Patent Application
20240391220
This invention relates to materials and methods of manufacturing materials.
The use of composite materials has grown in industries as diverse as but not limited to wind energy, aerospace, automobile, medical, and civil engineering. Void free composites is a key goal in high grade composite manufacturing as voids cause a severe drop in mechanical properties. As such, lots of time and effort is spent on manufacturing parts with a minimal to no void content and verifying this fact. Defective parts that have voids often have to be scrapped and are deemed unusable. To reduce void quantity, commonly, an autoclave or heated pressure vessel is used. Autoclave based manufacturing techniques are extremely capital intensive, have limited in production rates and part size, have slow cycle time, and require high labor intensity. As composites have expanded their use in industry and make up a larger percentage of the product cost has become a more influential factor.
In one aspect, a composite can include a matrix including a plurality of fibers, and a nanoporous material adjacent at least a portion of the fibers or the matrix. The composite can be a layered materials, for example, a laminate.
In another aspect, a preform for a composite can include a plurality of fibers, and a nanoporous material adjacent to at least a portion of the fibers. The preform can include a matrix. The matrix can includes a polymer, a metal or a ceramic. The nanoporous material can be adjacent to the matrix.
In another aspect, a method of manufacturing a composite article can include providing a matrix and a preform, applying a reduced pressure to the preform, and heating the preform to densify the preform to the composite article. The matrix can includes a polymer, a metal or a ceramic. The nanoporous material can be adjacent to the polymer matrix. The preform can include a plurality of fibers and a nanoporous material adjacent to at least a portion of the fibers or the polymer matrix.
In certain circumstances, the nanoporous material can be a nanoporous network.
In certain circumstances, the nanoporous material can include a network of capillary pores extending along a horizontal dimension of the composite. The network of capillary pores can be an open pore network with porosity in the vertical dimension and the horizontal dimension of the composite. The nanoporous material can have open porosity.
In certain circumstances, the nanoporous material can be a layer.
In certain circumstances, the nanoporous material can be a mat.
In certain circumstances, the nanoporous material can include an aerogel.
In certain circumstances, nanoporous material can include polymer nanofibers, for example, electrospun nanofibers.
In certain circumstances, the nanoporous material can have a pore size of less than 100 nm, less than 80 nm, less than 60 nm, less than 40 nm, less than 20 nm, less than 10 nm, or less than 5 nm.
In certain circumstances, voids can be present at 1 vol % or greater in the absence of the nanoporous material and voids can be substantially undetectable when the nanoporous material is present.
In certain circumstances, the nanoporous material can have a porosity of at least 70 vol %, at least 75 vol %, at least 80 vol %, at least 85 vol %, at least 90 vol %, at least 95 vol %, or at least 98 vol % prior to incorporation into the composite. For example, an aerogel can have a porosity of at least 99 vol %.
In certain circumstances, the composite or preform can be a laminate. For example, the laminate can be a prepreg.
In certain circumstances, the composite can be a multilayer laminate including two or more layers of the plurality of fibers and matrix with the nanoporous material between neighboring pairs of the two or more layers.
In certain circumstances, the laminate can include layers of unidirectional fibers on unidirectional fibers, unidirectional fibers on woven fibers, or woven fibers on woven fibers, or combinations thereof.
In certain circumstances, the matrix and the nanoporous material can be composed of the same material.
In certain circumstances, heating includes moving a source of the heat laterally across the preform.
In certain circumstances, the preform or composite can include a contoured surface with a bend. For example, the preform or composite can be a complex surface, which can be a surface having multiple curvatures in different directions.
In certain circumstances, at least a portion of the heating does not take place within an autoclave.
In certain circumstances, the composite article can have a volume fraction of nanoporous material of greater than or equal to 0.001.
Unexpectedly, the composite can be substantially free of voids when cured.
Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.
In general, nanoporous network (NPN) materials based on polymers can be used in layered or laminate structures in which the NPN is absorbed into the host matrix after it does its capillary-induced wetting job during curing of a laminate, such as a multilayer laminate.
A preform can include fibers and an NPN that can be infused with a polymer, such as by infusion or similar process (resin transfer molding (RTM), resin film infusion (RFI), or other method).
Driven by manufacturing challenges, there has been development of a prepreg material system fittingly called Out-of-Autoclave prepreg where the prepreg morphology is changed as dry fiber channels are left in the prepreg enabling gas to evacuate the material system more easily. These materials are designed to cure in an oven and therefore do not have some of the same cost and manufacturing challenges. However, this material system can have many drawbacks. The cycle times may actually be longer due to the time spend debulking or edge-breathing required for low void content, there are surface quality issues for sandwich structures such as honeycomb composite structures, and there are immense challenges in using automation in such layups. These material systems require specific material and environmental parameters and are very sensitive to deviations from ideal conditions with not-full void evacuation being the consequence. Furthermore, the matrix material rheology is often altered from the autoclave standard.
The approach described herein applies various nanomaterial systems with nanoscale porosity, the nanoporous network (NPN) materials, which are applied to each ply-ply interface to remove voids by encouraging resin infusion. The inclusion of the NPN removes the need for applied autoclave pressure to a composite structure and enables oven-curing of autoclave grade materials, or out-of-oven (OoO) curing using conductive heaters. This can reduce or eliminate the drawbacks of autoclave manufacturing without the need to alter the prepreg material system. By applying a nanoporous network ply-ply interface, a capillary effect can be induced, which draws the resin into dry areas of the fiber, collapsing bubbles of trapped air or resin volatiles. Without an NPN in the structure, if a laminate is cured just under vacuum pressure in an oven or on a hot plate significant voids occur. However, when the nanoporous network is applied, there are no observable voids. While it is possible to use aligned carbon nanotube (CNT) systems as textured nanostructures with aligned capillaries for interlaminar void removal, more affordable and scaled NPN materials described herein can be more efficiently and effectively used to reduce or eliminate the need to apply pressure during curing to avoid formation of voids. See, for example, US 2019/0085138, U.S. Application Ser. No. 16/056,745, filed Aug. 7, 2018, which is incorporated by reference in its entirety. A variety of NPN materials, described below, can be used in the systems described herein. Initial investigations (proof-of-concept) focused on a polyimide (PI) aerogel material system (NPN Alternative1) and a Nyon66 electrospun nanofiber fiber film system (NPN Alternative2) as examples. Industry aerogel and electrospun fiber systems are available with various thicknesses, material compositions, and material properties (porosity etc.) that give the NPN composite materials and methods of manufacture described herein scale and affordability, creating an easier path forward for implementation.
For example, tested specimens were laid up in a 16 ply quasi isotropic fashion utilizing carbon fiber reinforced epoxy composite prepreg. The void analysis completed was done primarily via μCT but no voids were found visually after polishing, during microscopy or SEM in samples that underwent those techniques. As described below, the void removal can be most clearly seen in the images where there are clearly no voids in interlaminar layers with the NPN and voids in the interlaminar region without the NPN. The results show full void removal in flat composite material. Voids can also be reduced or substantially eliminated in curved laminate structures. Strength of the interface was assessed for the electospun nylon 66 NPN via short beam shear (SBS) strength (ASTM 2344) with strength maintained. Examples of the technology described herein has been demonstrated on small 1″×1″ panel sizes, 6″×6″ panel sizes and 60 cm×60 cm panel sizes. The layup process for adding the NPN material is similar to that of prepreg systems and would be feasible for integration into conventional hand layup or automated layup systems. The NPN material described herein has a clear pathway for industrial implementation due to it removing autoclave manufacturing challenges without alteration of material composition and/or morphology.
Referring to
A composite can include a matrix including a plurality of fibers, and a nanoporous material adjacent at least a portion of the fibers. In another example, a preform for a composite can include a plurality of fibers, and a nanoporous material adjacent to at least a portion of the fibers. The preform can include a matrix. The matrix can includes a polymer, a metal or a ceramic. The polymer can include, but is not limited to, epoxies (e.g., rubber strengthened epoxy), bis-malemides (BMI), polyesters, vinylesters, polyamides, polyimides, polyarylene sulfides, polyetherimide, polyesterimides, polyarylenes, polysulfones, polyethersulfones, polyphenylene sulfides, polyetherimide, polypropylenes, polyolefins, polyketones, polyetherketones, polyetherketoneketone, polyetheretherketones, and/or polyesters. The metal can include, but is not limited to, aluminum, copper, stainless steel, molybdenum, titanium, or iron. The ceramic can include a metal oxide, a metal nitride or a metal carbide.
The preform can be processed into a composite by creating the matrix around the plurality of fibers and the nanoporous material. For example, a preform is a preliminary structure that is processed into a composite material. As described herein, a method of manufacturing a composite article can include providing a polymer and a preform, applying a reduced pressure to the preform, and heating the preform to densify the preform to the composite article. The preform can include a plurality of fibers and a nanoporous material adjacent to at least a portion of the fibers. In certain examples, the preform or composite can include a contoured surface with a bend, such as an L shape or other sharp curve. For example, the preform or composite can be a complex surface, which can be a surface having multiple curvatures in different directions. In certain examples, the complex surface can be an airfoil or wing, which can have a surface with varying curvature direction and radius of curvature.
In certain circumstances, the polymer matrix and the nanoporous material can be the same composition, for example, the same polymer.
The polymer can be, for example, an organic polymer. In some embodiments, the polymer is a thermoset polymer (e.g., a thermoset resin). In certain embodiments, the polymer is a thermoplastic polymer (e.g., a thermoplastic resin). Examples of suitable polymers include, but are not limited to, epoxies (e.g., rubber strengthened epoxy), bis-malemides (BMI), polyesters, vinylesters, polyamides, polyimides, polyarylene sulfides, polyetherimide, polyesterimides, polyarylenes, polysulfones, polyethersulfones, polyphenylene sulfides, polyetherimide, polypropylenes, polyolefins, polyketones, polyetherketones, polyetherketoneketone, polyetheretherketones, and/or polyesters. In some embodiments, the thermoset material can include epoxy, rubber strengthened epoxy, BMI, PMK-15, polyesters, and/or vinylesters. In certain embodiments, the thermoplastic material can include polyamides, polyimides, polyarylene sulfides, polyetherimides, polyesterimides, polyarylenes, polysulfones, polyethersulfones, polyphenylene sulfides, polyetherimides, polypropylenes, polyolefins, polyketones, polyetherketones, polyetherketoneketone, polyetheretherketones, and/or polyesters.
