Technical Field

The present invention is directed to fiber tows that are coated with binder particles, and in particular, to dry carbon fiber tape and sheet products having a non- tacky binder coated thereon. The present invention is further directed to carbon fiber tape and sheet products used in dry preforms that are infused with resin for making composite products.

Background

Continuous fibers, such as carbon fibers, glass fibers and aramid fibers, are often used to produce two-dimensional and three-dimensional preforms for use in resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), or resin injection molding (RIM). The process for making such preforms typically involves stacking or assembling woven or stitched fabrics, forming the assembly into a predetermined near-net-shape, and holding this shape by applying a chemical binder to the shape. Another approach that is used is to place and stitch fiber bundles or tows in automated or semi-automated equipment to produce two- dimensional preforms that will conform to a three-dimensional shape. It has also been proposed to produce such preforms using a dry fiber tape with a tacky film adhesive attached to the tape. However, such tacky fiber tapes are difficult to handle and often necessitate use of a release liner or release coating, which must be removed before use. In addition, the tackiness of the fiber tapes makes subsequent processing unfeasible, such as for weaving or braiding.

Summary

In accordance with a first aspect of the present invention, there is provided a dry fiber material that includes a fiber tow form having a planar shape of

predetermined width and having a first major exterior surface and a second major exterior surface, the fiber tow form comprising a plurality of fiber filaments; and a discontinuous binder resin layer fused to at least the first major exterior surface of the fiber tow form. The binder resin layer is not tacky at room temperature, and the amount of binder resin fused to the exterior surface of the fiber tow form is less than 20% by weight, based on the weight of the fiber tow form. The dry fiber material is capable of being impregnated with a matrix resin.

In one embodiment, the fibers of the tow form are chosen from among carbon, glass, ceramic, quartz, aramid and metal fibers.

In one embodiment, the binder resin is a thermosetting resin. The

thermosetting resin may be chosen from among polyesters, polyimides, epoxies, phenolics and polyurethanes.

In another embodiment, the binder resin is a thermoplastic resin. The thermoplastic resin may be chosen from among polyurethane, polyurea, polyimide, polyetherimide, polyamide, polyamideimide, polyesters, polycarbonate, polysulfone, polyethersulfone, polyphenylene sulfide, polyketones, polyolefins, (meth)acrylates, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, acrylonitrile-styrene-acrylate and combinations thereof.

In one embodiment, the fiber tow form is a tape. In another embodiment, the fiber tow form is a sheet.

The dry fiber material may further include a discontinuous layer of binder resin fused to the second major exterior surface of the fiber tow form.

In one embodiment, the resin includes an epoxy and the fibers of the tow are carbon fibers.

The layer of resin may be applied to the fiber tow form as powder particles.

In accordance with a second aspect of the present invention, a method of making a dry fiber material includes the steps of: arranging a fiber tow into a planar tow form having a predetermined width, and having a first major exterior surface and a second major exterior surface, the fiber tow including a plurality of fiber filaments; depositing a plurality of binder resin particles onto at least the first major exterior surface of the fiber tow form; and sintering the binder resin particles onto the tow form to form a discontinuous layer of binder resin. The binder resin layer is not tacky at room temperature, and the amount of binder resin sintered onto the tow form is less than 20% by weight, based on the weight of the fiber tow form.

In one embodiment the method further includes depositing and sintering a plurality of binder resin particles onto the second major exterior surface of the fiber tow form. Brief Description of the Drawings

FIG. 1 is a schematic illustration of a fiber tape having a binder particle sintered to the surface of the fiber tape in accordance with an embodiment of the present invention.

FIG. 2A is a plan view of a segment of a fiber tape having a random

distribution of discontinuous binder particles attached thereto in accordance with the present invention.

FIG. 2B is a cross-sectional view of the fiber tape of FIG. 2A.

FIG. 3 is a schematic diagram of the process steps for manufacturing a bindered fiber tape in accordance with an embodiment of the present invention.

FIG. 4A is a schematic illustration of an exemplary apparatus for depositing binder particles onto the surface of a fiber tape using pneumatic cascade deposition in accordance with an embodiment of the present invention.

