Edge coating for honeycomb used in panels with composite face sheets

The present invention generally relates to sandwich panels and other related structural composites. Sandwich panels are typically composed of surface sheets (also referred to as "skins") that are adhesively bonded to opposite sides of a core material, such as a honeycomb, to form a sandwich panel. In particular, the present invention relates to a honeycomb treatment for improving the bonding between the honeycomb and the surface sheet. The present invention is particularly applicable to sandwich panels in which the surface sheet is made from a composite material.

Sandwich panels are used in a variety of applications that require high strength and light weight. The core used in most sandwich panels is either lightweight honeycomb, rigid foam, paper or wood. Honeycomb is a popular core material because of its high strength and weight ratio, and its high fatigue resistance fracture resistance. Various products, including metals and composites, are used to make honeycomb cores.

Further, the surface sheet bonded to the side surface (also referred to as "edge") of the honeycomb is manufactured from various materials including metal and composite material. An important point of view in all sandwich panel configurations is the method of joining the surface sheet to the honeycomb. Adhesives are usually used to bond the surface sheet to the core. In order for the load to be transferred from one surface material to the other and the structure to accept the commonly used stress calculation methods and meet the suggested conditions, the adhesive must firmly adhere the surface material or epidermis to the core. It doesn't become. When the adhesive fails, the strength of the panel becomes extremely weak. Adhesives are especially important in sandwich panels that use honeycombs as cores. The reason is that the area where the edge of the honeycomb contacts the surface sheet is relatively small.

One method of attaching the composite surface sheet to the honeycomb comprises forming a prepreg sheet containing at least one fiber reinforced layer and an uncured resin matrix. Prepreg is a technical term used in the composites industry to refer to mats, wovens, non-woven materials, tows, or rovings that have been pre-impregnated with resin and can be finally cured. Typically, a film adhesive is added to the prepreg core assembly and then joined by curing the prepreg resin and the adhesive resin at elevated temperatures. The film adhesive can be attached as a separate ply layer or as an integral part of the prepreg sheet.

Honeycomb sandwich panels are used in many applications where the stiffness and structural strength of the panel are fundamental requirements. In addition, honeycomb sandwich panels are also widely used in the aerospace industry, where panel weight is fundamentally important. As a result, planned efforts have been made and continued to reduce the weight of honeycomb sandwich panels without sacrificing structural strength. One area of study for weight reduction is the reduction of individual adhesive layers. This was achieved by making surface sheets from self-adhesive composites. The resin used in such adhesive prepregs must meet the dual requirement of providing adequate structural strength while providing adequate adhesion to the honeycomb. Representative adhesive surface sheets are described in published European Patent Application Nos. EP0927737A1 and EP0819723A1 and US Pat. Nos. 6,440,257 and 6,508,910.

An alternative method of joining the surface sheet to the honeycomb involves applying an adhesive to the edges of the honeycomb. This adhesive is usually applied by "immersing" the edges of the honeycomb in the adhesive. The adhesive used for this type of bonding is commonly referred to as a "immersion" resin or adhesive. The advantage of this method is that the adhesive is not distributed over the entire surface sheet, but only where the honeycombs are in contact with the surface sheet. Generally, this method is used to join non-adhesive surface sheets such as aluminum and other metal surface sheets to honeycombs.

