US3821013A

US3821013A - Surface modification of graphite fibers

Info

Publication number: US3821013A
Application number: US00223975A
Authority: US
Inventors: L Daley; G Ferment
Original assignee: Celanese Corp
Current assignee: SUBJECT TO AGREEMENT RECITED SEE DOCUMENT FOR DETAILS; BASF SE; BASF Corp
Priority date: 1972-02-07
Filing date: 1972-02-07
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28

Abstract

Graphite fibers exhibiting an enhanced ability to bond to a matrix material (e.g., a thermosetting resinous material or a metallic material) are produced wherein a film of amorphous carbon in intimate association with titanium is provided upon the surface of the same. A predominantly graphitic carbonaceous fibrous material is coated with a film of a dihydropyridacene polymer in intimate association with a hydrolyzable organotitanium compound, the organotitanium compound hydrolyzed to form titanium dioxide, and the polymer portion of the film carbonized to a predominantly amorphous form. Composite articles which incorporate the carbon fibers produced by the present process exhibit improved physical properties, such as an improved interlaminar shear strength.

Description

United States Patent [191 Daley et al,

[111 3,821,013 1 June 28, 1974 SURFACE MODIFICATION OF GRAPHITE FIBERS Inventors: Lawrence R. Daley; George R.

Ferment, both of Dover, NJ.

Assignee: Celanese Corporation, New York,

Filed: Feb. 7,1972

Appl. No.: 223,975

11.8. CI. 117/46 CC, 8/1156, 117/169 R, 117/226, 117/228, 423/447, 264/D1G. 19, 1l7/DIG. 11

Int. Cl 344d 5/12 Fieldof Search 117/46 CB, 46 CC, 169 R, 117/226, 160 R, 161 ZB, 228, DIG. 11; 423/447;8/1l5.6; 264/29, DIG. 19

References Cited UNITED STATES PATENTS 1/1973 Leeds 117/46 CC Primary Examiner-Wi11iam D. Martin Assistant Examiner-Janyce A. Bell Attorney, Agent, or Firm-Thomas J. Morgan [57] ABSTRACT dioxide, and the polymer portion of the film carbon ized to a predominantly amorphous form. Composite articles which incorporatethe carbon fibers produced by the present process exhibit improved physical propll erties, such as an improved interlammar shear strength.

14 Claims, 2 Drawing Figures 1 SURFACE MODIFICATION OF GRAPHITE FIBERS BACKGROUND OF THE INVENTION In the search for high performance materials, considerable interest has been focused upon graphitic carbon fibers. Graphite fibers are defined herein as fibers which consist substantially of carbon and have a pre dominant x-ray, diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand,

are defined as fibers in which the bulk of the fiber I weight can be attributed to carbon and which exhibit a predominantly amorphous x-ray diffraction pattern.

Graphite fibers generally have a higher Youngs modulus than do amorphous carbon fibers and in addition are more electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and graphitic carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature re sistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature.

'Uses for graphitic carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use as possible matrices in which graphitic carbon fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is selected is commonly a thermosetting resinous material and is commonly selected because of its ability to also withstand highly elevated temperatures. Metallic matrix materials may also be utilized.

While it has been possible in the past to provide graphitic carbon fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantages of such properties in the resulting carbon fiber reinforced composite article; Such inability to capitalize upon the superior single filament properties of the reinforcing fiber has been traced to inadequate adhesion between the fiber and the matrix in the resulting composite article.

Various techniques have been proposed in the past for modifying the fiber properties of a previously formed carbon fiber in order to make possible improved adhesion when present in a composite article. See, for instance, US. Pat. No. 3,476,703 and British Pat. No. 1,180,441 to Nicholas J. Wadsworth and William Watt wherein it is taught to heat a carbon fiber normally within the range of 350C. to 850C. (e.g. 500 to 600C.) in an oxidizing atmosphere such as air for an appreciable period of time. Other atmospheres contemplated for use in the process include an oxygen rich atmosphere, pure oxygen, or an atmosphere containing an oxide of nitrogen from which free oxygen becomes available such as nitrous oxide and nitrogen dioxide. Improved carbon fiber surface modification processes are disclosed in commonly assigned U.S. Ser. Nos. 65,454 (now US. Pat. No. 3,734,957) and 65,456, (now U.S. Pat. No. 3,732,150), filed Aug. 20, 1970; and U5. Ser. No. 99,169 (now US. Pat. No. 3,745,104), filed Dec. 17, 1970.

It is an object of the invention to provide a continuous process for modifying the surface characteristics of graphitic carbon fibers possessing an enhanced ability to bond to a matrix material.

It is an object of the invention to provide graphitic carbon fibers possessing modified surface characteristics which eliminates the need for heating the same in an oxidizing atmosphere as commonly conducted in the prior art.

It is another object of the invention to provide a process for producing carbon fibers possessing an enhanced ability to bond to a matrix material without degradation of the graphitic carbon fiber tensile properties.

It is another object of the invention to provide composite articles reinforced with graphitic carbon fibers exhibiting an improved interlaminar shear strength.

These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for the surface modification of a predominantly graphitic carbonaceous fibrous material containing at least 90 per cent carbon by weight comprises: i

a. coating the fibrous material with a film of a dihydropyridacene polymer which is substantially free of inter-molecular cross-linking consisting of to 100 mol per cent of acrylonitrile units wherein the pendant nitrilegroups thereof are at least about percent cyclized, and 0 to 15 mol per cent of copolymerized monovinyl units, with the dihydropyridacene polymer film being in intimate association with a hydrolyzable organotitanium compound capable of yielding titanium dioxide upon hydrolysis,

b. hydrolyzing the hydrolyzable organotitanium compound in intimate association with the dihydropyridacene polymer film to form titanium dioxide, and

c. carbonizing the dihydropyridacene polymer portion of the film present upon the fibrous material to a predominantly amorphous carbon form containing at least about 90 per cent carbon by weight by heating in an inert gaseous atmosphere at a temperature of at least about 900C, but not exceeding about 1,800C. to produce a predominantly graphitic carbonaceous fibrous material which possesses an enhanced ability to bond to a matrix material.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph made with the aid of a scanning electron microscope of a representative graphitic carbon fiber surface modified in accordance with the present invention at a magnification of 6600X possessing an enhanced ability to bond to a matrix material.

