Lubricant Additives

288 Lubricant Additives: Chemistry and Applications

10.4.1 SOLUTION PROCESS

The most common method for manufacturing OCP viscosity modifi ers is the solution process as

described in Figure 10.4 [65]. It is made up of four sections—polymerization, polymer isolation,

distillation, and packaging. In the polymerization section, monomers, an organic solvent such as

hexane, and a soluble catalyst are introduced into a continuously stirred polymerization reactor.

During polymerization, the polymer remains in solution and causes the bulk viscosity of the reac-

tion medium to increase. To maintain good agitation, monomer diffusion, and thermal control,

polymer concentration in the polymerization reactor is typically limited to 5–6 wt% [27]. Up to

fi ve reactors arranged in series have been reported in the literature [66]. The effl uent from the last

reactor is contacted with an aqueous shortstop solution to terminate polymerization and wash away

the catalyst, although this step is often omitted when using metallocene catalysts due to their high

reactivity and, therefore, low concentration [22]. While the copolymer is still in solution, extender

oils or antioxidants can be added.

In the isolation section, three techniques have been described in the literature. In the most

common method (shown in Figure 10.4), the polymer is fl occulated with steam, and the solvent and

unreacted monomers are recovered, purifi ed, and recycled. The aqueous polymer slurry is mechani-

cally dewatered, granulated, and air-dried. A second nonaqueous method for isolating the polymer

has been described in which the polymer is concentrated in a series of solvent removal steps [67,68].

The fi nal step may be conducted in a devolatilizing extruder. A third technique does not isolate the

polymer as a solid; rather, it mixes the polymer solution into mineral oil and distills off the solvent,

producing a fi nished liquid OCP product [2].

Another type of solution polymerization process that has received a great deal of attention has

been Exxon’s tubular reactor technology. Its purpose is to generate a polymer with long blocks

differing in monomer composition for improved performance as a viscosity-improving polymer

[3,69–71]. A schematic of this process is shown in Figure 10.5. Monomers and solvent are premixed

FIGURE 10.4. Solution process for manufacturing EPDM. (Adapted from Hydrocarbon Processing, 164,

November 1981.)

Propylene

Shortstop

Waste water

Extender

oil

AO

Steam

High-boils

Fresh

solvent

Solvent

Water

Baling

wrapping

Expeller

Expander

Off

gas

Solvent

Polymerization Dewatering

Distillation

Termonomer

Catalyst

Ethylene

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