UV-resistant surfacing for composite parts

Abstract

A surface material that can provide improved UV resistance while providing good surface properties. The surface material includes a resin layer formed from a curing agent resin composition containing: (A) one or more cycloaliphatic epoxy resins, each having two or more epoxy groups per molecule; (B) an epoxy-amine adduct having two or more epoxy groups per molecule and obtained by reacting (i) an epoxy compound having two or more cycloaliphatic epoxy groups per molecule with (ii) an amine compound having two or more amino groups per molecule; (C) a curing agent and / or catalyst; (D) ceramic microspheres; and (E) a flow control agent in the form of inorganic particles that are not ceramic microspheres.

Description

DETAILED DESCRIPTION OF THE INVENTION

[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, watercraft hulls, and bicycle frames. Composite structural parts for aerospace applications typically include a protective outer film, often referred to as a "surfacing film," to provide the required performance characteristics to the composite structure before painting. Such surfacing films are used to improve the surface quality of structural parts while reducing labor, time, and cost. The surfacing film is typically co-cured with the fiber-reinforced polymer matrix composite during the manufacturing of the structural part.

[0002] Most conventional epoxy-based surface films are based on aromatic epoxy resins due to their excellent mechanical properties. Aromatic epoxy resins are characterized by the presence of an aromatic benzene ring structure in the epoxy. Bisphenol-based epoxy resins, known as bisphenol A diglycidyl ether (BADGE, or DGEBA), are the most common epoxy resins based on the reaction of epichlorohydrin (ECH) with bisphenol A. Novolac epoxy resins are another common aromatic epoxy derived from the reaction of epichlorohydrin with novolac, such as epoxy phenol novolac (EPN) or epoxy cresol novolac (ECN).

[0003] Such conventional surfacing films have been found to lack ultraviolet (UV) resistance, such that they exhibit color change and / or surface degradation, i.e., chalking and loss of paint adhesion, after exposure to UV radiation. Surface materials based on aromatic epoxy resins are believed to be susceptible to degradation from ultraviolet (UV) exposure due to the fact that aromatic epoxy resins are good chromophores. Previous efforts to improve the UV resistance of these aromatic epoxy-based films through the inclusion of UV absorbers and stabilizers have been limited in success due to the inherent UV sensitivity of the aromatic structure in the epoxy molecule. Aliphatic epoxies have also been used to improve resistance to UV exposure, but for aerospace applications, the use of such aliphatic epoxies has been found to result in inferior mechanical performance in terms of lower glass transition temperatures (Tg), lower strength and toughness, and poor long-term durability.

[0004] A known solution to prevent UV exposure and damage is to coat all exposed surfaces of a composite structure with a paint primer containing UV protective additives prior to final part assembly or painting. Typically, such a primer is applied immediately after the composite part is demolded from the molding tool. Disadvantages of using paint primers include high labor and maintenance costs, added weight, and adverse environmental impacts due to the organic solvents typically used in paint primers.

[0005] The use of other protective coatings, such as gel coats or paints with UV protective properties, has been a more effective method of protecting surface films from UV exposure. However, both gel coats and paints require regular reapplication and are not considered permanent fixations. Pigmented thermoplastic films, such as PTFE films, can provide UV protection and ease of use, but lack drapeability over contoured areas of structural components.

[0006] Disclosed herein is a multi-functional facing that can provide improved UV resistance while providing good surface properties such as smoothness and lack of porosity. Furthermore, upon curing, the multi-functional facing is defect-free and "ready to paint," i.e., does not require additional surface treatment before painting. Lightning strike protection (LSP) can be achieved when electrical conductors are incorporated into the facing. The multi-functional facing is lightweight (areal weights of 300 gsm (g / m)). 2 ) and its manufacturing costs are reduced compared to conventional facings. In some embodiments, the facing's areal weight is in the range of 50 to 200 gsm, or 0.05 to 0.04 psf.

[0007] In one embodiment, the multifunctional surface material is (A) one or more alicyclic epoxy resins, each having at least one alicyclic skeleton and two or more epoxy groups per molecule; (B) an epoxy-amine adduct having two or more epoxy groups per molecule and obtained by reacting (i) an epoxy compound having two or more alicyclic epoxy groups per molecule with (ii) an amine compound having two or more amino groups per molecule; (C) a curing agent and / or catalyst; (D) with ceramic microspheres; (E) a flow control agent in the form of inorganic particles different from component (D); The film is made of a single resin film formed from a curable resin composition containing the above.

[0008] The epoxy-amine adduct contains a terminal epoxy functional group that can react with a curing agent or catalyst due to the reactive epoxy group on its chemical structure.

[0009] The combined components A and B constitute greater than 40% by weight (wt%), based on the total weight of the curable resin composition. The relative amounts of components A and B, based on the total weight of the curable resin composition, can be as follows: 5-30 wt% component (A) and 15-55 wt% component (B). Alternatively, the relative amounts of components (A) and (B) can be described as: 5-80 wt%, or 10-75 wt%, or 20-65 wt% A per 100 wt% of the combined A and B (i.e., A+B).

[0010] The amount of curing agent and / or catalyst in the curable resin composition will vary depending on the selection. The curing agent and catalyst will react with the epoxy resin in the composition to provide a crosslinked thermoset matrix. The curing agent generally reacts with the available epoxy resin and becomes incorporated into the cured thermoset network. The catalyst initiates polymerization of the epoxy groups to form the cured thermoset network.

[0011] The amount of ceramic microspheres () in the curable resin composition can be in the range of 10 to 30 wt %, and the amount of flow control agent can be in the range of 2 to 8 wt %, based on the total weight of the curable resin composition.

[0012] In a preferred embodiment, the curable resin composition contains components A through E discussed above and an inorganic pigment (F) selected from metal oxides and carbon black. Preferred metal oxides include white titanium oxide (TiO) and red iron oxide. The amount of component F can be in the range of 5 to 30 wt. %, based on the total weight of the curable resin composition.

[0013] In another preferred embodiment, the curable resin composition contains components A-E or A-F discussed above plus a toughening agent (G) that is not an epoxy-amine adduct, i.e., different from component B. The toughening agent (G) may be selected from thermoplastic polymers, elastomeric polymers, and core-shell rubber (CSR) particles. Preferred toughening agents are selected from (i) polyvinyl acetal resin (PVB); (ii) polyvinyl formal resin; (iii) copolymers of polyethersulfone (PES) and polyetherethersulfone (PEES); (iv) core-shell rubber (CSR) particles; and combinations thereof. The amount of component G may be in the range of 1 to 15 wt %, based on the total weight of the curable resin composition.

[0014] The curable resin composition, according to any one of the embodiments disclosed herein, may further contain additional components other than those mentioned above to further modify the properties of the surfacing.

[0015] In a preferred embodiment, the curable resin composition does not contain any aromatic epoxy resins, as aromatic epoxy resins are susceptible to degradation from UV exposure. Such aromatic epoxy resins include bisphenol-based epoxy resins, novolac epoxies, and other multifunctional aromatic epoxy resins such as EPON™ 828 (bisphenol-A diglycidyl ether), DER™ 331 (liquid reaction product of epichlorohydrin and bisphenol A), Araldite® PY 306 (bisphenol-F diglycidyl ether), DEN™ 439 (epoxy novolac resin - reaction product of epichlorohydrin and phenol-formaldehyde novolac), DEN™ 431 (epoxy novolac resin - reaction product of epichlorohydrin and phenol-formaldehyde novolac), Araldite® MY 0510 (diglycidyl ether of para-aminophenol), and Araldite® MY 721 (N,N,N′,N′-tetraglycidyl-4,4′-methylenebisbenzenamine).

[0016] After curing a resin film formed from the curable resin compositions described herein, the cured film is considered to be "UV stable," meaning that after UV exposure, it exhibits no apparent surface color shift or has a total color shift ΔE* of less than 1 or 2.5.

[0017] Alicyclic epoxy resin Alicyclic epoxy resins (Component A) are characterized by non-aromatic saturated rings in their molecular structure. The term "epoxy resin" as used herein refers to monomers, oligomers, and polymers containing epoxy groups. Alicyclic epoxy resins (A) for use in the curable resin compositions disclosed herein are compounds having at least one alicyclic backbone (i.e., saturated carbon ring) and two or more epoxy groups per molecule. Suitable cyclic epoxy resins include those having the following general formula (I): [ka] A compound represented by the formula:

[0018] In general formula (I), X represents a single bond or a linking group. Examples of linking groups include divalent hydrocarbon groups, carbonyl groups (-CO-), ether bonds (-O-), ester bonds (-COO-), amide bonds (-CONH-), carbonate bonds (-OCOO-), and groups containing two or more of these linked together. Examples of divalent hydrocarbon groups include linear or branched alkylene groups having 1 to 18 carbon atoms (of which, those having 1 to 6 carbon atoms are more preferred); and divalent alicyclic hydrocarbon groups (of which, divalent cycloalkylene groups are more preferred). Examples of linear or branched alkylene groups include methylene, methylmethylene, dimethylmethylene, ethylene, propylene, and trimethylene groups. Divalent alicyclic hydrocarbon groups are exemplified by 1,2-cyclopentylene, 1,3-cyclopentylene, cyclopentylidene, 1,2-cyclohexylene, 1,3-cyclohexylene, 1,4-cyclohexylene, and cyclohexylidene groups.

