Lubricant Additives

284 Lubricant Additives: Chemistry and Applications

and physical properties of OCPs continue to evolve to achieve improvements in low-temperature

rheology, thickening effi ciency, and bulk handling characteristics.

Several excellent reviews of OCP viscosity modifi ers have been published [1–3]. This chapter

serves as an update and current compilation of information relating to the chemistry, properties, and

performance characteristics of this important class of lubricant additives.

10.2 CLASSES OF OLEFIN COPOLYMERS

There are many ways to classify OCP viscosity modifi ers. From a user’s perspective, OCPs are mar-

keted as either solids or liquid concentrates. The physical state of the solids depends on several fac-

tors, primarily on the ethylene/propylene (E/P) mass ratio. When E/P is in the 45/55–55/45 range,

the material is amorphous and cold fl ows at room temperature. Thus, OCPs of this composition are

most commonly sold as bales, packaged in rigid boxes to maintain bale shape. When E/P is higher

than 60/40, the copolymer becomes semicrystalline in nature and does not cold fl ow under ambient

conditions. Thus, both bales and pellets can be produced.

Liquid concentrates of OCP in mineral oil contain enough rubber to raise the kinematic

viscosity (KV) to 500–1500 cSt (mPa s) range at 100°C. A typical viscosity/concentration rela-

tionship is shown in Figure 10.1.

From the preceding discussion, OCPs can also be classifi ed according to crystallinity, which is

measured by x-ray diffraction or differential scanning calorimetry. The infl uence of crystallinity on

rheological performance will be discussed in Section 10.5.

Shear stability is another parameter by which OCP viscosity modifi ers are categorized. The higher

the molecular weight of a polymer, the more prone it is to mechanical degradation when elongational

forces are imposed by the fl uid fl ow fi eld. This subject is dealt with in detail in Section 10.5.2.2.3.

Finally, chemical functional groups can be grafted to the OCP backbone, providing added

dispersancy, antioxidant activity, and low-temperature viscosity enhancement. A number of

chemical routes for functionalizing OCPs are described in Section 10.3.3.

10.3 CHEMISTRY

10.3.1 SYNTHESIS BY ZIEGLER–NATTA POLYMERIZATION

Although methods for synthesizing high-molecular-weight polymers of ethylene were commercial-

ized in the 1930s (the Imperial Chemical Industries (ICI) PLC, currently a division of AkzoNobel

high-pressure process), the polymers contained a signifi cant number of short- and long-chain

FIGURE 10.1 Kinematic viscosity of 50 permanent shear stability index (PSSI) amorphous OCP dissolved in

100N mineral oil. (Minick, J., A. Moet, A. Hiltner, E. Baer and S.P. Chum, J. Appl. Poly. Sci., 58, 1371–1384,

1995. Reprinted with permission of John Wiley & Sons, Inc.)

0

1000

2000

3000

4000

0510

OCP concentration, mass %

Viscosity (cSt) at 100

°C

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