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494 Lubricant Additives: Chemistry and Applications

automotive industries. In fact, many of the antioxidants that are used in plastics are also used to

stabilize automotive and industrial lubricants.

Today, most additive (and lubricant) companies have corporate websites that provide informa-

tion. Some provide only cursory information and would do well to expend the effort to provide the

scientist, formulator, and potential customer with more useful information. Other companies publish

almost encyclopedic detail on all of their commercially available additives. Some include detailed

chemistry, properties and performance characteristics, and examples of formulations. The internet

over the past several years has become a repository of information on additives and lubricants for

the lubricant industry. A list of additive suppliers and the Uniform Resource Locators (URLs) for

the corresponding website can be found in Chapter 31 and in the CD accompanying this book. In

addition to websites from the individual companies, industry organizations such as Noria, Society

of Tribologists and Lubrication Engineers (STLE), and others provide information on lubrication,

which includes discussion of additive interactions and depletion during use. Book publishers have

also placed online, the contents of entire books that can be purchased to download in their entirety

or only the desired chapter(s). Several recent examples of books speci cally related to additives and

lubricant  uids have been published [1–3].

20.2 HISTORY OF ADDITIVES

The Egyptians around 1500 BC may have used animal fats with or without water-based additives to

move the huge stones and statues to the building sites of the early pyramids on lubricated wooden

sledges [4]. Most certainly, there were earlier applications involving the lubrication of wheels and

axles with mixtures of animal fats and natural oils. The earliest reported evidence of solid  lm lubri-

cation were metal inserts in wooden implements found in the Middle Ages, around AD 500 [5].

The use of additives and additive industry has grown as a result of the larger use of lubricants

and the more stringent requirements placed on lubricating  uids. The Industrial Revolution, ∼1700

to 1800, resulted in the greater need for both lubricants and lubricant additives. Addition of inor-

ganic chemicals to water and the use of animal fats, vegetable, and  sh oils were common.

The discovery of oil at Titusville, Pennsylvania, in 1859 resulted in not only the growth of a new

petroleum-based industry but also a need for an additives industry to supply chemical components

that would impart improved properties and performance to the oils being used as lubricants at the

time [6]. Early use of additives to improve lubricating oils was in metalworking  uids required by

the rapid expansion of the railroad and automobile industries [7]. Animal fats,  sh oils, vegetable

oils, and blown rapeseed oils were added as friction modi ers. Rosin oil, mica, and wool yarn

were among the early grease additives. Sulfur was added to metalworking oils as early as 1916 and

shortly thereafter phosphorus additives came into use.

Petroleum re ning continued to grow in scale and in process technology during the beginning

of the twentieth century. At  rst, re ning operations were aimed at improvements in the quality and

quantity of fuel, heating oils, and asphaltic components for road paving. Between these ends of the

spectrum, there were petroleum distillates that found their way into use as lubricants. These were

high-boiling components and contained various amounts of sulfur and nitrogen that were later found

to retard oxidation and wear when these  uids were used as lubricants. So one might consider the

natural nonhydrocarbon components of petroleum as the  rst modern lubricant additives. Chemists

at the major oil companies worked to isolate these materials and identify their structures so that

they could be made as desired and added in optimum concentrations depending on the application

requirements. There is an analogy here to the pharmaceutical industry where natural plant sub-

stances are extracted, isolated, and characterized as a prelude to direct chemical synthesis of these

materials or improved analogues. Process engineers at the same time were developing processes to

eliminate these materials so as to produce more stable, clearer lubricants. Synthetic chemists were

also working to develop synthetic oils formed from basic chemicals derived from petroleum and

other sources. All of these resulted in lubricant  uids that contained lower concentrations of naturally

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Additives for Industrial Lubricant Applications 495

occurring antioxidants and antiwear (AW) and corrosion components. This made these oils more

susceptible to degradation and less-effective lubricants, and therefore there arose a need to develop

speci c additives to protect these new  uids from all of the degradation processes that occur under

the newer, more severe operating conditions. It should be noted that in general for petroleum-derived

lubricants, the greater the extent of hydroprocessing the lower the amount of heteroatom components,

and therefore the more nonpolar and hydrocarbon-like the  uid.

