Two-part curable liquid shimming composition with high compression strength

Abstract

Two-part curable shimming compositions include Part A: a curative part including at least one multi-functional amine, and at least one SiC (silicon carbide) fdler, and Part B: a resin part including at least one multi-functional epoxy resin and at least one SiC (silicon carbide) fdler. The two-part curable shimming composition, upon mixing cures at room temperature or at elevated temperature to form a shim. The shim is a high compression layer having a compression of greater than 150 MegaPascals, and has a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals.

Description

[0001] PA103213W002

[0002] TWO-PART CURABLE LIQUID SHIMMING COMPOSITION WITH HIGH COMPRESSION STRENGTH

[0003] Summary

[0004] Disclosed herein are two-part liquid shimming compositions that upon curing form solid materials with high compression strength that are suitable for use as solid shims. Also disclosed are methods for bonding aircraft assemblies using these two-part liquid shimming compositions.

[0005] The disclosure includes two-part curable shimming compositions. In some embodiments, the two-part curable shimming composition comprises Part A: a curative part comprising at least one multi-functional amine, and at least one SiC (silicon carbide) filler, and Part B: a resin part comprising at least one multi-functional epoxy resin and at least one SiC (silicon carbide) filler. The two-part curable shimming composition, upon mixing cures at room temperature or at elevated temperature to form a shim. The shim comprises a high compression layer having a compression of greater than 150 MegaPascals, and has a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals.

[0006] Also disclosed are methods of bonding an aircraft assembly comprised of a skin and a substructure. In some embodiments, the method comprises mixing a two-part curable shimming composition to form a reactive mixture, dispensing the reactive mixture onto a surface of either the skin or the substructure, mating the skin or the substructure so that the reactive mixture is disposed therebetween, securing the skin and the substructure to each other using at least one mechanical fastener, and allowing the reactive mixture to cure to form a solid shim. The two-part curable shimming composition is described above.

[0007] Detailed Description

[0008] A shim is a thin piece of material used to fill small gaps or spaces between parts to be joined together. Shims assume the shape of the gap and can support a compressive load to prevent undue distortion and damage to structural parts when fastened together, typically using mechanical fasteners. Shims are used in industrial applications, such as in automotive and aerospace manufacturing. Various forms of shims are also used in residential and commercial construction.

[0009] In the aerospace field, the precision of shim dimensions is especially critical for proper assembly of parts and ensuring structural integrity of the aircraft. Mounting an aerodynamic surface or skin to the internal substructure requires fitting the parts together at mating surfaces without leaving any gaps between the mating surfaces greater than a predetermined allowance. Gaps greater than the predetermined allowance must be filled with a shim to provide a desirable aerodynamic performance and structural integrity. Conventional predictive shimming involves multiple iterations of assembly and disassembly to ensure each gap is shimmed properly per specifications. This procedure can be regarded as very inefficient, time consuming and cost intensive for preparing aerospace constructions.

[0010] Various types of shims are available. Solid shims may be made of the same material as the interfacing parts. Laminated peelable shims may be made of foil layers that can be removed one-by-one until a favorable fit is achieved. Liquid shim materials work well in filling irregular or tapered interfaces and are typically used to fill gaps less than 0.7 millimeters in width.

[0011] For example, assembling large CFRP (carbon fiber reinforced polymers) based aerostructures (e.g. wings) inevitable result in gaps between joints due to discrepancies in dimensional inaccuracies. In load bearing structures, shimming materials are required to have flexibility in order to prevent premature fatigue cracking by filling these gaps. Typically, the methods to close gaps between CFRP structures use shimming methods with liquid and / or solid shims. Each type of shim, solid or liquid, has advantages and disadvantages. Solid shims are typically made of the same or similar material as the interfacing parts and are designed for specific gap widths and geometries. Therefore, the manufacturer needs hundreds of bespoke sizes for shimming, which makes their use costly, complex and labor-intensive. The advantage of solid shims is that extremely high compression loads can be taken by solid shims. After processing, the solid shim is commonly adhered by means of a suitable adhesive to its final position. Liquid shims are easily applied thixotropic pastes with a variety of desirable properties including providing leveling of surface imperfections and ease of dispensing to increase manufacturing productivity. Typically, epoxy-amine based liquid shims are used that have moderate to good adhesion properties. Compression strength and fatigue / creep resistance is adjusted by selection of proper epoxies, amine curatives, tougheners and fdlers. However, the curable liquid fdlers have a limit to the compression strength that can be achieved with them.