FIG. 2 is a photograph made with the aid of a scanning electron microscope of a control representative graphitic carbon fiber at a magnification of 6400X which underwent no form or surface modification.

DESCRIPTION OF PREFERRED EMBODIMENTS The Starting Material The carbonaceous fibers which are modified in accordance with the process of the present invention contain at least about 90 per cent carbon by weight and exhibit a predominantly graphitic x-ray diffraction pattern. In a preferred embodiment of the process the graphitized carbonaceous fibers which undergo surface modification contain at least about 95 per cent carbon by weight (e.g., at least about 99 per cent carbon by weight).

The graphitized carbonaceous fibrous materials are preferably provided in a continuous length in any one of a variety of physical configurations provided substantial access to the fiber surface is possible during the surface modification treatment described hereafter. For instance, the fibrous materials may assume the configuration of a continuous length of a multifilament yarn, tape, tow, strand, cable, or similar fibrous assemblage. In a preferred embodiment of the process the fibrous material is one or more continuous multifilament yarn'or a tow. When a plurality of multifilament yarns are surface treated simultaneously (as described hereafter), they may be continuously processed while in parallel and in the form of a flat ribbon, as may a flat tow.

The previously graphitized carbonaceous fibrous material which is treated in the present process optionally may be provided with a twist which tends to improve the handling characteristics. For instance, a twist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi, may be imparted to a multifilament yam. Also, a false twist may be used instead of or in addition to a real twist. Alternatively, one may select continuous bundles of fibrous material which possess essentially no twist.

The graphitized carbonaceous fibers which serve as the starting material in the present process may be formed in accordance with a variety of techniques as will be apparent to those skilled in the art. For instance, organic polymeric fibrous materials which are capable of undergoing thermal stabilization may be initially stabilized by treatment in an appropriate atmosphere at a moderate temperature (e.g. 200 to 400C), and subsequently heated in an inert atmosphere at a more highly elevated temperature until a carbonized and graphitized fibrous material is formed. For instance, the thermally stabilized material may be carbonized by heating in an inert atmosphere at a temperature of about 900 to 1,000C. and subsequently heated to a maximum temperature of 2,000 to 3,IOOC. (preferably 2,400 to 3,100C.) in an inert atmosphere for a sufficient residence time to produce substantial amounts of graphitic carbon.

The exact temperature and atmosphere utilized during the initial stabilization of an organic polymeric fibrous material commonly vary with the composition of the precursor as will be apparent to those skilled in the art. During the carbonization reaction elements present in the fibrous material other than carbon (e.g., oxygen and hydrogen) are substantially expelled. Suitable organic polymeric fibrous materials from which the graphitized carbonaceous fibrous materials may be derived include an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole', polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric materials are particularly suited for use as precursors in the formation of graphitized carbonaceous fibrous materials. Illustrative examples of suitable cellulosic materials include the natural and regenerated forms of cellulose, e.g., rayon. Illustrative examples of suitable polyamide materials include the aromatic polyamides, such as nylon 6T, which is formed by the condensation of hexamethylenediamine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly- 2,2-m-phenylene-5,5'-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may be formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should contain not less than about mol per cent of recurring acrylonitrile units with not more than about 15 mol per cent of a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monovinyl compounds.

During the formation of a preferred graphitized carbonaceous fibrous material for use in the present process multifilament bundles of an acrylic fibrous material may be initially stabilized in an oxygen-containing atmosphere (i.e., preoxidized) on a continuous basis in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E. Stuetz, which is assigned to the same assignee as the present invention and is herein incorporated by reference. More specifically, the acrylic fibrous material should be either an acrylonitrile homopolymer or an acrylonitrile copolymer which contains no more than about 5 mol per cent of one or more monovinyl comonomers copolymerized with acrylonitrile. In a particularly preferred embodiment of the process the fibrous material is derived from an acylonitrile homopolymer. The stabilized acrylic fibrous material which is preoxidized in an oxygen-containing atmosphere is black in appearance, contains a bound oxygen content of at least about 7 per cent by weight as determined by the Unter' zaucher analysis, retains its original fibrous configuration essentially intact, and is non-burning when subjected to an ordinary match flame.

In preferred technique for forming the starting material for the present process a stabilized acrylic fibrous material is carbonized and graphitized while passing through a temperature gradient present in a heating zone in accordance with the procedures described in commonly assigned U.S. ,Ser. Nos. 777,275, filed Nov. 20, 1968 (now abandoned) of Charles M. Clarke; 17,780, filed March 9, 1970 (now U.S. Pat. No. 3,677,705) of Charles M. Clarke, Michael .1. Ram, and John P. Riggs; and 17,832, filed March 9, 1970 .of Charles M. Clarke, Michael 1. Ram, and Arnold J. Rosenthal (now U.S. Pat. No. 3,775,520). Each of these disclosures is herein incorporated by reference.

