Lubricating Grease: A Chemical Primer - ACS Publications

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George B. Kauffman California State University Fresno, CA 93740

Lubricating Grease: A Chemical Primer

Craig J. Donahue Department of Natural Sciences, University of Michigan–Dearborn, Dearborn, MI 48128; [email protected]

Undergraduate chemistry textbooks rarely discuss lubricating grease. One exception is the recently published textbook by Gesser (1). This topic has not been addressed in JCE either. This primer was assembled to provide the instructor and student background information on the function, composition, preparation, and properties of lubricating greases. This topic is part of the larger topic of lubricants, which in turn is part of the field of tribology, the science of friction, lubrication, and wear. We have incorporated the topic of lubricating grease into a new general chemistry curriculum targeted at undergraduate engineering students. To make chemistry more interesting and relevant to this audience the overarching theme of “chemistry and the automobile” is used and integrated into each topic covered in this two-semester sequence. In the first half of the second semester, when an introduction to organic chemistry and synthetic polymers is presented, petroleum refining, petrochemicals, fuels, and lubricants are used as examples. Lab experiments (2–4) have also been developed to support this curriculum. Grease is one of several lubricant types needed to operate an automobile (5, 6). Engine oil and automatic transmission fluid are two other extremely important automotive lubricants. Brake fluid and power steering fluid also perform at least some lubricant functions. Although grease consumption represents only 2–4% of the total lubricant use (7), it has the distinction that it is used in more locations in a vehicle than any other lubricant. The automobile industry is the biggest user of grease, accounting for approximately 40% of total grease consumption (7).

Grease is designed to serve many of the same functions as a liquid lubricant. The base oil in grease lubricates moving metal parts because greases are thixotropic gels, gels that flow under shearing forces and then resolidify when the force is removed. Some of the important differences between grease and a liquid lubricant are the following (1, 6, 9–16):

General Functions of Lubricants and Greases

Consistency Although grease is a semisolid that is formulated to stay put when applied, it nonetheless needs to exhibit the property of pumpability. Grease is pumped into containers at the end of the manufacturing process or into large tanker trucks for delivery to the user. The user often pumps the grease through lines in the factory where it is applied to parts on an assembly line. The National Lubricating Grease Institute (NLGI) has created a classification scheme to communicate the consistency of the grease that is based on the depth to which a metal cone penetrates the grease at 25 ⬚C (15, 17). The NLGI numbers range from 000, 00, 0, 1 to 6. Grease with a 000 NLGI number is the softest and grease with a 6 NLGI number is the hardest. Depending on the manufacturing methods and the formulation, the texture of the grease may be smooth, buttery, stringy, fibrous, spongy, or rubbery.

A fully-formulated liquid lubricant will provide many of the following functions (1, 5, 8): • It reduces friction by forming a continuous fluid film to prevent direct contact between moving parts under pressure (hydrodynamic lubrication). • It provides wear protection by depositing a chemical film on surfaces when the lubricant film becomes too thin to give full fluid film separation of rubbing surfaces (boundary lubrication1). • It reduces corrosion (and inhibits rust formation). • It reduces the formation and buildup of sludge, varnish, and deposits. • It transfers heat away from hot surfaces and supplies heat to cold surfaces. • It suspends and transports away particles, debris, and contaminants. • It seals by providing a fluid barrier and by delivering chemical additives that cause seal-swell. 862

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• Grease, once applied stays put—it does not wash away or drip off. • Grease functions poorly as a coolant—it is ineffective at dissipating heat. • Grease does not wash away debris and contaminants; however, it does prevent contaminants from entering the system. • Some greases can absorb considerable quantities of water and still lubricate satisfactorily. • Grease is the lubricant of choice for machinery that operates intermittently or under extreme conditions. • Grease is the lubricant of choice for parts that are not easily accessible and parts that will go a long time before being supplied with fresh lubricant or are lubricated for life when assembled.

Typical Properties of Greases Among the important properties associated with greases are consistency (or hardness), high- and low- temperature operating ranges, stability in the presence of water and oxygen, shear stability, and dropping point. A battery of tests, employing special test equipment, is used to measure these and other properties (17).

Dropping Point The thermal stability of grease is communicated by reporting its dropping point even though grease is never used at that high a temperature. Grease does not have a sharp melt-

