This invention relates to matrix composites.

A fiber-reinforced composite, or matrix composite, is an article comprising a plurality of fibers (the reinforcement) embedded in a plastic (the matrix). Typically, the fibers give strength and/or stiffness to the composite, and the matrix maintains fiber alignment and transfers load around broken fibers Matrix composites are described in detail in numerous references, such as Kirk-Othmer Ency. Chem. , Tech. - Supp., Composites, High Performance, at 260-281 (J. Wiley & Sons 1984).

A number of fibers are available for use in matrix composites, each having different combinations of tensile and compressive strength and modulus, temperature stability, creep, cost, and other properties. Suitable fibers may contain, for example, aramid (such as Kevlar™ fibers), boron, glass, carbon, gel-spun polyethylenes (such as Spectra™ fiber), polybenzoxazole, polybenzothiazole, or polybenz- imidazole. Suitable fibers and processes for their

fabrication are described in numerous references, such as U.S. Patent 4,533.693; 3 Kirk-Othmer Ency. Chem. Tech., Aramid Fibers, 213 (J. Wiley & Sons 1978); Kirk- Othmer Ency. Chem., Tech. - Supp., Composites, High Performance, at 261-263; 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988) and W. W. Adams et a!., The Materials Science and Engineering of Rigid-Rod Polymers, at 245- 312 (Materials Research Society 1989).

A number of matrix materials are also available for use in matrix composites. Examples of polymer matrix materials include polyesters, epoxy resins, polycyanates, polybutadienes, vinyl ester resins and polyimides _* Some carbon matrix composites have been made. Metal and ceramic matrix composites are also know .

There is a need for improved matrix materials in advanced matrix composites. For example, the nonflammability, chemical resistance, solvent resistance and thermal stability of many polymer matrix materials is much poorer than the same properties of the fiber reinforcement. Stronger polymer matrices could yield stronger composites using the same amount of fiber. Metal matrix materials are heavier than polymers. Ceramic and carbon matrix materials are expensive and brittle. An objective of the present invention is to provide a iber-reinforced composite having new polymer matrix materials that can improve one or more of the qualities of flame resistance, chemical resistance, solvent resistance, thermal stability, tensile strength and tensile modulus.

The present invention is a fiber-reinforced composite that contains:

(1) a plurality of reinforcing fibers in a number sufficient to reinforce the composite; and j- (2) a matrix resin in a quantity sufficient to bind the reinforcing fibers together and maintain the alignment of the fibers,

characterized in that the matrix resin is a 10 polybenzoxazole or polybenzothiazole polymer containing repeating mer units that are represented by any one of the Formulae:

25 wherein each Z is independently an oxygen or a sulfur atom; each Ar contains an aromatic group selected so that the matrix polymer is not thermoplastic; and DM is oø * a bond or an aromatic group selected so that the matrix polymer is not thermoplastic.

The polybenzoxazole or polybenzothiazole matrix in the composite can be selected to provide any one of low flammability, low smoke generation, high temperature

-it-

stability, high chemical resistance, high solvent resistance, high strength and/or modulus or a combination of those properties. Composites of the present invention are useful for structural materials and parts.

The present invention uses fibers, such as those previously described. The fibers should be a type whose properties are not substantially degraded by contact with the solution of polymer or copolymer and its solvent. The fiber is preferably aramid, carbon, polybenzoxazole or polybenzothiazole. It is most preferably carbon or polybenzoxazole. Polybenzoxazole and polybenzothiazole fibers are preferably heat treated. The tensile strength of the fiber is preferably at least 2.5 GPa, more preferably at least

3.0 GPa and most preferably at least 3-5 GPa. The tensile modulus of the fiber is preferably at least 135

GPa, more preferably at least 200 GPa and most preferably at least 270 GPa.

The fibers may have dimensions that are usual for reinforcing materials in matrix composites. Their average diameter is preferably no more than about 40 μ and more preferably no more than about 20 μ. The fiber may be, for instance, in the form of a cloth or in the form of long strands or in the form of a short fiber or fiber pulp suitable for making random fiber composites. A mixture of fibers may be used. For instance, the fibers may contain a mixture of at least one fiber having high tensile properties, such as aramid or polybenzazole, and another fiber having high compressive properties, such as quartz.

The present invention also uses matrix materials containing a non-thermoplastic polybenzoxazole (PBO) or polybenzothiazole (PBT) or copolymers thereof. PBO, PBT and random, sequential and block copolymers of PBO and PBT are described in references such as Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,703,103 (October 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzo- thiazole) Compositions, Process and Products, U.S. Pat¬ ent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystal¬ line Polymer Compositions, Process and Products, U.S. Patent 4,533,693 (August 6, 1985); Evers, Thermoxada- tively Stable Articulated p-Benzobisoxazole and p-Benzo- bisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Patent 4,578,432 (March 25, 1986); 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988) and W. W. Adams et al., The Materials Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989).

