WO1993024558A1

WO1993024558A1 - Method to synthesize carbon-carbon composites

Info

Publication number: WO1993024558A1
Application number: PCT/US1993/005303
Authority: WO
Inventors: Peter E. Pierini; Ritchie A. Wessling; Charles A. Nielsen
Original assignee: The Dow Chemical Company
Priority date: 1992-06-04
Filing date: 1993-06-02
Publication date: 1993-12-09
Also published as: KR950701948A; EP0643738A1; JPH07507533A

Abstract

A matrix composite that contains carbon or polybenzazole fibers in a polybenzazole matrix is carbonized to make a carbon-carbon composite.

Description

METHOD TO SYNTHESIZE CARBON-CARBON COMPOSITES

This invention relates to carbon-carbon composites and processes for making them.

Carbon-carbon composites contain reinforcing carbon fibers in a carbon matrix.

They are used in structural applications because of their low weight, good strength and excellent thermal properties.

Carbon-carbon composites suffer in that they are difficult and time consuming to fabricate. Fabrication and properties of carbon-carbon composites are explained in numerous published references, such as: Savage, "Carbon-carbon Composite Materials," Metals and Materials 544-548 (1988) and Schmidt, "Carbon/carbon Composites", SAMPE Journal 9-19 (1972). The two most common methods of making carbon-carbon composites are chemical vapor deposition and liquid impregnation.

In chemical vapor deposition, a substrate of carbon fibers is heated by induction and then contacted with hydrocarbon gas, so that the gas decomposes to a deposited carbon (which is deposited on the substrate) and hydrogen gas. The temperature of the substrate is usually 590°C to 1500°C. The hydrocarbon gas is frequently a C₁-C₆ alkane, such as methane. The technique requires many repeated cycles of heating and deposition, so that total process times of 2000 hours are not uncommon.

In liquid impregnation, the substrate is impregnated with a carbonizable resin

(such as a phenolic resin, pitch or tar) to form an organic matrix composite. Then the matrix material is carbonized by heating the composite to very high temperatures (1000°C to 2800°C). The process of impregnation and carbonization is usually repeated 4 to 12 times to eliminate voids, since the char yield of the carbonizable resin is usually very low. Since impregnation frequently requires 20 hours, the process usually takes several days.

If a very high quantity of carbon can be generated on the first cycle, then the number of subsequent cycles can be reduced, and the time to make a good composite can be shortened. What is needed is a process to make a carbon-carbon composite that can deposit very high level of carbon in a single step.

The present invention is a process to make a carbon-carbon composite in which a matrix composite, which contains fibers embedded in an organic matrix material, is heated to a temperature and for a time sufficient to convert the organic matrix material to carbon, characterized in that the organic matrix material is a lyotropic liquid-crystalline

polybenzoxazole or polybenzothiazole polymer or copolymer.

The method can be used to synthesize carbon-carbon composites with very high char yields in the first step. This can reduced the need for subsequent reimpregnation and carbonization steps. Moreover, the fibers may be polybenzazole, and may be carbonized simultaneously with the matrix resin. The composite can be used for ordinary structural and friction purposes of carbon-carbon composites, such as brake shoes.

The present invention uses a matrix composite that contains fibers and a matrix. The fibers may be carbon or a lyotropic liquid crystalline polybenzoxazole or polybenzothiazole polymer or copolymer (which are collectively referred to as polybenzazole polymers in this application). The description and preferred embodiments of the PBO or PBT polymer are the same for both the fiber and the matrix, although the fiber and the matrix may be different polymers.

Suitable carbon fibers are well-known and commercially available. Suitable polybenzazole fibers are also known and are described in numerous references such as in Wolfe et al., U.S. Patent 4,533,693 (August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 1 1 , 1989); and Ledbetter et al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Engineering of Rigid-Rod Polymers at 253-64 (Materials Res. Soc. 1989).

