WO2012087406A2

WO2012087406A2 - Fabric based laminar composite and method for manufacture thereof

Info

Publication number: WO2012087406A2
Application number: PCT/US2011/055409
Authority: WO
Inventors: John M. Rice; Yong K. Kim; Armand F. Lewis
Original assignee: U Mass Dartmouth-Central
Priority date: 2010-10-07
Filing date: 2011-10-07
Publication date: 2012-06-28
Also published as: WO2012087406A3

Abstract

Fiber based z-directional reinforced composites having enhanced inter-laminar strength, impact toughness, energy transmission properties (electrical and thermal conduction) and coefficient of thermal expansion are provided. The composites include at least two substrates separated by a reinforcement zone that includes a plurality of fibers disposed in a binder resin. At least some, and in one embodiment, a majority, of the fibers are oriented so as to be substantially perpendicular to the substrates. Multi- layered composites having more than two substrate layers can also be formed. Methods for forming such composites are also provided.

Description

FABRIC BASED LAMINAR COMPOSITE AND METHOD FOR

MANUFACTURE THEREOF

FIELD OF USE The present disclosure relates to fabric based laminar composites showing high interlaminar strength, in particular to z-directional fiber reinforced composites.

BACKGROUND

Delamination of layered fabric-reinforced composites represents one of the most prevalent structural, life-limiting failure modes of such materials. As an example, Organic Polymer Laminar Composites ("OPLC") materials based on layered fabrics have many advantageous property and processing features. However, one structural drawback is their generally poor interlaminar shear strength. Layered OPLCs have little or no fiber reinforcement in the thickness direction. Therefore, their inter-ply strength is less than their longitudinal strength which can result in poor impact and/or interlaminar flexural fatigue strength.

Various techniques have been introduced to enhance the interlaminar strength of layered composite materials. A common technique is to use a rubber-toughened matrix material resin. However, these resins are generally not thermally durable. An alternative approach is to manufacture special pre-forms using advanced textile technologies such as 3-D knitting/weaving/braiding or through -the-fabric

stitching/pinning processes. However, these methods are slow, inefficient, and expensive. While fabricated pre-forms may include yarns in a z-directional orientation, these reinforcements are generally not conducive to an optimized stress distribution in the mechanically functioning structure component. Such 3-D structures are prone to stress concentrations under mechanical service leading to poor fatigue resistance. These approaches appear to work in their primary goal, but they degrade the composite's in- plane properties.

Furthermore, Kim et al., "Fracture Toughness of Flock Reinforced Layered Composites," Proceedings of 1^st Industrial Simulation Conference 2003, June 9-11, UPV, Valencia, Spain, p. 477-482 (2003) and Kim et al., "Through-Thickness Reinforcement of Laminar Composites," Journal of Advanced Materials," Vol. 36, no. 3, July 2004, pp 25-31, the entirety of these references hereby incorporated herein by reference, disclose that composites reinforced with z-directional fibers appear to have the potential to exhibit improved inter-laminar strength. However, z-directional reinforcement remains highly unpredictable due to the large number of variables (e.g., fiber type, flock fiber density (the number of perpendicularly oriented flock fibers per unit area of interface between the substrates), fiber denier (mass in grams per 9000 m length), fiber length, binder resin type, bonding strength between fiber and binder resin, etc.) present in such a composite. As a result, many such composites do not show improved interlaminar shear properties and/or suffer a decrease in toughness.

Therefore, there is a need in the art for a composite showing improved characteristics such as inter-laminar shear strength and/or fracture toughness.

SUMMARY Various embodiments of a z-directional fiber reinforced composite exhibiting enhanced properties (e.g., inter-laminar strength, toughness, etc.) and a method of fabrication thereof are provided herein. For example, a method for fabricating a reinforced laminar composite can include providing a plurality of dry fibrous substrates, flocking a plurality of reinforcing fibers onto each of the substrates, placing the plurality of substrates one upon the other to form a stack of substrates, and applying a fluid matrix resin to impregnate the stack of substrates. In some embodiments, applying the fluid matrix resin to the stack of substrates can include applying the fluid matrix resin to substantially saturate the substrates.

The method can further include curing the plurality of substrates after they are substantially saturated with the fluid matrix resin. In some embodiments, applying a fluid matrix resin to the stack of substrates can also include applying the fluid matrix resin to a top layer of the stack of substrates so that it penetrates through all of the substrate layers. There are many ways to cause the fluid matrix resin to penetrate through all of the substrate layers. For example, a vacuum can be applied to the bottom of the substrates to pull the fluid matrix resin on top of the fibrous layers through the stack of substrates so that it penetrates through all of the layers. In some embodiments, the fluid matrix resin can be an epoxy. Further, flocking the plurality of reinforcing fibers can include electrostatically flocking the fibers into interstices within each of the substrates. The fibers can be flocked, for example, onto a first side of each of the substrates. The first flocked side of the side substrate can then be placed adjacent to a second, unflocked side of another of the substrates. Another approach could involve alternately laying together a double side flocked layer of fibrous material along with a non-flocked layer of fibrous material. This would accomplish the same effect as stacking together one side flocked fibrous layers as described above. Any sort or type of fiber known in the art can be used as the flock fiber or the fibrous layers including, but not limited to, polymer-based fibers, glass fibers, carbon fibers, natural fibers, and metal fibers.

In other aspects, exemplary reinforced composites are provided. In one embodiment, a composite can include first and second substrates stacked one on top of the other and having a plurality of z-reinforcing fibers disposed therebetween. The reinforcing fibers can provide the resulting composite with an increased thickness in the range of about 3% to about 20% relative to a thickness of the first and second substrates without the reinforcing fibers. The composite can also include a fluid matrix resin that is disposed throughout the first and second substrates and that is cured thereafter. The plurality of reinforcing fibers can have any length as needed, but on average they can have a length in the range of about 0.5 mm to about 2 mm. In some embodiments, the z- axis oriented reinforcing fibers can provide the first and second substrates with a toughness in the range of about 3 times to about 8 times with respect to non flock fiber z- reinforced laminar composites (in the range of about 0.9 KJ/m² to about 8.0 KJ/m²) and with a strength retention after impact damage in the range of about 60% to about 90

% of non-impact damaged composites. The types of fabric/fiber substrates for individual layers that are used to fabricate the composite include unidirectional filament sheets, woven structures of plain square weave, twill, and sateen weave, and random fiber mats having a porosity of about 50% to about 90% made of organic polymer fibers (nylon 66, nylon 6, PET polyester, PPTA, PBO, UHMWPE etc.) or inorganic (carbon, glass, ceramic etc.) and fibrous metal plies.

In one aspect, a reinforced laminar composite is provided and can include a plurality of substrates stacked together and having a plurality of reinforcing fibers disposed between each substrate within a cured epoxy resin composite. The reinforcing fibers can be configured to provide the composite with a fracture toughness in the range of about 0.06 KJ/m² to about 0.25 KJ/m² for typical carbon/epoxy. The composite can have any suitable flock fiber density, and in one embodiment, the flock fiber density can be in the range of about 100 fibers/mm² to about 500 fibers/mm². In addition, the plurality of reinforcing fibers can provide the composite with an increased thickness in the range of about 3% to about 20% relative to a thickness of the plurality of substrates without the z-axis reinforcing fibers. Any sort of fiber known in the art can be used in the composite including, but not limited to, polymer-based fibers, glass fibers, carbon fibers, natural fibers, and metal fibers. The reinforcing fibers can provide the plurality of substrates with a toughness in the range of about 0.9 KJ/m² to about 8.0 KJ/m².

In another embodiment, a method of fabricating a flocked thin veil or scrim of fabric is provided and includes providing an uncoated, dry substrate having a thickness in the range of about 0.125 mm to 0.25 mm, and applying an uncured layer of sizing resin to opposed surfaces of the substrate. The sizing resin being applied at a concentration level less than about 1% of the weight of the substrate. The method can further include flocking a plurality of reinforcing fibers onto opposed surfaces of the substrate having the sizing resin thereon, and curing the substrate with the fibers and the sizing resin to form a free-standing interlayer substrate.

