WO2015011549A1 - Fiber reinforced polymer composition with interlaminar hybridized tougheners - Google Patents

Abstract

The present application provides an innovative fiber reinforced polymer composition comprising a plurality of reinforcing fibers impregnated by an adhesive composition comprising at least an interlayer toughening material and a high-aspect-ratio material. Both the high-aspect-ratio material and the interlayer toughening material may be localized on the surface of the plurality of reinforcing fibers and a substantial amount of the high-aspect ratio material may be located closer to the plurality of reinforcing fibers than is the interlayer toughening material. The high-aspect-ratio material forms a barrier layer to keep the interlayer toughening material away from the vicinity of the plurality of reinforcing fibers and compacted in an interlayer between two pluralities of the reinforcing fibers of the fiber reinforced polymer composition during curing; such a spatial distribution in the interlayer improves at least mode II interlaminar fracture toughness of the cured fiber reinforced polymer composition substantially.

Description

FIBER REINFORCED POLYMER COMPOSITION

WITH INTERLAMINAR HYBRIDIZED TOUGHENERS

Cross-Reference to Related Applications

This application claims priority to United States Provisional Application No.

61/857,744, filed July 24, 2013, and United States Provisional Application No. 61/946,322, filed February 28, 2014, the disclosures of each of which are incorporated herein by reference in their entirety for all purposes.

Field of the Invention

The present invention provides an innovative fiber reinforced polymer composition comprising a plurality of reinforcing fibers, an interlayer toughening layer comprising at least an interlayer toughening material and a barrier layer comprising at least a high-aspect-ratio material, wherein the barrier layer is sandwiched between the plurality of reinforcing fibers and the interlayer toughening layer. The barrier layer is adapted to hinder infusion of the interlayer toughening material into the plurality of reinforcing fibers. The barrier layer may be configured to allow at least a portion of one or more components of the interlayer toughening layer other than the interlayer toughening material to infuse into the barrier layer and possibly also the plurality of reinforcing fibers upon curing. This results in an increase in the concentration of the interlayer toughening material in the interlayer toughening layer when the fiber reinforced polymer composition is cured. Both the high-aspect-ratio material and the interlayer toughening material may be localized on the surface of the plurality of reinforcing fibers and a substantial amount of the high-aspect ratio material may be found closer to the plurality of reinforcing fibers than is the interlayer toughening material. The present invention permits the mode II interlaminar fracture toughness of the cured fiber reinforced polymer composition to be enhanced substantially, as compared to analogous compositions that do not contain such a barrier layer.

Background of the Invention

To increase fracture toughness of a fiber reinforced polymer composite, specifically mode I interlaminar fracture toughness Gic, a conventional approach is to toughen the matrix with a submicrometer-sized or smaller soft polymeric toughening agent. Upon curing of the fiber reinforced polymer composite, the toughening agent is most likely spatially located inside the fiber bed/matrix region, called the intraply as opposed to the resin-rich region between two plies, called the interply. Uniform distribution of the toughening agent is often expected to maximize Gic. Examples of such resin compositions include those described in the following documents: US6063839 (Oosedo et al., Toray Industries, Inc., 2000), EP2256163A1 (Kamae et al., Toray Industries, Inc., 2009) with rubbery soft core/hard shell particles, US6878776B1 (Pascault et al., Cray Valley S.A., 2005) for reactive polymeric particles, US68941 13B2 (Court el al., Atofina, 2005) for block copolymers and US20100280151 Al (Nguyen et al., Toray Industries Inc., 2010) for reactive hard core/soft shell particles. For these cases, since a soft material was incorporated in the resin in a large amount either by weight or volume, Gic increased substantially at the expense of the resin' s modulus and subsequently compressive properties of the composite.

To increase mode II fracture toughness of the composite, an interlayer toughening technique could be utilized. Typically, a thermoplastic additive (e.g., polyimide, polyamide) having a particle size from 2μηι-50μιη could be confined in the interlayer area or the resin zone between two bundles of fibers. Guc is a measure of how well the composite part resists impact loads as opposed to tensile loads in Gic. In this case, cracks generated due to quasi- static bending of the part experience in-plane shear load, which tends to slide one crack face with respect to the other. Examples of such interlayer toughening techniques include the techniques described in US 5,413,847 (Kishi et al., Toray Industries, Inc., Japan) or US 5,605,745 (Recker et al., Cytec Technology Corp., U.S.). It has been shown that the higher the amount of interlayer particles localizing in the interlayer, the higher the interlayer thickness and subsequently the higher Gnc and compression after impact (CAI) strength. However, weight will be increased and compressive properties could be reduced accordingly. As a result, an optimized amount of interlayer particles is needed to balance these properties.

Summary of the Invention

An embodiment of the present invention relates to a fiber reinforced polymer composition comprising a plurality of reinforcing fibers, an interlayer toughening layer comprising at least an interlayer toughening material and a barrier layer comprising at least a high-aspect-ratio material, wherein the barrier layer is sandwiched between the plurality of reinforcing fibers and the interlayer toughening layer and is adapted to hinder infusion of the interlayer toughening material into the plurality of reinforcing fibers upon curing of the fiber reinforced polymer composition. The barrier layer may be additionally adapted to allow at least a portion of the interlayer toughening layer other than the interlayer toughening material (e.g., thermosetting resin, curing agent) to infuse into at least the barrier layer upon curing of the fiber reinforced polymer composition. Both the high-aspect-ratio material and the interlayer toughening material may be localized on the surface of the plurality of reinforcing fibers and a substantial amount of the high-aspect ratio material may be found closer to the plurality of reinforcing fibers than is the interlayer toughening material.

The layer of high-aspect-ratio material (e.g., carbon nanotubes) thus may act as a barrier layer in that it prevents the interlayer toughening material (which may be in the form of particles) from infiltrating into the plurality of reinforcing fibers, but at least certain other components of the matrix in which the interlayer toughener material is initially present, such as thermosetting resin and curing agent, are able to pass into and possibly through the barrier layer. Thus, the barrier layer acts like a sieve or filter, retaining the interlayer toughener material in the interlayer toughening layer while allowing at least a portion of the interlayer toughening layer other than the interlayer toughener material to infiltrate the barrier layer and possibly also the reinforcing fiber layer. The loss of such components from the initially present interlayer toughening layer reduces the volume (thickness) of the interlayer toughening layer, thereby increasing the concentration of interlayer toughener material in the interlayer toughening layer upon curing (where the interlayer toughener material is in particulate form, the spacing between the retained particles of interlayer toughener material is thus decreased when the fiber reinforced polymer composition is cured).

The interlayer toughening layer may comprise at least a thermosetting resin, a curing agent, and a micron-sized interlayer toughener. The barrier layer may comprise at least a high- aspect-ratio material comprising a nanofiber, wherein the nanofiber could be impregnated by an adhesive composition comprising components of the same kinds or different kinds from those found in the interlayer toughening layer or the plurality of reinforcing fibers (if the plurality of reinforcing fibers has been impregnated, e.g., with an adhesive composition).

Another embodiment of the invention relates to a fiber reinforced polymer composition comprising a plurality of reinforcing fibers impregnated by one or more adhesive

compositions, wherein the one or more adhesive compositions comprise at least a

thermosetting resin, a curing agent, a nanofiber and an interlayer toughening material, such that a layer of the nanofiber is located closer to the plurality of reinforcing fibers than is a layer of the interlayer toughening material. The nanofiber may be selected from the group consisting of carbon nanotubes, carbon nanofibers, oxide nanofibers, metal nanofibers, ceramic nanofibers, halloysite nanofibers, other types of organic or inorganic nanofibers, and assemblies thereof (in the context of the present invention, "assemblies" includes mats, sheets, forests, arrays and the like). The assemblies might be impregnated by an adhesive composition whose components are of the same kinds or different kinds from those found in the plurality of reinforcing fibers (where the plurality has been impregnated with an adhesive composition) or the layer of the interlayer toughening material.

Another embodiment of the invention relates to a fiber reinforced polymer composition comprising an interlayer region, between two pluralities of reinforcing fibers, comprising a nanofiber and an interlayer toughening particle such that a layer of the nanofiber is located somewhere spatially in the interlayer region such that spaces among the interlayer toughening particles and the thickness of a interlayer toughening layer comprising a substantial amount of the interlayer toughening particles are significantly reduced after the fiber reinforced polymer composition is cured. The layer of the nanofiber (or the barrier layer) could be closer to the reinforcing fiber than is the layer of the interlayer toughening particle (or the interlayer toughening layer). Alternatively, one or more barrier layers could be placed in between two interlayer toughening layers for a similar purpose. The interlayer could further include a conductive material distributed in the barrier layer, the interlayer toughening layer or both.

Other embodiments of the invention relate to a prepreg comprising one of the above- described fiber reinforced polymer compositions.

Other embodiments of the invention relate to a method of manufacturing one of the above-described fiber reinforced polymer compositions.

