CA1239750A - Fiber-reinforced syntactic foam composites and method of forming same - Google Patents

Abstract

ABSTRACT
Fiber-reinforced syntactic foam composites having a low specific gravity and a low coefficient of thermal expansion suitable for forming lightweight structures for spacecraft applications are prepared from a mixture of a heat curable thermosetting resin, hollow microspheres having a diameter of about 5 to 200 micrometers and fibers having a length less than or equal to 250 micrometers.

Description

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FI~ER-REINFORCED SYNTACTIC
ROAM COMPOSITES
AND cathode OF ~ORMIIaG MOE

BACKGROUND OF TIE I~YENTION
Jo ' 1. Field of the Invention The present invention relates, in general, to 5 syntactic foam composites and, more particularly, to fiber-reinforced thennosetting resin toed 3ynta~tic foam composites exhibiting a low pacific gravity and a low Coffey iciest of herbal expansion, 10 discretion of the Prior art A continuing objective in the development of satellites us to optimize satellite payload weight I: One mean; of achieving this objective is Jo reduce the intrinsic weight of Roy operatiorlal en within 15 eye spacecraft.., It has been recognized by the art that the desired weigh reduction could be realized by repl~cirlg conventional Motorola, such as aluminum, with lower density synthetic composites possessing requisite mech~n~c~sl, 'thermal and chemical stability.
20 Included in thus low density 6yne-hetic compv~ites is a group ox trials referred Jo in the art a ~yntactlc ohm .

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Joy I syntactic foams are produced by diapering microscopic rigid, hollow ox solid particles in a liquid or semi-liquid thermose~tin~ resin end then hardening thy system by curing The particles are s generally spheres or ~icroballoons of carbon, polystyrene t finlike ruin; urea-formaldehyde resin, Gus or silica, ranging fry 20 o 2~0 micrometer diameter.
Commercial ionospheres have specific gravities raying from 00033 Jo 0.33 for hollow spheres end up to 2.3 for solid glass pharisee The liquid resins used are the ~-~ usual resin used in molding reinforced articles, e.g., - Epoxy resin, polyesters, end urea-for~aldehyde Rosen,.
In order to form such foams, the resin containing a curing agent thoroughfare, and micro spheres my be mixed to form a paste which is then cat into the desired hype and lured to form the foam The latter method, us well a other known methods for forming syntactic foams, is described by Patrimony en at in the publication entitled syntactic Foams I. Preparation, Structure, 20 end Propriety Jo in the Journal of Solely r Plastics, July/Augu~t 1980, pages 22302~9, when fabricated in Lowry block form, such foams possess a compressive strength which has made the suitably for use in submerged structure In addition the more pliable versions of the foam ore utilized a fuller materials which, after hardening, function machinable, local-densification substance in applications such us automobile repair and the filling of structural honeycombs. Despite these characteristic of adequate compressive strength, good machinability, and light weight, Bush foams luck the degree of dimensional end thermal stability required to render them applicable for the spacecraft environment More specifically, syntactic foam systems tend Jo exhibit varying filler orientation and distributions within the geometrical areas in a molded intricate structure, which limits the structural intricacy thought I

con ye achieved, as jell US reducing dimensional stubbles If syntactic foam yummy are too highly fillet, sacrifices are lade in moldability coefficient of thinly ~xpan3ion, ~'cr~ngth, density, dln~ensional stability and typhoons Harvard such foams tend to . `
exhibit poor adhesion to Metallic plzltin~a which it required to form the desired product; such as an antenna compote nut ., In order for the syntactic foam to be useful as a $UI:)5titute for aluminum in antellna end antenna I- microwave ~or~ponen~s in a cpa~ecraft~ the foam just hove the feloniously charaeteristicsr ( I The material snowsuit have a pacify to gravity ox 1.00 or lets, as compared to a specific gravity of 2 .7 for ~lumlnum.
( 2) The maternal must h21ve linear Coffey i-kink of thermal expansion ( I or CUTE) comparable . o that of aluminum, preferably close to 13 x 10-5 in/in~F t 23 x 10 6 cm/cm/C) or less .
Thermal distortion of antenna components subjected to thermal cycling in the extremes of the space I- environment to a major contributing factor to julienne loss, pointing error, and phase shifts.
( 3 ) The material Utah meet the National Aeronautics and Space l~dmini~tration (NASA) outguessing requirement to insure that the atonal doe not release gaseous component ~ub~t~nc~s which undesirably accumulate on other spacecraft parts in the outer space vacuum.
g 4 ) The material mutt have lony-term s'caS~ility, us required for porks exposed to thy space temperature environment ego -luff to 250F or -73C to 121C3 for extended periods of time, arch 10 years 4 75 [3 (5) The material must be capable of being cast into complex configurations in order to form component parts for antenna structures, such as wave guides or antenna feed distribution networks.
The art, until the present invention, has been unable -to satisfy these requirements and particularly the requirement for a low coefficient of thermal expansion (a). Thus, known epoxy resin based syntactic foams filled with 10 to 30% by volume hollow micro spheres generally have a in the range of 17 to 36 x 10 6 in/in/F
(30 to 65 x 10-6 cm/cm/C).
A need, unsatisfied by existing technology, has thus developed for a syntactic foam material which is both lightweight and of sufficient mechanical, thermal and chemical stability to enable it to be substituted for aluminum in physically demanding satellite environments.

SUMMARY OF THE INVENTION
A fiber-reinforced syntactic foam composite having a specific gravity less than 1.0 and a coefficient of thermal expansion of about 9.0 x 10 6 in/in/F (16.2 x 10 6 cm/cm/C) or less, the composite being prepared from an admixture of: -a) a heat curable thermoset-ting resin comprising:
an uncured polyglycidyl aromatic amine, a polycarboxylic acid android curing agent, and a curing accelerator selected from -the group consisting of substituted imidazole compounds and organometallic compounds;
d) hollow micro spheres having a diameter in the range of about 5 to about 200 micrometers; and c) fibers having a length of less than or equal to 250 micrometers.
The syntactic foam composites of the present invention can be cast as complex structure which contain lightweight hollow micro spheres having fibers such as graphite fibers, in the voids between the micro spheres, with the micro spheres and fibers being . . .

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1 bonded together by the heat cured resin ~atrixO The composites of the present invention readily meet the specific ~ravityV coefficient of thermal e~p~n~i~n and NASH outguessing requirements, which easily qualify the S composites us aluminum substitutes for spacecraft use In order to for the fiber-r~sin-microsphere composite of the present invention hiving the d0sirecl density end coefficient of thermal expansion, each of I`- the three components must be selected 80 that the resulting combination thereof provides a mixture mendable to being cast into the desired configuration, AS well as providing a final product having the required structural and physical properties. Acceptable mixtures just ho viscosity that produces an accurate, void-free casting with uniform material properties. In addition, the proportion of giber in the composite must provide the required thermal expansion, stren~th9 and stiffness properties. further, the ionosphere component just be chosen to provide the required lo density in ( the composite. Finally, each of the components must be capable of being combined with the other components end the fact of each on the other in the mixture thereof, US a well a in the final composite mutt be taken into account. In particular the properties of the composite are influenced by the properties, relative volume ratios, and interactions of the individual components.
More specifically d~ns~y, strength, Stephens brutalness, coefficient of thermal expansion and proces~ibility art strong functions of filler and fiber type volume ratios and micro packing. the following discus in provides a more detailed consideration of these various actors It should be noted that in the Jo , I 75~

1 following discussion the term syntactic foam is used herein to denote a filled polymer made by dispersing rigid, microscopic particles in a fluid polymer or resin and then curing the resin, as it known in the art. The term fiber reinforced syntactic foam composite it used herein to denote the cured product formed from the mixture of resin, ~icroballoon~, and reinforcing fibers in accordance with the present invention.

