EPOXY RESIN COMPOSITION

This invention relates to methods of making epoxy resin composites.

Composite materials such as carbon fiber composites are increasingly in demand in the land vehicle and aerospace markets as lightweight replacements for sheet metal or even structural steel.

Composites such as these are often manufactured in a multi-step process that separates the step of infusing the fibers with the resin compositions to form a semifinished product from the final curing step. The semifinished product is often referred to as a "prepreg", "sheet molding compound" ("SMC"), or "bulk molding compound" ("BMC"), depending on the particular manufacturing method. The semifinished product is usually subjected to a B- staging step, in which the epoxy resin is partially cured. The final product is made by molding this semifinished product at elevated temperature and pressure to further cure the resin and form a composite having cured resin phase that has a high T _g .

There are several requirements the epoxy resin composition must fulfill in order to be applicable in these manufacturing processes. First of all, the epoxy resin composition should have a low viscosity so it can penetrate between and wet out the fibers. It must be able to partially cure to form a B-staged material. The B-staged material needs to be heat-softenable and essentially free of gels. The viscosity of the epoxy resin composition can be reduced by increasing the temperature during the fiber impregnation. However, this increases the rate of the curing reaction, which in turn causes viscosity to increase over time. Consequently, the temperature conditions during the fiber impregnation step must balance the viscosity of the formulation with its reactivity, so the viscosity of the formulation remains low enough during this step to wet out the fibers.

Once the fibers are impregnated, the epoxy resin formulation is partially cured to form a B-staged material. During this step, the resin formulation is cured so it solidifies without significant gelling. The glass transition temperature of the partially cured material is typically at or above the temperatures at which the B-staged material will be stored and transported, so the semifinished product can be handled as a solid and the resin does not flow away from the fibers. However, the glass transition temperature achieved in this step is typically no greater than 60°C, to prevent gel formation during the B-staging step and to facilitate the flow of the material during the subsequent molding step.

The B-staging is often performed at ambient or only slightly elevated temperatures, such as from 20°C to 60°C. Under these conditions, the T _g can increase to above the B-staging temperature. When that happens, the resin solidifies ("vitrifies") and becomes glassy.

The B-staged material should be chemically stable at the temperatures it encounters during storage and transportation, until such time as it is subsequently molded into a part. This stability eliminates or reduces the need for refrigerate the semi-finished products. The requirement for stability further requires the formulation to exhibit a latent cure, so the formulation has the necessary stability at room temperature (23°C), but cures fast at elevated temperature to produce a cured product with a high glass transition temperature.

Because the B-staged material is not gelled, it can still flow under heat and pressure. During the subsequent molding step, these properties allow the B-staged material to flow and fill the mold before it gels and cures. Under the molding conditions, the B-staged material cures further to form a high T _g composite. (Unless otherwise indicated herein, all glass transition temperatures refer to those of the resin phase of the composite or B-staged material, i.e., to the T _g of the B-staged epoxy resin composition and/or fully cured epoxy resin composition, as the case may be.) Short molding times are essential for high productivity and to avoid the need for additional post-demold curing steps.

An obstacle to greater penetration into those and other markets is the lack of low viscosity, inexpensive resin epoxy resin formulations that can be B-staged easily to form a B-staged material that is highly stable at room temperature, yet cures rapidly at elevated temperatures to form high T _g cured resins.

An epoxy resin system that is widely used for prepregging contains dicyanamide, a latent hardener supplied in the form of a particulate solid. The curing catalyst also is often a solid material such as 2,4-diamino-6-[2'-methylimidazolyl-(l')]-ethyl-s-triazine. The latency of this system is attributable mainly to the fact that the hardener or catalyst, or both, are solids which must be dissolved and/or melted for curing to take place.

A problem with epoxy resin systems that contain solid hardeners or catalysts is that the fibers can filter those materials out of the resin during the infusion step. Portions of the resin that penetrate farther into the fibers become depleted of the solid components, and therefore do not cure completely. This leads to inhomogeneity and a very significant loss of properties.

Some liquid-phase epoxy formulations are known. In these, the hardeners and catalysts can be liquid compounds and/or dissolved in some solvent. U.S. Patent No. 4,447,586, for example, discloses an aqueous copper tetrafluoroborate solution that can accelerate curing of a curable composition containing an epoxy resin and a hindered aromatic diamine. U.S. Patent No. 6,359, 147 describes chromium (III) carboxylates that can catalyze the polymerization of an epoxy with a carboxylic acid, an anhydride, an imide, a lactone and/or a carbonate ester, to produce prepolymers, polymers, and thermosets.

What is desired a B-stagable epoxy resin formulation and a method of forming B- staged materials. The epoxy resin composition should be, prior to B-staging, a low viscosity liquid to provide excellent fiber wetting and impregnation. The epoxy resin composition needs to be capable of partially curing, without gelation, to form a B-staged material having a glass transition temperature of 20°C to 60°C. The B-staging reaction preferably takes place at a temperature of 20°C to 60°C. The B-staged material must be storage- stable at room temperature for a significant period of time, to allow for storage and/or transportation before the semifinished product is molded and fully cured. Additionally, the B-staged material must be rapidly curable at temperature of 130° to 160°C to form a composite that has a glass transition temperature above 140°C.

