Issue Date: October 18, 2010
Metallocenes Rise Again
A decade ago, every sizable polymer producer had an aggressive program under way to develop metallocene and other single-site catalysts for polyolefins. The precision these new catalysts permit in polymerization gave the polymers never-before-seen strength and clarity. Industry insiders expected the new plastics to take the polymer world by storm, even rendering most existing forms of polyethylene and polypropylene obsolete.
Things didn’t quite work out according to plan. The hype died down, almost as if metallocenes were another late-1990s fad in the mold of Enron or the Y2K bug.
It turned out that the advantages came with trade-offs. Metallocene catalysts were expensive, and resin producers demanded a hefty premium for the products. Converters turning the resins into finished products expected additional value to justify the expense. Metallocene resins such as linear low-density polyethylene (LLDPE) for film were harder to process than traditional Ziegler-Natta catalyst-based LLDPE or low-density polyethylene (LDPE). Many mom-and-pop resin converters decided the benefits weren’t worth it.
But metallocene-based resins didn’t disappear. The hype may have died down, but behind the scenes, chemists and engineers were hard at work establishing a toehold for them in the market. Today, millions of tons of metallocene resins are made every year, and they enjoy more robust growth than traditional plastics.
With advances now being made at a sure pace and more new developments on the horizon, resin producers are gaining confidence and again talking about a bright future for the catalysts. As these firms surmount longtime technical hurdles, new activator technologies are making metallocene catalysts cheaper to use.
Metallocene catalysts are metal complexes with two cyclopentadienyl (Cp) or substituted Cp groups. Standard Ziegler-Natta catalysts, in contrast, are typically built from titanium and chlorine.
Chemists have been aware of metallocene structures since the 1950s. But their commercial utility didn’t become apparent until 1977, when Walter Kaminsky at the University of Hamburg demonstrated that metallocenes, with the help of a methyl aluminoxane cocatalyst, can be useful in polymerizing olefins. The active site, sandwiched between the constrained geometries of the Cp structures, had the potential to knit together olefin monomers with pinpoint accuracy.
Dozens of chemical companies raced to develop an ever-increasing number of catalyst structures, yielding an enormous stable of new materials. The foundational R&D work reached a crescendo in 2002. According to Science IP, the patent search arm of Chemical Abstracts Service, the number of patents published for metallocene polyolefin catalysts peaked that year at 387 globally. By 2009, the number had shrunk to 236.
A couple of practical reasons explain the decline, says Ken Sinclair, principal of Franklinton, N.C.-based STA Research. One is the economic downturn at the beginning of the 2000s that forced companies to pull back on R&D. Another is consolidation among the firms doing the research: Dow Chemical purchased Union Carbide, Exxon merged with Mobil, and Phillips and Chevron formed the Chevron Phillips Chemical joint venture. “Instead of double the innovation, you were left with only one of these companies continuing,” he says.
Still, by 2004, there were more than enough new resins on the market that the industry could turn to application development. “The fabricators and the processors started dealing with qualifying these resins,” says John J. Murphy, president of Catalyst Group Resources, a consulting firm in Spring House, Pa. “They were determining whether the resins were in fact necessary for the applications and the degree to which they should be substituted for traditional Ziegler-Natta-based materials.”
Metallocene resins have made respectable headway since then. Some 5 million tons of metallocene LLDPE was consumed in 2009, nearly 25% of the global LLDPE market, according to Murphy. He says the annual growth rate ranged between 10 and 15% from 2000 to 2005 and has settled at 7 to 10% since, largely owing to the recession. This is a far cry from the 25% growth rates that resin producers had once hoped for, but it’s not bad compared with the rest of the plastics industry. “Growth rates for metallocene resins are high single digits as opposed to traditional resins, which have witnessed reduced growth patterns over the last couple of years,” Murphy says.
