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254 Lubricant Additives: Chemistry and Applications

had been strongly pushed, leading to the development of synthetic rubber (Buna). Subsequently,

because it was cheap and based on locally available raw materials, factis had found increasing use

as rubber substitute and rubber diluent.

9.2.2 FIRST APPLICATION IN METALWORKING OILS (1920–1930)

In the very early days of modern lubrication, it had become known that sulfur is an important ele-

ment to improve frictional properties and prevent seizure under high loads. Free sulfur and sulfur-

containing heterocyclic molecules are known as part of natural crude oil (thiophenes, thioethers,

etc.). In early re ning technology, they were not removed effectively, especially from the higher-

viscosity oils that were typically used for gear oils, which had up to 3–4% sulfur. This natural sulfur

contributed to mild EP performance (antiwelding). After the positive effects of sulfur for lubricant

oil formulations were recognized, the next step was to physically dissolve sulfur  ower into the

lubricant oil at elevated temperatures. This sulfur, however, is very reactive and corrosive against

copper and its alloys. Also, sulfur  ower has a limited solubility in mineral oil, which limits its

maximum dosage and  nal EP performance achievable.

Sulfurized esters were  rst used in metalworking. For heavy-duty operations with a high degree

of boundary lubrication conditions, it was realized that the addition of oil-soluble sulfur compounds

had a tremendous effect on the performance. The  rst milestone literature that reports this effect

was published in 1918 by the E.F. Houghton Corporation [3] for cutting oils. It is claimed that a mix-

ture of lard oil, mineral oil, and wool fat treated with sulfur  ower at elevated temperatures results

in a sulfurized product that increases the performance of cutting oils enormously. In particular, the

tool life is extended, and smoking of the coolant is reduced greatly due to friction and temperature

reduction. These observations are still valid today and may be considered the starting point of the

application of sulfur carriers as additives for lubricants.

In comparison with the solid, rubberlike material factis, which has been commercialized for

several decades, Houghton Corporation’s breakthrough was to produce a liquid fatty material that

was soluble in mineral base oil at any ratio. It overcomes the solubility limits of sulfur  ower and

allows the adjustment of the EP performance level according to treat rate. They achieved this simply

by using nonreactive mineral oil and wool wax as chain-breaking agents and diluent to control the

polymerization reaction of lard oil to keep it liquid. From thereon, the use of sulfurized oils has

become quite common in metalworking.

9.2.3 SULFURIZED COMPOUNDS FOR GEAR OILS AND OTHER LUBRICANTS (1930–1945)

Some years later, the idea of improving load-carrying capacity under high-pressure and high-tem-

perature conditions had been picked up by automotive lubricant researchers and applied to oils for

the newly constructed hypoid gear boxes. With the advent of hypoid gears in automotive applica-

tions in the 1920s and 1930s, wear and seizure under high-load conditions became a major technical

problem that lubricant companies needed to solve. Most of the development work had been done

within these lubricant companies, and the new technology had not been published in detail. How-

ever, the number of patents on sulfur compounds for lubricants developed very rapidly throughout

the 1930s and 1940s.

1936 First patent review on EP lubricants [4]

1940 Patent review 1938–1939 [5]

1941 General publication on lubricating additives including an extensive patent

bibliography [6]

1946 Review article on sulfurization of unsaturated compounds [7]

This clearly indicates that the ideas that had been invented and  rst applied by the metalworking

people also worked for gears. Combinations of sulfurized products with lead soaps and lubricity

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Sulfur Carriers 255

esters were the  rst high EP performance technology in gear oils. Many years later, Musgrave [8]

stated in an article on hypoid gear oils that it was just by chance that the synergistic effect of sulfur

with lead soaps had been discovered in the early 1930s.