[0012] Therefore, a need remains for a shim that has the convenience of a liquid shim, but also that behaves like a solid shim having a high compression strength. In other words, the liquid shim cures, but does not adhere strongly to the interface materials, and is able to withstand high compressive loads like a solid shim. It is desirable that the solid shim does not adhere strongly to the interface materials like the liquid shims do so that, if necessary, the solid shim can readily be removed and reworked. In this way the convenience and ease of use of a liquid shim is combined with the resistance to compression of a solid shim.

[0013] Disclosed herein are liquid shimming materials that cure to form a high compressive strength shim but does not adhere strongly to the materials with which they interface as do conventional liquid shims. These curable materials are epoxy / amine curing systems that contain SiC (silicon carbide) fillers. Also disclosed are methods for using the curable liquid shimming materials in aerospace manufacturing.

[0014] The terms "room temperature" and "ambient temperature" are used interchangeably to mean temperatures in the range of 20°C to 25°C.

[0015] The term “adjacent” as used herein when referring to two layers means that the two layers are in proximity with one another with no intervening open space between them. They may be in direct contact with one another (e.g. laminated together) or there may be intervening layers.

[0016] The terms “polymer” and “macromolecule” are used herein consistent with their common usage in chemistry. Polymers and macromolecules are composed of many repeated subunits. As used herein, the term “macromolecule” is used to describe a group attached to a monomer that has multiple repeating units. The term “polymer” is used to describe the resultant material formed from a polymerization reaction.

[0017] The term “aliphatic” as used herein, refers to organic compounds containing carbon and hydrogen joined together in straight chains, branched chains, or non-aromatic rings. The term “aromatic” as used herein, refers to organic ring compounds that have a delocalized conjugated n system, most commonly an arrangement of alternating single and double bonds.

[0018] The term “alkyl” refers to a monovalent group that is a radical of an alkane, which is a saturated hydrocarbon. The alkyl can be linear, branched, cyclic, or combinations thereof and typically has 1 to 20 carbon atoms. In some embodiments, the alkyl group contains 1 to 18, 1 to 12, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, cyclohexyl, n-heptyl, n-octyl, and ethylhexyl.

[0019] The term “aryl” refers to a monovalent group that is aromatic and carbocyclic. The aryl can have one to five rings that are connected to or fused to the aromatic ring. The other ring structures can be aromatic, non-aromatic, or combinations thereof. Examples of aryl groups include, but are not limited to, phenyl, biphenyl, terphenyl, anthryl, naphthyl, acenaphthyl, anthraquinonyl, phenanthryl, anthracenyl, pyrenyl, perylenyl, and fluorenyl.

[0020] The term “alkylene” refers to a divalent group that is a radical of an alkane. The alkylene can be straight-chained, branched, cyclic, or combinations thereof. The alkylene often has 1 to 20 carbon atoms. In some embodiments, the alkylene contains 1 to 18, 1 to 12, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. The radical centers of the alkylene can be on the same carbon atom (i.e., an alkylidene) or on different carbon atoms.

[0021] The term “arylene” refers to a divalent group that is carbocyclic and aromatic. The group has one to five rings that are connected, fused, or combinations thereof. The other rings can be aromatic, non-aromatic, or combinations thereof. In some embodiments, the arylene group has up to 5 rings, up to 4 rings, up to 3 rings, up to 2 rings, or one aromatic ring. For example, the arylene group can be phenylene.

