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314 Lubricant Additives: Chemistry and Applications
118. Rossi, A., “Re nery/Additive Technologies and Low Temperature Pumpability,” Soc. Automot. Eng.
Tech. Paper Ser. No. 881665, 1988.
119. Mac Alpine, G.A. and C.J. May, “Compositional Effects on the Low Temperature Pumpability of Engine
Oils,” Soc. Automot. Eng. Tech. Paper Ser. No. 870404, 1987.
120. Reddy, S.R. and M.L. McMillan, “Understanding the Effectiveness of Diesel Fuel Flow Improvers,”
Soc. Automot. Eng. Tech. Paper Ser. No., 1981.
121. Xiong, C-X, “The Structure and Activity of Polyalphaole ns as Pour-Point Depressants,” Lubr. Eng.,
49(3), 196–200, 1993.
122. Rubin, I.D., M.K. Mishra and R.D. Pugliese, “Pouir Point and Flow Improvement in Lubes: The Interaction
of Waxes and Methacrylate Polymers,” Soc. Automot. Eng. Tech. Paper Ser. No. 912409, 1991.
123. Webber, R.M., “Low Temperature Rheology of Lubricating Mineral Oils: Effects of Cooling Rate and
Wax Crystallization on Flow Properties of Base Oils,” J. Rheol., 43(4), 911–931, 1999.
124. Kleiser, W.M, H.M Walker and J.A. Rutherford, “Determination of Lubricating Oil Additive Effects in
Taxicab Service,” Soc. Automot. Eng. Tech. Paper Ser. No. 912386, 1991.
125. Carroll, D.R. and R. Robson, “Engine Dynamometer Evaluation of Oil Formulation Factors for Improved
Field Sludge Protection,” Soc. Automot. Engr. Tech. Paper Ser. No. 872124, 1987.
126. R.J. Chang and M. Yoneyama, “CEH Marketing Research Report, Ethylene-Propylene Elastomers,”
525.2600, Chemical Economics Handbook—SRI Consulting, Mento Park, California, January 2006.
127. Petroleum Additives Product Approval Code of Practice, American Chemistry Council, Appendix I,
Arlington, Virginia, April 2005.
128. ATC Code of Practice, Technical Committee of Petroleum Additive Manufacturers in Europe, Section H,
Brussels, Belgium, October 2006.
129 Code of Practice for Developing Engine Oils Meeting the Requirements of the ACEA Oil Sequences,
ATIEL, Technical Association of the European Lubricants Industry, Issue No. 13, Appendix C, Brussels,
Belgium, November 28, 2005.
130. U.S. Patent 4169063, 1979.
131. U.S. Patent 4132661, 1979.
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315
11
Polymethacrylate Viscosity
Modifi ers and Pour
Point Depressants
Bernard G. Kinker
CONTENTS
11.1 Historical Development....................................................................................................... 315
11.1.1 First Synthesis ........................................................................................................ 315
11.1.2 First Application .................................................................................................... 316
11.1.3 First Manufacture and Large-Scale Application ................................................... 316
11.1.4 Development of Other Applications ...................................................................... 317
11.2 Chemistry ............................................................................................................................ 317
11.2.1 General Product Structure ..................................................................................... 317
11.2.2 Monomer Chemistry .............................................................................................. 318
11.2.3 Traditional Polymer Chemistry ............................................................................. 319
11.2.4 Patent Review ......................................................................................................... 321
11.3 Properties and Performance Characteristics .......................................................................323
11.3.1 Chemical Properties...............................................................................................323
11.3.1.1 Hydrolysis ............................................................................................... 323
11.3.1.2 Thermal Reactions: Unzipping and Ester Pyrolysis ...............................323
11.3.1.3 Oxidative Scissioning .............................................................................324
11.3.1.4 Mechanical Shearing and Free Radical Generation ............................... 325
11.3.2 Physical Properties ................................................................................................. 325
11.3.2.1 Pour Point Depressants ........................................................................... 325
11.3.2.2 Viscosity Index Improvers ...................................................................... 327
