High-Temperature Surface Films for Composite Substrates

Abstract

A high temperature surface film formed from a curable resin composition containing: (i) at least one bismaleimide (BMI) monomer; (ii) at least one comonomer reactive with the BMI monomer; (iii) a pre-reacted adduct that enhances film forming properties and improves toughness; (iv) inorganic microspheres; and (v) a flow modifier in the form of a particulate inorganic filler that is not microspheres.

Description

[Technical Field]

[0001] Fiber-reinforced polymer matrix composites (PMCs) are high-performance structural materials commonly used in applications requiring resistance to aggressive environments, high strength, and / or light weight. Examples of such applications include aircraft components (e.g., tails, wings, fuselages, and propellers), high-performance automobiles, ship hulls, and bicycle frames. Composite structural parts for aerospace applications typically include a surface film to impart required performance characteristics to the composite structure before painting. Such surface films are used to improve the surface quality of structural parts while reducing labor, time, and cost. The surface film is typically co-cured with the PMC material during the manufacturing of the structural part. Such surface films are also commonly combined with lightning strike protection (LSP) materials to provide an integrated surface and LSP solution.

[0002] Currently, the majority of surface films for aerospace applications are formed from epoxy-based resin compositions containing epoxy resin as the thermosetting resin component. Such epoxy-based surface films can be cured together with the epoxy-based composite substrate at temperatures ranging from 121°C to 176°C (i.e., 250°F to 350°F). The resulting epoxy-based surface films typically have a glass transition temperature (T) of about 350°F (about 176°C) or less. g After being cured together, the resulting cured surface film eliminates surface defects such as pinholes and pits on the composite substrate.

[0003] Surfaces for aerospace applications typically contain epoxy resins because they offer excellent mechanical properties, allow for a wide operating temperature range, and are easy to apply during part manufacturing. The maximum operating temperature is defined as the highest temperature at which a material can be used for an extended period of time without significant changes in properties or degradation. Epoxy-based surfaces cannot be used in extreme environments, such as high-temperature applications above approximately 180°C. Therefore, their maximum operating temperature of approximately 180°C precludes their use in the high-temperature applications common in aerospace applications.

[0004] Bismaleimide (BMI) resins have been used in aerospace applications where service temperatures exceeding the capabilities of epoxy resins are required. Current BMI-based composites can be used at service temperatures ranging from 149°C to 265°C and provide excellent mechanical properties, such as resistance to microcracking, over long periods of time at such temperatures. BMI resins can be initially cured at relatively low temperatures (e.g., 350°F or 176°C) and then post-cured at higher temperatures (e.g., 450°F to 510°F or 232°C to 265°C) to complete the polymerization reaction and produce a highly cross-linked network with a high glass transition temperature. There remains a need for surface materials that can be co-cured with BMI-based composites. Such surface materials must be able to withstand the post-cure conditions of BMI resins.

[0005] Disclosed herein is a surfacing film for PMCs that can be used at high service temperatures exceeding 176°C and can even be co-cured with high temperature thermoset composites, such as BMI-based composites. This surfacing film is referred to herein as the "HT surfacing film." DETAILED DESCRIPTION OF THE INVENTION

[0006] The HT surface film is formed from a curable resin composition containing (i) at least one bismaleimide (BMI) monomer, (ii) at least one comonomer reactive with the BMI monomer, (iii) a pre-reacted adduct that enhances film-forming properties and improves toughness, (iv) inorganic microspheres or microballoons to improve the surface smoothness of the film, and (v) a flow modifier in the form of a particulate inorganic filler that is not microspheres or microballoons. The curable resin composition may further include a pigment or dye to impart color to the HT surface film.

[0007] The combination of BMI monomer and comonomer comprises, in weight percent, greater than 45%, and in some embodiments greater than 50%, of the total weight of the curable resin composition. In a preferred embodiment, the curable resin composition does not contain any epoxy resin other than the epoxy resin used to form the pre-reacted adduct.

[0008] After curing, the cured HT surface film has a glass transition temperature (T) above 176°C, e.g., 270°C to 300°C (i.e., 518°F to 572°F). g The cured HT surface film can be used at service temperatures above 176°C (i.e., above 350°F), for example, from 200°C (392°F) to 265°C (510°F).