This invention is in one aspect a method for making a B-staged composite, comprising impregnating a fiber mass with a liquid epoxy resin composition that includes a) a liquid epoxy resin or liquid mixture of epoxy resins, b) at least one liquid aromatic amine or hindered cycloaliphatic amine curing agent, compatible with the liquid epoxy resin or liquid mixture of epoxy resins, and c) a catalyst mixture comprising at least one BF ₄ ^" , PF ₆ ^" , AsF ₆ -, SbF ₆ ^" , FeCk, SnCle ^" , Bids ^" , A1F ₆ ^" , GaCk, InF ₄ ^" , TtfV, ZrF ₆ ^" , CIO4 ^" or R ⁴ B- (where each R is independently hydrocarbyl) salt and at least one compound of a transition metal and an oxygen donor ligand, and curing the liquid epoxy resin composition in the impregnated fiber mass at a temperature of 80°C or less to produce a B-staged composite having a resin phase characterized by a glass transition temperature of 20°C to 60°C and a gel content of 5% or less by weight.

In second aspect, this invention is a B-staged composite comprising a fiber mass impregnated with resin phase that includes a partially cured epoxy resin composition, the partially cured epoxy resin composition being a reaction product of a liquid epoxy resin composition that includes a) a liquid epoxy resin or liquid mixture of epoxy resins, b) at least one liquid aromatic amine or hindered cycloaliphatic amine curing agent compatible with the liquid epoxy resin or liquid mixture of epoxy resins, and c) a catalyst mixture comprising at least one BF4 ^" , PF6 ^" , AsF6 ^" , SbF6 ^" , FeCLr, SnCk ^" , Bids ^" , A1F ₆ -, GaCLr, InF ₄ -, TiF ₆ -, ZrF ₆ ^" , CIO4- or R ⁴ B ^" (where each R is independently hydrocarbyl) salt and at least one compound of a transition metal and an oxygen donor ligand, wherein the resin phase has a glass transition temperature of 20°C to 60°C and a gel content of less than 5% by weight.

The invention is also a method of forming a cured composite, comprising impregnating a fiber mass with a liquid epoxy resin composition that includes a) a liquid epoxy resin or liquid mixture of epoxy resins, b) at least one liquid aromatic amine or hindered cycloaliphatic amine curing agent compatible with the liquid epoxy resin or liquid mixture of epoxy resins, and c) a catalyst mixture comprising at least one BF4 ^" , PF ₆ -, AsF ₆ -, SbF ₆ ^" , FeCLr, SnCle ^" , Bids ^" , A1F ₆ ^" , GaCk, InF ₄ ^" , TiF ₆ ^" , ZrF ₆ ^" , CIO4- or R ⁴ B ^" (where each R is independently hydrocarbyl) salt and at least compound of a one transition metal and an oxygen donor ligand, B-staging the impregnated fiber mass by subjecting the impregnated fiber mass to a temperature of up to 80°C to cure the epoxy resin composition to produce a B-staged composite having a resin phase characterized by a glass transition temperature of 20°C to 60°C and a gel content of less than 5% by weight, and then curing the B-staged composite at a temperature of at least 120°C to form a composite having a resin phase characterized by a glass transition temperature of at least 140°C.

All references to "liquid" materials herein refer to the aggregate state of such material at room temperature (22°C) unless another temperature is explicitly stated.

The catalyst system used in the invention has been found to possess unusual and highly beneficial properties. The catalyst provides latency, that is, at ambient and moderately elevated temperatures, this catalyst system promotes a slow and partial cure. The partial cure leads to the development of a glass transition temperature to 20 to 60°C, when vitrification occurs and subsequent curing becomes very slow unless the material is exposed to higher temperatures. Surprisingly, the curing reaction ceases, and epoxy advancement stops or almost stops, before significant cross-linking reactions that result in gelation take place. This behavior permits B-staging to take place at ambient or moderately elevated temperatures, forming a B-staged material that is storage- stable at room temperature for periods of a month or more. B-staging can also take place at slightly elevated temperatures such as 30-80°C for short times such as 0.1 to 3 hours. The B-staged material then cures rapidly when heated to some temperature of 140° C or more to form a high T _g material.

The catalyst component useful in the present invention is a combination of two components.

The first component is a salt represented by the general formula M ⁿ⁺ X _n> where M represents a cation having valence n and X is an anion selected from BF4 ^" , PF6 ^" , AsF6 ^" , SbF ₆ -, FeCk, SnCk, BiCls ^" , A1F ₆ -, GaCk, InF ₄ ^" , TtfV, ZrFe and CIC ^" . R ⁴ B ^" salts include salts in which the R groups each are independently alkyl such as methyl, ethyl, n-butyl, t-butyl and the like; or phenyl or alkyl-substituted phenyl. Specific examples include (CH ₃ )2(C ₆ H5)2B-, (C ₆ H ₅ )4B- and (C ₄ H ₇ )4B-.

