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624 Lubricant Additives: Chemistry and Applications
TABLE 24.5 (Continued)
Additive Function Types of Additives Related Performance Tests
Soluble EP additive Sulfurized esters [26] ASTM D 5182, Standard test method for evaluating the scuf ng load capacity of oils (FZG visual method)
ASTM D 2782, Standard test method for measurement of extreme-pressure properties of lubricating uids
(timken method)
ASTM D 2783, Standard test method for measurement of extreme-pressure properties of lubricating uids
(four-ball method)
Sulfurized ole ns [26]
Diaryl disul des [26]
Dialkyldithiophosphate esters
Lead naphthenate
Bismuth naphthenate [27]
Antimony dialkyldithiocarbamate
Antimony dialkyldithiophosphate
Ammonium salts of phosphate esters
Chlorinated waxes
High-molecular-weight complex esters [28]
Borate esters [29]
Solid EP additive Molybdenum disul de See the above test method for EP properties and wear preventive properties
Graphite
Tacki er Polyisobutylenes No ASTM standard test method
Demulsi ers Polyalkoxylated phenols ASTM D 1401, Standard test method for water separability of petroleum oils and synthetic uids
ASTM D 2711, Standard test method for demulsibility characteristics of lubricating oilsPolyalkoxylated polyols
Polyalkoxylated polyamines
Friction modi ers Acidless tallow ASTM D 5183, Standard test method for determination of coef cient of friction of lubricants using
the four-ball wear test machine
PPDs Poly (alkylmethacrylates) ASTM D 97, Standard test method for pour point of petroleum products
ASTM D 5949, Standard test method for pour point of petroleum products
Foam inhibitors Polydimethylsiloxanes ASTM D 892, Standard test method for foaming characteristics of lubricating oils
Shear stable VI improvers Poly(alkylmethacrylates) Severe shear tests are under development
Ole n copolymers
Styrene diene copolymers
Antimisting additives Polyisobutylenes [30–32] ASTM D 3705, Standard test method for misting properties of lubricating uids
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Long-Term Trends in Industrial Lubricant Additives 625
24.4.2.2 Friction Modifi ers
For applications involving primarily sliding contact, such as worm gears, friction modi ers such
as acidless tallow may be used. In these applications, friction modi ers can be more effective than
sulfur-containing EP additives, which might promote corrosion of bronze worm gears.
24.4.2.3 Viscosity Index Improvers
Because gear oils can be subjected to even greater mechanical shear than hydraulic oils, the poly-
mers used in VI improvers in gear oils must possess exceptional shear stability. It is preferable
to use low-molecular-weight VI improvers in gear oils as low-molecular-weight polymers display
greater shear stability than high-molecular-weight polymers. The use of lower-molecular-weight
VI improvers increases formulation costs as they must be used at higher percentages than high-
molecular-weight VI improvers to achieve the same VI increase. Despite the higher costs associated
with the use of lower-molecular-weight VI improvers, this approach is necessary to maintain gear
oil viscosity grade and lubricating lm thickness during operation.
24.4.2.4 Antimisting Additives
Mist lubrication is an effective means of lubricating gears and other elements; however, care must
be taken to minimize stray mist, which could cause health problems for those working in the vicinity.
Polymeric additives, such as polyisobutenes, can be used to promote coalescence of oil drops and
reduce the formation of stray mist [30–32].
24.4.3 ADDITIVES FOR OPEN GEAR LUBRICANTS
For large, slow-moving gears that cannot practically be enclosed in oil-tight casings, an adhesive,
high-viscosity material is required to stick to the gears. Such open gear lubricants are generally
applied by spraying or brushing. In cold climates, a synthetic base oil may be required to provide a
high-viscosity base uid that can be sprayed at low temperatures [35,36]. Such lubricants generally
contain the following core components:
1. High-viscosity base oil
2. Oil-soluble EP additives
3. Solid EP additives
4. Tacki er
5. Anticorrosion additive
24.4.4 ENVIRONMENTAL CONCERNS
In the past, gear oils have contained organic derivatives of lead for EP performance, and open gear
lubricants have contained chlorinated solvents to improve pumpability. Over the last decade, envi-
ronmental considerations have essentially resulted in the elimination of these components. Newer
open gear lubricant formulations also tend to avoid the use of asphalt on the basis of environmental
and performance considerations [35,36].
