United States Patent [19J
`Oldham
`
`[II) Patent Number:
`[45] Date of Patent:
`
`4,681,718
`Jul. 21, 1987
`
`[54] METHOD OF FABRICATING COMPOSITE
`OR ENCAPSULATED ARTICLES
`
`[75]
`
`Inventor: Susan L Oldham, Long Beach, Calif.
`
`[73] Assignee: Hughes Aircraft Company, Los
`Angeles, Calif.
`
`[21] Appl. No.: 770,917
`
`[22] Filed:
`
`Aug. 30, 198S
`
`Related U.S. Application Data
`[63] Continuation-in-part ofScr. No. 608,614, May 9, 1984,
`abandoned.
`
`Int. 0.4
`[51)
`. ..................... B29C 39/ 10; B29C 39/24;
`B29C 39/42
`[52) u.s. a ..................................... 264/ 102; 264/500;
`264/548; 264/272.11; 264/272.13; 264/331.12;
`264/33 1.16
`[58) Field of Search ........... 264/102, 500, 548, 272.11,
`264/ 272.13, 331.12, 331.16
`
`[56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,028,284 4/1962 Reeves ... ............................. 156/213
`3,541,194 11/1970 Resnick ................................. 264/71
`4,289,722 9/1981 Tranbarger ......................... 264/102
`4,321,418 3/1982 Dran et al. ...................... 264/102 X
`4,333,900 6/1982 Carey .............................. 264/102 X
`4,374,080 2/1983 Schroeder ........................... 264/ 102
`
`FOREIGN PATENT DOCUMENTS
`1305243 8/1962 France .
`1330854 5/1963 France .
`47-30797 I 1/1972 Japan .
`502886 3/1971 Switzerland .
`903734 8/1962 United Kingdom .
`1108595 4/1968 United Kingdom .
`1111436 4/1968 United Kingdom .
`
`OTHER PUBLICATIONS
`Katz, H. S. and J. V. Milewski, Handbook of Fillers and
`
`Reinforcements for Plastics. New York, Van Nostrand
`Reinhold, @ 1978, pp. 66-78; 326-330.
`Primary Examiner-Philip Anderson
`Attorney, Agent, or Firm-M. E. Lachman; A. W.
`Karamebelas
`[57)
`ABSTRACT
`A method for encapsulating an electrical component or
`forming a resin based filler reinforce{! composite article
`which comprises providing a rigid mold having a cavity
`and an opening in one surface of the mold connected to
`the cavity, and a chamber in which the mold is placed
`for applying heat and varying pressure to the mold
`contents. The chamber is preheated to the curing tem(cid:173)
`perature of the resin and is maintained at this tempera(cid:173)
`ture. The component or filler is loaded into the mold
`cavity and the mold is placed in the preheated chamber.
`Then the mold cavity is filled with a low viscosity heat
`curable, thermosetting resin such as an epoxy resin.
`Next, the mold cavity is evacuated to a subatmospheric
`pressure to impose a vacuum on the mold to impregnate
`the component or filler with the resin, degas the mold
`cavity contents, and expand any voids in the resin. The
`vacuum is released to atmospheric pressure to collapse
`any gas bubbles remaining in the mold contents. Then,
`a superatmospheric pressure is applied to the mold to
`burst any gas bubbles and cause the resin to compact
`and encapsulate the component or filler. The tempera(cid:173)
`ture and the superatmospheric 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 further heat(cid:173)
`ing to completely cure the resin. The method may be
`performed in cyclic fashion to form continuously in
`sequence a plurality of encapsulated components or
`composite articles.
`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 high voltage resistance.
`
`25 Claims, 1 Drawing Figure
`
`lOAO FillER OR C)MPONEIT
`INTO MOLD
`
`PlACE MOLD 11TH
`flllfl OR COMPOIEWT 1---i
`IN PREHEATED
`PRESS / AUTOCLAVE
`
`APPLY SUPERATNOSPHEAIC
`I PIESSURE TO MOl& TO PARTIALlT
`.
`CURE RESIN
`
`001
`
`SONY 1014
`
`

`

`LOAD fiLLER OR COMPONENT
`INTO NOLO
`
`PREHEAT PRESS/
`AUTOCLAVE
`
`' . ~
`
`PLACE MOLD WITH
`fiLLER OR COMPONENT
`IN PREHEATED
`PRESS I AUTOCLAVE
`
`'
`
`REPEAT fOR
`CYCLIC PROCESS
`
`fiLL NOLO CAVITY
`WITH RESIN
`
`r-
`
`'
`AT 1-4 mm Hg
`fOR COMPONENTJ
`
`APPLY VACUUM TO
`MOLD fOR 1-5 MIN.