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

1(a) AB

Kb) AA/BB

wherein:

Each Ar represents an aromatic group. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system, but is preferably a single six-membered ring. Size is not critical, but the aromatic group preferably contains no more than about 18 carbon atoms, more preferably no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties and biphenylene moieties. Ar in AA/BB-PBZ mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof. Ar in AB-PBZ 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 DM is independently a bond or an aromatic group. DM is preferably an aromatic group. It is most preferably a 1,4-phenylene 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 11 Ency. Poly. Sci. & Eng., supra, at 602.

Each Ar and DM is selected so that the polymer is not thermoplastic.

The polymer preferably consists essentially of either AB-PBZ mer units or AA/BB-PBZ mer units, and more preferably consists essentially of AA/BB-PBZ mer units. It is most preferably a rigid rod AA/BB-PBZ polymer. Azole rings within the polymer are preferably oxazole rings (Z = 0) so that the polymer is a polybenzoxazole. Preferred mer units are illustrated in Formulae 2 (a)- -(h). The polymer more preferably consists of mer units selected from those illustrated in 2(a)-(h), and most preferably consists of a plurality of identical units selected from those illustrated in 2(a)-(c).

*___

(b)

Each polymer preferably contains on average at least 25 mer units, more preferably at least 50 mer units and most preferably at least 100 mer units. The intrinsic viscosity of rigid AA/BB-PBZ polymers in methanesulfonic acid at 25°C is preferably at least 10 dL/g, more preferably at least 15 dL/g and most preferably at least 20 dL/g. For some purposes, an intrinsic viscosity of at least 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 40 dL/g. The intrinsic viscosity of semi- -rigid AB-PBZ polymers is preferably at least 5 dL/g, more preferably at least 10 dL/g and most preferably at least 15 dL/g.

The polymer or copolymer is not thermoplastic - i.e., it does not become flowable or moldable at any temperature below its decomposition temperature. The polymer or copolymer is preferably essentially insoluble in common organic solvents such as halogenated hydrocarbons, alkanes, benzene or toluene. The polymer or copolymer is preferably insoluble in non-acidic aqueous solvents.

The polymer or copolymer is dissolved in a solvent to form a solution or dope. Some polybenz¬ oxazole and polybenzothiazole polymers are soluble in cresol, but the solvent is preferably an acid capable of

dissolving the polymer. The acid is preferably non- -oxidizing. Examples of suitable acids include pol phosphoric acid, methanesulfonic acid and sul uric acid and mixtures of those acids. The acid is preferably polyphosphoric acid and/or methanesulfonic acid, and is more preferably polyphosphoric acid. The fiber should be chosen so that its properties do not degrade upon contact with the acid.

The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a solid article. When the polymer is lyotropic liquid crystalline, then 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 7 weight percent, more preferably at least 10 weight percent and most preferably at least 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 20 weight percent.

Suitable polymers or copolymers and dopes can be synthesized by known procedures, such as those described- 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 11, 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). In summary, suitable monomers (AA-monomers and BB- -monomers or AB-monomers) are reacted in a solution of

nonoxidiziπg and dehydrating acid under nonoxidizing atmosphere with vigorous mixing and high shear at a temperature that is increased in step-wise or ramped fashion from no more than 120°C to at least 190°C. Examples of suitable AA-monomers include terephthalic acid and analogs thereof. Examples of suitable BB-monomers include 4,6-diaminoresorcinol, 2,5- -diaminohydroquinone, 2,5-diamino-1 ,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples of suitable AB-monomers include 3-amino-4- -hydroxybenzoic acid, 3-hydroxy-4-aminobenzoic acid, 3-amino-4-thiobenzoic acid, 3-thio-4-aminobenzoic acid and analogs thereof, typically stored as acid salts.

The fiber is prepregged with the dope. The optimum procedure for prepregging the fiber in the dope will vary depending upon the fiber, the dope and the desired composite. A less viscous dope, whose viscosity is similar to that of other uncured matrix resins, may be prepregged according to processes used for known matrix resins. Likewise, a fiber or fiber tow or a group of tows may be prepregged with a viscous dope 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.

Such processes ordinarily form a prepregged tape that can be laid up in a desired orientation and shape. Many different fiber configurations are known and may be used. The fibers may run in a single direction to form a unidirectional composite, having great strength in one direction but poorer properties in other directions. The fibers may be laid out in layers directed at different angles with respect to each other

to form a multidirectional composite. The prepreg may be laid out flat or filament wound to form a shaped article.

