BACKGROUND OF THE INVENTION Composite materials comprised of fibers in resinous matrices are taught rather extensively in the art, and are widely used for diverse applications. The reinforcing fibers contained in such composites may range in length from very short to continuous, and they may be incorporated as single strands, as multifilament bundles or yarns, and as woven or nonwoven fabrics and webs. Regardless of form, the fibers will generally be impregnated by saturating them with a dispersion, solution, or melt of the matrix-forming resin. Circuit boards, used for printed circuit applications, are especially well suited for fabrication from composite materials of the kind described above; this use is disclosed for example in Medney et al, United States patent No. 4,943,334. Such boards are conventionally manufactured by laying up a number of plies of impregnated fiberglass cloth, and converting the resultant mass to a coherent structure through the application of heat and pressure. The impregnant will usually be either a phenolic or an epoxy thermosetting resin, and consequently the process must be carefully controlled to ensure that the thermo- set material does not cure beyond its so-called "B-stage" con¬ dition; full cross-linking of the resin is reserved for subse¬ quent laminating steps.

In common practice, the fiberglass cloth is saturated by drawing it through a solvent solution of the matrix-forming polymer. Suitable mechanical means is then used to remove excess polymer solution, and the thus-impregnated material is thereafter drawn through an oven to drive off the solvents and to flow out the resin to a "tack-free" condition; plies of the product may then be laminated to produce a multilayer circuit board structure.

The solvents for the impregnating resins used are normally toxic, and frequently they are also highly flammable; means is therefore usually required for the recycle, disposal, or de¬ struction of the solvent after removal from the cloth. More- over, coating processes of the kind described require constant monitoring, both to adjust the concentration of the polymer and

also to control the mechanical resin-removal system.

Dry techniques for producing fiber/resin composites have been proposed in which solid particles of resin are deposited from an electrostatic fluidized bed upon strands of reinforcing filaments. As taught in the art, however, such particle-depo¬ sition methods require spreading of the fibers of a strand or bundle to permit sufficient penetration of the resin particles for adequate encapsulation; in this regard, United States patents Nos. 3,817,211 and 3,919,437, to Brown et al, are nota- ble.

Brown et al employ electrostatic charging techniques both for strand opening and also for particle impregnation. It is taught that the invention is applicable in the production of mats, webs, or fabrics, but clearly the patents do not conte - plate the impregnation of preformed fabrics, in which the con¬ stituent filaments may lie in fixed relative positions close to one another.

The following art (all patents listed being of the United States) may also be of interest to the present invention: Hug, No. 2,706,963, discloses the electrostatic flock coating of a string while moving it through a container.

Swann, No. 2,730,455, provides a method for coating glass fibers which involves spreading of the fibers by the use of electrostatic effects. In accordance with Harmon et al. No. 2,820,716, a nonwoven fabric is produced by electrostatically distributing dry adhe¬ sive particles throughout a loose, fibrous web, the particles subsequently being rendered adherent.

Adhesive-coated sheeting material is made, pursuant to the teachings of Dickey et al No. 2,869,511, by use of an electro¬ static deposition method.

Terrell et al, No. 3,354,013, provides a process in which continuous filament tow is impregnated with a particulate addi¬ tive from a fluidized bed; the prior use of induced electro- static charges, for applying powder to fibers, is mentioned. Williams, Jr. et al. No. 3,549,403, teaches an electro¬ static deposition method for applying a powdered thermoplastic

resin to uncalendered paper, followed by hot compression roll¬ ing to produce an adherent polymeric film on the paper.

Spencer, No. 3,673,027, provides a method for forming coated fibers, in which a strand of continuous filaments is bent over a guide to flatten and spread them as they enter a fluidized bed of particles.

Lamanche et al. No. 3,703,396, teaches the impregnation of strands by passing them through an electrostatic fluidized bed; separation of the fibers may be achieved by electrostatic re- pulsion or by air current effects, and the strands may be dis¬ posed in a parallel relationship to one another to provide a sheet of threads.

In accordance with Mayer, Jr., et al. No. 3,707,024, yarn is bulked electrostatically and is impregnated with charged resin particles.

Price, No. 3,742,106, provides a fiber-reinforced thermo¬ plastic body by impregnating a roving in a fluidized bed, the strands of the roving being separated by a comb or by ribbed rods or bars; the use of electrostatic effects for causing penetration of the polymer is expressly discounted.

In No. 4,086,872, Pan describes an electrostatic method for coating a continuous web, which web may be conductive or nonconductive.

