Abstract:
An acceleration cell for use in coating substrates with plastic resin particles. The cell includes a housing that has an air inlet port, an air outlet port, and a particle feed port, the latter in association with a resin particle source. The housing receives a carrier airflow for taking up resin particles so that the particles are suspended in the carrier flow. The air outlet port has a configuration having a predetermined width, which generally corresponds to the width of the substrate. The cell also contains at least one electrostatic charger for charging the suspended resin particles and at least one apparatus for accelerating the carrier flow and the suspended particles. Finally, the cell includes at least one flow-modifying apparatus for modifying the resin particle outflow, producing a uniform delivery of the particles across the substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus, system and method for electrostatically coating substrates with resins for use as prepregs in producing composite materials.  
         BACKGROUND OF THE INVENTION  
         [0002]    A prepreg is a substrate pre-impregnated with a matrix resin that binds together the fibers of the substrate. Prepregs are precursor materials that can be used to make finished composite components for inclusion in a wide range of applications, such as airplane structures, medical products, printed circuit boards, industrial components, recreational products and commercial vehicles. In general, composites have advantages over competing materials such as metals. Among other attributes, prepregs generally have higher specific strength, better corrosion resistance, and allow for faster assembly.  
           [0003]    The use of composite components made from advanced thermoplastic prepregs is relatively recent. Composites are available in a wide range of substrates and thermoplastic resins. The substrate is often a carbon, glass or aramide substrate, while typical resins include polyethylene (PE), polypropylene (PP), polyetheretherketone (PEEK), polyethersulfone (PES), polyphenylsulfone (PPS), polyimide (PI), polyamides (PA), polycarbonate (PC), polyethylene terephthalate (PET), polyurethane (PU), polyester and fluoropolymers. Thermoplastic prepreg fabrics typically have inherent toughness, good viscoelastic damping, indefinite shelf life, chemical resistance, assembly flexibility and recycling capabilities.  
           [0004]    Thermoplastic prepregs can be prepared using solvent impregnation, hot melt coating, film stacking, as well as other methods. However, chemical resistance of the resin often makes solvent impregnation difficult. Hot melt coating, a process similar to pultrusion, requires resins with moderate to high viscosity and melt temperatures. In addition, it often requires high-pressure pumps and resin meters.  
           [0005]    Film stacking uses thin films of dry thermoplastic resins that are sandwiched or stacked together with the fabric. After sandwiching, the stack is consolidated under heat and pressure. While this method is clean and solvent-free, consolidation must be carefully carried out to fully impregnate the fabric. The cost of these thin film resins is often relatively high, especially when resins like PEEK and PPS are employed.  
           [0006]    Dry powder deposition methods, primarily the electrostatic fluidized bed (EFB) method, are at least 30 years old. Their use obviates the processing difficulties of wet systems (wetting, flow, and homogeneity). In the EFB method, powdered resin particles are aerated in a fluidized chamber and are electrostatically charged by ionized air forced through a porous plate at the base of the chamber. As the powder particles are charged, they repel each other to such a degree that they rise above the chamber forming a low-velocity, essentially uniform cloud of charged particles.  
           [0007]    When a substrate is passed over or conveyed through this cloud, the charged powder particles are attached to it because of the potential difference between the particles and substrate. As the particles become attached to the substrate, the particles form a coating whose thickness and deposition rates are controlled both by the magnitude of the applied voltage in the air ionization process and by the exposure time of the substrate to the cloud. Because of the large potential difference between the charging media and most substrates, even natural insulators can be coated. Once coated with particles, the substrate is transported through an oven where the powder melts, flowing over the substrate.  
           [0008]    Reference is now made to FIG. 1 where a schematic illustration of a typical prior art EFB coating apparatus  110  is presented. It is composed of a dry air input  12  through which dry air enters into an air plenum chamber  14 . The latter is situated under a charging medium (plate)  16  that is connected to a high-voltage DC power supply  18 . The incoming dry air is blown past charging medium  16  and through porous plate  20  on which powdered resin is placed. The charged air transfers charge to the powdered resin and forms a low-velocity cloud of charged particles  22  that attaches itself to a grounded substrate  24 .  
           [0009]    While FIG. 1 shows an object being electrostatically coated, it is readily apparent to one skilled in the art that fabric, tow, tube, tape or fiber substrates can also be coated when such substrates are drawn between two fluidized beds disposed symmetrically on either side of the substrate. FIG. 1 does not show the heating apparatus that melts the polymer resin particles electrostatically attached to the substrate. Typical substrates that can be coated by such an apparatus are fiberglass, carbon fibers and aramide materials.  
           [0010]    There are drawbacks to the EFB method. Difficulties exist because the porous plate in fluidized bed coating systems often becomes blocked, resulting in a non-uniform distribution of the charged powder across the coated substrate. In addition, the holes in EFB porous plates can never be fabricated with sufficient uniformity to ensure homogeneity of the coating. Moreover, low-velocity particles generally coat only the surface of a substrate and cannot penetrate into the spaces or interstices of the substrate. Prepregs produced by this method have relatively high resin coating loads. As a result, when such coated fabrics are used to form composites, the composite layers do not adhere to each other uniformly and the composites are generally of low quality.  
         Definitions  
         [0011]    Except where noted otherwise, in what is discussed herein, the following terms will be used with the following meanings:  
           [0012]    Substrate—fabric, often a web-type fabric, fiber, strand or tow material. In certain instances, the word “fabric” may be used to indicate any type of substrate.  
           [0013]    Tow—a bundle of untwisted continuous filaments.  
           [0014]    Strand—twisted continuous filaments.  
           [0015]    Prepreg—a substrate pre-impregnated with a matrix resin, the resin acting to bind together the fibers of the substrate.  
           [0016]    Composite—two or more layers of prepregs to which heat and pressure have been applied, thereby causing the matrix resin in the several prepreg layers to fuse and form an integral object.  
           [0017]    Resin load—the mass of resin deposited per unit area or per unit mass of substrate.  
