具体实施方式：
Composite Sheet
[0023]This invention is directed to a fiber-resin composite sheet comprising a reinforcing fibrous substrate and a resin coated onto or into the substrate.
[0024]The substrate may be in the form of a paper or fabric.
Substrate
[0025]Preferably the reinforcing fibers of the substrate have a filament tenacity of from 3 to 60 grams per dtex and a filament diameter of from 5 to 200 micrometers. In some embodiments fibers having filament diameters of from 7 to 32 micrometers is preferred. In other embodiments, the filament tenacity is from 8 to 60 grams per dtex. In some embodiments the substrate is a paper.
[0026]A preferred paper contains both high strength fibers and binder. In one embodiment, the paper comprises from 10 to 100 weight percent fibers and correspondingly from 0 to 90 weight percent binder. In another embodiment the paper comprises from 10 to 85 weight percent fibers and from 15 to 90 weight percent binder. In yet another embodiment the paper comprises from 50 to 100 weight percent fibers and from 0 to 50 weight percent binder.
[0027]The high strength fibers of the paper have an initial Young's modulus of at least 180 grams per dtex (200 grams per denier) and a tenacity of from 11 grams per dtex (10 grams per denier) to 56 grams per dtex (50 grams per denier). In one embodiment, the length of the fibers in the paper is from 0.5 to 26 mm. In another embodiment, the length of the fibers is from 1 to 8 mm and in yet another embodiment, the length of the fibers is from 1.5 to 6 mm.
[0028]The reinforcing substrate can also include fibers of lower strength and modulus blended with the higher modulus fibers. The amount of lower strength fiber in the blend will vary on a case by case basis depending on the desired strength of the core structure. The higher the amount of low strength fiber, the lower will be the strength of the core structure. In a preferred embodiment, the amount of lower strength fiber should not exceed 30%. An example of such lower strength fiber is poly (ethylene terephtalamide) fiber or cellulose.
[0029]The reinforcing substrate can contain small amounts of inorganic particles and representative particles include mica, vermiculite, and the like; the addition of these performance enhancing additives being to impart properties such as improved fire resistance, thermal conductivity, dimensional stability and the like to the substrate and hence the final core structure.
[0030]In some embodiments, the paper thickness is from 12 to 1270 micrometers (0.5 to 50 mils) and the paper basis weight is from 10 to 900 grams per square meter (0.29 to 230 ounces per square yard). The paper may be a single sheet or a plurality of sheets that have been laminated together.
[0031]Fibers comprising the paper may be in the form of cut fiber (floc) or pulp either used alone or in combination.
[0032]Floc is generally made by cutting continuous spun filaments into specific-length pieces. If the floc length is less than 0.5 millimeters, it is generally too short to provide a paper with adequate strength. If the floc length is more than 26 millimeters, it is very difficult to form uniform wet-laid substrates. Floc having a diameter of less than 5 micrometers, and especially less than 3 micrometers, is difficult to produce with adequate cross sectional uniformity and reproducibility. If the floc diameter is more than 20 micrometers it is very difficult to form uniform papers of light to medium basis weights.
[0033]The term “pulp”, as used herein, means particles of fibrous material having a stalk and fibrils extending generally therefrom, wherein the stalk is generally columnar and about 10 to 50 micrometers in diameter and the fibrils are fine, hair-like members generally attached to the stalk measuring only a fraction of a micrometer or a few micrometers in diameter and about 10 to 100 micrometers long. One possible illustrative process for making aramid pulp is disclosed in U.S. Pat. No. 5,084,136.
