US3853600A - Improved carbon fiber reinforced composite article - Google Patents

Abstract

A process is provided for modifying the surface characteristics of a carbonaceous fibrous material (i.e., either amorphous carbon or graphitic carbon) and to thereby facilitate enhanced adhesion between the carbonaceous fibrous material and a matrix material. The carbonaceous fibrous material is coated with a compact polyphenylene polymer coating which is deposited thereon upon contact with an excited gas species generated by applying high frequency electrical energy in pulsed form to an inert gas in the presence of at least one aromatic compound. The coating step is efficiently conducted at a temperature of about 0* to 150*C. and at a pressure within the coating zone of about 1 to 3 atmospheres. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

Description

Ilnited tates atertt [191 IMPROVED CARBON FIBER REINFORCED COMPOSITE ARTICLE.

[75] Inventor: Kenneth C. Hou, Whippany, NJ.

[73] Assignee: Celanese Corporation, New York,

[22] Filed: Feb. 28, 1973 [21] Appl. No.: 336,868

Related U.S. Application Data [62] Division of Ser. No. 142,656, May 12, 1971, Pat. No.

[56] References Cited UNITED STATES PATENTS 3,002,850 10/1961 Fischer r 117/33 3,108,018 10/1963 Lewis ll7/l6l 3,238,054 3/1966 Bickedike et a1. 117/46 3,421,930 l/l969 Knox et a1. 1l7/93.1 3,429,739 2/1969 Tittmann et a1. 117/106 [111 Dec. 10, 11974 Rohl et a1. 117/46 Primary ExaminerRalph S. Kendall Assistant Examiner-P. E. Willis 5 7 ABSTRACT A process is provided for modifying the surface characteristics of a carbonaceous fibrous material (i.e., either amorphous carbon or graphitic carbon) and to thereby facilitate enhanced adhesion between the carbonaceous fibrous material and a matrix material. The carbonaceous fibrous material is coated with a compact polyphenylene polymer coating which is deposited thereon upon contact with an excited gas species generated by applying high frequency electrical energy in pulsed form to an inert gas in the presence of at least one aromatic compound. The coating step is efficiently conducted at a temperature of about 0 to 150C. and at a pressure within the coating zone of about 1 to 3 atmospheres. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

9 Claims, 8 Drawing Figures GENERATOR PATENIEB 308531.600

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SOUQCE PULSE GENERATOR PATENTEB BEE? 0 I974 SHEEF 2 [IF A /Z m M fi -ll M Q F a U F W P L/ 7 7 C v 4 a K c K f a W TUNE h WM M BACKGROUND OF THE INVENTION In the search for high performance materials, considerable interest has been focused upon carbon fibers. The term carbon fibers" is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant 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 weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Youngs modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and 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 resistance, 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 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 carbon fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is utilized is commonly a thermosetting resinous material and is commonly selected because of its ability to also withstand highly elevated temperatures.

While it has been possible in the past to provide carbon fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantage 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 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 instnce. 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 (e.g., 1 hour or more).

It has also been proposed in the past that coupling agents be deposited upon carbon fibers via an electric discharge plasma producing reaction which is conducted at a pressure substantially below atmospheric (e.g., 2 mm. Hg). Such techniques have proven to be tedious and not readily adaptable to commercial utilization because of the necessity of operating at a substantially reduced pressure.

It is an object of the invention to provide a process for efficiently modifying the surface characteristics of carbon fibers with no substantial reduction in the single filament tensile properties.

It is an object of the invention to provide a process for improving the ability of carbon fibers to bond to a resinuous matrix material.

It is an object of the invention to provide a process for modifying the surface characteristics of carbon fibers which may be conducted relatively rapidly at moderate temperatures at atmospheric pressure.

