Giving the Old Chrome Catalyst New Life

By Jan H. Schut

The SPE’s annual Polyolefins conference in Houston is always a good place to look for catalyst news, and this year is no exception. Polyolefins 2012, held February 26-29 at the Hilton Houston North Hotel (www.spe-stx.org/conference.php), includes a presentation on an advance in control of long chain branching with a very traditional catalyst—chromium. Eric Schwerdtfeger, senior research chemist at Chevron Philips Research Center in Bartlesville, Okla. (www.cpchem.com), will present a patent pending new highly long-chain-branched HDPE made by a new variation of Phillips’ chromium on silica catalyst.

At the Polyolefins Conference, Chevron Phillips will present work on a new chromium catalyst for HDPE that controls long chain branching, a trick normally done with LDPE using metallocene catalysts. This chart shows that the new catalyst produces more LCB with lower concentrations of ethylene in the reactor.

Chromium catalysts are known for making HDPE with broad molecular weight distributions (MWD) and a high molecular weight tail of long linear chains, but low levels of long chain branching–much, much lower than in  LDPE, for example. Broad MWD and long linear chains are needed for strength, especially against stress cracking. Long chain branches, which create Y-shaped chains, are generally bad for physical properties, but help processability.

This modified chromium catalyst is different because it can control production of long chain branching even on HDPE. In fact, it can dramatically increase the frequency of LCB to more than 40 per million total carbon atoms vs. only a fractional amount per million in HDPE normally, Schwerdtfeger says. Even at ppm levels LCB improve processability and melt strength, reduce sag and prevent ovality in large diameter pipe, improve shear thinning, and reduce die swell, Schwerdtfeger adds.

The catalyst research team, headed by Max McDaniel, a senior fellow at Chevron Phillips, used an advanced research tool, known as SEC-MALS (size exclusion chromatography coupled to multi-angle light scattering) with its chromium catalyst to determine the location of long chain branches within the MWD. MALS measures radius of gyration, or the “size” of a molecule, by passing laser light at different angles through a column of molecules in solution. Combined with size exclusion chromatography (SEC), also known as gel-permeation chromatography, it shows where long chain branches are concentrated in the molecular weight peaks of the polymer.

Early inventors at Phillips celebrate the 35th anniversary of inventor Max McDaniel two years ago. From left, Don Norwood invented the slurry loop reactor; Marvin Johnson invented refining catalysts; Paul Hogan co-invented chromium on silica catalyst; Joe Shveima invented chromium on silica catalyst for HMW film; and McDaniel invented some 350 patents including the highly significant SSA cocatalyst for metallocenes.

At the Polyolefins Conference, a younger inventor at Chevron Phillips, Eric Schwerdtfeger, will present their latest development in chromium catalyst technology with controlled long chain branching, allowing high levels of LCB in HDPE for the first time.

Polymer chains with long chain branches “occupy a smaller volume in solution than do linear chains of the same MW. SEC-MALS measures the average size of solvated molecules. It has allowed us to see for the first time the placement of LCB within the MW distribution,” McDaniel writes in a technical paper on the same work to be published in the “Journal of Polymer Science Part A: Polymer Chemistry.” SEC-MALS was used previously to measure long chain branching in metallocene polymers, but this is believed to be its first use with a chromium catalyst.

“Max has unlocked the nature of the chromium catalyst and how it relates to long chain branching,” explains Michael Jensen, a senior scientist at Grace Davison Specialty Catalysts, Columbia, Md. (www.grace.com), which develops catalysts with Chevron Phillips and other resin producers. “Typically with a chromium on silica catalyst, the way to control long chain branching (in the polymer) is through tailoring and treating the silica support. Now Chevron Phillips can create either a very low or very high level of long chain branching with traditional Phillips catalyst. Before, trying to tune long chain branch formation was primarily empirical.” This highly chain branched HDPE has only been produced in a lab scale reactor so far, but the technology is expected to be applicable to Phillips loop reactors. Higher levels of control of long chain branching in HDPE has potential for large diameter pipe, geomembrane, and some types of blow molding.

Chevron Phillips has already used a chromium catalyst with LCB manipulation to make two PE100 pipe resins (H524 and H525) since 2003 and 2004 respectively. PE100 pressure pipe resins are generally bimodal or have broad MWD to meet high performance requirements with good processability. They typically contain long linear molecule chains, which span more amorphous regions in the polymer than Y-shaped long chain branches of the same molecular weight. PE100 is a European ISO standard for pipe resins, rated to hold 100 bar of pressure at 20 degrees C. The U.S. pipe standard is ASTM 4710 with PN (pressure nominal) grades up to PN 16 for pipe, rated for a maximum pressure of 16 bar.

BIMODAL HDPE IN A SINGLE LOOP REACTOR

Very tight control of molecular chain development is usually associated with metallocene catalysts and narrow MWD polymers, not with chromium catalyst and broad MWD polymers. But McDaniel, who doesn’t use the title of doctor if he can help it and is seldom listed first on some 350 patents with his name, holds major patents in both catalyst groups. McDaniel has worked 37 years at Phillips, starting out under Paul Hogan and Robert Banks, the original inventors of the chromium catalyst for PP and HDPE. So it’s fitting that he should be the one to update the chromium franchise.

On the metallocene catalyst side, McDaniel’s research team also recently developed single-reactor bimodal HDPE technology (U.S. Pat. # 7619047 in 2009; U.S. Pat. Applic. # 20110201770 in 2011). This technology uses a dual-metallocene catalyst with Phillips SSA (solid super acid) activator and can also create broad MWD bimodal or multi-modal HDPE in a single Phillips loop reactor.