[0034]A preferred binder is fibrids. The term “fibrids” as used herein, means a very finely-divided polymer product of small, filmy, essentially two-dimensional particles having a length and width on the order of 100 to 1000 micrometers and a thickness on the order of 0.1 to 1 micrometer. Fibrids are typically made by streaming a polymer solution into a coagulating bath of liquid that is immiscible with the solvent of the solution. The stream of polymer solution is subjected to strenuous shearing forces and turbulence as the polymer is coagulated. Preparation of fibrids is taught in U.S. Pat. No. 3,756,908 with a general discussion of processes to be found in U.S. Pat. No. 2,999,788. The fibrids should be refined in accordance with the teachings of U.S. Pat. No. 3,756,908 only to the extent useful to permit permanent densification and saturability of the final paper. Preferable polymers for fibrids in this invention include aramids (poly (m-phenylene isophthalamide) and poly (p-phenylene terephthalamide)). Other binders include polysulfonamide (PSA), poly-phenylene sulfide (PPS), and polyimides. Other binder materials are in the general form of resins and can be epoxy resins, phenolic resins, polyureas, polyurethanes, melamine formaldehyde resins, polyesters, polyvinyl acetates, polyacrylonitriles, alkyd resins, and the like. Preferred resins are water dispersible and thermosetting. Most preferred resin binders comprise water-dispersible epoxy resins. The binder may also be derived from a biological source. An example of such a polymer is one based on 1,3-propanediol, the diol component being manufactured via a fermentation process from corn sugar. Soy is another source of biological binder material.
[0035]The composition of both fibers and fibrids can vary. Preferable types of fibers include aromatic polyamide, liquid crystal polyester, polybenzazole, polypyridazole, polysulfonamide, polyphenylene sulfide, polyolefins, carbon, glass, ceramic, basalt and other inorganic fibers or mixture thereof.
[0036]Preferable types of fibrids include aromatic polyamide, aliphatic polyamide, polysulfonamide (PSA), poly-phenylene sulfide (PPS), polyimide and blends thereof.
[0037]Suitable aromatic polyamides are meta-aramid and para-aramid. A suitable meta-aramid polymer is poly (m-phenylene isophthalamide) and a suitable para-aramid polymer is poly (p-phenylene terephthalamide).
[0038]Papers made using fibrids and short fibers have been described in U.S. Pat. No. 3,756,908, to Gross and U.S. Pat. No. 5,137,768 to Lin.
[0039]A commercially available p-aramid high modulus high strength fiber reinforcing paper substrate for the production of core structures is KEVLAR® N636 paper sold by E. I. DuPont de Nemours and Company, Wilmington, Del. Core structures can also be made from m-aramid fiber nonwoven substrate also available from DuPont under the tradename NOMEX®.
[0040]A paper substrate may also comprise cellulose as exemplified by a Kraft paper. Cellulose may also be present in a paper comprising a blend of p-aramid and/or m-aramid and cellulosic fibers. A paper may also comprise polyester or glass fibers either alone or in combination with other fibers.
[0041]Once the paper is formed, it is calendered to the desired density or left uncalendered depending on the target final density.
[0042]In some embodiments, the fibrous reinforcement substrate is a fabric material comprising continuous filament yarns. By fabric is meant structures that are may be woven, unidirectional, may be multiaxial, 3-dimensional or a nonwoven randomly oriented discontinuous fiber mat. Each of these fabric styles is well known in the art. A multitude of different fabric weave patterns including plain, twill, satin, crowfoot satin, plain derivative, leno and mock leno may be used. Plain weave patterns are preferred. Carbon, ceramic, basalt or glass fibers are preferred fibers for the fabrics. In some embodiments the fabric filaments are of aromatic polyamide or aromatic copolyamide. The yarns can be intertwined and/or twisted. For purposes herein, the term “filament” is defined as a relatively flexible, macroscopically homogeneous body having a high ratio of length to width across its cross-sectional area perpendicular to its length. The filament cross section can be any shape, but is typically circular or bean shaped. Herein, the term “fiber” is used interchangeably with the term “filament”. A “yarn” is a plurality of filaments. The filaments can be any length. Multifilament yarn spun onto a bobbin in a package contains a plurality of continuous filaments. The multifilament yarn can be cut into staple fibers and made into a spun staple yarn suitable for use in the present invention. The staple fiber can have a length of about 1.5 to about 5 inches (about 3.8 cm to about 12.7 cm). The staple fiber can be straight (i.e., non crimped) or crimped to have a saw tooth shaped crimp along its length, with a crimp (or repeating bend) frequency of about 3.5 to about 18 crimps per inch (about 1.4 to about 7.1 crimps per cm).
[0043]Other suitable fiber forms for some of the fabrics include stretch broken or comingled yarns.