Other embodiments of the invention relate to a method of manufacturing a composite article comprising curing one of the above-described fiber reinforced polymer compositions. Brief Descriptions of the Drawings

FIG. 1 shows a schematic of a fiber reinforced polymer composition in a prepreg form consisting of a reinforcing fiber layer, a barrier layer, and an interlayer toughening layer. The interlayer toughening layer as shown is impregnated by an adhesive composition comprising at least an interlayer toughener. Either the reinforcing fiber, the barrier layer, or both could be impregnated by another adhesive composition of the same kind or different kind from each another (not shown).

FIG. 2 shows a schematic of an interlayer between two plies of the prepreg in Fig. 1 stacked together. During curing under heat and pressure, the stack is consolidated such that the barrier layers restrict the movement of the interlayer tougheners in the interlayer, leading to reduced spacing among the interlayer tougheners (i.e., the interlayer toughener domains or particles are closer together). On the other hand, some of the components in the adhesive composition could infuse the barrier layers, leading to an increase in their thicknesses. Detailed Description of the Invention

An embodiment of the invention relates to a fiber reinforced polymer composition comprising a plurality of reinforcing fibers, an interlayer toughening layer comprising at least an interlayer toughening material and a barrier layer comprising at least a high-aspect-ratio material, wherein the barrier layer is sandwiched between the plurality of reinforcing fibers and the interlayer toughening layer and is adapted to hinder infusion of the interlayer toughening material into the plurality of reinforcing fibers upon curing of the fiber reinforced polymer composition. The barrier layer may be additionally adapted to allow at least a portion of the interlayer toughening layer other than the interlayer toughening material (e.g., thermosetting resin, curing agent) to infuse into at least the barrier layer upon curing of the fiber reinforced polymer composition. Both the high-aspect-ratio material and the interlayer toughening material are localized on the surface of the reinforcing fibers and a substantial amount of the high-aspect ratio material is found closer to the reinforcing fibers than the interlayer toughening material.

In the above embodiment, there are no specific limitations or restrictions on the choice of a reinforcing fiber, as long as the effects of the invention are not deteriorated. Examples include carbon fibers, organic fibers such as aramid fibers, silicon carbide fibers, metal fibers (e.g., alumina fibers), boron fibers, tungsten carbide fibers, glass fibers (e.g., S glass, S-l glass, S-2 glass, S-3 glass, E-glass, L-glass from AGY), and natural/bio fibers. Carbon fiber in particular is used to provide the cured fiber reinforced polymer composition exceptionally high strength and stiffness as well as light weight. Of all carbon fibers, those with a strength of 2000 MPa or higher, an elongation of 0.5% or higher, and modulus of 200 GPa or higher are preferably used. Examples of carbon fibers are those from Toray Industries having a standard modulus of about 200-250 GPa (Torayca ® T300, T300J, T400H, T600S, T700S, T700G), an intermediate modulus of about 250-300 GPa (Torayca ® T800H, T800S, T1000G, M30S, M30G), or a high modulus of greater than 300 GPa (Torayca ® M40, M35J, M40J, M46J, M50J, M55J, M60J).

The form and the arrangement of a plurality of reinforcing fibers used are not specifically limited. Any of the forms and spatial arrangements of the reinforcing fibers known in the art such as long fibers in a direction, chopped fibers in random orientation, single tow, narrow tow, woven fabrics, mats, knitted fabrics, and braids can be employed. The term "long fiber" as used herein refers to a single fiber that is substantially continuous over 10 mm or longer or a fiber bundle comprising the single fibers. The term "short fibers" as used herein refers to a fiber bundle comprising fibers that are cut into lengths of shorter than 10 mm.

Particularly in the end use applications for which high specific strength and high specific elastic modulus are required, a form wherein a reinforcing fiber bundle is arranged in one direction may be most suitable. From the viewpoint of ease of handling, a cloth-like (woven fabric) form is also suitable for the present invention.

Higher adhesion between the reinforcing fiber and an adhesive composition could provide higher interlaminar fracture toughness and compression after impact (CAI). In order to achieve better adhesion, the reinforcing fiber could have a non-polar surface energy at 30 °C of at least 30 mJ/m , at least 40 mJ/m , or even at least 50 mJ/m and/or a polar surface energy at 30 °C of at least 2 mJ/m², at least 5 mJ/m², or even at least 10 mJ/m². High surface energies are needed to promote wetting of the adhesive composition on the reinforcing fiber. This condition is necessary to promote good bonds from the adhesive composition.

Non-polar and polar surface energies could be measured by an inverse gas

chromatography (IGC) method using vapors of probe liquids and their saturated vapor pressures. IGC can be performed according to Sun and Berg's publications (Advances in Colloid and Interface Science 105 (2003) 151-175 and Journal of Chromatography A, 969 (2002) 59-72). A brief summary is described as follows. Vapors of known liquid probes are carried into a tube packed with solid materials of unknown surface energy and interacted with the surface. Based on the time that a gas traverses through the tube and the retention volume of the gas, the free energy of adsorption can be determined. Hence, the non-polar surface energy can be determined from a series of alkane probes, whereas the polar surface energy can be roughly estimated using two acid/base probes.

Instead of using surface energies as described above for a selection of suitable reinforcing fibers for bonding with the adhesive composition, an interfacial shear strength (IFSS) value of at least 10 MPa, at least 20 MPa, at least 25 MPa, or even at least 30 MPa, determined in a single fiber fragmentation test (SFFT) according to Rich et al. in "Round Robin Assessment of the Single Fiber Fragmentation Test" in Proceeding of the American Society for Composites: 17th Technical conference (2002), paper 158 could be needed. A brief description of SFFT is described as follows. A single fiber composite coupon having a single fiber embedded in the center of a dog-boned cured resin is strained without breaking the coupon until the set fiber length no longer produces fragments. IFSS is determined from the fiber strength, the fiber diameter, and the critical fragment length determined by the set fiber length divided by the number of fragments. Good adhesion between the adhesive composition and the reinforcing fiber herein is referred to as "good bonds" in that one or more components of the adhesive composition chemically react with functional groups found on the reinforcing fiber's surface to form crosslinks. Good bonds in one embodiment can be documented by examining the cured fiber reinforced polymer composition after being fractured under a scanning electron microscope (SEM) for failure modes. Adhesive failure refers to a fracture failure at the interface between the reinforcing fiber and the cured adhesive composition, exposing the fiber's surface with little or no adhesive found on the surface. Cohesive failure refers to a fracture failure which occurs in the cured adhesive composition, wherein the fiber's surface is mainly covered with the adhesive composition. Note that cohesive failure in the fiber may occur, but it is not referred to in the invention herein. The coverage of the fiber surface with the cured adhesive composition could be about 50 % or more, or about 70 % or more. Mixed mode failure refers to a combination of adhesive failure and cohesive failure. Adhesive failure refers to weak adhesion and cohesive failure is strong adhesion, while mixed mode failure results in adhesion somewhere in between weak adhesion and strong adhesion and typically has a coverage of the fiber surface by the cured adhesive composition of about 20 % or more. Mixed mode and cohesive failures herein are referred to as a good bond between the cured adhesive composition and the fiber surface while adhesive failure constitutes a poor bond.

For applications of interest using a carbon fiber, in order to achieve high IFSS, the carbon fiber typically is oxidized or surface treated by an available method in the art (e.g., plasma treatment, UV treatment, plasma assisted microwave treatment, and/or wet chemical- electrical oxidization) to increase its concentration of oxygen to carbon (O/C). The O/C concentration can be measured by an X-ray photoelectron spectroscopy (XPS). A desired O/C concentration may be at least 0.05, at least 0.1 , or even at least 0.15. The oxidized carbon fiber is coated with a sizing material such as an organic material or organic/inorganic material such as a silane coupling agent or a silane network or a polymer composition compatible and/ or chemically reactive with the adhesive composition to improve bonding strengths. For example, if the adhesive resin composition comprises an epoxy, the sizing material could have functional groups such as epoxy groups, amine groups, amide groups, carboxylic groups, carbonyl groups, hydroxyl groups, and other suitable oxygen-containing or nitrogen-containing groups. Both the O/C concentration on the surface of the carbon fiber and the sizing material collectively are selected to promote adhesion of the adhesive composition to the carbon fiber. There is no restriction on the possible choices of the sizing material and a desired O/C concentration as long as the requirement of surface energies of the carbon fiber is met and/or the sizing promotes good bonds.

To have good bonds between carbon fibers and the cured adhesive composition an IFSS value of at least 15 MPa could be needed. Alternatively, a measurement of fiber-matrix adhesion could be obtained by interlaminar shear strength (ILSS) described by ASTM D-2344 of the cured fiber reinforced polymer composition. Good bonds could refer to an IFSS of at least 20 MPa, at least 25 MPa, at least 30 MPa or even 35 MPa and/or a value of ILSS of at least 13, at least 14 ksi, at least 15 ksi, at least 16 ksi, or even at least 17 ksi. Ideally, both an observation of failure modes and an IFSS value are needed to confirm good bonds. However, generally, when either observations of failure modes or an IFSS value cannot be obtained, an ILSS value between 13-14 ksi could indicate a mixed mode failure while an ILSS value above 16 ksi could indicate a cohesive failure and an ILSS value between 14-15 ksi could indicate either mixed mode or cohesive failure, depending on the reinforcing fiber and the adhesive composition.