FIG. 4B is a schematic illustration of an exemplary apparatus for depositing binder particles onto the surface of a fiber tape using mechanical deposition in accordance with another embodiment of the present invention.

FIG. 5A is a schematic illustration of a process for sintering binder particles onto the surface of a fiber tape using infrared radiation in accordance with an embodiment of the present invention.

FIG. 5B is a schematic illustration of a process for sintering binder particles onto the surface of a fiber tape using chemically induced sintering in accordance with another embodiment of the present invention.

Detailed Description

The present invention is directed to a dry fiber tape or sheet and a method for producing the dry fiber tape or sheet by applying a powder binder coating to a bundle of fibers having a controlled width. The method may include the steps of arranging fiber tows into a planar form having a predetermined width, such as a flat tape or sheet, coating only the outer fibers of the tow form with binder particles while leaving the interior of fibers of the tow form uncoated, sintering the binder particles onto the tow form, and winding the resulting coated fiber tape or sheet onto a take-up bobbin or roll. The amount of binder applied to the tow form is generally less than 20% by weight, based on the weight of the fiber tow form. The resulting dry coated fiber tape or sheet can be further processed to form complex dry fiber preforms. While the fiber product and method of manufacture is described herein with reference to "fiber tape", it is to be understood that other planar forms of fiber tows and fiber ravings, including sheets, may be produced in accordance with the present invention. The thickness of the planar form may be in the range of about 0.001 in. to about 0.1 in. (0.0254 - 2.54 mm), or about 0.005 in. to about 0.05 in. (0.127 - 1 .27 mm), or about 0.005 in. to about 0.015 in. (0.127 - 0.381 mm).

The present invention is further directed to a dry fiber tape product that has a powder or other form of binder particles applied to exterior of one or both sides of the tape, such that the binder is non-tacky at typical ambient conditions. As used herein, typical ambient conditions means at room temperature (i.e., 20 ± 5°C) in the presence of air and typical atmospheric pressure. With the application of heat, the binder becomes tacky, allowing the tape to be placed by automated or semi- automated equipment to produce a two-dimensional or three-dimensional preform. The dry fiber tape is held in position in this preform by the binder. By subsequent build up of this dry tape, a complex dry fiber preform can be produced.

As used herein, the term "tow" means a large grouping of parallel fiber filaments. A small tow refers to a fiber tow that contains 24,000 or fewer filaments. A large tow refers to a fiber tow that contains on the order of 48,000 to 320,000 filaments or more.

The term "axial" is used in reference to a characteristic in the direction of the fiber orientation. The term "transverse" is used in reference to a characteristic in a direction 90° to the fiber orientation.

As used herein, the term "preform" means an assembly of dry tapes or sheets which has been prepared for a matrix addition process. A preform is typically stabilized in some way to maintain its shape before final processing.

The dry fiber tape produced in accordance with the process of the present invention is distinguishable from a "towpreg". A conventional towpreg is made up of thousands of filaments impregnated with a continuous mass of matrix resin. The amount of resin added to the fiber tow in a typical towpreg is at least 35% by weight, based on the weight of the fiber tow.

By the process of the present invention, the binder does not enter into the fiber tape, but only coats a portion of the outer surface of the fiber tape. The discontinuous binder coating permits the coated fiber tape and the preform

constructed from the coated fiber tape to be subsequently impregnated with a resin to form a structural composite. That is, the binder does not render the fiber tape or preform impenetrable, but allows matrix resin to fill the interstices of the fiber tape in a subsequent operation to produce a fiber reinforced composite structure.

In an exemplary embodiment of the process of the present invention, a dry powder binder is applied to the surface of a controlled width dry fiber tape, and then sintered onto the exterior surface of the tape. In another embodiment, the binder particles are applied to the surface of the dry fiber tape by a liquid spraying

operation. The amount of binder applied to the dry fiber tape is less than 20% by weight, based on the weight of the dry fiber tape (i.e., the uncoated fiber tape). In one embodiment, the amount of binder applied to the dry fiber tape is less than 15% by weight, or less than 10% by weight, based on the weight of the dry fiber tape.