11.4 Manufacturers, Marketers, and Economics ........................................................................ 332
11.4.1 Manufacturers and Marketers ................................................................................ 332
11.4.2 Economics and Cost-Effectiveness ........................................................................ 333
11.4.3 Other Incentives ..................................................................................................... 334
11.5 Outlook and Trends ............................................................................................................. 334
11.5.1 Current and Near-Term Outlook ............................................................................ 334
11.5.2 Long-Term Outlook ................................................................................................ 335
References ...................................................................................................................................... 336
11.1 HISTORICAL DEVELOPMENT
11.1.1 F
IRST SYNTHESIS
The rst synthesis of a polymethacrylate (PMA) intended for potential use in the eld of lubricant
additives took place in the mid-1930s. The original work was conducted under the supervision of
Herman Bruson, who was in the employ of the Rohm and Haas Company (a parent of RohMax), and
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316 Lubricant Additives: Chemistry and Applications
it was conducted in Rohm and Haas’ Philadelphia Research Laboratories. Bruson was exploring
the synthesis and possible applications of longer alkyl side chain methacrylates [1]. He had pro-
posed poly lauryl methacrylate as a product that might serve as a potential thickener or viscosity
index improver (VII) for mineral oils. The result of the work was the 1937 issuance of two U.S.
patents, for “Composition of matter and process” [2] and for “Process for preparation of esters and
products” [3].
11.1.2 FIRST APPLICATION
Bruson’s invention did indeed thicken mineral oils, and it was effective in increasing viscosity at
higher temperatures more so than at lower, colder temperatures. Since this behavior in uences
the viscosity–temperature properties or VI of a uid, these materials eventually became known
as viscosity index improvers (VIIs). Although PMAs successfully thickened oils, there were
other competitive thickeners of that time, which increased the viscosity of mineral oils; these
were based on poly isobutylene and alkylated polystyrene. The commercial success of PMA
was not at all assured. The driving force behind PMA eventually eclipsing the other commer-
cial thickeners of the era was PMA’s value as a VII rather than as a simple thickener of oils. In
other words, PMAs have the ability to contribute relatively little viscosity at colder temperatures
such as those that might be encountered at equipment start-up, but have a much higher contri-
bution to viscosity at hotter temperatures at which equipment tends to operate. This desirable
behavior enabled oil formulators to prepare multigrade oils that could meet a broader range of
operating-temperature requirements. The positive enhancement of VI ensured the future success
of PMAs.
11.1.3 FIRST MANUFACTURE AND LARGE-SCALE APPLICATION
The commercial development of PMAs as VIIs lagged until the beginning of World War II, when
the U.S. government board rediscovered Bruson’s VII invention. The board was charged with
searching the scienti c literature for useful inventions that might aid the war effort. When con-
sidering potential utility, they hypothesized about a PMA VII providing more uniform viscosity
properties over a very broad range of temperatures, particularly in aircraft hydraulic uids. The
uids of that era were judged to be de cient particularly in ghter aircraft because of the exag-
gerated temperature/time cycles experienced. On the ground, the uids could experience high-
temperature ambient conditions and engine waste heat; and then after a rapid climb to high, very
cold altitudes, the uid might experience temperatures below −40°C. After successful trials of
the multigrade aircraft hydraulic uid concept, Rohm and Haas, in cooperation with the National
Research Defense Committee, rapidly proceeded to commercialize PMA VIIs and delivered the
rst product, Acryloid HF, in 1942. These multigraded hydraulic uids were quickly adopted by
the U.S. Army Air Corps and were followed by other multigraded hydraulic uids and lubricants
in ground vehicles that incorporated VIIs.
After the war, Rohm and Haas introduced PMA VIIs to general industrial and automotive
applications. Early passenger car engine oil VIIs were rst introduced to the market in 1946.