An interesting historical dimension was added to the EP gear oil development during World

War II. Most of the EP gear oil development occurred in the United States due to the great importance

of automotive industry in the 1920s and 1930s. German gear oil technology was not as advanced as

that of United States. Eyewitnesses report of frequent gear box failures of German tanks and heavy

equipment during the attack against Russia. The reason was that in autumn Russian roads were

turning into mud and the heavy vehicles were operated most of the time at maximum power. The

only mildly additized gear oils were not just good enough to prevent scoring and welding.

9.2.4 SCIENTIFIC RESEARCH ON CHEMISTRY AND APPLICATION (1930–1949)

Between 1930 and 1950, the important basics of sulfur carrier technology had been developed. Patents

from this period include most of today’s raw materials and reaction pathways. Raw materials used

were animal oils [3], vegetable oils/organic acids [9], pine oils [10,11], whale oil (sperm oil), acrylates,

ole ns [12], alcohols [13], synthetic esters [14], and salycilates [15]. Even thiocarbonates [16] and xan-

thogenates [17] were synthesized and used as organic, oil-soluble sulfur-containing EP additives.

Reaction pathways mentioned in patents mentioned earlier include

Sulfur  ower reaction with and without H

S, aminic, and other suitable catalysts

Sulfur chlorination with S

[18]

Organic halides with alkalipolysul des

Mercaptan route [19]

Important product properties that are still part of today’s development work were also mentioned

in that period:

Stability of sulfurized products, for example, diisobutene [20]

Active and inactive sulfur compounds

Corrosive and noncorrosive compounds

Light- and dark-colored derivatives

High- and low-odor products

Parallel to new chemistry, the development of test machines for tribological research progressed

quickly along with publications on mechanistic studies of additives. In 1931, at an API meeting,

Mougay and Almen [21] presented the  rst chemical interpretation for the load-carrying capacity of

sulfur-containing EP additives and their synergy with lead soaps. They attributed the performance

to the formation of a separating  lm between the frictional partners—a theory generally accepted

today in tribological science. In 1939, this  lm-forming theory of sulfur compounds was proven

using the four-ball tester [22]. In 1938, Schallbock et al. published [23] standard-setting results on

investigations in the  eld of metalworking. Empirical correlations were found among cutting speed,

temperature, and tool life that are still valid. In 1946, synergistic effects of chlorinated additives

with sulfur additives were explained based on a chemical reaction theory [24] under the aspect

of newest generation hypoid gear formulations. Phosphorus additives (tricresylphophate [25], zinc

dialkyldithiophosphates [ZnDTP]), primary antioxidants (AOs) (phenyl-α-naphthylamine, butylated

hydroxytoluene [BHT]), and detergent/dispersants [26] (salycilates) also joined the world of lubri-

cant additives during this period and have been used since in combination with sulfur carriers.

Most of the development work at that time had been done in a deductive way in a trial-and-error

approach. Theoretical explanations, tribological, and chemical modeling always trailed behind (look-

ing back from today’s point of view, it is quite astonishing that not so much has changed in 70 years).

•

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256 Lubricant Additives: Chemistry and Applications

9.2.5 SUMMARY OF THE PAST 50 YEARS

With the fundamentals of sulfur carriers being explored so early in lubricant additive history, the

literature from 1950 until today concentrates around improvements in production procedures, com-

binations and synergies with other additives, improvement in product qualities, and search for spe-

cial applications. The use of sulfur carriers has been extended from metalworking and automotive

engine to industrial oils and greases. Ashless hydraulic oils may contain sulfurized EP additives for

special applications. Now this product group is used throughout the lubricant oil industry.

Tribology has been de ned as a particular  eld of scienti c research, and several basic models

of additive response have been worked out. A review article [27] in 1970 summarizes the state of

the art at that time, including many literature references.

Until the 1950s, sulfur carriers were mostly made by the lubricant manufacturers. However,

increasing environmental awareness, growing market, and the need for more specialized products

brought about change. As the sulfurization process involves deep chemical knowledge and produc-

tion know-how and includes extremely high safety risks, specialty chemical companies became

active in this area. It is expected that the few lubricant companies that still produce a small quantity

of black sulfurized fats in-house may discontinue sooner or later.

