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344 Lubricant Additives: Chemistry and Applications
12.4 POUR POINT DEPRESSANT PERFORMANCE TESTING
Mineral oil is typically non-Newtonian at low temperatures; therefore, no single test can ensure
that a lubricant blended with mineral oil will remain uid over a wide range of conditions. Thermal
history, including temperature cycling and cooling rates, can also affect the ow behavior of the oil.
Various test methods have been developed over the years to evaluate oils under conditions that are
experienced during operation.
A test, to be meaningful in predicting wax crystal growth, must incorporate three critical fac-
tors: a low-temperature end point, a de ned cooling pro le, and a low shear rate. Obviously, wax
crystal growth is a low-temperature phenomenon, so a low-temperature end point is required.
The low temperature nally achieved in a test needs to re ect the temperature requirements of
the application. Not all waxy molecules in a uid are crystalline at the same temperature, and
changing the temperature point for a measurement will greatly impact the quantity of wax present
as a crystal; hence, making an appropriate choice of a nal test temperature is critical. The rate
of cooling affects the competing factors of crystal growth and nucleation of new crystals, thus
affecting the number of crystals formed and their relative sizes. The existence of temperature
cycling within the cooling pro le can further affect the number or size of wax crystals. Clearly,
FIGURE 12.7 Crystallization of a PPD with a needle-like structure.
FIGURE 12.8 PPD prevention of gel network formation.
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Pour Point Depressants 345
a de ned cooling rate is critical in understanding wax crystal growth. Lastly, low shear rate is
also a critical part of characterizing low-temperature behavior. Wax networks are fragile and eas-
ily disrupted under higher shear conditions. To understand whether wax crystal networks have
formed, it is important to study the uid under low shear conditions, allowing any network that
formed to be preserved.
A comparison of the cooling and shear rates for the different low-shear test methods is given
in Table 12.1. The ASTM D 97 [6] test for pour point employs a rapid and unrealistic cooling
rate for wax crystal growth. Quick cooling does not allow the wax crystals that form to fully
associate. However, this test has been used by the industry for many years, and its quickness
certainly has bene t in quality control situations. The stable pour point [7] is more realistic than
the ASTM D 97 pour point because of its longer cooling time. ASTM D 3829 [8] and ASTM D
4684 have comparable shear rates, but ASTM D 3829 employs a programmed cooling rate over a
shorter time period, on average 2°C/h over 16 h, versus 0.33°C/h over ∼48 h for ASTM D 4684.
Figure 12.9 illustrates the cooling rates for several pumpability bench tests. Because of the differ-
ences in the tests, analysis of several test method results is critical to selecting the best PPD for a
TABLE 12.1
Comparison of Low-Temperature and Low-Shear Rate Tests
Test Cooling Rate (ºC) Shear Rate (s
−1
) Test Duration
ASTM D 97 (pour point) 0.6/min 0.1–0.2 2 h
Stable pour point 0 to –40 over 7 days 0.1–0.2 7 days
ASTM D 3829 (MRV) 2/h 17.5 16 h
ASTM D 4684 (MRV TP-1) 0.33/h 17.5 48 h
ASTM D 2983 (Brook eld
viscosity)
Shock 0.1–12 16 h
ASTM D 5133 (scanning
brook eld viscosity and
gelation index)
1/h 0.25 35 h
Note: MRV = mini-rotary viscometer; MRV TP-1 = mini-rotary viscometer temperature pro le 1.
0
−10
−20
−30
−40
Temperature (°C)
0 5 10 15 20 25 30 35 40
Brookfield
MRV TP-1
Time (h)
Pour point
Scanning
Brookfield
FIGURE 12.9 Cooling rates of pumpability bench tests. MRV TP-1 = mini-rotary viscometer temperature
pro le 1.
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346 Lubricant Additives: Chemistry and Applications
lubricant formulation. A PPD that gives satisfactory performance at both rapid and slow cooling
rates under low shear conditions is presumed to be better able to deliver performance over all
foreseeable conditions than a PPD that already shows performance de cits under one of the limit-
ing test conditions. In the end, it is impossible to test all conditions that might possibly arise.