These and other aspects of the present disclosure are described in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS The presently disclosed composites and methods will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 schematically illustrates an exemplary embodiment of a prior art z- directional fiber based reinforced composite; FIG. 2A schematically illustrates an exemplary embodiment of a dry flocked laid up composite without a fluid matrix resin;

FIG. 2B schematically illustrates the composite of FIG. 2 A with the resin disposed throughout; FIG. 3 A is a schematic diagram illustrating one embodiment of a flocking apparatus;

FIG. 3B is a schematic diagram illustrating an embodiment of a hand held flocking apparatus;

FIG. 4A is an image of test samples cut from a laminate plate; FIG. 4B is an image of the test samples of FIG. 4A deliberately cracked to form an open end;

FIG. 4C is a close-up image showing the split in the test sample of FIG. 4B;

FIG. 4D is an enlarged image of the split shown in FIG. 4C;

FIG. 4E is diagram illustrating a test apparatus for measuring thermal conductivity;

FIG. 5 is a graphical representation of a thin dry flocked veil for use in a composite;

FIG. 6 is a cross-sectional photograph of a surface zone of a dry flocked substrate; FIG. 7 is a graph showing toughness versus delamination length for a dry flocked composite;

FIG. 8 is a graphical representation of a dry flocked composite formed using VectorPly®;

FIG. 9 illustrates one embodiment of a vacuum assisted apparatus; FIG. 10 illustrates one embodiment of a beater bar apparatus; FIGS. 11 A is a diagram illustrating a flocked ply according to a "Z-link" process;

FIG. 1 IB is a diagram illustrating an unflocked ply having a vacuum force applied thereto, according to a "Z-link" process;

FIG. 11C is a diagram illustrating the plies of FIGS. 11 A and 1 IB combined, according to a "Z-link" process;

FIG. 1 ID illustrates one embodiment of a laminate configuration;

FIG. 12A illustrates one embodiment of a two ply laminate base made according to a "Z-link" process:

FIG. 12B illustrates one embodiment of a three ply laminate base made according to a "Z-link" process;

FIG. 12C illustrates one embodiment of an N ply laminate base made according to a "Z-link" process; and FIG. 13 is a graph showing thermal conductivity versus fiber flock density.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the composites and methods of fabrication disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the composites and methods specifically described herein and illustrated in the accompanying drawings are non-limiting embodiments and that the scope of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure.

The present disclosure generally provides fiber based z-directional reinforced composites specifically configured and optimized to exhibit a number of desired properties and/or characteristics. Composites are provided having two or more substrates with a reinforcement zone between adjacent substrates. The reinforcement zone can include a plurality of fibers that extend substantially perpendicularly to the substrates (i.e., in the z-direction) to provide reinforcement for improved delamination resistance between the two substrates. In some embodiments, the z-directional fibers can be flocked on a substrate using a process called "dry flocking." Dry flocking involves flocking the z-directional fibers onto a dry substrate that has not been coated with any binding agent or wet resin flock adhesive. After the fibers are flocked onto the substrate, a wet resin or other binding agent can be applied and then cured. Flocking fibers onto a dry substrate allows the fibers to more readily penetrate into the depth of the substrate and virtually lock themselves into the weave or mesh of the substrate. As described below, the strength, performance, and properties of a composite can be optimized by using the technique of dry flocking compared with traditional wet flocking techniques. Further optimization can occur through the selection of

fiber/binder/substrate combinations and/or optimization of numerous other variables.

Before turning to the dry flocking method in detail, FIG. 1 illustrates an exemplary prior art composite 10 having a reinforcement zone 16 formed by a plurality of z-directional fibers 20 disposed in a binder resin 18 between two substrates 12, 14. In general, the use of the z-directional fibers 20 between the substrates 12, 14 results in improved delamination resistance of the composite 10. For example, FIG. 1 illustrates a crack 22 propagating from a distal end of the composite 10 to a proximal end. As the crack 22 attempts to grow, energy must be used to pull the fibers 20 out of the substrate(s) 12, 14 and/or the binder resin 18. Only after this initial energy barrier has been overcome may the crack 22 grow thereby causing delamination. As such, increasing the amount of energy necessary to pull the fibers 20 out of the binder matrix 18 and/or the substrate(s) 12, 14 will slow crack growth.

With traditional and/or standard flocking techniques, short textile fibers are generally electro-statically propelled and impinged onto a layer of wet adhesive coated substrate. The wet adhesive and/or binder matrix 18 is applied to the substrate(s) 12, 14 before the fibers 20 are flocked onto the substrate. The traditional flocking technique normally known simply as "flocking" will be referred to herein as "wet flocking" because each substrate is wet with the binder matrix 18 as the fibers are flocked thereon. Exemplary composite forming processes using wet flocking and characteristics of such composites are described in detail in U.S. Patent No. 7,981,495 entitled "Materials

Methodology to Improve the Delamination Strength of Laminar Composites," filed October 31, 2007, which is hereby incorporated by reference in its entirety.

Exemplary methods and techniques associated with the current invention include flocking fibers onto a substrate before applying a binder matrix to the substrate. This technique is referred to herein as "dry" flocking because the substrate is dry without any binder matrix thereon as the fibers are flocked onto the substrate. The binder resin is then applied after the fibers have been flocked onto the substrate. For example, as shown in FIG. 2A, a plurality of z-reinforcing fibers 30 can be flocked onto substrates 32a, 32b, 32c, 32d containing no resin or other binding matrix. In some embodiments, the flock fibers 30 can be electrostatically thrust into the interstices of each dry, open fibrous structure substrate 32a-d. Because each substrate 32a-d is dry, the flock fibers 30 are able to more readily penetrate into the depth of the substrate spaces or interstices and virtually lock themselves into the weave or mesh of the substrate 32a-d.

To fabricate a dry flocked laminar composite structure 40, several layers of already dry flocked substrates 32a-d can be individually laid upon each other, as shown in FIG. 2A, such that air is disposed between the substrates 32a-d and fibers 30. After the lay-up of these dry flocked plies 32a-d, a fluid matrix resin 34, such as an epoxy resin, can be applied to the laid-up dry flocked multiple-layer laminar structure 40. For example, the fluid matrix resin 34 can be placed on a top surface 36 of the assembled lay-up structure 40. The fluid matrix resin 34 can then be allowed to wick into and penetrate through the interstices of the lay-up through the multiple substrate layers 32a- d.

Any mechanism known in the art can be used to cause the resin matrix 34 to penetrate through the substrates 32a-d. For example, the fluid resin matrix 34 can be allowed to soak through the substrates or it can be forced through by a hand pressure roller, a pressure assisted mechanism, and/or a vacuum assisted mechanism to force the fluid resin matrix through the substrate layers 32a-d. In some embodiments, the laid-up composite structure can be partially soaked with the matrix resin 34. In other embodiments, it can be substantially saturated with the matrix resin 34. Once soaked with the resin, the composite can be consolidated and cured. There are many methods for curing known in the art, including room temperature curing and/or heat curing at any temperature above room temperature. In addition, curing can occur for any length of time as needed, including in the range of a few seconds to 24 hours or longer. There can also be a number of stages of curing in which a composite is cured for a certain amount of time at a first temperature and then cured for another period of time at a second temperature. Those skilled in the art will recognize that various methods of curing are within the spirit and scope of the present disclosure. Those skilled in the art will also appreciate that any or all of the various steps described above can be automated (e.g., in a continuous manufacturing process). Various fibers are suitable for use within the reinforcement zones 38, shown in

FIG. 2B. As will be apparent to those skilled in the art, any such fiber capable of providing a composite having the desired properties is within the spirit and scope of the present disclosure. The type of fiber will impact the selection of fiber dimensions, the flock density found in a given reinforcement zone, the type of binder resin, etc. For example, the fibers can be polymer-based fibers, glass fibers, carbon fibers, natural fibers, metal fibers, or any combination thereof. Exemplary polymer-based fibers include those made from polyester (e.g, polyethylene terephthalate ("PET") fiber), polybutylene terephthalate ("PBT")), nylons (nylon 6, 6-6, 3, 6-10), rayons, cellulosic fibers, polyvinylacetate fibers, polyimide and polyaramides (e.g., Nomex® or Kevlar®). Exemplary natural fibers include cotton, jute and other bast fibers. Examples of metal fibers include stainless steel fibers, titanium fibers, nickel fibers, copper fibers, brass fibers, bronze fibers, or any such alloys. In one embodiment, the fibers are

nanostructures. In some embodiments, such nanostructures can include a magnetic material (e.g., nickel, cobalt-nickel, etc.) capable of responding to a magnetic field. As will be appreciated by those skilled in the art, the fibers can have a wide range of dimensions. However, as indicated above, careful selection and optimization of such dimensions in relation to various other variables (e.g., type of fiber, type of binder resin, type of substrate, etc.) can provide a desired range of properties for a resulting composite. In exemplary embodiments, the fibers can have a length to denier ratio (measured as length to diameter ratio for certain fibers) in the range of about 1 to about

10, about 2 to about 9, about 3 to about 8, about 4 to about 7, and about 5 to about 6. Exemplary fibers have an average denier in the range of about 0.2 to about 25 and an average length in the range of about 0.5 mm to about 5 mm. In some embodiments, the average denier can be in the range of about 0.5 to about 20, 1 to about 15, 5 to about 10, etc., and the average length can be in the range of about 1 mm to about 4 mm, about 2 mm to about 3 mm, etc.