In some embodiments, polymer of the polymer matrix may soften and/or melt during at least a portion of the heating (e.g., the portion of the heating during which the pressure of the environment is reduced). The softening and/or melting of the polymer may cause the polymer to become more compliant and/or less viscous. In certain embodiments, the softening and/or melting of the polymer may reduce the absolute viscosity of the polymer (e.g., from greater than or equal to 1000 Poise, 2000 Poise, 3000 Poise, or greater) to less than or equal to 100 Poise, less than or equal to 50 Poise, less than or equal to 20 Poise, or less than or equal to 10 Poise. In some cases, the softening and/or melting of the polymer may cause the polymer to flow within plurality of fibers and/or through the nanoporous material arranged therebetween (e.g., into one or more channels or pores present in the nanoporous material). One or both of these effects may cause one or more nanostructures within the nanoporous material to penetrate into the space around the plurality of fibers (e.g., into the polymer(s) therein). In some cases, one or more nanostructures within the nanoporous material may become at least partially embedded in the plurality of fibers (e.g., during a heating step). The embedded nanoporous material and the plurality of fibers may together form a composite article.
As described herein, two components (e.g., a nanoporous material and a plurality of fibers) are directly adjacent when they are adjacent and there is no intervening component positioned between them. Two components that are adjacent may be directly adjacent, or may have one or more intervening components positioned between them (e.g., the plurality of fibers may be adjacent to a second plurality of fibers when the nanoporous material is positioned between the plurality of fibers may be adjacent to a second plurality of fibers). It should also be understood that when a component is referred to as being “adjacent” or “between” another component(s), it may be adjacent or between the entire component(s) or adjacent or between a part of the component(s). For example, the collection of nanostructures may be arranged between the entirety of the plurality of fibers and a second plurality of fibers, may be arranged between a portion of the plurality of fibers and the entirety a second plurality of fibers, or may be arranged between a portion of the plurality of fibers and a portion of the second plurality of fibers.
The nanoporous material can be an object having at least one cross-sectional dimension of less than 20 microns. For example, the nanoporous material can be a porous gel or fiber mat having a thickness of less than 15 microns, less than 10 microns, less than 8 microns or less than 5 microns. In some embodiments, the nanostructure has at least one cross-sectional dimension of less than 500 nm, less than 250 nm, less than 100 nm, less than 75 nm, less than 50 nm, less than 25 nm, less than 10 nm, or, in some cases, less than 1 nm. The solid portion of the nanoporous material described herein may have, in some cases, a maximum cross-sectional dimension of less than 1 micron, less than 500 nm, less than 250 nm, less than 100 nm, less than 75 nm, less than 50 nm, less than 25 nm, less than 10 nm, or, in some cases, less than 1 nm.
In some cases, a nanoporous material may comprise a high volume fraction of nanostructures. For example, the volume fraction of the nanostructures within the nanoporous material may be at least 0.001, at least 0.002, at least 0.005, at least 0.01, at least 0.02, at least 0.05, at least 0.1, at least 0.2, at least 0.4, at least 0.6, at least 0.7, at least 0.75, at least 0.78, or higher. The volume fraction of the nanostructures may be less than or equal to 0.8, less than or equal to 0.78, less than or equal to 0.75, less than or equal to 0.7, less than or equal to 0.6, less than or equal to 0.4, less than or equal to 0.2, less than or equal to 0.1, less than or equal to 0.05, or less than or equal to 0.02. Combinations of the above-referenced ranges are also possible (e.g., at least 0.001 and less than or equal to 0.8). Other ranges are also possible. (Those of ordinary skill in the art would understand that these volume fractions expressed in decimals would be multiplied by 100% to determine percent volume. For example, a component having a volume fraction of 0.8 within an article would make up 80% of the volume of that article.)
In some embodiments, polymers may make up a volume fraction of a nanoporous material prior to a step in which the collection of nanostructures is embedded in a substrate. In certain circumstances, the nanoporous material can have a polymer composition that is the same as the polymer composition of the polymer matrix. For instance, polymers may make up a volume fraction of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 of the composition of the composite.
The nanoporous material is a material with an open pore structure. This allows liquid to penetrate the entirety of the nanoporous material. In some embodiments, the nanoporous material can include channels or pores that make up a volume fraction of the nanoporous material prior to a step in which the nanoporous material is surrounded by the polymer matrix, for example, in an embedding step of manufacture. Prior to an embedding step, channels or pores may make up a volume fraction of the collection of nanostructures of less than or equal to 0.999, less than or equal to 0.998, less than or equal to 0.995, less than or equal to 0.99, less than or equal to 0.98, less than or equal to 0.95, less than or equal to 0.9, less than or equal to 0.8, less than or equal to 0.6, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.25, less than or equal to 0.22, or lower. Prior to an embedding step, channels or pores may make up a volume fraction of the collection of nanostructures of greater than or equal to 0.2, greater than or equal to 0.22, greater than or equal to 0.25, greater than or equal to 0.3, greater than or equal to 0.4, greater than or equal to 0.6, greater than or equal to 0.8, greater than or equal to 0.9, greater than or equal to 0.95, greater than or equal to 0.98, greater than or equal to 0.99, greater than or equal to 0.995, or greater than or equal to 0.998. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 0.999 and greater than or equal to 0.2). Other ranges are also possible.
The nanoporous material can be a nanoporous network. The nanoporous network can form a sheet, a mat, or layer of material. The mat can be a woven or unwoven mat or fabric. For example, the nanoporous network can be an assembly of nanofibers, polymer nanoparticles, or a porous gel such as an aerogel. The nanoporous network can be made of a polymer as described herein. The nanoporous material can be a layer, sheet or mat. In preferred embodiments, the nanoporous material includes an aerogel or an electrospun nanofiber. For example, the composite can be a multilayer laminate including two or more layers of the plurality of fibers with the nanoporous material between neighboring pairs of the two or more layers.
The composite can be a layered structure. The layered structure can have plys or films, which have a ply or film direction and a thickness direction. The ply or film direction can be referred to as a planar dimension, also referred to herein as a horizontal dimension. This dimension description can apply to curved and complex structures. The layered structure can form a curved surface or other complex structure. The nanoporous material can have a porosity in the horizontal dimension. The nanoporous material can have a material in the ply or film direction, or the horizontal dimension.
The nanoporous material can include a nanofiber material such as a polymer nanofiber. Nanofiber materials can have a high surface area to volume ratio, high porosity/tortuosity, and high permeability. Nanofibers can be manufactured using one of several techniques, such as nanolithography, phase separation, self-assembly, electrospinning, melt-blowing, and template synthesis have been used for the production of nanofibers. Electrospinning can be a simple, versatile, and flexible processes for producing nanofibers. For example, electrospun nanofibers can be manufactured using a syringe, a flat tip needle, a high voltage power supply, and a conducting collector. The material can be a solution or melt. Electrospinning is able to fabricate continuous nanofibers from a wide range of materials such as polymers, composites, semiconductors, or ceramics.
The nanofibers can have an average diameter of about 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 50 nm, 25 nm, 20 nm, 10 nm, 5 nm, or smaller. Examples of suitable polymers for nanofibers, including electrospun nanofibers, include, but are not limited to, epoxies (e.g., rubber strengthened epoxy), bis-malemides (BMI), polyesters, vinylesters, polyamides, polyimides, polyarylene sulfides, polyetherimide, polyesterimides, polyarylenes, polysulfones, polyethersulfones, polyphenylene sulfides, polyetherimide, polypropylenes, polyolefins, polyketones, polyetherketones, polyetherketoneketone, polyetheretherketones, and/or polyesters.
The nanoporous material can include an aerogel. An aerogel can have a very low density, for example, between about 0.001 to about 0.5 g cm−3. Aerogel can have high porosity content. The porosity content of an aerogel can be greater than 80%, greater than 90%, greater than 95%, greater than 97%, greater than 98%, greater than 99% or greater than 99.5% by volume. Aerogels can have open porosity (that is, the gas in the aerogel is not trapped inside solid pockets). Aerogels can have average pore diameters of about 1 nanometer, about 10 nm, about 15 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 90 nm, or about 100 nanometers.
In general, an aerogel can be considered a dry, low-density, porous, solid framework of a gel that can be isolated intact from a liquid component of a gel (the part that makes up most of the volume of the gel). An aerogel can be manufactured by drying a gel, for example, by supercritical drying of a gel. The aerogel can be a monolith, a sheet, or a composite of an aerogel in a matrix material. The aerogel can be a polymer aerogel. Examples of suitable polymers for aerogels, include, but are not limited to, epoxies (e.g., rubber strengthened epoxy), bis-malemides (BMI), polyesters, vinylesters, polyamides, polyimides, polyarylene sulfides, polyetherimide, polyesterimides, polyarylenes, polysulfones, polyethersulfones, polyphenylene sulfides, polyetherimide, polypropylenes, polyolefins, polyketones, polyetherketones, polyetherketoneketone, polyetheretherketones, and/or polyesters.
In certain circumstances, the nanoporous material can have a pore size of less than 100 nm, less than 80 nm, less than 60 nm, less than 40 nm, less than 20 nm, less than 10 nm, or less than 5 nm. Because the nanoporous material is an open cell material, the pores, or other channels are contiguous openings through the bulk of the material.
In certain circumstances, the nanoporous material can have a porosity of at least 70 vol %, at least 75 vol %, at least 80 vol %, at least 85 vol %, or at least 90 vol % prior to incorporation into the composite.