<p> According to the present invention, there is provided a honeycomb sandwich panel comprising an inner surface joined to the honeycomb core and at least one surface sheet having an outer surface. This surface sheet contains at least one fiber layer and a resin matrix. The honeycomb comprises a wall that contours the plurality of honeycomb cells, the wall having at least one edge joined to the inner surface of the surface sheet. As one feature of the present invention, an adhesive containing a polyamide (a common example commonly referred to as "nylon") is used to bond the surface sheet to the honeycomb. It has been found that the use of a polyamide-containing adhesive provides a particularly strong bond between the honeycomb and the surface sheet when the surface sheet contains a resin matrix. It has been found that the use of the polyamide adhesive according to the present invention is particularly effective in increasing the bonding strength between the honeycomb and the prepreg surface sheet.</p><p> As a further feature of the present invention, when the resin matrix used for the prepreg contains a thermosetting resin, the polyamide-containing adhesive enhances the bonding strength between the honeycomb and the prepreg surface sheet. When the prepreg resin matrix contains a thermosetting resin reinforced with rubber and / or a thermoplastic resin, the use of a polyamide adhesive is particularly effective in increasing the bonding strength of the adhesive prepreg surface sheet. In one embodiment of the invention, the bonding strength provided by the polyamide adhesive is that the honeycomb wall is torn when the surface sheet is separated from the core (in a drum peeling test), while the bonding between the surface sheet and the honeycomb It is strong enough to remain intact. In such an embodiment, when the surface sheet is peeled off the panel, the adhesive bond is stronger than the core, so the sandwich panel undergoes core failure rather than cohesive failure.</p><p> As an additional feature of the present invention, adhesives containing polyamides are particularly well suited for joining surface sheets to honeycombs made from composite materials. In the honeycomb manufacturing process, honeycomb sections are usually cut out from large blocks of honeycomb. Cutting out the honeycomb from the block tends to expose the fibers and form "jagged" or "fluffy" edges, which tends to make it difficult to bond to the surface sheet. Therefore, in order to improve the bonding strength, the edges of the composite honeycomb are generally treated by polishing, machining or other surface treatment methods prior to bonding to remove exposed fibers and smooth the honeycomb edges. To. Polyamide adhesives according to the present invention provide good bonding strength even when fluffy or smooth honeycombs are used. Therefore, if desired, the time-consuming step of removing fluffy edges from the composite honeycomb can be eliminated.</p><p> The present invention not only includes a sandwich panel as described above, which is made using an edge-coated honeycomb with a polyamide adhesive, but also joins a surface sheet to the honeycomb to make such a sandwich panel. It also includes methods. The method comprises providing a surface sheet comprising an inner surface and an outer surface for joining to the honeycomb core, wherein the surface sheet comprises at least one fiber layer and a resin matrix. A honeycomb is provided that includes a wall that contours the plurality of honeycomb cells, wherein the wall has at least one edge for joining to the inner surface of the surface sheet. An adhesive containing nylon is applied to the honeycomb edges to provide an edge coated honeycomb, which is then joined to the surface sheet.</p><p> Finished sandwich panels manufactured according to the present invention can be used in a variety of situations where lightweight and structurally strong materials are required. However, the present invention is particularly well suited for use in aerospace applications where a number of stringent mechanical and chemical requirements must be met and at the same time weight limits must not be exceeded. The above-mentioned features of the present invention, many other features and the advantages of the attachment will be better understood by referring to the detailed description below in connection with the accompanying drawings.</p><p> FIG. 1 is a perspective view of a preferred representative honeycomb core and two composite surface sheets before joining the honeycomb to the surface sheet using a nylon adhesive (polyamide or modified polyamide) according to the present invention.</p><p> FIG. 2 is a perspective view of a preferred representative sandwich panel in which the constituent materials shown in FIG. 1 are joined together and cured to form the final honeycomb sandwich panel.</p><p> FIG. 3 is a partial diagram of a representative method according to the invention showing a single edge coated honeycomb wall joined to a surface sheet.</p><p> FIG. 4 is a partial diagram showing an embodiment of the present invention in which the sandwich panel suffers core fracture when the surface sheet is forcibly peeled from the honeycomb.</p><p> For aerospace applications where weight reduction and structural strength are particularly important design criteria by improving the bonding between composite surface sheets (especially adhesive prepregs) and honeycomb cores using nylon (polyamide) adhesives according to the present invention. It is preferable to produce a lightweight structural panel. However, those skilled in the art will appreciate that the present invention is not limited to aerospace applications and can be used in any situation where there is a need for honeycomb sandwich panels with surface sheets made from composite materials. ..</p><p> Three basic building blocks of a typical honeycomb sandwich panel preferred for aerospace applications are shown in Figure 1 prior to panel fabrication. This constituent material includes a honeycomb core 12 having walls 11 forming a plurality of honeycomb cells 13. The wall has edges that form the surface or edges of the honeycombs represented by 14 and 16. The other two constituent materials are prepreg surface sheets 17 and 19. Surface sheets 17 and 19 include internal surfaces 21 and 23 for joining to the honeycomb edges, respectively. Further, the surface sheets 17 and 19 include outer surfaces 25 and 27, respectively. According to the present invention, the edges 14 and 16 of the honeycomb 12 are coated with an adhesive containing polyamide as an essential component (not recognizable in FIG. 1) to prepare an edge-coated honeycomb. The use of a polyamide adhesive (polyamide coating) improves the bonding between the surface sheets 17 and 19 and the honeycomb 12, while increasing the weight of the resulting panel very little. The uncured prepreg surface sheets or skins 17 and 19 are attached to the edge coated honeycomb 12 and then cured to prepare surface sheets 18 and 20 as shown in FIG.</p><p> FIG. 3 is a partial diagram illustrating a method of joining a honeycomb to one of the surface sheets 19 according to the present invention. Briefly, FIG. 3 shows the joining of a single wall 16 where the entire edge 16 of the honeycomb is coated with a polyamide adhesive (coating). As shown in FIG. 3, the lower edge of the honeycomb wall 11 is coated with a polyamide adhesive (polyamide coating) 15 and then joined to the lower part of the uncured prepreg surface sheet 19. As indicated by arrow 30, edge-coated honeycombs are placed against the surface sheet 19 to form fillets 28 and 29 such that the polyamide adhesive (polyamide coating) partially flows over the prepreg 19. The prepreg then cures to form the final sandwich panel 10 containing the cured surface sheet 20, as indicated by arrow 40.</p><p> The honeycomb core 12 can be manufactured from any material used to form the honeycomb core. Representative honeycomb core materials include aluminum, aramid, carbon or fiberglass composites, resin impregnated paper, non-woven spunbonded polypropylene, spunbonded nylon, spunbonded polyethylene terephthalate (PET) and the like. A typical preferred honeycomb material is an aramid-based substrate, EIDuPont de Nemours &amp; Materials sold under the trade name NOMEX® available from Company (Wilmington, Delaware). Honeycomb cores manufactured from NOMEX® are sold by Hexcel, Dublin, CA. Preferred representative NOMEX® honeycombs include HRH® 10 available from Hexcel. Another preferred honeycomb material is KEVLAR®. Preferred representative KEVLAR® honeycombs are available from Hexcel under the trade name HRH® 36. Honeycombs made from carbon or glass composites are preferred and typically contain carbon or glass fiber and a phenolic resin and / or polyimide matrix. The honeycomb is typically supplied in a cured form and is bonded to the epidermis or surface sheet without any further treatment other than applying the polyamide adhesive described below. If desired, the core can be polished or machined to remove "fluffy" edges that may be present due to fiber exposure when cutting out honeycomb sections from large blocks of honeycomb. .. However, as mentioned earlier, the use of a polyamide adhesive according to the present invention as an edge coating either omits the removal of fibers exposed on the honeycomb edges during manufacturing, or at least substantially reduces the need thereof.</p><p> The size of the honeycomb can be changed significantly. For aerospace applications, honeycomb cores are typically 1/8 to 1/2 inch (3.2 to 12.7 mm) cells (ie, diameter / cross section), and this core is 1/4 inch (6.4). It has a thickness of 2 inches (50.8 mm) from mm) (distance between honeycomb edges). Also, the thickness of the honeycomb wall can vary, with a typical honeycomb wall having a thickness of about 0.001 inch (0.25 mm) to 0.005 inch (0.13 mm). The combination of cell size, wall thickness, and material density used determines the core weight in pounds / cubic feet (pcf). Composite honeycombs having a weight of about 2 pcf to 8 pcf are preferred.