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264 Lubricant Additives: Chemistry and Applications
9.3.4 RAW MATERIALS
In principle, any single- or multi-double-bond-containing molecule may be sulfurized. Therefore, the
list of ole nic raw materials is long. The list of sulfur-containing materials is rather short. It is mainly
sulfur ower (S
8
), hydrogen sul de gas, some S
2
Cl
2
, and some alkali polysul de (e.g., NaS
x
).
On the ole n side, patent literature reports of the following:
Vegetable oils (soybean, canola, rapeseed, cottonseed, rice peel, sun ower, palm, tall oil,
terpenes, etc.)
Animal fats and oils ( sh oils, lard oil, tallow oil, sperm oil, etc.)
Fatty acids
Synthetic esters
Ole ns (isobutene, diisobutene, triisobutene, tripropylene, tertapropylene, α-ole ns, n-ole-
ns, cyclohexene, styrene, polyisobutene, etc.)
Acrylates, methacrylates
Succinic acid derivatives, and more
The choice of commercially applied raw material is certainly limited to those compounds that have
a reasonable price level and give certain performance bene ts. Sulfur carriers based on low-boiling
ole ns (e.g., C4-types) are limited to closed lubricating systems due to the volatility of the decom-
position products and associated offensive smell. For water-based lubricant oil systems, sulfurized
fatty acids that can be easily emulsi ed and active types of ole ns that cannot be hydrolyzed are
preferred. In oil applications, one can nd the full range of products.
9.4 PROPERTIES AND PERFORMANCE CHARACTERISTICS
9.4.1 C
HEMICAL PROPERTIES
9.4.1.1 Effect of Additive Structure on Performance
9.4.1.1.1 Raw Materials
The additive structure is mainly in uenced by the choice of the raw materials and the sulfurization
method. A general overview of the performance properties of sulfurized products based on differ-
ent raw materials is shown in Table 9.1.
9.4.1.1.2 Infl uence of Raw Materials on Extreme Pressure and Antiwear
The raw material determines the polarity and, therefore, the af nity of the product to a metal surface
[33]. With increasing polarity, an increasing EP performance can be observed. Straight sulfurized
•
•
•
•
•
•
•
TABLE 9.1
Performance Properties of Sulfurized Products
Ester Triglyceride Olefi ns
Inactive Active Inactive Active Inactive Active
Extreme pressure Fair Good Good Very good Low Fair
Antiwear Good Low Very good Low Good Poor
Reactivity Low High Low High Low Very high
Cu corrosion Low High Low High Low High
Antioxidant Good Low Good Poor Good Poor
Lubricity Fair Fair Very high Very high Poor Poor
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Sulfur Carriers 265
ole ns are nonpolar and show a relative poor af nity for metal surfaces (see Section 9.4.2.1.4). As
the polarity increases from ole n < ester < triglyceride, the EP performance increases in the same
order. This behavior is demonstrated in a simple four-ball EP test. Chart 9.1 shows the four-ball weld
load (DIN 51350 Part 2) of sulfurized additives over the sulfur level in oil. The products with a high
polarity (C and D) show considerably higher EP loads than the nonpolar additives (A and B).
The content of active sulfur is only of minor importance on the EP performance, but the polarity
and chemical structure play a major role.
9.4.1.1.3 Activity
Active sulfur is the amount of sulfur available for a reaction at a certain temperature. A common
method for its determination is ASTM D-1662 [34]. The amount of active sulfur is determined by
reacting copper powder with the sulfurized product for 1 h at 149°C. Depending on the raw mate-
rials and on the sulfurization method, the active sulfur content can vary greatly. The activity is a
function of the temperature. Chart 9.2 shows typical active sulfur contents of sulfurized products
based on different chemistry and sulfurization methods.