[0009] As used herein, the terms "cure" and "curing" refer to the irreversible solidification of a prepolymer material or resin precursor caused by heating at elevated temperatures, ultraviolet and radiation exposure, or chemical additives. The term "curable" means capable of being cured into a solidified material.

[0010] Bismaleimide (BMI) BMI monomers are generally prepared by the reaction of maleic anhydride or substituted maleic anhydride with an appropriate diamine. Both aromatic and aliphatic diamines are suitable for preparing BMI. The curable resin composition of the HT surface film can contain either aromatic or aliphatic BMI monomers.

[0011] Eutectic mixtures of two or more different bismaleimide monomers may be used, whereby the melting point of the bismaleimide component is significantly reduced compared to the melting points of the individual bismaleimide monomers.

[0012] Suitable BMI monomers include, but are not limited to, 4,4-bismaleimido-diphenylmethane (BMI-H), 2,2-bismaleimidotoluene m-xylylene bismaleimide (MXBI), p-xylylene bismaleimide, 1,6-hexamethylenediamine bismaleimide (HMDA-BMI), 1,2-bis(maleimido)ethane, 1,4-di(maleimido)butane, 2,2-bis[4-(4-maleimidophenoxy)-phenyl]propane, N,N'-(1,3-phenylene)dimaleimide, N,N'-(1,4-phenylene)dimaleimide, 4,4-bismaleimidodiphenyl ether, bismaleimide methyl ether, bis(4-maleimidophenyl)sulfone, N,N'-4,4'-3,3'-dichloro-diphenylmethane-bismaleimide, and N-phenylmaleimide.

[0013] Comonomers The bismaleimide monomers are reacted with comonomers to improve processability, impart toughness, and form highly crosslinked networks. Suitable comonomers are selected from allyl compounds, aromatic amines, and propenyl benzophenones (or propenyl phenol ether compounds).

[0014] Suitable allyl compounds are characterized by the presence of two or more allyl or methallyl groups per aromatic ring nucleus. Preferred allyl compounds include 2,2'-diallyl bisphenol A or o,o'-diallyl bisphenol A (commercially available from Huntsman as Matrimid® 5292B and from Evonik Corp. as Compimide® TM124), diallyl ethers of bisphenol A, 2,2'-diallyl-4,4'-biphenol, 3'-allyl-4'hydroxyacetophenone, diallyl phthalate, triallyl isocyanurate, triallyl cyanurate, and triallyl trimellitate.

[0015] Preferred aromatic amines include 4,4'-methylenedianiline (MDA), 4,4'-diaminodiphenyl sulfone (DDS), m- or p-phenylenediamine (PD).

[0016] Propenylbenzophenones (or propenylphenol ether compounds) are liquids at room temperature and are characterized by having two or more propenylphenoxy groups per aromatic ring nucleus. An example of a suitable propenylbenzophenone is 4,4'-bis(o-propenylphenoxy)benzophenone (commercially available as Compimide® TM123 from Evonik Corp.), which reacts with BMI to form a tough, heat-resistant polymer network.

[0017] Pre-reacted adduct One drawback of BMI resins is their brittleness. The presence of the pre-reacted adduct improves the film-forming properties of BMI-based curable compositions and the flexibility (or drapeability) of BMI-based films. The presence of the pre-reacted adduct also improves the toughness and resistance to microcracking of the cured BMI polymer.

[0018] In one embodiment, the pre-reacted adduct is the reaction product of a difunctional epoxy resin, epoxydicyclopentadiene (DCPD), and an elastomer. In another embodiment, the pre-reacted adduct is the reaction product of core-shell rubber (CSR) particles, one or more multifunctional epoxy resins, epoxydicyclopentadiene (DCPD), and an elastomer. Multifunctional epoxy resins include difunctional, trifunctional, and tetrafunctional epoxy resins. The reaction is preferably carried out in the presence of an accelerator, such as triphenylphosphine.

[0019] Multifunctional epoxy resins refer to polyepoxides with two or more epoxy functional groups per molecule, difunctional epoxy resins refer to polyepoxides with two epoxy functional groups per molecule, trifunctional epoxy resins refer to polyepoxides with three epoxy functional groups per molecule, and tetrafunctional epoxy resins refer to polyepoxides with four epoxy functional groups per molecule.