         SUMMARY OF THE PRESENT INVENTION  
         [0018]    Applicant has realized that an apparatus, herein called an “acceleration cell,” emitting charged resin powder at high velocity (“forced flow”), that does not include a porous plate and has a wide aperture, solves many of the problems found in the prior art. Applicant has determined that such a cell produces a uniform coating with lower resin loads, as well as increased resin powder penetration of the substrate. The cell can employ either frictional or high-voltage direct current (DC) power source methods to charge the resin powder. Alternatively, a single acceleration cell can use both methods simultaneously. Systems using a plurality of such cells can employ both power source charging and friction-charging concurrently. A coating method using such cells is described.  
           [0019]    It is an object of the present invention to provide an apparatus, system and method for preparing uniformly coated prepreg substrates to be used in producing composites.  
           [0020]    It is yet a further object of the invention to prepare prepregs with the coating penetrating more deeply into the substrate.  
           [0021]    It is yet another object of the invention to form prepregs with resin loads smaller than those in prepregs prepared by other dry methods, particularly the electrostatic fluidized bed method.  
           [0022]    It is yet another object of the invention to provide large-area coated substrates having uniform coatings, smaller resin loads and deeper coating penetration.  
           [0023]    It is a further object of the present invention to more readily use micron-size resin particles in fabricating prepregs.  
           [0024]    Other objects of the present invention will become apparent from the following embodiments of the present invention.  
           [0025]    There is thus provided in accordance with the present invention an acceleration cell for coating a substrate with plastic resin particles which includes a housing having first and second ends, the first end containing an air inlet port and the second end an air outlet port. The housing further includes a particle feed port, which is formed in a wall of the housing between the inlet and outlet ports. The feed port is connected to a plastic resin particle source. The housing receives a carrier flow of air from the inlet port, which exits through the outlet port. The carrier flow takes up the resin particles delivered via the particle feed port, so that there is an outflow of the resin particles suspended in the carrier flow. The outlet port has a generally wide configuration with a width that is predetermined so as to correspond to the width of a substrate being coated. This allows the suspended resin particle outflow to deliver the resin particles across the entire width of the substrate. The acceleration cell also contains at least one electrostatic charger positioned in the housing which charges the particles suspended in the carrier flow. In addition, associated with the housing is at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow through the housing. Additionally, the cell includes at least one flow-modifying apparatus disposed within the housing for modifying the suspended resin particle outflow so as to cause a uniform spatial distribution of the resin particles exiting from the cell, thereby producing a uniform spatial delivery of particles across the substrate.  
           [0026]    In accordance with one embodiment of the present invention, the at least one flow-modifying apparatus is a turbulence-producing means. In some embodiments the turbulence-producing means is a plurality of deflectors; in other embodiments, the turbulence-producing means is a plurality of baffle-like elements producing sufficient turbulence to ensure the desired degree of uniformity in the spatial distribution of the exiting particles.  
           [0027]    In further embodiments, the at least one flow-modifying apparatus is a plurality of airflow vanes. In some embodiments the length of these vanes is about 3 to 7 times the distance between adjacent vanes, while in other embodiments their length is about 4 to 6 times the distance between nearest neighbors.  
           [0028]    In yet another embodiment, the length to height ratio (L/H) of the housing is between about 1 to about 10, where length L is the distance between the side of the at least one flow-modifying apparatus distal to the proximate side of a nozzle region of the housing, and the proximate side of the nozzle region. The height H is the distance between opposite surfaces of the housing in the region defining length L; the height H is taken along a direction generally parallel to the shorter side of the air outlet port. In another embodiment, the length to height (L/H) ratio is between 3 to 5.  
           [0029]    Additionally, in another embodiment of the invention the at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow is at least one sloped wall of the housing, the sloped wall narrowing the housing in the direction of the air outlet port. In some embodiments of the invention, the sloped wall of the housing has a slope that can range up to about 40 degrees, while in other embodiments the slope can range up to 15 degrees.  
           [0030]    In a further embodiment of the invention, the slope of the at least one sloped wall is discontinuous as the wall proceeds in the direction of the air outlet port.  
           [0031]    In yet another embodiment, the at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow is a Venturi constriction, the Venturi constriction producing a pressure differential between the area in, and adjacent to, the constriction and the plastic resin particle source, thereby bringing the resin particles into the housing through the particle feed port.  
           [0032]    Additionally, in an embodiment of the present invention, the at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow is at least one electrically charged surface having a charge opposite to the charged particles.  
           [0033]    In still another embodiment, the at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow further includes a means for generating a magnetic field, the field increasing the uniformity of the spatial distribution of the particles exiting from the air outlet port.  
           [0034]    In other embodiments the at least one apparatus for accelerating the carrier flow and charged particles suspended in the flow is a blower.  
           [0035]    In a further embodiment of the invention, the air outlet port is a rectangular slot aperture characterized by at least one of the following: an aspect ratio ranging from about 1 to about 3000, and a length of at least 2 mm. In another embodiment of the invention, the air outlet port is a rectangular slot aperture characterized by at least one of the following: an aspect ratio ranging from about 1 to about 200, and a length of at least 50 mm.  
           [0036]    In still another embodiment of the invention, the air outlet port is a conic section shaped aperture, where the aperture is characterized by at least one of the following: a major to minor axis ratio ranging from about 1 to about 3000, and a major axis of at least 2 mm. In another embodiment of the invention, the air outlet port is a conic section shaped aperture, where the aperture is characterized by at least one of the following: a major to minor axis ratio ranging from about 1 to about 200, and a major axis of at least 50 mm.  
           [0037]    In yet another embodiment of the invention, the at least one electrostatic charger includes a high-voltage power source that applies voltage to at least one chargeable surface, the chargeable surface providing charge to the carrier flow of air in the housing, the charge then being transferred to the resin particles. In an embodiment of the invention, the at least one chargeable surface is at least one brush. In yet another embodiment of the invention, the at least one charger is at least one friction-charging surface.  
           [0038]    Additionally, in another embodiment of the invention, at least one friction-charging surface includes at least one surface selected from the following list of surfaces: at least one planar surface, at least one undulating surface, at least one roughened surface, and at least one smooth surface.  
           [0039]    In a further embodiment of the invention, the cell includes both at least one friction-charging surface and at least one high-voltage power source that applies voltage to at least one chargeable surface, the chargeable surface providing charge to the carrier flow of air in the housing, which is then transferred to the resin particles. In some embodiments these components can be used in series and in others in parallel.  