To facilitate the infusion of resin using such processes as resin transfer molding (RTM), resin injection molding (RIM), or other related processes, the binder applied to the dry fiber tape should not impede the infusion of the subsequently applied resin within the preform. The process aspect of the present invention addresses this issue in two ways. Firstly, the binder product is applied to the fiber tape uniformly along its length (i.e., in the axial direction) and across its width (i.e., in the transverse direction). One method for achieving this uniformity is by passing the fiber tape through a "waterfall" or cascade of powder created by either mechanical or pneumatic means. The speed with which the fiber tape is passed through the cascade determines the amount (or weight) of material applied. Secondly, the binder powder is fused (or attached) to the fiber tape in such a manner that fusion is limited to the actual contact between a binder particle and individual filaments of the fiber tape. That is, the binder particles are sintered onto the fiber tape, rather than melted to the fiber tape. This sintering provides sufficient adhesion of the binder particles to the fiber tape to hold the fiber tape, which is made of many individual filaments, together laterally (i.e., maintain its lateral integrity) without melting into or onto the fiber tape and impeding subsequent resin infusion by reducing the

permeability of the preform.

Sintering of the binder particles to the fiber tape can be achieved by several means, depending on the fiber tape and binder involved, including chemical, thermal, and other means. In a particular aspect of the invention utilizing carbon fiber tow, the binder particles may be sintered to the fiber tape using of infrared radiation (IR). Because the carbon fiber tow is opaque to IR, absorption of the IR causes heating of only the exposed carbon filaments, i.e., those at the outer surface of the fiber tape. The binder particles are essentially transparent to the IR. The heated carbon filaments cause the binder particle, at the point of contact with the heated carbon filament(s), to reach the melting point of the binder, thereby effecting sintering of the particle to the filament(s). In another embodiment of the invention, a solvent is used to effect a surface melting of the binder particles, which results in a similar sintering upon evaporation of the solvent under the influence of heat, vacuum, etc.

The dry, bindered fiber product produced in accordance with the method of the present invention includes a binder layer made up of discrete particles, each of which is attached to multiple and overlapping filaments. With this binder layer, a laterally constrained and stable band, sheet, or tape is produced with minimal impedance to the flow of resin in the axial direction, the transverse direction and in the direction of the thickness of the band, sheet or tape during subsequent molding operations. The distributed and discontinuous binder layer has minimal effect on the permeability of preforms constructed from the dry, bindered fiber product.

The binder applied to the surface of the fiber tape may be a thermoplastic resin or a thermosetting resin. Examples of thermoplastic materials include, but are not limited to, thermoplastic polyurethane, polyurea, polyimide, polyetherimide, polyamide, polyamideimide, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polycarbonate, polysulfone, polyethersulfone, polyphenylene sulfide, polyketones such as polyether ketone, polyetherether ketone and polyetherketoneketone, polyolefins such as polypropylene, (meth)acrylates, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, acrylonitrile-styrene-acrylate and combinations thereof (e.g., blends and/or alloys of at least two thereof).

The fiber tape may also be coated with a thermosetting resin. Non-limiting examples of such thermosetting polymers include polyesters, polyimides, epoxies, phenolics and polyurethanes.

The binder layer on the external surface of the tape is formed by partial melting of the binder particles, resulting in the fusing of the binder particles onto the fiber tape. Once the melted binder particles have cooled, the resulting coating is non-tacky. The resulting tape or sheet can then be wound onto a take-up package. By subsequently heating the coated dry fiber tape or sheet, the surface of the fiber tape or sheet may become tacky. In addition to providing the ability to build up the preform by holding the dry fiber tapes in place, the binder also provides stability and desirable handling characteristics for the tape product of sheet.

The fiber tows may be composed of carbon fiber, glass fiber, aramid fiber, ceramic fiber, quartz fiber, or metallic fiber such as aluminum, steel, or stainless steel fiber, or combinations of two of more types of fibers.

Because the binder is non-tacky at room temperature conditions, the tape or sheet can be spooled without sticking to itself. Upon application of heat, the tape or sheet can be made to stick to a preform carrier material and subsequently layers of tape or sheet may be built up to produce the complex preform.

