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334 Lubricant Additives: Chemistry and Applications
automatic transmission uids (ATF), multigrade hydraulics, and the lower SAE W grade gear oil
grades. In these situations, it may often be impractical to employ other VII chemistries because of
some technical de ciency related to the aforementioned properties.
In engine oils where these technical criteria are not as stringent, PMA VIIs are often at a cost
disadvantage. Fundamentally, PMAs are less-ef cient thickeners than many competitive hydrocar-
bon VIIs such as ethylene–propylene copolymers or isoprene-based polymers as described earlier.
VII thickening is a function of polymer backbone molecular weight (actually the unperturbed root
mean-square end-to-end distance), but only a minor portion (∼15%) of a typical PMA’s molecular
weight is in the backbone compared to an ole n polymer with ∼80 to 90% backbone molecular
weight. Recall that a large majority of the molecular weight of a PMA is found in the pendant side
chain simply to provide oil solubility. Therefore, when comparing a PMA to a polyole n of similar
molecular weight, the PMA clearly has a shorter end-to-end distance and is clearly a less ef cient
thickener. In situations where a PMA may bring some unique value to the formulation such as addi-
tional dispersancy (i.e., soot control in Heavy Duty Diesel Engine Oil) or superior low-temperature
properties, these technical advantages may make PMA more economic.
For PPDs, it is again dif cult to assign a typical treat rate. PPD concentrations may vary widely
due to numerous factors including the types of base stocks employed, the impact of other additives
on desired properties, and even the types of cold-temperature tests used for a given application.
11.4.3 OTHER INCENTIVES
In addition, if the VII brings value-added features such as pour point depressancy or dispersancy,
then the economic credit for these features must be accounted for in the net treat cost.
PPDs economics also involve the commercial reality that as few PPD products as possible, more
likely a single PPD, would be favored at a blend plant. Thus, performance robustness in various
formulations is highly leveraged. Methacrylates with their inherent exible chemistry can be well
tuned to meet this objective and thus often provide a further degree of cost-effectiveness. Overall,
it is dif cult to quantify cost-effectiveness in a simple way; however, PMA PPDs are believed to be
the most widely used chemistry in this application and command over 50% of the world market.
11.5 OUTLOOK AND TRENDS
11.5.1 C
URRENT AND NEAR-TERM OUTLOOK
The commercial presence of PMAs over so many decades is a compliment to adaptability and the
innovative effort to evolve the chemistry to meet more and more demanding rheological require-
ments. Competitive pressures from other chemistries will always remain high in the modern, global
economy, thus the continued use of PMAs depends on the market’s continuing and evolving needs
for higher rheological performance. Given this, PMA compositions will almost assuredly evolve, as
well. At the same time, raw materials and processes will be under constant investigation to create
more cost-effective materials. Finally, advanced polymer processes are being developed to provide
unique polymer architectures that provide enhanced rheological properties.
The outlook for PMA VIIs is, to a large degree, tied to transportation and industrial multigrade
uid viscosity requirements that are driven by equipment and operating condition needs. Both equip-
ment and operating conditions are in a time of ux. The evolution of automatic transmission equip-
ment offers a pronounced example of such change with a concomitant change to ATF requirements.
This has created need in ATFs for more stringent, low-temperature viscometrics (cold temperature
operation), lower high-temperature viscosity and higher VIs (improved fuel economy contribution),
better shear stability performance (long uid life), while maintaining the traditional properties of
uid lm strength and thermal/oxidative stability. PMAs offer the best commercial route to low-
temperature viscometric performance and high VI contribution at the same time as being entirely
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Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 335
adaptable relative to shear stability needs. Obviously, the evolution of ATFs has favored the continu-
ing use of PMA VII.
Fluid requirements may often appear to be contradictory. For instance, modern ATFs frequently
require low overall viscosity but high VI for fuel economy reasons. Another example is the con-
stant push to better low-temperature viscometric performance at the same time as improved shear
stability; typically, better shear stability implies higher polymer treat rates, which imply higher
low-temperature viscosity. In so much as these contrasting requirements emphasize one or more of
the desirable properties of PMAs, these factors further reinforce PMA.
