Composite having interlayer reinforcing particles and method for producing the composite

Abstract

A fiber-reinforced polymer composite structure having chemically active thermoset particles positioned in the interlaminar regions between adjacent layers of reinforcing fibers, and a method for manufacturing the composite structure. During curing of the composite structure, the chemically active functional groups of the thermoset particles form covalent bonds with the matrix resin surrounding the particles. In one embodiment, the particles are comprised of a partially cured thermoset polymer having a degree of cure less than 100%. In another embodiment, the particles are derived from a thermoset resin composition, with a stoichiometric ratio that is insufficient or in excess of the amount of curing agent required to react with 100% of the thermoset resin components. In some embodiments, the composition of the chemically active thermoset particles is the same or nearly the same as the composition of the matrix resin of the composite structure. [Selection diagram] None

Description

Technical Field

[0001] The present invention generally relates to fiber reinforced polymer (FRP) composites having reinforcing particles and methods of manufacturing such composites.

Brief Description of the Drawings

[0002]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0003] Fiber reinforced polymer (FRP) composites have been used as high-strength, lightweight industrial materials to replace metals in aerospace structures such as the main structures of aircraft. Important properties of such composite materials are high strength, stiffness and light weight.

[0004]

[0005] Multilayers of prepreg plies are typically used to form structural composite parts having a laminated structure. Delamination between such composite parts is a critical failure mode. Delamination occurs when two layers peel away from each other. Critical factors that limit the design include both the energy required to initiate delamination and the energy required to propagate delamination. Ic and G IIc ) are improved.

[0006] CAI measures the ability of a composite material to withstand damage. In the test for measuring CAI, after applying an impact of a given energy to the composite material, a compressive load is applied. The damaged area and the depth of the indentation are measured before and after the impact and before the compression test. During this test, the composite material is restrained to ensure that elastic instability does not occur, and the strength of the composite material is recorded.

[0007] Fracture toughness is a property that represents the ability of a material containing cracks to withstand fracture and is one of the most important properties of materials for aerospace applications. Fracture toughness is a quantitative method that represents the resistance of a material to brittle fracture in the presence of cracks.

[0008] Fracture toughness can be quantified as the strain energy release rate (G c ), which is the energy dissipated during fracture per newly created unit of fracture area. G c is either G Ic (mode 1 - opening mode) or G IIc (mode II - in-plane shear). The subscript "IC" means a mode I crack opening formed under a perpendicular tensile stress perpendicular to the crack, and the subscript "IIC" means a mode II crack caused by a shear stress acting parallel to the plane of the crack and perpendicular to the crack front. The initiation and growth of delamination are often determined by examining the mode I and mode II fracture toughnesses.

[0009] The CAI performance of fiber-reinforced polymer composites can be improved by two main techniques. The first technique involves the use of high-strength reinforcing fibers with a relatively high fracture strain. These fibers are thought to absorb large amounts of energy without fracturing and thereby redistribute the energy over a larger area of the composite laminate.

[0010] Interlaminar toughness (G IC and G IIC) Moreover, the CAI performance of the fiber-reinforced polymer composite can also be improved by incorporating specific reinforcing particles into the interlayer region of the multilayer composite laminate. The "interlayer region" refers to the region between two adjacent structural layers of reinforcing fibers in the composite laminate. The presence of the reinforcing particles in the composite laminate creates a resin-rich intermediate layer that aids in accommodating crack propagation in this interlayer region.

[0011] Conventionally, thermoplastic particles such as polyamide (PA) have been incorporated into the interlayer region of the composite laminate to improve CAI. The "interlayer region" refers to the region between adjacent layers of reinforcing fibers in a multilayer composite laminate. However, polyamide-based thermoplastic particles can pose problems such as a low melting point (Tm), especially for polyamides with long aliphatic chains, or extremely high hygroscopicity, such as for polyamides with short aliphatic chains. Amorphous thermoplastic particles such as amorphous PA or PI can pose problems such as insufficient solvent resistance. Another common problem is the formation of an inadequate interface between the thermoplastic particles and the thermoset matrix, where the particles are embedded due to a mismatch in the coefficient of thermal expansion (CTE) between the particles and the surrounding resin matrix. Such CTE mismatch can cause delamination during thermal cycling tests, which is commonly referred to as microcracking and is a major concern in the aerospace industry.

[0012] The present invention relates to the use of chemically active or "live" thermoset particles as interlayer reinforcing particles for improving the damage tolerance and fracture toughness of fiber-reinforced polymer composites. More specifically, the particles possess chemical functional groups that can react with the thermoset resin matrix, and they disperse and form covalent bonds during the curing of the resin matrix.

