METHOD OF FABRICATING COMPOSITE OR ENCAPSULATED ARTICLES

Abstract: A method for molding composite articles or encapsulating an article wherein an article to be encapsulated or molded and a heat curable thermosetting resin are loaded into the cavity of a mold, which has been preheated to the curing temperature of the resin. The mold is then evacuated to a subatmospheric pressure to impose a vacuum on the mold in order to impregnate the article with the resin and to degas the mold contents. The vacuum is thereafter released to burst any gas bubbles remaining in the mold contents. Superatmospheric pressure is then applied to the mold contents to cause the resin to encapsulate the article. The temperature and the superatmospheric pressure conditions are maintained for a time sufficient to partially cure the resin and form a unitary structure which is ejected from the mold. The structure ejected from the mold is subjected to a further heating cycle to completely cure the resin. Composite articles so formed are useful as structures, such as antenna waveguides, for space applications. Electrical components encapsulated by the process of the present invention have a high-reliability passivation layer.

Description: METHOD OF FABRICATING COMPOSITE OR ENCAPSULATED ARTICLES BACKGROUND ' OF THE ' INVENTION 1. Field of the Invention This invention relates to a method of fabricating lightweight, composite materials. More particularly, this invention relates to a method of preparing epoxy resin based filler reinforced composite members exhibiting improved performance characteristics such as reduced weight and thermal expansion coefficients, as well as to a method of encapsulating electrical components to provide increased resistance to electrical stress. 2. Description of the Prior Art The successful utilization of filler reinforced composites in aerospace applications, such as antenna fabrication, imposes several important requirements on the filler reinforced composite. A feature of primary importance is that the composite have strength and dimensional stability characteristics (e.g. a low linear coefficient of thermal expansion) calculated to withstand the rigors of environmental temperature cycling. In addition, for complex structures such as antennae, structures molded from epoxy resin based composites are preferred over metal structures since the composite can be molded directly, thereby avoiding costly machining operations. Glass and graphite fiber reinforced epoxy based composites are finding growing use in aerospace applications because of their high strength-to-weight ratio. These composite materials, because of their relatively low coefficient of thermal expansion (α), find wide application in structural components such as antenna used in space. Such materials are described, for example, by H. S. Katz and J. V. Milewski, in the book entitled "Handbook of Fillers and Reinforcements for Plastics," Chapter 19, Hollow Spherical Fillers, Van Nostrand Reinhold Company, New York, 1978, pages 326 to 330. In a space environment where one face of the structural member is subjected to constant sunlight while the opposite face of the member is in darkness, the face exposed to sunlight is heated considerably more than the opposed face. Such non-uniform heating causes uneven expansions of the structural members making up the antenna with the resultant distortion of the antenna from its desired shape. Thus, aerospace use requires that some critical antenna structural components exhibit a dimensional stability over a ten year lifetime in the operating range of 54° to 115°F (12 to 46°C) and have densities equal to or less than 0.9 grams/cm-- 1 . When the structural components are electroplated with a metal layer to provide, for example, utility as a electromagnetic interference (EMI) shielding or a conductive path in an antenna waveguide structure, the metal coating layer must also withstand thermal cycling in the extremes of the space environment without loss of adhesion. Furthermore, in another area of current interest, it is recognized that high voltage power supplies and pulse forming networks for aerospace use must meet high standards of performance and reliability for long periods under extreme environmental conditions. To assure trouble-free operation of components within the assembly, the components, such as magnetic coils. capacitors, diode arrays, transformers, stator generators, and resistor networks, are commonly encapsulated with synthetic resin materials to provide electrical insulation to the components. Such encapsulants for electrical and electronic components are described, for example, in the book by Katz et al, previously referenced, at page 327. Because of their excellent adhesion, good mechanical, humidity and chemical properties, epoxy resins are used extensively both as encapsulants for electronic components and in combination with glass and graphite fibers and microballoons in the manufacture of the previously discussed high performance reinforced composite structures, such as antenna. The encapsulated article or filled epoxy resin composite is molded using conventional molding techniques, such as compression molding ' or transfer molding, in which the required materials, including the resin are loaded into a cylinder, and curing of the resin is effected under increased pressure, using hydrostatic gas pressure or hydraulic pressure, respectively. Problems encountered in the molding of the epoxy systems and especially filled epoxy resin systems have limited the use of these resins as encapsulants, as well as for the fabrication of structural composites useful in a space environment. In molding filler reinforced composites, the problem is aggravated due to the fact that high filler loadings, e.g. 40 to 60 percent by weight, are required in the epoxy resin based composite structures to achieve the required low α value and corresponding high dimensional