The polymer in a polybenzazole fiber may contain AB-mer units, as represented in Formula 1 (a), and/or AA/BB-mer units, as represented in Formula 1 (b)

wherein:

Each Ar represents an aromatic group selected such that the polymer is a lyotropic liquid crystalline polymer. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. Size is not critical, but the aromatic group preferably contains no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Ar¹ in AA/BB-mer units is preferably a 1,2,4,5-phenylene moiety or an analog thereof. Ar in AB-mer units is preferably a 1 ,3,4-phenylene moiety or an analog thereof.

Each Z is independently an oxygen atom or a sulfur atom. Each Z is preferably an oxygen atom.

Each DM is independently a bond or a divalent organic moiety selected so that the polymer is lyotropic liquid crystalline and DM does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety is preferably an aromatic group (Ar) as previously described. It is most preferably a 1 ,4-phenyiene moiety or an analog thereof.

The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed.

The azole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 1 1 Ency. Poly. Sci. & Eng., supra, at 602.

The PBZ polymer in a PBZ fiber selected should be graphitisable, meaning that it can be converted to a carbon or "graphite" that is suitable for use in carbon-carbon composites. The PBZ polymer is preferably selected so that it yields at least about 45 percent char, more preferably at least about 50 percent, more highly preferably at least about 55 percent and most preferably at least about 60 percent, based on the original weight of polymer that is carbonized. The char preferably contains at least about 90 weight percent carbon, more preferably at least about 95 weight percent, and most preferably at least about 99 weight percent. The remainder of the char usually consists of varying amounts of nitrogen, oxygen and sulfur, depending upon the polymer and the conditions under which it is carbonized. The yield of char is limited by the quantity of carbon in the polymer, and cannot ordinarily exceed about 72 percent by weight for typical cis-PBO as shown in Formula 2(a). The actual yield is typically somewhat lower than the theoretical maximum.

The polymer in the PBZ fiber preferably comprises, and more preferably consists essentially of, mer units selected from those illustrated in 2(a)-(h). It most preferably consists essentially of units illustrated in 2(a), (b) or (d). ,

,

,

,

,

.

, and

.

Each polymer in the PBZ fiber preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units. The intrinsic viscosity of rigid AA/BB-PBZ polymers in methanesulfonic acid at 25°C is preferably at least about 10 dL/g, more preferably at least about 15 dL/g and most preferably at least about 20 dL/g. For some purposes, an intrinsic viscosity of at least about 25 dL/g or 30 dL/g may be best. Intrinsic viscosity of 60 dL/g or higher is possible, but the intrinsic viscosity is preferably no more than about 40 dL/g. The intrinsic viscosity of semi-rigid AA-PBZ polymers is preferably at least about 5 dL/g, more preferably at least about 10 dL/g and most preferably at least about 15 dL/g.

The fiber may be laid out in any form suitable for acting as a reinforcement in a composite, such as in the form of aligned fibers, fabrics, braids or random mats of short fiber or fiber pulps. Examples of some suitable fiber layouts are described in 14 Encyclopedia Poly. Sci. & Eng., Reinforcement, at 404 (J. Wiley & Sons 1989).

The fibers are held in a matrix that predominantly contains (and preferably consists essentially of) a graphitisable lyotropic liquid crystalline polybenzazole polymer. The polymer in the matrix has the same general limitations and preferred embodiments that are previously set out for polymer used in fibers.

Suitable matrix composites and processes to make them are described in detail in Pierini et al., PCT Publication W092/10364 (June 25, 1992). The fibers are prepregged using a dope that contains the matrix polymer dissolved in a solvent acid. Then, the prepregged composite is contacted with a coagulant to coagulate the matrix polymer. The solvent is preferably an acid capable of dissolving the polymer. The acid is preferably non-oxidizing. Examples of suitable acids include polyphosphoric acid, methanesulfonic acid and mixtures of those acids. The acid is more preferably polyphosphoric acid.

The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a solid article. The concentration of polymer in the dope is preferably high enough to provide a liquid crystalline dope. The concentration of the polymer is preferably at least about 7 weight percent, more preferably at least about 10 weight percent and most preferably at least about 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and dope viscosity. The concentration of polymer is seldom more than 30 weight percent, and usually no more than about 20 weight percent.