The cation may be, for example, a metal, or an onium ion. The metal ion may be a Group 8, 9 or 10 metal ion such as iron, cobalt or nickel ions; a Group 11 metal ion such as copper or silver ions, or a Group 12 metal ion such as zinc or cadmium ions. Preferred metal cations include zinc and copper. Onium cations include ammonium ions and phosphonium ion. The onium ions may be quaternary ammonium or quaternary phosphonium cations in which the central nitrogen or phosphorus atoms are bonded to alkyl, aralkyl and/or aryl groups.

Among the BF ₄ ^" , PF ₆ ^" , AsF ₆ -, SbF ₆ ^" , FeCk, SnCk, BiCls ^" , A1F ₆ ^" , GaCk, InF ₄ ^" , TiF ₆ ,

ZrF6 ^" , CIO4 ^" or R ⁴ B ^~ salts, tetrafluroborate salts are generally preferred. Zn(BF4)2 and Cu(BF4)2 salts are especially preferred.

Mixtures of any two or more of the foregoing salts can be used as the first component.

The second component of the catalyst is a compound of a transition metal and at least one oxygen donor ligand. By "oxygen donor ligand", it is meant an organic species that bonds to the transition metal via an oxygen atom. The bond may be ionic or covalent, or may be a coordination bond. The oxygen donor ligand may be a carboxylate have, for example, one to twenty or more carbon atoms. Useful carboxylates include, for example, C2-C20, substituted and unsubstituted, straight and branch-chained, alkyl, aryl, and aralkyl carboxylate ions, such as acetate, propionate, hexanoate, heptanoate, octanoate, 2-ethylhexanoate, n-decanoate, laurate, stearate and benzoate. The oxygen donor ligand may be a beta-dicarbonyl compound such as acetylacetonate, or other enolate. The transition metal may be, for example, a Group 6 metal such as chromium or molybdenum, or a group 12 metal such as zinc.

Specific examples of transition metal carboxylates are chromium (III) octoate, chromium (III) 2-ethylhexanoate, chromium (III) acetate, chromium (III) heptanoate, chromium (III) acetylacetonate, zinc (II) octoate, Zn (II) 2-ethylhexanoate, molybdenum (III) 2-ethylhexanoate and mixtures of any two or more thereof. A useful commercially available chromium carboxylate product is Hycat 3000S or Hycat 3100S from Dimension Technology Chemical Systems. This product contains an activated Cr (III) carboxylate along with phenolic and amine components.

The amount of the BF ₄ ^" , PF ₆ ^" , AsF ₆ -, SbF ₆ ^" , FeCk, SnCle ^" , Bids ^" , A1F ₆ ^" , GaCk,

InF4 ^" , TiF6 ^" , ZrF6 ^" , CIC ^" or R ⁴ B ^" (where each R is independently hydrocarbyl) salt may be, for example, from 0.05 to 5 percent of the total weight of the epoxy resin formulation. A preferred amount is at least 0.1 weight-%. A preferred upper amount is 3.0 weight-% and a more preferred upper amount is 1 weight-%. The amount of the compound of the transition metal and oxygen donor ligand may be, for example from 0.25 to 10 percent of the total weight of the epoxy resin formulation. A preferred amount is at least 0.5 weight-% and a more preferred amount is at least 2 weight-%.

The catalyst components are compatible in the epoxy resin composition at the concentrations thereof that are present. By "compatible", it is meant the catalyst components do not separate from the epoxy resin composition to form a separate phase or layer. More preferably, the catalysts are soluble in the epoxy resin or mixture by itself, and/or in the aromatic amine or hindered cycloaliphatic amine hardener by itself, so the catalysts can be formulated into one or more of the components of the epoxy resin composition easily.

If desired or necessary, the catalysts may be dissolved in a solvent to facilitate dispersal into the epoxy resin composition or a component thereof. Such a solvent may be, for example, an alcohol, an ester, a glycol ether, a ketone, a polyalkylene glycol, a polyalkylene glycol monoethers, or mixture of two or more thereof. Specific solvents include, for example, methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, tertiary butanol, tertiary amyl alcohol, glycerin, acetone, methyl ethyl ketone, methyl isobutyl ketone, butylene glycol methyl ether, ethylene glycol methyl ether, ethylene glycol ethyl ether, ethylene glycol n-butyl ether, ethylene glycol phenyl ether, diethylene glycol n-butyl ether, diethylene glycol ethyl ether, diethylene glycol methyl ether, propylene glycol methyl ether, propylene glycol ethyl ether, propylene glycol n-butyl ether, propylene glycol phenyl ether, dipropylene glycol methyl ether, dipropylene glycol n-butyl ether, tripropylene glycol methyl ether, polyethylene glycol, polypropylene glycol, poly(ethylene glycol) methyl ether, and any combination thereof; and the like. If such a solvent is present, it is preferably present in an amount of up to 20 weight-% more preferably up to 5 weight-% and still more preferably up to 2 weight-%, based on the total weight of the B-stagable epoxy resin composition.