It is another object of the invention to provide composite articles reinforced with carbon fibers exhibiting 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 an improved process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about percent carbon by weight comprises: (a) providing in a coating zone at a pressure of about 1 to 3 atmospheres an inert gas and at least one aromatic compound having from 1 to 4 six member carbon rings which is capable of undergoing polymerization to form a polyphenylene polymer, (b) applying high frequency electrical power in pulsed from to the inert gas sufficient to establish an excited gas species within the coating zone while maintaining the temperature of the zone at about 0 to C, and (c) contacting the carbonaceous fibrous material while present in the coating zone with the excited gas species until a. compact coating of a polyphenylene polymer is deposited on the carbonaceous fibrous material having a thickness of about 25 to 800 angstrom units.

The resulting carbon fibers 'may be incorporated in a resinuous matrix material to form a composite article exhibiting enhanced interlaminar shear strength.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration of a representative apparatus arrangement for modifying the surface characteristics of a carbonaceous fibrous material in accordance with the present invention.

FIGS. IA and IB are schematic illustrations of alternative means for capacitively exciting the inert gas in the coating zone of FIG. 1.

FIG. IC is a schematic illustration of means for inductively exciting the inert gas in the coating zone of FIG. I.

FIG. 2 is a schematic illustration of a further representative apparatus arrangement for modifying the surface characteristics of a carbonaceous fibrous material in accordance with the present process wherein a mercury pool surrounds the surface modification zone.

FIG. 3 is a photograph made with the aid of a scan ning electron microscope of a portion of a graphic filament which has not undergone surface modification. (5500X) FIG. 41 is a photograph made with the aid of a scanning electron microscope of a portion of graphite filament bearing a compact coating of a polyphenylene polymer which is representative of the appearance of carbon fibers which are surface modified in accordance with the present process. (4900X) FIG. 5 is a photograph made with the aid of a scanning electron microscope of a portion of a graphite filament bearing a non-compact coating of a polyphenylene polymer which is representative of the appearance of carbon fibers which are surface modified while providing the coating zone at an excessive temperature. (SSOOX) DESCRIPTION OF PREFERRED EMBODIMENTS The Starting Material The fibers which are surface modified in accordance with the present process are carbonaceous and contain at least about 90 per cent carbon by weight. Such carbon fibers may exhibit either an amorphous carbon or a predominantly graphitic carbon x-ray diffraction pattern. In a preferred embodiment of the process the carbonaceous fiber which undergo surface coating contain at least about 95 per cent carbon by weight, and at least about 99 per cent carbon by weight in a particularly preferred embodiment of the process.

The carbonaceous fibrous material may be provided as either a continuous or a discontinuous length. In a preferred embodiment of the process the carbonaceous fibrous material is a continuous length which may be any one of a variety of physical configuration provided substantial access to the fiber surface is possible during the surface coating treatment-described hereafter. For instance, the carbonaceous fibrous material may assume the configuration of a continuous length of a multifilament yarn, tow, tape, strand, cable, or similar fibrous assemblage. In a preferred embodiment of the process the carbonaceous fibrous material is one or more continuous multifilament yarn. When a plurality of multifilament yarns are surface coated simultaneously, they may be continuously passed through the heating zone while in parallel and in the form of a flat ribbon.

The 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 yarn. 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 a which possess essentially no twist.

. mally stabilized material is heated to a maximum temperature of 2,000 to 3,100C. (preferably 2,400 to 3,100C.) in an inert atmosphere, substantial amounts of graphitic carbon are commonly detected in the resulting carbon fiber, otherwise the carbon fiber will commonly exhibit an essentially amorphous x-ray diffraction pattern.

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 fibrous material capable of undergoing carbonization 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 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 hexamethylenediarnine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly-2,2-mphenylene-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 carbonaceous fibers 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 United States Ser. No. 749,957, filed Aug. 5, 1968 (now abandoned), 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 acrylonitrile homopolymer. The stabilized acrylic fibrous material which is preoxidized in an oxygen-containing atmosphere is black in appearance, commonly contains a bound oxygen content of at least about 7 per cent by weight as determined by the Unterzaucher analysis, retains its original fibrous configuration essentially intact, and is nonbuming when subjected to an ordinary match flame.