The cold cranking simulator (CCS) test (ASTM D 5293 [9], which replaced ASTM D 2602 [10])
is a common oil testing method that is inappropriate for wax gel testing because it uses a rapid cool-
ing rate and high shear rate. One set of results from ASTM D 2602 testing to determine whether
PPD addition can affect the CCS viscosity of a Society of Automotive Engineers (SAE) 5W-30 oil
is depicted in Table 12.2. As the PPD content was increased from 0 to 0.3%, virtually no change in
the viscosity was observed. This result is contrary to what many hope for with the addition of a PPD
and is explained by the high shear nature of the CCS test. PPDs are added to a uid to control wax
crystal growth. Wax crystal networks are fragile. Any wax network that formed would be broken
up under the high shear conditions of the CCS test and as a result PPDs cannot improve the CCS
viscosity of an oil.
12.5 PRINCIPLES OF POUR POINT DEPRESSANT SELECTION
12.5.1 T
REAT RATE AND POUR POINT REVERSION
Using a PPD to improve the pour point means the addition of a waxy material to the lubricant. The
treat rate, or concentration, of a PPD should be optimized so that the PPD itself does not cause crys-
tallization. The effect of the PPD treat rate on the pour point of Group I 100N base oil is shown in
Figure 12.10. The addition of a PPD at 0.2% by weight reduces the pour point from −15 to −33°C.
Doubling the concentration to 0.4% produces only an incremental decrease of ∼3°C. An additional
increase in the PPD content does not reduce the pour point any further. For this particular oil–PPD
system, the optimum treat rate is ∼0.2%.
Continued increases in the PPD content result in a phenomenon known as pour point reversion.
The addition of PPD beyond the optimum treat rate is in effect adding wax to the system and conse-
quently reversing the bene t of the PPD by contributing to the formation of crystals or PPD polymer
networks. An example of PPD overtreatment of the oil is shown in Figure 12.11. At a treat rate of
0.7; the pour point begins to increase at a treat rate of 0.7% from its minimum value at the treat rate
of 0.3%. At the highest concentration tested, 1.2%, the pour point was similar to the untreated oil.
Pour point reversion can also occur with improper selection of a PPD with a WIF that is too high
for the oil being treated.
Figure 12.12 is a typical response curve demonstrating the effect of a PPD as a function of the
WIF in one speci c base oil. The lowest treat rate of 0.05% (top curve of Figure 12.12) indicates
that a speci c WIF is responsible for optimum performance. As the treat rate increases, the size of
the response window increases. If a less-speci c response is desired, then a higher treat rate can be
used. Also, WIF and treat rate can, to some degree, be traded off, meaning that a nonoptimum WIF
can be offset by a higher treat rate and vice versa. This exibility can allow for one PPD to function
well in multiple oils or across plant systems.
TABLE 12.2
CCS Viscosity of SAE 5W-30 Oil with PPD
Percentage PPD As is 0.15% 0.3%
Viscosity at –25ºC (mPa s) 2900 2950 2850
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Pour Point Depressants 347
12.5.2 EFFECT OF PERFORMANCE ADDITIVES
12.5.2.1 Base Oil Wax Chemistry and Content
The wax content of base oils from which lubricants are formulated varies with the source of the
crude oil, the re ning process used for the crude oil, the dewaxing process used for the re ned oil,
and the nal viscosity grade of the base oil. The wax removal process is certainly a key determinant
0
−10
−15
−20
−25
−30
−35
−40
−45
0 0.2 0.4 0.6 0.8
Wt% PPD
Pour point (°C)
FIGURE 12.10 Effect of PPD treat rate on the pour point of Group I 100N base oil.
0
−5
−10
−15
−20
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Wt% PPD
Pour point (°C)
−25
−30
−35
−40
−45
FIGURE 12.11 Pour point reversion.
Treat
rate
0.05%
0.10%
0.20%
0.50%
Wax interaction factor
Low temperature response
FIGURE 12.12 Low-temperature response of PPD as a function of the WIF.