           [0040]    Additionally, in an embodiment of the invention, the second end of the housing is a detachable sleeve with the sleeve being replaceable with another sleeve having an air outlet port of a different size. In other embodiments, the second end of the housing is a sleeve with an air outlet port, the size of the outlet port being variable.  
           [0041]    In an embodiment of the invention, the cell further includes a humidity controller.  
           [0042]    Additionally, in yet another embodiment of the invention, the average velocity of the particles is at least 0.1 m/s as they exit the air outlet port of the cell, while in still another embodiment, the average velocity of the particles is at least 0.5 m/s as they exit the air outlet port of the cell.  
           [0043]    Additionally, there is provided in accordance with the present invention a system for coating a substrate with plastic resin particles, the system including a coating chamber and at least one acceleration cell constructed according to any one of the previous embodiments. The at least one cell jets charged resin particles at high velocities into the coating chamber through an air outlet port of the acceleration cell. The system also includes a substrate positioned in the coating chamber on which the jetted high-velocity charged resin particles are deposited. In addition, the system contains a heat source for melting the resin particles deposited on the substrate, whereby the melted resin coats the substrate.  
           [0044]    In an embodiment of the present invention, the substrate positioned in the chamber is moving.  
           [0045]    In a further embodiment of the present invention, the average velocity of the jetted particles as they exit the air outlet ports is at least 0.1 m/s. In other embodiments, the velocity is at least 0.5 m/s.  
           [0046]    Further, in accordance with another embodiment of the present invention, the at least one acceleration cell is at least two acceleration cells. In some embodiments, at least one of the at least two acceleration cells charges the particles by friction and at least one of the at least two acceleration cells charges the resin particles by using a high-voltage power source.  
           [0047]    Additionally, in another embodiment of the present invention, the at least one acceleration cell charges the resin particles by friction.  
           [0048]    In another embodiment of the present invention, the at least one acceleration cell charges the resin particles by using at least one high-voltage power source.  
           [0049]    Further, in an embodiment of the present invention, the at least one acceleration cell includes both friction-charging components and high-voltage power source charging components, and the cell charges the resin particles by at least one of these methods. Additionally, in an embodiment of the invention, the frictional and high-voltage components are used in series, while in another embodiment they are used in parallel.  
           [0050]    In still another embodiment of the present invention, the substrate is charged so as to attract the jetted charged particles entering the coating chamber from the at least one acceleration cell, thereby further accelerating the particles. In some embodiments, the substrate is charged by moving it past at least one contacting plastic body, while in others it is charged by a power source.  
           [0051]    In a further embodiment of the present invention, the coating chamber further includes at least one charged element positioned substantially opposite the air outlet port of the at least one acceleration cell so as to attract and accelerate the jetted charged particles emitted from the acceleration cell.  
           [0052]    In still another embodiment of the present invention, the system further includes a computerized control system for control of the active elements which regulate at least one of the following parameters: charging voltage, speed of conveyance of the substrate, speed of the carrier flow in the acceleration cells, size of the air outlet port, quantity of resin particles brought into the cell, output voltage and output current. The control system is in communication with sensors in the system, the sensors sensing the values of at least one of the above parameters. Based on the sensed values, the computer adjusts the values of the parameters by communicating the optimizing values to the active elements.  
           [0053]    In yet another embodiment, the system further includes a humidity controller.  
           [0054]    In a further embodiment, the orientation of the at least one acceleration cell is such that the particles emitted from the air outlet port of the cell impinge the substrate substantially perpendicularly.  
           [0055]    In still another embodiment of the present invention, the orientation of the at least one acceleration cell is such that the particles emitted from the air outlet port of the cell impinge the substrate at a generally non-perpendicular angle.  
           [0056]    Further, in accordance with the present invention, a plane containing the air outlet port of the acceleration cell makes an angle of between about 60 and about −60 degrees with respect to the normal to a plane of the substrate, the plane of the substrate being the plane being coated.  
           [0057]    Additionally, there is provided in accordance with the present invention a method for coating a large-area substrate where the method includes the steps of:  
           [0058]    positioning the substrate in a coating chamber;  
           [0059]    accelerating charged resin particles through an air outlet port of at least one acceleration cell, the acceleration cell being constructed as described above, the particles impinging and depositing on a wide swath of the substrate, the particles moving with a velocity of at least 0.1 m/s as they exit the air outlet port; and  
           [0060]    melting the deposited resin particles, thereby coating the substrate.  
           [0061]    In another embodiment of the invention, the positioning step of the method includes moving the substrate through the chamber.  
           [0062]    Further, in accordance with another embodiment of the present invention, during the accelerating step of the method, the particles coat continuous wide swaths of a continuously moving substrate.  
           [0063]    In still another embodiment of the invention, the method also includes the step of attracting the charged particles toward the substrate.  
           [0064]    In yet another embodiment of the method of the present invention, the method includes a second accelerating step where the first accelerating step accelerates particles having diameters equal to or less than a predetermined diameter, while the second accelerating step accelerates particles having diameters greater than the predetermined diameter. In some embodiments, this predetermined diameter is 5 microns.  
           [0065]    In a further embodiment of the invention, the positioning step of the method includes positioning a web-like substrate that is moving through the coating chamber.  
           [0066]    In another embodiment of the invention, the particles exit the air outlet port with a velocity of at least 0.5 m/s. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0067]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
         [0068]    [0068]FIG. 1 is a schematic cross-sectional view of a typical prior art fluidized bed coating apparatus;  
         [0069]    [0069]FIG. 2 is a schematic side view illustration of a coating line incorporating a coating apparatus and system constructed in accordance with a preferred embodiment of the present invention;  
         [0070]    [0070]FIG. 3 is a side view illustration of a coating chamber constructed and operative according to an embodiment of the present invention;  
         [0071]    [0071]FIGS. 4A and 4B are isometric views of a high-voltage charging acceleration cell and coating chamber constructed in accordance with a preferred embodiment of the present invention;  
         [0072]    [0072]FIGS. 5A and 5B are schematic side and top views, respectively, of an acceleration cell using high-voltage to charge resin powder, constructed in accordance with a preferred embodiment of the present invention;  
         [0073]    [0073]FIGS. 6A and 6B are schematic side and top views, respectively, of an acceleration cell using friction to charge resin powder, constructed according in accordance with a preferred embodiment of the present invention;  
         [0074]    [0074]FIGS. 7A-7C are respectively top-side, top and side schematic views of a nozzle suitable for use in acceleration cells constructed according to embodiments of the present invention;  
         [0075]    [0075]FIG. 8 is a schematic cut-away, top-side view of a portion of an acceleration cell constructed in accordance with a preferred embodiment of the present invention; and  
         [0076]    [0076]FIGS. 9A, 9B and  9 C are top-side and top views, respectively, of turbulence-producing elements for use with the embodiment shown in FIG. 8.  
