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464 Lubricant Additives: Chemistry and Applications
Over many years, dispersants have played a major part in keeping the engine clean, and they
continue to play an important part in the oil formulation. In modern top-quality engine oil formula-
tions, dispersant will range from 3 to 6% by weight. Typically, this would account for ~50% of the
total oil additive in the lubricant.
19.3.2 DISPERSANT STRUCTURE
Dispersants consist of an oil-soluble portion and a polar head group. The polar group is attached to
the oil-soluble group by means of a hook. The schematic in Figure 19.7 gives a simple representation
of a dispersant structure based on the reaction of a polyisobutenyl succinic anhydride (PIBSA) with
either a polyamine or a polyol.
Many different types of dispersants have been used in lubricant additive packages. Over the
years, these have evolved as the lubricant requirements for both OEMs and the testing organizations
have become more demanding. The following list, although not exhaustive, covers the most popular
types of dispersant chemistries in use today:
Polyisobutenyl succinimide
Polyisobutenyl succinate ester
Mannich dispersants
Dispersant viscosity modi ers (VMs) (e.g., dispersant ole n copolymer [dOCP], dispersant
polymethacrylate [dPMA])
By far the most common group of dispersants is polyisobutenyl succinimides. These are the
preferred dispersants for tackling the black sludge problem that occurred in gasoline engines during
the 1980s. During the 1990s and up to the present day, they have also been used in ever increasing
levels to reduce oil thickening, which is the result of high levels of soot in diesel engine oils.
To prepare a polyisobutenyl succinimide dispersant, a hook is attached by reacting maleic
anhydride (MA) with PIB to make PIBSA. The PIBSA is then typically neutralized with a polyamine
mixture to yield the succinimide dispersant (Figure 19.8).
The reaction between the PIB and the MA occurs through the unsaturated end group of the PIB.
This can be achieved by directly reacting the PIB with MA at temperatures in excess of 200°C (direct
alkylation or DA for short). Once the PIBSA is formed, it can react again to make the disuccinated
product. The reaction of PIB and MA depends on the unsaturated end groups of the PIB being reac-
tive enough to add to the MA. As the reaction proceeds, the addition of MA to the PIB slows as all
•
•
•
•
Oil-soluble function
(e.g., polyisobutene)
Connector
(succinic anhydride)
Polar head group
(polyamine/alcohol)
FIGURE 19.7 Schematic of a dispersant molecule.
O
O
O
PIB
NH
2
−R
N
H
O
O
PIB
R
OH
N
O
O
PIB
R
−H
2
O
PIBSA
Succinimide
dispersant
FIGURE 19.8 Imidation of a PIBSA.
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Additives for Crankcase Lubricant Applications 465
the reactive end groups in the PIB are used up. With some types of PIBs, this will leave a relatively
large amount of unreacted PIB. In recent years, this problem has been overcome by using PIBs with
very high levels of terminal vinylidene groups, in some cases >80%. These vinylidene groups are
more reactive through the DA route, enabling DA PIBSAs to be prepared with relatively low levels
of unreacted PIB (see Figure 19.9).
The key to the PIB–MA reaction is to ensure that the PIB is completely converted into PIBSA.
Any unreacted PIB in the nished lubricant will result in a formulation with poor low-temperature
viscometric properties. In addition, the dispersant will be less effective in preventing sludge forma-
tion and in dispersing soot since not all the oil-soluble PIB will contain active dispersant chemistry.
For these reasons, it is bene cial to maximize the conversion of the PIB to the succinic
anhydride. The addition of MA to the PIB is made easier by using chlorine, which catalyzes the
reaction through a PIB diene intermediate that reacts more readily with the MA yielding a highly
converted PIB with minimal unreacted PIB. A schematic of the reaction is shown in Figure 19.10.
H
O
O
O
n
H
n
O
O
O
H
n
O
O
O
H
n
O
O
O
O
OO
+
Disuccinated products
Alder-ene reaction
FIGURE 19.9 Direct alkylation reaction of PIB with MA.
n
Cl
2
−2HCl
O
O
O
n
n
O
O
O
Cl
2
O
O
O
Disuccinated products
FIGURE 19.10 Chlorination/succination of PIB.
