Lubricant Additives

162 Lubricant Additives: Chemistry and Applications

viscosity [110]. Cranking viscosity is an indication of how easily the engine will turn over in
extremely cold weather conditions. Pumping viscosity is the ability of the lubricant to be pumped
to reach various parts of the engine. For cold weather operation, low to moderate cranking and
pumping viscosities are highly desirable. Although pumping viscosity and the pour point can be
lowered by the use of additives, called pour point depressants [3,13], lowering cranking viscosity
is not easy. In the case of base oils, this is usually achieved by blending carefully selected base
stocks. An ideal polymeric dispersant must provide high-temperature viscosity advantage with-
out adversely affecting the cold-cranking viscosity of the lubricant. Dispersant polymers have
the same requirement. Good high-temperature viscosity to cranking viscosity ratio in polymeric
dispersants can be achieved by

Carefully balancing the type and the molecular weight of the hydrocarbon chain [111]

Choosing the optimum olefi n to maleic anhydride molar ratio [112]

Selecting the type and the amount of the polyamine used

In dispersant polymers this can be achieved by selecting (1) a polymer of correct molecular weight
and branching and (2) a suitable pendant group. Dispersant polymers derived from medium-
molecular-weight, highly branched structures, and ester-type pendant groups are best suited for use
as additives. Examples include polyacrylate, PMA, and styrene ester–derived dispersants. These
additives not only act as viscosity modifi ers and dispersants but also act as pour point depressants,
thereby improving the low-temperature properties of the lubricant.
A number of patents pertaining to dispersants with balanced high-temperature viscos-
ity and low-temperature properties are reported in the patent literature [113–117]. A Mannich
(alkylphenol) dispersant, derived from ethylene/1-butene polymers of Mn 1500–7500, has been
claimed to possess improved dispersancy and pour point [113]. Another patent claiming the syn-
thesis of a dispersant with superior dispersancy and pour point depressing properties has also
been issued [114]. The dispersant is based on the reaction of maleic anhydride/lauryl methacry-
late/stearyl methacrylate terpolymer with dimethylaminopropylamine, and a Mannich base was
obtained by reacting N- aminoethylpiperazine, paraformaldehyde, and 2,6-di-t-butyl phenol. A
number of patents describe the use of ethylene/α-olefi n/diene interpolymers to make dispersants
[115–117]. These dispersants are claimed to possess excellent high- and low-temperature viscosi-
ties, as defi ned by VR ́/VR. Here VR ́ pertains to the dispersant and VR pertains to the precursor,
such as alkylphenol or alkenylsuccinic anhydride. VR ́ is the ratio of the –20°C cold-cranking
simulator (CCS) viscosity (cP) of a 2% solution of dispersant in a reference oil to the 100°C kine-
matic viscosity (cSt) of the dispersant. VR is the ratio of the –20°C CCS viscosity (cP) of a 2%
solution of precursor in the reference oil to the 100°C kinematic viscosity (cSt) of the precursor.
The values of 2.0–3.9 for VR and VR ́ and of <1.11 for VR ́/VR are considered suitable for bal-
anced low- and high-temperature viscosities.

5.7.4 SEAL PERFORMANCE

Seals in automotive equipment are used for many purposes, the most prominent of which are to
have easy access to malfunctioning parts to perform repair and to minimize contamination and loss
of lubricant. Various polymeric materials are used to make seals. These include fl uoroelastomers,
nitrile rubber, polyacrylates, and polysiloxanes (silicones). Maintaining the integrity of seals is criti-
cal; otherwise, the lubricant will be lost, and wear damage and equipment failure will occur. The
seals fail in a number of ways. They can shrink, elongate, or become brittle and thus deteriorate.
The damage to elastomer seals is assessed by examining volume, hardness, tensile strength change,
and the tendency to elongate and rupture [118]. Two primary mechanisms by which seal damage can
occur include abrasion due to particulate matter in the lubricant and the attack of various lubricant
components on the seals. The lubricant-related damage can occur when some of its components