         [0077]    Similar elements in the Figures are numbered with similar reference numerals. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0078]    Prepregs currently used to form composite materials are often characterized by very non-uniform plastic resin coatings, high resin loads and little penetration of the substrate by the resin. Applicant has realized that the use of high-velocity (“forced flow”) charged resin particles ejected from an acceleration cell that electrostatically charges such particles can obviate these problems. Applicant has developed a cell for coating wide area substrates where the cell has a uniformly charged resin particle discharge stream. The particles constituting the discharge stream are traveling at relatively high velocities compared to prior art dry coating systems. Uniformity and velocity are maintained by means which include, but are not limited to, blowers, Venturi constrictions, turbulence-producing baffles, air control vanes, and a decreasing internal cross-sectional area of the cell in the direction of the cell&#39;s wide aperture. The acceleration cells discussed hereinbelow can employ, separately or concurrently, either high-voltage power source electrical charging or friction-charging methods. The acceleration cells can be used in coating systems described herein; a method for using these cells and systems for coating large-area, continuously moving substrates, is also described. The system is particularly useful for use with small micron-size resin particles, the fabrication of which has recently been improved, and for which increased future usage is expected.  
         [0079]    Reference is now made to FIG. 2 in which is illustrated a schematic view of a typical coating line, referenced generally  210 , incorporating a coating apparatus and system constructed and operative in accordance with a preferred embodiment of the present invention. A substrate referenced  38  is led from a pay-off roller  32  to a take-up roller  34 . Optionally, the substrate can be passed through a wetting station  30 , which moistens substrate  38 , improving the subsequent attachment of charged powder to substrate  38 . Wet station  30  will most beneficially be used when substrate  38  is an aramide or glass substrate. The substrate is then passed through a coating chamber  36  and a heating means  28 . Substrate  38 , typically a carbon, glass, or aramide substrate such as Kevlar®, is guided along line  210  by a plurality of control rollers  40 , some of which are nip rollers  40 A. Nip rollers  40 A also assist in controlling the speed of substrate  38  as it traverses coating line  210 .  
         [0080]    Two electrostatic acceleration cells  12 A and  12 B, having wide apertures  46 A and  46 B respectively, are positioned substantially opposite each other in coating chamber  36 . Acceleration cells  12 A and  12 B charge resin powder particles brought into the cell as described below. While not readily seen in FIG. 2, acceleration cells  12 A and  12 B protrude into chamber  36 ; this can be better seen in FIGS. 3, 4A and  4 B discussed hereinbelow. The charged powder exiting from acceleration cells  12 A and  12 B at apertures  46 A and  46 B, enters coating chamber  36 , impinges on moving substrate  38  at high velocities, and adheres electrostatically to substrate  38 .  
         [0081]    In FIG. 2, apertures  46 A and  46 B of acceleration cells  12 A and  12 B are shown to be substantially co-linear with each other and perpendicular to the path of the substrate. In other embodiments, while the main portion of each of acceleration cells  12 A and  12 B may be independently oriented perpendicularly to the path of the substrate, nozzles  23 A and  23 B of cells  12 A and  12 B can be angularly displaced with respect thereto. Preferably, however, nozzles  23 A and  23 B are oriented so as to project particles perpendicularly to the path of the substrate.  
         [0082]    In FIG. 2, wide aperture acceleration cells  12 A and  12 B use high-voltage supplied by DC power supplies  14 A and  14 B to charge a preselected resin powder stored at powder storage boxes  24 A and  24 B. Powdered resin  25 A and  25 B is brought into cells  12 A and  12 B through powder tubes  50 A and  50 B from powder boxes  24 A and  24 B at Venturi constrictions  22 A and  22 B formed in respective cells  12 A and  12 B. As air is accelerated in cells  12 A and  12 B, as by use of a pair of air blowers  18 A and  18 B, past Venturi constrictions  22 A and  22 B, a drop in pressure is produced at constrictions  22 A and  22 B. This decrease in pressure causes a pressure differential to exist between constrictions  22 A and  22 B and the interior of powder boxes  24 A and  24 B, thereby drawing powder up into cells  12 A and  12 B.  
         [0083]    Blowers  18 A and  18 B blow dry air into cells  12 A and  12 B via inlets, respectively referenced  15 A and  15 B, past brushes, respectively referenced  16 A and  16 B, mounted within cells  12 A and  12 B, as shown. Brushes  16 A and  16 B, typically made of brass or iron, are connected to high-voltage DC power supplies  14 A and  14 B. Brushes  16 A and  16 B facilitate the charging of the moving air, which in turn transfers charge to the powdered resin. The charged air and resin particles are accelerated toward coating chamber  36  as they pass through Venturi constrictions  22 A and  22 B. Between Venturi constrictions  22 A and  22 B and apertures  46 A and  46 B, at least part of the charged air transfers charge to the powdered resin. While the air-moving means driving air through inlets  15 A and  15 B have been exemplified as air blowers, other suitable means could also be used, in accordance with alternative embodiments of the present invention.  
         [0084]    Coating chamber  36  is typically a plastic cylindrical chamber, into which acceleration cells  12 A and  12 B protrude, and has formed therewith a powder basin  58  into which unattached resin powder falls. The powder collected in powder basin  58  may then be returned via intermediate powder storage boxes (not shown) and a filtration device (also not shown) to powder boxes  24 A and  24 B from which it is again drawn into acceleration cells  12 A and  12 B.  