In certain circumstances, the nanoporous material can include a network of capillary pores extending along a horizontal dimension of the composite. For example, when the composite is a multilayer laminate or prepreg, the capillary pores can extend along the planar dimension, also referred to as the horizontal dimension, of the layers of the laminate or prepreg. The capillary pores can be randomly arranged vertically, horizontally, or in both horizontal and vertical directions. The horizontal dimensions of the capillary pores can be a network providing lateral mobility of the materials. This mobility of the materials can improved the final quality of the material when in a bent configuration (for example, an L shape) by reducing or substantially eliminating the creation of voids in the layers of the cured product. For example, each layer is a sheet with long dimensions (usually x-y or curvilinear plane, also referred to herein as a horizontal dimension) and a thickness, usually z-direction (also referred to here as a vertical dimension. The layers are stacked in the thickness or ply direction. The pores in NPN are both vertical (z-direction) and horizontal direction. More than 20%, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, or more than 80% of the capillary pores can be substantially oriented along a horizontal, or planar, dimension of the composite. Less than 80%, less than 70%, less than 60%, less than 50%, less than 40%, less than 30%, or less than 20% of the capillary pores can be substantially oriented along a vertical, or thickness, dimension of the composite.
As described above, certain embodiments relate to methods of forming composite articles by heating one or more substrates. In some embodiments, one or more of the substrates (e.g., a first substrate and/or a second substrate) comprises a prepreg. As used herein, the term “prepreg” refers to one or more layers of polymer (e.g., thermoset or thermoplastic resin) containing embedded fibers, for example fibers of carbon, glass, silicon carbide, and the like. According to certain embodiments, the prepreg includes fibers that are aligned and/or interlaced (woven or braided). In some embodiments, the prepregs are arranged such the fibers of many layers are not aligned with fibers of other layers, the arrangement being dictated by directional stiffness requirements of the article to be formed. In certain embodiments, the fibers cannot be stretched appreciably longitudinally, and thus, each layer cannot be stretched appreciably in the direction along which its fibers are arranged. Exemplary prepregs include thin-ply prepregs, non-crimp fabric prepregs, TORLON thermoplastic laminate, PEEK (polyether etherketone, Imperial Chemical Industries, PLC, England), PEKK (polyetherketone ketone, DuPont) thermoplastic, T800H/3900 2 thermoset from Toray (Japan), and AS4/3501 6 thermoset from Hercules (Magna, Utah), IMA from Hexcel (Magna, Utah), IM7/M21 from Hexcel (Magna, Utah), IM7/977-3 from Hexcel (Magna, Utah), Cycom 5320-1 from Cytec (Woodland Park. N.J.), and AS4/3501 6 thermoset from Hexcel (Magna, Utah).
As also described above, one or more substrate(s) (e.g., a first substrate and/or a second substrate) may comprise a prepreg that is an autoclave prepreg. As used herein, an autoclave prepreg is a prepreg that is configured to be cured in an autoclave (i.e., it would be believed by one of ordinary skill in the art to have inferior properties if cured outside of an autoclave and/or at a pressure less than 3 bar). One of ordinary skill in the art would be capable of distinguishing autoclave prepregs from other prepregs. Non-limiting examples of autoclave prepregs include certain prepregs manufactured by Hexcel (e.g., part nos. M76, 913, 8551-7, ZM91, M21, 8552, M18, M18/1, 922-1, HT93, 200, M65, F655, 996, 954-3, 954-6, M35-4, M47, and M81), certain prepregs manufactured by Tencate (e.g., part nos. EX-1515, TC410, TC890, BTCy-2, EX-1522, C640, C740, E650, E720, E721-FR, E722, E726, E731, E732, RS-1, E745, E750, RS-17B, RS-3, E760, 8020, RS-51, 8020 Rapi-Ply, 8020-FR, BTCy-1, BTCy-1A, TC380, RS-8HT), certain prepregs manufactured by Cytec Solvay Group (e.g., part nos. CYCOM 381, CYCOM 919, CYCOM 934, CYCOM 950, CYCOM 970, CYCOM 985, CYCOM 997, CYCOM 2237, CYCOM 5216, CYCOM 7668, CYCOM 7701, CYCOM 7714, Avimid RB, CYCOM 5250-4, CYCOM 5250MC, CYCOM 5276-1, CYCOM 7714A, CYCOM 937A, CYCOM 5575-2, CYCOM 950-1, CYCOM 977-2, CYCOM 977-3, CYCOM 985 LV, Avimid N, Avimid R), certain prepregs manufactured by Toray (e.g., part nos. #2500, #2580-14, #2573, #2574, #2592, #3631-2, #3633, #3900-2B), and certain prepregs manufactured by Gurit (e.g., part nos. SC 110 (T2), SC 160). Non-limiting examples of prepregs that are not autoclave prepregs include certain prepregs manufactured by Hexcel (e.g., part nos. M56, M26T, M92, M20, HT93, M9.X, M103, M104, M34, M49, M77, M79, 3H04), certain prepregs manufactured by Tencate (e.g., part nos. BT250E-1, BT250E-6, TC250, TC275-1, TC350-1, TC420), certain prepregs manufactured by Cytec Solvay Group (e.g., part nos. CYCOM 6101, CYCOM 5320-1, CYCOM 5215, MTM45-1, MTM44-1), certain prepregs manufactured by Toray (e.g., part nos. #2510, #2511), and certain prepregs manufactured by Gurit (e.g., part nos. SE70, SE84LV, SE84 Nano, Sparpreg™, WE 91-1, WE 91-2). The lists above should not be taken to be exhaustive; it should be understood that there are numerous autoclave prepregs that are not listed above, and numerous prepregs that are not autoclave prepregs that are not listed above. In some embodiments, one or more substrate(s) (e.g., a first substrate and/or a second substrate, either or both of which may be a prepreg) does not include a channel in the surface that is adjacent to the nanoporous materials. For example, out of autoclave prepregs generally include channels in their surfaces that allow for the transport of gas (e.g., air) away from the surface that is being bonded. In some embodiments, layers of fibers in prepregs or laminates can be free of such channels. In some such embodiments, layers of fibers in prepregs or laminates are free of such channels at their interface surfaces while still forming a strong, low-void bond between each other.
In certain circumstances, the composite or preform can be a laminate. For example, the laminate can be a prepreg. The prepreg or laminate can have 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 layers. The prepreg or laminate can include layers of unidirectional fibers on unidirectional fibers, unidirectional fibers on woven fibers, or woven fibers on woven fibers, or combinations thereof.
Polymers may make up any suitable volume fraction of a nanoporous material after a step in which the nanoporous material becomes at least partially embedded in the plurality of fibers. After the nanoporous material becomes at least partially embedded in the plurality of fibers, polymers may make up a volume fraction of the nanoporous material that is complementary to the volume fraction of nanostructures in the nanoporous material prior to a step in which the nanoporous material has become embedded in the plurality of fibers. In other words, the nanoporous material may include mainly nanostructures and polymer after the nanostructures have become embedded in the plurality of fibers. After an embedding step, polymers may make up a volume fraction of the nanoporous material of less than or equal to 0.999, less than or equal to 0.998, less than or equal to 0.995, less than or equal to 0.99, less than or equal to 0.98, less than or equal to 0.95, less than or equal to 0.9, less than or equal to 0.8, less than or equal to 0.6, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.25, less than or equal to 0.22, or lower. After an embedding step, polymers may make up a volume fraction of the nanoporous material of greater than or equal to 0.2, greater than or equal to 0.22, greater than or equal to 0.25, greater than or equal to 0.3, greater than or equal to 0.4, greater than or equal to 0.6, greater than or equal to 0.8, greater than or equal to 0.9, greater than or equal to 0.95, greater than or equal to 0.98, greater than or equal to 0.99, greater than or equal to 0.995, or greater than or equal to 0.998. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 0.999 and greater than or equal to 0.2). Other ranges are also possible.
The suitable nanoporous material may have a size and/or arrangement, as described in further detail below, which promotes capillary flow such that the polymer matrix surrounds the plurality of fibers and the nanoporous material. The capillary forces may cause the polymer to flow through nanoporous material via channels or pores or combinations thereof. The channels may also provide a direction along which any gas or other trapped material can escape. In some embodiments, the flow may be in a relatively uniform direction, which may promote relatively even filling of the channels. In certain cases, the embedded nanostructures may have a morphology that exerts sufficiently strong capillary forces on the polymer so that the channels may be completely (or almost completely) filled when capillary forces make up a relatively large percentage of the total force the polymer is subject to. For example, in some cases adjacent layers of fibers can be joined together with relatively few voids without supplying external pressure during manufacture. In certain embodiments, reduced pressure or vacuum can be applied.
In certain embodiments, the method comprises locally heating the preform. For example, in some embodiments, the preform is not located within an autoclave or any other type of oven during the heating process. In some embodiments, vacuum bag curing is employed (e.g., in which the first substrate and the second substrate are located within a vacuum bag during the heating process). In some embodiments, less than 30% (or less than 20%, less than 10%, less than 5%, or less) of the energy used to heat the first substrate and the second substrate is transferred to the substrates via convective heat transfer.
It is believed that the nanoporous material may provide one or more advantages. For example, when certain portions of the nanoporous material become embedded sequentially, relatively few or substantially no voids may be present at the conclusion of the embedding process. Any voids that form as a given portion of the nanoporous material becomes embedded may be formed around the edges of the embedded portions. As these portions then become heated and the nanostructures therein become embedded in the fibers, the voids (if any) may move towards the edges of newly embedded portions. If this process continues throughout the embedding process, any voids formed during embedding may move to the last portion(s) of the substrate(s) into which the nanoporous material becomes embedded. If these portions are on one or more edges and/or corners of the fibers, the voids may be eliminated through the edges and/or corners of the fibers when the embedding concludes. By contrast, if the preform is heated uniformly, the nanoporous material may become embedded in the fibers in a random and/or nonuniform manner. Voids may form at any and/or multiple location(s) within the substrate(s), and may become trapped if there is not a pathway for their escape.
Through this approach, interlaminar and intralaminar voids can be present at 1 vol % or greater in the absence of the nanoporous material and voids can be substantially undetectable when the nanoporous material is present. One test for comparing structures with and without voids can be to construct an article with and without the nanoporous material and comparing the occurrence of voids, for example, by the optical techniques or x-ray computed tomography, or methods described herein. In other words, the nanoporous material contributes to the reduction or elimination of voids in the composite material when manufactured at atmospheric pressure or reduced pressure. The NPN can reduce or eliminate voids in the interlaminar region of the composite. The NPN can impact voices in the intralaminar region of the composite by gas diffusion or physical connection of spaces prior to densification.
