WO1997035715A1

WO1997035715A1 - Composites of thermosetting resins and carbon fibers having aliphatic polyamide sizings

Info

Publication number: WO1997035715A1
Application number: PCT/US1997/003850
Authority: WO
Inventors: John L. Lesko; Judy S. Riffle; Anand Rau
Original assignee: Virginia Tech Intellectual Properties, Inc.
Priority date: 1996-03-27
Filing date: 1997-03-12
Publication date: 1997-10-02
Also published as: AU2206297A

Abstract

Composite material having carbon fibers embedded in a polymeric matrix comprising a thermoset resin, where the carbon fibers are precoated with a sizing agent comprising an aliphatic polyamide before being embedded in the resin. The invention also contemplates processes for making this composite material and intermediate products thereof.

Description

COMPOSITES OF THERMOSETTING RESINS AND CARBON FIBERS HAVING ALIPHATIC POLYAMIDE

SIZINGS

DESCRIPTION

BACKGROUND OF THE INVENTION

Field of the Inven tion

This invention generally relates to composites of thermosetting resins and carbon fibers having aliphatic polyamide sizings located at the interface (interphase region) of the fiber surfaces and the resin.

Description of the Prior Art Polymeric matrix composites, also known as "fiber-reinforced plastics", involve products in which a polymeric matrix is combined with reinforcing fibers. Composites advantageously combine strength, light weight, versatility in shape selection, and corrosion resistance, among other things. Composites formed of glass fiber-reinforced thermosetting resins have been widely used for a number of years. However, in the case of such prior glass fiber-reinforced composites, a problem stems from the poor moisture stability of the glass fibers. That is to say, the glass fibers sustain structural damage when moisture (liquid or vapor infiltrating the resin matrix) contacts the fiber surfaces and the moisture is alternately frozen and thawed. This scenario arises when the glass fiber-reinforced composite part is subjected to regular outdoor exposures in a moist climate including temperature swings or temperature cycling over the freezing temperature of water. As a consequence, the glass fibers become fatigued, embrittled and ultimately fail. Therefore, despite the relative low cost of glass fiber-reinforced composites, they remain ill-suited for many outdoor applications available for composites.

Carbon fibers have been explored as an alternative type of high strength, fibrous reinforcement for resin matrices of composites. Carbon fibers have better environmental resistance against moisture, lighter weight (lower density) , and higher stiffness as compared to glass fibers. However, the problem encountered with usage of carbon fibers in prior composites has been poor product durability. This poor durability of the composite has been attributed to bonding failure occurring at the interphase region of the resin matrix and the carbon fiber surfaces. As a consequence, the resin matrix eventually jiggles loose from the carbon fibers, whether in discrete fiber form or in fabric form, to cause failure of the composite. Carbon fibers and vinyl ester resins, in particular, have a compatibility problem which has proved a considerable prior obstacle to the use of these systems. This incompatibility has frustrated prior efforts to achieve an appreciable bond between this matrix system and carbon fiber surfaces per se. This has been disappointing because considerable cost advantages can be garnered from the carbon fiber-reinforced vinyl ester system due to the initial low material cost of the vinyl ester polymer, and the short time and low temperature required to fully cure these systems. The short hardening times and lower cure temperatures associated with this resin system translate into reduced production cycles which further reduces production costs.

In light of such fiber and resin compatibility problems, the character of the bond between fiber and matrix in composite materials has attracted considerable prior interest and scrutiny in the field, as indicated by the following patents.

U.S. Pat. 4,781,947 to Saito et al . teaches a sizing agent applied as a precoating to a carbon fiber, where the sizing agent is an unsaturated urethane compound produced by a reaction of an unsaturated alcohol with an isocyanate which is able to couple the carbon fiber with an unsaturated polyester resin or vinylester resin. U.S. Pat. 4,904,818 to Minami et al . describes various bisphenol-polyalkylene glycol etherester copolymers used as sizing agents for carbon fibers in a thermosetting matrix that can include vinyl ester resins.

U.S. Pat. 4,269,876 to Lind et al . describe a composite material in which the sizing agent is a polymer coating, viz., polyethylene, selected to behave as a physical barrier to chemical bonding between functional groups on the surfaces of carbon fibers exposed by a prior surface removal step and the resin matrix material.

U.S. Pat. 4,764,427 to Hara et al . teach a fiber or fiber bundle having a porous coating of a thermoplastic resin. The fiber can be carbon fiber and the thermoplastic resin used to form the porous coating on the fibers can be polybutylene and polyethylene terephthalate, polycarbonate, various nylons such as nylon 6, 6.6, 6.10, 11 and 12 nylons, polyoxymethylene, polyarylates, polyarylene ether sulfones, polyamideimides, polyether imides, thermoplastic polyimides, polyether ketone, poly(phenyl sulfide) , polyvinyl chloride, poly(meth) acrylates, polystyrenes, modified polyethylene resins such as ethylene/vinyl acetate copolymer, and polyester elastomers.

U.S. Pat. 5,455,107 to Homma et al . describe carbon fiber woven fabrics impregnated with thermosetting resins, such as epoxy resin and vinyl ester resin, or thermoplastic resins. Homma et al . teach sized carbon fibers without identifying or otherwise elaborating on the type and function(s) of sizing agent applied to the carbon fibers.