Transmission oils satisfying lubricant needs for more complex step transmissions, continuously
variable transmissions, and double clutch transmissions are not the only multigrade uids
undergoing change. Transportation gear oils will evolve to better meet fuel economy expectations
and more thermally demanding environments. In industrial and tractor hydraulics, higher-pressure
hydraulic pumps and the ubiquitous desire for fuel economy improvement will drive uid change.
In so much as these changes involve higher VI, better low-temperature viscometrics, and enhanced
shear stability, PMA VII will be involved.
It seems quite probable that such equipment trends and usually more severe operating environ-
ments will continue to evolve over the near and mid-term future and thus require further enhancement
of uid viscometric properties. In those applications where PMA VIIs provide valued advantages, this
should not only reinforce their use but also spur further innovation. In lubricants such as engine oils
where PMA VIIs are not necessarily needed for value-added features and are not cost- competitive, it
is not easy to foresee signi cant future use unless requirements change appreciably.
The outlook for PMA PPDs is obviously tied to low-temperature rheology speci cations as
well as to the wax characteristics of lubricant uids. Certainly, low-temperature speci cations have
trended to become more restrictive for many lubricants in addition to the ATF situation discussed
earlier. North American engine oil speci cations have now added used oil low-temperature require-
ments to deal with oxidative thickening (light-duty engines) and soot-related thickening (heavy-
duty engines). The existence of these requirements adds a new dimension to PPD evaluations since
used oils must now be evaluated. The latter point on wax characteristics of lubricant uids deals
with wax chemistry and wax concentration in base stocks as these factors will continue to be of
paramount importance to the future use of PPDs. As base oil processing moves forward to provide
more advanced oils, these will likely contain different combinations of wax structures than past
base oils. PPDs will need to be developed to control the resulting different wax crystallization phe-
nomena. In addition, the interaction of base stock wax with other additives that contribute waxlike
character (i.e., detergents) provides a complex overlay that often impacts the choice of PPD chem-
istry. PMA PPD structure can be modi ed to provide appropriate crystallization interaction with
the various types and levels of wax in any uid. Given the ability of PMA to meet both current and
future wax-related needs, their future as PPDs seems assured.
For PMA dispersant VIIs (disregarding thickening effects), the outlook is not as clear. In those
applications where these materials are currently employed, their use will probably continue for
the foreseeable future with little change in terms of chemistry or commercial volumes. There are
potentially important commercial possibilities if industry needs for dispersancy reach higher levels.
An example is enhanced soot dispersancy in future diesel engine; however, the advent of diesel
particulate lters would seem to reduce the possibility of this need.
11.5.2 LONG-TERM OUTLOOK
Just as for many other additives and base stocks, the future of PMAs depends on the rheological
appetites of future equipment. In the absence of revolutionary equipment development, longer-term
lubricant trends will continue to be driven by the concerns mentioned earlier: emissions control, fuel
ef ciency, and longer uid life times. These three concerns certainly apply in varying degrees to all
multigrades and will cause uid requirements to continue their migration to enhanced rheological
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336 Lubricant Additives: Chemistry and Applications
properties. This should solidify the case for PMA in those applications where it holds promi-
nent technical advantages. At the same time, PMAs will also continue to evolve to better provide
improvements in shear stability, low-temperature viscometrics, wax interaction, VI, and perhaps,
dispersancy. Current polymer architecture research noted in Section 11.2.4 certainly points in this
direction.
Another important factor for future use of PMAs depends intimately on future base stocks. For
instance, should entirely synthetic polyalphaole n (PAO) base stocks be the choice of the future to
provide enhanced properties, then this would obviate the use of any PPDs, as these base stocks are
wax free. However, additives such as PMA PPDs may be an enabling factor in the use of newer base
stocks such as API group II or API group III base stocks in meeting performance goals that match
or are quite similar to those of API group IV (PAO) base stocks.
For the case of revolutionary equipment changes such as ceramic engine parts or vehicles based
on batteries, fuel cells, or even clean burn systems (e.g., hydrogen or natural gas), lubricant require-
ments will surely change and lubricants themselves will undergo a similar revolution. Most of these
changes would radically alter the chemical and physical characteristics and volumes of lubricants
since the interesting but hostile environment of an internal combustion engine would no longer
dictate the complex chemistry of engine lubricants. If engine lubricant volumes decline because of
radical new materials or power sources, then surely all PPDs and VIIs would experience a propor-
tional decline. However, in other applications, such as hydraulics, where PMA VIIs are primarily
employed, such radical equipment changes do not seem to be on the horizon.
