K'98 Report: The Future of Metallocene Catalysts Less Than Certain
There has been no lack of enthusiastic publicity, and the future has been painted in particularly bright colors. Metallocene catalysts were among the star exhibits in the raw materials sector three years ago at K'95, offering exciting glimpses of their many qualities. Announced as a truly revolutionary development, these catalysts promised polyolefins, in particular, a prosperous future.
Yet the revolution has been somewhat derailed as the future of this new form of catalysis has yet to fully dawn. In fact, broad acceptance of these fascinating chemical facilitators is still being obstructed by high costs and heretofore unresolved processing problems. Thus their application has been rather limited to date and still remains far removed from the consumption figures forecast at the beginning of the 1990s. In the medium term, too, the "catalytic effect" of the metallocene alternative is likely to produce no more than a rather damped reaction. The traditional Ziegler/Natta solutions are therefore likely to continue to be preeminent in the foreseeable future. This does not necessarily mean, however, that the outlook for metallocenes and their derivative polymers is bleak. But the fact remains that the euphoria associated with them a few years ago has subsided appreciably.
In October 1995 at K'95, Düsseldorf's 13th International Trade Fair Plastics + Rubber, metallocene catalysts were still regarded as an exciting — albeit largely unproven — prospect for the future; their commercial exploitation had only just begun. The range of plastics based on this polymerization aid was still relatively limited. Nevertheless, the experience gathered up to that time brought about considerable interest, if not visions of a brave new plastics world and hope for even more dynamic growth. Because, unlike catalysts such as those developed during the mid 1950s by Karl Ziegler and and his Italian colleague Giulio Natta, metallocenes provided the prospect of hitherto unimaginable progress — and profits.
Owing to their origin, conventional Ziegler/Natta catalysts — which are inorganic compounds of transition metals — are difficult to analyze and, therefore, to modify. By contrast, the structure of the new metallocene competitors can be analyzed in every detail. This would appear to indicate that they lend themselves to effective experimentation in the manner of building blocks in a construction kit. The required properties can thus be defined with considerable accuracy and efficacy.
Plastics Leave Steel in Their Wake
When Hermann Staudinger coined the term "macromolecule" in 1920, he laid the foundation for the development of a completely new, versatile class of materials: namely plastics. From around the 1950s, these have been increasingly replacing conventional materials such as metal, glass, wood and even paper. Thanks to their unusual combinations of properties, they have also opened up the way to new technologies. Large-scale production of plastics also began during the 1950s, and staggering growth followed. While in 1960, world plastics production was in the range of five to six million tons, in the interim 40 years output levels have risen to a staggering 120 million tons. Plastics have thus long overtaken global steel production in volume terms.
Around half the world's plastics output is accounted for by the two polymer standards: polyethylene (PE) and polypropylene (PP), generically classified as polyolefins. According to Rolf Mühlhaupt, professor at Freiburg University in Germany and head of its Institute for Macromolecular Chemistry, the basis for the rapid growth of these two materials in particular is derived in large part from innovations that have occurred in the field of catalytic technology. Over the last two decades, these developments have contributed significantly to increasing the performance of these two materials. Today, using conventional catalysts in large-scale production processes, activities can be performed which were previously regarded as the exclusive domain of enzymatic reactions in the biosphere. For example, 1,020 kg of ethylene monomer, the starting stock for PE, will produce 1,000 kg of final product. For the same reason, claims Mühlhaupt, a modern polyolefin plant can now be safely regarded as exceptionally environmentally friendly.
However, innovative catalysts excel not only in terms of their high performance. They also open up the way to completely new polyolefin materials through, for example, effective control of the molecular weight distributions, the microstructure and the polymer morphology — in other words the molecular structure of the material. And while, until the end of the 1970s, there was an unwritten law that, for example, isotactic PP could only be obtained using the catalysts discovered by Ziegler and Natta, this no longer applies. This is because with metallocenes such as those introduced for the first time at the beginning of the 1980s by H. H. Brintzinger, professor at Konstanz University, it also became possible to achieve outstanding results in the synthesis of isotactic PP in combination with methyl aluminoxane as the activator. It was also found that every conceivable PP structure could be obtained by varying the specific metallocene structure. Moreover, according to Professor Mühlhaupt, extremely narrow molecular weight distributions can be obtained with this technology.
A Question of Price
Werner Schöne from BASF AG, Ludwigshafen/Germany, is also convinced that metallocenes are going to usher in a new and exciting future for PP. However, in his opinion, intensive efforts will be necessary in the next few years in order to develop those products and applications for which metallocene PP will be able to assume the position of technical and economic market leader.
Compared with metallocene PE, the commercialization of PP using the same catalyst base has only just begun. It is Schöne's belief that much research work is still needed in order to enable metallocene-polymerized PP to become widely established in the marketplace. It would appear, therefore, that these promising developments are still only in the starting phase.
At K'95, the prognosis was quite different — or at least a good deal more optimistic. Even at the third Metallocene Congress, held last year in Düsseldorf, confidence abounded to such an extent that highly optimistic forecasts were made. While, in the course of this conference metallocene catalysis was celebrated as a "quantum leap in the production of polyolefins," there were some words of caution — to the extent that metallocenes were called "highly sensitive chemical compounds," which meant that manufacturers needed to have a great deal of skill and expertise. In fact, many process stages and a large slice of experience are necessary in order to succeed in producing metallocenes properly. This, of course, is then reflected in the high price that industry has to pay for them. However, this should have been immeasurably offset by the fact that metallocenes, — with all their impressive properties — could be easily incorporated into existing processes.
