Lubricant Additives

300 Lubricant Additives: Chemistry and Applications

states that “introduction of a termonomer ... can ... detrimentally affect shear and oxidation stability,

dependent on the monomer,” but he offers no data. Others [5,27] cite high-temperature aging experi-

ments on solid rubber specimens, which demonstrate that EP copolymers are more stable (in terms

of tensile properties) than EPDM terpolymers of similar E/P ratio. Copolymers containing higher

levels of ethylene are claimed to have better thermal/ oxidative stability than more propylene-rich

copolymers, presumably due to the lower concentration of oxidatively labile tertiary protons contrib-

uted by the propylene monomer. High thermal stresses are suffi cient to promote hydrogen abstraction

by a free radical mechanism. The relative susceptibility of protons to hydrogen abstraction follows

the classical order tertiary > secondary > primary. In the presence of oxygen, peroxy radicals are

formed, which can accelerate the degradation process.

Despite these suggestions that diene-containing E/P copolymers may be less thermally stabile

than EP copolymers, the author is not aware of any defi nitive studies that have shown that EPDM

viscosity modifi ers are more likely to degrade in service than EP copolymers. Indeed, engine oils

formulated with both types have been on the market for years.

10.5.4 COMPARATIVE RHEOLOGICAL PERFORMANCE IN ENGINE OILS

The most infl uential factors governing the rheological performance of OCPs in engine oils are

molecular weight and monomer composition. The effects of molecular weight and molecular

weight distribution were discussed in Section 10.5.2.3, and the infl uence of E/P ratio on low-

temperature rheology was covered in Section 10.5.2.1. In this section, two comparative rheologi-

cal studies are presented to further illustrate the links between OCP structure and rheological

performance.

10.5.4.1 Comparative Study of OCP Viscosity Modifi ers in a

Fixed SAE 5W-30 Engine Oil Formulation

There are two ways to compare the relative performance of several viscosity modifi ers. One is to

choose a fi xed engine oil formulation where the base oil composition and additive concentrations

are held constant, and the VM level is adjusted to achieve a certain 100°C KV target. The other is

to adjust both base oil composition and VM concentration to achieve predetermined KV and CCS

viscosity targets. Section 10.5.4.1 offers an example of the fi rst approach and Section 10.5.4.2 illus-

trates the second approach.

Four OCP viscosity modifi ers were blended into an SAE 5W-30 engine oil composition con-

sisting of a 95/5 w/w blend of Canadian 100N/250N mineral base stocks, an ILSAC GF-2 quality

performance additive, and a polyalkylmethacrylate PPD. The viscosity modifi ers are described in

Table 10.5. OCP1 and OCP2 are high SSI polymers differing in both E/P ratio and diene content.

OCP3 and OCP4 are progressively more shear stable and have essentially 0% crystallinity.

Rheological data are summarized in Table 10.6. Comparing OCP1 and OCP2, the former is a

more effi cient thickener because it contains no long-chain branching (no diene monomer) and it has

TABLE 10.5

Properties of OCP Viscosity Modifi ers Used in Table 10.6

Viscosity Modifi er Code

Shear Stability Index

(ASTM D6278) Copolymer Type

OCP1 55 EP, LTOCP

OCP2 50 EPDM, amorphous

OCP3 37 EP, amorphous

OCP4 25 EP, amorphous

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