I Heat Curably resin Jo The heat curable thermosetting resins used to I- prepare the syntactic foam composites of the present invention can be any heat curable thermosetting resin having appropriate viscosity for casting (e.g., less than 1000 centipoise), pot life (cog., greater than 2 hours), coefficient of thermal expansion, and thermal stability in the temperature range of -100F to 250F
~-73C to 121C) required in the space environment.
The resin material contains a curing agent which reacts 2Q with the resin to produce a hardened material. Curing agents end other additives will of course affect thy - viscosity and other properties of the final mixture I-- from which the composite is formed. Examples of suitable resins include low viscosity, polymerixable liquid polyester resins which comprise thy product of the reaction of at least one pol~meri~able ethylenically unsaturated ~olycarboxylic acid, such as malefic acid or its android, and a polyhydric alcohol, such as, for example, propylene glycol and optionally, one or more saturated polycarboxylic acids, such as for example, phthalic acid or its android. Other suitable resin include condensates of formaldehyde such as urea-formaldehyde, melamine-formaldehyde and phenol-formaldehyde resins Preferred resins for use in the 1 practice of the present invention are epoxy resins having I epoxy groups or mixtures of such resin, end include cycloaliphatic epoxy resins such a the gladly ethers of polyphenols, liquid Bisphenol-A
diglycidyl ether epoxy resins (such as whose sold under the-trademarks Eon 815, Eon 825, Eon 828 by Shell Chemical Company) f phenolfsrmaldehyde novolac polyglycidyl ether epoxy resins (such as those sold under the trademarks DEN 431, DEN 438 and DEN 439 by Dow Chemical Company), and epoxy crossly novel 5 (such as those sold under the trademarks EON 1235, EON 1773, EON 1280 and EON 1299 by Cuba Products Company).
The particular epoxy resins proofread in the practice of the present invention are polyglycidyl aromatic amine, i.e. N-glycidyl amino compounds prepared by reacting a halohydrin such us epichlorohydrin with an amine. Examples of the Yost preferred polyglycidyl aromatic mines include diglycidylaniline, diglycidyl orthotoluidine, tetraglycidyl ether of ethylene dianiline and tetraglycidyl ~etaxylene Damon, or mixtures thereof The epoxy resins which are preferably in liquid I- form at room temperature are admixed with polyfunctional curing agents to provide heat curable epoxy resins which are cross-linkable at a moderate temperature, go about 100C, to form thermo~et articles. Suitable polyfunctional curing agents for epoxy resins include ~liphatic polyamides of which diethylene thiamine and triethylene tetramine are exemplary; aromatic amine of I which ethylene dianiline, mote phenylene Damon, 4~4' diaminodiphenyl ~ulfone are exemplary; end polycarboxylic acid androids of which pyromellitic dianhydride, ~enzophenone tetracarboxylic dianhydridef hexahydrophthalic android, nadir methyl android (malefic android adduce of methyl cyclopentadiene), I

1 methyl tetrahydrophthalic android and methyl hexahydrophthalic android are exemplary Polycarboxylic acid android compounds ens preferred curing gents for the above note preferred epoxy resins, with the three compounds last noted being most preferred.
In preparing teat curable, thermosetting, epoxy resins compositions, the epoxy resin is mixed with the curing agent in proportions from about 0.6 to about 1.0 of the stoirhiometric proportions, which provides suffix client ~nhydride groups and carboxylic acid groups to react with from bout 60 to 90 percent of the epoxide groups.
The term cowering as used herein denotes the conversion of the thrusting resin into an insoluble and infusible cro~s-llnked product and, in particular, as a rule, with simultaneous molding to give shaped articles.
In addition curing accelerators may be added to the epoxy resins, as it known in the art, to provide a low curing temperature. Preferred accelerators for the above-noted preferred polyglycidyl aromatic amine resins are substituted imidazoles, such as 2-ethyl-4-methyl i~idazole, and organo~etallic compounds such as stuns octet, cobalt octet and dibutyl tin dilaurate which are incorporated at a concentration of zero to about 3 parts by weight per 100 parts resin.
US Moreover, other materials may be added to 'eke epoxy material in order to improve certain properties thereof t as is known in the art. For example, the tendency of the resin to separate from the mixture can be minimized by the addition of f ire particulate f idler such as Cab-O-Sil (a fumed silica manufactured by Cabot Corporation), acicular fibers, such us talc, or short chopped or milled fiber In addition, resin penetration of the filler may be enhanced by the addition of titan ate wetting, agent, such as CRUSOE, an isopropyl tritdioctylpyrophosphate) titan ate, available from Enrich Petrochemical Co.
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1 A par ocularly useful resin compssi~ion for forming the composites of the present invention comprises a poly~lycidyl aromatic amine, a polycarboxylic acid android curing agent, and a curing accelerator Examples 3 and 4 herein no directed to the use of this preferred resin formulation in thy practice of the present invention

2. hollow Micro spheres The syntactic foam composites prepared in accordance . with the prevent invention contain a relatively uniform I-- distribution of hollow microspheresO These hollow micro spheres are usually hollow thermoplastic spheres composed of acrylic-type resins such as polymethyl-methacrylate, acrylic modified styrenes polyvinylidene chloride or copolymers ox styrenes and methyl methacrylate;
finlike resins; or hollow glass, silica or carbon spheres that are very light in weight and act as a lightweight filler in the syntactic foam. These micro spheres preferably have a diameter in the range of about 5 to about 200 micrometers Methods for the production of these hollow micro spheres are well known in the art and are discusser for example, by Harry S. Katz and John I. Milwaukee in the book entitled, handbook of 25 Tillers and Reinforcements for Plastics Chapter 19:
Hollow Spherical Fillers, Van Nostrand Reinhold, 1978, the teachings of which are incorporated herein by reference Such micro spheres are readily available commercially. These hollow micro spheres can be compressed somewhat when subjected to external pressure.

1 however, they are relatively fragile end will collapse or fracture at high pressures. Therefore, there is a pressure range under which the micro spheres can effectively operate. It ha been determined that when hollow glass micro spheres are employed in the practice of the present invention/ syntactic foam composites can be molded at pressures up Jo the limit Jo the hollow ionospheres without fracture, with molding pressure in the range of about 700 to about 900 psi 10( 00102 to 0.131 Pascal) being preferred.
I- By controlling the amount of hollow micro spheres `- added to the syntactic foam, it it possible to control the specify to gravity of the foam. A imply mixture of an epoxy material and hollow micro spheres tends to separate on tendon with the micro balloons rising to the surface ox the epoxy. However, it has been found that with an increased volume of micro balloons added to the epoxy, there is a decreased tendency to separate into discrete phases. Moreover, it has been found I that at a sufficiently high Lydia of micro balloons, namely about 65% by volume for micro balloons, the tendency to separate into discrete places is minimized.
To achieve specific gravities of lets than 1.0, the hollow micro spheres are included in the syntactic foam in up to 65~ by volume and generally in a range of about 35 to about 65~ by volume and preferably about 50 to about 65% by volume The volume percentage of hollow micro spheres it adjusted based on the composition of the hollow micro sphere selected, the brand of micro-I spheres and the size of the micro spheres. Therefore it may be necessary to select the proper mixture of heat curable resin material and hollow micro sphere for preparation of the syntactic foam on a trial and error basis, For example, the C15/250 series of lass miexospheres available from the EM Company has a specific gravity of 0~15 and a mean diameter of 50 micrometers , .