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466 Lubricant Additives: Chemistry and Applications
19.3.3 POLYISOBUTENE SYNTHESIS
PIB is produced by cationic polymerization of either pure isobutene or C
4
stream from an oil re nery.
The isobutene in the C
4
re nery stream reacts preferentially, whereas other compounds such as
n-butenes and butanes do not. Typical catalysts for these reactions include AlCl
3
and BF
3
.
The molecular weight of the PIB is very important and can have a signi cant effect on the
dispersant performance. Typically, the number average molecular weight (M
n
) of PIB ranges from
500 to 3000 although there are instances where PIBs outside this range may be used. The higher-
molecular-weight PIB dispersants have VM properties and aid the formulation of multigrade oils.
In addition, they are much more effective in the handing of black sludge and soot. Although lower-
molecular-weight dispersants have found use as sludge and soot dispersants, they are clearly less
effective. Dispersants made using higher-molecular-weight PIB (M
n
> 2500) have an adverse effect
on the low-temperature viscosity properties of the nished lubricant, particularly the cold crank
viscosity, and for this reason are not normally used. In addition, they are more dif cult to react with
MA due to their higher viscosity.
19.3.4 DISPERSANT BASICITY
The amount of polyamines added to the PIBSA will determine the basicity of the dispersant. This
is referred to as TBN and is measured by the ASTM D-2896 method. The unit of measurement for
TBN is milligrams of KOH per gram. The TBN of the dispersant will give a good indication of
the dispersant structure. Higher levels of polyamines, that is, more base, will predominately yield
a monosuccinimide structure, which is the most basic of the PIBSA dispersants (see Figure 19.11).
When there is a molar excess of PIBSA more of the bis and tris structures will predominate
(see Figure 19.12).
19.3.5 SUCCINATE ESTER DISPERSANTS
These dispersants are prepared from a PIBSA and a polyol as shown in Figure 19.13. Ester-based
dispersants are used to reduce sludge and piston deposits and function in a similar manner to the
succinimide-based dispersants.
N
O
O
PIB
NH
HN
NH
2
FIGURE 19.11 Example of monosuccinimide
from triethylenetetramine (TETA).
N
O
O
PIB
NH
HN
N
O
O
PIB
FIGURE 19.12 Example of bis-succinimide
from TETA.
O
O
O
O
O
O
PIB
R
O
HO−R−OH
R
OH
OH
PIB
FIGURE 19.13 Example of a simple succinate ester dispersant.
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Additives for Crankcase Lubricant Applications 467
19.3.6 MANNICH DISPERSANTS
These types of dispersants are prepared by reacting a PIB phenol with a polyamine in the presence
of formaldehyde. The resulting dispersant has some antioxidant properties (Figure 19.14). This
family of dispersants is typically used in gasoline engine oils.
19.3.7 SOOT CONTAMINATION IN DIESEL ENGINE OILS
The soot contaminants in heavy-duty diesel engines usually appear when the engine is operating
under very high loads or when the fuel injection is retarded (i.e., injected late in the cranking cycle).
Under speci c engine conditions, the small soot particles, typically <200 nm, will agglomerate
into a larger macrostructure leading to a signi cant rise in oil viscosity. The structure of the
dispersant is key to the lubricant’s ability to minimize these structures from forming. Thus, with
the correct choice of dispersant, it is possible to reduce the soot-related viscosity increase and
also to maintain cleanliness throughout the engine by preventing the large soot structures from
settling out.
Over the past 10–20 years, the main reason for adding dispersants to diesel engine oils has been
the increasing levels of soot, especially in the modern low-emission engines. During the 1990s, the
level of soot in diesel engine oil has trended higher as a result of new engine technologies designed
to reduce NO
x
and particulate emissions. This trend accelerated in the United States as older engine
designs were changed to meet the U.S. 1990 exhaust emission regulations and then more latterly
in Europe during 1992 for the Euro I regulations. During this period, retarded fuel injection has
been one of the main strategies employed by OEMs to reduce NO
x
emissions, which has resulted
in more unburned fuel entering the crankcase oil as soot particles. Combined with extended oil
drain intervals, it is not unusual to see soot levels as high as 5% in certain types of duty cycles. This
trend is set to continue as more legislation appears on the horizon aimed at reducing diesel exhaust
emissions.