REFERENCES
1. Hochheiser, S. Rohm and Haas, History of a Chemical Company. Philadelphia, PA: University of
Pennsylvania Press, 1986, p. 145.
2. U.S. Patent 2,001,627, Rohm and Haas Co., 1937.
3. U.S. Patent 2,100,993, Rohm and Haas Co., 1937.
4. Van Horne, W.L. Polymethacrylates as viscosity index improvers and pour point depressants. Ind Eng
Chem 41(5):952–959, 1949.
5. Billmeyer, F.W. Textbook of Polymer Science. New York: Wiley-Interscience, 1971, pp. 280–310RL
6. Stambaugh, R.L. Viscosity index improvers and thickeners. In R.M. Mortimer. S.T. Orszulik, eds.,
Chemistry and Technology of Lubricants. London: Blackie Academic & Professional, 1997, pp.
144–180.
7. U.S. Patent 2,737,496, E.1. Du Pont de Nernours Co., 1956.
8. U.S. Patent 3.506,574, Rohm and Haas Co., 1970.
9. U.S. Patent 3,732,334, Rohm GmbH, 1973.
10. British Patent 636,998, Rohm and Haas Co., 1964.
11. U.S. Patent 5.863,999, Rohm and Haas Co., 1998.
12. U.S. Patent 4,756,843, Institute Français du Petrol, 1998.
13. U.S. Patent 6,228,819, Rohr and Haas Co., 2001.
14. U.S. Patent Application 0,142,168, RohMax Additives, 2006.
15. U. S. Patent 3,249,545, Shell Oil, 1966.
16. U.S. Patent 3,052,648, Rohm and Haas Co., 1962.
17. U.S. Patent 4,790,948, Texaco Inc., 1988.
18. EP. Patent 569,639, Rohm and Haas Co., 1993.
19. Japanese Patent 4,704,952, Sumitomo Corp., 1972.
20. U.S. Patent 6,140,431, Rohm and Haas Co., 2000.
21. U.S. Patent 4.149.984, Rohm GmbH, 1979.
22. U.S. Patent 4,290,925, Rohm GmbH, 1981.
23. U.S. Patent 4,622,358, Rohm GmbH, 1986.
24. Great Britain Patent 1,559,952, Shell, Intl., 1977.
25. U.S. Patent 6,403,746, RohMax Additives GmbH, 1999.
26. U.S. Patent Application 2007/0082827, Arkema, 2003.
27. German Patent Application DE 103 14 776, RohMax Additives GmbH, 2003.
28. German Patent Application DE 10 2005 015 931, RohMax Additives GmbH, 2005.
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29. International Patent Application WO 2006/047398, The Lubrizol Corporation, 2006.
30. International Patent Application WO 2006/047393, The Lubrizol Corporation, 2006.
31. German Patent Application DE 10 2005 041 528, RohMax Additives GmbH, 2005.
32. German Patent Application DE 10 2005 031 244, RohMax Additives GmbH, 2005.
33. Arlie, J.P., J. Dennis, G. Parc. Viscosity index improvers 1. Mechanical and thermal stabilities of
polymethacrylates and polyole ns. IP Paper 75-005, Institute of Petroleum, London, 1975.
34. Branderup, J., E.H. Irmergul, eds. Polymer Handbook. New York: Interscience, 1966, pp. V-5–V-11.
35. March, J. Advanced Organic Chemistry, Reactions, Mechanisms and Structure. New York: McGraw-
Hill, 1968, p. 747.
36. Kinker, B., R. Romaszewski, J. Souchik. Pour point depressant robustness after severe use in passenger
car engines in the eld and in the Sequence IIIGA engine, SAE Paper 2005-01-2174.
37. Kinker, B.G. Automatic transmission uid shear stability trends and viscosity index improver trends,
Proceedings of 98th International Symposium of Tribology of Vehicle Transmissions, Yokohama, 1998,
pp. 5–10.
38. Kinker, B.G. Fluid viscosity and viscosity classi cation. In G. Totten, ed., Handbook of Hydraulic Fluid
Technology. New York: Marcel Dekker, 2000, pp. 318–329.
