More layers! Blow films push the coex limit.
* When Holmes Packaging in Rotorua, New Zealand, ordered its first coex blown-film line in 1995, it planned to extrude five layers. By the time Brampton Engineering began to build the machine, Holmes wanted six layers. When the machine was being crated for shipment, Holmes asked for a seventh layer. "We unpacked it, changed out the die, completely rebuilt the controls and replaced a 2.5-in. extruder with two 2-in. ones, which were shipped after the rest of the line," recalls Dean Ward, Brampton's v.p. of sales.
The next line Holmes bought from Brampton, delivered last October, had nine layers and set a world record for a commercially available system. Brampton is building a second nine-layer/nine-extruder line for Holmes, and Ward says, "We now have customers talking about 11 layers."
Five years ago, only a few dozen companies around the world made even five-layer blown films and only a handful went higher than that. Nowadays, hundreds of processors make five-layer films and dozens make six, seven, eight, or nine layers in a race to improve film properties and reduce costs.
Highly layered films are tailored to create specific levels of barrier protection for food packaging - keeping oxygen or even certain light wavelengths out, or flavors and perfumes in. Foods needing such protection include cereals and muffins with orange or blueberry flavors, smoked salmon, kangaroo meat, lamb, cheese, and bag-in-box wine.
Raise properties, trim costs
What can seven layers do that five can't? They can make "superbarrier" sandwiches of a slice of EVOH between slivers of nylon, each slice being only 3-7% of total film thickness. Both resins are polar, so no tie layer is needed. The sandwich reduces oxygen transmission 95% compared with a five-layer film with one nylon layer that's 20% of the total, according to a Brampton technical report.
Besides barrier, processors are using thinner layers to create new materials like soft thermoformable nylon. "Making films with the same exact content of nylon and tie resin in three, five, or seven layers results in three totally different films," Brampton's Ward notes. A film with three layers is rigid. One with five layers is flexible. And with seven layers separated by tie layers (nylon-tie-nylon-tie-nylon-tie-LDPE seal layer), the result is a soft film that can be more easily thermoformed with deep draws, which might otherwise lead to stress cracking. Hotdog packages are one application.
Processors are finding new medical and industrial markets for highly layered films. Seven layers are used in sterile medical packaging. Five-layer protective films are being used to protect the paint on new cars before delivery. And a giant five-layer, five-extruder line with a 52-in. die was recently installed in Japan to make greenhouse covers. Additives in different layers block certain wavelengths of light while letting other wavelengths through, tailored to the needs of specific plants.
Dividing films into thinner layers allows use of resins that don't normally have the melt strength for blown film. For example, polypropylene adds strength and stiffness to five-layer blown films. Its lack of melt strength isn't a problem because it is supported by other layers. Even high-melt-index HDPEs designed for cast films can be used as surface "scuff" layers, says James Krohn, marketing manager for packaging films for Equistar Chemicals LP.
Thinner layers also facilitate scrap recycling. "Below 10-15% nylon content you can recycle PE with nylon," notes Eric Hatfield, technical v.p. of FlexTech Packaging Ltd. in Cincinnati, a start-up company that is concentrating on five-layer barrier films.
A simple but economically important principle is that extruding more layers allows those layers to be thinner. High-layer blown film lines can save considerable resin cost by thinly slicing expensive materials and backing them with less-expensive resin, while maintaining or even improving performance.
For example, reducing a skin layer of expensive ionomer is possible by going from five layers to seven. "You can save $52,000 a month on a 40-in.-wide web while getting the same seal properties," says Lyle Hoegy, area sales manager for Brampton Engineering. In a five-layer film, 25% of the wall may be ionomer, backed by 10% tie, 20% EVOH, 10% tie, and 35% LDPE seal layer. Going to seven layers lets you trim the two surface layers, reducing the ionomer thickness to 15%, backed with 10% of less-expensive EVA, and cutting the LDPE seal layer to 20%, backed with 15% of less-expensive LLDPE. The LLDPE can be trimmed, too, with a layer of MDPE next to the tie layer to improve moisture barrier, says William Hellmuth, senior product manager at Battenfeld Gloucester Engineering.
"Barrier properties also improve simply by thin-layering. Using two thin layers of EVOH instead of one thick one reduces pinholes and improves consistency," says John Perdikoulias, a CAE specialist in coextrusion flow simulation and v.p. of software supplier Compuplast Canada. "If an EVOH layer is designed to be 5 microns thick, it may vary between 3 and 7 microns. If you use two thin EVOH layers, it's statistically unlikely that any two thin spots will line up."
New hardware, new features
A few large packagers with homegrown hardware systems are considered to be the pioneers of highly layered blown films. The likes of Cryovac (now Cryovac Sealed-Air Corp.) in Duncan, S.C., and Viskase in Chicago are believed to have gone beyond nine layers with their own proprietary equipment. Blown films have even been made with 11 and 14 layers, but only by self-laminating or "collapsed surface welding" (i.e., blocking) a tube of six or seven layers into a single flat web.
However, commercial machine suppliers are also busy developing and installing their most highly layered lines ever. Some are getting into high-barrier coex for the first time.
* Addex Inc. has a patent pending on a new pancake die, trademarked Redi (Regular Division). Addex president Rick von Kraus says the die design departs from conventional thinking by "dealing with the elasticity factor of the molten material as well as its shear history." Addex sees an advantage for its pancake die in larger diameters - up to 30 in. or more. That's because its design minimizes pressure between the die plates and shortens the melt path so sensitive resins like EVOH have less chance to degrade.