[0013] In one embodiment, the chemically active particles are derived from partially curing the thermosetting resin composition beyond its gel point in order to achieve “solid-like” properties, and then obtaining the desired particle size by polishing. Due to the partial curing, unreacted or non-crosslinked functional groups are present on the particle surface. The particles are sufficiently crosslinked beyond the gel point of the resin composition to maintain particle binding and ensure the formation of distinct interlayer regions during the curing of the composite laminate embedding the particles.

[0014] In another embodiment, the ratio of the thermosetting resin to the curing agent in the curable resin composition is adjusted such that the composition contains the thermosetting resin and the curing agent in a non-stoichiometric ratio, i.e., in an amount that is deficient or in excess of the amount of curing agent required to react with 100% of the thermosetting resin, and as a result, due to this deficiency or excess, unreacted or non-crosslinked functional groups from the thermosetting resin or the curing agent are present at the end of the complete curing cycle. After complete curing, the cured resin is pulverized to obtain particles having chemically active functional groups on the particle surface. In this embodiment, the resulting chemically active thermoset particles consist of a crosslinked thermosetting resin or thermoset polymer and chemically active functional groups capable of forming covalent bonds. When the particles are formed from a thermosetting resin composition containing an epoxy resin, the resulting chemically active thermoset particles consist of a crosslinked polyepoxide and non-crosslinked functional groups.

[0015] The partially cured particles can be formed from the same or substantially the same curable resin composition used to form the matrix resin of the fiber reinforced composite material, such as a prepreg. The term "substantially the same" means that more than 50% of the composition is the same. In one embodiment, some of the matrix resin used to form the composite material is taken to be partially cured and ground to form chemically active particles. The partially cured particles are then incorporated into the interlayer region of the composite laminate. In this way, the CTE of the particles will exactly match the CTE of the surrounding resin matrix, thus removing stress and microcracks in the cured composite laminate. Further, since the particles are made of the same or similar material as the matrix resin, the interfacial bond between the particles and the surrounding matrix resin will be strengthened after curing.

[0016] There is little or no CTE mismatch between the "active" (chemically active) thermoset particles and the surrounding resin matrix, and as a result, the cured composite laminate exhibits improved resistance to delamination and microcracks. Understand The use of the "active" thermoset particles disclosed herein , week surrounding matrix resin different chemical properties using thermoplastic or crosslinked thermoplastic reinforcing particles having as interlayer reinforcing particles a departure from conventional methods used in the aerospace industry .

[0017] The "active" thermoset particles disclosed herein do not swell during curing, as is the case with the crosslinked thermoplastic particles disclosed in U.S. Patent No. 8,846,818 and U.S. Patent No. 9,567,426. The swellable crosslinked thermoplastic particles disclosed in those patent documents are crosslinked and are derived from a composition consisting mainly of a thermoplastic polymer and usually do not have reactive functional groups remaining on the surface of the particles. As such, the swellable particles have low reactivity with the surrounding epoxy-based matrix of the composite in which the particles are dispersed.

[0018] Chemically active thermoset particles As used herein, the terms "cure" and "curing" include crosslinking of resin precursors or polymers brought about by mixing the main components, heating at high temperatures, and exposing to ultraviolet rays and radiation. "Complete cure" as used herein refers to a degree of cure of 100%. "Partially cured" as used herein refers to a degree of cure of less than 100%.

[0019] Partially cured particles are formed from a curable resin composition, which is cured to a degree of cure of less than 100%, for example, within the range of 50% to 99% of complete cure including 55% - 95%, 50% - 86%, 50% - 87%, 50% - 88%, 50% - 89%, 55% - 86%, 60% - 86%. The curable resin composition contains one or more thermosetting resins, at least one curing agent, and optionally additives such as a thermoplastic polymer, an elastomeric material, conductive fine particles, and an inorganic filler. At a degree of cure of 50% or more, the thermomechanical properties of the material are significantly modified and the material has "solid-like" properties.

[0020] To form partially cured particles, thermosetting is carried out past the gel point of the resin composition. Such a gel point can be defined as the intersection point of the G' curve and the G'' curve derived from rheological analysis during the curing cycle. G' represents the elastic modulus and G'' represents the viscosity coefficient.

[0021] The degree of cure of a thermosetting resin system can be determined by differential scanning calorimetry (DSC). The thermosetting resin system undergoes an irreversible chemical reaction during curing. As the components in the resin system cure, heat is generated by the resin, which is monitored by a DSC apparatus. The percent cure of the resin material may be determined using the heat of cure. As an example, the following simple calculation can provide this information: Cure % = [ΔH uncured - ΔH cured / [ΔH uncured X 100%

[0022] For example, when the particles are formed from a thermosetting resin composition containing an epoxy resin and an amine compound as a curing agent, the resulting chemically active thermosetting particles consist of crosslinked polyepoxide, uncrosslinked epoxy functional groups, and unreacted amine groups.