In some embodiments, the fibers can be subjected to a surface treatment thereby enhancing the performance of the composite. As will be apparent to those skilled in the art, any surface treatment capable of modifying the characteristics of the fiber and/or composite (e.g., interaction of the fibers with the binder resin) is within the spirit and scope of the present disclosure. For example, the surface treatment can include a surface electrical conductivity modifying agent and/or an adhesion promoting/degrading agent. The surface electrical conductivity modifying agents can be used as fiber surface activity agents that enhance the flockability of the fibers. Examples of such electrical activity agents include quaternary ammonium and poly-tannic acid compounds, metallic ionic compounds, and carbon black. These surface agents serve as humectants and ionic conduction compounds, which absorb moisture for changing the electrical conductivity of the flock fiber's surface thereby affecting the flock "activity" of the fiber. These humectant surface chemicals may assist in the electro-coating or flock processing of these z-direction reinforcement fibers.

As indicated above, the surface treatment agent can also include an adhesion promoting agent configured to increase the bonding strength between the fibers and binder resin (e.g., epoxy resin). It will be apparent to those skilled in the art that a wide range of such adhesion promoting agents are within the spirit and scope of the present disclosure. For example, the adhesion promoting agent can include strong oxidizing acids for carbon fibers, and coupling agents for specific resins such as epoxy functional silane compounds. Adhesion degrading agents can be used when it is desirable to reduce the fiber/binder matrix adhesion strengths. For this purpose, fluorocarbon based surface energy reducing agents can be used. Again, those skilled in the art will appreciate that a wide range of such adhesion degrading agents are within the spirit and scope of the present disclosure.

The substrates can be formed from a wide range of materials. By way of example, the substrate can be formed from a unidirectional filament sheet, a woven fabric, non-woven fibrous mat, glass fiber, carbon and/or any other type of advanced fiber, such as polyaramid. Exemplary binder resins can include any of a number of materials which exhibit adhesive properties. For example, the binder resin can be an epoxy resin, an unsaturated polyester resin, and/or a vinyl ester resin. In an exemplary embodiment, the binder resin is an epoxy resin. A useful epoxy resin can include about 100 parts Epon 826 (Shell Chemical Co.) mixed with about 26 parts of Epicure 3223 curing agent (Shell Chemical Co.). In another embodiment, the binder resin is Cycom 997 resin (commercially available from Cytech Industries). Other useful epoxy resins include amine cured (liquid) epoxy resins, Dicy cured epoxy resins, and anhydride cured (liquid) epoxy resins.

As noted above, the fibers can be delivered to the dry substrate by a flocking procedure which serves to embed fibers in the substrate. "Flocking" is the textile industry term for the process of electro-statically depositing short textile fibers so they become orthogonally oriented onto and/or embedded into substrate surfaces to create special fibrous coating effect. Flocked surfaces can be made to have a velvet, suede, or soft cushiony feel that is used for many applications, such as decorative textile fashion fabrics, headliners and glove-box interiors for automobiles, and greeting card artistic presentations, among many other applications.

While virtually any flocking apparatus can be utilized, FIG. 3 A illustrates one exemplary embodiment of a flocking apparatus 110 which is located in a closed chamber. As shown, the apparatus 110 generally includes a flock dispenser 112 having an electrode 114 connected to a proximal end thereof, and a flock fiber dispenser or hopper 116 at a distal end thereof that dispenses charged nylon fibers 118. The fibers

118 are deposited onto a substrate 120 which is positioned on a perforated table support 122. The table support 122 is grounded to an opposite electrode 124, and the table can be connected to a vacuum source 126 for applying suction to the substrate. As the fiber's traverse from the hopper to the grounded base electrode, the flock fibers are oriented parallel to the electrostatic field. The distance between the hopper and the table, i.e., between the two electrodes, can be changed to modify the electric field strength. As the fibers are delivered, these oriented fibers impact the substrate imbedding them in the surface of the substrate. Here the impinging textile fibers remain attached to the substrate in an orthogonal and for the most part a perpendicular orientation to the substrate's surface. A fluid matrix resin can then be applied to the substrate 120, and the vacuum can assist is pulling the fluid matrix resin through the substrate 120 to thereby adhere the fibers to the substrate. Once this occurs the coated textile fiber deposited surface is then cured whereby the electro-statically oriented short fibers are locked in place forming the completed electro-statically coated (dry flocked) surface. In an exemplary embodiment, the electrostatic flocking apparatus is a Model HEK100 Flocking Unit Magg Flockmaschinen GmbH (Gomaringen, Germany).

In another embodiment, a hand held flocking device 130 can be used, as shown in FIG. 3B. For example, fibers can be placed in a hopper unit 132 equipped with two rotating roller brushes, which push flock fibers down through a metal-mesh sieve electrode 134 installed on the base plate of the unit. The metal sieve electrode mesh hole size can be adjusted for different flock characteristics such as length and fiber denier. The electrodes are connected to a high-tension DC power supply (e.g. 20kV to 80 kV). Thus the fibers pushed through the metal mesh-electrode are directly charged. The uniform electric field formed between the metal hopper plate electrode and a ground electrode 136 underneath of the substrate carrier belt will transport the charged flock fibers with proper impinging forces into the surface of the substrate 138. The final stage of this continuous process is the vacuuming off through a suction column 140 of any excess and loose flock fibers from the flocked surface.

In one exemplary method for manufacturing a "dry flocked" composite, the procedure starts with electro-statically depositing short textile fibers onto/into the surface of "bare," uncoated, as-received, dry fibrous fabric or mat. Flocking equipment is used in this electro-static z-axis fiber deposition process. First the particular "bare" fibrous ply layer to be flocked is weighed. After "flocking," these "dry" fabric ply layers are inverted and shaken slightly to remove any un-flocked loose fibers. The flocked ply layer is then weighed in order to determine the amount of flock fiber that has been deposited onto/into the fibrous surface. These "dry" flocked fibrous plies can then be stored individually or stacked together for later impregnation with liquid epoxy matrix resin. One advantage of such a "dry flocking" procedure is that reinforced laminar plies can be prepared by the "dry" flocking step and later, virtually no time limit, the resin matrix impregnation step can be carried out at a separate time and place. For "dry flocked" treated fibrous plies, matrix resin impregnation of the (initially

"dry") configurationally stacked, laminar fibrous plies can be carried out by pouring resin over the stack of laminar plies and applying pressure with a small (paint) roller to force the resin into the interstices of the fibrous layers, and placing the stacked fibrous layers in a vacuum assist resin impregnation system, or adapt the commercially available resin infusion processing technique to force and saturate the (liquid) matrix resin into the stacked-up fibrous ply assembly. After the resin matrix is impregnated into the fibrous layers, the sample lay-up is treated exactly like a z-axis "wet" prepared sample configuration, namely, the sample is placed overnight in a flat-press and post cured in an 80° C oven for 2 hours. In other embodiments, two ancillary flock processing modifications can be used, namely vacuum assisted flocking and vibration (or 'beater bar') assisted flocking.