Improvements in the performance of industrial lubricants since the late 1930s, for the most part,

resulted from the development and use of synthetic additives. These developments occurred initially by

researchers in major oil companies, but as the industry developed, many additive companies came into

existence and  ourished. A chronology of the development of some additives is given in Table 20.1.

Several reviews of lubricant additives and their applications have been published [3,8–12].

Companies interested in protecting their research investments and technology patented their new

innovations, and by the 1950s, over 3000 additive patents had been issued. The rapid growth of the

additive market in the 1900s has slowed in recent years because the world’s  nished lubricant mar-

ket continues to remain  at. It is predicted that global additive demand will grow at only a 1–1.2%

per year until about 2015 [13].

TABLE 20.1

Chronological History of Additive Developments

Period Application Additive Comments

BC Wheels Animal fats, oils Chariots, carts

Construction Water Pyramids, moving heavy stones

1500 Wire drawing Wax Au, Ag metals

1750 Industrial Water Industrial revolution, cooling

1800 Industrial, metalworking Water and inorganics Rust prevention

1900 Industrial Phosphorus, sulfur Oxidation, wear protection grease

1920 Metalworking Emulsi ers Soluble oils

Automotive, engine oils Fatty acids Friction reduction

1930 Engine oils Isoparaf ns, polymethacrylates Pour point depressants

Gears Lead soaps EP/wear protection

1940 Engine oils Calcium carboxylates Detergents

Engine oils Zinc dithiophosphates AW/antioxidant

Engine oils Sulfonates, phosphonates, phenates Detergents, dispersants

1950 Metalworking Soluble oils Cooling capability

Engine oils Viscosity modi ers, basic sulfonates Temperature–viscosity improvers,

detergents

Overbasing, salicylates Dispersants, acidity, oxidation control

1970 Industrial Solid additives EP/wear protection

Phosphate glass

Boron nitride

Fluorocarbons Te on-like additives

Low toxicity

Biodegradable Environmentally friendly

1990 Industrial Low volatility Reduced emissions

Food grade Safety in food processing/handling

Nanomaterials Improved microlubrication

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496 Lubricant Additives: Chemistry and Applications

20.3 THE FUNCTION OF ADDITIVES

As evidenced by the plethora of structures for additives described in the  rst 19 chapters of this

book, the lubricants industry has had the advantage of the talents of many excellent organic chem-

ists able to provide new organic and inorganic components as additives. Many of these structures, or

slight modi cations, are used to stabilize similar materials in other industries. Naturally occurring

materials or synthetic versions are also used. For example, alpha-tocopherol, vitamin E, is used as

an antioxidant both in food products and in lubricant products to maintain the stability of highly

isoparaf nic base  uids that are used in food-grade and nonfood-grade industrial and automotive

lubricants. Antioxidants, in particular, are employed to protect raw materials used in lubricant for-

mulations as well as to protect the fully formulated oil during storage and use.

Although most industries have speci c performance requirements related to their use of addi-

tives, there are some general requirements of additives that apply to all applications.

The overriding criterion for additives in lubricant formulations is that it be effective for the

application. That said, there are certain criteria that make this possible.

These include the following:

1. Solubility. The additives must all be soluble in the formulated oil, and in some applications,

this may require solubility in water. Additive solubility is critical in a lubricant through-

out the temperature range that the lubricant will experience. This includes conditions of

storage as well as the lowest and highest temperatures the oil will experience during use.

Lubricants must be formulated such that the additives remain in solution as the oil is used

so that the additives do not drop out and lower their effective concentration in the lubricant

and so that they do not cause any deleterious effects by virtue of their insolubility.

2. Stability. The additives should have good stability to ensure acceptable performance over

the useful life of the lubricant product. Lack of additive stability can affect the color and

odor of the product. Breakdown of additive components caused by heat or light or moisture

can result in reduced product stability and therefore performance. Many additives, espe-

cially arylamine antioxidants, are susceptible to darkening on exposure to light. This can

affect product performance in terms of stability and also affects personal acceptance of the

product in terms of quality.