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Chemistry for Everyone

ing point. The dropping point is measured by packing the grease in a standardized thimble with a small hole in the bottom of it (17). The temperature at which the grease has softened sufficiently that a drop of liquid falls out the hole is the dropping point of the grease. General Composition and Structure of Greases It is useful to review the distinction between liquid lubricants and greases. Liquid lubricants consist of base oil, either mineral oil or synthetic oil, and a collection of additives that must be soluble in the base oil to perform properly. Typical multipurpose automotive grease consists of base oil made from mineral oil, a soap-thickening agent, and an extensive additive package often containing solid lubricants (molybdenum disulfide, MoS2, or graphite). Greases are unique in that insoluble additives (e.g., MoS2 or graphite) can be added and still perform their function. Such multipurpose greases are estimated to cover 80% of all applications requiring grease lubrication (9). Grease is distinguished from other lubricants by its physical state—it is a semisolid or solid. Grease is unique among lubricants for the presence of a thickener or gelling agent. Typical multipurpose grease is 10 wt % thickener. Metal fattyacid soaps, derived from animal fats and vegetable oils, constitute the most important class of thickener. Other categories of grease are complex soap-thickened greases and the nonsoap-thickened greases. Grease has been likened to a sponge soaked with liquid lubricant (9). Grease is not a chemical solution, but rather a three-dimensional matrix of soap fibers dispersed throughout a liquid lubricant—minimally a two-phase system. This structure has been confirmed by electron microscopy, which reveals an elaborate fibrous matrix created by the fatty-acid soaps in soap-thickened and complex-soap thickened greases (6, 10, 15, 18, 19). The base oil is thought to be bound to the framework of the stringlike fibrils by three mechanisms: (i) molecular attraction between polar regions on the soap fibers and the base oil molecules, (ii) capillary forces, and (iii) mechanical occlusion (6). Since grease’s behavior can be described as both solidlike and liquidlike it has been labeled a viscoelastic plastic solid (9). The rheological behavior of grease is quite complex. While base oils are Newtonian fluids, greases are non-Newtonian fluids. For Newtonian fluids, flow rate (shear rate) is proportional to the applied pressure (shear stress) at a given temperature. Grease does not begin to flow until a shear stress exceeding the yield point is applied. Apparent viscosity, the label given to the behavior of a non-Newtonian material, varies with both temperature and shear rate. Properties, Composition, and Origin of Petroleum-Derived Base Oils (20–23) Base oil represents between 65% and 95% by weight of the typical grease. The base oil used is nearly always a mineral oil, as opposed to synthetic oil. Mineral oil is a product of petroleum refining. Synthetic base oils possess superior properties (e.g., low pour points, high temperature stability, oxidative resistance) compared to mineral oils, but are more expensive and therefore seldom used in multipurpose greases. Some specialty greases rely on synthetic base oil. Types of synwww.JCE.DivCHED.org

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thetic base oils include synthetic hydrocarbons (hydrogenated polyalphaolefins), diesters, silicones, phosphate esters, perfluoropolyethers, and fluorinated silicones (24). Base oil viscosity is an important parameter in grease formulation and has some bearing on the consistency (hardness) of the final grease. The nature of the soap thickener and the quantity used has the most influence on the consistency of the grease. To thicken properly, the soap thickener should be neither completely soluble in the base oil, nor completely insoluble. The base oil should have some affinity for the soapthickener; this is referred to as solvency.

Petroleum-Derived Base Oil Properties The base oil used in grease is similar to that used in liquid lubricants (e.g., motor oil). This base oil is composed of hydrocarbon molecules possessing between 25 and 45 carbon atoms, a molecular weight range of 350 to greater than 700, and a boiling point range of 350 ⬚C to greater than 500 ⬚C (20, 23). As a point of reference, the molecular weights and boiling points of two long-chain n-alkanes are: n-C20 (282 g mol᎑1, 287 ⬚C) and n-C44 (619 g mol᎑1, 555 ⬚C). Just as crude oils are often characterized as paraffinic or naphthenic depending on the relative quantities of paraffins and naphthenes present; so base oils are also labeled paraffinic or naphthenic reflecting their origin or content.2 Paraffinic oil will contain predominantly paraffins, but a sizeable fraction of the oil may be naphthenes. Naphthenic oil contains predominantly naphthenes, but also contains paraffins. The aromatic content of the base oil is strongly dependent on the type of refining processes used, but is usually low, less than one percent in some cases. The most important property of lubricating base oil is its viscosity. This quantity is often reported as a dimensionless number referred to as viscosity index (VI). The kinematic viscosity of a sample is measured at 40 ⬚C and at 100 ⬚C and then these values are compared to an empirical reference scale first devised in 1929 by Dean and Davis (21). One reference oil was assigned a VI of 100 and another a VI of 0. Base oils with high VI values exhibit only small changes in viscosity with temperature while base oils with low VI values exhibit large changes in viscosity with temperature. Base oils with high VI values (80–120) are desirable, because they thin less at high temperatures. Paraffinic base oils exhibit good viscosity–temperature profiles (reflected by high VI values), adequate pour points, and good thermal and oxidative stability. Naphthenic base oils, in contrast, exhibit poorer viscosity–temperature profiles (with lower VI values), superior (lower) pour points, and good solvency properties. Aromatics are removed from base oils because they exhibit poor viscosity–temperature profiles and poor oxidative stability. The toxicity of some aromatic species, especially the polycyclic aromatics, is also a concern. Aromatics display superior solvency for additives. One group of desirable aromatics and naphthenes consists of single ring structures with long alkyl side chains. The low temperature properties of base oils are also important. If the base oil is paraffinic, cooling will produce wax crystals when the high melting point n-paraffins begin to crystallize out. The onset of wax formation is referred to as the cloud point. The pour point is the lowest temperature at which an oil sample will flow under the influence of gravity. In paraffinic base oils, the pour point is reached when the

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matrix of wax crystals becomes sufficiently dense to prevent the oil from pouring. Solvent dewaxing, the removal of nparaffins by extraction or catalytic dewaxing is applied to crude oils high in paraffins to ensure that the base oil has a sufficiently low pour point. Typical pour points for base oils fall in the range of ᎑10 ⬚C to ᎑20 ⬚C.

