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214 Lubricant Additives: Chemistry and Applications

8.1 INTRODUCTION

To optimize the balance between low wear and low friction, machine designers specify a lubricant

with a viscosity suf cient to generate hydrodynamic or elastohydrodynamic oil  lms that sepa-

rate the machine’s interacting surfaces, but not too high to induce excessive viscous drag loss. In

practice, the various contact types in a machine, the incidence of operating conditions beyond the

design range, and the pressure to improve ef ciency by reducing oil viscosity conspire to reduce oil

 lm thickness below the optimum. The high spots, or asperities, on the interacting surfaces then

start to interact with one another, initially through micro-elastohydrodynamic lubrication (EHL)

 lms, and at the end through direct surface contact, resulting in increased friction and the likelihood

of surface damage. Antiwear and extreme-pressure (EP) additives are added to lubricating oils to

decrease wear and prevent seizure under such conditions.

A common way to demonstrate the viscosity optimization is shown in Figure 8.1; this is known

as a Stribeck curve. The curve is a composite of a boundary friction curve and a viscous friction

curve that decreases as viscosity and, therefore,  lm thickness increase and that increases as vis-

cosity and speed increase, respectively. A good operating target is represented slightly to the right of

the minimum in the curve. Improving the surface  nish of contacting surfaces can move the mini-

mum in the curve to a lower viscosity range, saving energy but increasing the cost of components.

The hardening or coating of surfaces can increase their durability under increased levels of contact

with lower viscosity, but again at an increase in component cost. Notwithstanding these component

manufacturing improvements, the need for antiwear and EP additives will continue, but the nature

of their chemistry is likely to change due to environmental constraints, component material devel-

opments, and the continuing increase in severity of machine operating conditions.

The distinction between antiwear and EP additives is not clear-cut. Some are classed as

antiwear in one application and EP in another, and some have both antiwear and EP proper-

ties. To add to the confusion, EP additives come in mild and strong  avors, and some are only

effective in low-speed, high-load situations and others only in high-speed, high-temperature

applications. Generally, antiwear additives are designed to deposit surface  lms under normal

operating conditions and thereby reduce the rate of continuous, moderate wear, whereas EP

additives are expected to react rapidly with a surface under severe distress and prevent more

catastrophic modes of failure such as scuf ng (scoring), galling, and seizure. Recently, it has

been suggested that EP additives be renamed as antiscuf ng additives, since there is no pressure

Fluid film

Hydrodynamic

regime

Boundary

friction

0.15

Coefficient

of friction

0.001−0.002

Minimum fluid

friction

Z = viscosity of oil

N = speed of sliding

P = pressure between surfaces (load)

Mixed

lubrication

Boundary

lubrication

FIGURE 8.1 Regions of lubrication—Stribeck curve.

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Ashless Antiwear and Extreme-Pressure Additives 215

distinction between them and antiwear additives; only an expectation of a performance boost

under severe conditions. EP/antiscuf ng additives tend to be very reactive, and some can have

adverse effects on oxidative stability of oils, can be corrosive to nonferrous materials, and can

reduce the fatigue life of bearing and gear surfaces. They should only be used when severe dis-

tress is a distinct possibility.

Antiwear additives function in various ways. Some deposit multilayer  lms thick enough to

supplement marginal hydrodynamic  lms and prevent asperity contact altogether. Some develop

easily replenishable monolayer  lms that reduce the local shear stress between contacting asperi-

ties and are preferentially removed in place of surface material. Others bond chemically with the

surface and slowly modify surface asperity geometry by controlled surface material removal until

conditions conducive to hydrodynamic  lm generation reappear.

EP additives are designed to prevent metal–metal adhesion or welding when the degree of

surface contact is such that the natural protective oxide  lms are removed and other surface-active

species in the oil are not reactive enough to deposit a protective  lm. This is most likely to occur

under conditions of high-speed, high-load, or high-temperature operation. EP additives function

by reacting with the metal surface to form a metal compound such as iron sul de. They act in a

manner similar to that of antiwear additives, but their rate of reaction with the metal surface and

therefore the rate of EP  lm formation are higher and the  lm itself is tougher. Some EP additives

prevent scoring and seizure at high speed and under shock loads; others prevent ridging and rippling

in high-torque, low-speed operations. In both cases, EP additives and surface metal are consumed,

and a smoother surface is created with an improved chance of hydrodynamic action, resulting in

less local distress and lower friction. In the absence of such additives, heavy wear and distress well

beyond the scale of surface asperities would occur, accompanied by very high friction.

