First technical details on some next-generation polyolefins.

A new palette of ethylene copolymers made by a patented "constrained-geometry" metallocene catalyst system is being produced in developmental quantities by Dow Plastics in Freeport, Texas. Described in Dow's 58-page patent as pseudo-random ethylene/styrene copolymers with up to 50% styrene content, the new materials are the first such copolymers of ethylene and styrene reported in technical or patent literature. One earlier attempt in 1988 is described as a copolymerization of ethylene with only 1% styrene, compared to 48% styrene in the Dow material. The new material is surprisingly elastic in both solid and melted states, making it good for films, foams and articles fabricated, for example, by blow molding, Dow's patent says.

NOVEL STRUCTURE

A "pseudo-random" copolymer is different from either a random or block structure typical of PE/PP copolymers. Pseudo-random comonomer units only attach to each other in certain directions to form a polymer chain. In Dow's ethylene/styrene copolymer, for example, ethylene units can attach in any order either to other ethylenes or to styrene monomer units, while styrene units are "hindered," meaning they can only link tail-to-tail to another styrene. They cannot link head-to-tail or head-to-head. One effect of this quirk is that Dow's new constrained-geometry catalysts cannot make either a styrene homopolymer or a styrene/ethylene copolymer containing over 50% styrene.

Dow's patent also mentions several terpolymers of ethylene/styrene with varying amounts of butene or vinyl benzocyclobutane (BCB) and up to 22% styrene content.

NEW ELASTIC PE COPOLYMERS

Dow's constrained-geometry metallocene catalysts also make a range of ethylene/1-octene copolymers described as having higher melt elastic moduli at high melt indices than conventional polymers. Patent data indicate that these materials have high melt strength and a balance of rubbery and plastic behavior not typical of polyolefins. The patent says these elastic PE resins can have melt indices ranging TABULAR DATA OMITTED from 0.5 to 125 g/10 min, densities of 0.85-0.97 g/cc, melting points of 122-275 F, and a wide range of molecular-weight distributions.

These polymers are also marked by high clarity--"Especially lower haze than typical PE, making them especially well suited for film and injection molding applications," the patent says. And they're readily crosslinked and grafted to other copolymers.

Production of highly elastic PE copolymers of ethylene/octene or ethylene/hexene, with up to 20% by weight of hexene or octene, is also described. For these, Dow coined a marketing name of "plastomers," meaning they're rubbery but more rigid than elastomers. Materials with over 20% by weight of octene comonomer, Dow calls elastomers. And when ethylene monomer or hexene monomer feedstock is used without a comonomer, a PE or polyhexene homopolymer can be made.

All these copolymers are made by a variety of one-stage high-temperature solution polymerization processes. These materials are quite different from previously published results with metallocene, or "single-site," catalysts, in that they have broader molecular-weight distributions.

POTENTIAL MARKETS

Since Dow's announcement last December of its new catalyst technology (see PT, Jan. '92, p. 13), there has been speculation as to whether these materials would resemble typical thermoplastics or would be elastomeric. Dow says the plastomer/elastomer (ethylene/1-octene) materials will compete with PVC, EPR and EVA. Other materials may resemble polybutylene, HDPE or possibly toughened PP. A question now raised by knowledgeable polymer chemists is whether Dow's new "pseudo-random" styrene/ethylene copolymers might compete with various styrenic block copolymer TP elastomers like Kraton D or G from Shell Chemical Co., Vector from Dow's new Dexco Polymers joint venture, or Sol T polymers from EniChem America. These sources note that the new Dow materials are made with Ziegler-Natta chemistry, which is usually a less expensive route than the anionic polymerization used to make current styrenic block copolymers.

Marketing questions will be answered soon because Dow expects to begin commercial production of the first elastomer/plastomer materials next year. "We'll be using a modified version of an existing continuous solution plant," says Michael Levinson, Dow's project manager of constrained-geometry catalyst resins. "But the market development will be in specific target areas. These won't be commodities."