From Crude Oil to Base Oil (20–23) This section reviews the processes involved in converting crude oil to base oil or mineral oil. Most of the traditional methods involved physical techniques (e.g., solvent extraction and solvent dewaxing) and a final chemical process (mild hydrofinishing). Recently, most of these physical processes have been supplanted by chemical processes (catalytic dewaxing, isodewaxing, and severe hydrotreatment). The refining of crude oil to yield lubricating base oil begins with the residuum from the atmospheric distillation unit. The fractions obtained from the atmospheric distillation unit, known collectively as light ends and used in gasoline production are too volatile for use in lubricating base oil. The atmospheric residuum is fed to a vacuum distillation unit and several fractions of increasing boiling point range are collected plus a vacuum distillation residue. The thick, black residue remaining after vacuum distillation can be solvent extracted to produce oil known as brightstock by a process known as deasphalting. This is accomplished using excess propane or sometimes butane or pentane. Propane kept near its critical point is used in a countercurrent extraction tower. Removal of the propane yields deasphalted oil, which needs further treatment (solvent extraction and dewaxing) before it is suitable base oil. The purpose of solvent extraction is to remove undesirable aromatics from the vacuum distillates and deasphalted oil. This process has replaced the older technique of using sulfuric acid (acid refining) to remove unsaturated compounds from these fractions. Furfural, 1, and N-methylpyrrolidone, 2, are two solvents widely used today as extraction solvents (Figure 1). Previously sulfur dioxide had been used, and the use of phenol is on the decline. Adjusting the temperature, solvent-to-oil ratio, and contact time controls the severity of the process. After removal of the extraction solvent, two phases are obtained, an aromatic oil used by the chemical and rubber industries and the saturated hydrocarbon oil or raffinate.3 Solvent dewaxing involves the removal of large n-paraffin molecules labeled wax from the oil by filtration after the oil is chilled in the presence of a solvent or solvent pair (e.g., methyl ethyl ketone兾toluene). As the severity of solvent

dewaxing increases (to achieve very low pour points) the cost of the operation increases, thus a compromise between the desired base oil properties and the cost of the operation is usually struck. Unlike deasphalting, solvent extraction, and solvent dewaxing, which are physical separation techniques, hydrofinishing or hydrotreating is a chemical process. The function of mild hydrotreating is removal of organic nitrogen-, oxygen-, and some of the sulfur-containing compounds from the solvent-extracted, solvent-dewaxed oil. During mild hydrotreating, the chemically bound nitrogen and sulfur escape as ammonia and hydrogen sulfide, respectively. Mild hydrotreating does not convert aromatics to naphthenes. This process improves the color of the oil and its thermal and oxidative stability. The refining processes described above represent the conventional or older processes used to produce lubricating base oil. These processes are being supplanted by newer catalytic processes—catalytic dewaxing, isodewaxing, and severe hydrotreatment. Catalytic dewaxing employs zeolite catalysts whose channels only admit linear alkanes or long alkyl groups. These alkanes are cracked to yield low molecular weight, unsaturated byproducts, which if left untreated can deactivate the catalyst through coke formation or can polymerize. This problem is addressed by incorporation of a hydrogenation catalyst (e.g., Pd or Pt) in the catalyst package. Although catalytic dewaxing produces base oils with a VI lower than solventdewaxed base oils with the same pour points, the operating costs of the catalytic dewaxing operation are substantially lower than those for solvent dewaxing. Isodewaxing represents newer technology than catalytic dewaxing. In this approach, n-alkanes are isomerized to isoalkanes, which remain within the boiling point range of the base oil. This produces base oils with high VI values and reduces yield loss of the base oil. Severe hydrotreatment or hydrocracking, which employs metal sulfide (Mo, W, Ni, Co) catalysts supported on highsurface-area, high-acidity alumina or silica-alumina, involves hydrogenation of aromatics to naphthenes, ring-opening reactions, and severe denitrogenation and desulfuration reactions. Because this process involves the elimination of unwanted species (e.g., aromatics) and their conversion to desirable species (e.g., naphthenes), naphthenic base oils can be created from paraffinic crude oils and crude oil otherwise unsuitable for lubricating base oil production can be used. The base oil produced from severe hydrotreatment has high VI values, very low nitrogen and sulfur content, and a narrower range of hydrocarbon types than conventionally refined base oils. Grease Thickeners

O O

N

O

H 1

CH3 2

Figure 1. Furfural, 1, and N-methylpyrrolidone, 2, compounds used as extraction solvents.