In accordance with a particularly preferred carbonization and graphitization technique a continuous length of stabilized acrylic fibrous material which is non-buming when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith is converted to a graphitic fibrous material while preserving the original fibrous configuration essentially intact while passing containing an inert gaseous atmosphere and a temperature gradient in which the fibrous material is raised within a period of about 20 to about 300 seconds from about 800C. to a temperature of about 1,600C. to form a continuous length of carbonized fibrous material, and in which the carbonized fibrous material is subsequently raised from about 1,600C. to a maximum temperature of at least about 2,400C. within a period of about 3 to 300 seconds where it is maintained for about seconds to about 200 seconds to form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to produce the graphitized carbonaceous starting material may be varied as will be apparent to those skilled in the art. It is essential that the apparatus selected be capable of producing the required temperature while excluding the presence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous material undergoing carbonization and graphitization is heated by use of an induction furnace. In such a procedure the fibrous material may be passed in the direction of its length through a hollow graphitetube or other susceptor which is situated within the windings of an induction coil. By varying the length of the graphite tube, the length of the induction coil, and the rate at which the fibrous material is passed through the graphite tube, many apparatus arrangements capable of producing carbonization and graphitization may be selected. For large scale production, it isof course preferred that relatively long tubes or susceptors be used so that the fibrous material may be passed through the same at a more rapid rate while being carbonized and graphitized. The temperature gradient of a given apparatus may be determined by conventional optical pyrometer measurements as will be apparent to those skilled in the art. The fibrous material because of its small mass and relatively large surface area instantaneously assumes substantially the same temperature as that of the carbonization/graphitization heating zone through which it is continuously passed.

The Surface Modification The graphitic carbonaceous fibrous material is coated with a film of a dihydropyridacene polymer which is substantially free of inter-molecular crosslinking consisting of 85 to 100 per cent of acrylonitrile units wherein the pendant nitrile groups thereof are at least about 90 per cent cyclized (preferably fully cyclized), and 0 to mol percent of copolymerized monovinyl units. Representative monovinyl units include sytrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such comonomers. In a preferred embodiment of the invention the dihydropyridacene polymer is substantially free of intar-molecular cross-linking and consists of 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized.

The dihydropyridacene polymer may be derived from 1 an acrylonitrile homopolymer, or (2) an acrylonitrile copolymer containing at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith. Preferred dihydropyridacene polymers are derived from an acrylonitrile homopolymer, or from acrylonitrile copolymers which contain at least about mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith. Preferred dihydropyridacene polymer derived from an acrylonitrile homopolymer consists of recurring units of the structural formula indi cated below where (I) represents and recurring structure of the acrylonitrile homopolymer (e.g., 4 acrylonitrile units), and (II) represents the structure of the fully cyclized dihydropyridacene polymer.

(1) CHa CH1 When dihydropyridacene copolymers are formed containing 0 to 15 mol per cent of copolymerized monovinyl units the structural formula of the polymer is directly analogous to that of (II) with the exception that the monovinyl units are randomly dispersed within the polymer chain.

The dihydropyridacene polymer utilized in the process may be formed in accordance with the procedures described in commonly assigned U.S. Ser. Nos. 88,487 (now U.S. Pat. No. 3,736,309), and 88,489 (now U.S. Pat. No. 3,736,310), filed Nov. 10-, 1970, of Klaus H. Gump and Dagobert E. Stuetz which are herein incorporated by reference. More specifically, as described in U.S. Ser. No. 88,487, an acrylic polymer selected from the group consisting of anacrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of monovinyl units copolymerized therewith may be converted to the desired dihydropyridacene polymer in the absence of intermolecular cross-linking by (a) providing a solution of said acrylic polymer which contains: a catalytic quantity of an organic cyclization promoting agent selected from the group consisting of a carboxylic acid, a sulfonic acid, and a phenol, (b) heating the solution while present in an essentially oxygen-free zone at a temperature of about to 240C. for about 45 minutes to 16 hours, and (c) recovering the resulting cyclized dihy dropyridacene polymer. Alternatively, as described in U.S. Ser. No. 88,489, an acrylic polymer selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol per cent of acrylonitrile units and up to about 15. mol per cent of monovinyl units copolymerized therewith may be converted to a cyclized dihydropyridacene polymer in the absence of intermolecular cross-linking by (a) providing a mixture of the acrylic polymer and 2-pyrrolidinone, (b) heating the mixture while present in an essentially oxygen-free zone at a temperature of about 130 to 220C. for about 2 minutes to 16 hours, and (c) recovering the resulting cyclized dihydropyridacene polymer.

The film of dihydropyridacene polymer is preferably applied to the graphitic fibrous material by contact with a solution containing the polymer dissolved in the solvent incapable of destroying the original fibrous configuration of the fibrous material, and the solvent evaporated. Contact may be conveniently carried out by immersing the fibrous material in the solution. Alternatively, contact may be made by spraying, etc. Preferred solvents for the dihydropyridacene polymer are formic acid, sulfuric acid, and polyphosphoric acid. Other representative solvents are trifluoroacetic acid, or mixtures of the foregoing acids with acetonitrile, methanol, acetone, ethylene glycol, or 2-pyrrolidone (e.g., equal parts by weight solvent mixtures). When the pendant nitrile groups of the dihydropyridacene polymer are not fully cyclized, then more common acrylic solvents, such as N,N-dimethylacetamide may be selected. The dihydropyridacene polymer may be provided in the solution in a concentration of about 0.01 to 10 per cent by weight based upon the weight of the total solution, and preferably in a concentration of about 0.1 to 5 per cent by weight. The solution is preferably at a temperature of about to 50C. when contacted with the graphitic fibrous material. Evaporation of the solvent from the solution of dihydropyridacene polymer in contact with the fibrous material is preferably conducted in any convenient manner, such as by heating in air at about 30 to 100C. for a few minutes. The film of the dihydropyridacene polymer present upon the surface of the graphitic fibrous material preferably has a thickness of about 5 to 1,000 angstroms, and most preferably a thickness of about 100 to 500 angstroms.

