Close up on processing metallocene resins.

The first processors to extrude metallocene-catalyzed polyolefins have gotten by so far with standard LLDPE equipment. What has been lacking is understanding of the subtle processing nuances of ethylene plastomers and metallocene LLDPE (mLLDPE) and the equipment modifications needed to accommodate them. In the year since our first report on optimizing equipment for metallocene resins (PT, Feb. '95, p.44), new research has answered many of the outstanding questions.

To bring extrusion machinery makers up to date, Exxon Chemical Co., Houston, invited them to a private symposium in April. Exxon, one of only two commercial U.S. producers of metallocene resins, presented its research on what equipment designs and processing techniques work best with hexene-based Exceed mLLDPE and butene- or hexene-based Exact plastomers. In this exclusive report, we pass on to you the insight that will result in improved machinery for these enhanced film materials.

Exxon senior development engineer Craig Erderly and consultant Werner Kurzbuch presented the company's latest advice on design of screws, barrels, dies, air rings, and downstream equipment for blown film. Less attention was given to cast film because the downstream equipment and processing techniques are virtually identical to those for standard LLDPE, Erderly says.

METALLOCENE'S DIFFERENCES

"Exceed mLLDPE resins process very much like Ziegler-Natta LLDPE's. However, the Exact plastomers are slightly harder to process in monolayer structures," Erderly says. "Both grades process with fewer problems in a coextrusion." Exceed and Exact grades typically extrude hotter and exhibit slightly lower melt strength. The resins are slightly less shear sensitive than conventional LLDPE due to their narrow molecular-weight distribution. Erderly says these material properties result in the following extrusion characteristics:

* Higher viscosity at typical extrusion rates.

* Greater shear with existing screws, thus requiring lower barrel temperatures.

* Need for 5-15% higher torque and motor-load requirements.

* Slightly lower melt strength and bubble stability.

The foregoing may lead to lower extrusion rates if your blown film system has insufficient cooling capacity. One corrective measure is to blend the metallocene resin with 3-10% of conventional LDPE to add long-chain branching and increase melt stability. Exxon also suggests using screws with a larger barrier clearance, air rings with enhanced bubble stabilizing features, lower pressure dies, and coarser screen packs in order to alleviate output limitations in a monolayer film.

GROOVED VS. SMOOTH BORE

Extruders for Exxon's metallocene resins are generally the same as are used for standard LLDPE. The major challenge in designing a smooth-bore extruder for metallocene resins is controlling the melt temperature. For a grooved-feed extruder, the key challenge is feeding the soft metallocene pellets without groove fouling, Erderly says. "Although grooved-feed machines have been used successfully to process mLLDPE with densities greater than 0.918, the processing of plastomers is more of a challenge on most grooved-feed machines," he notes. "Our experience to date has been that [grooved-feed extruders] usually produce lower specific throughput (lb/hr/hp) because of the softer pellets, this being more evident with the plastomers."

As a result, Exxon prefers using smooth-bore extruders in the range of 24-30:1 L/D. (Most of its monolayer film trials were done on 3.5-in. machines.) Smooth-bore extruders can also process metallocene-resin regrind "fluff," a feat Erderly says cannot be accomplished easily on a grooved-feed machine.

In Exxon's tests, a smooth-bore extruder processing Exceed mLLDPE yielded output/rpm similar to that obtained with conventional LLDPE but showed a 15 [degrees] F higher melt temperature and 5-10% lower energy efficiency. However, the smooth-bore extruder processing Exact plastomer showed 10-15% higher output/rpm, lower melt temperature, and higher energy efficiency than with standard LLDPE.

Exxon says that a grooved-feed extruder with the proper screw gives similar results with mLLDPE and conventional LLDPE. But a plastomer resin on the grooved-feed machine gives about 50% lower output/rpm than with LLDPE.

OPTIMIZING SCREW DESIGN

To process its metallocene resin in smooth-bore extruders, Exxon prefers to use a low-shear, barrier-type mixing screw having at least a 0.050-in. undercut. Compared with screws for standard LLDPE, metallocene resins require slightly larger barrier clearances and slightly greater metering depth. The screw must be able to accommodate blending with LDPE, Erderly says. Very "low-work" screws with 0.065-0.075 in. undercuts reduce melt temperature and increase output rates for 100% metallocene polymers (plastomers or mLLDPE), but such a screw design is probably inadequate for blends, since the more shear-sensitive LDPE could create melt-uniformity problems.