Vacuum assisted flocking is preferably employed only during the "dry" flocking of fibrous plies. Here, a vacuum is applied under the "bare" fibrous fabric or mat material being flocked. From this, it is presumed that the added air flow caused by the underneath vacuum feature will cause the electro-statically applied flock fibers to be injected (by suction) more deeply into the network and interstices of the fibrous laminar ply. A diagram of this vacuum assist flocking arrangement is shown in Figure 7. Here, the vacuum assist flock substrate mount assembly is placed onto the table base of the flocking booth. Beater bar assisted flocking is widely use in the commercial

manufacture of flocked textile goods. The principle here is that when the flock fibers impinge on the "wet" or "dry" substrate surface, the flock fibers are simultaneously vibrated, causing them to become more deeply imbedded into the surface being flocked. The vibrating surface also causes the flock fibers to become more uniformly distributed onto the flocked surface. This leads to a more firmly attached distribution of flocked fibers on a surface. The excess, loose flock fibers are then more effectively removed by suction from the freshly flocked surface. A sketch of the laboratory vibration or beater- bar assisted flocking apparatus is presented in Figure 9. In this apparatus, the substrate to be flocked is clamped onto a wire screen frame. This sample and frame is then positioned so that it is hinge mounted so it rubs against a rotatable shaft having a number of flat sides, e.g., six. When the shaft is rotated, the flat irregularities in the shaft cause the hinged sample mounted frame to vibrate up and down. The frequency and amplitude of these beater-bar induced vibrations on the sample being flocked can be modified as desired. Just as in the vacuum assisted flock apparatus, the vibration assisted flocking jig can be mounted in the flocking booth during all the fibrous substrate flocking sample ply preparations. In another exemplary embodiment, the flocking procedure can apply a magnetic field to a plurality of magnetic nanoparticle fibers. The above-identified steps of flocking and applying a binder may be repeated several times to produce additional layers. Each time an additional layer is produced it is stacked upon previously formed layers until a composite with the desired number of layers is formed. In another exemplary embodiment, a free-standing substrate for use in a laminar composite is provided, as shown in FIG. 5. Such a substrate can be used for increasing the interlaminar strength, i.e., toughness, of laminar composites. The substrate can be a thin, flocked layer of a fibrous veil 100 that can be added between the plies of a laid-up laminar composite before it is cured. By way of non-limiting example, the thin veil can have a thickness in the range of about 0.125 mm to 0.25 mm. More particularly, fibers

102 can be dry flocked using, for example, electrostatic flocking, onto a very thin, planar gauze or fibrous material of the veil 100 such that the flock fibers 102 are made to penetrate half way through the veil 100, as shown in FIG. 5. In one embodiment, the veil 100 can be precoated with a thin uncured layer of sizing resin such that the resin facilitates adhesion of the fibers to the substrate. In an exemplary embodiment, the sizing resin can be applied at a concentration level that is less than about 1% of the weight of the thin veil base fabric, and more preferably that is less than about 0.5% of the weight of the thin veil base fabric. Alternatively, or in addition, a polymer resin or other binding agent can then be sprayed onto this assembly to lock in the flock fibers 102 within the veil 100, resulting in a pre-flocked veil 104. The substrate having the fibers and resin thereon can be cured to thereby form a composite ply material that can be subsequently used, or used at some point in time in the future, as an added inter-ply for a laminar composite lay-up material structure.

The pre-flocked veil 104 can be used as interply lay-up layers when multi-ply laminar composites are being fabricated and can impart z-direction reinforcement to an assembled laminate composite. The pre-flocked veil 104 can be added between each layer of composite ply fabric as it is being laid up and subsequently cured. The added pre-flocked veil 104 can be used for both wet and pre-preg laminate fabrications.

TEST METHODS

Mechanical/physical tests were chosen based on the need for evaluating the overall fracture toughness of z-axis reinforced composites. These include: Mode I

Fracture Toughness (DCB) (ASTM D 5528); Tensile Strength (ASTM 3039-93); Inter- laminar Shear (ASTM D-2344); Falling Weight Impact test (ASTM F736); Photo- microscopic examination/study composite fracture; and Void Content/Material density. Some of the more involved testing procedures are detailed in the following. DCB Testing

A most important evaluation of a composite's inter-laminar fracture toughness is the Double Cantilever Beam (DCB) test; ASTM D 5528. For this test, the prepared laminar composite test panel was pre-fitted with a one-inch wide thin Teflon® film that was placed between the geometrically middle two plies of the laid up composite. This Teflon® release strip served as the deliberate crack opening site from which the DCB test crack was initiated.

For each test, five (5) 12.7 mm wide by 178 mm long DCB test samples were cut from the laminate plate. These DCB test coupons are shown in Figure 4A . These test samples were then edge-coated with a thin layer of water-based white paint from just ahead of the 'crack-opening' insert to the end of the sample. Next, vertical tick-lines were scribed on these white painted edges every 2 mm (using a fine point marker pen) to a length of 30 mm along both side edges of the test strip.

Piano hinges were then attached to the top and bottom surfaces of the test-strip samples at the deliberate crack opening end (see 4B). The samples were tested by mounting the end-attached piano hinge tabs in the grips of the pull testing machine (Instron 5569, Norwood, MA) in displacement control. According to ASTM test D 5528, the test sample was split apart at a crosshead speed was 0.5mm per minute. The load versus (opening mode) displacement is traced on an X-Y chart recorder. The delamination length is the distance from the loading line to the end of the (deliberate) insert plus the increment determined by the tick marks; this is observed visually. The opening displacements are recorded when the delamination grows from the end of the insert in the sequence a₀,ai,a_2, etc. When the delamination extends to the 14^th mark (ai₃), the sample is unloaded and the test is stopped. In some of the experiments, during the test, a high-speed camera with a magnification of 64X {INfocus KC microscope with a

Cooke SensiCam, Edmund Optics Inc, Barrington, NJ) was positioned on one side the sample to observe the delamination front as it extended along one edge of the sample during the test.

Double cantilever beam specimens as shown in FIG. 4A were used to determine the mode I fracture toughness for these composites. FIG. 4B is a photograph of an ongoing DCB test showing the (between-the-plies) splitting of the composite sample. FIGS. 4C and 4D show a close-up (using the Cooke SensiCam) of the nature of the split at the crack tip. Bridging by the z-axis reinforcing fibers are clearly shown. DCB test data are interpreted by two experimental parameters: (1) the initial force needed to initiate the splitting (cleavage) of the laminar sample, and (2) the highest splitting force ( a peak) reached after the crack starts to propagate.

Tensile Testing

Tensile testing was carried out according to the ASTM 3039-93 test. Here the composite test panels are cut into 254 mm X 25.4 mm test coupons. 50.8 mm x 25.4 mm lengths of composite reinforcing tabs were bonded (epoxy resin) to each side and at each end of the individual test coupon. Biaxial precision strain gages (C2A-13-125LT- 350, Vishay Micro-Measurements, Vishay Americas, Shelton, CT) were attached to the middle of the test sample. With these strain gages in place on the sides of the samples, an accurate measure of strain and the Poisson's ratio of the composite material test coupon can be measured. For tensile testing, the ends of the test specimens are clamped in the grips of an Instron 4400 tensile testing machine and pulled apart in tension at a crosshead speed of 0.05 mm/minute.

Thermal Conductivity

The thermal conductivity test was achieved by following the standard, ASTM C518-98 "Standard Test Method for Steady-State Thermal Transmission Properties by

Means of the Heat Flow Meter Apparatus." The test apparatus, as shown in FIG. 4E, consisted of a hot plate heat source, two 2" thick aluminum blocks to uniformly store heat, two heat flux/temperature transducers, a small air flow fan to move heat upward and minimize the horizontal heat loss through the sides and a carbon composite test sample. The hot plate heat source was set to 100°^* C and then allowed to reach a steady- state temperature. It took about two hours for thermal equilibrium to be reached before recording the data. The transducers were calibrated with a "standard" low carbon (0.5%) carbon steel plate with a thermal conductivity of 52 watts per meter Kelvin (W/(m-K)). EXAMPLES

The following examples provide experimental data to further illustrate certain aspects of the present disclosure. These examples are in no way meant to limit or define the scope of any of the embodiments described above.

Example 1 The following experiment compared a dry flocked composite with a wet flocked composite and a non-flocked composite (no z-directional reinforcing fibers). In this experiment, three laminar composite panels were prepared from 12" X 18" sheets of glass mat having an areal density of 400 g/m² with bright white 3 denier nylon flock fiber having a length of 1.8 mm (0.070 in.). The matrix resin used was an unsaturated polyester/styrene catalyzed with methylethylketone peroxide (a free radial initiator). The flocking device utilized was a hand held HEK 100-type D.C. mini-flocker by Maag Flockmaschinen GmbH, used with a green hopper screen for nylon flock having 2 mm² holes.

Three laminates were made employing standards known in the art of ply lay-up and resin impregnation procedures. The descriptions of these samples are presented below. All samples were cured at room temperature on a smooth release-film coated table top surface. All samples were tested at least 2 weeks after they were prepared and were suitably cured at room temperature.

Control

1. Six ply glass mat (no flock added between the plies) Z-Reinforcement Flocked with Nylon Flock

2. Six ply glass mat, 10 grams flocked (100 fibers/mm²), five layers of flock (wet flocked as taught by U. S. Patent Application No.

2008/02743240, incorporated by reference herein)

3. Six ply of glass mat, dry flocked on five of the layers, 10 grams (100 fibers/mm²). Post impregnation of assembly with resin from bottom and from top of layed-up composite system as disclosed herein. These three samples were comparison tested and the results are presented in Table 1.