3. Volatility. Low volatility is generally required for most industrial lubricant applications.

This reduces the nonuniform loss of oil from the system and therefore thickening of the oil

caused by the heavy components remaining in the system. Furthermore, this is often not

considered when formulating; the additives that are chosen must be nonvolatile under the

conditions of use to maintain the concentration of these additives in the lubricating  uid.

This is especially true of lubricants that are to be used in high-temperature applications. In

some particular applications, there is a need for a certain degree of volatility of the additive,

for example, vapor-phase corrosion inhibitors must have a controlled rate of volatility.

4. Compatibility. Additives must be compatible with other components in the systems, includ-

ing base oils and other additives. One of the most important considerations is compatibility

with the base  uids being used in the formulation. This also involves compatibility with

system components such as seals, gaskets, and hoses. Incompatibility is initially observed

as a haze or cloudiness during formulation. This should be the  rst clue that something is

insoluble or reacting. Bench tests that include visual observation of the oil after testing can

also provide information as to the compatibility of additives in the formulated oil.

5. Odor. Odor must be acceptable for the particular application. This is especially true in

food-processing applications in which any odor from the base oil or additives can affect

food product quality due to adsorption of odorous oil components, including additives. The

same may be true for lubricants that are used in the manufacture of textile or paper or other

 brous materials that can absorb odors.

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Additives for Industrial Lubricant Applications 497

6. Activity. The additive must have functional activity over the lifetime of the application. For

example, a very reactive antioxidant that is depleted in a short time period will render a

lubricant susceptible to oxidation sooner than a product formulated with long-term acting

antioxidants as part of the additive chemistry in the product.

7. Environmental compatibility. Three issues to consider here are biodegradability of the

additive, disposal constraints, and toxicity. These issues are important both from the stand-

point of use and proper disposal and also in the event of a spill. Many industries are now

using life cycle analysis to understand and control the use of all components that enter

and leave their products and processes [14–16]. Green chemistry is also applied to the

manufacture of additives to minimize the environmental impact of these processes and the

disposal of waste products.

8. Health and safety issues. These issues are concerned with any of the aspects of human

contact and transportation of the additives as raw materials or as components in  nished

products. Health and safety issues are particularly important for additives when used in

food-grade lubricants for use in food processing plants. Every additive that is allowed for

use in food-grade H-1 (incidental contact) lubricants has an upper concentration limit.

Lubricants formulated with quantities higher than these limits are not considered food

grade because of the potential for unsafe levels of the additive to contact the food.

Before describing the chemistry of the various classes of additives used in the wide variety of

industrial applications, it is appropriate to consider the additive types in terms of how they func-

tion as part of the formulated oil, that is, physical interaction or chemical interaction. Additives

that function by physical interaction act through physical adsorption–desorption phenomena. These

additives include pour point depressants, additives that impart lubricity, color stabilizers, additives

that change in structural form with changes in temperature (viscosity index improver [VII]), addi-

tives that cause changes in surface or interfacial tension (antifoam and emulsi ers), those that cause

the formation of structures that trap base  uid (tacki ers, thickeners, and  llers), antimisting addi-

tives, water repellants, or additives that affect the perceptive qualities of a lubricant (odorants, color

stabilizers or dyes, and chemical marking agents) (Table 20.2).

Table 20.3 describes those additive types that act by chemically reacting with surfaces and are

consumed or chemically changed during use. Chemical processes can also change the physical

TABLE 20.2

Additives That Function by Physical Interaction

Additive Function

Antifoam Prevents the formation of stable foam

Antimisting Reduces the tendency to mist or aerosol

Color stabilizer Slows darkening of  uids

Demulsi er Enhances the separation of water and oil by promoting drop-coalescence

and gravity-induced phase separation

Dyes Impart color, mask color, product identi cation

Emulsi er Reduces interfacial tension and allows dispersion of water

Friction modi er Associate with the metal surface and improve sliding between surfaces

Odor control Prevent or mask undesirable odors or maintain odor level

Pour point depressant Lowers low-temperature  uidity by reducing the formation of wax crystals

Tackiness Improve cohesion of  uid and nondrip quality

Thickener, solid  ller Converts oil into solid or semisolid lubricant

VII Improve viscosity–temperature characteristics

Water repellant Impart water resistance to greases and other lubricants

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498 Lubricant Additives: Chemistry and Applications

properties of the lubricant. For example, oxidation of the base  uid in a formulation results in deg-

radation, producing smaller molecules that are more volatile than the original molecules. This has

the effect of increasing the viscosity of the oil as the lower-molecular-weight components volatilize

and can result in a decrease in oil volume if the process continues.