Metallocene catalyst successes have been achieved largely in LLDPE. High-density polyethylene has only been targeted more recently; plastomers and elastomers saw gains early in the era. Together these polymers amount to another 3 million tons of metallocene resin demand, Murphy says.
Even in the LLDPE market, establishing metallocene resins has been a hard-fought battle. The first that were launched in the mid-1990s had strong properties to offer converters, says Steven F. Stanley, commercial vice president at Univation Technologies, a joint venture between ExxonMobil and Dow that licenses technology and sells metallocene and conventional catalysts for the Unipol gas-phase polyethylene process.
For resins made, for example, with the company’s XCAT HP catalyst these properties include clarity and dart impact strength—an important attribute for polyethylene films that is tested by dropping a dart on trash bags, stretch wrap, and other films. “The big improvement in the dart impact over conventional resins is one of the clear areas where metallocene resins have delivered on what was hyped,” Stanley says.
However, the first metallocene LLDPE resins were more difficult to process in traditional film-blowing equipment than Ziegler-Natta-based LLDPE. This was because the narrow molecular-weight distribution of the metallocene-based resins affected the rheology, or flow properties, of the resin during processing. “For existing converter process equipment, you couldn’t get the same throughput with early-generation metallocene as you could with conventional polyethylene,” Stanley says.
“It was and is very difficult to process,” adds STA Research’s Sinclair. “That meant the market access was essentially confined to those processors that were the best equipped and that were the most sophisticated.” Thousands of other processors, he says, were forced to blend LDPE with the metallocene-based LLDPE. LDPE is more expensive than Ziegler-Natta LLDPE and using it dilutes the metallocene LLDPE’s exceptional properties.
F. Gregory Stakem, vice president of technology at Univation, has been with the company since it was formed in the late-1990s. Before then, he worked for Union Carbide’s Unipol licensing arm. “In the beginning, people thought metallocenes would penetrate a lot quicker than they did,” he recalls. He compares the situation to the 1980s, when LLDPE, then in its infancy, would supposedly spell the end of LDPE made in high-pressure reactors. “LDPE is bumping along with 2% growth rates every year.”
However, the plastics industry has been chipping away at metallocene-resin-processing problems, Stakem says. Converters have modernized their equipment, and resin companies have come out with new products and developed new applications. Machinery for multilayer film has taken off in recent years. This technology allows metallocene resins to be layered with cheaper, conventional LLDPE. The thinner gauge of the film is also easier to process. “Really, with most modern equipment that is sold nowadays, processibility is not as much of an issue as it was back in the beginning,” he says.
Resin producers have also pioneered new uses, Stakem points out. Large companies helped blaze a trail for other smaller resin producers by defining new applications such as heavy-duty shipping bags and agricultural film and providing commercial quantities of resins to serve them. Smaller polymer makers are more likely to license metallocene catalysts than they were 10 years ago, when it was a much riskier proposition. “There is nothing that helps your sale better than a customer being able to buy commercial metallocene film and resin somewhere and look at them,” Stakem says.
And chemical companies have been introducing new metallocene resins that directly address the old processing issues. In 2008, ExxonMobil introduced its Enable line of metallocene polyethylenes, which are meant to do the work of LDPE/metallocene LLDPE blends in a single resin. The company boasts that Enable can increase output by 20% versus the older blends. And by adding toughness, the resin allows converters to reduce the film’s thickness, referred to as down-gauging.
In an interview with C&EN in March, Theodore J. Wojnar Jr., senior vice president of ExxonMobil Chemical, intimated that the new product line was gaining traction in the marketplace, even through the economic downturn. “Metallocene products have continued to grow throughout this entire time period just because of the value they bring and the increased strength and the ability to down-gauge,” he said. “They are growing much faster than overall polyethylene is growing.”
Sinclair agrees that Enable has been a winning proposition. “This is stuff that is bought by thousands of simple converters because they can use it in the same machines they always had and they can down-gauge by 25%,” he says.