[0023] In an alternative embodiment, the ratio of the thermosetting resin to the curing agent in the curable resin composition is adjusted to contain either an amount less than or in excess of the amount of curing agent required for the composition to react with 100% of the thermosetting resin, such that as a result of this deficiency or excess, unreacted or uncrosslinked functional groups from the thermosetting resin material are present at the end of a given curing cycle. For example, if an amount X of curing agent is required to achieve 100% cure in a given curing cycle, chemically active particles may be realized by using an amount less than X in the resin composition, such as 50% - 80% of X or 60% - 70% up to 90% of X. Alternatively, if an amount X of curing agent is required to achieve 100% cure in a given curing cycle, chemically active particles may be realized by using an amount in excess of X in the resin composition, such as 120% - 150% of X or 130% - 140% including at least 110% of X.

[0024] To determine the lowest possible amount of hardener (i.e., curing agent) within a useful range, the simplified Carothers equation described in Introduction to Polymers, Third Edition, by Robert J. Young, Peter A. Lovell, pp. 46 - 47 (CRC Press, Jun 27, 2011) can be applied. The simplified Carothers equation is a method for predicting the amount of conversion (degree of reaction progress) required for a given epoxy or other thermosetting resin with a functionality of, for example, 2, 3, 4, etc. to reach the gel point when reacting with a hardener having a given functionality, such as 2, 3, 4, etc. The gel point for the functionality available for crosslinking is defined as n = 2 / (2 - pf), where n = number average degree of polymerization, p = decimal reaction progress (where 1 represents 100% reacted), and f = total number of functional groups undergoing the crosslinking reaction. Usually, the gel point is defined when n = infinity. For a trifunctional epoxy and a bifunctional hardener, the total functionality is 5, and thus, when p = 0.4, or 40% conversion, n = infinity. For a tetrafunctional epoxy and a tetrafunctional hardener (e.g., di - primary amine), f = 8, and gelation is predicted to occur at 25% conversion, etc.

[0025] Upon complete curing of the resin composition, the cured material contains unreacted / non - crosslinked functional groups that are a source of chemically active functional groups capable of forming covalent bonds. For example, when using an epoxy resin and an amine hardener and the amine compound is insufficient, the resulting cured particles contain unreacted / non - crosslinked epoxy functional groups. Conversely, when the amine compound is in excess, the resulting cured particles contain unreacted amine groups.

[0026] As an example, when the particles are formed from a thermosetting resin composition containing an epoxy resin and an amine compound as a hardener and the amine compound is insufficient, the resulting chemically active thermosetting particles consist of crosslinked polyepoxide and non - crosslinked epoxy functional groups due to an excess amount of epoxy resin.

[0027] The chemically active particles may have an average particle size (d50) of less than about 100 μm, such as 10 - 70 μm, 15 - 50 μm, or 15 - 30 μm, or 20 - 25 μm. The average particle size disclosed herein can be measured by laser diffraction technology using, for example, a Malvern Mastersizer 2000 operating in the range of 0.002 nanometers to 2000 microns. "d50" represents the median of the particle size distribution, or alternatively, is the value of a distribution such that 50% of the particles have a particle size less than this value.

[0028] Suitable thermosetting resins for forming the particles include, but are not limited to, epoxies, phenols, phenol, cyanate esters, bismaleimides, benzoxazines, polybenzoxazines, polybenzoxazoles, combinations thereof, and precursors thereof.

[0029] Polyfunctional epoxy resins (or polyepoxides) having multiple epoxide functional groups per molecule are particularly suitable. The polyepoxide may be a saturated, unsaturated, cyclic, or acyclic, aliphatic, aromatic, or heterocyclic polyepoxide compound. Examples of suitable polyepoxides include polyglycidyl ethers prepared by the reaction of epichlorohydrin or epibromohydrin with polyphenols in the presence of an alkali. Thus, suitable polyphenols are, for example, resorcinol, pyrocatechol, hydroquinone, bisphenol A (bis(4-hydroxyphenyl)-2,2-propane), bisphenol F (bis(4-hydroxyphenyl)methane), fluorine 4,4'-dihydroxybenzophenone, bisphenol Z (4,4'-cyclohexylidenebisphenol), and 1,5-dihydroxynaphthalene. Other suitable polyphenols as the base of the polyglycidyl ether are known condensation products of the novolak resin type of phenol with formaldehyde or acetaldehyde.