[0019] Among the alicyclic epoxy compounds represented by general formula (I), those which are typically prepared by oxidizing the corresponding alicyclic olefin compound with an aliphatic peroxycarboxylic acid are preferred, and the aliphatic peroxycarboxylic acid is substantially anhydrous.

[0020] Examples of suitable difunctional cycloaliphatic epoxy compounds include those having the following chemical structure: [ka] (wherein n represents an integer of 1 to 30). Examples of the epoxy include those having the formula:

[0021] In addition to the above compounds, exemplary alicyclic epoxy compounds that can be used herein as component A further include glycidyl ether compounds in which the epoxy group does not contain a carbon atom that constitutes an alicyclic skeleton (e.g., glycidyl ether epoxy compounds each having at least one alicyclic skeleton and a glycidyl ether group). These compounds specifically include the following compounds: [ka] Hexahydrophthalic acid diglycidyl ether [ka] Cyclohexanedimethanol diglycidyl ether This is exemplified by:

[0022] Compounds 8 and 9 are available from CVC Thermoset Specialties as EPALLOY® 5000 / 5001 and 5200, respectively. Compound 10 is available from Hexion as Heloxy™ 107.

[0023] Exemplary cycloaliphatic epoxy compounds that can be used herein as component (A) further include multifunctional epoxy compounds having three or more epoxy groups, which can be represented by the following chemical formula 11: [ka] (wherein R represents a group equivalent to a q-valent (q-hydric) alcohol R—(OH)q except for removing q —OH groups therefrom; p is an integer from 1 to 50; and q is an integer from 1 to 10). The numbers for p and q may be the same or different.

[0024] Additional cycloaliphatic epoxy compounds with three or more epoxy groups include those of the following structures 12 and 13: [ka] (In the formula, a, b, c, d, e, and f each independently represent an integer of 0 to 30.) The compound represented by the formula:

[0025] Component A may contain a single cycloaliphatic epoxy resin or a combination of different cycloaliphatic epoxy resins. Preferably, Component A is comprised of a combination of cycloaliphatic epoxy resins with different functionalities. In a preferred embodiment, Component A contains a combination of (a) a difunctional cycloaliphatic epoxy resin and (b) a multifunctional cycloaliphatic epoxy resin with a functionality greater than two, such as a trifunctional or tetrafunctional epoxy resin.

[0026] A cycloaliphatic epoxy resin having a low viscosity can function as a reactive diluent when used in combination with another cycloaliphatic epoxy resin in Component A. Suitable low-viscosity compounds include cycloalkylene glycol diglycidyl ethers having a viscosity of 2500 mPa·s or less at 25°C, as measured by Brookfield Viscomete, preferably 500 mPa·s or less at 25°C. Such cycloalkylene glycol diglycidyl ethers include cyclohexanedimethanol diglycidyl ether, cyclohexanediol diglycidyl ether, and hydrogenated bisphenol A epoxy resins. Examples of commercially available low viscosity cycloaliphatic epoxy resins are Celloxide™ 2021P (3′,4′-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate), Celloxide™ 2081 (epsilon-caprolactone modified 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate), Celloxide™ 8000 and 8010 (3′,4′-epoxycyclohexyl)-3,4-epoxycyclohexane from Daicel Corporation; Araldite™ CY 184 (hexahydrophthalic acid diglycidyl ester) from Huntsman Corp.; EPALLOY™ 5000, 5001, and 5200 (discussed above), and Heloxy™ 107 (discussed above).

[0027] Epoxy-amine adducts The epoxy-amine adduct (component B) in the curable resin composition has two or more epoxy groups per molecule and is obtained by reacting (i) an epoxy compound having two or more alicyclic epoxy groups per molecule with (ii) an amine compound having two or more amino groups per molecule.

[0028] For example, the epoxy-amine adduct may have 2 to 10 or 2 to 4 epoxy groups. The epoxy groups in the epoxy-amine adduct may be located at any position, but are generally located at the ends of the molecular chain of the epoxy-amine adduct. In particular, when the epoxy-amine adduct is a linear epoxy-amine adduct, the epoxy groups are generally located at both ends of the molecular chain of the epoxy-amine adduct. However, the epoxy group positions are not limited thereto.

[0029] In one embodiment, the epoxy-amine adduct has Formula II: [ka] wherein X, at each occurrence, is independently selected from a single bond and a divalent group having at least one atom, as exemplified by X in formula (I); R 2 is a divalent organic group having a carbon atom at each bonding site with the nitrogen atom as defined in the formula: It is expressed as:

[0030] The epoxy compound (i) used in the epoxy-amine reaction may be selected from the cycloaliphatic epoxy compounds described above for component A.

[0031] The amine compound (ii), which serves as a starting material (or precursor) for forming the epoxy-amine adduct, is a polyamine compound having two or more amino groups (-NH), preferably unsubstituted amino groups, per molecule. As used herein, the term "amino group" refers to -NH (unsubstituted amino group) unless otherwise specified; the term "-NH- group" does not include unsubstituted amino groups (-NH). The amine compound (ii) is represented by the general formula III: R 2 (NH2) r (III) (wherein r is an integer of 2 or more, preferably 2 to 6 or 2 to 5, more preferably 2 or 3). It is expressed as:

[0032] R in Formula III2 represents an organic group (organic residue) having a valence of r and having a carbon atom at each bonding site with the nitrogen atom as defined in the formula. 2 is exemplified by an r-valent linear or branched aliphatic hydrocarbon group; an r-valent cyclic aliphatic hydrocarbon group; an r-valent aromatic hydrocarbon group; and an r-valent group containing two or more of these groups linked directly or via a heteroatom-containing linking group (divalent group).

[0033] Heteroatom-containing bonding groups (divalent groups) include -CO- (carbonyl group), -O- (ether bond), -CO-O- (ester bond), -O-CO-O- (carbonate group), -CO-NH- (amide group), and -CO-NR a -(substituted amide group) (wherein R a represents an alkyl group), -NH-, -NR b -(where R b represents an alkyl group), -SO-, and -SO2-, each containing one or more heteroatoms (e.g., oxygen, nitrogen, and sulfur atoms); and divalent groups each containing two or more thereof bonded to each other.

[0034] In some embodiments, the amine compound contains 2 or 3 amino groups. If the number of amino groups is greater than 6, it may be difficult to blend the epoxy-amine adduct (formed by reacting the amine compound with the epoxy compound) with other components in the resin composition.

[0035] The amine compound (ii) may have a number average molecular weight (Mn) of 80 to 15000, or 100 to 12000, or 200 to 10000. The number average molecular weight (Mn) can be measured by gel permeation chromatography (GPC).

[0036] Suitable amine compounds are represented by the following formula IV: [ka] (In the formula, p represents the number of repetitions of the structural unit in the parentheses and represents an integer of 1 or more.) The repeating number p is preferably 1 to 100, such as 1 to 70 and 1 to 30.

[0037] R in Formula IV 3 is selected from divalent linear, branched, or cyclic aliphatic hydrocarbon groups, and divalent groups containing one or more linear or branched aliphatic hydrocarbon groups and one or more cyclic aliphatic hydrocarbon groups bonded to each other, and such divalent linear, branched, or cyclic aliphatic groups may have one or more substituents. Such substituents are exemplified by the substituents that the r-valent linear or branched aliphatic hydrocarbon groups may have. 3 is preferably a C2 to C6 linear or branched alkylene group, or a C2 to C4 linear or branched alkylene chain, preferably an ethylene, trimethylene, or propylene group.

[0038] R in Formula IV 4 In each occurrence, R is independently selected from divalent linear, branched, or cyclic aliphatic hydrocarbon groups; and divalent groups containing one or more linear or branched aliphatic hydrocarbon groups and one or more cyclic aliphatic hydrocarbon groups bonded to each other. The divalent linear, branched, or cyclic aliphatic hydrocarbon groups may have one or more substituents. The substituents are exemplified by the substituents that the r-valent linear or branched aliphatic hydrocarbon groups may have. 4 is preferably a C2 to C6 linear or branched alkylene group or a C2 to C4 linear or branched alkylene group, in particular an ethylene, trimethylene, or propylene group.

[0039] When p in Formula IV is an integer equal to or greater than 2, R in each pair of parentheses 4 (i.e., R in more than one occurrence 4 ) may be the same or different. R in two or more occurrences 4 When are different from each other, the structures in each pair of parentheses of p can be added (polymerized) in a random manner or in a block manner.

[0040] In Formula IV, R3 and R 4 may be the same or different in each occurrence.

[0041] An exemplary amine compound of formula IV has the following chemical structure 14: [ka] (wherein n is the degree of polymerization and is an integer of 50 to 150) A specific example is poly(oxy-1,4-butanediyl), alpha(4-amino-butyl)-omega-(4-aminobutoxy) having an average molecular weight (MW) of about 10,000 g / mol.

[0042] The amine compound (ii) can also be represented by the following formula V: [ka] (In the formula, s represents an integer of 1 or more, preferably an integer of 1 to 100, 1 to 70, or 1 to 30; t is R 6 represents the number of structures bonded to the aryl group, and represents an integer of 3 or more, preferably 3 to 6 or 3 to 5, and in some embodiments, 3 or 4. The polyetheramine may be represented by the formula:

[0043] R in Formula V 5 is independently selected from, in each occurrence, a divalent linear, branched, or cyclic aliphatic hydrocarbon group; and a divalent group comprising one or more linear or branched aliphatic hydrocarbon groups and one or more cyclic aliphatic hydrocarbon groups bonded together. The divalent linear, branched, or cyclic aliphatic hydrocarbon group is as described above for R.

