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134 Lubricant Additives: Chemistry and Applications

As mentioned, common metals that can be used to make neutral or basic detergents include

sodium, potassium, magnesium, calcium, and barium. Calcium and magnesium  nd most extensive

use as lubricant additives, with a preference for calcium due to its lower cost. The use of barium-

derived detergents is being curbed due to concerns for barium’s toxicity. Technically, one can use

metal oxides, hydroxides, and carbonates to manufacture neutral (nonoverbased) detergents; for

nonoverbased detergents, oxides and hydroxides are the preferred bases. Sodium hydroxide, cal-

cium hydroxide, and barium hydroxide are often used for sodium, calcium, and barium detergents.

For magnesium detergents, however, magnesium oxide is the preferred base.

During the synthesis of calcium detergents, overbasing is usually stopped before all the metal

base is converted into calcium carbonate. As a result, the excess base is present as a mixture of

calcium hydroxide and calcium carbonate. The calcium carbonate predominates because, if the

reaction is overblown with carbon dioxide, the amorphous calcium carbonate, which is desired, is

converted into crystalline calcium carbonate. Of low solubility in the overbased system, crystalline

calcium carbonate falls out of solution, and one obtains an oil-insoluble gel-like product. Although

such products are of little use as lubricant additives, they are useful as rheology control agents in

coatings. The challenge is to make them on a consistent basis. Lubrizol supplies such products

derived from alkylbenzenesulfonic acids as its Ircogel

product line. Gelled carboxylates and solid

calcium micellar complexes have also been reported in the patent literature [56–58].

Basic detergents contain reserve base, which is entrained into the detergent in a colloidal form.

The base, such as the carbonate, is believed to be encapsulated by soap molecules. In this arrange-

ment, the polar head group (sulfonate, phenate, or carboxylate) of the soap associates with the car-

bonate, and the hydrocarbon portion of the soap associates with the oil (see Figure 4.11). The base

neutralizes acids that result from oxidation of the fuel and the lubricant and from the oxidation and

thermal decomposition of thermally labile additives.

Some detergents are marketed as neutral or nonoverbased. However, most of them have a small

amount of reserve base present. In other words, they are overbased to some degree. This implies that

no effort was made to overbase them, and their reserve base is due to the presence of the unreacted

base used to make them. For example, commercially available neutral sulfonates have a TBN of 30

or less, and the base is commonly present as a hydroxide such as calcium hydroxide. Conversely,

basic or overbased detergents have a much higher base number, that is, they typically have a TBN

of 200–500, and the base is commonly present as a metal carbonate.

Calcium-based phenate detergents are easier to make than magnesium-based detergents

because alkylphenols are weak acids, and their reaction with magnesium oxide, a weak base,

is not facile. To make the neutral salt, one must react the alkylphenol with a strong base such as

FIGURE 4.11 Micelle structure of detergents.

Hydrocarbon group

Sulfonate head group

Neutral calcium sulfonate

CaCO

Basic sulfonate inverse micelle structure; size: 100−150

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Detergents 135

magnesium alkoxide. This reagent can be prepared by reacting magnesium metal with an excess

of highly reactive alcohol such as methanol. However, this method is hazardous because of the

hydrogen gas by-product and costly because of the price of the magnesium metal. Once the neutral

salt or soap formation occurs, the excess alcohol is exchanged for an inert solvent, such as toluene

or mineral oil, before overbasing. Alternatively, one can use a high-temperature overbasing pro-

cedure using a low-molecular-weight alkylphenol as a promoter [59]. In the case of methylene or

sulfur-bridged phenols that are more acidic than regular alkylphenols, the reactivity toward mag-

nesium oxide is not a problem. And these compounds form neutral and overbased magnesium salts

without dif culty. Neutral and basic calcium phenates from alkylphenols, bridged or unbridged,

are easy to make because calcium hydroxide, being a stronger base, reacts with them readily. Other

acids, that is, alkylsalicylic acids, fatty carboxylic acids, and alkenylphosphonic acids, react with

calcium and magnesium bases without any problem.

The synthetic sequences used to make common types of neutral and carbonate overbased deter-

gents are outlined in Figures 4.12 through 4.17. Sulfonate, salicylate, and carboxylate detergents are

commercially available as calcium and magnesium salts, and phosphonates are available as calcium

salts. Some specialty sulfonates, for example, NA-SUL BSB

, are also available as barium salts.

Phenate detergents are commonly available as calcium salts, and phosphonate detergents are avail-

able as both calcium and barium salts. Basic calcium sulfonates make up ~65% of the total detergent

market, followed by phenates at ~31%.

4.6 TESTING

Detergents are used in engine lubricant formulations to perform two key functions. One is to neu-

tralize the acidic by-products of lubricant oxidation and thermal decomposition, and the other is to

suspend neutral but highly polar-oxygenated species in the bulk lubricant.

FIGURE 4.12 Synthesis of neutral and basic metal sulfonates.