Chevron Phillips in this mLLDPE packaging film controls strength properties with metallocene catalysts. Now Phillips traditional chromium catalyst can have similar control of long chain branching in broad MWD HDPE. Potential markets include large diameter pipe, shown with Chevron Phillips technical service engineer Pam Maeger.

Chevron Phillips, however, isn’t the first company with a single-reactor bimodal. Univation Technologies LLC in Houston, Texas (www.univation.com), a joint venture between ExxonMobil Corp. in Texas (www.exxonmobil.com), and Dow Chemical Co., Midland, Mich. (www.dow.com), launched its “Prodigy” metallocene catalyst technology for bimodal HDPE made in a single gas-phase reactor in 2007.

But Chevron Phillips’s bimodal catalyst technology may offer more real-time flexibility because Chevron Phillips’s SSA cocatalyst system is formulated on site. “The CPChem activator supports are a real breakthrough for single-reactor bimodals,” says Kenneth Sinclair, principle of polyolefin research firm STA*Research, Foothill Ranch, Calif. (staresearch@juno.com). “With the activator support you can prepare the catalyst in line as you are feeding the reactor and tailor MWD dynamically.”

The single reactor bimodal technology may become available to Phillips licensees, but only with an additional fee. Philips licensees account for about 40% of global HDPE production capacity, according to Chevron Phillips, so the licensing potential could be enormous.

The SSA activator, or cocatalyst, was invented by McDaniel and Grace’s Jensen over a decade ago, and was the first low-cost support/activator for metallocene resins. (McDaniel and Jensen won Phillips’s Shield Innovation Awards for it 1999.) The SSA cocatalyst can make the total metallocene catalyst package significantly less expensive than the traditional metallocene package with MAO (methylalumoxane) or boron as cocatalyst, which are used in many commercial metallocene polymers. Cocatalyst is a big cost item, since anywhere from 100:1 up to 1000:1 or more of it is used than of metallocene, and that doesn’t count the silica that may be used to support the cocatalyst.

The acronym SSA, by the way, is fuzzy. Originally it stood for “solid super acid,” a chemically treated solid oxide used in petrochemical cracking, but not in polymer production before. SSA in polymer production has come to stand for “supported” or “solid single-site activator” or “solid support activator.”  There are almost 100 Chevron Phillips patents involved, starting with a Patent Cooperation Treaty in 1999. Three significant patents are U.S. Pat. # 6239059 in 2001; U.S. Pat. #6613852 in 2003; and U.S. Patent # 7148298 in 2006.

SSA isn’t the only way to reduce the total cost for metallocenes anymore either. In 2008, Albemarle Corp., Baton Rouge, La. (www.albemarle.com), introduced its ActivCat activation technology, which reportedly doubles the activity of silica-supported metallocenes used with MAO cocatalyst. Raghu Menon, business development manager, performance catalyst solutions at Albemarle, will present an update at the Polyolefins conference on “Albemarle’s ActivCat Activation Technology that Enables Polyolefin Producers to adopt Metallocene Catalysts.”

Other polymer companies are also working on developments in support activators for metallocenes for both HDPE and PP: Total Petrochemicals in Brussels, Belgium (www.totalpetrochemicals.com) for HDPE and Lummus Technology (part of CB&I Chicago Bridge and Iron Co.) in The Woodlands, Texas (www.cbi.com); ExxonMobil; and Sumitomo Chemical in Japan (www.sumitomo-chem.co.jp) for PP, says STA*Research’s Sinclair.

A PLACE IN CATALYST HISTORY

McDaniel’s SSA cocatalyst technologies may have played a significant role in allowing Phillips to negotiate a settlement with ExxonMobil after prolonged patent infringement lawsuits. Throughout the 1990s, Exxon aggressively defended several patents, especially the “’800 patent” (U.S. Pat. # 5324800 in 1991 issued to Howard Welborn Jr. and John Ewen for a “Process and catalyst for polyolefin density and molecular weight control”). The ‘800 patent covers a substituted bis-cyclopentadiene metallocene catalyst with MAO cocatalyst.

Exxon sued Mobil in late ’96 on the basis of the ‘800 patent and won its metallocene patent infringement suit in 1998, when a federal court in Texas ordered Mobil to pay $171 million to Exxon. Instead, the two companies signed an agreement to merge, which they did in 1999, forming the biggest company in the world at the time. Dow and Exxon, whose historic patents (on mono-cyclopentadiene metallocene catalysts) were applied for and issued within a few days of each other, settled their disputes in 1999.

Shortly after winning the judgement against Mobil, Exxon sued Phillips in 1998, also on the basis of the ‘800 patent. Phillips won the first round on a technicality without going to trial. Exxon appealed and the appellate court ruled in Exxon’s favor and ordered a trial. In October 2001, before the case went to trial, the two companies announced that they had cross-licensed each other’s technologies. Exactly what metallocene technology Chevron Phillips Chemical licensed ExxonMobil is confidential, but a big part of the settlement is believed to have been McDaniel’s SSA cocatalyst inventions.

There are probably fewer than a dozen inventors of truly major polyolefin catalysts and cocatalysts. Hogan and Banks for the chromium catalyst for PE and PP; Karl Ziegler and Giulio Natta for the titanium catalyst for PE and PP; Walter Kaminsky and John Ewen for the alumoxane cocatalyst for metallocene for PP and PE; and the unassuming inventor Max McDaniel for the SSA cocatalyst for metallocenes.

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