For applications in which industrial gear oils are used in environmentally sensitive areas such
as forests and near waterways, biodegradable gear oils based on natural and synthetic esters have
been developed [37].
24.4.5 FUTURE TRENDS
A failure phenomenon known as micropitting has surfaced as a problem in certain applications
such as wind turbine gearboxes [38]. Although the failure mode is not completely understood, it is
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626 Lubricant Additives: Chemistry and Applications
believed to be related to the surface nish of the gears. Test methods have been developed to mea-
sure the ability of gear oils to protect against micropitting [39]. There is more to learn in terms of
understanding micropitting and minimizing its occurrence.
As in the case of hydraulic pumps, gearbox ef ciencies are improved to provide lower operating
costs for the end users. This will result in higher gear speeds and loads and higher oil operating tem-
peratures. The challenge for gear oil formulators will be to use base oils and additives that provide
1. Improved thermal stability and cleanliness
2. Improved high-temperature EP performance
3. Improved oxidation resistance
4. Extended demulsibility life
5. Lower friction
6. Improved foam resistance
7. Improved surface fatigue protection
24.5 ADDITIVES FOR COMPRESSOR OILS
24.5.1 P
ERFORMANCE REQUIREMENTS FOR COMPRESSOR OILS
It is dif cult to generalize the lubrication requirements of compressors as requirements depend on a
number of parameters such as the type of compressor, the properties of the gas being compressed,
and designed discharge temperatures and pressures [40,41].
Compressors can be classi ed into two types, positive displacement and dynamic, depending
on the process by which they compress gas. Compressors based on positive displacement trap the
gas, reduce its volume, and then discharge the compressed gas. Dynamic compressors accelerate
the gas and convert velocity to increased pressure. Positive-displacement compressors are useful for
high-pressure applications with one limiting factor being the discharge gas temperature. Dynamic
compressors are useful for high-volume, low-pressure applications.
Positive-displacement compressors can be further classi ed into reciprocating and rotary com-
pressors. Reciprocating compressors utilize single- or double-acting pistons. Rotary types include
rotary vane and rotary screw compressors.
Principal sites for lubrication in reciprocating compressors include the crankshaft and associ-
ated bearings, connecting roads, wrist pins, pistons, cylinders, piston rings, and valves. Rotary
screw compressor lubrication sites include the rotors, bearings, and shaft seals. Rotary vane com-
pressors require lubrication of bearings and shafts [41].
Dynamic centrifugal compressors require lubrication of bearings, gear reducers, and seals [41].
Critical lubrication requirements of the various types of compressors include the following:
1. Oxidation resistance
2. Thermal stability
3. Low carbon deposits at high temperatures (high-pressure reciprocating compressor)
4. Compatibility with gas being compressed
5. Viscosity retention (resistance to dilution by gas being compressed)
6. Antiwear properties (rotary vane)
7. Gas seal provided
8. General lubrication
9. Heat removal
10. Corrosion prevention
11. Compatibility with compressor components
12. Good demulsibility
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Long-Term Trends in Industrial Lubricant Additives 627
13. General cleanliness
14. Low-foaming tendencies
15. Low ammability
24.5.2 COMPRESSOR OIL FORMULATION
High-pressure reciprocating compressors require that lubricants should be stable at high tempera-
tures because discharge temperatures can range from 350 to 500°F. Lubricants for such reciprocat-
ing compressors must not generate heavy carbon deposits at hot valve areas as this increases the
risk for res and explosions. Consequently, lubricants for these applications are usually based on
synthetic base oils with low carbon-forming tendencies such as diesters and polyol esters.