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348 Lubricant Additives: Chemistry and Applications
of the low-temperature low-shear (LTLS) behavior of an oil. Different types of hydrocarbons
remain in the oil after re ning, depending on the process. Dewaxing of the re ned crude oil by
solvent extraction removes the long-chain hydrocarbons. In contrast, catalytic dewaxing changes
the structure of the hydrocarbons by either cracking them into smaller pieces or isomerizing them,
rather than removing them. Lastly, the viscosity grade of the base oil is directly dependent on the
wax content of the oil given that the higher-viscosity oils are produced by including longer-chain
hydrocarbons.
The effect of two different PPDs on the viscosity of solvent-extracted and catalytically dewaxed
SAE 10W-30 formulated oils is shown in Figure 12.13. ASTM D 4684 was used to determine the
viscosity of the four PPD–oil formulations. Both oils had the same DI and VII, and PPD 1 had a
lower WIF than PPD 2. For the solvent-extracted oil, the viscosity remained fairly low for both
PPDs. However, the catalytically dewaxed oils had a higher viscosity with PPD 2 than with PPD 1
and extremely high YS of 105 Pa with PPD 2. To pass current North American engine oil speci -
cations, YS must be less than 35 Pa. The higher WIF of PPD 2 most likely caused the increase in
viscosity and YS in the catalytically dewaxed oil. One explanation for this is that the catalytically
dewaxed stock contains fewer linear molecules of C14 or greater. The waxy side chains of the
PPD, nding limited natural waxes to interact with, self-associate and built viscosity and structure
through polymer-to-polymer interactions.
The effect of the WIF of a PPD on different viscosity grades of Group I base oils is shown in
Figure 12.14. For the lower viscosity grades of 100N and 150N, increasing the WIF from 1 to 2 had
PPD 1
PPD 2
120,000
100,000
80,000
60,000
40,000
20,000
Solvent
extracted
Catalytic
dewaxed
YS = 105 Pa
ASTM D 4684 ( mPa
s)
SAE 10W-30 PPD treat rate = 0.1% b
y
wei
g
ht
0
FIGURE 12.13 Solvent-extracted versus catalytically dewaxed oil.
−5
−10
−15
−20
−25
−30
−35
−40
100N 150N 325N 600N
WIF 1.0
WIF 2.0
PPD treat rate = 0.1% b
y
wei
g
ht
ASTM D 97 (°C)
FIGURE 12.14 Viscosity grade effects of Group I base oils.
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Pour Point Depressants 349
a small effect on decreasing the pour point temperature. For the higher viscosity grades of 325N
and 600N, the result was reversed, and the higher WIF decreases the pour point temperature sig-
ni cantly. This result can be explained by the interaction of the PPD with the longer-chain hydro-
carbons present in the higher viscosity grades.
12.5.2.2 Other Wax Sources
Other wax sources include DI packages and friction modi ers that may themselves contain waxy
hydrocarbon chains. Also, VIIs, which can typically contain long ethylene sequences, should be
considered. These waxy components can affect the LTLS performance of the fully formulated oil,
both positively and negatively, and therefore it is important to understand their contribution when
selecting a PPD for a lubricant.
Detergent effects using two different PPDs in SAE 15W-40 are shown in Figure 12.15.
The viscosity of two oils was tested by ASTM 4684. PPD A had a lower wax content than
PPD B. In the oil with detergent A, PPD A was able to control the viscosity to 13,000 mPa s
(100 mPa s = 100 cP = 1 P), which was below the test speci cation of 30,000 mPa s.
However, PPD A could not control the YS, which, at 105 Pa, exceeded the test speci cation of
<35 Pa. The excessive YS indicated that a gel network was forming. One explanation for this is
that detergent A was waxy in nature, and the waxy side chains of PPD A interacted with detergent
A leaving fewer PPD side chains available for interaction with the wax in the oil. PPD B, a higher
WIF PPD than PPD A, controlled the wax crystallization. The results for both PPDs were compa-
rable in detergent B testing, passing both viscosity and YS speci cations.