[0044] R in Formula V 6 represents an organic group having a valence of t and having a carbon atom at each bonding site with the oxygen atom as defined in the formula. The organic group R6 is as described above for R2.

[0045] Specific examples of amine compounds encompassed by Formula V are available as commercial products under the trade name JEFFAMINE. Some of the JEFFAMINE products include the following examples: JEFFAMINE D-230, polypropylene glycol 130 bis(2-aminopropyl ether), structure 15 below: [ka] a difunctional primary amine characterized by repeating oxypropylene units in the backbone as shown by: JEFFAMINE EDR-148, triethylene glycol diamine, structure 16 below: [ka] Symmetrical, unhindered diamines represented by the formula: JEFFAMINE T-403, trimethylolpropane polyoxypropylene triamine, structure 17 below: [ka] (wherein x+y+z=5 to 6) A symmetrical, trifunctional primary amine represented by Includes:

[0046] The epoxy-amine adduct disclosed herein can be produced by reacting an epoxy compound (i) with an amine compound (ii). More specifically, the alicyclic epoxy group of the epoxy compound (i) and the amino group of the amine compound (ii) react with each other to produce an epoxy-amine adduct (reaction product).

[0047] A single epoxy compound or a combination of different epoxy compounds can be used to prepare the epoxy-amine adducts according to the present disclosure. Similarly, a single amine compound or a combination of different amine compounds can be used to form the epoxy-amine adducts.

[0048] The reaction between the epoxy compound (i) and the amine compound (ii) can be carried out in the presence of a solvent or in the absence of a solvent (i.e., without the use of a solvent), for example, in a hot melt process.

[0049] The solvent is not limited, but is preferably one in which the epoxy compound (i) and the amine compound (ii) can be dissolved or uniformly dispersed. More specifically, the solvent may be selected from aliphatic hydrocarbons such as hexane, heptane, and octane; alicyclic hydrocarbons such as cyclohexane; aromatic hydrocarbons such as benzene, toluene, xylene, and ethylbenzene; halogenated hydrocarbons such as chloroform, dichloromethane, and 1,2-dichloroethane; ethers such as diethyl ether, dimethoxyethane, tetrahydrofuran, and dioxane; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; esters such as methyl acetate, ethyl acetate, isopropyl acetate, and butyl acetate; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; nitriles such as acetonitrile, propionitrile, and benzonitrile; alcohols such as methanol, ethanol, isopropyl alcohol, and butanol; and dimethyl sulfoxide. Each of the different solvents may be used alone or in combination.

[0050] The reaction between the epoxy compound (i) and the amine compound (ii) can be carried out in the presence or absence of a catalyst. More particularly, when a non-aromatic amine compound is used, the reaction is preferably carried out in the absence of a catalyst.

[0051] When a non-aromatic amine compound is used as the amine compound (ii), the catalyst may be used in an amount of less than 1 part by weight (but greater than 0) per 100 parts by weight of the epoxy compound (i). In some embodiments, the amount of catalyst may be less than 0.5 parts by weight, and in other embodiments, less than 0.3 parts by weight, per 100 parts by weight of the epoxy compound (i).

[0052] When an aromatic amine compound is used as the amine compound (ii), the catalyst may be used in an amount of 0.1 to 10 parts by weight, or 0.5 to 8 parts by weight, or 1 to 5 parts by weight, per 100 parts by weight of the epoxy compound (i).

[0053] The molar ratio of the epoxy compound (i) to the amine compound (ii) subjected to the reaction is not limited and can be adjusted so that the molar ratio of cycloaliphatic epoxy groups to amino groups in the reaction is 1 to 0.05, or 0.95 to 0.10, or 0.90 to 0.15. When the epoxy-amine reaction is carried out at a molar ratio of cycloaliphatic epoxy groups to amino groups greater than 1, the excess epoxy compound unreacted during the epoxy-amine reaction will be available as part of the epoxy composition in the curable composition disclosed herein.

[0054] The epoxy-amine reaction can be carried out at a temperature (reaction temperature) of 30°C to 250°C, or 80°C to 200°C, or 120°C to 180°C.

[0055] The epoxy-amine reaction can be carried out for a time sufficient to react all of the amine compound (reaction time). In some embodiments, the reaction time is 0.2 to 20 hours, such as 0.5 to 10 hours, and 1 to 5 hours.

[0056] The epoxy-amine reaction can be carried out under any pressure, such as under normal atmospheric pressure, under pressure (under load), or under reduced pressure. The reaction can also be carried out in any atmosphere, such as an inert gas (e.g., nitrogen or argon) or air atmosphere.

[0057] The epoxy-amine reaction can be carried out in any system selected from, without limitation, batch, semi-batch, and continuous flow systems. For example, when the reaction is carried out in a batch system, the reaction can be carried out by typically charging the epoxy compound (i), the amine compound (ii), and other optional components such as a solvent as needed, into a batch reactor, and optionally further heating and / or stirring the mixture as needed.

[0058] Curing Agents and Catalysts Suitable curing agents and catalysts (component C) for use in the curable resin composition of the present disclosure can be selected from amine-based compounds, Lewis acid-amine complexes, anhydrides, and cationic catalysts. The term "amine-based," as used herein, means that the compound contains one or more amine groups in its chemical formula. Combinations of one or more curing agents and catalysts can be used, depending on the curing agent or catalyst selected.

[0059] Lewis acid-amine complexes (curing catalysts) are complexes of Lewis acids and amine compounds. The acid is combined with a basic amine to form the Lewis acid-amine complex.

[0060] Examples of Lewis acids that constitute the Lewis acid-amine complex include, without limitation, conventional Lewis acids, such as metal halides (e.g., titanium tetrachloride, tin tetrachloride, zirconium chloride, aluminum chloride, iron chloride, zinc chloride, copper chloride, antimony chloride, and zinc bromide); boron halide compounds (e.g., boron trifluoride, boron trichloride, and boron tribromide); phosphorus halide compounds (e.g., phosphorus pentafluoride and phosphorus pentachloride); arsenic halide compounds (e.g., arsenic pentafluoride); antimony halide compounds (e.g., antimony pentafluoride); and metal triflate compounds (e.g., trimethylsilyl trifluoromethanesulfonate, scandium triflate, yttrium triflate, and zinc triflate). In a preferred embodiment, a boron halide compound is used to form the complex.

[0061] Examples of amines constituting the Lewis acid-amine complex include, without limitation, conventional amines, such as ammonia, aliphatic amines, alicyclic amines, aromatic amines, and heterocyclic amines. More specific examples thereof include ammonia, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, isopropylamine, n-butylamine, pentylamine, n-hexylamine, octylamine, dodecylamine, laurylamine, 4,4-diaminodiphenylamine, isophoronediamine, triethylenetetramine, benzylamine, piperidine, aniline, monoethanolamine, diethanolamine, triethanolamine, cyclohexylamine, cyclohexane ... Examples of suitable curing agents include silylamine, morpholine, guanidine, N-alkylmorpholines, 1,8-diazabicyclo[5.4.0]undecene-7,6-dibutylamino-1,8-diazabicyclo[5.4.0]undecene-7, 1,5-diazabicyclo[4.3.0]nonene-5,1,4-diazabicyclo[2.2.2]octane, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, pyridine, and N-alkylpiperidines. Among these, aliphatic amines are preferred because they allow the curing reaction to proceed at low temperatures to reduce the heat generated by the curing reaction.

[0062] More specific examples of Lewis acid-amine complexes include BF3·n-hexylamine, BF3·monoethylamine, BF3·benzylamine, BF3·diethylamine, BF3·piperidine, BF3·triethylamine, BF3·aniline, BF4·n-hexylamine, BF4·monoethylamine, BF4·benzylamine, BF4·diethylamine, BF4·piperidine, BF4·triethylamine, BF4·aniline, PF5·ethylamine, PF5·isopropylamine, PF5·butylamine, PF5·laurylamine, PF5·benzylamine, and AsPF5·laurylamine. In a preferred embodiment, an amine complex of boron trifluoride, particularly an aliphatic amine complex of boron trifluoride, is used as the curing catalyst.

[0063] If selected, the total amount of Lewis acid-amine complex (a single complex or a combination of different complexes) is in the range of 1 to 8 weight percent, based on the total weight of the curable resin composition. Alternatively, the total amount of Lewis acid-amine complex can be 0.5 to 20 parts by weight, or 1 to 15 parts by weight, based on 100 parts by weight of the total amount of epoxy component (A) and epoxy-amine adduct component (B), i.e., the total amount of A+B, in the curable resin composition.

[0064] The anhydride used as a curing agent may be an acid anhydride. Such an acid anhydride may be selected from those conventionally used as curing agents for epoxy resins. The acid anhydride is preferably one that is liquid at room temperature, such as methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, dodecylsuccinic anhydride, methyl-endomethylene-tetrahydrophthalic anhydride, and mixtures thereof. Alternatively, an acid anhydride that is solid at room temperature may be used as long as it does not adversely affect the homogeneous blending of the epoxy resin in the composition. Suitable solid acid anhydrides include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride, carbic anhydride, methylcyclohexene dicarboxylic anhydride, and mixtures thereof.

