Composite structures having hollow portions and method of making the same

Abstract

A method for forming a composite structure having at least one hollow portion is disclosed. The method includes: placing a stack comprising two composite skins separated by a non-adherent three-dimensional (3D) insert in a compression molding tool; compression molding the stack of composite skins and non-adherent 3D insert while applying heat to the tool, wherein compression molding of the stack is carried out until each composite skin conforms to and adhere to the molding surface adjacent to it but the thermoset resin in each composite skin is not fully cure; opening the compression molding tool, wherein each composite skin remains adhered to the adjacent molding surface; removing the non-adherent 3D insert from the compression molding tool; closing the molding tool to allow portions of the skins to contact each other, creating at least one empty space between the non-contacting portions of the skins; and applying compressive pressure and heat to affect full curing of the skins.

Description

COMPOSITE STRUCTURES HAVING HOLLOW PORTIONS AND METHOD OF MAKINGTHE SAME

[0001] The present disclosure relates generally to composite structures with hollow portions and method of making the same.BRIEF DESCRIPTION OF THE DRAWINGS

[0002] FIGS. 1-8 illustrate the different stages of a press forming method to form a composite structure with at least one hollow portion according to an embodiment of the present disclosure.

[0003] FIG. 9 shows a two-part compression molding tool with a non-adherent insert for forming a composite structure with hollow portions.

[0004] FIG. 10 shows the geometry of the upper and lower composite skins after compression molding using the molding tool and the non-adherent insert shown in FIG. 9.

[0005] FIG. 11 shows the composite structure formed by compression molding using the molding tool and non-adherent insert shown in FIG. 9.DETAILED DESCRIPTION

[0006] Attempts have been made to produce hollow composite structures using bladders, meltable material, and collapsible mandrels. Such processing materials are placed on the inside of the hollow composite structures during the manufacturing process in order to provide back pressure necessary for the composite materials to consolidate during its cure. However, those processing materials have to be removed from the final composite structures. As a result, the hollow sections are traditionally limited to be constant sections by design, and to be open-ended, which limits their use to just simple Omega-Stringer configuration. If the hollow sections need sealing, it is necessary to add multiple parts madeout of various materials via supplementary operations. The manufacture of hollow parts is usually highly complex due to the need to reach inside the parts, which incurs quality risks, poor repeatability, lengthened cycle times, and greater labor costs.

[0007] Although highly desirable, lightweight hollow composite structures for aerospace applications are not commonly used at the present time due to the process limitations of the traditional manufacturing methods. Consequently, the hollow composite structures have been found to be both too costly and too slow to produce. The traditional methods also limit the design of hollow structures that can be produced. There remains a need for a fabrication method that can produce composite structures with hollow portions at high processing rate and low production costs and also allow possibilities for new hollow structure designs that can be produced.

[0008] Disclosed herein is a method for forming a composite structure having at least one hollow portion by compression molding (hereafter referred to as “press forming” method) at a relatively short cycle time, e.g. less than 1 hour, and at low cost.

[0009] Generally, the press forming method of the present disclosure includes: placing a stack comprising two thermosettable composite skins separated by a non-adherent, three- dimensional (3D) insert in a compression molding tool; applying compressive pressure and heat to the stack for a time period sufficient to mold and partially cure the skins; removing the non-adherent 3D insert from the molding tool while leaving the skins adhered to the molding surfaces of the tool; optionally, applying an adhesive film on one of the skins; closing the molding tool to allow portions of the skins to contact each other, creating at least one empty space between non-contacting portions of the skins; applying compressive pressure and heat to the skins in the tool to affect (or cause) full curing of the composite skins and bonding of the composite skins where the skins contact each other. The nonadherent 3D insert has a configuration that defines at least one hollow portion in the final composite structure, and is designed to prevent the two composite skins from contactingeach other when the insert is placed between the composite skins. The resulting structure after the press forming method described herein is a composite structure with at least one hollow portion. The press forming method disclosed herein can produce a composite structure with a hollow core or a composite structure having two or more hollow portions.