U.S. Pat. 4,328,151 to Robinson teaches carbon fiber reinforced poly(vinylidene fluoride) resin compositions prepared by extrusion blending carbon fibers, which have been precoated with poly (vinylidene fluoride) resin, with a poly (vinylidene fluoride) resin matrix (PVDF) . The poly(vinylidene fluoride) resin matrix is a crystalline, non-polar thermoplastic material. Robinson describes commercially available carbon fibers coated with polyvinylpyrrolidone or an epoxy resin in order to improve their bulk density and handling characteristics. Robinson comparatively teaches composites of thermoplastic poly(vinylidene fluoride) matrix resin and carbon fibers precoated with polyvinyl pyrrolidone or an epoxy resin.

U.S. Pat. 5,080,968 to Riew et al . teaches fiber reinforced composites of thermoplastic vinyl resin compositions, preferably thermoplastic vinyl halide resin compositions, and fibrous reinforcement material which is coated with a uniformly thin and continuous cured elastomeric coating. The fiber coating is prepared from an aqueous dispersion containing a reactive liquid polymer composition, an epoxy resin and a curing agent. The fibers can be graphite fibers, among other types of fibers described by Riew et al.

U.S. Pat. 4,943,472 to Dyksterhouse et al . teaches a composite article having a plurality of adjoining substantially parallel reinforcing filaments impregnated and bonded in place by a matrix-forming thermosetting resin. The thermosetting resin is derived from an impregnation bath in which solid particles of matrix-forming thermosetting resin are initially dispersed in an aqueous medium containing an effective amount of a dissolved polymeric binding agent, and the viscosity is increased adequate to provide a substantially uniform suspension of the thermosetting resin particles. Consequently, the binding agent will be dispersed throughout the matrix. The polymeric binding agent is described as functioning to increase the viscosity of the dispersion of thermosetting particles in the aqueous medium, and also as causing the adjoining filaments to adhere well together and to exhibit tacky properties in the presence of the aqueous medium. Representative water-soluble polymeric binding agents having the requisite properties are described as polyacrylic acid binding agents possessing a cross-linked molecular structure. Polyvinyl alcohol and polyvinyl pyrrolidone are indicated as being other representative types of the water-soluble polymeric binding agent. Dyksterhouse et al . explicitly describe usage of carbon filaments in unsized form, and provision of heating and/or solvent means to remove any sizing agent that may be present on the carbon filaments prior to the impregnation treatment. The approach to making composites described by Dyksterhouse et al . is not concerned with nor adaptable to significant closed-mold processing techniques such as resin transfer molding (RTM) . Types of thermosetting resins taught by Dyksterhouse et al . include solid particulates of phenolic resins, polyester resins, polyimide resins, polyurethane resins, epoxy resins, vinyl ester resins, and cyanate ester resins, among others. The fibrous material is described as carbon, glass, silicon carbide, silicon nitride, boron nitride, and synthetic polymers capable of use at high temperatures.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a composite of thermosetting resin and carbon fibers having improved durability and moisture stability.

According to the invention, there is a composite material having carbon fibers embedded in a polymeric matrix comprising a thermoset resin, where the carbon fibers are precoated with a sizing solution containing an aliphatic polyamide sizing agent before being embedded in the resin.

The aliphatic polyamide sizing agents usable in the practice of this invention are thermoplastic materials that are compatible with the carbon fiber surfaces and miscible with the resinous matrix material. The aliphatic polyamide sizing agent forms a thin, ductile, thermoplastic coating film on the exposed surfaces of carbon fibers which promotes adhesion between the polymeric matrix and carbon fibers, thereby endowing the composite product with enhanced durability.

A sizing solution containing the sizing agent is applied to the exterior surfaces of carbon fibers in a convenient manner, then dried to fuse the sizing agent to the fiber surfaces; thereby providing a sized carbon fiber having enhanced capability to couple to thermosetting resins. In this way, the sizing agent is effectively confined to the interfacial (interphase) region between the surfaces of the carbon fibers and the resin matrix at the time of curing the composite. An improvement in composite performance results from the enhanced bond achieved between the carbon fibers and resinous matrix. This enhanced fiber-to-resin matrix bond serves to reduce the initiation and propagation of damage to the fibers, resin matrix and/or fiber-resin interphase region, which retards the overall degradation of the composite during the course of fatigue cycling. Consequently, the durability of the inventive composite product under cyclically applied stresses is significantly improved as compared to composites reinforced with carbon fibers lacking the sizing treatment of this invention. In one preferred embodiment, the sizing agent is poly(vinyl pyrrolidone) . Also, the types of thermosetting resins which can be afforded improved bonding efficacy to carbon fibers is not particularly limited in this invention. For instance, suitable thermosetting resins include unsaturated polyesters. In one advantageous embodiment of the invention, the thermosetting resin matrix of the composite is based upon a vinyl ester resin and the sizing agent is poly(vinyl pyrrolidone) . The composites of this invention are well- suited for many applications inclusive of aggressive outdoor environments. The inventive composites can be formed into virtually any desired configuration and shape. For example, the carbon fiber-reinforced vinyl ester composites made by this invention are desirable for many applications, including civil infrastructure, marine uses, and the like, where high strength is needed but the operating temperatures are relatively mild. The inventive carbon-fiber-reinforced vinyl ester composites systems also are suited to applications in the automotive industry where lightweight durable exterior body parts are needed. Also contemplated are heavy construction applications including wrapping concrete structures in the inventive composite to reduce weathering, such as on bridges and overhead highways . This invention also encompasses intermediate products related to composite precursor materials developed during processing where the sized carbon fibers are embedded in a thermosetting resin system that is not yet fully cross-linked (e.g., a B-stage resin) . For example, certain prepreg materials developed by the process of the invention can be conveniently handled and/or appropriately stored until a later time when it is desired to shape and then fully cure and harden (solidify) the composite resin.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the drawings, in which: Figure 1 is a graph summarizing measurements of amplitude to wavelength compared as distributions based on cross sections of the weave geometry of axial fibers in inventive carbon fiber/vinyl ester composites and control carbon fiber/vinyl ester composites.