In preferred techniques 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 United States Pat. Nos. 777,275, filed Nov. 20, 1968 of Charles M. Clarke (now abandoned); 17,780, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and John P. Riggs (now U.S. Pat. No. 3,677,705); and 17,832, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and Arnold J. Rosenthal (now US. 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-burning 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 mole 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 through a carbonization/graphitization heating zone containing an inert gaseous atmosphere and a temperature gradient in which the fibrous material is raised within a period of about 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 10 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 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 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 or carbonization and graphitization 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 being carbonized or 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 essentially the same temperature as that of the zone through which it is continuously passed.

The Contents of the Coating Zone Within the coating zone is provided in inert gas at a pressure of about I to 3 atmospheres as well as aromatic compound capable of undergoing polymerization to form a polyphenylene polymer. The necessity of operating at reduced pressure conditions and the concomitant disadvantages associated therewith are accordingly avoided in the present process. The coating zone is conveniently provided at substantially atmospheric pressure.

Suitable inert gases for inclusion in the coating zone include nitrogen, helium, argon, neon, krypton, and xenon, and mixtures of the foregoing. The preferred inert gases are monoatomic, e.g., helium, argon, neon, krypton, and xenon since these tend to undergo excitation more readily. The particlarly preferred monoatomic inert gases for use in the process are helium and argon. The relatively high current costs of neon, krypton, and xenon militate against their selection. When present in the coating zone, the inert gas undergoes excitation upon application of the high frequency electrical power in pulsed form (described hereafter) and aids in the generation of an excited gas species capable of promoting the formation of polyphenylene polymer. In the absence of the appreciable presence of the inert gas in the gaseous mixture, the desired surface coating of polyphenylene polymer is not accomplished because of the inability to achieve the requisite degree of excitation while maintaining moderate coating conditions, e.g., temperature. It is recommended that the inert gas within the coating zone as well as the aromatic compound withinthe coating zone be either intermittently or continuously replenished (e.g., by the continuous introduction of each).

The aromatic compound which undergoes polymerization in the coating zone to form a polyphenylene polymer coating or film upon the surface of the carbon fiber has from 1 to 4 six member carbon rings (i.e., substituted or unsubstituted benzene rings). When the aromatic compound contains more than one six member carbon ring, the rings present in the compound optionally may be fused (i.e., condensed). The nature of the atoms or groups of atoms (i.e., functional groups) which are additionally bonded to the carbon atoms of the six member carbon ring is not critical to the operation of the present process. Other atoms or groups of atoms may optionally be substituted for the hydrogen atoms normally bonded to the ring carbon atoms of such aromatic hydrocarbons. The presence of functional groups bonded to the six member carbon ring of the aromatic compound may, however, serve as a means for introducing functional groups into the resulting polyphenylene polymer which may ultimately serve to further enhance the adhesive bond between a carbon fiber and a specific resin matrix material.

Illustrative examples of unsubstituted aromatic compounds are benzene, naphthalene, anthracene, phenanthrene, pyrene, and chrysene.

Illustrative examples of alkyl substituted aromatic compounds are toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, n-propylbenzene, cumene, nbutylbenzene, isobutylbenzene, p-ethyltoluene, diphenylmethane, l,2-diphenylethane, mesitylene, pentamethylbenzene, and hexamethylbenzene.

Illustrative examples of halogen substituted aromatic compounds are chlorobenzne, bormobenzene, fluorobenzene, o-dichlorobenzene, m-dichlorobenzene, pdichlorobenzene, l-bromo-4-chlorobenzene, pdibromobenzene, l ,2,3-trichlorobenzene, l ,3,5- trichlorobenzene, 1,2,4-trichlorobenzene, hexafluorobenzene, hexachlorobenzene, and l chloronaphthalene.

Illustrative examples of additional monosubstituted aromatic compounds include diphenyl, nitrobenzene, aniline, phenol, styrene, divinylbenzene, benzaldehyde, benzyl acetate, and benzoic acid.

Illustrative examples of disubstituted aromatic compounds having mixed functional groups are 0- chlorotoluene, m-chlorotoluene, p-chlorotoluene, o-

bromotoluene, p-bromotoluene, m-nitrobenzoic acid, 2,4-dinitrobenzoic acid, and p-bromoaniline.

