dispersants can be achieved by

Carefully balancing the type and the molecular weight of the hydrocarbon chain [111]

Choosing the optimum ole n to maleic anhydride molar ratio [112]

Selecting the type and the amount of the polyamine used

In dispersant polymers this can be achieved by selecting (1) a polymer of correct molecular weight

and branching and (2) a suitable pendant group. Dispersant polymers derived from medium-

molecular-weight, highly branched structures, and ester-type pendant groups are best suited for use

as additives. Examples include polyacrylate, PMA, and styrene ester–derived dispersants. These

additives not only act as viscosity modi ers and dispersants but also act as pour point depressants,

thereby improving the low-temperature properties of the lubricant.

A number of patents pertaining to dispersants with balanced high-temperature viscos-

ity and low-temperature properties are reported in the patent literature [113–117]. A Mannich

(alkylphenol) dispersant, derived from ethylene/1-butene polymers of Mn 1500–7500, has been

claimed to possess improved dispersancy and pour point [113]. Another patent claiming the syn-

thesis of a dispersant with superior dispersancy and pour point depressing properties has also

been issued [114]. The dispersant is based on the reaction of maleic anhydride/lauryl methacry-

late/stearyl methacrylate terpolymer with dimethylaminopropylamine, and a Mannich base was

obtained by reacting N- aminoethylpiperazine, paraformaldehyde, and 2,6-di-t-butyl phenol. A

number of patents describe the use of ethylene/α-ole n/diene interpolymers to make dispersants

[115–117]. These dispersants are claimed to possess excellent high- and low-temperature viscosi-

ties, as de ned by VR´/VR. Here VR´ pertains to the dispersant and VR pertains to the precursor,

such as alkylphenol or alkenylsuccinic anhydride. VR´ is the ratio of the –20°C cold-cranking

simulator (CCS) viscosity (cP) of a 2% solution of dispersant in a reference oil to the 100°C kine-

matic viscosity (cSt) of the dispersant. VR is the ratio of the –20°C CCS viscosity (cP) of a 2%

solution of precursor in the reference oil to the 100°C kinematic viscosity (cSt) of the precursor.

The values of 2.0–3.9 for VR and VR´ and of <1.11 for VR´/VR are considered suitable for bal-

anced low- and high-temperature viscosities.

5.7.4 SEAL PERFORMANCE

Seals in automotive equipment are used for many purposes, the most prominent of which are to

have easy access to malfunctioning parts to perform repair and to minimize contamination and loss

of lubricant. Various polymeric materials are used to make seals. These include  uoroelastomers,

nitrile rubber, polyacrylates, and polysiloxanes (silicones). Maintaining the integrity of seals is criti-

cal; otherwise, the lubricant will be lost, and wear damage and equipment failure will occur. The

seals fail in a number of ways. They can shrink, elongate, or become brittle and thus deteriorate.

The damage to elastomer seals is assessed by examining volume, hardness, tensile strength change,

and the tendency to elongate and rupture [118]. Two primary mechanisms by which seal damage can

occur include abrasion due to particulate matter in the lubricant and the attack of various lubricant

components on the seals. The lubricant-related damage can occur when some of its components

•

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Dispersants 163

diffuse into the seals. This will either cause a change in the seal’s hardness, thereby leading to

swelling and or elongation, or extract the plasticizer, an agent used to impart  exibility and strength

to polymeric materials.

Abrasive damage is not common since most equipment has an installed lubricant  ltration sys-

tem. The lubricant-related damage, however, is of primary interest to us. The lubricant is a blend of

base stocks and an additive package. Certain base stocks, such as those of high aromatics content

or those that are of the ester type, have the tendency to extract the plasticizer because of their high

polarity. Additives, however, have the ability to diffuse into the seal material and alter its properties

as well as remove the plasticizer. Among additives, dispersants are the most implicated in causing

seal damage, especially to  uoroelastomer (Viton

) seals. Although in many cases seal failure can

be corrected by the use of additives, called the seal-swell agents, it is wise to eliminate such damage

by prevention. Elastomer compatibility requirements are a part of the current United States, Asso-

ciation des Contsructeurs Européens de l’Automobile (ACEA), and Japanese standards for engine

oils and worldwide automotive transmission and tractor hydraulic  uid speci cations [119]. Damage

to seals is prevalent in the case of nitrogen dispersants. In general, the higher the nitrogen content,

the higher the seal problems [118]. Rationally, these problems occur due to the presence of low-

molecular-weight molecules in the dispersant. These include free amine either as such or in a labile

form, such as an alkylammonium salt, or low-molecular-weight succinimides and succinamides.

Because of their high polarity and smaller size, these molecules are more likely to diffuse into the

seal material and alter its physical and mechanical properties [120]. It is believed that in the case of

Viton seals, the loss of  uoride ions is responsible for seal deterioration. Removal of the free amine

and of low-molecular-weight succinimides will improve seal performance. Alternatively, one can

posttreat dispersants with reagents, such as boric acid and epoxides, which will either make

such species innocuous or hinder their diffusion into the seal material. Many chemical treat-

ments of dispersants, covered in Section 5.7.1, claim to improve seal performance of dispersants

and crankcase lubricants that use them. These reagents react with seal-damaging amines and low-

molecular-weight succinimides to make them harmless. Strategies other than those listed earlier are

also reported in the patent literature [121–125].

5.8 PERFORMANCE TESTING

Engine oils account for almost 80% of the automatic transmission dispersant use. Other applica-

tions that use these additives include automatic transmission  uids, gear lubricants, hydraulic  uids,

and re nery processes as antifoulants. Dispersants of relatively lower molecular weight are also

used in fuels to control injector and combustion chamber deposits [126,127]. Such dispersants usu-

ally contain a polyether functionality [128].

Succinimide and succinate ester–type polymeric dispersants are used in gasoline and die-

sel engine oils, but the use of alkylphenol-derived dispersants, that is, of the Mannich type, is

limited to gasoline engine oils. Dispersant polymers derived from ethylene–propylene rubbers,

styrene–diene copolymers, and PMAs are also used in both gasoline and diesel engine oils. As

mentioned earlier, dispersant polymers lack suf cient dispersancy to be used alone and hence are

used in combination with polymeric dispersants. The PMA and styrene ester–derived dispersant

polymers are used in automatic transmission  uids, in power-steering  uids, and, to a limited

extent, in gear oils.

Additive manufacturers use various laboratory screen tests and engine tests to evaluate a

dispersant’s effectiveness. Many of the screen tests are proprietary, but all are developed around

evaluating performance in terms of a dispersant’s ability to disperse lamp black or used engine oil

sludge. The laboratory engine tests are industry-required tests and include both gasoline engine

and diesel engine tests. These are listed in International Lubricant Standardization and Approval

Committee (ILSAC), American Petroleum Institute (API), ACEA 2002, Japanese Automobile

Standards Organization (JASO), and Bureau of Indian Standards (BIS) standards. It is important

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