Figure 2 is a graph plotting data obtained for inventive carbon fiber/vinyl ester composites and for control carbon fiber/vinyl ester composites as described in the examples herein in terms of applied cyclic stress level (Ksi) versus life (cycles) .

Figure 3 is a graph plotting data obtained for inventive carbon fiber/vinyl ester composites and control carbon fiber/vinyl ester composites in the examples described herein in terms of the ratio of total secant compression stiffness reduction versus applied cyclic load level (% UCS) .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Forms of carbon fiber-reinforced composites

(also known as "carbon fiber-reinforced plastics") encompassed by the invention include fiber reinforcement, fiber preforms, fiber prepregs, and fiber molding compounds. For purposes of this invention, all such composite forms are associated with products in which a thermosetting polymeric matrix is combined with carbon reinforcing fibers pretreated with an aliphatic polyamide sizing agent. The starting carbon fiber usable in this invention include carbon or graphite filamentary material. In general, the carbon fiber is prepared from organic, carbon-rich filamentary material, referred to as the "carbon fiber precursor", which is subjected to combinations of stretching and carbonization in inert 12

atmospheres at high temperatures according to conventional processing techniques. The surface of the carbon filaments produced is preferably oxidized to further promote adhesion to polymeric matrices. Otherwise, the carbon fiber starting materials preferably are unsized before treatment with the sizing agent of the present invention.

Suitable carbon fiber precursors include polyacrylonitrile (PAN) -based fibers. Suitable carbon fibers derived from PAN fibers include those commercially-available from ZOLTEK and from Hercules, Corp, which are oxidized, unsized carbon fibers. Carbon fibers can also be derived from petroleum pitch-based fibers. Also, graphite fibers can be derived from rayon-based fibers.

An important aspect of the present invention is that the carbon fiber is sized with a solution containing an aliphatic polyamide, as the sizing agent, before the carbon fiber is contacted with a thermosetting resin in the production of the composite. The sizing agent can be applied so as to form a substantially uniform, continuous surface film on the bare exterior surfaces of the carbon fibers.

The aliphatic polyamide sizing agents of this invention include thermoplastic polyamide compounds that are ope -chain compounds and their alicyclic analogs (viz. the aliphatic cyclic hydrocarbon analogs thereof) . The aliphatic polyamide sizing agent compounds are devoid of aromatic groups (e.g., phenyl groups, benzyl groups, styryl groups, and the like) . In the case of polyamides that are aliphatic cyclic hydrocarbons, the rings include the amide group. In general, the aliphatic polyamides have the basic structural unit of one of the formulae (A) - (C) below:

where x is a positive integer of 1 or greater, and R independently is hydrogen or an alkyl group (preferably a lower alkyl group of 1-8 carbon atoms) . Therefore, the basic structural with the amide linkage may be present in an open chain segment of an aliphatic polyamide as in formulae (B) and (C) , or in a closed ring structure of an alicyclic polyamide as in formula (A) . Examples of aliphatic polyamides within the scope of the present invention include poly(vinyl pyrrolidone) , poly(alkyloxazoline) , poly(N,N-dialkyl acrylamide) , or poly(N,N-diallylmethacrylamide.

Poly(vinyl pyrrolidone) , [CAS: 9003-39-8] , is one type of alicyclic polyamide sizing agent useful in practicing this invention. Poly(vinyl pyrrolidone) , referred to occasionally herein in abbreviated form as "PVP", is a water-soluble, white, free-flowing, amorphous powder under normal conditions. PVP is formed of the basic repeat structural unit indicated in formula (A) above .

PVP is used in a number-average molecular weight ranging from about 10,000 to about 360,000. In the present invention, PVP is dissolved and dispersed in a volatizable liquid vehicle so that it can be uniformly coated upon the exterior surfaces of carbon fibers. The vehicle favorably is water to provide an aqueous PVP solution, but use may also be made of other appropriate organic solvents. In general, the range at which the sizing agent, such as PVP, is coated upon the carbon fibers is at least a minimal amount effective to promote adhesion between the sized carbon fibers and the resin matrix of the composite. If the amount of add-on of sizing agent to the fiber surfaces becomes too great, no additional benefit is achieved. The coating thickness of the aliphatic polyamide sizing agent on the carbon fibers was provided so as to be related to the concentration of aliphatic polyamide sizing agent in the sizing solution bath, as generally understood and practiced in the carbon fiber industry. The concentration of the sizing agent in the sizing solution generally will be about 0.1 to 5.0 wt . % with respect to the carbon fibers, preferably 2.0 to 5.0 wt.% in the case of aqueous PVP solutions. The amount of the aliphatic sizing agent, such as PVP, on the sized fibers will generally be about 0.1 to 5.0 wt.% with respect to the carbon fibers, preferably 2.0 to 5.0 wt.% in the case of PVP sizing agent.

