By Jan H. Schut
Trimodal polyolefins, primarily HDPE, are considered to be the next big polymer advance since metallocene single-site catalysts revolutionized LLDPE in the ‘90s, though little has been written about them. Compared to metallocenes, trimodal technology can optimize three different resin properties at once—stiffness, impact and processability—using three reactors in series, where metallocene technology typically optimizes only one. Trimodal polyolefins are already commercially available in Europe, the Middle East and Asia, where several new plants are being built or modified. But they’re not available so far in North America, although one European producer is considering making them here.
Trimodal polymers have three distinct molecular weight contents—low, medium and high–instead of two MW peaks for bimodals, though a trimodal MWD profile might not have three distinct peaks. Medium and high MW content improve mechanical strength, low MW content improves viscosity, processability and throughput. The big advantage of trimodals over bimodals is that trimodals are easier to process and yield higher throughputs. In blown film trimodals have higher melt strength for a more stable bubble with a lower stalk and higher throughput, resin producers say. Trimodals enjoy the same mechanical property advantages over conventional unimodal PE as bimodals, i.e., as density and stiffness go up, ESCR and impact strength don’t go down.
Low and medium density PE and PP can also be trimodal, but the early commercial markets are for trimodal HDPE for blown film for standup pouches, extrusion blow molded bottles and articles, and pressure pipe meeting the European PE100 standard (www.pe100plus.com), which China has also adopted. PE100 pipe is rated to withstand up to 10 MPa (100 bar) of stress for 50 years at 20 degrees C, where the U.S. pipe standard goes only up to 80 bar. In many markets, trimodal resins sell at a premium over bimodals, which sell at a premium over conventional unimodal HDPE.
WHERE THE TRIMODALS ARE
Trimodal PE is made commercially in Europe, the Middle East and Asia by the Hostalen ACP (advanced cascade process) from LyondellBasell (www.lyondellbasell.com); the Borstar PE 2G (second generation) process from Borealis (www.borealisgroup.com); and possibly the CX process from Mitsui Chemicals (www.mitsuichem.com).
Actual production of trimodals isn’t known because it’s all made on swing capacity. LyondellBasell has announced potential capacity for 4.4 billion pounds/year of trimodal HDPE from four plants in Germany, Poland and Saudi Arabia including licensees. But these plants can also produce bi- and monomodal PE. Borealis’s Borstar trimodal PE can be made in any of its existing bimodal PE plants.
Trimodal PE is made in a cascade of three reactors using the same catalyst particle throughout. Each reactor is fed different monomers and comonomers to produce the three distinct molecular weight fractions. Typically the first reactor makes homopolymer PE. Then other comonomers are fed into the two subsequent slurry reactors or a slurry reactor followed by a gas phase reactor to make two other distinct MW fractions. One molecular weight fraction can also be very small—in the 2-5% range or less, according to a recent patent application (U.S. Applic. 20090252910).
Simple blending or compounding of three different molecular weight polymers can’t achieve the same mechanical properties because polymer chains aren’t dispersed as finely as they are when made insitu. Compounding also uses higher cost ingredients like hexene copolymer instead of lower cost monomers, so it adds too much cost to be practical for packaging and pipe resin.
LyondellBasell’s process is the only trimodal available for license. It uses three slurry loop reactors in series. The first produces low molecular weight homopolymer PE; the second produces higher MW ethylene-butene-1 copolymer, and the third makes an ultra high molecular weight copolymer. LyondellBasell made its first commercial trimodal for blow molding in 2000, added pipe resin for PE100 pipe in 2002 and film resin in 2004.
Borealis’s second generation Borstar process for trimodals uses the same equipment as its original bimodal process, which happens to have three reactors: a small pre-polymerization slurry reactor, followed by a large slurry reactor, followed by a gas phase reactor. Borstar technology isn’t available for license, but is shared with a partner, Borouge, in Abu Dabi. Based on recent patent applications (U.S. Applic. 2008026942 and 20090252910), Borealis creates “three different ethylene polymer components” by using the small prepolymer reactor to make the third component, a small amount of high MW PE homopolymer. Then it adds low MW comonomer in the large slurry reactor and medium MW comonomer in the gas phase reactor. The medium and low MW fractions from the large reactors are each 20%-60% of the finished product, according to the patent, while the high MW fraction is very small– less than 5%. Borealis doesn’t call Borstar PE 2G resins trimodal because they have broader MWD than the bimodals do, but not three distinct peaks. Alternatively, Borealis also describes mechanically blending an insitu bimodal with a third polymer to achieve trimodality.
Mitsui Chemical’s CX bimodal process could also be used with the addition of a third slurry vessel to make trimodal PE. Mitsui Chemicals also has a gas phase Evolue H process for bimodal m-LLDPE made in two reactors. A Mitsui technical paper indicates that its FI (Furuyama invented) phenoxy-imine-based catalyst can make trimodals in a gas phase reactor. Mitsui’s trimodal technology isn’t commercially available through Prime Polymers (www.primepolymer.co.jp), the technology licensing partnership of Mitsui Chemicals and Idemitsu Kosan (www.idemitsu.co.jp).
A fourth potential contender is the Prodigy technology from Univation Technologies LLC (www.univation.com), a joint technology licensing venture between Dow Chemical and Exxon, including former Union Carbide patents. Prodigy currently makes bimodal PE, and Univation holds a considerable patent estate for making multimodal materials in a single gas phase reactor. A Univation patent application (U.S. Applic. 20080312380) describes a hybrid catalyst of hafnium combined with supported metallocene single-site catalyst able to make three molecular weight materials in one reactor, which would have significant cost advantages over any multi-reactor process. Univation’s developments have been tested internally on a pilot scale so far, with no customer sampling.
PROPERTIES OF MULTIMODAL HDPE
Producer LyondellBasell LyondellBasell Borealis MitsuiChem
Grade CRP 100 Hostalen 4731B HE3490-LS SP7005
Type Trimodal Trimodal Trimodal Bimodal
Application PE100 pipe Heating pipe PE100 pipe Bottles
Density, g/cm3 0.959 0.947 0.959 0.963
MFR, g/10 min. 6.4* 12.4* 0.25** 0.26**
@190 C 21.6 k
Tens. Mod., MPa 900 850 1,100 1,600
@ 23 C
Tens. Stress @ 23 22 25 30
Charpy Notched 13 8 — —
@ -30 C
Shore D Hard. 63 59 — 65
* 21.6 kg, ** 0.5 kg