One of the most important factors in dispersing soot is the dispersant TBN. Typically, as the
base number of the succinimide dispersant tends to higher values, the dispersant molecular weight
tends to lower values as more of the monosuccinimide structure prevails with more primary nitro-
gen. This type of dispersant is the most ef cient for use in diesel engines, especially modern low-
emissions engines where there are high levels of oil soot. The effectiveness of the dispersant is
determined by its ability to separate soot particles and thus prevent them from agglomerating. The
mechanism whereby this occurs can be represented by the basic dispersant molecule attaching itself
to an acidic site on the surface of the soot particle. Assuming that there are suf cient dispersant
molecules attached to the soot particle, the PIB chains will prevent the soot particles from coming
together. The stability of the soot dispersion is in uenced by the molecular weight of the PIB and
the level of reactive amines (primary and secondary) per molecule of PIB. A very simplistic repre-
sentation of a soot–dispersant interaction is given in Figure 19.15. It is fair to say that a lot of energy
has been spent nding the most optimum dispersant for dispersing soot as the patent literature dem-
onstrates. Each individual additive company will have their favored structures.
OH
R
N
H
H
N
N
H
OH
R
x
FIGURE 19.14 Mannich dispersant.
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468 Lubricant Additives: Chemistry and Applications
19.3.8 SOOT-THICKENING TESTS
As the soot levels of heavy-duty diesel engine oils increased, OEMs, particularly in North America,
introduced engine tests to evaluate the oils’ ability to disperse soot. One of the tests used to evalu-
ate soot-mediated oil thickening for a low-emissions engine was the Mack T-7. This is included
into the Mack EO-K and API CF-4 speci cations. As the engine technologies progressed to lower-
emission levels, more severe tests were introduced into API CG-4 and API CH-4 speci cations.
Table 19.1 summarizes the progression of the Mack engine tests and the limits applied to the API
C categories.
For light-duty diesel applications, Peugeot introduced the XUD-11 soot-thickening test for the
Association of European Automotive Manufacturers (ACEA) passenger car diesel oil approvals.
This test, now in its second generation and called the XUD11 BTE, is used to measure oil thickening
at soot levels up to 4%. Because the level of soot is much higher compared with what is typically
found in European passenger car diesels pre-Euro 2 (1996), the test makes a good measure of the
ability of the oil to minimize oxidative soot thickening for small European diesel engines. Criteria
have also been applied to the Mercedes OM602A, OM364LA, and OM441LA diesel engine tests
in respect of soot-related oil thickening and sludge control. Despite all these tests, the engines that
have driven oil formulations to ever-higher levels of basic dispersant are still the Mack engines.
A comparison of an API CG-4 formulation and an API CH-4 formulation in the Mack T-8 is shown
in Figure 19.16. This demonstrates the progression of dispersant characteristics from API CG-4 to
API CH-4.
FIGURE 19.15 Dispersant–soot interaction.
TABLE 19.1
Mack Engine Tests to Evaluate Soot-Mediated Oil Thickening
Engine Oil Category Engine Test Limits
API CF-4 Mack T-7 ≤0.04 cSt/h (last 50 h of test)
API CG-4 Mack T-8 ≤11.5 cSt at 3.8% soot
API CH-4 Mack T-8E ≤2.1 × sheared viscosity
a
at 4.8% soot
a
Sheared viscosity for Mack T-8E = (KV
initial
+ KV
DIN sheared
)/2.
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Additives for Crankcase Lubricant Applications 469
19.3.9 SEAL TESTING
The balance between soot handling and seal compatibility has provided lubricant formulators with
signi cant challenges over the past 10 years especially as seal testing is a major part of the oil
approval process in Europe.