Referring to FIG. 1 , a single binder particle 14 is 'sintered' to a fiber tape 10 consisting of a plurality of small filaments 12. The representative binder particle 14 is shown with a diameter (D) of 50 microns, or about 7 times larger than the diameter (d) of an individual filament (7 microns). As shown, the filaments 12 in direct contact with the particle 14 have become partially embedded into the melted surface of the particle 14 in the local melt zone 18. In this particular example, the filaments 12 have absorbed infrared (IR) radiation, which freely passes through the IR transparent binder 14. This absorbed energy is converted into heat, thereby raising the temperature of the filament 12. The IR absorption is controlled by the intensity of the IR radiation and the time of exposure to that radiation so as to increase the filament temperature to a point above the melting point of the binder. As the binder particle 14 melts in the immediate vicinity of the filament contact (i.e., the melt zone 18), energy (heat) is removed from the filament 12 until the filament 12 and the adjacent melt zone 18 cool below the melt temperature. At this point, the binder particle 14 re-solidifies, creating an attachment to the filament 12. As the individual binder particles 14 are significantly larger than the filaments 12, each binder particle 14 attaches to a number of filaments 12 in this "sintering" process.

Referring to FIG. 2A, a segment of fiber tape 10, consisting of a plurality of individual filaments 12, includes a random distribution of binder particles 14 sintered thereto. Each binder particle 14 remains substantially separate from other binder particles such that spaces exist between particles 14. This discontinuous dispersion of binder particles 14 ensures that the bindered fiber product is permeable to infused resin in all directions, particularly in the direction normal to the plane of the fiber tape. As each binder particle 14 is attached to multiple filaments 12, the random placement of binder particles 14 across the width of the fiber tape 10 or band ensures that or most or all of the filaments 12 are secured to a sufficient number of other filaments to fix the width of the fiber tape 10 in place.

Referring to FIG. 2B, the view across the segment of fiber tape 10 of FIG. 2A shows that randomly distributed particles 14 overlap each other along the fiber length and bind the filaments 12 together across the fiber tape width W into fixed positions relative to each other, resulting in a stable width band.

Referring to FIG. 3, an exemplary process 30 for manufacturing a dry bindered fiber tape or sheet is illustrated. The process 30 may include the following steps or operations. In the creel operation 32, single or multiple tows, ravings, or otherwise denoted bundles of filamentary material are drawn from a fiber supply station. A desired level of fiber tension to the material supplied and may be used to arrange multiple fibers into a desired position. In the tow spreading and bandwidth control operation 34, the fiber tow can be spread to form a tape, band or sheet having the desired width, as may be required for specific applications for the fiber product. In the binder deposition operation 36, the desired quantity of powdered binder is deposited on the upper surface of the moving fiber tape. The method of deposition of the binder can occur through a variety of different means. Exemplary configurations based on pneumatic and mechanical means are described herein with reference to FIGS. 4A and 4B. In the binder content verification and control operation 38, verification that desired quantity of binder has been deposited atop the fiber tape is performed. In addition, feedback to other parts of the process, as necessary, is provided. In the sintering operation 40, the binder particles are affixed or sintered to the fiber tape using either heat (in the form of IR radiation) or chemical means to attach binder particles to the individual filaments of the fiber tape at separate points of contact. When heat is used in the sintering operation 40, latent heat must be removed from the product before packaging. In the cooling operation 42, the desired degree of cooling is provided by air exposure, either ambient exposure or forced (blown) air. In the spooler and packaging operation 44, a spooling capability for narrow fiber products or a means to package the wider product forms into rolls is provided, depending on the width of the produced fiber product.