[0010] The thermosettable composite skins are composed of reinforcement fibers impregnated or infused with a curable thermoset resin composition. The optional adhesive film is formed from a curable resin containing one or more thermoset resin(s) and a curing agent. During the second compression molding in which portions of the skins are in contact, the application of compressive pressure and heat results in full curing of the composite skins. The entire press forming method can be carried out in less than one hour, and in some embodiments, less than 30 minutes.

[0011] FIGS. 1-8 illustrate the different stages of a press forming method according to an embodiment of the present disclosure.

[0012] Referring to FIG. 1, a compression molding tool 10 having at least a first (upper) mold portion 11 and a second (lower) mold portion 12 is provided. As shown, the molding tool is in an open position. The first and second mold portions 11 and 12 are movable relative each other, and have opposing molding surfaces 11a and 12a that cooperate to define a mold gap or mold cavity when the compression molding tool is in a closed position. The mold gap or mold cavity defines the geometrical shape of the final structure to be molded.

[0013] Referring to FIG. 2, a stack of two thermosettable composite skins 13a and 13b separated by a non-adherent 3D insert 14 is placed on the lower mold portion 12 while the molding tool is in an open position. Each thermosettable composite skin comprises reinforcement fibers impregnated or infused with a curable thermoset resin composition. At this stage, the resin composition in the composite skins is uncured, thus, the compositeskins are very tacky and pliable. The non-adherent 3D insert 14 is arranged between the composite skins (13a, 13b) and is configured to prevent the skins from sticking to or contacting each other. The non-adherent 3D insert 14 has a configuration that defines at least one hollow portion in the final structure. As shown in FIG. 2, the 3D insert 14 has a convex upper surface that defines the hollow portion, and the upper mold portion 11 has a concave surface that mates with the convex upper surface of the 3D insert 14.

[0014] Referring to FIG. 3, the compression molding tool is closed by moving the upper mold portion 11 toward the lower mold portion 12, causing the stack of composite skins and non-adherent 3D insert to be compressed between the molding surfaces of the upper and lower mold portions (11, 12). Compressive pressure and heat are applied during compression molding for a duration sufficient to cause each composite skin to conform to and adhere to the molding surface adjacent to it but not to fully cure the curable resin composition in the composite skin. At this stage, the composite skins went through minimum consolidation and the resin compositions therein are only partially cured. The compressive pressure applied during compression molding may be in the range of 100 kPa to 10,000 kPa. The temperature applied during compression molding may be in the range of 120°C to 200°C. The duration of the compression molding may be less than 20 minutes. In one embodiment, compression molding is carried out for 7 to 10 minutes at 180°C.

[0015] The expression “partially cured” refers to a material state that is less than 100% degree of cure, but greater than 0%. As an example, the degree of cure of the partially cured resin in the composite skins may be in the range of 30% to 90%. In some embodiments, the degree of cure of the partially cured resin is in the range of 60% to 65%.

[0016] The degree of cure of a thermoset resin can be determined by Differential Scanning Calorimetry (DSC). A thermoset resin composition undergoes an irreversible chemical reaction during curing. As the components in the resin composition cure, heat is evolved by the resin, which is monitored by the DSC instrument. The heat of cure may beused to determine the percent cure of the resin. As an example, the following simple calculation can provide the degree of cure:% Cure = [A H uncured AH cure d] / [AH uncure d] X 100%.

[0017] Referring to FIG. 4, the compression molding tool is opened by moving the upper mold portion 11 away from the lower mold portion 12, thereby allowing the non-adherent 3D insert 14 to be removed from the tool while the composite skin 13a remains adhered to the molding surface of the upper mold portion 11 and the composite skin 13b remains adhered to the molding surface of the lower mold portion 12.