stability. Thus, when using transfer molding or batch encapsulation techniques, the use of high filler contents in the epoxy system impedes adequate resin flow into the mold because of the high viscosity of such a resin system. This, in turn, results in varying filler orientations and distributions within the geometrical areas in the complex structures being molded, with a resultant loss in o and ultimate dimensional stability, as well as erratic adhesion of metal layers plated on the surfaces of the structure. This problem is further aggravated when hollow particulates such as glass or graphite microballoons are used as filler materials since the high viscosity filled epoxy resin may have unpredictable densities due to mlcroballoon fractures and voids in the filler/resin mixture. Further, high viscosity resins are unable to penetrate and impregnate the filler to provide homogenous systems. In addition, with regard to encapsulants for electrical components, state-of-the-art heat curable epoxy resin systems used as encapsulating resins have the disadvantage that the viscosity of the resin at working temperatures, e.g. 100°C, (75°F) , are quite high, e.g.., 500. centipoise (cps) , and such resins are not intruded completely into crevices present in the electrical component. Thus, the insulation is often incomplete and defective. It is the primary object of the present invention to provide a method for molding filled thermosetting resins such as polyimides, bismaleimides, and parti- cularly epoxy resins, to provide resin based filler reinforced composite structural components which have a low coefficient of thermal expansion rendering the composite relatively insensitive to environmental temperature cycling while possessing desirable performance properties of low density, high strength, and amenability to the plating of adherent metal films and coatings. It is a further object of the present invention to provide a method for encapsulating electrical components or devices to provide components or devices having improved reliability and improved electrical properties. SUMMARY OF THE INVENTION The above objectives are achieved in accordance with the present invention, which is directed to a method for encapsulating and impregnating an article which comprises first loading the article in a mold cavity and then filling the mold cavity with a low viscosity heat curable, thermosetting resin such as an epoxy resin. The mold cavity is preheated to the curing temperature of the resin and maintained at this temperature. Next, the mold cavity is evacuated to a subat ospheric pressure to impose a vacuum on the mold to impregnate the article with the resin and degas the mold cavity contents. The vacuum is released to atmo¬ spheric pressure to burst any gas bubbles remaining in the mold contents. Then, a superatmospheric pressure is applied- to the mold to cause the resin to encapsulate the article. The temperature and the supera'tmospheric pressure are maintained for- a time sufficient to partially cure the resin and form a unitary structure which can be ejected from the mold. The ejected structure is then subjected to a further heating cycle to completely cure the resin. DETAILED DESCRIPTION OF THE INVENTION The heat curable, thermosetting resins employed in the molding compositions used in this invention can be any low-viscosity heat curable resin. Preferred heat curable resins are low-viscosity epoxy resins having 1,2 epoxy groups or mixtures of such resins, and include cycloaliphatic epoxy resins, such as the glycidyl ethers of polyphenols, epoxy resins, liquid bisphenol-A diglycidyl ether epoxy resins (such as that sold under trademark as EPON 815 by Shell Chemical Company) . In addition, other resin compositions, such as polyimide compositions and b ' ismaleimide compositions, having appropriate viscosities, gel times, green strengths, and dielectric properties may be used in the practice of the present invention. Epoxy resins preferred in the practice of the method of the present invention are polyglycidyl aromatic amines, i.e. N-glycidyl a ino compounds conventionally prepared by reacting a halohydrin such as epichlorohydrin with an amine. Examples of those preferred polyglycidyl aromatic amines include diglycidyl aniline, diglycidyl orthotoluidine and tetraglycidyl etaxylene diamine. The epoxy resins are admixed with polyfunctional curing agents to provide heat curable epoxy resins which are cross-linkable at a moderate temperature, e.g. about 100°C, to form thermoset articles. Suitable polyfunctional curing agents include polyearbox lie acid anhydrides of which nadic methyl anhydride (i.e. a maleic anhydride adduct of methyl cyclopentadiene) , methyl tetrahydrophthalic anhydride and methyl hexa- hydrophthalic anhydride are exemplary. Polycarbox lic acid anhydride compounds are preferred curing agents for polyglydicyl aromatic amine based epoxy resins. In preparing heat curable, thermosetting epoxy resin compositions, the epoxy resin is mixed with the curing agent in proportions from about 0.6 to about 1.0 of the stoichiometric proportions. Cure accelerators can be employed in preparing the heat curable epoxy resin formulations " , and in particular when using polycarboxylic acid anhydrides as curing agents, the preferred accelerators include substituted imidazoles such as 2-ethyl-4-methyl imidazole and organometallic compounds such as stannous octoate, cobalt octoate, and dibutyl tin dilaurate. Preferred epoxy resin compositions useful in the practice of the present invention and comprising the above-described preferred epoxy resin, curing agent, and accelerator are disclosed in my copending patent application Serial No. (PD-82339), assigned to the present assignee. The term "curing" as used herein denotes the conversion of the thermosetting resin into an insoluble and infusible crosslinked product and, as a rule, with simultaneous shaping to give shaped articles. To prepare