MOS

Divalent metal salt of

alkylbenzenesulfonic acid (soap)

Overbased or basic sulfonate

M = Ca, Mg, Ba

x MCO

Natural sodium sulfonate

MOS

Excess base

Metal halide

Alkylbenzenesulfonic acid

Base (stoichiometric amount)

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136 Lubricant Additives: Chemistry and Applications

All components of a lubricant—base oil, additives, and viscosity modi er—oxidize because

of their organic nature. Oxidation of the API group I base oils is more facile than that of the group

II and group III base oils, primarily because of the presence of aromatic and sometimes ole nic

components. These compounds oxidize to form hydroperoxides and radicals [6]. These species are

highly reactive and start the oxidation chain reaction. The result is the oxidative breakdown of all

components of the lubricant to highly oxygenated polar species, which are of low lubricant solubility.

Because of this, these materials, both acidic and neutral, tend to separate on surfaces, thereby

FIGURE 4.13 Synthesis of neutral and basic metal phenates.

O M O

Alkylphenol

Base

Stoichiometric

amount

Divalent metal salt of

alkylphenol (soap)

Excess base

x MCO

Overbased or basic phenate

M = Ca, Mg

FIGURE 4.14 Synthesis of neutral and basic bridged metal phenates.

OHOH

O O

Divalent metal salt of sulfur- or

methylene-bridged alkylphenol (soap)

Base

Stoichiometric

amount

Sulfur- or methylene-bridged

alkylphenol

Y = S, CH

M = Ca, Mg

Excess base CO

x MCO

Overbased or basic bridged phenate

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Detergents 137

FIGURE 4.15 Synthesis of neutral and basic metal salicylates.

COOK

O M O

Potassium

alkylsalicylate

COOH

Alkylsalicylic

acid

HCI

n MCO

Basic or overbased metal salicylate

M = Ca or M

KCI

Neutral metal salicylate

(soap)

Excess base

Base

Stoichiometric

amount

Metal halide

FIGURE 4.16 Synthesis of neutral and basic phosphonates.

O M O P

Overbased or basic alkenylphosphonate

Base

Stoichiometric

amount

Polyisobutenylphosphonic

acid

X = O or S

R = H or alkyl

M = Ca, Ba

Divalent metal salt of

polyisobutenylphosphonic acid (soap)

Excess base CO

x MCO

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138 Lubricant Additives: Chemistry and Applications

impairing the proper functioning of the various equipment parts. The acidic materials such as sulfur

oxides, and organic acids resulting from lubricant oxidation can attack metal surfaces and cause

corrosion. Sulfur oxides and sulfuric acid result from the combustion of fuel sulfur or the oxidation

of sulfur-containing additives, such as sulfurized ole ns or zinc dialkyldithiophosphates. The func-

tion of the detergent in this case is to neutralize these acids, thus causing corrosion. The reserve base

in detergents primarily performs this function. The soap portion keeps oil-insoluble polar products

suspended in oil.

Phenates, sulfurized phenates, and salicylates can also act as oxidation inhibitors because of the

presence of the phenol functionality. Phenols are well known for their oxidation-inhibiting action

[5]. Detergents are also effective corrosion inhibitors, especially basic detergents [25,26], because

they not only neutralize corrosive acidic products but also form surface  lms that isolate metal

surfaces from corrosive agents [6]. The carbonate portion in the detergent performs acid neutraliza-

tion, and the soap portion forms the protective surface  lm. NA-SUL 729

and NA-SUL CA-50

are examples of commercial corrosion inhibitors belonging to this class of additives. The tests that

evaluate rust and corrosion are described elsewhere [3,6]. Detergents derived from fatty carboxylic

acids are good friction modi ers, primarily because of the linear structure of their soaps [6].

Detergents  nd primary use in engine oils, which are responsible for more than 75% of the total

consumption. The detergent level in marine diesel engine oils is the highest because marine engines

use high-sulfur fuel, which leads to highly acidic combustion products such as sulfuric acid. The

lubricants for these engines therefore require the base reserve of highly overbased detergents.

A variety of proprietary and industry-established tests are used to determine a detergent’s effec-

tiveness in lubricants. For gasoline engine oils to be used in North America, these include the CRC

L-38, TEOST (Thermo-Oxidation Engine Oil Simulation Test [60]), ASTM Sequence IIIE/IIIF,

and ASTM Sequence VE/VG tests. For European gasoline engines, in addition to performance

in the ASTM Sequence IIIE/IIIF and VE/VG Engine tests, performance in Peugeot TU3M High-

Temperature Test and MB M111 Black Sludge Test is also required. These tests are part of the

ACEA 2002 Standard. The standard also includes a sulfated ash limit that directly affects the

amount of detergent used in formulations since it is the primary contributor to sulfated ash.

The ef cacy of diesel engine oils for North American use is evaluated by the use of both the

single-cylinder and the multicylinder engine tests. Single-cylinder tests include CRC L-38 and

FIGURE 4.17 Synthesis of neutral and basic fatty carboxylates.