[0018] Referring to FIG. 5, an adhesive film 15 is optionally placed on the composite skin 13b at least at the surface areas that will come into contact with the upper composite skin 13a. The adhesive film 15 may be added to improve the bonding strength between the composite skins. The adhesive film 15 is formed of a curable thermoset resin composition and, optionally, includes a carrier embedded therein. The curable thermoset resin composition of the adhesive may contain one or more thermoset resin(s), e.g., epoxy resin(s), and a curing agent, e.g., an amine curative such as dicyandiamide (DICY). The carrier may be a woven or knitted mat, or a nonwoven mat, formed from glass, polyester, nylon, or other suitable polymeric materials. Such carrier is useful for controlling the bondline thickness. The adhesive film may have a thickness of 0.05 mm to 0.50 mm. Suitable adhesives include FM® 209-1 and FM®309-1 available from Syensqo.

[0019] Referring to FIG. 6, the compression molding tool is closed to carry out a second compression molding. Closing the molding tool allows portions of the composite skins to come into contact each other and creates an empty space between the non-contacting portions of the skins. Compressive pressure and heat are applied during the second compression molding for a duration sufficient to cause bonding between contacting portions of the composite skins and full curing of the resin composition in each composite skin. Thecompressive pressure applied at this stage may be 100 kPa to 10,000 kPa. The temperature applied during the second compression molding may be in the range of 120°C to 200°C. The duration of the second compression molding may be less than 20 minutes. In one embodiment, the second compression molding is carried out for 8 to 10 minutes at 180°C.

[0020] Referring to FIG. 7, the compression molding tool is opened, yielding a sealed composite structure 16 having a hollow portion. At FIG. 8, the composite structure 16 is removed from the compression molding tool.Composite Skins

[0021] As discussed above, each thermosettable composite skin comprises reinforcement fibers impregnated or infused with a curable thermoset resin. The term “impregnate” as used in this disclosure refers to the introduction of a curable resin to reinforcement fibers so as to partially or fully encapsulate the fibers with the resin.

[0022] In one embodiment, the composite skin is a sheet of composite material, which contains a layer of reinforcement fibers embedded in a layer of curable matrix resin. As used herein, the term “matrix resin” refers to a mass of resin, and the expression “embedded in a matrix resin” means firmly fixed or positioned within a surrounding mass of resin.

[0023] The layer of reinforcement fibers may be in the form of unilaterally aligned continuous fibers or a woven fabric. The sheet of composite material is also referred herein as a ply of “prepreg” in the present disclosure. The skin may be a laminate of multiple plies of prepregs, also called a “prepreg layup”.

[0024] The curable thermoset resin composition for impregnating or infusing the reinforcement fibers is a hardenable or thermosettable resin containing one or more uncured thermoset resins, which include, but are not limited to, epoxy resins, bismaleimide, vinylester resins, cyanate ester resins, isocyanate modified epoxy resins, phenolic resins, furanic resins, benzoxazines, formaldehyde condensate resins (such as with urea, melamine or phenol), polyesters, acrylics, hybrids, blends and combinations thereof. Upon thermal curing by heat application, the thermoset resin composition undergoes crosslinking and becomes irreversibly harden, resulting in a hardened material that can no longer be reshaped by thermoforming (a process that includes heating a material and shaping the thus heated material so as to obtain the desired shaped object).

[0025] The term “curing” or “cure” in the present disclosure refers to the hardening of a polymeric material by the chemical cross-linking of the polymer chains. The term “curable” in context of curable composition means that the composition is capable of being subjected to conditions that will render the composition to a hardened or thermoset state.

[0026] In preferred embodiments, the thermoset resin composition contains one or more epoxy resins and one or more curing agents and / or catalyst(s).

[0027] Suitable epoxy resins include polyglycidyl derivatives of aromatic diamine, aromatic mono primary amines, aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylic acids. Examples of suitable epoxy resins include polyglycidyl ethers of the bisphenols such as bisphenol A, bisphenol F, bisphenol S and bisphenol K; and polyglycidyl ethers of cresol and phenol based novolacs.