One should note that the chemical structure of the base  uid and that of the additives used

in a lubricant formulation are all crucial to the properties and performance of the lubricant. The

polarity of the base  uids and the additives is very important because each component of the

mixture is competing for the metal surfaces. A polar additive that would normally adsorb and

desorb reversibly from a metal surface in a nonpolar base  uid and has a reasonable steady-state

concentration on the surface might have a much lower, even ineffective, concentration on the

surface when in a formulation that contains a high concentration of polar base  uid such as ester

or vegetable oil.

The AW performance of tricresyl phosphate has been studied in 11 different base  uids. When

molecular weight, dielectric constant, and solubility parameters were considered, correlations for

optimum concentration of additive were obtained [17].

20.4 CLASSES OF ADDITIVES

20.4.1 A

NTIOXIDANTS

Antioxidants chemically interact with free radicals and hydroperoxides to slow the chain reac-

tions that result in oxidation of the base  uid. The two most common classes of compounds that

are used as antioxidants are hindered phenols and aromatic nitrogen compounds. Some phenolic

antioxidants commonly used in industrial applications include di-tert-butyl p-cresol (BHT) and

3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid, C7–C9 branched alkyl ester. Examples of aryl

amines include diphenylamine, dioctyl diphenylamine, styrenated diphenylamine, phenyl-alpha-

naphthylamine (PANA), and polymerized versions of trimethylquinoline. These types of additives

are described in detail in Chapter 1. In general, these types of antioxidants are used in gear oils,

turbine oils, hydraulic oils, compressor oils, and various greases. Some additive molecules contain-

ing different chemical structures, such as phenothiazines, are sometimes used in ester formulations

and can react many times with oxygen molecules before they are depleted. Other additives such as

TABLE 20.3

Additives That Function by Chemical Interaction

Additive Function

Antibacteria (biocides) Prevents or slows growth of bacteria in systems

Anticorrosion (corrosion inhibitors) Protects surfaces against chemical attack

Antioxidant Reduces the rate of lubricant oxidation and deterioration

and increases oil and machine life

Antirust Eliminates rusting due to water or moisture

AW Reduces thin- lm, boundary wear

Basicity control Neutralizes acids from oxidation processes

Detergent Maintains surface cleanliness

Dispersant Suspends and disperses undesirable combustion, wear, and

oxidation products

EP (temperature) Prevents seizing and increases load-carrying ability

Friction modi ers Reduce friction and increase lubricity

Metal deactivator Counteracts catalytic effects of surfaces by passivating

surfaces

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Additives for Industrial Lubricant Applications 499

zinc dithiophosphates (ZDTPs) and ashless phosphorus-containing additives can also function as

antioxidants in addition to other functions. These are described in Chapters 2 and 3, respectively.

20.4.2 ANTIWEAR AND EXTREME-PRESSURE (ANTISCUFFING) ADDITIVES

AW- and extreme-pressure (EP)-type (antiscuf ng) additives react with metal surfaces to form a

chemical  lm that is sheared more easily than the moving metal surfaces. Additives that perform

these functions, ZDTPs, ashless phosphorus additives, ashless AW and EP additives, and sulfur car-

riers, are described in detail in Chapters 2, 3, 8, and 9, respectively. Two criteria differentiate AW

from EP additive. The differences depend on the load the additive  lm formed can carry and how it

survives the high temperatures generated by the friction forces. Although bulk oil temperatures may

be low, the temperatures at the metal-to-metal interfaces in the contact zone can exceed 300ºC. At

extreme loads, the frictional heat can exceed the melting temperature of the metal. In fact, during

four-ball EP testing, the weld load is that load that caused the balls to actually weld to each other.

