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Polypropylenes will be next wave of metallocene-catalyzed polyolefins.

Metcon '94, the second Worldwide Metallocene Conference, held recently in Houston, introduced one brand-new family of polyolefin resins made by "single-site" metallocene catalysts and provided glimpses of several more families still in development. The technical papers also reflected three emerging trends in metallocene resin development:

* While the first commercial families of single-site resins have been primarily ethylene-based plastomers and elastomers, the next wave to emerge from the lab will be isotactic and syndiotactic polypropylenes. We won't see these resins for at least another year.

* Though initial metallocene resins have been aimed at higher-value specialty uses, some suppliers are now looking to upgrade high-volume commodity-type polyolefins with metallocene catalysts.

* Although metallocene resins appear to process reasonably well on conventional equipment, there is definitely opportunity for hardware optimization in order to realize the full potential of the new materials.

ENHANCED PROCESSABILITY

Exxon Chemical Co., Houston, claims to have pushed its Exxpol metallocene technology a step ahead with the addition of "APT" (Advanced Performance Terpolymers) to its Exact plastomer line. They were developed to improve on the blown-film processability of preceding Exact plastomers without sacrificing any of their high physical properties. In his Metcon paper, Dirk J. Michiels, Exact film technology team leader, conceded that the narrow molecular-weight distribution of earlier Exact resins produces high melt viscosity and low melt strength, which limit output rates in monolayer blown film extrusion. Although dry-blend addition of as little as 5-10% of high-pressure LDPE enhances melt strength and bubble stability, it detracts from the excellent mechanical and optical properties of narrow-MDW resins. No blending is needed to run the APT resins on standard LLDPE equipment.

The first commercial grade of the new family is Exact SLP-9042 APT plastomer (0.902 g/cc). It will be followed by other commercial grades (all of 0.900 density or higher) in the next six months. These materials have the same narrow MWD as other Exact resins. Exact APT resins will be aimed mainly at food packaging.

Michiels reported that the APT resin matches the processability of Affinity plastomers from Dow Plastics, Midland, Mich., despite the lack of "long-chain branching," which Dow says improves the processability of its metallocene resins. To demonstrate this, both SLP-9042 APT and Affinity PL-1880 (0.902 g/cc, 1.0 M.I.) were extruded into 1.25-mil film on a 2.5-in. LLDPE blown-film line using a 60-mil die gap, 2.2 BUR, and output of 7 lb/hr/in. or more. Also run under the same conditions was Exact 3033, a previously introduced terpolymer (1.2 M.I., 0.900 g/cc). As measured by extruder backpressure, motor amps, specific output (lb/hr/rpm), and maximum output (lb/hr/in.), Michiels' data show "no significant difference" in processability of APT and Affinity resins.

On the other hand, the APT resin had higher dart impact, MD and TD tensile strength, MD tear strength, and MD/TD balance of properties than the Affinity grade (which was higher only in TD tear strength). Exact 3033 was slightly higher in tensile strength, dart impact, and MD/TD balance than the APT resin, but was significantly lower in Elmendorf tear strength. Compared with Dow's Affinity resin, Michiels also noted much lower haze and volatiles content for both Exact resins, as well as comparable or better heat-seal strength and hot tack.

SINGLE-SITE PPs COMING

Exxon also presented at the conference the first details on its metallocene-based "Exxpol PP" technology. As reported last month (PT, July '94, p. 77), Exxon and Hoechst AG of Germany are cooperating on metallocene-PP catalyst development. They are concentrating on isotactic PP because they consider it to have broader applicability than the syndiotactic PP being produced by some other firms. The range of potential products is extremely broad--homopolymers and copolymers from less than 0.1 MFR up to 10,000 MFR.

First commercial tests are expected next year and full commercialization may not come until 1996. Exxon and Hoechst note that they have only scratched the surface of catalyst development for PP. However some potential advantages of metallocene PP are already evident. The narrow MWD provides greater melt extensibility, which has permitted test production of finer and stronger nonwoven fibers for diapers, hospital gowns and drapes, apparel, geotextiles, and filtration media. Better filtration efficiency is one end-use benefit. To achieve these properties, metallocene PP needs no peroxide treatment nor oil or wax additives. Added to this is the absence of atactic byproducts, resulting in lower extractables than current "CR" or "vis-broken" resins, as well as lower die drool. In metallocene random copolymers, narrow composition distribution means less of the sticky high-comonomer component, allowing production of softer resins than are possible today.

What's more, metallocene PP shows unexpected thermomechanical behavior. Although first-generation products have lower melting points than conventional PP, they actually show higher HDTs, indicating the potential for higher service temperatures while processing at lower temperatures. This could be advantageous both for molded goods and BOPP films. As the catalyst technology matures, Exxon expects to raise the melting points and HDTs of metallocene PP and to exceed the stiffness of conventional PP.