[0044]In other embodiments, the fabric is a non-woven mat comprising randomly oriented discontinuous filaments in which the filaments are bonded or interlocked. Example of a nonwoven fabric mats include felts and spunlace or spunbonded sheets.
Polymeric Resin Coating
[0045]A polymeric resin is coated onto or into the reinforcing substrate. In some embodiments, the resin only partly impregnates into the substrate. The coating resin comprises a first thermoplastic polymer and a second thermoplastic polymer. The first and second polymers form a two phase blend. The first polymer comprises from 35 to 99 weight percent of the blend of first and second polymers. In some embodiments, the first polymer comprises from 45 to 85 weight percent or even 45 to 70 weight percent of the blend of first and second polymers. The composite sheet may optionally comprise a third polymer.
[0046]In addition, the first or second polymers may optionally comprise, either alone or in combination, reactive or non-reactive additives such as colorants, diluants, processing agents, UV additives, fire retardants, mineral fillers, organic fillers, bonding additives, surfactants, pulp, antioxidants, antistatics, slip agents and tackifiers. A suitable pulp is aramid pulp. Methods for incorporation of these additives into the polymer are well known.
[0047]Suitable fire retardants include brominated flame retardants, red phosphorus, asbestos, antimony trioxide, borates, metal hydrates, metal hydroxides, Tetrakis(hydroxymethyl)phosphonium salts, fluorocarbons or combinations thereof.
[0048]At least one plasticizer can optionally be added to the polymer, preferably to the second polymer. Suitable examples include phthalate-based plasticisers, trimellitate-based plasticisers, adipate-based plasticisers, sebacate-based plasticisers, maleate-based plasticisers, organophosphate-based plasticisers, sulfonamide-based plasticisers, benzoate-based plasticisers, epoxidised vegetable oils, poly(ethylene oxide) or combinations thereof. In some embodiments, the at least one plasticizer is a plasticizer having a reactive group such as an epoxidised vegetable oil. Examples of epoxidised vegetable oils are epoxidized soybean oil (ESO), epoxidized linseed oil (ELO), epoxidized tallate or combinations thereof.
[0049]The polymeric resin coating as described herein provides a coated substrate that is amenable to processing by laser welding techniques.
First Polymer
[0050]Both the first and second polymers belong to a group of polymers having good mechanical properties and good chemical resistance. Such resins are frequently referred to in the trade as High Performance Polymers or Engineered Thermoplastics. Some rubbers and elastomers also fit into this category of material.
[0051]The first polymer is a thermoplastic polymer having a melting point of from 75 to 400 degrees C. In some embodiments the first polymer has a melting point of from 110° C. to 300° C. or even from 140° C. to 230° C. The first polymer forms a continuous or co-continuous phase with the second polymer. A continuous phase, as defined by the International Union of Pure and Applied Chemistry (IUPAC), refers to a matrix in which a second phase is dispersed in the form of particles. A co-continuous phase, as described by IUPAC, is a matrix that is either a semi-interpenetrating polymer network (SIPN) or an interpenetrating polymer network (IPN). A semi-interpenetrating polymer network is a polymer comprising one or more polymer network(s) and one or more linear or branched polymer(s) characterized by the penetration on a molecular scale of at least one of the networks by at least some of the linear or branched chains. An interpenetrating polymer network is a polymer comprising two or more networks which are at least partially interlaced on a molecular scale but not covalently bonded to each other and cannot be separated unless chemical bonds are broken. The second polymer forms a dispersion within the first polymer or a co-continuous network within the first polymer. The first polymer provides the major contribution to the thermal and mechanical performance of the composite. Preferably, the first polymer should have a melting point that is higher than the peak operating temperature of the article comprising the composite sheet. The peak operating temperature is defined as the maximum temperature to which the article is exposed when in service. The peak operating temperature will vary according to the particular application for which the polymer is used. Other factors affecting the peak operating temperature are climatic situation, geographical zone, and/or seasonal fluctuations encountered as well as proximity to a heat source. In some embodiments, the first polymer should have a melting point that is at least 5° C. higher than the peak operating temperature. In other embodiments, the first polymer should have a melting point that is at least 10° C. higher than the peak operating temperature.
[0052]Preferably, the first polymer is polyolefin, polycondensate, or an elastomeric block copolymer.