</p><p> The surface sheet bonded to the honeycomb can be manufactured from any material used in the composite sandwich panel structure. Usually, such composites include one or more fibers and a resin matrix. The fibers used in the prepreg surface sheets 17 and 19 can be any fiber material used to form the composite laminate. Representative fiber materials include glass, aramid, carbon, ceramics and composites thereof. The fibers may be in the form of woven, unidirectional or random fiber mats. Woven carbon fibers are preferred, such as plain weave, harness satin, aya, and basket weave styles, having an area weight of 80 to 600 gsm, but more preferably 190 to 300 gsm. The carbon fibers can have 3,000 to 40,000 filaments / tow, more preferably 3,000 to 12,000 filaments / tow. All of these are commercially available. Further, glass fibers of the same type can be used, and most universally, 303 gsm is 7781 and 107 gsm is 120. When using a unidirectional structure, a typical ply weight is 150 gsm for carbon and 250 gsm for glass.</p><p> The composite material may be pre-cured prior to joining to the honeycomb, but it is preferred that the resin matrix is uncured or partially cured prior to joining. The uncured surface sheet is uncured because it combines curing and bonding of the surface sheet and honeycomb in a single step, and the polyamide adhesive present on the honeycomb interacts with the uncured resin matrix during the bonding process. A surface sheet is preferred. A preferred form is the type of prepreg, or uncured surface sheet.</p><p> The resin matrix for the prepreg must contain a thermosetting resin such as epoxy, cyanate ester, phenol, benzoxazine, polyimide, or bismaleimide. A prepreg with certain peculiar adhesiveness (adhesive prepreg) is particularly preferred. The types of prepregs described in U.S. Pat. Nos. 6,440,257 and 6,508,910 promulgated are examples of adhesive prepregs that can be joined to honeycombs to make suitable sandwich panels without the use of adhesives. is there. This prepreg contains a resin matrix that is a combination of thermosetting resin and thermoplastic resin. The use of nylon adhesive applied to the honeycomb edges has been found to unexpectedly improve the bond strength between the honeycomb and this type of adhesive surface sheet.</p><p> Preferred resins used to form the resin matrix include epoxy and / or cyanate esters and / or bismaleimide resins, and one or more curing agents. The resin matrix also preferably contains a viscosity modifier and thermoplastic fillet-forming particles, as described in US Pat. Nos. 6,440,257 and 6,508,910. When the viscosity modifier and fillet-forming particles are contained in the resin matrix, the resin is first mixed with the viscosity modifier to form a resin mixture. If necessary, the mixture is heated to ensure that the viscosity modifier is completely dissolved. Next, a curing agent and a fillet forming agent are added to the resin mixture. The final resin mixture is kept at a temperature lower than the temperature at which the fillet-forming particles dissolve in the resin. As a result, at this stage, the fillet-forming particles are preferably uniformly mixed throughout the resin, and the fillet-forming particles are not dissolved to a substantial extent, so that the resin viscosity is high to an unacceptable level. Never become. The viscosity of the resin mixture is important because the resin must be able to penetrate the fibers and form prepregs. For the purposes of this specification, it is considered that particles that maintain at least 90% by weight of the original particle weight are not dissolved to a substantial extent. If less than 10% by weight of the particles are initially present in the resin as-is, it is considered that the particles are substantially dissolved.</p><p> The viscosity of the final resin mixture used to make the prepreg containing the fillet-forming particles should be between 150 and 1,500 poises. Preferred viscosities are between 300 and 1,200 poises. The above viscosity range is the final before prepreg production when the viscosity is measured by Rheometrics RDA2 at 2 ° C / min, 10 rad / sec, and a gap setting of 0.8-1.0 mm. It represents the minimum viscosity for the resin mixture. During the curing / bonding step, the viscosity of the resin mixture gradually increases as the fillet-forming particles dissolve.</p><p> The fillet-forming resin tends to be concentrated towards the surface of the prepreg during the prepreg manufacturing process due to the inherent filtration of particles by the fiber layer. As an alternative method, fillet-forming particles may be applied to the resin surface after the resin has been molded into a prepreg film or after the resin has been impregnated into the fiber layer (eg, by the powder deposition method). In this way, the fillet-forming particles are substantially distributed on the surface of the prepreg. In each case, the resin temperature is maintained at a sufficiently low level to prevent the fillet-forming particles from melting until the prepreg is attached to the core material and cured.</p><p> After bonding to the edge-coated core, the prepreg is heated to a sufficient level for a sufficient amount of time to substantially dissolve the fillet-forming particles, cure the prepreg, and cure the polyamide adhesive. The melted thermoplastic particles increase the tenacity of the bond and interact with the polyamide adhesive (also the thermosetting resin present in the prepreg matrix resin) to further strengthen the bond.</p><p> Representative thermosetting resins that can be used to make prepreg matrix resins include phenols, benzoxazines, polyimides, epoxies, cyanate esters, and bismaleimide resins. Typical epoxy and cyanate ester resins are glycidyl amine type epoxy resins such as glycidyl-p-aminophenol and tetraglycidyl diaminodiphenyl-methane; bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, phenol novolac. Glycidyl ether type epoxies such as type epoxies, cresol novolac type epoxies (eg ECN1299 available from Huntsman Advanced Materials, Brusta, New York) and resorcinol type epoxies; 1,1'-bis (4-shi) Anatophenyl) Etan (Huntsman Advanced Materials, Brusta, AroCy L-10 available from New York), 1,3-Bis (4-Cyanatophenyl-1-1- (1-methylethylidene)) benzene (Huntsman) Includes cyanate esters such as RTX366) available from Advanced Materials, Brusta, NY. Epoxy resin is preferred. Particularly preferred epoxy blends include a mixture of trifunctional epoxy and bifunctional bis-F epoxy.</p><p> Further, in order to prepare a basic prepreg matrix resin, a curing agent and a viscosity modifier (optionally selected) are added to the thermosetting resin. The curing agent is preferably an amine curing agent, and the viscosity modifier is preferably a thermoplastic material that dissolves in a thermosetting resin.</p><p> Various matrix resins can be used for the composite surface sheet, but matrix resins based on epoxy and cyanate ester formulations are preferred. In addition, sandwich panels with two prepreg layers on each surface of the honeycomb are preferred. The two layers are preferably arranged in the longitudinal direction with two layers (0/90) or two layers (± 45, 0/90) in the longitudinal direction of the honeycomb. Those skilled in the art will recognize that the present invention can also be applied to multilayer surface sheets containing two or more fiber layers.</p><p> Typical preferred surface sheet matrix resin formulations are: (1) 1 to 70 parts by weight of epoxy, 5-40 parts by weight amine hardener, 1 to 30 parts by weight of viscosity modifier, and 5 to 50 parts by weight of thermoplastic fillet-forming particles, (2) 10 to 40 parts by weight of trifunctional epoxy resin, 10-40 parts by weight bifunctional epoxy resin, 11 to 25 parts by weight aromatic hardener, 0 to 3 parts by weight of non-aromatic hardener, 5 to 15 parts by weight of viscosity modifier, and 8 to 30 parts by weight of thermoplastic fillet-forming particles.</p><p> Epoxy consists of trifunctional epoxies, bifunctional epoxies, and many combinations of trifunctional and bifunctional epoxies. A tetrafunctional epoxy can also be used. Typical trifunctional epoxies include triglycidyl-p-aminophenol and N, N-diglycidyl-4-glycidyloxyaniline (MY-0510 or MY-0500 available from Huntsman Advanced Materials, Brusta, NY). .. Representative bifunctional epoxies that can be used with this resin include bis-F epoxies such as GY-281, LY-9703, and GY-285 available from Huntsman Advanced Materials, Brusta, NY. Bis-A epoxies such as GY-6010 (Huntsman Advanced Materials, Brusta, NY) and DER 331 (Dow Chemical, Midland, Michigan) are suitable bisphenol-A epoxies and can be used. A typical tetrafunctional epoxy is tetraglycidyl diaminodiphenylmethane (Huntsman Advanced). MY-721, MY-720 and MY-9512) available from Materials, Brusta, NY. Other suitable epoxies include phenol novolac epoxies, cresol novolac epoxies, resorcinol epoxies. Preferred bis-F epoxies include GY-281 and GY-285 available from Huntsman Advanced Materials, Brusta, NY.</p><p> Typical curing agents are amino or glycidylsilanes such as dicyandiamide, 3,3-diaminodiphenylsulfone (3,3-DDS), 3-aminopropyltriethoxysilane, CuAcAc / nonylphenol (1 / 0.1), 4,4'. -Diaminodiphenyl sulfone (4,4'-DDS), 4,4'-methylenebis (2-isopropyl-6-methylaniline), eg Lonzacure M-MIPA (Lonza, Ferrone, New Jersey), 4,4 Includes'-methylenebis (2,6-diisopropylaniline), such as Lonzacure M-DIPA (Lonza, Ferrone, New Jersey). Dicyandiamide and 3,3-DDS are the preferred curing agents. A combination of 3,3-DDS and dicyandiamide is particularly preferred.