Methods for forming composite articles from tailorable polyimide resin systems using RTM and RI techniques

BACKGROUND OF THE INVENTION

0001This invention relates generally to resin transfer molding (RTM) and resin infusion (RI) methods applicable to polyimide resin systems, resin systems processible using RTM and RI methods, and to reinforced composite articles obtained therefrom.

0002Fiber-reinforced composite materials, which are made up of reinforcing fibers and matrix resins, are lightweight and may exhibit excellent mechanical properties. As such, these composite materials have been widely used in a variety of structural and non-structural applications in the aerospace industry.

0003Various methods or techniques, such as prepreg, hand lay-up, filament winding, pull-trusion, RTM and RI, have been used to produce reinforced composite materials. In the RTM method, a preform structure made up of reinforcing material is placed in a mold, a resin poured therein to impregnate the preform, and the impregnated preform structure cured to produce a molded product. The RTM method offers the advantage that a large component having a complicated shape can be molded in a short period of time.

0004The preform structure may include a tackifier which, when heated, will fuse onto the surface of the reinforcing material and then solidify upon cooling. Layers of the reinforcing materials with the tackifier can be stacked together, the tackifier heated, fusing the plies together under an appropriate pressure, and then cooled to form the net-shaped preform structure. The multilayered preform structure is placed into a mold, the matrix resin added, and the composite formed using usual resin transfer molding processes. <patcit id="pcit0001" dnum="US5766534A"><text>U.S. Patent 5,766,534</text></patcit> discloses a process for preparing a matrix resin composite utilizing a preform comprising two or more layers of reinforcing material and a tackifier of a curable resin applied to at least one layer of a reinforcing material. The layered assembly is compressed while the tackifier is at least partially crosslinked. Thereafter, the preform is contacted with the matrix resin, which may be the same or different from the tackifier resin. The matrix resin and tackifier are finally cured to form the matrix resin composite.

0005As disclosed in <patcit id="pcit0002" dnum="US7129318B"><text>US Patent No. 7,129,318</text></patcit>, the use of composite materials having polyimide resin matrices is increasing because of their lightweight and load-bearing characteristics and their oxidative stability at elevated temperatures. However, polyimide resin systems present challenges for use with RTM and RI techniques. Fiber-reinforced composite materials that use polyimide resins as the matrix resin are generally prepared using prepreg methods. For example, poly(amid) acid solutions may be processed into prepreg with various reinforcing fibers. The poly(amide) acid solutions have low solids content and high viscosity, presenting processing problems. This material is then hand-laid into composites in a labor-intensive operation.

0006Polyimide resin systems for use in RTM processes generally use preimidized polyimides with molecular weights ranging from 800 to 1100 g/mol. The preimidized powder may be melted and injected into a dry fiber preform. However, current RTM polyimide parts suffer from microcracking, poor thermal stability, or offer only limited temperature capability. The low viscosity/low molecular weight needed for injection often does not create favorable end properties. There currently does not exist a tackifier system suitable for use with polyimides.

0007Accordingly, it would be desirable to provide an RTM and RI methods that utilize the benefits of a tackifier, suitable for polyimide resin systems that provide reinforced composite structures that exhibit good mechanical and thermal properties.

0008<patcit id="pcit0003" dnum="WO9426493A"><text>WO 94/26493</text></patcit> discloses an improved process to make composites comprises the steps of: (1) contacting a substrate with a thermoplastic partially-cured curable tackifier resin at above the glass-transition temperature to form a perform; (2) adding a matrix resin that can dissolve the tackifier or that is essentially identical to the tackifier and co-curing the tackifier and matrix resin to form a composite. The resulting composites are useful in structural application.

BRIEF DESCRIPTION OF THE INVENTION

0009In one embodiment the method comprises tackifying a preform structure with a first prepolymer component comprising at least one of a monomeric blend or a reaction product of an end-capping component, at least one dianhydride or derivative thereof, and at least one diamine. Using resin infusion or resin transfer molding techniques, the tackified preform structure is contacted with a second prepolymer component, different from the first prepolymer component, including at least one of a monomeric blend or a reaction product of an end-capping component, at least one dianhydride or derivative thereof, and at least one diamine. A polyimide resin system is comprised of the first prepolymer component and the second prepolymer component. The polyimide resin system is cured under suitable conditions such that the first and second prepolymer components are mixed and react to provide a polyimide composite structure.

DETAILED DESCRIPTION OF THE INVENTION

0010Embodiments disclosed herein provide polyimide systems that simultaneously offer low toxicity, a high glass transition temperature, excellent thermal oxidative stability, and the ability to be processed using RTM and RI methods. Furthermore, embodiments disclosed herein provide tailorable polyimide systems wherein relative amounts of starting materials may be altered to achieve desired outcomes.

0011In an exemplary embodiment, a polyimide matrix of a reinforced composite article is the reaction product of a mixture of monomeric reactants, polyimide-precursor reaction products, oligomers, and mixtures thereof. The composite articles are formed by RTM or RI methods.

0012Exemplary embodiments include a pre-polymer polyimide resin system that includes a first prepolymer component and a second prepolymer component. The first prepolymer component may be a mixture of monomers, a mixture of oligomers, or a pre-imidized component. The second prepolymer component may be a mixture of monomers, a mixture of oligomers, or a pre-imidized component. The first and second prepolymer components are capable of reacting to provide the cured polyimide matrix for a composite article.