Although the requirements for compressor oils vary signi cantly depending on a number of
parameters and a substantial portion of compressor oils are based on synthetic uids, many formu-
lations still rely on the types of additives listed in Tables 24.2 and 24.3 for turbine oils and hydrau-
lic oils. Rotary vane compressors require lubricants that minimize vane wear and vane-sticking
problems. In addition to the types of additives already mentioned for turbine and hydraulic oils,
detergent additives may be used in some rotary vane compressor oils to help prevent deposit forma-
tion and vane sticking.
The formulations of compressor oils can vary signi cantly depending on the type of compres-
sor and the type of gas being compressed. For example, mineral oils are generally not used for
compressing natural gas because natural gas is soluble in mineral oil. Dilution of mineral oil–based
compressor oil with natural gas would decrease the viscosity of the oil below an effective lubricating
level. In this case, a polyglycol-based oil with limited solubility for natural gas is more suitable.
24.5.3 COMPRESSOR OIL TRENDS
Although mineral oil–based lubricants may be suitable for normal duty with nonreactive gases, a
variety of synthetic-based lubricants are used for severe-duty, high-temperature, and high- pressure
applications and for gases that are reactive with mineral oils. As a general trend, ester-based com-
pressor lubricants are often used for reciprocating compressors in severe service duty due to their
lower tendency to form carbon deposits that can lead to res in reciprocating compressors. For
rotary screw and rotary vane compressors, synthetic polyalphaole ns are growing in popularity
due to their good thermal stability and lower cost relative to ester-based uids. The formulation of
compressors oils for severe service is so dependent on equipment design and the chemical nature of
the gas being compressed that the compressor manufacturers generally designate the compressor oil
formulations that are suitable for their equipment.
24.6 ADDITIVES FOR LUBRICATING GREASES
24.6.1 P
ERFORMANCE REQUIREMENTS FOR LUBRICATING GREASES
A lubricating grease consists of a lubricating oil, performance additives, and a thickener, which
is a ne dispersion of an insoluble solid that is capable of forming a matrix that retains the oil in
a semisolid state. Owing to their semisolid consistency, lubricating greases hold some advantages
over lubricating oils. A grease is more easily sealed into a bearing than an oil; a grease does not
require a circulating system; and in wet or dusty environments, a grease can act as a seal against
contaminants. For these reasons, greases are often the preferred lubricant for ball and roller bear-
ings in electric motors, household appliances, mobile equipment wheel and chassis lubrication, and
some industrial plain and roller bearings.
Owing to the wide variety of applications in which a lubricating grease may be used, there are
many different speci cations for lubricating greases. Many multipurpose greases meet the LB-GC
requirements of ASTM D 4950, Standard Classi cation and Speci cation for Automotive Service
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628 Lubricant Additives: Chemistry and Applications
Greases, which is a classi cation system that was developed by a cooperative effort of the ASTM,
the NLGI, and the Society of Automotive Engineers (SAE). The ASTM D 4950 speci cation covers
lubricating greases suitable for the periodic relubrication of chassis systems and wheel bearings of
passenger cars, trucks, and other vehicles. In the ASTM D 4950 classi cation system, the letters
LB signify service typical of lubrication of chassis components and universal joints in passenger
cars, trucks, and other vehicles under mild to severe duty. The letters GC signify service typical of
lubrication of wheel bearings operating in passenger cars, trucks, and other vehicles under mild to
severe duty. It is possible to formulate lubricating greases that meet both the LB and GC speci ca-
tions, and such greases may display the NLGI GC-LB certi cation mark [42].
Many military grease speci cations have been written [2], and greases meeting particular
military speci cations have been designated for critical applications such as aviation applications.
Additionally, many original equipment manufacturers, such as automobile manufacturers, issue
speci cations for greases that are speci c for the equipment that they build.