</p><p> Typical viscosity modifiers are thermoplastic polyetherimides such as ULTEM® 1000P available from General Electric (Pittsfield, Mass.) And pulverizations such as 5003P available from Sumitomo Chemical (Osaka, Japan). Includes polyethersulfone, HRI-1 available from Hexcel (Dublin, CA), and polyimide MATRIMID® 9725 available from Huntsman Advanced Materials (Brüsta, NY). ULTEM® 1000P and micronized PES are preferred. Micronized PES is particularly preferred. Rheometrics If the minimum viscosity of the final resin mixture is maintained between 150 and 1,500 poise when the viscosity is measured using RDA2) at 2 ° C / min, 10 rad / sec, and 0.8-1.0 mm gap settings. For example, the amount and type of viscosity modifier added to the epoxy resin mixture can be changed. As mentioned earlier, a mixture with a minimum viscosity between 300 and 1,200 poise is preferred. The viscosity of the prepreg resin before adding the fillet-forming particles should be between about 50 and 2,000 poises at room temperature. The preferred viscosity range is from about 100 poises to 1,500 poises at room temperature.</p><p> Densified polyether sulfone (PES) and densified polyetherimide particles can be used as suitable fillet-forming particles. Dense PES particles are preferred. Preferably, the densified polyether sulfone (PES) particles are produced according to the technique of US Pat. No. 4,945,154. This content is incorporated herein by reference. The average particle size of PES particles ranges from 1 to 150 microns. An average particle size of 1 to 50 microns is preferred, with an average particle size of 10 to 25 microns being particularly preferred. The microspheres are generally spherical in shape, and the densified microsphere powder is passed through a micron sieve for classification. The glass transition temperature (Tg) of these particles is preferably over 200 ° C.</p><p> If desired, the PES used can be "miniaturized". Miniaturized PES, called PES particles, have the characteristics of a rough surface produced by grinding the particles or using other polishing manufacturing techniques known in the industry. In addition, the miniaturized PES particles can be produced using a spraying and drying method known in the industry. The miniaturized PES particles are preferably less than 120 microns in diameter. Particles with a diameter of less than 50 microns are particularly preferred, with a range of 10 to 25 microns being particularly preferred.</p><p> The prepreg resin is produced by first mixing the epoxy components with each other and then gradually adding polyetherimide or a miniaturized PES viscosity modifier. The resulting mixture is heated to about 130 ° C. and mixed for a sufficient time to dissolve the polyetherimide / PES particles. Once the polyetherimide / PES is dissolved, the mixture is cooled to about 75 ° C. Next, an aromatic amine curing agent and fillet-forming densified PES particles are added to the mixture. The resin must be maintained at a temperature below about 70 ° C to 75 ° C while the curing agent and densified PES particles are mixed with the resin. The final resin is between 150 and 1,500 poise when measured in viscosity using Rheometrics RDA2 at 2 ° C / min, 10 rad / sec, and 0.8-1.0 mm gap settings. Has a minimum viscosity of. The preferred viscosity range is 300 to 1,200 poise.</p><p> The finished resin is applied to the desired fiber dough to form a prepreg. The resin content of the prepreg can be varied depending on many different factors to achieve the desired mechanical and structural properties for the sandwich panel. The prepreg preferably has a resin content of 35 to 45% by weight.</p><p> The prepreg is bonded to the edge coated honeycomb using vacuum and / or pressure and heated to cure the prepreg and polyamide adhesive to form a tightly bonded cured surface sheet to the honeycomb. The amount of vacuum, pressure and heat required to cure the prepreg and join it to the edge-coated honeycomb is the specific matrix resin formulation, the specific polyamide adhesive formulation, the amount of resin matrix in the prepreg, and the polyamide on the honeycomb edge. It can be changed according to the amount of adhesive. In general, sufficient pressure must be applied to the prepreg to ensure good contact between the honeycomb and the prepreg surface sheet and proper bonding. When fillet-forming particles are present in the matrix resin, the temperature and other curing conditions are selected so that the densified PES particles are substantially dissolved during the curing / bonding step.</p><p> According to the present invention, the polyamide adhesive is applied to the edges of the honeycomb before joining with the composite surface sheet. This adhesive contains polyamide (nylon) as the main component. This type of adhesive is referred to herein as a "nylon adhesive" or "polyamide adhesive" and is any known adhesive containing nylon (at least 40% by weight, preferably at least 60% by weight) as the main component. Contains adhesive compounds. Polyamide adhesives are commercially available and typically contain an amount of polyamide in the range of 60-100% by weight. In addition, other components such as epoxy may be mixed in the polyamide adhesive at a concentration of, for example, 40 to 60% by weight of the total polyamide adhesive. Typical polyamide types that can be used include nylon 66, nylon 612, nylon 6.</p><p> Polyamide adhesives are sold by EIDuPont de Nemours &amp; Company (Wilmington, Delaware) under the trade names NYSOL (formerly ZYTEL) and ELVAMIDE. NYSOL and ELVAMIDE are preferred polyamide (nylon) adhesives for edge coating. Other typical commercially available polyamide adhesives are ORGASOL, available from Atofina Chemical (Philadelphia, PA), and GRILAMID TR55, available from EMS-Chime (Sumter, South Carolina). Representative preferred polyamide adhesives include ELVAMIDE 8061, 8066 and 8023R, and NYSOL 1020.</p><p> The above-mentioned commercially available polyamide adhesives are usually provided as solids. This solid polyamide adhesive is dissolved in a suitable solvent such as a mixture of water and alcohol to prepare a solution to be applied to the honeycomb edge. The solvent evaporates, leaving a polyamide coating on the honeycomb edges. Ethanol, benzyl alcohol, and a mixture of ethanol and benzyl alcohol are suitable solvents. The amount of solvent used to dissolve the polyamide can be varied to provide a solution with the desired viscosity for application to the honeycomb edges. Generally, the polyamide adhesive solution will contain 10 to 50% by weight of polyamide.</p><p> The final honeycomb sandwich panel may be designed for use in high temperature (approximately 160 ° F / 71 ° C and above) and / or high humidity (80% relative humidity and above) environments. In such situations, it is preferred that the polyamide be combined with one or more thermosetting resins to produce a polyamide adhesive that works well under such high temperature and / or high humidity conditions. Typical thermosetting resin types that can be combined with polyamides to make high temperature / high humidity polyamide adhesives according to the present invention are epoxies, cyanate esters, bismaleimides, and phenolic resins. Epoxy and phenolic resins are preferred. This resin can be used alone or in combination. Typical epoxies are as described above. Typical phenolic resins are available from Durez, Texas, 23-983. Includes Resin and any commercially available phenolic resin such as 23056 available from Durez, Texas, USA. Preferred high temperature / high humidity polyamide adhesives include an amount of polyamide in the range of 40 to 90% by weight and one or more thermosetting resins in an amount in the range of 10 to 6% by weight. As is well known in the industry, suitable hardeners for this resin need to be included. As is well known in the industry, coupling agents and other conventional additives may be included. In addition, to make a high temperature / high humidity polyamide adhesive solution, make a resin solution to be combined with a polyamide (as described above) solution using a solvent for thermosetting resins such as toluene and xylene. Can be done. The amount of thermosetting resin in such a solution will typically be in the range of 10-80% by weight.</p><p> The polyamide adhesive can be applied to the honeycomb edges using any technique that allows uniform edge coating. A preferred method is to immerse or press the honeycomb edges into a layer of polyamide adhesive solution. It is preferred to form the adhesive layer by pouring the polyamide adhesive solution onto a smooth surface and spreading the adhesive with a Gardner knife or other suitable blade. The purpose is to form a uniform and thick layer with a thickness of 0.001 inch (0.0254 mm) to 0.050 inch (1.3 mm). The preferred thickness range for this adhesive layer is 0.003 (0.08 mm) inches to 0.008 (0.20 mm) inches. If desired, the thickness of this layer can be increased or decreased beyond this range, depending on the thickness of the core, the cell size, and the thickness of the wall, and the particular surface sheet bonded to the honeycomb. The thickness of this adhesive solution layer determines the depth of coating on the honeycomb edges (see "D" in Figure 3). The preferred depth of edge coating for any given honeycomb-surface sheet combination is a conventional experimental method that includes varying the depth of the edge coating and measuring the bonding (peeling) strength of the resulting honeycomb-surface sheet junction. Can be decided by.