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244 Lubricant Additives: Chemistry and Applications
The impetus for signi cantly improved lubricant additives is found on a number of fronts.
Governmental and regulatory requirements continue to challenge the industry for improved products
with lower toxicity. New engine developments, such as increased use of diamond-like carbon (DLC)–
coated engine parts and ceramic components for wear resistance and higher contact temperatures, are
on the horizon and present opportunities for antiwear additives that can function at very high operating
temperatures. Space technology and other advanced transportation needs present new challenges to the
industry. And, of course, there will always be a need for low product costs and ease of production.
Four particular developments may have a major impact on the lubricant industry in the near
term: (1) a move toward low-sulfur hydroprocessed (groups II and III), sulfur-free gas-to-liquid
(GTL) and synthetic (groups IV and V) base stocks; (2) the imminent trend toward lower ash, sulfur,
and phosphorus in engine oils; (3) a desire to reduce or eliminate chlorine in lubricants, particularly
in metalworking uids; and (4) a move to eliminate heavy metals and achieve low ash or even ash-
free in both engine and industrial oils.
To meet the growing needs for better thermal/oxidative stability and better viscometrics, synthetic
base stocks such as polyalpha ole ns, together with hydrotreated petroleum base stocks and GTLs are
continuing to expand in all lubricant sectors. These types of materials have no aromatic hydrocarbons
or greatly reduced amounts of aromatic hydrocarbons, which are potentially problematic for additive
solvency; as a result of removing these solubilizing aromatics, the additives tend to precipitate out
of the oil. This is particularly true for surface-active, polar components such as antiwear additives.
Therefore, greater compatibility with nonconventional base stocks (groups II–IV) will be an essen-
tial requirement for all ashless antiwear/EP additives. Meanwhile, there are noticeable synergies
identi ed among certain ashless antiwear/EP additives and nonconventional base stocks in a number
of lubricant applications. Therefore, the choice of proper ashless additives will be vitally important.
Because of the large number of automobiles equipped with catalytic converters that are sensitive
to phosphorus derived from zinc dithiophosphates in the crankcase oil (possible reduction of cata-
lytic ef ciency), strong needs exist for engine oils with lower phosphorus content. Initially, ILSAC
GF-4 aimed to reduce phosphorus levels to as low as 0.05% (about one-half of the former GF-3 level),
but settled on a maximum phosphorus level of 0.08% instead (a 20% reduction). In addition to phos-
phorus limits, GF-4 oils also offered improved oxidation stability (including nitration control), high-
temperature wear discrimination, high-temperature deposits control and used oil pumpability [103].
As vehicle emission regulations become more challenging, increasing restrictions are likely to
be placed on other lubricant elements besides phosphorus that can impact emission control systems.
Sulfur and metals are also under scrutiny as sulfur is suspected as a poison of DeNO
x
catalysts,
and ash (from metals) may plug after-treatment particulate traps. Modern engine oils rely heavily
on ZDDP to provide antiwear, antioxidation, and anticorrosion protection. Since ZDDP is rich in
phosphorus, sulfur, and zinc, it becomes an obvious target for emission control. In fact, at former use
level, ZDDP was almost solely accountable for more than two-thirds of sulfur and all the phosphorus
and zinc present in engine oils, excluding sulfur from base oils. Oils with ZDDP at former levels
could make it dif cult for OEMs to optimize (for cost and life) their exhaust after-treatment systems.
Therefore, the future trend will likely be toward further reduced ZDDP in engine oils providing that
the performance integrity can be maintained through the use of alternate additives [104].
To satisfy performance requirements in terms of oxidation control and deposit levels, more
antioxidants could be added to the engine oil formulation. These ashless antioxidants (hindered
phenols and arylamines) may effectively compensate for the loss of oxidation protection due to the
reduction in ZDDP concentrations. However, since ZDDP is such a cost-effective additive and is
the sole antiwear component used in many engine oils, a reduction in ZDDP treat levels may not
provide the needed wear protection. Recently, engine builders are requiring even greater antiwear
protection, and more demanding test protocols are being put in place to ensure that lubricants can
meet these more stringent speci cations. Therefore, there is a strong need for advanced ashless
antiwear systems to replace or supplement ZDDP to satisfy emission regulations while ensuring
high levels of wear protection [105,106].