The film of dihydropyridacene polymer present upon the surface of the graphitic carbonaceous fibrous material is provided in intimate association with a hydrolyzable organotitanium compound capable of yielding titanium dioxide upon hydrolysis. The hydrolyzable organotitanium compound is preferably also applied to the surface of the graphitic carbonaceous fibrous material as a film in a discrete coating step subsequent to the application of the film of dihydropyridacene polymer. The dihydropyridacene polymer coated fibrous material may be contacted with a solution containing the hydrolyzable organotitanium compound dissolved in a solvent incapable of destroying the original fibrous configuration of the graphitic carbonaceous fibrous material or deleteriously influencing the film of dihydropyridacene polymer, and the solvent evaporated.

The chemical structure of the hydrolyzable organotitanium compound selected for use in the present invention may be varied widely as will be apparent to those skilled in the chemistry of organotitanium compounds. For instance, representative classes of hydrolyzable organotitanium compounds include the simple alkyl titanates. polymerized alkyl titanates, alkyl titanate chelates, polymeric titanium phosphinates, etc.

The alkyl titanates suitable for use in the present process possess at least one alkyl group having one to eight carbon atoms. The alkyl group may optionally be of the cycloalkyl type. The inclusion in the alkyl titanate of at least one alkyl group having one to eight carbon atoms imparts to the organotitanium compounds the capacity to readily undergo hydolysis upon exposure to water, such as water vapor present in air. In a preferred embodiment of the invention the alkyl titanate utilized is a tetraalkyl titanate having one to eight carbon atoms in each alkyl group. Such tetraalkyl titanates commonly possess a formula of Ti(OR)., where R is an alkyl group containing one to eight carbon atoms. Illustrative examples of hydrolyzable tetraalkyl titanates possessing a symmetrical molecular configuration include: tetramethyl titanate, tetraethyl titanate, tetrapropyl tita nate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate, tetrapentyl titanate, tetra(2- ethylhexyl) titanate, tetraoctyl titanate, and the like. The lower alkyl titanates containing one to four carbon atoms per alkyl group are particularly preferred. Such compounds are commonly light yellow liquids. Mixed tetraalkyl titanates may also be utilized in which at least a portion of the alkyl groups of each molecule exceed eight carbon atoms in length. For instance, compounds such as isopropylstearyl titanate may be employed. Representative commercially available simple alkyl titanates are Tyzor TPl", Tyzor TBT, Tyzor TOT, and Tyzor AP organic titanates which are provided by the Du Pont C0.

It is also possible for the alkyl titanates discussed above to be partially condensed or polymerized to form relatively low molecular weight polytitanates. As is well known in the chemistry of organotitanium compounds, such condensation or polymerization products may result from the reaction of the alkyl titanate with less than the stoichiometric amount of water. Condensed esters of varying degrees of hydrolysis from hexaalkoxy dititanates, [(RO) Ti] O, to dialkoxy polytitanates, RO[- Ti(OR) O-],R, can be formed by the addition of the required amount of water. See U.S. Pat. No. 2,689,858. For example, hexaisopropyl dititanate, or hexabutyl dititanate possessing I structural configurations of (C H O) Ti-O-Ti(OC l-l and (C H O) Ti-O-Ti(OC l-l respectively, may be selected. A representative commercially available polymerized alkyl titanate is a polymerized butyl titanate designated as Tyzor PB organic titanate which is provided by the Du Pont Co.

Alkyl titanate chelates may be formed by reacting either a beta-diketone (e.g., acetylacetone) or a ketoester (e.g., ethyl acetoacetate, diethyl malonate, and malononitrile) with an alkyl titanate having two to four carbon atoms in each alkyl group (e.g., tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetran-butyl titanate, and tetraisobutyl titanate). A preferred chelating agent is acetylacetone, and a preferred alkyl titanate coreactant is tetraisopropyl titanate. The titanium chelates may be formed by simply mixing the chelating agent with the alkyl titanate coreactant in a mol ratio of l to 4 mols of chelating agent per 1 mol of the alkyl titanate. During the reaction, which is exothermic, a proportionate number of the alkoxy groups of the alkyl titanate are replaced, and may be recovered if desired as the corresponding alcohol by suitable distillation techniques, such as by distillation at relatively low temperatures under reduced pressure. If more drastic distillation procedures are attempted, polymeric insoluble condensation products tend to form. Distillation maybe terminated when the stoichiometric quantity of alcohol is recovered. Particularly preferred organotitanium reaction products for use in the present invention are prepared by reacting approximately 2 mols of chelating agent per 1 mol of the alkyl titanate, so that approximately one-half of the alkoxy groups are replaced on each alkyl titanate molecule. For instance, if acetylacetone and tetraisopropyl titanate are the coreactants, the reaction product is believed to be largely di-isopropoxytitanium bis-(acetyl acetonate). A representative commercially available titanium chelate formed by the use of an acetylacetone chelating agent is Tyzor AA organic titanate which is provided by the Du Pont Co.

Polymeric titanium phosphinates may be formed as described by B. P. Block in Inorganic Macromolecules Reviews, Vol. 1, Pages ll-l25 (1970). The preferred polymeric titanium phosphinate is poly(bis-diphenyl phosphenyl) titanate having recurring units of the for mula The solvent utilized in the formation of the solution containing the hydrolyzable organotitanium compound may be varied widely as will be apparent to those skilled in the art. It is essential, however, that the solvent be incapable of destroying the original fibrous configuration of the graphitic carbonaceous fibrous .material or otherwise adversely influencing its properties or the properties of the dihydropyridacene polymer film. Additionally, the solvent must be incapable of producing any substantial hydrolysis of the organotitanium compound dissolved therein. The particular organotitanium compound utilized may influence the solvent which is selected. Representativesolvents may be selected from the following: benzene, carbon tetrachloride, isopropyl alcohol, n-butyl alcohol, nheptane. octane, trichloroethylene, dioxane, petroleum ether, xylol. and the like. in addition when the hydrolyzable organotitanium compound is an alkyl titanate chelate the solvent may even be water which is adjusted to a pH of about 3 (e.g., with acetic acid).