The easy-process approach is hardly unique to ExxonMobil. Univation has been offering next-generation catalysts for easy-processing metallocene LLDPE for years. “At the end of the day, converters can get more throughput out of it than they could with first-generation metallocene resins,” Stanley says. “They are reducing or eliminating any kind of rate penalty that may have existed for metallocenes.”
Last year, Total Petrochemicals introduced a new generation of metallocene polyethylenes and polypropylenes under the Lumicene tradename. The product line includes next-generation medium-density polyethylene (MDPE) grades for easy processing. “With our technology, the polyethylene gives you an improvement in rheology,” says Mark Miller, market manager for flexible-packaging applications at Total Petrochemicals USA. “It is something our customers appreciate, as the resins offer a combination of clarity, stiffness, and strength.”
In contrast to their success in polyethylene, metallocene catalysts have struggled to capture much of the polypropylene market. About 1.4 million metric tons of metallocene polypropylene was purchased last year, according to Catalyst Group Resources’ Murphy, representing only about 2 to 3% of the world’s polypropylene demand.
Murphy says metallocene catalysts don’t have as much to offer the polypropylene world. “I don’t think there is as big a gap in polypropylene between needs and the ability to meet those needs as there is with linear low,” he says.
In addition, STA Research’s Sinclair says metallocene catalysts had more difficulty overcoming their limitations with polypropylene. Propylene polymerization yields a lot of by-product hydrogen when it is catalyzed with metallocenes. Hydrogen terminates the polymerization prematurely, leading to low-molecular-weight polymers with viscosity that is too low for many applications. “That meant that fiber and high-flow injection molding were about the only things that you could do with it,” he says.
However, Mike Musgrave, market manager for molded applications at Total Petrochemicals USA, says his firm’s new Lumicene metallocene polypropylenes have lower melt flow than earlier metallocene polypropylenes. This advance, he says, has helped open up blow molding and thermoforming applications, such as disposable drinking cups to metallocene polypropylene. In that market, metallocene polypropylene has clarity and gloss advantages that allow polypropylene to compete with polyethylene terephthalate.
Developers of Ziegler-Natta catalysts aren’t taking the advances lying down and are challenging metallocene polypropylene with their own new technologies. In the mid-2000s, for example, Dow set to work on improved electron donors for Ziegler-Natta catalysts. Donors are typically silane-based compounds that help control the active site. Using high-throughput experimentation, the company screened thousands of molecules to develop its Advanced Donor Technology.
The new donors increased the activity of the site, which improves the cost-effectiveness of the catalyst. The new donors also enhance polymer properties, says Duane Thompson, senior R&D manager for Unipol polypropylene licensing at Dow. They allow for stiffer plastic articles that emit fewer volatile organic compounds, a benefit in markets such as automotive interiors. Some 70% of Dow’s Unipol polypropylene licensees are using the technology.
Advanced donors have enhanced the standing of Ziegler-Natta catalysts in polypropylene, STA Research’s Sinclair concurs. “It is a great negative as far as metallocenes are concerned because you are achieving a lot of the things that metallocenes offer.” He adds that similar developments in Ziegler-Natta catalysts for polyethylene have helped them remain competitive in that market as well.
In turn, developers of metallocene catalyst technology are pushing back on the traditional cost advantage of Ziegler-Natta catalysts. Catalyst suppliers have been focusing on the methyl aluminoxane (MAO) catalyst activators, which can cost as much as the metallocene catalysts themselves.
In 2008, Albemarle, which supplies catalysts and MAO activators to polymer companies, launched its ActivCat activator technology. Amy Motto, vice president of polyolefins and chemical catalysts at Albemarle, declines to describe the activator in detail other than to say it is “based on MAO-type technology.” She claims it can double catalyst activity without adversely influencing resin properties.