A wide variety of antiwear and EP additives are commercially available, and many other chemi-

cals with antiwear and EP functionality have been reported in the literature and in patents. To be

commercially viable, additives must be adequately soluble in lubricant formulations and reasonable

in cost, must neither overly reduce the lubricant’s oxidative stability nor increase the corrosivity of

metals contacted by the lubricant [1–3]. Lead naphthenates were extensively used early in the indus-

try’s history, but environmental concerns have led to their virtual disappearance. Similarly, chlorine-

containing additives are in decline. Zinc dialkyldithiophosphates (ZDDPs or ZnDTPs) are the best

known and most widely used antiwear additives in engine oils, transmission  uids, and hydraulic

oils. However, the concern for phosphorus poisoning of automotive catalysts and for zinc as an envi-

ronmental contaminant has resulted in a pressure to  nd metal- and phosphorus-free replacements

for both automotive and industrial applications. This has resulted in a move toward ashless antiwear

and EP additives, and this chapter covers these additives in terms of their chemistry, properties, and

performance characteristics, applications, marketing, sales, and outlook.

8.2 CHEMISTRY, PROPERTIES, AND PERFORMANCE

(CLASSIFIED BY ELEMENTS)

8.2.1 S

ULFUR ADDITIVES

Sulfur-containing additives are used to provide protection against high pressure, metal-to-metal

contacts in boundary lubrication. The magnitude of the EP activity is a function of the sulfur content

of the additive; high-sulfur-content additives are usually more effective EP agents than are low-

sulfur-content additives. The sulfur content of the additive must be balanced against requirements for

thermal stability and noncorrosiveness toward copper-containing alloys. The additive’s composition

and structure represent a chemical compromise between con icting performance requirements. In

general, any compound that can break down under an energy-input stress, such as heat, and allow

for a free sulfur valence to combine with iron would do well as an antiwear and EP additive. Sulfur

additives are probably the earliest known, widely used EP compounds in lubricants.

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216 Lubricant Additives: Chemistry and Applications

Sulfurization by addition of sulfur compounds [elemental sulfur, hydrogen sul de, and mercap-

tans] to unsaturated compounds has been known to the chemical industry for years [4–8]. The two

most common classes of additives are called sulfurized ole ns [9,10] and sulfurized fatty acid esters

[11], because they are produced from reactions of ole ns and naturally occurring or synthetic fatty

acid esters with sulfur compounds.

In the absence of initiators, the addition to simple ole ns is by an electrophilic mechanism;

and Markovnikov’s rule is followed. However, this reaction is usually very slow and often cannot

be done or requires very severe conditions unless an acid catalyst is used. In the presence of free

radical initiators, H

S and mercaptans add to double and triple bonds by a free radical mechanism,

and the orientation is anti-Markovnikov. By any mechanism, the initial product of addition of H

to a double bond is a mercaptan, which is capable of adding to a second molecule of ole n, so that

sul des are often produced (reaction 8.1):

HC C S C CH

(8.1)

8.2.1.1 Sulfurized Oleﬁ ns

Sulfurized ole ns are prepared by treating an ole n with a sulfur source under proper reaction con-

ditions. The more the sulfur used, the higher is the sulfur content. Suitable ole ns preferably include

terminal ole ns and internal ole ns, mono-ole ns and polyole ns. However, to provide adequate

oil solubility, the ole n should provide a carbon chain of at least four carbon atoms. Accordingly,

suitable alpha ole ns are butenes, pentenes, hexenes, and preferably higher alpha ole ns such as

octenes, nonenes, and decenes. Isobutylene is a very unique ole n that not only exhibits very high

reactivity toward sulfur reagents (high conversion rate) but also can produce sulfurized products

having very good stability and lubricant compatibility. Therefore, sulfurized isobutylene (SIB) has

been by far the most cost-effective, widely used EP additive in lubricants.