The concentration of polybenzazole polymer in the dope is preferably as high as possible, in order to maximize the carbon deposited during the carbonization step. In order to get the concentration of polymer beyond ordinary solubility limits, it may be desirable to add polybenzazole powder to the dope.

The optimum procedure for prepregging the fiber in the dope will vary depending upon the fiber, the dope and the desired composite. Less viscous dopes may be applied to the fiber before it is laid up by dipping, spraying and other ordinary prepregging methods. More viscous dopes may be applied to the fiber before it is laid up by known means for putting viscous coatings on fibers or wires, such as by extruding the dope on the fiber using a cross-head die.

Alternatively, the fibers may be prepregged with a viscous dope after the fibers are laid up in the desired pattern, by (1) making the dope into one or more dope films, and (2) either pressing the fibers into a single film of dope or pressing the fibers between at least two films of dope. Several alternating layers of fiber and dope film may be pressed together to form a composite having several layers of fiber. The dope film may be thicker to form a "resin-rich" composite or thinner to form a "resin-starved" composite. The dope film is preferably on average at least about 25 μm thick. The temperature should be high enough for the fibers to embed in the dope and for the dope sheets to consolidate. The temperature is usually 25°C to 150°C.

Suitable dope films for this type of prepregging can be made by known processes, such as those described in Pierini et al, et al., PCT Publication W092/10527 (June 25, 1992) ; Chenevey, U.S. Patent 4,487,735 (December 1 1 , 1984); Lusignea et al., U.S. Patent 4,871 ,595 (October 3, 1989); Chenevey, U.S. Patent 4,898,924 (February 6, 1990); Harvey et al., U.S. Patent 4,939,235 (July 3, 1990); Harvey et al., U.S. Patent 4,963,428 (October 16, 1990); and Lusignea et al., U.S. Patent 4,966,806 (October 30, 1990). For instance, the dope may be extruded from a slit die, after which it is preferably mechanically stretched before coagulation to impart biaxial orientation. Alternatively, the dope may be extruded in a tubular film that is preferably stretched biaxially by a bubble process to impart biaxial orientation.

After prepregging is accomplished and the prepregs are laid up in the desired shape and configuration, the composite is hardened by contacting the dope with a fluid

(usually a liquid) that causes the polymer to coagulate. Ordinarily, the fluid is a nonsolvent for the polymer that dilutes the solvent. Many nonsolvent liquids have been studied and their effects on polybenzazole coagulation reported. The nonsolvent liquid is preferably volatile. The nonsolvent liquid may be an organic compound, such as an alcohol or a ketone containing no more than about 4 carbon atoms. The nonsolvent liquid is preferably aqueous, and more preferably consists essentially of water, at least at the commencement of the coagulation. When the solvent is volatile or contains a volatile component, such as methanesulfonic acid, then the volatile component can be at least partially removed by evaporation to concentrate the polymer before coagulation.

The coagulated polymer is preferably washed for a period of time sufficient to remove substantially all of the remaining solvent. Washing may be accomplished with a liquid (such as water) or a vapor (such as steam). The washed matrix resin preferably contains no more than about 3000 ppm residual solvent, more preferably no more than about 2000 ppm, more highly preferably no more than about 1000 ppm, and most preferably no more than about 500 ppm. Usually, it is feasible to reach residual levels below about 50 ppm. The composite may be dried. It is preferably restrained from shrinking as it is dried. After drying, the composite may be heat treated. Heat treatment is preferably carried out under pressure. The finished composite may be machined into a desired final shape.

The resulting composite has fibers of carbon or graphitisable PBZ polymer embedded in a matrix resin containing graphitisable PBZ polymer. The composite preferably contains at least about 20 volume percent fiber, more preferably at least about 40 volume percent fiber and most preferably at least about 50 volume percent fiber. It preferably contains at least about 20 volume percent matrix and more preferably at least about 35 volume percent matrix.