         [0085]    Coating chamber  36  is also formed with ports  39 A and  39 B through which substrate  38  enters and exits coating chamber  36 . Near exit port  39 B there is a vacuum port  56  connected to vacuum powder collector  26  that collects the loose, excess powder in chamber  36 . The vacuum can be used to fine tune the resin load on substrate  38 , as by thinning out the resin particle layer on substrate  38  by removing poorly attached resin powder from substrate  38  as substrate  38  exits chamber  36 .  
         [0086]    Substrate  38 , covered with electrostatically attached powdered resin, then advances to heating means  28  where the resin is melted, allowing the resin to flow over substrate  38 . Typically, but without being limiting, heating means  28  can be any of the large number of commercially available hot air or IR ovens. Substrate  38  is then led to take-up roller  34  via a pair of nip rollers  40 A.  
         [0087]    Acceleration cells  12 A and  12 B employ high-voltage DC power supplies  14 A and  14 B to charge the resin particles. The cells and their operation are described in more detail in conjunction with FIGS. 5A and 5B below. In other embodiments, acceleration cells employing friction-charging means can be used to charge the resin powder. Such cells are similar to the ones described above and are described in more detail in conjunction with FIGS. 6A and 6B below.  
         [0088]    Referring now to FIGS. 3, 4A and  4 B, there is seen a coating apparatus  310 , constructed in accordance with a preferred embodiment of the present invention. The illustrated components are similar to those shown and described above in conjunction with FIG. 2. Similar components are therefore referenced by similar numerals, and are not specifically described again except as may be necessary to gain a further understanding of the present embodiment. Acceleration cell  12 A uses a high-voltage DC power source (not shown) to charge resin powder. The cell has a wide aperture  46  through which powder is projected into coating chamber  36 . While shown in FIGS. 3 and 4A, second acceleration cell  12 B is truncated and not presented in a cut-away view. Powder basin  58  catches powder that enters chamber  36  but which fails to attach to the substrate. A vacuum apparatus (not shown) removes all resin powder that does not adhere tightly to substrate  38  and that is found loose within chamber  36  through vacuum port  56 .  
         [0089]    Referring now to FIGS. 5A and 5B, there is shown, in schematic form, the acceleration cell  12  as shown and described above in conjunction with the embodiment of FIGS. 2-4B, in accordance with a preferred embodiment of the invention. Acceleration cell  12  charges dry air using a high-voltage DC power source (not shown). Cell  12  includes brushes  16  attached to leads  57 ; the brushes increase the efficiency of charging the air as it is forcibly blown through cell  12  by a blower (not shown). The dry ionized air blown through cell  12  flows through Venturi constriction  22  where it is accelerated toward aperture  46 .  
         [0090]    Powder is introduced into cell  12 , substantially as described above in conjunction with FIGS. 2, 3,  4 A and  4 B, from a powder box  24  (FIGS. 2 and 3) through powder tube  50  (FIGS. 2 and 3) via powder feed ports, referenced  52 , formed proximate to Venturi constriction  22 . Powder feed ports  52  are most clearly seen in FIG. 5B.  
         [0091]    After entering cell  12 , the resin powder acquires electrostatic charge from the ionized air, the latter also serving as a carrier medium for the charged powdered resin. A series of airflow control vanes  54 , most clearly seen in FIG. 5B, is located in the forward part of cell  12  that lies between the Venturi constriction  22  and aperture  46 . Typically, but not necessarily, the vanes are positioned in the nozzle portion  23  of cell  12 . Vanes  54  are important to assure a uniform discharge stream of particles as the particles exit cell  12  and enter coating chamber  36 . In order to improve uniformity, the length of the vanes is typically 3 to 7 times the distance between adjacent vanes, preferably 4 to 6 times the distance between nearest neighbor vanes.  
         [0092]    Another embodiment of an acceleration cell includes several smaller vanes (not shown) formed between vanes  54 , shown in FIG. 5B, in a region close to aperture  46 . In yet other embodiments, vanes  54  extend from nozzle portion  23  in the direction of Venturi constriction  22 , reaching mixing region  27  discussed below.  
         [0093]    As seen in FIGS. 5A and 5B, a plurality of deflectors  53  is formed on a base portion  51 , thereby to define within cell  12  a mixing region, referenced generally  27 . The provision of the deflectors  53  gives rise to turbulent flow, thereby to improve the uniformity of the spatial distribution of the particles. These deflectors are shown and described in greater detail below with reference to FIGS. 8, 9A and  9 B. While deflectors have been described as the turbulence-producing means above, any baffle-like elements, or other turbulence-generating means disposed in any manner could also be used, provided the desired degree of uniformity is attained. More generally, any means can be used that produces a uniform distribution of particles in the discharge stream exiting from the cell through its wide aperture.  
         [0094]    Acceleration cells  12 , as depicted in FIGS. 5A and 5B, show the interior walls of a stabilization region  29  to be formed as a first sloped portion S 3 , and a second, more sharply sloped portion S 2  contiguous therewith, formed within nozzle  23 , proximate to aperture  46 . These sloped portions are described hereinbelow with reference to FIGS. 7A-7C,  8 ,  9 A and  9 B.  
         [0095]    As described above, Venturi constriction  22  is provided so as to generate a pressure reduction in the region of the constriction that allows for the introduction of resin powder into acceleration cell  12 , accelerating the powder therein. It will be appreciated that the Venturi constriction  22  can be located at any position along the length of acceleration cell  12  between brushes  16  and mixing region  27 . Furthermore, it will be readily apparent to one skilled in the art that other methods for introducing the resin into the cell are also possible. Examples of such other methods include the placement of powder in a powder box above acceleration cell  12 , the powder box being shaken so as to cause a gravity feed into the cell. Additionally, any vacuum-producing device attached to the powder box could be used to draw powder into the cell.  