8.2.1.1.1 Chemistry and Manufacture

Initially, sulfurized ole ns were synthesized through a two-step chloride process, and often, the

products were referred to as “conventional sulfurized ole ns.” Sulfur monochloride and sulfur

dichloride were used in the  rst step to produce chlorinated adducts, and then the adducts were

treated with an alkali metal sul de in the presence of free sulfur in an alcohol–water solvent,

followed by further treatment with an inorganic base (reactions 8.2 and 8.3) [12]. The  nal product

is a light yellow–colored  uid with oligomeric monosul des and disul des as the main composi-

tions, as typi ed by reactions 8.2 and 8.3.

CH3

Cl-C(CH

)

-CH

-S-S-CH

-C(CH

)

-Cl

Major adduct

(8.2)

NaSH

SIB + 2 NaCl

Cl-C(CH

)

-CH

-S-S-CH

-C(CH

)

-Cl

SIB =

S-C(CH

)

-CH

-S-S-CH

-C(CH

)

-S

(8.3)

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Ashless Antiwear and Extreme-Pressure Additives 217

The manufacture of conventional sulfurized ole ns involves sulfur monochloride, and the  nal

product contains some residual chlorine. The process also generates aqueous waste with halogen-

and sulfur-containing by-products that must be disposed of. Chlorine in lubricants and other

materials is increasingly becoming an environmental concern because chlorinated dioxins can be

formed when chlorine-containing materials are incinerated. Chlorinated waxes have been elimi-

nated from many lubricants for this reason. Residual chlorine content is also becoming a major con-

cern in many areas of the world. Germany currently has a 50 ppm maximum limit on the chlorine

content of automotive gear oils. This requirement is a problem for automotive gear oil suppliers as

well as additive suppliers if their technology is based on conventional sulfurized ole ns, since the

residual chlorine content is a consequence of the chemistry required to manufacture conventional

sulfurized ole ns. By  ne-tuning the manufacturing process, the chlorine content of conventional

sulfurized ole ns may be reduced from a typical 1500 ppm to <500 ppm. However, manufacturing

changes to reduce the residual chlorine content will probably slow the production process, require

additional capital investments, and possibly generate more aqueous waste.

In the late 1970s, the high-pressure sulfurized isobutylene (HPSIB) process was developed to

replace the conventional, low-pressure chlorine process. HPSIBs are usually mixtures of di-tert-butyl

trisul des, tetrasul des, and higher-order polysul des [13–16]. Some HPSIBs contain oligomeric

polysul des of poorly de ned composition or other materials such as 4-methyl-1, 2-dithiole-3-thione

(Structure A, [8,17], and reaction 8.4). The higher-order polysul des generally favor EP activity at the

expense of oxidative stability and copper corrosivity compared to the monosul des and disul des of

conventional sulfurized ole ns. In the absence of other reagents, the straight reaction of elemental

sulfur and isobutylene results in a dark-colored liquid that contains a signi cant amount of dithiolethi-

ones (thiocarbonates). 4-Methyl-1, 2-dithiole-3-thione is a pseudoaromatic heterocyclic compound.

Owing to its rigid ring structure, dithiolethiones can be easily precipitated as yellowish solids that

cause severe staining problems. Therefore, dithiolethione is often not a desirable side product in SIB.

Sulfur

High pressure

Dithiolethione

Polysulfides

Oligomeric polysulfide

(8.4)

STRUCTURE A

In the presence of various catalysts (or basic materials), such as aqueous ammonia, alkali metal

sul des, or metal dithiocarbamates, amounts of dithiolethiones (Structure A) and oligomeric

polysul des can be reduced, and low-molecular-weight polysul des (X = 2 to 6 in Structure B) are

the predominant products [18].

(CH

)

C(CH

)

STRUCTURE B

The use of hydrogen sul de in the high-pressure sulfurized ole n process can ease the reac-

tion complexity and also yield high-quality, low-molecular-weight polysul des. The compositions

of products prepared from this process usually have good clarity, low odor, light color, and high

EP activity. Hydrogen sul de is a very foul smelling and toxic gas. It leads to collapse, coma, and

death as result of respiratory failure within a few seconds after one or two inhalations. Lique ed

hydrogen sul de has a high vapor pressure that requires additional, adequate protective equipment.