Some additives, for example, ZDTPs and alkyl amines, perform multiple functions. These addi-

tives can function as antioxidants and AW additives or as AW and surface deactivators. ZDTPs are

some of the most economical and effective additives. The largest application is in the formulation

of automotive and diesel engine oils; however, they are used in some industrial hydraulic  uids. The

temperature at which zinc dialkyldithiophosphates (ZDDPs) are most effective depend on the mix

of hydrocarbon group, that is, primary alkyl, secondary alkyl, or aryl. Therefore, both the amount

and structure of the ZDDP components affect the performance of the fully formulated oil.

ZDTP has come under scrutiny from an environmental viewpoint in automotive engine oils.

Phosphorus and sulfur in the additive are suspected of being detrimental to the effectiveness of

after-treatment catalysts in spark (gasoline) and compression ignition (diesel) engines. Research is

ongoing to develop a cost-effective chemical alternative to ZDDPs. Alternative additives, evaluated

to date, are not as effective and are signi cantly more expensive than the ZDTPs. As combustion

in the presence of lubricants and after-treatment catalysts are not factors in industrial applications,

there is no problem with use of ZDDPs.

The adsorption and desorption of dibenzyl disul de and dibenzyl sul de on steel have been

investigated. This involved the study of the rate of uptake of labeled additives onto stainless steel

disks. The kinetics of both adsorption of these additives and the desorption from the metals was

examined. Two processes, reversible physisorption and irreversible chemisorption, were identi ed,

and the implications in AW and EP behavior were described [18].

20.4.3 FRICTION MODIFIERS

Friction modi ers or lubricity additives (see Chapter 7) absorb on the surface to form  lms that

reduce the friction between the moving surfaces. These additives are generally polar molecules

having a polar functional group (alcohol, aldehyde, ketone, ester, and carboxylic acid) and a non-

polar hydrocarbon tail. The polar portion of the molecule adsorbs onto the surface with the long

hydrocarbon chains exposed to the moving surfaces, reducing the friction. They can also contain

polar elements that adsorb and chemically react with the surface to form a protective  lm. Vegetable

oils, fats, and animal based have structures that make these materials excellent friction modi ers.

In addition to liquid organic friction modi ers, there are various solid lubricants that reduce fric-

tion. These materials are reviewed in Chapter 6. A recent example from the literature reports on the

in uence of graphite, molybdenum disul de, and per uoropolyalkylethers (PFPAE) on changes in

the shear stress values in lubricating greases [19].

20.4.4 VISCOSITY MODIFIERS/VISCOSITY INDEX IMPROVERS

VIIs are very high-molecular-weight compounds (polymers) that function by physically changing

form with changes in temperature. VIIs can be depleted chemically through thermal or oxidative

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500 Lubricant Additives: Chemistry and Applications

breaking of bonds. Higher-molecular-weight versions can be mechanically sheared to form smaller,

less-effective molecules. The stability of the VII depends on both the chemical structure and the size

of the molecule. There are two main structural types of molecules used as VIIs—the nonpolar ole n

copolymers (Chapter 10) and the polar polymethacrylates and related compounds (Chapter 11).

20.4.5 TACKINESS AND ANTIMISTING ADDITIVES

Tackiness and antimisting additives are used in several applications. In particular, tackiness is

important in maintaining contact between the lubricant and a metal surface. Lubricants can have

the tendency to migrate away from the surfaces that need lubrication depending on the lubricant

viscosity and the geometry of the equipment. Antimisting is important in terms of reducing the ten-

dency of lubricants to become airborne aerosols or  ne mists. The materials that are used to reduce

these phenomena are described in Chapter 13.

20.4.6 SEAL SWELL ADDITIVES

Seal swell additives are critical in maintaining contact between rubber or other types of seal mate-

rials and the metal surfaces in equipment. These seals keep lubricant where it is needed and away

from areas where it is not desired. Seal swell agents are adsorbed by the seal material and prevent

the seals from cracking and deteriorating. These additives are described in Chapter 14.