The entire composite is converted to carbon-carbon composite at temperatures suitable to convert both the matrix and any PBZ fiber to carbon. Carbonization of PBZ polymers is described in references, such as Stuetz, U.S. Patent 4,460,708 (July 17, 1984);

Murakami, U.S. Patent 4,876,077 (October 24, 1989); Murakami, U.S. Patent 4,915,984 (April 10, 1990); and Sandor, "Polybenzimidazole (PBI) as a Matrix Resin Precursor for Carbon/Carbon Composites," 22nd Int'l SAMPE Tech. Conf. 647-657 (1990).

The temperature of carbonization may be held steady or started at a lower temperature and increased during the process. The temperature preferably reaches at least about 900°C, more preferably at least about 1200°C, and most preferably at least about 1600°C. The maximum temperature is governed primarily by practical considerations, such as the stability of the composite. It is preferably no more than about 8000°C, and more preferably no more than about 2500°C.

The composite is preferably carbonized under restraint in order to prevent deformation. Restraint may be applied by numerous methods, such as by attaching two or more edges of the composite to a frame which maintains tension on the composite during the carbonization process. The atmosphere during carbonization should not interfere with the process. It is preferably vacuum except for volatile components formed by the carbonization process itself.

The optimum time for carbonization will vary depending upon the polymer, the article being carbonized and the conditions (such as temperature) under which carbonization occurs. The composite should be heated until polybenzazole polymer in the composite is converted to at least about 90 percent carbon. The polybenzazole polymer is preferably converted to at least about 95 percent pure carbon and more preferably at least about 99 percent pure carbon. The time is usually 1 to 100 hours. The carbonization process preferably requires no more than about 10 hours of heating, and more preferably no more than about 5 hours.

The resulting carbon-carbon composite may be too porous for best use after only one cycle. In that case, additional carbon may be added by running cycles of ordinary liquid impregnation or chemical vapor deposition. Because the polybenzazole polymer has a very high yield of carbon, fewer cycles are required. If the dope used to make prepregs contains very high levels of polybenzazole polymer, a single cycle may be sufficient to make an acceptable polymer. Because of the viscosity of most polybenzazole dopes, subsequent cycles preferably use chemical vapor deposition or liquid impregnation with a lower-viscosity liquid, such as pitch.

Carbon-carbon composites made by the present invention can be used for all ordinary uses for such composites, such as structural materials, brake linings and high-temperature applications. Some suitable uses are described in Fitzer, "The Future of Carbon-Carbon Composites," 25 Carbon 163-190 (1987).

The process is better illustrated by the following Example:

The following examples are given to illustrate the invention and should not be interpreted as limiting the Specification or the Claims. Unless stated otherwise, all parts and percentages are given by weight.

Example 1 - Carbonization of a Composite that Contains Polybenzazole Fiber and

Polybenzoxazole Matrix.

A dope containing 14 weight percent cis-polybenzoxazole (consisting essentially of mer units illustrated in Formula 2(a) - intrinsic viscosity of about 25 dL/g to 45 dL/g in methanesulfonic acid at about 25°C) in polyphosphoric acid is extruded from a slit die as a 10 mil thick sheet between two sheets of 2 mil thick Teflon™ fluoropolymer. Two 4 inch by 4 inch squares of the dope film are cut, and the Teflon™ sheet is stripped off of one side of each sheet.

A 5 mil thick piece of cloth woven from PBO fibers containing a similar PBO polymer is cut to 4 inches long and 0.75 inches wide. The cut cloth is placed between the two dope film samples, with the dope sides against the fiber cloth. The article is pressed at 160°C under 2 to 4 tons of pressure for twenty minutes to form a prepreg. The prepreg is cooled to room temperature, and the Teflon™ sheet is stripped off of each side of the prepreg. The prepreg is clamped between two heavy stainless steel screens to prevent shrinkage. The framed prepreg is placed in four liters of water, left in the water for two days, removed from the frame and dried in air at ambient temperature. The resulting composite contains about 59 percent PBO fiber reinforcement and about 41 percent PBO matrix.