Gray et al. No. 4,098,927, provides a method in which con- tinuous glass roving is impregnated with resin powder in a fluidized bed, using a series of bars to separate the roving into bundles of filaments.

Garner, Nos. 4,205,515 and 4,252,583, describe an opening member that can be positioned in a fluidized bed to assist filling of cables with powder.

In accordance with Siggen et al, No. 4,338,882, a rope of fibers, spread to form a sheet, is treated with plasticizer by depositing it as a mist of electrically charged particles.

Yamada et al. No. 4,427,482, provides a method for pro- ducing prepreg rovings and fiber-reinforced plastic articles, in which polyester reactants are impregnated into the roving, which may then be consolidated with monomer-impregnated sheet

plies.

Drain et al. No. 4,892,764, discloses fiber/resin compos¬ ites whose resin phase is comprised of two components, at least one of which is curable by actinic radiation. Fay et al. No. 4,913,956, teaches a moldable blanket made by depositing a liquid binder and a dry binder (the latter in the form of a powered resin) upon glass fiber, which may con¬ tain other reinforcing fibers, as well.

Pettit, Jr., No. 4,921,913, concerns the electrostatic de- position of powder to various substrates, by use of spraying or fluidized bed apparatus; the substrates include fiber-rein¬ forced plastics.

Asensio et al. No. 4,927,684, discloses the use of a woven fabric for reinforcing a multilayer plastic article. In accordance with Gauchel et al. No. 4,929,651, prepregs are formed by impregnating a woven roving with a solution of thermoset polyester, and are molded to produce ballistic- resistant composites.

The above-mentioned patent to Medney et al. No. 4,943,334, discloses a method for making reinforced plastic laminates for use in the production of circuit boards, in which sets of strands are maintained under a controlled tension during im¬ pregnation and cure of the resin; vacuum-assisted impregnation, using solvent-free formulations, is taught. An electrostatic technique is disclosed in Smyser, No.

Re.22,419, for coating opposite sides of fabric, paper, or the like to produce adhesive articles.

In a paper entitled "Electrostatic Prepregging of Thermo¬ plastic Matrices," and believed to have been published in January of 1989, Muzzy et al describe the production of thermo¬ plastic towpregs by electrostatic deposition of charged and fluidized polymer powders on spread continuous fiber tows.

SUMMARY OF THE INVENTION Notwithstanding the level of activity in the art evidenced by the foregoing, the need remains for a practical and commer¬ cially attractive method for the production of a composite web, comprised of a fabric of multi-filament bundles in a matrix of

synthetic resinous material, wherein the filaments are main¬ tained in substantially fixed relative positions of close mutu- al contact during coating and impregnation of the fabric.

Accordingly, it is the broad object of the present invention to provide a method to satisfy the need described.

It is also an object of the invention is to provide such a method in which deep penetration of the resin into the fibrous component, and ultimately full impregnation, are achieved ef¬ fectively, with facility, and at atmospheric pressure (i.e., without need for pressure assistance) , to produce a composite web that is free from voids.

Another object is to provide such a method by which any of numerous species of fibrous materials and resinous materials can readily be combined with one another, which method is in addition highly advantageous from economic as well as environ¬ mental standpoints. A further specific object of the invention is to provide a method having the foregoing features and advantages, which is especially well suited for the production of a formed article of manufacture, and particularly laminate structures.

It has now been found that the foregoing and related ob- jects of the invention are attained by the provision of a method for the production of a composite web, utilizing a con¬ tinuous-length fabric of multi-filament bundles, in which the filaments lie in substantially fixed relative positions of close mutual contact. In carrying out the method, the fibrous substrate is transported sequentially through a wetting station, a coating station, and a heating station, without substantial opening or spreading of the filaments. Moisture is applied to the fabric at the wetting station, and solid parti¬ cles of a fusible synthetic resinous material are applied at the coating station.

DESCRIPTION OF THE DRAWING Figure 1 is a plan view of a section, generally designated by the numeral 10, of a composite web produced by the method of the invention, in which the fibers of a plain weave fabric are

encapsulated within a matrix of synthetic resinous material.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In carrying out the instant method, moisture is applied to the fabric at the wetting station to enhance penetration of the fused particulate material. The fabric will normally exit from the wetting station with a moisture content in the range 5 to 25 weight percent, based upon the weight of the dry fabric, as may result by merely spraying its surfaces with water or by treating the fabric with steam to effect moistening. Prefer¬ ably, the moisture content of the exiting fabric will be at least 9 percent (on the same basis) , and 12 percent will gener¬ ally represent an optimal upper limit.

