AND THERMOPLASTIC FIBRETS

The present invention relates to the manufacture of high modulus media capable of being thermoformed into thermoplastic structures. In particular, the present invention relates to thermoplastic media used in component parts of aircraft, automobiles, and other structures requiring high tensile modulus properties. The present invention also relates to media that are suitable replacements for metal in parts, thus reducing the weight of the item containing the parts.

BACKGROUND

High modulus structural parts have been previously formed by laminating random distribution carbon fiber webs with thermoplastic polymer fibers or films made of, for example, polyethersulfone, and thermoforming these laminates into composite parts. The carbon webs contained a binder which had to be removed by solvent extraction to prevent decomposition products from forming voids during the thermoforming process. These voids promoted crack propagation and failure.

The decomposition products were formed when the laminates were thermoformed at processing temperatures of approximately 600-650°F, causing either the binder material to decompose or the solvent to be released. Removing the binder to prevent the formation of decomposition products was difficult and residual binder and/or solvent in the laminate resulted in voids forming. On the other hand, carbon fiber has no bonding sites, thus a binder was necessary to form the prior art materials used in the thermoforming process.

SUMMARY OF THE INVENTION

The present invention provides a process which eliminates the need for a binder and for subsequent solvent extraction. The present invention further provides a high modulus structural media which do not form voids upon thermoforming. The present process also avoids the need for other additional materials which can decompose in the molding process.

In particular, the process of the present invention provides a homogeneous dispersion of fibrets and high modulus tibers that have sufficient strength and stability to be handled in a vacuum forming or pultrusion process and formed into a high modulus composite structure without the use of any binder or adhesive materials.

The present invention provides a process for forming high modulus media comprising: a) forming thermoplastic polymer material into fibrets; b) mixing the fibrets with high modulus fibers in an aqueous suspension; c) removing the mixture of the high modulus fibers and fibrets from the aqueous suspension to form a web of the fibret and the high modulus fibers; and d) drying the web to form media.

The present invention further provides high modulus structural media comprising a high temperature/high performance thermoplastic polymer material in the form of fibrets and high modulus fibers.

The present invention further provides high modulus composite structures comprising a substantially homogeneous and substantially void free

mixture of thermoplastic polymer material in the form of fibrets and high modulus fibers.

DESCRIPTION OF THE DRAWING

Figure 1 shows a preferred embodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Any type of thermoplastic polymer material which can be formed into fibrets can be used in the present invention. Combinations of thermoplastic polymers can also be used. Preferred thermoplastic polymer material include high temperature/high performance thermoplastic materials. These high temperature/high performance materials are well known in the art and are referred to as "engineering plastics." Such polymers can be selected from fluoropolymers, liquid crystal polymers, modified aromatic polyamides, polyamide, polyamideimide, polyarylates, polyarylene ketones, polyarylene sulfides, polyaryletherketones, polyarylethersulfones, polyarylsulfones, polybenzimidazoles, polyetheretherketones, polyetherimides , polyetherketones, polyetherketoneketones, polyethersulfones , polyethersulfoneamides, polyi ides, polyimidesulfones, polyketones, polyketone sulfides, polyphenylene sulfides, polyphenylsulfones, and polysulfones.

Most preferred thermoplastic materials are polysuleones, polyarylsulfones, polyethersulfones, and polypherilrlene sulfide sulfones.

The formation of fibrets is known in the art. Fibrets are generally made by flash spinning the polymer. Such a process is described in Smith,

Jeanne, "Cellulose Acetate Fibrets and Their Applications," World Pupp & Paper Technology 1990. p. 79-81 which is hereby incorporated by reference. Other methods can be used to form the fibrets as long as the fibrous structure produced is less than 5 micrometers in diameter and has a large surface area per unit weight. A typical fibret, currently available in the market, such as the cellulose acetate fibrets of Hoechst Celanese Corporation, is highly fibrillated and has a surface area of about 20 m ² /g. This is 60-70 times the surface area of a staple fiber in the 1.5 to 3.0 denier range. The fibrets have a length generally ranging from about 20 to 200 μm. It is believed that the desirable media of the claimed invention are obtained due to the fibrillar structure of the fibrets. The fibrillar structure of the fibret in combination with the paper making process used in the present invention enables the fibrets to act as the binder with the high modulus fibers by entangling the fibers.