20.5 DYES

Dyes are often used to identify  uids by application. Hydraulic  uids were often dyed red and

coolants green during World War II to make it easier for technicians to select the correct lubricant

and thus properly maintain equipment. MIL-L-Spec.5605 Red Oil was used as an aircraft hydraulic

 uid in World War II in many aircraft. Dyes are also used for product branding and are used to

identify that a particular supplier’s oil has been used instead of a lower-cost-competitive oil.

20.6 BIOCIDES

Biocides are used to reduce the growth of bacteria in lubricants used in metalworking. These appli-

cations by nature use large volumes of lubricants that are exposed to air and the environment and

consequently have the tendency to decompose by natural processes. To maintain lubricant quality

and performance, biocides are added to these lubricants. These are described in Chapter 15.

20.7 SURFACTANTS

Surfactants are one of the most widely applied and diverse group of materials. They have been

applied to lubrication for centuries and form the basis for a combination of performance features

that include reducing friction and wear and cooling during lubrication. As these materials can solu-

bilize polar and nonpolar components, surfactants make possible many of the  uids used in met-

alworking and other areas of lubrication. Surfactants adsorb to the metal surface and provide a

cushion or molecular boundary between moving surfaces to protect them from wear. The chemistry

of surfactants is described in Chapter 16.

20.8 CORROSION INHIBITORS

Corrosion is potentially devastating to industrial equipment that comes in contact with water and

other corrosive materials. Even water in the form of atmospheric humidity, that can enter equipment

or react with exterior surfaces, can reduce the lifetime of expensive industrial machines. Corrosion

inhibitors are summarized in Chapter 17.

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Additives for Industrial Lubricant Applications 501

20.9 ADDITIVES FOR FOOD-GRADE LUBRICANTS

Lubrication is obviously needed in the food production and the preparation industries for the same

reasons as are needed to lubricate machinery in other industries. An additional requirement, how-

ever, in the food industry is the need to be certain that the chemistries used are safe in the event

that any of the lubricant makes contact with the food product. There are speci c additives that are

allowed and limiting concentrations of these additives permitted in food-grade additives. Additives

for food-grade lubricants are described in detail in Chapter 21.

20.10 MAGNETIC DISK DRIVE ADDITIVES

Magnetic disk drives are a critical component of our modern technology. These components are

evolving in terms of the rate or rotation of the magnetic disk, and precision and smaller tolerances

in the rotating spindle. Lubricants are designed to provide adequate lubrication, and additives must

maintain performance at both the spindle motor and the head–disk interface. Additives protect

against oxidation of the base  uid and control conductivity and other properties necessary for long-

term performance.

Additives for magnetic disk recording applications are described in Chapter 22.

20.11 ADDITIVES FOR GREASE

Additives for use in grease are in general similar to those used in other industrial lubricants, but their

concentrations and combinations can be quite different. These additives are described in Chapter 23.

20.12 ADDITIVES USED IN BIODEGRADABLE LUBRICANTS

Bioderived and biodegradable lubricants are becoming more important due to increased interest in

protecting the environment and due to regulations imposed by many governments around the world.

For lubricants to meet the environmental criteria of biodegradability and toxicity, the additives must

perform needed functions without being toxic themselves. These additives are sometimes the same

as those currently in use in other applications, but more and more, they are speci cally developed

to meet the performance criteria and be nontoxic and biodegradable. The performance of some of

these materials is described in Chapter 18. One example of newly synthesized additives directed at

biodegradable esters is reported [20]. These additives were reported to form stable lubricating  lms

on the surface of stainless steel balls used in four-ball wear testing.

Additives improve the properties and performance of lubricants by modifying chemical and

physical characteristics of the oil. The mechanisms of action of additives depend on the chemical

structure of the additive, the chemical structure of the base  uids, the temperature of use, and the

mechanical pressures applied to the formulated oil. Many of these mechanisms are well understood

and have been described in the literature. Some aspects of the additive chemistry of the more com-

mon additives are brie y reviewed in the following section.

20.13 ADDITIVE CHEMISTRY

An understanding of additive chemistry is necessary both for the synthesis of additives and to

develop an understanding of how lubricants and additives react, perform, and degrade during use.