`
`r---
`I--
`
`RELEASE VACUUM TO
`ATMOSPHERIC PRESSURE
`
`-
`
`AT l-200mm Hg
`fOR fiLLER
`
`EJECT PARTIALLY
`
`APPLY SUPERATMOSPHERIC
`
`CURED STRUCTURE - PRESSURE TO MOLD TO PARTIALLY f+
`
`fROM MOLD
`
`COMPLETE CURE
`Of RESIN
`BY HEAT
`
`CURE RESIN
`t
`FOR 0.5-3 HRS
`AT 50-100 PSI
`fOR COMPONENT
`
`-
`
`fOR 1-2 HRS
`AT 100-1000 P.SI
`fOR fiLLER
`
`~
`00
`•
`
`t-C a a
`
`c....
`~
`N
`......
`...
`......
`\0
`00
`........
`
`...
`.&:;:.
`0'\
`00
`~ ...
`......:.I
`~
`00.
`
`002
`
`

`

`1
`
`4,681,718
`
`METHOD OF FABRICATING COMPOSITE OR
`ENCAPSULATED ARTICLES
`
`This application is a continuation-in-part of patent 5
`application Ser. No. 608,614, filed May 9, 1984 now
`abandoned.
`
`2
`bly, the components, such as magnetic coils, capacitors,
`diode arrays, transformers, stator generators, and resis(cid:173)
`tor networks, are commonly encapsulated with syn(cid:173)
`thetic 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 a!, previously referenced, at page
`327. The conventional encapsulation processes used are
`BACKGROUND OF THE INVENTION
`batch processes, i.e., processes which are not in continu-
`10 ous production, but rather are carried out on a limited
`1. Field of the Invention
`This invention relates to a method of fabricating
`number of items at one time.
`Because of their excellent adhesion, good mechani-
`lightweight, composite materials, as well as to a method
`cal, humidity and chemical properties, epoxy resins are
`of encapsulating electrical components to provide in-
`used extensively both as encapsulants for electronic
`creased resistance to electrical stress. More particularly
`with regard to the former, this invention relates to a IS components and in combination with glass and graphite
`fibers and microballoons in the manufacture of the pre-
`method of preparing epoxy resin based filler reinforced
`composite members exhibiting improved performance
`viously discussed high performance reinforced compos-
`characteristics such as reduced weight and thermal
`ite structures, such as antenna. The encapsulated article
`or filled epoxy resin composite is molded using conven-
`expansion coefficients.
`2. Description of the Prior Art
`20 tiona! molding techniques, such as compression mold-
`The successful utilization of filler reinforced compos-
`ing or transfer molding, in which the required materials
`ites in aerospace applications, such as antenna fabrica-
`including the resin are loaded into a mold, and curing of
`tion, imposes several important requirements on the
`the resin is effected under increased pressure, with the
`filler reinforced composite. A feature of primary impor-
`load being applied directly to the mold by the action of
`tance is that the composite have strength and dimen- 25 a contained media, namely a gas for pneumatic pressure
`or a liquid for hydraulic pressure. In compression mold-
`sional stability characteristics (e.g. a low linear coeffici-
`ent of thermal expansion) calculated to withstand the
`ing, the pressure is applied to a platen which then
`rigors of environmental temperature cycling.