39. RohMax Publication RM-96 1202. Pour point depressants, a treatise on performance and selection,
Horsham, PA, 1996.
40. International Lubricant Standardization and Approval Committee, ILSAC GF-4 standard for passenger
car engine oils, January 14, 2004.
41. Bartko, M., D. Florkowski, V. Ebeling, R. Geilbach, L. Williams. Lubricant requirements of an advanced
designed high performance, fuel ef cient low emissions V-6 engine, SAE Paper 2001-01-1899.
42. Neudoer , P. State of the art in the use of polymethacrylate VI improvers in lubricating oils. In
W.J. Bartz, ed., 5th International Colloquium, Additives for Lubricants and Operations Fluids, vol. 11.
Technische Akademic Esslingen, Ost cldcrn, 1986, pp. 8.2-1–8.2-15.
43. Selby, T.W. The non-Newtonian characteristics of lubricating oil. Trans ASLE 1:68–81, 1958.
44. Mueller, H.G., G. Leidigkeit. Mechanism of action of viscosity index improvers. Tribol Intl 1(3):
189–192, 1978.
45. Kopko, R.J., R.L. Stambaugh. Effect of VI improver on the in-service viscosity of hydraulic uids. SAE
Paper 75.069_319, 1975.
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339
12
Pour Point Depressants
Joan Souchik
CONTENTS
12.1 Introduction ........................................................................................................................ 339
12.2 Fluid Mechanics and Pour Point Depressant Mechanism of Action ..................................340
12.2.1 Bingham Fluid Behavior .......................................................................................340
12.2.2 High-Viscosity Behavior .......................................................................................342
12.2.3 Pour Point Depressant Mechanism of Action ....................................................... 342
12.3 Pour Point Depressant Chemistry .......................................................................................342
12.4 Pour Point Depressant Performance Testing ......................................................................344
12.5 Principles of Pour Point Depressant Selection ...................................................................346
12.5.1 Treat Rate and Pour Point Reversion .....................................................................346
12.5.2 Effect of Performance Additives ...........................................................................347
12.5.2.1 Base Oil Wax Chemistry and Content ..................................................347
12.5.2.2 Other Wax Sources ...............................................................................349
12.5.2.3 Fully Formulated Oil ............................................................................ 350
12.6 Pour Point Depressant Robustness ..................................................................................... 351
12.7 Lubricant Applications ....................................................................................................... 352
12.7.1 Automotive Engine Oils ......................................................................................... 352
12.7.2 Automotive Transmission Oils .............................................................................. 352
12.7.3 Industrial Oils ........................................................................................................ 352
12.7.4 Biodegradable Fluids ............................................................................................. 353
References ...................................................................................................................................... 353
12.1 INTRODUCTION
Pour point depressants (PPDs), also known as low-temperature ow improvers and wax crystal
modi ers, are polymeric molecules that are added to mineral oil-based lubricants to improve their
cold ow properties. Without the addition of a PPD, many lubricants at cold temperatures would be
too viscous to ow easily, or might even be gelled, and the result would be little or no lubricant mov-
ing through the system or machine requiring lubrication. For various lubricant applications such as
automatic transmission uids (ATFs), engine oils, gear oils, and hydraulic uids, paraf nic base
stocks are the preferred lubricant. These paraf nic base stocks are typically derived from crude
petroleum and are composed of nonaromatic saturated hydrocarbons. Paraf nic stocks, also called
base oils, make excellent lubricants because they are chemically stable, resistant to oxidation, and
have good viscosity index values. However, paraf nic stocks, by their very nature, contain molecu-
lar species that have linear carbon chains of 14 carbons or more. These species, recognized as waxy
materials, can cause oil pumpability failures at low temperatures. In addition to the inherent waxy
base stock components, other sources of waxy material are added to the lubricant as part of its
product-speci c formulation. These additional components include some viscosity index improvers
(VII) and components of the detergent-inhibitor (DI) package.
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340 Lubricant Additives: Chemistry and Applications
12.2 FLUID MECHANICS AND POUR POINT
DEPRESSANT MECHANISM OF ACTION
Mineral oils are commonly understood to be Newtonian uids, meaning that they behave according
to the following equation:
Shear stress = shear rate × viscosity
Experiments show that this equation holds for mineral oils as long as the temperature is above the
cloud point of that oil. The cloud point is the temperature at which some of the waxy components
of a mineral oil start to crystallize and precipitate from solution, leading to a hazy appearance.