The adoption of “all season” oils in the commercial market was greatly in uenced by two events.
First, the automotive manufacturers’ viscosity speci cation introduction of the new designation
“W” (for winter grades); and then by Van Horne’s publication [4] pointing to the possibility of
making and marketing cross-graded oils such as the now well-known “10W-30” as well as other
cross-grades. By the early 1950s, use of multigrade passenger car oils became widespread in the
consumer market. Methacrylates played a major role in enabling the formulation of that era’s
multigrade engine oils. The use of PMA VIIs has since been extended to gear oils, transmission
uids, and a broad array of industrial and mobile hydraulic uids in addition to the early usage in
aircraft hydraulic uids.
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Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 317
11.1.4 DEVELOPMENT OF OTHER APPLICATIONS
Another important application area for PMA chemistry is in the eld of pour point depressants (PPDs).
When a methacrylate polymer includes at least some longer alkyl side chains, relatively similar to the
chain length of waxes normally present in mineral oil, it can interact with growing wax crystals at suf-
ciently low temperatures. Wax-like side chains can be incorporated into a growing wax crystal and
disrupt its growth. The net effect is to prevent congealing of wax in the oil at the temperature where
it would have occurred in the absence of a PPD. Early PMA PPDs were used rst by the military and
later by civilian industry when Rohm and Haas offered such products to the industrial and automotive
markets in 1946. Although PMAs were not the rst materials used as PPDs (alkylated naphthalenes
were), PMAs are probably the predominant products in this particular application now.
Another use of wax-interactive PMAs is as re nery dewaxing aids. The process of dewaxing
is carried out primarily to remove wax from paraf nic raf nates to lower the pour point of the
resulting lube oil base stocks. PMA dewaxing aids are extremely interactive with waxes found in
raf nates and thus function as nucleation agents to seed wax crystallization and promote the growth
of relatively large crystals. The larger crystals are more easily ltered from the remaining liquid
so that lube oil throughputs and yields are improved, whereas pour points are lowered by virtue of
lower wax concentrations.
Incorporating monomers more polar than alkyl methacrylates into a PMA provides products
useful as ashless dispersants or dispersant VIIs. The polar monomer typically contains nitrogen
and oxygen (other than the oxygen present in the ester group), and its inclusion in suf cient con-
centration creates hydrophilic zones along the otherwise oleophilic polymer chain. The resulting
dispersant PMAs (d-PMAs) are useful in lubricants since they can suspend in solution what might
otherwise be harmful materials ranging from highly oxidized small molecules to soot particles.
PMAs have also been used in a number of other petroleum-based applications albeit in rela-
tively minor volumes. An abbreviated list would include asphalt modi ers, grease thickeners,
demulsi ers, emulsi ers, antifoamants, and crude oil ow improvers. PMAs have been present in
lubricants for about 65 years now, and their longevity stems from the exibility of PMA chemistry
in terms of composition and process. Evolution of the original lauryl methacrylate composition
to include various alkyl methacrylates and nonmethacrylates has brought additional functionality
and an expanded list of applications. Process chemistry has also evolved such that it can produce
polymers of almost any desired molecular weights (shear stability) or allow the synthesis of complex
polymer architectures. The evolution of ef cient processes for controlled radical polymerization in
the 1990s has led to the development of taper and block copolymers and has permitted the develop-
ment of products with narrower molecular weight distribution.
11.2 CHEMISTRY
11.2.1 G
ENERAL PRODUCT STRUCTURE
Typically, a methacrylate VII is a linear polymer constructed from three classes (three dis-
tinct lengths) of hydrocarbon side chains. These would be short, intermediate, and long-chain
lengths. A more extensive discussion is given in Section 11.3, but an abbreviated description
is given here to better understand the synthesis and chemistry of methacrylate monomers and
polymers.