[0028] Specific examples are tetraglycidyl derivatives of 4,4’-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, bromobisphenol F diglycidyl ether, tetraglycidyl derivatives of diaminodiphenylmethane, trihydroxyphenyl methane triglycidyl ether, polyglycidylether of phenol-formaldehyde novolac, polyglycidylether of o-cresol novolac or tetraglycidyl ether of tetraphenylethane.

[0029] Commercially available epoxy resins suitable for use in the composite skin include N, N, N', N'-tetraglycidyl diamino diphenylmethane (e.g. MY 9663, MY 720, and MY721 from Huntsman); N,N,N',N'-tetraglycidyl-bis(4-aminophenyl)-1,4-diiso-propylbenzene (e.g. EPON 1071 from Momentive); N,N,N',N'-tetraclycidyl-bis(4-amino-3,5-dimethylphenyl)- 1,4-diisopropylbenzene, (e.g. EPON 1072 from Momentive); triglycidyl ethers of p- aminophenol (e.g. MY 0510 from Hunstman); triglycidyl ethers of m-aminophenol (e.g. MY 0610 from Hunstman); diglycidyl ethers of bisphenol A based materials such as 2,2-bis(4,4'- dihydroxy phenyl) propane (e.g. DER 661 from Dow, or EPON 828 from Momentive), and Novolac resins, preferably of viscosity 8-20 Pa s at 25°C; glycidyl ethers of phenol Novolac resins (e.g. DEN 431 or DEN 438 from Dow); di-cyclopentadiene-based phenolic novolac (e.g. Tactix 556 from Huntsman); diglycidyl derivative of dihydroxy diphenyl methane (Bisphenol F) (e.g. PY 306 from Huntsman). Other epoxy resins include cycloaliphatics such as 3',4'-epoxycyclohexyl-3,4-epoxycyclohexane carboxylate (e.g. CY 179 from Huntsman).

[0030] The addition of curing agent(s) and / or catalyst(s) in the curable thermoset resin composition is optional, but the use of such may increase the cure rate and / or reduce the cure temperatures, if desired. The curing agent is suitably selected from known curing agents, for example, aromatic or aliphatic amines, or guanidine derivatives. An aromatic amine curing agent is preferred, preferably an aromatic amine having at least two amino groups per molecule, and particularly preferable are diaminodiphenyl sulphones, for instance where the amino groups are in the meta- or in the para-positions with respect to the sulphone group. Particular examples are 3,3'- and 4-,4'-diaminodiphenylsulphone (DDS); methylenedianiline; bis(4-amino-3,5-dimethylphenyl)-1 ,4-diisopropylbenzene; bis(4- aminophenyl)-1,4-diisopropylbenzene; 4,4’methylenebis-(2,6-diethyl)-aniline (MDEA from Lonza); 4,4’methylenebis-(3-chloro, 2,6-diethyl)-aniline (MCDEA from Lonza);4,4’methylenebis-(2,6-diisopropyl)-aniline (M-DIPA from Lonza); 3,5-diethyl toluene-2, 4 / 2,6- diamine (D-ETDA 80 from Lonza); 4,4’methylenebis-(2-isopropyl-6-methyl)-aniline (M-MIPA from Lonza); 4-chlorophenyl-N,N-dimethyl-urea (e.g. Monuron); 3,4-dichlorophenyl-N,N- dimethyl-urea (e.g. DIURON TM) and dicyanodiamide (e.g. AMICURE TM CG 1200 from Pacific Anchor Chemical).

[0031] Suitable curing agents also include anhydrides, particularly polycarboxylic anhydrides, such as nadic anhydride, methylnadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, endomethylenetetra- hydrophtalic anhydride, and trimellitic anhydride.

