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Enter a new generation of polyolefins.

A new family of polyethylenes, made with new catalysts, is being sampled by Exxon Chemical Co., Houston, to about 50 plastics processors for specialty applications from blends and coextruded cast films and wire/cable coatings to injection molded medical and automotive parts that are both tough and stiff. Each processor is sampling (under a nonanalysis secrecy agreement) a different resin, tailored for its application.

Exxon's new resins are said to be the first commercial production to date using a homogeneous metallocene (or single-site) catalyst, a technology that was first patented by Hoechst AG in Germany 15 years ago, but is currently the hottest topic in polyolefins research and is potentially applicable to HDPE, LLDPE and PP production.


Traditional heterogeneous catalysts for olefin polymerization produce their own characteristic mix of molecular-weight species. It takes a lot of trial-and-error research to develop a catalyst that gives that particular mix one is looking for, and properties, are frequently a trade-off. But a single-site catalyst produces molecular chains of more uniform length with more evenly spaced comonomer. The result is extraordinarily narrow and predictable molecular-weight distribution composition.

By arranging a multistep polymerization process with a succession of different single-site catalyst, virtually any final molecular-weight distribution can be specified quite exactly. That's how the new technology can readily customize resin properties like tear and impact strength, flex modulus, and heat-seal initiation temperature.

At NPE in June, Exxon first spoke publicly--but only in general terms--about its new resins, trade-named Exact and produced with its single-site-catalyst technology, which goes by the name Exxpol. "Super PE" resins, also made with single-site catalyst, were announced by Mitsui Petrochemical Co. of Japan earlier this year, along with plans for a 220 million 1b/yr "Super PE" plant in Japan to start up by late 1993. Hoechst and Fina Oil & Chemical Co., Dallas, also holders of metallocene catalyst parents, are actively pursuing new resins as well. Hoechst, Mitsui, Fina, and Chisso Corp. of Japan (which has its own patents) are considered to be farther along than Exxon in PP polymerization with the new catalysts.

These are only the tip of an R&D iceberg that includes most major polyolefin companies, over 700 catalyst-related patents and patent applications, and already a patent lawsuit or two. All four of the major catalyst-patent holders have the capability to put these resins into full production by 1995, says Dr. Ludwig Boehm, manager of polyolefin R&D at Hoechst. "In the first stage these will be specialty resins. Later on it will be possible to make many polyolefins in this way," Boehm says.

"The dream that can come true eventually is that you can tailor the production of your polyolefin to the customer's needs," says Michel Daumerie, general manager of technology at Fina, about metallocene-catalyst R&D. (Metallocene catalyst precursors are organometallic compounds. Ligands attached to the transition metal are typically cyclopentadienes.)

Becuase these catalysts can customize resins with nearly unlimited combinations of properties, some researchers say the technology has the potential in the coming decade to become the dominant force in polyolefin manufacture. The significance of singlesite catalyst technology was suggested by the keynote speaker and in two other technical papers introducing the topic at the SPO '91 specialty polyolefins conference in Houston in September. In his address, Kenneth Sinclair, senior consultant at SRI International, Menlo Park, Calif., said: "Future [polyolefin industry] development will continue to be led by specialties, but on an increasingly broad front to the extent that specialties may become the dominant characteristic of the industry." (Proceedings are available from the sponsor, Schotland Business Research, Inc., Princeton, N.J.)


Exxon appears to have an early lead in the U.S. in single-site catalyst PE production with a new 30-million-lb/yr pilot plant in Baton Rouge, La., which started up in June (see PT, Nov. '89, p. 14; Feb. '90, p. 88; June '90, p. 93). The $40 million plant was built with process technology from Mitsubishi Petrochemical Co. for market-development purposes. "Baton Rouge is a start of our growth strategy for Exxpol catalyst, and we expect to file many more method and applications patents in coming months," says Michael Jeffries, venture manager for the new catalyst project.

Actually, Exxon began sampling some customers as long as three years ago from a pilot plant in Antwerp, Belgium. In June, when Exxon first announced its single-site catalyst polymers, it said the Baton Rouge Exxpol plant would be capable of making a wide range of polymers from elastomeric types to HDPE. Exxon's Jeffries says, "The pace of our product and technology developments is accelerating more than we would have believed."

The products sampler ranges from extremely viscous HMW resins to low-molecular-weight liquids and from highly crystalline, stiff materials to low-modulus amorphous ones.

"Exact" LDPE resins include "plastomers" with densities as low as 0.870-0.899 g/cc, VLDPE with 0.900-0.914 density and LLDPE with 0.915 density. Melt indexes ranging from 0.2 to over 30 g/10 min were cited in a paper at an wire and cable coatings given at an IEEE meeting in Dallas in September by Exxon's Monica Hendewerk.

Her paper compares an Exact wire/cable resin with a conventional (Ziegler-catalyst) LLDPE, both having 0.920 density and 5.0 MI. The Exact LLDPE is tailored for a tear strength of 530 lb/in. and 3% hexane extractables, compared with 150 lb/in. and 22% extractables for the conventional resin.

Melting point in Exact resins also varies directly with density, unlike conventional resins, which "tend to retain a high melting peak over a wide density range" because of the presence of high-ethylene-containing molecular species, said Hendewerk. A conventional 0.908-density LLDPE might have a melting point of 248 F, while a customized Exact resin of the same density has a melting peak of 220 F. "Melting point can be adjusted to meet the requirements of a given application by altering density or combining these narrowly distributed polymers," Hendewerk said. At lower densities, copolymers "start processing more like elastomers than plastics... This can provide a larger processing window to operate without scorching and requires less energy."

In data supplied to PLASTICS TECHNOLOGY, Exxon shows that a 3-mil film of an Exact LLDPE with 1.7 MI and 0.90 density has a seal-initiation temperature of 194 F vs. 248 F for a conventional LLDPE resin of the same MI and density. (Tests were conducted at 102 psi and dwell time of 0.5 sec.)

Stanley Speed, technology manager for Exxon's single-site catalyst research, said in a technical paper at the SPE Polyolefins VII conference in Houston this past February that the absence of long or short molecules with irregular comonomer clumps in Exact resins makes haze very low compared with conventional PE.

Exact plastomers are said to crosslink "exceptionally well" with radiation or peroxides and to accept carbon black and clay fillers well, so that electrical compounds may be produced similar to those based on EP rubber.

Fresh details on Exact resin capabilities are shown in Tables 1 and 2. Table 1 compares films of conventional and Exact LLDPE of the same MI and density. Note the improved toughness, lower extractables, lower heat-seal temperature, higher clarity, and higher stiffness. Table 2 compares conventional and Exact LLDPE injection molding resins. One Exact grade shows comparable physical properties at higher flow and lower density.

Exxon and Mitsui can already make a new PE grade for each new customer application, at least in sectors of cast film and injection molding. For now, the high shear and low melt strength of the new resins, caused by their very narrow MWD, means that optimized blow molding and monolayer blown film grades aren't yet possible, Exxon says. Besides, the new resins are expected to be two-to-three times more expensive than standard polyethylenes, so initial applications are likely to be in layers and blends.

But all told, the evolution seems to be toward a polyolefins market in which large processors can order any material they want with virtually any combination of properties.
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Title Annotation:Technology News
Author:Schut, Jan H.
Publication:Plastics Technology
Date:Nov 1, 1991
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