Polyimide Fibers

This application claims the benefit of U.S. Provisional Application No. 60/021,206, filed Jul. 3, 1996, now abandoned.

This invention relates generally to the extrusion preparation of fibers and relates specifically to the preparation of polyimide fibers having enhanced flow and high mechanical properties.

High performance polyimides are used in the aerospace industry, for example, in joining metals to metals, or metals to composite structures. In addition, polyimides are rapidly finding new uses as matrix resins for composites, molding powders and films. These materials display a number of performance characteristics such as high temperature and solvent resistance, improved flow for better wetting and bonding, high modulus, chemical and hot water resistance, and the like. Another area of application is in the manufacture of lighter and stronger aircraft and spacecraft structures.

U.S. Pat. No. 5,147,966 (St. Clair, et al) discloses a polyimide that can be melt processed into various useful forms such as coatings, adhesives, composite matrix resins and film. This polyimide is prepared from 3,4'-oxydianiline (3,4'-ODA) and 4,4'-oxydiphthalic anhydride (ODPA) in various organic solvents. The use of phthalic anhydride as an endcapping agent is also disclosed in this patent to control the molecular weight of the polymer and, in turn, to make it easier to process in molten form.

In the present invention, a polyimide fiber is produced from the polyimide powder that is prepared from the 3,4'-ODA and ODPA monomers. This polyimide powder was endcapped with phthalic anhydride, as described in U.S. Pat. No. 5,147,966, to control the number average molecular weight in the range of 10,000 g/mole to 20,000 g/mole.

The need for polyimide fibers of this type are apparent. Since this polyimide is fire resistant, these fibers would be useful in manufacturing articles which might be exposed to flames/fire to prevent their burning. These articles would range from clothing articles to aerospace components. In addition, fibers from this polyimide, since they are thermoplastic, could be co-mingled with reinforcing media such as glass fiber, graphite fiber, inert fillers, and the like, and be subsequently melted, in-situ, to form composite structures which would have utility in fire-resistant applications in the automotive and aerospace industries.

A primary requirement for such fibers is that they have adequate textile properties to permit processing using normal textile equipment. The two primary measures of fiber quality are tensile strength and elongation-to-break percentage. Adequate strength must be developed to afford handling properties during processing and to afford adequate strength in the final form. Additionally, the fiber must possess adequate elongation-to-break properties to provide the required toughness in the processing operations and in the final components.

The invention is a polyimide fiber formed from the reaction product of 3,4'-oxydianiline (3,4'-ODA) and 4,4'-oxydiphthalic anhydride (ODPA) in 2-methoxyethyl ether (diglyme), or other suitable solvent, and endcapped with phthalic anhydride to control the number average molecular weight in the range of 10,000 g/mole to 20,000 g/mole.

It is an object of the present invention to provide a polyimide fiber that has the textile physical property characteristics to permit processing into useful articles using normal textile equipment.

Another object of the present invention is polyimide fiber having high strength and high elongation-to-break property characteristics.

A further object of the present invention is a process for preparing polyimide fibers via melt extrusion of a molten polyimide.

An additional object of the present invention is a process for making polyimide fibers over a range of processing conditions which produce fibers having high strength and high elongation-to-break properties.

Another object of the present invention is a process for preparing polyimide fibers that exhibit mechanical properties that permits them to be used in various commercial applications.

Still another object of the present invention is to provide polyimide fibers having fire resistant properties.

The invention and its advantages are illustrated by the specific examples given below. Conditions for melt spinning/extrusion are detailed in the individual examples and a comparison of resulting mechanical properties are shown in the data Tables 1-3.

The polyimide powder employed in each of the specific examples was the reaction product of 3,4'-ODA and 4,4'-ODPA endcapped with phthalic anhydride to control the number average molecular weight of the polymer powder in the range of 10,000 g/mole to 20,000 g/mole. This polyimide powder was obtained commercially as LaRC.TM.-IA from IMITEC, Inc., 1990 Maxon Road, Schenectady, N.Y. 12308.

In each example below, the LaRC.TM.-IA polyimide powder, as received, was dried in an air oven at approximately 200.degree. C. for twenty-four hours prior to use to drive off any residual solvent and/or moisture. The melt extrusion was performed using a Barbender, single screw extruder, equipped with a NASA-LaRC drive unit, and provided with a 8-filament, 0.0135 inch vertical die. The extrusion rate employed was 10 revolutions per minute (rpm).

Polyimide fibers were extruded from three heights to vary the diameter of the fiber: (1) 364.5 inches, (2) 209 inches, and (3) 100.5 inches. Three processing temperatures were also utilized: 340.degree. C., 350.degree. C. and 360.degree. C.

The following specific Examples are provided for purposes of illustration, and are not intended to serve as limitations of the invention.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 340.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 30 feet, 4.5 inches (364.5 inches). The average final fiber diameter was 0.0094 inch. Mean tensile strength, modulus, and elongation were 22.7 ksi, 412 ksi, and 103%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 350.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 30 feet, 4.5 inches (364.5 inches). The average final fiber diameter was 0.0068 inch. Mean tensile strength, modulus, and elongation were 23.1 ksi, 436 ksi, and 102%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 360.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 30 feet, 4.5 inches (364.5 inches). The average fiber diameter was 0.0071. Mean tensile strength, modulus, and elongation were 20.8 ksi, 415 ksi, and 84%, respectively.

The mechanical properties of the melt-extruded fibers obtained from a height of 364.5 inches in Examples I-III are tabulated in TABLE I.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 340.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 17 feet, 5 inches (209 inches). The average final fiber diameter was 0.0094 inch. Mean tensile strength, modulus, and elongation were 20.9 ksi, 406 ksi, and 79%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 350.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 17 feet, 5 inches (209 inches). The average final fiber diameter was 0.0101 inch. Mean tensile strength, modulus, and elongation were 20.6 ksi, 448 ksi, and 65%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 360.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 17 feet, 5 inches (209 inches). The average final fiber diameter was 0.0117 inch. Mean tensile strength, modulus, and elongation were 17.7 ksi, 441 ksi, and 34%, respectively.

The mechanical properties of the melt-extruded fibers obtained from a height of 209 inches in Examples IV-VI are tabulated in TABLE II.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 340.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 8 feet, 4.5 inches (100.5 inches). The average fiber diameter was 0.0147 inch. Mean tensile strength, modulus, and elongation were 19.9 ksi, 433 ksi, and 50%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 350.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 8 feet, 4.5 inches (100.5 inches). The average final fiber diameter was 0.0118 inch. Mean tensile strength, modulus, and elongation were 17.3 ksi, 463 ksi, and 28%, respectively.

The oven dried LaRC.TM.-IA powder was placed in the sample feed of the single screw melt extruder. The extrusion temperature was 360.degree. C. with an extrusion rate of 10 rpm. The fiber exited through the 0.0135 inch diameter slit and fell 8 feet, 4.5 inches (100.5 inches). The average final fiber diameter was 0.0115 inch. Mean tensile strength, modulus, and elongation were 15.8 ksi, 465 ksi, and 14%, respectively.

The mechanical properties of the melt-extruded fibers obtained from a height of 100.5 inches in Examples VII-IX are tabulated in TABLE III.

The foregoing specific Examples are given to illustrate the principal of the invention and are not intended to serve as limitations thereof. There are many variations and modifications of the invention that will be readily apparent to those skilled in the art in the light of the above teachings. It is therefore to be understood that, within the scope of the pending claims, the invention may be practiced other than as specifically described herein.