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Ashless Antiwear and Extreme-Pressure Additives 245
Not all phosphorus-containing additives behave the same in engine oils. Furthermore, even
within the same ZDDP family, not all ZDDP respond the same to after-treatment devices as evi-
denced by their relative volatility performance. Data indicated that volatilized phosphorus showed
very low statistical dependence on either oil volatility or phosphorus concentration in the fresh oil.
Rather the data seemed to indicate that the chemistries of the phosphorus-containing additives and
their formulation with other additives were the controlling cause of phosphorus volatility and, by
extension, emission level. Selby’s Phosphorus Emission Index (PEI) and Sulfur Emission Index
(SEI) shed some insights into the volatility impact on emission issues and better S/P volatility con-
trol than the current ZDDP that is highly desired for future ashless antiwear additives [107–109].
Chlorine in lubricants and other materials is becoming an increasing environmental concern.
Legislation around the industrial world limits the chlorine content of many lubricant products
to 50 ppm or less. The Montreal Protocol mandated a gradual phase-out of the use of chlorine-
containing refrigerants, such as hydrochloro uorocarbon (HCFC) and chloro uorocarbon (CFC),
and replacement with alternative hydro uorocarbons (HFC). Increased wear occurred in the refrig-
eration compressor when HFC refrigerants were substituted for CFC, and the cause of this increased
wear was believed to be inferior antiwear capability of the alternative HFC refrigerant as the envi-
ronmental gas, compared to that for CFC [110]. This offers some opportunities for the development
of new ashless antiwear additives for refrigeration compressor oils.
The cutting oil industry is facing similar ecological pressures, and future changes to reduce
or eliminate chlorine are expected. The most signi cant opportunity is perhaps driven by human
health and waste disposal issues concerning the use of chlorinated paraf ns. Chlorinated paraf ns
are used extensively as EP additives in metalworking uids. The National Toxicology Program
(NTP) listed chlorinated paraf ns, derived from C12 feedstocks and chlorinated at 60%, as a
suspect carcinogen. Although few metalworking uids are formulated with this class of chlorinated
paraf n, the image of chlorinated paraf ns in general has suffered due to uncertainties about future
NTP reclassi cation of all such additives.
Gear additives are another area of concern. Because of the problems associated with chlorine
additives, their use in gear oils has been greatly reduced. However, a number of processes for
making gear additives utilize chlorine or chlorine-containing reagents at some point in the reaction
sequence. Small amounts of chlorine still remain in the nal product. The complete removal of
chlorine is therefore expected to become an important priority, but will be dif cult to attain in the
near future.
Finally, the use of metallic antiwear/EP additives is diminishing due to the in uence of environ-
mental concerns. Heavy metals are considered pollutants, and their presence is no longer welcomed
in the environment. Given equal performance and costs, ashless antiwear additives will be preferred
for many future lubricants.
In the future, the lubricant additive business will continue to grow and will need more ash-
less antiwear/EP additives [111]. Possible new markets include biodegradable lubricants, biodiesel
fuel–friendly lubricants, advanced transportation lubricants, robotics, ceramics, and space technol-
ogy lubricants. Traditional markets in engine oils, ATFs, marine, aviation, gear, hydraulic, circu-
lating oils, metalworking, and other industrial lubricants are also expanding. Healthy growth for
nonconventional base oils (groups II–V) is expected in many of these areas. Clearly advanced ashless
antiwear additives with environment-friendly features, excellent stability, and unique performance
properties, especially for nonconventional base oils, will be the additives of choice for increasingly
demanding lubricant applications.
ACKNOWLEDGMENT
I thank Pat Dedert and Elvin Hoel for assistance in the literature search and also many of my
colleagues, especially Dr. Douglas Deckman, Dr. David Blain, Dr. Steven Kennedy, Dr. Andy
Horodysky, and Dr. Andy Jackson, for their valuable comments.
