Background of the Invention Carbon fibers are fibers with a uniform carbon content throughout a cross-section of the fiber. Carbon fibers are made from precursor fibers of polyacrylonitrile or multipolymers of acrylonitrile and less commonly from pitch or rayon. Currently, commercial production of acrylic fibers is based on either wet or dry spinning technology. In both instances, the acrylic polymer is dissolve in a solvent, and the fibers are formed when the acrylic polymer solution is extruded through a spinnerette into a coagulating liquid for wet spinning or a hot gaseous environment for dry spinning. In both wet and dry spinning, a solvent is used, and the solvent must diffuse through the filament and into a coagulating bath solution for wet spinning or evaporate into the spinning chamber for dry spinning. Current commercial acrylonitrile carbon fiber precursors, are spun from solution using solvents because they cannot survive a conventional melt spinning process due to polymer decomposition as is disclosed in USPN 4,107,252.

Carbon sheets are sheets with a uniform carbon content throughout a cross-section of the sheet. Carbon sheets are made from precursor sheets of polyacrylonitrile or multipolymers of acrylonitrile. Currently, commercial production of acrylic sheets is based on solvent casting technology. The acrylic polymer is dissolved in a solvent and the sheets are formed when the acrylic polymer solution is cast and then drawn into a chamber pressurized with gas or air-water vapor.

The production of carbon fibers or sheets from solution is undesirable due to the problem of solvent removal. Removal of the solvent from the fiber and sheet is not always complete, nor uniform, resulting in voids and lack of homogeneity in the fiber and sheet structure.

It is advantageous to produce a carbon fiber and/or sheet by a waterless, solventless process that employs a melt-processable high nitrile multipolymer. It is desirable to produce carbon fiber and/or sheet from a melt process because the resulting fiber and/or sheet is homogenous throughout and substantially void free. It is advantageous to produce carbon fiber and/or sheet from a melt process and not a solvent-based process wherein the solvent needs to be removed and recovered.

It is understood that the term multipolymer is a multipolymer of acrylonitrile and olefinically unsaturated monomer (s) and includes copolymers, terpolymers and multipolymers throughout the specification.

It is understood that the term acrylic herein means multipolymers composed of 85% or more acrylonitrile with olefinically unsaturated monomer (s).

Summary of the Invention It has been discovered that carbon fibers and sheets can be obtained from melt-processable, solventless, waterless multipolymers of acrylonitrile and olefinically unsaturated monomers. In the instant invention, carbon fibers are produced from a process comprising: (a) preparing a melt-processable multipolymer of acrylonitrile and olefinically unsaturated monomers; (b) melt spinning the multipolymer of acrylonitrile and olefinically unsaturated monomers in the absence of solvent and water comprising the steps of: (i) melt spinning the multipolymer into a fiber at a temperature higher than the glass transition temperature of the multipolymer to about 300°C; (ii) drawing the fiber at a temperature in the range of about ambient temperature to about 260°C; (c) oxidizing the multipolymer fiber in an oxidizing environment at a temperature in the range of 100°C to about 500°C for about 0.1 hour to about 24 hours; and

(d) carbonizing the oxidized fiber in an inert carbonizing environment for about 0.1 hour to 24 hours, the carbonizing environment having a temperature in excess of about 500°C.

In the instant invention, carbon sheets are produced from a process comprising: (a) preparing a melt-processable multipolymer of acrylonitrile and olefinically unsaturated monomers; (b) melt extruding or compression molding the multipolymer of acrylonitrile and olefinically unsaturated monomers into a sheet in the absence of solvent and water at a temperature higher than the glass temperature to about 300°C; (c) drawing the sheet uniaxially or biaxially at a temperature in the range of about ambient temperature to about 260°C; (d) oxidizing the multipolymer sheet in an oxidizing environment at a temperature in the range of 100°C to about 500°C for about 0.1 hour to about 24 hours; and (e) carbonizing the oxidized sheet in an inert carbonizing environment for about 0.1 hour to about 24 hours, the carbonizing environment having a temperature in excess of about 500°C.

The present invention provides for novel carbon fibers and/or carbon sheets which are homogeneous with a high degree of orientation and are useful for composite systems as reinforcement for high performance composite materials. The carbon fibers or carbon sheets are homogeneous throughout and substantially void free.

