Lubricant Additives

Olefi n Copolymer Viscosity Modifi ers 289

with a highly active Ziegler–Natta polymerization catalyst and metered into a plug fl ow reactor

under conditions that minimize chain transfer and termination reactions. Ethylene or propylene is

injected into the tube at different points to adjust the local monomer concentration and, thereby, the

monomer composition along the growing polymer chain. In comparing the rheological properties

of different A-B-A type block compositions, Ver Strate and Struglinski reported [12] that “chains

with high ethylene section in the center of the chain ... associate at low temperature with little inter-

molecular connectivity.” When the high ethylene (crystallizable) segments are at the ends, polymer

networks can form at low temperatures, which can impart a gelatinous texture to the solution.

10.4.2 SUSPENSION PROCESS

Ethylene and a nonconjugated diene, if desired, are contacted with liquid propylene, which acts

both as a monomer and a reaction medium [27,72]. In the presence of a suitable catalyst, polymer-

ization takes place rapidly, producing a suspension of copolymer granules that are insoluble in the

reaction medium. The heat liberated during the polymerization reaction is dissipated by propylene

evaporation, thus providing a convenient mechanism for temperature control. In addition, since the

polymer is not soluble in the reaction medium, viscosity remains low. Thus, relative to typical solu-

tion processes, the polymer concentration in a suspension reactor can be fi ve to six times higher.

Upon exiting the polymerization reactor, the polymer suspension is contacted with steam to strip

off unreacted propylene that is then recycled. According to Corbelli and Milani [72], the Dutral®

process does not include a catalyst washing step. The copolymer product, in aqueous suspension, is

dewatered, dried, and packaged in a similar fashion to polymer made by the solution process.

10.4.3 POSTPOLYMERIZATION PROCESSES

There are two main types of packaging processes in practice today [64]. In one, the isolated poly-

mer is mechanically compressed into rectangular bales. The bales are often wrapped in a polyolefi n

packaging fi lm to prevent the bales from adhering to one another during storage and foreign matter

from sticking to the tacky rubber surface. Typical types of polyolefi ns fi lms include poly(ethylene-

co-vinyl acetate), low-density polyethylene, and ethylene/α-Olefi n Copolymers (OCP). Another

method for packaging solid OCP rubber is to extrude the polymer, pass the melt stream into a

water-cooled pelletizer, and dry the fi nal product. The pellets may be packaged in bags or boxes or

may be compressed into rectangular bales.

FIGURE 10.5 Tubular reactor process for preparation of multiblock ethylene–propylene copolymers.

(U.S. Patent 4874820, 1989; U.S. Patent 4804794, 1989; U.S. Patent 5798420, 1998.)

Catalyst

Solvent

Cocatalyst Solvent/monomers

To

collection

Solvent/

monomers

Solvent/

monomers

Solvent/

monomers

Tubular reactor

Temperature-

controlled

catalyst

premixing

device

Mixing

zone Polymerization zone

CRC_59645_Ch010.indd 289CRC_59645_Ch010.indd 289 12/6/2008 10:10:12 AM12/6/2008 10:10:12 AM