[0065] If selected, the curable resin composition may contain the anhydride curing agent in an amount of 50 to 150 parts by weight, or 55 to 140 parts by weight, per 100 parts by weight of the total amount of epoxy component (A) and epoxy-amine adduct (B). Alternatively, the amount of anhydride may be in the range of 20 to 40% by weight, based on the total weight of the curable resin composition. More generally, the curable resin composition preferably contains the anhydride curing agent in an amount sufficient to cause crosslinking of the epoxy resin, and more specifically, in an amount sufficient to provide 0.5 to 1.5 acid anhydride equivalents per equivalent of epoxy groups in the epoxy component (A) and epoxy-amine adduct (B).

[0066] When an acid anhydride is selected, it is preferably used in combination with a curing accelerator for the acid anhydride. The curing accelerator is a compound having the function of accelerating the curing reaction when the epoxy resin is cured by the action of the acid anhydride.

[0067] Such accelerators may be, but are not limited to, diazabicycloundecene accelerators or phosphorus accelerators. Such accelerators may be used alone or in combination as a mixture with one or more other accelerators for epoxy resins, such as tertiary or quaternary amines.

[0068] An example of a diazabicycloundecene curing accelerator is the compound having the following chemical structure (18): [ka] 1,8-Diazabicyclo[5.4.0]undecene-7(DBU) Examples of the compound include 1,8-diazabicyclo-[5.4.0]-undecene-7(DBU) represented by the following formula: and salts thereof.

[0069] Examples of other cure accelerators that can be used herein together with the acid anhydrides as catalysts include tertiary amines such as benzyldimethylamine and 2,4,6-tris(dimethylaminomethyl)phenol; imidazoles such as 2-ethyl-4-methylimidazole and 1-cyanomethyl-2-ethyl-4-methylimidazole; organic phosphine compounds such as triphenylphosphine; tertiary amine salts; quaternary ammonium salts; phosphonium salts; and metal salts such as stannous octoate and zinc octoate.

[0070] The total amount of cure accelerator in the curable resin composition can be in the range of 0.05 to 5 parts by weight, or 0.1 to 3 parts by weight, or 0.2 to 3 parts by weight, or 0.25 to 2.5 parts by weight, per 100 parts by weight of the total amount of Components A and B. The diazabicycloundecene or phosphorus cure accelerator, when used together with other accelerators, preferably constitutes at least 50% by weight of the total amount of cure accelerator.

[0071] A catalyst for use in the curable resin composition may be a cationic polymerization initiator, which is an initiator that, upon heating, releases a substance that starts cationic polymerization.

[0072] Exemplary cationic polymerization initiators include aryl diazonium salts, aryliodonium salts, aryl sulfonium salts, and blocked acids. Specific examples include triarylsulfonium hexafluoroantimonate; (4-hydroxyphenyl)methyl(1-naphthylmethyl)sulfonium hexafluoroantimonate; (4-hydroxyphenyl)methyl(2-methylbenzyl)sulfonium hexafluoroantimonate; benzyl(4-hydroxyphenyl)methylsulfonium hexafluoroantimonate; and benzyl(4-hydroxyphenyl)sulfonium hexafluoroantimonate; ammonium hexafluoroantimonate; and ammonium salts of triflic acid.

[0073] If selected, the cationic catalyst may be present in an amount of 0.01 to 15 parts by weight, or 0.05 to 12 parts by weight, or 0.1 to 10 parts by weight, per 100 parts by weight of the total amount of Components A and B. Alternatively, the amount of cationic catalyst may be in the range of 0.1 to 2% by weight, based on the total weight of the curable resin composition.

[0074] Amine-based curing agents for reacting with the epoxy resin in the resin composition include latent amine curing agents that are activated at elevated temperatures (e.g., temperatures above 150°F (65°C)). The term "amine-based," in reference to a curing agent, means that the curing agent contains one or more amine groups in its chemical formula. Examples of suitable latent amine curing agents include dicyandiamide (DICY), guanamine, guanidine, aminoguanidine, and derivatives thereof. The amine-based curing agent may be present in an amount ranging from 1 to 5 wt. %, based on the total weight of the curable resin composition. A cure accelerator may be used in combination with the latent amine curing agent to accelerate the curing reaction between the epoxy resin and the amine-based curing agent. Suitable accelerators include imidazoles, dihydrazides, and aliphatic polyamines. Such cure accelerators may be present in an amount ranging from 0.5 to 8 wt. %, based on the total weight of the curable resin composition.

[0075] Suitable cure accelerators for latent amine curing agents include alkyl- and aryl-substituted ureas (such as aromatic or cycloaliphatic dimethyl ureas) and bis-ureas based on toluenediamine or methylenedianiline. One example of a bis-urea is 4,4'-methylenebis(phenyldimethylurea) (commercially available as Omicure® U-52 or CA 152 from CVC Chemicals), which is a suitable accelerator for dicyandiamide. Another example is 2,4-toluenebis(dimethylurea) (commercially available as Omicure® U-24 or CA 150 from CVC Chemicals). Another example is Omicure® U-35M / U-35, a cycloaliphatic substituted urea commercially available from CVC Chemicals. A further example of a substituted urea is Technicure® IPDU-8, N-3-(dimethylamino)carbonylaminomethyl-3,5,5-trimethyl-cyclohexyl-N,N-dimethyl-urea, commercially available from A&C Catalyst.

[0076] Suitable aliphatic polyamines include Ancamine® 2014, Ancamine® 2337S (aliphatic amine-phenolic resin reaction products), and Ancamine® 2441 (alicyclic amine); suitable dihydrazides include adipic dihydrazide (ADH), isophthalic dihydrazide (IDH), and valine dihydrazide (VDH); and suitable imidazoles include 2-methylimidazole azine and Technicure LC-80 (encapsulated modified imidazole). These accelerators can be used in combination with DICY or alone.

[0077] Ceramic Microspheres Ceramic microspheres (Component D) are added to curable resin compositions to improve the surface smoothness of surface films formed from such compositions. In one embodiment, hollow ceramic microspheres made from an inert silica-alumina ceramic material are used. The ceramic microspheres can have a crush strength greater than 60,000 psi, a dielectric constant of about 3.7 to 4.6, a softening point in the range of 1000°C to 1100°C (i.e., 1832°F to 2012°F), and a particle size in the range of 0.1 micron to 50 microns, or 1 to 50 microns. Such particle size can be measured by scanning electron microscopy (SEM). The high softening point of ceramic microspheres allows them to be non-absorbent to solvents, flame-retardant, and highly chemically resistant. Microspheres having diameters in the range of about 0.1 to about 20 microns, preferably about 1 to about 15 microns, have been found to be particularly suitable. Examples of commercially available ceramic microspheres that are particularly suitable for use in the present surface film composition 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. In certain embodiments, the amount of ceramic microspheres is at least 10 wt. %, preferably at least 15 wt. %, based on the total weight of the curable resin composition. In some embodiments, the amount of ceramic microspheres can be in the range of 10 to 30 wt. %, based on the total weight of the curable resin composition.

[0078] Flow Control Agents A flow control agent (Component E) in the form of inorganic particles, e.g., powder, is added to the curable resin composition to control the flow of the resin composition and prevent agglomeration therein. Suitable inorganic fillers that can be used in the curable resin 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 inorganic filler can be in the range of 1 to 8 wt %, based on the total weight of the curable resin composition.

[0079] Pigments and dyes Pigments and / or dyes can be added to the curable resin composition to impart color to the resin system. Examples include, but are not limited to, red iron oxide, green chromium, carbon black, and titanium dioxide. In a preferred embodiment, titanium dioxide (TiO2) white pigment is added to the curable resin composition. Titanium dioxide is commercially available in two crystalline structures: anatase and rutile. Rutile TiO2 pigments are preferred because they scatter light more efficiently, are more stable, and are more durable than anatase pigments. Among them, rutile TiO2 with particle sizes of 0.2 to 0.3 microns in diameter, such as Ti-Pure TS-6200, R-902+, or R-706 pigments manufactured by DuPont (now Chemours), are most preferred. In another embodiment, carbon black pigment is added.

[0080] Additional reinforcers In addition to the epoxy-amine adducts described above, one or more secondary toughening agents may be added to the curable resin compositions disclosed herein to act synergistically to maximize the toughness performance of the curable compositions. The additional toughening agents are selected from (i) polyvinyl acetal resins (PVB); (ii) polyvinyl formal resins; (iii) thermoplastic copolymers of polyethersulfone (PES) and polyetherethersulfone (PEES); and (iv) core-shell rubber (CSR) particles, and combinations thereof.

[0081] Suitable polyvinyl acetal resins include vinyl butyral acetal polymers (PVB) having number average molecular weights (Mn) ranging from 30,000 to 45,000. Mn can be measured by gel permeation chromatography (GPC) using polystyrene molecular weight standards.

[0082] Suitable polyvinyl formal resins include Vinylec (polyvinyl formal) or Vinylec-C (polyvinyl formal-co-acrylic acid) polymers having Mn in the range of 44,000 to 134,000.

[0083] The amount of additional toughening agent can range from about 1 to 15 weight percent, based on the total weight of the curable resin composition.

[0084] Copolymers of PES and PEES are as described in US Pat. No. 7,084,213 (assigned to Cytec Technology Corp.).