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The thickeners employed in greases are typically subdivided into three broad categories: soaps, complex soaps, and nonsoap thickeners. Soap-thickened greases are the most common (about 46% of the North American market), followed by the complex soap-thickened greases (about 40% of the same market). The various nonsoap-thickened greases constitute about 14% of the North American market (25). The use of complex soap greases is increasing.

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Soap Thickeners The soap content of soap-thickened greases ranges from 5% to 20% by weight. Increasing the soap content increases the stiffness of the grease. Both the soap cation and anion influence grease properties. The soap cation influences the thickening power of the soap in the grease, as well as the water resistance and the dropping point of the grease. The chain length of the fatty acid anion determines the solubility of the soap in the base oil and influences the oxidation stability of the grease and its surface boundary characteristics. Eighteencarbon atom fatty acids are the most common in grease (6). Eighty percent of soap and complex soap-thickened greases use lithium soap (25). Historically, hydrated calcium soap was the first soap used to thicken grease (13). It was introduced in the 1880s and dominated the market for the next fifty years. Sodium soaps were introduced in the 1930s and although they could withstand higher temperatures than the calcium soap-thickened greases, they had one major drawback—their high water solubility. The introduction of lithium soap-thickened greases in the 1940s and 1950s revolutionized the grease industry and led to the introduction of multipurpose greases. The two most common lithium soaps employed in lubricating greases are lithium stearate and lithium 12-hydroxystearate. Stearic acid is found in many animal fats and vegetable oils. In addition, complete hydrogenation of the eighteen-carbon atom unsaturated fatty acids—oleic acid, linoleic acid, and linolenic acid—leads to stearic acid. However, there is only one common source for 12-hydroxystearic acid, 3, and that is castor oil (26–29). Castor oil is unusual on three counts. First, it is the only readily available source of the fatty acid, ricinoleic acid, (Z )12-hydroxy-9-octadecenoic acid, 4, the precursor to 12hydroxystearic acid (Figure 2). Hydrogenation converts ricinoleic acid to 12-hydroxystearic acid. Second, approximately 90% of the fatty-acid content of the triglyceride castor oil is ricinoleic acid—most triglycerides are not this rich in a single fatty acid. And third, ricinoleic acid is an unusual fatty acid in that it contains a hydroxyl group—most fatty acids do not. Grease manufacturers purchase hydrogenated castor oil (HCO) or methyl 12-hydroxystearate and react it with LiOH to obtain lithium 12-hydroxystearate. India is

responsible for about two-thirds of the world’s castor oil production (29). Worldwide, over 50% of the lithium grease production is based on lithium 12-hydroxystearate (28). Greases made from lithium 12-hydroxystearate have a higher dropping point than greases made from lithium stearate. The former greases also exhibit superior shear stability. These improvements in properties are attributed to the ability of the 12-hydroxystearate anion to hydrogen bond. Lithium 12hydroxystearate has been described as the “foundation stone of the current generation of lubricating greases” (9). Lithium soaps dominate soap-thickened greases because they yield greases with superior properties. Calcium and aluminum soap-thickened greases are the second most important—each has about 6–8% of the market share. Small quantities of sodium soap-thickened greases are also produced (25). Aluminum soap-thickened greases find use in marine applications because they exhibit excellent water resistance. Table 1 (15, 26) provides a comparison of the effect of changing the soap cation from Ca2+ to Na+ to Li+ and the soap anion from stearate to 12-hydroxystearate on three important grease properties: dropping point, shear stability, and water resistance.

Complex Soap Thickeners Greases with these thickeners contain the same soaps that are used to make soap-thickened greases, but in addition, the salt of a second acid is added. Often the preparation of this thickener is carried out in two steps. First, the main fattyacid soap is formed by saponification and then the second acid is neutralized to give a metal salt. One of three types of acids are generally added to make a complex grease—inorganic acids (e.g., boric and phosphoric acid), short-chain carboxylic acids (e.g., acetic acid or benzoic acid) or dicarboxylic acids (e.g., azelaic acid, 5; sebacic acid, 6; and terephthalic acid, 7; Figure 3; ref 6 ). The most common lithium complex grease is based on 12-hydroxystearate and azelaic acid (6, 30). Calcium and barium complex greases employ acetic acid as the complementary acid and aluminum complex greases utilize benzoic acid (6). Complex grease offers the advantage of a higher dropping point than the conventional

O

O H3C

OH

O OH

HO 5

OH

3

O

HO

OH HO

OH

HO

CH3

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O

O

4 Figure 2. The lithium salt of 3 is used in lubricating greases. Compound 4 is the precursor to 3.

•

O

6

O

OH 7

Figure 3. Examples of acids added to make a complex grease.