[0020] Difunctional epoxy resins suitable for forming the pre-reacted adduct include diglycidyl ethers of bisphenol A (e.g., Hexion's Epon™ 828 (liquid epoxy resin), Dow Chemical Co.'s DER331, DER661 (solid epoxy resins), Huntsman Advanced Materials' Tactix 123 and Araldite® 184). Additional difunctional epoxy resins for forming the pre-reacted adduct can include diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol S, diglycidyl ethers of bisphenol Z, diglycidyl ethers of tetrabromobisphenol A, and diepoxides of hydrogenated bisphenol A.

[0021] Suitable trifunctional epoxy resins for forming the pre-reacted adduct include triglycidyl ethers of aminophenols. Specific examples of commercially available trifunctional epoxy resins are Araldite® MY0510, MY0500, MY0600, and MY0610, supplied by Huntsman Advanced Materials.

[0022] Suitable tetrafunctional aromatic epoxy resins are polyepoxides having at least one glycidylamine group, examples of which have the following general chemical structure: [ka] It is a tetraglycidyl ether of methylenedianiline having the formula:

[0023] The amine groups in the structures are shown in the para- or 4,4' position of the aromatic ring structure; however, it should be understood that other isomers such as 2,1', 2,3', 2,4', 3,3', 3,4', etc. are possible alternatives. Examples of commercially available tetrafunctional epoxy resins are Araldite® MY9663, MY9634, MY9655, MY-721, MY-720, MY-725 supplied by Huntsman Advanced Materials.

[0024] Epoxydicyclopentadiene (DCPD) is an epoxy resin based on dicyclopentadiene, a multifunctional hydrocarbon epoxy novolac, with the following chemical formula / structure: [ka] (In the formula, [ka] where n=1 to 3).

[0025] The CSR particles for forming the pre-reacted adduct may have a particle size of 300 nm or less. Particle size can be measured, for example, by laser diffraction using a Malvern Mastersizer 2000 instrument. The CSR particles may be either core-shell particles, in which a soft core is surrounded by a hard shell. Preferred CSR particles are those having a polybutadiene rubber core or a butadiene-acrylonitrile rubber core and a polyacrylate shell. However, CSR particles with a hard core surrounded by a soft shell can also be used. The CSR particles may be supplied as a suspension containing 25% to 40% by weight of CSR particles dispersed in a liquid epoxy resin, such as Kane Ace™ MX120, MX125, or MX156 (25% to 37% by weight of CSR particles in DER™ 331 epoxy resin), available from Kaneka.

[0026] The elastomer for forming the pre-reacted adduct is preferably an elastomeric polymer having carboxyl or amine functionality. Suitable elastomers for forming the pre-reacted adduct include, but are not limited to, rubbers such as amine-terminated butadiene acrylonitrile (ATBN), carboxyl-terminated butadiene acrylonitrile (CTBN), carboxyl-terminated butadiene (CTB), fluorocarbon elastomers, silicone elastomers, styrene-butadiene polymers, and the like. In one embodiment, the elastomer used to form the pre-reacted adduct is CTBN or CTB.

[0027] In one embodiment, the pre-reacted adduct is formed by reacting a difunctional epoxy resin and epoxydicyclopentadiene (DCPD) with an elastomeric polymer (preferably one containing carboxyl or amine groups) in the presence of a catalyst such as triphenylphosphine (TPP) at about 300°F (i.e., 148.9°C) to chain link the epoxy resin and elastomer and form a high viscosity, film-forming, high molecular weight epoxy-based pre-reacted adduct. The pre-reacted adduct is then mixed with the remaining components of the curable composition to form the HT surface film.

[0028] In another embodiment, the pre-reacted adduct is formed by reacting a suspension of CSR particles in a liquid difunctional epoxy resin with DCPD, an elastomer, and optionally a tri- or tetra-functional epoxy resin in the presence of the catalyst described above. The presence of the CSR particles imparts additional toughness to the cured surface film. Such increased toughness is advantageous when dealing with inherently brittle BMI resins. The presence of the tri- or tetra-functional epoxy resin improves the strength of the cured surface film, increasing its T g will rise further.

[0029] The amount of pre-reacted adduct in the curable composition is about 8% to 30% by weight based on the total weight of the curable composition.