Beginning in the 1940s, the severe operating conditions of military equipment during World War II

led to the investigations of these areas in much greater detail than had previously been done. More

severe operating conditions required greater thermal and oxidative stability, less friction, and better

wear protection from the lubricants. As lubricant base  uids provide only part of the required prop-

erties and performance, the additives used in formulations needed to provide greater performance

than ever before.

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502 Lubricant Additives: Chemistry and Applications

Research on oxidation mechanisms and oxidation inhibitor additives have been investigated

by Ingold [21], Watson [22], Mahoney et al. [23], Chao et al. [24], Jensen et al. [25], Booser et al.

[26–28], Dennison and Condit [29] and Zuidema [30], and others.

Antioxidants reduce oxidation by trapping free radicals or by decomposition of hydroperoxides

generated during the oxidation process. As discussed earlier, substances containing amines or

phenolic groups are effective antioxidants. Antioxidants that contain sulfur and combinations

of phosphorus and sulfur are effective against hydroperoxides [31,32]. Some metal-containing

compounds such as copper naphthenates can be either prooxidants or affect antioxidants depending

on the formulation and the additive concentration [33,34].

A review of the effects of metals and the effect of hard-coated metals on the thermooxidative

stability of branched per uoroalkylether lubricants have also been reported [35,36].

Metal deactivators reduce the effect of metals and their salts on the rate of oxidation of the bulk

 uid [37]. Some of these, such as ethylenediaminetetraacetic acid (EDTA), complex with the metal

particles suspended in the  uid rendering them inactive.

AW additives form protective  lms by chemical or physical reaction with the surface [38–42].

They minimize the removal of metal through formation of lower shear strength  lms that reduce

friction, decreasing the contact temperatures or by increasing the contact surface thereby reducing

the effective load. The most effective AW additives contain sulfur or phosphorus or both. Studies of

the performance and mechanisms of action of tricresyl phosphate (TCP) and ZDDP have been well

documented in the literature [43–48].

EP, or more correctly extreme-temperature additives, also contain phosphorus or sulfur com-

pounds. Chlorinated compounds are also effective [49]. Use of phosphorus compounds started

around the time of World War I, and addition of sulfur and chlorine to gear oils began in the 1930s

[50–53]. EP gear oils in the 1960s and 1970s were formulated using combinations of sulfur-, phos-

phorus-, and chlorine-containing additives. The use of chlorine-containing AW and EP compounds

is gradually declining due to environmental concerns with some suppliers developing new replace-

ment products [54].

Rust and corrosion inhibitors adsorb on the surface forming a barrier hydrocarbon  lm against

water and corrosion-causing materials. These are often basic amine-type compounds that have the

ability to neutralize acids. They may also passivate the metal surface reducing the catalytic effect

on oxidation of the oil [55,56]. When combined with antioxidants in an additive package, this com-

bination is typically referred to as rust and oxidation package or rust and oxidation oil (R&O)

and generally provides protection to industrial oils that are used under relatively mild operating

conditions.

Detergents can neutralize acids produced by oxidation of the oil or, in the case of internal com-

bustion engines, combustion. The combustion products get into the  uid as a result of blowby. Deter-

gents can be phenates, sulfonates, and salicylates [38,57]. Typical dispersants are succinimides,

succinates, and Mannich-type reaction products. Dispersants slow the formation of deposits and

sludge by dispersing the precursors and insoluble particles in the  uid. These, like detergents, are

large molecules with a polar end and a nonpolar portion. Dispersants and detergents are typically

used in engine oils and not in industrial oils. These two categories of additives represent the largest

category of additives sold. Normally, dispersants and detergents are not required in industrial gear

oils, turbine oils, or industrial hydraulic  uids.