Note that the above ranges of IFSS and ILSS for carbon fibers could be used to screen other reinforcing fibers for good bond formation by the adhesive composition. However, depending on their strengths some deviations might be possible.

There are no specific limitations or restrictions on the choice of components to create an interlayer toughening layer comprising at least an interlayer toughening material, hereafter also referred to as an interlayer toughener, as long as the effects of the invention are not deteriorated. That is, the interlayer toughening material may be localized on the surface of the plurality of reinforcing fibers (meaning that little or no interlayer toughening material is present within the plurality of reinforcing fibers, upon curing of the fiber reinforced polymer composition) or localized in an interlayer region between two pluralities of reinforcing fibers of the fiber reinforced polymer composition (hereafter referred to as an interlayer). The interlayer toughening material is suitable to provide significant impact resistance and tolerance to the cured fiber reinforced polymer composition versus the control composition without the interlayer toughening material. One of the key measurable properties is mode II fracture toughness (by shear mode) or Gnc- For higher Gnc, it is important to keep a crack propagating in the interlayer toughening layer. Another key property is CAI. In most cases, CAI relates to Gnc such that the higher Gnc leads to the higher CAI. In some cases, the interlayer toughener might be able to increase mode I fracture toughness (by opening mode). The interlayer toughener could be one or more thermoplastics, one or more elastomers, or combinations of one or more elastomers and one or more thermoplastics, or combinations of an elastomer and an inorganic material such as glass, or pluralities of nanofibers or

micro nfibers. The interlayer toughener could be in the form of a particulate or a sheet with a desired thickness (e.g., film, a mat, a woven or a non-woven fabric/veil). In some cases, the sheet form is preferred for ease of manufacturing the fiber reinforced polymer composition. If the interlayer toughener is in particulate form, the average particle size of the interlayer tougheners could be no more than 100 μη , or 5-50 μηι, to keep them in the interlayer after curing to provide maximum toughness enhancement. Such particles are generally employed in amounts of up to about 30%, or up to about 15% by weight (based upon the weight of total resin content in the composite composition). The resulting interlayer thickness may be at most 200 μιτι, at most 100 μηι or even at most 50 μηι. The amount of the interlayer toughener and/or the thickness of the interlayer depend on desired mechanical properties versus weight of the fiber reinforced polymer composition. For instance, a higher amount of the interlayer tougheners could be needed to increase Gnc and C AI, but at the expense of compressive properties such as open-hole compression (OHC). Examples of suitable thermoplastic materials include polyamides. Known polyamide particles include SP-500, produced by Toray Industries, Inc., "Orgasol^®" produced by Arkema, and Grilamid^® TR-55 produced by EMS- Grivory, nylon-6, nylon- 12, nylon 6/12, nylon 6/6, and Trogamid^® CX by Evonik.

The interlayer toughener could be a conductive material or coated with a conductive material (e.g., Micropearl^®AU215, AU225 [Ni and Au-plated polymeric particles] from Sekisui Chemical Co., Ltd. or Bellpearl^® C-2000 from Air Water Inc. as described in US Patent 7931958 B2) or combination of a conductive material and a non-conductive material to regain z-direction electrical and/or thermal conductivity of the cured fiber reinforced polymer composition that was lost by the introduction of the resin-rich interlayers. "Conductive," as used herein, refers to the electrical conductivity of a material. In some cases, it may also refer to the thermal conductivity, or collectively refers to both electrical and thermal conductivities of the material, or its thermoelectric property, i.e., its capability to generate an electric potential from a temperature difference, or heat from an electric potential difference. An electrically conductive material herein refers to a material having an electrical conductivity of at least 10^"13 S/m, at least 10^"10 S/m, 10^"5 S/m, or even at least 10^"1 S/m, while a non-conductive material is a material having an electrical conductivity of less than 10^"13 S/m. Examples of conductive interlayer particles include but are not limited to carbon particles and conductive material coated particles. The conductive interlayer particles could be larger and have a narrower size distribution than the interlayer toughener to provide better contact between two pluralities of the reinforcing fibers or two barrier layers. Typical amounts of the conductive particles could be up to 50 % of the total interlayer tougheners.

The interlayer toughener could be deposited directly on either surface of the plurality of reinforcing fibers impregnated by an adhesive composition, or incorporated in another adhesive composition if it is in a particulate form or a fibrous form, or impregnated by this adhesive composition if it is an assembly of a fibrous material. The interlayer toughener could further comprise a curable functional group, such as an epoxy group, an amine group, an amide group, a carboxylic group, a carbonyl group, other suitable oxygen- or nitrogen-containing group, or a combination thereof, that reacts with either the adhesive compositions or the reinforcing fibers to further enhance fracture toughness and CAI.

During curing of the fiber reinforced polymer composition comprising an interlayer toughener, due to a lower viscosity of the adhesive composition as applied heat increases under a compaction pressure, the interlayer toughener could move around in an interlayer region between two pluralities of reinforcing fibers (e.g., between two rovings, mats or layers of reinforcing fibers) and even might be forced to penetrate into the pluralities of the reinforcing fibers. This often leads to an increase in a space (herein referred to as a resin rich region) among these individual tougheners or groups of individual tougheners having a particulate form or an elongated form resulted from heating of the fiber reinforced polymer composition. Such resin rich regions are prone to not effectively resist crack propagation such that cracks could emerge into a crack front propagating into a plurality of reinforcing fibers (herein referred to as intraply de lamination) and causing a premature failure or low Gnc of the cured fiber reinforced polymer composition. In such cases, it is inevitable to minimize the spacing among the interlayer tougheners. The present invention proposes a solution to this problem wherein a barrier layer comprising at least a high-aspect-ratio material is placed between the plurality of reinforcing fibers and the interlayer toughening layer such that this barrier layer helps to minimize the movement of the interlayer toughener in the interlayer and keep it from approaching a vicinity of the plurality of reinforcing fibers during curing under heat and pressure.

The barrier layer could comprise at least a high-aspect-ratio material comprising a nanofiber or a plate-like nanomaterial. Such a material is also referred to as a nanotoughener having at least one dimension of at least 1 nm and not more than 1000 nm. The barrier layer comprising a nanotoughener is also referred to as a nanotoughener layer.

The nanotoughener could be a plate-like material. In the context of the present invention, "plate-like" means having the general shape of a plate having two large dimensions (length and width) and one small dimension (thickness). Examples of a plate-like material include but are not limited to clay, graphene, graphene oxide, graphene nanoplatelet or other materials having a thickness less than 10 nm, less than 100 nm or even less than 1000 nm. The nanotougheners could be nanofibers having an aspect ratio of length to diameter greater than one and a diameter of at most 1000 nm, at most 500 nm, or even at most 100 nm. Since the nanotoughener is intended to be localized on the surface of the plurality of reinforcing fibers, one of the nanotoughener dimensions such as its length could be at least 1 μηι, at least 3 μη or even at least 10 μηι. For an assembly of the nanotoughener, the longer the length is, the better its integrity can be kept. However, when the nanotoughener is to be incorporated into an adhesive composition, the longer the length is, the higher the viscosity of the adhesive composition, which might lead to difficulties in processing. In this case, a smaller amount of the nanotoughener could be used. Suitable nanofibers could be carbon nanotubes (sometimes referred to as CNT), carbon nanofibers, oxide nanofibers (e.g. alumina, silica or glass), ceramic nanofibers, metal nanofibers (e.g., nickel strands), halloysite nanofibers, other suitable types of organic or inorganic nanofibers, or combinations thereof. The nanotougheners could be blended with a polymer or a polymer blend.

The barrier layer could comprise a preformed assembly of the nanofibers. The assembly of the nanofibers could have a substantial amount of the nanofibers aligned in a direction or in random orientations. The assembly could have a thickness of at least 10 nm, at least 100 nm, at least 1 μπι or even at least 10 μηι, and/or an area weight of at least 0.01 g/m², at least 0.1 g/m² or even at least 1 g/m². The assembly can be further impregnated by an adhesive composition having components similar to or different from those found in the adhesive composition used to impregnate the plurality of reinforcing fibers, or components found in the adhesive composition of the interlayer toughening layer. Such an impregnated nanofiber layer is called a nanofiber prepreg.

The barrier layer could have a thickness of at most 100 %, at most 50 %, or even at most 25 % of the thickness of the interlayer toughening layer, and/or the loading of

nanotougheners in the barrier layer could be at most 100 wt%, at most 50 wt% or even at most 25 wt%, as long as the barrier layer could be capable of restricting the movement of the interlayer toughening material in the interlayer during curing. The thinner and/or the lighter the barrier layer is, the more likely it is that the desired weight requirement of the fiber reinforced polymer composition will be achieved. However, increasing the total interlayer thickness between two pluralities of reinforcing fibers often leads to higher Gnc and subsequently CAI. In addition, the thicker the barrier layer is, the greater the chances that a crack will be confined in the barrier layer as opposed to the interlayer toughener layer as discussed above.