</p><p> The honeycomb edge is coated with an adhesive by simply placing the honeycomb edge on the adhesive layer and moving the honeycomb downward to bring it into contact with the basal plane. The honeycomb is then pulled up from the basal plane with an adhesive solution that remains only at the site of the honeycomb wall immersed in the adhesive layer. If the solvent remains, the solvent is evaporated leaving a coating of polyamide adhesive on the honeycomb edges. Edge coating of honeycombs by this method is a well-known and preferred method as it provides a simple and effective method for uniformly coating the honeycomb edges with an adhesive. Other methods, including powder coating of adhesives, spraying, roller or brushing, can also be used if desired, especially for edge coating of relatively large honeycomb cores.</p><p> The amount of polyamide adhesive applied to the honeycomb edges can be varied as long as the desired degree of bonding strength is obtained without adding excessive weight to the panel structure. Further, the amount of the polyamide adhesive that realizes the optimum combination of high bonding strength and weight reduction can be determined by a usual test method. This can be achieved not only by changing the depth (D) of the edge coating, but also by changing the viscosity and tackiness of the polyamide adhesive solution. Also, after evaporation of the solvent, the core can be reimmersed to remove additional adhesive. This reimmersion step can be repeated as many times as necessary to achieve the desired level of adhesive application. The drying temperature depends on the solvent used for the polyamide solution. For water-based coatings, the drying time is typically 4 minutes at 121 ° C. The amount of polyamide adhesive applied to the honeycomb edges is preferably 1 g / ft.²And 10g / ft²Between. However, the amount of adhesive used can vary widely depending on the intended use of the honeycomb sandwich panel and the combination of desired panel weight and surface sheet peel strength.</p><p> The bond formed by the polyamide adhesive and the surface sheet resin matrix preferably has a strength equal to or greater than the structural strength of the honeycomb. For example (when the surface sheet is forcibly peeled from the honeycomb), the honeycomb body (shown by 50 in FIG. 4) is removed from the surface sheet 53 before the joint 54 breaks between the honeycomb and the surface sheet joints. Polyamide adhesives, prepreg surface sheets and honeycombs are preferably selected so that they are separated (see arrow 52). In accordance with the present invention, the combined use of the polyamide adhesive edge coating and the composite surface sheet provides a sufficiently high bond strength between the surface sheet and the honeycomb, so that at least the core portion before the bond breaks during the peel test. It is usually possible to produce sandwich panels that are structurally disrupted. The particularly high joint strength achieved by the present invention makes it possible to obtain maximum structural strength from the panel structure such that the overall strength of the panel is constrained by the structural strength of the core rather than the joint between the surface sheet and the core. To do.</p><p> Examples will be described below.</p><p>(Comparative example 1) The prepreg matrix resin was manufactured with the following formulation: 23% by weight MY-0510 (N, N-diglycidyl-4-glycidyloxyaniline, 25% by weight GY281 (bis-F epoxy), 19% by weight 3,3-diaminodiphenyl sulfone (3,3-DDS), 7% by weight ULTEM® 1000P (polyetherimide), and 26% by weight densified PES.</p><p> This densified PES was manufactured from PES 5003P available from Sumitomo Chemical Co., Ltd. (Osaka, Japan). PES has been refined in accordance with US Pat. No. 4,945,154. MY0510 and GY281 were first mixed in a mixing vessel and heated to 70 ° C for about 10 minutes. Next, ULTEM® 1000P particles were added and the resulting mixture was heated for about 75 minutes while mixing to 130 ° C. to fully dissolve the ULTEM® 1000P particles. The mixture was then cooled to 75 ° C, 3,3-DDS was added and mixed for about 15 minutes. Subsequently, the densified PES was gradually added and mixed for about 10 minutes to realize the final resin mixture. Using a rheometric dynamic analysis method, uniform resin viscosities were measured over the entire curing temperature range (ie, 20 ° C to 177 ° C), as described above. The minimum viscosity of the resin was 900 poise.</p><p> Surface sheet is 1m containing 193gsm 3K PW woven fabric²It was produced by first producing 138 g of resin prepreg per unit. The prepreg was prepared as follows.</p><p> Using a reverse roll coater, apply the resin on the release paper at about 175 ° F (79 ° C) and 69 g / m.²A film containing the above was prepared. This resin film is 193g / m²It was impregnated with carbon fibers (in the form of woven fabric) having an area weight of.</p><p> This prepreg is attached to a 1/8 inch (0.31 cm) cell, 1/2 inch (1.27 cm) thick HRH® 10 core under reduced pressure of 22 inch (56 cm) Hg and 45 psi. It was cured at 177 ° C for 2 hours under the pressure of 177 ° C, degassed at a pressure of 20 psi, and inclined cooled at a rate of 2 ° C / min. The obtained test piece was subjected to a peeling test according to ASTM D1781. All surface sheets exhibited peel strength ranging from 20 to 33 inches-pounds / 3 inches wide.</p><p>(Comparative example 2) A prepreg matrix resin was produced in the same manner as in Comparative Example 1, except that the following components were used to produce the resin: 21 parts by weight MY-0510, 21 parts by weight AcroCy® L-10, 21 parts by weight GY281, 9 parts by weight ULTEM® 1000P 1.5 parts by weight of CuAcAc / nonylphenol (1 / 0.1), and 26.5 parts by weight densified PES.</p><p> The minimum viscosity of this uniform resin mixture was found to be about 500 poise. The viscosity of the final resin mixture was measured as described in Comparative Example 1. Using the final resin mixture, a prepreg was prepared and attached to an HRH® 10 core in the same manner as in Comparative Example 1. The peel strength of the resulting surface sheet was 26 inches-pounds / 3 inches wide.</p><p>(Comparative example 3) A prepreg matrix resin was made using the following formulation: 27.0% by weight MY-0510 (N, N-diglycidyl-4-glycidyloxyaniline), 24.9% by weight GY285 (bis-F epoxy), 15.8% by weight 3,3'-diaminodiphenyl sulfone, 1.3% by weight dicyandiamide, 13.5% by weight of miniaturized polyether sulfone (PES), and 17.5% by weight densified polyether sulfone (PES).</p><p> The amounts of MY-0510, GY281 and 3,3-DDS can be changed up to ± 15% to make resin formulations. Also, the amount of both types of PES can be changed up to ± 40%. The amount of dicyandiamide can be changed up to ± 50%.</p><p> The densified PES was the same as that used in Comparative Examples 1 and 2. The average particle size was 10 to 25 microns, with less than 13% by weight being smaller than 5 microns and less than 4% by weight being larger than 40 microns. 24.9 parts by weight of GY285 and 6.0 parts by weight of MY-0510 were mixed in a resin reaction kettle and heated to 65 ° C with stirring. When this temperature is reached, 13.5 parts by weight of miniaturized PES 500 3P is added to the resin reaction kettle. The mixture is then heated to 128 ± 2 ° C and maintained at this temperature for 75 minutes. At the end of 75 minutes, remove the heat and add 21 parts by weight MY-0510 to the reaction kettle. Continue stirring to cool the mixture to 65 ° C. Add 15.8 parts of 3,3-DDS and mix for 15 minutes. Then 1.3 parts of dicyandiamide is added and the mixture is stirred at 65 ° C for 5 minutes. Finally, add 17.5 parts of densified PES and stir for 10 minutes. The minimum viscosity of this resin was measured as described in Comparative Example 1 and found to be about 370 poise. 1m containing 193gsm 3K PW carbon woven fabric²Panels were manufactured by first producing 70 g of resin prepreg per unit. The prepreg was made as follows:</p><p> Using a reverse roll coater, apply the resin on the release paper at about 165 ° F (74 ° C) and 70 g / m.²A film containing the above was prepared. This resin film is 193g / m²The carbon fiber woven fabric having the area weight of was impregnated. Next, this prepreg was attached to an HRH® 10 core and cured in the same manner as in Comparative Example 1. The peel strength was about 32 inches-pounds / 3 inches wide on the 3-pound core and about 31 inches-pounds / 3 inches wide on the 8-pound cores.</p><p>(Comparative example 4) A sandwich panel was produced in the same manner as in Comparative Example 3 except that KEVLAR® 2.5 pcf HRH® 36 was used as the core to replace HRH® 10. Peel strength was found to be 12 to 18 inches-pounds / 3 inches wide.</p><p>(Comparative example 5) A sandwich panel was produced in the same manner as in Comparative Example 4, except that the HRH (registered trademark) 36 core was edge-coated with various coatings containing no polyamide adhesive. One of the materials was A-22, a commercially available anionic surfactant available from Cytee Industry (West Paterson, NJ). As described in US Pat. No. 00/5575882A, a significant increase in honeycomb modulus was observed after immersing the core in a 2 wt% solution of A-22 anionic surfactant. The second edge coating material was 23056, a typical phenolic resin for non-metallic cores such as HRH® 10 cores. The 23056 is sold by Durez, Addison, Texas. The third material was Phenolic Coating 94-917, a modified phenolic resin used as a can coating. Phenolic Coating 94-917 is sold by Durez, Addison, Texas. Honeycomb edge is about 6.3g / ft²It was applied with each coating of. All of the resulting panel peel strengths were less than 11 inches-pounds / 3 inches wide.</p>