[0022] The term “heteroalkylene” refers to a divalent group that includes at least two alkylene groups connected by a thio, oxy, or -NR- where R is alkyl. The heteroalkylene can be linear, branched, cyclic, substituted with alkyl groups, or combinations thereof. Some heteroalkylenes are poloxyyalkylenes where the heteroatom is oxygen such as for example,

[0023] -CH2CH2(OCH2CH2)nOCH2CH2-.

[0024] Disclosed herein are liquid shimming materials that are two-part curable compositions that cure to form a high compressive strength shim, but do not adhere strongly to the materials with which they interface. These curable materials are epoxy / amine curing systems that contain SiC (silicon carbide) fillers. Prior to the development of the two-part curable compositions of this disclosure, it was unclear whether SiC filler could be used in such compositions, or if they could be used whether they would provide a dramatic increase in the compression strength of the cured compositions.

[0025] Disclosed herein are two-part curable shimming compositions. In some embodiments, the two-part curable shimming composition comprises Part A: a curative part comprising at least one multi-functional amine, and at least one SiC (silicon carbide) filler, and Part B: a resin part comprising at least one multi-functional epoxy resin and at least one SiC (silicon carbide) filler. The two-part curable epoxy composition, upon mixing cures at room temperature or at elevated temperature to form a shim. The cured shim may have a variety of sizes and shapes. In some embodiments, the shim is capable of filling a gap of from 1 micrometer to 1.5 millimeters.

[0026] The shims formed upon curing comprises a high compression layer having a compression of greater than 150 MegaPascals and a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals. Typically the test substrate for OLS is aluminum. In some embodiments, the cured shim has a Shore D hardness of 85-95. These properties are measured according to the test methods described in the Examples section.

[0027] Part A, the curative part of the two-part curable composition comprises at least one multi-functional amine and at least one SiC filler. The multi-functional amines function as curatives for the epoxy resins in the resin part.

[0028] Useful curatives include cyclic compounds that contain at least one cyclic moiety, which may either be aliphatic or aromatic. The cyclic moiety may be substituted by primary amino groups, i.e. the primary amino groups may be bonded directly to the ring. Generally, the cyclic moiety is substituted by one or more residues carrying the primary amino group, optionally at a terminal position. That residue may be, for example, a linear or branched aminoalkyl group, typically with the primary amino group at the terminal position.

[0029] The curative contain at least two primary amino groups (-NH2 groups) at a terminal position. The at least one cyclic moiety is typically a five- or six-membered ring, which may be an alkylene ring or a heteroalkylene ring. The heteroalkylene ring typically contains one, or more than one, heteroatom selected from nitrogen and oxygen atoms.

[0030] Examples of suitable curatives include but are not limited to cyclohexanes containing terminal primary amino groups and / or aminoalkyl residues with terminal primary amino groups, piperazines containing terminal primary amino groups and / or aminoalkyl residues with terminal primary amino groups, and morpholines containing terminal primary amino and / or aminoalkyl groups with terminal primary amino groups. Particular examples include but are not limited to bis or tris aminoalkyl piperazines or morpholines. Specific examples include, but are not limited to, l-amino-3 -aminomethyl - 3, 5, 5 -trimethylcyclohexane (also called isophorone diamine) and N,N-bis(3 aminopropyl) piperazine.

[0031] The curatives may be aliphatic cyclic (poly)amines as described above or they may be adducts of such aliphatic cyclic polyamines with one or more epoxy resins, with the proviso that the aliphatic cyclic polyamines are used in molar excess to ensure the adducts contain at least two primary amine groups, generally at a terminal position of the adduct. Typically, the epoxy resin used to form the adduct is the same or a similar one to one of the epoxy resins used in the resin part.

[0032] Useful aliphatic amines need not be cyclic and can include linear and / or branched polyetheramines. In some embodiments the curative composition comprises at least one adduct of an excess of a difunctional unbranched poly etheramine with an epoxy resin, typically a molar excess of from 200% to 800%, from 300% to 600%, from 400% to 500%, or in some embodiments, less than, equal to, or greater than 200%, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800%.