Furthermore, a higher amount of a nanotoughener in a polymer, a polymer blend or a formulated resin often leads to difficulty in processing due to high viscosity. An appropriate amount of the nanotoughener as well as the dimensions of the nanotoughener could be selected for easy of processing. Alternatively, one could use a preformed assembly of the

nanotoughener with or without a binding material.

The adhesive compositions of the aforementioned could comprise at least a

thermosetting resin and/or a thermoplastic resin. The thermosetting resin may be defined herein as any resin which can be cured with a curing agent or a cross-linker compound by means of an externally supplied source of energy (e.g., heat, light, electromagnetic waves such as microwaves, UV, electron beam, or other suitable methods) to form a three dimensional crosslinked network having the required resin modulus. The thermosetting resin may be selected from, but is not limited to, epoxy resins, epoxy novolac resins, ester resins, vinyl ester resins, cyanate ester resins, maleimide resins, bismaleimide resins, bismaleimide-triazine resins, phenolic resins, novolac resins, resorcinolic resins, unsaturated polyester resins, diallylphthalate resins, urea resins, melamine resins, benzoxazine resins, polyurethanes, and mixtures thereof, as long as it does not deteriorate the effects of the invention.

From the viewpoint of an exceptional balance of strength, strain, modulus and environmental effect resistance, of the above thermosetting resins, epoxy resins could be used, including mono-, di-functional, and higher functional (or multifunctional) epoxy resins and mixtures thereof. Multifunctional epoxy resins are preferably selected as they provide excellent glass transition temperature (Tg), modulus and even high adhesion to a reinforcing fiber. These epoxies are prepared from precursors such as amines (e.g., epoxy resins prepared using diamines and compounds containing at least one amine group and at least one hydroxyl group such as tetraglycidyl diaminodiphenyl methane, triglycidyl-p-aminophenol, triglycidyl- m-aminophenol, triglycidyl aminocresol and tetraglycidyl xylylenediamine and their isomers), phenols (e.g., bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol S epoxy resins, bisphenol R epoxy resins, phenol-novolac epoxy resins, cresol-novolac epoxy resins and resorcinol epoxy resins), naphthalene epoxy resins, dicyclopentadiene epoxy resins, epoxy resins having a biphenyl skeleton, tris(hydroxyphenol)methane based epoxies (Tactix^® 742 by Huntsman), tetraglycidyl ether of glyoxal phenol novolac, fluorene based epoxies, isocyanate- modified epoxy resins and compounds having a carbon-carbon double bond (e.g., alicyclic epoxy resins). It should be noted that the epoxy resins are not restricted to the examples above. Halogenated epoxy resins prepared by halogenating these epoxy resins can also be used.

Furthermore, mixtures of two or more of these epoxy resins, and compounds having one epoxy group or monoepoxy compounds such as glycidylaniline, glycidyl toluidine or other glycidylamines (particularly glycidylaromatic amines) can be employed in the formulation of the thermosetting resin matrix.

Examples of commercially available products of bisphenol A epoxy resins include "jER (registered trademark)" 825, "jER (registered trademark)" 828, "jER (registered trademark)" 834, "jER (registered trademark)" 1001 , "jER (registered trademark)" 1002, "jER (registered trademark)" 1003, "jER (registered trademark)" 1003F, "jER (registered trademark)" 1004, "jER (registered trademark)" 1004AF, "jER (registered trademark)" 1005F, "jER (registered trademark)" 1006FS, "jER (registered trademark)" 1007, "jER (registered trademark)" 1009, "jER (registered trademark)" 1010 (which are manufactured by Mitsubishi Chemical

Corporation), EPON^® 825 and EPON^® 828 (from Momentive). Examples of commercially available products of the brominated bisphenol A epoxy resin include "jER (registered trademark)" 505, "jER (registered trademark)" 5050, "jER (registered trademark)" 5051, "jER (registered trademark)" 5054 and "jER (registered trademark)" 5057 (which are manufactured by Mitsubishi Chemical Corporation). Examples of commercially available products of the hydrogenated bisphenol A epoxy resin include ST5080, ST4000D, ST4100D and ST5100 (which are manufactured by Nippon Steel Chemical Co., Ltd.).

Examples of commercially available products of bisphenol F epoxy resins include "jER

(registered trademark)" 806, "jER (registered trademark)" 807, "jER (registered trademark)" 4002P, "jER (registered trademark)" 4004P, "jER (registered trademark)" 4007P, "jER

(registered trademark)" 4009P and "jER (registered trademark)" 401 OP (which are

manufactured by Mitsubishi Chemical Corporation), and "Epotohto (registered trademark)" YDF2001 , "Epotohto (registered trademark)" YDF2004 (which are manufactured by Nippon Steel Chemical Co., Ltd.), Epiclon^® 830 (from Dinippon Ink and Chemicals, Inc.). An example of a commercially available product of the tetramethyl-bisphenol F epoxy resin is YSLV-80XY (manufactured by Nippon Steel Chemical Co., Ltd.). An example of a bisphenol S epoxy resin is "Epiclon (registered trademark)" EXA-154 (manufactured by DIC Corporation).

Examples of commercially available products of tetraglycidyl diaminodiphenyl methane resins include "Sumiepoxy (registered trademark)" ELM434 (manufactured by Sumitomo Chemical Co., Ltd.), YH434L (manufactured by Nippon Steel Chemical Co., Ltd.), "jER (registered trademark)" 604 (manufactured by Mitsubishi Chemical Corporation), and "Araldite (registered trademark)" MY720, MY721 , and MY722 (which are manufactured by Huntsman Advanced Materials). Examples of commercially available products of triglycidyl aminophenol or triglycidyl aminocresol resins include "Sumiepoxy (registered trademark)" ELM100 (manufactured by Sumitomo Chemical Co., Ltd.), "Araldite (registered trademark)" MY0500, MY0510 and MY0600, MY0610 (which are manufactured by Huntsman Advanced Materials) and "jER (registered trademark)" 630 (manufactured by Mitsubishi Chemical Corporation). Examples of commercially available products of tetraglycidyl xylylenediamine and hydrogenated products thereof include TETRAD-X and TETRAD-C (which are manufactured by Mitsubishi Gas Chemical Company, Inc.).

Examples of commercially available products of phenol-novolac epoxy resins include "jER (registered trademark)" 152 and "jER (registered trademark)" 154 (which are

manufactured by Mitsubishi Chemical Corporation), and "Epiclon (registered trademark)" N- 740, N-770 and N-775 (which are manufactured by DIC Corporation).

Examples of commercially available products of cresol-novolac epoxy resins include

"Epiclon (registered trademark)" N-660, N-665, N-670, N-673 and N-695 (which are manufactured by DIC Corporation), and EOCN-1020, EOCN-102S and EOCN-104S (which are manufactured by Nippon Kayaku Co., Ltd.).

An example of a commercially available product of a resorcinol epoxy resin is

"Denacol (registered trademark)" EX-201 (manufactured by Nagase chemteX Corporation).

Examples of commercially available products of naphthalene epoxy resins include HP- 4032, HP4032D, HP-4700, HP-4710, HP-4770, HP-5000, EXA-4701 , EXA-4750, EXA-7240 (which are manufactured by DIC Corporation) and MY0816 (which is manufactured by Huntsman).

Examples of commercially available products of dicyclopentadiene epoxy resins include "Epiclon (registered trademark)" HP7200, HP7200L, HP7200H and HP7200HH (which are manufactured by DIC Corporation), "Tactix (registered trademark)" 558

(manufactured by Huntsman Advanced Material), and XD-1000-1L and XD-1000-2L (which are manufactured by Nippon Kayaku Co., Ltd.).

Examples of commercially available products of epoxy resins having a biphenyl skeleton include "jER (registered trademark)" YX4000H, YX4000 and YL6616 (which are manufactured by Mitsubishi Chemical Corporation), and NC-3000 (manufactured by Nippon Kayaku Co., Ltd.).

Examples of commercially available products of isocyanate-modified epoxy resins include AER4152 (manufactured by Asahi Kasei Epoxy Co., Ltd.) and ACR1348

(manufactured by ADEKA Corporation) each of which has an oxazolidone ring.

The thermosetting resin may comprise both a tetrafunctional epoxy resin (in particular, a tetraglycidyl diaminodiphenyl methane epoxy resin) and a difunctional glycidylamine, in particular a difunctional glycidyl aromatic amine such as glycidyl aniline or glycidyl toluidine from the viewpoint of the required resin modulus. Another difunctional epoxy resin, such as a difunctional bisphenol A or F/epichlorohydrin epoxy resin could be used to provide an increase in a flexural deflection of the cured adhesive composition; the average epoxy equivalent weight (EEW) of the difunctional epoxy resin may be, from 177 to 1500, for example. For example, the thermosetting resin may comprise 50 to 70 weight % tetrafunctional epoxy resin, 10 to 30 weight percent difunctional bisphenol A or F/epichlorohydrin epoxy resin, and 10 to 30 weight percent difunctional glycidyl aromatic amine.