In certain embodiments, the reduction or elimination of voids using NPN structures can be at a curve or other non-planar portion of the composite structure. For example, a non-planar section can have an arc or a radius of curvature that can be measured, such as a bend, 90 degree angle, an L-shape, or another angled shape. The NPN can reduce or eliminate voids at a bend, 90 degree angle, an L-shape, or other angled shape of the composite.
Certain methods described herein may comprise one or more steps in which one or more substrate(s) are heated. The substrate(s) may be heated to any suitable temperature. In some embodiments, the substrate(s) may be heated to a temperature of less than or equal to 1200° C., less than or equal to 1000° C., less than or equal to 750° C., less than or equal to 500° C., less than or equal to 400° C., less than or equal to 300° C., or less than or equal to 200° C., less than or equal to 100° C. In some embodiments, the substrate(s) may be heated to a temperature of greater than or equal to 25° C., greater than or equal to 100° C., greater than or equal to 200° C., greater than or equal to 300° C., greater than or equal to 400° C., greater than or equal to 500° C., or greater than or equal to 750° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 25° C. and less than or equal to 1000° C., or greater than or equal to 25° C. and less than or equal to 1200° C.). Other ranges are also possible.
In some embodiments, one or more substrate(s) may comprise a polymer that softens and/or melts during at least a portion of a heating step. In some embodiments, the polymer may soften and/or melt (e.g., experience a reduction in absolute viscosity to less than or equal to 100 Poise) at a temperature of less than or equal to 350° C., less than or equal to 300° C., less than or equal to 250° C., less than or equal to 200° C., less than or equal to 150° C., less than or equal to 110° C., less than or equal to 100° C., less than or equal to 90° C., less than or equal to 80° C., or less than or equal to 70° C. The polymer may soften and/or melt at a temperature of greater than or equal to 60° C., greater than or equal to 70° C., greater than or equal to 80° C., greater than or equal to 90° C., greater than or equal to 100° C., greater than or equal to 110° C., greater than or equal to 150° C., greater than or equal to 200° C., greater than or equal to 250° C., or greater than or equal to 300° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 60° C. and less than or equal to 350° C., or greater than or equal to 60° C. and less than or equal to 110° C.). Other ranges are also possible.
As described above, certain embodiments relate to methods for forming composite articles. The composite articles formed by these methods may have one or more advantageous properties. For example, the composite articles may have a relatively small number voids. In some embodiments, the percentage of the composite article occupied by voids may be less than or equal to 10%, less than or equal to 5%, less than or equal to 2%, less than or equal to 1%, less than or equal to 0.5%, or less than or equal to 0.2%. The percentage of the composite article occupied by voids may be greater than or equal to 0.1%, greater than or equal to 0.2%, greater than or equal to 0.5%, greater than or equal to 1%, greater than or equal to 2%, or greater than or equal to 5%. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.1% and less than or equal to 5%, greater than or equal to 0.1% and less than or equal to 2%, or greater than or equal to 0.1% and less than or equal to 10%). Other ranges are also possible. The percentage of the composite article occupied by voids may be determined by analyzing computerized X-ray tomography images. Briefly, three dimensional computerized X-ray tomography images are taken of the composite and voids in the images are identified by image processing software.
The use of carbon fiber reinforced polymer (CFRP) composite systems has grown in the aerospace industry due to their high mass-specific stiffness and strength and other properties. Yet, many advanced aerospace-grade CFRPs require autoclave pressure vessels to thermally process composites, which has numerous drawbacks such as high capital costs, hours-long heating and cooling cycles, high energy use, and bottlenecks due to fixed size. This work investigates various nanomaterial systems with nanoscale porosity, termed nanoporous network (NPN) materials, which are applied to each ply-ply interface in the composite prepreg laminates to remove voids by encouraging resin infusion through capillary effects. This removes the need for applied autoclave pressure and enables vacuum-bag-only (VBO) curing of autoclave-grade composites using either conductive or convective heating under vacuum. Aligned carbon nanotubes (A-CNTs) have been successfully utilized as textured NPNs with aligned capillaries, but alternative, particularly scaled and lower-cost, NPN materials are of interest. Commercially-available electrospun polymer nanofiber (EPN) films with different fiber diameters, film thickness, and polymer material are investigated extensively and a bespoke-commercial polyimide (PI) aerogel is preliminarily investigated. EPN NPN interlayers are able to create void-free laminates as revealed by micro-computed tomography (μCT) on flat aerospace-grade CFRP laminates using unidirectional epoxy-based prepreg plies. The VBO cured EPN NPN laminates are also found to have the same interlaminar shear strength as the autoclave-cured baseline laminates. It is found that polymer (polyimide) aerogels with a given porosity of 96 vol % are also a viable NPN. L-shape geometries with autoclave-grade laminates are preliminary examined with the investigation revealing void elimination, including in the problematic curved section using EPN NPN interlayers when cured under VBO conditions. Investigations using interlaminar NPNs to reduce voids in laminates that contain woven-woven and unidirectional-woven prepreg interfaces reveal significant void reductions, but not elimination, utilizing various NPNs, such that continuing challenges exist for full void elimination. Future work includes parametric studies of various NPN materials to further increase the breadth of NPNs available as well as quantifying the precise capillary pressures needed in different prepreg systems to eliminate interlaminar voids.
The motivation for this thesis was to ascertain a system that would remove interlaminar voids without external pressure while utilizing standard previously certified aerospace-grade autoclave prepreg. Various nanomaterial systems with nanoscale porosity, termed nanoporous network (NPN) materials, are applied to each ply-ply interface to remove voids by encouraging resin infusion.
Composite material is a broad term. However, as described herein, the type of composites of concern are aerospace-grade advanced composites comprised of carbon fibers in a (thermoset) polymer matrix, known as carbon fiber reinforced polymers (CFRPs). As discussed above, such composites have grown in usage across a variety of industries, especially aerospace, due to their advantaged mass-specific properties. CFRPs are susceptible to more complex failure modes compared to homogeneous materials as failure in composites can reflect fiber dominated failure, matrix dominated failure or interfacial dominated failure between the two constituents [Ref. 11]. This description provides a background in composite curing utilizing both autoclave curing methods and non-autoclave methods, typical manufacturing defects, in particular voids, the utilization of capillary effect for void removal, an introduction to two different polymer nanoporous network (NPN) materials of interest in this thesis, and challenges with L-shape or complex geometry parts.
Various alternative methods of manufacture have been investigated to try and circumvent the autoclave with numerous heat sources (microwave, induction heating, joule heating, lasers, hot presses, blankets, etc. [Refs. 14, 15, 16] as well as prepreg modifications In particular, a prepreg system termed out-of-autoclave (OoA) has shown that it is viable to produce parts using only vacuum bag consolidation without the applied pressure that an autoclave provides. This allows for a significant reduction in operation and acquisition costs, enabling lower cost cure set ups and can increasing the number of composite manufacturers, particularly smaller ones [Ref. 3]. The challenge for OoA prepregs, however, has been its propensity to have voids (particularly in the already low-performance interlaminar regions between the plies) in the laminate and disputes over the true cost saving of utilizing these materials in part due to their longer preparation time and higher material costs [Ref. 17].
Defects in composite manufacturing, particularly voids, under VBO conditions, tend to come from two sources: the presence of entrapped air or released volatiles (gas-induced voids) or the failure of the dry fibers to be fully infiltrate by the resin (flow-induced voids). Modeling of gas induced voids show that the evacuation of voids decreases linearly with dry fiber permeability, but increases quadratically with part size [Ref. 25]. Gas-induced voids are evacuated primarily through the in-plane permeability, enhanced by the dry fiber regions in OoA prepreg, as the transverse plane or thickness permeability is four orders of magnitude larger. With an increase in plies, the air flow in the transverse plane becomes near zero, in part because as additional plies are added, the likelihood of complete overlap of dry fibers decreases through the thickness. Additionally, unlike in autoclave cured prepreg, increased humidity has a large effect on void content as gas pressure within moisture-induced voids can be greater than the resin pressure without the autoclave to apply pressure during manufacturing. These conditions of larger part sizes and humidity effect can be offset by increased debulking time where vacuum is pulled, and/or increased vacuum pressure applied to ensure higher levels of air and volatile evacuation in VBO cures [Refs. 25, 26].
Flow induced voids often occur due to partial impregnation of the dry fiber pathways and are due to the fiber and matrix properties as opposed to manufacturing error in vacuum bagging or otherwise. A main driving factor is long out time where the prepreg material is left at room temperature or if the laminate is cured at lower temperatures. In these cases, voids occur as viscosity is higher than expected which hinders resin flow into the dry fiber region [Ref. 23].
Woven fabrics can present additional challenges. In order to make certain the dry pathways are contiguous for void evacuation, the entire tow cross section should to be unimpregnated. If not, the entrapped air may not be able to fully evacuate [Ref. 7]. Bothwoven and unidirectional VBO prepreg systems have been shown to have mechanical properties of near equivalence to autoclave-cured specimens [Ref. 27]. However, in a study done with IM7/8552 and IM7/MTM45 representing autoclave and out of autoclave material systems respectively, when there was out time, drops in drape and tack were observed with the MTM45 followed by a decrease in mechanical properties and an increase in void content. This out time sensitivity corresponds to the importance of resin properties, particularly viscosity, to void percentage of OoA prepregs and their effect on the dry fiber evacuation channels. When there was no out time effect mechanical performance of VBO and autoclave cured specimens was similar [Ref. 28].
