'Plastomer' resins spark changes in blown film equipment.

Not since the advent of LLDPE has any new material caused so much rethinking of extrusion equipment design. From the feedthroat to the winder, some changes are in store.

Equipment designers are just starting to figure out what it will take to get the most out of the new metallocene-catalyzed polyolefins. The new resins - called "plastomers" because of their mix of plastic and elastomer characteristics - have some distinctive processing characteristics that will certainly demand equipment optimization.

What's also clear is that these machinery modifications will not be as radical as those that followed the introduction of LLDPEs in the late 1970s.

"If you're set up to run LLDPE, you're in good shape to run the plastomers," says Frank Goffreda, v.p. of sales at Filmaster Inc.

However, industry sources agree that equipment designed for high-pressure LDPEs is not suitable for running plastomers.

While plastomer users are getting by with standard LLDPE equipment, they may be sacrificing output rates a bit or giving up some of the film properties of these expensive high-performance resins. That's why processors and materials and equipment suppliers agree that some tweaking of current LLDPE designs is needed.

PLASTOMERS ARE DIFFERENT

Film-grade plastomers, supplied by Dow Plastics and Exxon Chemical, are linear copolymers or terpolymers of ethylene with butene, hexene, or octene. They have very low densities - 0.895 to 0.915 g/cc. Narrow molecular-weight distributions and high comonomer contents give plastomers their unusual performance properties and also make them process differently from LLDPE.

Plastomers tend to be faster melting and to have lower melting points, poorer melt strength, and higher viscosity. John Perdikoulias, R&D engineer at Brampton Engineering, says these characteristics can lead to problems when processing the new resins on LLDPE equipment. "A typical film processor will experience higher motor amperage, increased tendency toward temperature override, higher die pressures, and lower bubble stability - all possibly resulting in lower production rates."

R&D on solving these problems is veiled in secrecy. However, processors and suppliers clearly indicate the main areas in which you'll see equipment changes:

* More torque may be needed to push the plastomers through the extruder due to the resins' higher viscosity.

* A redesigned screw would facilitate better melting and mixing.

* A change in die design should be considered to improve output rates at lower pressures and temperatures.

* Dual-lip air rings are necessary for bubble stability and higher output because of the plastomers' low melt strength.

* Roller-type collapsing frames are preferable to older slat types in order to avoid problems with film tackiness.

Two other aspects of equipment design are debated among machinery builders for both plastomers and standard polyethylenes: One concerns the merits of grooved feed sections versus smooth bores for achieving better output; the other involves the pros and cons of longer vs. shorter screws.

MORE TORQUE OR LESS OUTPUT

"Compared to a machine equipped to run LDPE, the motor will need a lot more torque [to run the plastomers]," says Kevin Slusarz, process-development engineer at Egan, a division of Davis-Standard. Slusarz says an LDPE extruder could require a new motor and gearbox capable of 150-170% higher torque in order to process plastomers. Extruders designed for LLDPE can probably generate enough torque if run at lower screw speeds, he adds.

Filmaster's Goffreda agrees: For example, an LLDPE machine that normally operates at 100 hp and 135 rpm could run the new polyolefins effectively at 85 rpm, he explains. More generally, Goffreda says extruders of 2.5-3.5 in. diam. with 100-155 hp motors would generate enough torque to run plastomers at 75-90 rpm. A 4.5-in. extruder with a 200-250 hp motor would run plastomers at 75-85 rpm.

"I would guess that one might need 10-20% more horsepower and 10-20% lower screw speed [for plastomers than for LLDPE]," says William Hellmuth, senior product manager of blown film systems at Battenfeld Gloucester. He also notes that plastomers increase the motor amperage draw by an average of 20%, though the figure can vary widely depending on the resin.

TAMING OF THE SCREW

Battenfeld Gloucester, Egan, Filmaster, Reifenhauser, and Windmoeller & Hoelscher all recommend using some type of barrier screw to run the plastomers. Today's designs are not optimal, however.