The curing agent is also referred to as a cross-linker compound. There are no specific limitations or restrictions on the choice of a compound as the curing agent, as long as it has at least one active group which reacts with the thermosetting resin and collectively provides the required resin modulus and/or promotes adhesion. For the above epoxy resins, examples of suitable curing agents include polyamides, dicyandiamide [DICY], amidoamines (e.g., aromatic amidoamines such as aminobenzamides, aminobenzanilides, and

aminobenzenesulfonamides), aromatic diamines (e.g., diaminodiphenylmethane,

diaminodiphenylsulfone [DDS] such as Aradur^® 9664-1 from Huntsman), aminobenzoates (e.g., trimethylene glycol di-p-aminobenzoate and neopentyl glycol di-p-amino-benzoate), aliphatic amines (e.g., triethylenetetramine, isophoronediamine), cycloaliphatic amines (e.g., isophorone diamine), imidazole derivatives, guanidines such as tetramethylguanidine, carboxylic acid anhydrides (e.g., methylhexahydrophthalic anhydride), carboxylic acid hydrazides (e.g., adipic acid hydrazide), phenol-no volac resins and cresol-novolac resins, carboxylic acid amides, polyphenol compounds, polysulfides and mercaptans, and Lewis acids and bases (e.g., boron trifluoride ethylamine, tris-(diethylaminomethyl) phenol). Depending on the desired properties of the cured fiber reinforced epoxy composition, a suitable curing agent or suitable combination of curing agents is selected from the above list. For example, if dicyandiamide is used, it will generally provide the product with good elevated-temperature properties, good chemical resistance, and a good combination of tensile and peel strength. Aromatic diamines, on the other hand, will typically give high heat and chemical resistance and high modulus. Aminobenzoates will generally provide excellent tensile elongation though they often provide inferior heat resistance compared to aromatic diamines. Acid anhydrides generally provide the resin matrix with low viscosity and excellent workability, and, subsequently, high heat resistance after curing. Phenol-novolac resins and cresol-novolac resins provide moisture resistance due to the formation of ether bonds, which have excellent resistance to hydrolysis. Note that a mixture of two or more above curing agents could be employed. For example, by using DDS together with DICY as the hardener, the reinforcing fiber and the adhesive composition could adhere more firmly, and in particular, the heat resistance, the mechanical properties such as compressive strength, and the environmental resistance of the fiber reinforced composite material obtained may be markedly enhanced. In another example when DDS is combined with an aromatic amidoamine (e.g., 3- aminobenzamide), an excellent balance of thermal and mechanical properties and

environmental resistance could be achieved.

The curing agent can be employed in an amount up to about 75 parts by weight per 100 parts by weight of total thermosetting resin (75 phr). The curing agent might also be used in an amount higher or lower than a stoichiometric ratio between the thermosetting resin equivalent weight and the curing agent equivalent weight to increase resin modulus or glass transition temperature or both. In such cases, an equivalent weight of the curing agent is varied by the number of reaction sites or active hydrogen atoms and is calculated by dividing its molecular weight by the number of active hydrogen atoms. For example, an amine equivalent weight of 2-aminobenzamide (molecular weight of 136) could be 68 for 2 functionality, 45.3 for 3 functionality, 34 for 4 functionality, and 27.2 for 5 functionality.

A benzoxazine resin could be used as a curing agent for an epoxy resin. An accelerator as discussed below, however, might be used to speed up the curing process. Examples of suitable benzoxazine resins include, but are not limited to, multi-functional n-phenyl benzoxazine resins such as phenolphthaleine based, thiodiphenyl based, bisphenol A based, bisphenol F based, and/or dicyclopentadiene based benzoxazines. When an epoxy resin or a mixture of epoxy resins with different functionalities is used with a benzoxazine resin or a mixture of benzoxazine resins of different kinds, the weight ratio of the epoxy resin(s) to the benzoxazine resin(s) could be between 0.01 and 100. The combination typically improves processability of the benzoxazine resin and achieves exceptional resin modulus, heat resistance and hot-wet properties owing to the benzoxazine resin.

An accelerator optionally could be used with a curing agent as described above to speed up the reaction. There are no specific limitations or restrictions on the choice of a compound or combination of compounds as the accelerator, as long as it can accelerate reactions between the resin and the curing agent and does not deteriorate the effects of the invention. Examples include urea compounds, sulfonate compounds, boron trifluoride piperidine, p-t-butylcatechol, sulfonate compounds, tertiary amines or salts thereof, imidazoles or salts thereof, phosphorus curing accelerators, metal carboxylates and Lewis or Bronsted acids or salts thereof. Examples of suitable urea compounds include Ν,Ν-dimethyl- N'- (3,4-dichlorophenyl) urea, toluene bis(dimethylurea), 4,4' -methylene bis (phenyl dimethylurea), and 3-phenyl- 1 , 1 -dimethylurea. Commercial examples of such urea compounds include DCMU99 (manufactured by Hodogaya Chemical Co., Ltd.), and Omicure (registered trademark) 24, 52 and 94 (all manufactured by CVC Specialty Chemicals, Inc.). Commercial products of an imidazole compound or derivative thereof include 2MZ, 2PZ and 2E4MZ (all manufactured by Shikoku Chemicals Corporation). Examples of suitable Lewis acid catalysts include complexes of a boron trihalide and a base, such as a boron trifluoride piperidine complex, boron trifluoride monoethyl amine complex, boron trifluoride triethanol amine complex, and boron trichloride octyl amine complex. Examples of sulfonate compounds include methyl p-toluenesulfonate, ethyl p- toluenesulfonate and isopropyl p-toluenesulfonate.

The thermoplastic resin in the adhesive compositions, if optionally used with a thermosetting resin, is typically selected to modify the viscosity of the adhesive composition for processing purposes, and/or enhance its toughness. The thermoplastic resin, when present, may be employed in any amount up to 50 phr, or even up to 35 phr for ease of processing. One could use, but is not limited to, the following thermoplastic resins such as polyvinyl formals, polyamides, polycarbonates, polyacetals, polyphenyleneoxides, polyphenylene sulfides, polyarylates, polyesters, polyamideimides, polyimides, polyetherimides, polyimides having phenyltrimethylindane structure, polysulfones, polyethersulfones (e.g., Sumikaexcel^®

PES5003P from Sumimoto Chemical Co., Ltd., Virantage^®) VW-10700RP from Solvay), polyetherketones, polyetheretherketones, polyaramids, polyethernitriles, polybenzimidazoles, their derivatives and their mixtures thereof. One could use an aromatic thermoplastic resin which does not impair the high thermal resistance and high elastic modulus of the adhesive composition. The selected thermoplastic resin could be soluble in the thermosetting resin to a large extent to form a homogeneous mixture. The thermoplastic resins could be compounds having aromatic skeletons which are selected from the group consisting of polysulfones, polyethersulfones, polyamides, polyamideimides, polyimides, polyetherimides,

polyetherketones, and polyetheretherketones, their derivatives, the alike or similar polymers, and mixtures thereof. Polyethersulfones, polyimides, polyetherimides and mixtures thereof could be of interest due to their high heat resistance and toughness. Suitable polyethersulfones, for example, may have a number average molecular weight of from about 10,000 to about 75,000. Note that the aforementioned thermoplastic resins could be used alone in one of the above adhesive compositions as long as the effects of the invention are not deteriorated.

Furthermore, all the adhesive compositions optionally may contain a filler to further improve mechanical properties such as toughness or strength or physical/thermal properties of the cured fiber reinforced polymer composition as long as the effects of the present invention are not deteriorated. If the filler is intended to toughen the thermosetting resin inside the plurality of reinforcing fibers (hereafter referred to intralayer toughener), its longest dimension could be no more than 1 μηι. A filtering effect in that particles could be concentrated outside a plurality of reinforcing fibers could result if the longest dimension is greater than 1 μηι. One or more polymeric and/or inorganic tougheners can be used. The intralayer toughener could be a conductive material or a non-conductive material, The intralayer toughener may be uniformly distributed in the form of particles in the cured fiber reinforced polymer composition to maximize its effects on the intended purpose(s). Such intralayer tougheners include, but are not limited to, elastomers, branched polymers, hyperbranched polymers, dendrimers, rubbery polymers, rubbery copolymers, block copolymers, core-shell particles, oxides or inorganic materials such as clay, polyhedral oligomeric silsesquioxanes (POSS), carbonaceous materials (e.g., carbon black, carbon nanotubes, carbon nanofibers, fullerenes), ceramics and silicon carbides, with or without surface modification or functionalization. Examples of block copolymers include the copolymers whose composition is described in US 68941 13 (Court et al., Atofina, 2005) and include "Nanostrength^®" SBM (polystyrene-polybutadiene- polymethacrylate), and AMA (polymethacrylate-polybutylacrylate-polymethacrylate), both produced by Arkema. Other suitable block copolymers include Fortegra^® and the amphiphilic block copolymers described in US 7820760B2, assigned to Dow Chemical. Examples of known core-shell particles include the core-shell (dendrimer) particles whose compositions are described in US20100280151A1 (Nguyen et al., Toray Industries, Inc., 2010) for an amine branched polymer as a shell grafted to a core polymer polymerized from polymerizable monomers containing unsaturated carbon-carbon bonds, core-shell rubber particles whose compositions are described in EP 1632533A1 and EP 212371 1 Al by aneka Corporation, and the "KaneAce MX" product line of such particle/epoxy blends whose particles have a polymeric core polymerized from polymerizable monomers such as butadiene, styrene, other unsaturated carbon-carbon bond monomer(s), or their combinations, and a polymeric shell compatible with the epoxy, typically polymethylmethacrylate, polyglycidylmethacrylate, polyacrylonitrile or similar polymers. Also suitable as block copolymers in the present invention are the "JSR SX" series of carboxylated polystyrene/polydivinylbenzenes produced by JSR Corporation; "Kureha Paraloid" EXL-2655 (produced by Kureha Chemical Industry Co., Ltd.), which is a butadiene alkyl methacrylate styrene copolymer; "Stafiloid" AC-3355 and TR-2122 (both produced by Takeda Chemical Industries, Ltd.), each of which are acrylate methacrylate copolymers; and "PARALOID" EXL-261 1 and EXL-3387 (both produced by Rohm & Haas), each of which are butyl acrylate methyl methacrylate copolymers. Examples of suitable oxide particles include Nanopox^® produced by nanoresins AG. This is a master blend of functionalized nanosilica particles and an epoxy.