Make the honeycomb edge NYSOL 1020 (also ZYTEL) so that the adhesive depth "D" is between 0.005 inch (0.13 mm) and 0.008 inch (0.20 mm). Sandwich panels are produced in the same manner as in Comparative Examples 1 and 2, except that they are coated with an aqueous solution of a polyamide adhesive (also known as FE310018). Apply adhesive (coating) to the honeycomb edges as follows: (1) Clean the smooth, horizontal glass table surface with a degreasing solvent to remove all debris and dirt; (2) Place the Gardner knife on the table Place on a surface and adjust the gaps set at both ends of the knife to achieve the desired film thickness; (3) Pour the polyamide solution on the table surface parallel to the Gardner knife and apply even pressure to both ends of the knife. Sprinkle and drag the knife across the adhesive to make a thin film slightly larger than the core area; (4) Visually inspect the resulting film to confirm that a uniform film of constant thickness has been formed; (5) ) Place the core on the film surface and manually press down with a uniform pressure to evenly apply the adhesive film to the core; (6) Then lift the core from the table surface and in an electric oven, Dry at 250 ° F (121 ° C) for 4 minutes; (7) Weigh the core before and after coating and measure the amount of nylon adhesive remaining on the core edge; and (8) Desired polyamide coating amount ( 3 and 10 g / ft in this example²Repeat this step if necessary to obtain). The panel manufactured in this example has a peel strength of about 90 inches-pounds / 3 inches wide. The adhesive bond between the core edge and the epidermis or surface sheet is strong enough that during the peel test, the honeycomb wall (not the core-skin junction) structurally breaks above at least part of the honeycomb edge. Will do.