The main ingredient that differentiates a grease from an oil is the grease thickener. Most grease
thickeners are soaps composed of the reaction product of a fatty acid derivative and a metallic
hydroxide. Other salts may be combined with the soap thickeners to impart higher grease drop-
ping points (temperature at which the grease lique es). Such greases are called complex greases.
A smaller proportion of lubricating greases are manufactured with nonsoap thickeners such as
organoclays or polyurea compounds. Some of the more common thickeners are as follows:
1. Lithium salt of hydrogenated castor oil fatty acid
2. Calcium stearate (hydrous)
3. Calcium stearate (anhydrous)
4. Lithium complex
5. Aluminum complex
6. Calcium complex
7. Organoclay
8. Polyurea
9. Silica
24.6.2 ADDITIVE
Performance additives for greases are similar to those used in lubricating oils; however, it is impor-
tant that the grease additives also be compatible with the grease thickener. Lack of compatibility
between an additive and a grease thickener could lead to disruption of the thickener matrix that
holds the oil in a semisolid state. This would result in an uncontrolled softening or hardening of the
grease or a decrease in the grease dropping point.
Various grease thickeners behave differently with respect to their compatibility with perfor-
mance additives. Generally, lithium soap thickener is compatible with a much wider variety of
grease additives than organoclay thickener. Consequently, an anticorrosion additive such as cal-
cium sulfonate may perform quite well in a lithium soap-thickened grease, whereas it could cause
signi cant softening of an organoclay-thickened grease. Therefore, performance additives must be
carefully matched with the type of grease thickener.
The levels at which additives are used in lubricating greases are generally higher than in lubri-
cating oils. Several factors can contribute to the need for higher additive addition rates in lubricating
greases as compared with lubricating oils.
1. The grease thickener may compete with the performance additive for adsorption sites on
the metal surfaces.
2. The grease thickener may adsorb additive so that less is available for performing the
desired additive function.
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Long-Term Trends in Industrial Lubricant Additives 629
3. Compared with lubrication by circulating oil, grease lubrication does not provide a mecha-
nism for heat removal, and the reserve of lubricant is much less.
Another difference between lubricating oils and greases, in terms of additive use, is that lubricating
greases can more readily utilize solid additives. Although attempts have been made to suspend vari-
ous dispersions of solid additives in lubricating oils, such lubricants generally suffer from a tendency
to separate or plug lters. However, the semisolid consistency of greases is such that they can easily
keep ne solid lubricants, such as graphite and molybdenum disul de, suspended inde nitely.
The types of additives used in greases are listed in Table 24.6 along with corresponding test
methods used to evaluate the performance properties these additives impart to the grease. Some
of the additives mentioned in Table 24.6 are used much less frequently than in the past with the
increasing HSE concerns over the use of heavy metals, chlorinated compounds, and components
that may react to give potential carcinogens. National Health Safety and Environment (HSE) regu-
lations as well as equipment manufacturer requirements are making the selection and treat rate of
additive components a challenging process. See Section 24.6.3 for more details.
24.6.2.1 Rust and Oxidation Inhibitors
Greases for lightly loaded and high-speed applications rely on elastohydrodynamic lubrication and
generally require only rust and oxidation inhibitors and antiwear additives. The types of additives
used to achieve these performance properties are similar to those used in lubricating oils (Table 24.6).
As mentioned earlier, the additives may be used at higher percentages in lubricating grease than are
typically used in lubricating oils.
Unlike oil formulators, grease formulators are not restricted to performance additives that give
clear and bright solutions in oil. Even insoluble additives, such as sodium nitrite and sodium seba-
cate, have been used as rust inhibitors in greases. A drawback of solid additives is that they can
contribute to higher noise levels in grease-lubricated bearings [51].