[0085] Core-shell rubber (CSR) particles can be any core-shell particle with a soft core surrounded by a hard shell. Preferred CSR particles have a polybutadiene rubber core or a styrene-butadiene rubber core and a polyacrylate shell. However, CSR particles with a hard core surrounded by a soft shell can also be used. CSR particles can be supplied as 25-40% CSR particles dispersed in a liquid epoxy resin. CSR particles with a particle size of 300 nm or less are preferred. Such particle size can be measured by scanning transmission electron microscopy (STEM).

[0086] CSR particles having a rubber core and a polyacrylate shell are commercially available from Kaneka Texas Corporation (Houston, Texas) under the trade name Kane Ace® MX. It is preferred, but not required, that the core-shell rubber particles be added to the curable resin composition as a suspension of particles in a suitable liquid epoxy resin. Kane Ace® MX 553 and MX 554 are suitable sources of core-shell rubber particles, each of which is a suspension of 25-30% by weight of core-shell rubber particles in a cycloaliphatic epoxy resin. Kane Ace® MX 411 is a suspension of 25% by weight of core-shell rubber particles in MY 721 epoxy resin. Kane Ace® MX 120, MX 125, or MX 156, which contain 25-37% by weight of the same core-shell rubber particles dispersed in DER™ 331 resin, are also suitable sources of core-shell rubber particles. Other suitable sources of core-shell rubber particles may also be used, such as MX 257, MX 215, and MX 451. Another commercial source of core-shell rubber particles is Paraloid™ EXL-2691 (methacrylate-butadiene-styrene CSR particles of about 200 nm average particle size) manufactured by Dow Chemical Co.

[0087] conductor Electrical conductors in particulate form, e.g., particles or flakes, may also be added to the curable resin composition to impart electrical conductivity to the final surface film formed from the curable resin composition. The amount of conductor may range from 0.5 to 3 wt. % for carbon-based conductive particles, or greater than 35 wt. % for metal-based conductive particles, based on the total weight of the curable resin composition.

[0088] Examples of suitable conductors include metals in flake or particle form, such as silver, gold, nickel, copper, aluminum, and their alloys. Carbon-based nano-sized materials, such as carbon nanotubes (single-walled or multi-walled), carbon nanofibers, and graphene flakes, can also be used as conductive components to impart electrical conductivity to the resin film. The nanofibers can have diameters ranging from 70 to 200 nanometers and lengths of about 50 to 200 microns. The nanotubes can have an outer diameter of about 10 nanometers, lengths of about 10,000 nanometers, and an aspect ratio (L / D) of about 1000.

[0089] UV stabilizers or absorbers One or more UV stabilizers or absorbers may be added to the curable resin composition. The total amount of UV stabilizer may range from about 0.5 to 5% by weight, based on the total weight of the resin composition.

[0090] Examples include butylated hydroxytoluene (BHT); pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; 2-hydroxy-4-methoxy-benzophenone; 2,4-bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine; phenolic antioxidants such as 3,5-di-tert-butyl-4-hydroxybenzoic acid, n-hexadecyl ester; 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol, and methyl Liquid hindered amine light stabilizers (HALS) such as 1,2,2,6,6-pentamethyl-4-piperidyl sebacate are also suitable. Decanedioic acid, bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl ester) and 2-[4-[(2-hydroxy-3-tridecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine may also be used as suitable UV stabilizers.

[0091] Table 1 below lists various embodiments of curable resin compositions for forming surfacings of the present disclosure, with amounts given in weight percentages (wt %) based on the total weight of the overall composition.

[0092] Table 1A shows the ingredients for forming the epoxy-amine adduct (component B) in each of the embodiments shown in Table 1. The embodiments in Table 1A correspond to the embodiments in Table 1. The ingredients shown in Table 1A are pre-reacted to form the epoxy-amine adduct before being incorporated into the curable resin composition of Table 1. The amounts are shown in weight percentages (wt%) based on the total weight of all ingredients for the adduct.

[0093] [Table 1]

[0094] [Table 2]

[0095] The ingredients of the surface film composition can be added to a mixing vessel equipped for mixing, heating, and / or cooling the ingredients. Additionally, one or more organic solvents can also be added to the mixture, if desired, to facilitate mixing of the ingredients. A surface film is then formed from the surface film composition using conventional film-forming processes.

[0096] Resin layers formed from the curable compositions disclosed herein can be prepared by a hot-melt process or a solution process. For the hot-melt process, a resin film is formed from the curable resin composition without a solvent. For the solution process, one or more solvents are added to the curable resin composition to achieve a workable solids content suitable for coating, e.g., 65% to 85%, preferably 70% to 80%. Examples of such solvents include, but are not limited to, methyl ethyl ketone (MEK), acetone, dimethylacetamide, and N-methylpyrrolidone.

[0097] To facilitate handling of the resin layer as a surface material, the resin layer may be combined with a carrier by pressing the carrier into the resin layer under high pressure to embed the carrier into the resin film. Such carriers may be selected from nonwoven veils or woven fabrics made of thermoplastic polymer fibers, glass fibers, or carbon fibers, or combinations thereof; knitted veils or fabrics composed of polymer fibers, etc. Examples of metal screens or foils include expanded metal screens or foils and metal-coated veils. Such screens and foils may include copper, aluminum, silver, nickel, and alloys thereof. Examples of nonwoven mats, woven, or knitted backings include carbon mats, polymer mats, and metal-coated carbon, glass, or polymer-glass veils. Nonwoven mats, woven, or knitted backings may be coated with copper, aluminum, silver, nickel, and alloys thereof.

[0098] The resin layers (with or without the carrier) used as the surface layer may have a total area weight of less than about 500 gsm, for example, from about 50 gsm to about 250 gsm, or from 50 to 200 gsm. The carrier may have an area weight of from about 5 gsm to about 20 gsm.

[0099] As discussed above, the facing can consist of a single resin layer and, optionally, a carrier embedded therein. In other embodiments, the facing is a multilayer structure including a resin layer formed from the curable resin composition described above and a conductive layer. When such a conductive layer is incorporated into the facing, lightning strike protection (LSP) can be achieved. The conductive layer can be a continuous, non-porous layer of a metallic or non-metallic conductor having a thickness of less than about 102 μm, in some embodiments, from about 5 μm to about 75 μm, or from 10 μm to 40 μm, or from 16 μm to 38 μm. Alternatively, the conductive layer can be a porous layer, such as a metal screen or a stamped metal foil. Such metal screens can have an areal weight in the range of about 60 gsm to about 820 gsm, or from 60 gsm to about 350 gsm. When the conductive layer is a stamped metal foil, it can have a thickness in the range of about 10 μm to about 75 μm.

[0100] When the conductive layer is a metal layer, the metal may be selected from copper, aluminum, bronze, titanium, and alloys thereof. The conductive layer may be formed from an inherently electrically conductive non-metallic material, such as carbon in sheet form, such as graphene sheets and carbon nanotube (CNT) paper. A specific example of CNT paper is flexible CNT Bucky paper.

[0101] The curable resin composition can be applied to one or both sides of a conductive layer (e.g., a solid metal foil) using conventional coating techniques to form a two-layer or three-layer structure, respectively. Alternatively, a conductive surface can be produced by laminating a prefabricated resin film onto one side of a conductive layer to form a two-layer structure, or by laminating two prefabricated resin films onto opposite sides of a conductive layer to form a three-layer structure. The conductive layer can also be embedded in a resin layer formed from the curable resin composition.

[0102] Figure 1 shows a facing composed of a resin layer 10 and a conductive layer 11 laminated to a composite substrate 20, which may be a prepreg lay-up. Figure 2 shows a facing composed of a resin layer 13 having a conductive layer 14 embedded therein, laminated to a composite substrate 20, which may be a prepreg lay-up.

[0103] Resin layers formed from the curable resin compositions disclosed herein can be used to form multi-layer facings including a self-releasing layer and a conductive layer to form mold-releasing facings as disclosed in WO 2017 / 112766 A1 (published June 29, 2017) and corresponding U.S. Patent Application No. 2018370083 A1 (published December 27, 2018). The UV-stable feature of the facings disclosed herein is expected to significantly reduce tool preparation time and eliminate the use of in-mold UV protective primers, thereby increasing production speeds and significantly lowering manufacturing costs for producing composite parts.

[0104] Resin layers formed from the curable resin compositions disclosed herein can be used to form multilayer surfaces including a barrier film. Such multilayer surfaces can be comprised of a barrier film sandwiched between two resin films formed from the curable resin compositions disclosed herein. Such multilayer surfaces can further include conductive layers adjacent to the barrier film and between the two resin films. The barrier film can be as described in WO 2017 / 095810 (published June 8, 2017).

[0105] The barrier film is formed from a polymeric material that is impermeable to liquids. Preferred properties for the polymeric material include a high Tg, for example, within 140°C to 170°C, or 150°C to 162°C, as measured by DSC at a ramp rate of 10°C / min according to ASTM D3418. Another preferred property is a crystallinity of 3% to 50%, such as 10% to 40%, or 20% to 30%, as measured by DSC at a ramp rate of 10°C / min according to ASTM D3418. By way of example, the barrier film may be made of a thermoplastic material selected from polyaryletherketone (PAEK) polymers, polyamides, polyimides, polyetherimides (PEI), polyamide-imides, polyesters, and combinations thereof. In some embodiments, the composition of the barrier film comprises a PAEK polymer selected from poly(ether ketone) ("PEK"), poly(ether ether ketone) ("PEEK"), poly(ether ketone ketone) ("PEKK"), poly(ether ketone ether ketone ketone) ("PEKEKK"), poly(ether ether ketone ether ether ketone) ("PEEKEEK"), poly(ether diphenyl ketone) ("PEDK"), poly(ether diphenyl ether ketone) ("PEDEK"), poly(ether diphenyl ether ketone ketone) ("PEDEKK"), and poly(ether ketone ether naphthalene) ("PEKEN"), and combinations thereof.