Since the 1950s, the sulfur carrier market was split into two segments: automotive and indus-

trial. In automotive gear oil applications, sulfurized isobutene (SIB) soon became the standard EP

product because it is high in sulfur content but low in corrosivity. The typical, rather strong smell of

SIB is no real problem in this  eld of application because gear boxes are totally closed systems. In

any open lube system, this EP technology is not acceptable. The big oil companies had their petro-

chemical subsidiaries (Mobil Chemical, BP Chemical, Shell Chemical, Exxon Chemical, Chevron

Chemical, etc.) and added additive manufacturing as the market grew including SIB production

units. So SIB production has always been the target for those companies with focus on automotive

additives and lubricants. Over the decades, the SIBs have gone through changes in chlorine level

due to environmental requirements [28]. Starting at 2–3% in the early days, today high qualities no

longer contain chlorine because of a chlorine-free, high-pressure H

S production processes. Also,

the amount of active sulfur in SIBs has been reduced to improve the long-term abrasive wear of

gear oil formulations in bearings and to meet today’s  l l - f o r - l i f e requirements. But in principle, in

automotive applications, the same sulfur chemistry is in use today as it was some 60 years ago.

The other big  eld of application of sulfurized EP additives is industrial lubrication. Tradition-

ally this area is less regulated and restricted by Original Equipment Manufacturer (OEM) approv-

als, general speci cations, and standards—it is particularly true for the metalworking market. Here

much more differentiated, problem-solving additives have been and still are in use. This environ-

ment has generated a greater variety of smaller volume sulfur carriers that address strongly differ-

entiated technical requirements of metalworking processes and grease applications. Subsequently

smaller, more specialized chemical companies entered the lube additive business. The  rst products

were dark in color, but as early as 1962, Rhein Chemie commercialized its  rst light-colored, low-

odor sulfurized synthetic ester based on chlorine-free production technology (see Section 9.3.2).

A big milestone in the history of sulfur carriers has been the international banning of sperm oil

(whale oil) in 1971. Up to this year, sperm oil and lard oil (pig fat) have been the dominant fatty raw

materials for sulfur carriers. The sperm-oil-based products in particular showed excellent solubility

and lubricity in addition to their sulfur-related EP properties. The extensive research activities of

this period resulted in various patents [29,30]. The new raw materials turned out to be vegetable oils

in combination with either synthetic esters or ole ns.

Another aspect that strongly in uenced the sulfur carrier market has been the change of re n-

ery technology for base oils. The driving force behind was the necessity to improve environmental

as well as health and safety aspects of the major re nery products: fuels. These requirements led

to drastic reduction of aromatic components and sulfur content in fuels and subsequently of base

oils. From a lubricant point of view, the reduction of aromaticity of the base oils had strong negative

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Sulfur Carriers 257

impact on their solvency for additives and thus triggered intensive adjustment work on the additive

producer’s side including sulfur carrier manufacturers.

The reduction in sulfur content however has contributed substantially to the market growth of

sulfurized additives. The reduction of naturally occurring sulfur in base oils through the desulfur-

ization units now needs to be balanced for speci c applications through the addition of synthetic,

oil-soluble sulfur components to keep EP/AW properties as well as AO performance. This trend

started in the 1970s but is getting stronger today with the increase in the availability of XHVI base

oils/groups II and III as well as completely synthetic basestocks (polyalphaole ns [PAOs]).