The activity depends mainly on the sulfur chain in the molecule. Mono- and disul des are
not aggressive against yellow metals. Pentasul des are highly reactive and, therefore, suitable for
heavy-duty machining of steel The long-term inhibition of these products against yellow metals is
hardly possible. Long-chain sulfur bridges in polysul des (A) are thermally less stable than short
sulfur bridges, where sulfur is linked to the carbon atom of the raw material. For this reason,
the reaction with the metal surface is possible at relatively low temperatures. Mono and disul des
show only a medium activity, because sulfur will be released only at higher temperatures [35]. The
active sulfur at a given temperature is an indication of the ability of the product to provide suf -
cient reactive sulfur to form metal sul des. Published work on the mechanism of the in uence of
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
6500
7000
7500
0 0.5 1.5 2.5 3 3.5 4.5 5 5.5 6.5
% Sulfur in oil
Four-ball weld load (N)
A
B
C
D
Type Total Sulfur Active Sulfur
Activity (%)
A Olefin
40 36 90
B Ester
17 8 47
C Triglyceride
10 0.5 5
D Triglyceride
18 9 50
1642
CHART 9.1 In uence of raw materials on EP performance.
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266 Lubricant Additives: Chemistry and Applications
organosulfur compounds on the load-carrying properties of lubricating oils indicates that this is due
to their ability to form sul de lms that are more easily sheared than the metallic junctions under
EP conditions [36].
Therefore, active sulfur has a signi cant in uence on the AW performance. Higher sulfur activ-
ity results in faster formation of the metal sul de and higher wear. This performance is visualized
in Chart 9.3. The chart shows the four-ball wear scar (DIN 51350 Part 3) of sulfurized products with
various activities.
9.4.1.1.4 Copper Corrosion
ASTM D-130 [37] is a common method to determine the copper corrosion of additives. This copper
corrosion does not necessarily re ect the activity of a sulfurized product, because very often yellow
metal deactivators are used to mask the active sulfur.
The degree of copper corrosion depends on the amount of active sulfur and the presence of yel-
low metal deactivators. Inactive sulfurized products will show a long-term inactivity toward yellow
metals, whereas active sulfur, masked with yellow metal deactivators, will react with the yellow
metal as soon as the deactivator is consumed/reacted.
Therefore, the only statement that can be made is that a product will not stain copper under the
given test parameters. This method is not suitable to determine the activity that is of major relevance
for the performance of a sulfurized product (see Section 9.4.1.1.3).
0
10
20
30
40
50
60
70
80
90
50 70 90 110 130 150
Temperature (°C)
Active sulfur (% of total sulfur).
A
B
C
D
E
Type Total Sulfur
Active Sulfur
at 149°C
A
Olefin 40 36
B
Olefin 20 5
C
Triglyceride 10 0.5
D
Triglyceride 18 10.5
E
Olefin/Triglyceride 15 4.5
CHART 9.2 Active sulfur of various sulfurized products.
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Sulfur Carriers 267
9.4.1.1.5 Antioxidant
Sulfurized products with low active sulfur content are suitable to improve the AO behavior of
lubricants. This is particularly important if hydrocracked, almost sulfur-free base uids are used.
During the synthesis of these oils, the natural sulfur (mainly heterocycles, inactive) is removed. The
reintroduction of inactive sulfur carriers improves the oxidation stability, especially in combination
with other secondary AOs.
9.4.1.1.6 Lubricity
Lubricity can be described as friction reduction under low-pressure conditions. Under these condi-
tions, physical adsorbed lubricating lms are effective (see Section 9.4.2.1.4). Inactive sulfurized
triglycerides are widely used to improve the lubricity of a lubricant. In general, the lubricity of sul-
fur carriers increases with the polarity. Sulfurized ole n (no lubricity) < sulfurized ester (medium
lubricity) < sulfurized triglyceride (high lubricity). Special products with enhanced lubricity are
based on synergistic raw material blends such as triglyceride/long-chain alcohol, triglyceride/fatty
acid, and triglyceride/ole n.
