METALLOCENES BROADEN PP PROCESSING

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HOUSTON — Metallocene-catalyzed polypropylenes can lead to higher line speeds and fewer breaks in biaxially oriented film extrusion, a researcher told delegates at Polyolefins X in Houston.

Aspy Mehta, senior staff engineer at Exxon Chemical Co.'s Baytown, Texas, Polymers Center, said specially tailored PP can have a broader processing window than conventional resins.

Exxon created a resin with broad molecular weight distribution and a high-molecular-weight component. Such a resin can overcome processing problems inherent in metallocene-catalyzed PP with narrow molecular weight distribution, and it can match or exceed conventional PP made with Ziegler-Natta catalysts.

Exxon created the broad-molecular-weight resin by using a mixed-metallocene catalyst that can create two or more polymer backbones simultaneously.

Mehta said metallocene PP also promises better performance in BOPP shrink film by providing control over a greater range of shrinkage and shrink tension levels, compared with conventional PP.

Cast film applications for Achieve homopolymer metallocene PP have shown good results, but researchers obtained even better results with a blend of a metallocene-based propylene/ethylene/hexene random copolymer and a standard Ziegler-Natta resin, according to Mehta. The 1:1 blend gave dart impact strength values about eight times higher than either resin alone because of the synergy of their different strength mechanisms.

Exxon catalyst researcher Anthony Speca claimed mixed metallocene catalysts can create more than one polymer simultaneously. This technique creates a resin with broad molecular weight distribution, which improves melt processing compared with metallocene resins with narrow molecular weight distribution, he said. It also contains no molecular weight waxes, which are present in a conventional broad-molecular-weight resin, he said.

Speca described propylene polymerization Exxon did using catalysts his firm developed with Hoechst AG. Different metallocene catalysts are compatible on a silica carrier, allowing simultaneous production of more than one tailored polymer. The simplest property to control is molecular weight. However, since metallocene catalysts differ in how they respond to process conditions, higher levels of molecular design are possible, he said.

Gregory McPike, president and chief executive officer of Exxon and Union Carbide Corp.'s joint venture, said the venture has plenty of catalyst capacity to meet needs of technology licensees. The partners can make enough metallocene catalyst to feed 3 billion pounds of PE production annually, he said.

Officials of Abermarle Corp. of Baton Rouge, La., and Witco Corp. of Memphis, Tenn., said they are major suppliers of metallocene catalyst and co-catalyst ingredients to Exxon and other polyolefin firms. They agreed catalyst supply is not a problem.