The epoxy resin component of the liquid B-stagable resin composition is a liquid epoxy resin or a liquid mixture of epoxy resins. The epoxy resin or mixture of resins should contain an average of at least 1.8, preferably at least, two epoxide groups per molecule. The epoxy resin(s) may be aliphatic, cycloaliphatic, aromatic, cyclic, heterocyclic or mixtures thereof.

Epoxy resins useful in the present invention include those described, for example, in U.S. Patent Nos. 3,018,262, 5, 137,990, 6,451,898, 7, 163,973, 6,887,574, 6,632,893, 6,242,083, 7,037,958, 6,572,971, 6, 153,719, and 5,405,688; WO 2006/052727; U.S. Patent Published Patent Applications 2006-0293172 and 2005-0171237, each of which is hereby incorporated herein by reference.

Among the useful epoxy resins are glycidyl ethers of polyphenols. The polyphenol may be, for example, bisphenol; a halogenated bisphenol such as tetramethyl-tetrabromobisphenol or tetramethyltribromobisphenol; a bisphenol such as bisphenol A, bisphenol AP (l, l-bis(4-hydroxyphenyl)- l-phenyl ethane), bisphenol F, or bisphenol K; an alkylated bisphenol; a trisphenol; A; a novolac resin; a cresol novolac resin, a phenol novolac resin; or a mixture of any two or more thereof.

Other useful aromatic epoxy resins include polyglycidyl ethers of aromatic di- and polyamines and aminophenols.

Other useful epoxy resins include polyglycidyl ethers of aliphatic alcohols, carboxylic acids and amines such as polyglycols, polyalkylene glycols, cycloaliphatic polyols, poly carboxylic acids.

Epoxy resins are room temperature solids that by themselves are useful in the present invention if dissolved one or more other liquid components of the B-stagable resin composition. These solid epoxy resins include type I, II and IV bisphenol A advanced solid resins, or epoxy terminated oxazolidone resins.

Suitable commercially available epoxy resin compounds useful in the composition of the present invention may be, for example, epoxy resins commercially available from Olin Corporation such as the D.E.R.™ 300 series of liquid epoxy resins, the D.E.N.™ 400 series of epoxy novolac resins, the D.E.R.™ 500 series, the D.E.R.™ 600 series, the D.E.R.™ 700 series of solid epoxy resins, and DER 858 and DER™ 6508 epoxy terminated oxazolidone resins.

The useful epoxy resins include non-commercial and commercially available epoxy resins.

Each epoxy resin may have an epoxy equivalent weight of, for example, 100 to 5000, provided that the epoxy resin or mixture of resins is a room temperature liquid. If a single epoxy resin is present, a preferred epoxy equivalent weight is 125 to 300, and a more preferred epoxy equivalent weight is 150 to 250. A mixture of epoxy resins preferably includes at least one epoxy resin with an epoxy equivalent weight of 125 to 300, more preferably 150 to 250, and the mixture preferably has an epoxy equivalent weight of 125 to 1000, more preferably 125 to 500 and still more preferably 150 to 300.

The epoxy resin most preferably is one or more room temperature liquid, 170 to 225 epoxy equivalent weight, diglycidyl ethers of a bisphenol or a liquid mixture of one or more room temperature liquid diglycidyl ethers of a bisphenol and one or more room temperature solid diglycidyl ethers of a bisphenol, or a mixture of one or more room temperature liquid, 170 to 225 epoxy equivalent weight, diglycidyl ethers of a bisphenol with one or more epoxy novolac and/or epoxy cresol novolac resins.

The B-stagable epoxy resin composition contains at least one liquid (at room temperature) aromatic amine and/or hindered cycloaliphatic amine hardener. The hardener(s) should be soluble in the epoxy resin or mixture of epoxy resins at room temperature. A suitable liquid aromatic amine hardener has at least two primary amino groups, each bonded directly to an aromatic ring carbon. Examples of such aromatic amine hardeners include diethyltoluenediamine, 4,4'-diaminodiphenyl ether 3,3'-diaminodiphenyl sulfone, 1,2-, 1,3-, and 1,4-benzenediamine; bis(4- aminophenyl)methane; bis(4-aminophenyl)sulfone; xylenediamine, l,2-diamino-3,5- dimethyl benzene; 4,4'-diamino-3,3'-dimethylbiphenyl; 4,4'-methylenebis(2,6- dimethylaniline); l,3-bis-(m-aminophenoxy)benzene; 9,9-bis(4- aminophenyl)fluorene,3,3'-diaminodiphenylsulfone; 4,4'-diaminodiphenylsulfide; 1,4- bis(p-aminophenoxy)benzene, l,4-bis(p-aminophenoxy)benzene, l,3-propanediol-bis(4- aminobenzoate); and mixtures of any two or more thereof.

