Lubricant Additives

296 Lubricant Additives: Chemistry and Applications

Long-chain branching has a directionally detrimental effect on thickening effi ciency for poly-

mers of equal molecular weight. This is not surprising, since the average chain end-to-end distance

of a random coil in solution is controlled, in large part, by the length of the main chain. Branching

essentially shortens the chain and lowers its hydrodynamic radius. For example, Table 10.3 contains

thickening effi ciency data for two sets of noncrystalline OCP viscosity modifi ers, one linear and the

other containing 2% branching agent, each set differing only in molecular weight.

10.5.2.2.2 High-Temperature, High-Shear Rate Viscosity

Since concentric journal bearings operate in the hydrodynamic or elastohydrodynamic lubrication

regimes, oil fi lm thickness is a critical factor infl uencing wear [108,109]. For this reason, SAE J300

specifi es a minimum HTHS viscosity for each viscosity grade [104]. HTHS viscosity is measured

at very high shear rates and temperatures (10^6 s–1 and 150°C, respectively), which is similar to the

TABLE 10.2

Mark–Houwink K′ Constants for E/P Copolymers Containing

Different Mole Percentages of Ethylene (a = 0.74)

Mol% ethylene K′ × 104 Mol% ethylene K′ × 104

5 2.020 55 3.020

10 2.115 60 3.140

15 2.205 65 3.260

20 2.295 70 3.385

25 2.390 75 3.515

30 2.485 80 3.645

35 2.585 85 3.790

40 2.690 90 3.940

45 2.795 95 4.240

50 2.910 ——

Source: Adapted from Crespi, G., Valvassori, A., and Flisi, U., Stereo Rubbers,

W.M. Saltman, ed., Wiley-Interscience, New York, NY, 365–431, 1977.

FIGURE 10.13 Polymer concentration needed to raise the kinematic viscosity of a 130N base oil to 11.5 cSt.

(Data from Kapuscinski M.M., Sen, A., and Rubin, I.D., Soc. Automot. Eng. Tech. Paper Ser. No. 892152, 1989.)

0

0. 5

1

1. 5

2

0 100 200 300 400 500

Mw × 10 −^3

Polymer Concentration (wt%)

60 mol%

ethylene

80 mol%

ethylene

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