[0030] Examples of suitable epoxy resins include the diglycidyl ethers of bisphenol A or bisphenol F, such as EPON™ 828 (liquid epoxy resin), D.E.R 331, D.E.R. 661 (solid epoxy resins) available from Dow Chemical Co.; the triglycidyl ethers of aminophenol, such as ARALDITE® MY 0510, MY 0500, MY 0600, MY 0610 manufactured by Huntsman Corp. Additional examples include phenolic novolak epoxy resins commercially available from Dow Chemical Co. as DEN428, DEN431, DEN438, DEN439, and DEN485; cresol novolak epoxy resins commercially available from Ciba-Geigy Corp. as ECN1235, ECN1273, and ECN1299; and hydrocarbon novolak epoxy resins commercially available from Huntsman Corp. as TACTIX® 71756, TACTIX® 556, and TACTIX® 756.

[0031] The curing agent for the curable resin composition may be selected from known curing agents, such as aromatic or aliphatic amines, or guanidine derivatives. Aromatic amine curing agents are preferred, preferably aromatic amines having at least two amino groups per molecule. For example, diaminodiphenyl sulfone in which the amino group is in the meta or para position relative to the sulfone group is particularly preferred. Specific examples are 3,3'- and 4,4'-diaminodiphenyl sulfone (DDS); methylenedianiline; bis(4-amino-3,5-dimethylphenyl)-1,4-diisopropylbenzene; bis(4-aminophenyl)-1,4-diisopropylbenzene; 4,4'-methylenebis-(2,6-diethyl)-aniline (MDEA manufactured by Lonza); 4,4'-methylenebis-(3-chloro,2,6-diethyl)-aniline (MCDEA manufactured by Lonza); 4,4'-methylenebis-(2,6-diisopropyl)-aniline (M-DIPA manufactured by Lonza); 3,5-diethyltoluene-2,4 / 2,6-diamine (D-ETDA 80 manufactured by Lonza); 4,4'-methylenebis-(2-isopropyl-6-methyl)-aniline (M-MIPA manufactured by Lonza); 4-chlorophenyl-N,N-dimethyl-urea (e.g., Monuron); 3,4-dichlorophenyl-N,N-dimethyl-urea (e.g., Diuron TM) and dicyandiamide (e.g., Amicure TM CG 1200 manufactured by Pacific Anchor Chemical).

[0032] Suitable curing agents also include acid anhydrides, particularly polycarboxylic acid anhydrides, such as nadic anhydride, methyl nadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, and trimellitic anhydride, etc.

[0033] To form partially cured particles, the curing agent can be present in a stoichiometric ratio such that there are reactive groups from the curing agent in an amount sufficient to react with the reactive groups of the thermosetting resin, for example, in a ratio such as 1 mole of amine curing agent per 1 mole of epoxy resin. To form particles that are fully cured but chemically reactive, the stoichiometric ratio is such that there are reactive groups from the curing agent in an amount insufficient to react with the reactive groups of the thermosetting resin, for example, in a ratio such as 0.5 to 0.9 moles of amine curing agent per 1 mole of epoxy resin. Alternatively, to form particles that are fully cured but chemically reactive, the stoichiometric ratio is such that there are reactive groups from the curing agent in an excessive amount to react with the reactive groups of the thermosetting resin, for example, in a ratio such as 1.1 to 1.5 moles of amine curing agent per 1 mole of epoxy resin.

[0034] Additives that can optionally be incorporated into the curable resin composition include thermoplastic polymers, elastomers, and combinations thereof. The thermoplastic polymer may be selected from polyamide; polyetherimide (PEI); polysulfone such as polyether sulfone (PES), polyether ether sulfone (PEES); polyphenylene oxide (PPO); poly(ethylene oxide) (PEO), phenoxy (a thermoplastic copolymer of bisphenol A and epichlorohydrin), polyimide (PI), polyamideimide (PAI), polysulfone (Psu) copolymer, and combinations thereof. The elastomer may be selected from rubbers such as amine-terminated butadiene acrylonitrile (ATBN), carboxyl-terminated butadiene acrylonitrile (CTBN), carboxyl-terminated butadiene (CTB); fluorocarbon elastomers, styrene butadiene polymers. The amount of the thermoplastic polymer and / or elastomer, when present, is less than 40% by weight, for example, 5% to 35% by weight of the thermoplastic polymer, based on the total weight of the resin composition, so that the particles retain their thermosetting properties.