Metallocene PP is also much more resistant to free-radical attack, Exxon says. That suggests longer life in outdoor uses from carpets to car bumpers, better resistance to radiation sterilization in medical products, and also greater recyclability.

The lower and very sharp melting point of metallocene PP is attractive in heat-seal layers for packaging films. Lower-temperature seal capability of an Exxpol random copolymer is shown in Fig. 2. Superior hot tack is also said to be exhibited by this product.

Unlike conventional PP processes, metallocene catalysts allow incorporation of higher-alpha-olefin comonomers, leading to the possibility of novel random copolymers. For example, a hexene random copolymer requires less than one-third as much comonomer to achieve the same softness as an Exxpol ethylene copolymer. The result is higher heat and creep resistance coupled with good flexibility, attractive for uses such as medical tubing and meat-packaging films.

The state of development of metallocene-catalyzed syndiotactic PP (sPP) was summarized by Dr. Edwar S. Shamshoum, manager of polymer R&D for Fina Oil & Chemical Co., Dallas. He characterized the resins and their unusual processing behavior, much of which has been reported previously. He also said that commercialization of sPP was two to three years off, although the company made its first commercial production run in April '93 and is currently sampling two grades to 40-50 customers.

BASF AG of Germany also discussed its work on metallocene isotactic PP, concentrating on impact copolymers. Although this work is still in its early stages, BASF's Novolen gas-phase process has used metallocene catalysts to make in-reactor PP copolymers. What's unusual about the results is that the EP rubber phase is entirely amorphous, whereas standard catalysts produce crystalline PE blocks inside the rubber particles. Improved impact modification is expected to result. BASF says toughness should be further enhanced by improved interaction between the rubber and PP matrix at the phase boundaries.

'COMMODITY' METALLOCENE PE

While Dow and Exxon have focused on higher-value specialty applications for metallocene polyolefins, Quantum Chemical Co., Cincinnati, is looking for inexpensive ways to upgrade commodity-type PEs with combinations of metallocene and standard catalysts in existing reactors. Pursuing what it calls the "best of both worlds," Quantum seeks to upgrade performance properties of standard LLDPE without sacrificing processability. In fact, processability can actually improve, Quantum found. Using both catalysts produces bimodal combinations of two types of resins simultaneously in an intimate mixture and in proportions that could be readily tailored. Quantum is in the pilot-plant stage of work with butene copolymers in a gas-phase process.

Research engineer Mark A. Wendorf stated in his Metcon paper that with mixed catalysts, "a resin can be obtained which has equivalent properties to standard LLDPE with improved processing, or with equivalent processing and improved properties. These mixed-catalyst resins do not have the performance of straight metallocene resins, but bridge the gap between standard and metallocene LLDPE."

Quantum made 1.25-mil blown films of several different resins using both a narrow (40-mil) die gap typical of LDPE and a wide (100-mil) die gap typical of LLDPE. In both cases, the extruder amps and barrel pressure were much lower with the mixed-catalyst resin than with straight butene or hexene LLDPE or with an LL/LDPE blend. Melt temperatures were similar for all resins; output (lb/hr/rpm) for the mixed-catalyst resin was slightly less than for the butene LLDPE but higher than for hexene LLDPE or LL/LDPE blend. The LLDPEs experienced significant melt fracture when extruded with the narrow die gap, while melt fracture was "virtually eliminated" with the mixed-catalyst resins.

Finally, the mixed-catalyst resin compared favorably in mechanical and optical properties to butene LLDPE and LL/LDPE blend with both die gaps.

AN EQUIPMENT MAKER'S VIEW

While suppliers of metallocene PEs generally insist that they run pretty well on existing LLDPE-type film equipment, spokesmen for Dow and Exxon acknowledged that some equipment tailoring would help get the most out of these resins. Exxon's Michiels said his firm is working actively in this area. He noted that Exact resins can give problems of bubble stability and can be harder to cool, both of which commonly result in small reductions in output rate--up to 10%.

Frank Goffreda, v.p. of sales for Filmaster Inc., Fairfield, N.J., gave what may be the first public presentation by a machinery supplier on processing metallocene PE. He found Dow's material easier to process than Exxon's, though the latter had better film properties. He noted that Affinity resins may actually run at lower temperatures, pressures and amps than standard LLDPE.

Goffreda also noted some small modifications in blown-film equipment design and operating conditions that have worked well with metallocene PEs:

* Slightly narrower die gaps and annular clearances in the die spirals can be used than are typical for LLDPE.

* Barrier screw designs of recent vintage work well, though the barrier flight perhaps should be cut a little deeper than usual.

* Clearances in the Maddock mixing section were also opened up for metallocene PE.

* Because of the materials' tendency toward stickiness when hot, an air collapsing frame may work best.

* Because of these resins' low melting points, it's essential to keep the extruder feed section cool.
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Author:Naitove, Matthew H.
Publication:Plastics Technology
Date:Aug 1, 1994
Words:1703
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