Preimpregnated reinforcements and high strength composites therefrom

Abstract: Described herein are compositions which contain a specific group of hardeners and epoxy compounds which when combined with structural fbers produce composites which have improved tensile properties and high compres­ sive strengths.

Description: Background of the Invention Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix. Most advanced composites are fabricated from prepreg. a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State of the art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, since most epoxy formulations used in prepreg are brittle, these composites have low toughness, which results in poor impact resistance and tensile properties which do not fully translate the properties of the reinforcing fiber. French Patent No. 2,427,910 describes the preparation of preimpregnated reinforcements and high strength composites therefrom using a composition consisting of a rigid epoxy resin of low viscosity, a flexibilizing agent, e.g. a polyepoxy resin having a long aliphatic chain, a hardening agent and an aromatic polyamide fibre. After the hardening of said composition of a diluent is added to said composition to favour the moistening of the fibres in an amount necessary to adjust the viscosity of the system between 500 and 1500 cPo. As hardening agent among others polymethylene-polyphenylamine without any further definition of it is mentioned. No data concerning tensile properties or compressive strengths of the composition are mentioned. The use of composites as aircraft parts, automotive part, pressure vessels etc. requires a combination of good tensile properties and compressive strenghts. There is a need to improve materials known for this purpose in this respect. Subject matter of the present invention is the use of a composition comprising: (a) 5 to 60 weight percent of a diamine hardener represented by the following general formula: wherein the X's are independently selected from a direct bond, O, S, SO₂, CO, COO, C(CF₃)₂, C(R₁,R₂) wherein R₁ and R₂ are independently hydrogen or alkyl of 1 to 4 carbon atoms; (b) 10 to 60 weight percent of an epoxy resin selected from (2,3-epoxycyclopentyl) ether, copolymers of bis(2,3-epoxycyclopentyl) ether with ethylene glycol, mixtures of bis(2,3-epoxycyclopentyl) ether with a bisphenol A epoxy resin of the formula where n has a value from about 0 to about 15; or with epoxidized novolak resins of the formula where n is 0.1 to 8 and R is H or CH₃ or with N,N-diglycidyl aniline or N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane; (c) 3 to 85 weight percent of a structural fiber having a tensile strength of greater than 6.9 x 10² MPa, a tensile modulus of greater than 13,8 x 10³ MPa, and a decomposition temperature of greater than 200° C as such or in form of prepregs or composites, for producing aircraft parts, automotive parts, pressure vessels, tanks, pipes, protective armor on military vehicles and sporting goods. The preferred diamine hardeners are selected from one or more of the following compounds: These diamines may be used in combination with conventional aromatic diamines. Examples of conventional diamines include 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl methane. 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodipenyl sulfone, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl sulfide, 1,4-bis(p-aminophenoxy)benzene, 1,4-bis(m-aminophenoxy)benzene, 1,3-bis(m-aminophenoxy)benzene, and 1,3,bis(p-aminophenoxy)benzene. The preferred diamine hardeners are selected from one or more of the following compounds: The copolymers of the bis(2,3-epoxycyclopentyl)ether with ethylene glycol can be those which are described in US-Patent 3,398,102. Regarding the components that can be mixed with the bis(2,3-epoxycyclopentyl)ether the following should be mentioned: Useful polyglycidyl compounds are made via condensation reactions with 2,2-bis(4-hydroxyphenyl)propane, also known as bisphenol A, and have structure II: where n has a value from about 0 to about 15. These epoxides are available commercially under the trade names such as "Epon 828", "Epon 1001", and "Epon 1009" from Shell Chemical Co., and as "DER 331" and "DER 334" from Dow Chemical Co. The most preferred bisphenol A epoxy resins have an "n" value between 0 and 10. Polyglycidyl derivatives of phenolformaldehyde novolaks III where n 0.1 to 8 and cresolformaldehyde novolaks IV where n = 0.1 to 8 are also usable. III R = H IV R = CH₃ The former are commercially available as D.E.N. 431, D.E.N. 438, and D.E.N. 485 (obtained from Dow Chemical Co.). The latter are available as, for example, ECN 1235, ECN 1273, and ECN 1299 (obtained from Ciba Geigy Corporation Ardsley, NY). The composition may additionally contain an accelerator to increase the rate of cure. Accelerators which may be used herein include Lewis acid:amine complexes such as BF₃₋monoethylamine, BF₃₋piperidine, BF₃₋2-methylimidazole; amines, such as imidazole, 