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Table 1. Properties of Metal Soap Thickened Greases Grease Type

Dropping Point/⬚F

Shear Stability

Water Resistance

Calcium stearate

205–220

Fair

Excellent

Calcium 12-hydroxystearate

275–290

Fair/Good

Excellent

Sodium stearate

325–350

Fair/Good

Very Poor

Lithium stearate

350

Good

Very Good

Lithium 12-hydroxystearate

375–400

Excellent

Very Good

Calcium complex 12-hydroxystearate

500+

Fair/Good

Fair/Excellent

Lithium complex 12-hydroxystearate

500+

Good/Excellent

Good/Excellent

soap-thickened grease (Table 1). These greases are often referred to as simply complex greases.

Nonsoap Thickeners Among the various types of nonsoap thickeners, only the two major types are mentioned here—organo-clay and polyurea. To use clay as a thickener, it is first chemically modified by reacting or coating it with a quaternary ammonium compound. This treatment renders it hydrophobic. To manufacture organo-clay grease, the organo-clay is mixed with the base oil and a pregelling agent (e.g., acetone or ethanol) and the mixture is intensively milled. Polyurea grease thickeners are produced in situ in the base oil by the reaction of bifunctional isocyanates and amines to yield a polymer referred to as a polyurea. Polyurea grease is the preferred grease for lubrication of ball bearings (e.g., in electric motors). Unlike the rest of the world, Japan relies heavily on polyurea-thickened greases instead of complex greases (31). Lubricant Additives Additives are present in lubricants for a variety of reasons. Additives can be differentiated based on whether they target the physical or chemical properties of the base oil or whether they modify the metal surface the lubricant contacts (32). Not all additives used in lubricating oils (e.g., detergents, dispersants) are appropriate for greases. One reason for this is that the polar groups in the soap of soap-thickened greases compete with metal surfaces for the additive. Typical multipurpose grease contains the following additives (6, 16): • antioxidants • metal deactivators and corrosion inhibitors

tion, propagation, branching, and termination. The oxidation process begins with the generation of alkyl free radicals under harsh conditions (high temperature, UV light, or metal catalyst). Reaction of the alkyl radical with molecular oxygen yields the peroxy radical, which in turn abstracts a hydrogen atom from a hydrocarbon molecule to yield a hydroperoxide radical and a second alkyl radical. Other free radical reactions ensue, producing additional free radical species including alkoxy and hydroxyl radicals. Termination reactions produce long-chain hydrocarbons and a host of oxygen-containing species including alcohols, aldehydes, ketones, and carboxylic acids. Antioxidants are sacrificial compounds that disrupt either the propagation or branching steps of free radical degradation. Antioxidants are classified according to their mode of action as radical scavengers and as hydroperoxide decomposers. Because radical scavengers interfere with propagation steps they are referred to as primary antioxidants and because hydroperoxide decomposers disrupt branching processes they are labeled secondary antioxidants. Figure 4 shows examples of two classes of compounds that are typically used in lubricating grease as primary antioxidants: hindered phenols (2,6-di-tert-butyl-4-methylphenol, 8) and aromatic amines (diphenylamine, N-phenyl-1-naphthylamine, 9). At high temperatures aromatic amines are found to be more effective antioxidants than hindered phenols. There are two classes of secondary antioxidants (Figure 5)— sulfur compounds (e.g., zinc dithiophosphates, 10, and zinc dithiocarbamates, 11) and organophosphorus compounds (e.g., aryl phosphites, 12). It is common practice to employ different types of antioxidants as well as different derivatives of a single family (e.g., diphenylamine and N-phenyl-1-naphthylamine) in a grease formulation because of the synergistic effects that are achieved (33).

• polymer additives • friction modifier (FM), anti-wear (AW), and extreme pressure (EP) additives • solid lubricants

Studying the effect of an additive on lubricant performance is complicated by the presence of other additives and it is often most appropriate to consider the additive package, since one additive can have a synergistic effect or an antagonistic effect on another additive (32).

Antioxidants (33, 34) These additives protect the base oil and thickener from the ravages of oxidation. Oxidation is a free radical process that mechanistically is broken down into four steps: initia866


•

OH (CH3 )3C

C(CH3)3

NH

CH3 8

9

Figure 4. Examples of two classes of compounds that are typically used in lubricating grease as primary antioxidants.

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Metal Deactivators and Corrosion Inhibitors (33, 34) Metal deactivators protect yellow metals (copper, brass, and bronze) from corrosion caused by organic acids (Figure 6). They are subdivided into two groups: surface passivating agents and chelating agents. Surface passivators (e.g., benzotriazole, 13) work by bonding through a heteroatom to the surface of the metal, while chelating agents (e.g., N, N´disalicylidene-1,2-diaminopropane, 14) bind metal ions (e.g., Cu2+) in the body of the grease (33). Corrosion inhibitors prevent rust formation on the surface of ferrous alloys. The alkali or alkaline earth metal hydroxides are excellent corrosion inhibitors—they neutralize acidic products formed by thermal and oxidative decomposition of the lubricant. Consequently, in the manufacture of a soap-thickened grease, an excess of metal hydroxide is often used (6). In addition, other inhibitors are also used and typically employ amphiphilic species, which are polar at one end and contain a long nonpolar chain at the other end. These molecules adsorb to the surface of the metal through the polar group, while the nonpolar end creates a barrier between the metal surface and water molecules. Typical corrosion inhibitors (Figure 7) include dodecenyl succinic acid, 15, oleoyl sarcosine, 16, imidazoline derivatives, 17, and metal (e.g., calcium) sulfonates, 18. Polymer Additives (32, 35, 36) Polymers are used in several ways to modify the properties of lubricants and sometimes greases. Pour point depres-