The most dif cult issue to contend with is that the highly basic dispersants, used in diesel oil
formulations to disperse soot, are aggressive toward uoroelastomer seals. There are many uoro-
elastomer seal tests, one common example being the Volkswagen PV3344 test, which is a require-
ment for Volkswagen oil approvals. There are also seal test requirements for most of the European
engine manufacturers, for example, Mercedes-Benz, MAN, and MTU and collectively through the
ACEA. A comparison of three formulations with different dispersants in a Mercedes-Benz seal test
is summarized in Table 19.2. This clearly shows that as the soot dispersancy is increased for API
CH-4, the MB uoroelastomer seal test worsens.
19.3.10 CORROSION
Another potential drawback with high TBN dispersants is that they tend to be more aggressive toward
Cu/Pb bearings. This has led some additive companies to treat their dispersants with boron compounds,
0
10
20
30
40
50
60
70
80
90
100
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.
0
Percentage soot by TGA
API CG-4
API CH-4
Viscosity increase (cSt)
FIGURE 19.16 Comparison of an API CG-4 formulation versus an API CH-4 formulation in the Mack T-8
engine test.
TABLE 19.2
Effect of Dispersant TBN on Mercedes-Benz Fluoroelastomer Seals
Formulation A B C
Max variation of AK6 Viton
Hardness (Shore A) −30 1
Volume (%) 1 1 0.9
Tensile strength (%) −8 −32 −46
Elongation rupture (%) −15 −33 −38
Pass Pass Borderline fail
Note: A, European passenger car gasoline/diesel formulation; B, North American
heavy-duty diesel formulation (API CG-4); C, North American heavy-duty diesel
formulation (API CH-4).
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470 Lubricant Additives: Chemistry and Applications
for example, boric acid to reduce Cu/Pb corrosion. In some cases, this can improve the antiwear prop-
erties but usually reduces the effectiveness of the dispersant at dispersing soot. Clearly a balance has
to be found, and formulators will use different dispersants to meet the various requirements.
19.3.11 SLUDGE
Under certain conditions, sludge will accumulate in an internal combustion engine. During the
1980s, this problem intensi ed throughout the world, particularly in gasoline engines in Germany,
the United Kingdom, and the United States. The origins of the problem are most likely related to fuel
quality, drive cycles, extended oil-drain intervals, and the redirection of blowby gas into the rocker
cover. Sludge build up, if left unchecked, can spread throughout the engine, leading to reduced oil
ow through the lter and drain-back holes on the valve-deck. In extreme situations, this will lead to
engine seizure. The problems were not linked to poor-quality lubricants. However, as lubricant tech-
nology evolved, it was found that newer lubricant formulations could help alleviate the problem.
Sludges can be split into low-temperature sludges and high-temperature sludges. Sludges
formed at low temperatures will tend to be soft and easily removable from surfaces by wiping. As
previously mentioned, the sludge-forming mechanism is thought to be accelerated by the transfer
of blowby gases into the oil. Blowby gases contain water, acids, and partially burned hydrocarbons
in the form of oxygenates and ole ns. The ole ns react further with nitrogen oxides to form oil-
insoluble products. Fuels with high-end boiling points or with high aromatics also tend to contrib-
ute to more sludge. Once formed, highway driving will exacerbate the problem as the engine runs
hotter, thereby causing the sludge to bake on.
Sludge formation is reduced by the addition of basic succinimide dispersants particularly the
high-molecular-weight types. The addition of boric acid to a succinimide dispersant to make a lower
TBN dispersant will reduce the effectiveness of a dispersant in dispersing sludge [7]. Although
this indicates that basic dispersants are required to neutralize sludge precursors, it has been found
that very high levels of basic nitrogen are not necessarily required for sludge dispersion in gasoline
engines. For Sequence VE performance, high-molecular-weight PIB bis-succinimide dispersants
have demonstrated excellent performance. This would appear to indicate that the mechanism of
sludge formation in a gasoline engine is different from the agglomeration of soot in a diesel engine,
which do require high levels of basic nitrogen especially in low-saturate mineral base stocks.