The hydrolyzable organotitanium compound is provided in the solution in a concentration of about 0.] to 5 per cent by weight based upon the total weight of the solution, and preferably in a concentration of about 0.1 to l per cent by weight based upon the total weight of the solution. The solution is preferably provided at a temperature of about 0 to 50C. when contacted with the graphitic carbonaceous fibrous material, and most preferably at a temperature of about to C. Contact of the graphitic carbonaceous fibrous material with the solution may be accomplished on a batch or a continuous basis. For instance, the fibrous material may be wound upon a support and immersed in the solution. A continuous length of the fibrous material may be conveniently passed on a continuous basis through a vessel containing the solution. Alternatively, the stabilized acrylic fibrous material may be sprayed with the solution. When contact is made via immersion in the solution, residence times of about 1 to 10 seconds may be. conveniently utilized. The temperature of the solution and the duration of the contact are generally not critical to the operation of the process. The concentration of the organotitanium compound in the solution will influence the thickness of the coating achieved upon evaporation of the solvent.

The solvent of the solution in contact with the fibrous material is next evaporated so that a film of the hydrolyzable organotitanium compoundis deposited upon the surface of the fibrous material. Evaporation of the solvent may be conducted in a circulating gaseous atmosphere, e.g.. air. The film is preferably substantially uniform and provided in a thickness of about 4 to 200 angstroms, and most preferably in a thickness of about 4 to 40 angstroms.

The film of hydrolyzable organotitanium compound isnext hydrolyzed to form a corresponding film of titanium dioxide upon the surface of the fibrous material. The film of titanium dioxide is preferably provided in a thickness of about 4 to 200 angstroms, and most preferably in a thickness of about 4 r040 angstroms. The

hydrolysis is preferably carried out by heating in a gaseous atmosphere which contains water vapor until the hydrolysis reaction is substantially complete. The period of time required to complete the hydrolysis reaction varies with the specific organotitanium compound involved, the temperature of the gaseous atmosphere, the thickness of the film, and the concentration of water vapor in the gaseous atmosphere. If desired the evaporation step described above may be conducted in the same zone in which hydrolysis is carried out. Hydrolysis treatment times employing air (e.g., of about 5 to 100 per cent relative humidity) at about 5 to 400C. commonly range from about 0.1 to 60 minutes. Preferred hydrolysis reactions utilize air of about 5 to 60 per cent relative humidity at about 20 to 300C. for a treatment time of about 0.25 to 60 minutes. Alternatively, the hydrolysis of the film of organotitanium compound may be similarly accomplished by contact with a strong acid bath,by contact with steam, etc. Hyrolysis carried out in air at about 200 to 300C. additionally serves to oxidatively cross-link the dihy' dropyridacene polymer and thereby to further enhance its thermal stability.

The resulting fibrous material bearing a film of dihydropyridacene polymer in intimate association with titanium dioxide upon its surface is heated in an inert (i.e. non-oxidizing)gaseous atmosphere at a temperature of at least about 900C, but not exceeding l,800C.. until the dihydropyridacene polymer portion of the film is carbonized to a predominantly amorphous carbon form containing at least about per cent carbonby weight is formed (preferably at least per cent carbon by weight) to produce a. predominantly graphitic carbonaceous fibrous material which exhibits an enhanced ability to bond to a matrix material. Suitable inert gaseous atmospheres include nitrogen, argon, and helium. At temperatures much below about 900C. the carbonization reaction is inordinately slow. At processing temperatures much above about l,800C. a fiber is produced exhibiting no substantial enhancement in its ability to bond to a matrix material as a result of the intermediate processing heretofore described. The. maximum carbonization temperature is preferably about l,200C. A carbonization temperature gradient optionally may be employed wherein the fibrous material is heated at gradually increasing temperatures above 900 C. up to about l,800C. (preferably up to about l,200C.). The carbonization reaction may be carried out in accordance with known techniques on either a batch or a continuous basis in an. inert gaseousatmosphere provided the maximum temperature of about l,800C. is not exceeded. carbonization residence times at the temperatures indicated commonly range from about I to 10 minutes. If desired, considerably longer carbonization residence times may be selected.

In a preferred technique a continuous length of the coated graphitic carbonaceous fibous material undergoing carbonization is heated by use of an induction furnace. In such a procedure the fibrous material may be passed in the direction of its length through a hollow graphite tube or other susceptor which is situated within the windings of an induction coil. By varying the length of the graphite tube, the length of the induction coil, and the rate at which the fibrous material is passed through the graphite tube, many apparatus arrangements capable of producing carbonization may be selected. For large scale production, it is of course preferred that relatively long tubes or susceptors be used so that the fibrous material may be passed through the same at a more rapid rate while the dihydropyridacene polymer is carbonized. The temperature gradient of a given apparatus may be determined by conventional optical pyrometer measurements as will be apparent to those skilled in the art. The fibrous material because of its small mass, and relatively large surface area instantaneously assumes essentially the same temperature as that of the zone through which it is continuously passed. Resistance heated carbonization heating zones may also be utilized in another preferred embodiment of the process.