Referring to FIG. 4A, an apparatus 50 for depositing binder particles 14 onto a moving fiber tape 10 in accordance with binder deposition operation 36 is illustrated. Apparatus 50 includes pneumatic means for depositing the binder particles 14 by forming a "cascade" 58, or waterfall, of binder particles. To create the cascade 58, a controlled amount of binder 14 is fed from a binder hopper 52 through a feed control valve 54 into a flowing air stream 56. This air stream 56 is contained within a plenum 60 of desired width and height so as to entrain the binder particles 14 within the air stream 56, thereby conveying the binder particles along the plenum 60 in a substantially uniform distribution due to the velocity of the air stream. The plenum 60 then feeds into the deposition chamber 62, where the rapid increase in available volume results in a rapid decrease in air velocity. The binder particles 14, being heavier than air, can no longer be carried by the now slower air and fall out of the airstream in a somewhat random cascade 58 onto the moving fiber tape 10 traveling in the direction of arrow 68. The air stream 56 exits the deposition chamber 62 through a filtered vent 64. Excess binder particles 66 accumulate in the deposition chamber 62 and may be recirculated to the binder hopper 52.

Referring to FIG. 4B, an apparatus 70 for mechanically depositing controlled amounts of binder particles 14 atop a moving fiber tape 10 includes a supply hopper 70 for containing powdered binder 14 and a rotary wheel 74 or drum. The direction of movement of the fiber tape is illustrated by arrow 78. The binder particles 14 are fed to rotary wheel 74 having a plurality of vanes 76 arranged on the outer circumference of the wheel 74. The height, stiffness, and circumferential spacing of the vanes 76, as well as the rotating speed of the wheel 74 determine the amount of binder 14 passed from the hopper 70 and dropped onto the moving fiber tape 10. In the illustrated embodiment, the vanes 76 are configured to contact the inner surface of the hopper 72 to only allow the binder material entrained between vanes to pass and be dropped onto the fiber tape 10. Other suitable apparatus and methods of binder deposition can be adapted to suit the present invention, provided such apparatus and methods have sufficient robustness, accuracy, and controllability of the binder deposition.

After the binder particles are deposited onto the fiber tape, the binder particles are fused or "sintered" to the fiber tape. In one embodiment illustrated in FIG. 5A, the individual binder particles 14 are affixed, or sintered, onto the fiber tape 10 by heating the individual filaments 12 on the surface of the fiber tape 10 through the application of infrared (IR) radiation from an IR source 80. In this embodiment, the fiber tape 10 with the binder particles 14 deposited thereon passes under an elongated IR source 80. The intensity of the IR radiation may be controlled by separation distance from the fiber tape or by direct electronic means. The IR radiation passes through the IR-transparent binder particles 14 and is absorbed by the surface filaments 12 of a carbon fiber tape 10, creating heat. When the individual filaments 12 reach the melting temperature of the binder particle 14, the heated filaments 12 cause localized melting of the binder particle 14 at the point of contact. The action of melting the binder particle 14 removes heat from the filaments 12, eventually lowering the temperature at the contact point to below the binder melting point. This allows the binder particle 14 to re-solidify at the point of contact, thereby affixing the binder particle 14 to several individual filaments 12. In this embodiment, the IR intensity is controlled to allow for different binder particle content levels and processing speeds.

In another embodiment of the invention, a chemically-based sintering operation is used with fibers that are not receptive to IR heating. Non-limiting examples of such fibers include glass and aramid fibers. Referring to FIG. 5B, in a chemically-based sintering operation 82, the fiber tape 10, onto which has been deposited the desired amount of binder particles 14, is passed through an atomized spray 82 of a volatile solvent. The solvent chosen is suitable for the particular binder 14 used. A fine solvent spray 82 coats the individual binder particles 14, and also wets the surface filaments 12. The solvent dissolves the surface of the binder particle 14, allowing the contacts between the binder particle 14 and the several filaments 12 to become more intimate. Upon removal of the solvent by evaporation in an evaporation operation 43, the dissolved binder particle 14 re-solidifies, which affixes, or chemically sinters the individual particle 14 to numerous filaments 12. The evaporated solvent may be recovered and reused.

Example:

A dry coated carbon fiber tape product is produced using a 50,000 filament Zoltek™ carbon fiber tow that has been spread to a consistent width, and a dry powder epoxy binder applied and sintered onto the upper surface of the tape.

While the invention has been explained in relation to various embodiments, it is to be understood that various modifications thereof will be apparent to those skilled in the art upon reading the specification. The features of the various embodiments of the articles described herein may be combined within an article. Therefore, it is to be understood that the invention described herein is intended to cover such modifications as fall within the scope of the appended claims.