S

RO

HOOC

P

HOOC

10 R N

Zn

R⵮

N

H3C

O

N

H3C

O

R⬙

S

S

HO

15

R′

S

S

C12H23

OR⬙

S

S

Friction Modifier (FM), Anti-Wear (AW), and Extreme Pressure (EP) Additives (1, 32, 37) These additives are referred to as tribological additives, and they reduce friction and wear when operating conditions become more severe (e.g., high load, low speed). Fluid film lubrication (hydrodynamic lubrication) is replaced with mixed film and then boundary lubrication when the thickness of the lubricating film is less than surface irregularities (called asperites) in the moving metal parts. Wear is the physical loss of material from a metal surface. Four wear mechanisms are delineated: adhesion, abrasion, corrosion, and contact fatigue (37). The mode of action from FM additives to AW additives

OR′

S Zn

P R⵮O

sants (e.g., polymethacrylates, 19, and ethylene vinyl acetate copolymers, 20, lower the pour point of the lubricant by changing the morphology of the wax crystals (Figure 8). They interfere with the agglomeration process and prevent the formation of the extended three-dimensional network, which means the oil can still flow even though the wax crystals are present. Viscosity index improvers are polymers that extend the useful liquid range of the base oil at high temperatures. At low temperatures these high molecular weight star polymers exhibit low solubility in the base oil and under these conditions curl in on themselves and consequently have little effect on oil viscosity. At high temperatures the polymer’s solubility in the base oil increases and it extends its arms and thickens the oil.

16

11 HO

P

O

N

R′

R

N

Ca

C(CH3)3

SO3ⴚ

C17H33

(CH3 )3C Figure 5. Examples of compounds that are typically used in lubricating grease as secondary antioxidants.

2

18

17

3

12

Figure 7. Typical corrosion inhibitors.

CH3 N N N

CH3

OH

N H 13

O

14

•

CH2 x

O(CH2)11CH3 19

H C O

H3C

n

Figure 6. Compounds that protect yellow metals from corrosion caused by organic acids.

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CH2

CH2 C

HO

N

CH2

y

O

20

Figure 8. Examples of pour point depressants.

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and finally EP additives represents a shift toward increasingly harsh operating conditions—mixed lubrication to boundary lubrication. FM additives reduce friction and include fatty acids, fatty alcohols, and fatty amines and amides. The polar functional groups of these species adsorb on metal surfaces and separate sliding metal surfaces. AW additives (e.g., zinc dialkyldithiophosphate and cresyl phosphate, 21) operate in the mixed lubrication domain under moderate stress conditions (Figure 9). They chemically react with the metal surface creating an anti-wear film (e.g., iron phosphides or sulfides) that is less shear resistant. EP additives [e.g., bis(diamyldithiocarbamate)zinc and ashless methylene-bis(di-n-butyldithiocarbamate), 22] target high stress conditions and like the AW additives, form tribochemical reaction layers. EP additives have been described as a form of controlled corrosive attack that replaces uncontrolled abrasive wear (1).

Solid Lubricants (1, 38, 39) A qualitative comparison of three common solid lubricants—molybdenum disulfide (MoS 2 ), graphite, and poly(tetrafluoroethylene), PTFE—is found in Table 2 (38). Molybdenum disulfide is a good-to-excellent lubricant under a wide variety of conditions (high stress, low stress, and vacuum). Molybdenum disulfide is a natural lubricant possessing a layered or lamellar structure. In the MoS2 structure, two layers of sulfur atoms are in direct contact with each other and consequently these like-charged layers repel each other and slide by each other with relative ease. The distance separating these layers is 349 pm. Graphite also has a layered structure; each carbon atom in a planar layer is part of a six-membered ring with sp2 hybridization. The in-plane C–C bond distance is 142 pm,