The rst class is short-chain alkyl methacrylate of one to seven carbons in length. The inclusion
of such short-chain materials in uences polymer coil size particularly at colder temperatures and
thus in uences the viscosity index of the polymer in oil solutions. The intermediate class contains
8–13 carbons, and these serve to give the polymer its solubility in hydrocarbon solutions. The long-
chain class contains 14 or more carbons and is included to interact with wax during its crystalliza-
tion and thus provide pour point depressing properties.
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318 Lubricant Additives: Chemistry and Applications
The structure of PMAs used as PPDs differs from that of a VII by virtue of normally contain-
ing only two of these sets of components. These are the long-chain, wax crystallization interactive
materials, and intermediate chain lengths.
The selected monomers are mixed together in a speci c ratio to provide an overall balance of
the aforementioned properties. This mixture is then polymerized to provide a copolymer structure
in which R represents different alkyl groups and x indicates various degrees of polymerization. The
simpli ed structure is shown in Figure 11.1.
11.2.2 MONOMER CHEMISTRY
Before discussing lubricant additives based on PMA, it is necessary to give an introduction to the
chemistry of their parent monomers. The basic structure of a methacrylate monomer is shown in
Figure 11.2.
The four salient features of this vinyl compound are as follow:
1. The carbon–carbon double bond that is the reactive site in addition polymerization
reactions.
2. The ester functionality adjacent to the double bond, which polarizes and thus activates the
double bond in polymerization reactions.
3. The pendent side chain attached to the ester (designated as R). These chains may range from
an all-hydrocarbon chain to a more complex structure containing heteroatoms. A signi cant
portion of the bene cial properties of PMAs is derived from the pendant side chain.
4. The pendant methyl group adjacent to the double bond, which serves to shield the ester
group from chemical attack, particularly as it relates to hydrolytic stability.
As mentioned earlier, various methacrylate monomers, differing by length of the pendant side
chains, are normally used to construct PMA additives. The synthesis chemistry of these monomers
falls into two categories: shorter chains with four or fewer carbons and longer chains with ve or
more carbons. The commercial processes used to prepare each type are quite different.
The short-chain monomers are often mass produced because of their usefulness in applications
other than lubricant additives. For instance, methyl methacrylate is produced in large volumes and
used primarily in the production of Plexiglas
®
acrylic plastic sheet and as a component in emulsion
paints and adhesives. It is also used in PMA lubricant additives, but the volumes in this application
pale in comparison with its use in other product areas. Methyl methacrylate is generally produced
by either of two synthetic routes. The more prevalent starts with acetone, then proceeds through
its conversion to acetone cyanohydrin, followed by its hydrolysis and esteri cation. The other route
is oxidation of butylenes followed by subsequent hydrolysis and esteri cation. Recently, additional
processes for methyl methacrylate have been introduced based on propylene and propyne chemistry,
but their use remains comparatively small.
x
(
)
O
RO
CH
3
C
C
H
2
C
H
FIGURE 11.1 Generalized struc-
ture of polyalkylmethacry late.
OR
O
C
CH
3
CH
2
C
FIGURE 11.2 Alkyl
methacrylate monomer.
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Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 319
The long-chain monomers are typically but not exclusively used in lubricant additives and can be
produced by either of two commercial processes. The rst is direct esteri cation of an appropriate
alcohol with methacrylic acid. This well-known reaction is often used as a laboratory model of
chemical strategies used to ef ciently drive a reaction to high yield. These strategies involve a
catalyst, usually an acid; an excess of one reagent to shift the equilibrium to product; and removal
of at least one of the products, typically water of esteri cation, again to shift the equilibrium.
The relevant chemical equation is given in Figure 11.3.
A second commercial route to longer-side-chain methacrylate monomers is transesteri cation
of methyl methacrylate with an appropriate alcohol. The reaction employs a basic compound or
a Lewis base as a catalyst. The equilibrium is shifted to product by use of an excess of methyl
methacrylate and by removal of a reaction product, that is, methanol (if methyl methacrylate is used
as a reactant). Figure 11.4 shows the reaction equation.