`presses on an opening in the mold which communicates
`In addition, for complex structures such as antennae,
`with the mold cavity. In transfer molding, the pressure
`structures molded from epoxy resin based composites 30 is applied to a transfer ram which then presses on the
`are preferred over metal structures since the composite
`mold contents through the opening in the mold. Prob-
`can be molded directly, thereby avoiding costly rna-
`]ems encountered in the molding of the epoxy systems
`chining operations. Glass and graphite fiber reinforced
`and especially filled epoxy resin systems have limited
`the use of these resins as encapsulants, as well as for the
`epoxy based composites are finding growing use in
`aerospace applications because of their high strength- 35 fabrication of structural composites useful in a space
`to-weight ratio. These composite materials, because of
`environment. In molding filler reinforced composites,
`their relatively low coefficient of thermal expansion (a),
`the problem is aggravated due to the fact that high filler
`loadings, e.g. 40 to 60 percent by weight, are required in
`find wide application in structural components such as
`antenna used in space. Such materials are described, for
`the epoxy resin based composite structures to achieve
`example, by H. S. Katz and J. V. Milewski, in the book 40 the required low a value and corresponding high di-
`entitled "Handbook of Fillers and Reinforcements for
`mensional stability. Thus, when using transfer molding
`Plastics," Chapter 19, Hollow Spherical Fillers, Van
`or batch encapsulation techniques, the use of high filler
`Nostrand Reinhold Company, New York, 1978, pages
`contents in the epoxy system impedes adequate resin
`326 to 330. In a space environment where one face of
`flow into the mold because of the high viscosity of such
`the structural member is subjected to constant sunlight 45 a resin system. This, in turn, results in varying filler
`orientations and distributions within the geometrical
`while the opposite face of the member is in darkness, the
`face exposed to sunlight is heated considerably more
`areas in the complex structures being molded, with a
`resultant Joss in a and ultimate dimensional stability, as
`than the opposed face. Such non-uniform heating causes
`uneven expansions of the structural members making up
`well as erratic adhesion of metal layers plated on the
`the antenna with the resultant distortion of the antenna 50 surfaces of the structure. This problem is further aggra-
`from its desired shape. Thus, aerospace use requires that
`vated when hollow particulates such as glass or graph-
`some critical antenna structural components exhibit a
`ite microballoons are used as filler materials since the
`dimensional stability over a ten year lifetime in the
`high viscosity filled epoxy resin may have unpredict-
`operating range of 54° to 115° F. (12" to 46° C.) and
`able densities due to microballoon fractures and voids in
`have densities equal to or less than 0.9 grams/cm3. 55 the filler/resin mixture. Further, high viscosity resins
`When the structural components are electroplated with
`are unable to penetrate and impregnate the filler to
`a metal layer to provide, for example, utility as a elec-
`provide homogenous systems.
`tromagnetic interference (EMI) shielding or a conduc-
`In addition, with regard to encapsulants for electrical
`tive path in an antenna waveguide structure, the metal
`components, state-of-the-art heat curable epoxy resin
`coating layer must also withstand thermal cycling in the 60 systems used as encapsulating resins have the disadvan(cid:173)
`tage that the viscosity of the resin at working tempera(cid:173)
`extremes of the space environment without loss of adhe-
`tures, e.g. 100° C., (75° F.), are quite high, e.g., 500
`sion.
`centipoise (cps), and such resins are not intruded com(cid:173)
`Furthermore, in another area of current interest, it is
`recognized that high voltage power supplies and pulse
`pletely into crevices present in the electrical compo(cid:173)
`nent. Thus, the insulation is often incomplete and defec(cid:173)
`forming networks for aerospace use must meet high 65
`tive.
`standards of performance and reliability for long peri(cid:173)
`It is the primary object of the present invention to
`ods under extreme environmental conditions. To assure
`provide a method for molding filled thermosetting res-
`trouble-free operation of components within the assem-
`
`003
`
`

`

`4,681,718
`
`BRIEF DESCRIPTION OF THE DRAWING
`The FIGURE presents a flow chart representation of
`the process steps for two preferred embodiments of the
`present invention.
`
`50
`
`SUMMARY OF THE INVENTION
`The above objectives are achieved in accordance
`with the present invention, which is directed to a
`method for encapsulating an electrical component or 20
`forming a resin based filler reinforced composite article
`which comprises providing a rigid mold having a cavity
`and an opening in one surface of the mold connected to
`the cavity, and a chamber in which the mold is placed
`for applying heat and varying pressure to the mold 25
`contents. The chamber is preheated to the curing tem(cid:173)
`perature of the resin and is maintained at this tempera(cid:173)
`ture. The component or filler is loaded into the mold
`cavity and the mold is placed in the preheated chamber.
`Then the mold cavity is filled with a low viscosity heat 30
`curable, thermosetting resin such as an epoxy resin.