At the coating station, solid particles of a fusible syn- thetic resinous material are deposited upon the substrate by any suitable means, e.g., by simple gravity feed, mechanical spreading, electrostatic attraction, etc. The fabric may be stationary or moving during coating, and it may be disposed in any appropriate attitude (e.g., horizontal or vertical); the fabric itself may be of any suitable form, including discrete pieces and continuous lengths.

At the heating station, the fabric is heated to an elevat¬ ed temperature above ambient using any appropriate source of thermal energy, including radiant and/or convection heating. The coated substrate is maintained at temperature for a period sufficient to effect fusion of the resinous material and move¬ ment thereof into the bundles, thereby to achieve encapsulation of the filaments; the resin is believed to infuse into the fibrous material through capillary action, displacing any air or moisture that may be present in the interstices. Although at least adequate levels of penetration and encapsulation of the filaments can usually be achieved under atmospheric pres¬ sure conditions, applied pressure assistance may be desirable in certain instances. Needless to say, the oven, bank of lamps, or other form of heating station employed must be of a sufficient length to enable both effects to occur before the resin loses its requisite fluidity, and therefore it is the

heating station that will usually represent the limiting factor in attaining maximum throughput rates.

It will also be appreciated that the resinous material must remain fusible at the elevated temperature to which the substrate is initially heated. Thermoplastic resins will of course inherently remain fusible; to maintain thermosetting resins in that state, however, care must be taken not to prema¬ turely heat the resin to a temperature above that at which it will exist in its B-stage (cross-linked, but less than fully cured) condition. The resin must exhibit a melt viscosity at the temperature of operation which is sufficiently low to per¬ mit a degree of penetration that is adequate to achieve the objectives set forth.

It is believed that any conventional fabric style or weave known to those skilled in the art (e.g., basket, plain, crow¬ foot satin, harness satin) can be utilized in the practice of the present invention. Greatest benefit will however often be realized when the fabric has a weave that is sufficiently tight as to substantially immobilize the filaments of the constituent bundles against relative displacement from their established positions of intimate mutual contact. The filaments of which the fibrous substrate is comprised will advantageously be selected from the group consisting of cellulosic fibers, fiber¬ glass, carbon fibers, graphite fibers, polyester fibers, aramids (e.g., Kevlar) and mixtures thereof. By way of example, the filament diameter will desirably be in the range 6 to 10 microns, and the denier of the warp and filling yarns (e.g., of a Kevlar fabric) will typically have a value in the range 195 to 3000; the yarn count (warp x filling, per inch) will typically be in the range 17x17 to 50x50, and the fabric will desirably be about 5 to 30 mils thick.

The synthetic resinous material used for coating may, as noted, be either a thermosetting resin or a thermoplastic resin; suitable classes of resins include epoxy resins, pheno- lie resins, polyester resins, polyether resins, alkyd resins, polyamide resins, polyamide resins, bismaleimide resins, poly- olefins, vinyl resins, etc., as will be evident to those

skilled in the art. The resin particles will normally be of a size in the range 10 to 200 microns. It should be noted that the ability to effectively encapsulate fine filaments (e.g., of 6 to 10 microns diameter) using a resin of substantially larger average particle size is regarded to be a surprising benefit of the instant method. The deposit on the substrate will advanta¬ geously be sufficient to cause substantially complete encapsu¬ lation of the filaments throughout the full thickness of the fabric; normally, sufficient resin will be deposited to provide about 35 to 70 percent thereof, based upon the weight of the composite web.

The method of the invention will usually include the addi¬ tional step of cooling the composite to solidify the fused resinous material, and in many instances it will advantageously include the further step of forming the resultant web, under heat and pressure, to produce an article of manufacture. More par-ticularly, a plurality of plies of a fabric-reinforced com¬ posite web may be stacked and then subjected to the forming step, to produce a laminate structure suitable for use in the manufacture of circuit boards. It will be appreciated that, when a thermosetting matrix resin is employed, the temperature attained in the forming step will be higher than that at which fusion is effected in the initial heating step, and will be sufficient to convert the resinous material to its fully cured condition. Thermosetting resins used must of course have a glass transition temperature that is low enough to permit flow- out at the temperature of initial heating, without effecting excessive crosslinking.

Exemplary of the efficacy of the present invention is the following Example. The tests reported therein demonstrate the surprising and most advantageous effect of water in increasing fabric penetration by fused resins.