0013An exemplary first prepolymer component includes a first monomeric mixture, or repeat units from monomers including at least one end-capping agent, at least one aromatic dianhydride or a derivative thereof (e.g., the ester product formed from the dianhydride and alcoholic solvent), and at least one diamine. The second prepolymer component includes a second monomeric mixture, or repeat units from monomers including at least one end-capping agent, at least one aromatic dianhydride, and at least one diamine. The at least one aromatic dianhydride or the at least one diamine in the first prepolymer component is different from the at least one aromatic dianhydride or derivative thereof, or the at least one diamine in the second prepolymer component. The selection of the end-capping agent(s), aromatic dianhydrides, diamines, and their relative molar ratios, are considered with respect to the desired property outcomes such as molecular weight, processibility, high temperature performance, and the like.

0014End-group components may include structures that are capable of forming oligomer compounds and capable of crosslinking in an addition polymerization reaction to form a crosslinked polyimide structure. Crosslinkable-group-containing end blocking agents of various kinds are usable depending on the synthesis process of the polyimide, including monoamines and dicarboxylic acid anhydrides as representative examples. A variety of crosslinkable groups may be selected in accordance with molding or forming conditions.

0015The crosslinkable group structures contained in the end groups may include ethynyl groups, benzocyclobuten-4'-yl groups, vinyl groups, allyl groups, cyano groups, isocyanate groups, nitrilo groups, amino groups, isopropenyl groups, vinylene groups, vinylidene groups, and ethynylidene groups.

0016The above described, crosslinkable-group-containing end blocking agents can be used either singly or in combination. Some or all of the hydrogen atoms on one or more of the aromatic rings of the end group containing material may be replaced by a like number of substituent groups selected from halogen groups, alkyl groups, alkoxy groups, and combinations thereof.

0017Exemplary end group components may include, but are not limited to, the following end group structures: <ul id="ul0001" list-style="none" compact="compact"><li>nadic end groups, including, but not limited to the following formula: <chemistry id="chem0001" num="0001"><img file="EP2075111B1_D0001.tif" /></chemistry> vinyl end groups including, but not limited to the following formula: <chemistry id="chem0002" num="0002"><img file="EP2075111B1_D0002.tif" /></chemistry></li><li>acetylene end groups including, but not limited to the following formula: <chemistry id="chem0003" num="0003"><img file="EP2075111B1_D0003.tif" /></chemistry> phenylethynyl end groups including, but not limited to the following formula: <chemistry id="chem0004" num="0004"><img file="EP2075111B1_D0004.tif" /></chemistry></li></ul> and mixtures thereof.

0018Ar as shown above in the nadic and phenylenthynyl end group structures may include aromatic groups, such as substituted or unsubstituted aromatic monocyclic or polycyclic linking structures. Substitutions in the linking structures may include, but are not limited to ethers, epoxides, amides, esters and combinations thereof.

0019The dianhydride component may include, but is not limited to, monomers having an anhydride structure, wherein an exemplary structure includes a tetracarboxylic acid dianhydride structure. The dianhydride component employed may be any suitable dianhydride for forming crosslinkable or crosslinked polyimide prepolymer, polymer or copolymer. For example, tetracarboxylic acid dianhydrides, singly or in combination, may be utilized, as desired.

0020Illustrative examples of aromatic dianhydrides suitable for use include: 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane dianhydride; 4,4'-bis(3,4-dicarboxyphenoxy) diphenyl ether dianhydride; 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride; 4,4'-bis(3,4-dicarboxyphenoxy)benzophenone dianhydride; 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride; 2,2-bis(4-(2,3-dicarboxyphenoxy) phenyl)propane dianhydride; 4,4'-bis(2,3-dicarboxyphenoxy)diphenyl ether dianhydride; 4,4'-bis(2,3-dicarboxyphenoxy)diphenyl sulfide dianhydride; 4,4'-bis (2,3-dicarboxyphenoxy)benzophenone dianhydride; 4,4'-bis(2,3-dicarboxyphenoxy) diphenyl sulfone dianhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy) diphenyl-2,2-propane dianhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl ether dianhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)benzophenone dianhydride and 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride, 1,2,4,5-benzenetatracarboxylic dianhydride as well as mixtures comprising one of the foregoing dianhydrides.

0021Exemplary dianhydride components include the following dianhydride compounds: <ul id="ul0002" list-style="none" compact="compact"><li>3,4,3',4'-biphenyltetracarboxylic dianhydrides (BPDA) having the following formula: <chemistry id="chem0005" num="0005"><img file="EP2075111B1_D0005.tif" /></chemistry></li><li>3,4,3',4'-benzophenonetetracarboxylic dianhydrides (BTDA) having the following formula: <chemistry id="chem0006" num="0006"><img file="EP2075111B1_D0006.tif" /></chemistry></li><li>2,2-bis(3',4'-dicarboxyphenyl) hexafluoropropane dianhydrides having the following formula: <chemistry id="chem0007" num="0007"><img file="EP2075111B1_D0007.tif" /></chemistry> pyromellitic dianhydrides having the following formula: <chemistry id="chem0008" num="0008"><img file="EP2075111B1_D0008.tif" /></chemistry> and mixtures thereof.</li></ul>

0022Depending on the fabrication process, tetracarboxylic acid monoanhydrides, tetracarboxylic compounds other than anhydrides, or their derivatives such as salts may also be used as desired instead of the above-recited dianhydrides. The dianhydride components, as described above, may be used either singly or in combination as needed.