The high modulus fibers used in this invention can comprise any type of high modulus fiber. Typical fibers include glass fibers, mica flakes, ceramic fibers, carbon fibers derived from polyacrylonitrile or pitch, and other carbon fibers. The preferred fibers are carbon fibers. Combinations of these fibers can also be used.

Preferred carbon fibers include Hercules® Graphite Fiber AS4 and Thornel® Carbon Fiber T300. Other preferred fibers include Kevlar® fibers and Nomexa® fibers, both of which are polyaramid fibers. Although any high modulus fibers can be used in the present invention, to be suitable for use in, for example, high performance aircraft components, the

high modulus fibers generally must have a tensile modulus greater than about 20 msi. Preferably, the tensile modulus ranges from about 20 to about 65 msi. This range can vary depending upon the performance desired and the cost of the part. A higher tensile modulus fiber will provide a higher performance part, but will increase the cost of the part.

Any conventional diameter high modulus fibers can be used in the present invention. The fibers generally range in diameter from about 5 micrometers to about 20 micrometers in diameter, with a preferred diameter of about 8 micrometers to it about 15 micrometers. A broad range of high modulus fiber lengths can be used in the present invention. Long fibers, however, are difficult to handle in the process of the present invention. On the other hand, end users prefer long fibers to get maximum aspect ratio. In general, the fiber length can range from about 1/32 inch to about two inches. Preferably, the fiber length ranges from about 1/4 inch to about 3/4 inch.

The thermoplastic polymer fibrets can be used in any amounts in the present invention, as long as the desired performance is achieved. Generally, the thermoplastic polymer fibret is present in an amount ranging from about 30 to about 90 percent, based upon the volume of the medium. Preferably, the thermoplastic polymer fibret is present in an amount ranging from about 60 to about 80 percent, based upon volume, with a most preferred amount being about 70 percent.

The remainder of the medium can consist of a single type o high modulus fiber, or a combination of high modulus fibers. It can further contain other

additives which can be used to modify physical properties or improve the economics of the medium.

The medium is formed using a paper-making technique known as a wet-laid process. This process is shown in Figure 1.

Referring to Figure 1, the thermoplastic polymer fibrets 1 and the high modulus fibers 2 are added to a hydrapulper 3 where they are dispersed in water 4 and intermixed. The thermoplastic polymer fibrets 1 and the high modulus fibers 2 are added at a solids level ranging from about 4 to about 6 percent, on a weight basis. A moderate shear mixing is used for a sufficient time to break up and disperse the fibers. High shear mixing over an extended time will tend to break the fibers thus lowering the average aspect ratio.

The mixture is then transferred to a storage chest 5 prior to delivery to a paper machine 8. The storage chest 5 uses a low shear to prevent settling of the fibers from the water. In contrast to standard paper making processes, the fiber/water mixture, known as a furnish, is not refined prior to the transfer to the storage chest 5 due to the brittleness of the high modulus fibers. The furnish is then diluted by the addition of dilution water 6 and then delivered to the paper machine 8. Preferably, the solids level is reduced to about 0.5 to about 2 percent solids, on the basis of the medium desired. The furnish is received in the head box 7 of the paper machine 8 and a woven screen 9 removes the fibers from the mixture in a dewatering process 10. The thus formed web passes over a series of vacuum boxes 11 to form a wet web having about 30% by weight solids. The web is then dried in a conventional

manner, such as through the use of steam dryer cans 12, a tunnel drier or an IR drier. The dried medium can then be rolled 13 or sheeted.

The dried medium is a homogeneous mixture of fibers and fibrets having a random orientation, rather than a unidirectional orientation.

The formed medium of thermoplastic polymer and high modulus fibers can then be formed into a high modulus composite structure using a thermoforming process. In the preferred process, the medium is first cut to size and then placed against a mold and then heated to a temperature generally ranging from about 3000° to about 7000°F, depending upon the choice of the thermoplastic polymer and its melt flow characteristics. The medium can then be held at a pressure to the mold until the material is set. This again depends upon the flow characteristics of the formed medium.

In a preferred embodiment, where the medium is composed of about 30 volume percent carbon fibers and 70 volume percent polyarylsulfone fibrets, the homogeneous structure has a tensile strength, before thermoforming, of about 6 psi.

The media are self supporting and capable of being handled. The media contain randomly oriented fibers and fibrets. The are particularly useful for forming high performance aircraft parts.

The above describes some of the preferred aspects of the nvention. However, the invention is not limited tc .his description, but includes all variations which would occur from this description t one of ordinary skill in the art. The invention is particularly pointed out by the attached claims.