Lubricant Additives

Olefi n Copolymer Viscosity Modifi ers 293

where

c = polymer concentration (g/dL)

ηsp = specifi c viscosity (η – η 0 )/η 0

η = solution viscosity

ηo = solvent viscosity

k′ = constant

Thus, for a specifi c lubricant composition, c and ηo are fi xed, and the temperature dependence of the

solution viscosity is directly related to that of the intrinsic viscosity.

Low-temperature viscosity is an important rheological feature of automotive lubricants. For the

vehicle to start in cold weather, the lubricant viscosity in the bearings should be below a critical value

as determined by low-temperature engine startability experiments [7] and defi ned within SAE J300

[104] for all “W” grades. The cold cranking simulator (CCS) test, ASTM D5293, is a high-shear rate

rheometer operating at fi xed subambient temperatures, designed to simulate oil fl ow in automotive

bearings during start-up. After the engine starts, the lubricant must also be able to freely fl ow into

the oil pump and throughout the internal oil distribution channels of the engine. This is the other

half of the low-temperature viscosity specifi cation for motor oils [104]. The mini-rotary viscometer

(MRV), ASTM D4684, is a low-shear rate rheometer designed to simulate pumpability characteristics

of a multigrade oil in a vehicle that was sitting idle for about 2 days in cold weather. SAE J300 also

contains upper viscosity limits for MRV viscosity and yield stress for all W grades.

Thus, the mechanism of intermolecular crystallization, which leads to molecular size contraction

in solution, affords high ethylene OCP viscosity modifi ers the opportunity to contribute less to

FIGURE 10.9 Intrinsic viscosities of EP copolymers in SNO-100 base oil at various temperatures. EPC3,

EPB1, EPA2, and EPA1 have 73, 61, 50, and 50 wt% ethylene, respectively. Differences in intrinsic viscosity

above 20°C are attributable to differences in molecular weight. (Rubin, I.D. and Sen, A., J. Appl. Poly. Sci.,

40, 523–530, 1990. Reprinted with permission of John Wiley & Sons, Inc.)

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0.00

− 20 − 10 0 10 20 30 40 50 60

EPA1

EPA2

EPB1

EPC3

Intrinsic viscosity (dL /g)

Temperature (°C)

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