[0032] The curable thermoset resin composition may contain other additives such as comonomers, rheology control agents, tackifiers, inorganic or organic fillers, thermoplastic and / or elastomeric polymers as toughening agents, stabilizers, inhibitors, pigments, dyes, flame retardants, reactive diluents, and other additives well known to those skilled in the art for modifying the properties of the matrix resin before or after curing.

[0033] Suitable toughening agents for the curable resin composition include but are not limited to homopolymers or copolymers, either alone or in combination, of polyamides, copolyamides, polyimides, aramids, polyketones, polyetherimides (PEI), polyetherketones (PEK), polyetherketoneketone (PEKK), polyetheretherketones (PEEK), polyethersulfones (PES), polyetherethersulfones (PEES), polyesters, polyurethanes, polysulphones, polysulphides, polyphenylene oxide (PPO) and modified PPO, poly(ethylene oxide) (PEO) and polypropylene oxide, polystyrenes, polybutadienes, polyacrylates, polymethacrylates, polyacrylics, polyphenylsulfone, high performance hydrocarbon polymers, liquid crystal polymers, elastomers and segmented elastomers. If present, the total amount of toughening agent(s) in the resin composition is less than 25% (in weight percentage) based on the total weight of the resin composition.Non-Adherent Insert

[0034] The non-adherent 3D insert is formed of any material that does not form a permanent, chemical bond with the curable matrix resin in the composite skins. As examples, the non-adherent layer is formed of a high-temperature tolerant material selected from: silicone; rubber; hydrophobic fluoropolymers, including polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), fluorinated ethylene-propylene (FEP), polyethylenetetrafluoroethylene (ETFE), polyethylene-chlorotrifluoro-ethylene (ECTFE), perfluoropolyether (PFPE); and combinations thereof. Alternatively, the nonadherent 3D insert may be made of a metal or metal alloy or a composite material coated on opposite sides with a polymer such as PTFE. The composite material may be a pre-cured composite material or prepreg composed of fibers, e.g. carbon fibers, embedded in a cured thermoset resin, e.g., cured epoxy. Such composite material has a low expansion coefficient such that it does not expand during compression molding.EXAMPLE

[0035] A multilayered assembly composed of two composite skins separated by a rigid metallic shim made of aluminum was placed in a two-part compression molding tool. FIG. 9 shows the molding tool with an upper mold part 17 and a lower mold part 18, and a metallic shim 19 between the mold parts. As shown in FIG. 9, the metallic shim 19 functions as a non-adherent insert and has two convex portions that define two hollow portions in the final composite structure. The molding tool included top and bottom metal molding parts with molding surfaces that mate with the top and bottom surfaces of the metallic shim. The rigid metallic shim was configured in such a way that the two composite skins can be consolidated against the top and the bottom surface of the tool without being in contact.Each composite skin was composed of two prepreg plies. Each prepreg ply was composed of a carbon fabric impregnated with CYCOM® EP2750 (epoxy-based resin) from Syensqo.

[0036] An initial compression molding of the multilayered assembly with the metallic shim was carried out for 17.5 minutes at 30 bar and 165°C. The tool was opened and the metallic shim was removed from the tool, leaving the skins adhered to the opposing surfaces of the tool. FIG. 10 shows the geometry of the upper composite skin 20 and the lower composite skin 21 after initial compression molding. For illustration purposes only, the upperand lower composite skins (20, 21) are shown as being separated from the upper and lower molding surfaces in FIG. 10.

[0037] After the metallic shim was removed, the tool was closed again and a second compression molding was carried out for 25 minutes at 30 bar and 165°C to bond the upper skin to the lower skin where they contact each other, thereby creating two empty spaces between the skins. The total processing time was 42.5 minutes. FIG. 11 shows the final cured structure with two hollow portions. The hollow portions were created by the geometry of the metallic shim 19 shown in FIG. 9.

[0038] Based on visual observations, the cured part with hollow sections contained no porosities on the skin outer surfaces. A cross-sectional scan report showed an absence of defect through the thickness of the cured structure and at the bondline.

