Smooth vs. grooved.

Experts Duel Over the Best Barrel & Screw Designs for Metallocene Resins

Are smooth-bore or grooved-feed extruders best for making blown film from the new metallocene-catalyzed poly-olefins? This question, which seemed less pressing than matters of screw and die design 18 months ago, has recently mushroomed into a major issue of debate. "More customers are using metallocene resin as a specification on a new line. They want to make sure the equipment is designed to run it," comments William Hellmuth, senior product manager for blown film systems at Battenfeld Gloucester Engineering.

Coming up with proper equipment designs for metallocene resins takes on added urgency when one considers the magnitude of the predicted "metallocene revolution" and how fast it is approaching. U.S. demand for metallocene resins is forecast to rise more than 26 times current consumption levels within the next 10 years. It is expected to reach 4 billion lb/yr by 2005, versus 150 million lb this year. Film extrusion is expected to account for 70-80% of that demand. These figures come from a new report by Chem Systems Inc., a consulting firm in Tarrytown, N.Y.

The smooth-versus-grooved extruder controversy gained intensity after a two-day symposium in April sponsored by Exxon Chemical, which invited blown-film equipment suppliers to hear its lab findings and recommendations on equipment changes for optimal processing of the company's metallocene resins. A report presented at the meeting by Exxon senior R&D engineer Craig Erderly and consultant Werner Kurzbuch noted a preference for smooth-bore extruders over grooved-feed machines. This was due principally to a reported tendency of the soft metallocene resins to plug up the barrel grooves, resulting in lower throughput rates (see PT, June '96, p. 49). Exxon's results were derived from tests using equipment manufactured by Alpine American, Battenfeld Gloucester Engineering, Black Clawson-Sano, Brampton Engineering, and Macro Engineering & Technology, Erderly says.

Exxon's report found support from firms that sell primarily smooth-bore extruders, such as Battenfeld Gloucester, Black Clawson-Sano, Egan Davis-Standard, and Addex. But other equipment makers vigorously dispute Exxon's contention that grooved feed throats are less preferable for processing metallocene resins. In fact, Windmoeller & Hoelscher ran Exxon's metallocene resins successfully on its grooved-feed extruders during the K'95 show in Dusseldorf, Germany, says Andrew Wheeler, W&H senior sales manager. Reifenhauser v.p. of sales Matthew Bangert reports that several customers have used his company's grooved-feed machines to process Exxon's resins successfully at output rates approaching 70% of those for conventional LLDPE. "We find Exxon's comments to be true in the narrowest sense - for the U.S. market exclusively," Bangert says. In his opinion, Exxon's additive package for its metallocene resins is designed for smooth-bore extruders, since they represent 80-90% of film machines sold in the U.S. for LDPE and LLDPE film. He notes that in Europe, grooved feed predominates by a similar margin. "I think Exxon's done a poor job of tailoring their material for grooved feed," Bangert adds. However, Erderly says Exxon resins are designed for any extruder.

It's worth noting that Dow Plastics, the only other commercial U.S. supplier of metallocene resins, has no preference for one type of feed section over the other, according to development associate Tom Butler.

It's not surprising that opinions clash about these resins' processing behavior. Until recently they were in limited supply and in a state of continuous and rapid evolution. "Designing an extruder to perform optimally was difficult because the material was not only in short supply, but was a moving target. The material has become more consistent, so now we have a design target," says Reifenhauser's Bangert.

Some hardware suppliers, such as Kiefel Inc., haven't been able to get their hands on enough material to conduct extensive studies. And even those that have tested the resins are bound by secrecy agreements with resin makers not to discuss the results of their research. Nevertheless, several equipment makers that have experience with the metallocenes agreed to speak in general terms for this article.

PLASTOMERS ARE THE REAL TEST

Equipment and resin suppliers agree that optimizing current LLDPE machine designs for metallocenes depends on four factors: the type and density of metallocene resin, whether the resin is used alone or in a blend, the blend ratio, and whether it's processed in a mono-layer or coex structure.

Suppliers of smooth-bore and grooved-feed equipment say processing metallocene LLDPE (mLLDPE) requires little or no change to existing LLDPE equipment. The real test is in processing metallocene plastomers. "The mLLDPEs and the plastomers have to be dealt with as separate issues," says Jay Ragusa, engineering manager at Alpine American. "Plastomers are more of a challenge, especially when you go to the lower densities - under 0.915 g/cc."