24.6.2.2 Antiwear and Extreme-Pressure Additives
For mobile equipment and industrial bearings in which heavier loads are involved and the oil lm may
not be suf cient to prevent metal-to-metal contact, EP and antiwear additives, such as those listed in
Table 24.6, are used. These additives include most of the types used in EP gear oil formulations.
As indicated by the EP test methods listed in Table 24.6, there are several methods for measur-
ing the EP properties of greases. These test methods differ in the way that they evaluate antiwear
and EP performance, and, therefore, it is desirable to develop grease formulations that perform well
in all these test methods when designing multipurpose industrial greases. This is usually accom-
plished by using more than one additive and taking advantage of synergistic effects between the
additives [52]. It has been reported that the performance of some EP additives may be affected by
the temperature at which they are added to the grease. For example, the Timken OK load (ASTM D
2509) of a lithium base grease appeared to increase from 35 to 55 lb when the temperature of addi-
tion of the sulfurized ole n EP additive was raised from 90 to 120°C [53].
Applications in which shock loading may occur, such as the bearings of large shovels and certain
conveyor bearings, the rapid application of load could cause scoring of metal surfaces before soluble
EP additives could react to form a sacri cial layer. Solid lubricants are used in such applications to
provide a physical separation of metal surfaces during shock loading. Such lubricants are usually
nely dispersed solids, which have the ability to form lms on metal surfaces that decrease sliding
friction. Various solid lubricants that have been used in lubricating greases are listed in Table 24.6.
Perhaps, the two most commonly used solid lubricants in greases are graphite and molybdenum
disul de. Synergistic EP effects between these two solid lubricants have been exploited in greases
and open gear lubricants for a number of years [54]. It has also been claimed that the EP/antiwear
performance of graphite can be enhanced by the treatment of graphite with polar agents such as
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630 Lubricant Additives: Chemistry and Applications
TABLE 24.6
Additives Used in Lubricating Greases
Additive Function Types of Additives Related Performance Tests
Antioxidant Hindered phenols ASTM D 5483, Standard test method for oxidation induction time of lubricating greases by pressure
differential scanning calorimetry
ASTM D 942, Standard test method for oxidation stability of lubricating grease by the oxygen
pressure vessel method
Diaryl amines
Phenothiazine
Oligomers of trimethyldihydroquinoline
Metal dialkyldithiocarbamates
Ashless dialkyldithiocarbamates
Metal dialkyldithiophosphates
Rust inhibitor Alkylsuccinic acid derivatives ASTM D 1743, Standard test method for corrosion preventive properties of lubricating greases
ASTM D 5969, Standard test method for corrosion preventive properties of lubricating greases in the
presence of dilute synthetic sea water environments
ASTM D 6138, Standard test method for determination of corrosion preventive properties of
lubricating grease under dynamic wet conditions (Emcor test)
Ethoxylated phenols
Nitrogen-containing heterocyclic compounds
Fatty amines
Salts of fatty acids and amines
Salts of phosphate esters and amines
Metal sulfonates [43,44]
Ammonium sulfonates
Substituted imidazolines
Lead naphthenate
Bismuth naphthenate
Sodium nitrite
Sodium sebacate
Metal deactivator Disalicylidenepropanediamine ASTM D 4048, Standard test method for detection of copper corrosion from lubricating greases
Derivatives of 2,5-dimercapto-1,3,4-thiadizole
Antiwear additive Alkylphosphoric acid esters and salts ASTM D 5707, Standard test method for measuring friction and wear properties of lubricating grease
using a high-frequency, linear-oscillating (SRV) test machine
ASTM D 2266, Standard test method for wear preventive characteristics of lubricating grease
(four-ball method)
Dialkyldithiophosphates
Metal dialkyldithiocarbamates
Phosphate esters
Dithiophosphate esters
Derivatives of 2,5-dimercapto-1,3,4-thiadiazoles
Molybdenum carboxylates
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Long-Term Trends in Industrial Lubricant Additives 631
Soluble EP additive Sulfurized esters [26] ASTM D 5706, Standard test method for determining extreme properties of lubricating greases