[0033] The unbranched poly etheramine can have a molecular weight of from 130 g / mol to 500 g / mol, from 180 g / mol to 400 g / mol, from 200 g / mol to 300 g / mol, or in some embodiments, less than, equal to, or greater than 130 g / mol, 140, 150, 160, 170, 180, 190, 200, 220, 250, 270, 300, 320, 350, 370, 400, 420, 450, 470, or 500 g / mol.

[0034] The unbranched polyetheramine can have between one and four ether oxygens and more typically two or three ether oxygens. In some embodiments the unbranched polyetheramine may be a compound according to Formula 1 :

[0035] H2N-[(CH2)xO]y-(CH2)x-NH2 Formula 1 wherein y is selected from 1, 2, 3 or 4, and wherein each x is independently selected from 2, 3, or 4.

[0036] In some embodiments the unbranched polyetheramine may be 4,7,10-trioxa tridecane 1,13 -diamine (TTD). In some embodiments the unbranched polyetheramine may be 4,7-dioxadecane 1,10-diamine, commercially available as JEFFAMINE EDR 176.

[0037] Suitable amounts of the polyetheramine can be from 5 percent to 30 percent, from 5 percent to 20 percent, from 5 percent to 15 percent, or in some embodiments, less than, equal to, or greater than 5 percent, 10, 15, 20, 25, or 30 percent, relative to the overall weight of the shimming composition prior to curing.

[0038] In some instances, Part A further includes a secondary curative. Secondary curatives can be imidazoles or salts thereof, imidazolines or salts thereof, or phenols substituted with tertiary amino groups. An exemplary secondary curative is tris-2,4, 6- (dimethylaminomethyl)phenol that is commercially available under the trade designation ANCAMINE K54 from Evonik Industries AG, Essen, Germany.

[0039] Part A further comprises at least one SiC fdler. SiC fdler is carbide ceramic material. The class of carbide ceramics are extremely resistant against high temperature, abrasion and corrosion. They show high hardness, have a high thermal and variable electrical conductivity. These ceramics are typically used in mechanical engineering, chemical-Zpower engineering and microelectronics. By far the most important of the carbide ceramics are materials based on silicon carbide (SiC). SiC materials show many excellent features such as high-temperature strength, good wear resistance, high hardness, chemical corrosion resistance, and etc. As an abrasive, it can be used as grinding tool, such as grinding wheel, oil stone and grinding head.

[0040] In the current disclosure, it was found that the use of readily available SiC results in cured materials having very high compression strength compared to the use of common inorganic fdler systems. The current disclosure is not limited to the single type of SiC as fdler, but also discloses mixtures of SiC and other fdler types to obtain the desired product properties. A wide range of SiC fdlers are suitable. Particularly suitable are ultrafme SiC particles. Examples of suitable ultrafme particles include the silicon carbide particles with a ds50-value of 2.0 ± 0.4 micrometers available as F 1500 Half Sludge from 3M Company.

[0041] In addition to the curative and SiC fdler, Part A may contain additional optional additives as long as these additives do not adversely affect the curing of the two-part composition or the final properties of the cured composition. In some embodiments, Part A further comprises at least one additional additive selected from catalysts, thixotropic agents, wetting agents, dispersing agents, or combinations thereof. Examples of suitable catalysts include the secondary curatives described above. Suitable thixotropic agents include fumed silica, examples of suitable wetting agents include silane coupling agents, and suitable dispersing agents include surfactants.

[0042] The two-part curable shimming composition also comprises Part B a resin part. Part B comprises at least one epoxy resin. Typically, Part B is comprised of a blend of epoxy resins. Generally, the blend includes a difunctional epoxy resin and a multifunctional epoxy resin having an epoxide functionality of at least two and generally three or more.