Another embodiment of the invention relates to a fiber reinforced polymer composition comprising a plurality of reinforcing fibers impregnated by one or more adhesive

compositions, wherein the one or more adhesive compositions comprising at least a high- aspect-ratio material and an interlayer toughening material, such that a layer of the high-aspect ratio material is found to be closer to the plurality of reinforcing fibers than a layer of the interlayer toughening material.

In this embodiment, a reinforcing fiber and an adhesive composition are required. There are no specific limitations or restrictions on the choice of the reinforcing fiber as long as the effects of the present invention are not deteriorated. Examples of suitable reinforcing fibers were discussed above.

One or more adhesive compositions of the fiber reinforced polymer composition comprising at least a thermosetting resin, a curing agent, a high-aspect-ratio material and an interlayer toughening material and the one or more adhesion compositions are used to impregnate the plurality of reinforcing fibers such that a layer of the high-aspect-ratio material is found closer to the plurality of reinforcing fibers than a layer of the interlayer toughening material. Such spatial distribution allows compaction of the interlayer toughening material in an interlayer between two pluralities of the reinforcing fiber during curing of the fiber reinforced polymer composition. This keeps crack propagation contained within the interlayer as a load is applied, leading to substantial increase of Gnc and possibly CAI.

The thermosetting resin and the curing agent in one or more adhesive compositions could be of the same kinds or different kinds from one another. Examples of such

thermosetting resins and curing agents were discussed above. Optionally, the one or more adhesive compositions could comprise at least one of an accelerator, a thermoplastic resin, a filler, or a combination thereof.

The interlayer toughening material could have the same composition as the high- aspect-ratio material comprising a nanotoughener as defined above or a different composition from the nanotoughener and could be in a form of a micron-sized particle, an interleaf, a mat, a film or a veil. The nanotougheners could be a plate-like material. Examples of a plate-like material include but are not limited to clay, graphene, graphene oxide, graphene nanoplatelet or other materials having a thickness less than 10 nm, less than 100 nm or even less than 1000 nm. The nanotougheners could also be nanofibers having an aspect ratio of length to diameter greater than one and a diameter of at most 1000 nm, at most 500 nm, or even at most 100 nm. Since the nanotoughener is intended to be localized on the surface of the plurality of reinforcing fibers, one of the dimensions such as its length could be at least 1 μιη, at least 3 μηι or even at least 10 μιη. For an assembly of the nanotoughener, the longer the length is, the better its integrity can be kept; However, when the nanotoughener is to be incorporated into an adhesive composition, the longer the length is, the higher the viscosity of the adhesive composition, which might lead to processing difficulties. In this case, a smaller amount of the nanotoughener could be used. Such nanofibers could be carbon nanotubes (sometimes referred to as CNT), carbon nanofibers, oxide nanofibers (e.g., alumina, silica or glass), ceramic nanofibers, metal nanofibers, halloysite nanofibers, other types of organic or inorganic nanofibers, or combinations thereof. The nanofibers could be a preformed assembly of a nanofiber such as a CNT mat or buckypaper. The assembly could have a thickness of at least 10 nm, at least 100 nm, at least 1 μηα or even at least 10 μιη, and/or an area weight of at least 0.01 g/m², at least 0.1 g/m² or even at least 1 g/m². The assembly could have a substantial amount of the nanofibers aligned in a direction such as an aligned CNT sheet.

Another embodiment relates to a fiber reinforced polymer composition comprising an interlayer region, located between two pluralities of reinforcing fibers, comprising a nanofiber and an interlayer toughening particle such that a layer of the nanofiber is located somewhere spatially in the interlayer region such that the spaces between the interlayer toughening particles and the thickness of a interlayer toughening layer comprising a substantial amount of the interlayer toughening particles are significantly reduced after the fiber reinforced polymer composition is cured.

In the above embodiment, there is no restriction on choices of the reinforcing fiber, the nanofiber and the interlayer toughening particles, as long as the effects of the inventions are not deteriorated. Examples of these components were discussed previously.

The layer of the nanofibers (or the barrier layer) could be closer to the plurality of reinforcing fibers than the layer of the interlayer toughening particles (or the interlayer toughening layer). That is, the layer of the nanofibers (or the barrier layer) may be positioned between the layer of the interlayer toughening particles (or the interlayer toughening layer) and the plurality of reinforcing fibers. Such spatial arrangement allows the interlayer toughening particles to be compacted in the interlayer during curing of the fiber reinforced polymer composition. This is important to keep a crack confined in the interlayer rather than escaping into the inside of the pluralities of reinforcing fibers, causing delamination and premature failure of the cured fiber reinforced polymer composition. Alternatively, one or more barrier layers could be placed in between two interlayer toughening layers for a similar purpose. In any case, the thickness of the barrier(s) would be optimized to avoid a crack propagating in the barrier layer rather than the interlayer toughening layer.

Additionally, the interlayer might be required to include z-direction conductive paths to improve electrical or thermal conductivity of the cured fiber reinforced polymer composition. In such cases the barrier layer, the interlayer toughening layer, or both could further comprise a conductive material. Examples of suitable conductive materials include, but are not limited to, metals and transition metals (e.g., nickel, copper, silver, zinc, gold, platinum, cobalt, tin, titanium, iron, chromium, aluminum), metal alloys (e.g., aluminum alloys, magnesium alloys, lithium aluminum alloys), carbonaceous materials (e.g., carbon nanotubes, carbon black, carbon nanofibers, graphite, graphene, graphene oxides, graphite nanoplatelets), conductive coated materials, and conductive oxides (e.g., indium tin oxides) and mixtures thereof. The form, size, shape, and amount of the conductive material in either the barrier layer or the interlayer toughening layer could be selected to achieve desired mechanical, thermal and conductivity properties.

There are no specific limitations or restrictions on the choice of a method of making a fiber reinforced polymer composition as long as the effects of the present invention are not deteriorated.

One embodiment of the present invention relates to a manufacturing method to combine fibers and resin matrix (adhesive composition) to produce a curable fiber reinforced polymer composition (sometimes referred to as a "prepreg") which is subsequently cured to produce a composite article. Employable is a wet method in which a plurality of reinforcing fibers is soaked in a bath of the resin matrix dissolved in a solvent such as methyl ethyl ketone or methanol, and withdrawn from the bath to remove solvent. For adhesive compositions having a low viscosity, use of a solvent is not necessary. Such a procedure is called an impregnation. The high-aspect-ratio material could be incorporated into the resin bath. If not, after a 1 ^st impregnation the high-aspect-ratio material followed by an interlayer toughening material can be applied onto the surface(s) of the impregnated plurality of reinforcing fibers. A direct application for a preformed assembly, a spraying method, an application from a nozzle/ orifice or similar could be utilized. Optionally, the impregnated plurality of reinforcing fibers could be wound onto a drum surface as it rotates and translates.

Another suitable method is a hot melt method, where an adhesive composition is heated to lower its viscosity, directly applied to the reinforcing fibers to obtain a resin-impregnated prepreg; or alternatively, as another method, the adhesive composition is coated on a release paper to obtain a thin film. The film is consolidated onto both surfaces of a sheet of reinforcing fibers by heat and pressure. One embodiment relates to a manufacturing method for one or more the above fiber reinforced polymer compositions comprising (1) impregnating a first adhesive composition comprising at least a thermosetting resin, a curing agent, and a high- aspect-ratio material onto one side or both sides of a plurality of reinforcing fibers, (2) applying a second adhesive composition comprising at least an interlayer toughening material on one side or both sides of the impregnated plurality of reinforcing fibers. In another embodiment, a manufacturing method for one or more of the above fiber reinforced polymer composition comprises (1) impregnating a first adhesive composition comprising at least a thermosetting resin and a curing agent onto one side or both sides of a plurality of reinforcing fibers, (2) applying a preformed assembly of a high-aspect-ratio material onto one side or both sides of the impregnated plurality of reinforcing fibers; wherein the high-aspect-ratio material could be in combination with a binder material or no binder material, (3) applying a second adhesive composition comprising at least an interlayer toughening material onto one side or both sides of the impregnated plurality of reinforcing fibers. In yet another embodiment a manufacturing method for the fiber reinforced polymer composition comprises (1) impregnating a first adhesive composition comprising at least a thermosetting resin and a curing agent onto one side or both sides of a plurality of reinforcing fibers, (2) applying an assembly of a high-aspect-ratio material onto one side of a second adhesive composition film comprising at least an interlayer toughening material, (3) applying the side of the second adhesive composition film containing the high-aspect-ratio material onto one side or both sides of the impregnated plurality of reinforcing fibers.

