Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 323
11.3 PROPERTIES AND PERFORMANCE CHARACTERISTICS
- 3.1 CHEMICAL 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 defi 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 signifi 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 suffi 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 olefi n from the pendant side chain. The olefi 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.