Coextrusion makes LCP competitive in barrier packaging.

A recently formed New England company is looking to develop commercial packaging applications for coextruded film and sheet that utilize the superior barrier and heat-resistance properties of thermoplastic liquid-crystal polymers (LCPs).

Superex Polymer Inc., Waltham, Mass., is utilizing technology introduced three years ago by its parent firm, Foster-Miller Inc. of Waltham, for processing biaxial and multiaxial oriented LCP film (see PT, Feb. '91, p. 35). This process was designed to overcome the major processing drawback for LCPs: that the material tends to orient itself only in the direction of flow, creating weakness in transverse directions.

As revealed at a technical conference in late 1990, Foster-Miller's technique involves downward blown-film extrusion using a special annular die in which concentric inner and outer mandrels rotate in opposite directions. Shear produced by this counterrotation produces some orientation, and the film is further oriented by stretching after it leaves the die. Melt is fed from the extruder to a metering pump and filter, and then to a distribution block that houses the counterrotating die.

Superex was established nearly two years ago and is currently in pilot production of coextruded thermoplastic sheet, film, tubing, and thermoformed articles. The technology incorporates the Foster-Miller LCP sheet method with a unique three-layer coextrusion process. The coextruded sheet is composed of an LCP layer, a recently developed, proprietary thermoplastic tie layer, and a PET layer. The tie layer is the crucial discovery that allows LCP to be coextruded with other plastics.

Current total film thickness runs 2-4 mils with the LCP layer accounting for about 20-30% of that thickness. A near-term goal will be to reach a film thickness of 1 mil. The Foster-Miller process is said to offer a more uniform distribution of film layers than conventional coextrusion methods. Superex indicates a typical range for LCP thickness across the film width is [+ or -]10%.

Additional coextruded materials in development for the near term will match LCP with layers of nylon and polycarbonate and include coextrusions with more than three layers. A longer-range development goal is to coextrude sheet of polyolefins with LCP.

Richard W. Lusignea, president of Superex and formerly the manager of polymer composites for Foster-Miller's Materials Technology Div., says his firm is targeting three application niches: high-barrier film and sheet for food packaging and medical markets; electronic substrates; and lightweight structural tubing. The company is operating two pilot production lines: one for film (layflat widths of 10-20 in.) and one for tubing (0.25- to 2-in. diam.).

In sheet and film, the LCP imparts an off-white appearance, suitable for translucent applications such as blood bags, fluid/fuel containers, lid stocks, cap seals, food containers, and hot-fill retort packaging. The main performance advantage of the coextruded material lies in the LCP layer, which offers superior moisture and oxygen barrier, chemical and fuel resistance, and high-temperature (400 F) resistance.

COST COMPETITIVE

Though typically considered a high-priced ($10/lb) engineering resin, the Superex process for coextruding LCP film enables it to be competitive with traditional barrier resins, such as EVOH and PVDC, on a cost/performance basis, Lusignea says. "We don't expect to replace existing packaging materials in every application, but we feel we can be competitive in certain niches, especially those that have stringent requirements for oxygen and moisture," he says. It also provides the ability to downgauge existing packaging film and sheet, due to the LCP's superior gas-barrier performance.

The company offers the following as a cost/performance comparison for LCP vs. traditional packaging resins. Superex says that in order to achieve an oxygen permeation rate of 0.1 cc/100 [in..sup.2]-day-atm, the cost of LCP resin needed is 9[cents]/1000 [in..sup.2]. For equivalent performance, the cost of EVOH would be 10-20[cents] and that of PVDC would be 15[cents]. Oxygen permeability of Superex films based on monolayer LCP is as low as 0.015 cc-mil/100 [in..sup.2]-day-atm.

Lusignea anticipates the company will reach annual production volume greater than 1 million lb within two years. That volume will include coextruded sheet, structural tubing, and thermoformed molded articles.

Production of the coextruded LCP sheet is only one aspect of Superex's business plan, says Lusignea. The company also seeks to license the film and sheet production technology, as well as manufacture and sell the extrusion equipment. As of last month, Superex had not yet struck any licensing agreements.