[0030] inorganic microspheres Microspheres or microballoons are added to the curable composition to improve the surface smoothness of the surface film. The microspheres are small, spherical, hollow bodies. Each microsphere has an outer shell surrounding a hollow core. Inorganic microspheres can be made from a variety of materials, including glass, silica (SiO2), and ceramic. Microspheres having diameters ranging from about 0.1 μm to about 20 μm, preferably from about 1 μm to about 15 μm, have been found to be particularly suitable.

[0031] In a preferred embodiment, the inorganic microspheres are hollow ceramic microspheres, such as microspheres made from an inert silica-alumina ceramic material. The ceramic microspheres may have a crush strength of greater than 60,000 psi, a dielectric constant of about 3.7 to 4.6, a softening point in the range of 1000 to 1100°C (i.e., 1832 to 2012°F), and a particle size in the range of 0.1 to 50 microns, or 1 to 50 microns. The high softening point of ceramic microspheres allows them to be non-absorbent to solvents, flame-retardant, and highly chemically resistant. Examples of commercially available ceramic microspheres particularly suitable for use in surface film compositions are sold by Zeelan Industries, Inc. under the trade name Zeeospheres®, e.g., G-200, G210, and W-200. These are thick-walled, odorless, and light gray hollow silica-alumina spheres.

[0032] The amount of inorganic microspheres is at least 3 wt % based on the total weight of the curable composition. For example, the amount of ceramic microspheres in weight percent can be in the range of about 5 wt % to about 15 wt %, or about 10 wt % to about 30 wt %, or about 20 wt % to about 40 wt % based on the total weight of the curable composition.

[0033] Flow adjuster To control the flow of the resin composition and prevent the components therein from agglomerating, inorganic fillers in the form of particles other than inorganic microspheres or microballoons may be added to the curable composition as flow control agents or rheology-adjusting components. Filler particles include powders and particles of any shape. Suitable inorganic fillers that can be used in the curable composition include talc, mica, calcium carbonate, alumina, and silica. In one embodiment, hydrophobic fumed silica (e.g., Cab-O-Sil TS-720) is used as the inorganic filler. The amount of flow control agent can be in the range of about 0.5 wt % to about 5 wt %, based on the total weight of the curable resin composition.

[0034] Optional Additives Pigments and / or dyes known in the art for imparting color to resin systems may be added to the curable composition. Examples of pigments and / or dyes include, but are not limited to, red iron oxide, green chromium, carbon black, and titanium dioxide. In one embodiment, titanium dioxide (white) pigment is added to the resin composition. In another embodiment, carbon black pigment is added. Such pigments / dyes may be added in amounts of 0.5 to 10 wt. % based on the total weight of the curable composition.

[0035] Illustrative Embodiments Some embodiments of the curable compositions for forming the HT surface films of the present disclosure are provided below.

[0036] [Table 1]

[0037] The amounts in the above embodiments are weight percent (wt %) based on the total weight of the entire composition.

[0038] In one embodiment, the pre-reactant is formed by reacting the following components in weight percent (wt%): 1 to 5 wt. % of a carboxylated nitrile elastomer; 1 to 5 wt. % CTBN or CTB elastomer; 1-15 wt% epoxydicyclopentadiene (DCPD); 5 to 15% by weight of bisphenol A diglycidyl ether; 1-2 wt % of a promoter (preferably triphenylphosphine).

[0039] In another form, the pre-reactant is formed by reacting the following components in weight percent (wt%): 1 to 5 wt. % of a carboxylated nitrile elastomer; 5-15 wt% liquid difunctional epoxy resin (e.g., MX120, MX156) containing 25-40 wt% CSR particles; 1-15 wt% epoxydicyclopentadiene (DCPD); 1 to 15% by weight of a trifunctional epoxy resin (e.g., MY510) or a tetrafunctional epoxy resin (e.g., MY721, MY9663); 1-2 wt % of a promoter (preferably triphenylphosphine).

[0040] Manufacturing method and uses To form the surface film, the components of the curable composition are placed in a mixing vessel and blended at room temperature (23°C to 25°C) using a solution process. One or more organic solvents may be added to facilitate component mixing and film formation. Possible solvents include methyl ethyl ketone (MEK), acetone, N-methylpyrrolidone (NMP), ethanol, dioxalane, and propylene carbonate. The surface film is then formed from the curable composition using conventional film-forming processes. The resulting surface film may have a basis weight of 0.01 to 0.045 psf (i.e., 48 gsm to 220 gsm), depending on the intended use.