Table 1

Sample Laminate Increase in Tensile Stress Flex Toughness

Description Thickness Laminate at Maximum Modulus Factor

(inches) Thickness Due Load (psi x (psi x

to Flock Layer 1,000) 10,000)

(1) Control 0.210 +/- Not Applicable 22.0 +/-1.0 129.0 56

(no flock) 0.003

(2) Wet 0.281 +/- 34% 13.5 +/-1.6 93.0 34

Flocked 0.010

(3) Dry 0.216 +/- 3% 33.3 +Λ2.4 232.0 100

Flocked 0.010 From the Table 1 data, it is clear that the dry flocked glass mat laminate has superior mechanical strength and toughness compared to the control and the wet flocked samples. Also, the thickness of the dry flocked test sample only increased in thickness by 3%. This suggests that in the dry flocking process, the flock fibers indeed penetrate into the surface and interstices of the fibrous mat structure. In the wet flocking process, penetration of the flock into the fibrous structure does not occur at all, or occurs to a much lesser extent. In the wet flocking process, it is likely that the fibers only embed in the resin, without penetrating the interstices. When the plies of this wet flocked laminate are laid over each other, the perpendicular orientation of the flock fibers are likely severely disturbed. Most the of the flock fibers are likely bent or "washed" over on their side within the interlayer as the wet flocked plies are laid over each other. This compresses the fibers resulting in a chopped fiber reinforced resin interlayer between each laminar ply layer. As shown in the example in Table 1, this causes an increase in laminate thickness of over 30%, which is an undesirable feature for industrial applications. Providing an increase in laminar composite strength and toughness without any increase in overall laminate thickness is ideal.

Example 2

The following example compares the toughness and thickness of a dry flocked composite with a composite made with no flock. In this experiment, two laminar composite panels were prepared from 9" X 12" sheets of glass mat having an areal density of 400 g/m² with bright white 20 denier nylon flock fiber having a length of 5.08 mm (0.2 in.). The matrix resin used was amine cured epoxy resin matrix from

FibreGlast Company 2000/2060 resin system. The flocking device utilized was a hand held HEK 100-type D.C. mini-flocker by Maag Flockmaschinen GmbH used with a green hopper screen for nylon flock having 2 mm² holes.

Before fabricating the composites, a study was conducted to determine the depth of fiber penetration and orientation. To ensure maximum inter-layer bonding, the flocked fibers should be as perpendicular as possible to the fiber mat layer. A sample of mat was dry flocked and a picture was taken along an edge of the mat, as shown in FIG.

6. As shown, most of the flocked fibers are upright, confirming that the fibers are disposed appropriately within the interstices of the mat before resin is applied.

Two laminates were then prepared employing standards known in the art of ply lay-up and resin impregnation procedures. First, a six ply 9" X 12" glass mat was "dry flocked" with 2.5 grams of "dry" flock nylon fibers (3 fibers/mm²). A "dry flocked" laminate was fabricated by pouring the epoxy on a large plastic sheet and placing all six "dry flocked" glass mat layers on the wet epoxy. These mats were then rolled, so that the epoxy could more readily penetrate and evenly seep into and through the six "dry flocked" layers. The assembled laminate was then placed in a 12" X 12" platen press. The press pressure was raised to 125 PSI and the laminate was left to cure in the press overnight at room temperature. The next day, the composite panel was then post-cured (in the press) for two hours at 125 psi at 80°C.

To complete the study, a "control" 9" X 12" glass mat/epoxy matrix laminar composite panel was made without nylon flock. In this case, the lay-up and curing procedure was identical to the dry flocked laminate just described.

These two samples were comparison tested using the Double Cantilever Beam ("DCB") Method. The results showed that the toughness of the unflocked control laminate was found to be 2.5 KJ/m² while the toughness of the z-reinforced dry flocked laminate was found to be 4.5 KJ/m². The results show that the dry flock methodology improves the toughness by about 80%. This is similar to the toughness increase found in Example 1 above. In comparing these two samples the addition of dry flock to the five glass mat interlay ers increased the thickness of the z-reinforced laminate by only 15.4 %.

Example 3

The following example compares various attributes of dry flocked composites with wet flocked and no flocked composites. A glass mat laminate using the same material composition as described in Example 2 above with the exception that the laminate was composed of ten plies of the glass fiber mat (400 g/m² areal density), with an additional 2 layers of fiberglass fabric one on the top and another on the bottom of the mat laminate for added stiffness and thickness. Each of the inner ten glass mat plies was dry flocked with 3 denier, 1.8 mm (0.070") long nylon flock. This twelve ply laminate was then impregnated with the FiberGlast 2000/2060 Epoxy resin system. The prepared laminate was cured in a flat press at 125 psi overnight at room temperature. The sample was then post cured in an oven for 2 hours at 80 C.

Following an identical procedure to the above, a second ten ply laminate was prepared such that each of the nine mat plies were dry flocked with 18 denier, 4.57 mm (0.18") long nylon flock. A third twelve ply laminate was then prepared in which each of the ten inner mat plies were wet flocked according to the wet flocking procedure described herein. Finally, a comparable ten ply non-flocked "control" sample was prepared in a manner identical to the dry flocked and wet flocked laminated panels.

DCB toughness tests were performed on the four composites. In all of the DCB tests, a crack opening was induced to occur in the middle of the fabricated ten ply laminate between the # 5 and #6 layer of the glass fabric mats. The results of these DCB tests are presented in Table 2.

Table 2

(*) This represents the % increase in thickness relative to the Control.

In the last two columns of Table 2, the first value toughness and initial peak toughness for different laminate configuration are listed. These values are shown in FIG. 7 for a typical DCB test. Point A is the first toughness data point (crack initiation force) collected immediately as the DCB test starts and called the "first value toughness." As the toughness increases with increasing delamination length, point B is the first significant peak (crack propagation force) and is called the "initial peak toughness." In Table 2, the first value toughness for both the dry flocked 18 denier and wet flocked laminates are at least 4 times tougher then the control laminate. While the wet flocked laminate is slightly tougher than the 18 denier (about 9% tougher), both have a significantly higher toughness than the dry flocked 3 denier and the control laminates. It is likely that because the 18 denier fiber is longer than the 3 denier fiber, it is able to embed itself deeper into the mat and better bridge pairs of mat layers, while the epoxy in the wet flock laminate facilitates a bridge between pairs of layers.

Referring again to Table 2, for the initial peak toughness, the control and 3 denier laminates have about the same toughness (about 12% difference), while the wet flock and 18 denier laminates are about 2.5 times and 1.5 times tougher respectively than the 3 denier and the control laminates. Again, the reason for this is likely that the 18 denier fiber is longer than the 3 denier fiber and is thus able to embed itself deeper into the mat and better bridge pairs of layers. Also the epoxy in the wet flock may provide a bridge between pairs of layers.

Example 4 A series of studies were carried out on the z-reinforcement of a carbon fabric laminar structure called "VectorPly®" (VectorPly Corporation, Phenix City, Alabama). This type of carbon fabric composite configuration is used extensively in military panel structures, as well as in pultrusion processing and wet "lay-up" procedures. It is composed of four unidirectionally oriented carbon yarn fabric layers that have been oriented at a 0/+45/90/-45 degree quadraxil lay-up sequence (VectorPly® CQX-2300).

These four plies are then loosely stitched (loosely sewn together) so that they can hold together during lay-up or pultrusion processing. When two of the four ply layers of carbon yarn are laid together back-to-back, they form a balanced 8 ply quasi-isotropic laminar construction (0/+45/90/-45 //+45/90/-45/0). This construction has great structural utility as a light weight, highly stiff laminar composite structure (areal density of each VectorPly layer is 811 g/m²). Several experiments were carried out on these 8 ply constructions to evaluate the efficacy of using the dry flocking and wet flocking techniques to improve its

interlaminar Z-direction toughness. Samples of the VectorPly® CQX-2300 laminates were prepared for subsequent DCB toughness testing, however the back-to-back fabrication was too thin for proper DCB toughness testing. It was then decided to support the two VectorPly® layers with layers of square weave glass fabric. Four layers of glass fabric 602 (areal density of each single glass fabric ply was 193 g/m²) were added to both sides of the two plies of VectorPly® 604, as shown in FIG. 8.

Several types of the VectorPly® laminates were fabricated using Z-reinforced nylon flock fibers between the layers, as shown below in Table 3. The DCB toughness test results are also presented in Table 3.