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246 Lubricant Additives: Chemistry and Applications
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251
9
Sulfur Carriers
Thomas Rossrucker and Achim Fessenbecker
CONTENTS
9.1 Introduction ........................................................................................................................... 252
9.2 History ................................................................................................................................... 253
9.2.1 First Synthesis and Application (1890–1918) ............................................................253
9.2.2 First Application in Metalworking Oils (1920–1930) ...............................................254
9.2.3 Sulfurized Compounds for Gear Oils and Other Lubricants (1930–1945) ...............254
9.2.4 Scienti c Research on Chemistry and Application (1930–1949) ..............................255
9.2.5 Summary of the Past 50 Years ..................................................................................256
9.3 Chemistry ..............................................................................................................................257
9.3.1 Chemical Structure of Sulfur Carriers ......................................................................257
9.3.1.1 Sulfurized Isobutene ...................................................................................258
9.3.1.2 Active-Type Sulfurized Ole ns ...................................................................258
9.3.1.3 Inactive Sulfurized α-Ole ns ......................................................................258
9.3.1.4 Sulfurized Synthetic Esters (Light Color) ..................................................258
9.3.1.5 Sulfurized Fatty Oil (Black Color) .............................................................259
9.3.1.6 Sulfurized Fatty Oil/Ole n Mixture (Light Color) .....................................259
9.3.2 Current Commercial Production Processes ..............................................................259
9.3.2.1 General Aspects ..........................................................................................259
9.3.2.2 Black Sulfurization .....................................................................................260
9.3.2.3 High-Pressure H
2
S Reaction ....................................................................... 261
9.3.2.4 Mercaptan Route .........................................................................................262
9.3.3 Other Synthetic Routes ..............................................................................................262
9.3.3.1 Sulfurchlorination Route ............................................................................. 262
9.3.3.2 Alkylhalogenide/NaS
x
................................................................................263
9.3.4 Raw Materials ............................................................................................................264
9.4 Properties and Performance Characteristics .........................................................................264
9.4.1 Chemical Properties ..................................................................................................264
9.4.1.1 Effect of Additive Structure on Performance .............................................264
9.4.2 Physical Properties ....................................................................................................267
9.4.2.1 Effect of Additive Structure on Properties .................................................267
9.5 Comparative Performance Data in Pertinent Application Areas ..........................................269
9.5.1 Metalworking ............................................................................................................269
9.5.1.1 Cutting/Forming .........................................................................................269
9.5.1.2 Contribution of Sulfur Carriers to Metalworking .......................................269
9.5.1.3 Replacement of Sulfur Flowers ...................................................................269
9.5.1.4 Copper Corrosion ........................................................................................270
9.5.1.5 Substitutes for Chlorinated Paraf ns ..........................................................270
9.5.1.6 Substitute for Heavy Metals ........................................................................ 270
9.5.1.7 Carbon Residue Reducing in Rolling Oils ..................................................270
9.5.1.8 Water Miscible Metalworking Products .....................................................270
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252 Lubricant Additives: Chemistry and Applications
9.5.2 Grease ........................................................................................................................ 271
9.5.3 Industrial Oils ............................................................................................................ 271
9.5.3.1 Industrial Gear Oils ....................................................................................272
9.5.3.2 Slideway Oils .............................................................................................. 272
9.5.3.3 Hydraulic Fluids ..........................................................................................272
9.5.3.4 Multifunctional Lubricants ......................................................................... 272
9.5.3.5 Agricultural Applications ...........................................................................272
9.5.3.6 Automotive Applications ............................................................................ 272
9.5.4 Synergies/Compatibility with Other Additives ......................................................... 273
9.5.4.1 Zinc Dialkyldithiophosphates .....................................................................273
9.5.4.2 Basic Alkali Metal Salts ............................................................................. 274
9.5.4.3 Antioxidants ................................................................................................ 274
9.5.4.4 Esters/Triglycerides .................................................................................... 274
9.5.5 Cost-Effectiveness ..................................................................................................... 275
9.6 Manufacture and Marketing Economics ............................................................................... 275
9.6.1 Manufacturers ............................................................................................................275
9.6.2 Marketers ................................................................................................................... 276
9.6.3 Economics ................................................................................................................. 276
9.6.4 Government Regulations ...........................................................................................276
9.6.4.1 Competitive Pressures ................................................................................. 276
9.6.4.2 Product Differentiation ............................................................................... 276
9.7 Outlook .................................................................................................................................. 276
9.7.1 Crankcase/Automotive Applications ......................................................................... 276
9.7.2 Industrial Applications ..............................................................................................277
9.8 Trends ....................................................................................................................................277
9.8.1 Current Equipment/Speci cation ..............................................................................277
9.8.1.1 Types of Equipment ....................................................................................277
9.8.1.2 Additives in Use ..........................................................................................277
9.8.1.3 De ciencies in Current Additives ...............................................................277
9.9 Medium-Term Trends ............................................................................................................278
9.9.1 Metalworking ............................................................................................................ 278
9.9.2 Industrial Oils ............................................................................................................ 278
References ......................................................................................................................................278
9.1 INTRODUCTION
In the lubricant industry, a great variety of sulfur-containing additives are known and in use today.