9.4.1.1.7 Color
The color of sulfurized compounds is mainly in uenced by the production method and virtue of the
raw materials. Light color is not only a matter of cosmetics but also a quality feature. Light-colored
products manufactured with high-pressure hydrogen sul de processes or by mercaptan oxidation
do not have remaining unsaturated double bonds, and therefore, they show better oxidation stability
in general.
9.4.2 PHYSICAL PROPERTIES
9.4.2.1 Effect of Additive Structure on Properties
9.4.2.1.1 Raw Materials
The selection of the raw materials and the production process determine the chemical structure
of the compound. The physical properties of a sulfurized product are dependent on the chemical
structure. An overview is given in Table 9.2.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.5 1 1.5 2 2.5 3 3.5 4
% Sulfur in oil
Four-ball wear scar (mm)
A
B
C
D
Type Total Sulfur
Active
Sulfur
Activity (ASTM D-1662)(%)
A
Hydrocarbon
40
36
90
B
Hydrocarbon 20 5
25
C
Triglyceride 10
0.5
5
D
Triglyceride 18 10.5
58
CHART 9.3 In uence of activity on AW performance.
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268 Lubricant Additives: Chemistry and Applications
9.4.2.1.2 Polymerization
During the sulfurization process, the molecules of the raw materials are linked through sulfur.
Depending on the structure of the raw material, two or more raw material molecules will be linked.
Triglycerides such as lard oil and soybean oil do polymerize and form solid, rubberlike products, if
the polymerization is not controlled through chain terminators such as esters or ole ns, containing
only one double bond. Ole ns with only one double bond do not polymerize. Two molecules are
linked by a sulfur chain where length depends on the production process. Esters behave in a similar
way but due to varying amounts of multiple unsaturated compounds in natural esters, some polym-
erization takes place. Dark sulfurized products not only show less oxidation stability compared
with light-colored, completely saturated compounds but will also resume polymerization after the
production process is nished.
9.4.2.1.3 Solubility
The solubility is mainly a function of the polarity of the product. As the polarity increases from
ole n < ester < triglyceride, the solubility decreases. Polarity as well as the grade of polymeriza-
tion determines the solubility. In general, sulfurized ole ns have excellent solubility in solvents and
all mineral oils. Depending on the sulfurization method, esters can exhibit good solubility even in
group II and group III base oils if their polymerization grade is controlled during production. Sul-
furized triglycerides are, in general, limited in their solubility due to their high polarity. But even
more, the grade of polymerization plays a predominant role. A controlled reaction/polymerization
can lead to light-colored products that will be soluble in paraf nic base oils, whereas uncontrolled
polymerization will lead to dark-colored products, soluble only in oils with higher polarity and
aromatic content such as, for example, naphthenic base oils.
9.4.2.1.4 Polarity
Polarity determines the adhesion of a sulfurized product to the metal surface. The polarity depends
on the raw materials used for the sulfurization. The organic portion of the molecule is responsible
for the polarity and the af nity of the sulfurized product to the metal surface [35]. As the polarity
increases from sulfurized hydrocarbon < sulfurized ester < sulfurized triglyceride, the af nity
(physical adsorption) to metal surfaces also increases. Therefore, sulfurized products based on tri-
glycerides, fatty acids, or alcohols provide superior lubricity compared with sulfur carriers based
on less polar esters or nonpolar ole ns.
9.4.2.1.5 Viscosity
Viscosity of a sulfurized product depends on the type of raw material used for the sulfurization
and polymerization grade. A higher degree of polymerization (molecular weight) results in higher
viscosity. The raw materials determine the viscosity index (VI) of a sulfur carrier. Short-chain
sulfurized ole ns show low VIs, whereas sulfurized triglycerides have VI above 200.