[0041] To facilitate handling of the surface film, a carrier can be embedded in the film, which can be selected from fiber sheets made from thermoplastic polymer fibers or carbon fibers, metal screens or foils, nonwoven mats, random mats, knitted carriers, metal-coated carbon veils, etc.

[0042] The surface film may be combined with a conductive layer to provide lightning strike protection (LSP). The conductive layer may be selected from a variety of expanded metal screens or foils for use as a surface and lightning strike protection material for aircraft composite parts. The metal screens or foils may include expanded metal screens or foils and metalized veils.

[0043] The curable composition for forming the HT surface film can be applied to one or both sides of the conductive layer using conventional coating techniques to form a two-layer or three-layer structure, respectively. Alternatively, a prefabricated surface film can be laminated to one side of the conductive layer to form a two-layer structure, or two prefabricated surface films can be laminated to opposite sides of the conductive layer to form a three-layer structure. The conductive layer can also be embedded in the surface film.

[0044] The HT surface films disclosed herein can be co-cured with BMI-based fiber reinforced composite substrates at temperatures ranging from 300°F to 380°F (i.e., 148°C to 193°C). For BMI-based composite substrates, a post-cure is required to impart high temperature properties. This post-cure can be carried out at temperatures ranging from above 350°F (176.66°C) to 510°F (265°C).

[0045] BMI-based fiber-reinforced composite substrates are composed of reinforcing fibers impregnated or embedded in a matrix resin. The matrix resin includes one or more BMI resins and an optional epoxy resin. The composite substrates may be in the form of prepreg plies or prepreg lay-ups. Prepreg plies are composed of reinforcing fibers in the form of resin-impregnated cloth or directionally aligned continuous fibers. The directionally aligned fibers may be unidirectional or multidirectional. Prepreg lay-ups are composed of multiple prepreg plies arranged in a stacking order.

[0046] Typically, an uncured HT surface film is applied to an uncured or partially cured fiber-reinforced composite substrate and then cured together to form a fully cured composite structure bonded to the thermoset (hardened) surface film. After curing, the surface film becomes the outermost layer of the composite structure.

[0047] The resulting surface film is highly crosslinked and has a high T gThe cured surfacing film provides a paintable surface that is free of surface defects such as pinholes, pits, or porosity. If the surfacing film is additionally combined with a metal screen, foil, or metallized veil, the surface will also have sufficient lightning strike protection properties.

[0048] The HT surface film of the present disclosure has a higher glass transition temperature (T) compared to current epoxy-based surface films currently available on the market. g ) has the T of existing epoxy-based surface films. g is about 180°C, whereas T exceeds 215°C (e.g., up to 300°C) depending on the BMI comonomer used. g is achievable.

[0049] The thermal stability of the cured HT surface film, as measured by thermogravimetric analysis (TGA), defined as the temperature at which 5% weight loss occurs, was found to be between 330°C and 390°C, compared to the thermal stability of current commercially available epoxy-based surface films, defined as the temperature at which 5% weight loss occurs, which is approximately between 270°C and 285°C. [Example]

[0050] The following examples illustrate specific embodiments of HT surface films formed in accordance with the present disclosure, but are not intended to limit the scope of the disclosure.

[0051] Example 1 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 1. The amounts shown in Table 1 are in weight percent (wt%) based on the total weight of the entire composition. Table 1A shows the ingredients for forming the pre-reacted adduct of Table 1. The ingredients shown in Table 1A are pre-reacted to form the pre-reacted adduct before being formulated into the curable resin composition of Table 1. The amounts in Table 1A are shown in weight percent (wt%) based on the total weight of all components of the adduct.

[0052] In Table 1, BMI-H represents N,N'-(4,4'-diphenylmethane)bismaleimide.