11.2.3 TRADITIONAL POLYMER CHEMISTRY
A combination of alkyl methacrylate monomers chosen for a given product is mixed together in
speci c ratios and then polymerized by a solution, free radical–initiated addition polymerization
process that produces a random copolymer. The reaction follows the classic pathways and tech-
niques of addition polymerization to produce commercial materials [5].
Commercial polymers are currently synthesized through the use of free radical initiators. The
initiator may be from either oxygen or nitrogen-based families of thermally unstable compounds
that decompose to yield two free radicals. The oxygen-based initiators, that is, peroxides, hydro-
peroxides, peresters, or other compounds containing an oxygen–oxygen covalent bond, thermally
decompose through homolytic cleavage to form two oxygen-centered free radicals. Nitrogen-based
initiators also thermally decompose to form two free radicals, but these materials quickly evolve a
mole of nitrogen gas and thus form carbon-centered radicals. In any event, the free radicals attack
the less hindered, relatively positive side of the methacrylate vinyl double bond. These two reactions
are the classic initiation and propagation steps of free radical addition polymerization and are shown
in Figures 11.5 and 11.6.
The reaction temperature is chosen in concert with the initiator’s half-life and may range
from 60 to 140°C. Generally, a temperature–initiator combination would be selected to provide
an economic, facile conversion of monomer to polymer and avoid potential side reactions. Other
temperature-dependent factors are taken into consideration. Chief among these might be a need to
maintain a reasonable viscosity of the polymer in the reactor as it is being synthesized. Obviously,
H
2
O
CH
3
H
2
C
C
C
O
OH
+
CH
3
ROH
H
2
C
C
OR
+
C
O
FIGURE 11.3 Direct esteri cation of methacrylic acid and alcohol.
H
2
CC
CH
3
C
O
+ CH
3
OH
CH
3
ROH
+
H
2
CC
OCH
3
C
OR
O
FIGURE 11.4 Transesteri cation of methyl methacrylate and alcohol.
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320 Lubricant Additives: Chemistry and Applications
the temperature can be utilized (as well as solvent) to maintain viscosity at a level appropriate for
the mechanical agitation and pumping systems within a production unit. Excessive temperatures
must be avoided to avoid the ceiling temperature of the polymerization, which is the temperature
where the depolymerization reaction commences (see Section 11.3.1.2).
Normally, a mixture of alkyl methacrylate monomers is used to produce a random copolymer.
No special reaction techniques are needed to avoid composition drift over the course of the reaction
since reactivity ratios of alkyl methacrylates are quite similar [6].
The most important concern during a synthesis reaction is to provide polymer at a given
molecular weight so as to produce commercial product of suitable shear stability. As normal for
vinyl addition polymerizations, methacrylates can undergo the usual termination reactions: combi-
nation, disproportionation, and chain transfer. Chain transfer agents (CTA), often mercaptans, are
the most commonly chosen strategy to control molecular weight. Selection of the type and amount
of CTA must be done carefully and with an understanding that many other factors in uence molec-
ular weight. Numerous factors can impact the degree of polymerization: initiator concentration,
radical ux, solvent concentration, and opportunistic chain transfer with compounds other than
the CTA. An undesirable opportunistic chain transfer possibility is hydrogen abstraction at random
sites along the polymer chain leading to branched polymers that are less ef cient thickeners than
strictly linear chains. The mercaptan chain transfer reaction is shown in Figure 11.7. In addition to
chain transfer, the other usual termination reactions of chain combination or disproportionation can
occur with methacrylates.
Commercial products cover a broad range of polymer molecular weights ranging from ∼20,000
to ∼750,000 Da. Molecular weight is carefully controlled and targeted to produce products that
achieve suitable shear stability for a given application.
H
2
CC
CH
3
C
O
OR
+
H
2
CC
C
CH
3
I
I
O
RO
FIGURE 11.5 Free radical initiation of methacrylate polymerization.