[0053]Examples of elastomeric block copolymers are acrylonitrile-butadiene-styrene, polyisopropene-polyethylene-butylene-polystyrene or polystyrene-polyisoprene-polystyrene block copolymers, polyether-ester block copolymers, or combinations thereof.
[0054]Other suitable polymers include polyamides, polyamide copolymers, polyimides, polyesters, polyurethanes, polyurethane copolymers, polyacrylics, polyacrylonitrils, polysulfones, silicone copolymers.
[0055]In some embodiments it is preferred that the first polymer is polyamide, polyester, polyester copolymers or combinations thereof. A preferred polyolefin is polypropylene.
[0056]In some other embodiments, it is preferred that the first polymer is a polyamide such as an aliphatic polyamide or a semi-aromatic polyamide. Preferred polyamides are polyamides having an amine-end content of at least 30%, more preferably of at least 50%, most preferably of at least 70%. Preferably, suitable aliphatic polyamides are Nylon 6, Nylon 66, Nylon 6/66, Nylon 11, Nylon 12, Nylon 612, Nylon 13, Nylon 1010, or combinations thereof. More preferably, suitable aliphatic polyamides are Nylon 6, Nylon 11, Nylon 12, Nylon 612, Nylon 13, Nylon 1010, or combinations thereof.
[0057]Preferable semi-aromatic polyamides are Nylon 6T, Nylon 6/6T, Nylon 3T, Nylon 6/3T, Nylon 66/6T, Nylon 10/6T, Nylon 12/6T, Nylon 10/3T, Nylon 12/3T, and/or combinations thereof.
[0058]Amorphous polyamides can preferably be used in a range up to 10 weight percent based on the total weight of the polyamides. Preferred is the use of crystalline, semi-crystalline polyamides or combinations thereof.
Second Polymer
[0059]The second polymer is dispersed in the continuous or co-continuous phase of the first polymer and has an effective diameter of from 0.01 to 15 micrometers.
[0060]Where the dispersed second thermoplastic polymer is present as spherical particles, the effective diameter is the diameter of the particle. Where the dispersed second thermoplastic polymer is present as non-spherical particles such as elongated spheroid shapes, ellipsoids, or a network of branched filament-like structures, the effective diameter is the diameter that can be traced around a plane of the smallest cross sectional area of the particle.
[0061]The second polymer has a melting point of from 25 to 350 degrees C. In some embodiments, the melting point of the second polymer is from 50 to 200 degrees C. Preferably the second polymer has a melting point at least 5 degrees C. lower than the melting point of the first polymer. In some embodiments the second polymer has a melting point at least 10 degrees C., 20 degrees C. or even 30, 50, 75, 100 or 120 degrees C. lower than the melting point of the first polymer. The second polymer facilitates processing ease and speed when the composite sheet is being formed into a composite article, for example during laser welding of a core structure. The second polymer also enhances the bond strength between successive fibrous substrates.
[0062]In some embodiments, the major dimension of the particles is of the same order of magnitude as the smallest dimension of the filaments comprising the reinforcing substrate.
[0063]In other embodiments, the major dimension of the particles is less than the smallest dimension of the filaments comprising the reinforcing substrate. The effective diameter of the particles of the second polymer is from 0.01 to 15 micrometers. In some embodiments, the diameter is from 0.01 to 5 micrometers or even from 0.01 to 1 micrometer. By effective diameter is meant the smallest circular diameter that can be circumscribed around the cross section of the particle.
[0064]Preferably, the second polymer is polyolefin, polycondensate, or an elastomeric block copolymer.
[0065]Examples of polyolefin polymers are polyethylene, polyethylene copolymers, polypropylene, polypropylene copolymers, polybutylene and polybutylene copolymers.
[0066]Suitable polyethylene polymers include low density polyethylene, very low density polyethylene, metallocene polyethylene and polyethylene copolymers such as ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymers and ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymers partially neutralized with metal salts.
[0067]Where the second polymer is an ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer, the α,β-unsaturated C3-C8 carboxylic acid can be chosen from acrylic acid or methacrylic acid.
[0068]The ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer is preferably a terpolymer of ethylene, α,β-unsaturated C3-C8 carboxylic acid and α,β-unsaturated C3-C8 dicarboxylic acid.