[0106] Multilayer facings with such fluid barrier films are particularly useful for composite sandwich panels containing porous cores. Such composite sandwich panels have widespread applications in aerospace structural components due to their high stiffness-to-weight and strength-to-weight ratios. Composite sandwich panels typically have composite skins adhesively co-cured to the core by autoclaving. The core material can take various forms, such as lightweight honeycomb, rigid foam, paper, or wood. An exemplary configuration of a composite sandwich panel with a honeycomb core includes first and second prepreg skins adhesively bonded to the honeycomb core, and an outermost facing laminated to each of the prepreg skins. Each prepreg skin may be composed of several curable prepreg plies. The outermost facing may be a multilayer structure with a barrier film incorporated therein.

[0107] The facings disclosed herein are designed to be co-cured with fiber-reinforced, resin-matrix composite substrates at temperatures above 150°F (65°C), more particularly within the range of 120°C to 185°C. The fiber-reinforced, resin-matrix composite substrates are composed of reinforcing fibers impregnated or infused with a matrix resin. The matrix resin may include one or more thermosetting resins, such as epoxy resins. 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 fabrics or directionally aligned continuous fibers impregnated with a resin, such as an epoxy resin. The directionally aligned fibers may be unidirectional or multidirectional. A prepreg lay-up is composed of multiple prepreg plies arranged in a stacking sequence. Generally, the uncured facing is applied onto a fiber-reinforced, resin-matrix composite substrate in an uncured or partially cured state and subsequently co-cured to form a fully cured composite structure having a thermoset (cured) facing layer bonded thereto as the outermost layer. In some embodiments, facings are applied onto a mold surface, and then multiple prepreg plies are laid up onto the facings in a stacked arrangement. The tool surface can be planar or non-planar (e.g., a curved surface or some other three-dimensional shape). Alternatively, prepreg plies are first laid up onto the mold surface, and then facings are applied onto the prepreg layup. Heat and pressure are then applied to co-cure the prepreg layup and facings into a final, cured composite structure of a selected shape.

[0108] The facing material of the present disclosure can be in the form of a lightweight, flexible tape; configured for automated placement processes, such as automated tape laying (ATL) or automated fiber placement (AFP). The facing tape can have a width of about 0.125 inches to about 12 inches (i.e., about 3.17 mm to about 305 mm). In one embodiment, the facing tape has a width of about 0.125 inches to about 1.5 inches (i.e., about 3.17 mm to about 38.1 mm), such as about 0.25 inches to about 0.50 inches (i.e., about 6.35 mm to about 12.77 mm). In another embodiment, the facing tape has a width of about 6 inches to about 12 inches (i.e., about 152 mm to about 305 mm). The length of the tape can be continuous or very long relative to its width, for example, at least 10 times its width, more particularly, 100 to 100,000 times its width. In continuous form, the facing tape can be wound up into a roll for storage prior to its application in an automated process.

[0109] Continuous surface tapes can be incorporated into an ATL / AFP process, which automatically lays continuous, resin-impregnated prepreg tapes to form composite structures. Each prepreg tape is composed of unidirectional reinforcing fibers, such as carbon fibers, embedded in a curable resin matrix, such as an epoxy-based matrix. When surface tapes are used in such an automated placement process, they are first laid onto the mold surface. The tapes are dispensed side-by-side to produce surface layers of the desired width and length. Individual prepreg tapes are then laid onto the surface layer, and additional layers are then built onto the preceding layer to provide a prepreg lay-up of the desired thickness. Subsequent tapes can be oriented at different angles relative to the preceding tape. All tapes are laid at high speeds using one or more numerically controlled placement heads to dispense, clamp, cut, and restart each tape during placement. Such ATL / AFP processes are routinely used for the manufacture of large composite aerospace structures, such as aircraft fuselage sections or wing skins. This automated placement process eliminates some of the intermediate processing steps that are typical in the conventional method of manually applying large surface films onto existing prepreg lay-ups. [Example]

[0110] The following examples serve to provide specific embodiments of surface films formed in accordance with the present disclosure, but are not intended to limit the scope of the disclosure in any way.

[0111] Eight surface films were prepared based on the formulations shown in Tables 2 to 9. For comparison, a comparative surface film was prepared according to the formulation shown in Table 11.

[0112] Example 1 A curable resin composition for forming a surface film was prepared according to Table 2. An epoxy-amine adduct (B) was prepared by reacting 8.8 g (grams) of 3',4'-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate with 20.6 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 8.8 g of polytetrahydrofuranamine (elastomer) at 150°C for 1 hour.

[0113] Each epoxy-amine adduct was prepared by adding the ingredients to a mixing vessel and mixing the ingredients using a high-shear laboratory mixer. The epoxy resin and amine compound were added to the mixing vessel, and the mixture was gradually heated under shear mixing (1000-3000 rpm) to the specified adduct reaction temperature and held at the reaction temperature for the specified period. After the holding period, the reaction product was cooled to approximately 30°C-40°C, and then the epoxy-amine adduct was discharged for further use in preparing the curable compositions in Table 2.

[0114] [Table 3]

[0115] The surface films were prepared by adding the ingredients disclosed in Table 2 to a mixing vessel and mixing the ingredients using a high-shear laboratory mixer. The epoxy resin (A) and epoxy-amine adduct (B) were added first. If necessary, methyl ethyl ketone (MEK) solvent was added to about 80% solids by weight to control the viscosity of the mix. The temperature of the composition was maintained below about 70°C throughout mixing.

[0116] To form a surface film, the prepared composition was strained, degassed, and deposited as a resin film. Straining was performed using a filtration medium (nylon mesh screen). Degassing was performed so that the solids content of the composition was approximately 80% by weight. The strained and degassed composition was then coated as a film having a film weight of approximately 50 to 200 gsm using a film coater, and then dried to achieve a volatile content of less than 2% by weight. A carrier (glass or polyester type carrier with an area weight of 10 to 20 gsm) was pressed into the resin film under light pressure to embed the carrier into the film.

[0117] Example 2 A curable resin composition for forming a surface layer was prepared according to Table 3. An epoxy-amine adduct (B) was prepared by reacting 9.6 g of epoxidized hydrogenated bisphenol A with 22.5 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 9.6 g of polytetrahydrofuranamine (elastomer) at 135° C. for 1.5 hours. The epoxy-amine adduct was prepared by the same procedure as described for the epoxy-amine adduct in Example 1.

[0118] [Table 4]

[0119] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 3 as described in Example 1. The carrier used was a 10 gsm glass scrim.

[0120] Example 3 A curable resin composition for forming a surface layer was prepared according to Table 4. Epoxy-amine adduct (B) was prepared by reacting 8.6 g of epoxidized hydrogenated bisphenol A with 19.9 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 8.6 g of polytetrahydrofuranamine (elastomer) at 135° C. for 1.5 hours. The epoxy-amine adduct was prepared by the same procedure as described in Example 1.

[0121] [Table 5]

[0122] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 4 as described in Example 1. The carrier used was a 10 gsm glass scrim.

[0123] Example 4 A curable resin composition for forming a surface layer was prepared according to Table 5. An epoxy-amine adduct (B) was prepared by reacting 8.8 g of epoxidized hydrogenated bisphenol A with 20.5 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 8.8 g of polytetrahydrofuranamine (elastomer) at 135° C. for 1.5 hours. The procedure for forming the epoxy-amine adduct was the same as that described in Example 1.

[0124] [Table 6]

[0125] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 5 using the method as described in Example 1. The carrier used was a 10 gsm glass scrim.

[0126] Example 5 A curable resin composition for forming a surface layer was prepared according to Table 6. An epoxy-amine adduct (B) was prepared by reacting 8.8 g of (3',4'-epoxycyclohexyl)-3,4-epoxycyclohexane) with 20.6 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 8.8 g of polytetrahydrofuranamine (amine-terminated elastomer) at 150°C for 1 hour.

[0127] [Table 7]

[0128] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 6 using the method described in Example 1. The carrier used was a 10 gsm glass scrim.

[0129] Example 6 A curable resin composition for forming a surface layer was prepared according to Table 7. Epoxy-amine adduct (B) was prepared by reacting 5.0 g of (3',4'-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate with 4.9 g of (3',4'-epoxycyclohexyl)-3,4-epoxycyclohexane, 23.2 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether, and 9.9 g of polytetrahydrofuranamine (elastomer) at 150°C for 1 hour.

[0130] [Table 8]

[0131] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 7 using the method described in Example 1. The carrier used was a 10 gsm glass scrim.

[0132] Example 7 A curable resin composition for forming a surface layer was prepared according to Table 8. An epoxy-amine adduct (B) was prepared by reacting 8.8 g of (3',4'-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate) with 20.6 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 8.8 g of polytetrahydrofuranamine (amine-terminated elastomer) at 150°C for 1 hour.

