By Jan H. Schut
High value microlayer films have been made commercially for over a decade, with up to 500 microlayers. But they’re all cast films made horizontally through flat dies. Microlayer blown films, made with round dies, have been under development for almost as long, but are much more difficult because the flow paths through a round die disrupt the microlayers. Microlayer blown film would have certain efficiencies over microlayer cast film, such as no edge trim waste, more efficient product changovers, and more even biaxial orientation.
Over the past three years, however, a major packaging player has quietly commercialized the world’s first microlayer films made with a round die. During 2009 and 2010, the Cryovac division of Sealed Air in Duncan, S.C., (www.cryovac.com) began making microlayer shrink films, called CT-301 and CT-501. CT stands for Cryovac Technology Platform referring to the microlayer core. The first Material Safety Data Sheets for CT-301 and 501 are dated May 20, 2009, so testing may have begun then.
CT-301 was announced in France in February 2010 and in September 2010 in the U.S. CT-501 and a third microlayer film, CT-701, were announced in October 2010 in the U.S. The MSDS for CT-701 is dated Dec. 15, 2010. None of the announcements, however, generated any reports on the technical significance of the films.
The three films are “based on the patent-pending Sealed Air micro-layering technology platform,” according to Cryovac. They are believed to have 25 microlayers plus two other layers on either side, or 29 layers in all. Cryovac’s published patent application describes creating a microlayer core on a separate “forming stem.” This microlayer fluid mass then enters the flow between other layers inside the coex die.
Cryovac’s epic achievement is the first commercial extrusion of anything with more than 11 layers through a round die. CT-301 and CT-501 are attention getting for a combination of thin gauge (0.30 mil), high Elmendorf tear strength and ability to work in automatic shrink wrap machinery, which no shrink film could do before, according to Cryovac’s patent application (U.S. Pat. Applic. # 2010/0227136 Sept. 9, 2010). The application for “Multilayer, Heat-Shrinkable Film Comprising a Plurality of Microlayers” describes such significant improvement in Elmendorf tear that the films can be down-gauged 50% and have the same properties (Elmendorf tear of at least 10 grams) and the same performance.
Thinner, lighter weight films mean source reduction, longer rolls, longer between roll changes, lower transportation and storage cost for customers. A CT film distributor, for example, advertises that a roll of CT-301 is 2667 meters long vs 1332 meters long for a roll of standard shrink film with the same performance. CT-301 also has moisture and oxygen barrier properties (see table). CT-501 is described in Cryovac literature as a “breakthrough” shrink film, which is “softer shrinking.” CT-701 has oxygen and moisture barrier like CT-301, but is stiffer for high-speed automatic wrapping.
FOUR BIG SURPRISES
The scale-up to commercial production must have been exciting. Even allowing for dramatic flair on the part of the patent’s authors, the wording of the shrink film patent application exudes a sense of genuine surprise not found in Cryovac’s original patent application on the microlayer blown film die, which published only six months earlier.
The inventors use the words ‘surprisingly’ and ‘unexpected’ a half a dozen times. “Surprisingly, it was discovered that the inclusion of a plurality of microlayers in a heat-shrinkable film enabled the thickness, and therefore polymer usage, of such film to be reduced by up to 50%, yet still perform as well as an otherwise identical film having twice the thickness and twice the polymer usage,” the inventors say.
Shrink films are made by a two-step process in which a thick tape-like film is extruded downward, quenched in a water bath, then reheated and inflated to orient it and set a certain level of shrink tension into it. Cryovac’s process patent application (WO 2010/101895 A1, Sept. 10, 2010) and microlayer die patent application describe a discontinuous double-bubble process, blowing an initial tape-like bubble, then winding it on rolls to be oriented later to finished size.
“Surprisingly” the inventors found the microlayer shrink films could be stretched a second time up to 36 times their original size, whereas the same film without microlayers could be stretched only up to 25 times. Maximum width for CT films is 1610 mm.
More surprises were in store for the inventors. “A further unexpected benefit,” the inventors say, was that microlayers allowed relatively inexpensive polyethylene to be used and still “unexpectedly improved the Elmendorf Tear of such films, thereby eliminating the need to use expensive and exotic resins (like octenes) to achieve high performance.” Finally, microlayers allow scrap to be reprocessed even in 0.3 mil shrink films, which wasn’t possible at all before. “Surprisingly, the inventors discovered” that they could use 50% reprocessed micro-layer film and 50% LLDPE as one of the alternating microlayers and achieve an overall scrap content of 12.5 wt% of the entire film structure.