Table 3

The results in Table 3 reflect a characteristic of the VectoPly® itself. These results can be interpreted by examining the failure surface of the DCB (fracture peeled) test specimens. Recalling that these VectorPly® layers are actually a combination of four (independent) fibrous layers that are loosely stitched together. Therefore, when the Z-Axis fibers are flocked onto the surface of this four (4) layer combination, the Z - reinforcement effect does not penetrate through all four layers in the VectorPly® structure. Only the outer layer ply of the four (4) ply lay-up is Z-reinforced. By examining the fracture surfaces of the DCB test specimens, it was found that the splitting failure of these VectorPly® specimens occurred away from the Z-Axis reinforced ply. Failure between the ply layers one interface away from the z-axis reinforced interface. In other words, the z-reinforced interface is interfacially reinforced so failure occurs between the nearest un-reinforce inter layer zone. This fracture surface behavior and pattern was observed in both the dry-flocked and wet-flocked processed laminates. The fabric weave of the VectorPly® component layers were too closely packed to allow complete four layer penetration of the z-axis applied dry flock fibers, resulting in the control sample having the best overall toughness properties. This suggests that in order to improve the fracture toughness of VectorPly® fabricated laminates some means of z-axis reinforcing all four plies of the VectorPly® must be devised. Dry flocking each of the four (4) component plies of the VectorPly configuration, before the material is stitched together, would be the way to proceed.

Example 5

This example tests the alternative embodiment discussed above with regard to the pre-flocked veil laminar insert. Laminar constructions were prepared using the carbon yarn VectorPly® fabric layups such as those described in Example 4. These test panels also included a dry flocked carbon veil inserted at the interface of interest, as shown in FIG. 8. The areal density of the nonwoven veil was 7 g/m² with a thickness of 0.0011 " or 0.028 mm. No additional flocking was performed during the preparation of these laminates. It is presumed that the flocked veil configuration will provide the Z- direction reinforcing fibers at the interface of interest in order to DCB "toughen" the laminate. DCB toughness test results are presented in Table 4 on several laminar configurations where the flocked carbon veil insert method of z-reinforcement was used.

Table 4

The data in Table 4 clearly indicates that the presence of the thin carbon fiber "veil" at the interface between the two VectorPly® layers reduces the overall cleavage toughness of the laminar composite. This fibrous layer serves as an inter-ply layer that can direct the crack opening position. The particular carbon fiber veil that was used in this experiment was detrimental to the cleavage strength of these VectorPly® laminates (compare the control sample (no veil) in Table 3 and the control sample (with veil) in Table 4). Of interest here is that adding dry flocked nylon fibers to the veil before it is laminated together increases the crack propagation strength of the laminate. The dry- flocked veil interlayer samples have a DCB Initial Peak (crack opening) toughness of 1.51 KJ/m² and 1.60 KJ/m² compared with the control (non-flocked veil at interface) which had a DCB Initial Peak (crack opening) toughness of only 1.07 KJ/m².

Example 6A

Studies Involving Glass Mat and Epoxy Resin Laminar Structures with Vacuum

Assisted Flocking

Two (2) glass mat (cored) laminates using the same material composition as described in Example 3 were each composed of Ten (10) plies of the glass fiber mat (400 grams/ meter square areal density), with an additional 2 layers of glass fiber fabric one on the top and another on the bottom of the mat laminate for added stiffness and thickness needed for the DCB test. Each of the inner ten (10) glass mat plies was "Dry Flocked" with 15 denier, 3.8 mm (0.15") long nylon flock. During the flocking process, a vacuum assist was used to pull more flock deeper into the dry mat's interstices, as shown in FIG. 9. The applied vacuum produced an air flow of approximately 2 m/s through the to-be-flocked glass fiber mat ply. A 70kV electric field was found to be the optimum (flock hopper) voltage to drive and orient the flock fibers into the glass fiber mat for this and another (flocked but not vacuum assisted) laminate sample that will be used as the no-vacuum assist control.

After applying the flock to each glass fiber mat ply using the dry flocking method, each ply was turned upside down over a waste barrel and shook systematically to discharge any loose, excess flock fibers that did not get trapped or embed into the surface of the fibrous mat ply. Each twelve-ply laminate was then impregnated with a FiberGlast 2000/2120 Epoxy resin system. Each prepared laminate was then placed in a flat press at 125 psi and cured overnight at room temperature. Each laminate was then post-cured in an oven for 2 hours at 80°C. Next, DCB toughness tests were performed on these two (2) laminates. In all the DCB tests, the crack opening was induced to occur in the middle 2-ply zone of the fabricated ten-ply laminate; that is between the middle two (fifth and sixth) plies of each laminate. The results of these DCB tests are presented in Table 5.

Example 6B

Studies Involving Glass Mat and Epoxy Resin Laminar Structures with Beater Bar Assisted Flocking Another two (2) glass fiber mat laminates were fabricated as described in

Example 6A, except instead of using a vacuum assist, a "beater bar" assist was used to up and down vibrate a glass fiber mat ply as it was being flocked. This up and down vibration serves to agitate the flock thereby driving or pulling the flock deeper into the glass fiber dry mat's interstices, as shown in FIG. 10. As discussed above, beater-Bar assisted flocking is commonly used in industrial flock processing. In our present experiment, a 40kV electric field was found to be the optimum voltage to orient and drive the flock into the fibrous mat for this sample. In another "control" sample, a "duplicate" laminate fabricated with no beater-bar assisted flocking procedure was prepared. The sample was used as the no beater bar control. Each laminate was then impregnated with epoxy, and subjected to the same curing schedule and DCB tests described in Example 6A. The results of these beater-bar assisted flocked composite samples are presented in Table 5.

Example 6C

Studies Involving a Vacuum Assisted/Z-linked Flock Fiber Reinforced Glass Mat Epoxy Resin Laminar Structures

A glass fiber mat laminate was fabricated as described in Example 6A, except a novel vacuum assist methodology was used to "fiber-link" laminar ply pairs. This conceived novel process variation has been referred to and called "Z-linking". In "Z- linking" contiguous pairs of laminar plies of the glass fiber mat laminate were intimately linked together by a vacuum assist process during the stepwise lay-up of the laminate.

This vacuum assist, when applied to an already "dry" flocked ply layer and an un- flocked ply layer, is able to orient and more deeply embed and distribute the flock fibers across the interface between the contiguously placed plies. The details of this Z-linking procedure for glass fiber mat ply pairs with flock fibers are presented: 1. Flocking 1 of the 2 ply pairs using the vacuum assisted technique described in Example 6A.

2. Setting this one flocked ply aside, flock side up, for a moment, as shown in FIG. 11 A for ply 1.

3. Placing an unflocked ply on the vacuum (table with a wire screen surface and a vacuum pull from underneath) apparatus in the flocking booth with vacuum applied, as shown in FIG. 1 IB for ply 2.

4. Next turn ply 1 (flocked) so that the flock side is now pointing downward. This flock faced laminar ply is then placed downward onto the surface of un-flocked ply 2. Since this un-flocked glass mat ply layer is on the "vacuum table", air vacuum sucking through the porous glass mat (ply 2 structure) the flock on the surface of ply 1 gets partially sucked into ply 2 in the z-direction. A mechanical, entanglement bonding is therefore induced between the two glass mat ply layers.

5. The flock is now shared and embedded between the two "Z- linked" plies in the z-direction, as shown in FIG. 1 1C.

The configuration of a laminate is shown in FIG. 1 ID, where the middle ply pair (location of induced crack) is adjacent to a ply pair above and below it. The rest of the plies are flocked only on one side. Three ply pairs were fabricated and the one with the most fibers was used as the middle pair.

During the flocking process, a 70kV electric field was found to be the optimum voltage to orient and drive the flock into a ply. After the "dry" lay-up of all the laminar plies, each laminate was impregnated with epoxy. These samples were then subjected to the same curing schedule and DCB tests as described in Example 6A. The results are presented in Table 5.

Table 5

Comparing the Toughness of "Dry" Flocked Ten Ply Glass Mat Epoxy Laminates (15D Nylon Flock) and the Non-Flocked Control (no vacuum or beater bar assist)

In columns 4 and 5 of Table 5 the first value toughness and initial peak toughness for different laminate configuration are listed, respectively. These toughness levels are defined in Example 3.

From the data presented in Table 5 it appears that the goal of using vacuum assist and beater bar flock processing variations to increase the flock density and the number of effective z-axis inter-ply flock fibers have been achieved. A significant increase in flock fiber density was observed when applying the vacuum and beater bar assists. The increase is 553% for the Z-link/vacuum assisted laminate compared to the no vacuum control laminate. This dramatic increase in flock fiber is due to intentionally shaking the upside down, just flocked, ply less vigorously than in Examples 6A and 6B so as to discard less excess fiber knowing that any excess fiber would be sucked into the bottom unflocked ply.