Chemistry effects of VIIs and a PPD in SAE 5W-30 oil are shown in Figure 12.16. The oils
containing both VII 1 and VII 2 had comparable performance, and the oils met the test speci ca-
tion of <30,000 mPa s at −30°C. As the temperature was lowered to −35°C, a much larger change
in viscosity was observed in the oil containing VII 2. The oil containing VII 1 doubled in viscosity,
whereas the oil containing VII 2 tripled in viscosity. Although both VIIs met the test speci cation
of <60,000 mPa s at −35°C, this PPD was not able to control viscosity growth in the oil with VII 2.
With proper PPD selection, VII 2 certainly can show equal performance to VII 1. This demonstrates
the effect that different cold-temperature conditions can have on PPD selection and how seemingly
small changes in an oil formulation, VII 1 or VII 2, can change the optimum PPD for a lubricant.
Ole n copolymers (OCPs) are used commonly as VIIs. The ethylene content of the OCP
can greatly affect the choice of PPD for optimum viscosity and structure control of a lubricant.
The importance of testing PPDs and considering all pertinent tests is summarized in Table 12.3,
PPD A
PPD B
14,000
12,000
10,000
8,000
6,000
4,000
2,000
YS = 105 Pa
DET A
DET B
ASTM D 4684 at −20°C (mPa
s)
0
FIGURE 12.15 Detergent effects.
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350 Lubricant Additives: Chemistry and Applications
which presents the results of testing two different PPDs in SAE 10W-30 oil containing either a
traditional OCP or a higher ethylene (HE) OCP. Using PPD 1, a lower-WIF PPD, the oil contain-
ing the HE OCP had results similar to the oil containing traditional OCP for all the tests except
the gelation index, which approached the speci cation limit of 12, indicating a structure is start-
ing to develop. When PPD 2, a higher-WIF PPD, was used with the HE OCP, the results were
similar to or better than the oil containing traditional OCP and PPD 1. Assuming that the longer
ethylene sequences in the HE OCP can contribute to wax interactions, these data demonstrate
that a high-WIF PPD is needed to disrupt those waxy VII crystals as well as to interact with the
waxy species in the base oil.
12.5.2.3 Fully Formulated Oil
It should be obvious from the previous discussion that another important aspect of PPD selection
is the viscosity behavior of fully formulated oil versus base stocks. The fully formulated oil almost
always contains additives and wax sources not present in the base oil, and these waxes may affect
the viscosity behavior. The effect of two different PPDs on a 150N base oil and a SAE 10W-40,
formulated with that same 150N oil, is shown in Figure 12.17. PPD 2 has a higher WIF than PPD
1. PPD 1 and PPD 2 gave similar viscosity results when tested in the base oil alone; however, YS
was detected with PPD 2. These results indicate that PPD 1 would be a better candidate for the
150N base oil. With the fully formulated oil, however, PPD 1 resulted in a higher viscosity and a
TABLE 12.3
VII Effects of HE OCP
VII Traditional HE OCP HE OCP
PPD 1 1 2
Properties
ASTM D 97 (ºC) −30 −27 −30
ASTM D 4684, viscosity (mPa s) 13,700 13,900 9,700
ASTM D 5133 (ºC at 30,000 mPa s) −28.8 −25.9 −30.2
Gelation index 4.6 9.8 4.8
Note: Oil source, SAE 10W-30, treat rate = 0.1% weight.
FIGURE 12.16 VII chemistry effects.
−30°C
−35°C
60,000
50,000
40,000
30,000
20,000
10,000
0
ASTM D 4684 (mPa
s)
VII 1
VII 2
SAE 5W-30 PPD treat rate = 0.1% by weight
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Pour Point Depressants 351
YS of 105 Pa, which exceeded the test speci cations of <35 Pa. In this case, PPD 2 is the better
choice because the additional waxy components from the DI or VII in the SAE 10W-40 formulated
with that some 150N oil formulation required more waxy sites on the PPD polymer to effectively
control the LTLS properties of the fully formulated oil.
