A perspective on some recent developments in metallocene catalysts.

The rubber industry has a long history, which can be traced back through a timeline that includes many scientific advances. Examples of these advances can be divided into many categories, including developments pertaining to polymers and their manufacture, as well as developments of other compounding ingredients, and of course, the art of rubber compounding itself. The focus here will be on the first area -- scientific advances that change and advance the science of polymerization.

Polymers are, by definition, long molecules comprised of repeating units. In the production of polymers, including those having elastomeric properties, a reaction is initiated and sustained to induce the building block molecules, or monomers, into forming a chain. This reaction is sustained by catalysts -- chemical species which cause the building block molecules to be changed from their stable state into a more reactive state, to join these energized molecules together, and to continue this reaction using a supply of fresh monomers to form molecular chains. The actual choice of catalyst used in a specific polymerization is determined by the nature of the monomers used, and the structure desired in the resultant polymer. From a practical standpoint, the catalyst system must permit a controlled reaction, and function within a process which can be operated both safely and economically.

The most significant achievement in the commercial polyolefin industry was the discovery, development and implementation of a group of catalysts known as Zeigler-Natta catalysts. These catalysts were the first to effect the polymerization of ethylene and also of propylene, two widely available and economical monomers (ref. 1). These basic catalyst systems were used to build the polyolefin industries that exist today, including polyethylene in all of its forms (HDPE, LLDPE, VLDPE, ULDPE) and syndiotactic polypropylene. In the rubber industry, the greatest impact of Zeigler-Natta catalysts has been in the commercial growth of EPM/EPDM (refs. 2 and 3). This class of elastomer combines both ethylene and propylene monomers, and in many cases a third monomer for the sake of adding unsaturation to the polymer chain, and thus allowing sulfur vulcanization. The combination of ethylene and propylene produces an amorphous, elastomeric polymer, which has minimal crystallinity when compared to polymers produced using ethylene or propylene alone. The first commercial appearance of EPDM was in the early 1960s; current industry capacity for this polymer is 1.5 billion pounds per year (ref. 4).

In more recent years, a second class of catalysts has been making an increasing impact in the polyolefin world, the class of metallocene catalysts. Initially, the development activity for this class of catalysts has been focused on semicrystalline thermoplastic materials, particularly polyethylene. Commercial examples where metallocene catalysts are being used include polymers such as Affinity, based on constrained geometry catalyst technology from Dow Chemical, and Exact from Exxon. These polymers have offered several improvements over traditional polyethylene, which is made using multi-site Zeigler-Natta catalysts. The most compelling improvement is the ability to combine softness and toughness in the same polymer. In addition, these specific metallocene catalysts have proven to be capable of polymerizing a wide choice of monomers and offering a greater control over molecular structure. Others have been used to produce PE homo-, co- and tar-polymers, isotactic, syndiotactic and atactic PP, syndiotactic polystyrene and cyclic olefin copolymers (ref. 5). An excellent review of the development and intricacies of metallocene catalysts for thermoplastic polyolefins is provided by John Ewen (ref. 1). Obviously, these catalyst systems have become the major force for improvements in the thermoplastics industry. The extent of their impact on the rubber industry is yet to be seen and can be actively debated, but there certainly will be an impact.

Metallocene catalyst systems

The term metallocene has come to be regarded as a generic term which refers to a large class of materials. The use of these molecules as reactive catalysts was first explored in the 1950s. On a very basic level, metallocenes are molecules that contain both an organic and a metallic component. By definition, the organic component can be any organic structure, but the most useful structures have been found to contain ringed structures, typically a cyclopentadienyl group. One example of a metallocene structure is the iron sandwich, where two organic rings align on opposite sides of a molecule of iron. Another example would be a half sandwich structure where only one organic ring is found paired with a metal. In the earliest studies, the metallic component was iron, but the most extensive use to date has been with a variety of transition metals.

One proprietary subset of metallocene catalysts has become known as constrained geometry catalysts. This term has been used to refer to a class of catalysts that has been specifically designed to have a highly accessible or open, single reactive site. A representation of the specific constrained geometry catalyst is shown in figure 1 (ref. 6).

[Figure 1 ILLUSTRATION OMITTED]

The reactive site has been enhanced through the use of a bridging structure, which is used to strictly define and highlight the reactive site. By using this engineered geometric structure, the reactivity of the catalyst to the chosen monomers can be enhanced, and its selectivity to only those monomers is reinforced. The implications of this structure for the ensuing polymers are numerous. This enhanced structure increases reactivity to all of the specific monomers consumed in the reaction. The reactivity is very predictable, and therefore the reaction itself is very predictable. The ability to design the structure of these catalysts and the flexibility in the catalyst molecule allows the polymerization of many monomers which cannot be produced using Zeigler-Natta catalysts on a commercial scale. For instance, constrained geometry catalysts have been used to produce copolymers of ethylene and octene. The addition of octene to a chain of polyethylene provides increased flexibility and softness in the polymer, as compared to previously available grades of polyethylene. Other monomer combinations are only now being explored.