The toughness increased significantly for several cases compared to the no flock control, e.g., the Z-link/vacuum assisted laminate is 3.3 and 1.8 times tougher for the first value of toughness and the initial peak toughness, respectively. Mechanistically, the vacuum assist during Z-linking of the two center plies of the laminate would help to mechanically reinforce this center interface in two ways (1) the loose (un-bonded) dry flock fibers are physically drawn more deeply into the unflocked contiguous ply, and (2) the flock fibers are more favorably oriented in the z-direction at the interface by the vacuum-assist action.

Example 7A

Studies Involving Glass Fiber Fabric and Epoxy Resin Laminar Structures with Beater Bar Assisted Flocking.

Two (2) glass fiber fabric laminates were prepared and composed of twelve (12) plies of the glass fiber fabric (200 grams/ meter square areal density). Eleven (11) inner plies were "Dry Flocked" with 22Denier 0.7 mm (0.027") long black polyester flock. During the flocking process a beater bar assist, as shown in Figure 8, was used to pull or otherwise 'lock' more fibers deeply into the dry glass fabric plies of one of the laminates. A 40kV electric field was found to be the optimum voltage to orient and drive the flock fiber into the glass fiber fabric. Each laminate was impregnated with epoxy resin and subjected to the same curing schedule and DCB testing as described in Example 6A. The results are presented in Table 6.

Example 7B Studies Involving Z-Linked Glass Fiber Fabric and Epoxy Resin Laminar Structures with Vacuum Assisted Flocking

A glass fiber fabric laminate was fabricated as described in Example 7A except that the same Z-link (vacuum assist) method described in Example 6C was used during the lay-up of ply pairs. Each laminate was impregnated with epoxy and subjected to the same curing schedule and DCB testing as were described in Example 6A. The results are presented in Table 6.

Table 6

Comparing the Toughness of "Dry" Flocked Twelve Ply Glass Fabric Epoxy Laminates Using Beater Bar and Vacuum Assisted Z-Linking Procedures.

(*) 22 Denier, 0.7mm long Polyester Flock used for all laminates

(**) toughness values from Liang and Hoskote, reference.

As shown in Table 6, the goal of using the beater bar to increase the flock density was achieved. A significant increase (178 %) in DCB fracture toughness was observed compared to the no beater bar control laminate. Furthermore, the initial peak toughness of the laminate increased significantly for all cases compared to the ideal non-flocked, control, e.g., the Z-link, vacuum assisted laminate was measured to be 4.8 times tougher than the ideal non- flocked, control.

A practical Z-link laminate would be Z-linked between all the inner plies. To fabricate the laminate one could: 1. Z-link the bottom two plies as shown in FIG. 12 A, and set it aside.

2. Flocking another single ply using the vacuum assisted technique described in Example 6A, and set it aside.

3. Place the Z-link plies from step 1 on the vacuum apparatus in the flocking booth with a stronger vacuum applied, and turn the flocked single ply from step 2 so that the flock side is now pointing down (flock now points toward the unflocked layer) and placing it on the z-linked plies in the vacuum apparatus in the flocking booth so that the flock from the single ply gets partially sucked into the Z-linked plies in the z-direction, as shown in FIG. 12B.

4. Keep repeating steps 2 and 3, as shown in FIG. 12C, until the laminate is completed.

Example 8

In recent work, Colon, Rice, Kim and Chalivendra [Hector O. Colon, John M. Rice, Yong K. Kim, Vijaya B. Chalivendra, "Projectile Impact Behavior of Flock Fiber Z-Reinforced Composites," EVIPLAST Conference, Providence, RI, October 12, 2010] have studied the impact behavior of z-axis reinforced glass mat and glass fabric composites by striking them with a spherically tipped 11.45 mm diameter copper jacket/lead core metal projectile backed by a plastic sabot (combined weight of 16.1 grams). The overall objective of this study was to characterize the projectile impact resistance of non-flock fiber "control" laminates and compare the results with the projectile impact resistance of "wet" and "dry" flock process, fiber reinforced composites. The purpose of this study was to quantify any improvements in impact fracture toughness due to the z-axis introduced fiber reinforcement. Here, the glass fiber mat was a standard 458 grams/m² chopped strand material. Eight layers of this glass mat were used to prepare the test samples. For the glass fabric, a plain weave fabric of areal density 203 grams/m² was used. Ten layers of this fabric were used to prepare this set of test samples. All the samples used a Fiber Glast Inc, Brookville, Ohio, epoxy resin matrix system; Epoxy 2000/2120. Three sets of laminate test samples were fabricated for each fibrous structural layer type; fibrous glass mat and glass fiber weave. The three sets of samples were: (1) "control" samples, not z-axis reinforced, (2) samples z-axis fiber reinforced using the "wet" flock process, and, (3) samples z-axis fiber reinforced using the "dry" flock process. Each fabricated sample was cured overnight at room temperature under a 125 psi pressure followed by a two-hour 80° C oven cure. In the impact testing, a compressed gas gun was used to shoot projectiles perpendicular to the frame mounted samples. The test involved shooting the metal projectiles at a 4" X 4" size target composite sample at an increasing velocities until penetration of the target sample is accomplished. A new, unused test sample was the target for each of the target velocities tested. Only the projectile impact results for sample penetration are reported here. In these measurements, it was determined that it took a velocity of 145.1 meters/second for the projectile to pass completely through the glass mat laminate and a projectile velocity of 110.1 meters/second to pass through the glass fabric laminate. From this, the energy absorbed by the composite during the projectile's impact and through penetration was calculated by measuring the difference in the projectile's velocity before and after it penetrated through the composite target.

Table 7 presents data on the energy absorbed by the specified test projectile during its penetration of the composite sample. Data for z-axis fiber reinforced composites (glass mat and fabric) and their non-z-axis reinforced controls are compared. The overall energy absorbed by the glass mat during penetration impact is almost 350% higher than the penetration absorption energy for the glass fabric laminates. Comparing the "wet" and "dry" flocked sample results we see that both the "wet" and "dry" flock reinforced glass mat sample configurations had higher energy absorption at penetration than the control. Also, comparing the projectile impact energy absorption values reveals that the "dry" flocked laminate absorbed about 5% more projectile penetration impact energy than the glass fabric laminate. For the glass fabric laminate samples the projectile penetration energy absorption value for the "dry" flocked laminar specimen was only about 2% lower than the value determined for the comparable "wet" flocked composite sample.

Another parameter studied in this projectile penetration impact study was the amount of (radial) damage that surrounds the hole made by the projectile. Table 7 lists this information in terms of area of radial damage in the composites after projectile impact testing. First we can conclude that the glass mat samples have a much larger overall radial damage area than the glass fabric laminates. This is because of the structural nature of each of the tested laminar fabrics. The glass mat is a random assembly of chopped glass fiber strands with little or no lateral structure. Note here that z-reinforcement of this glass mat laminates did indeed result in a slightly smaller radial damage zone area. This suggests the z-reinforcement in this composite was suppressing somewhat the radial shock wave of the impacting and penetrating projectile. The glass fabric composite, however, while having a much smaller radial (penetration) damage area, the radial damage area here was larger for the z-reinforced composite than for the non reinforced control. This again has to do with the inherent lateral structure of the woven glass fabric. Woven fabric structures can absorb impact energy in a lateral mode and the more structure it has in this lateral direction the more impact energy it can withstand. This increase in radial damage area in the glass fabric laminate is probably why these same z-axis fiber reinforced samples have a higher absorption energy for projectile, impact penetration. Comparing "wet" and "dry" flocked reinforced laminated fabric samples, we observe that the damage radius of the "dry" flocked laminate is slightly smaller than the "wet" flocked reinforced laminate. From these ballistic impact, projectile penetration experiments, we have further demonstrated the merits of inter- laminar organic polymer laminar composite reinforcement by z-axis flock fiber placement.

Table 7

Measured Energy Absorption During the Projectile Penetration of Composite Test Samples and also the Resulting Radial Damage Area

Sample Target Sample Sample Energy Absorbed Radial Damage

Description Thickness (mm) Thickness (Joules) Area

Change (cm²) Glass Mat Control 4.0 Not 122 49.3

(not flocked) Applicable

Glass Mat Z-axis 4.3 7.5 % increase 133 45.1

Fiber Reinforced

"WET" Flocked

Glass Mat Z-axis 4.2 5.0% increase 140 48.0

Fiber Reinforced

"DRY" Flocked

Glass Fabric 2.5 Not 31 4.7

Control Applicable

(not flocked)

Glass Fabric 3.2 28% increase 43 5.8

Z-axis Fiber

Reinforced

"WET" Flocked

Glass Fabric Z- 3.0 20% increase 42 5.3 axis Fiber

Reinforced

"DRY" Flocked

Example 9

Combining the beater bar and vacuum assist flock inter-laminar toughness results presented in Examples 6A, 6B, 7A and 7B, it was a natural conclusion to the study of the "dry" flocking processes by simultaneously applying the vacuum assist and beater bar flock processing modifications to the flocking process. In some preliminary DCB experiments, twelve (12) ply glass fabric test specimens prepared using the combined vacuum and beater bar flock processes were observed to fail way before the crack fully developed. In other words, the glass fabric composite was so strong that the first peak toughness was never reached. Therefore, it was decided to construct a Z-link panel using twenty layers for the glass fabric samples. Results for glass mat and glass fabric (z-axis reinforced) laminates prepared using the (simultaneous) combined vacuum and beater bar assist flock process are presented in Tables 8 for the glass mat and glass fabric epoxy laminates.

