Lubricant Additives

Olefi n Copolymer Viscosity Modifi ers 297

fl ow environment in an operating crankshaft bearing at steady state. At these rates of deformation,

most high-molecular-weight polymers will align with the fl ow fi eld [110], and a temporary reduction

in viscosity is measured. The difference between low-shear rate viscosity and HTHS at 150°C is

termed temporary viscosity loss (TVL) or percent temporary viscosity loss (relative to the low-shear

rate KV). As is true for most polymers [110], TVL is proportional to molecular weight [1] as seen

in Table 10.4. For polymers of equal weight average molecular weight, those with narrow molecular

weight distributions undergo less TVL than those with broad Mw / Mn values [1].

HTHS viscosity can be adjusted by increasing the viscosity of the base oil or by increasing the

viscosity modifi er concentration, as shown in Figure 10.14. Since the formulation also has to meet

KV and CCS viscosity limits, there is often only limited fl exibility to adjust HTHS viscosity within

the bounds of a given set of base oils and additives.

10.5.2.2.3 Permanent Shear Stability

The tendency of an OCP molecule to undergo chain scission when subjected to mechanical forces

is dictated by its molecular weight, molecular weight distribution, ethylene content, and degree of

long-chain branching. Mechanical forces that break polymer chains into lower-molecular-weight

fragments are elongational in nature, causing the molecule to stretch until it can no longer bear the

load. This loss in polymer chain length leads to a permanent degradation of lubricant viscosity at

all temperatures. In contrast to temporary shear loss, permanent viscosity loss (PVL) represents an

irreversible degradation of the lubricant and must be taken into account when designing engine oil

for commercial use.

PVL is similar to TVL, except that the viscosity loss is measured by KV before and after shear.

Permanent shear stability is more commonly defi ned by the permanent shear stability index (PSSI)

or simply SSI, according to ASTM D6022 as follows:

PSSI SSI (^100) ()()V 00

VSbV V
TABLE 10.3
Thickening Effi ciency of Linear and Branched
OCP Viscosity Modifi ers
Mw Linear Branched
230,000 13.50 12.03
180,000 11.17 10.87
Note: Thickening effi ciency is defi ned as the kinematic viscosity (at
100°C) of a 1.0 wt% polymer solution in 6.05 cSt mineral oil.
TABLE 10.4
Rheological Comparison of Lubricants Containing OCP Viscosity Modifi ers Differing in
Molecular Weight
Viscosity Modifi er
Weight Average
Molecular Weight PSSI
Capillary Viscosity
(cP) at 150°C
HTHS (cP)
at 150°C % TVL
OCP1 160,000 45 5.33 3.43 36
OCP2 80,000 30 5.33 3.77 29
OCP3 50,000 22 5.33 3.88 27
Source: Adapted from Spiess, G.T., Johnston, J.E., and VerStrate, G., Addit. Schmierst. Arbeitsfl uessigkeiten, Int. Kolloq.,
5th, 2, 8.10-1, Tech. Akad. Esslingen, 1986.
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