0023The aromatic dianhydrides can be prepared by any suitable fabricating method known in the art. One suitable fabrication method for fabricating aromatic dianhydrides may include hydrolysis, followed by dehydration, of the reaction product of a nitro substituted phenyl dinitrile with a metal salt of dihydric phenol compound in the presence of a dipolar, aprotic solvent.

0024The diamine component may include, but is not limited to, an aromatic diamine monomer having the following formula:         H₂N-Ar-NH₂

0025Ar as used in this formula preferably includes aromatic compounds, including substituted aromatic compounds and compounds having multiple aromatic rings. Substituent groups for substitution in the Ar group may include any suitable functional group, including, but not limited to halogen groups, alkyl groups, alkoxy groups, and combination thereof.

0026Examples of suitable diamine components may include, but are not limited to: 1,3-bis(aminophenoxy)benzene, 1,4-bis(aminophenoxy)benzene, 1,4-phenylenediamine ("para-PDA" or "pPDA"), 1,3-phenylene diamine ("meta-PDA" or "mPDA"), 4,4'-[1,3-phenylene bis (1-methyl-ethylidene)] bisaniline ("Bis aniline M" or "Bis-M"), ethylenediamine, propylenediamine, trimethylenediamine, diethylenetriamine, triethylenetetramine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, 1,12-dodecanediamine, 1,18-octadecanediamine, 3-methylheptamethylenediamine, 4,4-dimethylheptamethylenediamine, 4-methylnonamethylenediamine, 5-methylnonamethylenediamine, 2,5-dimethylhexamethylenediamine, 2,5-dimethylheptamethylenediamine, 2,2-dimethylpropylenediamine, N-methyl-bis (3-aminopropyl) amine, 3-methoxyhexamethylenediamine, 1,2-bis(3-aminopropoxy) ethane, bis(3-aminopropyl) sulfide, 1,4-cyclohexanediamine, bis-(4-aminocyclohexyl) methane, m-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, m-xylylenediamine, p-xylylenediamine, 2-methyl-4,6-diethyl-1,3-phenylene-diamine, 5-methyl-4,6-diethyl-1,3-phenylene-diamine, benzidine, 3,3"-dimethylbenzidine, 3,3"dimethoxybenzidine, 1,5-diaminonaphthalene, bis(4-aminophenyl) methane, bis(2-chloro-4-amino-3,5-diethylphenyl) methane, bis(4-aminophenyl) propane, 2,4-bis(b-amino-t-butyl) toluene, bis(p-b-amino-t-butylphenyl) ether, bis(p-b-methyl-o-aminophenyl) benzene, bis(p-b-methyl-o-aminopentyl) benzene, 1,3-diamino-4-isopropylbenzene, bis(4-aminophenyl) sulfide, bis (4-aminophenyl) sulfone, bis(4-aminophenyl) ether, 1,3-bis(3-aminopropyl) tetramethyldisiloxane and mixtures comprising at least one of the foregoing organic diamines.

0027Further, these diamines are also usable in place of some or all of the hydrogen atoms on one or more of the aromatic ring(s) of each of the diamines. A like number of ethynyl groups, benzocyclobuten-4'-yl groups, vinyl groups, allyl groups, cyano groups, isocyanate groups, nitrilo groups and/or isopropenyl groups, which can act as crosslinking points, may also be introduced as substituent groups on the aromatic rings, preferably to an extent not impairing the moldability or formability.

0028Glass transition temperature (Tg) is a measure of the ability of the polymer to maintain properties at elevated temperatures. Because bulk motion of the polymer is restricted below the Tg, the higher the Tg a material displays, typically, the higher the temperature capability of that material. Therefore, Tg of the crosslinked polyimide matrix may be a driving consideration in the make up of the prepolymer blend.

0029Melt viscosity is a measure of a fluids resistance to flow at temperatures above the melt point. For processing composites, it is generally desirable to have melt viscosities below 100,000 centipoise (cps) with the preferred range or 40,000 cps - 800 cps wherein the melt viscosity is dependent upon the processing utilized. If the melt viscosity is not sufficiently low, processing requires excessive pressures in order to make the resin flow. Lower melt viscosities generally lead to greater processing options due to decreased pressure needs. Thus, a desired melt viscosity of the prepolymer blend may influence the respective amounts of the components in the prepolymer blend.