The sizing solution can be applied to the carbon fibers in any convenient manner, such as by impregnation bath, wash coating, and the like. The carbon fiber or fabric made therefrom can be pulled or drawn through an impregnation bath filled with the sizing solution by equipment arrangements conventionally used and available for applying sizings to fibers/fabrics. Full immersion techniques are preferred for application of the sizing solution to facilitate formation of a uniform, continuous coating of the sizing agent on the exposed fiber surfaces. The amount of sizing solution, and thus sizing agent, finally applied to carbon fibers before drying can be controlled by mangle roller after applying the sizing solution, by rotational velocity of an oiling roller and the coating density, or by conveying the fiber or fabric through the nip of opposing dies (or rollers) after application of the sizing solution. After application of the sizing solution to the fiber surfaces, any appropriate drying method can be used for drying the sizing coating to volatize the liquid vehicle and leave the sizing agent, e.g., PVP, as a film residue attached to the fiber surfaces. Drying of the sizing solution coated upon the carbon fibers can be accomplished by heat and/or air drying methods. By applying and drying the sizing agent upon the carbon fiber surfaces before any contact with the matrix resin, it is possible to effectively confine the sizing agent to the interfacial region between the carbon fiber surfaces and the matrix resin. The matrix resin, in its bulk, thus is devoid of aliphatic polyamide sizing agent in this invention. That is, the sizing and matrix materials will interdiffuse following application of the matrix resin, thus forming a thin interphase region with a graded sizing/matrix composition distribution. The sizing will be effectively confined to this "interphase region."

In addition to the aliphatic polyamide, the sizing solution according to the invention may also contain, if necessary, other adjuvants such as a lubricant, an emulsifier, a conventional sizing agent, and so forth, as long as the effects achievable by the present invention are not frustrated. Examples of such other adjuvants include those described in U.S. Pat. 4,904,818, which description is incorporated herein by reference.

The carbon filaments, when sized according to the invention, can be treated in continuous or chopped monofilamentary form. Alternatively, the carbon filaments can be sized as continuous or discontinuous lengths of staple fibers, yarns, tows, or fabrics (woven or nonwoven) formed of the carbon filaments. Preferably, the carbon fibers are sized in continuous, filamentary form or woven fabric form.

The polymeric matrices prepared in the composites of this invention are thermosetting type. Classes of thermosetting resins useful in the practice of the invention include, for example, unsaturated polyesters (e.g., vinyl esters) , thermosetting polyimides, phenolic resins, and polyurethane resins. The effects of the invention are optimal where the thermosetting resin is miscible with the aliphatic sizing agent, such as PVP. Therefore, favored thermosetting resin types for this invention include unsaturated polyesters, such as vinyl ester resins, phenolic resins and polyurethane resins. Thermoset resins are derived from thermosetting resin systems which form highly cross-linked (thermoset) polymers when cured. The thermosetting resin systems encountered in this invention generally include a combination of appropriate monomers for forming the thermoset resin polymer desired, and this combination of monomers constitutes a liquid thermosetting resin precursor solution. The thermosetting resin systems are applied to the sized carbon fibers or sized carbon fiber fabrics in the B-stage. The thermosetting resins are not fully cross-linked until after the composite material has been configured into any desired permanent shape. The unsaturated polyesters are unsaturated resins with a polyester linkage and include , β-unsaturated polyester resins and vinyl ester resins. The , β-unsaturated polyester resins are obtained by dissolving an unsaturated polyester, which is obtained by condensation of an , β-unsaturated dicarboxylic acid (or anhydride thereof) , with or without a second dicarboxylic acid (or anhydride thereof) , and glycol, in an olefinic unsaturated monomer capable of polymerizing and reacting with unsaturations in polyester molecules to form a three-dimensional network. Useful examples of these materials are described in U.S. Pat. No. 4,904,818, which description is incorporated herein by reference. The typical molecular weight of the unsaturated polyester molecules formed from the condensation of the α, β-unsaturated dicarboxylic acids (or anhydride thereof) with or without a second dicarboxylate acid monomer, and glycol is about 1000 - 3000. The α, β-unsaturated dicarboxylic acids or anhydrides include, for example, maleic acid or anhydride, fumaric acid (unsaturated) , and itaconic acid. Supplementary saturated (aromatic) dicarboxylic acids also can be used as difunctional acids, such as o- phthalic acid or anhydride, isophthalic acid, terephthalic acid, and adipic acid (saturated) . The glycols include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and glycerol . The olefinic unsaturated monomer is a styrene monomer or another vinyl monomer such as -methyl styrene, vinyltoluene, methylmethacrylate, diallylphthalate, and triallylcyanurate . Minor amounts of olefinic unsaturated comonomers may also be incorporated into these systems.

The α, β-unsaturated polyester resin systems can include other adjuvants, such as curing inhibitors, initiators (catalysts) , accelerators, extenders, fillers, and so forth, known in the field. As understood in the field, the density of unsaturations in the unsaturated polyester resins, as well as the proportion of polymerizable monomer, determine the final cross-link density, or the "tightness" of the molecular network and, hence, its stiffness and hardness.

Useful vinyl ester resins involve short linear molecules resulting from the esterification reaction of epoxide group- terminated molecules, such as diglycidyl ether of bisphenol-A with unsaturated acids, such as acrylic acid or methacrylic acid. Vinyl ester resins feature ester linkages and vinyl-type unsaturations, hence the name vinyl ester resins. The diglycidyl ether of bisphenol-A can be derived from bisphenol A and epichlorohydrin, cresol-novolac epoxy resins and phenol-novolac resins, including those described in U.S. Pat. No. 4,904,818, which description is incorporated herein by reference. That is, the novolac resins are produced by reacting phenol or a substituted phenol with formaldehyde in acid solution. The novolacs suitable for reaction with epichlorohydrin contain from about 2 to 6 phenolic hydroxyl groups. These vinyl ester resins have acrylate or methacrylate groups on its terminals. The main chain of these vinyl ester resins is constituted from bis-phenol or novolac molecular structure.