Not Suitable for All Applications
A study carried out by the international market research company Frost & Sullivan of Frankfurt into metallocene technology and its place in the European market came to the conclusion that there are two prominent problems that need attention: not only is the manufacture of the new polymers expensive, their further processing into final products — e.g., in the extrusion of films — is often beset with problems. Frost & Sullivan assessed that metallocene plastics are not as suitable for all types of application as their traditional competitor materials. Consequently, there were likely to be few changes in relation to the manufacture of low-density PE, for instance. David Cornforth, market researcher within the firm, also points to the fact that many companies "have simply ignored the hullabaloo about metallocenes which has appeared recently in the technical press." Rather, they have continued to concentrate in their research and development activities on modern Ziegler/Natta polymerization techniques.
Towards the end of 1996, on the other hand, chemist Andreas Mayer presented a paper at a VDI (German Association of Engineers) conference in Wiesbaden dealing with the subject of metallocene polyolefins (m-polyolefins). In the view of Mayer, an employee at the European headquarters of the U.S. group Dow Chemical in Horgen, Switzerland, the use of metallocene catalysts combined with suitable manufacturing processes is destined to result in a larger range of ethylene-based plastics becoming available. The production of homogeneous raw materials and the expansion of the density range which can be applied under large-scale manufacturing conditions, will, in Mayer's judgement, lead to improved and perhaps even completely new levels of performance in many fields of application.
Initial practical experience had already shown that even relatively contradictory performance requirements could be reconciled within a previously unknown framework of parameters. These advantages mean that the potential outlook for such m-polyolefins is extremely promising indeed. Predictions from Dow indicate that the production capacity for PE raw materials manufactured on the basis of metallocene catalysis will rise from virtually zero in 1995 to 3.5 million tons by the turn of the century.
Searching for the Optimum
There is no denying that the large-scale breakthrough of innovative metallocene systems based on transition metal compounds involving, for example, zirconium or titanium occurred relatively quickly. As a primary feature, such catalyst molecules contain just a single active center. In the relevant literature, they are therefore described as "single-site" catalysts. These facilitate the production of particularly homogeneous molecules and polymers. The construction of corresponding production facilities for the polymerization of ethylene polymers then quickly followed, with activity in this sector being particularly intensive in the United States. By 1992, commercial polymers were already available from Exxon Chemical, these being manufactured on a metallocene basis using the Exxpol process.
In spite of these rapid initial successes and the commercial application of the process, the research conducted by Frost & Sullivan indicates that there are still a large number of questions unanswered. In addition to the price, which is unlikely to come down until volume sales increase significantly to secure revenue, improving the properties of the polymers produced is a main concern voiced by the entire potential market for metallocene technology. Dow and BP Chemicals have announced the development of a metallocene plastic which will reportedly offer the same excellent processing properties of conventional LDPE. More than that, even films extruded from this material will, it is claimed, be as ideal for manufacturing processes as the conventional LDPE films.
Perhaps K'98, occurring October 22-29 in Düsseldorf, will provide more information in this matter. This year's International Trade Fair Plastics + Rubber will also be offering an abundance of indicators as to where the journey into the elusive metallocene future is heading. On display at the exhibition from the raw materials side will be a number of innovations that can credit their existence to modern metallocene catalysis. One example is Metocene X50081 from Targor GmbH, a joint venture of two chemical giants in the polypropylene field, BASF and Hoechst. This product represents the first metallocene PP to be commercially produced in Europe. The material is particularly suitable for the injection molding of thin-walled, highly transparent components for packaging applications. In this sector, it is likely to serve as an alternative to polystyrene (PS). The new metallocene material offers particularly high fracture resistance combined with a comparable level of contact transparency.
According to Hans-Jürgen Kablitz, head of the injection molding division at Targor, the development and manufacture of this new metallocene PP have also provided evidence of how synergy effects arising from the cooperation between these two companies can be utilized in the PP sector. Hoechst is able to provide expert knowledge and worldwide patent protection in respect of its metallocene catalysts, while BASF has successfully developed catalysts for its own gas phase process. BASF is also the owner of a major pilot and semi-commercial-scale plant to facilitate the product trial and introduction phases.
300 Patents Already in Existence by 1995
A number of alliances have already been formed, such as the one between Hoechst and Exxon, or are likely to be formed in the potentially lucrative field of metallocene technology. There is a cooperation agreement between Hoechst and U.S.-based Fina involving patent exchange, particularly in the field of PE catalysis. Exxon has also teamed up with the U.S. group Union Carbide, forming with them a subsidiary to market the Unipol gas phase process, Exxpol metallocene technology and the corresponding metallocene catalysts on a worldwide scale.
The years to come should bring much more collaborative activity. Therefore it is quite possible that the metallocene alternative will be spared relegation to the status of mere footnote in polymer evolution. Certainly, the level of activity demonstrated by those companies involved in the development of metallocene technology would appear to indicate that there is still a good deal of promise. By 1995, nearly 300 patents had already been granted in matters regarding metallocene. Of this figure, Hoechst, Exxon and the Japanese company Chemietrust Mitsui alone accounted for two-thirds of the applications.
However, in the final analysis, what matters most is which technology makes that vital breakthrough in the marketplace; and also how quickly and at what price the associated products will become available. In the meantime, the revolution — once so close — will remain just over the horizon.
Editor's Note: This article was provided by the K'98 Press Office. For more information: Eva Rugenstein, Tel:++49/211/4560-240 or Anne Meerboth-Maltz, Tel: 312-781-5180.