1 ~Carbosphere~ carbon micro spheres available from the Yersar Corporation have a specific gravity of 0.32 and a mean diameter of I micrometers Desirably a inure of two or Gore types of hollow micro sphere may be employed in the practice of the present invention. the glass micro spheres provide the syntactic foam with improved s rectorial strength, while those of carbon advantageously contribute to both a lowered coefficient of thermal expansion and greater amenability to subsequent metal-plating operations. When using a combination of glass and carbon micro spheres in preparing the composites to of the present invention, the ratio of glass micro spheres to carbon ionospheres is about 1:4 to 1:1.
Furthermore, it has been found by using packing theory that an increased volume percent solids in the resin mixture can be achieved Packing theory it based on the concept that, wince the largest particle size filler in a particular reinforcement system packs to produce the gross volume of the system, the addition of ~ucceedingly 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 i : is discussed by awry S. Katz and John TV Mollusk t in thy book entitled handbook of Fillers and Reinforcements US for Plastic 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.
o This theory applies to the use of solid particulate 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 micro spheres.

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1 To overcome this problem the mold is pry packed with the dry Miller it a mixture of mierospheres and f gibers ) . By applying packing theory as described above, the filler can be packed at high density and Jo what segregation of ingredients does not occur.
Finally the ionospheres may be advantageously treated with a coupling and wetting agent: to unable the resin to wet the sphere surfaces and pronto good filler-resin adhesion, as discussed in greater detail below with regard to similar treatment of the fibers used in the present invention I,

3. Fix ens The f gibers used in the practice of the present invention must be compatible with the selected resin in order to provide good coupling between the fiber and resin" Fibers such as graphite, glass, ~evlar (an aromatic polyamide material obtains prom E. I . Dupont and Company) nylon, or carbon are added to the syntactic 20 foam composite of the present invention to improve the strength and dimensional stability of the composite.
over thy contribution of the fiber to the coefficient I: of thermal expansion of the composite product end to the viscosity of the mixture of components must also be considered. Graphite gibers have been found to be particularly useful since they provide the desired strength in the composite, while also reducing the coefficient of expansion of the composite. An additional factor to consider is fiber length. While Shorter fixers (ego having a length-to-diameter ratio of less than 100:1) provide less rein~orcemeng per fiber than do longer fibers shorter fibers have less impact on the viscosity of the mixture. Thus a greater volume friction ox shorter fibers can be incorporated into a 1 mixture at a given level of viscosity which provides a higher level of reinforcement at that viscosity level by shorter fibers. In addition, the use of shorter fibers improves the uniformity of the mix Trust fibers useful in the composite of the present invention have a length lets ken or equal to 250 micrometers and generally in the range of about 50 to about 250 micrometers Fibers having a length about 150 to about 250 micrometers were found to provide the best compromise between viscosity and reinforcement as I-- discussed previously. When graphite fiber, the preferred fiber material, is used, the diameter of the graphite fibers is in the range of about 5 to about 10 micrometers.
Moreover, the interaction of the fibers with the micro spheres discussed previously just be considered.
It ha been determined by micro packing theory, as described in Chapter 4 of the book by Katz and Milwaukee, previously reverenced, that the optimum ratio of fibers-to-spher~s Aries with the length/diameter ratio IL/D) of the fibers and with the ratio of the sphere-diameter to the fiber-diameter I For each value of L/D, there is one R value where thy packing efficiency is `` zero; and as R increases or decreases on either wide of this minimum, packing efficiency increases It has been found most desirable in the practice of the present invention to use graphite fibers of the micrometer lengths discussed above, which have a length to diameter ratio (L/D) of about 5:1 to about 30:1 and preferably about OWE o about 30:1, and a ~ph~re-diameter to fiber-diame or ratio OR) of at least about 6:1 and preferably about 15:1.
Graphite fiber used in the practice of the present invention art selected Jo have high strength and low Dante Sullenness JOY graphite fiber and US Chortled HUMS graphite fiber are especially suitable.

1 Sullenness GYP 70 fiber is 8 micrometers in dotter has a tensile trying of 7~,000 pounds per square inch (5.24 x owe Pa), a specific gravity of 1083 ~m/cm3 end on of -0.3 x 10 6 in/in/~F. Courtaulds MY
graphite fibers have diameter of 8 micrometers, a tensile strength of ~0,000 psi (3.45 x 10~ Pa), a specific gravity of 1,91 ~m~cm3 and a longitudinal of -1 x 10-6 in/in/PO The graphite fiber are commercially available as con~inuous-fiber tows, For example, Sullenness JOY fiber consists of 384 fibrous The fiber ow are reducible to required lengths on the order of between about 50 micrometers and 250 micrometers by ball milling or from commercial processing concerns such as the Chortled Company of the United Kingdom.
The amount of fiber incorporated in the resin-micro sphere admixture generally ranges from about 3 to about 10 volume percent and preferably from about 3 to bout 5 volume percent in order to achieve composites having values of 25 x 1~-6 in/in/F ~45 x 10-6 cm/cm/C~
or lest As the amount of hollow micro sphere and fibers incorporated in the heat curable resin increases, there is a corresponding increase in the viscosity of the resin. High viscosity prevents uniform dispersion of the micro spheres and fibers and interferes with the processing of the resin-microsph~re-fib~r mixture during molding operation. however, in order to reduce the viscosity of the mixture, the surfaces of the micro spheres and fibers may be provided with a thin layer of coupling and wetting gents. The micro sphere and fiber surfaces are treated with a solution containing a Solon coupling agent such as Solon Alibi (beta~-3,4-epoxy cyclohexyl)~
ethyltrimethoxy Solon), Solon A-1120 (n-beta-~aminoethyl)-gamma~aminopropyl tri-methoxy-silane) or a Jo 3 9 Jo 1 titan ate coupling agent such as di(dioctylpyrophos phato)ethylene titan ate (~R238M volubly prom enrich Petrochemical Company of Bayonne, New Jersey; or twitter diallyloxymethyl-l~butoxy)titaniuM di(ditridecyl phosphitc) (CRY available from Enrich?; or titanium di(cumylp-~enylate) oxyac~tate ~XR134S available from Renrich1; or isopropyl tridodecylbenzene~ulfonyl (RR9S available from enrich). The coupling amen s enable the resin to wet the sphere and fiber surfaces, and promote a stronger bond between the resin, micro spheres I and fibers without increasing the viscosity appreciably.
The coupling agents may be applied by simply dissolving the agents in the resin-microsphere-fiber blent. Optionally, these agents may be applied by first dissolving the agents at a concentration of 0.1 - OHS% of the filler weight in water or an organic solvent such as isopropanol or Freon TO pa f fluorocarbon compound volubly from ELI. Dupont and Company; and thin immersing the micro spheres and fibers which have been premixed in predetermined proportions in the solution for a period of 5 to 30 minutes followed by , filtering and drying the mixture The microsphere~fiber mixture may then be blended with the heat curable resin preparatory to fabricating the syntactic foam composite.