while the distance between the layers of carbon atoms is 335 pm. Graphite is not a natural lubricant. Only when certain contaminants (e.g., water) are intercalated between the carbon layers does it exhibit low friction—thus it has poor lubricating ability under vacuum. Over the last fifty years, molybdenum disulfide has supplanted graphite as the solid lubricant of choice, in part because the load-carrying capacity of MoS2 is superior to that of graphite. Nonetheless both these solid lubricants are often added to grease together because it is believed they perform better in combination. Although insoluble in grease, these solid lubricants are added to improve the performance of the grease under boundary or mixed lubrication conditions. They function by being distributed between asperites and adhering strongly to the surfaces of the moving metal parts. Because the adhesion of the solid lubricant to the metal surfaces is stronger than the shear energy of the interlaminar layers these layers slide over each other reducing the coefficient of friction of the moving (sliding) metal surfaces and provide lubrication. Grease containing “moly” forms a film of MoS2 on the metal surfaces that is replenished when removed by wear; this reduces wear to the metal surfaces. Typical concentrations of MoS2 in grease are 3% to 10%. Above 20% MoS2, the flow properties of grease deteriorate. At high concentrations of MoS2 (up to 60%), the lubricant behaves like a paste. These pastes find use as anti-seize compounds. One drawback to both MoS2 and graphite as solid lubricants is their black color, which is unacceptable in certain applications. Although much more expensive, PFTE is the most common white polymeric lubricant in greases; high or ultra-high polyolefins powders are making inroads as solid lubricants. Manufacture of Grease (6, 15, 16)

O

P

O

CH3 3

21 S C4H9

N

S S

S

C4H9

N

C4H9

C4H9 22

Figure 9. Examples of anti-wear, 21, and extreme pressure, 22, additives.

Table 2. A Qualitative Comparison of Solid Lubricants Property

MoS2

Graphite

PTFE

Color

Black

Black

White

Max. service temp.

345 ⬚C

425 ⬚C

260 ⬚C

Corrosion resistance

Good

Excellent

Excellent

Surface adhesion

Good

Fair

Poor Poor

High stress lubrication

Excellent

Good

Low stress lubrication

Good

Good

Excellent

Vacuum lubrication

Excellent

Poor

Good

868


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The manufacture of grease has evolved from a craft to more of a science. Although some grease is made in a continuous plant operation (30), it is still common to make grease in batches in large kettles of 1500 to 2000 gallon capacity (40). Some or all of the following six steps are involved in the manufacture of grease: saponification, dehydration, cutback, milling, deaeration, and filtration. These steps apply to the preparation of grease containing a simple soap thickener made batchwise. In some instances the use of a preformed soap may eliminate the need for the saponification step. However, typically a fat (e.g., from beef tallow) or fatty acids are dispersed or dissolved in a small quantity of the base oil at high temperature (up to 90 ⬚C). Next, the metal hydroxide (e.g., lithium hydroxide) is added to the mixture as a solution or suspension in water and saponification occurs. This reaction transforms the fats to metal fatty-acid soaps and glycerol. After completion of the saponification reaction, the mixture is often heated to a higher temperature (> 200 ⬚C) to drive off the water (dehydration) and to melt the soap. As the dispersion cools, the soaps crystallize into a network of fibrils, which in turn causes the grease to thicken. During the finishing process, the grease is homogenized, and more base oil is added to adjust the consistency. The slow addition of the remaining base oil is referred to as cutback. Additives are introduced at this stage. Since most additives are temperature sensitive and many are attacked by strong alkali, they cannot be added earlier in the process. To obtain the

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correct consistency, the grease may be subjected to milling, that is, chopping and mixing. Milling takes large soap fibers and reduces their size, which leads to an increased surface area, a larger number of interactions between the soap and base oil and a firmer grease. As part of the finishing steps, the grease is deaerated to remove trapped air. Just prior to placing the grease into containers it is filtered to remove any solid raw materials, impurities, or contaminants. Acknowledgments The author gratefully acknowledges the financial support of this work from a NSF Course, Curriculum, and Laboratory Improvement grant, DUE-0088729 and thanks the staff of the Scientific Research Laboratory, Ford Motor Company for access to their library and for helpful discussions while he was on sabbatical leave there. Notes 1. A region labeled mixed lubrication separates hydrodynamic lubrication and boundary lubrication. These three regions are illustrated in a Stribeck curve. 2. The labels commonly used in the petroleum industry to describe classes of hydrocarbons are used in this article. n-Paraffins are n-alkanes, isoparaffins are branched alkanes, and naphthenes are cycloalkanes. 3. In a solvent extraction process, raffinate refers to that portion of the remaining liquid mixture not extracted by the selective solvent.

Literature Cited 1. Gesser, H. D. Applied Chemistry: A Textbook for Engineers and Technologists; Kluwer Academic/Plenum Publishers: New York, 2002; Chapter 8. 2. Donahue, C. J. J. Chem. Educ. 2002, 79, 721–723. 3. Donahue, C. J.; D’Amico, T.; Exline, J. A. J. Chem. Educ. 2002, 79, 724–726. 4. Donahue, C. J.; Exline, J. A.; Warner, C. J. Chem. Educ. 2003, 80, 79–82. 5. Schwartz, S. E. Automotive Lubricants; General Motors R&D Publication #8968, January 2000; pp 1–16. 6. Dresel, W.; Heckler, R. Lubricating Greases. In Lubricants and Lubrications; Mang, T., Dresel, W., Eds.; Wiley-VCH: Weihheim, Germany, 2001; Chapter 16. 7. Fuchs, M. NLGI Spokeman 1997, 61 (5), 25–40. 8. Lansdown, A. R. Lubrication and Lubricant Selection: A Practical Guide; Mechanical Engineering Publications: London, 1996; Chapters 1 and 2. 9. Gow, G. Lubricating Grease. In Chemistry and Technology of Lubricants, 2nd ed.; Mortier, R. M., Orszulik, S. T., Eds.; Blackie Academic and Professional: London, 1997; Chapter 11. 10. Boner, C. J. Manufacture and Application of Lubricating Greases; Reinhold: New York, 1954. 11. Boner, C. J. Modern Lubricating Greases; Scientific Publica-