The cross-linking of these vinyl ester reactants into three-dimensional networks, like regular unsaturated polyester resins such as described herein, usually incorporates a monomer such as styrene. Therefore, in one embodiment, a vinyl ester polymer of the invention includes a thermoset resin derived from diacrylate oligomers co-polymerized with styrene monomers or the like.

Useful phenolic type matrix resins for the present invention include thermosetting polymers obtained by the condensation of phenol or substituted phenols with aldehydes such as formaldehyde, acetaldehyde, and furfural and phenol. Epoxidized novolak resin is another type of phenolic resin useful in the invention.

The matrix resins of this invention can also contain other functional additives, if necessary, such as impact modifiers, pigments, heat stabilizers, lubricants, processing aids, fillers, and plasticizers, as long as the effects achievable by the present invention are not frustrated. The types of compounds useful for performing these desired functions are known in the art . The (pre) sized carbon fibers according to this invention can be combined with the matrix- forming resin in a number of forms including continuous filaments, or as chopped fibers, as woven fabrics, or nonwoven webs made of continuous filaments or chopped fibers.

The proportion of sized carbon fibers mixed with and embedded in the resin matrix is not particularly limited and will depend, at least in part, on the ultimate end use envisaged for the finished composite material with consideration of the product strength and stiffness desired as tailored by adjusting the fiber content. The sized carbon fibers will generally constitute about 3 wt.% to about 80 wt.% of the total weight of the composite material product, more typically about 3 wt.% to about 70 wt.%, the balance being constituted by the resin matrix and its constituents.

Any number of known techniques for manufacturing fiber-reinforced plastic parts can be used to impart a desired shape to the carbon fiber-reinforced composites of this invention during their production. In general, the (pre) sized carbon fibers can be embedded in the resinous polymer through either open-tool processing or closed-mold processing.

For example, open-tool processing generally involves use of a single tool surface to give a part its shape. Conventional open-tool techniques adaptable to the practice of this invention include contact molding by hand lay- up, spray-up, or vacuum bag; filament winding; centrifugal casting; pultrusion; continuous laminating; and formation of laminated parts by combination of the carbon fiber-reinforced thermosetting resins (as backing) with rigidized thermoformed sheets (as skin) . Closed-mold processing, also referred to as "matched-mold" or "matched die" processing, generally involves formation of a shaped part in the cavity of a mold. As known, the fiber/resin system can be managed in two basic ways in closed-mold processing. In one basic approach, the (pre) sized fiber-reinforcement and resin are combined prior to their introduction into the mold. The combination of sized fibers and resin can be accomplished just prior to their introduction into the mold as a "pre-mix" , or alternatively, the combination of sized fibers and resin can be prepared by specialized cσmpounders and supplied as ready-to-mold stock as "compounds" . In a second basic approach, the sized fiber and resin are combined within the mold cavity (at-press or in-mold combination) . In this case, the sized fiber reinforcement is placed in the mold, either in the form of cut pieces of mat or woven fabric, or as preforms prepared in advance. A suitable amount of resin can be placed under, or poured over the fiber reinforcement just prior to closing the mold (hot or cold press molding) , or the resin can be injected into the closed mold in low viscosity form to at least partly if not fully engulf and embed the fibers .

Conventional closed-mold techniques adaptable to the practice of this invention generally include hot-mold processing techniques (i.e., using temperature-activated thermosetting resins) or, alternatively, cold mold processing (i.e., using a catalyst-activated resin. More specific categories of conventional closed-mold techniques that can be used in the practice of this invention include, for example, resin transfer molding (RTM) , structural reactive injection molding (SRIM) , cold press molding, compression molding, batch laminating, transfer molding, injection molding, elastic reservoir molding (ERM) , thermal expansion resin transfer molding (TERTM) , ultimately reinforced thermoset reaction injection (URTRI) , and resin infusion molding (RFI) .

The equipment necessary for preparing composite materials in the various manners described hereinabove are widely available and the appropriate techniques for operating and using such equipment will be appreciated by one of skill in the field. One basic scheme for making a composite material of the invention is as follows. A carbon fiber, or a fabric made of same, is coated with a sizing solution in which an aliphatic polyamide sizing agent described herein is dispersed in an aqueous solution. The sizing coating, after application to the fiber or fibrous fabric, is dried to eliminate the aqueous medium and fuse the sizing agent to the fiber surfaces. A fabric formed from the sized fibers, or sized fabric per se, is cut to desired dimensions as needed. Layers of such fabric are stacked and placed inside a mold and a thermosetting resin system is injected into the mold cavity, permeating the fabric and filling the mold. The resin permeating the fabric is then cured (cross-linked) by heat and/or catalytically sufficient to harden and solidify the resin. The sizing agent located at the interface between the carbon fiber surfaces and the resin promotes adhesion between the fibers and resin matrix, thereby enhancing the durability of the composite produced.

More specifically, a process for making the carbon fiber-reinforced composite material of this invention can be summarized as involving the following steps, in this sequence, of:

(a) coating carbon fibers with an aqueous sizing solution containing an aliphatic polyamide dispersed in a volatizable liquid vehicle; (b) drying the aqueous sizing solution effective to volatize at least substantially all the liquid vehicle to form sized carbon fibers;

(c) embedding the sized carbon fibers in a thermosetting resin system to provide an intermediate product; and

(d) heating the intermediate product effective to cross-link the thermosetting resin system to form a solidified, thermoset resin matrix attached to the carbon fibers. This process of the invention is adaptable to either open-tool processing or closed-mold processing techniques. The composite materials of the invention can be readily shaped into desired shapes up until a time when the resin is fully cross-linked to fix the shape.