4. Optional Micro beads Solid micro beads may optionally be incorporated in the composite of the present invention in order to increase packing efficiency. Advantageously, such micro beads were also found to decrease the viscosity of the formulation, improve its pour ability, and increase composite uniformity. In a preferred practice of the present invention, about 2 to about 8 percent by volume of solid inert material, such as glass or silica micro-beads having a diameter of about 2 to about 8 micrometers 16 ~.~ 3 I

1 and a specific gravity of 202 to 2.4 are incorporate din the resin-micrssphere-fiber admixture volume percentages in excess of 8% increase the viscosity of the uncured filled heat curable resin formulation to a level at which it is unworkable for molding purposes.
In addition, it way found that large filler volume fractions vowel of micro balloons, fiber and micro beads treater than 60 percent) had a reduced coefficient of thermal expansion, but the viscosity of the mix was unworkable Small volume fractions of filler live.
volume of microballoonst fiber, and micro beads less then 40 percent) were found to improve process ability, but increased the coefficient of thermal expansion to an unacceptable level. However by choosing a filler I combination that maximized filler volume yet minimized filler surface urea, both viscosity and the coefficient ox thermal expansion were reduced. Such a combination was used in the reinforced syntactic foam RSF~34E~ shown in Table III, which was process able, uniform, had good physical properties, end was successfully cast in a metal mold In preparing syntactic foams by the method of the prevent invention, the hollow microphones and graphite fiber, and optionally the solid ~icrobeads, are admixed with the heat curable resin in any conventional fashion using a suitable mixing device such as a Waring blender.
The homogeneous admixture is then debased as by applying vacuum Then the mixture it loaded into a mold of suitable configuration from a reservoir or by using an air gun or other conventional loading device The shape of the mold will of course determine the shape of the cured product and may be chosen a required to form a desired structure such as an antenna wave guide. Molding i& then accomplished in an autoclave at the temperature I

, 1 it which the resin it curable ~09. to 250F to 350F
(121~C to 17~C), for epoxy resins generally and about 150F to 250qF (So to 121C) for the preferred epoxy composition described herein, it 50 to lo psi (2586 Jo 5171 em Hug or (3 45 to 6.90 x 10 Pa for about 2 to about 4 Horace Molding of the filled heat curable rosin formula-lions to form syntactic foam composites of the present invention Jay alto be effected by other conventional molding methods including transfer molding and compression molding procedures wherein the heat curable formulation is cured at the abQve-noted curing temperatures, using pressures on the order of 800 to 1000 psi 41308 to 51710 mm Hug or 5.52 to 6.90 x lo Pascal) for 1 to 2 hours.
It has been found particularly advantageous to form the filled heat curable resin mixtures into the syntactic foam composites of the present invention by vacuum liquid transfer molding process. In this procedure, the mold is first loaded with the micro sphere/
fiber filler which has been mechanically or manually premixed in predetermined proportions and pretreated with a sizing gent a previously described. Next, the mold Jay optionally be vibrated to promote a uniform distribution of the filler in the mold ego.
about 5 minutes on a vibration twill Then the mold cavity is filled with the heat curable resin. The mold us a sealable pressure vessel constructed to support the vacuum/pressure sequence described below. To prepare for the molding process, the mold cavity is preheated to bring the cavity up to the temperature at which thy heat curable resin is curable. R vacuum is then drawn on the mold to degas the mold cavity contents and to impregnate the filler with the resin.
The vacuum it released to atmospheric pressure to I., ",:

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1 burst any was bubbles remaining in the told contents Then a ~uperatmospheric pressure, such as 100 to 1000 psi (6u9o x 105 to 6.90 x owe Pascal), is applies to the told to cause the resin to encapsulate the fuller Thy elevated temperature and super atmospheric pressure are maintained for a time suffiei~nt to partially rune the resin arid form a unitary structure which can be ejected from the told. The ejected structure is then subjected to further heating cycle to completely cure the resin By the practice of the present invention, reinforced syntactic roam composites art obtained which hove a coefficient of thermal expansion of about 25 x 10~6 $n/in/F (45 x 10-6 cm/cm/C) or less sod a density of less than 1.0 gm/cm3, as well us lon~-tenm thermal stability, amenability to being molded in various configurations, end ability to meet the NASA outguessing requirements Using the preferred epoxy resin formulation described herein, composites are obtained which have a coefficient of thermal expansion of bout 9.0 x 10 5 in/in/F (16~2 x 10~6cm~cm/ C) or less. In addition, the mechanical properties of these composites are repeatable This combination of properties makes the composites of the present invention particularly well suited for use as a substitute for Lyman in antenna and antenna microwave components used in space applications. In particular, heat curable epoxy resins comprised of mixtures of tetrafunctional aromatic epoxy resins and liquid android when heated to 150F are sufficiently low in viscosity to accept loadings of micro spheres up to 65 percent of the volume of the system fiber loadings of up to 10 percent, and bead loadings up Jo 65 percent.
These microsphere/fiber/b4ad filled epoxy resins are readily curable and when surged produce syntactic foam composites having specific gravities of between ORB and 0~9 and coefficients of thermal expansion approximating ~3~t75j~

1 that of aluminum or twill Depending on the filler fiber volume used in the composite of the prevent invention composites may be tailored to have coefficients of thermal expansion ranging from that of the unfilled resin to that of steel cause of their relatively low coefficient of thermal expansion epoxy resin based syntactic foam composites prepared in accordance with the present invention have been determined to be especially amenable I to conventional metal plating processes, such us electron f- less plating, when the surfaces thereof are prepared for playing by plasma treatment. The relatively high adhesion of petal deposits to the surface of the prevent composite is believed to be a function of both the topography of the plasma-treated surface plus the mechanical integrity of the remaining surface. The plasma removes the resin skyline from the comFositle, leaving the graphite fiber/microballoon filler exposed, to provide a surface which is readily playable. Such metal plating of the composite of the present invention may be required in forming antenna component in which I; an electrically conductive surface or path is required, ```` as is known in the art.
To of foot plasma treatment in preparation for plating 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 thy filler, as discussed above.
Normal plasma etching conditions known to the art are used. For example, for a plasma excitation energy of 200 wettest of composite, an Owner yes source of approximately 1000 ml/minute, a vacuum pressure of 200 micrometer Hug, and one hour duration art used.

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1 When a or deposit is required, as in an antenna wave guide structure it is advantageous to first form a layer of an electroless or vapor deposited metal such us copper Jo provide a conductive surface which can then be built up with additional electrolytic plating such as copper or silver plate to produce a smooth reface finish. Electrolytic silver plating may readily be formed on the electrolytic copper surface to provide a silver plated surface with good adhesion to the underlying composite material Electroless plating of the plasma-treated composite . surface can be accomplished by standard procedures such as by dipping the plasma treated composite in the plating solution for a time sufficient to achieve a continuous buildup of metal on the etched surface Metals that can be plated on the molded epoxy resin based composites prepared in accordance with the present invention include, for example, copper, silver, nickel, cobalt, nickel/iron, nickel/cobalt, other nickel alloys, and told. For electroless copper playing, an aqueous path of Shipley Co. ~328 copper plating solution may be used, which contains copper sulfate, sodium potassium . tart rate 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 I in thy range of about 15C to about 95C Ahab 59F to 203F~. Metal adhesion of this electroless copper plating has been determined to be excellent even after exposure ox the plated composite to cycles of widely different typewriter, as described in Examples 3 and 4 heroin.

