Lubricant Additives

Olefi n Copolymer Viscosity Modifi ers 295

10.5.2.2 High-Temperature Rheology

Copolymer composition has less infl uence on high-temperature rheological behavior than it has at

low temperatures, partly because lightly crystalline OCPs have melting points well below 100°C

[97]. Since both high-temperature KV and high-temperature high-shear rate viscosity (HTHS) are

used to classify multigrade engine oils [104], it is important to understand how copolymer composi-

tion and molecular weight infl uence these key parameters.

10.5.2.2.1 Kinematic Viscosity

For both economic and performance reasons, it is desirable to limit the amount of polymer needed to

achieve a given set of rheological targets. Therefore, it is important to quantify the effects of molecular

weight, molecular weight distribution, and branching on thickening effi ciency. Thickening effi ciency

has been defi ned in many ways, but the most common defi nitions are (1) the amount of polymer nec-

essary to increase the KV of a reference oil to a certain value or (2) the KV or specifi c viscosity (see

Section 10.5.2.1) of a given polymer concentration in a reference oil. For polymers of equal molecular

weight, thickening effi ciency increases with ethylene content and is highest for copolymers with nar-

row molecular weight distributions [1,2]. In Figure 10.12, a plot of intrinsic viscosity versus weight

average molecular weight (M) demonstrates the familiar Mark–Houwink power law relationship:

[]
KM′ a

where K′ and a are constants.

Assuming a single value for the power law constant a = 0.74, Crespi et al. [5] published a table of

K′ values as a function of ethylene content, which is reproduced in Table 10.2. This clearly shows

that thickening effi ciency can be improved by increasing ethylene concentration.

This is further illustrated by plotting data from Kapuscinski et al. [98] in Figure 10.13. The

80 mol% ethylene copolymer requires less polymer to achieve a target viscosity than a 60 mol%

copolymer; therefore the former has a higher thickening effi ciency than the latter.

FIGURE 10.12 Intrinsic viscosity as a function of weight average molecular weight for EP copolymers

of narrow polydispersity (Mw / Mn ∼ 2). (Spiess, G.T., Johnston, J.E., and VerStrate, G., Addit. Schmierst.

Arbeitsfl uessigkeiten, Int. Kolloq., 5th, 2, 8.10-1, Tech. Akad. Esslingen, 1986. Reproduced with permission

of ExxonMobil Chemical Company.)

10

1.0

0.1

0.01

103 104 105 106

Weight average molecular weight

Intrinsic viscosity (dL /g) Decalin 135

°C

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