More on properties of next-generation polyolefins.

Houston's biennial Polyolefins conference has grown from a local chapter gathering of the Society of Plastics Engineers into a world-class gathering of research chemists to discuss new plastics in the R&D pipeline. Polyolefins VIII last month drew record attendance of over 800 people to hear some 50 papers on new resins, new molding techniques and new additives among other topics. But above all, Polyolefins VIII was a barometer of excitement about new materials on the horizon.

The keynote paper, given by consultant Ken Sinclair of SRI International, Menlo Park, Calif., described some of the astonishing attributes of new-generation polyolefins made with "single-site" TABULAR DATA OMITTED metallocene catalysts. Sinclair estimated that over 60 companies are active in the race to commercialize metallocene-catalyst resins and identified over 580 distinct metallocene patents.

Sinclair discussed metallocene catalyst developments for PE from Exxon Chemical Co., Houston, and Dow Plastics, Midland, Mich., and metallocene catalysts for PP used by Hoechst AG in Germany and Fina SA in Belgium. Of the two papers given by Dow on its new "Insite" metallocene catalyst technology, one from senior research scientist Bill Knight shed more light on the phenomenon by which Dow simultaneously achieves narrow molecular-weight and comonomer distribution together with good processability. Knight described "formation of a long-chain branch in the final molecule" of the ethylene-octene polymer chain, which confers processability at the same time that the material's narrow MWD gives it greater strength and bubble stability.

Although metallocene-catalyzed PE has been much in the limelight lately, PP made with such catalysts is now starting to draw similar attention (see Technology Newsfocus). Metallocenes are being used to make both conventional isotactic PP and novel syndiotactic versions (see PT, March '92, p. 29).

PP FROM METALLOCENES

Syndiotactic and isotactic PP have very different molecular shapes. Isotactic PP has pendant methyl groups on only one side of the polymer backbone like teeth on a comb, while syndiotactic PP has methyl groups on alternating sides in a regular pattern. Isotactic PP made with metallocene catalysts is said to exhibit somewhat different properties from PP made with conventional high-activity catalysts.

One of syndiotactic PP's outstanding features is its high clarity. Sheet made with syndiotactic PP homopolymer or copolymer has one-fourth the haze level of sheet made with highly isotactic PP homopolymer or copolymer, though isotactic PP is stiffer.

At Polyolefins VIII, Sinclair of SRI described an intriguing and unexplained synergy found in a patent of Mitsui Toatsu in Japan, which is also developing metallocene catalysts for syndiotactic PP. When high-clarity syndiotactic PP homopolymer is blended with conventionally made PP homopolymer, the blend of 30% conventional PP with 70% syndiotactic is much clearer than either material alone. The blend shows as little as one-third as much haze as any currently commercial PP, even with the best nucleating agents. Sinclair notes that "clarity is one of the most sought-after properties for PP," and therefore this development is "bound to have a significant effect on the competitive position of PP against PET and PVC in packaging."

Syndiotactic PP is also not degraded by radiation sterilization--in fact, its properties may actually improve, whereas conventional PP degrades (as noted in another Mitsui Toatsu patent). Sinclair speculates that the phenomenon of improved properties is caused by "controlled unsaturation" in the PP carbon chain, which lets some polymer chains crosslink, reducing the effect of the chain breakage caused by radiation.

And unlike conventional PP, syndiotactic PP reportedly is not degraded during reactive extrusion processing, so Sinclair thinks syndiotactic polymers could significantly improve performance of grafted PP adhesives and high-performance mineral-reinforced PP compounds and alloys. The receptivity of syndiotactic PP molecules to grafting allows controlled addition of a range of other monomers.

Not all the attributes of metallocene PP are good, however. Its lower melting point is a problem for many applications--but not, of course, for heat-sealing layers. Syndiotactic PP tends to have 300-320 F melt temperatures vs. 320-330 for conventionally made PP. Isotactic PP made with metallocene catalyst, however, has extra stiffness--30% more than is achieved even with special high-crystallinity versions of isotactic PP. Sinclair notes that there is no explanation offered in the patent literature for this odd combination of low melt temperature with higher stiffness.

The high cost of metallocene catalysts ($1400-$2500/lb) may not pose a problem to their commercialization since the activity level of these catalysts is so high that they may add less than 1|cent~/lb to PP costs. Syndiotactic PP will cost in the area of 80|cents~-$1/lb, Sinclair expects. He also expects announcements very soon on full-scale commercial use of syndiotactic and isotactic PP from metallocene catalysts.

The 724-page book of technical papers from Polyolefins VIII is available for $100. Until May 1, contact Charles Shedd at H. Muehlstein & Co. in Houston. After May 1, contact SPE headquarters in Brookfield, Conn.

More papers on metallocene-made PP will be given at two upcoming conferences. Dr. Walter Spaleck of Hoechst will speak at MetCon '93 in Houston May 26-28, sponsored by Catalyst Consultants Inc., Spring House, Pa. And Dr. Edwar S. Shamshoum of Fina Oil & Chemical Co., Dallas, and Dr. Abbas Razavi of Fina Research in Belgium will speak on metallocene-catalyst PP at SPO '93 in Houston on Sept. 21-23. That conference is sponsored by Schotland Business Research Inc., Princeton, N.J.