[0043] The difunctional epoxy resin has an epoxide functionality of exactly two and can be miscibly blended with the multifunctional epoxy resin. The multifunctional epoxy resin can have an epoxide functionality of at least 2.1, at least 2.2, at least 2.3, at least 2.4, at least 2.5, at least 2.6, at least 2.7, at least 2.8, at least 2.9, or at least 3. The multifunctional epoxy resin may also be comprised of two or more resins having respective epoxide functionalities greater than two.

[0044] Epoxy resins include glycidated resins, cycloaliphatic resins, and epoxidized oils. The glycidated resins can be the reaction product of a glycidyl ether, such as epichlorohydrin, and a bisphenol compound such as bisphenol A. Various examples of epoxy resins include C4-C28 alkyl glycidyl ethers; C2-C28 alkyl-and alkenyl-glycidyl esters; C1-C28 alkyl-, mono- and poly-phenol glycidyl ethers; polyglycidyl ethers of pyrocatechol, resorcinol, hydroquinone, 4,4'-dihydroxydiphenyl methane (or bisphenol F), 4,4'-dihydroxy-3,3'-dimethyldiphenyl methane, 4,4'-dihydroxydiphenyl dimethyl methane (or bisphenol A), 4,4 '-dihydroxydiphenyl methyl methane, 4,4'-dihydroxydiphenyl cyclohexane, 4,4'-dihydroxy-3,3'-dimethyldiphenyl propane, 4,4'-dihydroxydiphenyl sulfone, and tris (4-hydroxyphynyl)methane; polyglycidyl ethers of the chlorination and bromination products of the above-mentioned diphenols; polyglycidyl ethers of novolacs; polyglycidyl ethers of diphenols obtained by esterifying ethers of diphenols obtained by esterifying salts of an aromatic hydrocarboxylic acid with a dihaloalkane or dihalogen dialkyl ether; polyglycidyl ethers of polyphenols obtained by condensing phenols and long-chain halogen paraffins containing at least two halogen atoms; N,N'-diglycidyl- aniline; N,N'-dimethyl-N,N'-diglycidyl-4,4'-diaminodiphenyl methane; N,N,N',N'- tetraglycidyl-4,4'-diaminodiphenyl methane; N,N'-diglycidyl-4-aminophenyl glycidyl ether; N,N,N',N'-tetraglycidyl-l,3-propylene bis-4-aminobenzoate; phenol novolac epoxy resin; cresol novolac epoxy resin; and combinations thereof.

[0045] Representative non -limiting examples of suitable epoxy resins include bis-4,4'-(l- methylethylidene) phenol diglycidyl ether and (chloromethyl) epoxide bisphenol A diglycidyl ether. Commercially available epoxy resins that can be used in the practice of this discloaure include those sold under the trade designation ARALDITE by Huntsman Corporation, The Woodlands, TX and EPON by Hexion Inc., Columbus, OH. Suitable epoxy resins also include glycidyl ethers of trihydric phenols such as tris(hydroxyphenyl) methane. Such resins are commercially available under the trade designation TACTIX by Huntsman Corporation, The Woodlands, TX.

[0046] In some embodiments, an epoxy novolac resin may be used. In some embodiments, the multifunctional epoxy resins include a tetrafimctional epoxy resin based on meta-xylenediamine, such as those sold under the trade designation ERISYS by Emerald Performance Materials LLC, Vancouver, WA.

[0047] It can be advantageous to use a mixture of epoxy resins whose constituents are selected to provide the desired viscosity characteristics before curing. In some embodiments, the multifunctional epoxy resin includes a trifunctional epoxy resin, such as triphenyl methane triglycidyl ether, or other glycidyl ether with three or more epoxide groups per molecule. The trifunctional epoxy resin is, in some instances, a solid epoxy resin at ambient temperature. Optionally the trifunctional epoxy resin is blended with a tetrafimctional epoxy resin, such as 4,4'-methylenebis(N,N-diglycidylaniline). The difunctional epoxy resin can be a bisphenol A / epichlorohydrin derived liquid epoxy resin, or other glycidyl ether with two epoxide groups per molecule.