1 Next a copper plating is built up to any desired thickness on the ~lectroless copper by known electrolytic plating methods, using commercially available electron--deposit copper plating solution. finally an electrolytic silver plate is formed to the desired thickness on the electrolytic copper plate by known methods, using commercially available silver plating solution formula-lions. silver plating of a composite owe the present invention is described in Example 5 The following examples illustrate but do not limit the present invention.

This example illustrates a process for forming one type of fiber-reinforced syntactic foam composite in accordance with one process embodiment of the present invention.
The components of the syntactic foam formulation designated ~S-61~ are shown in Table I. The following ~-~ details regarding the components of S-61 apply to Table I.
a. MOE is a tetraglycidyl ethylene dianiline manufactured by Cuba Geigy.
b. HOWE is a nadir methyl android hardener manufactured by Cobb Geigy.
c. BDMA is benzyldimethylamine accelerator available from Eve Roberts or Cuba Geigy.
d. D3~/4500 micro spheres ore borosilicate micro spheres having a mean diameter :3 75 micrometers, a specific grovel of 0.32, and a compressive strength of 4500 psi, available from the EM Company.

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en YO-YO fibers are graphite fibers milled Jo a length of about 150 cryometry end having diameter of about B micrometers, available from the Sullenness Corporation.
f RUSS I;R55J and ~R9S are titrate coupling and wetting agents, available from Enrich Petrochemical Company, Bayonne, New Jersey 9. AFT is a surfac~ant, available from Foreign Comma cay 1 Co .

,~, , , .
TABLE I
COMPOSITION OF E~ORMVLATION So 1 . . .. Jo Component _ FRY i iota ( us 11. Resin MOE epoxy resin 100 ~00 yo-yo hardener }00 1.0 . - BUM acre aerator O . 25 4 . O
RR38S 190 4.0 . Micro spheres RR55 0.3 1.2 AFT (Optional) 0.2 008 3 . Fiber Milled JOY 20 BY
CRUSOE 0.2 0.8 .
I I_ .
_______ PUP is parts per hundred epoxy resin I

1 Preparation biers The JOY graphite fibers in continuous tow form wore cut into lengths of approximately Lob inch to 1/2 inch (0.32 Jo 1.27 centimeters), using if paper cutter.
Batches of the chopped fibers approximately 80 grams each) were loaded into a ball mill jar having a on-gallon capacity and sufficient Freon TO was added to cover the ceramic balls to verve as a suspension medium.
The fibers were milled for 24 hours . Scanning electron ~icrographs of the milled fibers showed them to be f broken iJlto Molly fragments ranging from approximately ' 2 to 10 micrometers in length.
The milled fibers and Freon were poured into a shallow stainless steel pan, and the Freon was allowed lo to evaporate. The fibers were then dried 4 hour in an air-c~rculating oven it at 250F l1~1C) and sited on a vibration plate to pass a 325 mesh screen. The dried, sifted fibers were stored in a desiccator box until ready for use.
Composite Formation The formulation S61 was prepared as follows A
online ho told pot for a Waring blender was heated to 140F (60C) using a temperature-eontrolled water bath The remeasured amount of the HOWE hardener was put in the blender and the mixer speed was adjusted using a Variac variable potentiometer so that the hardener was just barely agitated. With the blender on slow" jetting, the Variac was twirl to 79 percent of I full speed. The resin, which had been preheated to 150F ~71~C~, was added to the pot and the contents of the pot were mixed until the mixture appeared homogeneous (about 5 minutes), and then cooled to root temperature , .
.. . .

I I

1 Next, there was gradually added to the pot thy ~R38S, AFT (optional), and 25 percent of the milled fibers which had been previously dried overnight in an oven at 200 (93C) and fluffed by running in Lowe blender on Lowe speed at 70 percent of the full Variac speed for byway 15 seconds for 5 grams of fiber. The mixture was mixed for about 5 minutes Next, 10 percent of the micro spheres which had been dried overnight in an oven at 2009F ~93C) was gradually added and thy contents of the pot were mixed until streaks of micro spheres disap-purred The remaining amount of fiber and the US
were gradually added and the pot cQnten~s mixed for about 30 minutes. Next, the DAM was added slowly, followed by the RR55 and the remaining amount of micro-spheres. The pot contents were mixed until strealcs of micro spheres disappeared Then, the pot was covered and a vacuum pump was attached to the pot with the pump jet to pull a vacuum of 22 inches ~559 mm) of mercury. The mixer was run for 45 minutes under vacuum or until there were no black streaks of fibers in the mixture. Finally, the mixture was carefully poured Jo as to minimize air ;' : entrapment into a preheated stainless steel test specimen mold which had been prepared by: cleaning with methyl ethyl tone solvent, baking at 300 (149C) for on Monterey brushing with a fluorocarbon mold release agent to provide three coats of the release agent with 30 minutes air drying or each coat, end preheating to 140F (60C). (Optionally, the formulation was injected with an air gun into the mulled After pouring the mixture into the mold, the mold was vibrated on a vibrating table for 5 minutes at the maximum safe speed with a large flat, 0.5 inch thick aluminum plate placed on top of the mold D Next, the mold was 1 placed in an oven preheated to EYE (135~C~ end a thermocouple was placed on/in each of the following. on the mold in the oven, and in the mold contents through hole in the side Hall of the mold When the thermos couple in thy mold contents registered 27~F (13~C), the following cure cycle was runs 10 minutes at 275F
(135C); 10 minutes at 3009F (149~C~; 120 minutes at 350F (177C). The maximum oven rate way used for changing temperatures The mold was removed from the oven and was disassembled, and the part was removed from the mold I- while the mold was still hot, being sure to keep the thermocouple embedded in the syntactic foam. The part was~deflashed as necessary with a file. For the post-cure, the remolded part was placed in an oven preheated to 400F (204C) between 0.5 inch thick aluminum plates, with 2-5 kilograms weight on the top plate when the thermocouple in the syntactic foam registered 400F
(kiwi the following post-cure cycle was run: 1 hour at 400F (204C); 1 hour at 425F (218C); 1 hour at 450~F (232~C), and 1 hour at 475~F (246C) Finally, the part was removed from the oven . The fiber reinforced syntactic foam composite formed as described above was found to have the properties I shown in Table II. With regard to Table II, the following test requirements apply:
a. CUTE was determined using a quartz dilatometer to Moser the change in length as a function of temperature.
b. Specific gravity was measured using pycnometerO
C. viscosity was measured with a Brook field Visco~eter~

Of. Shrinkage was measured by determining the dimensional difference between the molded product and the mold e. Gel time was determined qualitatively us ho tile required for the liquid resin to -- form a gel.
. Pot life was deten~ined qualitatively us the time required for the liquid resin to in Russ in viscosity to the point of lo being unworkably.
9. Doria of exothenn was determined by using a differential scanning caloriraeter.