TABLE 9.2
Physical Properties of Sulfurized Products
Ester Triglyceride Olefi n
Polymerization Low High Very low
Solubility Good Fair–good Very good
Polarity Moderate High Low
Viscosity Low High Very low
Biodegradability Good Excellent Poor
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Sulfur Carriers 269
9.4.2.1.6 Biodegradability
Depending on the raw materials and on the sulfurization process, sulfurized products cover the
whole range from nonbiodegradable to readily biodegradable. Besides the raw material, the produc-
tion technology plays a predominant role. Catalysts used, impurities in the raw materials, and side
components formed during the synthesis have a strong in uence on the biodegradability. Therefore,
biodegradability cannot be predicted but has to be tested for every single product. Biodegradable
sulfur carriers are available for various applications [38].
9.4.2.1.7 Stability
Storage stability is obtained by total reaction of the double bonds in the sulfur carrier and in elimi-
nating H
2
S and mercaptans. Especially mercaptans, but also H
2
S, are left over from the sulfuriza-
tion process. If H
2
S or mercaptans are not removed completely, they will evaporate under severe
conditions in the nal application or even under unfavorable storage conditions. Mercaptans can
react with the polysulfanes and thereby release H
2
S. Depending on the raw materials and the type of
sulfurization process, some sulfurized products continue to polymerize during storage. Especially
triglycerides, sulfurized with ower of sulfur under atmospheric pressure, show a steady and some-
times very strong polymerization during storage.
9.5 COMPARATIVE PERFORMANCE DATA
IN PERTINENT APPLICATION AREAS
9.5.1 M
ETALWORKING
9.5.1.1 Cutting/Forming
In principle, we have to deal in all cutting processes with abrasive wear (i.e., cutting) and adhesive
wear (i.e., build up edges). Depending on the particular process, the machining parameters, one of
these wear types, play a dominant role. At low machining speeds, like in most of the forming opera-
tions, adhesive wear (cold welding), the formation of build up edges, and wear on the ank of the
cutting edge are very often the limiting factors for tool life. At high machine speeds and increasing
contact temperatures, the abrasive wear determines the tool life. The reactivity of additives depends
on temperature and pressure. Field and laboratory tests showed that different types of sulfur carriers
(same sulfur content but varying raw materials or production processes) lead to signi cantly differ-
ent results in a metalworking operation [39].
9.5.1.2 Contribution of Sulfur Carriers to Metalworking
Sulfurized products can be designed to meet technical and ecological requirements in metalwork-
ing processes. They are used successfully for more than 80 years to avoid abrasive and adhesive
wear and enhance lubricity. In cutting operations, their main function is to support the cut and to
prevent wear of the tool, whereas in forming processes, sulfurized products should form a pressure-
stable lubricant lm and prevent adhesive wear.
9.5.1.3 Replacement of Sulfur Flowers
In the past, it was very common to dissolve sulfur owers in metalworking uids to obtain a high
reactivity and good EP properties. This procedure is very cost-intensive because it has to be done
under controlled temperature conditions below the melting point of sulfur and has several disad-
vantages such as limited solubility (maximum 0.8% S), limited stability (sulfur dropout), and high
corrosivity toward yellow metals. Also, there is a risk of H
2
S generation, a highly toxic gas well
known because of its rotten-egg odor. Today, this process is widely substituted by using sulfurized
products. If just reactivity is required, sulfurized ole ns with high total and active sulfur content are
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270 Lubricant Additives: Chemistry and Applications
used, although it is possible to adjust almost any activity/lubricity ratio while using the combination
of appropriate sulfurized products.
9.5.1.4 Copper Corrosion
Depending on the process and on the metals machined, corrosion control toward yellow metals can
be a requirement. If inactivity (no staining) toward yellow metals is required, it is important to use
either absolute inactive sulfurized products or medium-active sulfur carriers in combination with
yellow metal deactivators. Active sulfurized products can be inhibited short term but will, in the
long term, turn active again.
9.5.1.5 Substitutes for Chlorinated Paraffi ns
The driving forces for the replacement of chlorinated paraf ns are mainly ecological and toxico-
logical reasons. Users and waste oil disposal facilities have additional concerns over the corrosivity
of the chlorinated paraf n decomposition products, primarily hydrochloric acid.