[0053] [Table 2]

[0054] [Table 3]

[0055] The pre-reacted adduct was prepared by mixing the ingredients in Table 1A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0056] The resin compositions were prepared by placing the ingredients listed in Table 1 into a mixing vessel and mixing the ingredients using a high-shear laboratory mixer. The BMI-H resin and diallyl comonomer were added first. MEK was added as a solvent to the BMI resin and comonomer mixture to adjust the rheology and solids content of the mixture. The pre-reacted adduct was then added to the mixing vessel. Zeeospheres, fumed silica, and carbon black were also added to the mixer. Additional MEK solvent was added to control the viscosity of the composition to approximately 90% solids by weight. The composition ingredients were mixed at 2000 rpm for 50 minutes. The temperature of the composition was maintained at 75°F (23°C) during mixing. Additional MEK was added to achieve a solids content of 90% by weight.

[0057] To form a surface film, the prepared resin composition was filtered, degassed, and deposited as a resin film. Filtration was performed through a nylon mesh. Degassing was performed so that the solids content of the composition was approximately 90% by weight. The filtered and degassed composition was then coated on a film coater as a film with a film weight of 0.020 psf (i.e., 97.6 gsm), and then dried to a volatile content of less than 1%.

[0058] Example 2 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 2. The formulations of the pre-reacted adducts are disclosed in Table 2A, where the amounts are given in weight percent (wt%).

[0059] [Table 4]

[0060] [Table 5]

[0061] The pre-reacted adduct was prepared by mixing the ingredients in Table 2A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0062] A resin composition was prepared by placing the ingredients disclosed in Table 2 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 85% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.045 psf (i.e., 220 gsm).

[0063] Example 3 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 3. The formulations of the pre-reacted adducts are disclosed in Table 3A, where the amounts are given in weight percent (wt%).

[0064] [Table 6]

[0065] [Table 7]

[0066] A pre-reacted adduct was prepared by mixing the ingredients in Table 3A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0067] A resin composition was prepared by placing the ingredients disclosed in Table 3 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 95% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.015 psf (i.e., 73 gsm).

[0068] Example 4 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 4. The formulations of the pre-reacted adducts are disclosed in Table 4A, where the amounts are given in weight percent (wt%).

[0069] [Table 8]

[0070] [Table 9]

[0071] The pre-reacted adduct was prepared by mixing the ingredients in Table 4A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0072] A resin composition was prepared by placing the ingredients disclosed in Table 4 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 95% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.030 psf (i.e., 146 gsm).

[0073] Example 5 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 5. The formulations of the pre-reacted adducts are disclosed in Table 5A, where the amounts are given in weight percent (wt%).

[0074] [Table 10]

[0075] [Table 11]

[0076] A pre-reacted adduct was prepared by mixing the ingredients in Table 5A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0077] A resin composition was prepared by placing the ingredients disclosed in Table 5 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 90% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.010 psf (i.e., 49 gsm).

[0078] Example 6 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 6. The formulations of the pre-reacted adducts are disclosed in Table 6A, where the amounts are given in weight percent (wt%).

[0079] [Table 12]

[0080] [Table 13]

[0081] A pre-reacted adduct was prepared by mixing the ingredients in Table 6A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0082] A resin composition was prepared by placing the ingredients disclosed in Table 6 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 90% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.040 psf (i.e., 195 gsm).

[0083] Example 7 Curable resin compositions for forming HT surface films were prepared based on the formulations shown in Table 7. The formulations of the pre-reacted adducts are disclosed in Table 7A. The amounts in the table are given in weight percent (wt%).

[0084] [Table 14]

[0085] [Table 15]

[0086] In Table 7, HMDA BMI stands for 1,6-hexamethylenediamine bismaleimide.

[0087] A pre-reacted adduct was prepared by mixing the ingredients in Table 7A and heating the mixture to 300°F (ie, 148.9°C) for 1 hour.

[0088] A resin composition was prepared by placing the ingredients disclosed in Table 7 into a mixing vessel and mixing the ingredients using a high shear laboratory mixer. The mixing conditions were as described in Example 1. The resulting resin composition after mixing had a solids content of 85% by weight. A resin film was formed from the resin composition by the film-forming method described in Example 1. The dried resin film had a film weight of 0.020 psf (i.e., 98 gsm).

[0089] Example 8 Properties of the cured surface film Each resin film made in Examples 1-7 was spread into a mold and cured using the following autoclave cure cycle: ramp 3°F / min (approximately 2°C / min) to 350°F (176.7°C), hold at 350°F for 360 minutes, then post-cure at 440°F (226.7°C) for 360 minutes.