Description of the Invention In accordance with the present invention, carbon fiber and/or carbon sheet is produced from a novel solventless, waterless melt processable multipolymer of acrylonitrile and olefinically unsaturated monomers. Further, in accordance with the present invention, a method for providing a homogeneous and substantially uniform carbon fiber and/or sheet comprises melt extruding a high nitrile multipolymer

containing polymerized acrylonitrile monomers and olefinically unsaturated monomer (s); spinning the molten high nitrile multipolymer through a spinnerette into a fiber or extruding the molten high nitrile multipolymer into a sheet; drawing the fiber or sheet; oxidizing the fiber or sheet; and carbonizing the fiber or sheet into its respective carbon fiber or carbon sheet.

The melt-processable multipolymer used in this invention for producing carbon fiber and/or sheet is of a composition that is homogeneous with a substantially uniform microstructure and can be obtained according to USPN 5,618,901 entitled"A Process for Making a High Nitrile Multipolymer Prepared From Acrylonitrile Olefinically Unsaturated Monomers" ; USPN 5,602,222 entitled"Process for Making an Acrylonitrile, Methacrylonitrile and Olefinically Unsaturated Monomers"and USPN 5,596,058 entitled"Process for Making an Acrylonitrile/Methacrylonitrile Copolymers", all incorporated herein.

The high nitrile multipolymer comprises about 76% to about 98%, preferably about 80% to about 95% and most preferably about 85% to about 92% of polymerized acrylonitrile monomer and at least one of about 2% to about 24%, preferably about 5% to 20% and most preferably 8% to about 15% polymerized olefinically unsaturated monomer.

It will be readily apparent to one skilled in the art that the multipolymer may be further modified by the addition of lubricants, dyes, leaching agents, plasticizers, pseudoplasticizers, pigments, delustering agents, stabilizers, static control agents, antioxidants, reinforcing agents such as fillers and the like. It is understood that any additive possessing the ability to function in such a manner can be used as long as it does not have a deleterious effect on the melt and/or thermal characteristics of the high nitrile multipolymer.

The olefinically unsaturated monomer employed in the high nitrile multipolymer is one or more of an olefinically unsaturated monomer with a C=C double bond polymerizable with acrylonitrile. The olefinically unsaturated monomer employed in the multimonomer mixture can be a single polymerizable monomer resulting in a copolymer or a combination of polymerizable monomers resulting in a multipolymer.

The choice of olefinically unsaturated monomer or combination of monomers

depends on the properties desired to be imparted to the resulting high nitrile multipolymer and its fiber or sheet end use.

The olefinically unsaturated monomer generally includes but is not limited to acrylates, methacrylates, acrylamide and its derivatives, methacrylamide and its derivatives, maleic acid and its derivatives, vinyl esters, vinyl ethers, vinyl amides, vinyl ketones, styrenes, halogen containing monomers, ionic monomers, acid containing monomers, base containing monomers, olefins and the like.

The acrylates include but are not limited to C, to C12 alkyl, aryl and cyclic acrylates such as methyl acrylate, ethyl acrylate and functional derivatives of the acrylates such as 2-hydroxyethyl acrylate, 2-chloroethyl acrylate and the like. The preferred acrylates are methyl acrylate and ethyl acrylate.

The methacrylates include but are not limited to C, to C12 alkyl, aryl and cyclic methacrylates such as methyl methacrylate, ethyl methacrylate, phenyl methacrylate, butyl methacrylate, isobornyl methacrylate, 2-ethylhexyl methacrylate and functional derivatives of the methacrylates such as 2-hydroxyethyl methacrylate, 2-chloroethyl methacrylate and the like. The preferred methacrylate is methyl methacrylate.

The acrylamides and methacrylamides and each of their N-substituted alkyl and aryl derivatives include but are not limited to acrylamide, methacrylamide, N-methyl acrylamide, N, N-dimethyl acrylamide and the like.

The maleic acid monomers include but are not limited to maleic acid monododecyl maleate, didodecyl maleate, maleimide, N-phenyl maleimide.

The vinyl esters include but are not limited to C, to Ce vinyl ethers such as ethyl vinyl ether, butyl vinyl ether and the like.

The vinyl esters include but are not limited to vinyl acetate, propionate, butyrate and the like. The preferred vinyl ester is vinyl actetate.

The vinyl amides include but are not limited to vinyl pyrrolidone and the like.

The vinyl ketones include but are not limited to C, to Cg vinyl ketones such as ethyl vinyl ketone, butyl vinyl ketone and the like.