`Next, the mold cavity is evacuated to a subatmospheric
`pressure to impose a vacuum on the mold to impregnate
`the component or filler with the resin, degas the mold
`cavity contents, and expand any voids in the resin. The 35
`vacuum is released to atmospheric pressure to collapse
`any gas bubbles remaining in the mold contents. Then,
`a superatmospheric pressure is applied to the mold to
`burst any gas bubbles and cause the resin to compact
`and encapsulate the component or filler. The tempera- 40
`.. ture and the superatmospheric 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 further heat(cid:173)
`ing to completely cure the resin. The method may be 45
`performed in cyclic fashion to form continuously in
`sequence a plurality of encapsulated components or
`composite articles.
`
`4
`3
`dielectric properties may be used in the practice of the
`ins such as polyimides, bismaleimides, and particularly
`present invention.
`epoxy resins, to provide resin based filler reinforced
`Epoxy resins preferred in the practice of the method
`composite structural components which have a low
`coefficient of thermal expansion rendering the compos-
`of the present invention are polyglycidyl aromatic
`ite relatively insensitive to environmental temperature 5 amines, i.e. N-glycidyl amino compounds convention-
`ally prepared by reacting a halohydrin such as epichlo-
`cycling while possessing desirable performance proper-
`ties of low density, high strength, and amenability to the
`rohydrin with an amine. Examples of those preferred
`plating of adherent metal films and coatings.
`polyglycidyl aromatic amines include diglycidyl ani-
`It is a further object of the present invention to pro-
`line, diglycidyl orthotoluidine and tetraglycidyl meta-
`vide a method for encapsulating electrical components 10 xylylene diamine.
`or devices to provide components or devices having
`The epoxy resins are admixed with polyfunctional
`improved reliability and improved electrical properties.
`curing agents to provide heat curable epoxy resins
`Another object of the present invention is to provide
`which are cross-linkable at a moderate temperature, e.g.
`about too· C., to form thermoset articles. Suitable poly-
`methods of the type described above which can be
`performed in cyclic and continuous fashion.
`15 functional curing agents include polycarboxylic 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(cid:173)
`ylic 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 or(cid:173)
`ganometallic compounds such as stannous octoate, co-
`balt 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 U.S. Pat. No.
`4,559,272, assigned to the present assignee. This resin is
`particularly useful for providing high electric stress(cid:173)
`resistant encapsulated components.
`The term "curing" as used herein denotes the conver-
`sion of the thermosetting resin into an insoluble and
`infusible crosslinked product and, as a rule, with simul(cid:173)
`taneous shaping to give shaped articles.
`To prepare resin based filler reinforced composite
`articles in accordance with one embodiment of the pres(cid:173)
`ent invention, the epoxy resin in admixture with the
`curing agent and accelerator may be further mixed
`before curing with fillers and reinforcing agents, as for
`example, glass fibers, carbon fibers, graphite fibers,
`Kevlar fibers, ceramic fibers such as AI203 or Ab0J/(cid:173)
`Si02, ceramic whiskers such as silicon carbide or silicon
`nitride, glass microballoons, carbon microballons, phe(cid:173)
`nolic microballoons, ceramic particles, or glass parti(cid:173)
`cles, and the mixture is loaded into the mold. However,
`55 it is preferred in the practice of the method of the pres(cid:173)
`ent 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 pre-
`mixed 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 tita(cid:173)
`nate or zirconate wetting agent and/or a silane or zir(cid:173)
`coaluminate 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,
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The heat curable, thermosetting resins employed in
`the molding compositions used in this invention can be
`any low-viscosity, liquid heat curable resin. Preferred
`heat curable resins are low-viscosity epoxy resins hav- 60
`ing 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). 65
`In addition, other resin compositions, such as polyimide
`compositions and bismaleimide compositions, having
`appropriate viscosities, gel times, green strengths, and
`
`004
`
`

`

`4,681,718
`
`5
`the mold may optionally be vibrated to promote a uni(cid:173)
`form 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 5
`largest particle size filler in a particular reinforcement
`system packs to produce the gross volume of the sys(cid:173)
`tem, 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 10
`volume. This theory is discussed by Harry S. Katz and
`John V. Milewski, in the book entitled "Handbook of
`Fillers and Reinforcements for Plastic," Chapter 4,
`Packing Concepts in Utilization of Filler and Reinforce(cid:173)
`ment Combinations, Van Nostrand Reinhold, 1978. The 15
`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 20
`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 micro(cid:173)
`spheres, fibers, and/or whiskers). By applying packing
`theory as described above, the filler can be packed at a 25
`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 filler-matrix adhesion) results in a ma(cid:173)
`terial which, when vacuum liquid transfer molded as 30
`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 35
`foam composite," which denotes a composite of a syn(cid:173)
`tactic foam (i.e. a non-porous structure made by incor(cid:173)
`porating hollow spheres in a liquid resin) and reinforc(cid:173)
`ing fibers dispersed throughout the syntactic foam. The
`composite article formed in accordance with the pres- 40
`ent 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, fi- 45
`hers, and microspheres used to form fiber-reinforced
`syntactic foam composites in accordance with one em(cid:173)
`bodiment of the present are discussed in detail in co(cid:173)
`pending patent application Ser. No. 607,847 and U.S.