A sample of composite 3.88 inches long and 0.49 inches wide is cut from the composite. The sample is 0.0053 inches thick and weighs 0.1264 g. The sample hung in a vertical tube furnace with a 120 g weight attached to the bottom. The sample is heated in a nitrogen atmosphere from 20ºCto 600°C at a rate of 20°C per minute, and then heated at 600°C for five minutes. The temperature is raised from 600°C to 725°C at 2°C, and maintained at 725°C for 5 minutes. The temperature is raised from 725°C to 1200°C at 10°C per minute and maintained at 1200ºC for 15 minutes. Thereafter, the sample is cooled to room temperature. A carbon-carbon composite results that retains 63 percent of its original mass. The composite has a density of about 1 .80 g/cm³ as measured by a helium pycnometer.

Example 2 - Carbonization of Composite that Contains Carbon Fiber.

A 3 inch by 3 inch composite is made as described in Example 1 , except that: (1) The fibers are Type PWB-6™ carbon fiber obtained from Stackpole Fibers Co.

(2) The dope films are 15 mil thick; and

(3) The composite is pressed together at 10,000 pounds pressure and 150°C for one minute.

A rectangular strip that is 10.6 cm long, 1.2 cm wide and 0.065 cm thick is hung in a vertical tube furnace with a 120 g weight attached to the bottom. The strip is heated in a nitrogen atmosphere according to the following temperature profile:

a) Temperature increased from 20°C to 600°C at 20°Ominute; b) Temperature held at 600°C for 78 minutes;

c) Temperature increased from 600°C to 626°C at 10°C/min,

d) Temperature held at 626°C for 50 minutes;

e) Temperature increased from 626°C to 642°C at 10°C/minute; f) Temperature held at 642°C for 30 minutes;

g) Temperature increased from 642°C to 664°C at 10°Ominute; h) Temperature held at 664°C for 78 minutes;

i) Temperature increased from 664°C to 1000°C at 20°C/minute; and j) Temperature held at 1000°C for 30 minutes.

The resulting carbon-carbon composite contains 97 percent carbon, 2.6 percent nitrogen and the remainder mostly hydrogen. Example 3 - Carbonization of Composite that Contains Carbon Fiber.

The process of Example 2 is repeated, except that the heating profile is as follows: a) Temperature raised from 20°C to 600°C at 20°C/minute;

b) Temperature raised from 600°C to 725°C at re/minute;

c) Temperature held at 725ºC for 10 minutes;

d) Temperature raised from 725°C to 1200ºC at 2.5°C/minute; and e) Temperature held at 1200°C for 80 minutes.

Four elemental analyses show that the composite contains between 97.5 and 99.1 weight percent carbon.

Claims

1. A process to make a carbon-carbon composite in which a matrix composite, which contains fibers embedded in an organic matrix material, is heated to a temperature and for a time sufficient to convert the organic matrix material to carbon, characterized in that the organic matrix material is a lyotropic liquid-crystalline polybenzoxazole or polybenzothiazole polymer or copolymer.

2. A process as described in Claim 1 wherein the matrix composite is made by the steps of:

(a) impregnating a substrate that contains carbon or polybenzazole fibers with a dope that contains polybenzoxazole or polybenzothiazole polymer or copolymer dissolved in a solvent to make a prepreg; and

(b) washing the prepreg with one or more fluids to remove substantially all of the residual solvent from the composite.

3. A process as described in any of the preceding Claims wherein the polybenzoxazole or polybenzothiazole polymer or copolymer yields at least about 50 percent char by weight.

4. A process as described in any of the preceding Claims wherein the polybenzoxazole or polybenzothiazole polymer or copolymer yields at least about 60 percent char by weight.

5. A process as described in any of the preceding Claims wherein further carbon is added by subsequent cycles of chemical vapor deposition or liquid impregnation and carbonization.

6. A process as described in any of the preceding Claims wherein the polybenzoxazole or polybenzothiazole polymer or copolymer contains any of the following repeating units:

,

,

,

,

,

.

, a nd

.

7. A process as described in any of the preceding Claims wherein the temperature of the carbonization step reaches 900°C to 3000°C.

8. A process as described in any of the preceding Claims wherein the carbonization process takes no more than about 10 hours.

9. A process as described in any of the preceding Claims wherein the fibers in the matrix composite are carbon fibers.

10. A process as described in any of the preceding Claims wherein the fibers in the matrix composite are polybenzazole fibers.