The following non-limiting examples will further illustrate the present invention. All parts, ratios and percentages are based upon weight unless otherwise specified.

EXAMPLES

Example i Carbon fiber-reinforced composites were fabricated as described below to investigate the workability and durabilities of the composites of the present invention as compared to controls which used carbon fibers lacking a sizing treatment according to the invention.

Panels of composites (6.0 inches width x 6.0 inches length x 1/4 to 3/8 inches inches ave. thickness) were fabricated as follows with and without fibers sized according to the invention. Woven carbon fiber fabrics were obtained from Fabric Development Inc . of Quakertown, PA. , which were made of unsized tows of "AS-4" fabric strips to an aerial weight 194 grams/m². The woven fabrics were sized by a washcoat process in which the fabrics were passed through an aqueous solution of the sizing material, then dried with air heaters while being passed over rollers.

One sample of the woven fabric was coated with a sizing solution containing 4 wt.% poly(vinyl pyrrolidone) . The PVP was obtained from BASF Cosmetic Chemicals division, Mount Olive, NJ, under the trade designation "K-17" . This PVP had an intrinsic viscosity of 0.08 (as measured in chloroform, 25^CC) ; a T_g (°C) of 133 (2nd scan after 1st run up to 300°C) ; and a molecular weight (M_v) of 1.22 x IO⁴. Continuous movement of the size coated fabric was performed during the drying process to affect the separation of the fibers upon drying of the sizing. The pick-up level of the PVP sizing agent on the dried fabric was 2.9 wt.% based on the dry fabric weight .

To produce a hardened composite, a resin transfer molding process was employed. The dried, sized fabric was stacked inside a mold to obtain an approximately 60 volume percent fabric in the composite. A vinyl ester was injected into the mold sufficient to completely infiltrate and embed the fabric. Upon complete infiltration the mold temperature was raised to 130°C at a rate of 3-4 °C/min and held for 30 minutes. The laminate was subsequently cooled to room temperature at 4°C/min. The vinyl ester used was obtained under the trade name DERAKANE

441-400 from Dow Chemical, Freeport TX. The glass transition temperature of the fully cured resin, measured by DSC, was 135°C and the neat resin possessed a K_lc of 0.61 ± 0.13 Mpa //n. The composite made by the above process was representative of the present invention and was designated composite El.

Another composite sample representing the present invention was made in the same manner as composite El except for the differences that a sample of the woven fabric was coated with a sizing solution containing 3 wt.% poly(vinyl pyrrolidone) . The pick-up level of the PVP sizing agent on the dried fabric was 3.3 wt.%. The PVP used in this sample was obtained from BASF Cosmetic Chemicals division, Mount Olive, NJ, under the trade designation "K-90". This PVP had an intrinsic viscosity of 1.50 (as measured in chloroform, 25°C) ; a T_g (°C) of 177 (2nd scan after 1st run up to 300°C) ; and a molecular weight (M of

1.19 x IO⁶. This resulting composite was designated composite E2.

A comparative composite was made in the same manner as composite El except that the fabric was not sized before, to provide a control. The control was designated composite Cl . The composite products El, E2 and Cl were compared by optical microscopy and by scanning electron microscopy (SEM) . The composite panels of samples El and E2 were of good quality and very low void content . Polished cross sections revealed complete infiltration of the vinyl ester polymer into the architecture of the fabric. SEM of the sized fabrics showed relatively uniform coverage by the sizing agent . This was confirmed by XPS showing that the elemental percentages were very close to those predicted for a PVP coating. For instance, the atomic percentage of carbon, oxygen, and nitrogen in bulk PVP is 75%, 12.5%, and 12.5%, respectively. The coated surface of the fabric used in composite El had atomic percentages measured of carbon (79.5%) , oxygen (13.4%) , and nitrogen (7.2%) . The coating fabric for composite E2 had carbon (76.5%) , oxygen (13.7%) , and nitrogen (9.8%) . This indicates that the coatings on the fiber surfaces were comprised of PVP. The fibers of samples El and E2 did not appear to be substantially adhered together; only a few of the fibers appeared connected by a bridge of polymer. The durabilities of inventive composite El and E2, and control composite Cl were investigated and compared as described hereinafter.

Mechanical testing

Two specimens (0.75 inches in width and 6.0 inches in length) were cut from the panels of each of composites El, E2, and Cl for characterization of the virgin unnotched properties. The measured compression strength was subsequently used to determine the load level for fatigue testing. The remaining specimens were reserved for fatigue tests described below. The static strengths provided a reference from which to select the fatigue load levels.

Aluminum V-notch tabs were adhered to each specimen surface with a silicone adhesive to accommodate the knife edges of a one inch gage length, +4% strain extensometer. The extensometer was secured to the specimen with rubber bands which held the knife edges in the slotted tabs. Two layers of 60 grit sand paper were secured with masking tape to both faces of the specimen ends, leaving approximately two inches of unexposed specimen at the center. The grit side of the sand paper was positioned to face the specimen surface. This was done to minimize potential damage from gripping and to aid in clinching the specimen ends within the grip. A grip pressure of 1000 psi was selected which prevented slip during testing and did not damage the specimen.