Chlorinated paraf ns work because of their ability to form a highly persistent lubricating lm
even at low temperatures or moderate pressure. At high-temperature/high-pressure conditions, they
decompose, and the formed hydrogen chloride forms metal chloride with the metals involved in the
process [39].
Chlorinated paraf ns can be substituted with sulfurized products. Depending on the main
function of the chlorinated compound in the particular process used, lubricity, or activity, suitable
sulfurized products are available, which can function as alternatives. The lubricity performance
is mainly covered by highly polar, inactive sulfur compounds (see Section 9.4.1.1.6), whereas the
activity will be covered by reactive sulfurized ole ns or mixed sulfurized ole ns/triglycerides.
9.5.1.6 Substitute for Heavy Metals
Heavy metals, particularly antimony, molybdenum, and zinc compounds are used as EP and AW
additives in severe metalworking processes. Sulfur carriers have proven to be suitable substitutes,
particularly when used with synergistic compounds such as polymer esters, phosphates, phosphites,
dialkyldithiophosphates, and sulfonates.
9.5.1.7 Carbon Residue Reducing in Rolling Oils
Sulfur carriers are used in cold rolling of steel to prevent carbon residues build up on the surface of
the metal sheets during the annealing process. Carbon residues are generated by oxidation/polym-
erization of additives used in rolling oils. Therefore, typical rolling oils contain sulfur-based anti-
snakey edge and carbon-reducing additives [40]. Clean burning of the lubricant is important to
obtain a clean metal surface that can be evenly coated in subsequent process steps.
Typical products used for this application are sulfurized ole ns with low to medium activity or
inactive sulfurized triglycerides.
9.5.1.8 Water Miscible Metalworking Products
Sulfurized products are used in water-miscible metalworking systems to provide EP performance
and, depending on the type of sulfur carrier, lubricity. By far the biggest applications are soluble
oils or emulsions. Standard sulfurized products are not water-soluble. Surfactants must be used to
keep the sulfur carrier in the emulsion. Compared to applications in non-water-based systems, the
water-based systems require hydrolytically stable products that can react at relative low temperature.
Therefore, active sulfurized ole ns, preferably pentasul des, are widely used for this application.
Sulfurized esters and triglycerides are also used, especially if additional lubricity is required. Spe-
cialty sulfur carriers are reaction products of sulfurized fatty acids and sulfurized ole ns. These
sulfur carriers have good emulsifying properties with relatively high hydrolytic stability and activity.
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Sulfur Carriers 271
Straight sulfurized fatty acids such as sulfurized oleic acid, are used in semisynthetic metalworking
uids. The sulfurized fatty acid will be reacted with alkaline compounds such as amines or potassium
hydroxide to form a soap. This soap is water-dispersible and needs much less emulsi ers than a sulfu-
rized ole n. However, hard water stability can become a problem with this type of sulfur carrier.
9.5.2 GREASE
High demands on load-carrying capacity of machine parts require the use of EP and AW additives
to avoid material loss and the destruction of the surfaces of the friction partners. Older technology
still uses typical gear oil sulfur carriers based on short-chain ole ns such as isobutene. These sulfur
carriers provide a high sulfur content, but their distinct, strong odor prohibits their use in open
lubricating systems.
As it is almost impossible to mask the activity of sulfurized products in greases by using yellow
metal deactivators or sulfur scavengers, inactive sulfurized products are widely used as EP addi-
tives in greases. Especially if the grease is designed for a wide application range, it is imperative
that truly inactive sulfur carriers are used, because yellow metals are widely present as friction part-
ners (e.g., brass cages in bearings). In addition, there are increasing demands on high-temperature
stability for various grease types. This also calls for inactive, oxidation-stable sulfurized products.
Typical sulfur carriers for greases are shown in Table 9.3.