resin based filler reinforced composite articles in accordance with one embodiment of the present invention, the epoxy resin in admixture with the curing agent and accelerator are further mixed before curing with fillers and reinforcing agents, as for example, glass fibers, carbon fibers, graphite fibers, Kevlar fibers, glass microballoons, carbon microballons, ceramic, particles, or glass particles, and the mixture is loaded into the mold. However, it is preferred in the practice of the method of the present invention that the mold be prepacked with the " filler, i.e. the filler is added first to the mold, followed by the addition of the heat curable resin. If the filler is premixed with the resin and then loaded into the mold, a cured composite with varying filler orientations and distributions within the different geometrical areas of the mold is produced. In the preferred embodiment of the present invention, the components of the filler, such as fibers and microballoons, are pretreated with a titanate sizing agent as discussed below and are mixed mechanically or manually to form a homogenous mixture, which is then added to the mold cavity. Next, the mold may optionally be vibrated to promote a uniform distribution of the filler in the mold. It has been found by using packing theory that an increased volume percent solids in the resin mixture can be achieved. Packing theory is based on the concept that, since the largest particle size filler in a particular reinforcement system packs to produce the gross volume of the system, the addition of succeedingly smaller particles can be done in such a way as to simply occupy the voids between the larger filler without expanding the total volume. This theory is discussed by Harry S. Katz and John V. Milewski, in the book entitled "Handbook of Fillers and Reinforcements for Plastics," Chapter 4, Packing Concepts in Utilization of Filler and Reinforcement Combinations, Van Nostrand Reinhold, 1978. The fillers used in the present invention are chosen on the basis of particle size, shape, and contribution to overall composite properties. This theory applies to the use of solid particulates as well as hollow spheres. Because of the high viscosity of such a highly loaded resin, the mixture could not flow into the mold without damaging the microspheres. To overcome this problem, the mold is pre-packed with the dry filler (i.e. a mixture of microspheres and fibers) . By applying packing theory as described above, the filler can be packed at a high density and so that segregation of ingredients does not occur. The use of maximum filler loading with a very low viscosity resin and appropriate coupling agents (for good iller-matrix adhesion) results in a material which, when vacuum liquid transfer molded as described herein, provides homogenous, void-free structures. The composite article formed in accordance with the preferred process embodiment of the present invention is referred to herein as a "fiber reinforced syntactic foam composite," which denotes a composite of a syntactic foam (i.e. a foam made by incorporating hollow spheres in a liquid resin) and reinforcing fibers dispersed throughout the syntactic foam. The composite article formed in accordance with the present invention is also referred to herein in a more general sense as a "resin based filler reinforced composite," which indicates a composite of a resin and reinforcing filler material. The compositions of preferred mixtures of resin, fibers, and microspheres used to form fiber-reinforced syntactic foam composites in accordance with one embodi¬ ment of the present are discussed in detail in copending patent applications Serial Nos. (PD-81188 and PD-81188A) , assigned to the present assignee. Preferred filler materials are fibers of predetermined length and hollow microspheres of predetermined diameter as discussed in the above-noted copending patent applications. The fibers may be formed of graphite, glass, carbon or polyamide, while the hollow microspheres may be formed of glass, silica or carbon. Carbon fibers useful as reinforcing agents in molding the present filler reinforced composites are high strength, high modulus fibers composed essentially of amorphous carbon but more preferably of graphite or pyrolytic graphite and generally referred to as graphite fiber. One type of graphite fiber which may be used in reinforcing the composites are HM-S graphite fibers (available from the Courtaulds Company of the United Kingdom) which have the following dimensions and physical properties: Modulus 50 million psi ( stress/strain ) ( 7.25 x 103 pascals ) Length 50 micrometers ( u ) Density 1.83 gm/crn-^ Glass microballoons or hollow microspheres used as fillers are composed essentially of silica and typical glass microballoons which may be used have the following dimensions and physical properties: . Diameter 10-200y Density 0.15 to 0.32 gm/cm2 Carbon microballoons are composed primarily of thin-walled carbon balloons and typical ones which may be used have the following dimensions and physical properties: Diameter 40μ Density ' 0.32 gm/cm^ As is known to the art, wetting of fillers, such as graphite fibers, by epoxy resins can be facilitated by the use of titanate sizing agents such as di(dioctyl- pyrophosphato)eth lene titanate (KR238M available from Kenrich Petrochemical Company of Bayonne, New Jersey), tetra(2,2-diallyloxymethyl 1-butoxy) titanium di(ditridecyl) phosphite (KR55 available from Kenrich) or titanium di(cumylphenylate)oxyacetate (KR134S, available from Kenrich) , whereby compaction and resin penetration of the filler by the resin is enhanced by the presence of such agents, and the use of such sizing agents represents a preferred practice of the present invention. The sizing agents may be dissolved in the mixture of resin, microspheres and fibers; or, preferably they are applied to a mixture of the microspheres and fibers in predetermined proportions by immersing this mixture in a solution of the sizing agent, followed by filtering and drying the mixture to provide a pretreated microsphere/fiber mixture. The following formulations show the range of proportions, in percent by weight and percent by volume, of the components which a filled heat curable epoxy resin formulation may suitably contain for use in the molding of filler reinforced composite articles in accordance with the process of the present invention. It should be noted that other resin-fiber-microsphere mixtures besides those specifically shown may also be 'used and may be adjusted to meet particular end use requirements. Component Wt.