[0035] A conductive material in the form of particles, such as particles or flakes, may be added to the curable resin composition to impart a through-thickness conductivity, also known as Z-conductivity, to the final composite laminate. Examples of suitable conductive materials include metals in the form of flakes or particles, such as silver, gold, nickel, copper, aluminum, and their alloys, carbon powder, carbon-based nanomaterials, such as carbon nanotubes (single-walled carbon nanotubes or multi-walled carbon nanotubes), and carbon nanofibers. As used herein, the term "nanomaterials" refers to materials having at least one dimension smaller than about 0.1 micrometer (< 100 nanometers). Carbon nanotubes (CNTs) are tubular strand-like structures having an outer diameter in the range of about 0.4 nm to about 100 nm. For example, the outer diameter may be less than about 50 nm or less than about 25 nm, and the aspect ratio may be from 100:1 to 5000:1. Nanofibers may have a diameter in the range of 70 nm to 200 nm and a length in the range of 50 to 200 micrometers. The amount of the conductive material, if present, is less than 10% by weight, for example, 1% to 4%, based on the total weight of the resin composition.

[0036] A flame retardant may be added to the curable resin composition to impart increased flame retardancy to the final composite laminate. For example, Strujtol Polydis handling products commercialized by Schill+Seilacher. Other commercially available flame retardants will be apparent to those skilled in the art.

[0037] In one embodiment, the particles are formed from a curable resin composition containing (a) one or more polyfunctional epoxy resins, (b) at least one amine curing agent, and (c) a thermoplastic or elastomeric reinforcing agent. The amounts of components (a)-(c) may be (a) 100 parts, (b) 5-70 parts, and (c) 5-50 parts as follows.

[0038] In another embodiment, the resin composition further comprises conductive particles such as carbon nanotubes (CNT), carbon powder, metal particles, and combinations thereof. The amount of conductive particles, when present, is up to 10% by weight, for example, 1% - 10%, 2% - 5% based on the total weight of the resin composition.

[0039] It should be understood by those skilled in the art that instead of grinding after partial curing, the chemically active thermosetting particles of the present disclosure can be formed by other processes capable of producing such particles.

[0040] Composites and laminates The chemically active particles of the present disclosure can be used as interlayer particles between the layers of reinforcing fibers in a composite laminate, i.e., the particles are located in the interlayer region of the composite laminate. The "interlayer region" refers to the region between adjacent layers of reinforcing fibers in a multi-layer composite laminate.

[0041] In some embodiments, the chemically active particles are dispersed in the interlayer region formed between adjacent layers of reinforcing fibers at a content of about 2% - about 20% by weight, including about 5% - about 15%, and about 8% - about 12% based on the total weight of the matrix resin contained in the composite laminate.

[0042] Composite laminates containing interlayer particles may be manufactured using different processes. In one embodiment, particles are deposited on the surface of a prepreg ply before a number of prepreg plies are laminated together to form a stack or "prepreg layup". The prepreg plies in the layup may be positioned relative to each other in a selected orientation such as 0°, ±45°, 90°, etc. When the prepreg plies are stacked together to form a laminate, the particles remain in the interlayer region of the laminate. Once properly placed, the prepreg layup is consolidated and cured under heat and pressure to achieve the desired fiber volume fraction with minimal voids.

[0043] Particles can be deposited on the prepreg by any conventional technique such as dispersion, electrostatic precipitation, spray coating, spray distribution, and any other technique known to those skilled in the art. The distributed composite particles adhere to the surface of the prepreg due to the adhesiveness of the matrix resin.

[0044] In another embodiment, prior to the manufacture of the prepreg, a specific amount of particles is mixed with the curable resin composition. In such an embodiment, the resin film is first manufactured by coating the particle-containing resin mixture onto the release paper. Next, with the assistance of heat and pressure, the obtained resin film is laminated onto a layer of fibers, for example unidirectional fibers, to impregnate the fibers, thereby forming a prepreg ply having a specific fiber weight per unit area and resin content. During the impregnation process, due to the fact that the size of the particles is larger than the spacing between the fibers, the particles are filtered out and remain outside the fiber layer. Subsequently, when stacking two layers of prepreg containing particles in sequence for layup, the particles are positioned in the interlayer region of the prepreg layup.

[0045] In an alternative embodiment, a curable resin composition without particles is coated onto the release paper to form a resin film, and then the film is brought into contact with one or both of the opposing surfaces of the non-impregnated fiber layer. The resin impregnates the fibers, and little or no resin remains on the outer surface of the fiber layer. Subsequently, a second film of the particle-containing curable resin is brought into contact with the outer surface of the resin-impregnated fiber layer. An additional film of the particle-containing curable resin is brought into contact with the opposing outer surface of the resin-impregnated fiber layer to form a sandwich structure. As a result, the resin layer rich in particles remains outside the impregnated fiber layer and does not further impregnate the fibers. A plurality of such structures are laminated together to form a composite structure having particles in the interlayer region.