The only other known microlayer blown film technology is the Modular Disk Die developed by BBS Corp., Spartanburg, S.C., in the late ‘90s, invented by Henry Schirmer (email@example.com), a retired director of new technology from Cryovac. Schirmer’s Modular Disk Die goes up to 30 layers including 25 microlayers. Both his and Cryovac’s patents use horizontal plates to flow successive microlayers onto each other. BBS has also invented a Layer Sequence Repeater, which can be used inside the Modular Disk Die to insert nanolayer sections between microlayer sections. BBS has built a developmental Modular Disk Die as big as 18 in. in diameter and partnered since last year with Jin Ming North America (www.stjm.com) in Tryon, N.C., to offer commercial scale trials.
MICROLAYER BLOWN FILM WITHOUT A MICROLAYER DIE?
The second game-changing new development in round microlayer dies is that you might not need one to make microlayer blown film. Dow Chemical Co. (www.dow.com) made microlayer blown film with over 100 layers through a modified crosshead-style blown film die, forming the microlayer structure in the melt before it reaches the die. Dow R&D fellow Joseph Dooley presented the patent-pending new technology (U.S. Pat. Applic. # 20100215879 Aug. 26, 2010) for the first time at the SPE’s recent ANTEC conference in May (see blog for May 2011).
Dow’s technology forms a multilayer structure in a coex feed block, then passes the coex structure through Dow’s previously patented Layer Multiplier to form the microlayers. The Layer Multiplier is based on the inventions of Walter Schrenk at Dow in the 1970s. The microlayer section then goes through an encapsulation feedblock where it is coated with a protective layer top and bottom. Dow’s encapsulation technology was originally developed to coat its corrosive PVDC resin before going into a blown film die.
So far it’s all historical Dow technology and all horizontal. Then the encapsulated microlayer stack is split in two and fed into a round crosshead die manifold. The manifold is modified so the two flows don’t meet abruptly on the other side, which would disrupt the continuity of the microlayers. Instead the two flow paths overlap, the way a stovepipe is put together. Overlapping the flow doubles the number of microlayers in the overlap area of the bubble and avoids a weld line, Dow says.
The modified manifold can be combined with other manifolds in a conventional multilayer blown film die, for example replacing the middle manifold in a five-layer die. This leaves the other four manifolds for additional layers (like adhesive or skin). Dow can thus make microlayer film in a conventional blown film die with one modified manifold. This approach also neatly stays out of the way of both the Schirmer and Cryovac patents on microlayer blown film dies.
Dow ran tests on a 7-in. lab blown film line making 34-layer PE blown film with 27 microlayers, two encapsulation layers, and five coex layers, two on one side of the microlayer core and three on the other. Dooley at first used Dow’s 501 LDPE in all 27 microlayers, pigmenting alternate microlayers to be able to see them and check continuity. The 27-microlayer core was about 25% of the finished film thickness, he reports.
Dow then added another Layer Multiplier unit to produce 108 microlayers, alternating LDPE and plastomer, plus two encapsulation layers and four coex layers for 114-layer blown film. Benefits to Dow’s microlayer blown film vs microlayer cast film also include the ability to alternate foam and film structures, according to Dow’s patent application.
Sam Crabtree, a research scientist in materials and parts processing at Dow, also presented a similar technology combination at ANTEC, but for microlayer blow molding. It consists of a coex feedblock, Dow’s Layer Multiplier, encapsulation and a modified extrusion blow molding head, based on the same patent application (U.S. Pat. Applic. # 20100215879 Aug. 26, 2010). Crabtree describes what is believed to be the first microlayer blow molding ever. All papers from the ANTEC conference are available from the SPE for $150 for non-attendees (www.4spe.org). The world of microlayers, which has been flat for the past 30 years, is now going round!
Properties of world’s first microlayer films made with a round die
Film CT-301 CT-501 CT-701
Gauge 30 30 30
Yield, sq.in./lb 99,500 98,911 NA
Impact Strength Peak
Load, lbs 7.5 5.8 4.2
Haze, % 3.2 5.8 2.7
Clarity, % 85.5 82 88.5
Gloss, % 86 70 82
Coefficient of Frict. (film-
to-film, kinetic) 0.17 0.30 0.25
100%RH, 100 F) 2.2 NA 2.0
OTR (cc/sq.m/24hrs @
73 F, 1 atm) 17,000 NA 13,000
Tensile Strength, kpsi 20.9/21.6 18.7/19.2 14.5/15.5
Elong. @ Break, % 88/105 92/110 75/90
Modulus, kpsi 69.0/69.6 92/83 105/105
Elmendorf Tear, g 15.9/11.8 17/16.6 8.0/3.0
Shrink Tension, psi
@ 200 F 476/664 481/503 481/503
@ 220 F 499/641 518/475 518/475
@ 240 F 548/578 486/424 486/424
@ 260 F NA 405/338 405/338
Free Shrink, %
@ 200 F 11/17 19/22 19/22
@ 220 F 20/30 36/42 36/42
@ 240 F 53/57 71/70 71/70
@ 260 F NA 79/77 79/77
Source: Cryovac/Sealed Air