[0048] The relative amounts of multifunctional epoxy resin and the difunctional epoxy resin can be adjusted to obtain suitable crosslink density, which in turn affects important adhesive properties such as glass transition temperature, tensile strength, and shear strength. Low viscosity difunctional epoxy resins in suitable amounts can also help the uncured adhesive flow and wet the bonding surfaces of a substrate for improved bond strength. In the provided shimming adhesives, the multifunctional epoxy resin and the difimctional epoxy resin can be present in a relative weight ratio of from 1: 1 to 6: 1, from 1: 1 to 4: 1, from 1: 1 to 2: 1, or in some embodiments, less than, equal to, or greater than 1: 1, 2: 1, 3: 1, 4: 1, 5: 1, or 6: 1, relative to each other.

[0049] In some embodiments, the multifunctional epoxy resin includes both a trifunctional epoxy resin and tetrafunctional epoxy resin in relative amounts that balance the competing properties of stiffness and adhesiveness in the cured shimming adhesive. It was discovered that certain epoxy resins, such as solid or semi-solid triphenyl methane triglycidyl ethers, were found to increase adhesive stiffness, while others such as the liquid tetrafunctional 4,4'-methylenebis(N,N-diglycidylaniline) were found to enhance adhesive strength.

[0050] In keeping with these considerations, the trifunctional epoxy resin and the tetrafunctional epoxy resin can be present in a relative weight ratio of from 1: 1 to 8: 1, from 1: 1 to 6: 1, from 1: 1 to 4: 1, or in some embodiments, less than, equal to, or greater than 1: 1, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1, 7: 1, or 8: 1, relative to each other.

[0051] The epoxy resin or resins in the base part can have any suitable molecular weight. The weight average molecular weight can be from 100 g / mol to 50,000 g / mol, from 175 g / mol to 20,000 g / mol, from 250 g / mol to 10,000 g / mol, or in some embodiments less than, equal to, or greater than 100 g / mol; 125; 150; 175; 200; 250; 300; 350; 400; 450; 500; 550; 600; 650; 700; 750; 800; 850; 900; 950; 1000; 2000; 5000; 7000; 10,000; 20,000; 30,000; 40,000; or 50,000 g / mol.

[0052] Part B, like Part A also comprises a SiC fdler. The SiC materials are described above.

[0053] Besides the epoxy resin or resins and the SiC fillers, Part B may contain a variety of additional optional additives as long as these additives do not adversely affect the curing of the two-part composition or the final properties of the cured composition. In some embodiments, Part B further comprises at least one additional additive selected from reactive diluents, thixotropic agents, wetting agents, dispersing agents, carbon fibers, or combinations thereof. Reactive diluents, which lower the viscosity of the epoxy resin components, are generally epoxy resins having either a branched aliphatic backbone that is saturated or a cyclic backbone. Examples of reactive diluents include, but are not limited to, the diglycidyl ether of resorcinol, the diglycidyl ether of cyclohexane dimethanol, the diglycidyl ether of neopentyl glycol, and the triglycidyl ether of trimethylolpropane. Diglycidyl ethers of cyclohexane dimethanol are commercially available under the trade designation HELOXY MODIFIER 107 from Hexion Specialty Chemicals in Columbus, OH and under the trade designation EPODIL 757 from Evonik Industries AG, Essen, Germany.

[0054] Reactive diluents may be added in suitable amounts to obtain a desired viscosity profde for the uncured shimming adhesive. Typical amounts can be from 1 percent to 12 percent by weight based on the total weight of the epoxy component. Further details of reactive diluents can be found in, for example, PCT Publication No. WO 2014 / 210298 (Elgimiabi et al.).

[0055] Suitable thixotropic agents include fumed silica, examples of suitable wetting agents include silane coupling agents, and suitable dispersing agents include surfactants. Carbon fibers, if used are typically sufficiently short so at to permit the ready mixing of Part A with Part B.