[0046] In another embodiment, two films of the curable resin composition free of particles are brought into contact with two opposing surfaces of the non-impregnated fiber layer. The resin impregnates the fibers, and little or no resin remains on the outer surface of the fiber layer. Subsequently, two films of the curable resin containing particles are brought into contact with the opposing surfaces of the fiber layer previously impregnated with the resin. A plurality of such structures are laminated together to form a composite structure having particles in the interlayer region. Such a technique is preferred because it tends to provide a regular laminate produced by particles that do not interfere with the arrangement of the fibers.

[0047] In the embodiments disclosed herein, the term "prepreg" refers to a layer of fiber material impregnated or infused with a curable matrix resin (in that form, a unidirectional fiber, nonwoven mat, or fabric ply). The term "impregnate" as used in this disclosure refers to the introduction of a curable resin into reinforcing fibers to partially or completely encapsulate the fibers with the matrix resin.

[0048] The matrix resin of the prepreg may have the same composition or a similar composition as the composition of the chemically active particles. As such, the thermosetting resins, curing agents, and additives already disclosed in relation to the particles are equally applicable to the matrix resin of the prepreg.

[0049] The fiber reinforcement material may be in the form of a woven fabric ply or a nonwoven fabric ply, or a unidirectional tape made of unidirectional fibers. "Unidirectional fibers" refer to a single layer of reinforcing fibers that are aligned in the same direction. The prepreg plies in the layup may be positioned relative to each other in a selected orientation such as 0°, ±45°, 90°, etc.

[0050] The reinforcing fibers of the composite laminate and prepreg can take the form of chopped fibers, continuous fibers, filaments, tows, bundles, sheets, plies, and combinations thereof. Continuous fibers can further adopt not only swirl mats, felt mats, and chopped mat structures, but also any unidirectional configuration (aligned in one direction), multidirectional configuration (aligned in different directions), nonwoven configuration, woven configuration, knitted configuration, stitched configuration, wound configuration, and braided configuration. The woven fiber structure may include a plurality of woven tows, and each tow may consist of a plurality of filaments, for example, thousands of filaments. In a further embodiment, the tows can be held in place by a small amount of resin binder such as cross-tow stitch, weft insertion knitted stitch, or thermoplastic resin.

[0051] Examples of fiber materials include, but are not limited to, glass (including electrical or E-glass), carbon, graphite, aramid, polyamide, high modulus polyethylene (PE), polyester, poly-p-phenylene-benzoxazole (PBO), boron, quartz, basalt, ceramic, and combinations thereof.

[0052] Regarding the production of high-strength composite materials such as materials for aerospace and automotive applications, it is preferable that the reinforcing fibers have a tensile strength exceeding 3500 MPa (in accordance with the ASTM D4018 test method).

Examples

[0053] Example 1 A resin-based U (「resin U」) containing no reinforcing particles was prepared based on the formulation shown in Table 1.

[0054] TIFF0007687824000001.tif44170

[0055] Resin U was prepared by mixing epoxy precursors Araldite® MY0510 and Araldite® PY306 at a temperature in the range of 60°C to 90°C. Araldite® MY0510 is triglycidyl p-aminophenol, and Araldite® PY306 is the diglycidyl ether of bisphenol-F, and both are manufactured by Huntsman Advanced Materials Inc. Sumikaexcel 5003P, a polyethersulfone (manufactured by Sumitomo Chemical), was added to the epoxy mixture and then dissolved at a temperature in the range of 110°C to 130°C. Next, the aromatic amine hardener Aradur® 9664-1, 4,4'-diaminodiphenyl sulfone (4,4'-DDS) (manufactured by Huntsman Advanced Materials Inc.) was added and mixed at a temperature in the range of 60°C to 90°C.

[0056] Next, the resin U thus produced was thinned on a release paper to a nominal aerial weight of 23.4 gsm (grams per square meter). Medium modulus carbon fibers were spread with a conventional prepreg device to form a fiber web of unidirectional layer fibers having a nominal aerial weight of 190 gsm. Next, the formed fiber web was sandwiched between two films of resin U to obtain prepreg U having a nominal fiber areal weight (FAW) of 190 gsm and a nominal resin content of 19.8 wt%.

[0057] Four resin compositions P.1 to P.4 were prepared based on the formulations shown in Table 2, one of which contains no particles and three of which contain thermosetting reinforcing particles with different activities. All amounts are in wt%.