TALE II

Property value . CUTE 19-22 x 10 6 c~c~/C
. 10 . 6012 . 7 x 10-6 in/in/F
Specific gravity aye ViRco~ity at 150F (65.61'C) 35,000 centipoise Shrinkage O . 8 S
Gel 'cite > 60 minutes Sty life >360 nuts Doria of exotherm 15F (8.3C) __ _ __ __ --T

This example illustrates a process for funning iber-reinforc:ed syntactic foam composites of various compositions in accordance with the present irlv~ntiorlO
The components of 'eke various fQr3llulations designa~c~d us the ~RSF series art shown in table III .
Thea following detail regarding the ~peaific components apply to Tall III~, a. epoxy is a mixture of 70 parts Glyamin~ 135 Sdi~lycidyl orth9 Teledyne) and 30 porks Glyamine 120 (tetraglycidyl ethylene I- . dianiline ), both materials obtained from FIX
Resins of Ann Franciscs), California, mixed with about 115 part nadir methyl android hardener end about 0.25 parts berlzyldimethyl-z~nilin~ accelerator.
b I, Zeeospheres 0/8 are sol id gloss spheres having a median diameter of 3 micrometers, avow fable from Zillion Industries of it. Paul, Minnesota .
c. C:arbospheres Type A z~rs hollow carbon spheres having an average diameter of 50 micrometers, available from Versar of Springfield, Virsinia4 I` d. EM A 32/2500 glass bubbles are glass micro-spheres having a mean diameter of 50 muckrake-eaters a specific gravity of 0.32, and a compressive ~trens~th of 2500 psi, available fryer the EM Company of Minnesota eon EM A 16/500 are glass micro spheres having a err donator of 75 micrometers, a specific gravity of 0,,169 and a compressive strength of 500 psi, avzlilable from the EM Company f Eccospheres SO ore hollow Luke microphones having a diameter off 45-125 micrometers, available from Emerson and Curing Inc. of Canton, Masochist .

~3975~D
I

., E;refco 213 R40 brads art solid Lucy micro-spheres having a doomsayer of 3 8 ~icrt~meters, available from ~refco Inc. of Torrance, California ., h I. SIMS 50 ( 501J ) are graphite f gibers having a - length of bout 50 micrometers end a diameter of tout 8 micrometer available from Courtaulds Coo of the lJnited ~in~dom.
i . US 5û ( yo-yo ) graphite are graphite f gibers having a length of about 250 micrometers end I diameter off about 8 micrometers, zlvailab:Le from Cc>urt~ulds of the United kingdom.
j O O 0 063" ~MS-50 ( 1/16" ) ore ~araphi~e ibis having a length of bout 1600 micrometers end a diameter ox about 8 micrometerfi, availably Finn end tram of Sun Valley, California .

Using Mach of the fonnulations of the RSF series 20 designated in Table III, a fiber-reinforced syntactic foam composite was wormed following the general procedure set forth in Example 1. The properties off each owe these ~ompo item is shown in Table IV. The following test requirements were applied for top measurements in I Table IV.

a . Density was determined by pyenometerO
by CUTE was determined using a quartz dilato~eter to measure thy change in length ( A ) as a Enchain of temperature ,, S c.... Compressive strength was determined using the Marconi Society for 'reseizing and littorals (ASTM) Standard No DS95.
d. Compressive modulus was determined using ASTM D695, using crusade speed in place of strain Gus e. Ilniformity was determined by visual inspection .
f O Viscosity was measured with a I~rc~okf told Viscometer .

I

TABLE III
CQMPOSITIOM OF FORMULATIONS OF RSF SERIES

: VOLUME RATIO OF FOAM FILLERS
. . . I.
MICRO SPHERES FIBERS
.. , _ _ I
Z I 6J~ Jo IA
O lye O OWE LIT Us _ I: I
-r I) Lbl =
I T a. ELI I tax O Lo _ I
Us ad O :~- _ I .=
Tao I I ail_ I LO I CLUE I
G O LO O QC I Ll.1 lo g I O
O I Jo J IT K
I lay PI C3 it W N to I
of ' . _ _ _ _ -- .... ..
3 o .405 o .098 o .~71 by 4 0.375 0.048 0.514 0,064 0.401 0.500 0.059 0~040 6 3 .4~1 o Sue ooze o .034~
7 0.536 0.057 0.400 0.007 n~30 0.057 0.395 0.018 13 0.53~ 0.~57 0~,40~ 0.~07 14 o .530 0.057 0.39~ o .018 Lo 0~37 0~05 O'er 0~06 it 0~322 O ~0~1 0~586 00051) I O ~37~i O ~048 Jo ~514 OOZE
23 0 ~583 n ~023 O ~368 011~02~
I 0~503 0~022 0~45Q 0~025 26 Ox 3 Owe Owe Owe28 0~583 00023 0~368 0~026 29 O ~583 O Allah O ~368 O ~32~
31 O Do 13 702~ 0 ~457 assay 33 O .503 û~.022 "450 O .025 34 0.394 O .026 I. 550 .03û
34F O .410 O .025 O . 530 O .,035 û.353 D ~0~8 D . 588~* 0 ~032 I 0.383 O aye _ _ . 0.560 0.03 Plus 0.007 of AS 50 (250 ) graphite .

TABLE I V
PROPERTIES OF OOMPOSITFS OF FORMULATION OF RSF SERIES
. . ___ __ a Lo o a, * ,~:
:>- _ Us ~-~
LO a ~--~ ô o Of: =:1 Clue it L I O L I V
O to O Z Al _-I _ ____ _ ___ ___ . _ 3 O owe owe 15 ~30~1 394 3 I 5 4 0 ~88113 ~81 16 ~300 I 4 I 4 0~87215~10 16 ~400 ~06 I 4 6 O ~6922 ~16 14 ~300 ~07 3 I I
7 0~96820 ~82 15~100 394 I I
8 0 ~982;~1 ~39 18 ~400 410 4 7 13 1~00025~4~i 14~900 38~i Jo 7 14 1 ~01925~55 15~400 405 1~3 6 19 0 ~85214 ~06 13 ~200 ~11 4 I 3 ZOO 0 ~69414 ~09 En ~600 335 2 I 2 21 O ~56117 ~02 14 OWE 439 3 I 3 23 0~9912213 ~69 16 ~300 384 5~5 7 ~038730 51 19 ~00~) 423 __ 6 26 1 ~00521 ~73 15~700 39~; -1 ox !;
EYE 0~98220~10 17 ~10(~ 411 6 I 5 29 1 ~00220 ~70 17 ,801~ 393 or ~6!5 31 ~)~88823~90 17 ~800 400 6 I 8 33 EYE 13~300 343 3~9 7 34 O r82414 59 17 51~0 394 4 I 6 34F 0 84214.24 17,300 425 __ Jo 0 738~7.23 10,200 3~3 4.0 6 36 O .745~7 I 12 ,100 33~ 3 I 5 _. __, . _ . _ 1 pus guy x 103 Pascal .
.

