Lubricant Additives

298 Lubricant Additives: Chemistry and Applications

where

Vo = viscosity of unsheared oil

Vs = viscosity of sheared oil

Vb = viscosity of the base fl uid (without polymer)

SSI represents the fraction of viscosity contributed by the viscosity modifi er that is lost during

shear. SSI is proportional to log 10 molecular weight (Mw), as shown in Figure 10.15. Commercial

OCP viscosity modifi ers have SSI values in the range of 23–55.

FIGURE 10.14 The effects of base oil composition and viscosity modifi er concentration on HTHS viscosity.

SAE 15W-40 engine oil consisting of European API Group I base oils (150N + 600N), an ACEA A3-98/B3-98

quality performance additive, an oil diluted amorphous OCP viscosity modifi er, and a pour point depressant.

Bars marked with an asterisk comply with ASTM D445 and D5293 limits for SAE 15W-40 oils.

49.2 36.5

20.0 11.2

0.0 4.85.8

6.8

7.8

8.8

3.5

3.75

4

4.25

4.5

4.75

5

HTHS Viscosity (cP)

% 600N Oil

%Viscosity modifier

*

*

*

*

*

FIGURE 10.15 Relationship between weight average molecular weight and SSI (ASTM D3945) for a series

of OCP viscosity modifi ers.

0

10

20

30

40

50

60

5.0 5.1 5.2 5.3 5.4 5.5 5. 6

Log weight average molecular weight

Shear stability index

CRC_59645_Ch010.indd 298CRC_59645_Ch010.indd 298 12/6/2008 10:10:15 AM12/6/2008 10:10:15 AM