"In an ideal world, the extruder screw would be designed for each grade of resin," says Ricky Keller, industry director of film systems at Egan. But screw technology has leaned toward multi-resin versatility. Today's multi-use barrier screw can run LLDPE, LDPE, EVA, and ionomers. This hybrid design can generate problems when processing plastomers. Because of their low melt temperature, plastomers are prone to stagnate at the beginning and end of the barrier section of a typical multi-resin screw design. "If the ratio of the separation of the solid/melt channel isn't correct at the first barrier flight, then you can have problems," warns Egan's Keller. Stagnant flow of material reduces output and promotes degradation, which necessitates downtime for cleaning.

"What's unusual is that a plastomer resin can run fine for an hour or days or weeks, but when it stagnates it can take days to clean," Keller notes.

Stagnation and/or poor melting and mixing can cause the plastomer to form gels, clogging the flow channels and increasing the risk of crosslinking. In addition, sources say these processing obstacles can add to melt pressures that are already 10-15% higher because of plastomers' high viscosity. High pressure in turn raises melt temperature, leading to poorer melt strength and increased cooling requirements downstream.

Dow technicians say the problem with conventional LLDPE barrier screws is that they impart low shear. For plastomers, moderate to high shear is recommended.

Equipment suppliers are considering several possible modifications of barrier screws: altering the solid/melt channel depth ratio in the barrier section, changing the pitch of the primary and barrier flights, redesigning the barrier clearance or undercut, and increasing the mixing gaps to improve melting and mixing.

On the other hand, Bill Van Volkenburgh, project manager at Dow's laboratories in Freeport, Texas, says a single-flight screw works best of all. "The single-flight screw is designed for better mixing and homogenization. The barrier is designed for high output," he explains.

Van Volkenburg adds that a single-flight screw may be better for producing plastomer films with good optics and excellent uniformity. Dow recommends that a single-flight screw for plastomers have a longer transition or compression section than a conventional LDPE or LLDPE screw.

TABLE 1 - PLASTOMERS AVAILABLE FOR BLOWN FILM

Supplier &                    Melt Index,   Density,
Trade Name         Grade      g/10 min      g/cc        Type

Dow Plastics
Affinity           FM1570        1.0         0.915     Octene
Affinity           PF1140        1.6         0.895     Octene
Affinity           PL1840        1.0         0.908     Octene
Affinity           PL1880        1.0         0.902     Octene

Exxon Chemical
APT                SLP-9042      2.0         0.902     Terpolymer

Exact              3025          1.2         0.910     Butene
Exact              3028          1.2         0.900     Butene
Exact              3033          1.2         0.900     Terpolymer
Exact              4015          1.5         0.896     Butene

Whether you use a barrier or a single-flight screw, all suppliers strongly recommend the addition of a mixing section.

One supplier offers a means of converting existing screws for plastomer extrusion. Alpine's modular, single-flight screw design consists of multiple segments that slide onto a central shaft. This construction permits customizing screws with more or less shearing or mixing, as required by the resin.

TABLE 2 - PROCESSING DOW AFFINITY PLASTOMER VS. LLDPE(a)

                               Affinity      LLDPE

Melt Index (I2)                1.0           1.0

Density, g/cc                  0.920         0.920

Melt Temperature,
F                              440           440
C                              227           227

Screw rpm                      60            62

Drive amps                     59            63

Pressure,
psi                            2845          3313
MPa                            20            23

Output,
lb/hr                          150           151
kg/hr                          68            68

a Results obtained with a 2.5-in. extruder, 24:1 L/D, equipped with
Barr ET screw, 6-in. LLDPE low-pressure die, 70-mil (1.8-mm) die
gap. (Source: Dow Plastics)

HOW LONG IS ENOUGH?

The long-running debate over longer vs. shorter screws for various types of PE film extrusion is carrying over into discussions of plastomer screw designs (see PT, Feb. '92, p. 37; Dec. '94, p. 25). Some equipment suppliers see longer screws as a way to impart sufficient shear and aid in melting and mixing of plastomers.

Reifenhauser, W&H, and Battenfeld Gloucester recommend a longer screw - around 30:1 L/D. They argue that shorter screws apply too much shear and don't provide effective mixing and melting. A longer screw may feature changing flight pitch or channel depth in order to achieve a more uniform melt at a lower temperature.