Electrospun polymer nanofibers (EPNs) are a material class that has a multitude of industrial usages from biomedical applications, such as tissue engineering, to functional structural materials such as filtration or composite reinforcement [Refs. 40, 41]. The nanofiber mats or films are manufactured by a high voltage electric force drawing charged threads of polymer solution. The high voltage is applied to the liquid droplet, charging it, and electrostatic forces stretch the droplet counteracting the surface tension. The liquid dries as it is deposited on the surface. This process enables nanometer-scale polymer fibers to be created. The fibers, when collected on a surface, form veils, mats, or films [Ref. 42]. The fiber morphological properties can be tuned by the polymer characteristics including surface tension and viscosity, the process parameters including voltage, federate capillary diameter, and shape, as well as environmental conditions including pressure, temperature, and humidity [Refs. 42, 41, 43]. The thickness of the film is altered by how fast the deposit surface is removed. Thermoplastic materials are typically used as there is a good bond between the fibers and matrix for composite applications which facilitates improved fracture toughness. When utilizing thermoset resins for the EPN, there is less improvement in fracture toughness [Refs. 43, 44, 45]. A popular EPN used is polyamide (PA) which is known commercially as Nylon. Polyamide is inexpensive, has relatively good mechanical properties in bulk and as individual nanofibers, a high melting temperature (compared to thermoset matrix cure temperatures), and is easily dissolved in a wide range of solvents [Ref. 43]. By having the melting temperature higher than cure temperature, when the CFRP laminate cures, the nanofibers in the EPN film maintain their shape and therefore can be used for reinforcement.
EPN films are used for mechanical reinforcement of glass and carbon reinforced polymer composites. The EPN films are used due to their ability to be manufactured in thin film only a few microns thick, their low density, their high porosity enabling-resin infiltration, and low volume. The low volume enables low interlaminar thickness changes of only a few percent, as the material is additionally compressed when pressure is applied to the laminate during curing, helping minimize the interlaminar thickness. Additionally, the high surface area promotes good bonding [Refs. 43, 46].
EPNs, when placed in the interlaminar region as an interlayer have been shown to improve mechanical properties including Mode I, Mode II interlaminar fracture toughness, compression after impact fatigue resistance, and vibrational damping. 1.5 gsm polyamide 66 was shown to improve Mode II critical strain energy release rate by 29% [Ref. 47].
Thermosetting resins like epoxy tend to be brittle materials with little resistance to crack propagation. During delamination failure modes, cracks propagate through the matrix material. Two options to help address this are toughening the matrix with the addition of particles and inserting nanofiber mats as in the ply-ply interlayer. The nanofiber mat was seen to encourage nanofiber bridging [Ref. 48], resin reinforcement [Ref. 49], and crack path modification [Ref. 50] Toughening particles increase resin viscosity which can increase void content when curing in vacuum-bag-only conditions with OoA prepreg systems, but are a more common technology as they are included in the surface in CFRP prepreg in several aerospace autoclave CFRP systems in production of primary structures.
Aerogels are a class of micro porous solids where the dispersed phase is a gas. They have ultra low densities, large surface areas, high porosity, low values of thermal conductivity and form three dimensional solid networks that can be highly cross linked. They are often formed by supercritical drying to avoid structural collapse during manufacturing and have an air volume of 80-99% [Refs. 51, 52]. Their large surface areas and nanoporosity (pores are 1-100 nm) make them good candidates as NPNs for polymer composites. Aerogels can be made out of a variety of materials including CNTs, graphene, polymers, chalcogenides, silica, etc. The aerogel can be compressible. For example the thickness of an aerogel sheet can be compressed by two times, three, time, four times, five times, or more. Traditional silica aerogel nanocomposites have been attempted [Refs. 53, 54]. However, their properties were lost when polymer was mixed into the aerogel. When a carbon aerogel was integrated into the matrix system of a CFRP laminate, modulus, strength, and interfacial properties increased. Mode I tests with a carbon 0.1 wt % aerogel showed delamination fraction energy increase from 265 J m−2 to 346 J m−2. The delamination fracture energy caused by Mode II loading was increased from 655 J m−2 for the unmodified laminate to 1088 J m−2 for laminate modified with 0.5 wt % aerogel [Ref. 55]. Aerogels form three dimensional hierarchical morphologies with large surface areas, porosities, and have ultra-low densities with some demonstrated reinforcement properties. Polymer aerogels are exemplified herein.
Composites can typically be categorized into three categories: flat panels, sandwich structures and complex geometries. Complex geometries are commonly used as stiffeners or to attach composite flat panels together, but also includes geometries comprised of single- and multiple complex curvatures such as shells, such as in a wing, fuselage, or pressure vessel. Stiffer cross sections come in various shapes. L, T, J, and hat shaped are most common and can be seen in
Interlaminar fiber wrinkling is the most prevalent manufacturing defect which directly influences the interlaminar voids in complex non-flat geometries. This occurs along sharp curvatures of complex composites and in laminates that include 90° plies as these transverse plies have little resistance to the deviatoric stresses from the mold geometry curvature or the pressure intensifier. Resin accumulation in the corners occurs in complex shaped laminated when laid up in a female mold as the corner is a low-pressure region which can cause low compaction and resin flow to that region causing corner thickening [Refs. 59, 8]. To ensure a more uniform pressure distribution a caul sheet or a rubber pressure intensifier is often used [Refs. 60, 61]. The amount of thickening changes based on layup orientation as well as mold geometry. A higher radius to thickness ratio corresponds to higher thickness variation [Ref. 62]. There are two main drivers of corner thickening or thinning. The first is pressure and the second is friction (or lack of friction which is beneficial to corner quality) [Ref. 63] and they impact compaction. To reduce thickness variation and defects and thick laminates, ply-by-ply debulking every 3-5 plies is seen as critical to reduce void content [Ref. 64]. Drapability or mold conformability is also an important factor. Laminates made with unidirectional (UD) prepreg have a tendency for fiber wrinkling due to low drapability. Therefore, plain and satin woven fabric has been seen as the more suitable prepreg system [Ref. 65].
While traditionally aerospace-grade composites are cured with capital- and operational-intensive thermal pressure vessels or autoclaves [Ref. 17], a next generation processing technique, where voids are evacuated from a composite laminate, not by external pressure but by capillary pressure, is presented utilizing polymer based nanoporous networks (NPNs). The technique enables conduction or convection vacuum-bag-only (VBO) curing of traditional autoclave-grade prepreg material systems, and as discussed above, has been demonstrated using a conduction cure with aligned carbon nanotubes (A-CNTs) as an NPN in a lab scale environment [Ref. 10]. Utilizing autoclave-grade prepreg systems enables a diverse selection of certified and widely studied materials to be manufactured in vacuum-bag-only conditions. These traditional materials do not contain morphology or polymer rheology modifications that comprise out-of-autoclave (OoA) prepeg systems [Ref. 3], which are limited in the number of certified materials due to the additional testing required that costs millions of dollars and spans 15-20 years to complete [Ref. 66]. This description demonstrates a technique utilizing two NPN material systems: commercial electrospun polymer nanofiber (EPN) films with polyamide (PA) nanofibers and a bespoke polyimide (PI) aerogel, both having commercial scale capabilities. These polymer-based NPN systems have different morphologies and manufacturing techniques. The EPN material group utilizes two different polyamide variants: common PA 66 (Nylon 66), and PA XD10 (Lexter®), the latter of which is a thermoplastic bio-inspired polyamide resin developed by Mitsubishi Gas and Chemical Company. The XD10 resin, 8000 variant, has a melting point of 190° C., a bending modulus of 3.2 GPa, bending strength of 136 MPa, density of 1.13 g/cm3, and saturated water gain of 2.4% [Refs. 67, 68]. μCT images show interlaminar void elimination when the NPN systems are applied to the interlaminar region as an interlayer while an autoclave-grade prepreg laminate is cured under VBO conditions. Presented is the manufacturing process of the NPN applied composites, SEM images and characterization of the NPN, void content of the cured composites, mechanical testing of specimens made with the electrospun polyamide NPNs, and an interlaminar region thickness analysis. Results were compared to laminates cured via VBO without NPN interlayers and to autoclave-processed composites as a baseline for the same material system. The alternative-NPN polymer materials and the NPN approach shows great capability as the NPN material system can be transferred directly to a prepreg before layup, or likely applied in an automated layup capable manner, thereby enabling practical autoclave-less curing of autoclave-grade prepreg.
Diverse industries including aerospace, wind, solar, automobile, and civil have been utilizing the lightweight but strong material performance of advanced fiber reinforced polymer (FRP) composites in large and small structures [Refs. 7, 3, 10]. A keystone manufacturing technique for carbon fiber FRPs (CFRPs) has been the utilization of a material system designated as a prepreg, in which continuous fibers (carbon, glass, etc.) were pre-impregnated with a resin (thermoset or thermoplastic) to form a layer of, typically, between 0.1 and 0.3 mm thick. These layers were then cut to the desired shape and laid up or stacked on one another, after rotating the orientation of the fibers as needed, to achieve a laminate with desired properties. Prepreg is often used due to the high fiber volumes, reproducibility, and low void content of the laminates [Ref. 12]. The standard prepreg is autoclave-grade, meaning it is engineered to be cured under vacuum at up to −0.1 MPa (−1 bar), with applied pressure of 0.5-1.4 MPa (5-14 bar) in an autoclave, with the most common being 0.7 MPa (7 bar). The applied vacuum causes an atmospheric pressure of 0.1 MPa (1 bar) to be applied on the laminate [Refs. 5, 69]. This manufacturing technique obtains repeatable high quality laminates as the applied pressure, in combination with the part under vacuum, removes the entrapped voids or air pockets that form during the prepreg and layup processes while suppressing new voids from developing [Ref. 3]. For aerospace applications, a void allowance of around 1% is considered acceptable, with autoclave-processed composites typically generating voids that are immeasurably small given common techniques such as ultrasonic C-scan. In other commercial applications, a void percentage of 3-5% can be acceptable [Ref. 70]. Voids can lead to severe degradation of composite part performance and lifespan. Specifically, they degrade the composite's polymer matrix dominated properties such as interlaminar shear, transverse tensile strength, and fatigue resistance [Ref. 29]. While the void percentage specifications appear to be minimal at first glance, composites have strict requirements as the effects of voids can be significant. Only a 1-3% increase in void content can decrease the mechanical properties by up to 20% [Ref. 71]. Voids act as crack initiation sites and can allow moisture penetration [Refs. 29, 11]. While providing the highest-quality and repeatable laminates, autoclave manufacturing has some major commercial drawbacks. In particular, the autoclave pressure vessels require high capital cost, long heating and cooling cycles limiting cycle frequency, high energy use, inert gas (often nitrogen) to pressurize, and are size-fixed, which limits maximum part size and run efficiencies when not filled [3]. Due to these drawbacks, there has been research on alternative methods to man-ufacture high quality composites without an autoclave, in a VBO cure, where only vacuum is applied with no external pressure. The most common of these currently is the OoA prepreg material system that is engineered to be cured without a pressure vessel.