This visual evidence of the onset of wax crystals can be tested using American Society for Testing
and Materials (ASTM) D 2500 [1]. A plot of log–log viscosity versus log temperature is shown in
Figure 12.1. Above the cloud point, denoted by the data point in Figure 12.1, the viscosity decreases
proportionally with temperature. At temperatures below the cloud point, the viscosity increases
steeply as the temperature decreases. Below the cloud point, it is not uncommon to observe one of
the two non-Newtonian behaviors in these otherwise Newtonian uids: Bingham uid behavior or
unpredictably high viscosity.
12.2.1 BINGHAM FLUID BEHAVIOR
Bingham uid behavior is the failure of a uid to move under low-shear conditions, that is, unless
some energy is added to the system.
The following equation describes Bingham uid behavior:
Shear stress = (shear rate – yield stress) × viscosity
The relationship between shear stress and shear rate for a Bingham uid is shown in Figure 12.2.
40
0
100
log temperature (°C)
Cloud point
log−log viscosity
FIGURE 12.1 Relationship between viscosity and temperature for a mineral oil.
0
0
Shear stress
Shear rate
Yield
Stress
FIGURE 12.2 Bingham uid behavior.
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Pour Point Depressants 341
This failure to ow is similar to the initial ow observed on opening a bottle of ketchup. When a
bottle of ketchup is turned over, the ketchup does not ow out of the bottle due to weak associations
among some molecular components of the ketchup. However, adding energy to the system by tapping
the bottom of the bottle triggers the ketchup to move. Likewise, mineral oils at cold temperatures
often do not ow because they contain molecules that have a crystalline nature at low temperatures.
First, two-dimensional crystals (platelets) form and then ultimately three-dimensional needle-like
structures form. The needle-like structure containing platelets is shown in Figure 12.3. The needle-
like structures layer on one another, forming a network of crystals that trap the noncrystalline oil
molecules within the gel network, which impedes the ow of the oil (see Figure 12.4). This process
is known as gelation, and the source of the yield stress (YS) in the preceding equation is the wax–gel
matrix in which the noncrystalline oil molecules are immobilized.
Bingham uid behavior can be observed by running two widely recognized industry tests:
ASTM D 4684 [2] and ASTM D 5133 [3]. A YS observation in the ASTM D 4684 test or a high
gelation index (>12) measurement in the ASTM D 5133 test indicates the formation of a crystal
network and quanti es the extent of it.
Air binding is an example of problematic Bingham uid behavior that can occur in an automo-
bile engine. When the engine is at rest, the oil drains and collects in the oil pan. At cold tempera-
tures, the waxy materials crystallize in the oil creating a gel network. When the engine is started,
FIGURE 12.3 Three-dimensional needle-like structure of crystal platelets.
FIGURE 12.4 Gel network of needle-like structures.
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342 Lubricant Additives: Chemistry and Applications
there is enough energy created by the oil pump to break apart the fragile gel structure just at the
oil lter. A small bit of oil is pumped into the engine leaving an air gap at the lter. It is possible
that the oil in the pan is of a viscosity that it could pump, but is locked in place by the wax crystal
network that has formed. At this point, air is being pumped into the engine and as air is a very poor
lubricant, this can lead to engine failure.
12.2.2 HIGH-VISCOSITY BEHAVIOR
The second non-Newtonian behavior often observed in a mineral oil at temperatures below its cloud
point is unpredictably high viscosity. As waxy molecules in the oil crystallize, they cocrystallize but
possibly do not form a strongly organized network. However, these cocrystals have enough hydro-
dynamic volume that they impede the ow of the noncrystalline oil molecules, greatly increasing
the viscosity of the oil. In this situation, the oil is so viscous that it cannot be pumped at all into the
engine, creating a serious situation where the engine runs with limited, if any, lubrication.