Sinclair agrees that a doubling of activity is a reasonable estimate of what ActivCat can do. And he notes that other chemical companies, such as ExxonMobil and Chevron Phillips, have also made substantial progress in catalyst activation. “It’s plain astonishing that you can get such large step-changes at this stage in the technology,” he marvels.
Motto says ActivCat brings the cost of using metallocene catalysts to near parity with Ziegler-Natta LLDPE catalysts and slightly higher than the cost of catalysts for HDPE. “Before, Ziegler-Natta catalysts were so effective that a metallocene for HDPE didn’t make sense,” she says. The activators can help bring down the cost of metallocene catalysts for polypropylene as well, she adds.
New, cost-effective activators have spurred metallocene resin demand, Motto says. In addition, the market is opening up because key metallocene patents are expiring. Also important, she says, is that companies in the U.S., Europe, South Korea, and Japan worry about the wave of commodity polyolefin capacity in the Middle East and China and want to use the catalysts to differentiate their product lines. “They are asking, ‘Please, do you have any catalysts that we can use to help us out?’ ” she explains.
The result is a growing market for metallocene catalysts, Motto says. Albemarle expanded its flagship catalyst facility in Baton Rouge, La., last year and is planning another series of expansions this year and next. The company is also building a new catalyst and activator facility in Yeosu, South Korea.
Albemarle’s rival in catalyst supply, W.R. Grace, has been expanding for the same reasons. “It took a while to develop, but now metallocene catalysts are on a sustainable growth path,” says Anthony J. Dondero, vice president and general manager of specialty catalysts and process technologies at Grace Davison.
Last month, the company signed an agreement to purchase the fine chemicals maker Synthetech for $19.2 million. Synthetech makes chiral and peptide pharmaceutical intermediates at its facility in Albany, Ore. Grace’s aim is to make Ziegler-Natta and metallocene catalysts at the facility, eliminating the need to build a new plant. “Some modifications will be required to get the assets in line with the specialty molecules that we want to make,” Dondero says. Grace was already a Synthetech customer, though, so the firm has some familiarity with organometallic chemistry, he adds.
The acquisition is the latest in a series of polyolefin catalyst expansions at Grace. The company is expanding its Worms, Germany, polypropylene catalyst site. In 2005, Grace purchased the Chester, Pa.-based firm Single-Site Catalysts. The next year, it purchased a catalyst facility in Edison, N.J., from LyondellBasell.
Another sign the metallocene catalyst industry is building steam is that resin developers are once again adventurous in introducing new platforms, many outside their comfort zone of LLDPE for film.
For example, Total’s Lumicene platform has resins for cast and heat-sealable polypropylene film, used in packaging; HDPE for caps and enclosures; MDPE for rotational molding, used to make large, hollow objects such as fuel tanks; and MDPE for water pipe. The company has also unveiled a metallocene MDPE product for artificial turf that it says is easier to process and is more durable than conventional LLDPE.
Univation’s Stanley believes the market penetration of metallocene polyethylene will double by 2020, largely because of emerging opportunities in HDPE, in addition to growth in LLDPE. Univation aims to put HDPE in stronger focus with the XCAT VP platform of catalysts.
The original focus for the new catalysts was to improve the tear-resistance of metallocene LLDPE film. Since then, Univation has been applying the XCAT VP catalysts to HDPE, particularly in injection- and rotational-molding applications. Stakem says the advantages the resins bring to the table include strength and reduced cycle times for fabricating plastic parts. They will be so-called multisite catalysts, meaning that several catalyst sites are on a single support. This is the same approach Univation used for its Prodigy line of metallocene catalysts for making bimodal HDPE, used in pipe fabrication, in a single reactor.
Like all new products, the resins will enter into the long application development phase. The industry may have overestimated the time and effort to introduce new metallocene resins in the past, but it isn’t likely to again. Having seen this process play out before, Stakem is prepared. “The resin market is big, and it is really hard to turn a big boat,” he says.
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