X MCO

Oleic acid

Base

Stoichiometric

amount

Divalent metal oleate (soap)

Excess base

Overbased or basic carboxylate

M = Ca, M

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Detergents 139

Caterpillar 1K, 1M-PC, 1N, 1P, and IR engine tests. CRC L-38’s viscosity requirement, an impre-

cise measure of a detergent’s effectiveness, is a part of the API CG-4 standard. Caterpillar 1K test

is a part of the API CF-4, API CH-4, and API CI-4 standards. Caterpillar 1M-PC is a part of the

API CF, CF-2, and the U.S. Military’s MIL-PRF-2104G speci cations. Caterpillar 1N is a part of

the API CG-4, API CI-4, and the U.S. Military’s MIL-PRF-2104G speci cations. Caterpillar 1P

is a part of the API CH-4 speci cation, and caterpillar 1R is a part of the API CI-4 speci cation.

The pass ratings for these tests require meeting the overall appearance of the cylinder and its parts,

which are expressed in terms of weighted demerits, percent top groove  ll, ring side clearance

loss, top land heavy carbon, oil consumption, and stuck rings. All these parameters are related to

deposits on the piston and its parts. The multicylinder tests that determine the effectiveness of a

detergent include Detroit Diesel 6V92TA engine test (a part of the API CF-2 and MIL-PRF-2104G

speci cations) and Mack M11 engine test (a part of the API CH-4 and CI-4 speci cations). These

two tests evaluate a detergent’s ability to prevent deposit-related port plugging and engine sludge,

respectively. For European use, oils must pass the requirements of tests such as VW1.6L TC diesel,

XUD11BTE, VWDI, MB OM 364 LA, and MB OM 441 LA. These tests evaluate ring sticking,

piston cleanliness, viscosity increase, and  lter plugging. It is important to note that technically

most of these tests evaluate the combined effectiveness of the detergent and the dispersant. The per-

formance of the individual additive is hard to unravel. Detergents  nd additional use in automatic

transmission  uids and tractor hydraulic  uids. In this case, the primary function of these additives

is not to neutralize acids or to minimize deposit formation, but to alter the frictional properties of

these  uids. This is critical if the  uids are to perform effectively as driveline lubricants.

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140 Lubricant Additives: Chemistry and Applications

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Detergents 141

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143

Dispersants

Syed Q. A. Rizvi

CONTENTS

5.1 Introduction ........................................................................................................................... 143

5.2 Nature of Deposits and Mode of Their Formation ............................................................... 144

5.3 Deposit Control by Dispersants ............................................................................................ 147

5.4 Desirable Dispersant Properties ............................................................................................147

5.5 Dispersant Structure ............................................................................................................. 147

5.6 Dispersant Synthesis ............................................................................................................. 148

5.6.1 The Hydrocarbon Group ........................................................................................... 149

5.6.2 The Connecting Group .............................................................................................. 151

5.6.3 The Polar Moiety ....................................................................................................... 152

5.7 Dispersant Properties ............................................................................................................ 157

5.7.1 Dispersancy ............................................................................................................... 157

5.7.2 Thermal and Oxidative Stability ............................................................................... 160

5.7.3 Viscosity Characteristics ........................................................................................... 161

5.7.4 Seal Performance....................................................................................................... 162

5.8 Performance Testing ............................................................................................................. 163

References ...................................................................................................................................... 166

5.1 INTRODUCTION

Lubricants are composed of a base  uid and additives. The base  uid can be mineral, synthetic, or

biological in origin. In terms of use, petroleum-derived (mineral) base  uids top the list, followed

by synthetic  uids. Base oils of biological origin, that is, vegetable and animal oils, have not gained

much popularity except in environmentally compatible lubricants. This is because of the inherent

drawbacks these base oils have pertaining to their oxidation stability and low-temperature proper-

ties. Additives are added to the base  uid either to enhance an already-existing property, such as

viscosity, of a base oil or to impart a new property, such as detergency, lacking in the base oil.

The lubricants are designed to perform a number of functions, including lubrication, cooling, pro-

tection against corrosion, and keeping the equipment components clean by suspending ordinarily

insoluble contaminants in the bulk lubricant [1]. Although for automotive applications all functions

are important, suspending the insoluble contaminants and keeping the surfaces clean are the most

critical. As mentioned in Chapter 4 on “detergents,” this is achieved by the combined action of the

detergents and the dispersants present in the lubricant. Dispersants differ from detergents in three

signi cant ways:

1. Dispersants are metal-free, but detergents contain metals, such as magnesium, calcium,

and sometimes barium [2]. This means that on combustion detergents will lead to ash

formation and dispersants will not.

2. Dispersants have little or no acid-neutralizing ability, but detergents do. This is because

dispersants have either no basicity, as is the case in ester dispersants, or low basicity, as

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