148 Lubricant Additives: Chemistry and Applications
in nature, and depending on its molecular weight, dispersants can be classifi ed into polymeric
dispersants and dispersant polymers. Polymeric dispersants are of lower molecular weight than
dispersant polymers. The molecular weight of polymeric dispersants ranges between 3,000 and
7,000 as compared to dispersant polymers, which have a molecular weight of 25,000 and higher.
Although various olefi ns, such as polyisobutylene, polypropylene, polyalphaolefi ns, and mixtures
thereof, can be used to make polymeric dispersants, the polyisobutylene-derived dispersants are the
most common. The number average molecular weight (Mn) of polyisobutylene ranges between 500
and 3000, with an Mn of 1000–2000 being typical [29]. In addition to Mn, other polyisobutylene
parameters, such as molecular weight distribution and the length and degree of branching, are also
important in determining the overall effectiveness of a dispersant.
Substances obtained through a polymerization reaction, especially those made by using an acid
catalyst or a free-radical initiator, often contain molecules of different sizes. Molecular weight
distribution, or polydispersity index, is commonly used to assess the heterogeneity in molecular
size. Polydispersity index is the ratio of weight average molecular weight (Mw) and Mn, or Mw/Mn
[30–32]. These molecular weights are determined by subjecting the polymer to gel permeation
chromatography (GPC). The method separates molecules based on size [33]. The larger molecules
come out fi rst, followed by the next size. When the molecules are of the same size, Mw/Mn equals
1 and the polymer is called a monodisperse polymer. The polymers with an index >1 are called
polydisperse polymers. For most applications, monodispersity is desired. Polyisobutylene, derived
from acid-catalyzed polymerization reaction, typically has a polydispersity index between 2 and 3.
This will impact many of the dispersant properties described below.
Dispersant polymers, also called dispersant viscosity modifi ers (DVMs) and dispersant viscos-
ity index improvers (DVIIs), are derived from hydrocarbon polymers of molecular weights between
25,000 and 500,000. Polymer substrates used to make DVMs include high-molecular-weight olefi n
copolymers (OCPs), such as ethylene–propylene copolymers (EPRs), ethylene–propylene–diene
copolymers (EPDMs), polymethacrylates (PMAs), styrene–diene rubbers (SDRs) of both linear and
star confi gurations, and styrene–ester polymers (SEs).
The polar group is usually nitrogen- or oxygen-derived. Nitrogen-based groups are derived
from amines and are usually basic in character. Oxygen-based groups are alcohol-derived and are
neutral. The amines commonly used to synthesize dispersants are polyalkylene polyamines such
as diethylenetriamine and triethylenetetramine. In the case of DVMs or dispersant polymers, the
polar group is introduced by direct grafting, copolymerization, or by introducing a reactable func-
tionality. The compounds used for this purpose include monomers such as 2- or 4-vinylpyridine,
N-vinylpyrrolidinone, and N,N-dialkylaminoalkyl acrylate and unsaturated anhydrides and acids
such as maleic anhydride, acrylic acid, and glyoxylic acid. The details of these reactions are
described in Section 5.6, which deals with the dispersant synthesis. Amine-derived dispersants are
called nitrogen or amine dispersants, and those that are alcohol-derived are called oxygen or ester
dispersants [28]. Oxygen-derived phosphonate ester dispersants were popular at one time, but their
use in engine oils is now restrained because of the phosphorus limit. Phosphorus limit pertains
to its tendency to poison noble metal catalysts used in catalytic converters. Formulators prefer to
take advantage of the phosphorus limit by using zinc dialkyldithiophosphates, which are excellent
oxidation inhibitors and antiwear agents. In the case of amine dispersants, it is customary to leave
some of the amino groups unreacted to impart basicity to the dispersant. The reasons for this are
described in Section 5.7.
5.6 DISPERSANT SYNTHESIS
Since it is not easy to attach the polar group directly to the hydrocarbon group, except in the case
of olefi ns that are used to make DVMs, the need for a connecting group or a link arises. Although
many such groups can be used, the two common ones are phenol and succinic anhydride. Olefi n, such
as polyisobutylene, is reacted either with phenol to form an alkylphenol or with maleic anhydride