As with all polymerization catalysts, a mixture of ingredients is required, as opposed to a single component. The main catalyst is the central ingredient, but activators are also employed both to activate the catalyst molecule and to maintain the optimum conditions for the reaction to occur. During the initial exploratory work on the use of metallocenes as catalysts, the catalysts, efficiencies of the order necessary to make them practical were not obtained. It was the discovery of methylaluminoxane (MAO) as a co-catalyst by Walter Kaminsky at the University of Hamburg in 1980 which was the critical breakthrough, and which increased the reactivities of these catalysts systems to a level approaching commercial viability (ref. 7). Since that discovery, the activities around the refinements and engineering of metallocene catalysts systems have been furious, leading to the systems responsible for the dramatic achievements in the thermoplastic arena.

There are some parallels between metallocene systems and the Zeigler-Natta catalysts which have evolved during use in the numerous polyolefin commercial processes developed throughout the 1960s, '70s and '80s. Both systems contain a transitional metal component, which is typically based on vanadium or titanium. The Zeigler-Natta catalysts are classified as either supported or non-supported. A supported catalyst needs to be placed on a more efficient structure in order to be dispersed into the reaction medium. A non-supported catalyst is soluble in the reaction medium itself and can be dispersed on its own as a stable structure; therefore it does not require a support. When the catalyst requires a supporting structure, the actual support material can be either inert or can actually participate in the catalyst chemistry. An inert material, such as silica, simply provides a platform for the catalyst. A participating support, such as magnesium chloride, will form a complex with the catalysts. Metallocene catalysts are non-supported.

Zeigler-Natta catalyst systems can be single site or multiple site. This distinction refers to whether the catalyst complex contains a single point within itself that is the active catalyst site, or whether each complex contains more than one active site that will promote polymerization. A supported catalyst which employs a participating support will be a multiple site catalyst complex. In comparison, metallocene catalysts are single site, soluble systems.

The catalyst systems, whether Zeigler-Natta or metallocene, which are used for polyethylene are different from those used for polypropylene, which are in turn different from the one used for EPDM. Also, each manufacturer uses a different system based on their individual process and the nature of the polymers that they are targeting. For example, current Zeigler-Natta catalysts for the polymerization of EPDM are based on Group I-III organometallic (alkyl) compounds and a group IV-VII transition-metal compound, usually a chloride. Vanadium systems, based on [VCl.sub.4], [VOCl.sub.3] or other derivatives are the most common. Literature reports of efficiencies between 800 and 8,000 g polymer per g vanadium possibly understate the latest developments with vanadium systems; however, it is generally accepted that significantly higher polymer yields are achieved with titanium (ref. 8). The constrained geometry catalyst used to polymerize EPDM has demonstrated efficiencies on the order of 1 x [10.sup.6] g of polymer produced per gram of metal, the reactive site on the catalyst molecule.

Commercial application of metallocene catalysts to EPDM

The first elastomeric material produced using a constrained geometry catalyst system offered to the rubber industry was ethylene-octane copolymer, available under the tradename of Engage from DuPont Dow Elastomers. Although these polymers cannot be sulfur cured, they do provide a flexibility and softness not associated with traditional polyolefins. The use of the octene monomer also provides an improved high temperature heat resistance over its closest cousin in the rubber industry, EPM. The use of this product in a crosslinked part requires peroxide curing.

The second application of this catalyst system, which represents the most significant and direct comparison of elastomeric polymers produced using this new technology versus commercial elastomers produced using Zeigler-Natta systems, occurs in the realm of EPDM. The commercial history of EPDM extends over the past 35 years. In the early '60s, the first large scale production of EPDM elastomer was undertaken simultaneously by Exxon, Enichem, DuPont and Uniroyal. Later, these manufacturers were joined by DSM, JSR, Mitsui and Bayer. Several of the manufacturing facilities have changed ownership over the years.

Since the earliest facilities were started up, the impact of EPDM has grown across a broad range of applications. Interestingly, no single feature of EPDM is responsible for its emergence as a major component of the worldwide elastomer market, except perhaps its value in use. EPDM has shown, on balance, the highest performance/cost ratios among commercial elastomers, and it has been referred to as the best value for properties delivered of any elastomeric product. This balance refers to, for example, its outstanding resistance to aging, good mechanical properties, high extensibility and an ease of processing, all of which contribute to minimize costs for the compounder and processor. The polymer is particularly valued by the largest and perhaps most demanding market sector, the automotive industry. In addition to a rapidly growing use in automotive, further growth in construction and other sectors is projected, and by 1999, EPDM consumption is expected to become the largest of all elastomers in non-tire applications, overtaking natural rubber (ref. 9).