Table 8

Toughness of "Dry" Flocked Glass Mat and Glass Fabric Epoxy Laminates using Simultaneously Combined Vacuum and Beater Bar Assist Flocking Process (a)

Description* Thickness Flock Density DCB - First DCB -Initial

* 60 kV electric field, 15 Denier, 3.8 mm Nylon flock used for both the Glass Mat and Glass Fabric Test Specimens

** Glass Mat composites were ten (10) ply laminates (compare results with Table 5).

Comparing the results in Table 8 to the glass mat control (flocked, no vacuum or beater bar assist) in Table 5 there is a significant increase in toughness. Compared to this control, where the First Value Toughness (FVT) was 0.29 KJ/m² and the Initial Peak Toughness (IPT) was 1.1 KJ/m², there is an increase in FVT and IPT of 134% and 93%, respectively, for the glass mat, vacuum and beater bar assisted laminates; and 79% and 46%) for the Z-linked, glass mat, vacuum and beater bar assisted laminates.

Comparing the results in Table 8 to the glass fabric control (Flocked, no vacuum or beater bar assist) in Table 6 there is a significant increase in toughness. Compared to this control with no z-axis flock, where for a twelve ply glass fabric laminate the First Value Toughness (FVT)was 0.1 KJ/m² and the Initial Peak Toughness (IPT) was 0.69 KJ/m² , there is an increase in FVT and IPT of 500% and 90%, respectively, for the vacuum and beater bar assisted , Z-link, laminates. From this example it is shown that the combined effect of the vacuum assist and beater bar assist are advantageous to the "dry" flocking processing of both glass mat and glass fabric laminar composites.

Example 10

Tests were conducted to show thermal conductivity improvement in carbon pre- preg laminates by employing the z-axis dry and wet flocking methods. The purpose of this study was to quantify improvements in thermal conductivity due to z-axis fiber reinforcement.

Carbon pre-preg laminates have poor conductivity perpendicular to the plane of the laminate, i.e., in the z direction. To improve this property, carbon fibers were flocked perpendicular to all the interior pre-preg plies of a laminate. In the direction perpendicular to the axis of a carbon fiber the thermal conductivity is generally poor, e.g., less than 1 W/(m-K), however in the direction of the fiber axis the thermal conductivity can be very high, e.g., Nippon pitch based fiber XN100 has reported conductivities as high as 900 W/(m-K). Two carbon fibers were tested in this study: polyacrylonitrile (PAN) based fiber, Toray T300, ½ mm in length, with a conductivity of approximately 100 W/(m-K) in the axial direction; and a pitch based fiber P-120, 1 mm in length, with a conductivity of approximately 600 W/(m-K) in the axial direction.

Eight (8) ply, 6" X 6" square, carbon pre-preg laminates were fabricated for testing following the thermal conductivity standard ASTM C518-98 "Standard Test

Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus." A set of three (3) non-flocked laminates for the control and several sets of three (3) at various flock densities in a range up to 800 fibers/sq. mm were tested. The test apparatus, shown in Fig. 4E, was calibrated with a 304 stainless steel plate with a known conductivity of 16 W/(m-K). Some test results have been achieved thus far as shown in FIG. 13.

Comparing the results in FIG. 13 at a flock density of 200 fiber/sq. mm, the P- 120 dry flocked laminates exhibit the highest average conductivity at 8.7 W/(m-K) and compared to its control at 6.4 W/(m-K), this represents an increase of 36% in thermal conductivity. At a flock density of 800 fiber/sq. mm, the P-120 wet flocked laminates exhibit the highest overall average conductivity at 12.4 W/(m-K) and compared to its control at 6.2 W/(m-K), this represents an increase of 100% in thermal conductivity.

One skilled in the art will appreciate further features and advantages of the present disclosure based on the above-described embodiments. Accordingly, the present disclosure is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

What is claimed is:

Claims

CLAIMS:

1. A method for fabricating a reinforced laminar composite, comprising:

providing a plurality of uncoated, dry substrates;

flocking a plurality of reinforcing fibers onto each of the plurality of substrates; placing the plurality of substrates one upon the other to form a stack of substrates; and

applying a fluid matrix resin to the stack of substrates.

2. The method of claim 1, wherein applying the fluid matrix resin to the stack of substrates includes applying the fluid matrix resin to substantially saturate the substrates.

3. The method of claim 2, further comprising curing the plurality of substrates after they are substantially saturated with the fluid matrix resin.

4. The method of claim 1, wherein applying a fluid matrix resin to the stack of substrates comprises applying the fluid matrix resin to a top layer of the stack of substrates so that it penetrates through all of the layers.

5. The method of claim 1, wherein applying a fluid matrix resin to the stack of substrates includes applying a pressure to the substrates to force the fluid matrix resin through the stack of substrates so that it penetrates through all of the substrate layers.

6. The method of claim 1, wherein the fluid matrix resin is an epoxy.

7. The method of claim 1, wherein flocking the plurality of reinforcing fibers comprises electrostatically coating by a flocking process the fibers into interstices within each of the plurality of substrates.

8. The method of claim 1, wherein flocking the plurality of reinforcing fibers onto each of the plurality of substrates includes electrostatically depositing the reinforcing fibers onto a first side of each of the substrates.

9. The method of claim 8, wherein placing the plurality of substrates one upon the other to form a stack of substrates includes placing the first side of at least one of the plurality of substrates adjacent to a second, unflocked side of another of the plurality of substrates.

10. The composite of claim 1, wherein the plurality of reinforcing fibers are selected from a group consisting of polymer-based fibers, glass fibers, carbon fibers, natural fibers, and metal fibers.

11. A reinforced laminar composite, comprising:

first and second substrates stacked one on top of the other and having a plurality of reinforcing fibers disposed therebetween, the plurality of reinforcing fibers providing the composite with an increased thickness in the range of about 3% to about 20% relative to a thickness of the first and second substrates without the reinforcing fibers.

12. The composite of claim 11, further comprising a cured matrix resin disposed throughout the first and second substrates.

13. The composite of claim 12, wherein the plurality of reinforcing fibers provide the first and second substrates with a Mode I toughness in the range of about 0.9 KJ/m² to about 8.0 KJ/m².

14. The composite of claim 11, wherein the plurality of reinforcing fibers have an average length in the range of about 0.5 mm to about 2 mm.

15. A reinforced laminar composite, comprising:

a plurality of substrates stacked together and having a plurality of reinforcing fibers disposed between each substrate within a cured epoxy resin, the reinforcing fibers configured to provide the composite with a fracture toughness in the range of about 0.06 KJ/m² to about 0.25 KJ/m² for typical carbon/epoxy.

16. The composite of claim 15, wherein the flock fiber density is in the range of about 2 fibers/mm² to about 500 fibers/mm².

17. The composite of claim 15, wherein the plurality of reinforcing fibers provide the composite with an increased thickness in the range of about 3% to about 20% relative to a thickness of the plurality of substrates without the reinforcing fibers.

18. The composite of claim 15, wherein the plurality of reinforcing fibers are selected from a group consisting of polymer-based fibers, glass fibers, carbon fibers, natural fibers, and metal fibers.

19. The composite of claim 15, wherein the plurality of reinforcing fibers provide the plurality of substrates with a toughness in the range of about 0.9 KJ/m² to about 8.0 KJ/m².

20. A method of fabricating a flocked thin veil or scrim of fabric, comprising:

providing an uncoated, dry substrate having a thickness in the range of about

0.125 mm to 0.25 mm;

applying an un cured layer of sizing resin to opposed surfaces of the substrate, the sizing resin being applied at a concentration level less than about 1% of the weight of the substrate;

flocking a plurality of reinforcing fibers onto opposed surfaces of the substrate having the sizing resin thereon; and

curing the substrate with the fibers and the sizing resin to form a free-standing interlayer substrate.