0030Thermal Oxidative Stability (TOS) is the ability of the polymer to withstand elevated temperatures in an oxygen-containing environment, such as air, with minimal loss of weight and/or properties. Turbine engine components often operate in high pressure as well as high temperature environments and the high pressure acts to increase the concentration of oxygen accelerating the deterioration of composite properties. Since, in a composite, compression strength is a resin-dominated property, the retention of compression strength after long-time exposures to high temperatures is monitored as a measure of TOS. Weight loss over time is also used as a measure. Polymers degrade through mechanisms, such as volatilization, resulting in a composite having reduced mass due to this loss of polymer. One test used herein to measure TOS includes placing a plaque of polymeric or composite material in a chamber, increasing the temperature and pressure within the chamber to a predetermined temperature and pressure, and holding these conditions for up to 150 hrs with multiple atmospheric changes over the course of the test. The plaques are then removed and tested for weight loss and retention of compression strength. The weight loss and retention of compression strength reflect service conditions in a turbine engine and provide a measure of the longer-term stability of the polymer material. A higher TOS is important for material that will be placed in a high temperature environment for long periods of time. The crosslinked polyimide copolymer preferably has a TOS of less than about 2.0% weight loss.

0031One embodiment includes utilizing the prepolymer blends in a resin infusion (RI) process. In RI, a fiber containing preform is typically placed on a mold or other surface capable of providing the cured material with the desired geometry. A preferred fiber, particularly for aerospace applications, is carbon fiber. The fiber reinforcement of the preform is not limited to carbon fiber and may include any suitable fiber having high strength, sufficient stiffness, and relatively low density. The fiber for impregnation may be a fiber in any suitable form including, but not limited to uniaxial, braided, multi-layered, or woven forms. In addition, the fibers may be continuous fibers, chopped fiber, braided fiber, fiber fabric, woven fibers and noncrimp fabric, unitape fiber, fiber film or any suitable form of fiber that results in a reinforced composite material when cured. In addition, multiple types of fibers may be utilized in the preform.

0032An exemplary prepolymer blend may be placed as a film layer or layers on or within intermediate layers of the reinforcing fiber preform to cover all or a majority of the preform. Alternatively, a film material, including the prepolymer blend, may be provided as at least a portion of the preform, wherein the material provided includes fibers onto which the resin blend has been placed into contact. The prepolymer blend resin material may be applied onto the entire surface of the reinforcing fiber preform. Alternatively, the matrix material may be interleaved between layers of the preform to cover all the layers of reinforcing fiber preform. Sufficient prepolymer material is provided to impregnate the preform during a heated resin infusion phase. Typically, the RI method will include placing a barrier layer, such as a polytetrafluoroethylene barrier onto the prepolymer blend and/or prepreg material to assist in controlling the flow of resin. The preform and prepolymer blend may then be placed into a vacuum membrane or similar vacuum providing apparatus. The mold, fiber, resin, barrier layer and vacuum membrane may be placed into an autoclave or other controlled atmosphere device. The precise processing parameters utilized can vary and may depend upon the particular materials used as the first and second prepolymer components in the prepolymer blend.

0033In one embodiment, the temperature and pressure are increased within the autoclave, while simultaneously drawing a vacuum on the vacuum membrane. The increased temperature and vacuum facilitate the infiltration of the resin into the preform. The temperature and vacuum are maintained until the resin has sufficiently impregnated the preform to avoid the formation of voids. After infiltration, the temperature may be increased to begin crosslinking of the prepolymer blend. The specific parameters of the cure cycle vary and depend upon the particular materials used as the first and second prepolymer components in the prepolymer blend.

0034In another embodiment, the polyimide prepolymer blend may be processed using resin transfer molding (RTM). The materials utilized for the fiber reinforcement and the matrix are substantially the same as those used in the discussion of the RI process above. However, in RTM, an injection system is utilized to inject the prepolymer mixture into a mold by pressurization of the prepolymer mixture. The mold, which has the substantial geometry of the finished component, includes the fiber preform. The pressurized prepolymer blend impregnates the dry fibers of the fiber preform and is cured to crosslink the prepolymer mixture and form the final component. The specific parameters of the cure cycle vary and depend upon the particular materials used as the first and second prepolymer components in the prepolymer blend.

0035In some cases, the desired prepolymer blend (i.e., a blend that will provide desired qualities in a composite article) may not be amenable to conventional RTM or RI processing methods. Exemplary embodiments disclosed herein provide methods for obtaining the desired composite article properties, while utilizing RTM or RI techniques.

0036In an exemplary embodiment, the desired prepolymer blend includes first and second prepolymer components, which when suitably combined and cured will provide the desired composite article. For example, embodiments disclosed herein provide for use of the first prepolymer component as a tackifier for an RI or RTM method. For example, the preform structure may be impregnated with a suitable amount of, the first prepolymer component. The first prepolymer component may have a greater molecular weight than desired for conventional RI or RTM processing and thus, standing alone, may not be suitable for use with RI or RTM techniques. However, the first prepolymer component may impart desired qualities to the fully cured polyimide composite article. In order to incorporate the desired qualities into the composite article, while employing RTM or RI techniques, the first prepolymer component is utilized as a tackifier for the preform structure. The first prepolymer component may be a mixture of monomers, a blend of oligomers, or a pre-imidized reaction product.

0037The preform structure, tackified with the first prepolymer component, is then infused with a suitable amount of the second prepolymer component as in conventional RTM or RI processes. The first and second prepolymer components mix during processing and react under suitable reaction conditions to provide a polyimide composite article including a crosslinked polyimide matrix supported by the preform. In this example, the polyimide composite article may exhibit enhanced properties (i.e., Tg, void content, thermal oxidative stability, tensile strength) due to the incorporation of the first prepolymer component. Of course, other combinations of prepolymer components may be utilized following the principles taught herein. In other exemplary embodiments, the first and second prepolymer components may themselves be blends or mixtures of pre-polyimide components.