In the late 1970s and early 1980s, a new class of sulfur carrier has been introduced into the

industrial lubrication market: dialkylpolysul des. They are based on C8, C9, or C12 ole ns and

contain up to 40% sulfur in a very reactive form. They can be looked at as liquid, oil-soluble sul-

fur  ower. The starting point for the development of these additives was the requirement of many

lubricant-blending companies for an alternative to sulfurization of base oils with sulfur  ower. It is

a very time-consuming step and may generate toxic gas (hydrogen sul de, H

S) and sulfur dropout

during application. Sulfur  ower can be dissolved in mineral oil just above its melting point of

115°C in concentrations of typically 0.4–0.6% and is used if appropriate in heavy-duty metalwork-

ing applications or running-in gear oils (see Section 9.5.1.3). The solution that has been offered from

additive manufacturers has been the new class of organic polysul des of light color and rather low

odor. Diisobutenepentasul de and tert nonyl- and dodecylpentasul de have been introduced as easy

to blend liquids and substitutes for sulfur  ower. Today, these active type pentasul des have become

the most important and wide-spread class of sulfur carriers on the industrial oil side.

In 1985, it was found that sulfur carriers, preferably polysul de types, show a strong synergistic

EP/AW behavior when combined with high total base number (TBN) ASTM-D4739 sodium and

calcium sulfonates [31]. This has become known as the PEP technology (passive EP) in neat oil

metalworking. In the beginning, it was hoped that this combination would be a general and simple

solution to upcoming chlorinated paraf n replacement issue that started in Western Europe and

Scandinavia in the mid to late 1980s. But as it turns out today, the PEP technology can only partially

match the universal properties of chlorinated paraf n formulations especially under low-speed/

high-pressure operation conditions (for more details, see Section 9.5.4.2).

From the late 1980s to early 1990s, a totally new aspect of sulfurized esters and fats has gained

substantial ground—the toxicology and ecotoxicology of these chemicals. Workers’ safety, environ-

mental compatibility, biodegradability, and similar requirements need to be addressed in industrial

more than in automotive lubrication, because workers in machine shops often cannot avoid constant

direct contact with the lubricant. The fact that the use of natural, renewable raw materials and opti-

mized production procedures may give low-toxic and biodegradable sulfur carriers refreshed the

interest of development chemists in these special, environmentally safe but classic additives.

The twenty- rst century’s central question of additive and lubricant R&D departments is how

to further optimize energy ef ciency and reduce friction. Again, sulfur carriers play a role. Span-

ning from engine oils to wind turbine gear boxes, formulators take advantage of their multipurpose

character.

9.3 CHEMISTRY

9.3.1 C

HEMICAL STRUCTURE OF SULFUR CARRIERS

For the majority of sulfur carriers, discrete structures are very hard to sketch for several reasons:

The raw materials are very often mixtures of isomers: in the case of ole ns, for example,

diisobutene, there are  ve main isomers; tetrapropylene shows some 35 components in the

gas chromatogram (GC). Natural fatty oils have a distribution with mono-, double- and

triple unsaturated acids with unsaponi cable matter.

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258 Lubricant Additives: Chemistry and Applications

Depending on the temperature, inter- or intramolecular bonding of sulfur occurs preferably.

The catalyst directs the addition of sulfur in a certain way (Markovnikov, etc.).

It is a fact that sulfur carriers are technical products based on technical raw materials. In the follow-

ing, the most typical structures of sulfur carriers based on different, contemporary raw materials

are shown. Taking the rather complex reaction pathways of a sulfurization reaction into account,

they necessarily are simpli ed model structures.

9.3.1.1 Sulfurized Isobutene

This is the standard EP additive for gear oils with typical sulfur contents in the range of 40–50%

(Figure 9.1).

9.3.1.2 Active-Type Sulfurized Oleﬁ ns

These are the polysul de types of sulfur carriers that have been introduced as substitute for sulfu-

rization of base oil and are widely used today in metalworking applications (Figure 9.2).

9.3.1.3 Inactive Sulfurized α-Oleﬁ ns

These are used in noncorrosive lubricant applications ranging from metalworking, greases to even

engine oil applications (Figure 9.3).

9.3.1.4 Sulfurized Synthetic Esters (Light Color)

These are widely used in metalworking and grease applications. Depending on the type of synthetic

ester chosen, special properties such as low temperature stability/ uidity and low viscosity may be

achieved (Figure 9.4).