The preferred aromatic compounds are those which exist as a liquid at room temperature and which readily undergo volatilization.

The aromatic compound may be introduced into the coating zone by any one of a variety of techniques, and the techniques selected is not critical to the formation of the desired polyphenylene polymer coating. If the aromatic compound is a volatile liquid, it may be introduced into the coating zone as a gaseous stream (e.g., the inert gas may be passed through a vessel containing the liquid aromatic compound prior to its introduction into the coating zone). If desired, the temperature of the liquid aromatic compound may be elevated in order to increase its volatility. Alternatively, the aromatic compound may be introduced into the coating zone while present upon the carbonaceous fibrous material. For instance, the carbonaceous fibrous material may be initially immersed in a liquid aromatic compound whereby the carbonaceous fibrous material is impregnated with the same. Those aromatic compounds which normally exist as solids are conveniently dissolved in a solvent for the same and the carbonaceous fibrous material immersed in the solution prior to its introduction into the coating zone. When a solvent is employed, the solvent may optionally be volatized prior to introduction of the impregnated carbonaceous fibrous material into the coating zone. If an aromatic solvent is introduced into the coating zone, the solvent itself may also undergo the phenylene polymer formation reaction. The desired phenylene polymer formation reaction can be carried out (as described hereafter) regardless of whether the aromatic compound is introduced into the coating zone as a gas, a liquid, or as a solid. When the aromatic compound is introduced as a liquid or as a solid, it preferably is introduced while adhering to the carbonaceous fibrous material.

The Formation of the Polyphenylene Polymer Coating The coating of the surface of the carbonaceous fibrous material is accomplished by contacting the carbonaceous fibrous material while present in the coating zone with an excited gas species formed through the application of pulsed high frequency electrical power to the inert gas in the presence of the aromatic compound. The carbonaceous fibrous material may be statically suspended or otherwise positioned within the coating zone. In a preferred embodiment of the process a continuous length of the carbonaceous fibrous material is continuously passed, e.g., in the direction of its length, through the excited gas species present in the coating zone. For instance, a rotating feed roll may be provided at the entrance end of the coating zone, and a rotating take-up roll may be provided at the exit end of the coating zone.

The coating zone may be bounded by walls constructed of either a conductive or a non-conductive material. For instance, a tubular chamber constructed of transplant glass may be conveniently selected to define the bounds of the coating zone. In such an arrangement a continuous length of carbonaceous fibrous material may be axially suspended therein with free access of its surface to the excited gas species provided.

The excited gas species required to produce the desired polyphenylene polymer coating may be formed by inductively or capacitively coupling pulsed high frequency electric power to the contents of the coating zone. A combination of inductive and capacitive coupling may also be utilized. As shown in FIG. 1 (described in detail hereafter), the contents of the coating zone may be capacitively excited. Representative alternative apparatus arrangements wherein capacitive coupling also may be utilized are shown in FIG. 1A, FIG. 1B, and FIG. 2. In FIG. 1A the pulsed high frequency electrical power is applied to metallic rings which are oriented perpendicularly to the axis of an elongated coating zone and effectively surround the same. In FIG. 1B the pulsed high frequency electrical power is applied to a pair of mercury filled tubes oriented parallel to the axis of an elongated coating zone and positioned within the same. In FIG. 1C pulsed high frequency electrical power is inductively applied to an elongated coating zone through the use of a single coil which completely surrounds the same.

The term pulsed electrical power or electrical power in pulsed form as used herein is defined as pulses or bursts of high frequency electrical energy, e.g., pulsed rf energy. The power may be an ac. signal having an amplitude of about 500 v. to 10 Kv. peak-topeak and a frequency of about 0.5 KI-Iz, to 2,500 MHz. (preferably 1.0 KHz. to 30 MI-Iz.). The pulses may be from about 0.1 microseconds to 10 milliseconds duration (preferably 10 to 1,000 microseconds). The pulse repetition rate may be from about 0.1 KI-Iz. to 20 MHz. (preferably about 1.0 to KHz). The pulsed electrical power may be provided in accordance with techniques known to those skilled in the electrical arts, e. g., by gating a high frequency oscillator or klystron on and off to generate bursts of high frequency energy. The exact dimensions of the coating zone will influence the power requirement as will be apparent to those skilled in the art.