There are considerable risks associated with its routine use on an industrial scale, but hydrogen

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218 Lubricant Additives: Chemistry and Applications

sul de is a low-cost, commodity chemical, which can often offset the additional costs for safe

use. High- pressure sulfurized ole ns can also be prepared with reagents that generate hydrogen

sul de within the reactor during the course of the reaction. Direct handling of hydrogen sul de is

thus avoided, but there can be processing penalties, usually in the area of aqueous waste handling.

Performance wise, high-pressure sulfurized ole ns could replace conventional sulfurized ole ns in

suitable applications. A decision to manufacture high-pressure sulfurized ole ns by one process or

another will require a careful assessment of acceptable risks versus economic requirements.

Other ole ns or mixed ole ns are also used in the preparation of various sulfurized ole ns. Among

these, di-tert-nonyl and di-dodecyl trisul des and penta-sul des are very popular additives. Diisobutyl-

ene (2,4,4-trimethyl-1-pentene) is also used extensively to make higher-viscosity sulfurized products.

In addition, sulfurized hydrocarbons such as sulfurized terpene, sulfurized dicyclopentadiene, or

sulfurized dipentene ole n, and sulfurized wax are also widely used due to low raw material costs.

8.2.1.1.2 Applications and Performance Characteristics

Sulfurized ole ns played a key role in establishing superior ashless sulfur/phosphorus (S/P) additive

systems for lubricating automotive and industrial EP gear oils in the late 1960s [19–21]. The early

EP gear oil additives were clearly dominated by chlorine, zinc, and lead, which had dif culty in

adequately protecting heavy-duty equipment. On the contrary, the S/P gear oil additive technology,

based on ashless and chlorine-free components, possesses very good thermal-oxidative stability

and rust inhibition (CRC L-33 and ASTM D665B); therefore, this is a signi cant performance

improvement over the metal- and chlorine-based technologies.

Sulfurized ole ns function mainly through thermal decomposition mechanisms. Sulfur prevents

contact between interacting ferrous metal surfaces through the formation of an intermediate  lm of

iron sul de. By doing this, sulfur usually decreases the wear rate but accelerates the smoothing of

the surfaces. This smoothing actually helps reduce the wear rate. Furthermore, a higher percent

of active sulfur in a molecule increases the chances of reaction with the metal surface and favors

EP (antiseizure) more than antiwear properties. Thus, SIB is mainly a strong antiscuf ng addi-

tive, with outstanding scuf ng protection properties (e.g., CRC L-42 performance). Table 8.1 shows

coef cients of friction and dimensions with respect to metal surface, oil molecules, and sul de

layers. It can be seen that the friction coef cients of the sul de layers are about half of those for

metal-to-metal surfaces. The sul de layers retard the welding of the moving metal surfaces, but do

not prevent wearing. Particles of iron sul de are constantly sloughed off from the metal surface.

This wear can be determined by an analysis of the lubricating oils (residual iron content), and

subsequent sludge formation can be controlled by the use of dispersants.

Besides heavy-duty gear oil applications [22], sulfurized ole ns have also found usefulness in other

lubricant areas, such as metal processing oils, greases, marine oils, and tractor transmission oils.

TABLE 8.1

Typical Surface Characteristics

Surface Coefﬁ cient of Friction

Steel:steel 0.78

FeS:FeS 0.39

Copper:copper 1.21

CuS:CuS 0.74

Material Dimension (Å)

Size of oil molecules 50

Size of sul de layers 3000

Surfaces with super nish 1000

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Ashless Antiwear and Extreme-Pressure Additives 219

8.2.1.2 Sulfurized Esters and Sulfurized Oils

The oldest widely used sulfur-based additive that is still found in commercial lubricants is sulfu-

rized lard oil (SLO), a sulfurized animal triglyceride. In 1939, H. G. Smith made one of the most

important discoveries in the history of lubricant additives. He found that sulfurized sperm whale oil

(SSWO) was more soluble in paraf nic base oils, even at low temperatures, and had a much higher

thermal stability than SLO. Thus, over 60 years ago, he recognized that the improved stability of

sulfurized sperm oil resulted from its monoester structure, compared with the triester structure of

SLO. With long-chain monoesters, the sulfur has little potential to form bridges between the mol-

ecules, as it does when triglycerides are being sulfurized. SSWO is an excellent boundary lubricant

and is highly resistant to gumming, resin formation, or viscosity increase, when subjected to high

temperature and high pressure [23]. Unfortunately, from a lubricant cost-performance viewpoint,

this additive is no longer available due to restrictions on the use of sperm whale oil. Sulfurized

jojoba oil, an expensive alternative, is available; it is also a mixture of long-chain alcohol fatty acid

compounds. All these sulfurized fats or esters are usually manufactured to contain 10–15% sulfur

and are often good antiwear and mild EP agents.