FIGURE 9.1 Sulfurized isobutene.

0−4

FIGURE 9.2 Sulfurized diisobutene, sulfurized tetrapropylene.

FIGURE 9.3 Inactive sulfurized α-ole ns.

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Sulfur Carriers 259

9.3.1.5 Sulfurized Fatty Oil (Black Color)

See Section 9.3.2.2 (Figure 9.5).

9.3.1.6 Sulfurized Fatty Oil/Oleﬁ n Mixture (Light Color)

This special group of sulfur carriers is outstanding in properties as they combine the positive effects

of sulfurized ole ns (e.g., hydrolytic stability, high sulfur content) with the excellent lubricity and

 lm-forming properties of sulfurized fatty oils (Figure 9.6).

9.3.2 CURRENT COMMERCIAL PRODUCTION PROCESSES

9.3.2.1 General Aspects

In any current sulfurization process, the raw material is based on unsaturated compounds (ole ns

or unsaturated esters). All reactions are addition reactions to ole nic double bonds, and textbook

chemistry applies to this kind of reaction (catalytic conditions, mechanisms, addition patterns—

Markovnikov–anti-Markovnikov orientation).

Today’s large-scale production technology in general avoids any halogen-containing reaction

steps because there are low limits (maximum 30 ppm chlorine) in the  nal lubricants that may

not be exceeded, for example, in automotive gear oils. Also, expensive removal/workup steps, for

example, by washing, can be avoided if halogens are not used.

Chromophoric group

FIGURE 9.5 Sulfurized fatty oil (black color).

FIGURE 9.4 Sulfurized synthetic esters (light color).

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260 Lubricant Additives: Chemistry and Applications

Any sulfurization process involves either the possible formation or the actual use of hydrogen-

sul de (H

S), an extremely toxic and corrosive gas. The smell of H

S is generally that of rotten

eggs, which is offensive to the human nose. In earlier times, the H

S released during the sulfuriza-

tion reaction (see Section 9.3.2.2) was just vented through a chimney or, if at all, scarcely absorbed

in alkaline scrubbers. Today, this procedure is no longer tolerated almost worldwide, and smaller

factories that have not yet done so need to invest into expensive safety equipment. This is another

reason why the smaller lubricant oil companies that still do a little bit of sulfurization are consider-

ing stopping. The processes are either pure batch or semicontinuous processes. For industrial appli-

cations, the volumes and varieties would not completely justify continuous productions.

9.3.2.2 Black Sulfurization

This is the simplest and oldest of all production technologies for sulfur carriers. The  rst patented

sulfur carrier was done this way. The manufacturing equipment needs to withstand pressure above

1–2 bars (it may even be pressureless). Raw materials may be ole ns as well as natural or synthetic

esters with a certain degree of unsaturation. The other reactant introduced into the ole n-contain-

ing reaction vessel is sulfur  ower. The mixture is heated above the melting point of sulfur. An

uncatalyzed reaction starts to become exothermic above 150–160°C, with the evolution of substan-

tial amounts of H

S. Catalyzed reactions start just above the melting point of sulfur, in the range of

120–125°C. Typical catalysts are organic amines, metal oxides, and acids.

Mechanistic studies of this reaction have been reported [32] and are very complex. At tempera-

tures between 120 and 160°C, intermolecular reactions are preferred. At 160–190°C, intramolecu-

lar reactions may be detected.

The  rst reaction step in this so-called black sulfurization process is the ring opening of the

sulfur  ower (S

-ring structure) and the subsequent oxidative attack of the sulfur on the vinylic pro-

tons (Equation 9.1). This quite uncontrolled reaction ends in the release of H

S and the formation of

vinylic mercaptans, vinylic thioethers, vinylic alkyl- and dialkylpolysulfanes, vinylic thioketones,

and even sulfur-containing heterocycles (thiophenes, etc.).