Hindered cycloaliphatic amine hardeners have at least two primary amino groups, at least one of which is bonded to a carbon atom of a cycloaliphatic ring structure, and wherein at least one, and preferably both of the ring carbon atoms adjacent to the ring carbon atom(s) to which the amino groups are bonded are substituted. The substituent may be, for example, alkyl or substituted alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl, benzyl and the like. Examples of such hindered cycloaliphatic amine hardeners are 3,3'-dimethyl-4,4'- diaminodicyclohexyl- methane or 3,3',5,5'-tetramethyl-4,4'-diaminocyclohexylmethane.

Generally, the hardener is present in the epoxy resin composition in an amount sufficient to provide 0.7 to 1.8 equivalents of amine hydrogens per equivalent of epoxide group. In some embodiments, this ratio is 0.9 to 1.3 equivalents of amine hydrogens per equivalent of epoxide groups.

The epoxy resin composition may contain various optional ingredients. However, when the epoxy resin composition is to be used to make fiber-reinforced composites, it is preferred that all ingredients are room temperature liquids or are soluble in the remaining components of the epoxy resin composition such that the epoxy resin composition as a whole is liquid that does not contain suspended solids. However, very fine particulates such as, for example, core-shell rubber particles, can be tolerated if they are small enough to penetrate between fibers.

Optional ingredients include, for example, one or more other co-catalysts, one or more mold release agents; one or more solvents and/or diluents, one or more other resins such as a phenolic resin, one or more other epoxy hardeners, fillers, pigments or other colorants, toughening agents, flow modifiers, adhesion promoters, stabilizers, plasticizers, catalyst de-activators, flame retardants, and mixtures of any two or more thereof.

Among the suitable diluents are reactive diluents that contain a single epoxy group. Examples of these include for example, glycidyl ethers of phenolic compounds such as phenol, nonyl phenol, cresol, p-t-butyl phenol and resorcinol, such as ERISYS™ GE-10, GE-11, GE-12, GE-13 from CVC Thermoset Specialties.

Among the suitable other epoxy hardeners are monofunctional aromatic amines such as aniline or toluidine.

Generally, the amount of these optional ingredients, if present at all, may be for example, from 0 weight-% to about 70 weight-%, based on the entire weight of the epoxy resin composition. These other ingredients, if present at all, may constitute up to 40 weight-%, up to 10 weight-% or up to 5 weight-% of the epoxy resin composition.

The formulated epoxy resin composition, prior to B-staging, preferably has a Brookfield viscosity of no more than 3000 mPa · s at 40° C. The fibers are made of materials that are chemically and thermally stable at the temperature of the molding and curing step to form the final composite. Useful fibers include, for example, carbon fibers, graphite fibers, glass or other ceramic fibers, mineral fibers such as mineral wool, metal fibers, polymeric fibers, and the like. Preferred fibers include glass fibers and carbon fibers. Carbon fibers are especially preferred for making vehicular and/or aerospace composites.

The fibers may have diameters, for example, of 250 nm to 500 μπι. A more preferred diameter is 500 nm to 50 μπι and a still more preferred diameter is 1 to 10 μπι.

The fibers may be continuous fibers. Continuous fibers may be, for example, unidirectional, woven, knitted, braided or mechanically entangled. Alternatively, the fibers may be chopped; short fibers having lengths, for example, of up to 100 mm, preferably 3 to 50 mm. Chopped fibers can be held together with a binder to form fiber matts. In a specific embodiment, the fibers are randomly oriented chopped fibers having lengths of 12 to 25 mm.

The fiber content of the composite may be, for example 5 to 95% of the total weight of the composition. Preferred fiber levels are 20 to 90 weight-% or 35 to 70 weight-%, on the same basis.

The epoxy resin composition is made by mixing the foregoing components. The order of addition is in general not critical and determined by the composite fabrication process. It is often convenient to formulate the ingredients into separate resin and hardener components. In such a case, it is often convenient to formulate the catalysts into the hardener component.

The method of impregnating the fibers with the epoxy resin composition is not considered as particularly critical. The fibers can be applied to the epoxy resin composition and/or vice versa. The formulation of the current invention is suitable to make B-staged materials by using, for example, pultrusion, prepregging, SMC or BMC processes.

Prepregging processes involve infusing a unidirectional, woven or braided fabric to form a semifinished product known as a prepreg. Prepregs can be fabricated, for example, using a "film and tack" process, where the epoxy resin formulation is formed into a layer on a release film and the fabric is laid on top of the film. Due to the conveniently low viscosity of the epoxy resin formulation of the invention, it is often sufficient to contact only one side of the fabric. However, the epoxy resin formulation can be applied to both sides of the fabric if desired. In SMC and BMC processes, the epoxy resin formulation is applied to chopped fibers. In the SMC process, the formulation is formed into a layer on a release film or platform and the chopped fiber is sandwiched between two of these layers. In a BMC process, the epoxy resin formulation and the chopped fiber are compounded directly using mixing equipment such as a sigma mixer or an extruder.

In another suitable process, the epoxy resin composition can be sprayed or otherwise applied to a bed of the fibers or to a woven or braided fabric.