8.2.1.2.1 Chemistry and Manufacture

Sperm oil is a waxy mixture of esters of fatty alcohols and fatty acids with a small amount of

triglycerides. After the separation of solid waxes by  ltration or centrifuge, a liquid wax remains,

consisting mainly of an ester of oleic alcohol and oleic acid. Such a structure could not be better for

sulfurizing purposes.

Similar to sulfurized ole ns, sulfurized esters can be made by either direct sulfurization

with elemental sulfur or sulfurization with hydrogen sul de under superatmospheric pressures.

Nowadays, they are mainly made from vegetable oils having one or more double bonds. Sulfurized

esters are made from unsaturated fatty acids such as oleic acid, and esteri ed with an alcohol such

as methanol. Frequently the sulfurization of fats is made in the presence of an ole n, preferably of

long chain, and the resulting commercial product is a mixture of the two types. Equation 8.5 shows

a typical example of sulfurization of methyl oleate with elemental sulfur. When sulfurized with

hydrogen sul de, products usually possess light color and lower odor.

Heat

(CH

)

-CH

CH-(CH

)

COOCH

(CH

)

-CH--CH-(CH

)

COOCH

(CH

)

-CH--CH-(CH

)

COOCH

(CH

)

-CH-CH

-(CH

)

COOCH

(CH

)

-CH-CH

-(CH

)

COOCH

+ other products

x = 1, 2

(CH

)

-CH=CH-(CH

)

COOCH

(8.5)

8.2.1.2.2 Properties, Performance Characteristics, and Applications

The load-carrying property of sulfurized oils is directly linked to the amount of active sulfur in

the additive. Percent of active sulfur (which is believed to provide EP activity) and total sulfur can

be determined by proper analytical methods, and the difference is the percent of inactive sulfur.

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220 Lubricant Additives: Chemistry and Applications

The more the active sulfur present, the higher the load-carrying property. However, there is also

a direct correlation between active sulfur and copper corrosivity—the more the active sulfur, the

poorer the copper corrosion protection. More active sulfur can also lead to cleanliness and stability

challenges. Therefore, the ultimate product properties for a speci c lubricant product will dictate

which sulfurized products to use.

Although the sulfur content may not be as high as in many sulfurized ole ns, sulfurized esters

are attractive for their exceptionally good frictional properties in many applications. This is because

combining sulfur with fat in a lubricant additive provides a synergistic effect. In this instance, the

fat provides reduced friction, and sulfur provides wear and EP protection. Of all the elements,

sulfur probably gives the best synergistic results in combination with other components and organic

compounds. As to EP characteristics, sulfurized esters have a surface activity conferred by a small

amount of their normal free fatty acids. These are polar species that tend to be absorbed in layers of

molecular dimensions at the metal interface. The interposition of such  lms is effective in preventing

metal seizure under conditions of EP or under conditions tending to displace the lubricating  lm

between the bearing surfaces. Here,  lm strength and EP phenomena are often used synonymously.

Film strength implies that metal-to-metal contact and welding are prevented as a result of the  lm

formation (or replenishment) by the chemical reaction of the metal and an EP additive. Also, fatty

oils and sulfurized fatty oils because of their af nity for metal surfaces are less easily displaced

from metal surfaces by water than are mineral oils.

The ferrophilic ester groups improve the EP properties. Depending on the molecular structure

and its polarity, the surface activities vary. Since the surface activity or polarity of the substances

used for sulfurization plays an equally decisive role in lubricating action, it should be taken into

serious consideration when one formulates a product for a speci c application. Comparing sulfu-

rized triglycerides (e.g., SLO) with sulfurized monoesters (e.g., SSWO), the EP properties of the

triglycerides are better. Two factors may be responsible for this phenomenon: (1) as the triester

structure is more ferrophilic, hydrogen bridging may occur; (2) as triglycerides decompose at high

temperatures to form acrolein moieties during the lubrication process, the polymerized acrolein  lm

can add strength to the sul de  lm and improve the EP characteristics. However, this EP activity of

triglycerides has limited value due to their poor stability and oil solubility. Stability tests at elevated

temperatures show faster and heavier sludging for SLO than for SSWO. Therefore, a proper balance

of all properties is an essential part of product formulations.