% Vol. % Epoxy Composition 50-75 23-60 Filler (graphite fiber and microballoons) with density of 0.54 to 0.33 20-50 38-77 Sizing Agent (organic titanate) 0-3 0-2 To form the epoxy composition, the epoxy resin, curing agent and curing accelerator chosen for the molding resin can be mixed in any conventional fashion. The curing agent can be mixed into the epoxy resin at room temperature. A solid curing agent in powdered form also can be admixed in the epoxy resin at room temperature by continuous agitation prior to mixing with the chosen epoxy resin. The use of polyglycidyl aromatic amines as the epoxy resin component of the composite to be molded provides a low viscosity, solvent-free, liquid epoxy component. For example, polyglycidyl aromatic amines such as diglycidyl orthotoluidene combined with curing agents such as nadic ethylanhydride have viscosities in the range of 125 to 500 centipoise (cps) when measured at 75°F (24°C). By using such low viscosity resins in the molding process of the present invention, total wetting and complete impregnation of the filler and voids therein is readily accomplished, resulting in homogeneous, void-free, composite structures. To prepare composite articles in accordance with the method of the present invention, a suitable quantity of filler comprising, for example, premixed and pre- treated fibers and microspheres as previously described, is loaded in a suitable mold cavity and the mold may optionally be vibrated to promote a uniform distribution of the filler in the mold. The mold may be made, for example, of tool steel, internally coated with a release coating such as a fluorocarbon polymer, e.g., teflon, or sprayed prior to loading of the filler with a release agent such as polyvinyl alcohol. The shape of the mold cavity will, of course, determine the shape of the article molded therein. To prepare for the molding process, the mold cavity is preheated to a temperature at which the thermosetting resin formulation is curable and is maintained at this temperature. In the case of polyglycidyl aromatic amine based epoxy resin formu¬ lations, the mold is preheated to about 150 to 250°F (66 to 122°C). A suitable quantity of the uncured resin, such as an epoxy resin formulated as described above, is introduced into the mold cavity in a quantity sufficient to fill the mold. A subatmospheric pressure or vacuum is then drawn on the mold, generally about 1 to about 200 mm Hg and preferably about 5 - 150 mm Hg to impregnate the filler and to remove entrapped air or other bubbles from the resin contents during this "active vacuum" stage. The vacuum is drawn for about 1 to 5 minutes either on the autoclave, the vacuum bag and press, or the mold itself. The vacuum is then released to allow for collapse of any bubbles. This "passive vacuum" stage is performed for about 1 to 5 minutes before any further molding steps are initiated. Next, a superatmospheric pressure on the order of about 100 to about 1000 psi (about 1.45 x 10" 2 to 1.45 x 10"-*- pascals) is imposed on the mold to cause the filler to compact and the resin to encapsulate the filler. The increased pressure provides optimized flow of the resin into the interstices between the filler particles, as well as forcing excess resin out of the mold by use of a resin bleeding means. This latter pressurization step may be accomplished by placing the mold in a laminating or transfer press or an autoclave. When a laminating press or autoclave is used, a vacuum bag is constructed around the compression tooling. When a transfer press is used, the permanent mold of the transfer press acts as the vacuum pressure chamber, with pressurization being produced by activation of the transfer ram. Under this pressure, the mold temperature is maintained at the temperature at which the thermosetting resin is curable, e.g., about 66 to about 121°C (about 150°F to about 250°F) in the case of heat curable epoxy resins for a predetermined time period, e.g., about 1 to 2 hours, until the resin/ filler mixture has sufficiently cured or has sufficient mechanical (green) strength to be ejected from the mold cavity as a unitary structure. Thereafter, the molded, partially cured composite is post-cured at the curing temperature or higher (e.g., about 250 to about 350°F or about 121 to 177°C) for 1.5 to 4 hours to fully cure the resin, in a suitable heating device such as an oven. In the post-cure step for filler reinforced composites, the molded composite article is generally cured in an unrestrained state. To avoid deformation of the composite caused by thermally induced sagging of the unrestrained composite, it has been found advantageous to heat the composite upon its removal from the mold at a rate of 3 to 5°F (1.5 to 3°C) per minute until the post-cure temperature is reached, and then cure the composite in an unrestrained condition at the postcure temperature for about 2 to about 5 hours. In accordance with another embodiment of the present invention for encapsulating and impregnating an electrical component, the general procedure described above for forming a composite article is followed except that the electrical component, such as a magnetic coil, rather than a filler material, is loaded into the mold cavity. The subatmospheric pressure used to produce impregnation