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United States Patent [191 [11] Patent Number: 4,595,623 '

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United States Patent [191 [11] Patent Number: 4,595,623 ' UnitedO States Patent [191 [11] Patent Number: 4,595,623 ' Du Pont et a1; [45] Date of Patent: Jun. 17, 1986 [54] FIBER-REINFORCED SYNTACI‘IC FOAM 3,730,920 5/ 1973 D’Alessandro 521/54 COMPOSITES AND METHOD OF FORMING 3,849,350 11/1974 Matsko . .. 521/54 SAME 4,005,033 l/1977 Georgeau et a1. 521/54 4,031,059 6/1977 Strauss . .. 521/54 [75] Inventors: Preston S. Du Pont, Northridge; 4,107,134 8/1978 Dawans . .. 521/54 Janet E, Freeman, Pasadena; Robert 4,112,179 9/1978 Maccalous et al. 521/54 E. Ritter, Palos Verdes Estates; Alois 1:; gall?klf - - - - - - - - - - ' - - wlmna'm’- Palos Verdes’ an of cam‘' 4,363,883, , 12/1982 Gaglianiag 1am eet aal. .... .,1. 521/54 [73] Assignee: Hughes Aircraft Company, Los 4,410,639 10/1983 Bouley ........ .. 521/54 Angeles, Calif. 4,412,012 10/1983 Bouley et a1. 521/54 4,447,565 5/1984 Lula et a1. ........................... .. 521/54 [21] App]. No.: 607,847 ' Primary Examiner—-M0rton Foelak [22] Flled: May 79 1984 Attorney, Agent, or Firm-M. E. Lachman; A. W. [51] 1111.01.33 .............................................. .. B32B 3/00 Karambelas [52] US. Cl. .................................. .. 428/195; 428/209; [57] ABSTRACI‘ 521/54; 521/82; 521/99; 521/134; 521/135; _ _ _ _ _ 521/136; 521/138; 523/213; 523/219 Fiber-remforced syntactic foam composites having a [58] Field of Search ................ .. 521/54; 523/218, 219; 10W speci?c gravity and a low coefficient of thermal 423/195, 209 expansion suitable for forming lightweight structures _ for spacecraft applications are prepared from a mixture [56] References cued of a heat curable thermosetting resin, hollow micro U.S. PATENT DOCUMENTS spheres having a diameter of about 5 to 200 micrometers and ?bers having_a length less than or equal to 250 2,806,509 9/1957 Bozzacco et a1. .................. .. 521/54 3,230,184 1/1966 Alford ...................... .l 521/54 micrometers. 3,365,358 l/l968 Hutchins ................. .. 521/54 3,553,002 l/1971 Haraway, Jr. et a1. ............. .. 521/54 39 Claims, N0 Drawings s 4,595,623 1 2 In order for the syntactic foam to be useful as a substi FIBER-REINFORCED SYNTACI‘IC FOAM tute for aluminum in antenna and antenna microwave COMPOSITES AND METHOD OF FORMING components in a spacecraft, the foam must have the ' SAME following characteristics. (1) The material must have a speci?c gravity of 1.00 or BACKGROUND OF THE INVENTION less, as compared to a speci?c gravity of 2.7 for alu 1. Field of the Invention minum. The present invention relates, in general, to syntactic (2) The material must have a linear coefficient of ther foam composites and, more particularly, to ?ber-rein mal expansion (0. or CTE) comparable to that of forced thermosetting resin based syntactic foam com aluminum, preferably close to l3><10--6 in/in/“F. posites exhibiting a low speci?c gravity and a low coef (23><10-6 cm/cm/°C.) or less. Thermal distortion of ?cient of thermal expansion. antenna components subjected to thermal cycling in 2. Description of the Prior Art A continuing objective in the development of satel the extremes of the space environment is a major lites is to optimize satellite payload weight. One means 15 - contributing factor to gain loss, pointing errors, and of achieving this objective is to reduce the intrinsic phase shifts. weight of various operational elements within the (3) The material must meet the National Aeronautics spacecraft. It has been recognized by the art that the and Space Administration (NASA) outgassing re quirements to insure that the material does not release desired weight reduction could be realized by replacing 20 conventional materials, such as aluminum, with lower gaseous component substances which undesirably density synthetic composites possessing requisite me accumulate on other spacecraft parts in the outer chanical, thermal and chemical stability. Included in space vacuum. these low density synthetic composites is a group of (4) The material must have long-term stability, as re materials referred to in the art as syntactic foams. 