The styrenes include but are not limited to substituted styrenes, multiple -substituted styrenes, methylstyrenes, styrene, indene and the like. Styrene is of the formula:

wherein each of A, B, D and E is independently selected from hydrogen (H), Cl to C4 alkyl groups and halogen.

The halogen containing monomers include but are not limited to vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene bromide, vinylidene fluoride, halogen substituted propylene monomers and the like. The preferred halogen containing monomers are vinyl chloride, vinyl bromide and vinylidene chloride.

The ionic monomers include but are not limited to sodium vinyl sulfonate, sodium styrene sulfonate, sodium methallyl sulfonate, sodium acrylate, sodium methacrylate and the like. The preferred ionic monomers are sodium vinyl sulfonate, sodium styrene sulfonate and sodium methallyl sulfonate.

The acid containing monomers include but are not limited to acrylic acid, methacrylic acid, vinyl sulfonic acid, itaconic acid, styrene sulfonic acid and the like.

The preferred acid containing monomers are itaconic acid, styrene sulfonic acid and vinyl sulfonic acid.

The base containing monomers include but are not limited to vinyl pyridine, 2-aminoethyl-N-acrylamide, 3-aminopropyl-N-acrylamide, 2-aminoethyl acrylate, 2-aminoethyl methacrylate and the like.

The olefins include but are not limited to isoprene, butadiene, C2 toC, straight chained and branched alpha-olefins such as propylene, ethylene, isobutylene, 1-butene and the like.

The preferred multipolymer includes but is not limited to an acrylonitrile monomer polymerized with at least one monomer of methyl acrylate, ethyl acrylate,

vinyl acetate, methyl methacrylate, vinyl chloride, vinyl bromide, vinylidene chloride, sodium vinyl sulfonate, sodium styrene sulfonate, sodium methallyl sulfonate, itaconic acid, styrene, sulfonic acid, vinyl sulfonic acid, isobutylene, ethylene, propylene and the like.

The multipolymer is melt spun into a fiber or melt extruded into a sheet without solvent or water. A fiber can be obtained by the method disclosed in USSN 780,754 entitled"Melt Spun Acrylonitrile Olefinically Unsaturated Fibers and a Process to Make Fibers"incorporated herein. The melt spinning is conducted without the use of solvent or water. The multipolymer is placed in a conventional extruder and is generally employed as a powder or pellet. The high nitrile melt-processable multipolymer is extruded either by itself or with small amounts of stabilizer and/or processing aids. The multipolymer is extruded in the absence of solvent and in the absence of water. The temperature is sufficient to achieve melt flow and the multipolymer is melt spun at a temperature higher than its glass transition temperature to about 300°C, preferably about 150°C to about 280°C.

To produce carbon fiber precursors, the molten multipolymer may be pumped through a gear pump, which meters the high nitrile multipolymer melt at a constant rate through the spinnerette (s). A filtering device prior to the spinnerette (s) may be used to filter the melt and remove any impurities, contaminants, dust, and the like.

The filtering device includes, but is not limited to, screens, filters, sand and the like.

The extruded multipolymer goes through a spinnerette (s) hole (s) thereby forming fiber (s). Conventionally, a manifold is used to connect the extruder to multiple spinnerets.

The spinnerette (s) has from one to multiple thousand holes. Fiber size is dependent upon the melt rate from the gear pump to the spinnerette, the number of spinnerette holes and the take-up speed. The shape of the fiber cross-section is changed by employing any desired shape spinnerette hole and can be round, dog-boned, y-shaped, delta, tri-lobal, tetralobal, pentalobal, hexalobal, octalobal, or rectangular, hollow and the like. The high nitrile fiber from the spinnerette (s) is then taken up at a fixed speed.

The fiber is drawn at a temperature in the range of ambient temperature to about 260°C, preferably at about 80°C to about 200°C. Orientation can be achieved

either directly in line with or subsequent to the spinning operation by running the fiber over heated rolls at different speeds. Other conventional orientation methods may be employed.

To produce carbon sheets, the multipolymer is melt extruded in the absence of solvent and water at a temperature in the range of the glass transition temperature of the multipolymer to about 300°C, preferably at about 130°C to about 280°C. This is done using an extruder equipped with a slit die. Alternatively, the melt-processable multipolymer is placed in a preheated mold at a temperature in the range of the glass transition temperature of the multipolymer to about 300°C, preferably at about 150°C to about 280°C, and at a pressure in the range of 10 psi to 50,000 psi for a time period sufficient to achieve homogenous molding, followed by cooling under the applied pressure until it reaches a temperature to demold without warpage of the sheet. The thickness of the sheet can be further reduced by sequential pressing to achieve the desired thickness. The mold surface can be textured to provide various surfaces to the sheet.