         [0096]    Since ensuring coating uniformity is critical, acceleration cell  12  of FIGS. 5A and 5B is typically constructed so that the length (L) of the cell from the beginning of the mixing region to the beginning of the nozzle region is 1-10 times, and preferably 3-5 times, the height (H) of the cell. For purposes of this ratio, the height of the cell is defined as the distance along the y-axis as shown in FIGS. 5A and 5B in the region defined by L above. Similarly, uniformity typically requires an aspect ratio of wide aperture  46  of 1-3000, and preferably 1-200. The aspect ratio is herein defined as the ratio of the aperture&#39;s longer dimension to its shorter dimension e.g. length to width or major to minor axes. Typically, the aperture&#39;s longest dimension, its length, can range from at least 2 mm, preferably from at least 50 mm, to 1.8 meters, or even more.  
         [0097]    While slot-like apertures, i.e. rectangular apertures, are generally used and have been described in the embodiments above, elliptical apertures of suitable dimensions can also be used. Similarly, circular apertures of wide enough radii can be employed. Apertures having tooth-shaped baffles positioned across their face can also be used.  
         [0098]    Referring now to FIGS. 6A and 6B, there is shown, in schematic form, the acceleration cell  12  as shown and described above in conjunction with the embodiment of FIGS. 3-4B, in accordance with an alternative preferred embodiment of the invention, and in which resin powder is charged by friction. Arranged within cell  12  is a wave plate  59 , typically constructed from a plastic material like Teflon or nylon, which has an undulating surface  60 . Air is blown by a blower (not shown) from an opening  15  in end  64  of acceleration cell  12  past a Venturi constriction  22 , so as to cause a drop in pressure, generally as described above in conjunction with FIG. 2, thereby to cause resin powder to be drawn from a powder box  24  (FIG. 2) through tube  50  (FIG. 2) into cell  12 . The powder transported by the moving air moves past the undulating surface  60  of wave plate  59 , where the powder is charged by friction. The powder is then expelled through aperture  46  into coating chamber  36 , the latter best seen in FIGS. 2-4B. The likelihood of clogging in cell  12  is reduced because undulating surface  60  is spaced far enough away from the inside surface of housing  62 . Additionally, clogging is mitigated and the charged particle distribution made more uniform because undulating surface  60  provides for non-streamline flow.  
         [0099]    Typically, the inside surface of housing  62  is formed having a textured surface, while the surface  60  of wave plate  59  is made to be generally smooth. Both housing  62  and wave plate  59  are generally fabricated from plastic. The inside surface of housing  62 , or the housing  62  itself, and wave plate  59  can be made from the same or different plastics. The nature of the plastics employed determines whether the charge on the resin powder will be positive or negative. Typical plastics that can be used are Teflon®, nylon, propylene, and acrylics. The aforementioned list is exemplary only and not intended to be limiting. It is readily apparent to one skilled in the art that the speed of the particles across the friction-charging surfaces  60  and  62  is an important factor in determining the efficacy of charging.  
         [0100]    As in the embodiment of FIGS. 5A and 5B, the present embodiment also has a mixing region  27  having deflectors  53  positioned on a base  51 . Their construction and function are similar to deflectors  53  in mixing region  27  described with FIGS. 5A and 5B and discussed in greater detail with FIGS. 8, 9A and  9 B below. In addition, also as described in FIGS. 5A and 5B, FIG. 6A shows a slope S 3  in stabilization region  29  and an even sharper slope S 2  in nozzle  23  near aperture  46 . These slopes will be discussed further with reference to FIGS. 7A-7C,  8  and  9 A and  9 B.  
         [0101]    As is apparent from the descriptions of the embodiments associated with FIGS. 5A, 5B,  6 A and  6 B, the present invention uses a high-pressure, high-velocity stream (“forced flow”) of charged resin powder. This “forced flow” stream ensures greater coating uniformity and penetration of the substrate than is possible with low pressure, low-velocity charged resin clouds, such as those used in prior art fluidized bed coaters. Furthermore, the acceleration cells of the present invention have typically long, narrow apertures, which can continuously coat large moving swaths of substrate. Other high-velocity coating devices generally use small diameter circular apertures with narrow beam widths, making uniform coating of large-area substrates difficult. Penetration into the substrate is also improved because the acceleration cells constructed according to the present invention can employ micron-size particles. The velocity of the charged particles as they exit the wide aperture of the acceleration cell is at least 0.1 m/s, preferably between about 1 to about 10 m/s. The maximum velocity will generally be that velocity that begins to cause deterioration in the substrate.  
         [0102]    Electrostatic fluidized bed (EFB) coaters, such as the one shown in FIG. 1, employ particles that have low velocities. Clouds of such particles have a layered distribution. Heavier particles tend to settle and make up a greater percentage of the lower layers of an EFB particle cloud, while smaller particles make up a greater portion of the upper strata. As a result, it is readily apparent that when a substrate moves perpendicularly to the airflow in an EFB coater, the coating can never be entirely uniform. This situation does not occur with embodiments of the present invention.  
         [0103]    While in the embodiments of the system shown in FIGS. 2, 3,  4 A and  4 B two acceleration cells are used as described in FIGS. 5A-6B, three or more cells may also be used in accordance with further embodiments of the invention.  
         [0104]    Typically, both cells of the embodiments discussed with FIGS. 2-4B are of the same type, either frictional or electrical charging cells. However in other embodiments, the coating systems described herein employ at least one friction-charging cell and at least one electrical charging cell, concurrently.  
         [0105]    In yet other embodiments, the mechanisms for both types of charging can be positioned in a single cell housing and the two types of mechanisms can be used in parallel or serially. Typically, but without being limiting, when used in parallel, each of the two different charging mechanisms can be positioned side by side, parallel to the long axis of the cell.  
         [0106]    When used in series, the portion of the cell on the side of the Venturi constriction distal from the wide aperture is typically constructed as shown in FIGS. 5A and 5B with a brush element connected to a DC power source. The portion of the cell between the Venturi constriction and the wide aperture is constructed as in FIGS. 6A and 6B with a wave plate. Powder brought into the cell is thus first charged by ionized air previously charged by the brushes; the powder then undergoes charging by friction at the wave plate.  
         [0107]    In yet another embodiment, the two mechanisms can be used serially with the resin particles first charged by friction and then by electrically charged brushes. In such an embodiment, both the frictional wave plate and the charged brushes are typically placed between the Venturi constriction and the wide aperture of the cell. In this last embodiment, the brushes generally lie closer to the wide aperture and the wave plate closer to the Venturi constriction. It should be understood that the configurations in the embodiments describing serial and parallel usage hereinabove is exemplary only and not intended to be limiting.  