[0133] [Table 9]

[0134] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 8 using the method as described in Example 1. The carrier used was a 10 gsm glass scrim.

[0135] Example 8 A curable resin composition for forming a surface layer was prepared according to Table 9. An epoxy-amine adduct (B) was prepared by reacting 9.7 g of epoxidized hydrogenated bisphenol A with 22.6 g of poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether and 9.7 g of polytetrahydrofuranamine (elastomer) at 135°C for 1.5 hours.

[0136] [Table 10]

[0137] A curable surfacing film having an embedded carrier therein and an areal weight of approximately 150 gsm was formed from the resin composition of Table 9 using the method described in Example 1. The carrier used was a 10 gsm glass scrim.

[0138] Surface film evaluation Test panels were prepared using the facings produced according to Examples 1-8. Each test panel was a composite panel fabricated by placing the facing on a tool, followed by laying up prepreg plies (CYCOM 5276-1, a carbon fiber / epoxy prepreg from Cytec Industries Inc.) to form a prepreg layup. The prepreg layup was then cured in an autoclave at a temperature of approximately 177°C under 80 psi for 2 hours.

[0139] After curing, the composite panels surfaced with the surfacing film were inspected for surface appearance defects (pits, pinholes). The composite panels were then evaluated for their Tg, UV stability (color shift under UV exposure), paint stripper resistance, and wet and dry paint adhesion with and without UV exposure.

[0140] Table 10 shows the surface properties and test results for the test panels. The test panel number corresponds to the example number.

[0141] [Table 11]

[0142] All surface film examples, when tested alone, demonstrate excellent UV stability with minimal noticeable color change (ΔE*<1) after extended periods (>7+ days) of UV exposure. Most of the test panels surfaced with the curable UV-stable surface film also exhibited excellent UV stability at temperatures above 200°C. g It also showed good resistance to paint strippers.

[0143] The glass transition temperature (Tg) of the cured surface films was measured by using either a modulated DSC (TA 2910) or a thermomechanical analyzer (TMA 2940, TA Instruments) under nitrogen with a ramp of 10 °C / min within the temperature range of 30 °C to 300 °C.

[0144] UV Stability Test The UV resistance or UV stability of composite panels (in the form of 3" x 6" specimen size, 0.15 mm thick) surfaced with a surface film was measured according to ASTM G154, which refers to the Standard Test Method for Accelerated Weathering Testing of UV Stability of Coating Films by Exposing the Surface Under a Fluorescent UV Light Source. The cured test panels were irradiated with a UVA lamp (340 nm wavelength) at 1.55 W / m2 for various periods of time. The equipment used for UV testing was a QUV accelerated weathering test fixture.

[0145] Each test panel surface with a curable composition film was prepared and exposed to the specified QUV test conditions. The test panels were periodically removed and examined for color change. The color change (CIELAB ΔE*) of the test panels was measured using an X-Rite SP 62 Spectrophotometer. The color difference (ΔE*) before and after UV exposure was measured and reported. The smaller the color difference, the greater the UV stability of the test panel. A color difference (ΔE*) of less than 2.5 is considered good UV stability with barely noticeable color change.

[0146] Paint stripper resistance test The paint stripper resistance of unpainted, surface-finished composite panels (2-inch x 2-inch specimen size, 0.15 mm thick) was evaluated by measuring paint-removal fluid absorption and surface pencil hardness change over a period of immersion (up to 168 hours at ambient room temperature) in a benzyl alcohol-based paint stripper solution (Cee Bee 2012A, available from McGean, or Turco 1270-6, available from Henkel) used for aerospace composite structure paint stripping processes. The weight of each test panel was measured before and after paint-stripping immersion at intervals of 24 hours, 48 hours, and up to 168 hours (7 days). The paint-stripping fluid absorption (weight change over immersion time, expressed in weight percent) of the tested panel was measured at the same test intervals up to 168 hours (7 days) of immersion.

[0147] The surface of each unpainted test panel was immersed in a benzyl alcohol-based paint stripper solution at ambient room temperature for up to 168 hours and then tested for pencil hardness change during the immersion period according to ASTM D3363. ASTM D3363 refers to the Standard Test Method for Measuring the Surface Hardness of Clean, Pigmented Organic Coating Films on Substrates. The pencil hardness scale is as follows: 6B (softest), 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, 9H (hardest). The pencil hardness of the test panels was measured before and after immersion in the paint stripper at intervals of 24 hours, 48 hours, and up to 168 hours (7 days). A pencil hardness change of more than 2H level during 24 hours of immersion is not considered to have good paint stripper resistance.

[0148] Wet and dry paint adhesion with and without UV exposure Wet and dry scribe paint adhesion of painted composite panels (0.15 mm thick in the form of a 3" x 6" specimen size) surfaced with a surface film, with or without UV exposure prior to painting, was measured in accordance with ASTM D3359, which refers to the Standard Test Method for Evaluating the Surface Adhesion of a Coating Film to a Substrate by Applying and Removing a Pressure-Sensitive Tape Overcut Made in the Film (Cross-Hatch Scribe Tape Test). The cured test panels were subjected to zero (no UV), 200 kJ / m², and 1000 kJ / m² irradiation in accordance with AATCC Test Method 16, Option 3. 2 or 1000kJ / m 2 The panels were exposed to ultraviolet (UV-A) radiation. The equipment used for UV testing was a Xeno Weather-o-meter, such as an Atlas CI3000 FadeoMeter. Each test panel surface was prepared (cleaned with and without sanding) and coated with an exterior decorative paint coating used in aerospace painting (epoxy paint primer, followed by a polyurethane-based topcoat). Dry paint adhesion testing was then performed according to ASTM D3359. To determine wet paint adhesion, the UV-exposed test panels were painted and then immersed in deionized water at 75°F for 7 days. Wet paint adhesion testing was then performed according to ASTM D3359.

[0149] Example 9 (Comparative) For comparison, surface films were prepared based on the formulations shown in Table 11. Amounts are in weight percentage (wt%).

[0150] [Table 12]

[0151] Surfacing films based on the compositions of Table 11 were prepared using procedures similar to those described in Example 1. Composite test panels were prepared using the comparative surfacing films of Table 11. The cured test panels were tested to determine their Tg, UV stability, paint stripper resistance, and paint adhesion using the test procedures previously described. The comparative test results are shown in Table 12.

[0152] [Table 13]

[0153] As can be seen from Table 12, the aromatic epoxy-based surface film exhibits a significant color change under UV exposure, indicating its high UV sensitivity and is not UV resistant.

Claims

1. (A) one or more alicyclic epoxy resins, each having two or more epoxy groups per molecule; (B) an epoxy-amine adduct having two or more epoxy groups per molecule and obtained by reacting (i) an epoxy compound having two or more alicyclic epoxy groups per molecule with (ii) an amine compound having two or more amino groups per molecule; (C) a curing agent and / or catalyst; (D) ceramic microspheres; (E) a flow control agent in the form of inorganic particles that are not ceramic microspheres; A surface material comprising at least one resin layer formed from a curable resin composition comprising: The curable resin composition does not contain any aromatic epoxy resin.

2. The curable resin composition comprises (F) an inorganic pigment selected from metal oxides and carbon black; The facing of claim 1 further comprising:

3. The curable resin composition comprises (G) a toughening agent selected from thermoplastic polymers, elastomeric polymers, and core-shell rubber (CSR) particles, rather than an epoxy-amine adduct; The surface material of claim 1 or 2 further comprising:

4. The one or more cycloaliphatic epoxy resins may be represented by the following general formula I: 【Chemical 1】 (wherein X represents a single bond or a linking group; the linking group is selected from a divalent hydrocarbon group, a carbonyl group (—CO—), an ether bond (—O—), an ester bond (—COO—), an amide bond (—CONH—), a carbonate bond (—OCOO—), and a group containing two or more of the above groups linked to each other) The surface material according to any one of claims 1 to 3, represented by:

5. 5. The surface material of claim 4, wherein the divalent hydrocarbon group is selected from linear or branched alkylene groups having 1 to 18 carbon atoms, such as methylene, methylmethylene, dimethylmethylene, ethylene, propylene, and trimethylene groups; and divalent alicyclic hydrocarbon groups, such as 1,2-cyclopentylene, 1,3-cyclopentylene, cyclopentylidene, 1,2-cyclohexylene, 1,3-cyclohexylene, 1,4-cyclohexylene, and cyclohexylidene groups.

6. The one or more cycloaliphatic epoxy resins may have the following chemical structure: 【Chemistry 2】 (wherein n is an integer from 1 to 30) 【Chemistry 3】 (wherein a, b, c, d, e, and f independently represent integers of 0 to 30).

6. The surface material according to claim 1, wherein the surface material is selected from polyepoxides having the formula:

7. The one or more cycloaliphatic epoxy resins may have the following chemical structure: 【Chemistry 4】 (wherein R represents a group equivalent to a q-hydric (q-valent) alcohol R—(OH)q except for removing q —OH groups therefrom; p is an integer from 1 to 50; and q is an integer from 1 to 10).

6. The surface material according to claim 1, wherein the surface material is selected from polyepoxides having the formula:

8. 6. The surface material of claim 1, wherein the one or more cycloaliphatic epoxy resins are selected from cycloalkylene glycol diglycidyl ethers having a viscosity of 2500 mPa s or less at 25°C as measured by a Brookfield Viscometer.