.
3L~J~3~ ion This example illustrates the formation of fiber-reinforced ~yn~ac~ic foam composite using the preferred epoxy resin ~onmulation and preferred vacuum liquid reunifier molding prows described herein Thy heat curable epoxy resin formulation hod the following composition:

Diglycidyl ortho~oluidine 100 I Nadir methyl~nhydride 100 2-~thyl-4-~ethyl imidazole 2 This composition had a Mel time of 25 into, a vi8c3sity of 2~0 centipoise it 75F (24C)~ end a CUTE of 30.8 to 3~.3 x 10~6 in/in~F (55.8 to 58.1 x 10-6 cm/cm/~) A filler mixture was prepared having the composition shown below and a density of 0.543 gm/cm3. Carbospher~s are hollow carbon micro balloons having a mean dotter of about 50 micrometers, available from Yersar Inch of ~prin~field, Yearn HUMS graphite fibers are graphite fibers having a length of about 50 micrometer, available from Courtaulds Co. of the United Kingdom. Titan ate ~izinQ agents are available from Enrich Petrochemical Co. of Bayonne, New Jersey Carbosphere, 50 2nicro~eter~ 50 lit S fiber, 50 micrometers 50 Titan ate sizing sent ~R~3 8 M
Urn the above rioted resin end filler each of a errs ox resin/filler formulations shown in Table V
was processed as described below in ors3er to forgo the 10 composite of 'eke prevent ~nvent~on~, The filler composition (ire. a mixture of the fiber end lnicrospheres pretreated with the sizing agent a previously described herein ) was loaded into a cleaned 5.5 inch x 0.5 inch (14cm x 1.3cm) wide slab 15 mold internally coated with a polyvinyl alcohol release agent. The mold was preheated to 212F ~lû0C), the temperature at which hardening of the heat curable epoxy resin formulation was initiated. The epoxy resin formulation was poured in . o the mold keynote nine the 20 idler The mold was placed in a laminating press, a nylon vacuum beg way constructed around the Compression tooling of the press, end a vacuum pressure of 125 millilDeters Do mercury pressure (1.67 x 105 Pascal) was maintained on thy assembly err 2 loinutes to draw 25 down the Russell to impregn~e the filler and to degas the resin material in the mold. The vacuum was then released without removal ox the vacuum bag and the assembly held in this passive v~cuu~n state for an additional 2 mounts. Thereafter; a constant positive I pressure of appear tell 8Q0 pounds per square inch (41,360 mm Ho or 505 x 10~ Pascal way imposed on the rein filler mixture in thy mold for 2 hours at 212F
( Luke) 1 During this pressuriza~c~on stage, the resin was bled from the mold in the amount noted in Table V.
I.
r ~3~t7 I

The molded composite slab hod sufficient: çJreen strength to be ~jçcted from the mold, thereafter it was post cured for 4 hours unrestrained in on oven Set I 300F
CC),. The final void-free slab contained 'eke filler

5 ratio noted in Table V end was cut into appropriate slops for physical testing. The composite was found 'co hove the physical properties which are summarized in Table VI . As indicated by the values err CUTE riven in Table VI, unexpected signify leant improvement in the CUTE
10 of these composites was obtained using the preferred resin composition and filler compositions described herein .
In addition, a typical sample dyes tested in accordance with ASTM ESSAY and found to have a 15 collected volatile condensable material (CVCM) of lest than 0.1 percent and a total maws lows (TML) of less than 1 percent, which meets the NASA outguessing requirements .
Further, for Specimen 1 of Table 'Y, a portion Jo of the molded slab was surface plated with copper by subjecting the surface of the slab to on oxygen rich plasma treatment, as previously described. The treated I lo way when dipped iniquity) Shipley #328, elec~role~s copper placing solution, as previously described, and then dried it 248F (120C~ under 29 inches ~737mm) jig ( gauge pressure ) .
The plated composite was then evaluated for Audi on of the deposited copper layer unwise an ASTM
D3359 tap adhesion test before and after 25 cycles of I thermal hulk imposed on the plated surface by alternately zipping thy plated possum in liquid nitrogen (-3~0F
or -196~C) for 30 second and boiling water ( 212~F or lOQC~ for 10 seconds. No lost of copper was obs~r~led, I

TABLE V
COMPONENTS OF MOLDING COMPOSITION
__ _~_ _ Ryes 1 n Duff no Specimen Resin Miller Filler Ratio ~qoldinç
aye (gas. ) tams. ) vowel ) ___~ ___ _ 1 40 10 45 64 69 .
2 30 g .4 38 57 47 {I 3 31~ I 38 57 owe Ox 17.0* 5~5 37 57 45J3 AYE 6~0 3!3 59 50.1~;

6 I I 6 I 41 60 54 I

* RR134S sizing allegiant was substituted icon the previously noted sizing agent, f W -** Resin blend 8 t- we ) where I inlay resin weight .
We " resin displaced from the mold, using a bleeder cloth.
Vow . calculated from resin bleed varies bout 10-209; of the actual vow . value .
_ _ _ _ _ _ _ _ .. . .

~,q3~

TABLE VI
PHYSICAL PROPiERTIE5 OF MOLDED COMPOSITES

passional k n~s~Densit~
No.- ( ion ) gm/cm l10-~ cm/cm/C

1 0.~7g 0~743 US (3.6) 2 0.550 0.900 16.2 I
I-.; 3 00565 0.876 14.2 (8.1) 4 1.070 ago __ 1.042 0.915 _ _ 1.065 Ought __ determined using a quartz dilator~e~cer.

I, .

I

This example illustrates the ft)~nation of composite as jet forth in example 3 with the exception that thy co~pc1sition of . he Miller formulation Wow varied. The procedure set forth in Example 3 was followed except 'chat the filler compositions shown in Table VII were used. The following details regarding eye specific components apply to Table VI I
a, Carbospheres are carbon micro~,pher~æ having a specific gravity of 0.32 and mean diameter of I micrc~metersr available from Varier I--. Corporation.
'- b O En S 50 t 501~ ) graphite f gibers are graphite f gibers having a length of about So micrometers and a diameter of bout 8 micrometers, available iErom the Courtaulds Co,. of the United Random.
c. 1/4 mm MY 50 graphite fibers are graphite f giber having a length of about 250 micrometers and a diameter of bout 8 micrometers, available from the Courtaulds Cog of the United Kingdom.
do C15/250 glass micro balloons are composed of borosilicate glass, have a diameter of lQ~00 ~icrome~er~, density of 0.15 gnu and a compressive strength of 250 psi, available prom the EM Comparly of Minnesota, The physical properties of the molded composite Libya Jo formed are jet furl in Table VIII.

_ Us n En .
or; o 0 a y_-Jo I:
_ C I:
_ aye--En I
e ..-I
O I:
Us J ox I 0 0 0 I o In I
Us us u\ E SUE
O . .
'",,,~ us on a ED
I I
O
C~4Q
Go I _ _ o ^
Lo no lo Z own I us or H or En on a I
_ . _ .~' O
h O
O O
80~ us I
I
Us:

V I UP N en t) Us O

o o _ _ _ .

I
O-.

