Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 321
Higher-molecular-weight PMAs are rather diffi 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 suffi 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 suffi 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 infl 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 fi 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] identifi 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 benefi t of grafting is an ability to
incorporate polar monomers that do not readily copolymerize with methacrylates due to signifi -
cant differences in reactivity ratios. Grafting reactions are carried out after achieving high con-
version of the alkyl methacrylates to polymer. Bauer [10] identifi 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.
- 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 fi 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 fi nally, polymer architecture. A brief discussion of only a few of the more
important patents within the fi ve categories ensues.