9. 6. The surface material of claim 1, wherein the one or more cycloaliphatic epoxy resins are selected from cyclohexanedimethanol diglycidyl ether; cyclohexanediol diglycidyl ether; poly[(2-oxiranyl)-1,2-cyclohexanediol] 2-ethyl-2(hydroxymethyl)-1,3-propanediol ether; (3',4'-epoxycyclohexane)methyl 3,4-epoxy-cyclohexylcarboxylate; 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate; 3',4'-epoxycyclohexyl-3,4-epoxycyclohexane; and hexahydrophthalic acid diglycidyl ester.

10. 10. The surface material according to claim 1, wherein the epoxy-amine adduct has 2 to 10 epoxy groups per molecule, and the epoxy groups are located at molecular chain terminals of the epoxy-amine adduct.

11. The epoxy-amine adduct has the formula II: 【Chemistry 5】 wherein X, in each occurrence, is independently selected from a single bond and a divalent group having at least one atom; R ２ is a divalent organic group having a carbon atom at each bonding site; and n is an integer equal to or greater than 1. The surface material according to any one of claims 1 to 10, represented by:

12. The epoxy-amine adduct is a mixture of an alicyclic epoxy compound, polypropylene glycol 130 bis(2-aminopropyl ether); triethylene glycol diamine; 12. The surfacing of any one of claims 1 to 11, which is the reaction product of trimethylolpropane polyoxypropylene triamine and an amine selected from poly(oxy-1,4-butanediyl), alpha-(4-amino-butyl)-omega-(4-aminobutoxy).

13. The curing agent and / or catalyst (C) (i) an amine-containing compound; (ii) Lewis acid-amine complexes; (iii) acid anhydrides; and (iv) Cationic catalyst (or cationic polymerization initiator) The surface material according to any one of claims 1 to 12, selected from:

14. 14. The facing of claim 13, wherein the amine-containing compound is selected from dicyandiamide (DICY), guanamine, guanidine, aminoguanidine, and derivatives thereof.

15. 15. The surface material according to any one of claims 1 to 14, wherein the curable resin composition further comprises a curing accelerator selected from imidazoles, dihydrazides, aliphatic polyamines, alkyl- and aryl-substituted ureas (such as aromatic or alicyclic dimethyl ureas), and bis-ureas.

16. The Lewis acid-amine complex is BF ３ n-Hexylamine, BF ３ Monoethylamine, BF ３ Benzylamine, BF ３ Diethylamine, BF ３ Piperidine, BF ３ Triethylamine, BF ３ Aniline, BF ４ n-Hexylamine, BF ４ Monoethylamine, BF ４ Benzylamine, BF ４ Diethylamine, BF ４ Piperidine, BF ４ Triethylamine, BF ４ Aniline, PF ５ Ethylamine, PF ５ Isopropylamine, PF ５ ・Butylamine, PF ５ Laurylamine, PF ５ Benzylamine and AsPF ５ The surface material according to any one of claims 13 to 15, which is selected from laurylamines.

17. 17. The surface material according to any one of claims 13 to 16, wherein the acid anhydride is selected from methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, dodecenylsuccinic anhydride, methyl-endomethylene-tetrahydrophthalic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride, carbic anhydride, methylcyclohexene dicarboxylic anhydride, and mixtures thereof.

18. 18. The surface material of claim 17, wherein the acid anhydride is used in combination with a cure accelerator selected from diazabicycloundecene, phosphorus compounds, tertiary and quaternary amines.

19. The surface material according to any one of claims 13 to 18, wherein the cationic catalyst (or cationic polymerization initiator) is selected from aryl diazonium salts, aryl iodonium salts, aryl sulfonium salts, and blocked acids.

20. 20. The surface material of claim 19, wherein the cationic catalyst (cationic polymerization initiator) is selected from triarylsulfonium hexafluoroantimonate; (4-hydroxyphenyl)methyl(1-naphthylmethyl)sulfonium, hexafluoroantimonate; (4-hydroxyphenyl)methyl(2-methylbenzyl)sulfonium, hexafluoroantimonate; benzyl(4-hydroxyphenyl)methylsulfonium, hexafluoroantimonate; benzyl(4-hydroxyphenyl)sulfonium, hexafluoroantimonate; ammonium hexafluoroantimonate; and ammonium salts of triflic acid.

21. 21. The surfacing of any one of claims 1 to 20, wherein components A and B combined comprise more than 40% by weight, based on the total weight of the curable resin composition.

22. 22. The facing of any one of claims 1 to 21, wherein component A is present in an amount of 5 to 80 parts by weight per 100 parts by weight of A and B combined.

23. 23. The surfacing material of any one of claims 1 to 22, wherein the ceramic microspheres (D) are present in an amount of at least 10 wt%, based on the total weight of the curable resin composition.

24. 24. The surfacing material according to any one of the preceding claims, wherein the flow control agent (E) is selected from talc, mica, calcium carbonate, alumina, and silica.

25. 25. The surfacing of claim 24, wherein the flow control agent is present in an amount in the range of 1 to 8 weight percent, based on the total weight of the curable resin composition.

26. 26. The surface material according to any one of claims 2 to 25, wherein the inorganic pigment (F) is present in an amount ranging from 5 to 30 wt%, based on the total weight of the curable resin composition.

27. 27. The surfacing of any one of claims 3 to 26, wherein the toughening agent (G) is selected from (i) polyvinyl acetal resin (PVB); (ii) polyvinyl formal resin; (iii) thermoplastic copolymers of polyethersulfone (PES) and polyetherethersulfone (PEES); and (iv) core-shell rubber (CSR) particles.

28. 28. The surfacing material of any one of claims 3 to 27, wherein the toughening agent (G) is present in an amount in the range of 1 to 15 wt%, based on the total weight of the curable resin composition.

29. The surface material according to any one of claims 1 to 28, wherein the curable resin composition further comprises a conductor in the form of fine particles, carbon particles or carbon nanotubes.

30. 30. The surfacing of any one of claims 1 to 29, wherein the curable resin composition further comprises one or more UV stabilizers or absorbers.

31. The UV stabilizers include butylated hydroxytoluene (BHT); pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate); 2-hydroxy-4-methoxy-benzophenone; 2,4-bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine; 3,5-di-tert-butyl-4-hydroxybenzoic acid; n-hexadecyl ester; 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol, methyl 31. The surface material of claim 30, wherein the surface material is selected from 1,2,2,6,6-pentamethyl-4-piperidyl sebacate; decanedioic acid; bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl)ester; and 2-[4-[(2-hydroxy-3-tridecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine.

32. A facing according to any one of the preceding claims, wherein the resin layer has an areal weight in the range of 50 to 200 gsm.

33. 33. The facing of any one of claims 1 to 32, wherein the resin layer has a carrier embedded therein, the carrier being selected from a nonwoven veil or woven fabric comprising glass fibers, polymer fibers, carbon fibers or combinations thereof; and a knitted veil or fabric comprising polymer fibers.

34. 34. The facing of any one of claims 1 to 33, further comprising a conductive layer adjacent to or embedded in the resin layer.

35. 35. The facing of claim 34, wherein the conductive layer is selected from a non-porous, continuous layer of metal; a porous metal screen; a stamped metal foil; and a sheet of carbon.

36. 36. The facing of any one of claims 1 to 35, further comprising a barrier film formed from a thermoplastic material.

37. 32. A surface material according to any one of claims 1 to 31, comprising two resin layers formed from the curable resin composition and a barrier film between the resin layers, A facing material wherein the barrier film is formed from a thermoplastic material.

38. 38. The surface material of claim 37, further comprising a conductive layer between the two resin layers.

39. A facing according to any preceding claim in the form of a continuous or elongated tape suitable for automated placement.

40. 40. The facing of claim 39, wherein the tape has a width in the range of 0.125 inches to 12 inches (or 3.17 mm to 305 mm) and a length at least 10 times its width.

41. a composite substrate comprising reinforcing fibers and a curable matrix resin; The surface material according to any one of claims 1 to 40 applied onto the surface of the composite substrate. A composite structure including:

42. a prepreg lay-up comprising a plurality of prepreg plies arranged in a stacked arrangement, each prepreg ply comprising reinforcing fibers impregnated with or embedded in a curable matrix resin; The surface material according to any one of claims 1 to 40 applied onto the surface of the prepreg lay-up. A composite structure including:

43. placing the surface material according to any one of claims 1 to 40 on a mould surface; forming a prepreg lay-up of a plurality of prepreg plies on the facing, each prepreg ply comprising reinforcing fibers impregnated with or embedded in a curable matrix resin; co-curing the facing and the pre-preg lay-up to form a cured composite structure; removing the cured composite structure from the mold surface; A method of forming a composite structure comprising:

44. forming a prepreg lay-up of a plurality of prepreg plies on a mold surface, each prepreg ply comprising reinforcing fibers impregnated with or embedded in a curable matrix resin; placing a facing according to any one of claims 1 to 40 onto the prepreg lay-up; co-curing the facing and the pre-preg lay-up to form a cured composite structure; removing the cured composite structure from the mold surface; A method of forming a composite structure comprising:

45. 45. The method of claim 43 or 44, wherein the co-curing is carried out at a temperature in the range above 150°F (65°C).

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