On the other hand, Alpine says its screw design is successful with plastomers at only 21:1 or 25:1 L/D. Company sources add that a longer screw would increase polymer residence time, require tighter temperature control, and present greater risk of melt hang-up.

TABLE 3 - EXTRUSION PERFORMANCE USING EXXON EXACT PLASTOMER

                                Extruder Size (24:1 L/D)

                        2.5 in.     3.5 in.    4.5 in.   6 in.

Torque, hp/rpm          0.35-0.50   1.0-1.25   2.0-2.5   4.0-5.25

Drive Power, hp         40-60       100-125    200-250   300-400

Max. Screw Speed, rpm   115-125     100        80-100    75

Expected Extrusion
Rate, lb/hr             200-250     500-600    800-900   1400-1600

Although neither producer of plastomer resins will take a position on an "ideal" screw length, Exxon does say that screws from 24:1 to 32:1 L/D process its resins well.

Regardless of screw design or length, some equipment makers say you may pay a penalty in output when running plastomers. Reifenhauser v.p. of sales Matthew Bangert says production rates with plastomers tested on grooved-feed extruders of 70, 80, and 150 mm (all of 30:1 L/D) were 60% of LLDPE rates. Bangert hopes eventually to get output rates approaching 80% of LLDPE capability.

GROOVED OR SMOOTH?

As is the case with long or short screws, plastomer resins have been caught up in the ongoing debate about grooved-feed vs. smooth-bore extruders for PEs. Bangert claims that a grooved-feed design provides 20-25% more throughput capacity than a smooth-bore extruder of comparable size. But sources at Brampton say their 24:1 or 30:1 smooth-bore extruders process plastomers efficiently while maintaining low shear, which aids melt strength.

One point on which all sources agree is that care must be taken not to soften the low-melting plastomer resins too early in the feed section or else bridging may occur. Some machine builders say a water-cooled feed section is better than air cooling for the plastomers.

DIE REFINEMENTS

Though low-pressure dies designed for LLDPE can process the plastomers, knowledgeable sources say such dies may be too restrictive and will require some redesign. Running plastomers on standard LLDPE dies may trim output by at least 5%.

High-pressure conventional LDPE dies will not work with these new resins, says Macro Engineering president Mirek Planeta. High-pressure dies increase melt temperature, raising the possibility of melt fracture and poorer melt strength. Planeta and other sources agree that plastomers usually run well at system pressures of 2000-5000 psi, typical of LLDPE. These sources note that the die generates 30-40% of total extrusion pressure.

"Owing to the higher viscosity of metallocene resins, the depth and length of die ports and spirals will need more fine tuning to optimize pressure, advises Egan's Keller. "Too much pressure can cause stagnation and restriction of flow. Generally, the higher the viscosity of the material the bigger the die gap required."

Keller and Filmaster's Goffreda recommend widening the die gap to 0.080-0.120 in., as compared with typical die gaps for LLDPE of 0.050-0.084 in.

"Side-fed dies, high-pressure dies, or any die 15 years old or older are not suited to run plastomer resins," says Goffreda. LLDPE dies no more than eight yearn old are adequate to run plastomers, but may not do so at peak efficiency. He also recommends a reverse land, such as is often used with LLDPE.

Among Goffreda's other advice on plastomer film dies: Feed ports should range from 0.5 to 0.625 in. with a 180 [degrees] spiral. The spiral should wrap no more than 220 [degrees] around the die to limit the residence time and possible degradation of the plastomer.

Note that because plastomers have low melt strength, opening the die gap too wide or using too many feed ports can lower the die pressure and adversely affect bubble stability - perhaps even causing the bubble to collapse. Alpine engineering manager Jay Ragusa says one could use a larger die gap and switch to a smaller die diameter, but output may fall as a result. Goffreda says plastomers run in monolayer dies at typical rates of 8 lb/hr/in. of die circumference, vs. 10-12 lb/hr/in. for LLDPE.