There are currently 14 major EPDM polymerization facilities globally, all based on Zeigler-Natta catalyst systems. The first application of metallocene chemistry to a full product line of crosslinkable EPDM polymers is being offered by DuPont Dow Elastomers, and is called Nordel IP.

Impact of metallocene catalysts on EPDM

The characteristics of metallocene catalyst systems, described above, have a number of implications for the polymer produced using such a system. First of all, the steady and predictable nature of the catalysts allows an in-depth understanding of the polymerization reaction that has not traditionally been possible with Zeigler-Natta type catalysts. This understanding allows a predictable modeling of the reaction kinetics, which can in turn be translated into a mathematical model of the reaction itself. Such mathematical models are based on reaction kinetics. The models predict the impact of various reaction parameters on the polymerization. They are used to design the reaction conditions necessary to produce the desired polymer, i.e. polymer composition and rheology. This approach to understanding the polymerization reaction is a contrast to the more empirical approach typically used in Zeigler-Natta systems. Such an empirical approach tends to require a greater level of trial and error experimentation during development projects, as well as greater potential for variation in polymer properties during routine production. The control chart data presented in figure 2 show the variation in three key polymer variables during a routine production run in a developmental facility. These three variables are Mooney viscosity, wt. % ethylene and wt. % ENB. The control offered by the constrained geometry catalyst system will result in lesser variation of these parameters during commercial production, i.e., a more consistent performing polymer is produced.

[Figure 2 ILLUSTRATION OMITTED]

Secondly, the constrained geometry catalyst has a much greater reactivity to all three of the monomers used in the production of EPDM than do Zeigler-Natta catalyst systems. The implications of this are three-fold. Because of this greater reactivity, a much lower amount of catalyst is required to induce and maintain polymerization. In a typical Zeigler-Natta process, the spent catalysts and aluminum-based activator components are entrained in the system at high enough concentrations that they need to be removed from the polymer after the reaction is complete. This is achieved in a wash step, where the polymer is washed in a water system in order to remove the catalyst and activator components. Thus, water is introduced into the polymer, and although the vast majority is then removed during a drying step, residual internal moisture still remains. Also, this wash step does not remove all traces of the catalysts; a measurable amount of residual catalyst remains. These two contaminants to the EPDM polymer, namely moisture and catalyst residue, are typically noted in the specifications offered by the polymer producer, as residual moisture and residual metal content.

In contrast, the constrained geometry catalyst system is highly reactive, and present at a much lower concentration during polymerization. Since it has this enhanced efficiency, on the order of 1 x [10.sup.6] grams of polymer per gram of active metal, the concentration of spent catalyst in the final product is so low that a wash step is not required after the polymerization has been completed. Therefore, no internal moisture is introduced into the polymer. Also, despite the lack of such a wash step, the amount of residual catalyst found in the polymer is lower than the amount found in nearly all Zeigler-Natta produced polymers.

The lack of a water wash step has additional implications. If a water wash operation is not included in the production process, then a drying step is also not needed. During a traditional drying process, the polymer is subjected to elevated temperatures, during which the vast majority of the water is removed from the system. In addition, the polymer is exposed to oxygen and subjected to high shear. This drying step is known to be the source of some gel formation, particularly with high termonomer content EPDMs. Polymers made using a constrained geometry catalyst are not subjected to a drying operation.

The amount of metallic residue has a direct impact on the color of the polymer product. When using Zeigler-Natta catalysts, the vanadium residue often causes a yellowish or greenish tint to the polymer. Since there is a much lower level of metal residue in a polymer produced using the constrained geometry catalyst, the polymer has a much whiter and cleaner appearance. The measure of yellowness index of polymers produced using the constrained geometry catalyst versus those produced using traditional catalysts is shown in figure 3.

[Figure 3 ILLUSTRATION OMITTED]

Compounded properties of EPDM produced using a constrained geometry catalyst

At this point in time, the constrained geometry catalyst has been shown to produce EPDM polymers which can effectively perform in the applications where EPDM has been proven effective (ref. 10). This proprietary metallocene catalyst produces EPDM polymer having the range of Mooney viscosities and the range of compositional variables (ENB, ethylene and propylene contents) that have become widely accepted among consumers of EPDM. This benchmarking of the polymer produced using a new catalyst system against established EPDM polymer is a necessary first step to establish a baseline of performance.