The high frequency electrical power in pulsed form is applied to the inert gas in the presence of the aromatic compound in sufficient quantity to establish an excited gas species capable of forming a polyphenylene polymer coating while maintaining the temperature of the coating zone at about 0 to C, and preferably at about 20 to 100C, and most preferably at about 20 to 50C. When maintaining the temperture of the coating zone at the moderate temperatures indicated, a compact polyphenylene polymer coating is formed similar in appearance to that shown in FIG. 4. If the temperature of the coating zone is elevated much above about 150C., then a non-compact loosely adhering polyphenylene polymer coating results which is similar in appearance to that shown in FIG. 5. If desired, the maintenance of the desired temperature may be aided by immersion of the coating zone in a low dielectric liquid bath, such as silicon oil.

The carbonaceous fibrous material is contacted with the excited gas species present within the coating zone until a compact coating of a polyphenylene polymer having a thickness of about 25 to-800 angstrom units (preferably 50 to 250 angstrom units) is deposited thereon and its ability to bond to a matrix material is beneficially enhanced. Unlike many prior art surface modification techniques, the residence time required in the present process is relatively brief. For instance, residence times of about 0.2 to 20 minutes may be conveniently selected, and preferably residence times of about 1 to 4 minutes.

The surface modification process of the present invention offers the advantage of uniformly altering the surface characteristics of the carbonaceous fibrous materials to the substantial exclusion of adversely influencing the single filament tensile properties of the same, i.e., the tensile strength and Youngs modulus.

The surface modification of the present process makes possible improved adhesive bonding between the carbonaceous fibers, and a resinous matrix material. Accordingly, carbon fiber reinforced composite materials which incorporate fibers coated as heretofore described exhibit enhanced shear strength, fiexural strength, compressive strength, etc. The resinous matrix material employed in the formation of such composite material is commonly a polar thermosetting resin such as an epoxy, a polyimide, a polyester, phenolic, etc. The carbonaceous fibrous material is commonly provided in such resulting composite materials in ether an aligned or random fashion in a concentration of about 20 to 70 per cent by volume.

A representative apparatus arrangement for carrying out the surface modification process (i.e., coating process) of the invention is illustrated in FIG. 1. With reference to FIG. 1, the power unit includes a conventional variable dc. power supply 2, a conventional pulse generator 4 having a variable pulse repetition rate and a variable pulse width, a conventional signal amplifier 6, and a variable frequency oscillator 8. The output signal from the pulse generator 4 is applied to the oscillator 8 by way of the signal amplifier 6. Both a variable positive dc. voltage and a fixed negative bias voltage from the power supply 2 are applied to the oscillator 8.

The power supply 2 may be any conventional variable d.c. power supply, e.g., a Kepco Model 615B, -600 volt and negative 150 volt power supply. The pulse generator 4 may be any conventional pulse generaor of variable pulse repetition rate, e.g., a Hewlett Packard Model 3300A pulse generator, which provides pulses having a variable pulse repetition rate and either a constant or a selectably variable pulse width or duration. The amplifier 6 may be any conventional amplifier having an odd number of stages which amplifies and inverts the pulses from the pulse generator 4 and provides positive output pulses. The oscillator 6 may be any conventional variable high frequency oscillator which preferably generates an output signal in the radio frequency range above 1.0 KI-Iz., and which is capable of being gated or pulsed on and off to provide bursts of high frequency energy. In a preferred operation of the power unit this is accomplished by cutting off the oscillator by applying a negative 150 volt bias to the control grid of an oscillator tube (not shown) by way of an input terminal 110 and by periodically applying v positive pulses to the input terminal and thus the control grid of sufficient amplitude to drive the oscillator tube into conduction.