`Pat. No. 4,568,603 assigned to the present assignee. 50
`Preferred filler materials are fibers and/or whiskers of
`predetermined length and hollow microspheres of pre(cid:173)
`determined diameter as discussed in the above-noted
`copending patent applications. The fibers may be
`formed of graphite, glass, carbon, ceramic or polyam- 55
`ide, while the hollow microspheres may be formed of
`glass, silica, phenolic or carbon. The whiskers may be
`formed of such materials as metal oxides, carbides, ha(cid:173)
`lides or nitrides, such as silicon carbide, silicon nitride,
`sapphire (Ab03), MgO, MgO-Ab03, Fe203, BeO, 60
`Mo03, NiO, Cr203, ZnO, potassium titanate, or boron.
`The term "whiskers" is defined in Katz and Milewski,
`previously referenced, at pages 446-453 as "fibers
`grown under controlled conditions that lead to the
`formation of a single crystal" and having minimum 65
`length-to-diameter ratios of 10:1 and maximal cross-sec(cid:173)
`tional areas of 7.9X lQ-5 in.2 (corresponding to circular
`cross-sections of 0.010 in. diameter). Typical whiskers
`
`6
`have a diameter of less than one micrometer and high
`aspect ratios. The internal and surface perfection of
`. whiskers gives them high toughness and nonfriability in
`handling, high tensile strengths and Young's moduli as
`compared to fiberglass or polycrystalline fibers. For the
`discussion herein unless otherwise noted, whiskers are
`included in the term "fibers".
`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): (3.45 X toll pascals)
`Length: 50 micrometers (J.A,)
`Density: 1.83 gm/cm3
`Diameter: 8 micrometers
`Whiskers which are particularly useful in practising
`the present invention comprise silicon carbide. One
`type of such whiskers are Silar SC-9 (available from
`ARCO Metals Company, a Division of Atlantic Rich(cid:173)
`field Company) which have the following dimensions
`and physical properties:
`Modulus: 70 to 120 million psi
`(stress/strain): (4.83 X toll to 8.27 X toll pascals)
`Length: 10 to 80 micrometers
`Diameter: 0.8 micrometers
`Density: 3.2 gm/cm3
`Glass microballoons or hollow microspheres used as
`fillers are composed essentially of silica and typical
`glass microballoons which may be used have the fol(cid:173)
`lowing dimensions and physical properties:
`Diameter: 10-200J.A,
`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 physi(cid:173)
`cal properties:
`Diameter: 40J.t
`Density: 0.32 gm/cm3
`As is known to the art, wetting of fillers, such as
`graphite fibers, by epoxy resins can be facilitated by the
`use of titanate wetting agents such as di(dioctylpyro(cid:173)
`phosphato)ethylene titanate (KR238M available from
`Kenrich Petrochemical Company of Bayonne, N.J.),
`tetra(2,2-diallyloxymethyl 1-butoxy)
`titanium di(di-
`tridecyl) phosphite (KR55 available from Kenrich) or
`titanium di(cumylphenylate)oxyacetate (KRl34S, avail(cid:173)
`able 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 and
`wetting agents represents a preferred practice of the
`present invention. Coupling agents, such as silanes and
`zircoaluminates, may be used in conjunction with the
`titanates to achieve a chemical bond between the or-
`ganic resin and the inorganic fillers/fibers. The sizing
`and wetting 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 micros(cid:173)
`phere/fiber mixture.
`
`005
`
`

`

`4,681,718
`
`7
`The following formulations show the range of pro(cid:173)
`portions, 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 accor- 5
`dance with the process of the present invention. It
`should be noted that other resin-fiber-microsphere mix(cid:173)
`tures besides those specifically shown may also be used
`and may be adjusted to meet particular end use require(cid:173)
`ments.