Foam inhibitors prevent formation of foam by changing the surface tension that results in the

collapse of gas bubbles as they form [58]. The inhibitors are high-molecular-weight polymers that

are relatively insoluble in the oils. Antifoam additives are generally used at concentrations mea-

sured in parts per million and are usually blended into a more soluble solvent system to facilitate

the accuracy of addition of the small quantities required. Some commercial antifoam additives

supplied are already diluted in a suitable carrier  uid at concentrations that help to make addi-

tion of the material more accurate. Air entrainment is often mistaken for foaming in hydraulic

systems. Additives can sometimes help with air entrainment, but usually, mechanical changes

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Additives for Industrial Lubricant Applications 503

to the system to prevent air entering the system or modifying the base  uid is a better approach.

Polydimethylsiloxanes, polydiarylsiloxanes, mixed siloxanes, and polyglycol ethers are examples

of these additives.

Friction modi ers are used to lower the energy requirements of a system by reducing the

friction of the system. These additives are often long-chain hydrocarbon molecules with one end

of the molecule containing a polar group that is adsorbed on the surface [59–61]. Chain length and

branching of the carbon chain affect both the surface adsorption of these additives and the packing

density on the metal surface. Oleic acid and other organic fatty acids, alcohols, and amides are typi-

cal structures of commercial friction modi ers.

In addition to liquids, there are also solids that act as friction modi ers. Graphite or molyb-

denum compounds are examples of solid friction modi ers. These materials provide reduction in

friction and often help reduce wear. These materials are not soluble in hydrocarbon oils and are

therefore suspended in the lubricating  uid.

Rheological properties of industrial oils are important performance criteria [62–65].

Viscosity at normal operating temperatures is an important lubricant speci cation. The lubri-

cant  lm thickness under operating conditions needs to be considered to mitigate wear and damage

to equipment. Viscosity is a function of temperature and pressure (and shear rate for non-Newtonian

 uids). VIIs are used to modify the temperature–viscosity characteristics of an oil. VIIs are high-

molecular-weight polymers that are compact molecules at low temperatures and expand in size as

the temperature increases. Styrene-butadienes, acryloids, polymethacrylates, and various copoly-

mers are the main VIIs used in industrial oils today. Also VIIs usually increase the viscosity of the

lubricant and can be used in formulating a  uid to increase the viscosity of a base  uid.

Low-temperature viscosity is also an important speci cation for equipment that must start or

operate at low ambient temperatures. Paraf nic petroleum–derived base oils become very viscous

at low temperatures unless the high wax content is removed. Typically, isoparaf nic and naphthenic

base oils are  uid down to temperatures below –40ºC. Synthetic  uids generally have excellent

low-temperature properties. Natural oils, such as vegetable oils, are solids or become very viscous

by ∼0ºC.

Pour point depressant additives are often used to improve de ciencies in lubricants based on

petroleum-derived paraf nic base  uids. These additives interfere with the wax crystalline net-

work formed in these paraf nic oils at low temperatures. The effectiveness depends on both the

type of  uid and the chemical structure of the additive used. Evaluation of concentration levels and

 uid blending is required to achieve the maximum low-temperature properties for these additives.

Several Group II and Group III base  uids contain high levels of isoparaf nic molecules and have

acceptable pour points without the need of pour point depressants. Natural oils, on the contrary,

may require blending with synthetic  uids and pour point depressants to achieve the necessary vis-

cosity levels at lower temperatures. Some commercially available additives are effective in mineral

oils but have limited or no effect in natural oils. Acceptable pour point depends on the conditions

of use. Obviously,  uids to be used in arctic conditions must have  uidity at the lowest temperature

of use. Generally, this will be most important at start-up because some frictional heating will help

bring a lubricant to a lower viscosity as the equipment continues to run. This is critical for equip-

ment in remote locations such as space applications and remote measuring equipment. However,

these  uids are generally designed with synthetic base  uids that have low pour points and low

low-temperature viscosities.

System components such as seals and gaskets can shrink, swell, crack, or otherwise deterio-

rate if they are not compatible with the  uids used. Seal swell control is important to ensure  uid

compatibility with the proper seal materials to prevent oil leaks. Organic phosphates have been

used; however, the generally accepted approach, especially for synthetic  uids, is to blend the

base  uids to achieve the correct degree of seal swell. For example, blending mixtures of syn-

thetic hydrocarbons, such as polyalphaole ns, and esters to achieve desired seal swell results is a

common practice.

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