LCPs Break New Ground in Film Coextrusion and Thermoforming
Liquid-crystal polymer extrusion resins cost over $10/lb, but when used sparingly in 2-5 micron layers, they can be cost-effective in barrier packaging films. At least that's the hope of Ticona in Summit, N.J., which recently came out with a new family of LCP resins tailored specifically for coextruded film and sheet. Their first commercial use is in a coex industrial application, but a barrier packaging film containing LCP reportedly will go commercial in a few months.
LCPs offer a unique combination of high barrier to oxygen, aromas, and water vapor, together with chemical resistance superior to that of conventional barrier resins like EVOH or PVDC. Unlike EVOH, their oxygen barrier doesn't weaken with humidity, making them ideal for retort packaging at temperatures up to 250 F. And LCP reportedly can furnish the same barrier level as EVOH but in a layer only 10% as thick.
Previous commercial LCP extrusion activity has involved monolayer blown film for electronics uses and thin medical tubes. Both are extruded with a special rotating-die process developed by Foster-Miller, a technology development and licensing company in Waltham, Mass. Brampton Engineering in Brampton, Ont., builds the die under license from Foster-Miller. The firm has issued less than a dozen licenses since the late 1980s for this process.
For the moment, Ticona sources are revealing few details about what they say is the first commercial coex film with an LCP barrier layer. It was introduced in the U.S. last year for use in the communications field. It is a soft, flexible industrial cast film with five layers in a symmetrical structure of LDPE-tie-LCP-tie-LDPE. The LCP layer is 5 microns thick. The end user, which doesn't want to be identified, has applied for patents on use of an LCP barrier in this application.
LCP's upcoming debut in coex barrier packaging film involves a soft, five-layer structure based on EVA. Samples were run early last year on an 80-in.-wide cast-film line at Battenfeld Gloucester Engineering Co. in Gloucester, Mass.
Ticona hopes that late this year the first five-layer PP/LCP barrier packaging film will be launched in the U.S. This is a stiff film for bag-in-box applications, retorted food pouches, and thermoforming. It is designed to replace vacuum-metalized OPET. A single coextrusion of PP-tie-LCP-tie-PP with 5-10 microns of LCP in the core layer could replace a complex coated and laminated film, Ticona says. The FDA gave its go-ahead to Ticona's LCP on Feb. 21 of this year.
A production-scale run of rigid, five-layer PP/LCP film was made for Ticona on a cast-film line at a machinery supplier earlier this year. The product went out in late February to thermoformers and converters for preliminary trials on commercial packaging equipment. The PP/LCP film has good contact clarity, Ticona says.
Ticona says the new Vectran grades can also be blown into film and have been tested by several major blown-film equipment makers, including Hosokawa Alpine in Germany (U.S. office is in Natick, Mass.).
New ‘stretchable' grades
All three of the initial film applications use Ticona's new Vectran LCP extrusion grades, which are the first LCPs that can be coextruded in standard equipment with conventional packaging resins, according to Ticona. One reason is their lower processing temperatures—as low as 428 F. These are also claimed to be the first LCPs that stretch enough to be biaxially oriented and thermoformed. Although Celanese, Ticona's parent company, came out with LCPs that it claimed were extrudable and thermoformable back in 1986, company spokesmen now say the firm's earlier Vectra LCPs experienced too much strain hardening to permit much drawing or stretching.
The Vectran grades have been commercially available for two years, but were publicly announced only last November after the critical patent (U.S. Patent #6,132,884, Oct. 17, 2000) was issued, describing LCPs with "a high degree of stretchability."
Greater stretch is one of the key differences claimed for the new amorphous Vectran materials, which are chemically different from the Vectra family. Ticona's patent says they have 100% elongation at break.
Two of the new coextusion grades, Vectran V300P and V400P, have glass-transition temperatures of 230 F and are stretchable, Ticona says. Two more grades, V100P and V200P, have no defined Tg and are not thermoformable in very thin layers.
LCP molecules are partly stiff, partly flexible. The stiff parts align in the machine direction, whether the LCP is stretched in the molten or solid phase. This gives LCPs their inherent imbalance of properties—very strong in the machine direction, very weak and splitty in the transverse direction. TD weakness, both molten and solid, has always limited the utility of LCPs in a thin coex layer.
"They don't spread laterally into a thin cohesive layer but tend to puddle in one place," notes Rick Lusignea, president of the Superex Polymer div. of Foster-Miller. "Coextrusion would not be easy."
Dr. Arno Wolf, Ticona's marketing manager for new business development, says the new grades have higher molecular weight and higher viscosity. "We matched the viscosity of standard packaging resins, allowing layers to be extruded as thin as 2 microns in a coex cast film," he says.
More extrudable LCPs
At least one other resin producer has new LCP extrusion grades in development. DuPont Engineering Polymers, Wilmington, Del., also holds patents on extrudable LCP compositions that "form exceptionally tough films," according to the company. A developmental extrusion grade, called HX-8000, is in the "targeted sampling stage," DuPont says.
The new grade is said to be processable on conventional cast- or blown-film equipment.
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