[0090] The T of each cured resin sample was measured using a TA Instruments TMA Q400 at a heating rate of 10°C / min from room temperature to 350°C. g Thermomechanical analysis (TMA) was used to determine the thermal stability. Thermogravimetric analysis (TGA) was performed on each cured resin sample using a TGA Q50 (TA Instruments) to determine thermal stability at a temperature ramp of 10°C / min up to 500°C. Thermal stability was defined as a 5% weight loss of the cured material. The TMA and TGA results are shown in Table 8.

[0091] [Table 16]

[0092] The results in Table 8 demonstrate the high temperature properties and stability of the cured BMI-based surface films. These properties support their suitability for use at high temperatures (>180°C) and in harsh environments. The cured resins of Examples 1-7 exhibited T values ranging from 227.53°C to 310.10°C. g These values are typical T values of conventional epoxy-based surface film materials.g Such conventional epoxy-based surface film materials typically have a T of about 177°C or less. g The T of BMI-based surface film g The thermal stability also demonstrates the ability of these films to be cured with BMI-based composites, which can have post-cure temperatures of 275°C or higher. TGA, specifically the 5% weight loss of the cured material, is also much higher than conventional epoxy-based surfacing films, typically occurring at about 280°C to about 290°C. As a result, the BMI-based surfacing films of Examples 1-7 can more easily withstand thermal cycling in higher temperature environments.

Claims

1. A surface film formed from a curable resin composition, a) At least one bismaleimide (BMI) monomer, b) At least one comonomer having reactivity with a BMI monomer, c) Pre-reaction adduct and, d) an inorganic microsphere or microballoon, each having a hollow core, e) Inorganic particles that are not microspheres or microballoons, Includes, A surface film in which the preliminary reaction adduct is a reaction product of a bifunctional epoxy resin, epoxy dicyclopentadiene (DCPD), and one or more elastomers.

2. A surface film formed from a curable resin composition, a) At least one bismaleimide (BMI) monomer, b) At least one comonomer that is reactive with a BMI monomer, and (iii) c) Pre-reaction adduct and, d) Inorganic microspheres, each having a hollow core, e) Inorganic fillers in the form of particles other than microspheres, Includes, A surface film in which the preliminary reaction adduct is a reaction product of core-shell rubber (CSR) particles, one or more polyfunctional epoxy resins, epoxy dicyclopentadiene (DCPD), and one or more elastomers.

3. The surface film according to claim 2, wherein the preliminary reaction adduct is a reaction product of core-shell rubber (CSR) particles, a bifunctional epoxy resin, dicyclopentadiene (DCPD), an elastomer, and an optionally trifunctional or tetrafunctional epoxy resin.

4. The surface film according to claim 1 or 2, wherein the comonomer is selected from allyl compounds, aromatic amines, and propenylbenzophenone.

5. The surface film according to claim 4, wherein the comonomer is selected from 2,2'-diallylbisphenol A or o,o'-diallylbisphenol A; diallyl ether of bisphenol A; 2,2'-diallyl-4,4'-biphenol; 3'-allyl-4'hydroxyacetophenone; diallyl phthalate; triallyl isocyanurate; triallyl cyanurate; triallyl trimellitate; 4,4'-methylenedianiline (MDA), 4,4'-diaminodiphenylsulfone (DDS), m- or p-phenylenediamine (PD); and 4,4'-bis(o-propenylphenoxy)benzophenone.

6. The surface film according to claim 1 or 3, wherein the bifunctional epoxy resin is preferably selected from diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol S, diglycidyl ether of bisphenol Z, diglycidyl ether of tetrabromobisphenol A, and diepoxide of hydrogenated bisphenol A, and more preferably from diglycidyl ether of bisphenol A or F.

7. The surface film according to claim 3, wherein a trifunctional epoxy is included in the reaction to form a preliminary reaction adduct, and the trifunctional epoxy resin is an aminophenol triglycidyl ether.

8. The surface film according to claim 3, wherein a tetrafunctional aromatic epoxy resin is included in the reaction to form a preliminary reaction adduct, and the tetrafunctional aromatic epoxy resin is a tetraglycidyl ether of methylenedianiline.