0038In an exemplary embodiment, a prepolymer blend includes a first prepolymer component comprising at least a first polyimide oligomer having the formula E₁-[R₁]_n-E₁; and a second prepolymer component selected from the group consisting of M₁, a second polyimide oligomer having the formula E₂-[R₂]_n-E₂, and combinations thereof; wherein R₁ and R₂ independently comprise the following structure: <chemistry id="chem0009" num="0009"><img file="EP2075111B1_D0009.tif" /></chemistry> wherein n comprises from about 1 to about 5, wherein V is a tetravalent substituted or unsubstituted aromatic monocyclic or polycyclic linking structure, R is a substituted or unsubstituted divalent organic radical, E₁ and E₂ independently comprise crosslinkable functional groups, and wherein M₁ comprises a mixture of monomeric compounds including a diamine component comprising at least one diamine compound, a dianhydride component comprising at least one dianhydride compound, and an end group component comprising at least one end group compound.

0039Exemplary properties of the prepolymer blend, or the crosslinked polyimide matrix that may be varied include imidization temperature, maximum cure temperature, molecular weight distribution, tack, drape, ability to process using film infusion, ability to process using RTM, ability to modify the prepolymer blend with fillers or other agents, tensile strength, compression strength, inplane shear, and wet properties.

0040In other embodiments, prepolymer blends may include a plurality of preimidized reaction products. The preimidized reaction products may be blended in various ratios to optimize desired outcomes.

0041Using the processes described above, prepolymer blends can be readily tailored to provide desired property outcomes in the blends and the crosslinked matrices.

EXAMPLE

0042A prepolymer mixture was formed from a blend of dimethyl ester of 3,3', 4,4'-benzophenone tetracarboxylic dianhydride ("BTDA"), (4,4-[1,3-phenylene bis (1-methyl-ethylidene)] bisaniline) ("Bis Aniline M"), paraphenylene diamine ("para PDA"), norbornene 2,3-dicarboxylic acid ("NE") and 3,3',4,4'-biphenyl-tetracarboxylic dianhydride (BPDA). The above blend was further mixed with a solid powder second prepolymer component having a reaction product of NE, BTDA, metaphenylene diamine (meta PDA), and Bis-Aniline M.

0043The liquid prepolymer component included the following molar compositional concentrations of monomers: <ul id="ul0003" list-style="none"><li>30 mol% Bis Aniline M,</li><li>12.9 mol% p PDA,</li><li>28.6 mol% NE and</li></ul> varying mol% of BPDA and BTDA, as shown in TABLE 1, wherein the total mol% of the combination of BPDA and BTDA is 28.5 mol%. <tables id="tabl0001" num="0001"><table frame="all"><title><b>TABLE 1</b></title><tgroup cols="6"><colspec colnum="1" colname="col1" colwidth="18mm" /><colspec colnum="2" colname="col2" colwidth="14mm" /><colspec colnum="3" colname="col3" colwidth="14mm" /><colspec colnum="4" colname="col4" colwidth="24mm" /><colspec colnum="5" colname="col5" colwidth="14mm" /><colspec colnum="6" colname="col6" colwidth="14mm" /><thead><row><entry namest="col1" nameend="col6" align="center">MOLAR COMPOSITIONS OF EXAMPLES 1-12</entry></row><row><entry align="center">Example</entry><entry align="center">BTDA</entry><entry align="center">BPDA</entry><entry align="center">Bis Aniline M</entry><entry align="center">p PDA</entry><entry align="center">NE</entry></row></thead><tbody><row><entry align="center" valign="bottom">1</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">2</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">3</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">4</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1 %</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">5</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">6</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">7</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">8</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">9</entry><entry align="center" valign="bottom">24.2%</entry><entry align="center" valign="bottom">4.3%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">10</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">11</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row><row><entry align="center" valign="bottom">12</entry><entry align="center" valign="bottom">21.4%</entry><entry align="center" valign="bottom">7.1%</entry><entry align="center" valign="bottom">30.0%</entry><entry align="center" valign="bottom">12.9%</entry><entry align="center" valign="bottom">28.6%</entry></row></tbody></tgroup></table></tables>

0044A solid powder prepolymer component was added to the liquid monomer mixture in Examples 1-12. The solid powder prepolymer component included a reaction product of the following components: <ul id="ul0004" list-style="none"><li>40mol% NE,</li><li>20 mol% BTDA,</li><li>28 mol% 1,3-phenylene diamine (meta PDA), and</li><li>12 mol% bis-aniline M.</li></ul>

0045The reaction product forming the solid powder prepolymer component was a polyimide oligomer known in the art and is commercially available as a powder. One commercially available prepolymer corresponding to the above polyimide oligomer is MM 9.36 available from Maverick Corporation, Blue Ash, Ohio.

