Olefi n Copolymer Viscosity Modifi ers 299Although ASTM D6022 provides a defi nition for SSI, it is important to recognize that the only
component that is responsible for viscosity loss during shear is the high-molecular-weight poly-
mer. If the additive for which SSI is calculated happens to be a concentrated polymer solution in
oil, according to the strict defi nition of ASTM D6022, the composition of the base fl uid does not
include the viscosity modifi er (VM) diluent oil. Since the diluent oil viscosity is usually lower than
the base blend viscosity for most viscosity grades, Vb is higher than it would be if the VM diluent
oil viscosity was factored into Vb. For example, take an SAE 15W-40 engine oil formulated with
a liquid OCP concentrate containing 10 wt% polymer in a 5.1 cSt mineral oil. V 0 and Vs are 15.2
and 12.8 cSt, respectively. The base blend viscosity (when the VM component is a liquid) is 9.4 cSt.
When the VM component is defi ned as the solid polymer, Vb is 9.15 cSt. Calculated shear stability
index values are 41.4 and 39.7, respectively. Thus, the numerical value of SSI is dependent on the
defi nition of the polymeric additive in question.
The concept of “stay-in-grade” is generally used to refer to a lubricating oil, when tested in
vehicles or laboratory shearing devices, which maintains its KV within the limits of its original SAE
viscosity grade. The problem with viscosity measurements of engine drain oils is that many factors
other than permanent polymer shear infl uence viscosity—such as fuel dilution, oxidation, and soot
accumulation. Therefore, it is customary to measure PVL after shear in a laboratory rig, the most
common being the Kurt Orbahn test, ASTM D6278. Several reviews of methods for determining
the shear stability of polymer-containing lubricating oils have been published [111–113].
Selby devised a pictorial scheme for mapping the effects of shear rate and PVL on high-
temperature viscosity, the viscosity loss trapezoid (VLT) [114], shown in Figure 10.16. The corners
of the trapezoid are defi ned by viscosity data, and the points are connected by straight lines. Note
that the straight lines do not imply that there is a linear relationship between viscosity and shear
rate. The VLT is a convenient graphical representation of the temporary and permanent shear loss
characteristics of polymer-containing oils. Molecular weight degradation causes a permanent loss
in both KV and HTHS, but the magnitude of the former is always larger than the latter. The shape
of the VLT is the characteristic of polymer chemistry and molecular weight.
It is experimentally observed [115] that the Kurt Orbahn shear test breaks molecules above a
threshold molecular size; molecules smaller than the threshold value are resistant to degradation.
Selby [114] uses this observation to derive certain qualitative conclusions of the polymer molecular
weight distribution from the shape of the VLT.10.5.3 EFFECT OF DIENE ON THERMAL/OXIDATIVE STABILITY
There has been little solid scientifi c data published in the literature to compare the relative thermal/
oxidative stability of oil solutions containing E/P copolymers versus EPDM terpolymers. Marsden [2]FIGURE 10.16 Viscosity loss trapezoid, per Selby: (a) Fresh oil viscosities and (b) oil viscosities after per-
manent shear. (Selby, T.W., Soc. Automot. Eng. Tech. Paper Ser. No. 932836, 1993.)Shear rate High
(HTHS)Low
(KV)Absolute viscosity (cP)150 °C
(a)(b)CRC_59645_Ch010.indd 299CRC_59645_Ch010.indd 299 12/6/2008 10:10:16 AM12/6/2008 10:10:16 AM