The new die opens the way for the company to compete in more highly layered systems. Addex has a half-dozen five-layer dies or systems installed and two more under construction using its traditional spiral-mandrel dies. It has yet to build one for more than five layers.
* Alpha Marathon has built three seven-layer dies to produce barrier food packaging in Finland, Argentina, and the U.S. It has also built 10 five-layer dies (four of them supplied with complete lines) for customers in Latin America, South Africa, and Europe, as well as seven five-layer dies for an Italian extrusion machinery maker. Five- and seven-layer dies have been built using a brand-new low-pressure, conical stack design that has very short spirals, the company says. It is used for dies under 10 in. diam. The company offers IBC on spiral mandrel dies over 12 in. and on the new stack die down to 4 in. diam.
* Battenfeld Gloucester Engineering shipped its first nine-layer line in June to New Zealand. It has a 10-in. Optiflow spiral die and is the second commercial me-layer die anywhere and the first of a mandrel type. "The nine-layer film will be the same 2 mils thick as the present seven-layer film," says Hellmuth. The company has also built eight five-layer lines, two six-layer, 18 seven-layer (including 10 for one U.S. packaging maker), and one nine-layer. All are concentric spiral-mandrel types. In the past three years, Battenfeld has shortened its die height by 15 in. to reduce melt travel time, pressure loss, and combined flow length to minimize interfacial instability.
* BBS Corp. (Back to Basics), recently patented a low-pressure, modular die (U.S. Patent 5,762,971, issued June 9) that uses thin, "disposable" stainless-steel disks to distribute the melt. Within one tier of a die "cell," the disks divide the melt coming from one entry point into two, then four, then eight streams. A second tier spreads the melt from eight points to an annular layer. Die gaps are unusually large (0.030-in.), compared with conventional high-pressure stack dies. BBS has sold two 2-in. dies so far, one for seven layers to MS Intertech in Arlington Hts., Ill., and one to Cryovac Sealed Air. A 9-in. lab die can make up to 27 layers.
* Brampton Engineering has built a total of 45 five-layer, five six-layer, 11 seven-layer, six eight-layer, and one nine-layer line. A second nine-layer system is under construction, along with the first 10-layer die for Brampton's own R&D lab. These pancake stack dies range from 1 in. to over 52 in. diam. They use technology licensed from GE Plastics for thermal isolation of melt layers. It allows adjacent materials to run at temperatures up to 120 [degrees] F apart, so EVOH with a processing temperature of 440 F can be sandwiched between layers of nylon at 500 F or more with reduced risk of degradation.
Brampton offers a patented annealing stage that reheats asymmetric coex films to relax stresses that cause curling. Brampton also has applied for a patent on a new antiblocking device that reportedly reduces the need for antiblock additives in surface layers. Such additives reduce film clarity and add cost. A small tube carries chilled air from the IBC device all the way up to the primary nip, where it blows between the two sides of film as they come together. The first commercial application was last fall on a five-layer line in Europe that is making PVdC barrier film with EVA skins.
* Davis-Standard Egan will introduce its first stack die at K'98 in Dusseldorf in October. The new design can go up to nine layers, says Rick Keller, industry director of film systems. The die has a smaller wetted surface and lower residence time because it is side-fed, he says. Up to now, Egan has installed 36 five-layer blown film lines, including 14 originally from Western Polymer, which Davis-Standard acquired in 1994. All use spiral-mandrel dies.
* Kiefel Inc., which specializes in HDPE lines, has 25 five-layer systems installed worldwide. All use bottom-fed spiral-mandrel dies of 6-20 in. diam. "We will possibly be making a seven-layer system," says Kiefel president Steven Engel. "We'll either stay with a basic spiral die or go to a slightly modified design." A year and a half ago, Kiefel added IBC to raise outputs from its five-layer line.
* Hosokawa Alpine America, a new player in five-layer coex, delivered its first commercial system early this year. Alpine previously built a six-layer line for its lab in Germany and plans to install a five-layer line in its U.S. lab this month. Alpine's machine has stack dies with horizontal spirals. Each 8-in.-high stack consists of three plates for melt distribution, which allows it to process a wide range of polymers besides conventional ones, including nylon, EVOH, EVA, and even high-viscosity polymers like HMW-HDPE. "And it doesn't leak," says David Nunes, Alpine president.
* Macro Engineering & Technology showed its coex R&D line at May's Plast-Ex show in Toronto. It has five extruders and a five-layer, 18-in. conical stack die for making film up to 64 in. wide. Macro plans to convert the die later this year to seven layers, using the same five extruders. Macro previously built one six-layer line for Dow Chemical Co.'s R&D lab. It was used to make developmental quantities of film for customers using Dow's Saran PVdC.
* Reifenhauser in Germany has built 14 five-layer lines, including one for the U.S. It delivered its first eight-layer line this spring to a packaging company outside North America. Two years ago, Reifenhauser added the ability to put the same size extruders on every layer of high-layer coex systems for greater flexibility. "Now you can mix, match, and change layers around," says president Matt Bangert.
* Windmoeller & Hoelscher has 25 five-layer and three seven-layer systems installed worldwide. Its seven-layer lines all have conical stack dies under 14 in. diam. W&H recently introduced a new stabilizing cage around the bubble, which turns at the same speed as the oscillating haul-off. The first of these was installed four months ago on a five-layer coex line at World Class Films in Yonkers, N.Y., for making five-layer protective film.
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|Title Annotation:||coextrusion of plastic films|
|Author:||Schut, Jan H.|
|Article Type:||Cover Story|
|Date:||Aug 1, 1998|
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