We list only a few of the most common types:
Sulfur carriers (sulfurized ole ns, esters, and fatty oils)
Sulfur/phosphorus derivatives (dithiophosphates, thiophosphonates, thiophosphites, etc.)
Thiocarboxylic acid derivatives (dithiocarbamates, xanthogenates, etc.)
Heterocyclic sulfur (mercaptobenzothiazoles, thiadiazoles, etc.)
Sulfonates (Na-, Ca-salts of alkylbenzenesulfonic acids, nonylnaphthalenesulfonates, etc.)
Others (sulfated fatty oils/Turkish red oils, sulfurchlorinated fatty oils, sulfur-linked phe-
nols and phenates)
This long list gives a good impression of the versatility and importance of sulfur chemistry in lubri-
cants. But as versatile their chemistry is, the range of applications of sulfur-containing lubricant
additives is just as versatile. In this chapter, we try to review major aspects of a group of additives
commonly known as sulfur carriers. This is a generic name that has been accepted in the marketplace
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Sulfur Carriers 253
and used to summarize a group of additives that provide extreme-pressure (EP) and antiwear (AW)
properties and are used in gear oils, metalworking uids, greases, and engine oils. The vast majority
of them are sulfurized fats, esters, and ole ns. To distinguish them from other sulfur-containing
products and avoid misunderstandings, a suitable de nition of sulfur carriers is the following:
Sulfur carriers are a class of organic compounds that contain sulfur in its oxidation state 0 or –1,
where the sulfur atom is bound either to a hydrocarbon or to another sulfur atom
That does not contain other hetero atoms except oxygen
Produced by adding sulfur to all kinds of unsaturated, double-bond-containing compounds
such as ole ns, natural esters, and acrylates or by substitution reaction with reactive organic
halides and alike
Lubricant additives tting this de nition are the main focus of this chapter.
Owing to the overwhelming versatility of sulfur chemistry, other sulfur-containing product
groups cannot be discussed in depth but are mentioned in the context where appropriate.
Although this group of additives has been used in the lubricant industry for more than eight
decades, sulfur carriers are not at all an endangered species. In fact, we are still seeing increas-
ing usage today. This is partly due to continuous ongoing R&D work done in this area, which
brings about innovation and product improvement. Also, many chemical aspects and applications
are waiting to be discovered. Furthermore, sulfur carriers are essential additives for the solution
of upcoming lubricant market requirements such as chlorinated paraf n substitution; heavy metal
replacement; and health, safety, and environmental issues. Therefore, we expect to see substantial
future growth of light-colored, low-odor, and odor-free sulfur carriers.
9.2 HISTORY
As we look back on more than 100 years of sulfurized compounds, the authors had to rely on lit-
erature sources for the time before 1950s. During the literature studies, it turned out that one of
the most fruitful sources for the time period before 1950 is the review articles of Helen Sellei [1,2]
published in 1949. Much of what follows is based on their content, but we have tried to reinterpret
the information with today’s background knowledge.
9.2.1 FIRST SYNTHESIS AND APPLICATION (1890–1918)
Sulfurized fatty oils have been commercially produced for more than 100 years. Long before they
were used as additives in lubricants, they had become important additives for the rubber industry.
The addition of 4–8% sulfur to an unsaturated natural oil such as rapeseed oil at high temperatures
(120–180°C) gives a exible, gummy polymer called factis. Sulfur undergoes an addition reaction
to the double bonds of the natural oil and builds up a three-dimensional structure of sulfur bridges
between the triglyceride molecules. This is comparable to the vulcanization process of latex, which
results in rubber.
In the late nineteenth century, rubber was an expensive natural raw material, and with the rapid
industrialization in general and the growing automobile industry in particular, rubber tires were
needed in increasing amounts. It soon turned out that factis also provided special, positive proper-
ties to rubber goods during the vulcanization process. This was the starting point of smaller chemi-
cal factories producing additives for the rubber industry. In 1889, Carl Benz submitted the patent
for the world’s rst automobile in Mannheim, Germany. In the same year and city, Rhein Chemie
Rheinau GmbH was founded and started to produce sulfurized natural oils. Germany had seen a
special national aspect to the industrial history of sulfurized fats and rubber before 1914. Because
Germany had very few colonies, all rubber had to be imported. During the national tensions in
the rst decade of the twentieth century and subsequent trade boycotts, the search for alternatives
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