Most of the debate therefore focuses on optimizing the extruder for plastomers. Such optimization may reduce the machine's versatility for other uses. Opinions are mixed as to whether an extruder optimized for plastomers would do a good job of processing conventional LLDPEs or even mLLDPEs. For example, Egan Davis-Standard has developed a screw design dedicated to plastomers. That screw is not recommended even for blends of a plastomer with LLDPE, LDPE, or mLLDPE, says Rick Keller, Egan's industry director for film systems.

Compared with conventional LLDPE resins, plastomers are generally much softer due to their lower density and less shear sensitive because of their narrow molecular-weight distribution. Exxon says its Exact plastomers are typically extruded at a lower temperature due to their slightly lower melt strength than a conventional LLDPE. Dow's Butler says its Affinity plastomers, with their long-chain branching, exhibit more shear thinning and greater melt strength, which allows them to be run at a melt temperature more similar to conventional LLDPE.

Most sources interviewed agree that Dow's and Exxon's plastomers process differently. Says Wheeler of W&H, "Dow claims that their Affinity metallocenes should be run with a high-shear screw to avoid gels, while Exxon suggests running with very low shear, as their material is essentially gel-free and quite heat sensitive." Exxon and Dow concur with Wheeler's statement, although Dow notes that higher shear also contributes to better mixing.

Different densities of even the same brand of plastomers may run differently on the same machine. "It could present problems if you run a 0.90-g/cc material with a screw optimized for a 0.92 resin," Buffer notes.

A number of extruder suppliers agree with Exxon that plastomers present very different challenges to smooth- and grooved-feed machines designed for conventional LLDPE. [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] With a smooth bore, the key difficulty is controlling the melt temperature of the polymer - although Rick yon Kraus, president of Addex, says his company designs smooth-bore screws that accomplish good melt-temperature control. With grooved-feed machines, the main problem is achieving a satisfactory specific output rate (output per revolution, or lb/hr/R). "You need approximately 15% more horsepower with grooved-feed and smooth-bore machines alike to process these materials," notes von Kraus.

THE CASE FOR GROOVED FEED

As was reported in June, Exxon's Erderly found that the company's plastomer gave as much as 50% lower output/revolution than did standard LLDPE with a grooved feed. But with a smooth bore, plastomer actually ran better than LLDPE, Erderly reported. The plastomer yielded 10-15% higher output/revolution, lower melt temperature, and higher energy efficiency (lb/hr/hp).

Alpine's Ragusa acknowledges that throughput rates dropped about 50% from conventional levels in his firm's early trial runs of plastomers on grooved-feed equipment optimized for standard LLDPE. Alpine has achieved better rates since then through various hardware modifications. "Approximately 60% of our trials now involve metallocenes. We run metallocenes every week - in coextrusion lines, in blends, and 100%," Ragusa says.

"Grooved-feed extruders are advantageous with regards to pressure profile, pressure flow, shear-heat generation, melt-temperature control, and sensitivity to die pressure," says W&H's Wheeler. But he and other machine suppliers concede that the softer plastomer pellets can compact in the feed grooves and reduce the specific output rate. As a rule of thumb, Wheeler says, the softer the material (which correlates with lower density), the harder it is to run in grooved-feed barrels. Andreas Mayer, development associate at Dow Europe in Switzerland, says groove fouling can do more than reduce output - it can also result in cross-linking, unmelted particles, or both.

W&H reports that the reduction in specific output rate for a grooved-feed versus a smooth-bore machine depends on extruder size. In tests using its Varex extruder line, a 60-mm, 30:1 L/D model delivered a specific feed rate of 1.33 lb/hr/R with Exxon's Exact 4011 plastomer (0.885 g/cc, 2.2 MI). The result is a 30% lower rate than a comparable smooth-bore extruder can deliver with that same material. However, W&H finds that as the extruder size increases, the specific output rate improves relative to a smooth bore. A 70-mm, 30:1 model processes Exact plastomers at 1.9 lb/hr/R, only 15% below the smooth-bore rate. Wheeler adds, "The reduction in specific output can be made up by increasing screw speed, as backpressure on a grooved-feed extruder can be increased drastically without increasing the melt temperature."