`
`8
`the mold. The resin may be introduced under vacuum,
`such as less than about one millimeter of mercury, at
`atmospheric pressure, or under superatmospheric pres-
`sure. Introduction of the resin into the mold under vac(cid:173)
`uum is preferred. A subatmospheric pressure or vacuum
`is then applied to the press to draw on the mold, gener-
`ally about I to about 200 mm Hg and preferably about
`5-150 mm Hg to impregnate the filler and to expand
`bubbles of entrapped air or other gas in the resin con-
`to tents during this "active vacuum" stage. When a lami-
`nating press or autoclave is used, a bag is constructed
`around the compression tooling to provide a vacuum
`chamber. 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. (If a vacuum bag is used, the
`bag is vented to release the vacuum.) This "passive
`vacuum" stage is performed for about 1 to 5 minutes
`before any further molding steps are initiated. Next, a
`20 superatmospheric pressure on the order of about 100 to
`about 1000 psi (about 6.90X J05 to 6.90X J06 pascals) is
`imposed on the mold to burst any remaining bubbles
`and cause the filler to compact and the resin to encapsu-
`late the filler. The increased pressure provides opti(cid:173)
`mized 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 process. This latter
`pressurization step may be accomplished in a laminating
`or transfer press or an autoclave. When an autoclave is
`used, inert gas is pumped into the autoclave chamber,
`thereby pressurizing the mold. When a laminating press
`is used, the platens are moved into intimate contact with
`the mold and vented bag and a positive pressure is ap(cid:173)
`plied, either hydraulically or pneumatically. When a
`trausfer press is used, the permanent mold of the trans(cid:173)
`fer press acts as the vacuum pressure chamber, with
`pressurization being produced on the mold contents by
`activation of the transfer ram which exerts a compress-
`ing action that translates to the contents of the mold
`cavity. Under this pressure, the mold temperature is
`maintained at the temperature at which the thermoset(cid:173)
`ting 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. By ejecting the "green" structure from the
`heated assembly, the mold is at once available for the
`introduction of another quantity of filler material and
`continuation of the process of the present invention in a
`cyclic fashion. The advantages of such a cyclic process
`are discussed hereinbelow with regard to the encapsula-
`tion of a component in accordance with the process of
`the present invention. The molded, partially cured com-
`posite is post-cured at the curing temperature or higher
`(e.g., about 250° to about 350° F. or about 121° to l77o
`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 arti-
`cle is generally cured in an unrestrained state. To avoid
`deformation of tile composite caused by thermally in(cid:173)
`duced 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. (IS to 3°
`C.) per minute until the post-cure temperature is
`reached, and then cure the composite in an unrestrained
`
`Component
`Epoxy Composition
`Filler (graphite fiber,
`and/or whiskers, and
`microballoons) with
`overall density of
`0.33 to 0.54
`Sizing Agent (organic
`titanate)
`
`Wt.%
`
`50-75
`20-50
`
`Vol.%
`23-60
`38-77
`
`0-3
`
`0-2
`
`15
`
`;"',-
`
`40
`
`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 25
`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 ofpolyglycidyl aromatic amines as the epoxy
`resin component of the composite to be molded pro- 30
`vides a low viscosity, solvent-free, liquid epoxy compo(cid:173)
`nent. For example, polyglycidyl aromatic amines such
`as diglycidyl orthotoluidene combined with curing
`agents such as nadic methylanhydride have viscosities
`in the range of 125 to 500 centipoise (cps) when mea- 35
`sured at 75° F. (24° C.). By using such low viscosity
`t> • 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, as represented in the
`FIGURE, a suitable quantity of filler comprising, for
`example, premixed and pretreated fibers and/or whis(cid:173)
`kers, and microspheres as previously described, is 45
`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 is rigid and may be
`made, for example, of tool steel, internally coated with
`a release coating such as a fluorocarbon polymer, e.g., 50
`teflon, or sprayed prior to loading of the filler with a
`release agent such as polyvinyl alcohol. The mold may
`optionally be formed of another hard tooling (i.e. non(cid:173)
`disposable) metal which is capable of withstanding the
`increased and decreased pressures used in the process of 55
`the present invention. The mold may be a single unit or
`a two-part mold in which the parts are separable. The
`shape of the mold cavity will, of course, determine the
`shape of the article molded therein. To prepare for the
`molding process, the press is preheated to a temperature 60
`at which the thermosetting