[0056] The amounts or Part A, Part B and the total mixture of Part A and Part B can vary widely. The total mixture of Part A and Part B generally includes at least 20 weight percent epoxy resin based on a combined weight of Part A and Part B. For example, the shimming composition can include at least 25 weight percent, at least 30 weight percent, at least 40 weight percent, or at least 50 weight percent epoxy resin. The shimming composition can include up to 75 weight percent epoxy resin.

[0057] The amount of SiC filler in the two-part curable shimming composition can also vary. Typically, the SiC is present in an amount of 20% or greater by weight based on 100 parts by weight of the total composition.

[0058] As mentioned above, the two-part curable shimming compositions are mixed and cured to form shims. This curing can be carried out in a variety of ways. In some embodiments, after Part A and Part B are mixed and the mixture is disposed where desired, the composition is cured at room temperature for up to 7 days. In other embodiments, the composition is cured at a temperature of greater than room temperature for a time of less than 7 days.

[0059] Also disclosed are methods of shimming an aircraft assembly comprised of a skin and a substructure. In some embodiments, the method comprises mixing a two-part curable shimming composition to form a reactive mixture, dispensing the reactive mixture onto a surface of either the skin or the substructure, mating the skin or the substructure whereby the reactive mixture is disposed therebetween, securing the skin and the substructure to each other using at least one mechanical fastener, and allowing the reactive mixture to cure to form a solid shim. The two-part curable shimming composition has been described above and comprises Part A: a curative part comprising at least one multifunctional amine, and at least one SiC (silicon carbide) filler, and Part B: a resin part comprising at least one multi-functional epoxy resin, and at least one SiC (silicon carbide) filler. The two-part curable epoxy composition, upon mixing cures at room temperature or at elevated temperature to form a shim. The cured shim may have a variety of sizes and shapes. In some embodiments, the shim is capable of filling a gap of from 1 micrometer to 1.5 millimeters.

[0060] The shims prepared in this way behave as solid shims having a high compression strength and low adhesion to the skin and substructure. In some embodiments, the shim comprises a high compression layer having a compression strength of greater than 150 MegaPascals, and a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals. In some embodiments, the cured shim has a Shore D hardness of 85-95. The two-part curable shimming compositions have been described in detail above.

[0061] Examples

[0062] These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, unless noted otherwise. The following abbreviations are used: mm = millimeters; g = grams; ml = milliliters; min = minutes; Pa = Pascals; kPA = kiloPascals; MPa = MegaPascals. The terms “weight %”, “% by weight”, and “wt%” are used interchangeably. Table of Abbreviations

[0063] Test Methods

[0064] Compression Strength / Modulus Testing The compression properties of cured solid shims were tested according to ASTM

[0065] D695 at ambient temperature (23 °C) with a test speed of 1.27 mm / min. Overlap Shear

[0066] Overlap Shear (OLS; EN 2243-1) was exemplarily tested for selected compositions using AlClad 2024 T3 substrate, pretreated with PSA+BR127 with a test speed of 2.5 mm / min. To adjust the bondline thickness glass beads of 90-150 micrometers in diameter were added to the adhesive before application.

[0067] Shore D Hardness

[0068] Shore D hardness was tested according to ASTM D 2240.

[0069] Cured Density

[0070] Cured Density was determined by measuring the volume of a specimen divided by its weight.

[0071] Extrusion Rate

[0072] Extrudable amount (g) per minute by means of a pneumatic gun at 6 bar (600 kPa) pressure in combination with a static mixing nozzle MF1018 (Sulzer Mixpack).

[0073] Examples Exl-Ex4:

[0074] Sample curable shim compositions of Part A and Part B were prepared and hand- filled into a 200 ml double barrel cartridge (Sulzer Mixpack, F-Type) having the ratio 2: 1 by volume as shown in Table 1. The shim -mixtures were obtained via extrusion by means of a pneumatic gun at 6 bar (600 kPa) pressure in combination with a static mixing nozzle MF1018 (Sulzer Mixpack). The samples were dispensed and cured either at room temperature for 7 days or at 80°C for 2 hours. The samples were tested according to the above Test Methods. The results are shown in Table 2.