4-ethyl-2-methylimidazole, 1-methylimidazole, 2-methylimidazole and N,N-dimethylbenzylamine; acid salts of tertiary amines, such as the p-toluene sulfonic acid:imidazole complex; and dicyandiamide. The structural fibers which are useful in this invention include carbon, graphite, glass, silicon carbide, poly(benzothiazole), poly(benzimidazole), poly(benzoxazole), alumina titania, and other ceramic fibers, boron, and aromatic polyamide fibers. These fibers are characterized by a tensile strength of greater than 6,9,10² MPa, a tensile modulus of greater than 13.8.10³ MPa, and a decomposition temperature of greater than 200°C. The fibers may be used in the form of continuous tows (1000 to 400,000 filaments each), woven cloth, whiskers, chopped fiber or random mat. The preferred fibers are carbon fibers, aromatic polyamide fibers, such as Kevlar 49 fiber (obtaiined from E.I. duPont de Nemours, Inc., Wilmington, DE), and silicon carbide fibers. The preimpregnated reinforcement may be prepared by several techniques known in the art, such as wet winding or hot melt. To make impregnated tow or undirectional tape, the fiber is passed into a bath of the epoxy/hardener mixture. A non-reactive, volatile solvent such as dichloroethane may be opotionally included in the resin bath to reduce viscosity. After impregnation, the reinforcement is passed through a die to remove excess resin, sandwiched between plies of release paper, passed through a set of heated rollers, cooled, and taken up on a spool. It is used within a few days or may be stored for months at -17.8°C. During prepreg manufacture, the resin system "B-stages", or partially advances. In some applications it may be desirable prereact a portion of the epoxy resin with the hardener prior to prepreg manufacture. For example, if an epoxy blend containing 80 percent by weight of bis(2,3-epoxycyclopentyl)ether and 20 percent of a liquid bisphenol A resin is reacted with 4,4'-bis-(4-amino phenoxy)diphenyl sulfone, it is beneficial to prereact the hardener with the bisphenol A epoxy resin prior to addition of bis(2,3-epxoycyclopentyl)ether. The prereacted epoxy/hardener mixture has a lower melting point than the pure hardener, making it easier to blend with the remaining portion of the epoxy blend. Composites may be prepared by curing preimpregnated reinforcement using heat and pressure. Vacuum bag/autoclave cures work well with these compositions. Laminates may also be prepared via wet layup followed by compression molding, resin transfer molding, or by resin injection, as described in European Patent Application 0019149, published November 26, 1980. Typical cure temperatures are 37.8°C to 260°C preferably 82°C to 232°C. The compositions are well suited for filament winding. In this composite fabrication process, continuous reinforcement in the form of tape or two - either previously impregnated with resin or impregnated during winding - it is placed over a rotating and removable form or mandrel in a previously determined pattern. Generally the shape is a surface of revolution and contains end closures. When the proper number of layers are applied the wound form is cured in an oven or autoclave and the mandrel removed. For making preimpregnated tape with tack and drape, the preferred resin composition contains greater than 60 percent by weight of bis(2,3-epoxycyclopentyl)ether in the epoxide component and the hardener of Formula I (i.e., 4,4'-bis(3-aminophenoxy)diphenyl sulfone). If additional tack is required, glycidyl amines such as N,N N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane may be used as a coepoxide. In general, prepreg shelf life increases as the proportion of cycloaliphatic epoxy resin in the epoxy component increases. Coepoxides such as bisphenol A epoxy resins of Formula II wherein n = 0 to 5, or epoxidized novolak resins such as III or IV where n = 0 to 4 may be blended with bis(2,3- epoxycyclopentyl)ether for this application. For filament winding and wet layup. the preferred resin composition comprises epoxy resins selected from bis(2,3-epoxycyclopentyl)ether, bisphenol A epoxy resins of Formula II, where n = 0 to 6, and epoxidized novolak resins of Formulas III and IV, where n = 0 to 3. The preferred epoxy resin mixtures have viscosities less than 50 Pa.s. at 70°C. For all prepreg and composite formulations the preferred molar ratio of N-H groups in the hardener to 1,2-epoxide groups in the epoxy resin is 0.6 to 1.5. According to this invention the compositions are used as aircraft parts such as wing skins, wing-to-body fairings, floor panels, flaps, radomes; as automotive parts such as driveshafts, bumpers, and springs; and as pressure vessels, tanks and pipes. They are also suitable for protective armor on military vehides and sporting goods applications such as golf shafts, tennis rackets, and fishing rods. In addition to structural fibers, the composition may also contain particulate filler/reinforcement such as talc, mica, calcium carbonate, aluminum trihydrate, glass