0046As shown in Table 2, the solid powder prepolymer was blended with the liquid monomer prepolymer to form a mixture that has the Molecular Weight ("MW") and the structural unit size ("n") shown in the Examples. Examples 1-6 included a MW of 2100 g/mol and a structural unit size of 3. Examples 7-12 included a MW of 1600 g/mol and a structural unit size of 2. The ratio between BTDA and BPDA was varied as shown in Table 1 and the amount of powder added was varied, as shown in TABLE 2.

0047The mixture was cured at a temperature of about 600 °F (316 °C) and a pressure of 1379 Kpa (200 psi) for 4 hours. The glass transition temperature ("Tg") for the cured Examples are shown in TABLE 3. The cured sample was then subjected to a one of 2 post cures. The first post cure includes exposing the sample to a temperature of about 600 °F (316 °F) at ambient pressure for 12 hours. The Tg values for the first post cured Examples are shown in TABLE 3. The second post cure includes exposing the sample to a temperature of about 625 °F (329 °C) at ambient pressure for 12 hrs. The Tg values for the second post cured Examples are shown in TABLE 3.

0048In addition to the post curing, the samples were also measured for thermal oxidative stability (TOS). The TOS for Examples 1-12 are shown in TABLE 4. Likewise, the compression strength of the samples was measured after subjecting the samples to thermal cycling from room temperature to 550 °F (288 °C) for 380 cycles. The compression data is shown in TABLE 4.