The extruded or compressed mold sheets are then drawn uniaxially or biaxially. Uniaxially oriented sheets are prepared by drawing the sheets in one direction, allowing contraction to occur in the direction perpendicular to the draw direction. The sheets are drawn in hot air and/or with heated rolls, at a temperature in the range of about 50°C to 260°C, preferably 80°C to 200°C. While the sheets are still hot, they can be placed in a tenter frame to biaxially orient them by stretching in the perpendicular direction. The sheets can be sequentially drawn uniaxially to allow orientation in two directions.

Alternatively, to prepare extruded or compressed biaxially oriented sheets, they are drawn using a two-way stretch machine that allows simultaneous drawing along two perpendicular directions at a temperature in the range of about 50°C to about 260°C, preferably 80°C to about 200°C and at drawing speeds greater than 1 mm/sec in both directions.

The multipolymer fiber or sheet formed is thermally converted to form high quality carbon fiber or carbon sheet.

The fiber and/or sheet is held at constant dimension and heated in an oxidation chamber in an oxidizing environment at a temperature in the range of

about 100°C to about 500°C, preferably about 150°C to about 350°C, most preferably about 250°C to about 300°C for a period of time in the range of about 0.1 hour to about 24 hours, preferably in the range of about two hours to about 12 hours.

Subsequently, the oxidized fiber or sheet is carbonized in an inert atmosphere at a temperature in the range of about 500°C to about 2500°C or more preferably in the range of about 800°C to about 200°C for about 0.1 hour to about 24 hours to accomplis carbonization wherein the carbon fiber or sheet contains at least up to 60%, preferably 90%, and more preferably 92% carbon by weight. During carbonization, the existing carbon to carbon bonds are maintained and new carbon to carbon linkages are established while eliminating oxygen, hydrogen and nitrogen from the molecular structure of the fiber or sheet.

The carbonized fiber or sheet optionally may be heated at even higher temperature above 2500°C for a period of about 0.1 hour to about 24 hours in order to accomplis graphitization. The resulting carbon fiber or sheet has uniform carbon content throughout a cross-section of greater than 98% carbon by weight.

The carbon fibers and sheets are substantially void-free, resulting in substantially uniform fibers or sheets with improved strength due to fewer weak spots where voids would have been present due to the nonuniform removal of the solvent.

Additionally, the olefinically unsaturated comonomer (s) are uniformly interdispersed among the acrylonitrile units in the multipolymer. This minimizes the block sequencing of the comonomers and maximizes block sequencing of the acrylonitrile units, resulting in improved strength and dimensional stability and superior orientation in the carbon fiber or sheet.

Other improvements to the carbon fiber and sheet due to the high nitrile multipolymer is the uniformity and the homogeneity of composition and microstructure which results in improved and consistent mechanical properties of the final carbon fiber and sheet products.

The resulting carbon fiber and carbon sheet find utility in composite systems of the carbon fiber or the carbon sheet with a matrix. Representative matrices for such

fiber reinforcements include epoxy resins, maleimide resins, thermoplastic polymers, and the like. The carbon fiber and sheet are used with thermoplastic engineering polymer matrices and have found utility for sporting goods products. The carbon fiber and/or carbon sheet are useful in the manufacture of fire retarding and fire shielding assemblies or structural panels for use in vehicles, particularly airplanes or ships, or in installations and the like. Further, the carbon fiber and carbon sheet are useful for medical applications such as ligament and tendon replacement.

Examples The following examples are presented to illustrate the present invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

A multipolymer containing 85/15 acrylonitrile/methyl acrylate was prepared.

A 50-gallon stainless steel circulating hot water jacketing reactor was equipped with a reflux condenser, a thermocouple/controller, a turbin for agitation, which was set at about 150 to 250 rpm, a nitrogen purge, and a monomer feed mixture pump.

Multipolymer composition: The overall polymerization components for this example were as follows: Lbs.

Water 225 Dowfax 8390 (35% active) 8.57 Acrylonitrile (AN) 85 Methyl Acrylate 15 n-Dodecyl Mercaptan 1.8 Ammonium Persulfate 0.07 Dowfax is available from Dow Chemical Co.