OoA prepreg materials have their resin chemistry altered to prevent gas from being released during curing (out-gassing) and are engineered to have partially impregnated microstructures to become “breathable” with both dry and resin rich areas. The dry fiber regions, which act as void extraction channels, are next to resin-rich regions [Ref. 3]. The extraction channels increase the in-plane permeability and the dry fiber pockets enable the entrapped air voids to be extracted through these channels to the laminate edge. The voids are then evacuated from the vacuum bag by the vacuum pump. The dry regions become impregnated with resin as the resin flows from the resin-rich regions. This OoA prepreg technology allows the composite to be cured without additional pressure, enabling VBO oven curing, which enables lower manufacturing cost due to lower capital and operational costs [Ref. 17]. However, OoA prepreg systems have some key drawbacks. In particular, the morphology changes with the addition of the dry fiber void extraction channels and the altered resin chemistry [Ref. 3]. Additionally, the OoA prepreg often requires an extended debulking step where the laminate is subjected to vacuum pressure. This can last up to 16 hours and may need to be done every laid ply (i.e., ply-by-ply debulking) depending on the laminate and material system [Ref. 9]. These changes increase the layup processing time due to the extra de-bulking steps while limiting the variety of certified materials for aerospace use due to resin and morphology modifications which require lengthy certification testing. Here is presented the ability to cure composites without an autoclave, without the morphology changes, extra debulking steps, or resin modifications that OoA materials have, and rather utilizing autoclave-grade prepreg, which has a diverse selection of already certified material options with aerogel and EPN polymers. The NPN materials have porosities of 80% or above for void evacuation channels. Implementing the NPN at all ply interfaces enables a void evacuation channel to be a maximum of half a ply thickness away from any point in the laminate. As the resin's viscosity decreases with an increase in temperature during curing, the capillary pressure caused by the NPN encourages resin infiltration into the NPN in the interlaminar region. Due to the NPN's compressible nature, the NPN-comprised interlayer shrinks in thickness during manufacture and cure of the composite.
The effects of the integration of two NPN material systems, electrospun PA nanofibers and a PI aerogel, into a laminate's interlaminar regions were analyzed. The composite void content was found using X-ray micro-computed tomography (μCT), while the interlaminar region thickness was analyzed primarily by optical microscopy but also secondarily by scanning electron microscope (SEM) and μCT. Composite interlaminar shear strength was tested via short beam shear testing. Interlaminar region thickness was documented for comparison to non-NPN processed samples. The engineering goal was to achieve void-free laminates integrating novel commercially available nonporous material systems, with autoclave-grade prepreg cured without applied pressure, that have similar properties and performance similar to A-CNT NPNs [Refs. 72, 39].
The work presented herein utilizes unidirectional (UD) IM7/8552 which is an aerospace-grade carbon fiber prepreg designed to be cured with applied pressure under vacuum. It has a nominal cured ply thickness of 0.131 mm. The target fiber volume fraction is 57.7 vol % with a fiber density of 1.77 g/cm3 [Ref. 73]. Driven by the short beam shear standard, ASTM D2344, which utilizes a nominal 2 mm thickness, a 16-ply quasi-isotropic layup of [0/90/±45]s2 was used, giving a nominal thickness of 2.10 mm. The stacking sequence is seen in
The plies were cut to the desired specimen dimension. The first ply had its protective film removed and placed on a sheet of peel ply material in its desired orientation. A roller was then used to push firmly against the material before the backing sheet was removed. When an interface is to receive a NPN interlayer, the NPN was applied following the developed method: the NPN was cut larger than the specimen by 5 mm on each side, placed on the laminate, rolled firmly, and its backing paper removed. The oversized cut of NPN ensures full edge to edge coverage of the laminate. The process was repeated, ply-on-ply or ply-on-NPN, as determined by the stacking sequence and the desired NPN locations. After the last ply was placed, the edges were trimmed to ensure breathability (a common practice for OoA prepreg as well) and bagging materials were added to complete the vacuum bag layup process.
The bagging materials used include: guaranteed nonporous Teflon (GNPT) film (Airtech Release Ease 234 TFNP), breather (Airtech Release Ease 234 TFNP), 1 mm thick aluminum caul plate, 2.5 mm thick fiberglass plate, and a 2.25 mm tall cork dam. GNPT film was utilized as a base layer to prevent any resin that would bleed through the peel ply from attaching to either the hot plate or the caul plate. A caul plate on the laminate and an additional fiberglass plate (placed on top of the caul plate to make certain the bagging stacking sequence was higher than the cork dam) ensured that an even atmosphere pressure was exerted on the laminate (due to the setup being under vacuum). The cork dam prevents a pinching of laminate edges so as to make edges breathable while preventing resin bleed and the breather ensures vacuum is pulled from across the whole bag layup area, preventing a pinch in the bag from occurring, which would limit a uniform vacuum pull of the bagged area. These additional materials were added according to
A programmable hot plate (Torey Pines EchoTherm HS60A) was utilized as the thermal element for the flat plate VBO curing. The sample underwent a 180-minute (3 hr.) debulk at room temperature before the VBO cure cycle shown in
The electrospun nanofibers' diameters were analyzed using taken top-down SEM images of the nanofibers and the open-source software ImageJ, utilizing the DiameterJ toolbox [Ref. 74]. The diagram, showing the open-source code method of fiber diameter measurement, followed by the process of image segmentation and then the histogram of the fibers diameters. The PA XD10 nanofibers with manufacturer specified nanofiber diameter of 230 nm was found to have diameter of 248 nm with a standard error of ±0.92 nm. The PA 66 nanofibers with a manufacturer specified diameter of 150 nm was measured to have a diameter 153 nm with a standard error of ±0.32 nm.
Void content is important as it effects the strength and fatigue performance of composites [Ref. 71]. High resolution μCT imaging is a method of imaging the 3D location, size, and quantity of voids distributed throughout a sample. The non-destructive nature of this technique enables imaging of samples before mechanical tests are performed, enabling the correlation between mechanical properties and void percentage. μCT was conducted at the Institute for Soldier Nanotechnology (ISN) at MIT. The scans utilized the Zeiss Xradia Versa High-Resolution 3D X-ray Imaging System. The tungsten source was set to an X-ray energy of 80 kV and 7 W with no filter applied (apart from the ambient air). The scan isotropic pixel size is between 1.25 and 1.5 μm, giving a resolution of 4-5 μm. The specimens were rotated 360 degrees with 3201 projections taken per scan with the exposure time varying between 2.4-4.5 seconds depending on the sample thickness. The images were reconstructed utilizing the manufacture's reconstruction software and exported as 2D slices in .tiff format. The reconstructed slices were imported into ImageJ where void analysis was performed.
Void percentage analysis was conducted in ImageJ. The procedure is seen in
SBS testing is utilized for material performance comparison under shear like conditions and measures interlaminar shear strength (ILSS). ASTM D2344 was followed to obtain short-beam strength, a common measure of ILSS, of the specimens. The specimens' dimensions were nominally 12 mm×4 mm×2 mm (length×width×thickness). The specimens were cut from a 16-ply IM7/8552 laminate that was 15 cm×15 cm (6 in×6 in before edges were trimmed) composite laminate laid up as [0/90/±45]s2. A two-degree ply offset was used in the center, where two plies of the same orientation met, to limit the nesting of the carbon fibers of these two plies, which otherwise can give a non-conservative value of strength and contribute negatively to specimen-to-specimen variation/error. The samples were cut from the large laminate with a diamond coated band saw before being polished on a Struers TegraPol-21 grinder polisher. The short beam shear samples were polished at 320, 400, 500, 800, and 1200 American grit sandpaper (P400, P800, P1200, P2400, P1000 European grit) to remove rough edges and small cracks that might have occurred during the cutting of the laminate, or uneven surfaces. A three-point bend steel fixture with a loading nose diameter of 6 mm, support rods with diameters of 3 mm, and a span of four times the laminate thickness (nominally 8 mm) followed the ASTM D2344 standard. The loading nose was lowered at a rate of 1 mm per minute utilizing a mechanical testing machine (Zwick/Roell Z010). The thickness (tsbs) and width (wsbs) of each specimen was measured with a digital micrometer before being loaded on the test set-up. The maximum load observed during the test (Pm) was recorded. Short-beam strength (sbs) was calculated following Equation 4.1:
σsbs=0.75×Pm/tsbs×wsbs (4.1)
It must be noted that SBS presents shear like values and not necessarily pure interlaminar shear due to different failure modes such as irregular interlaminar shear, tension, plastic deformation, and compression. More complex methodologies such as the Iosipescu shear test or v-notched rail shear test should be utilized for pure shear strength values. However, SBS is commonly used as a benchmarking tool for shear strength comparison [Ref. 75].