using a high-frequency, linear-oscillation (SRV) test machine
ASTM D 2596, Standard test method for measurement of extreme pressure properties of lubricating
grease (four-ball method)
ASTM D 2509, Standard test method for measurement of load-carrying capacity of lubricating grease
(timken method)
Sulfurized ole ns [26]
Diaryl disul des [26]
Organic sulfur/phosphorus compounds
Lead naphthenate
Bismuth naphthenate [27,45]
Antimony dialkyldithiocarbamate
Antimony dialkyldithiophosphate
Ammonium salts of phosphate esters
Chlorinated waxes
High-molecular-weight complex esters
Chlorinated waxes
High-molecular-weight complex esters
Borate esters [46]
Solid EP additive Metal borates See the above test methods for EP properties and friction and wear properties
Molybdenum disul de [47]
Per uorinated polyole ns
Graphite [48]
Calcium carbonate
Calcium phosphate
Calcium acetate
Boron nitride
Boric acid
Metal powders
Phosphate glasses [49]
Tacki er Polyisobutylenes No standard test method
Polymers for water resistance Atactic polypropylene ASTM D 4049, Standard test method for determining the resistance of lubricating grease to water
spray
ASTM D 1264, Standard test method for determining the water washout characteristics
of lubricating greases
Polyethylene
Functionalized polyole ns [50]
Alkylsuccinimides
Polyisobutenes
Styrene–butadiene block copolymers
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632 Lubricant Additives: Chemistry and Applications
alkali molybdates and tungstates and alkali earth sulfates and phosphates. Treatment of graphite
with these polar inorganic materials helps the graphite to adhere to metal surfaces more strongly,
even in the absence of trace water, which is normally necessary for untreated graphite to adhere to
metal, and act as a friction-reducing lubricant [55].
Specialized semi uid grease containing solid lubricants have been developed for the lubrica-
tion of open gears that cannot be lubricated by means of an oil reservoir [56]. These greases are
required to produce a sturdy protective lm on heavily loaded gears that may also be subjected to
shock loading. The grease is usually applied at intervals through a spray nozzle and therefore must
be readily pumpable and sprayable. In the past, pumpability has been achieved by the addition of a
solvent such as 1,1,1-trichloroethane or a hydrocarbon solvent. More recently, solvent-free open gear
greases have been developed using synthetic base oils to achieve low-temperature performance and
pumpability [56]. Open gear greases generally contain combinations of soluble EP additives and
solid lubricants to achieve high EP performance. These lubricants may also contain high levels of
polymers to promote adhesion and water resistance. Open gear greases are usually gray to black in
color due to the presence of graphite and molybdenum disul de; however, light-colored open gear
greases have been developed through the use of colorless solid lubricants such as phosphate glass
[49]. With the increasing cost of raw materials, alternatives to molybdenum disul de are used where
technically suitable.
Thread compounds are another example of specialty greases that contain high levels of solid
lubricants. These compounds are generally composed of grease combined with a very high level
of metal powder and other solid lubricants. For example, API Bulletin 5A2 [57] describes a thread
compound composed of 36 wt% grease, 30.5 wt% lead powder, 12.2 wt% zinc powder, 3.3 wt%
copper powder, and 18 wt% natural graphite. Thread compounds may also be formulated with anti-
corrosion additive to protect pipe threads during storage.
24.6.2.3 Polymers for Water Resistance
Mobile equipment is often used in applications in which grease-lubricated bearings may be sub-
jected to water, as are bearings in certain industrial applications such as in steel and paper mills.
It has been found that the addition of various polymers to lubricating greases improves their water
resistance. Some of the types of polymers that have been used to impart water resistance to lubricat-
ing greases are listed in Table 24.6. Although most of these polymers improve water resistance by
raising the grease base uid viscosity, at least one functionalized polymer actually reacts with alkali
during the formation of the soap thickener and becomes a part of the grease thickener matrix [58].