No antibuckling guides were used in performing the compression tests. To ensure that buckling would not figure into the compression strength of these laminates, an unsupported length of 2.25 inches was selected. This unsupported length did not yield any failures which exhibited buckling. All tests were run in load control on a 22 kip servo-hydraulic test frame at a loading rate of 100 pounds per second. During the course of the test, the load, stroke, and extensometer strain were recorded by computer based data acquisition for analysis. The unnotched coupons described in the quasi-static characterization section were characterized in terms of their performance under R=-l fully reversed sinusoidal loading at 10 Hz. The fatigue tests were also conducted in load control. Preparation and gripping of these specimens was identical to that stated above. Again a 2.25 inch unsupported gage length was used and two layers of 60 grit sand paper were used to aid in effective gripping. However, the extensometer was not used during the fatigue tests. Real-time characterization of fatigue damage was made by monitoring the relationship between the applied load and resulting stroke. The preliminary quasi-static compression properties of the composites El, E2, and Cl which were collected for later reference in conducting the fatigue load levels, are summarized in Table 1.

Table 1 Co QS 3. te Compression E„ (Msi) Compression Strength

(Ksi)

El 8 . 9 42 . 0

E2 9 . 1 40 . 0

Cl 9 . 2 42 . 0

Cross sections of the three systems El, E2, and Cl were also analyzed to quantify the weave geometry of the axial fibers to permit an assessment of the affect of the sizing treatment, if any, on the fiber architecture. The data allowed an assessment of the warp tow geometry of the various composites of interest. Measurements of corresponding wavelength to amplitude were made and compared as distributions. The data collected is summarized in Figure

1 and it confirmed that the sizing process did not appear to have influenced or significantly disturbed the fiber architecture per se. The ratio of amplitude to wavelength (a/L) and of the woven tow is characterized showing similarity between the different fiber sizings assessed. The typical Weibull shape parameter ranges from 3-5 for the distributions shown in Figure 1. The corresponding extremes in weave angles range from 3-20°.

Fatiαue Test Results

The fully reversed fatigue of the unnotched materials for specimens of each of composites El, E2 and Cl revealed a significant effect of the fiber surface characteristics (i.e., sizing or no sizing) . Comparative composite Cl reinforced by unsized carbon fibers showed considerable scatter and reduced life as compared to composites El and E2 reinforced by the PVP sized carbon fibers, as shown in Figure 2. Figure 2 shows the applied R=-l load level versus life of the carbon/vinyl ester composites tested. All failures occurred in the compressive portion of the fatigue cycle. Failures were out of plane as would be expected due to the woven configuration of the warp tows which undulated out of plane of the respective specimen. The data is presented in terms of applied stress level and is directly comparable due to the similarity in the static compression strengths measured for all three systems tested. Th s, the improvements in fatigue performance observed can be attributed to increased durability of the composite materials El and E2 and are not attributable to any improved static compression strength. The fatigue performance of El and E2 was only slightly different even though two different PVP molecular weights were used in the sizings. On the other hand, the difference in fatigue performance between the unsized (Cl) and two sized fiber composites (El, E2) was significantly different to a level of confidence of 99%.

In order to assess the nature of the observed differences on fatigue performance due to the contrasting interphase conditions of El, E2 and Cl, compressive stiffness reduction was also analyzed in conjunction with the life of the specimen and the load level applied. Characterizing the total compressive stiffness reduction (i.e., the compressive secant stiffness at failure) , normalized by cycles to failure relative to the applied load level was postulated to illustrate differences in the failure process. The ratio of total compressive stiffness reduction to life, versus applied load level, as shown in Figure 3 , as determined for composites El, E2 and Cl, clearly shows a separation between the sized and unsized fiber composites. Figure 3 shows the ratio of total secant compression stiffness reduction versus applied cyclic load level measured for specimens of each of composites El, E2 and Cl . This representation of the damage process indicates that the loss of stiffness is cycle dependent. The damage mechanism has been postulated to most likely be the result of microcracking within the matrix and failure of the bond between the fiber and matrix. The combination of the tensile cycles was observed to significantly contribute to this degradation process. Thus, the longer the specimen was cycled the greater the degree of damage accumulation. While not desiring to be bound to any particular theory at this time, it is believed that the load level dependence seen in these experimental studies suggests that at a particular damage site, compressive failure of the axial tows in the woven fabric is possible due to reduction in support from the surrounding damaged matrix and off-axis tows. The fill tows were observed to become disconnected from the axial tows due to transverse shear matrix damage and thus provided little out of plane support to the warp bundles .

Thus, comparative composite Cl was found to have a higher rate of damage accumulation, while composites El and E2 of the invention had appreciably lower rates of damage accumulation. Inspections of the composites after fatigue testing by SEM indicated that the primary source of the improvements to performance in composites El and E2 was derived from enhanced bonding between the fiber and matrix, as no loose fibers were seen and only limited localized fiber surface damage could be seen typical of systems possessing some degree of fiber-to-matrix bond. By contrast, SEM inspection of surfaces of the unsized fibers in composite Cl after fatigue testing revealed a considerable amount of loose fibers and nonuniform, nonlocalized, extensive damage to the fiber surfaces.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Claims

CLAIMSWe claim:

1. A composite material, comprising: carbon fibers embedded in a polymeric matrix comprising a thermoset resin, wherein said carbon fibers are precoated with a sizing agent comprising an aliphatic polyamide before being embedded in said polymeric matrix.