         [0108]    The capability of using both methods of charging concurrently, as described in the preceding embodiments, is particularly advantageous. The ability of certain plastic resins to be charged by friction is more limited than others. Using high-voltage charging would obviate the difficulty. On the other hand some plastics are relatively easily charged by friction and high-voltage charging would be unnecessary. Additionally, small micron-size particles are more easily charged by friction than larger particles. The use of micron-size resin particles will become more prevalent because of recent improvements in their manufacture. If a resin with a wide particle size distribution is used, the capability of charging by both methods simultaneously, as described in the last embodiments, will make charging, and the entire coating system, more efficient.  
         [0109]    Since high particle velocity is important to ensure coating uniformity and particle penetration of the substrate, various means can be used to increase the velocity of the charged resin particles. Some of these means can be positioned in the acceleration cell, while others can be added to the coating system.  
         [0110]    Charging the substrate with a polarity opposite to that of the impinging charged resin particles can increase velocity. The substrate can be charged by contacting it with a plastic body, such as a plastic plate or plastic roller, as the substrate moves through the coating chamber. Alternatively, the substrate can be charged directly using a high-voltage power supply.  
         [0111]    Another means to increase particle velocity is best illustrated in the embodiment shown in FIG. 4A. Particle velocity can be enhanced by placing a conductive metal strip  47  in coating chamber  36 , substantially opposite wide aperture  46  of acceleration cell  12 . Strip  47  is charged oppositely to that of the resin particles via contacts  49  located on the outside of chamber  36 . Accordingly, strip  47  attracts and accelerates the particles toward the intervening substrate (not shown).  
         [0112]    Electrostatically charged plates, sometimes used in conjunction with magnetic fields, can be appropriately positioned within the acceleration cells or within the coating chamber to increase particle velocity. In addition to accelerating the particles, such plates and fields can be used to manipulate the particle beam, making it more uniform.  
         [0113]    Velocity enhancement can also be effected in the acceleration cells by using sloped walls inside the cells. This has been mentioned previously in the discussion of FIGS. 5A-6B and will be expanded upon below in a discussion of FIGS. 7A-9B.  
         [0114]    Yet another method for increasing the velocity of the charged resin particles includes altering the geometry of the Venturi constriction, particularly its slope on the wide aperture side of the constriction. Increasing the size of the powder inlets near the Venturi constriction, or using inlets of different sizes, also can increase the velocity of the charged particles.  
         [0115]    Reference is now made to FIGS. 7A-7C where three schematic views of a nozzle  23  of an acceleration cell  12  are shown. Nozzle  23  represents the end of an acceleration cell closest to the coating chamber. Nozzle  23  shown in FIGS. 7A-7C can be used with both the high-voltage and friction-charging type acceleration cells discussed above. The nozzle shown enhances particle beam uniformity and increases the velocity of the particles.  
         [0116]    A top-side schematic cut-away view of nozzle  23  of an acceleration cell constructed and operative according to the present invention is shown in FIG. 7A. Nozzle  23  contains four airflow control vanes  54 , which assist in controlling the spatial uniformity of the particle distribution. It is readily understood that more or less than four vanes can also be present. Vanes  54  can be constructed of any suitable plastic.  
         [0117]    In the embodiment of the present invention shown in FIGS. 7A-7C, nozzle  23  is constructed so that there are slopes (S 1  and S 2 ) in two dimensions of the nozzle. This can best be seen in FIGS. 7B and 7C which are schematic top and side views respectively of nozzle  23 . In yet other embodiments, a slope can be present in only a single dimension, such as the one shown in FIG. 7C, with a slope absent from the dimension best seen in FIG. 7B. In still other embodiments, shown in FIGS. 5A-6B, in addition to slopes S 1  and S 2  of nozzle  23 , acceleration cell  12  also contains slopes S 3  and S 4  extending back into the acceleration cell, almost reaching Venturi constriction  22  or mixing region  27 , the latter to be discussed below.  
         [0118]    The slope of acceleration cell  12  from wide aperture  46  to mixing region  27  or Venturi constriction  22  does not need to be a constant. As best illustrated in FIGS. 5A and 6A, the slope can be less in the stabilization region  29  extending from the mixing region  27  to nozzle  23  and greater in the region of nozzle  23 . Including a slope in the part of acceleration cell  12  closest to aperture  46  increases the uniformity of the charged particle distribution and accelerates the particles as they approach and exit aperture  46 . Typically, the angle of slopes S 1  and S 2  in the region of nozzle  23  can range up to about 40 degrees, preferably up to about 15 degrees and even more preferably up to 10 degrees.  
         [0119]    In the above discussion and Figures, we have used S 1 -S 4  as the four possible slopes of the various regions of the acceleration cell. The use of different designations  1 - 4  for the four slopes does not necessarily imply that they are all different. In some embodiments, some, or all, of the slopes may be identical.  
         [0120]    Reference is now made to FIG. 8 where a cut-away, top-side view of the region between the Venturi constriction  22  and the wide aperture  46  of a typical acceleration cell, constructed and operative according to a preferred embodiment of the present invention, is shown. This part of the cell includes several regions: a Venturi constriction  22 , a mixing region  27 , a stabilization region  29  and a nozzle region  23 . Nozzle region  23  has been discussed above with respect to FIGS. 7A, 7B and  7 C. Similarly, the Venturi constriction  22  has been discussed elsewhere. Mixing region  27  is meant to increase the uniformity of the charged particle distribution, while stabilization region  29  is intended to stabilize the flow as the particles approach nozzle region  23  where they are further accelerated by an increasingly sloped internal wall and a constantly decreasing cross-sectional area.  
         [0121]    Mixing region  27  can be constructed as shown in FIGS. 9A, 9B and  9 C to which reference is now made. In the embodiment shown, deflectors  53  introduce turbulence into the moving air and charged particles after they have traversed the Venturi constriction. This turbulence increases the uniformity of the particle distribution as the particles approach the nozzle region. As shown in FIGS. 9B and 9C, the orientation of deflectors  53 , attached to the bottom of the cell, are typically opposite to that of deflectors  531 , positioned on top of the cell. FIGS. 9B and 9C show top views of turbulence-inducing deflectors  53  and  53 ′, and their opposing displacements are clearly observable. In FIGS. 9A, 9B and  9 C, deflectors  53  and  53 ′ are mounted on bases  51  and  51 ′ respectively.  