0049As shown in Examples 1, 4, 7 and 10, a lower Tg and a higher TOS weight loss result from the presence of the liquid monomer mixture alone. The mixture of the liquid prepolymer component with the solid prepolymer component resulted in a Tg of greater than about 500 °F (260°C) in the cured state and a thermal oxidative stability having a TOS weight loss of less than 2.0%. In the post cured state, the Tg of Examples reached 600 °F (316 °C) or greater. <tables id="tabl0002" num="0002"><table frame="all"><title><b>TABLE 2</b></title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="46mm" /><colspec colnum="3" colname="col3" colwidth="10mm" /><colspec colnum="4" colname="col4" colwidth="45mm" /><colspec colnum="5" colname="col5" colwidth="48mm" /><thead><row><entry namest="col1" nameend="col5" align="center" valign="middle"><b>TAILORABLE POLYIMIDE RESINS NADIC END CAP</b></entry></row><row><entry align="center" valign="middle"><b>Example</b></entry><entry align="center" valign="middle"><b>Liquid Formulated MW (g/mol)</b></entry><entry align="center" valign="middle"><b>n=</b></entry><entry align="center" valign="middle"><b>Monomer Substitution in Liquid Prepolymer Component**</b></entry><entry align="center" valign="middle"><b>Powder Prepolymer Component Addition</b></entry></row></thead><tbody><row><entry align="center" valign="middle">1</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">0%</entry></row><row><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">15%</entry></row><row><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">30%</entry></row><row><entry align="center" valign="middle">4</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">0%</entry></row><row><entry align="center" valign="middle">5</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">15%</entry></row><row><entry align="center" valign="middle">6</entry><entry align="center" valign="middle">2100</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">30%</entry></row><row><entry align="center" valign="middle">7</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">0%</entry></row><row><entry align="center" valign="middle">8</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">15%</entry></row><row><entry align="center" valign="middle">9</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">15%</entry><entry align="center" valign="middle">30%</entry></row><row><entry align="center" valign="middle">10</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">0%</entry></row><row><entry align="center" valign="middle">11</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">15%</entry></row><row><entry align="center" valign="middle">12</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">25%</entry><entry align="center" valign="middle">30%</entry></row></tbody></tgroup><tgroup cols="5" rowsep="0"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="46mm" /><colspec colnum="3" colname="col3" colwidth="10mm" /><colspec colnum="4" colname="col4" colwidth="45mm" /><colspec colnum="5" colname="col5" colwidth="48mm" /><tbody><row><entry namest="col1" nameend="col5" align="justify">** percent of BTDA substituted by BPDA in liquid Resin MM 9.36 powder resin formulated MW = 936</entry></row></tbody></tgroup></table></tables><tables id="tabl0003" num="0003"><table frame="all"><title><b>TABLE 3</b></title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="37mm" /><colspec colnum="4" colname="col4" colwidth="37mm" /><thead><row><entry namest="col1" nameend="col4" align="center"><b>GLASS TRANSITION TEMPERATURE</b></entry></row><row><entry align="center"><b>Example</b></entry><entry align="center"><b>As Cured Tg (°F) °C</b></entry><entry align="center"><b>Post Cure 1 Tg (°F) °C</b></entry><entry align="center"><b>Post Cure 2 Tg (°F) °C</b></entry></row></thead><tbody><row><entry align="center" valign="bottom">1</entry><entry align="center" valign="bottom">(478) 248</entry><entry align="center" valign="bottom">(530) 277</entry><entry align="center" valign="bottom">(551) 288</entry></row><row><entry align="center" valign="bottom">2</entry><entry align="center" valign="bottom">(501) 261</entry><entry align="center" valign="bottom">(551) 288</entry><entry align="center" valign="bottom">(589) 309</entry></row><row><entry align="center" valign="bottom">3</entry><entry align="center" valign="bottom">(530) 277</entry><entry align="center" valign="bottom">(576) 302</entry><entry align="center" valign="bottom">(595) 313</entry></row><row><entry align="center" valign="bottom">4</entry><entry align="center" valign="bottom">(488) 253</entry><entry align="center" valign="bottom">(531) 277</entry><entry align="center" valign="bottom">(553) 289</entry></row><row><entry align="center" valign="bottom">5</entry><entry align="center" valign="bottom">(500) 260</entry><entry align="center" valign="bottom">(556) 291</entry><entry align="center" valign="bottom">(583) 306</entry></row><row><entry align="center" valign="bottom">6</entry><entry align="center" valign="bottom">(532) 278</entry><entry align="center" valign="bottom">(579) 304</entry><entry align="center" valign="bottom">(606) 319</entry></row><row><entry align="center" valign="bottom">7</entry><entry align="center" valign="bottom">(514) 268</entry><entry align="center" valign="bottom">(552) 289</entry><entry align="center" valign="bottom">(563) 295</entry></row><row><entry align="center" valign="bottom">8</entry><entry align="center" valign="bottom">(520) 271</entry><entry align="center" valign="bottom">(561) 294</entry><entry align="center" valign="bottom">(590) 310</entry></row><row><entry align="center" valign="bottom">9</entry><entry align="center" valign="bottom">(545) 285</entry><entry align="center" valign="bottom">(580) 304</entry><entry align="center" valign="bottom">(606) 319</entry></row><row><entry align="center" valign="bottom">10</entry><entry align="center" valign="bottom">(501) 261</entry><entry align="center" valign="bottom">(552) 289</entry><entry align="center" valign="bottom">(578) 303</entry></row><row><entry align="center" valign="bottom">11</entry><entry align="center" valign="bottom">(516) 269</entry><entry align="center" valign="bottom">(572) 300</entry><entry align="center" valign="bottom">(590) 310</entry></row><row><entry align="center" valign="bottom">12</entry><entry align="center" valign="bottom">(532) 278</entry><entry align="center" valign="bottom">(584) 307</entry><entry align="center" valign="bottom">(609) 321</entry></row></tbody></tgroup></table></tables><tables id="tabl0004" num="0004"><table frame="all"><title><b>TABLE 4</b></title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="56mm" /><colspec colnum="3" colname="col3" colwidth="47mm" /><thead><row><entry align="center" valign="middle" /><entry align="center" valign="middle"><b>THERMAL OXIDATIVE STABILITY</b></entry><entry align="center" valign="middle"><b>COMPRESSION STRENGTH</b></entry></row><row><entry align="center" valign="middle"><b>Example</b></entry><entry align="center" valign="middle"><b>TOS Weight Loss (%)</b></entry><entry align="center" valign="middle"><b>Compression (ksi) MPa</b></entry></row></thead><tbody><row><entry align="center" valign="middle">1</entry><entry align="center" valign="middle">4.83</entry><entry align="center" valign="middle">(56.95) 392.66</entry></row><row><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">1.42</entry><entry align="center" valign="middle">(89.75) 618.80</entry></row><row><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">1.62</entry><entry align="center" valign="middle">(78.94) 544.27</entry></row><row><entry align="center" valign="middle">4</entry><entry align="center" valign="middle">2.23</entry><entry align="center" valign="middle">(78.87) 543.79</entry></row><row><entry align="center" valign="middle">5</entry><entry align="center" valign="middle">1.39</entry><entry align="center" valign="middle">(85.16) 587.16</entry></row><row><entry align="center" valign="middle">6</entry><entry align="center" valign="middle">1.84</entry><entry align="center" valign="middle">(75.67) 521.73</entry></row><row><entry align="center" valign="middle">7</entry><entry align="center" valign="middle">2.8</entry><entry align="center" valign="middle">(90.57) 624.46</entry></row><row><entry align="center" valign="middle">8</entry><entry align="center" valign="middle">1.54</entry><entry align="center" valign="middle">(94.09) 648.73</entry></row><row><entry align="center" valign="middle">9</entry><entry align="center" valign="middle">1.91</entry><entry align="center" valign="middle">(92.9) 640. 52</entry></row><row><entry align="center" valign="middle">10</entry><entry align="center" valign="middle">1.25</entry><entry align="center" valign="middle">(97.76) 674.03</entry></row><row><entry align="center" valign="middle">11</entry><entry align="center" valign="middle">1.44</entry><entry align="center" valign="middle">(98.19) 676.99</entry></row><row><entry align="center" valign="middle">12</entry><entry align="center" valign="middle">1.67</entry><entry align="center" valign="middle">(91.61) 631.63</entry></row></tbody></tgroup></table></tables>

0050An optimized resin blend may include, in terms of molar ratio, about 2 (end group component):1.35 BTDA:0.35 BPDA:1.26 phenylene diamine (mPDA and pPDA):1.44 BisM. In an exemplary embodiment, the molar ratio may be 2 NE:1.35 BTDA:0.35 BPDA:0.42 mPDA:0.84 pPDA: 1.44 BisM. It is envisioned that other end capping groups may be successfully utilized in this and other exemplary formulations.