Procedure: The reactor was pre-charged with water and the surfactant which had been pre-dissolved at about 50°C with stirring at about 150-250 rpm. The reactor was heated to about 60°C under nitrogen purging. Ten percent of the comonomers and ten percent of the mercaptan were charged to the reactor. Ammonium persulfate was then added to the reactor to initiate the polymerization reaction.

The remaining multimonomer feed and mercaptan mixture was continuously pumped into the reactor at a constant rate. The multimonomer feed and mercaptan mixture stream was fed into the reactor over about 4 hours.

After the polymerization reaction was complete, the resulting multipolymer mulsion was filtered through a cloth filter bag to collect and separate any coagulum from the multipolymer. The latex was coagulated in water in a countercurrent train of three overflowing stirred tanks. These tanks were set at about 70-98°C and contained about 1 % aluminum sulfate based on the polymer in the latex. The washed and filtered multipolymer crumb was dried in a fluidized Fitzpatrick bed dryer at about 70°C for about 3 hours. The multipolymer was then analyzed and determined to be 85/15 acrylonitrile/methyl acrylate by NMR spectroscopy.

The high nitrile multipolymer resin comprising about 85% acrylonitrile and about 15% methyl acrylate was melt spun at about 235°C through a 128 hole spinnerette with about a 0.3 mm/hole diameter and 2 L/D. The winder take-up speed was about 1000 mpm.

The fiber birefringence was determined by polarized optical microscopy.

Fiber birefringence is given by An = R/d, and its determination involves the measurement of both the optical retardation, R, as well as the fiber diameter, d.

The intensity profile across a birefringent fiber was calculated, assuming weak optical retardation, and compared to measurements recorded by cross-polarized video microscopy. This method was corroborated by comparison with another technique which involved first coating the fiber with a metal. The optical retardation of the fiber was measured using a tilting compensator at 546 nm, and the birefringence of the fiber was calculated. The molecular orientational order parameter of these fibers was measured by x-ray diffraction and was found to correlate linearly with their birefringence.

Table I Fiber Draw Ratio Diameter of Fiber Birefringence (microns) (orientation) 85/15 acrylonitrile/methyl acrylate as spun 57.8 0.0016 85/15 acrylonitrile/methvl acrylate as spun 20.8 0.0027 85/15 acrylonitrile/methyl acrylate 1.46 18.8 0.0043 85/15 acrylonitrile/methvl acrylate 1.5 8 0.0068 Montefibre A* 2 40 0.001 Montefibre B* 14 15 0.002 BASF (carbon fiber precursor) 9.6 0.0045 Bayer 36.6 0.003 Courtauds 23.2 0.0028 Courtauds 19.93 0.0031 *Information obtained from Macromolecules, 1996,29,1830-1832, incorporated here, which states:.

Sample A is a commercial sample produced by Montefibre of Porto Marghera. It was produced by wet spinning from a 14% by weight dimethylacetamide solution. The fiber was stretch 14 times in a boiling water bath, the final diameter of each filament is close to 15 zm. Sample B is a laboratory sample prepared by the Montefibre Research Center of Porto Margera. This sample was also obtained by wet spinning from a 14% by weight dimethylacetamide solution, but the fiber was stretched only 2 times in a boiling water bath (the diameter of each filament is c/ose to 40 am). SamplesA and 8 present similar crystallinities but very different degrees of orientation. The half-widths of the equatorial main peaks along the azimuthal profile are 13 and 40 ° for-ample A and sample B respectively. The birefringence, determined using a Leitz polarizing microscope with an Ehringhaus rotary compensator of 5 orders, is 0 002 and 0.001 for samples A and B, respectively.

Table I compares the degree of orientation of solventless, waterless melt- processable multipolymer fibers and commercial acrylic fibers. The results demonstrate that the 85/15 acrylonitrile/methyl acrylate carbon fiber precursors have excellent orientation as shown by higher birefringence numbers than those of the commercial high nitrile fibers. Excellent orientation of the precursor is needed to make an excellent carbon fiber because the orientation is preserved upon oxidation and carbonization, and the better the orientation of the precursor fiber, the better and stronger the mechanical properties of the final carbon fiber.

From the above description and examples of the invention, those skilled in the art will perceive improvements, changes and modifications in the invention.

Such improvements, changes and modifications are intended to be covered by the claims.