9. The surface film according to claim 2, wherein the CSR particles for forming the pre-reaction adduct have a particle size of 300 nm or less as measured by laser diffraction, and preferably each CSR particle has a polybutadiene rubber core or a butadiene-acrylonitrile rubber core and a polyacrylate shell.

10. The surface film according to claim 1 or 2, wherein the elastomer for forming the pre-reaction adduct contains a carboxyl group or an amine functional group.

11. The surface film according to claim 1 or 2, wherein the elastomer for forming the pre-reaction adduct is selected from amine-terminated butadiene acrylonitrile (ATBN), carboxyl-terminated butadiene acrylonitrile (CTBN), carboxyl-terminated butadiene (CTB), fluorocarbon elastomer, silicone elastomer, and styrene-butadiene polymer, preferably CTBN or CTB.

12. The preliminary reactants consist of the following components in weight percentage (W%): 1-5% by weight of carboxylated nitrile elastomer; 1-5% by weight of CTBN or CTB elastomer; 1-15% by weight of epoxy dicyclopentadiene (DCPD); 5-15% by weight of diglycidyl ether of bisphenol A; 1-2% by weight of an accelerator (preferably triphenylphosphine) A surface film according to claim 1, formed by reacting the following.

13. The preliminary reactants consist of the following components in weight percentage (W%): 1-5% by weight of carboxylated nitrile elastomer; A liquid bifunctional epoxy resin containing 5 to 15% by weight of CSR particles, comprising 25 to 40% by weight; 1-15% by weight of epoxy dicyclopentadiene (DCPD); 1 to 15% by weight of a trifunctional or tetrafunctional epoxy resin; 1-2% by weight of an accelerator (preferably triphenylphosphine) A surface film according to claim 2, formed by reacting the following.

14. The surface film according to claim 1 or 2, wherein the combination of BMI monomer and comonomer accounts for more than 45% by weight of the total weight of the curable resin composition.

15. The surface film according to claim 1 or 2, wherein the amount of the pre-reaction adduct in the curable composition is about 8% to about 30% by weight based on the total weight of the curable resin composition.

16. The surface film according to claim 1 or 2, wherein the amount of inorganic microspheres is about 10% by weight to about 30% by weight based on the total weight of the curable resin composition.

17. The surface film according to claim 1 or 2, wherein the inorganic microspheres are manufactured from glass, silica, or ceramic and preferably have a diameter in the range of about 0.1 μm to about 20 μm.

18. The surface film according to claim 1 or 2, wherein the amount of inorganic filler is about 0.5% by weight to about 5% by weight based on the total weight of the curable resin composition.

19. The surface film according to claim 1 or 2, wherein the inorganic filler is manufactured from a material selected from talc, mica, calcium carbonate, alumina, and silica, preferably from fumed silica.

20. The surface film according to claim 1 or 2, wherein the curable resin composition further comprises a coloring pigment or dye.

21. A composite substrate containing reinforcing fibers impregnated or embedded in a curable matrix resin, A surface film according to claim 1 or 2 that contacts the outer surface of a composite substrate, A composite structure including, A composite structure in which the curable matrix resin of the composite substrate contains one or more bismaleimide (BMI) monomers.

22. A composite structure comprising a surface film according to claim 1 or 2 formed on a prepreg gray-up of a plurality of prepreg plies, A composite structure in which each prepreg ply contains reinforcing fibers impregnated or embedded in a curable matrix resin, the matrix resin containing one or more bismaleimide (BMI) monomers.

23. A conductive surface material comprising a conductive layer laminated or embedded on one side of the surface film according to claim 1 or 2.

24. Forming a prepreg gray-up of multiple prepreg plies, wherein each prepreg ply contains reinforcing fibers impregnated or embedded in a curable matrix resin; Bringing the surface film according to claim 1 or 2 into contact with the prepre-grain-up; To form a hardened composite structure by curing the surface material and prepre-grain up together; and Removing the hardened composite structure from the molding tool; A method for forming a composite structure including [a specific component].

25. The method according to claim 24, wherein the joint curing is performed at an initial temperature in the range of 300°F to 380°F (i.e., 148°C to 193°C), and then post-curing is performed at a temperature higher than the initial temperature, wherein the higher temperature is greater than 350°F (i.e., greater than 176°C) and up to 510°F (i.e., 265°C).