In operation, the pulse generator 4 generates a series of negative going pulses, the pulse repetition rate and- /or the pulse width of which may be varied to thereby vary the reoccurrence rate and/or the duration of the pulses. The signal from the pulse generator 4 is amplified and inverted by the amplifier 6 and the positive pulses from the amplifier 6 are applied to the oscillator 8. In the absence of a pulse from the amplifier 6, the oscillator 8 is cut off and does not provide an output signal. However, when a pulse from the pulse generator d is applied to the oscillator 8 by way of the amplifier 6, the oscillator 8 breaks into high frequency oscillations and provides an output signal for the duration of the applied pulse. The resultant pulsed high frequency signal may be coupled to the coating zone 20 through a conventional high frequency step-up coil 12, the primary winding of which may be utilized for both signal coupling and as a portion of the oscillator tank circuit. Lead 14 connects the coil 12 to coaxial electrode 21. Coaxial electrode 21 consists of a 10 inch length of copper tubing having an outer diameter of one-half inch and an inner diameter of seven-sixteenth inch. Situated in series with coaxial electrode 211 is a like coaxial electrode 22.

The amplitude of the output signal from the oscillator 6 may be varied by varying the voltage directly applied to the oscillator 8 from the power supply 2. The frequency of the output signal from the oscillator 8 may, of course, be varied in any suitable conventional manner, e.g., by varying the reactive avlue of an electn'cal component in a tank circuit (not shown). In addition, the relationship between the on" time and the off time of the output signal and the duration of the pulses of high frequency energy may be varied by adjusting the pulse repetition rate and/or width of the output pulses from the pulse generator 4. The pulse unit is thus capable of supplying bursts of electrical energy of a variable high frequency, the bursts occurring at a selectable burst repetition rate and having a variable burst width or duration.

Another representative pulsing unit which may be used to provide the pulsed high frequency signal to excite the inert gas in the coating zone is a Lepel Model No. T-53 high frequency power unit capable of delivering up to a 10 Kv. signal at a frequency of up to 30 MHz. pulsed by a grid pulse modulator Model 1414 available from Pulse Tronics Engineering Co.

By providing a pulsed frequency signal as described above, excessive heat buildup within the coating zone 20 may be prevented through variation of the pulse repetition rate, the pulse width or duration, or both of these parameters. The heat generated within the coating zone during the application of pulsed high frequency signal is allowed to dissipate to a great extent during the off period of the oscillator, i.e., between pulses of high frequency energy.

Since the signal amplitude, frequency, duration and repetition rate required for carrying out the process depend upon the diameter andlength of the coating zone, such parameters may vary widely. The temperature inside the coating zone 20 may be sensed by a thermocouple 23 and a visual temperature indication may be provided at meter 25. The temperature within the zone 20 may thus be easily regulated by visually monitoring the meter 25 and adjusting the pulse repetition rate and/or the pulse width of the high. frequency signal. The intensity of the excitation is controlled by the amplitude and duration of the pulses, the pulse repetition rate, the space gap between the electrodes, and the total length of the coating zone.

With a coating zone or chamber 20 of approximately 22 inches in length and seven-sixteenth inch in diameter, the process may be conveniently practiced utilizing a pulsed high frequency output signal from the oscillator 6 in the radio frequency range above 1.0 KHz, the particularly preferred range being from 1.0 Kl-Iz. to 30 MHZ. The signal may be pulsed at a repetition rate of Ill from about 1.0 to about 1,000 KHz. to 100 KHZ. being preferred) while the pulse width may be from 0.1 to 1,000 microseconds, (1.0 to 500 microseconds being preferred). The amplitude of the pulsed high frequency signal may be from 500 v. to 10 Kv. (1 to 5 Kv. being preferred).

The following examples are given as specific illustrations of the process of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples. EXAM- PLE I Reference is made to the apparatus of FIG. 1.

A high strength-high modulus continuous filament carbonaceous yarn derived from an acrylonitrile homopolymer in accordance with the procedures described in United States Ser. Nos. 749,957, filed Aug. 5, 1968, and 777,957, filed Nov. 20, 1968 (now abandoned) is selected as the starting material. The yarn consists of a 1,600 fil bundle having a total denier of about 1,000, has a carbon content in excess of 99 per cent by weight, exhibits a predominantly graphite x-ray diffraction pattern, a single filament tenacity of about 13 grams per denier and a single filament Youngs modulus of about 50 million psi.