[0058] TIFF0007687824000002.tif77170

[0059] The epoxy precursors Araldite® MY0510 and Araldite® PY306 were mixed at a temperature in the range of 60°C to 90°C to prepare each resin composition in Table 2. After adding Sumikaexcel 5003P (polyethersulfone), it was dissolved at a temperature in the range of 110°C to 130°C. Next, Aradur® 9664-1 (4,4’-DDS) and active thermosetting resin particles (LRTP) were added and mixed at a temperature in the range of 60°C to 90°C.

[0060] Each of the thus-produced resin compositions P was thinned to a nominal basis weight of 23.4 gsm on a release paper. Using a conventional prepreg apparatus, the prepreg U formed as described above was sandwiched between two resin films formed from the particle-containing resin composition P to obtain a prepreg P having a nominal fiber areal weight (FAW) of 190 gsm and a nominal total resin content of 33 wt%.

[0061] The different reinforcing particles used were labeled as VP-0X0, PK-0X0, and NT-0X0 in Table 2. Using the resin formulations shown in Table 3, these three types of reinforcing particles were prepared.

[0062] TIFF0007687824000003.tif68170

[0063] The epoxy precursors Tactix 123 and Araldite® PY306 were mixed at a temperature in the range of 60°C to 90°C to prepare the resins VP-0X0, PK-0X0, and NT-0X0. Tactix 123 is the diglycidyl ether of bisphenol A (manufactured by Huntsman Advanced Materials Inc.).

[0064] VP-0X0 Resin: Next, VP3619 and Aradur® 9664-1 were added and mixed at a temperature in the range of 70°C to 90°C. Struktol VP3619 is a nitrile rubber-modified epoxy prepolymer based on the diglycidyl ether of bisphenol-A (manufactured by Schill+Seilacher).

[0065] PK-0X0 Resin: PKHB100 from InChem, a polyhydroxy ether (i.e., phenoxy resin), was added to the epoxy mixture and then dissolved at a temperature in the range of 110 °C to 130 °C. Next, the aromatic amine hardener Aradur 9664-1 (4,4’-DDS) was added and mixed at a temperature of 60 °C to 90 °C.

[0066] NT-0X0 Resin: Multi-walled carbon nanotubes were pre-dispersed in the Tactix123 / PY306 blend. Next, the aromatic amine hardener Aradur 9664-1 (4,4’-DDS) was added and mixed at a temperature of 60 °C to 90 °C.

[0067] Three resins, VP-0X0, PK-0X0, and NT-0X0, were heated at 2 °C / min to 180 °C and then immediately cooled after reaching 180 °C to be partially cured, preparing three different particles (VP-0X0, PK-0X0, NT-0X0). The obtained partially cured resins were granulated before being ground by an ACM classifier mill (manufactured by Hosokawa). Differential scanning calorimetry (DSC) tests were carried out not only on the three kinds of particles VP-0X0, PK-0X0, NT-0X0 manufactured as described above but also on the initial three resins VP-0X0, PK-0X0, NT-0X0, and the conversion percentage of each of these three kinds of particles was determined using the following equation: Cure % = [ΔH uncured - ΔH cured / [ΔH uncured × 100%

[0068] The glass transition temperatures (T g ) of these three kinds of particles were also obtained from these DSC tests. Finally, the particle size distributions of these particles were measured by laser diffraction using a Mastersizer 3000 (manufactured by Malvern). The results are summarized in Table 4.

[0069] TIFF0007687824000004.tif63170

[0070] Figure 1 is a scanning electron microscope (SEM) image of the pulverized particles PK-0X0 disclosed in Table 4.

[0071] A plurality of prepregs P were laid up to form a composite laminate. The laminate was encapsulated in a conventional zero-bleed sealed vacuum bag and cured in an autoclave at 180 °C under a pressure of 85 psi (586 kPa or kilopascal) for 2 hours while maintaining a vacuum throughout the curing cycle.

[0072] Next, the cured panel was tested for damage resistance (CSAI) and for microcracks. The results are reported in Table 5.

[0073] TIFF0007687824000005.tif43170

[0074] The results shown in Table 5 illustrate the benefits of introducing these active thermosetting reinforcing particles that increase the impact performance by up to 50% without causing any particle microcracking problems.

[0075] Figure 2 shows a cross-section of the cured composite laminate with distinct interlayer regions.

[0076] After 1,200 thermal cycles between -55 °C and 70 °C, the resistance to thermal delamination was evaluated microscopically. No microcracks were found after this test.