The theory whereby the surface of the resulting graphitic carbonaceous fibers are rendered capable of enhanced adhesion to a matrix material is considered complex and incapable of simple explanation. It is believed, however, that the improved bonding characteristics can be traced to the presence of a substantially uniform film of predominantly amorphous carbon and titanium metal in intimate admixture (e.g. of to 1,000 angstroms and preferably 100 to 500 angstroms thickness) upon the surface of the resulting graphitic carbon fibers.

The surface modification of the present process exhibits an appreciable shelf life, and is not diminished to any substantial degree upon the passage of several weeks, or more.

The process of the present invention facilitates improved adhesive bonding between the predominantly graphitic carbonaceous fibers and a matrix material which may be either resinous (e.g., a thermosetting resinous material), or metallic. The composite articles of the resulting invention may be formed by conventional composite formation techniques. The resulting fiber reinforced composites which incorporate the graphite fibers of the present invention exhibit an enhanced interlaminar shear strength. Also, other composite properties such as flexural strength, compressive strength, etc., may be enhanced. The resinous matrix material employed in the formation of such composite materials is commonly a polar thermosetting resin such as an epoxy, a polyamide, a polyester, a phenolic, etc. The metallic matrix material employed in the formation of such composite materials may be aluminum, titanium,

-chromium, nickel, copper, silver, steel, etc.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific de tails set forth in the examples.

' EXAMPLE I The graphite carbonaceous yarn undergoing treatment was derived from acrylonitrile homopolymer yarn in accordance with procedures described in commonly assigned U.S. Ser. Nos. 749,957 (now abandoned), filed Aug. 5, 1968 and 777,275 (now abandoned) filed Nov. 20, 1968. The maximum graphitization temperature experienced by the yarn was 2,200C. The yarn consisted of a 9,500 fil bundle having a total denier of about 7,600, had a carbon content in excess of 99 per cent by weight, exhibited a predominantly graphitic x-ray diffraction pattern, a single filamenttenacity of about 13 grams per denier, and a single filament Youngs modulus of about 50 million psi.

The graphitic carbonaceous yarn was continuously passed through a 1.0 per cent by weight formic acid solution of dihydropyridacene polymer which was substantially free of inter-molecular cross-linking consisting of 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof were fully cyclized. The dihydropyridacene polymer was formed in accordance with the procedure described in commonly assigned U.S. Ser. No. 88,487 (now US. Pat. No. 3,736,309), filed Nov. 10, 1970 of Klaus H. Gump and Dagobert E. Stuetz. The yarn was immersed in the dihydropyridacene polymer solution provided at about 25 C. for about 10 seconds.

The formic acid solvent in contact with the yarn was evaporated by heating in air at C. for 30 minutes to form a substantially uniform film of dihydropyridacene polymer having a thickness of about angstroms, upon the surface of the graphitic carbonaceous fibrous material.

The resulting yarn was continuously passed through a 5.0 per cent by weight benzene solution of a polymerized alkyl titanate provided at about 25C. for about 10 seconds. More specifically, the organo-titanium compound was polymerized tetrabutyl titanate of the approximate formula C H O[Ti(OC l-l O] C H commercially available from the Du Pont Company under the designation Tyzor PB organic titanate.

The benzene solvent'was evaporated from the solution in contact with the yarn and the film of polymerized tetrabutyl titanate present on the surface of the yarn was simultaneously hydrolyzed by heating in air at 270C. having a relative humidity of about 5 per cent for minutes. A substantially uniform film of titanium dioxide having a thickness of about l00 angstroms was formed upon the surface of the graphitic carbonaceous yarn in intimate association with the film of dihydropyridacene polymer. The air heat treatment at 270C. also served to enhance the thermal stability of the dihydropyridacene polymer and to oxidatively cross-link the same.

The dihydropyridacene polymer film present upon the fibrous material was carbonized to a predominantly amorphous carbon form by continuous passage of the yarn through a muffle furnace provided with a nitrogen atmosphere having a temperature gradient to produce a continuous length of carbon fiber containing in excess of 95 per cent carbon by weight. While passing through the furnace the yarn was raised from room temperature (i.e. 25C.) 'to l,200C. in about seconds where it was maintained for about 75 seconds. Present upon the surface of the resulting predominantly graphitic carbonaceous yarn was a film consisting of predominantly amorphous carbon and titanium metal having a total thickness of about 200 angstroms.

P16. 1 is a photograph made with the aid of'a scanning electron microscope of the resulting predominantly graphitic carbonaceous fiber at a magnification of 6600X. FIG. 2 is a photograph made with the aid of a scanning electron microscope a control predominantly graphitic carbonaceous fiber at a magnification of 6400X which underwent no form of surface modification. Electrochemical surface area measurements indicated that the fiber of P16. 1 exhibited a ten fold surface area increase over the fiber of FIG. 2.

A composite article was next formed employing the surface modified yarn sample as a reinforcing medium in a resinous matrix. The composition article was a rectangular bar consisting of about 50 per cent by volume of the yarn and having dimensions of /s inch X A inch X inches. The composite article was formed by impregnation of the yarn in a liquid epoxy resinhardener mixture at 50C. followed by unidrectional layup of the required quantity of the impregnated yarn in a steel mold and compression molding of the layup for 2 hours at 93C., and 2.5 hours at 200C. in a heated platen press at about 100 psi pressure. The mold was cooled slowly to room temperature, and the composite article was removed from the mold cavity and cut to size for testing. The resinous matrix material used in the formation of the composite article was provided as a solventless system which contained 100 parts by weight of epoxy resin and 98 parts by weight of anhydride curing agent. t

The horizontal interlaminar. shear strength of the composite article was determined by short beam testing of the fiber reinforced composite according to the pro cedure of ASTM D2344-65T as modified for straight bar testing at a 4:1 span to depth ratio and was found to be 8,815 psi.