H
2
CC
CH
3
C
O
OR
+ X
H
2
CC
CH
3
RO
O
I
H
2
C
CH
3
RO
)
x
(
I
H
C
C
C
O
FIGURE 11.6 Monomer addition—propagation step.
+RSH
H
2
CC
C
CH
3
CH
3
(
R′
RS
RO
O
)
x
H
2
CC
)
(
H
R′
+
x
C
O
RO
FIGURE 11.7 Termination by chain transfer.
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Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 321
Higher-molecular-weight PMAs are rather dif cult to handle as neat polymers; hence, it is
necessary in almost all commercial cases to use a solvent to reduce viscosity to levels consistent
with reasonable handling properties. Additionally, it is important to maintain reasonable viscosity
during the polymerization reaction (although it always increases as monomer to polymer conver-
sion increases), so that suf cient agitation can be maintained. Thus, solvents are almost invari-
ably employed. An appropriate solvent would (1) be nonreactive, (2) be nonvolatile (at least at the
reaction temperature), (3) avoid chain transfer reactions, and (4) be consistent with the intended
application of the resulting product. It turns out that mineral oil meets the aforementioned criteria
reasonably well, so that a solvent choice can be made from higher- quality, lower-viscosity-grade
mineral oils. Nonreactivity demands relatively higher saturate contents; hence, better quality
American Petroleum Institute (API) Group I (or higher API group) mineral oil can be used. Choice
of solvent viscosity primarily depends on the end application; choices range from very-low-viscos-
ity oils of 35 SUS to light neutrals typically up to 150N. Alternatively, one can use a nonreactive
but volatile solvent when mineral oil might interfere with a sensitive polymerization and then do
a solvent exchange into a more suitable carrier oil. The amount of solvent added to commercial
PMA VIIs is suf cient to reduce viscosity to levels consistent with reasonable handling or con-
tainer pump out properties. This amount is dictated by polymer molecular weight, as this also
heavily in uences product viscosity. Generally, a higher-molecular-weight polymer requires more
solvent. Commercial products may thus contain polymer concentrations over a very broad range of
∼30 to 80 wt%.
d-PMAs were rst described by Catlin in a 1956 patent [7]. The patent claims the incorporation
of diethylaminoethyl methacrylate as a way of enhancing the dispersancy of VIIs and thus
providing improved deposit performance in engine tests of that era. The original dispersant
methacrylate polymers utilized monomers that copolymerized readily with alkyl methacrylates
and did not require different polymerization chemistry. Beyond these original random polymer-
izations, grafting is also an important synthetic route to incorporate desirable polar monomers
onto methacrylate polymers. Stambaugh [8] identi ed grafting of N- vinylpyrrolidinone onto a
PMA substrate as a route to improved dispersancy of VIIs. Another approach is to graft both
N-vinylpyrrolidinone and N-vinyl imidazole [9]. An obvious bene t of grafting is an ability to
incorporate polar monomers that do not readily copolymerize with methacrylates due to signi -
cant differences in reactivity ratios. Grafting reactions are carried out after achieving high con-
version of the alkyl methacrylates to polymer. Bauer [10] identi ed an alternate synthetic route
to incorporate dispersant functionality by providing reactive sites in the base polymer and then
carry out a postpolymerization reaction. For example, maleic anhydride copolymerized into or
grafted onto the polymer backbone can be reacted with compounds containing desirable chemi-
cal functionality such as amines. This strategy is a route to incorporate compounds that are
otherwise not susceptible to addition polymerization because they lack a reactive double bond.
This discussion characterizes most of the chemistry used to prepare the great majority of
commercial PMA products. Additional chemical strategies as well as some novel processes and
polymer blend strategies are reviewed in the literature and in the patent section below.