B-staging is performed by curing the impregnated fiber mass until the epoxy resin formulation has attained a glass transition temperature of 20°C to 60°C and a gel content of no greater than 5 weight-%.

B-staging is performed at a temperature of 80°C or lower. The B-staging reaction may take place at temperatures as low as about 10°C. A preferred temperature for the B-staging reaction is 20°C to 60°C. A more preferred temperature is 20°C to 45°C and still more preferred B-staging temperature is 20°C to 35°C. In general, higher temperatures within the foregoing ranges tend to accelerate the rate of the B-staging reaction.

A surprising and beneficial attribute of the invention is that the catalyst mixture accelerates the B-staging reaction, particularly at lower B-staging temperatures, but does not cause gelling at the B-staging temperatures mentioned above. The B-staging reaction therefore proceeds more rapidly using the catalysts compared to the uncatalyzed case.

The glass transition temperature reached in the B-staging step is high enough that the material becomes solid and therefore will not flow away from the fibers under force of gravity. Once the glass transition temperature reaches 20°C to 60°C (more typically 40°C to 55°C), the formulation vitrifies and further curing and development of glass transition temperature proceeds very slowly if at all. Similarly, gelation, i.e., the formation of an insoluble three-dimensional polymer network, does not occur even for prolonged periods.

The B- staged material contains less than 5% by weight of gels, preferably no more than 1% by weight gels and more preferably less than 0.1% by weight gels. Gel content is measured by dissolving the B-staged material in a solvent for the epoxy resin, and measuring the amount, if any, of material that remains undissolved. If a clear solution is obtained without precipitates or undissolved material, the gel content is essentially zero. The selection of particular catalysts can be made to tailor the rate of cure during the B-staging step, and/or to adapt the epoxy resin composition to be B-staged at a beneficial temperature. For example, at room temperature zinc octoate has been found to accelerate the B-staging reaction more than does chromium (III) octoate, without over-curing and forming gels, and would be favored in cases where a faster B-staging reaction is needed. On the other hand, chromium (III) octoate is favored if a slower B- staging reaction is needed. However, Zn-octoate alone cannot accelerate the final high temperature cure enough. Combinations of the Zn and Cr-octoates can be used to further tailor the B-staging curing rate, without adversely affecting the T _g of the final cured composite.

Because the B-staged material is very stable at room temperature (23°C), it can be prepared and then stored for a period of up to a month or more at that temperature. This storage stability allows the B-staged composites to be stored and/or transported for extended periods before being formed into a part.

Cured composite parts are formed from the B-staged material by curing the B- staged material at a temperature of at least 140°C to form a composite having a resin phase characterized by a glass transition temperature of at least 140°C. Because the B- staged resin composition is not gelled prior to the final curing step, it can be softened by heating it to above its glass transition temperature. This permits the B-staged composite to be shaped then molded and cured under pressure at temperatures of 120°C or more. A preferred temperature for the final curing step is 120°C to 200°C, especially 130°C to 160°C. During the molding process, the B-staged material flows and fills the mold, and then cures to form the final product.

The selection of particular catalysts and cycle time requirements may affect the necessary cure temperature. Higher temperatures promote faster curing rates. It is often beneficial to select a curing temperature at which a glass transition temperature in the cured resin phase attains a T _g of at least 140°C within 20 minutes, more preferably within 10 minutes, within 5 minutes or even within 1 minute. Even more preferably, the curing temperature is such that the cured resin phase attains a T _g of at least 150°C within 10 minutes, especially within 5 minutes, or even within 3 minutes.

In case of molding prepregs, several prepreg layers can be applied to obtain a cured composite with the proper thickness and thermal and mechanical properties. Due to the ability of the B-staged formulation to flow, B-staged SMC and BMC products can be used to fabricate parts having complex geometries. In these cases, the molten B-staged resin carries the chopped fiber along as it flows into and fills the mold.

Alternatively, the B-staged composite can be heat-softened prior to performing the curing step and preformed to a desired geometry. Thus, for example, the B-staged composite can be preformed, then transferred into a mold where it gains its final shape under heat and pressure and then cured.

The process of the invention is useful, for example, to make vehicle structural and non- structural parts, body panels, deck lids, for automobiles, trucks, train cars, golf carts, all-terrain vehicles, go-carts, farm equipment, lawn mowers and the like. It is useful to make aircraft skins. Other uses include luggage and consumer appliance panels.

Examples

The following examples are provided to illustrate the invention but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated. All materials are commercially available products.

In the following examples:

Epoxy Resin A is a liquid diglycidyl ether of bisphenol A, having an epoxy equivalent weight of about 180 and a viscosity of 9000-10,000 cps at 25°C.

Epoxy Resin B is an epoxy novolac resin having an epoxy equivalent weight of about 178 and a viscosity of 31,000-40,000 cps at 25°C.

DETDA is a mixture of diethyltoluenediamine isomers.

Cu(BF4)2 is copper (II) tetrafluoroborate hydrate and commercially available from Sigma-Aldrich Chemical.

