Lubricant Additives

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 specifi 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
simplifi 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 signifi cant
   portion of the benefi 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 fi 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 esterifi cation. The other route
is oxidation of butylenes followed by subsequent hydrolysis and esterifi 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

H 2 C C H

FIGURE 11.1 Generalized struc-

ture of polyalkylmethacry late.

OR

O

C

CH 3

H 2 C C

FIGURE 11.2 Alkyl

methacrylate monomer.