The carbonaceous yarn 24 is unwound from rotating feed roll 26 and is immersed in benzene 60 which is provided in vessel 62 by the aid of rollers 64, 66, and 68. The resulting benzene impregnated carbonaceous fibrous material next passes through neck 27, around pulley 28, through coating zone 20, through neck 32, around pulley 34, and is ultimately taken up upon rotating uptake roll 36. The carbonaceous yarn 24 passes through coating zone while axially suspended therein at a rate of 12 inches per minute.

The coating zone 20 is defined by tubular glass of about seven-sixteenth inch diameter and about 22 inches in length. Helium is introduced as the inert gas via inlet tube at a rate of 2,000 cc. per minute. Off

gases exit via exit tube 42. A 3,000 v. peak-to-peak a.c. signal having a frequency of 13.56 MHz. is applied to coaxial electrode 211 in pulses of 500 microseconds duration of p.r.r. (pulse repetition rate) of 100 KHz. An excited gas species is established throughout the length of the coating zone 20. The yarn 24 is in contact with the excited gas species for a residence time of about 2 minutes and a compact polyphenylene polymer coating is formed thereon having a thickness of about 100 angstrom units. Throughout the phenylene polymer coating treatment the temperature within zone 20 is maintained at approximately 25C. as measured by thermocouple 23 and indicated on meter 25. During the coating deposition the entire coating zone 20 is surrounded by cooling bath 46 of silicone oil, which is kept in circulation by a pump 48 connected to reservoir 50 via lines 52 and 54. The polyphenylene nature of the polymer coating is confirmed by infra red analysis. The carbonaceous yarn following surface treatment retains its original tenacity and Youngs modulus.

A composite article is next formed employing the polyphenyl polymer coated yarn sample as a reinforcing medium in a resinous matrix. The composite article is a rectangular bar consisting of about 65 per cent by volume of the yarn and having dimensions of A: inch X V4 inch X5 inches. The composite article is formed by impregnation of the coated yarn in a liquid epoxy resinhardener mixture at 50C. followed by unidirectional 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 is cooled slowly to room temperature, and the composite article is removed from the mold cavity and cut to size for testing. The resinous matrix material used in the formation of the composite article is provided as a solventless system which contains 100 parts by weight epoxy resin and 88 parts by weight of anhydride curing agent.

The horizontal interlaminar shear strength is determined by short beam testing of the carbon fiber reinforced composite according to the procedure of ASTM D2344-65T as modified for straight bar testing at a 4:1 span to depth ratio and found to be substantially greater than that of a control wherein an identical carbonaceous yarn serving as the fibrous reinforcement is never subjected to any form of surface modification.

EXAMPLE II Example I is repeated with the exception that the aromatic compound is toluene.

Substantially similar results are achieved.

EXAMPLE III Example I is repeated with the exception that the aromatic compound 60 is styrene.

Substantially similar results are achieved.

EXAMPLE IV Example I is repeated with the exception that the aromatic compound 60 is aniline.

Substantially similar results are achieved.

EXAMPLE V Example I is repeated with the exception that a mixture of aromatic compounds 60 is provided in vessel 62. The mixture of aromatic compounds consists of 10 per cent by weight solution of phenol dissolved in a benzene solvent.

EXAMPLE VI having a total denier of about 700.

When incorporated in a composite article (as described), the surface coated yam produces a composite article exhibiting an enhanced horizontal interlaminar shear strength when compared with that of a control wherein the yarn underwent no form of surface modification.

EXAMPLE VII Reference is made to the apparatus of FIG. 2 in which like numerals designate similar components to those previously described in connection with FIG. 1.

A carbonaceous multifilament yarn 24 identical to that employed in Example I is continuously unwound from feed roll 29 and passed through mercury seal 31 supplied from reservoir 33.