EXAMPLE ONE Except as hereinafter noted otherwise, in all tests ap¬ proximately 10 milligrams of a selected resin powder was depos¬ ited upon a horizontal swatch of tightly woven fabric; i.e., a

satin weave style 7781 fiberglass web of 8.95 oz/yd ² areal weight, 57 x 54 construction, 0.009 inch thick and with 642 type sizing. The coated fabric was laid upon a glass micro¬ scope slide, which was in turn placed upon the surface of a hotplate, heated to a temperature of approximately 170° Centi¬ grade; the response of the polymer, after 30 to 40 seconds, was observed. Two fabric samples were generally tested simulta¬ neously, one of which was dry and the other moistened by spray¬ ing with tap water. One of three polymers was used in each test: resin "pp" was a polypropylene resin having a melt temperature in the range 162° to 168° Centigrade, a density of 0.905 g/cm ³ , and a melt flow index of 180 g/10 min. ; resin "E" was a brominated epoxy resin having a melt temperature of about 50° Centigrade and a density of 1.42 g/cm ³ ; and resin "PE" was a polyester having a melt temperature of 38° Centigrade, and a density of 1.5 g/cm ³ . In the table below, the suffix to the test number indicates which resin was employed; the comments indicate any variations in the conditions applied, and the results observed. TABLE ONE

Test Comments 1-PP The wet sample took longer to melt than did the dry one, and it adhered to the slide; the dry sample did not adhere. 2-PP The exposed surfaces were covered by addi-tional slides. Water was drawn from the wetted fabric onto the top slide, thereby reducing its moisture content.

Neither fabric sample adhered to either slide.

3-PP A somewhat greater amount of polymer was placed on the dry sample than on the wet one, and the slides were kept on the hotplate for a timed period of 30 seconds. The wet sample adhered to the glass; the dry sample did not.

4-PP Test 3 was repeated, but the slides were maintained on the hotplate for 40 seconds. The dry sample turned brown, and did not stick to the slide; the wet

sa ple remained colorless, and did adhere. 5-PP Test 4 was repeated, but using somewhat less resin on the dry swatch than on the wet one. The results were the same. 6-PP One of the slides was pretreated by applying and evaporating water. Polymer, applied directly to the treated slide and to an untreated one melted and spread indistinguishably. 7-PP The polymer was applied directly (i.e., with no fab¬ ric) to wet and dry slides. When the water boiled, the power on the wet slide dispersed and covered a greater surface area than it did on the dry slide. 8-E The water on the dry slide appeared to boil before the epoxy sample melted completely. Residual water bubbled through the molten polymer, but craters left by the escaping gas smoothed quickly. The wet sample was stuck to the slide; the dry sample was not. 9-E The polymer was spread on the fabric before deposit¬ ing the swatches on the hotplate, forcing some of the powder into the fabric. The results were the same as in Test 8, but less bubbling was observed.

10-E Both resin deposits were melted on dry fabric. Water was added to one of the samples, which stuck to the slide after the water boiled off; the dry fabric did not stick. 11-E The polymer was put directly on a dry slide and a wet slide, without fabric. Rather than dispersing the powder, the molten material coalesced (more on the wet slide than on the dry one) . 12-PE Test 9 was repeated using the polyester; the results were the same.

13-PE Test 11 was repeated. The same results were observ¬ ed, except that the boiling water pitted the surface, and curing prevented the pits from flowing out. The foregoing tests show that water promotes dispersion of the fused polymer through the fabric with sufficient penetration to cause the polymer to adhere to the underlying glass slide. No

mechanis is known that would cause water to effect the spread¬ ing and dispersion observed, and indeed, one might have expect¬ ed the impurities in tap water to impede dispersion of the res¬ ins. Apart from the unique ability of the instant method to effect coating of fibrous substrates made from yarns in which the filaments are maintained in close mutual contact, one of its primary benefits resides in the avoidance of solvents. Also, the inherent tack-free nature of the composite webs (at room temperature) facilitates their subsequent processing, and the option of using thermoplastic resins can afford enhanced thermal properties in the products, virtually infinite self- life, and the ability to form shaped circuit boards.

Thus, it can be seen that the present invention provides a practical and commercially attractive method for the fabrica¬ tion of fiber-reinforced composite materials, using fabrics composed of multifilament bundles. Production can be carried out at high rates of speed, at ambient pressures and on commer¬ cially available equipment. Full impregnation of the resin into the fibrous component is readily and effectively achieved, and the method is highly advantageous from both economic and also environmental standpoints. Numerous combinations of fibers and resins can be used in the practice of the method, which is especially well suited for the production of laminate structures and other articles.