Sulfurized products are also used to substitute heavy-metal-containing compounds, which are
traditionally used as EP additives in greases. Besides their excellent performance, these heavy-metal-
containing compounds show some weak points. Antimony and bismuth compounds are known to have
some weakness regarding copper corrosion, and lead compounds are toxic. In the meantime, many
of these products have been replaced by special sulfurized products either as a direct replacement or
in combination with synergistic compounds such as zinc dialkyldithiophosphates, phosphate esters,
or overbased sulfonates [41]. Sulfurized products are also used in greases for constant velocity joints
(CVJ) [42]. They are very ef cient in combination with molybdenum compounds (e.g., molybdenum
dithiocarbamate [MoDTC], molybdenum dithiophosphate [MoDTP], Mo-organic salts) as a sulfur
source to support the formation of lubricating, active molybdenum disul de in the friction zone.
There is an increasing demand for EP greases for environmentally sensitive applications such as
railroad wheel ange lubrication, railroad switches, and agricultural equipment such as tractors or
cotton picker spindles. Some sulfurized products are biodegradable and show excellent ecological
data [39]. Therefore, these products are used rather than heavy-metal-containing compounds to
enhance EP and AW properties in such applications.
9.5.3 INDUSTRIAL OILS
An increasing variety of industrial uids use sulfurized products as EP and AW additives.
TABLE 9.3
Typical Sulfurized Products for Greases
Type Total Sulfur Active Sulfur Features
Triglyceride 8–12 0.5–3 Mainly inactive, limited EP performance
Triglyceride 13–15 4–7 Mainly active, hard to mask Cu corrosion
long term, good EP
Ole n 45 10–15 High EP performance, very distinct odor,
only for encapsulated systems
Triglyceride/ole n 15 4 Mainly inactive, high EP performance
Ester 9–11 1–3
Mainly inactive, limited EP performance,
excellent low-temperature pumpability
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272 Lubricant Additives: Chemistry and Applications
9.5.3.1 Industrial Gear Oils
Typical sulfur carriers for this application are short-chain sulfurized ole ns. Sulfurized Isobutene
(SIB) or Sulfurized Diisobutene are widely used as EP additive in industrial gear oils. SIB is used
for some decades as the EP additive of almost all industrial gear oil packages. The high sulfur
content, combined with a relative low active sulfur level, is ideally suited to match the require-
ments. Unfortunately, these products have a very distinct odor and, depending on the manufactur-
ing process, can contain chlorine compounds. Newer developments are based on sulfurized ole ns
with a longer chain length. Specialty products with additional demands on lubricity are based on
sulfurized triglycerides or mixtures of sulfurized ole ns and triglycerides.
9.5.3.2 Slideway Oils
Slideway oils are a special type of gear oils with very good anti-stick–slip properties. Besides the
austere requirements on coef cient of friction, there are also demands on compatibility and demulsi-
bility with metalworking emulsions. Inactive sulfurized triglycerides are suitable to reduce the coef-
cient of friction. Unfortunately, most of these products are easy to emulsify and will, therefore, not
meet the requirements on demulsibility without extensive formulation work. Modern slideway oils
are based on demulsifying sulfurized ole n/triglyceride-based products that have the advantages of
low coef cient of friction, good demulsibility, and high EP loads.
9.5.3.3 Hydraulic Fluids
It is possible to use inactive sulfur carriers in hydraulic systems with only moderate requirements on
thermal stability. Typical products are sulfurized ole ns and triglycerides or mixtures thereof.
9.5.3.4 Multifunctional Lubricants
Multifunctional lubricants cover more than just one lubrication application. There are increasing
demands for this lubricant type, especially in metalworking shops. As one lubricant will be used for
different applications with sometimes very different requirements, it is important that multifunc-
tional additives are used. Depending on the overall performance requirements, sulfur carriers are
used as EP, AW, or lubricity additive (see Tables 9.1 and 9.2). Multifunctional lubricants are almost
always a compromise in their formulation. For example, metalworking machines with combined
gear oil/process oil sump require a ne-tuned additive, especially on the EP side. Sulfur carriers
with a medium activity, additionally passivated with sulfur scavengers, are widely used for this
application.