I

TABLE VIII
PHYSICAL PROPERTIES OF MOLDED 5::0MPOSI~E rubs Pitter _ _ _ __ __ S Used in Poison Cut Molded Thickness Density Filler Ratio Bleed Coating Sample 1 in, 3 ( gm~cm3 ) (Wit 0 4 ) ( Vow . % ) Removed __ __ .. __ 0.4'13 1.0~039 I 51.0 < 5%

B 0.540 0.75326 47 56-77 61.9 5%
C 00850 ~.94827 541 16.~ 5-15 f I In addition, the surfaces of the molded composite slabs of table VIII were then subjected Jo plasma treatment under the following conditions: O2/inert gas source of proximately 1000 ml~minute~ vacuum pressure of 200~ go and one hour duration. The surfaces of the plasma etched slabs were then copper plated to a thickness of about 3-4 oils by dipping the etched slabs in an aqueous Shipley ~328 electroless copper plating both.
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 3. The adhesion rules are recorded in Table VIII, indicating the amount of copper coating on lattice removed by the tape.

, Jo A syntactic foam composite proper I SpecilTen No 2 of Table V described in Example 3 end molded in a slab told way plated with silver as ~Eollal~sO The surface of the slab was ~ubje ted to awn oxygen rich plasma etch which resulted in the removal of the surface polymer awaken as previously described O The etched slab was then metallized using the Shipley Company ~328 electroless copper plating solution process, a 10 previously described, and thoroughly rinsed and dried at 248~F (120C~ under 29 inches ~725 mm) Ho (gauge pressure ), to provide a layer of electroless coupler 20 ~icroin~hes (5.08 x 10-5 cm) thick.. Next, the slab was immersed in an acid copper electrolytic plating 15 bath Zloty 25C for 25 minutes Jo foreseen an electrode posited copper layer 100 micrGinche~ ( 2 .54 x 10-4 em) thick .
Finally the copper-plated slab was immured in on electrolytic E~ilv~r plating bath 5C for 25 minutes to form a layer of silver 300 micro inches (7~62 x 10~4 cm) 20 thick.
. The silver-plated slab way when evaluated for adhesion of the deposited layer using an STYMIE D3353 tape adhesion text before and after 25 cycles of thermal shock i~poYed on the plated surface by alternately I dipping the plated specimen in liquid nitrogen (~3;!8F
or -196~C~ for one minute and boiling water ( 212F or 10ûC) for one minute. No adhesion loss of silver was observed .
The ~ilv~r-plated syntactic foam had the tame low 30 Pc.F,, lows characteristics a aluminum when tested for insertion loss it 4.6 gigahertz using standard electronic test . Thus with the use of proper tooling for molding antenna wave guide structures may by formed from thy composite of the present ~nvent~on, which are effective 35 microwave or antenna component and which met thy .

I

1 requirement for use in space applications syntactic foams plated with petals us as silver and topper may err ~etal-plated core materials for both my rove co~pon0nts and microwave reflectors.
Thy fiberoreinforced syntactic foam composites of the prevent invention achieve twill reduction in eta in comparison with aluminum; which makes these components attractive for weight-sensi~ive applications in a spacecraft environment. At the same time, however, in tuitions elan for high volume production, the ,~. readily-~oldable nature of the reinforced foam mixture disclosed herein further offer the potential of sign-ficantly reduced cost in comparison with the machining traditionally employed for the production of conventional petal parts.
The preceding description has presented in Dwight exemplary preferred ways in which the concepts of the present invention my be applied. Those skilled in the art will recognize that numerous alternatives encompassing many variation may readily be employed without departing from the intention and keep of the invention jet forth in the pounded claims. In particular, thy present invention is not limited to the specific resin, gibers, or ~icroballoon~ set forth herein as examples. By felon the teachings provided herein relating to the effect of each component of the mixture on the final composite and the effect of the various components on each other, other suitable resin, fiber, and micro balloon materials may readily be determined. Further, by following the teaching provided herein, it may be determined how to form composite materials having a density or coefficient of thermal expansion other than those jet forth herein as require for the specifically ~ntioned end use in space applications.

MELstp

Claims (16)

WHAT IS CLAIMED IS:

1. A fiber-reinforced syntactic foam composite having a specific gravity less than 1.0 and a coefficient of thermal expansion of about 9.0 x 10-6 in/in/°F (16.2 x 10-6 cm/cm/°C) or less, the composite being prepared from an admixture of:
a) a heat curable thermosetting resin comprising:
an uncured polyglycidyl aromatic amine, a polycarboxylic acid anhydride curing agent, and a curing accelerator selected from the group consisting of substituted imidazole compounds and organometallic compounds;
d) hollow microspheres having a diameter in the range of about 5 to about 200 micrometers; and c) fibers having a length of less than or equal to 250 micrometers.

2. The composite of Claim 1 wherein the polyglycidyl aromatic amine is diglycidylaniline, diglycidyl ortho-toluidine, or tetraglycidyl metaxylylene diamine.

3. The composite of Claim 1 wherein the polycarboxylic acid anhydride is present in sufficient quantity to react with from about 60 to about 90 percent of the epoxide groups in said polyglycidyl aromatic amine.

4. The composite of Claim 1 wherein the polycarboxylic acid anhydride is nadic methyl anhydride, methyl tetra-hydrophthalic anhydride, or methyl hexahydrophthalic anhydride.

5. The composite of Claim 1 wherein said curing accelerator is present in the amount of about 0 to about 3 percent, by weight.

6. The composite of Claim 1 wherein said curing accelerator is 2-ethyl-4-methyl imidazole or stannous octoate.

7. The composite of Claim 1 wherein:
a) said uncured polyglycidyl aromatic amine is diglycidyl orthotoluidine and is present in the amount of about 100 parts per hundred resin by weight;
b) said curing agent is nadic methyl anhydride and is present in the amount of about 100 parts per hundred resin by weight; and c) said curing accelerator is 2-ethyl-4 methyl imidazole and is present in the amount of about 2 parts per hundred resin by weight.

8. A fiber-reinforced syntactic foam composite as set forth in Claim 1, comprising:
a) a heat curable theremosetting epoxy resin com-prising:
1) diglycidyl orthotoluidine in the amount of about 100 parts per hundred resin by weight;
2) nadic methyl anhydride in the amount of about 100 parts per hundred resin by weight; and 3) 2-ethyl-4-methyl imidazole in the amount of about 2 parts per hundred resin by weight;
b) hollow carbon microspheres having a diameter in the range of about 20 to about 200 micrometers; and c) graphite fibers having a length of less than or equal to 250 micrometers and a diameter of about 8 micrometers.

9. The composite of Claim 1 wherein the hollow microspheres are formed of glass, silica, carbon, acrylate resins or phenolic resins.

10. The composite of Claim 9 wherein the hollow microspheres are formed of glass and have an average diameter of about 50 micrometers.

11. The composite of Claim 9 wherein the hollow microspheres comprise a mixture of glass micro-spheres and carbon microspheres.

12. The composite of Claim 1 wherein the fibers are formed of graphite, glass, carbon, nylon, or poly-amide.

13. The composite of Claim 12 wherein the fibers are formed of graphite and have a length of about 50 micrometers and a diameter of about 8 micrometers.

14. The composite of Claim 1 wherein said admixture further includes a coupling and wetting agent.

15. The composite of Claim 1 wherein the admixture further includes solid microbeads.

16. The composite of Claim 1 which comprises about 35 to about 65 percent by volume microspheres and about 3 to about 10 percent by volume fibers, the balance being a matrix comprised of the heat cured resin throughout which the microspheres and fibers are dispersed and bonded together.