Although bis- and monosuccinimide dispersants are used in both gasoline and diesel engine oils,
it would be fair to say that, more recently, diesel engine oils have tended to higher levels of basic
dispersants, with correspondingly higher levels of PIB monosuccinimides compared with gasoline
engine oils. One of the big challenges for formulators since API CG-4 has been to develop lubri-
cants that meet the requirements of both gasoline and diesel engine oils, the so-called universal
engine oils. This requires careful formulation with both bis- and monosuccinimide dispersants.
When formulating engine oil, it is also important to consider the interactions between the
succinimide dispersant and other additives such as zinc dialkyldithiophosphate (ZDDP). A complex
between ZDDP and succinimide polyamine has been observed in laboratory tests especially where
there is a high basic nitrogen to ZDDP ratio [8]. As the level of basic dispersant is increased,
sludge and wear will improve. Above a critical concentration of dispersant, the antiwear properties
of the ZDDP will be dramatically reduced as the basic nitrogen forms a stable complex with the
ZDDP effectively reducing its antiwear capability. It has been demonstrated that the ZDDP–amine
complex retards the rate of peroxide decomposition by ZDDP [9], which in turn accelerates the
formation of sludge. Sludge will usually increase in line with wear, so clearly a high level of ZDDP
with a suf cient dispersant level to disperse the sludge is required. High levels of ZDDP permit
high levels of dispersant to be used in the lubricant thereby giving superior engine sludge and wear
performance. However, phosphorus has been implicated in exhaust catalyst poisoning. Because of
this, a maximum phosphorus limit of 0.1 wt% in the lubricant has been applied to recent API and
ILSAC speci cations. This means that where phosphorus limits are imposed for API SJ and ILSAC
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Additives for Crankcase Lubricant Applications 471
GF-3 lubricants, careful choice of dispersant and ZDDP are required. The phosphorus limits are set
to be reduced further for ILSAC GF-4 thereby providing more challenges for the lubricant to meet
the sludge and wear requirements in future engines.
19.3.12 SLUDGE ENGINE TESTS
Although sludge tests have been around for many years, the most signi cant lubricant tests to address
the sludge problems during the 1980s and 1990s were the Sequence VE and Mercedes M102E.
More recently, the Sequence VG and the MB M111E have superseded these tests and have become
the current benchmarks for measuring lubricant sludge-handling performance in Europe and North
America. Table 19.3 lists the test conditions for the various gasoline sludge tests. The Sequence IIIE
and Sequence IIIF are also included since high-temperature sludge is a rated parameter in these
gasoline engine tests.
High-temperature sludges are also present in diesel engines and are a rated parameter for
several engine tests, particularly the Cummins M11 for API CH-4. Dispersants based on high-
molecular-weight PIB are essential in minimizing sludge build up in this test. Sludge is also a
rated parameter for the Mercedes-Benz OM364LA and OM441LA engine test although it is not as
critical parameter as the Cummins M11 engine test.
19.4 ANTIWEAR
19.4.1 I
NTRODUCTION
As the power of engines has risen, the need for additives to prevent wear has become more impor-
tant. Initially, engines were lightly loaded and could withstand the loading on the bearings and
valve train. Corrosive protection of bearing metals was one of the early requirements for engine
oils. Fortunately, the additives used to protect bearings usually had mild antiwear properties. These
antiwear agents were compounds such as lead salts of long-chain carboxylic acids and were often
used in combination with sulfur-containing materials. Oil-soluble sulfur–phosphorus and chlori-
nated compounds also worked well as antiwear agents. However, the most important advance in
antiwear chemistry was made during the 1930s and 1940s with the discovery of ZDDPs [10–12].
These compounds were initially used to prevent bearing corrosion but were later found to have
exceptional antioxidant and antiwear properties. The antioxidant mechanism of the ZDDP was the
key to its ability to reduce bearing corrosion. Since the ZDDP suppresses the formation of perox-
ides, it prevents the corrosion of Cu/Pb bearings by organic acids.
19.4.2 WEAR MECHANISMS
Antiwear additives minimize wear in a mixed or partial lubricant lm operating under boundary
conditions (Figure 19.17). A classic example of boundary lubrication is in a nonconforming contact
such as a cam on a follower. The partial lubricant lm is most likely to occur when the oil is not
suf ciently viscous enough to separate the two surfaces completely.