[0069]The α,β-unsaturated C3-C8 dicarboxylic acid can be maleic acid, maleic anhydride, C1-C4 alkyl half esters of maleic acid, fumaric acid, itaconic acid and itaconic anhydride. Preferably, the α,β-unsaturated C3-C8 dicarboxylic acid is maleic anhydride, ethyl hydrogen maleate and methyl hydrogen maleate. More preferably, the α,β-unsaturated C3-C8 dicarboxylic acid is maleic anhydride, methyl hydrogen maleate or combinations thereof.
[0070]The ethylene/α,β-unsaturated C3-C8 carboxylic acid/α,β-unsaturated C3-C8 dicarboxylic acid polymer can further comprise up to 40 weight percent of a C1-C8 alkyl acrylate softening comonomer, which is preferably chosen from methyl (meth)acrylate, ethyl (meth)acrylate or n-butyl (meth)acrylate, more preferably from n-butyl acrylate or ethyl (meth)acrylate.
[0071]The term softening comonomer is well-known to those skilled in the art and refers to comonomers such as C1-C8 alkyl acrylate. The term (meth)acrylate covers both acrylate and methacrylate.
[0072]In the ethylene/α,β-unsaturated C3-C8 carboxylic acid/α,β-unsaturated C3-C8 dicarboxylic acid polymer, the α,β-unsaturated C3-C8 carboxylic acid can be present in a range of from 2 to 25 weight percent and the α,β-unsaturated C3-C8 dicarboxylic acid can be present in a range of from 0.1 to 15 weight percent with the proviso that the α,β-unsaturated C3-C8 carboxylic acid and the α,β-unsaturated C3-C8 dicarboxylic acid are present from 4 to 26 weight percent, and with the further proviso that the total comonomer content, including the C1-C8 alkyl acrylate softening comonomer, does not exceed 50 weight percent.
[0073]In other embodiments, the second polymer is an ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer partially neutralized with metal ions, which is commonly referred to as “ionomer”. The total percent neutralization is from 5 to 90 percent, preferably 10 to 70 percent and more preferably between 25 and 60 percent of the ionomer.
[0074]In the case where the second thermoplastic polymer is an ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer partially neutralized with metal ions, the α,β-unsaturated C3-C8 carboxylic acid can be acrylic acid or methacrylic acid. The ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer partially neutralized with metal ions is preferably a terpolymer of ethylene, α,β-unsaturated C3-C8 carboxylic acid and α,β-unsaturated C3-C8 dicarboxylic acid partially neutralized with metal ions. The α,β-unsaturated C3-C8 dicarboxylic acid can be chosen from the same components as described above.
[0075]The ethylene/α,β-unsaturated C3-C8 carboxylic acid/α,β-unsaturated C3-C8 dicarboxylic acid polymer partially neutralized with metal ions can further comprise up to 40 weight percent of an C1-C8 alkyl acrylate softening comonomer, which is preferably chosen among the same components as already described above.
[0076]In the ethylene/α,β-unsaturated C3-C8 carboxylic acid/α,β-unsaturated C3-C8 dicarboxylic acid polymer partially neutralized with metal ions, from 5 to 90 percent of the total number of α,β-unsaturated C3-C8 carboxylic acid units in the polymer are neutralized with metal ions, and the α,β-unsaturated C3-C8 carboxylic acid and the α,β-unsaturated C3-C8 dicarboxylic acid can be present in the same amounts as described above, with the same proviso regarding the α,β-unsaturated C3-C8 carboxylic acid and the α,β-unsaturated C3-C8 dicarboxylic acid and the same further proviso regarding the total comonomer content, including the C1-C8 alkyl acrylate softening comonomer, as described above.
[0077]The ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymer that is partially neutralized with metal ions which can be any metal ion of group I or group II of the periodic table. The preferred metal ions are sodium, zinc, lithium, magnesium, calcium or a mixture of any of these. More preferably, the ions are sodium, zinc, lithium or magnesium. Most preferably, the ion is zinc, lithium or combinations thereof.