For comparative purposes a composite article was formed as heretofore described employing the control predominantly graphitic carbonaceous yarn which was not subjected to any form of surface modification. The

T average horizontal interlaminar shear strength of the composite article was only 2,800 psi.

[n a comparative surface modification procedure wherein Example 1 was repeated with the exception that no organotitanium compound was applied to the yarn, the resulting composite article failed to exhibit a substantially improved interlaminar shear strength.

In a comparative surface modification procedure wherein Example I was repeated with the exception that no dihydropyridacene polymer was applied to the yarn, the resulting composite article failed to exhibit a substantially improved interlaminar shear strength.

EXAMPLE ll Example 1 is repeated with the exception that the yarn is immersed in a 0.1 per cent by weight formic acid solution of dihydropyridacene polymer, and following evaporation of the solvent in a 0.5 per cent by weight benzene solution of the polymerized tetrabutyl titanate.

The horizontal interlaminar shear strength of the resulting composite article is found to be 10,410 psi.

EXAMPLE 111 Example 1 is repeated with the exception that the hydrolyzable organotitanium compound utilized is an alkyl titanate chelate of the titanium acetylacetonate chemical type which is applied from a 1.0 per cent by weight aqueous solution wherein the pH of the solution is adjusted to about 3 by the presence of acetic acid. The alkyl titanate chelate is commercially available from the Du Pont Company under the designation of Tyzor AA organic titanate.

The resulting composite article exhibits a substantially enhanced horizontal interlaminar shear strength when compared with the control.

EXAMPLE lV Example I is repeated with theexception that the hy- EXAMPLE V Example I is repeated with the exception that the hydrolyzable organotitanium compound utilized is a simple alkyl titanate. More specifically, the organotitanium compound is tetraisopropyltitanate commercially available from the Du Pont Company under the designation Tyzor TPT. The tetraalkyl titanate is applied from a 5 per cent by weight solution inisopropanol.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

We claim:

l. A process for the surface modification of a predominantly graphitic carbonaceous fibrous material containing at least 90 per cent carbon by weight comprising: p

a. coating said fibrous material with a film of a dihydropyridacene polymer which is substantially free of inter-molecular cross-linking consisting of to 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are at least about per cent cyclized, and 0 to 15 mol per centof co polymerized monovinyl units, with said dihydropyridacene polymer film being in intimate association with a hydrolyzable organotitanium compound capable of yielding titanium dioxide upon hydrolysis,

b. hydrolyzing said hydrolyzable organotitanium compound in intimate association with said dihydroyridacene polymer film to form titanium dioxide. and

c. carbonizing said dihydropyridacene polymer portion of said film present upon said fibrous material to a predominantly amorphous carbon form containing at least about 90 per cent by weight by heating in an inert gaseous atmosphere at a temperature of at least about 900C, but not exceeding about l,800C. to produce :a predominantly graphitic carbonaceous fibrous material which possesses an enhanced ability to bond to a matrix material.

2. A process according to claim 1 wherein said dihydropyridacene polymer consists of mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized.

3. An improved process according to claim 1 wherein said hydrolyzable organotitanium compound capable of yielding titanium dioxide upon hydrolysis is selected from the group consisting of the simple alkyl titanates, the polymerized alkyl titanates, the alkyl titanate chelates, and the polymeric titanium phosphinates.

4. An improved process according to claim 1 wherein said hydrolyzable organotitanium compound in intimate association with said dihydropyridacene film is hydrolyzed by heating in air.

5. An improved process according to claim 1 wherein said inert gaseous atmosphere is provided at a temperature of at least about 900C, but not exceeding about 6. An improved process according to claim 1 wherein said inert gaseous atmosphere is selected from the group consisting of nitrogen, argon, and helium.

7. A process for the surface modification of a predominantly graphitic carbonaceous fibrous material containing at least 95 per cent carbon by weight comprising:

a. contacting said fibrous material with a solution containing a dihydropyridacene polymer which is substantially free of inter-molecular cross-linking consisting of 85 to l mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized, and 0 to 15 mol per cent of copolymerized monovinyl units which is dissolved in a solvent incapable of destroying the original fibrous configuration of said fibrous material,

. evaporating said solvent of said solution in contact with said fibrous material whereby a substantially uniform first film of said dihydropyridacene polymer having a thickness of about 5 to 1,000 angstroms is deposited upon the surface of said fibrous material,

c. contacting said resulting fibrous material bearing said first film of said dihydropyridacene polymer upon the surface thereof with a solution containing a hydrolyzable organotitanium compound selected from the group consisting of the simple alkyl titanates, the polymerized alkyl titanates, the alkyl titanate chelates, and the polymeric titanium phosphinates capable of yielding titanium dioxide upon hydrolysis which is dissolved in a solvent incapable of dissolving said first polymer film or destroying the original fibrous configuration of said fibrous material,

evaporating said solvent of said solution in contact with said fibrous material whereby a substantially uniform second film of said hydrolyzable organotitanium compound having a thickness of about 4 to 200 angstroms is deposited upon the surface of said fibrous material,

e. hydrolyzing said second film of said hydrolyzable organotitanium compound present upon the surface of said fibrous material by heating in air at about 200 to 300C. to form a substantially uniform film of titanium dioxide having a thickness of about 4 to 200 angstroms upon said fibrous material in intimate associated with said first film of said dihydropyridacene polymer while said first film of dihydropyridacene polymer is simultaneously oxidatively cross-linked, and

f. carbonizing said first film of dihydropyridacene polymer film present upon said fibrous material to an amorphous carbon form containing at least about per cent carbon by weight by heating in an inert gaseous atmosphere selected from the group consisting of nitrogen, argon, and helium at a temperature of at least about 900C, but not exceeding about l,200C. to produce a predominantly graphitic carbonaceous fibrous material which possesses an enhanced ability to bond to a matrix material.