Sulfurized fats or esters are used extensively in lubricants such as metalworking  uids, tractor-

transmission  uids, and greases.

8.2.1.3 Other Sulfur Additives

Elemental sulfur provides good EP properties; however, it leads to corrosion. It dissolves in mineral

oils up to certain levels depending on the type of base oils. Low polarity paraf nic/naphthenic type

group II and III base oils usually have very limited solubility of elemental sulfur. Sulfurized aromatics

such as dibenzyl disul de, butylphenol disul de, diphenyl disul de, or tetramethyldibenzyl disul de

generally containing less-active sulfur improve the EP characteristics of lubricants only moderately;

they are therefore used predominantly in combinations with other sulfur or phosphorus-containing

EP additives [24,25]. Other sulfur carriers such as sulfurized nonylphenol, dialkyl thiodipropionates

(S[CH

C(=O)OR]

), derivatives of thioglycolic acid esters (HS–CH

C(=O)OR), derivatives of

thiosalicylic acid, and trithians are also available [26]. However, materials with low sulfur content

are usually less active as antiwear/EP additives, but more effective as antioxidants.

8.2.2 PHOSPHORUS ADDITIVES

Phosphorus-containing additives are used to provide protection against moderate to high pressure,

metal-to-metal contacts in boundary lubrication and EHL. Unlike sulfur additives, where their

EP activity must be balanced against performance requirements for thermal stability and mild

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Ashless Antiwear and Extreme-Pressure Additives 221

corrosivity toward copper-containing alloys, phosphorus additives usually possess very good

corrosivity control. Owing to totally different mechanisms involved in surface  lm formation rates

and  lm strengths, phosphorus additives cannot replace sulfur additives in many applications and

vice versa. Typically, phosphorus additives are extremely effective in applications with slow sliding

speeds and high surface roughness.

8.2.2.1 Phosphate Esters

Phosphate esters have been produced commercially since the 1920s and have gained importance

as lubricant additives, plasticizers, and synthetic base  uids for compressor and hydraulic oils.

They are esters of alcohols and phenols with a general formula O=P(OR)

, where R represents alkyl,

aryl, alkylaryl, or very often, a mixture of alkyl and aryl components. The physical and chemical

properties of phosphate esters can be varied considerably depending on the choice of substituents,

and these can be selected to give optimum performance for a given application. Phosphate esters

are particularly used in applications that bene t from their high-temperature stability and excellent

 re-resistance properties in addition to their adequate antiwear properties [27].

8.2.2.1.1 Chemistry and Manufacture

Phosphate esters are produced by reaction of phosphoryl chloride with alcohols or phenols as shown

in reaction 8.6.

3ROH + POCl

⇒ O=P(OR)

+ HCl (8.6)

Early production of phosphate esters was based on the so-called crude cresylic acid fraction or tar acid

derived by distillation of coal tar residues. This feedstock is a complex mixture of cresols, xylenols,

and other heavy materials and includes signi cant quantities of ortho-cresol. The presence of high

concentrations of ortho-cresol results in an ester that has been associated with neurotoxic effects,

and this has led to the use of controlled coal tar fractions, in which the content of ortho-cresol and

other ortho-n-alkylphenols is greatly reduced. Phosphate esters using coal tar fractions are generally

referred to as natural as opposed to synthetic, where high-purity raw materials are used.

The vast majority of modern phosphate esters are synthetic, using materials derived from pet-

rochemical sources. For example, t-butylated phenols are produced from phenols by reaction with

butylene. The reaction of alcohol or phenol with phosphoryl chloride yields the crude product, which

is generally washed, distilled, dried, and decolorized to yield the  nished product. Low-molecular-

weight trialkyl esters are water-soluble, requiring the use of nonaqueous techniques. When mixed

alkylaryl esters are produced, the reactant phenol and alcohol are added separately. The reaction

is conducted in a stepwise process and the reaction temperature is kept as low as possible to avoid

transesteri cation reactions from taking place.