12.6 POUR POINT DEPRESSANT ROBUSTNESS
The usefulness of PPDs has been clearly established through laboratory testing. However, the ques-
tion of whether a PPD loses its effectiveness during use must be considered. The robustness of the
PPD can be determined from eld and laboratory tests. Experiments were performed to determine
whether the PPD response was deteriorated by severe eld and laboratory engine tests. The severe
eld test was a New York City 10,000 mi taxi test; the laboratory test was the Sequence IIIG test
(ASTM D 7320) [11], a 100-hour, high-temperature engine test. The pumping viscosity (ASTM D
4684 MRV TP-1) was measured to evaluate the PPD response. Figures 12.18 and 12.19 depict the
comparison of responses for fresh oil, used oil without the PPD, used oil with the PPD, and post-test
addition of the PPD to used oil in the taxi test and the Sequence IIIG test, respectively. All fresh oil
measurements were made at −35°C and used oil measurements at −30°C. Both tests demonstrate
that, without the PPD, the used oil had a signi cantly higher viscosity than the fresh oil, becoming
too viscous to measure during the testing at −30°C. On the contrary, the used oil containing PPD
from both tests easily passed the SAE J 300 limit of 60,000 mPa s with no YS. Additionally, when
PPD was added to the used oil without PPD from both taxi and Sequence IIIG tests, the used oil
60,000
50,000
40,000
30,000
20,000
10,000
0
YS = 0
YS = 0
YS = 35 Pa
YS = 105 Pa
150N oil
SAE 10W-40
PPD 1 PPD 2
ASDM D 4684 (mPa
s)
FIGURE 12.17 PPD selection in base oil versus fully formulated oil.
Solid
100,000
80,000
60,000
40,000
20,000
0
No PDD
With PDD
SAE
J 300
limit
Fresh
Used
PPD added after use
ASTM D 4684 (mPa
s)
FIGURE 12.18 PPD stability after taxi eld test. SAE 5W-30 as fresh oil (−35˚C), SAE 10W-40 as used oil
after 10,000 mi taxi service (−30˚C), PPD at 0.1%—when present.
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352 Lubricant Additives: Chemistry and Applications
viscosity was greatly improved, showing slightly higher but similar results to the used oil with PPD
results. Clearly, the PPD had not deteriorated during use as the PPD-pretreated used oil provides
good MRV TP-1 viscosity, the untreated used oil has failing MRV TP-1 viscosity, and the postadded
PPD used oil results are similar to the oil pretreated with PPD [12].
12.7 LUBRICANT APPLICATIONS
PPDs are selected and engineered for different lubricant applications ranging from automotive
engine and transmission oils to industrial oils and biodegradable uids. Engine oils tend to drive the
PPD selection process for most blenders as engine oils are required to meet several LTLS speci ca-
tions with severe limits, and they are often the high-volume products for a blend plant. Additionally,
other lubricants more often are less selective for PPD.
12.7.1 AUTOMOTIVE ENGINE OILS
A PPD is always needed for automotive engine oils formulated with mineral oil base stocks, and
given the complexity of these oils, the optimum PPD can usually only be identi ed by undertaking a
PPD study on the fully formulated oils. The LTLS tests that are most often used today to access the
pumpability of engine oils are ASTM D 4684, ASTM D 5133, and ASTM D 97. Various national,
international, and OEM-speci c engine oil speci cations exist, each varying in the low-temperature
test recommended or in the acceptable limits for a particular test.
12.7.2 AUTOMOTIVE TRANSMISSION OILS
Selection of PPDs for automotive transmission oils, including gear oils and ATFs, normally
entails testing with ASTM D 2983 [13] (Brook eld viscosity) at −12, −26, and −40°C. ASTM D
is also relevant. A PPD is always needed for mineral oil–based uids, but it may be a constituent of
the VII, if one is used.