Claims

1. A fiber-reinforced polymer composite structure comprising: Two or more layers of reinforcing fibers impregnated or injected with a curable matrix resin, the reinforcing fibers comprising one or more thermosetting resins and at least one curing agent; Chemically active thermosetting particles positioned in the interlayer region between adjacent layers of the reinforcing fibers and the chemically active thermosetting particles being derived from a thermosetting resin composition comprising one or more epoxy resins and at least one amine compound as a curing agent, the molar ratio of epoxy groups to amine groups being such that it is deficient or in excess of the amount of amine required to react with all of the epoxy groups at 100%, each of the chemically active thermosetting particles comprising either a crosslinked polyepoxide and a non-crosslinked epoxy functional group capable of forming a covalent bond or an unreacted amine group, the chemically active thermosetting particles not being particles comprising a core and a crosslinked thermosetting coating, a fiber-reinforced polymer composite structure.

2. A method of manufacturing a fiber-reinforced polymer composite structure, comprising: (a) forming thermosetting particles having chemically active functional groups on the surface of the particles; (b) forming a plurality of prepreg plies, each prepreg ply comprising reinforcing fibers impregnated or injected with a curable matrix resin; (c) depositing thermosetting particles on at least one surface of each prepreg ply; (d) laying up prepreg plies having particles thereon in a laminate arrangement such that particles are present between adjacent prepreg plies, thereby forming a prepreg layup; (e) consolidating the prepreg layup; and (f) curing the prepreg layup; wherein the thermosetting particles in (a) are formed by forming a thermosetting resin composition comprising one or more thermosetting resins and at least one curing agent, the molar ratio of the thermosetting resin to the curing agent being such that it is deficient or in excess of the amount of amine required to react with all of the epoxy resins at 100%, curing the thermosetting resin composition to form a crosslinked resin having chemically reactive functional groups, and pulverizing the crosslinked resin, during curing in (f), the chemically active functional groups of the thermosetting particles form covalent bonds with the matrix resin surrounding the particles a method.

3. A method of manufacturing a fiber-reinforced polymer composite structure, comprising: Step (a): forming thermosetting particles having chemically active functional groups on the surface of the particles; Step (b): forming a curable matrix resin composition comprising one or more thermosetting resins, at least one curing agent, and the thermosetting particles; Step (c): impregnating a plurality of layers of reinforcing fibers with the curable resin composition to form prepreg plies, each prepreg ply comprising reinforcing fibers impregnated with or injected with the curable matrix resin composition, and the thermosetting particles remaining on the outer surface of the reinforcing fiber layer; Step (d): laying up prepreg plies having particles thereon in a laminated arrangement, thereby forming a prepreg layup; Step (e): consolidating the prepreg layup; and Step (f): curing the prepreg layup; characterized in that the thermosetting particles in (a) form a thermosetting resin composition comprising one or more thermosetting resins and at least one curing agent, and the molar ratio of the thermosetting resin to the curing agent is such that it is insufficient or excessive with respect to the amount of amine required to react with all epoxy resin groups of 100%, and the thermosetting resin composition is cured to form a cured resin having chemically reactive functional groups, and is formed by pulverizing the cured resin, during curing in (f), the chemically active functional groups of the thermosetting particles form covalent bonds with the matrix resin surrounding the particles Method.

4. A method for manufacturing a fiber-reinforced polymer composite structure, comprising: Step (a): forming thermosetting particles having chemically active functional groups on the surface of the particles; Step (b): forming a resin film from a first curable resin composition not containing the thermosetting particles; Step (c): forming a resin film from a second curable resin composition comprising one or more thermosetting resins, at least one curing agent, and the thermosetting particles; Step (d): using heat and pressure to impregnate at least one resin film formed from the first curable resin composition into one layer of reinforcing fibers, thereby forming one layer of resin-impregnated reinforcing fibers; Step (e): bringing at least one resin film formed from the second curable resin composition into contact with one surface of the layer of resin-impregnated reinforcing fibers, thereby forming a particle-containing prepreg ply; Step (f): forming additional particle-containing prepreg plies according to steps (d) and (e); Step (g): laying up the prepreg plies in a laminated arrangement, thereby forming a prepreg layup; (h) a step of densifying the prepreg lay-up; and (i) a step of curing the prepreg lay-up; comprising the thermosetting particles in (a) form a thermosetting resin composition comprising one or more thermosetting resins and at least one curing agent, and the molar ratio of the thermosetting resin to the curing agent is such that it is less than or in excess of the amount of amine required to react with all of the epoxy resins of 100%, formed by curing the thermosetting resin composition to form a cured resin having chemically reactive functional groups and grinding the cured resin, and during curing in (i), the chemically active functional groups of the thermosetting particles form covalent bonds with the matrix resin surrounding the particles method.

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