8. A process according to claim 7: wherein said dihydropyridacene polymer consists of mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized.

9. A process according to claim 7 wherein said dihydropyridacene polymer is dissolved in a solvent selected from the group consisting of formic acid, sulfuric acid, and polyphosphoric acid.

10. A process according to claim 7 wherein said hydrolyzable organotitanium compound is a polymerized tetrabutyltitanate.

11. A process according to claim 7 wherein said by drolyzable organotitanium compound is titanium acetylacetonate.

12. An improved process according to claim 7 wherein said hydrolyzable organotitanium compound is a simple tetraalkyl titanate having 1 to 8 carbon atoms in each alkyl groupf I 13. A process according to claim 12 wherein said hydrolyzable organotitanium compound is tetraisopropyltitanate. I

14. A composite article exhibiting an enhanced interlaminar shear strength comprising a resinous matrix material having incorporated therein a predominantly graphitic carbonaceous fibrous material containing at least about 90 percent carbon by weight having a film of predominantly amorphous carbon containing at least about 90 percent carbon by weight upon the surface thereon in intimate association with metallic titanium formed in accordance with the process of claim 1, with said film of predominantly amorphous carbon in intimate association with metallic titanium having a thickness of about 10 to 1.000 angstroms.

Claims

2. A process according to claim 1 wherein said dihydropyridacene polymer consists of 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized.
3. An improved process according to claim 1 wherein said hydrolyzable organotitanium compound capable of yielding titanium dioxide upon hydrolysis is selected from the group consisting of the simple alkyl titanates, the polymerized alkyl titanates, the alkyl titanate chelates, and the polymeric titanium phosphinates.
4. An improved process according to claim 1 wherein said hydrolyzable organotitanium compound in intimate association with said dihydropyridacene film is hydrolyzed by heating in air.
5. An improved process according to claim 1 wherein said inert gaseous atmosphere is provided at a temperature of at least about 900*C., but not exceeding about 1,200*C.
6. An improved process according to claim 1 wherein said inert gaseous atmosphere is selected from the group consisting of nitrogen, argon, and helium.
7. A process for the surface modification of a predominantly graphitic carbonaceous fibrous material containing at least 95 per cent carbon by weight comprising: a. contacting said fibrous material with a solution containing a dihydropyridacene polymer which is substantially free of inter-molecular cross-linking consisting of 85 to 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized, and 0 to 15 mol per cent of copolymerized monovinyl units which is dissolved in a solvent incapable of destroying the original fibrous configuration of said fibrous material, b. evaporating said solvent of said solution in contact with said fibrous material whereby a substantially uniform first film of said dihydropyridacene polymer having a thickness of about 5 to 1,000 angstroms is deposited upon the surface of said fibrous material, c. contacting said resulting fibrous material bearing said first film of said dihydropyridacene polymer upon the surface thereof with a solution containing a hydrolyzable organotitanium compound selected from the group consisting of the simple alkyl titanates, the polymerized alkyl titanates, the alkyl titanate chelates, and the polymeric titanium phosphinates capable of yielding titanium dioxide upon hydrolysis which is dissolved in a solvent incapable of dissolving said first polymer film or destroying the original fibrous configuration of said fibrous material, d. evaporating Said solvent of said solution in contact with said fibrous material whereby a substantially uniform second film of said hydrolyzable organotitanium compound having a thickness of about 4 to 200 angstroms is deposited upon the surface of said fibrous material, e. hydrolyzing said second film of said hydrolyzable organotitanium compound present upon the surface of said fibrous material by heating in air at about 200* to 300*C. to form a substantially uniform film of titanium dioxide having a thickness of about 4 to 200 angstroms upon said fibrous material in intimate associated with said first film of said dihydropyridacene polymer while said first film of dihydropyridacene polymer is simultaneously oxidatively cross-linked, and f. carbonizing said first film of dihydropyridacene polymer film present upon said fibrous material to an amorphous carbon form containing at least about 95 per cent carbon by weight by heating in an inert gaseous atmosphere selected from the group consisting of nitrogen, argon, and helium at a temperature of at least about 900*C., but not exceeding about 1,200*C. to produce a predominantly graphitic carbonaceous fibrous material which possesses an enhanced ability to bond to a matrix material.
8. A process according to claim 7 wherein said dihydropyridacene polymer consists of 100 mol per cent of acrylonitrile units wherein the pendant nitrile groups thereof are fully cyclized.
9. A process according to claim 7 wherein said dihydropyridacene polymer is dissolved in a solvent selected from the group consisting of formic acid, sulfuric acid, and polyphosphoric acid.
10. A process according to claim 7 wherein said hydrolyzable organotitanium compound is a polymerized tetrabutyltitanate.
11. A process according to claim 7 wherein said hydrolyzable organotitanium compound is titanium acetylacetonate.
12. An improved process according to claim 7 wherein said hydrolyzable organotitanium compound is a simple tetraalkyl titanate having 1 to 8 carbon atoms in each alkyl group.
13. A process according to claim 12 wherein said hydrolyzable organotitanium compound is tetraisopropyltitanate.
14. A composite article exhibiting an enhanced interlaminar shear strength comprising a resinous matrix material having incorporated therein a predominantly graphitic carbonaceous fibrous material containing at least about 90 percent carbon by weight having a film of predominantly amorphous carbon containing at least about 90 percent carbon by weight upon the surface thereon in intimate association with metallic titanium formed in accordance with the process of claim 1, with said film of predominantly amorphous carbon in intimate association with metallic titanium having a thickness of about 10 to 1,000 angstroms.