Biaxial oriented film technique exploits properties of LCPs.

Biaxial Oriented Film Technique Exploits Properties of LCPs

A new method of processing biaxial and multiaxial oriented blown films utilizing liquid crystal polymer (LCP) as an in-situ "microcomposite" reinforcing agent has been introduced by Foster-Miller Inc., Waltham, Mass.

Unveiled by the company at the recent Ultralloy '90 conference in Houston (sponsored by Schotland Business Research Inc., Princeton, N.J.), the extruded film system was developed under contracts from the Ordered Polymers Program of the Materials Laboratory Polymer Group at Wright-Patterson Air Force Base in Dayton, Ohio. Foster-Miller first revealed its development work in thea area of LCP-based films early last year (see PT, April '90, p. 96); this new system respresents the next step in the ongoing refinement of the process.

The company recently won the second phase of a Small Business Innovation Research contract from the National Aeronautics and Space Administration (NASA). Phase one of the contract involved using the biaxial-orientation extrusion system to process an alloy film comprised of LCP and a thermoplastic polyimide known as LaRC-TPI, developed by NASA's Langley Research Center in Norfolk, Va. Foster-Miller executives believe this is the first time a thermoplastic polyimide has been extruded from a melt into biaxially oriented film. Phase two will continue development of the film extrusion system and involve research on thermoforming and blow molding complex-shaped structural parts using oriented-film sheets.

COUNTERROTATING DIE

Foster-Miller executives Adi R. Guzdar, v.p. of technology and systems development, and Richard W. Lusignea, Materials Technology Div. manager of polymer composites, say their goal was to develop a processing system that could exploit the microbrillar reinforement properties of LCPs blended with another thermoplastic, through biaxial orientation of the alloyed material. This was accomplished through a film extrusion system using a counterrotating annual die.

The counterrotating die is made up of concentric inner and outer cylinders (outer turning clockwise, inner turning counterclockwise). Melt is fed from the extruder to a metering pump and filter and then to a distribution block that house the die. It is pumped through the block and forced into holes along the outer rotating mandrel. Shear is induced by the counterrotation of the two cylinders, biaxially orienting the flow into a film. A transverse shear flow is imposed on the axial shear developed as the polymer melt is extruded through the die, and molecules are aligned along two distinct axes within a single film ply.

Upon exiting the die, the film is further oriented by stretching, and then wound on rolls.

Foster-Miller executives say the process offers flexibility to tailor properties of the oriented film. The first key variable is the initial alloying and compatibilization of the LCP and its partner thermoplastic. Though Foster-Miller's initial focus was an LCP blend using polyimide, the system can utilize various thermoplastics. Two immediate candidates appear to be polypropylene and thermoplastic polyester. Thus, the system potentially could impart the high mechanical strength, barrier capabilities, and heat resistance associated with LCPs into a matrix of a lower-priced commodity resin.

NARROW COMPATIBILIZER

WINDOW

Research so far has focused on unfilled alloys and has determined that between 10% and 30% LCP content provides optimum mechanical properties. However, that range could be altered in either direction, depending on the most important function that the LCP was intended to serve. For example, high barrier properties would require an LCP content over 30%.

Because of the acutely immiscible nature of LCPs as an alloy partner, proprietary block copolymers are introduced into the melt to promote interfacial adhesion of the materials. However, company officials say this is an especially delicate compatibilizing recipe, with a narrow window of error. While limited chemical interaction is necessary for a successful blend, too much miscibility will suppress the discrete formation of the LCP's fibrillar reinforcements.

NEXT STEP: STRUCTURAL PARTS

The new blown film processing method is significant in that it takes advantage of the "self-reinforcing" benefits of LCPs for larger, structural parts, say Guzdar and Lusignea. The concept for structural components would involve multilayer, laminated stacks of oriented film, which then could be processed as thermoformed sheet. Extrusion blow molding is another possibility for fabricating larger structural parts. While they decline to elaborate further on future developments, subsequent efforts will focus on extruding thicker films in order to provide greater application for thermoformed and extrusion blow molded parts, as well as tubular structures. The thicker films now under development will have full LCP orientation throughtout their cores, as well as their skins.

The biaxial film technology is currently at the laboratory stage of development. At present, Foster-Miller has built several lab-scale machines. Guzdar and Lusignea say the system is now under review by several potential licenses or joint-venture partners to develop it into commercial-scale production of unfilled, LCP-reinforced structural thermoplastic components.

PHOTO : Key to Foster-Miller's biaxial film extrusion systme for LCP-based blends is a counterrotating annular die made up of inner and outer cylinders, which receives the polymer melt flow.