2. The composite material according to claim 1, wherein said thermoset resin is selected from the group consisting of unsaturated polyester resins, vinyl ester resins, thermosetting polyimides, phenolic resins, epoxidized novolak, and polyurethane resins .

3. The composite material according to claim 1, wherein said thermoset resin comprises an unsaturated polyester resin.

4. The composite material according to claim 1, wherein said thermoset resin comprises a vinyl ester resin.

5. The composite material according to claim 1, wherein said thermoset resin comprises a phenolic resin selected from the group consisting of phenol-formaldehyde, phenol- furfural, phenol-acetaldehyde, and epoxidized novolak .

6. The composite material according to claim 1, wherein said sizing agent is selected from the group consisting of poly(vinyl pyrrolidone) , poly(alkyloxazoline) , poly(N,N-dialkyl acrylamide) , or poly(N,N-diallylmethacrylamide .

7. The composite material according to claim 1, wherein said sizing agent comprises poly(vinyl pyrrolidone) .

8. A carbon fiber-reinforced composite, comprising: presized carbon fibers embedded in a polymeric matrix with an interphase region located between surfaces of said carbon fibers and said polymeric matrix, wherein said carbon fibers are sized with poly(vinyl pyrrolidone) prior to being embedded in said vinyl ester matrix and at least substantially all of said poly(vinyl pyrrolidone) is located in said interphase region between said surfaces of said carbon fibers and said vinyl ester matrix.

9. A carbon fiber-reinforced composite precursor material, comprising: carbon fibers embedded in a B-stage thermosetting resin system, wherein said carbon fibers are precoated with a sizing agent comprising an aliphatic polyamide before being embedded in said thermosetting resin system.

10. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system is selected from the group consisting of unsaturated polyester resins, vinyl ester resins, epoxidized novolak, thermosetting polyimides, phenolic resins, and polyurethane resins.

11. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system comprises an unsaturated polyester resin.

12. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system comprises a vinyl ester resin.

13. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system resin comprises a phenolic resin selected from the group consisting of phenol-formaldehyde, phenol- furfural, phenol-acetaldehyde, and epoxidized novolak.

14. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said sizing agent is selected from the group consisting of poly(vinyl pyrrolidone) , poly(alkyloxazoline) , poly(N,N-dialkyl acrylamide) , or poly(N,N-diallylmethacrylamide .

15. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said sizing agent comprises poly(vinyl pyrrolidone) .

16. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system comprises vinyl ester and said sizing agent comprises poly(vinyl pyrrolidone) .

17. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said sizing agent comprises poly(vinyl pyrrolidone) .

18. The carbon fiber-reinforced composite precursor material according to claim 9, wherein said thermosetting resin system comprises vinyl ester and said sizing agent comprises poly(vinyl pyrrolidone) .

19. A process for making a carbon fiber- reinforced composite material, comprising the steps, in this sequence, of: (a) coating carbon fibers with an aqueous sizing solution containing an aliphatic polyamide dispersed in a volatizable liquid vehicle ; (b) drying said aqueous sizing solution effective to volatize at least substantially all said liquid vehicle to form sized carbon fibers; (c) embedding said sized carbon fibers in a thermosetting resin system to provide an intermediate product; and (d) heating said intermediate product effective to cross-link said thermosetting resin system to form a solidified, thermoset resin matrix attached to said carbon fibers.

20. The process according to claim 19, wherein said thermosetting resin system is selected from the group consisting of unsaturated polyester resins, vinyl ester resins, thermosetting polyimides, phenolic resins, epoxidized novolak, and polyurethane resins.

21. The process according to claim 19, wherein said thermosetting resin system comprises an unsaturated polyester resin.

22. The process according to claim 19, wherein said thermosetting resin system comprises a vinyl ester resin.

23. The process according to claim 19, wherein said thermosetting resin system comprises a phenolic resin selected from the group consisting of phenol-formaldehyde, phenol- furfural, phenol-acetaldehyde, and epoxidized novolak.

24. The process according to claim 19, wherein said sizing agent is selected from the group consisting of poly(vinyl pyrrolidone) , poly(alkyloxazoline) , poly(N,N-dialkyl acrylamide) , or poly(N,N-diallylmethacrylamide.

25. The process according to claim 19, wherein said sizing agent comprises poly(vinyl pyrrolidone) .

26. The process according to claim 25, wherein said poly(vinyl pyrrolidone) is attached to said carbon fibers, upon completion of drying step (b) , in an amount of 0.5 to 5 wt.% based on the weight of said carbon fibers.

27. The process according to claim 19, wherein said thermosetting resin system comprises vinyl ester resin and said sizing agent comprises poly(vinyl pyrrolidone) .

28. The process according to claim 19, wherein said embedding and heating steps are conducted within a closed mold.

29. The process according to claim 28, wherein said embedding and heating steps are conducted via molding selected from the group consisting of resin transfer molding (RTM) and resin infusion molding (RFI) .

30. The process according to claim 19, wherein said embedding step is conducted prior to placement of said resulting intermediate product within a closed mold wherein said heating step is conducted.

31. The process according to claim 19, wherein said embedding and heating steps are conducted via open-tool processing.

32. The process according to claim 19, wherein said embedding and heating steps are conducted via pultrusion.

33. The process according to claim 19, further comprising shaping said intermediate product prior to said forming of said solidified, thermoset resin matrix attached to said carbon fibers.

34. A carbon fiber-reinforced composite material produced by the process of claim 19.

35. A carbon fiber-reinforced composite material produced by the process of claim 25.

36. A carbon fiber-reinforced composite material produced by the process of claim 29.