[0075] Table 1: Formulations

[0076] Table 2: Measured Properties

Claims

What is claimed is:

1. A two-part curable shimming composition comprising:Part A: a curative part comprising at least one multi-functional amine, and at least one SiC (silicon carbide) filler;Part B: a resin part comprising at least one multi-functional epoxy resin and at least one SiC (silicon carbide) filler; wherein the two-part curable epoxy composition, upon mixing cures at room temperature or at elevated temperature to form a shim.

2. The two-part curable shimming composition of claim 1, wherein the shim comprises a high compression layer having a compression of greater than 150 MegaPascals, and wherein the cured shim has a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals.

3. The two-part curable shimming composition of claim 1, wherein the cured shim has a Shore D hardness of 85-95.

4. The two-part curable shimming composition of claim 1, wherein the Part A further comprises at least one additional additive selected from catalysts, thixotropic agents, wetting agents, dispersing agents, or combinations thereof.

5. The two-part curable shimming composition of claim 1, wherein the Part B further comprises at least one additional additive selected from reactive diluents, thixotropic agents, wetting agents, dispersing agents, carbon fibers, or combinations thereof.

6. The two-part curable shimming composition of claim 1, wherein the Part B further comprises at least one additional epoxy resin.

7. The two-part curable shimming composition of claim 1, wherein upon mixing to form a shim, the composition is cured at room temperature for up to 7 days.

8. The two-part curable shimming composition of claim 1, wherein upon mixing the to form a shim, the composition is cured at a temperature of greater than room temperature for a time of less than 7 days.

9. The two-part curable shimming composition of claim 1, wherein the SiC is present in an amount of 20% or greater by weight based on 100 parts by weight of the total composition.

10. The two-part curable shimming composition of claim 1, wherein upon mixing the composition cures to form a shim capable of fdling a gap of from 1 micrometer to 1.5 millimeters.

11. A method of bonding an aircraft assembly comprised of a skin and a substructure, the method comprising: mixing a two-part curable shimming composition to form a reactive mixture, the two-part curable shimming composition comprising:Part A: a curative part comprising at least one multi-functional amine, and at least oneSiC (silicon carbide) filler;Part B: a resin part comprising at least one multi-functional epoxy resin, and at least one SiC (silicon carbide) filler; wherein the two-part curable epoxy composition, upon mixing, cures at room temperature or at elevated temperature to form a shim; dispensing the reactive mixture onto a surface of either the skin or the substructure; mating the skin or the substructure whereby the reactive mixture is disposed therebetween; securing the skin and the substructure to each other using at least one mechanical fastener; and allowing the reactive mixture to cure to form a solid shim.

12. The method of claim 11, wherein the shim comprises a high compression layer having a compression of greater than 150 MegaPascals, and wherein the cured shim has a ShoreD hardness of 85-95, and wherein the cured has a low bonding as measured by OLS (overlap shear) of less than 10 MegaPascals.

13. The method of claim 11, wherein the Part A further comprises at least one additional additive selected from catalysts, thixotropic agents, wetting agents, dispersing agents, or combinations thereof.

14. The method of claim 11, wherein the Part B further comprises at least one additional additive selected from reactive diluents, thixotropic agents, wetting agents, dispersing agents, carbon fibers, or combinations thereof.

15. The method of claim 11, wherein the Part B further comprises at least one additional epoxy resin.

16. The method of claim 11, wherein upon mixing to form a shim, the composition is cured at room temperature for up to 7 days.

17. The method of claim 11, wherein upon mixing the to form a shim, the composition is cured at a temperature of greater than room temperature for a time of less than 7 days.

18. The method of claim 11, wherein the SiC is present in an amount of 20% or greater by weight based on 100 parts by weight of the total composition.

19. The method of claim 11, wherein upon mixing the composition cures to form a shim capable of filling a gap of from 1 micrometer to 1.5 millimeters.

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