Claims

CLAIMSWhat is claimed is:

1. A method for forming a composite structure having at least one hollow portion, the method comprising:(a) providing a compression molding tool having at least a first mold portion and a second mold portion, wherein the first and second mold portions have opposing molding surfaces that cooperate to define the geometry of a final composite structure;(b) placing a stack comprising two composite skins separated by a non-adherent three-dimensional (3D) insert on the molding surface of the first or second mold portion when the tool is in an open position, wherein the non-adherent 3D insert is configured to prevent the two composite skins from contacting each other and has at least one outer convex surface that defines at least one hollow portion in the final composite structure, and each composite skin comprises reinforcement fibers impregnated or infused with a curable thermoset resin;(c) closing the compression molding tool to compress the stack of composite skins and non-adherent 3D insert between the molding surfaces of the first and second mold portions while applying heat to the tool, wherein compression of the stack is carried out until each composite skin conforms to and adheres to the molding surface adjacent to it but the thermoset resin in said composite skin is not fully cured;(d) opening the compression molding tool, wherein each composite skin remains adhered to the adjacent molding surface;(e) removing the non-adherent 3D insert from the compression molding tool;(f) closing the compression molding tool to allow portions of the composite skins to come into contact with each other and to create at least one empty space between noncontacting portions of the composite skins;(g) applying heat and pressure to the compression molding tool until the thermosetresin in each composite skin is fully cured, thereby forming a composite structure with at least one hollow portion;(h) opening the compression molding tool; and(i) removing the composite structure from the compression molding tool.

2. The method of claim 1, further comprising applying an adhesive film onto one of the composite skins between step (e) and step (f).

3. The method of claim 1 or 2, wherein the curable thermoset resin in each composite skin comprises one or more thermoset resin(s) and a curing agent, preferably, the thermoset resin(s) is / are selected from epoxy resins, and the curing agent is an amine-containing compound.

4. The method according to any one of the preceding claims, wherein compression of the stack of composite skins and non-adherent 3D insert at (c) is carried out for a duration of less than 20 minutes at compressive pressure in the range of 100 kPa to 10,000 kPa while applying heat at a temperature in the range of 120°C to 200°C.

5. The method according to any one of the preceding claims, wherein heating at (c) is carried out to achieve partially cured composite skins with a degree of cure of greater than 0% but less than 100%, preferably, a degree of cure in the range of 30% to 90%, and in some embodiments, 60% to 65%.

6. The method according to any one of the preceding claims, wherein step (g) is carried out for a duration of less than 20 minutes and at a compressive pressure in the range of 100 kPa to 10,000 kPa while applying heat at a temperature in the range of 120°C to 200°C.

7. The method according to any one of the preceding claims, wherein the non-adherent 3D insert is formed of a material that does not form a permanent, chemical bond with the curable resin in the composite skins and is releasable from the composite skins.

8. The method according to any one of the preceding claims, wherein the non-adherent 3D insert is formed of a material selected from: silicone; rubber; hydrophobic fluoropolymers, including polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), fluorinated ethylene-propylene (FEP), polyethylenetetrafluoroethylene (ETFE), polyethylene-chlorotrifluoro-ethylene (ECTFE), perfluoropolyether (PFPE); and combinations thereof.

9. The method according to any one of claims 1 to 7, wherein the non-adherent 3D insert is formed of a metallic material or a composite material coated with a polymer.

15. The method according to any one of the preceding claims, wherein each composite skin is a prepreg ply or a multilayered laminate comprising two or more prepreg plies, each prepreg ply comprising reinforcement fibers embedded in a layer of curable matrix resin.

16. The method according to claim 15, wherein the reinforcement fibers in each prepreg ply is in the form of continuous unidirectional fibers or a woven fabric.

17. The method according to claim 15 or 16, wherein the reinforcement fibers in each prepreg ply are carbon fibers.

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