24.6.2.4 Grease Thickeners as Performance Additives
A recent development in the evolution of lubricating grease technology has been the use of solids
that act both as grease thickeners and as performance additives. For example, a number of patents
have been written concerning the preparation and use of calcium sulfonate complex greases in
which calcium carbonate dispersed by calcium sulfonate acts as a thickener, an anticorrosion addi-
tive, and an EP additive [59,60]. Similar multifunctional claims have been made for greases thick-
ened with specially designed titanium complex thickeners [61,62].
24.6.3 ENVIRONMENTAL CONCERNS
Environmental concerns and regulations have in uenced modern grease formulations, and
this trend will likely accelerate in the future. The disposal of lubricants that contain restricted
substances is onerous and expensive. Therefore, there has been a shift away from additives
that appear on restricted substance lists or that may appear on future restricted substance lists.
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Long-Term Trends in Industrial Lubricant Additives 633
Examples of formulation changes instigated primarily by environmental concerns include the
following:
1. Substitution of alternative EP and anticorrosion additives to replace lead naphthenates and
lead dialkyldithiocarbamates.
2. Removal of heavy-metal powders from thread compounds [63–65].
3. Removal of chlorinated solvents from open gear lubricants and greases [56].
4. Substitution of alternative EP and antiwear additives to replace chlorinated paraf ns
[28,66].
5. Substitution of alternative anticorrosion additives to replace sodium nitrite.
Concern was raised in California over the possible toxicity of the solid lubricant molybdenum disul-
de. This could have been a problem for the disposal of used grease containing molybdenum disul-
de. However, after toxicological review, the California Department of Toxic Substances Control
ruled that wastes containing molybdenum in the form of molybdenum disul de are excluded from
identi cation as hazardous waste [67].
With the increasingly global nature of the lubricant business, regulations such as those in the
European Union covering Registration, Evaluation, and Authorisation of Chemicals (REACh) are
affecting the lubricants and additive industries worldwide. The European Union’s new chemical
legislation came into effect on June 1, 2007. Although the details of its implementation are still to
be nalized, it is likely to have a signi cant impact on suppliers, producers, and consumers of more
than 30,000 chemicals.
Various original equipment manufacturers, such as automobile manufacturers, provide long
lists of restricted substances that are not allowed in materials, such as lubricants, which they pur-
chase. There is not much doubt that the list of restricted substances will grow in the future. Con-
cerns about environmental persistence will likely increase the demand for biodegradable greases.
Although the market for this type of grease is currently quite modest, the U.S. Army has written
a speci cation for biodegradable multipurpose grease that should help to advance the technology
[68]. Such greases usually contain natural or synthetic ester base oils that are biodegradable and
additives of low toxicity that, if not readily biodegradable themselves, at least do not impede the
ability of microorganisms to ultimately biodegrade organic portions of the grease to carbon dioxide,
water, and biomass. Studies have been performed to determine the effectiveness of various grease
additives in these types of base greases [69,70].
24.6.4 LUBRICATING GREASE TRENDS
As in the case of other industrial lubricants, the trend for lubricating greases is to develop
products with improved high-temperature performance leading to longer bearing lives. Grease
formulators are also searching for anticorrosion additives that provide better protection
against saltwater corrosion and that do not signi cantly affect the performance of the grease
thickener [44].
Because interaction of grease additives and grease thickener must be considered for the
development of optimum grease formulations, it follows that the importance of various grease
additives may change as the popularity of the various grease thickeners changes. Although the
versatility of lithium soap and lithium complex greases causes them to be the most widely used
grease types, polyurea greases are growing in popularity as they tend to give signi cantly longer
bearing lives in certain sealed bearing applications. It is expected that the popularity of poly-
urea- and diurea-thickened greases will continue to grow, and, as they do, more emphasis will be
placed on developing additives that perform optimally with these types of thickeners.
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