The most commonly used phosphate esters for antiwear performance features are tricresyl

phosphates (TCP), trixylenyl phosphates (TXP), and tributylphenyl phosphates (TBP).

8.2.2.1.2 Physical and Chemical Properties

The physical properties of phosphate esters vary considerably according to the mix and type of

organic substituents, the molecular weights, and structural symmetry, all proving to be particu-

larly signi cant. Consequently, phosphate esters range from low-viscosity, water-soluble liquids to

insoluble high-melting solids.

As mentioned previously, the use of phosphate esters as synthetic base  uids arises mostly from

their excellent  re resistance and superior lubricity, but is limited due to their hydrolytic and thermal

stability, low-temperature properties, and viscosity index. Although phosphate esters are widely used

as antiwear additives for lubricants, the concerns about hydrolytic stability, thermal stability, and of

course, satisfactory antiwear properties are equally important. In that sense, triaryl phosphates are

dominant over trialkyl phosphates, because their hydrolytic–thermal stability is much better.

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222 Lubricant Additives: Chemistry and Applications

The thermal stability of triaryl phosphates is considerably superior to that of the trialkyl esters,

which degrade thermally by a mechanism analogous to that of the carboxylic esters (reaction 8.7).

(8.7)

With respect to hydrolytic stability, aryl phosphate esters are superior to the alkyl esters.

Increasing chain length and degree of branching of the alkyl group leads to considerable improve-

ment in hydrolytic stability. However, the more the substituent is sterically hindered, the more

dif cult it is to prepare the ester. Alkylaryl phosphates tend to be more susceptible to hydrolysis

than the triaryl or trialkyl esters.

The low-temperature properties of phosphate esters containing one or more alkyl substituents

tend to be reasonably good. Many triaryl phosphates are fairly high-melting point solids, but an

acceptable pour point can be achieved by using a mixture of aryl components. Coal tar fractions,

used to make natural phosphate esters, are already complex mixtures and give esters with satisfac-

tory pour points.

Phosphate esters are very good solvents and are extremely aggressive toward paints and a wide

range of plastics and rubbers. When selecting suitable gasket and seal materials for use with these

esters, careful consideration is required. The solvency power of phosphate esters can be advanta-

geous in that it makes them compatible with most other common additives and enables them to be

used as carriers for other less-soluble additives to generate additive slurry.

8.2.2.2 Phosphites

Phosphites are the main organophosphorus compounds used to control oxidative degradation

of lubricants. They eliminate hydroperoxides and peroxy and alkoxy radicals, retard the dark-

ening of lubricants over time, and also limit photodegradation. In addition to their important

role as antioxidants, phosphites are also found to be useful antiwear additives. Dialkyl hydro-

gen phosphites and diaryl hydrogen phosphites are neutral esters of phosphorus acid. These

materials have two rapid equilibrating forms: the keto form, (RO)

P(=O)H, and the acid form,

(RO)

P–O–H. Physical measurements indicate that they exist substantially in the keto form,

associated in dimeric or trimeric groupings by hydrogen bonding. Trialkyl phosphites and tria-

ryl phosphites are neutral trivalent phosphorus esters. These materials are clear, mobile liquids

with characteristic odors.

8.2.2.2.1 Chemistry and Manufacture

Phosphites are produced by reaction of phosphorus trichloride with alcohols or phenols as given by

3ROH + PCl

+ 3NH

⇒ P(OR)

+ 3NH

Cl (8.8)

When mixed alkylaryl phosphites are produced, the reactants phenol and alcohol are added sepa-

rately with the reaction temperature being controlled carefully. High-molecular-weight phosphites

can be produced from transesteri cation reaction of either alcohols or phenols with trimethyl phos-

phite under catalytic (acidic) conditions.

P(OCH

)

+ 3ROH ⇒ P(OR)

+ 3CH

OH (8.9)

With acid catalyzed hydrolysis, dialkyl or diaryl hydrogen phosphites can be produced from trialkyl

or triaryl phosphites as shown in reactions 8.10 through 8.12.