The fiber reinforced polymer compositions of the present invention may, for example, be heat-curable or curable at room temperature. In other embodiments, the aforementioned fiber reinforced polymer compositions can be cured by a one-step cure to a final cure temperature, or by a multiple-step cure in which the fiber reinforced polymer composition is dwelled (maintained) at a certain dwell temperature for a certain period of dwell time to allow a good resin flow onto the reinforcing fiber's surface and removal of trapped air pockets and volatiles, and ramped up and cured at the final cure temperature for a desired period of time. The dwell temperature could be in a temperature range in which the adhesive composition has a low viscosity. The dwell time could be at least about five minutes. The final cure temperature of the adhesive resin composition could be set after the adhesive resin composition reaches a degree of cure of at least 20 % during the ramp up. The ramp rate could be at least 0.5 °C/min, at least 5 °C/min, at least 20 °C/min or even at least 50 °C/min. The final cure temperature could be about 220 °C or less, or about 180 °C or less. The fiber reinforced polymer composition could be kept at the final cure temperature until a degree of cure reaches at least 80 %. Vacuum and/or external pressure could be applied to the reinforced polymer composition during cure. Examples of these methods include autoclave, vacuum bag, pressure- press (i.e., one side of the article to be cured contacts a heated tool's surface while the other side is under pressurized air with or without a heat medium), or a similar method. Note that other curing methods using an energy source other than thermal, such as electron beam, conduction method, microwave oven, or plasma-assisted microwave oven, or a combination of such methods could be applied. In addition, other external pressure methods such as shrink wrap, bladder blowing, platens, or table rolling could be used.

To produce a composite article from the prepreg, for example, one or more plies are applied onto a tool surface or mandrel. This process is often referred to as tape- wrapping. Heat and pressure are needed to laminate the plies. The tool is collapsible or removed after curing. Curing methods such ^'as autoclave and vacuum bag in an oven equipped with a vacuum line could be used. A one-step cure cycle or multiple-step cure cycle in that each step is performed at a certain temperature for a period of time could be used to reach a cure temperature of about 220 °C or even 180 °C or less. However, other suitable methods such as conductive heating, microwave heating, electron beam heating and similar methods, can also be employed. In an autoclave method, pressure is provided to compact the plies, while a vacuum-bag method relies on the vacuum pressure introduced to the bag when the part is cured in an oven. Autoclave methods could be used for high quality composite parts. In other embodiments, any methods that provide suitable heating rates of at least 0.5 °C/min, at least 1 °C/min, at least 5 °C/min, or even at least 10 °C/min and vacuum and/or compaction pressures by an external means could be used.

Without forming prepregs, an adhesive composition may be directly applied to reinforcing fibers which are conformed onto a tool or mandrel for a desired part's shape, and cured under heat. Since this method does not involve an intermediate product, such as a prepreg, it has great potential for molding cost reduction and is advantageously used for the manufacture of structural materials for spacecraft, aircraft, rail vehicles, automobiles, marine vessels and so on. The methods include, but are not limited to, filament-winding and resin infusion such as resin injection molding, resin transfer molding, vacuum assisted resin transfer molding. In the former, application of a high-aspect-ratio material followed by an interlayer toughening material is similar to the wet method discussed above. The impregnated fibers are wrapped around a rotating metal core (mandrel) under tension at a predetermined angle. After the wraps of rovings reach a predetermined thickness, it is cured and then the metal core is removed.

In the resin transfer molding method the high-aspect-ratio material and the interlayer toughening material can be incorporated between two pluralities of reinforcing fibers to form a preform, which in turn is infused by an adhesive composition and cured. The adhesive composition comprises at least a thermosetting resin and a curing agent.

Additionally, a resin film infusion method could be used in that layers of the dry reinforcing fibers, the resin film containing the high-aspect-ratio material or the interlayer toughener are stacked in a desired sequence and heat and pressure are applied to consolidate the stack.

Composite articles comprises a hybridized interlaminar toughener system comprising a layer of high-aspect-ratio material and a layer of interlayer toughening material as described in the invention are advantageously used in applications, where high fracture toughness and CAI are needed, such as in aerospace and space. However, they also could be used in sports applications and general industrial applications. Concrete sports applications in which these materials are advantageously used include golf shafts, fishing rods, tennis or badminton rackets, hockey sticks and ski poles. Concrete general industrial applications in which these materials are advantageously used include structural materials for vehicles, such as automobiles, bicycles, marine vessels and rail vehicles, drive shafts, leaf springs, windmill blades, pressure vessels, flywheels, papermaking rollers, roofing materials, cables, and repair/reinforcement materials.

Tubular composite articles in accordance with the invention are advantageously used for golf shafts, fishing rods, and the like.

Examination of the interlayer in a cured composite article

Several methods are known to one skilled in the art to examine and locate the presence of the high-aspect-ratio material and interlayer toughening material and their distribution in a cured composite article. An example is to cut the composite at 90°, 45° or other angles of interest with respected to a fiber's axis to obtain a cross section. The cut cross-section is polished mechanically or by an ion beam such as argon, and examined under any type of high magnification light microscope and/or electron microscope such as SEM or TEM. The thickness of the interlayer made up by the barrier layer and the interlayer toughening layer could also be documented.

The sample can also be fractured to expose the fiber's surface and SEM or TEM can be used to document the distribution of the high-aspect-ratio material and interlayer toughening material in a laminate.

Examples

Next, certain embodiments of the invention are illustrated in detail by means of the following examples using the following components:

CB-X material was made from epoxy functionalized carbon black (CB) and a mixture of epoxies according to the below recipes in Table 1. CB from U.S. Research Nanomaterials was oxidized in concentrated nitric acid for 2 hr at 80 °C. The oxidized CB was placed in a solution of 3wt% GPS (Glycidoxypropyltrimethoxy silane from Gelest) in methanol/DI water

(95/5wt%) and stirred for 90 min. The solids were removed by a centrifuge and redispersed into fresh methanol. The procedure was repeated two times to obtain a final dispersion of epoxy-functionalized CB in methanol. The dispersion was mixed with the epoxy mixture and methanol was removed under heat and vacuum.

Comparative Examples 1-2

Comparative Examples 1 -2 show the effects of a barrier on compacting an interlayer tougher in the interlayer. T800S-10 fibers were used.

Appropriate amounts of each component of a resin composition as shown in Table 1 , except the curing agent, were charged into a mixer preheated at 100 °C. After charging, the temperature was increased to 160 °C while the mixture was agitated, and held for lhr. After that, the mixture was cooled to 65 °C and the curing agent was charged. The final resin mixture was agitated for 1 hr, then discharged and a portion was stored in a freezer.

To make a prepreg of Comparative Example 1 , the hot resins with (#1) and without

(#2) the interlayer toughener were first cast into a thin film using a knife coater onto a release paper. Note that the amount of the interlayer toughener listed in Table 1 was referenced to the total amount of the film #1 and film #2. The film #1 was consolidated onto a bed of fibers on both sides by heat and compaction pressure, followed by the film #2. A UD prepreg having a carbon fiber area weight of about 190 g/m² and resin content of about 35% by weight was obtained.

To make the prepreg for Comparative Example 2, after the fibers were impregnated with the film #1 , a sheet of carbon fiber veil was placed on the surfaces of impregnated fibers, followed by impregnation of film #2. A UD prepreg having a carbon fiber area weight of about 190 g/m² was obtained.

The prepregs were cut and hand laid up with the sequence listed in Table 2 for each type of mechanical test, followed an ASTM procedure and a z-direction electrical conductivity test. Panels were cured in an autoclave at 180 C for 2 hr with a ramp rate of 1.7 C/min and a pressure of 0.59 MPa.

Comparative Example 1 did not have a barrier. The interlayer thickness was found to be around 20-30 μιη in which the particles were sparse (i.e., the spaces between the particle were relatively large). GHC was found to be 12.5 lb. in/in². Similarly, Comparative Example 2 did not have a barrier layer as the binder material in the carbon fiber veil dissolved and exposed a small amount of the short carbon fibers originally in the veil. The interlayer thickness became significantly larger than that of Comparative Example 1 , largely due to the amount of the binder material of the veil. This allowed both the short carbon fibers and the interlayer tougheners to be sparser and randomly distributed in the interlayer, yet Guc was increased significantly.

Comparative Examples 3-4

Resins and prepreg were prepared and mechanical tests were performed using procedures as in previous Examples. These systems are high adhesion systems using T700G- 31 fibers.

As shown, use of a carbon veil did not improve Guc of Comparative Example 4 versus

Comparative Example 3. For this case, the barrier material did not form similar to Comparative Example 2.