A sandwich panel was produced in the same manner as in Example 1 except that the prepreg surface sheet or skin was the same as that used in Comparative Example 3. HRH® 10 (8 lbs) core edge coated with NYSOL 1020 nylon adhesive solution 4.3 g / ft²Achieved the amount of adhesive applied. The coating depth "D" on the core edge was about 0.005 inches (0.13 mm). The peel strength of this panel was 91 inches-pounds / 3 inches wide, with structural breakage within the honeycomb over substantially the entire portion of the honeycomb, but not at the core-skin junction.

Sandwich panels were manufactured in a manner similar to Example 2 except that HRH® 36 (2.5 pcf) was replaced in place of HRH® 10 cores. 6 individual core edges 1.1, 2.3, 3.4, 3.8, 4.0 and 6.2 g / ft²Was applied with a polyamide adhesive (ie, a 40% solid aqueous solution of NYSOL 1020 according to Example 1). The peel strength of the resulting panels was 26, 30, 59, 71, 71 and 76 inches-pounds / 3 inches wide, respectively. During the peeling test, the surface sheet was separated from the core at the junction between the surface sheet and the core. Based on this example, in this particular type of sandwich panel structure, the honeycomb edges are at least 3.0 g / ft.²Therefore, it is preferable that it is coated with a polyamide adhesive.