[0078]Partially neutralized ethylene/α,β-unsaturated C3-C8 carboxylic acid copolymers may be prepared by standard neutralization techniques such as is disclosed in U.S. Pat. No. 3,264,272. The resulting ionomers have an melt index (MI) of from 0.01 to 100 grams/10 minutes or more preferably from 0.1 to 30 grams/10 minutes, as measured using ASTM D-1238, condition E (190° C., 2160 gram weight).
[0079]The above ionomers can be prepared by free-radical copolymerization methods, using high pressure, operating in a continuous manner as described in U.S. Pat. Nos. 4,351,931; 5,028,674; 5,057,593 and 5,859,137. Exemplary examples of ionomeric materials include products available from DuPont under the tradename SURLYN, from Exxon under the tradename IOTEK and Dow under the tradename AMPLFY 10.
[0080]Examples of elastomeric block copolymers are acrylonitrile-butadiene-styrene, polyisopropene-polyethylene-butylene-polystyrene or polystyrene-polyisoprene-polystyrene block copolymers, polyether-ester block copolymers, or combinations thereof.
[0081]Other suitable polymers are polyamides, polyamide copolymers, polyimides, polyesters, polyurethanes, polyurethane copolymers, polyacrylics, polyacrylonitriles, polysulfones, silicone copolymers.
[0082]In some embodiments it is preferred that the second polymer is a thermoplastic elastomeric block copolymer such as polyisopropene-polyethylene-butylene-polystyrene or polystyrene-polyisoprene-polystyrene block copolymer.
[0083]The first and second polymers may be blended together and produced into various forms such as pellets, fibers, sheets, films, fabrics, hotmelts, powders, liquids or combinations thereof. As examples, the blending can be done by using a kneader, a single or twin screw extruder or a heated melt mixer at a temperature of between 80° C. to 420° C. The first polymer forms a continuous or co-continuous phase with the second polymer upon addition of the second polymer.
Third Polymer
[0084]In some embodiments, a third polymer may be present in an amount of from 0 to 99.7 weight percent based on the total weight of first, second and third polymers. The third polymer may also be a bimodal component of the first and second polymers. The third polymer may be polyetheretherketone (PEEK), polyetherketone (PEK), polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), polyetherimide (PEI), polysulphone (PSU), polyimide (PI) and polyphenylene oxide (PPO). The third polymer may be present as a separate layer on the substrate or it may replace the first polymer. In another embodiment the third polymer is blended with the first polymer. An example of such a blend is a polyamide first polymer and a polyimide third polymer.
Composite Article
[0085]The resin coated substrate described above may be incorporated into a composite article such as a structural core, an impact resistant article or a laminate.
[0086]It has been found that the use of a first and second resin as described above provides a number of advantages to a composite article.
[0087]When subjected to a shear or butt joint test, laminates show cohesive failure. Similar tests on laminates comprising an aliphatic polyamide resin such as nylon 6 or nylon 12 show adhesive failure. This result is indicative of a blend of first and second resin delivering (a) improved bending performance in a sandwich structure comprising core incorporating and (b) ease of expansion during manufacture of a honeycomb core.
[0088]Substrates coated with a first and second resin show improved shape retention when compared with similar substrates coated with nylon based resins. This can be demonstrated by placing a sample of coated substrate between two right angle shaped aluminum plates, placing the plate assembly for one minute in an oven at a range of temperatures from 50 to 325 degrees C., removing the plate assembly from the oven, cooling for 10 minutes and then removing the coated substrate from the plate assembly. The plates should be pre-heated to the required temperature before the substrate is positioned between the plates. After 24 hours storage at ambient conditions, the angle formed by the two sides of the substrate is measured. This is known as the retained angle. The closer the retained angle is to 90 degrees, the better is the shape retention property. By ambient conditions is meant a temperature of 23+/−1 degrees C. and a humidity of 50+/−10%. The smaller the difference in
[0089]The first and second resins permit a broader operating temperature range of from about 175 to 300 degrees C. when compared to a range of about 185 to 275 degrees C. for a nylon resin.
[0090]The first and second resins are amenable to laser welding of substrates to form a laminate as well as conventional bonding in a hot press, oven or autoclave. Such versatility is not possible with all resin systems.
[0091]A core structure may be in the form of a honeycomb or a folded core.