The polyamide nanofibers are manufactured by NanoLayr (formally known as Revolution Fibres until May 2021). The polyamide 66 (Nylon 66) came from the Xantu.Layr XLA Series, and the polyamide XD10 (poly-xylylenesebacamide, XD10, known as Lexter®) came from the Xantu Layr XLB Series. Both materials have a manufacturer given porosity of 80 vol % [Ref. 76]. The nanofibers form non-woven mats or films of continuous polymer nanofibers. The polyamide 66 (PA 66) nanofiber film and the polyamide XD10 (PA XD10) film have manufacturer stated nanofiber diameters of 150 nm and 230 nm respectively. The films, likewise, had measured nanofiber diameters of 153 nm and 248 nm, similar to the manufacturer's stated fiber diameter. SEM images were taken utilizing a JEOL JSM-6010LA machine under 15 kV. Before imaging, both the aerogel and the nanofibers were gold (Au) coated using a SC 7640 sputter with a discharge voltage of 2200 V for 90 seconds for a target thickness of 12 nm. The thickness of the PA 66 at 4.5 gsm was 120 μm and the thickness of the PA XD10 at 1.5 gsm was 9 μm as measured by SEM. Additionally, the thickness was measured by a micrometer (Mitutoyo 293-344-30 Digimatic Micrometer). The measured PA 66 at 4.5 gsm nanofiber mat was 23 μm and 1.5 gsm PA XD10 nanofiber mat was 8 μm. As an SEM image for the PA 66 at 4.5 gsm had out of focus areas with an in focus area of ˜20 μm, it is concluded that the 120 μm area is due to edge effects (both from the specimen preparation and the SEM focus) and not the true representation. As such, the thicknesses was determined to be 8 μm and 23 μm following the micrometer reading and is used throughout this description.
16-ply quasi-isotropic specimens were first manufactured with a planar area of 2.54 cm×2.54 cm (1 in×1 in) as described above in three different variations. The first variation had no NPN in any of the ply interfaces, the second had 4.5 gsm 23 μm PA 66 NPN at only the middle seven interfaces, and the third had the 4.5 gsm 23 μm PA 66 NPN at all 15 interfaces. The specimens were scanned with the μCT.
In the interfaces where the 4.5 gsm 23 μm PA 66 NPN was inserted, there were no interlaminar voids as seen in
Next, 15.24 cm×15.24 cm (6 in×6 in) samples were manufactured as described above with NPN at all interfaces and μCT scan was completed to assess voids at an isotropic voxel size of 1.3 μm. SBS testing was completed as described above following the ASTM D2344 standard. The results can be seen in
A bespoke PI aerogel was obtained from Aerogel Technologies LLC with a stated manufacturer porosity of 96%. The measured thickness of 20 μm and pores can be observed in SEM and optical images in
The laminate manufacturing procedure was the same as described above. However, due to strong adherence of the aerogel to the polyethylene terephthalate (PET) backing film, a heat-assisted method was used for application of the NPN to the prepreg. The prepreg plies were placed on a hot plate set to 30° C. for 3 minutes which increased their tack. The warmed plies were then placed face down on the aerogel. After 1 minute of resting the plies were removed. The heat-assisted transfer process is presented in
The laminate was cured under vacuum on a hot plate following the same cure cycle and bagging procedure as described above. In the μCT scan, there were zero voids (void percentage of 0.0 vol %) in the region where the NPN was applied as seen in
Interlaminar thickness was compared via microscopy as seen in
PI Aerogel with 200 μm Thickness was Also Obtained from Aerogel Technologies.
The laminate manufacturing procedure was the same as described above with a stacking sequence of [0/90/±45]2s with the NPN applied on all interfaces. The μCT image is seen in
Capillary Wetting from NPN Materials
The void elimination is obtained, in part, due to the capillary pressure applied by the NPN material. While polymer resins are not Newtonian fluids, the resin flow velocity is little when compared to the resin infusion [Ref. 79]. The low shear rate of resin flow during composite curing allows the process to be studied as Newtonian flow, following a similar procedure to Lee [Ref. 10] in a previous study. Darcy's law of capillary pressure, Equation 4.2, applies at the microscopic level to define the pressure at the interface between the resin and the gas. Vflow is flow velocity vector; K is the permeability tensor; μv is dynamic viscosity; and ∇PJ is the pressure gradient at front of the flow:
The pressure gradient predicts the front of the flow.
The void pressure gradient with NPN included is described in Equation 4.4, where ΔP is the void pressure gradient, Pr is resin pressure, PNPN is the capillary pressure of the NPN due to its pores, and Pv is void pressure.
A pre-factor of 2 was used in Equation 4.5 due to the nominal orthogonality of the electrospun fiber film to the resin system rather than pre-factor of 4 which would be used if the nanofibers ran parallel to the flow field. PNPN is NPN pressure, & is porosity, Df is fiber diameter, σ is resin surface tension, and θ is the contact angle between the resin and the nanofiber system. With a PA 66 NPN fiber diameter of 150 nm, a porosity of 80 vol %, both given by the manufacturer [Ref. 76] and an assumed resin surface tension of 35 mJ m−2 and=20° [Ref. 10], the capillary pressure from the NPN is estimated to be 0.11 MPa. The NPN capillary pressure, combined with the full vacuum pressure, creates a positive ΔP leading to void collapse. In this simplified model, when cured under full vacuum, only a minimal additional capillary pressure is needed with the contact angle <90° being the determining factor, as has to be only greater than zero and assumed positive values for other variables for a positive ΔP. However, more investigations need to be done to determine the minimum capillary pressure to obtain void removal and full wetting if the interlaminar region. If cured under partial vacuum, NPN traits such as fiber diameter, contact angle porosity, etc. have more governing effects. The capillary pressure of the PA XD10 nanofibers decreases to 0.07 MPa due to the increased fiber diameter of 230 nm but still leading to a positive ΔP.
In the case of no applied NPN in a vacuum-bag only-cure, the same Equation 4.5 can be used for intralaminar voids where the fiber diameter changes to 5.2 μm and porosity to 0.4 (based on fiber volume fraction). The pressure from the fibers would be 0.018 MPa. Due to the higher permeability of the carbon fibers, the void pressure is assumed to be zero. This leads to a positive pressure of 0.118 MPa as the resin pressure remains at 0.1 MPa. The 1D pressure modeling shows how intralaminar voids can be removed and helps explain why there are interlaminar voids but no intralaminar voids in hot plate cured laminates when no interlaminar NPN is added and helps explain why the interlaminar voids are more of a challenge than intralaminar voids in vacuum-bag-only cures and how the addition of NPN materials can enable VBO curing.
OoA prepreg systems are engineered to have alternating regions of dry fibers and resin-rich areas in-plane to the ply to enable VBO curing [Refs. 3, 81]. These pathways facilitate air evacuation before resin flows into the dry fiber area and, as the cure progresses, resin flows into these dry fiber areas. Lee found that A-CNT NPNs can remove interlaminar morphology variations due to the A-CNTs compliance and eliminate interlaminar voids in autoclave-grade prepreg cured VBO due to the capillary effects of the A-CNT NPNs [Ref. 39]. Polymer NPN systems act with an alike mechanism. Compared to the OoA prepreg the breathable pathway when NPNs are inserted into the interlinear region, is larger in area, the size of the whole laminate area, and the pathway is out of plane as it is an interlayer. The furthest a breathable pathway is to the NPN layer is half a ply thickness. For IM7/8552 UD prepreg, that is 0.0655 mm or 65 μm. NPNs presented here and previously are highly porous (80-99 vol %), enabling the evacuation of volatiles, entrapped air, and moisture before the resin impregnates the interlaminar region. As this void evacuation pathway is nearby all areas of the ply, entrapped air bubbles have little distance to travel which could reduce the amount of time the pathway needs to stay open. When the resin viscosity decreases as temperature rises, the capillary effect of the NPN systems encourages full resin infusion into the interlaminar region. Therefore, NPN+VBO manufacturing can be thought of as creating an OoA prepreg system with out of plane void evacuation channels made from NPNs that have inherent enhanced capillary properties which can encourage resin infusion and enable traditional autoclave prepreg systems to be cured VBO.
The usage of two polymer based commercially scaleable material systems as NPNs for use in a next generation composite manufacturing technique where there is no applied pressure during cure using autoclave-grade composites was described herein, along with no large manufacturing process changes or prepreg material changes (resin, morphology, etc.). The first explored NPN system is EPNs where two different versions of polyamide are demonstrated to remove voids: PA 66 (Nylon 66) and PA XD10 (Lexter®) each with a differing nanofiber diameter and film thickness. The second is a bespoke PI aerogel in different film thicknesses. The EPN specimens were demonstrated to be effective at the 15 cm×15 cm (6 in×6 in) scale. With all NPN materials, μCT analysis was done to show void-free samples of VBO cured autoclave-grade prepreg with NPN interlayers. Short-beam strength was also evaluated for the EPN NPN systems showing no degradation of composite properties with the addition of the 1.5 gsm 8 μm PA XD10 and 4.5 gsm 23 μm PA 66 NPN interlayers. The higher short-beam strength values is thought to be from the increased resin-rich in-terlaminar region with thermoplastic filler. The PI aerogel material, in addition to showing interlaminar void elimination with the 20 μm thickness, revealed that with too large of an NPN layer (200 μm thick) intralaminar voids occur due to resin flow into the NPN from the plies. Microscopy showed that the interlaminar region is still thickened with the thinnest utilized NPN systems: the 20 μm thick PI aerogel and the 8 μm thick 4.5 gsm PA XD10 EPN both had interlaminar regions of 5 to 7 μm as opposed to 1 to 2 μm regions of specimens cured without the NPN. Lastly, the mechanics of the capillary effects are discussed as well as the NPN layer acting as a whole ply area breathable pathway that is half a ply thickness away from any point before resin infiltration. This next generation composite cure methodology has the potential to eliminate autoclaves from the manufacturing process, improving the economics of composite manufacturing and revolutionizing aerospace-grade composite manufacturing.
NPN Integration into Geometric Complex L-Shaped Composites for Vacuum-Bag-Only Curing of Autoclave-Grade Prepreg
Composites manufactured in L-shape geometries have many use cases such as operating as stringers and enabling joining of flat plates in a T-configuration [Ref. 8]. In these complex geometries, however, there have been unresolved challenges in void formation, fiber wrinkling, and thickness variation in the curved region of the laminate [Refs. 8, 92]. This section focuses on the void elimination properties of NPNs and curing autoclave-grade prepreg in VBO conditions in the L-shape geometries.