Exxon discourages use of high-shear screws with its polymers, since such screws produce higher melt temperatures, reduce bubble stability, and limit output. Metallocene resins have low shear sensitivity and can be homogenized with moderate shear. Low-shear screws have been shown to produce lower melt temperature and higher energy efficiency than do conventional LLDPE screws. However, low-shear screws still result in a higher melt temperature and lower specific output than does conventional LLDPE with a conventional screw.

TABLE 1 - MACHINE REQUIREMENTS FOR mLLDPE EXTRUSION

                         Torque,      Drive Power,   Max. Screw
Extruder Size(a), in.    hp/rpm       hp             Speed, rpm

2.5                      0.5-0.6      50-75          100-125
3.5                      1.25-1.30    125-150        100-115
4.5                      2.5          200-250        80-100
6                        5.0-5.3      300-400        60-75

a 24:1 L/D.

[TABULAR DATA FOR TABLE 2 OMITTED]

TABLE 3 - LOW-SHEAR SCREW VS. LLDPE SCREW

                            3.5-IN., 24:1 BARRIER SCREWS
Parameter                  LLDPE Screw    Low-Shear Screw

Solids Channel             Deep, Narrow   Shallower, Wider

Barrier Flight Clearance   0.050 in.      0.065 in.

Maddock Mixer Barrier
Clearance                  0.050 in.      0.075 in.

Metering Zone Channel
Depth                      0.325 in.      0.320 in.

"Metering screws are not recommended for metallocene polyethylenes due to lower output and decreased melt homogenization and quality," Erderly warns. "Low- to medium-shear mixing devices, like a Maddock mixer, are recommended but should be placed at the screw tip, or no more than two turns from the end of the screw." He says static mixers can also improve melt homogeneity while minimizing heat generation.

Optimizing performance of a smooth-bore extruder requires higher power and torque. Improved feeding of the softer pellets is the main concern for optimum extrusion with a grooved feed throat. Exxon recommends greater cooling of the feed bushing to prevent plugging or fouling of the grooves.

A flatter pressure profile, particularly one that reduces the peak pressure in the feed section, is suggested. Exxon says a "hump" temperature profile can help bring about a more uniform melt than a "ramp" profile. The hump profile initiates melting much faster and results in a lower temperature differential between the bottom of the screw flight and the barrel wall.

The ideal melt temperature for film blowing is equal to or lower than that for conventional LLDPE. A 1-MI LLDPE should be processed at 400-420 F. A 1-MI Exceed mLLDPE should be processed in the same range, while a 1.2-MI Exact plastomer should run at 380-390 F.

DIES AND AIR RINGS

Erderly says dies with wider spiral clearances help reduce head pressure and the chance for non-uniform shear heating, which can lead to gauge variations around the film bubble. Die lips must be designed to minimize flow resistance and melt fracture. Erderly says 60-90 mil die gaps and 0.375-in. to 0.5-in. land lengths work well. He advises against using a "double-choked" die - having narrow entry and exit points - because it raises the risk of higher shear, higher pressures, and higher melt temperature, leading to lower melt stability.

With smooth-bore extruders, system pressures should range between 3500 and 5500 psi, though a narrower range of 4000-4500 psi is considered ideal. Exxon operates the blown-film die at 10 [degrees] F above barrel temperatures.

Erderly says the cooling phase may be the area of greatest opportunity for progress in blown-film processing. The fact that metallocene resins extrude at a higher melt temperature than conventional LLDPE but crystallize at a 9-11 [degrees] F lower temperature makes the heat-removal requirement greater. Erderly says dual-orifice air rings are mandatory, as the second ring improves bubble stability and increases productivity. However, a dual-lip air-ring could restrict the range of blow-up ratios (BURs). A shallow forming cone followed by a deeper forming cone in the air rings can increase cooling and may yield a wider BUR range. Erderly cautions that air volume must be controlled closely, since too much air on the bubble could cause it to flutter. Additional bubble-stabilizing devices, such as extended forming cones, shrouds, chimneys, and irises can also be beneficial.

"The devices allow stable operation at greater frost-line heights and thus provide opportunities for greater heat removal at moderate air velocities," he says. Internal bubble cooling is beneficial but not required. The use of chilled air in conjunction with the dual-orifice air ring is also a good approach, he says.

WATCH OUT FOR TACKINESS

Metallocene films are extremely tacky when hot, requiring the bubble collapsing frame to have low-friction surfaces. "Air bearings or PTFE rollers are the best," Erderly says.

Exxon prefers center winders with low lay-on pressure and taper tension, especially for plastomers, although gap winders can also suffice. "Handle the plastomers the same as a soft, elastic (e.g., high-EVA) film and wind the Exceed the same as a pallet stretch film," Erderly recommends.