[0092]FIG. 1A is a plan view illustration of one honeycomb 1 comprising a coated substrate and shows cells 2 formed by cell walls 3. FIG. 1B is an elevation view of the honeycomb shown in FIG. 1A and shows the two exterior surfaces, or faces 4 formed at both ends of the cell walls. The core also has edges 5. FIG. 2 is a three-dimensional view of the honeycomb. Shown is honeycomb 1 having hexagonal cells 2 and cell walls 3. The thickness of the honeycomb is shown at 10 in FIG. 2. Hexagonal cells are shown; however, other geometric arrangements are possible with square, over-expanded and flex-core cells being among the most common possible arrangements. Such cell types are well known in the art and reference can be made to Honeycomb Technology pages 14 to 20 by T. Bitzer (Chapman & Hall, publishers, 1997) for additional information on possible geometric cell types.
[0093]FIG. 3 shows a structural sandwich panel 5 assembled from a honeycomb core 6 with face sheets 7 and 8, attached to the two exterior surfaces of the core. The preferred face sheet material is a prepreg, a fibrous sheet impregnated with thermoset or thermoplastic resin, although face sheets of other material may also be utilized. Examples of other types of facesheet include metal, wood, ceramic and fiber-reinforced plastic. In some circumstances, an adhesive film 9 is also used to enhance the bonding of the facesheet to the core. Normally there are at least two prepreg skins on either side of the core.
[0094]FIG. 4 shows a folded core structure which is a 3-dimensional structure of folded geometric patterns folded from a relatively thin planar sheet material. Such folded or tessellated sheet structures are discussed in U.S. Pat. Nos. 6,935,997 B2 and 6,800,351 B1. A chevron is a common pattern for three dimensional folded tessellated core structures. Such structures are different from honeycomb structures. A preferred tessellated folded structure is of the type described in U.S. Pat. No. 6,913,570 B2 and United States patent publication number 20100048078. A corrugated sheet is another form of a folded core structure.
[0095]The core structure may optionally be coated with a fourth polymeric resin. Such a resin can provide additional flame resistance and mechanical strength to the core. Suitable fourth resins include phenolic, flame-retarded (FR) epoxy, FR polyester, polyamide, and polyimide resins. Phenolic resins normally comply with United States Military Specification MIL-R-9299C. Preferably, the resin is a phenol formaldehyde resin and may be a resole or a novolac resin. Other aldehydes, for example furfuraldehyde, may be used, and other phenols for example hydroquinone and p-cresol may also be used. The preparation of p-cresol and properties of such resins are described in “Phenolic Resins,” authors A. Knop and L. A. Pilato, Springer-Verlag, Berlin, 1985. A resole resin is cured simply by the application of heat whereas a novolac resin requires for its cure the additional presence of a formaldehyde generating substance, for example hexamethylenetetramine, also known as hexamine. Resole type resins are preferred. Suitable phenolic resins are available from companies such Hexion Specialty Chemicals, Columbus, Ohio or Durez Corporation, Detroit, Mich. When the coating of the substrate by the fourth resin is conducted prior to core forming it is preferred that the resin is partially cured. Such a partial curing process, known as B-staging, is well known in the composite materials industry. By B-stage we mean an intermediate stage in the polymerization reaction in which the resin softens with heat and is plastic and fusible but does not entirely dissolve or fuse. The B-staged reinforcing substrate is still capable of further processing into the desired core shape.
[0096]When the resin impregnation is conducted after the core has been formed, it is normally done in a sequence of repeating steps of dipping followed by solvent removal and curing of the resin. The preferred final core densities (nonwoven sheet plus resin) are in the range of from 5 to 500 kg/m3. In some embodiments the range is from 10 to 300 kg/m3 while in other embodiments it is from 15 to 200 kg/m3. During the resin impregnation process, resin is absorbed into and coated onto the reinforcing substrate. The coating resin is applied to the core in accordance with known block dipping or substrate coating procedures.
[0097]The resins may be used as solutions or dispersions in solvents or dispersion media, for example water, acetone, propan-2-ol, butanone, ethyl acetate, ethanol, and toluene. Mixtures of these solvents may be used to achieve acceptable evaporation rates of the solvent from the core. The amount of solvent used will vary widely depending upon a numb