The tendency to boundary lubrication increases as the temperature rises due to the viscosity–
temperature dependence of the lubricant. Low contact speeds, high contact pressures, and rough
surfaces will also contribute to more boundary lubrication. If these circumstances are taken to the
extreme and minimal or if no lubricant lm exists, then the maximum surface contact will exist
(Figure 19.18). This is de ned as an extreme-pressure (EP) contact and is usually associated with
very high temperatures and loads. Additives that prevent wear in an EP contact typically require
higher activation temperatures and load than an antiwear additive.
Antiwear and EP additives function by thermally decomposing to yield compounds that react
with the metal surface. These surface-active compounds form a thin layer that preferentially shears
under boundary lubrication conditions.
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472 Lubricant Additives: Chemistry and Applications
TABLE 19.3
Sludge Engine Test Comparison
Test
Cylinder
Confi guration
Displacement
(cc)
Test
Duration (h)
Test
Operation
Duration
(h)
Speed
(rpm)
Power
(kW)
Coolant
Temperature (ºC)
Oil Temperature
(ºC) Fuel Type
Sequence IIIE
(Buick)
V-6 3800 64 Steady speed 64 3000 50.6 115 149 Phillips GMR
leaded gasoline
Sequence IIIF
(Buick)
V-6 3800 80 Steady speed 80 3600 75.0 115 155 Howell EEE
unleaded
Sequence VE
(Ford)
I.L.-4 2290 288 Cyclic 2.00 2500 25.0 51.7 68.3 Phillips J
unleaded
gasoline
Slider 1.25 2500 25.0 85.0 98.9
Follower 0.75 750 0.75 46.1 46.1
Sequence VG
(Ford)
V-8 4600 216 Cyclic 2 1200 69 kPa 57 68 Unleaded
gasolineRoller 1.25 2900 66 kPa 85 100
Follower 0.75 700 Record 45 45
Mercedes- Benz
M-111E Sludge
I.L.-4 1998 224 Cyclic 48 h cold Idle to
5500
Idle to
maximum
40–98 45–130 CEC RF-86-T-94
(ULG)75 h WOT
100 h cyclic
Nissan VG-20E 2000 200 Cyclic 200 800–3500 2–9 kgf m 38–100 50–117 Unleaded
gasoline
Toyota 1G-FE 2000 48 Steady speed 48 4800 6 kgf m 120 149 Unleaded
gasoline
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Additives for Crankcase Lubricant Applications 473
After the discovery of ZDDP, it rapidly became the most widespread antiwear additive used
in lubricants. As a result, many interesting studies have been undertaken on ZDDP with many
mechanisms being proposed for the antiwear and antioxidant action [13–17]. The performance of
the ZDDP is strongly in uenced by the decomposition pathways. These pathways are thermoly-
sis, oxidation, and hydrolysis, which in turn depend on the conditions under which the ZDDP is
working. In general, it can safely be assumed that the degradation products of the ZDDP form a
lm on the metal surface, typically rich in phosphorus and oxygen and possibly polymeric. As men-
tioned earlier, this layer preferentially shears under boundary lubrication, thus reducing the wear on
the metal surfaces. As this layer needs to be constantly replenished, the concentration of ZDDP in
the lubricant is critical. It is therefore not unusual to nd up to 2 wt% of ZDDP in a modern lubri-
cant. This equates to a maximum level of ∼0.15% phosphorus in the lubricant.
19.4.3 ZDDP PREPARATION
ZDDP is prepared by reacting a dialkyldithiophosphoric acid with zinc oxide. The rst stage of the
process involves the preparation of the acid. The acid is prepared from an alcohol and phosphorus
pentasul de:
42
25 2
2
ROH P S RO P S SH
dialkyldithiophosphoric acid
HS
⫹ → ()()
Mixed film lubrication
Metal surface
Metal surface
FIGURE 19.17 Mixed lm lubrication.
Boundary lubrication
Metal surface
Metal surface
FIGURE 19.18 Boundary lubrication.
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