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Ashless Antiwear and Extreme-Pressure Additives 223

P(OR)

+ H

O ⇒ (RO)

P(=O)H + ROH (8.10)

P(OR)

+ HCl ⇒ (RO)

P(=O)H + RCl (8.11)

2P(OR)

+ HP(=O)(OH)

⇒ 3(RO)

P(=O)H (8.12)

By carrying out the preceding reactions in the presence of hydrogen chloride acceptors such as

pyridine, the isolation of mono, di, and trialkyl phosphites is feasible. However, with alcohols of

normal reactivity, the product is often mainly dialkyl hydrogen phosphite. This can be made in

up to 85% yield, by adding PCl

to a mixture of methanol and a higher alcohol at low tempera-

ture. The methyl and hydrogen chlorides are then removed by heating under reduced pressure on a

steambath.

PCl

+ 2ROH + CH

OH ⇒ (RO)

P(=O)H + CH

Cl + 2HCl (8.13)

The commonly used phosphites available in the marketplace are dimethyl hydrogen phosphite,

diethyl hydrogen phosphite, diisopropyl hydrogen phosphite, dibutyl hydrogen phosphite, bis(2-

ethylhexyl) hydrogen phosphite, dilauryl hydrogen phosphite, bis(tridecyl) hydrogen phosphite,

dioleyl hydrogen phosphite, trisnonylphenyl phosphite, triphenyl phosphite, triisopropyl phosphite,

tributyl phosphite, triisooctyl phosphite, tris(2-ethylhexyl) phosphite, trilauryl phosphite, triisodecyl

phosphite, diphenylisodecyl phosphite, diphenylisooctyl phosphite, phenyldiisodecyl phosphite,

ethylhexyl diphenyl phosphite, and diisodecyl pentaerythritol diphosphite.

8.2.2.2.2 Chemical and Physical Properties

Phosphites tend to hydrolyze when exposed to humidity in the air or moisture in the lubricant.

The extent of hydrolysis depends on the moisture content of the ambient atmosphere, the tempera-

ture, and the duration of exposure. Generally, liquid phosphites are more stable than solids because

of the reduced surface area available for moisture pickup. But hydrolysis can be minimized if proper

precautions, such as dry nitrogen atmosphere, cool storage, and use of tight seals, are observed. The

lower dialkyl hydrogen phosphites hydrolyze in both acidic and alkaline solutions to monoalkyl

esters and phosphorus acid. Rates of hydrolysis normally decrease with increasing molecular

weight. The lower esters of trialkyl phosphites are rapidly hydrolyzed by acids; however, they are

relatively stable in neutral or alkaline solutions. In general, the hydrolytic stability of the trialkyl

phosphites increases with molecular weight.

Since the dialkyl hydrogen phosphites are predominately in the keto form, they are somewhat

resistant to oxidation and do not complex with cuprous halides. Both of these reactions are charac-

teristic of trivalent organic phosphorus compounds [28–30]. These esters are relatively resistant to

reaction with oxygen and sulfur, but react quite readily with chlorine and bromine giving the cor-

responding dialkyl phosphorohalidates ((RO)

P(=O)X where X = Cl or Br) [27].

The hydrogen atom of the dialkyl hydrogen phosphites is replaceable by alkali but is not

acidic in the usual sense. The alkali salts are readily obtainable by reaction of ester with metals.

In contrast with the parent compound, these salts readily add sulfur to form the corresponding

phosphorothioates. Sodium salts of phosphites can be reacted with alkyl chlorides to produce alkyl

phosphonates. These salts react with halophosphites to produce pyrophosphites and with chlorine

or bromine to yield the corresponding hypophosphates. Dialkyl hydrogen phosphites add readily

to ketones, aldehydes, ole ns, and anhydrides, and these reactions are catalyzed by bases and free

radicals. This type of reaction provides an excellent method for preparing phosphonates.

Sulfur reacts readily with trialkyl or triaryl phosphites to form corresponding trialkyl or triaryl

phosphorothioates, which are also very useful antiwear additives. The reaction of trialkyl phosphites

with halogens is an excellent method for preparing dialkyl phosphorohalidates. Acyl halides and

most polyfunctional primary aliphatic halides can be used. Triisopropyl phosphite provides a unique

means for preparation of unsymmetrical phosphonates and diphosphonates because the by-product

isopropyl halide reacts very slowly and thereby does not compete with the primary reaction.

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