It is important to note that ow-limited behavior can occur in an oil for another reason, unre-
lated to wax crystal precipitation and growth. All uids will become more viscous as the tempera-
ture decreases, eventually reaching a viscosity where the uid cannot be pumped. The temperature
at which the uid no longer ows under gravity, not due to wax crystal growth, but simply due to the
viscosity contribution of the other molecular species present, is called the viscous pour point. The
addition of a PPD to a uid will not change its viscous pour point.
12.2.3 POUR POINT DEPRESSANT MECHANISM OF ACTION
A PPD can resolve both gel structure and high-viscosity modes of failure. PPDs work by controlling
the wax crystallization phenomenon in two principal ways: delaying the formation of wax–gel matrix
to signi cantly lower temperatures than would normally occur or reducing the viscosity contribution
of the crystal wax particles. PPDs act by interrupting the three-dimensional growth of wax crystals.
12.3 POUR POINT DEPRESSANT CHEMISTRY
Before the use of PPDs in the early 1930s, options were few for controlling wax crystallization. One
method was to use heat. For example, res were built under the oil reservoir of a vehicle. Another
technique was to increase the solvency of the lubricant uid portion by the addition of kerosene. The
kerosene would then evaporate during use. A third alternative was the use of naturally occurring
materials such as microcrystalline waxes or asphalt resin. Although these natural additives were
somewhat effective, they were often speci c to an application. This motivated the investigation
of synthetic PPDs with structures based on the hydrocarbon structures of the naturally occurring
PPDs. In 1931, small molecule chemistry efforts identi ed alkylated naphthalenes as one type of
PPD, and in 1937, polymer chemistry studies by Rohm and Haas established polymethacrylates as
the rst polymeric PPD [4].
Since the early 1930s, other synthetic PPDs have been introduced. However, polymeric PPDs
remain the most commercially viable option and include, but are not limited to, acrylates, alkylated
styrenes, alpha ole ns, ethylene/vinyl acetates, methacrylates, ole n/maleic anhydrides, styrene/
acrylates, styrene/maleic anhydrides, and vinyl acetate/fumarates. All of these polymers work
according to the same principles of operation.
The chemical structure of a PPD resembles a comb polymer, as shown in Figure 12.5. Long
waxy side chains are added into the polymer side chains and interspaced with short neutral (non-
wax interacting) side chains. The long waxy side chains interact with the wax in the oil in a manner
that is proportional to the length of the side chain. These side chains can be linear or branched and
should contain at least 14 carbon atoms for the PPD to interact with the wax in the oil. The short
neutral side chains act as inert diluents and help to control the extent of wax interaction. Also, a
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Pour Point Depressants 343
distribution of hydrocarbon side chain lengths can be utilized for best interaction with the wax in
the oil because the wax contains a range of molecule chain lengths. The backbone molecular weight
of a PPD, generally, can vary with little effect on performance; however, there is a minimum back-
bone size below which a PPD will become largely ineffective.
Figure 12.6 depicts the three-dimensional structure of a PPD. The long coil represents the back-
bone of the PPD, and the shorter zigzag pieces attached to the backbone represent the side chains.
Figure 12.7 shows the side chains of the PPD crystallizing with the oil crystals on the edges of the
platelets. This cocrystallization sterically inhibits the formation of the three-dimensional network
(see Figure 12.8), thus preserving the wax as a distribution of tiny crystals and ensuring the com-
plete uidity of the oil.
The intensity of interaction between the wax and the alkyl side chains is a function of the types
and amounts of side chains. This interaction can be expressed by a wax interaction factor (WIF).
WIF is a method of ranking PPDs by their wax nature, accounting for the amount of waxy side
chains in a PPD and their strength of interaction. A low-wax uid usually responds best to a low-
WIF PPD, and likewise, optimum performance per unit will be achieved in a more waxy uid with
a higher-WIF PPD. Because of the need to treat a vast variety of wax-related situations, a multitude
of PPD products have been developed [5].
As described, the mode of operation for a PPD is physical interference. In selecting an optimum
PPD, consideration needs to be given to the nature and amount of waxy molecules that may be present
in a uid. The waxy side chains present in the PPD need to be in balance with those waxy molecules
found in the uid. This balance is critical to blending oils with good low-temperature ow properties.
Polymer backbone
Lon
g
wax
y
side chains
Short neutral side chains
FIGURE 12.5 PPD comb polymer diagram.
FIGURE 12.6 Three-dimensional representation of a PPD.
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