The layup and integration of the specimens was completed as described above with a stacking sequence of [0°/90°]4s. L-shape geometries can be observed after they are cured. The μCT imaging and void analysis was completed as described above. Each sample was scanned in both the flat and curved section of the geometry midway through the laminate width at the midpoint of the curved and flat region. The approximate locations of the scan are seen in
In the flat region, the void content for the regions without NPN interlayers was 0.40 vol %. Similar to the work on flat plates described above, no voids were identified in the region where NPN was applied. However, in the curved region this was not the case. In the curved region where NPN interlayers were placed, interlaminar voids were reduced but not eliminated, and intralaminar voids were also observed. The void content percentage where NPNs were placed as interlayers was 0.95 vol %. This is an improvement from the non-NPN region with a void content of 4.18 vol %.
In both the flat and curved regions without NPN, voids were present. Upon further inspection, the region around the curve had a gloss like finish indicating a lack of full compaction as the peel ply layer would cause there to be a more matte finish to occur if otherwise. Subsequently, a 1 mm thick silicone rubber mat was used. A 1 mm thick A1 caul plate was used as described above for the plate specimens. Rubber was used in the L-shape specimens as opposed to bent aluminum due to its pliability, which enabled it to conform to the laminate along the curve without needing to be precisely machined. The thickness of only 1 mm also helped in that regard. The addition of this a caul sheet or pressure intensifier is not uncommon and has been seen in previous studies [60, 61]. Layups were completed with no NPN applied to the interlaminar regions and 4.5 gsm 23 μm PA 66 NPN applied to all interfaces.
Microscopy confirmed this result in both of the flat region and the curved region as seen in
Preliminary Investigations into Autoclave-grade Woven Prepreg with NPN Interlaminar Integration for VBO Curing
Unidirectional (UD) plies have fibers aligned in one direction. They are strong in the fiber direction (longitudinal direction) where the fiber properties dominate and weak in the orthogonal direction (transverse direction) where the matrix properties dominate. Laminates can be engineered to have different mechanical couplings by alternating the direction of stacked plies. Additionally, UD plies have higher static strength, allow for increased fiber volume fractions and elastic properties compared to woven fabrics [Ref. 6]. Woven fabrics, where fiber tows run in orthogonal directions and woven together, are used to create a fabric with more uniform in plane strength regardless of direction as seen in
The woven prepreg system utilized was IM7/8552 plain weave which had a warp to fill ratio of 50 to 50, a fiber volume percentage of 55.57 vol %, and a mass of 196 g/m2. An optical image of the woven fabric can be seen in
Interfaces between UD and woven plies occur in hybrid laminates where oftentimes a woven ply is placed on the outer layer of the laminate for abrasion, surface, and impact resistance [Ref. 6]. The natural morphology of the weave presents a larger challenge for void elimination with NPN's due to its varying interlaminar thickness. The μCT imaging and void analysis of the manufactured laminates was completed as described above. The greatest success in void reduction in UD-woven fabric specimens were with 70 to 80 μm A-CNTs, in both patterned and non-patterned configurations, where void content was found to be less than 0.3 vol % as depicted in
Additionally, A-CNTs with a height of 230 μm and the electrospun 4.5 gsm 23 μm PA 66, as described above, are utilized as NPN materials with a void content percentage of 2.03 vol % and 2.00 vol % respectively. While the introduction of NPN in the interlaminar region using VBO curing decreased void content from 3.62 vol %, more work needs to be done to ensure complete void removal. This result regardless might be might be promising for non-aerospace applications like wind and infrastructure.
In specimens made with woven fabric, two void prone areas are identified as depicted in
Cross sectional optical microscopy was completed to investigate the voids that occurred in the woven plies.
VBO curing of [0Fabric]4s laminates with NPNs integrated in the interlaminar region which has all Woven-Woven interfaces was investigated.
As described herein, investigations into expanding the next generation VBO curing with autoclave-grade prepreg materials into complex geometries and woven prepreg. Increased ply-ply slippage, due to the NPN, can improve ease of manufacture of complex shapes while conveying laminate consolidation benefits. Additionally, A-CNTs and patterned A-CNT arrays were utilized as NPN materials. L-shape geometric laminates were manufactured representing complex geometries that require a curve or a bend expanding VBO+NPN cures into non-2D plate geometries. The L-shape manufacturing process additionally utilized a convection heat source (an oven) as opposed to previous conduction heat sources (hot plate, CNT heater) for VBO+NPN curing. Convection based OoA curing is common in industry and the L-shape specimens verify its compatibility with VBO+NPN cures. The electrospun 4.5 gsm 23 μm PA 66 NPN was utilized as the NPN interlayer material and when cured with a 1 mm thick rubber caul plate, voids were eliminated in both the flat and curved region of the L as observed by both μCT and microscopy. Studies with the woven fabric showed a significant void reduction but not void elimination with the NPN materials tested, though voids were reduced compared to the non-NPN specimens. In the specimens with UD-woven interfaces, voids were suppressed but never eliminated. The best results of the specimens tested had A-CNTs (patterned and non-patterned) with a height of 70-80 μm which had void percentages of under 0.3 vol %. However, upon closer inspection of the μCT scans, rare but large (>200 μm) voids still occurred. In the laminates made with only woven fabric, void content was reduced but never below the aerospace-grade level of 1 vol %, much less eliminated. Two regions were identified as void prone: interweave regions between tows, and corner regions around the perimeter of the elliptical tows. In a specimen with CNTs as the NPN material, no CNTs are observed with microscopy in the interweave and the corner regions leading to the hypothesis that the lack of NPN penetration during transfer in these regions lead to the lack of void reduction. The next generation composite cure methodology of VBO+NPNs for curing autoclave-grade prepreg was demonstrated in convection cures and complex geometries successfully. While the integration of NPNs into the interlaminar regions of laminates with woven prepreg has been shown to reduce voids, it has not lead to complete void elimination yet and work in this area should continue in the future.
The work summarized here investigated various nanomaterial systems (nanoporous network, or NPNs), which have nanoporous properties that when applied to the interlaminar region of a laminate made from traditional aerospace-grade autoclave prepreg, void-free composites can be achieved even when curing under vacuum-bag-only (VBO) conditions without an autoclave. The enabling mechanism acts as follows: the NPN is applied to the ply-ply interface of the laminate during layup as an interlayer creating a full laminate planar area breathable pathway for entrapped air, volatiles, and moisture to be removed via vacuum, which is only a maximum half ply distance away from any point. Then, during cure, as the resin viscosity decreases, the capillary pressure of the NPNs encourage resin infiltration of the interlaminar region, further eliminating voids. Due to the NPN's compressible nature and the vacuum pressure, the NPN-comprised interlayer is compressed and is observed to reduce in thickness. This is demonstrated with two separate commercially available polymer NPN material systems, in conductive and convective cure environments, and with the most common utilized complex geometry (i.e., L-shape), which demonstrates curved laminates to also be effectively manufactured via VBO cure. Unidirectional (UD) prepreg systems were investigated extensively and woven fabrics preliminarily. While UD prepreg showed complete void elimination in flat and L-shape geometries, woven fabrics only showed void reduction, not full elimination in flat panel geometries. A-CNTs and patterned A-CNTs were also investigated with the woven fabric prepreg.
The two alternative, commercially available NPN material systems were investigated and demonstrated void removal: electrospun polymer nanofiber (EPN) films and a polymer aerogel. Two variants of the EPN films were utilized: Polyamide (PA) 66 and PA XD10 which additionally had different fiber diameters and film thickness. Both variants of EPN films, when integrated into a UD laminates' interlaminar region, enabled void-free VBO manufactured composites. The polyimide aerogel additionally demonstrated the same void elimination properties with laminates cured in VBO conditions. Specimens with EPN films were scaled void-free to the 15 cm×15 cm (6 in×6 in) scale. Short beam shear (SBS) tests showed no degradation of interlaminar shear strength when the EPN NPNs were integrated. Microscopy of the interlaminar region showed increased interlaminar thickness (5-7 μm vs 1-2 μm) when even the thinnest obtained EPN NPN was integrated (1.5 gsm 8 μm PA XD10 and 20 μm polyimide (PI) aerogel). It is important to note that these material systems can be manufactured in thinner forms which would reduce the interlaminar region further.
In addition to their void reduction capabilities, the NPN materials, being applied as interlayers between two tacky resin coated plies, act as a lubricating layer increasing manufacturing ease. While noted for flat plate geometries, for curved shape geometries it takes on importance, not just for increased drapability in areas with small radii of curvature, but also for laminate consolidation as the ply-ply friction opposes the ply sliding necessary for full laminate consolidation [Ref. 63]. The L-shapes manufactured with NPN in VBO conditions are not observed to have manufacturing defects such as fiber wrinkling, which are common in complex geometries. The results described herein show a diverse variety of scalable commercial (polymer) NPN material systems can be utilized to achieve void-free scaled composites cured VBO, even though they are autoclave-grade composites in various geometries, with no drop in interlaminar strength or thermophysical changes. The NPN can be applied to the prepreg directly pre-layup or as a layer during layup and is likely to be applied in an automated layup capable manner at industrial scale. Taken together, the demonstrated technology is an improvement over traditionally autoclave cured methods as it utilizes autoclave-grade prepreg of which there is a diverse certified material selection, but it eliminates the capital and operational-intensive autoclave. It is an improvement over the higher material cost OoA grade prepreg material as it does not require resin chemistry modifications or prepreg morphology modifications which drives lengthy and expensive certification testing. The NPN+VBO system improves the economics of composite manufacturing and is transformative for aerospace-grade composite manufacturing.
The composites were hot plate cured under vacuum-bag-only conditions.
The following formula was used to determine the capillary pressure with epoxy resin.
The following references, some of which are noted above in parenthetical “refs.” and many of which are cited herein, are each incorporated by reference in their entirety.
Details of one or more embodiments are set forth in the accompanying drawings and description. Other features, objects, and advantages will be apparent from the description, drawings, and claims. Although a number of embodiments of the invention have been described, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. It should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features and basic principles of the invention.
This application claims priority to U.S. Application No. 63/238,507, filed Aug. 30, 2021, which is incorporated by reference in its entirety.
This invention was made with Government support under Grant No. N68335-20-C-0213 awarded by the Naval Air Systems Command (NAVAIR). The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/041519 | 8/25/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63238507 | Aug 2021 | US |