11.2.4 PATENT REVIEW
A review of pertinent literature and patents shows PMAs to have been the subject of numerous
investigations over the course of years. A huge body of PMA patent literature exists, and a large sub-
set of it is related to lubricant additives. A summary from the additive-related patents suggests ve
major areas of investigation, which can be categorized as variation of polymer composition; incor-
poration of functionality to enhance properties other than rheology, that is, dispersancy; improved
processes to improve economics or enhance a performance property; polymer blends to provide
unique properties; and nally, polymer architecture. A brief discussion of only a few of the more
important patents within the ve categories ensues.
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322 Lubricant Additives: Chemistry and Applications
Since the rst PMA patents [2.3], there has been a continuing search for composition
modi cations to methacrylate polymers to improve some aspects of rheological performance.
As expected, much of the earlier work explored uses of various alkyl methacrylate monomers and
examined the ratios of one to another; this work is part of the well-established art. But, even more
modern patent literature includes teachings about PMA compositions. For instance, highly polar
PMA compositions made with high concentrations of short-chain alkyl methacrylates are useful
in polar synthetic uids such as phosphate esters to impart rheological advantages [11]. There are
numerous examples of incorporating nonmethacrylates into polymers; a good example is the use of
styrene [12] as a comonomer to impart improved shear stability. However, styrenic monomers have
different reactivity ratios than methacrylates, and the usual processes lead to relatively low conver-
sions of styrene. This can be overcome by a process utilizing additional amounts of methacrylate
monomers near the end of the process to drive the styrene to high conversion [13].
Incorporation of functional monomers to make dispersant versions of PMA has been discussed
in the preceding section on chemistry. Despite the well-known nature of d-PMA, it remains an area
of active research as exempli ed in Ref. 14, which describes a dispersant for modern diesel engine
oil soot. Although nitrogen-based dispersants are the focus of much research, oxygen-based dis-
persants such as hydroxyethyl methacrylate [15] and ether-containing methacrylates [16] have also
been claimed. In addition to incorporating dispersant functionality, signi cant efforts to incorpo-
rate other types of chemical functionality such as antioxidant moieties [17] have been made.
Novel processes have been developed to improve either economics or product properties. Tight
control of molecular-weight distribution and degree of polymerization can be achieved through
constant feedback of conversion information to a computer control system that adjusts monomer
and initiator feeds as well as temperature [18]. Coordinated polymerizations are useful in prepar-
ing alternating copolymers of methacrylates with other vinyl monomers [19]. A process has been
described to prepare continuously variable compositions, which can obviate the need to physically
blend polymers [20].
Polymer blends of PMA and ole n copolymer (OCP) VIIs provide properties intermediate to the
individual products with OCP imparting ef cient thickening (and economics) and PMA imparting
good low-temperature rheology. However, a physical mixture of the two VIIs in concentrated form
is incompatible. This problem is overcome by using a compatibilizer, actually a graft polymer of
PMA to OCP, to make an ∼70% PMA and ∼30% OCP mixture compatible [21]. Very-high-polymer
content products can be prepared by emulsifying the mixture, so that the PMA phase is continuous
in a slightly polar solvent, whereas the normally very viscous OCP phase is in micelles [22,23].
Blends of PMAs can provide synergistic thickening and pour depressing properties [24].
PMA polymer architecture is being very actively investigated today. Preparations of PMA
blocks, stars, combs, and narrow MWD polymers are all the subject of relatively recent patents
or patent applications. For instance, the newer polymerization technique of controlled radical
polymerization (CRP), speci cally atom transfer radical polymerization (ATRP), has been used
to prepare PMAs of very narrow molecular weight distribution to improve the thickening ef -
ciency/shear stability balance of the resulting product [25]. Similarly, a CRP nitroxide-mediated
polymerization (NMP) has been described [26] as providing products with similar improved prop-
erties. The ATRP technique has been used to prepare PMAs with functional (polar) monomers
in blocks to enhance physical attraction to metal surfaces and thus improve frictional properties
under low-speed conditions [27,28]. Star-shaped PMAs made through CRP processes including
reversible addition–fragmentation chain transfer (RAFT) polymerization, NMP, and ATRP have
been described as providing improved solution properties [29]. Star shapes and other polymer
architectures through various CRP processes are described [30,31] as having enhanced thicken-
ing ef ciency/shear stability and VI contribution relative to the more traditional linear polymers.
