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Syndiotactic PP is now for real.

For years, syndiotactic polypropylene was just a molecular oddity, an unusual three-dimensional crystalline form of the homopolymer, which held a tantalizing promise of inexpensive, high-performance resins. Syndiotactic PP has properties of impact resistance, flexibility and clarity akin to costlier "engineering" resins and PP copolymers, and can--at least in theory--be made in one reactor (which keeps the cost down compared with multi-reactor copolymerization). But skeptics said its wonderful properties might never be realized because of processing instability. "We talked to Exxon about syndiotactic PP back in 1985-86," recalls a design engineer at a large closure molder, "but it was all theory--they couldn't show us material."

Now attitudes are changing fast. Sooner than many polymer chemists expected, syndiotactic PP is close to commercial--in Japan. Chiba Group, a unit of Sumitomo Chemical, is said by knowledgeable industry sources to be developing a syndiotactic PP instrument panel for a 1994 car. The part is said to be molded by a new injection molding process developed by Sumitomo and others, which requires very high-flow material. Ford Motor Co., Mitsubishi Motors, Toyota and Nissan are known to be interested in the process and materials, which are protected by secrecy agreements. If reports are correct, this would be the first production part in the world made with this new polymer. Sumitomo refuses to discuss the project other than to say it is producing syndiotactic PP in developmental quantities. Sumitomo is said to have unique catalyst patents for syndiotactic PP.

Mitsui Toatsu Chemical has worked for years with syndiotactic PP from a pilot plant in Japan and has filed hundreds of patent applications on applications and processing of syndiotactic PP, including stabilizer and additive TABULAR DATA OMITTED formulations. It is said to be developing multilayer oriented stretch film with a syndiotactic center layer sandwiched between layers of standard PP. Mitsui Toatsu is said to exchange technical information with Petrofina S.A. of Belgium, whose U.S. unit, Dallas-based Fina Oil & Chemical Co., holds many U.S. patents on syndiotactic PP. Fina is also trying to create market interest in film and other applications for syndiotactic PP in the U.S. and is sampling a limited number of customers.

Also in laboratory-scale development is Exxon Chemical Co., Houston, which is looking to use its Exxpol metallocene catalysts in PP production, and hopes to develop new packaging and durable applications. Despite patent litigation between Exxon and Fina, U.S. commercial development of syndiotactic PP appears to be continuing. Hoechst AG in Germany and Mitsui Petrochemical in Japan also have pilot capabilities for syndiotactic PP as well. Chisso Corp. in Japan is another firm said to be active in product development with syndiotactic PP.

A DIFFERENT CRYSTAL LATTICE

The difference between syndiotactic and standard isotactic PP is its molecular structure. In isotactic propylene, pendant methyl groups hang off one side of the polymer backbone in a straight line, like the teeth of a comb. In syndiotactic PP, the side groups alternate on opposite sides of the backbone. The resulting lower crystallinity of syndiotactic material produces only 50% of the stiffness and hardness of conventional PP, but twice the impact strength, according to a recent Hoechst research paper.

Syndiotactic PP can only be made in sufficient yield and purity with new metallocene "single-site" catalysts, so called because they have one active polymerization site. (Metallocene catalysts are also sparking development of radically new forms of polyethylene and polystyrene. See PT, Nov. '91, p. 15; Jan. '92, p. 13 and this month's Technology Newsfocus).

In some applications, syndiotactic PP may be limited by its lower melt temperature and molecular weight than isotactic PP--though other applications may potentially benefit from these same properties. Because syndiotactic PP has the same glass-transition temperature as isotactic, it loses its impact advantage at low temperatures. Also, a tendency toward slow recrystallization as it cools makes processing "very different," says Walter Spaleck, who is in charge of metallocene-catalyzed PP research at Hoechst. "When isotactic PP cools, it solidifies in a stable crystalline order, but in syndiotactic the crystalline structure goes on changing. A processor would say, 'Oh no, this isn't PP.'"

However, brand-new versions of syndiotactic PP that correct these problems are available in the U.S., says a processor working with them, but "in limited quantities, and you have to know whom and how to ask."

WHAT ARE THE PROPERTIES

Hoechst's Spaleck recently released what may be the first published property data on syndiotactic PP. It describes two syndiotactic (designated G1 and G2) and three isotactic homopolymers (G4, GS6 and G8), all made with new metallocene catalysts, and compares them with standard commercial isotactic homopolymer. All the syndiotactic samples were made in a 16-liter batch bulk reactor and pelletized on a 28-mm twin-screw compounding extruder.

Melting points of syndiotactic material are around 275 F. Metallocene-catalyzed isotactic PP (for example GS6) melts at around 300 F, compared with 325 F for standard PP. According to a paper published by polymer chemist John Ewen, inventor of syndiotactic patents assigned to Fina, syndiotactic PP's melting point ranges 275-300 F. Of new metallocene-catalyst PPs, the syndiotactic are more different from conventional PP than isotactic. Syndiotactic G2, for instance, has only 39% crystallinity, versus 45% crystallinity for isotactic G4 and 61% for standard PP, according to WAXS-spectra tests performed on powder diffractometers. DSC-crystallinity tests show slightly different numbers for the same resin samples.

Some big U.S. processors are actively considering applications for syndiotactic PP. "We have assembled some of our best scientists in our consideration of syndiotactic PP, and there are several possibilities," says a researcher at 3M Co. in St. Paul, Minn. Film is an obvious area of interest, but "as a matter of corporate philosophy, we would expect to make new products, not even known today" he adds.
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Title Annotation:polypropylene
Author:Schut, Jan H.
Publication:Plastics Technology
Date:Mar 1, 1992
Words:957
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