Alpine has a new proprietary die coating that reportedly reduces friction in the die and allows a processor to keep the same die diameter while using a smaller die gap than would otherwise be necessary. With this coating, die gaps reportedly can be reduced by 25% for all resins, thereby providing better melt strength without increasing risk of melt fracture.

Bill Bode, an engineer at Battenfeld Gloucester, says it is necessary to nickel or chrome plate the die pin and ring, screen charger, adapter, and transfer piping to reduce the chance of plastomer resins hanging up in the die. Such plating is usually considered optional when processing traditional LLDPE, but not with plastomers, he says.

AIR RINGS: GO DUAL-LIP

For extruding plastomers, dual-lip air rings are considered a must. "When the plastomer emerges from the die it is pretty gooey and soupy and has very little melt strength," notes Goffreda. He adds that with a single-orifice air ring, bubble height may be insufficient to clear the ring's cone height. "So the processor may have to back down on the extrusion rate to get bubble stability. The plastomers need to be coaxed," Goffreda says.

A number of sources advocate cooling the bubble with air of lower velocity spread over a larger area. But slower bubble cooling may mean a lower output rate.

Robert Krycki, president of Future Design, disagrees. He says his company's Saturn air ring has been able to deliver outputs of plastomer film at 15-16 lb/hr/in., which is above the industry standard of 13 lb/hr/in. for LLDPE. Future Design moves the forming cone three times higher on the bubble than it would for LLDPE processing and adjusts the chimney to the bubble cone. The Saturn ring, which uses the company's Maximizer lip-set technology, has an adjustable flow cavity that can be used to increase the air volume while lowering its velocity, which gives the bubble more residence time in the primary air stream for greater cooling.

Besides a dual-lip air ring, internal bubble cooling (IBC) is also strongly recommended. Using IBC adds 20-30% more output for many PEs but typically increases plastomer output by approximately 10% because of screw or air-ring limitations, says Hellmuth of Battenfeld Gloucester. "Output rates of a plastomer in blown film using IBC is on par with the output rate of conventional LLDPE running without IBC," he says.

However, Mark Davis, director of product development at New England Extrusion, a film producer in Turners Falls, Mass., says you needn't rush out and buy an IBC system before you try to run the plastomers. "We've run them without IBC and had good results," he says. "But with IBC you get enhanced cooling and you could get additional bubble stability."

ATOP THE TOWER - AND AFTER

Many processors should upgrade their collapsing frame if they plan to process plastomers. Because of plastomers' tackiness, common wooden-slat A-flames create too much friction, potentially harming film clarity. At least one source says wear strips over the wooden slats work well to reduce friction and improve clarity. However, many equipment makers recommend upgrading to a roller-type collapsing frame if good clarity is important. And processors willing to spend the extra money could purchase a noncontacting air collapsing frame, which also provides additional cooling as it flattens the bubble.

In addition, equipment suppliers speculate that if the limitations on throughput rates are eliminated, and if processors take advantage of the down-gauging potential of these high-strength resins, then higher-speed winders may be needed. Printing and bag making might have to be moved off-line to accommodate optimized plastomer extrusion rates.

RELATED ARTICLE: FOR DETAILS ON ITEMS IN THIS ARTICLE, USE READER'S SERVICE CARD

Plastomer Resin Suppliers

Dow Plastics, Midland, Mich. Exxon Chemical, Houston

Blown Film Equipment Suppliers Experienced with Plastomers

Addex Inc., Boston Alpha Marathon Mfg. Inc., Mississauga, Ont. Alpine American, Plastics Div., Natick, Mass. Battenfeld Gloucester Engineering Co. Inc., Gloucester Mass. Black Clawson Sano, Amelia, Ohio Brampton Engineering Inc., Brampton, Ont. Davis-Standard Div., Crompton & Knowles Corp., Pawncatuck, Conn. Egan Davis-Standard, Somerville, N.J. Filmaster Inc., Fairfield, N.J. Future Design Inc., Brampton, Ont. Macro Engineering & Technology Inc., Mississauga, Ont. Merritt Davis Corp., Hamden, Conn. Reifenhauser Film Systems, Peabody, Mass. Windmoeller & Hoelscher Corp., Lincoln, R.I.