R HC CH CH R S R HC CH CH SR R HS H S

2 2

⫺⫺⫺ ⫺⫺⫺

⫺

⫽⫹⫽ ⫹⫹

→ ()

()1

(9.1)

FIGURE 9.6 Sulfurized fatty oil/ole n mixture (light color).

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Sulfur Carriers 261

A great part of the in situ generated H

S does not leave the reaction mixture but is directly adsorbed

by the double bonds, thus producing saturated alkylmercaptans (Equation 9.2a). They react further,

oxidatively, with sulfur  ower to generate alkyl- and dialkylsul des with the release of more H

(Equation 9.2b).

RHCCHR HS RHCCH(SH)R

Olefin

Alkylmercaptan interme

⫺⫺ ⫺⫺⫺⫽⫹→

ddiate

(9.2a)

2R H C CH(SH) R Sx R H C CHR S (S ) S CH

Alkylmercaptan

2x1

⫺⫺ ⫺ ⫺⫺ ⫺⫺ ⫺⫺

⫺

⫹ → RRCH R HS

Dialkylpolysulfide

⫺⫺⫹

(9.2b)

The  nal product consists of a full range of organic sulfur derivatives. Some of them are still

unsaturated, with isomerized double bonds and conjugated, chromophoric (color-deepening) sulfur

compounds such as thioketones and thiophenes, which cause the product to be dark black in color

and rather smelly. From an application point of view, these products exhibit EP/AW performance,

but because of their remaining double bonds, they have the following negative characteristics:

They will continue to polymerize during use and even under normal storage conditions.

They are easily oxidizable and form residues on fresh metal surfaces/discoloration.

They will cause a TAN increase within a short time in circulation systems and cause short

oil drain intervals.

They will even generate H

S/mercaptan during high-temperature usage in lubricant systems

(see Sections 9.4.2.1.7 and 9.4.2.1.2).

So today’s main use of these black sulfurized products are total loss lubricants in which long-term

stability and bad smell are not an issue. It is the cheapest way of making sulfurized additives.

9.3.2.3 High-Pressure H

S Reaction

High-quality sulfur carriers, which have improved properties compared to the black materials, are

produced today using high-pressure/high-temperature equipment. The handling of toxic H

S under

high-pressure conditions requires sophisticated handling techniques and safety measures. Further-

more, H

S is an expensive gas. All these aspects contribute to signi cantly higher production costs

compared to the simple black sulfurization.

In this process, the ole ns, sulfur, and H

S are added to a high-pressure-resistant reactor and

heated to 120–170°C. The reaction is also catalyzed by amines, metal oxides, acids, etc. For low-

boiling ole ns such as isobutene, the pressure may go up as high as 50–60 bar. For higher-boiling

ole ns such as diisobutene, typical pressure is in the range of 2–15 bar. The presence of H

S as

reducing agent and strong nucleophile makes a total difference to the black sulfurization process.

The oxidative attack of sulfur on the vinylic carbon–hydrogen (C–H) bond is effectively suppressed.

The side reaction of the black sulfurization process becomes the main reaction here: the addition of

S to the double bonds to form mercaptans (Equation 9.2a) which then quickly react with sulfur

in a redox reaction to form dialkyldi-, tri-, tetra-, and polysul des and release 1 mol equivalent of

S (Equation 9.2b).

This procedure gives much more controlled reaction conditions and  nally fewer side prod-

ucts. The most important effect of this reaction pathway is the fact that the double bonds are gone

after the reaction and no conjugated systems with chromophore (color-deepening) properties can

be formed. The sulfur carriers produced by this way are much more oxidatively stable, and they

are of light color. This one-step process is an advantage in terms of total production time and

turnover.

•

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262 Lubricant Additives: Chemistry and Applications

9.3.2.4 Mercaptan Route

Few producers synthesize sulfur carriers in a two-step process.