A sandwich panel was produced in the same manner as in Example 3 except that ELVAMIDE 8023R was used as a solution instead of NYSOL 1020 and applied to the honeycomb. The solution consisted of 64% by weight ELVAMIDE 8023R, 16% by weight ethanol, 10% by weight benzyl alcohol, and 9% by weight water. Apply sufficient solution to the edges of the three individual cores and apply 1.8, 2.9 and 3.7 g / ft of nylon adhesive²Achieved the coating amount of. The peel strength of the resulting panels was 37, 39 and 42 inches-pounds / 3 inches wide, respectively. During the peeling test, the surface sheet was separated from the core at the junction between the surface sheet and the core. Based on this example, in this particular type of sandwich panel structure, the honeycomb edges are at least 1.0 g / ft.²Therefore, it is preferable that it is coated with a polyamide adhesive coating.

A sandwich panel was produced in the same manner as in Example 3 except that the prepreg surface sheet was produced using F593 resin available from Hexcel. The peel strength of the panel is about 63 inches-pounds / 3 inches wide, with a core edge of about 5.5 g / ft.²It was the NYSOL 1020 polyamide coating weight. When the surface sheet was applied to the core without the polyamide edge coating, the peel strength was only about 5 inches-pounds / 3 inches wide.

Sandwich panels were produced in the same manner as in Example 3, except that a high temperature / high humidity polyamide adhesive was used in place of NYSOL 1020. This polyamide adhesive is preferably an epoxy and phenolic resin, an appropriate amount of curing agent and / or a catalyst for the resin (eg, 4,4'-diaminodiphenylsulfone (4-4-DDS)) and Dow-Corning. Consists of ELVAMIDE 8023R in combination with a coupling agent such as Z-6040 available from (Midland, Michigan). In a typical formulation, a 25-30 wt% ELVAMIDE 8023R solution (ethanol: benzyl alcohol = 1: 2, by weight) was first prepared. This polyamide solution is then added to a 15-25% by weight novolak epoxy solution (eg, 45-50% by weight ECN in toluene / xylene). Combined with 1299 epoxy) and 5 to 15% by weight phenolic resin (eg, 23-983 Resin available from Durez, Addison, Texas). Also, 0.5 to 2% by weight curing agent (eg 4-4-DDS), 0.1 to 0.5% by weight phenol urea catalyst (eg DIURON available from EIDuPont de Nemours &amp; Company (Wilmington, Delaware)), And 0.1 to 0.5% by weight of coupling agent (eg Z-6040) was included. Other additives such as AGERITE D Resin (RT Bander Built, Noor Walk, Connecticut) may be added in small amounts (eg, less than 1% by weight).

Manufactured with the above formulation, 2-4g / ft²The peel strength of sandwich panels with a polyamide adhesive coating will generally be about 32 inches-pounds / 3 inches wide when measured at room temperature under dry conditions. Under moist conditions (eg, above 95% relative humidity), when measured at an increased temperature of 71 ° C, the peel strength of the same sandwich panel generally falls to about 25 inches-pounds / 3 inches wide. Not too much. For polyamide adhesives containing pure nylon, nylon tends to adhere more water than epoxies, so a typical reading of peel strength is 37-42 inches-pounds / 3 inches at room temperature (dry). Width, 20 to 34 inches-pounds / 3 inches wide under increased temperature / wet conditions.

A sandwich panel was produced in the same manner as in Example 3 except that ELVAMIDE 8066 was used instead of NYSOL 1020. The solution applied to the honeycomb consisted of 16% by weight ELVAMIDE 8066, 38% by weight ethanol, 23% by weight benzyl alcohol, and 23% by weight water. Apply sufficient solution to the edges of the two separate cores, 1.3 and 3.9 g / ft²Achieved the edge coating amount of the polyamide adhesive. The peel strength of the resulting panels was 19.5 and 56.3 inches-pounds / 3 inches wide, respectively.

A sandwich panel was produced in the same manner as in Example 3 except that GRILAMID TR55 was used instead of NYSOL 1020 as the polyamide adhesive. Edges of two separate cores 2.3 and 4.8 g / ft²It was applied with the polyamide adhesive of. The peel strength of the resulting panels was 22.8 and 26.0 inches-pounds / 3 inches wide at room temperature, respectively. When the test was performed under wet conditions at 71 ° C, the peel strength of the two panels increased to 24.6 and 28.6 inches-pounds / 3 inches wide, respectively. It should be noted that GRILAMID TR55 is nylon 12 based and has a much higher glass transition temperature (Tg) than other polyamide adhesives such as nylon 6 based NYSOL 1020. Therefore, polymer adhesives such as GRILAMID TR55, which consist of nylon 12 and have a relatively high Tg, are more suitable for use in elevated temperatures and / or moist environments.

Thus, although the representative embodiments of the present invention have been described, those skilled in the art will note that the ones in the disclosure are only typical and that substitution, regulation and modification can be carried out within the scope of the present invention. Should be. Therefore, the present invention is not limited by the preferred embodiments described above, but only within the scope of the appended claims.

<figref num="1">It is explanatory drawing which shows the honeycomb core and the composite surface sheet before joining a honeycomb to a surface sheet by using an adhesive.</figref><figref num="2">It is explanatory drawing which shows the honeycomb sandwich panel manufactured by joining the constituent material shown in FIG.</figref><figref num="3">It is explanatory drawing which shows one edge-coated honeycomb wall joined to the surface sheet.</figref><figref num="4">It is explanatory drawing which shows that the core fracture occurs when the surface sheet is forcibly peeled from a honeycomb.</figref>