Examples 1-2

Resins and prepreg were prepared and mechanical tests were performed using procedures as in previous Examples. Instead of using aligned CNT sheets for a barrier layer, CNT mats were used. Similar effects on Guc compared to previous examples were observed. Yet, Gic values were significantly better, but still lower than the respective Comparative Examples 1 , 3.

Comparative Example 5 and Examples 3-5

These examples examined the possibilities for creating a barrier without using a preformed assembly of a high-aspect-ratio material seen in previous examples.

In these examples, resins whose components were listed in Table 1 were prepared using the same procedure as previous examples. However, for Comparative Example 5 random CNTs were incorporated in the resin #2 with the interlayer toughener, while for Examples 3-5 they were incorporated in the resin #1 without the interlayer toughener.

UD prepregs having a carbon fiber area weight of about 190 g/m and resin content of about 35% by weight were obtained using the same procedure as previous examples.

A barrier layer was not formed in Comparative Example 5 such that CNTs were partitioned in spaces among interlayer tougheners. As a result, Guc was found to be even lower than the systems without CNTs as seen in Comparative Examples 1 and 3. Surprisingly, a barrier layer was formed in Examples 3-5 and effectively compacted the interlayer toughener layer, leading to a significant increase in Gnc versus Comparative Example 1. As expected, CAI was also increased. In addition, Gic was also increased as the crack was not confined in the barrier layer as seen in previous examples where the barrier layers were formed by either aligned CNT sheets or CNT mats.

Examples 4-5 further showed that z-direction electrical conductivity of the composite of Example 3 was significantly improved by incorporating of a conductive interlayer toughener in the interlayer toughener layer (Example 4) and on top of that a conductive material (carbon black) is incorporated everywhere in the composite (Example 5). In these examples conductive materials provide conductive paths allowing electrons to flow in the z-direction.

The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

This application discloses several numerical range limitations. The numerical ranges disclosed inherently support any range within the disclosed numerical ranges though a precise range limitation is not stated verbatim in the specification because this invention can be practiced throughout the disclosed numerical ranges. Finally, the entire disclosures of the patents and publications referred in this application are hereby incorporated herein by reference.

Table 1

' Loading was calculated based on the total resin amount in a fiber reinforced polymer composition

Table 2

* Japanese Industrial Standard Test Procedure

** See the description below

Z-direction electrical conductivity measurement. 25 mm x 25 mm coupons (Wi x W₂) are prepared from the panel. Both top and bottom surfaces of a coupon are polished mechanically to remove the top layers up to 50 μηι and the coupon's thickness (t) is determined. A silver paint (Dotite D-550) is painted on polished surfaces, followed by applying a 25-mm wide copper tape (3M 1 181). The volumetric resistivity (RV) is determined from the below equation. Electrical conductivity is 1/RV t

where R is the resistance measured by a digital multimeter with four probe method between the top and bottom surfaces of the coupon (Advantest R6581).

Claims

What is claimed is:

1. A fiber reinforced polymer composition comprising a plurality of reinforcing fibers, an interlayer toughening layer comprising at least ah interlayer toughening material and a barrier layer comprising at least a high-aspect-ratio material, wherein the barrier layer is sandwiched between the plurality of reinforcing fibers and the interlayer toughening layer and is adapted to hinder infusion of the interlayer toughening material into the plurality of reinforcing fibers upon curing of the fiber reinforced polymer composition.

2. The fiber reinforced polymer composition of claim 1 , wherein the barrier layer is

additionally adapted to allow at least a portion of the interlayer toughening layer other than the interlayer toughening material to infuse into at least the barrier layer upon curing of the fiber reinforced polymer composition.

3. The fiber reinforced polymer composition of claim 1 , wherein both the high-aspect- ratio material and the interlayer toughening material are localized on the surface of the plurality of reinforcing fibers.

4. The fiber reinforced polymer composition of claim 1 , wherein the interlayer toughening layer comprises a resin film comprising at least a micron-sized particle, a fibrous material, or an assembly of a fibrous material.

5. The fiber reinforced polymer composition of claim 4, wherein the resin film comprises at least a thermosetting resin, a curing agent, and a micron-sized interlayer toughener.

6. The fiber reinforced polymer composition of claim 5, wherein the plurality of

reinforcing fibers is impregnated by an adhesive composition comprising at least a thermosetting resin and a curing agent that are the same as or different from at least one component of the similar kind present in the barrier layer or the interlayer toughening layer.

7. The fiber reinforced polymer composition of claim 1 , wherein the high-aspect-ratio material comprises a nanofiber or a plate-like nanomaterial.

8. The fiber reinforced polymer composition of claim 7, wherein the high-aspect ratio material includes at least one nanofiber selected from the group consisting of carbon nanotubes, carbon nanofibers, oxide nanofibers, metal nanofibers, ceramic nanofibers, halloysite nanofibers, organic nanofibers, and assemblies thereof.

9. The fiber reinforced polymer composition of claim 8, wherein the nanofiber is in the form of an assembly which is impregnated by an adhesive composition whose components are the same as or different from the components present in the interlayer toughening layer or the plurality of reinforcing fibers, where the plurality of reinforcing fibers has been impregnated.

10. The fiber reinforced polymer composition of claim 1, wherein the barrier layer

comprises an adhesive composition comprising at least a thermosetting resin, a curing agent, and nanofibers and the plurality of reinforcing fibers is impregnated with the adhesive composition such that a substantial amount of the nanofibers is localized on the surface of the plurality of reinforcing fibers, wherein the thermosetting resin and curing agent are the same as or different from the thermosetting resin(s) and curing agent(s) present in the interlayer toughening layer.

1 1. A fiber reinforced polymer composition comprising a plurality of reinforcing fibers impregnated by one or more adhesive compositions, wherein the one or more adhesive compositions comprise at least a high-aspect-ratio material and an interlayer

toughening material, such that a layer of the high-aspect ratio material is located closer to the plurality of reinforcing fibers than is a layer of the interlayer toughening material.

12. The fiber reinforced polymer composition of claim 1 1 , wherein the interlayer

toughening material comprises a micron-sized particle, a fibrous material, or an assembly of a fibrous material.

13. The fiber reinforced polymer composition of claim 12, wherein the high-aspect ratio material comprises a nanofiber or a plate-like nanomaterial.

14. The fiber reinforced polymer composition of claim 13, wherein the high-aspect-ratio material comprises a nanofiber and the nanofiber is selected from the group consisting of carbon nanotubes, carbon nanofibers, oxide nanofibers, metal nanofibers, ceramic nanofibers, halloysite nanofibers, organic nanofibers, and their assemblies and combinations thereof.

15. A prepreg comprising the fiber reinforced polymer composition of claim 1.

16. A prepreg comprising the fiber reinforced polymer composition of claim 1 1.

17. A manufacturing method for the fiber reinforced polymer composition of claim 1

comprising (1) impregnating a first adhesive composition comprising at least a thermosetting resin, a curing agent, and a high-aspect-ratio material onto one side or both sides of a plurality of reinforcing fibers, (2) applying a second adhesive

composition comprising at least a interlayer toughening material on one side or both sides of the impregnated plurality of reinforcing fibers.

18. A manufacturing method for the fiber reinforced polymer composition of claim 1 comprising (1) impregnating a first adhesive composition comprising at least a thermosetting resin and a curing agent onto one side or both sides of a plurality of reinforcing fibers, (2) applying an assembly of a high-aspect ratio material onto one side or both sides of the impregnated plurality of reinforcing fibers, (3) applying a second adhesive composition comprising at least a interlayer toughening material onto one side or both sides of the impregnated plurality of reinforcing fibers.

19. A manufacturing method for the fiber reinforced polymer composition of claim 1

comprising (1) impregnating a first adhesive composition comprising at least a thermosetting resin and a curing agent onto one side or both sides of a plurality of reinforcing fibers, (2) applying an assembly of a high-aspect-ratio material onto one side of a second adhesive composition film comprising at least an interlayer toughening material, (3) applying the side of the second adhesive composition film containing the nanofiber onto one side or both sides of the impregnated plurality of reinforcing fibers.

20. A method of manufacturing a composite article comprising curing the fiber reinforced polymer composition of claim 1.

21. A method of manufacturing a composite article comprising curing the fiber reinforced polymer composition of claim 11.

22. A fiber reinforced polymer composition comprising an interlayer region, between two pluralities of reinforcing fibers, comprising a nanofiber and an interlayer toughening particle such that a layer of the nanofiber is located somewhere spatially in the interlayer region such that spaces among the interlayer toughening particles and the thickness of a interlayer toughening layer comprising a substantial amount of the interlayer toughening particles are significantly reduced after the fiber reinforced polymer composition is cured.

23. The fiber reinforced polymer composition of claim 1 , wherein high-aspect-ratio

materials in the barrier layer are oriented in random directions.

24. The fiber reinforced polymer composition of claim 1 1 , wherein high-aspect-ratio

materials in the barrier layer are oriented in random directions.

25. The fiber reinforced polymer composition of claim 22, wherein high-aspect-ratio

materials in the barrier layer are oriented in random directions.