of the coil or other electrical component is preferably about 1 to about 4 millimeters of mercury. The preferred superatmospheric pressure is about 50 to about 100 pounds per square inch (about 7.25 x 10 **3 to 1.45 x 10~ 2 pascals), and the mold contents are subjected to this superatmospheric pressure at a temperature of about 66 to about 121°C (about 150 to about 250°F) for a heat curable epoxy resin for about 30 minutes to 3 hours. After ejection from the mold, the resin is post cured at about 275°F to 325°F (about 135 to 163°C) for about 2 to 4 hours to ensure total curing and cross-linking of the resin composition. In order to encapsulate a number of devices at one time, an individual mold having multiple cavities may be used, with one device being placed in each cavity, The resin is introduced into the preheated individual mold and then the individual mold or a number of individual molds are placed in the permanent mold or chamber of the transfer press for the application of vacuum or pressurization in accordance with the process of the present invention. Further, when the process of the present invention is used to encapsulate electrical devices, matched molds are used to provide net parts, or parts having the desired final configuration, with negligible finishing operations being required. In addition, the fast cure time of the preferred resins described herein substantially reduces the molding time as compared to batch encapsulation. These latter two factors provide substantial savings in labor costs by using the process of the present invention rather than known batch encapsulation processes. It should be noted that subat ospheric pressures and superatmospheric pressures other than those speci¬ fically noted above might be required in order to achieve impregnation and encapsulation of a particular device or material. For example, a tightly wound coil might require more extreme vacuum or pressurization than a less tightly wound coil. In addition, the viscosity or other physical properties of the particular resin used might also affect the subatmospheric pressure required to produce impregnation of the article or the superatmospheric pressure required to produce compaction and encapsulation of the article. Further, it is recognized that such process details as the curing temperature, dwell time and post- cure conditions, will depend, among other things, upon the composition of the particular resin used. The specific process details recited above for an epoxy resin are not intended to limit the present invention, but only to provide an example thereof. Moreover, it is recognized that the process of the present invention may be partially or fully automated for use in a production environment. In another alternative process embodiment of the present invention " for encapsulating an article, the mold is first loaded with a predetermined quantity of filler and then the article to be encapsulated, such as an electrical component, is placed within the filler material so as to be surrounded by the filler. Then, the resin is introduced into the mold and the previously described procedure for encapsulating an electrical . component is followed. The article so formed comprises the electrical component impregnated with the resin and having an overcoat a few mils thick of the resin based filler reinforced composite surrounding the component and integral therewith. Such a device has improved properties due to the filler overcoat which a resin coating alone cannot provide, such as improved thermal shock resistance and improved coefficient of thermal expansion. By practicing the method of the present invention as discussed above, dimensionally stable, lightweight intricate resin based filler reinforced composite structures having homogeneous filler distributions are readily and consistently molded. Such composites are particularly useful for forming structures, such as antenna waveguides, for space applications. In addition, in accordance with the present invention, electrical components can be encapsulated with an electrical insulation and passivation layer to provide high reliability components. Moreover, because of their relatively low coef- ficient of thermal expansion and high thermal conduc¬ tivity, filled epoxy resin composite structures prepared in accordance with the method of the present invention have been determined to be especially amenable to metal plating processes when the surfaces thereof are prepared for electroless plating by plasma treatment. The relatively high adhesion of metal deposits to the composite surface is believed to be a function of both the topography of the plasma-treated surface plus the mechanical integrity of the remaining surface. Such metal plating may be required, as previously noted, to provide a conductive path in an antenna waveguide structure. To effect plasma treatment, the surface of the filler reinforced epoxy resin based composite is subjected to a plasma process with a reaction gas containing a mixture of air, nitrogen, or argon with oxygen, water vapor, nitrous oxide, or other resin oxidizing source, to remove the polymer "skin" and expose the filler, which provides a surface that is readily platable. Normal plasma etching conditions known in the art are used. For example for a plasma excitation energy of 200 watts/ft 2 of composite, an 02/inert gas source of approximately 1000 ml/minute, a vacuum pressure of 200u Hg, and one hour duration are used. Metal plating of the etched epoxy resin composite surface can be conveniently accomplished by electroless plating of the plasma-treated composite surface by standard procedures such as by dipping the treated composite in the plating solution for a time sufficient to achieve a uniform buildup of metal on the treated surface. Metals that can be plated on the molded epoxy resin based composites prepared in accordance with the present invention include