9.5.3.5 Agricultural Applications
Lubricants in agricultural applications can be spilled on soil because of either the machine design
or leaks in hydraulic and gear systems. Therefore, there are increasing requirements on environ-
mentally compatible or less harmless lubricants. Sulfurized products are ideally suited for this
type of applications. They can be designed to meet performance and ecological requirements
(e.g., biodegradability). A wide range of lubricants exists for outdoor equipment based on veg-
etable oils (e.g., soybean, canola, rapeseed, and sun ower oil).
Sulfur carriers for these applications are mainly based on vegetable oils and synthesized in strictly
controlled manufacturing processes.
Typical sulfur carriers for agricultural applications are shown in Table 9.4.
9.5.3.6 Automotive Applications
It is disclosed in U.S. Patent Nos. 4,394,276 and 4,394,277 that various sulfur-containing alkane
diols may be formulated with lubricating oils to effectively reduce fuel consumption in an inter-
nal combustion engine. Sulfurized products in general and inactive sulfurized, oxidatively stable
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Sulfur Carriers 273
ole ns in particular are known to reduce friction ef ciently in engines. They provide not only AW
and antifriction but also antioxidation properties. However, they cannot substitute the multifunc-
tional zincdialkyldithiophosphates in this application.
Besides crankcase applications, the use of sulfurized products in automotive gear lubricants
is far more important. Since the middle of the twentieth century, almost every gear lubricant for
automotive applications has been formulated with SIB.
The advantages of SIB are its high sulfur content, oxidation stability, and low corrosivity, but
the very distinct odor and the low lubricity are its disadvantages.
Sulfurized esters and triglycerides are used in special transmission uids to adjust the stick–slip
properties. These sulfur carriers are also used in other lubricants in the automotive area such as
wheel bearing or CVJ grease.
9.5.4 SYNERGIES/COMPATIBILITY WITH OTHER ADDITIVES
Sulfurized products are compatible with most of the additives used in lubricants. Only strong acids
and bases must be avoided in combination with sulfur carriers.
9.5.4.1 Zinc Dialkyldithiophosphates
ZnDTPs are used in combination with sulfurized products in various applications. Besides their
primary functions as AW and AOs, there is a well-known synergistic effect in regard to stabiliza-
tion and improvement of copper corrosion of sulfur carriers. This behavior is demonstrated in the
ASTM D-130 copper corrosion test (Table 9.5).
In addition, ZnDTP can have a very positive effect on the odor of sulfur carriers.
TABLE 9.4
Sulfur Carriers for Agricultural Applications
Sulfur Carrier Type Application
Ester, inactive Gear greases (NLGI class 000), cotton picker
spindle lubricants
Triglyceride, inactive Gear lubricants, hydraulic uids, greases for
bearing lubrication, chassis lubricants
Triglyceride, medium-active Gear lubricants, chain saw, and bar saw lubricants
Ole n, inactive Gear greases (NLGI class 000), cotton picker
spindle lubricants
Triglyceride/ole n, inactive Gear lubricants, hydraulic uids, greases for
bearing lubrication, chassis lubricants
TABLE 9.5
Synergistic Effect of ZnDTP on Copper Corrosion (ASTM DISO)
Type Total Sulfur Active Sulfur
Treatment
Level (%)
Cu Corrosion
3 h at 100°C
Cu Corrosion
(+1.5% ZnDTP
a
)
3 h at 100°C
Triglyceride 18 10.5 5 4c 3b
Ester 17 8.5 5 3b 1b
Triglyceride 15 5 5 3a 1b
a
Thermally stabilized zinc dialkyldithiophosphate based on 2-ethylhexyl alcohol.
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