Another new polymer architecture of interest is comb polymers with polyole n and PMA elements
as described in Ref. 32; the same enhancement of properties as mentioned previously applies to
these structures.
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Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 323
11.3 PROPERTIES AND PERFORMANCE CHARACTERISTICS
11.3.1 C
HEMICAL PROPERTIES
PMAs are rather stable materials and do not normally undergo chemical reactions under moder-
ate to even relatively severe conditions. The chemical design of any VII or PPD clearly entails
avoiding reactive sites in their structure to provide as high a degree of stability as possible in the
harsh environments to which lubricants are exposed. It is expected that these PMA additives are
not chemically active, as they are added to alter only physical properties, that is, viscosity and
wax crystallization phenomena. When considering d-PMAs, which include chemistry other than
alkyl methacrylate, even these have essentially the same fundamental stability as the nondispersant
polyalkylmethacrylates. The most notable reaction of any VII, including PMA, is polymer shearing
brought about by mechanical mastication.
11.3.1.1 Hydrolysis
PMAs are not very susceptible to hydrolysis reactions; however, the question of hydrolytic stability
is often posed because of the presence of the ester group. In the polymeric form, methacrylate
ester groups are quite stable since they are well shielded by the surrounding polymer as well as
the pendant side chains. The immediate chemical environment surrounding the ester is de nitely
hydrophobic and not compatible with compounds that participate in or catalyze hydrolytic reactions.
Extraordinary measures can be used to induce hydrolysis; for instance, lithium aluminum hydride
hydrolyzes PMAs to yield the alcohols from the side chain. Nevertheless, there is no evidence that
PMA hydrolysis is of any signi cant consequence in lubricant applications.
11.3.1.2 Thermal Reactions: Unzipping and Ester Pyrolysis
These reactions are known to occur with PMAs, but very vigorous conditions are needed. Thus,
there appears to be no important consequences in lubricant applications since bulk oil temperatures
are usually lower than the onset temperatures of these reactions [33].
A purely thermal reaction of PMA is simple depolymerization (unzipping of polymer chains).
PMA chains at suf ciently high enough temperatures unzip to produce high yields of the original
monomers; the unzipping reaction is merely the reverse of the polymerization reaction. One con-
sequence of unzipping is that care must be taken to avoid depolymerization during polymer syn-
thesis by simply avoiding excessive temperatures. Thus, synthesis temperatures are designed to
be well below the ceiling temperature of the polymerization/depolymerization equilibrium. Onset
temperatures for unzipping of PMAs are on the order of 235°C—the temperature listed in the
literature for polymethylmethacrylate depolymerization [34]. There may be a minor dependence
on detailed side chain structure, but relatively little investigation has been done on longer-side-
chain alkyl methacrylate polymers useful in lubricants. It is also thought that terminal double
bonds on the polymer chain are the point where unzipping most readily starts. When terminal
double bonds are present in the structure, they are most likely the product of termination by
disproportionation.
Another potential reaction is the thermal decomposition of individual ester units within the
polymer chain. This reaction, usually termed beta ester pyrolysis, degrades polymer side chains
to yield an alpha ole n from the pendant side chain. The ole n is of the same length as the carbon
skeleton of the side chain (as long as the original alcohol is used to make the monomer). The other
reaction product is an acid that presumably remains in the polymer chain. The acid may react with
an adjacent ester to yield alcohol and a cyclic anhydride. Another possibility is the reaction of two
adjacent acid groups to form a cyclic anhydride with the elimination of a molecule of water. The
pathway for the pyrolysis reaction proceeds through the formation of a six-membered intermediate
ring formed from a hydrogen bond of side chain beta carbon hydrogen to the ester carbonyl oxygen.
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