microballoons or spheres, phenolic thermospheres, carbon black, asbestos, wollastonite, and kaolin. Up to half of the weight structural fiber in the composition may be replaced by filler and/or particulate reinforcement. Examples The following examples serve to give specific illustrations of the practice of this invention. Examples 1-17 relate to binary non-reinforced compositions suitable for preparing the ternary reinforced compositions within the scope of this invention. In the examples which follow, the epoxy equivalent weight (EEW) is defined as the grams of epoxy resin per mole of 1.2 epoxide group. Example 1 An epoxy resin blend was prepared by heating together 1600 g of a bisphenol A epoxy resin (EEW 189) with 2400 g of bis(2,3-epoxylcyclopentyl) ether at 50°C for 1 hour. A thermosetting epoxy resin formulation was prepared by combining 981 g of this solution with 900 g of 4,4'-bis(3-aminophenoxy)diphenyl sulfone. Example 2 An epoxy resin blend was prepared by combining 47.2 g of a bisphenol A epoxy resin (EEW 189) and 28.4 g of bis(2,3-epoxycyclopentyl)ether at 50°C. This solution was blended with 60 g of 4,4'-bis(3-aminophenoxy)diphenyl sulfone to make a thermosetting epoxy resin formulation. Example 3 An epoxy resin blend was prepared by combining 20.9 g of bis(2,3-epoxycyclopentyl)ether with 47.7 g of N,N,N',N'-tetraglycicyl-4,4'-diamino diphenyl methane (Araldite MY-720 obtained from Ciba-Geigy Corporation, Ardsley N.Y.). This blend was combined with 60.0 g of 4,4'-bis(3-aminophenoxy)diphenyl sulfone to make a thermosetting epoxy resin formulation. Example 4 An epoxy resin blend was prepared by combining 39.2 g of bis(2,3-epoxcyclopentyl)ether with 26.2 g of a bisphenol A epoxy resin (EEW 189) at 50°C. A thermosetting epoxy formulation was prepared by combining this blend with 60 g of 4,4',bis(4-aminophenoxy)diphenyl sulfone. Example 5 A thermosetting epoxy formulation was prepared by combining 55.0 g of bis(2,3-epoxycyclopentyl)ether with 65.1 g of 4,4'-bis(3-aminophenoxy)diphenyl sulfone. Example 6 A thermosetting epoxy formulation was prepared by combining 58.1 g of bis(2.3-epoxycyclopentyl)ether with 66.0 g of 4,4'-bis(4-aminophenoxy)diphenyl sulfone. Example 7 A 5 liter flask equipped with a paddle stirrer, nitrogen inlet, thermometer with a Therm-o-Watch controller, Claisen adapter, 304.8 mm jacketed Vigreux column. Barren trap, water cooled condenser, a nitrogen inlet, and an electric heating mantle was charged with the following materials: 451 g of potassium carbonate , 574 g of 4,4-dichlorodiphenyl sulfone 600 ml of toluene , and 1400 ml of N,N-dimethyl acetamide. The mixture was stirred, purged with nitrogen, and heated to 80°C. Then 229 g of m -aminophenol and 229 g of p -aminophenol were added. The temperature of the mixture was raised to 155-160°C and held there as toluene and a toluene/water azeotrope were collected in the trap. A small amount of toluene was continually recycled as the mixture was maintained at 160°C for 10 hours. The mixture was then cooled to 70°C and filtered. A 350 g portion of the filtrate was charged to a 5 liter flask containing 2 liters of methanol. This solution was heated and stirred at reflux as 1300 g of water was added over a 1 hour period. Following addition of water, heating was discontinued, but agitation continued as the contents cooled to room temperature (about 25°C). The cooled mixture contained a tan granular solid, which was recovered on a filter, washed once with hot water, and dried in a vacuum oven. The final product weighed 133 g and had a melting range of 150-163°C. It was a mixture of 4,4'-bis(4-aminophenoxy)diphenyl sulfone,. 4,4'-bis(3-aminophenoxy)diphenyl sulfone and 4-(3-aminophenoxy)-4'-(4-aminophenoxy)diphenyl sulfone. Example 8 An epoxy blend was prepared by combining 39.2 g of bis(2,3-epoxycyclopentyl)ether and 26.2 g of a bisphenol A epoxy resin (EEW 189) at 50°C. Athermosetting epoxy formulation was prepared by combining the blend with 60.0 g of the product of Example 8. Examples 9 to 17 Unreinforced castings were prepared from the formulations described in Examples 1 through6 and 8. Typical castings weighed 100 to 160 g and were made using the proportions given in the above Examples. Casting dimensions were 3.2 × 203.2 × 127 to 203.2 mm. The general procedure for making castings was as follows: The epoxy resin was charged to a 3-necked flask equipped with a paddle stirrer. The contents of the flask were heated to 120°C to 130°C and stirred as the amine hardener was added as a fine powder. The hardener dissolved in about five minutes. The resulting solution was subjected to a vacuum of about 635 mm of mercury for three minutres with agitation. followed by two minutes without agitation. It was then poured into a glass mold with a cavity of dimensions 3.2 × 203.2 × 203.2 mm and cured with a programmed heating cycle: 16 to 21 hours at 105°C, 8 hours at 140°C, and finally 16 hours at 175°C. Castings were tested to determine tensile properties, heat deflection