Zn(BF4)2 is zinc (II) tetrafluoroborate hydrate and commercially available from Sigma-Aldrich Chemical.

The zinc octoate is Acima™ Zinkoctoat 18, from the Dow Chemical Company.

The chromium octoate is Hycat 3000S, from Dimension Technology Chemical Systems.

Glass transition temperatures are measured by differential scanning calorimetry (DSC). DSC is performed using a Q2000 model DSC from TA Instruments equipped with an auto sampler and a refrigerated chiller system. 5-10 milligrams samples are transferred to a hermetic aluminum pan. Then pan is sealed and placed in the auto- sampler tray. The scan rate is 10°C/minute. Glass transition temperature is measured on the first scan, and determined using the half extrapolated tangents method.

Examples 1-16 and Comparative Samples A-L

Liquid epoxy resin formulations are prepared by blending Epoxy Resin A,

DETDA, Cu(BF4)2 and a second catalyst as indicated in Table 1, using a high-speed laboratory mixer. The Epoxy Resin A and DETDA are present in amounts to provide one equivalent of amine hydrogen atoms per equivalent of epoxy groups. The formulations all contain 0.6 weight percent Cu(BF4)2. The amounts of the second catalyst vary. The formulations are all 100% liquids and all have viscosities of less than 3000 mPa - s at 40°C.

Two 10-gram samples of each formulation are placed separately into shallow aluminum pans and allowed to B-stage at 25°C under air. The glass transition temperature is measured periodically by differential scanning calorimetry. As the samples B-stage, they harden to form solid materials. After 7 days, one of the samples is cured at 150°C for 5 minutes between two preheated steel plates, and the glass transition temperature of the heat-cured sample is measured by DSC. The remaining sample of each formulation is allowed to remain at room temperature. After 28 days, each sample is checked for gelation by immersing about 100 milligrams in about 10 mL of tetrahydrofuran overnight. Gelation is indicated by the formation of cloudy solutions and/or undissolved matter; clear solutions indicate that no gelling has taken place. Results are as indicated in Table 1.

Table 1- Octoate/Cu(BF4)2-Room temperature B-staging of Single-Epoxy

Resin System

Comparative Sample A illustrates the effect of Cu(BF4)2 by itself. At room temperature, Cu(BF4)2 promotes a slow cure at room temperature, as indicated by the increase in glass transition temperature to only 8°C after two days. Cu(BF4)2 exhibits the desirable feature of promoting only a limited cure at room temperature, even after an extended curing period of 28 days. Cu(BF4)2 is not very effective by itself as a high temperature curing catalyst; the T _g on the 150°C is only 87°C after five minutes.

Examples 1 and 2 show the effect of adding the zinc octoate catalyst. The room temperature cure is accelerated, as can be seen by comparing the 2-day glass transition temperature of Examples 1 and 2 with that of Comparative Sample A. Zinc octoate speeds up room temperature cure compared the chromium octoate as shown by examples 3 and 4. Despite the faster initial room temperature curing, the material soon reaches a degree of cure (as indicated by glass transition temperature) beyond which it no longer progresses. Examples 1 and 2 attain the same glass transition temperature after 28 days as does Comparative Sample A, and neither gels during that time. At 150°C, Examples 1 and 2 cure similarly to Comparative Sample A, indicating that neither the Cu(BF4)2 nor the zinc octoate catalyst, nor their combination, is a very effective catalyst at 150°C.

Examples 5-8 and Comparative Sample B are made and tested in an analogous manner. In these experiments, the epoxy resin is a mixture of 80% Epoxy Resin A and 20% Epoxy Resin B. Each system is liquid before curing. Results are as indicated in Table 2.

Table 2-Octoate/Cu(BF4)2-Room Temperature B-Staging of Mixed Resin

Systems

As the data in Table 2 shows, similar results are obtained with these catalyst systems when used to cure the mixed resins. The zinc octoate promotes partial curing at room temperature, but the degree of room temperature cure does not exceed that of Cu(BF4)2 by itself. The zinc octoate/ Cu(BF4)2 system does not promote fast curing at 150°C. The chromium octoate, on the other hand, provides a little acceleration of the room temperature cure (as shown by the 2 ^nd day T _g s of Examples 3, 4 and Comparative Sample B), but is very effective at 150°C.

Examples 9- 16 and Comparative Sample C and D are duplicates of Examples 1-8 and Comparative Samples A and B, respectively, except the Cu(BF4)2 is replaced with an equal quantity of Zn(BF4)2. Liquid formulations are obtained in each case. Curing and testing are performed as in previous examples, with results as indicated in Tables 3 and 4.