copper, silver, nickel, cobalt, nickel/iron, nickel/cobalt, other nickel alloys and gold. For electroless copper plating, for example, an aqueous bath of " Shipley Co. #328 copper plating solution may be used, which contains copper sulf te, sodium potassium tartrate, and sodium hydroxide. Other electroless copper plating formulations can also be employed. The plating bath is agitated or stirred prior to immersion of the plasma-treated composite. Preferred plating temperatures are in the range of about 15 to about 95°C (about 59°F to 203°F) . Metal adhesion is excellent even after exposure of the plated composite to cycles of widely different temperatures, as described in Examples 1 and 2 herein. After an electroless copper plate has been formed on the composite of the present invention to provide a conductive surface, the copper plating may be built up to any desired thickness to provide a smooth surface finish, using known electrolytic plating solutions and methods; and followed by electrolytic plating of a second metal, such as silver, on the electrolytic copper plate using known solutions and methods. Such a silver layer may advantageously be used in antenna waveguide structure. The following Examples illustrate, but do not limit the present invention. EXAMPLE 1 A heat curable epoxy resin formulation was prepared as disclosed in my copending patent application Serial No. (PD-82339) and had the following composition: Resin Component W . (g s.) Diglycidyl orthotoluidine 100 Nadic methylanhydride 100 2-ethyl-4-methyl i idazole 2 This composition had a gel time of 25 minutes and a viscosity of 220 centipoise at 75°F (24°C). A filler mixture was prepared having the composition shown below and a density of 0.543 gm/cm 3 . Filler Component * WT. (gms. ) Carbosphere hollow carbon micro¬ balloons, 50μ mean diameter 50 HM-S graphite fiber, ~50μ length 50 ' Titanate sizing agent 1 KR238M Carbospheres are available from Versar Inc.; HM-S graphite fibers are available from Courtaulds Company of the United Kingdom; and KR238M is available from Kenrich Petrochemical Company. Using the detailed procedure previously described, ten grams of the filler composition (i.e. a mixture of the fibers and microspheres pretreated with the sizing agent as previously discussed) were loaded into a cleaned 5.5 inch x 0.5 inch (14cm X 1.3cm) wide steel slab mold internally coated with a polyvinyl alcohol release agent and the mold was vibrated for about 5 minutes. The mold was preheated to 212°F (100°C), the temperature at which hardening of the heat curable epoxy resin formulation was initiated. Forty grams of the epoxy resin formulation were poured into the mold containing the filler. The mold was placed in a laminating press, a nylon vacuum bag was constructed around the compression tooling of the press, and a vacuum pressure of 125 millimeters (mm) mercury pressure was maintained on the assembly for 2 minutes to draw down the resin to impregnate the filler and to degas the resin materials in the mold. The vacuum was then released without removal of the vacuum bag and the assembly held in this passive vacuum state for an additional 2 minutes. Thereafter, a constant positive pressure of approximately 800 pounds per square inch (about 5.5 x * 10 6 pascals) was imposed on the resin/filler mixture in the mold for 75 minutes at 212°F (100°C). During this pressurization stage, 69% by weight of the resin was bled from the mold. The molded composite slab had sufficient green strength to be ejected from the mold, whereafter it was post cured for 4 hours unrestrained, in an oven set at 300°F (149°C). The final void-free slab contained 60% by weight (77% by volume) filler and was cut into appropriate shapes for physical testing. The composite was found to have the physical properties which are summarized in Table I. TABLE I Physical Properties of Composite (1) Determined by American Society for Testing and Materials Standard No. D695 (ASTM D695) . 1 psi = 1.45 x 10~ 4 pascals. (2) Determined by ASTM D695, using crosshead speed. (3) Determined using quartz dilatometer to measure the change in length as a function of temperature A portion of the molded slab was surface plated ' with copper by subjecting the surface of the slab to an oxygen rich plasma as previously described, which resulted in a surface weight loss of about 1.5 percent, taking into consideration moisture and resin removal. The plasma-treated slab was then dipped into Shipley #328, electroless copper plating solution, as previously described, and then dried at 248°F (120°C) under 29 inches (737mm) Hg (guage pressure). The plated composite was then evaluated for adhesion of the deposited copper layer using an ASTM D3359 tape adhesion test before and after 25 cycles of thermal shock imposed on the plated surface by alternately dipping the plated specimen in liquid nitrogen (-320°F or -196°C) for 30 seconds and boiling water (212°F or 100°C) for 10 seconds. No loss of copper was observed. EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the resin/filler mixture, was maintained at 212°F (100°C) in the molding press for 2 hours using varying resin/filler ratios. The components of the molding compositions and the physical properties of the molded composites are summarized in Tables II and III, respectively. The definitions of Table I apply also to Table III. *Resin bleed = 100 where W-^ =• initial reι?in weight W2 = resin displaced from the mold, using a bleeder cloth. Vol. % calculated from resin bleed varies about 10-20% of the actual vol. % value. TABLE III Physical Properties of Molded Composites EXAMPLE 3 The procedure of Example 2 was repeated with the exception that the titanate sizing agent used in the filler mixture was Kenrich KR 134S titanate. The use of KR 134S titanate was observed to provide better wetting of the filler mixture by the resin. The components of the molding compositions and the physical properties of the molded composites are