0051In an exemplary embodiment, some or all of the Bis M may be substituted by bis amino phenoxy benzene (APB). The Bis M may be substituted 1 for 1, maintaining the remaining molar ratios. In an exemplary embodiment, it may be desirable to increase the molar ratio of a phenylene diamine (mPDA, pPDA, or both) upon substitution of APB for Bis M. An exemplary molar ratio formulation includes about 2 NE: about 1.35 BTDA: about 0.35 BPDA: about 1.26 total (mPDA and pPDA): about 1.44 (Bis-M, APB or APB and Bis-M). In an exemplary embodiment, with a substitution of at least some of the Bis M with APB, an exemplary molar ratio formulation includes about 2 NE: about 1.35 BTDA: about 0.35 BPDA: about 1.2 total (mPDA and pPDA): about 1.5 (APB or APB and Bis M). An exemplary molar ratio includes about 2 NE: about 1.35 BTDA: about 0.35 BPDA: about 1.7 total (mPDA and pPDA): about 1.0 (APB or APB and Bis M).

0052The molar ratio of total phenylene diamine (mPDA and pPDA) to APB may be in the range of from about 1.2:1.5 to about 1.7:1.0. An increase in the molar ratio of total phenyl diamine to APB may be utilized to maintain the Tg of the cured polyimide matrix with respect to a comparable polyimide matrix formed from a prepolymer blend without APB substitution.

0053In an exemplary embodiment a tailorable polyimide prepolymer blend includes the end group component (e.g., NE), the dianhydride component (e.g., BTDA and BPDA) and the diamine component (e.g., mPDA, pPDA, and APB or APB and Bis-M). Within the diamine component, the molar ratio of total (mPDA and pPDA) to (APB or APB and Bis-M) is in the range of about 1.2-1.7 (mPDA and pPDA) : about 1.0 - 1.5 (APB or APB and Bis-M).

0054In other exemplary embodiments, the molar ratio of the end group component and/or the dianhydride component may also be varied to provide the desired tailorable properties of the prepolymer blend, the cure polyimide matrix, or both.

0055In an exemplary embodiment, a tailorable prepolymer blend has a molecular weight of between about 1,100 to about 2,100 g/mol. In an exemplary embodiment, a tailorable prepolymer blend has a molecular weight of between about 1,200 to about 1,600 g/mol.

0056Exemplary prepolymer components for use in RTM processes preferably have molecular weights of less than about 1000 g/mol. However, the desired high temperature properties may not be attained from such prepolymer components alone. To obtained the desired properties in the cured composite structure, the lower molecular weight component is mixed with another, higher molecular weight component.

0057In an exemplary embodiment, a preform structure is impregnated with the higher molecular weight component. During the RTM process, the lower molecular weight component contacts and mixes with the higher molecular weight component to form a suitable resin system. When cured, the composite article exhibits properties attained by utilizing the higher molecular weight component.

0058In an exemplary embodiment, the first and second prepolymer components may independently comprise a blend of two or more resins, a monomeric mixture; monomers and oligomers, and any combination thereof.

0059In an exemplary embodiment, the end cap component, the dianhydride component, and the diamine component are present in respective amounts such that, prior to cure, the prepolymer blend provides at least one predetermined prepolymer blend property, and when cured under suitable cure conditions, the prepolymer blend provides a crosslinked polyimide matrix having at least one predetermined crosslinked matrix property.

0060For example, in an exemplary embodiment, the predetermined prepolymer blend property may be selected from a melt viscosity of the prepolymer blend (between about 1,000-20,000 cps); a molecular weight (between about 1,100 to about 2,100 g/mol); a maximum cure temperature (about 343°C (650 °F)); suitable tack and/or drape for prepreg composites; processibility using RFI (with pressure at or below 1379KPa (200 psi) and temperatures at or below about 343°C (650 °F); processibility using RTM (with pressures at or below 1379KPa (200 psi) and temperatures at or below about 343°C (650 °F)), and combinations thereof.

0061Further, in an exemplary embodiment, the predetermined crosslinked matrix property may be selected from a thermal oxidative stability (less than 4% weight loss when exposed to 291°C (555 °F) and 344.1KPa (50 psi) for 1000 hours); a glass transition temperature (at least about 232°C (450 °F) or at least about 214°C (525 °F)) a void content (less than about 3%), room temperature tensile strength (at least about 689.5MPa (100 ksi)); room temperature compression strength (at least about 551.6 (MPa (80 ksi)); room temperature inplane shear (at least about 55.16KPa (8 ksi)), and combinations thereof.

0062In an exemplary embodiment, an article is formed from any of the exemplary tailorable polyimide prepolymer blends. The article may be a powder, a neat resin, a coating material, a film, an adhesive, a fiber, a composite, a laminate, a prepreg, a part, and combinations thereof.

0063Thus, embodiments disclosed herein provide processes suitable for forming polyimide composite articles. Exemplary processes include resin transfer molding (RTM) and resin infusion (RI). In an exemplary embodiment, the molar ratios of the various components may be varied to provide melt viscosities and molecular weights to provide RTM or RI processible prepolymer blends. In an exemplary process, a preform structure including reinforcing materials is tackified with a first prepolymer component of the polyimide resin system. A second prepolymer component of the polyimide resin system is forced into contact with the tackified preform. During the cure process, the first and second prepolymer components of the polyimide system intermingle and react to form the final crosslinked resin matrix.