Table 3-Octoate/Zn(BF4>2-Catalyzed B-staging of Single-Resin System

Comparative Samples C and D illustrate the effect of Zn(BF4)2 by itself. By itself, Zn(BF4)2 produces a somewhat faster room temperature cure than Cu(BF4)2, but the increase in glass transition temperature slows substantially after 2-7 days, and no gelling occurs even after 28 days. Zn(BF4)2 by itself is not very effective as a high temperature curing catalyst in either resin system. Adding zinc octoate slightly increases the rate of room temperature cure, but no gelling occurs even after 28 days in either resin system. The combination of Zn(BF4)2 and the zinc octoate catalyst is not a very effective catalyst at 150°C. Adding chromium octoate actually slows the initial curing at room temperature, but the mixture of chromium octoate and Zn(BF4)2 provides very effective catalysis at 150°C.

The ability of the inventive epoxy resin composition to B-stage at room temperature without gelling while still curing rapidly to a high T _g at high temperature is quite unusual, particularly for a low viscosity, all liquid system. To demonstrate this uniqueness, Epoxy Resin A/anhydride hardener compositions (Comparative Samples E- L) are prepared, with various catalyst systems being evaluated. The hardener is either methyl tetrahydrophthalic anhydride (MTHPA) or nadic methyl anhydride (NMA) and is used in an amount to provide an epoxide to anhydride equivalent ratio of 1.0. The catalyst is l-methylimidazole, a mixture of l-methylimidazole (1-M) and the Hycat 3000S chromium octoate, or a mixture of triphenylphosphine (PPI13) and the Hycat 3000S chromium octoate, as indicated in Table 5. In each case, the formulation is B- staged at room temperature for 1 or 2 days, until the T _g reaches at least 40°C, and the B-staged material is evaluated for gels. The B-staged Comparative Samples E, F and G are cured at 160°C for five minutes and evaluated for T _g .

Table 5

N.D.— not done.

The data in Table 5 demonstrates the difficulty in providing a system that B- stages without gelling. All of these anhydride-hardened systems gel during the B- staging. Comparative Sample E does not develop a high glass transition temperature quickly during the high temperature cure. Comparative Sample F develops a high T _g during the high temperature cure, but gels during B-staging. When the amount of catalyst is reduced in an attempt to avoid gelation during B-staging (Comparative Sample G), the sample does not attain a high T _g during the high temperature cure, and still gels during B-staging. All of the alternative catalyst systems evaluated in Comparative Samples H-L lead to gelation during B-staging.

Examples 17-22

Liquid epoxy resin formulations are prepared by blending a mixture of 80% Epoxy Resin A and 20% Epoxy Resin B, DETDA, either Cu(BF ₄ )2 or Zn(BF ₄ )2, and mixture of zinc octoate and chromium(III) octoate as indicated in Table 6. The epoxy resins and DETDA are present in amounts to provide one equivalent of amine hydrogen atoms per equivalent of epoxy groups. The formulations all contain 0.6 weight percent Cu(BF4)2 or Zn(BF4)2. Each of Examples 17-22 are cured and tested in the manner described for previous examples. Results are as indicated in Table 6.

Table 6

These examples further illustrate the ability to "tune" the rate of the room temperature B-staging reaction without over-curing and forming gels. The presence of the chromium octoate ensures a fast high-temperature cure.

A larger batch of Example 4 is prepared and used to make a carbon fiber composite on a pilot SMC production line. The Example 4 formulation is divided between two doctor boxes. The formulation from the first box is metered onto a plastic release film on a traveling belt to form a lower resin layer. Chopped carbon fibers are then applied on top of the lower resin layer at an areal weight of 1500 to 2500 grams/square meter. The formulation from the second doctor box is then metered on top of the carbon fibers, and a second release film applied to the top of the resulting sandwich assembly. The sandwich assembly is then compacted under heat to partially B-stage the resin formulation and wound up on rolls. The resulting composite is held for 24 hours at room temperature and then further B-staged by heating to 60°C for 1 hour in a convection oven.

The B-staged material is then cut into 21.6 cm X 21.6 squares. The squares each are sandwiched between 30.5 cm X 30.5 cm flat plaques and compression molded for 3 minutes on a 300 ton Lih Woei Thermoformer at a tool temperature of 150°C and a molding pressure of about 6 MPa, with no hold time.

The molded plaques are subjected to dynamic mechanical analysis to measure storage modulus against temperature in the range from 25°C to 200°C. The storage modulus of the material is approximately 15,000 MPa at 25°C. This value drops by about 10% as the material is heated to the glass transition temperature of about 145°C.

For comparison, a similar plaque is prepared using a resin formulation of 100 parts Epoxy Resin A, 4 parts of a mold release agent based on montanic acid esters, 3 parts of 2,4-diamino-6-[2'-methylimidazolyl-(l')]-ethyl-s-triazine catalyst, 2.5 parts of dicyanamide and 6 parts of a mixture of the 2,4- and 2,6 isomers of 1-methylcyclohexyl diamine. The storage modulus of this material is about 9,500 MPa at 25°C. When heated to its glass transition temperature of about 145-150°C, the storage modulus of this material drops by 53%. This steep drop in storage modulus is attributed to the inability of the solid components of the resin formulation (the dicyanamide and catalyst) to completely penetrate between the carbon fibers, so regions of incomplete curing exist within the composite.