         [0122]    It should be readily apparent to those skilled in the art that the number of deflectors can be more or less than that shown in the figures, the number being determined by the degree of agitation required for charged particle uniformity. It should further be apparent to one skilled in the art that turbulence-inducing elements of any shape, or the use of any turbulence-producing means, can be used as long as they produce a satisfactorily uniform particle distribution in the particle discharge stream. Moreover, any means—turbulence-producing or otherwise—that produces satisfactory uniformity in the particle distribution of the discharge stream can be used. One such means for improving uniformity would be the insertion of a plastic screen in the nozzle region of the acceleration cell. The screen would include a mesh large enough to prevent clogging and small enough to improve discharge stream uniformity.  
         [0123]    In embodiments of the present invention, the size of the aperture, that is its length and width, and the angle at which the projected charged powder impinges on the substrate, can be adjusted to produce a powder coating of a desired thickness and uniformity. Therefore, further embodiments of the present invention provide for acceleration cells in which the apertures are mechanically variable apertures. In these embodiments, the size of the aperture and/or the angle between the plane containing the wide aperture and a plane, or a “virtual” plane, of the substrate being coated can be varied. The “virtual” plane here refers to instances when the substrate is not necessarily planar; the plane then being coated is a “virtual” plane, which constitutes the surface being coated projected onto a plane.  
         [0124]    Alternatively, the aperture region of the cell can be enclosed in a detachable structure, the structure being replaceable with any of a series of similar structures, each such structure having an aperture of different dimensions, angle of incidence and/or shape. Depending on coating needs, the shapes of these structures can include conical structures such as those in FIG. 5A-6B, straight structures such as in FIGS. 3-4B and even round or rectangular horn-shaped structures similar to those found on loudspeakers.  
         [0125]    It is readily apparent that the uniformity of the coating depends on the uniformity of the particle beam emitted from the aperture. Preferably, the beam should be as narrow as possible when emerging from the cell. Accordingly, increasing the cell&#39;s aperture aspect ratio, that is the ratio of the aperture&#39;s length to width (or equivalently the ratio of its larger to its shorter dimension) and/or decreasing the aperture&#39;s cross-sectional area, typically enhances the uniformity of the particle discharge stream.  
         [0126]    Particle size also affects coating uniformity. Small particles of five microns or less have a greater surface area to volume ratio than larger particles. This results in a larger electrical charge to volume ratio, which increases particle velocity and enhances particle penetration of the substrate, leading to a more uniform coating and smaller resin loads. The fibers in composite substrates generally have a thickness of 5 to 20 microns and the inter-fiber spacings of such substrates are generally even smaller. As a result, it is readily apparent that particles of less than 5 microns can penetrate the spaces between such fibers more easily than conventional 50-100 micron resin particles. In addition, small micron-size particles, because of their high kinetic energy, can separate the fibers of the substrate. Finally, in addition to the penetration capability of small particles, they also charge more easily because of their greater surface area to volume ratio; accordingly, charging voltage can be reduced. As has been mentioned previously, recent improvements in the fabrication of micron-size resin particles will make the use of such small particles more commonplace. Mixed electrical/friction-charging cells or the concurrent use of both frictional and electrical charging cells in a single system as discussed above, will assist in assimilating such particles in prepreg manufacture.  
         [0127]    It should be appreciated that two-stage coating would be particularly advantageous when using small particles. The first stage of coating would employ small (5 microns or less) particles and would ensure good penetration of the substrate and thus better uniformity. In the second stage of coating, larger size resin particles would be deposited; this would lead to a faster overall deposition rate and reduce the time needed to coat a unit length of substrate.  
         [0128]    As can readily be concluded from the discussion above, achieving a uniform coating requires control of many variables. This includes controlling the charging voltage, air blower speed, pressure differential at the Venturi constriction and the amount of powdered resin carried per unit volume of airflow. Additional factors, which enter into the quality and uniformity of the coating, are the type, weave, fiber diameter and conductivity of the substrate. Additionally, the speed at which the substrate moves, the amount of powder used, the size distribution and density of the powder, the sizing used on the substrate, and the degree of ionization in the region of the substrate are important. The latter factor depends on charging voltage, humidity in the region of charging and the amount of charge lost in transit. Theoretically, as many of the above factors as possible should be monitored and, when necessary, adjusted to obtain an optimal coating.  
         [0129]    A computerized control system can be used with embodiments of the present invention. Variables such as air blower speed, substrate velocity, charging voltage, output voltage and output current can be measured by various sensors and transferred to a data acquisition unit, which is part of the computer used to control the coater system. The computer can include additional interface provisions for controlling the coater&#39;s active elements (high-voltage power source, air blowers, substrate conveyor, etc.). One typical interface architecture that could be used includes a general purpose interface bus (GPIB). At the direction of the computer, the output of the active elements can be adjusted via the interface to provide the charging voltage, air blower speed, substrate velocity, etc. that optimizes the coating.  
         [0130]    Prior to any control system being fully operational, data is gathered about as many of the key variables discussed above as possible, and a regression analysis for optimizing the coating is performed. This analysis and data are stored in the computer and used to analyze the values sensed by the above-mentioned sensors. Based on a comparison of the computer&#39;s stored data, regression analysis and the sensed data, the computer communicates, via the interface, to the active elements of the system the values required to optimize the coating.  
         [0131]    The definitions given above have been adhered to while discussing the construction and operation of the present invention. However, it should be readily apparent that the above-described invention can be applied to other substrates whenever a uniform, low load coating is required. These substrates need not necessarily be substrates used in forming prepregs for use in fabricating composites. Without being limiting, these substrates can include solid substrates such as metal, wood and Formica®, among others. Furthermore, the substrates defined hereinabove, which inter alia include carbon fibers, fabrics, tow and strands can also include tapes and tubes, particularly carbon tapes and tubes.  
         [0132]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the invention is defined by the claims that follow.