temperature and water sensitivity. For the latter, the change in weight on immersion of tensile bars in 71°C water after two weeks was recorded. Tensile properties were measured according to ASTM D-638 using a Type 1 dogbone specimen. Heat deflection temperatures were measured according to ASTM D-648 (1822 kPa stress). Table I summarizes the properties of unreinforced castings. These materials have low water sensitivity and high tensile moduli compared to castings of many other epoxy formulations. Example 16 describes the preparation of undirectional epoxy/graphite pregreg. Example 16 The resin formulation described in Example 1 was prepared by adding the hardner to the epoxy resin over a 20 minute period while maintaining the mixture at 110°C. The formulation was then cooled to 70°C and poured into a shallow pan on a pregreg machine. A 152.4 mm wide ribbon of 110 carbon fiber tows each containing 6,000 filaments, was impregnated with resin in the bath and then passed through a slot die with dimensions of 152.4 mm by about 0.356 mm in order to remove excess resin. The impregnated tape was sandwiched between release paper and passed through a prepreg machine. The finished tape contained about 55 percent by weight of fiber. The fiber was a PAN-based carbon fiber with a tensile strength of 34.5.10² MPa tensile modulus of 234.6.10³ MPa and a yield of 0.39 grams per meter. Example 17 describes the preparation and properties of a cured laminate. Example 17 A unidirectional laminate was prepared by stacking 8 plies of the preimpregnated tape made in Example 16 in a mold, covering them with a teflon impregnated spacer and bleeder cloths, and enclosing them in a nylon bag. The entire assembly was placed in an autoclave and cured. A control prepreg made with the same fiber but a different epoxy resin system was also cured in the autoclave using the manufacturer's recommended cure schedule. The laminates were tested to determine longitudinal tensile and compressive properties. Tensile properties were measured according to ASTM-D3039. Compressive properties were measured using a modified ASTM-D695 procedure. Unidirectional graphite/epoxy tabs were added to prevent the sample ends from crushing in a noncompressive failure mode. A gage length of approximately 4.826 mm was used. End tabs on compressive samples were adhered using FM-300 film adhesive (obtained from American Cyanamid, Havre de Grace, MD) which was cured at 177°C for 1 hr. Table II summarizes laminate properties. It is clear from this data that the prepreg composition of this invention afford composites with higher tensile and compressive properties than the Control. Furthermore, the hot/wet compressive strength of the composite of this invention is excellent.

Claims: The use of a composition comprising: (a) 5 to 60 weight percent of a diamine hardener represented by the following general formula: wherein the X's are independently selected from a direct bond, O, S, SO₂, CO, COO, C(CF₃)₂, C(R₁,R₂) wherein R₁ and R₂ are independently hydrogen or alkyl of 1 to 4 carbon atoms; (b) 10 to 60 weight percent of an epoxy resin selected from bis (2,3-epoxycyclopentyl) ether, copolymers of bis(2,3-epoxycyclopentyl) ether with ethylene glycol, mixtures of bis(2,3-epoxycyclopentyl) ether with a bisphenol A epoxy resin of the formula where n has a value from about 0 to about 15; or with an epoxidized novolak resin of the formula where n is 0.1 to 8 and R is H or CH₃ or with N,N-diglycidyl aniline or N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane; (c) 3 to 85 weight percent of a structural fiber having a tensile strength of greater than 6.9 x 10² MPa, a tensile modulus of greater than 13,8 x 10³ MPa, and a decomposition temperature of greater than 200°C as such or in form of prepregs or composites, for producing aircraft parts, automotive parts, pressure vessels, tanks, pipes, protective armor on military vehicles and sporting goods, The use as defined in claim 1 wherein the diamine is of the formula: The use as defined in claim 1 wherein the diamine is of the formula: The use as defined in claim 1 wherein the structural fibers are selected from carbon, graphite, glass, silicon carbide, poly(benzothiazole), poly(benz- imidazole), poly(benzoxazole), alumina, titania, boron, and aromatic polyamides. The use as defined in claim 1 wherein the composition contains an accelerator which increases the rate of cure. The use as defined in claim 1 wherein the composition contains 15 to 50 weight percent of component (a). The use as defined in claim 1 wherein the composition contains 15 to 50 weight percent of component (b). The use as defined in claim 1 wherein the composition contains 20 to 80 weight percent of component (c).

Inventors: BRODE GEORGE LEWIS, GARDNER HUGH CHESTER, MICHNO MICHAEL JOHN JR, BRODE, GEORGE LEWIS, GARDNER, HUGH CHESTER, MICHNO, MICHAEL JOHN, JR.
Applicants: AMOCO CORP, AMOCO CORPORATION
Application year: 1984
Main classification: C08G59
IPC codes: C08L77, C08G59, C08J5, C08K7, C08L63