Spokesmen for Alpine, Battenfeld Gloucester, Kiefel, Reifenhauser, and W&H say they have no intention of changing the geometry of their grooved-feed sections to accommodate plastomers. However, they do agree that it is best to have a water-cooled feed section for processing these resins.

Processing aids are recommended by virtually all equipment suppliers as a tool to improve extrudability of plastomers - though again it may depend on which plastomer you are processing. A 1% addition of processing aid can yield as much as a 20% increase in production, says Wheeler. The additive also helps reduce other potential plastomer processing headaches, such as melt fracture and poor melt strength.

Two other factors - blending and coextrusion - can ameliorate plastomer processing difficulties. Equipment makers believe that blends with LLDPE or LDPE will be much more popular than 100%-metallocene films. Machine suppliers say most of the benefits of plastomers' superior properties can be obtained at lower cost in blends with conventional polyethylenes. "Why go with 100% plastomer if you can get away with 20%? Even 5% can do wonders," Alpine's Ragusa says. Blending has the further advantage of lessening processing difficulties - as was discovered years ago with LLDPE and HMW-HDPE. Some new and still confidential experimental results indicate that blends narrow the difference in performance between smooth-bore and grooved-feed designs.

On the other hand, Dow's Butler warns that a plastomer/LLDPE blend will still process differently from an LLDPE/LDPE blend. And Keller at Egan Davis-Standard cautions that blending can create melt-temperature variations during processing, since the different PE materials tend to melt at different temperatures. This could lead to surging and gels as the percentage of plastomer in a blend rises, Keller says.

Equipment suppliers foresee plastomers being used mainly in coextruded structures (as is the case today) rather than in monolayer films. Restricting plastomer use to certain layers in a coextrusion reduces the impact of plastomers on film cost and processability.

DEBATE ON SCREW DESIGN

The main issue in plastomer screw design concerns the melting section of the screw. Most grooved-feed suppliers agree that a wider clearance between the screw flight and barrel wall is beneficial. Egan Davis-Standard's Keller says the firm's dedicated screw for metallocene resins features a larger barrier clearance and larger melt channels, which provide a gentler transition from pellet to molten polymer. Battenfeld Gloucester, which supplies both grooved and non-grooved extruders, agrees with Exxon's Erderly in recommending a 0.05-in.-wide barrier-flight clearance for plastomer blends and a 0.065-in.-wide clearance for running 100% plastomers. "If you increase the clearances, you don't overheat the material but you lose some mixing potential, which is an important element when you are running blends," cautions Battenfeld's Hellmuth.

Ragusa says Alpine has deepened its melt channels in the grooved-feed section to improve feeding of plastomers. In addition, the company is studying all aspects of plastomer screw design - including flight pitch, flight clearance, L/D, barrier design, and channel depth beyond the feed section.

Enlarged flight clearances further down the screw also help processability. Butler from Dow says his company has successfully processed its plastomers on both smooth-bore and grooved-feed extruders with mixing-gap clearances in a general range of 0.025-0.060 in.

Exxon and virtually all equipment makers prescribe use of a barrier screw to process plastomers. Butler notes that Dow's Affinity plastomers can be used with barrier or single-flighted screws so long as they are designed to impart moderate to high shear. Most barrier screws today, he cautions, are low-shear types. By contrast, Exxon specifically states that "a low- to medium-shear barrier screw is preferred for Exact plastomers, as well as Exceed mLLDPE."

Wheeler says W&H has come up with variants of its universal screw that are tailored for plastomers, including a high-shear design with a barrier section 12D to 22D long, and a lower-shear configuration with a barrier section ranging from 15D to 25D. Alpine's current screw design features a short (2D) dual-flighted shearing section. But Ragusa says Alpine is now researching a design for a true barrier screw.

Exxon's Erderly recommends that plastomer screws have a low- to medium-shear mixing device like a Maddock section. Egan Davis-Standard's Keller says a Maddock mixer with a twist is the way to go. But Battenfeld's Hellmuth says almost any well-designed dispersive mixer will do the job.

Reifenhauser and W&H agree that screw lengths of 30:1 L/D should be used for good melting and mixing. Exxon has shown a preference for 2430:1 L/D.

Keller from Egan Davis-Standard says film quality will benefit when a larger-diameter extruder is run at a slower screw speed versus a smaller extruder operating at a higher rpm. "It helps mixing, homogeneity, and thickness uniformity, and you get better mechanical properties," Keller says.