A group of fibers or tows may be prepregged with a dope that is viscous enough to form a film, by forming one or more dope films and either pressing the fibers into a single film of dope or pressing the fibers between 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 fibers pressed into the dope may have unidirectional or multidirectional orientation as previously described. They may be part of a cloth or a non-woven mat. 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 film may be uniaxially stretched to provide best properties in a single direction, but it is preferably biaxially stretched to provide good properties in at least two directions. The extrusion of dopes to form films is described in numerous references, such as in Chenevey, U.S. Patent 4,487,735 (December 11, 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.

The fibers may be short fibers or fiber pulps that are immersed in the dope to form a random fiber composite, similar to those described in U.S. Patents

4,426,470 and 4,550,131.

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 liquid that causes the polymer or copolymer to coagulate. Ordinarily, the liquid is a nonsolvent for the polymer or copolymer 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. 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 as previously described embedded in a matrix resin containing a polybenzoxazole or polybenzothiazole polymer or copolymer as previously described. The composite should contain a sufficient number of fibers to provide reinforcement for the composite. It should contain a sufficient quantity of matrix material to hold the fibers together and maintain fiber alignment, and preferably to transfer loads around broken fibers.

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.

Several variations on the basic composite are possible. For instance, the fiber may receive surface treatment or be coated with an adhesive to improve the adhesion of the fiber to the matrix. The matrix may contain a mixture of more than one polymer, such as several polybenzazole polymers or a mixture of the polybenzazole fiber and a non-polybenzazole polymer, as described in Uy, U.S. Patent 4,810,735 (March 7, 1989). The matrix preferably contains only a single polymer or copolymer. The fiber may be wrapped with another fiber to improve compressive strength, as described in U.S.

Patent 4,499,716 and in Ledbetter, Ser. No. 564,480 (filed August 8, 1990).

The preferred polybenzoxazole and polybenzo¬ thiazole matrix resins are essentially nonflammable and do not release smoke. They have good solvent resistance, good chemical resistance and high continuous use temperatures. They have a high strength per unit weight or volume. The composite may be fabricated into structural parts for many known uses.

Illustrative Examples

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 - Composite Containing Carbon Fiber and Polybenzoxazole Matrix.

A dope containing 14 weight percent cis-poly- benzoxazole (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 15 mil thick sheet between two sheets of 2 mil thick Teflon™ fluoropolymer. Two 3 inch by 3 inch squares of the dope film are cut, and the Teflon™ sheet is stripped off of one side of each sheet.

A 3 inch by 3 inch piece of Panex™ PWB-6 carbon fiber cloth, available from Stackpole Fibers Inc., is placed between the two dope film samples, with the dope sides against the carbon fiber cloth. The article is pressed at 150°C under 5 tons of pressure for one minute 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 placed in a "picture frame" holder to prevent shrinkage along the length and width of the sample but allow shrinkage in the thickness of the sample. The framed prepreg is placed in two liters of water, left in the water for two days, removed from the frame and dried in air at ambient temperature. A composite having carbon fiber reinforcement and a cis- -polybenzoxazole matrix results.

The composite is cut in half. One half is placed in a heated press at 150°C and 5000 lbs. pressure for one minute. It is golden yellow in color. One half is placed in a heated press at 300°C and 5000 lbs. pressure for one hour. It is darker yellow with a purple tinge. Both have smooth surfaces with no visible holes. The polybenzoxazole in both adheres firmly to the cloth.

Example 2 - Composite Containing Polybenzoxazole Fiber and Polybenzoxazole Matrix.

The procedure of Example 1 is followed, except that:

The cloth consists essentially of polybenz¬ oxazole fiber and has dimensions of about 5 inches by 5 inches;

The dope films have dimensions of about 5 inches by 5 inches;

The sample is coagulated in about 2 gallons of water.

The resulting composite appears similar to the composite of Example 1.

Example 3 - Composite Containing Two Layers of Carbon Fiber and Polybenzoxazole Matrix.

The procedure of Example 1 is followed, except that:

The cloth and dope film are laid up so that there is from top to bottom: a layer of Teflon ^™ film, a layer of polybenzoxazole dope, a layer of carbon fabric, a layer of polybenzoxazole dope, a layer of carbon fabric, a layer of polybenzoxazole dope, and a layer of Teflon™ film;

The prepregging is carried out at 150°C and 5000 lbs. pressure for 3 minutes;

The finished composite is pressed under 5000 lbs. pressure at a temperature ramped from room

te perature to 300°C over 90 minutes and held at 300°C for 30 minutes.

The resulting structures are much stiffer than the structures prepared in Example 1.