Dispersants 161
butadiene and styrene ester–derived dispersant polymers. Purely paraffi nic hydrocarbon groups that
contain tertiary hydrogen atoms, such as EPRs, oxidize at a faster rate than those that contain only
primary and secondary hydrogen atoms. Styrene isoprene–derived materials contain both benzylic
and tertiary hydrogen atoms. This implies that highly branched alkyl groups, such as polyisobutyl
and polyisobutenyl, have a higher susceptibility toward oxidation than linear or unbranched alkyl
groups. Dispersant polymers with built-in oxidation-inhibiting moieties are known in the literature
[77,78,96]. The polar moiety in an amine-derived dispersant is also likely to oxidize at a faster rate
than the oxygen-derived moiety because of the facile formation of the amine oxide functional group on
oxidation. Such groups are known to thermally undergo β-elimination [40], called the cope reaction, to
form an olefi n. This can oxidize at a faster rate as well as lead to deposit-forming polymeric products.
From a thermal stability perspective, the hydrocarbon group in the case of high-molecular-
weight dispersant polymers, such as those derived from OCPs, is more likely to break down (unzip)
than that derived from the low-molecular-weight polymers. Dispersants based on 1000–2000
molecular weight polyisobutylenes are relatively stable, except at very high temperatures that are
experienced in some engine parts, such as near the top of the piston [17,18]. Thermal breakdown of
the oxidized amine polar group is mentioned in the previous paragraph.
The chemical reactivity of certain dispersants toward water and other reactive chemicals pres-
ent in the lubricant formulation is an additional concern. The most likely reaction site is the con-
necting group. The common connecting groups are amide and imide in amine-derived dispersants
and ester in alcohol-derived dispersants. All three can hydrolyze in the presence of water [106], but
at different rates. Esters are easier to hydrolyze than amides and imides. The hydrolysis is facilitated
by the presence of bases and acids. Basic detergents are the source of the metal carbonate and metal
hydroxide bases, which at high temperatures catalyze the hydrolysis reaction. Additives, such as
zinc dialkyldithiophosphates, are a source of strong acids that result when these additives hydro-
lyze, thermally decompose, or oxidize. The fate of the ester-, amide-, and imide-type dispersant
polymers, such as those derived from polyacrylates, PMAs, and styrene ester substrates, is the same.
Some OCP-derived dispersant polymers, such as those obtained by grafting of monomers 2- or
4-vinylpyridine and 1-vinyl-2-pyrrolidinone [76,80], do not suffer from this problem since they do
not contain easily hydrolyzable groups. Reactivity toward other chemicals present in the formula-
tion is again prevalent in the case of ester-derived dispersants. Reaction with metal-containing addi-
tives, such as detergents and zinc dialkyldithiophosphates, can occur after hydrolysis to form metal
salts. This can destroy the polymeric structure of a dispersant and hence its effectiveness. Some
formulations contain amines or their salts as corrosion inhibitors or friction modifi ers. Depending
on the molecular weight and the ambient temperature, these can displace the polyol or sometimes
the polyamine, thereby altering the dispersant structure, hence its properties.
5.7.3 VISCOSITY CHARACTERISTICS
The amount of dispersant in automotive engine oils typically ranges between 3 and 7% by weight
[79], making it the highest among additives. In addition, dispersant is the highest molecular-weight
component except the viscosity improver [107]. Both of these factors can alter some physical prop-
erties, such as viscosity, of the lubricant. A boost in the viscosity of a lubricant at high temperatures
is desired, but at low temperatures it is a disadvantage. At high temperatures, the lubricant loses
some of its viscosity [108], hence its fi lm-forming ability, resulting in poor lubrication. Maintaining
good high-temperature viscosity of a lubricant is therefore imperative to minimize wear damage.
This is usually achieved by the use of polymeric viscosity modifi ers [3,109]. Some dispersants,
especially those that are based on high-molecular-weight polyolefi ns and have been oversuccinated
partly fulfi ll this need [44]. Therefore, the amount of polymeric viscosity modifi er necessary to
achieve specifi c high-temperature viscosity is reduced. Unfortunately, dispersants that provide a
viscosity advantage lead to a viscosity increase at low temperatures as well. The low-temperature
viscosity requirements for engine oils have two components: cranking viscosity and pumping