The carbonaceous yarn is passed through coating zone 20 at a rate of 16 inches per minute, and is axially suspended within the center of the zone by the aid of annular guide 30 and neck 32. Helium is introduced as the inert gas via inlet tube 70 at a rate of 2,000 0.0. per minute. A portion of the helium passes through conduits 72 and 80 at a rate of 1,600 c.c./min. as controlled by valve 7'6. The remaining portion of the helium passes through conduit 7d via valve 78 at a rate of 400 c.c./min. and is bubbled through benzene 84 provided at room temperature (i.e., about 25C.) in vessel 86. The helium together with volatilized benzene is next introduced into the coating zone 20 via inlet tube 40.

The gaseous helium and benzene present in the coating zone 20 is excited by means of the capacitance between mercury jacket l7 encircling the electrically grounded carbonaceous yarn.

A Pulse Tronics generator is used to control a Lepel Model T-3 high frequency signal generator to provide a 3,000 v. peak-to-peak a.c. signal at a frequency of 10 MHz. in pulses of 500 microseconds duration at a pm. of 10 KHz. An excited gas species is established throughout the length of the coating zone 20. The yarn 24 is in contact with the excited gas species for a residence time of about 1 minute during which time a compact coating of a polyphenylene polymer having a thickness of about 100 angstrom units is uniformly deposited thereon. Throughout the coating treatment the temperature within zone 20 is maintained at approximately 25 c. as measured by thermocouple 23 and indicated on meter 25.

Substantially similar surface modifications results are achieved.

The nature, scope, utility, and effectiveness of the present invention have been described and specifically exemplified in the foregoing specification. However, it should be understood that these examples are not in tended to be limiting and that the scope of the invention to be protected is particularly pointed out in the appended claims.

I claim:

1.. A composite article exhibiting enhanced interlaminar shear strength comprising a resinous matrix material derived from a thermosetting resin having incorporated therein a carbonaceous fibrous, material containing at least per cent carbon by weight which bears upon the surface of said carbonaceous fibrous material a compact coating of polyphenylene polymer having a thickness of about 25 to 800 angstrom units.

2. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim ll wherein said resinous matrix material is derived from a polar thermosetting resin selected from the group consisting essentially of an epoxy, a polyimide, a polyester, and a phenolic resin.

3. A composite article exhibiting; enhanced interlaminar shear strength in accordance with claim ll wherein said carbonaceous fibrous material contains at least per cent carbon by weight.

4. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said carbonaceous fibrous material includes graphitic carbon.

5. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim ll wherein said compact coating of a polyphenylene polymer is de rived from benzene.

6. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of a polyphenylene polymer is derived from toluene.

7. A composed article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of a polyphenylene polymer is derived from styrene.

8. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim It wherein said compact coating of polyphenylene polymer is de rived from aniline.

9. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of polyphenylene polymer has a thickness of about 50 to 250 angstrom units.

i= =l =i= =i

Claims (9)

1. A COMPOSITE ARTICLE EXHIBITING ENHANCED INTERLAMINAR SHEAR STRENGTH COMPRISING A RESINOUS MATRIX MATERIAL DERIVED FROM A THERMOSETTING RESIN HAVING INCORPORATED THEREIN A CARBONACEOUS FIBROUS MATERIAL CONTAINING AT LEAST 90 PER CENT CARBON BY WEIGHT WHICH BEARS UPON THE SURFACE OF SAID CARBO-

2. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said resinous matrix material is derived from a polar thermosetting resin selected from the group consisting essentially of an epoxy, a polyimide, a polyester, and a phenolic resin.

3. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said carbonaceous fibrous material contains at least 95 per cent carbon by weight.

4. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said carbonaceous fibrous material includes graphitic carbon.

5. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of a polyphenylene polymer is derived from benzene.

6. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of a polyphenylene polymer is derived from toluene.

7. A composed article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of a polyphenylene polymer is derived from styrene.

8. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of polyphenylene polymer is derived from aniline.

9. A composite article exhibiting enhanced interlaminar shear strength in accordance with claim 1 wherein said compact coating of polyphenylene polymer has a thickness of about 50 to 250 angstrom units.

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