summarized in Tables IV and V, respectively. The definitions of Table II apply also to Table IV, while the definitions of Table I apply to Table V. TABLE IV Components of Molding Compositions TABLE V Physical Properties of Molded Composites Compressive Compressive Specimen Thickness Density Strength Modulus* No. (in.) (gm/cm 3 ) (psi) (psi) 4 1.070 0.889 11,300 5 1.042 0.915 13,500 567,000 6 1.065 0.914 12,900 789,000 ♦ Qualitative measure only, calcuated from cross- head speed as indication of rate of compression. EXAMPLE 4 The procedure of Example 1 was repeated using a series of resin/filler formulations containing different fillers and filler ratios. The compositions of the filler formulations are summarized in Table VI, while the physical properties of the molded composite slabs are summarized in Table VII. The definitions of Table I apply to Table VII. With regard to Table VI, the following definitions apply: a. Carbospheres are carbon microspheres having a specific gravity of 0.32 and a mean diameter of 50 micrometers, available from Versar Corporation. b. HM-S 50 (50μ) graphite fibers are graphite fibers havng a length of about 50 micrometers and a diameter of about 8 micrometers, available from the Courtaulds Co. of the United Kingdom. c. 1/4 mm HM-S 50 graphite fibers are graphite fibers having a length of about 250 micrometers and a diameter of about 8 micrometers, available from the Courtaulds Co. of the United Kingdom. d. C15/250 glass microballoons are composed of borosilicate glass, have a diameter of 10-200 micro¬ meters, a density of 0.15 gm/cm 3 , and a compressive strength of 250 psi, available from the 3M Company of Minnesota. TABLE VI Composition of Filler Formulations TABLE VII Physical Properties of Molded Composite Slabs The surfaces of the molded composite slabs of Table VII were then subjected to plasma treatment under the following conditions: 02/i ert gas source of approximately 1000 ml/minute, vacuum pressure of 200μ Hg, and one hour duration. The surfaces of the-plasma- treated slabs were then copper plated to a thickness of about 3-4 mils by dipping the etched slabs in an aqueous Shipley #328 electroless copper plating bath. The plated composite was then evaluated for adhesion of the deposited copper layer using the ASTM D3359 tape adhesion test and thermal shock cycle of Example 1. The- adhesion results are recorded in Table VIII below. TABLE VIII Adhesion Results EXAMPLE 5 This example illustrates the encapsulation of an electrical component using the process of the present invention. A heat curable epoxy resin formulation was prepared as disclosed in my copending patent application Serial No. (PD-82339) and had the following composition: Resin Composition WT. (gm) Diglycidylaniline 100 Methyl tetrahydrophthalic 114 anhydride Stannous Octoate 3 A standard test magnetic coil comprising 2 layers of copper winding separated by 3 mils of insulation and having a length of 2 inches (5.08 cm) and a diameter of 1 inch (2.54 m) was loaded into a cleaned steel mold which was 3 inches (7.62 cm) wide and 7.25 inches (18.4 cm) long. The mold was preheated to and maintained at 212°F (100°C), the temperature at which hardening of the heat curable epoxy resin formulation was initiated. The mold was placed in a transfer press and two hundred grams of the epoxy resin formulation noted above were metered into the mold containing the coil. A vacuum of 1-4 millimeters (mm) mercury pressure was applied to the mold and maintained on the assembly for 2 minutes to impregnate the coil and degas the resin materials in the mold. The vacuum was then released and the assembly held in this passive vacuum state for an additional 2 minutes. Thereafter a constant positive pressure of approximately 90 psi (1.3 x 10~ 2 pascals) was imposed on the coil/resin system in the mold for 2 hours at 212°F (100°C). During this pressurization stage, it was estimated that about 50% by weight of the resin was bled from the mold. The encapsulated coil having a 2 mil layer of resin thereon was removed from the mold whereafter it was post cured for 3 hours in an oven set at 275°F (135°C). The resistance of the encapsulated coil to electrical stress is recorded in Table IX below. TABLE IX Parameter Value Breakdown voltage ( KRMS ) ( D 6.93 30 second counts at Peak Heights ( PC ) ( 2 ) 0 2. 0 0 5 . 0 0 10 . 0 0 20.0 0 50 . 0 0 Total counts (PC/sec) ( 3 ) 0 Electrical Stress (V/mil) 2310 (1) Breakdown voltage is the voltage at which the insulation breaks down, and is a measure of the short-term dielectric strength of the insulator. It is measured by applying an AC voltage until arc-over through the insulation occurs. (2) PC (picocoulombs) is a measure of energy and is measured with a Biddle Corona Tester and Discharge Energy Counter. The material is tested for a 30-second count, which is a convenient time period for establishing the discharge rate of picocoulombs/second. The number of peak heights of a given value of picocoulombs is measured for 30 seconds. A pass/fail criterion is used, with a value of over 100 failing. (3) Total counts is a calculated average of the values measured in the 30-second counts noted above. Although variations are shown in the present application, many modifications- and ramifications will occur to those skilled in the art upon a reading of the present disclosure. These, too, are intended to be included herein. In particular, other materials besides those specifically described herein may be used in the method of the present invention by adjusting certain process details, such as curing temperatures, dwell time, and post-cure conditions, in accordance with information known in the art about such materials. Further, the particular subatmospheric and superatmo¬ spheric pressures and the temperatures used in the method of the present invention may be altered to be compatible with the particular resin, filler material, or electrical component being processed.