tions: Broseley, United Kingdom, 1972. 12. Miller, R. W. Lubricants and Their Applications; McGraw-Hill: New York, 1993; Chapter 9. 13. Wilson, T. C. SAE Tech. Pap. Ser. 1984, No. 841212. 14. Musilli, T. G. Lubr. Engr. 1987, 43, 352–353. 15. Lubricating Grease Guide, 4th ed.; Ehrlich, M., Ed.; National Lubricating Grease Institute: Kansas City, KS, 1996. 16. Pirro, D. M.; Wessol, A. A. Lubrication Fundamentals, 2nd ed.; Marcel Dekker: New York, 2001; Chapter 4. 17. Rush, R. E. Lubr. Engr. 1997, 53, 17–26. 18. Hurley, S.; Cann, P. M. NLGI Spokeman 2001, 65 (5), 17–26. 19. Rizzo, N. W.; Irwin, L.; Foster, M. D.; Funk, M. R. NLGI Spokeman 1996, 60 (1), 24–25. 20. Mang, T. Base Oils. In Lubricants and Lubrications; Mang, T., Dresel, W., Eds.; Wiley-VCH: Weihheim, Germany, 2001; Chapter 4. 21. Prince, R. J. Base Oils From Petroleum. In Chemistry and Technology of Lubricants, 2nd ed.; Mortier, R. M., Orszulik, S. T. Eds.; Blackie Academic and Professional: London, 1997; Chapter 1. 22. Brock, D. V. Lubr. Engr. 2000, 56, 37–39. 23. Gresham, R. M. Lubr. Engr. 2002, 58, 7–8. 24. Synthetic Lubricants and High-Performance Functional Fluids; Shubkin, R. L., Ed.; Marcel Dekker: New York, 1993. 25. Fagan, G. L. NLGI Spokesman 2003, 66 (1), 21–39. 26. Hurd, P. W. NLGI Spokesman 1996, 60 (1), 14–23. 27. Kinnear, S.; Kranz, K. NLGI Spokesman 1998, 62 (5), 13–19. 28. Bhatia, J.; Sovani, D. R.; Dhawan, R. L. NLGI Spokesman 1992, 56 (7), 265–272. 29. Cherry, N. A. NLGI Spokesman 2000, 64 (1), 18–21. 30. Beret, S.; Boersig, T. J.; Loh, W.; Wong, P. K. NLGI Spokesman 1999, 62 (11), 14–17. 31. Todd, P. R. NLGI Spokesman 2002, 65 (10), 21–31. 32. Braun, J.; Omeis, J. Additives. In Lubricants and Lubrications; Mang, T., Dresel, W., Eds.; Wiley-VCH: Weihheim, Germany, 2001; Chapter 6. 33. Reyes-Gavilan, J. L.; Odorisio, P. NLGI Spokesman 2001, 64 (11), 22–23. 34. Rasberger, M. Oxidative Degradation and Stabilisation of Mineral Oil Based Lubricants. In Chemistry and Technology of Lubricants, 2nd ed.; Mortier, R. M., Orszulik, S. T., Eds.; Blackie Academic and Professional: London, 1997; Chapter 4. 35. Rizvi, S. Q. A. Lubr. Engr. 1999, 55, 33–39. 36. Liston, T. V. Lubr. Engr. 1992, 48, 389–397. 37. Bovington, C. H. Friction, Wear and the Role of Additives in Their Control. In Chemistry and Technology of Lubricants, 2nd ed.; Mortier, R. M., Orszulik, S. T., Eds.; Blackie Academic and Professional: London, 1997; Chapter 12. 38. Johnson, R. L. Bonded Lubricant Coatings; Climax Molybdenum Co.: Greenwich, CT, Bulletin L-79, 1979. 39. Landsdown, A. R. Molybdenum Disulphide Lubrication; Elsevier: Amsterdam, 1999. 40. Burkhalter, R. L.; Brocco, E. A. NLGI Spokesman 1999, 63 (2), 26–32.

The structures of a number of the molecules discussed in this article are available in fully manipulable Jmol and Chime format as JCE Featured Molecules in JCE Online (see page 882).

JCE Featured Molecules

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Lubricating Grease: A Chemical Primer - ACS Publications

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