Comparisons of the properties of compounded EPDM as produced using the constrained geometry catalyst versus commercial polymer produced in traditional processes are provided in figure 4.

[Figure 4 ILLUSTRATION OMITTED]

Metallocene catalysts for new elastomer systems

As mentioned above, metallocene catalysts were initially developed, explored and commercialized for the production of semicrystalline polyolefins materials (ref. 11) (i.e. polyethylene and polypropylene), as these polymers have an enormous commercial stature.

However, it was quickly noted that for traditional non-tactic ethylene, [Alpha]-olefin copolymers (i.e. LLDPE, ULDE), the properties of these metallocene derived copolymers not only possessed unique properties when compared to conventional thermoplastic copolymers derived from supported Zeigler-Natta catalyst systems (ref. 12), but the catalysts also exhibited enhanced reactivity towards [Alpha]-olefins that allowed the commercial production of high [Alpha]-olefin content copolymers that exhibited elastomeric properties (ref. 13). The initial commercial entries into this market were ethylene-octane copolymers produced by Dow Chemical and ethylene-butane and/or hexene based copolymers produced by Exxon (ref. 14).

Another important aspect of these new metallocene systems, when compared to conventional Zeigler-Natta supported systems, is their ability to efficiently incorporate non-traditional [Alpha]-olefins into the copolymer structure (ref. 15). These nontraditional monomers include cyclo-olefins and styrenic derivatives. Of the cyclo-olefins, norbornene derivatives offer the ability to produce a wide range of polymeric materials from glassy copolymers with ethylene to elastomeric terpolymers with ethylene and [Alpha]-olefins (ref. 16). The enhanced reactivity of metallocene catalysts towards ethylidene norbornene, over that observed with traditional Zeigler-Natta supported catalyst systems, allowed the commercialization of EPDM based on proprietary metallocene technology (refs. 17-19).

Another version of these metallocene systems can be used to produce ethylene-styrene random copolymer, which has not been possible with conventional Zeigler-Natta systems (ref. 20). Depending upon the styrene content, the random copolymer behavior can span the thermoplastic, elastomeric and glassy material ranges (refs. 21 and 22). These elastomeric ethylene-styrene copolymers offer a potential option to conventional styrene block copolymers in selected applications.

Still another aspect of metallocene derived elastomers is the unique polypropylene based thermoplastic elastomers produced utilizing an oscillating metallocene catalyst system (refs. 23 and 24). This unique metallocene system allows tailoring of the elastomeric properties of these polypropylenes and offers a glimpse of new propylene based elastomers possible in the future.

Finally, the renewed focus that metallocene catalyst systems brought to the thermoplastic and elastomeric polyolefins arena has opened the door to other new non-metallocene polymerization systems (ref. 25) that offer further insights into what is possible in the elastomeric field.

Summary

The chemistry of metallocene molecules as used for polymerization is currently being developed and refined at amazing rates. Their development has revolutionized the thermoplastics industry. The impact of these new discoveries on the rubber industry is only starting to be realized. Initial indications, based on experience in applying one particular catalyst to EPDM, are that polymers can be made that supply the same end use performance that has been established for EPDM produced using Zeigler-Natta catalysts systems, namely, excellent weathering, heat and water resistance, easy processing materials and high value to cost ratio. In addition, however, the constrained geometry catalyst produces polymers exhibiting significant improvements in product quality. Polymers having higher levels of quality will also impact the production and product quality of a rubber goods manufacturer. These improvements include lower levels of contaminants, such as moisture, catalyst residues (metals) and gel. A polymer having lower levels of contamination will directly provide a cured rubber part having lower levels of defects. Examples include a lower gel level impacting the surface qualities, or lower metal ion levels affecting electrical properties. In addition, the variability in the polymer's rheological and compositional characteristics are minimized through the greater reproducibility provided by the proprietary metallocene catalyst and subsequent control of the polymerization reaction. Greater consistency in polymer properties in turn leads to a compound which has more predictable flow behavior and cure characteristics, and performance that is consistent from one batch to another.

The application of such metallocene catalysts to polymers of interest to the rubber industry has just started. As a greater understanding of these catalysts is achieved, new metallocene species will be applied to other conventional elastomeric materials, as well as to polymerize monomers into combinations that have not been available before. The possibilities are numerous and exciting for the rubber industry!

Acknowledgements

"Advances in TPE polymer blend" is based on a paper given at the October, 1996 Rubber Division meeting.

"Styrenic TPEs -- the pathway to invention" is based on a paper given at the October, 1996 Rubber Division meeting.

"Effect of precipitated silicas in truck tire treads" is based on a paper given at the May, 1997 Rubber Division meeting.

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