12.7.3 INDUSTRIAL OILS
In contrast to the automotive oils, which typically require specialized low-temperature viscosity
testing, industrial oils often have just a simple pour point (ASTM D 97) requirement. Therefore, the
targets are often not dif cult, and a low treat level of a traditional PPD may be suitable. For these
oils, base stock choice is the primary driver for PPD selection. Some uids, such as multigraded
FIGURE 12.19 PPD stability after Sequence IIIGA testing. SAE 5W-30 as fresh oil (−35°C), SAE 10W-40
as used oil after sequence IIIGA (−30°C), PPD 0.3%—when present.
Solid
100,000
80,000
60,000
40,000
20,000
No PDD
With PPD
Fresh
Used
PPD added after use
SAE
J 300
limit
ASTM D 4684 (mPa s)
0
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Pour Point Depressants 353
hydraulic or tractor uids, have additional LTLS requirements, namely Brook eld viscosity. These
uids generally need a low dose of PPD to meet requirements, and a PPD lab study will identify the
optimum product and treat rate. Speci c industrial oils, such as refrigeration oils, may be blended
with synthetic or naphthenic stocks. These oils have excellent LTLS properties, and the addition of
a PPD will not generally give further improvement.
12.7.4 BIODEGRADABLE FLUIDS
Another application of PPDs is in biodegradable uids such as canola and soy oil. The PPDs for this
application have to be specially designed and engineered because the mechanism of interaction for
these biodegradable uids is different from that of traditional automotive lubricants. For these bio-
degradable uids, which consist almost exclusively of long-chain fatty acid triglycerides, the high
wax content is responsible for the observed low-temperature viscosity problems, and therefore, the
normal interference mechanism of small quantities of PPD containing long waxy side chains is not
suf cient to address the observed problems. This leads to the need for different PPD structures and
alternative approaches.
REFERENCES
1. ASTM Standard D 2500, Test Method for Cloud Point of Petroleum Products, ASTM International,
West Conshohocken, PA, www.astm.org.
2. ASTM Standard D 4684, Test Method for Determination of Yield Stress and Apparent Viscosity of
Engine Oils at Low Temperature, ASTM International, West Conshohocken, PA, www.astm.org.
3. ASTM Standard D 5133, Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/
Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique, ASTM Inter-
national, West Conshohocken, PA, www.astm.org.
4. Hochheiser, S., Rohm and Haas: History of a Chemical Company. Philadelphia, PA: University of
Pennsylvania Press, 1986.
5. RohMax Publication RM-96 1202, Pour Point Depressants, A Treatise on Performance and Selection,
Horsham, PA, 1996.
6. ASTM Standard D 97, Test Method for Pour Point of Petroleum Products, ASTM International, West
Conshohocken, PA, www.astm.org.
7. Federal Testing Method 203, Standard No. 791/Cycle C, Stable Pour Point, U.S. Military.
8. ASTM Standard D 3829, Test Method for Predicting the Borderline Pumping Temperature of Engine
Oil, ASTM International, West Conshohocken, PA, www.astm.org.
9. ASTM Standard D 5293, Test Method for Apparent Viscosity of Engine Oils between −5 and −35°C
Using the Cold-Cranking Simulator, ASTM International, West Conshohocken, PA, www.astm.org.
10. ASTM Standard D 2602, Test Method for Apparent Viscosity of Engine Oils between −5 and −35°C
Using the Cold-Cranking Simulator, ASTM International, West Conshohocken, PA, www.astm.org.
11. ASTM Standard D 7320, Test Method for Evaluation of Automotive Engine Oils in the Sequence IIIG,
Spark-Ignition Engine, ASTM International, West Conshohocken, PA, www.astm.org.
12. Kinker, B.G., R. Romaszewski, J. Souchik. Pour Point Depressant Robustness After Severe Use in
Passenger Car Engines in the Field and in the Sequence IIIGA Engine, SAE Paper 2005-01-2174,
2005.
13. ASTM Standard D 2983, Test Method for Low-Temperature Viscosity of Lubricants Measured by
Brook eld Viscometer, ASTM International, West Conshohocken, PA, www.astm.org.
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