1. In the  rst step, H

S is added to ole ns under the catalytic action of Lewis acids. If strong

activators such as BF

are used, the reaction takes place at as low as –20°C. Another pro-

cedure works at 60–90°C. The resulting alkylmercaptans are distilled from the reaction

mixture and isolated as intermediates (Equation 9.3). The nonreacted ole ns are circulated

back to the reaction vessel.

RHCCHR HS RHCCH(SH)R

Olefin

Alkylmercaptan

⫺⫺ ⫺⫺⫺⫽⫹→

(9.3)

2. The mercaptans are oxidized either with hydrogen peroxide (H

) (Equation 9.4a) to the

dialkyldisul des

2R H C CH(SH) R H O R H C CHR S S CHR CH R

222

Alkylmercaptan

⫺ ⫺ ⫺ ⫺ ⫺ ⫺⫺⫺ ⫺ ⫺⫹ →

DDialkyldisulfide

(9.4)

or by stoichiometric amounts of sulfur to trisul des (Equation 9.5) and polysul des (Equation 9.6)

22RHCCH(SH)R S RHCCHRSSSCHRCH R

Alkylmercaptan

⫺ ⫺ ⫺ ⫺ ⫺ ⫺⫺⫺⫺ ⫺ ⫺⫹ →

DDialkylpolysulfide

HS⫹

(9.5)

2 R H C CH(SH) R Sx R H C CHR S (S ) S CH

Alkylmercaptan

2x1

⫺⫺ ⫺ ⫺⫺ ⫺⫺ ⫺⫺

⫺

⫹ → RRCH R HS

Dialkylpolysulfide

⫺⫺⫹

(9.6)

This may be summarized in the reaction shown in Figure 9.7.

This process is mainly applied to ole n-based sulfur carriers based on tri- and tetrapropyl-

ene as starting material because the resulting tertiary dodecylmercaptan may be used chemical

intermediate and in other applications such as rubber processing as.

9.3.3 OTHER SYNTHETIC ROUTES

9.3.3.1 Sulfurchlorination Route

Sulfur carriers can be synthesized in a two-step process using disulfur dichloride and sodium sul-

 de solution (Figure 9.8). It had been widely used because it is a controlled way of adding discrete

-bridges to double bonds with little side reactions occurring.

FIGURE 9.7 Summary of two-step process.

+2 H

+ 2 S

− H

−20°C

Tertiary dodecyltrisulfide

Tetrapropene

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Sulfur Carriers 263

Step 1. Addition of disulfur dichloride to double bonds.

In case of fatty oil being used as ole n source, the resulting product is a sulfurchlorinated

fatty oil useful as chlorine and sulfur-containing EP additives in metalworking. From a

technical point of view, their biggest problem is the split-off of chlorine and subsequent

severe corrosion problems that are dif cult to control. From today’s point of view, the

presence of chlorine is not favorable anymore because of environmental concerns.

Step 2. Subsequent treatment of the sulfurchlorinated products with sodium sul de (NaS

)

solution in water. It is a substitution reaction of sulfur versus chlorine (Figure 9.9). Inter-

molecular linkage as well as ring closure may occur. The water-soluble sodium chloride

is washed out.

9.3.3.2 Alkylhalogenide/NaS

This process is closely related to the  rst step reaction discussed in Section 9.3.3.1. Starting materi-

als may be alkyl- or arylhalogenides. As shown in Figure 9.10, it is possible to substitute halogens

with sulfur using alkali sul des. If Na

S is used, monosul des are generated. In case alkali polysul-

 de is applied, alkyl- or arylpolysul des are the resulting derivatives.

This route has not found commercial interest as raw material costs are too high compared to

other synthetic methods.

+2Na

S −2 4NaCl

Sulfurized isobutene

FIGURE 9.9 Dechlorination reaction.

FIGURE 9.8 Sulfurchlorination reaction.

Arylhalogenide

(or Alkylhalogenide)

Sodium polysulfide

in water

−2 NaCl

Diarylpolysulfide

(or dialkylpolysulfide)

0−5

FIGURE 9.10 Dechlorination of arylhalogenides.

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