Claims: CLAIMS What is Claimed is: 1. A method for encapsulating and impregnating an article which comprises the steps of: (a) loading the article in a mold cavity; (b) heating the mold to a predetermined temperature and maintaining said temperature in said mold; (c) filling the mold cavity with a low viscosity heat curable thermosetting resin which cures at said predetermined temperature; (d) evacuating the mold cavity to a subatmospheric pressure to impose a vacuum on the mold to thereby impregnate the article with the resin and degas the mold contents; (e) releasing the vacuum to atmospheric pressure to burst any gas bubbles remaining in the mold contents; (f) applying a superatmospheric pressure to the mold to cause the heat curable resin to encapsulate the article loaded in the mold, the temperature and the superatmospheric pressure being maintained for a time sufficient to partially cure the resin and form a unitary structure; (g) ejecting the structure from the mold cavity; and (h) subjecting the ejected structure to a further heating cycle to completely cure- the resin. 2. The method of Claim 1 wherein the article is a quantity of a filler material whereby the resin encapsulates and impregnates particles of said filler in a manner which results in the formation of a resin based filler reinforced composite article. 3. The method of Claim 1 wherein the article is an electrical component whereby the resin encapsulates and impregnates the electrical component. 4. The method of Claim 1 wherein the heat curable resin is selected from the group consisting of an epoxy resin, a polyimide resin and a bismaleimide resin. 5. The method of Claim 1 wherein the heat curable resin is an epoxy resin comprising a mixture of a poly¬ glycidyl aromatic amine and a polycarboxylic acid anhydride curing agent. 6. The method of Claim 5 wherein the polyglycidyl aromatic amine is diglycidyl orthotoluidine and the polycarboxylic acid anhydride curing agent is nadic methyl anhydride. 7. The method of Claim 5 wherein the polyglycidyl aromatic amine is diglycidyl aniline and the polycarboxy¬ lic anhydride curing agent is methyl tetrahydrophthalic anhydride. 8. The method of Claim 5 wherein the structure ejected from the mold is heated at about 250 (121°C) to about 350°F (177°C) for about 1.25 to about 4 hours. 9. The method of Claim 2 wherein the filler material comprises fibers and hollow microspheres, the fibers being formed of a material selected from the group consisting of graphite, glass, carbon, and polyamide, and the microspheres being formed of a material selected from the group consisting of glass, silica, and carbon. 10. The method of Claim 2 wherein the subatmos¬ pheric pressure is about 1 to about 200 millimeters of mercury. 11. The method of Claim 2 wherein the super¬ atmospheric pressure applied to the mold is about 100 to about 1000 pounds per square inch (about 1.45 x 10~ 2 to 1.45 x 10""1 pascals). 12. The method of Claim 5 wherein the mold is heated to a curing temperature of about 150°F (66°C) to about 250°F (121°C) . 13. The method of Claim 2 wherein the heat curable resin is an epoxy resin and the completely cured epoxy resin unitary structure is subsequently electrolessly plated with copper. 14. The method of Claim 2 wherein the heat curable resin is an epoxy resin and the time sufficient to partially cure the resin is about 1 to 2 hours. 15. The method of Claim 3 wherein the superatmos¬ pheric pressure applied is about 50 to about 100 pounds per square inch (about 7.25 x 10~3 to 1.45 x 10~ 2 pascals) . 16. The method of Claim 3 wherein the subatmospheric pressure is about 1 to about 4 millimeters of mercury. 17. The method of Claim 3 wherein the heat curable resin is an epoxy resin and the time sufficient to partially cure the resin is about 0.5 to about 3 hours. 18. A method for molding an article comprising a low viscosity heat curable thermosetting resin and a filler material as set forth in Claim 2 wherein: a) said heating of said mold is performed prior to said loading of said article; and b) said mold cavity is filled simultaneously with said filler material and said resin. 19. An article formed by the method of Claim 1 comprising an electrical component in combination with and surrounded by a predetermined quantity of filler material whereby the electrical component is encapsulated and impregnated with the resin and coated with a layer of a resin based filler reinforced composite.

Inventors: OLDHAM SUSAN L, OLDHAM, SUSAN, L.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1984
Main classification: B29C39
IPC codes: B29C35, B29C39, B29C70, B29K101, B29K105, B29L31