25 quired for parts exposed to the space temperature Syntactic foams are produced by dispersing micro environment (e.g., -—100° F. to 250° F. or -—73‘’ C. to scopic rigid, hollow or solid particles in a liquid or 121° C.) for extended periods of time, such as 10 semi-liquid thermosetting resin and then hardening the years. system by curing. The particles are generally spheres or (5) The material must be capable of being cast into rnicroballoons of carbon, polystyrene, phenolic resin, 30 complex con?gurations in order to form component urea-formaldehyde resin, glass, or silica, ranging from parts for antenna structures, such as waveguides or 20 to 200 micrometers in diameter. Commercial rnicro antenna feed distribution networks. spheres have speci?c gravities ranging from 0.033 to The art, until the present invention, has been unable 0.33 for hollow spheres and up to 2.3 for solid glass to satisfy these requirements and particularly the re spheres. The liquid resins used are the usual resins used 35 in molding reinforced articles, e.g., epoxy resin, polyes quirement for a low coef?cient of thermal expansion ters, and urea-formaldehyde resins. ((1). Thus, known epoxy resin based syntactic foams In order to form such foams, the resin containing a ?lled with 10 to 30% by volume hollow microspheres curing agent therefor, and microspheres may be mixed generally have a a in the range of 17 to 36X 10-6 in to form a paste which is then cast into the desired shape 40 /in/°F. (30 to 65><10—6 cm/cm/°C.). and cured to form the foam. The latter method, as well A need, unsatis?ed by existing technology, has thus as other known methods for forming syntactic foams, is developed for a syntactic foam material which is both described by Puterman et al in the publication entitled lightweight and of suf?cient mechanical, thermal and “Syntactic Foams I. Preparation, Structure, and Prop chemical stability to enable it to be substituted for alu erties,” in the Journal of Cellular Plastics, July/August 45 minum in physically demanding satellite environments. 1980, pages 223-229. When fabricated in large-block form, such foams possess a compressive strength which SUMMARY OF THE INVENTION has made them suitable for use in submerged structures. The unresolved needs of the art are satis?ed by the In addition, the more pliable versions of the foam are present invention which provides thermally stable ?ber utilized as ?ller materials which, after hardening, func 50 reinforced syntactic foam composites having a speci?c tion as a machinable, local-densi?cation substance in gravity of less than 1.0 and a linear coef?cient of ther applications such as automobile repair and the filling of mal expansion of about 25X 10-6 in/in/°F. (45><10"6 structural honeycombs. Despite these characteristics of cm/cm/°C.) or less, which are prepared from an admix adequate compressive strength, good machineability, ture of a heat curable thermosetting resin, hollow mi and light weight, such foams lack the degree of dimen 55 'crospheres having a diameter between about 5 and sional and thermal stability required to render them applicable for the spacecraft environment. More speci? about 200 micrometers and ?bers having a length of cally, syntactic foam systems tend to exhibit varying about 50 to about 250 micrometers. ?ller orientation and distributions within the geometri The syntactic foam composites of the present inven cal areas in a molded intricate structure, which limits tion can be cast as complex structures which contain the structural intricacy that can be achieved, as well as lightweight hollow microspheres having ?bers, such as reducing dimensional stability. If syntactic foam sys graphite ?bers, in the voids between the microspheres, tems are too highly ?lled, sacri?ces are made in mold with the microspheres and ?bers being bonded together ability, coefficient of thermal expansion, strength, den by the heat cured resin matrix. The composites of the sity, dimensional stability and stiffness. Moreover, such 65 present invention readily meet the speci?c gravity, foams tend to exhibit poor adhesion to metallic plating coef?cient of thermal expansion and NASA outgassing which is required to form the desired product, such as requirements, which easily qualify the composites as an antenna component. aluminum substitutes for spacecraft use. 4,595,623 3 4 dehyde novolac polyglycidyl ether epoxy resins (such DETAILED DESCRIPTION OF THE as those sold under the trademarks DEN 431, DEN 438 INVENTION and DEN 439 by Dow Chemical Company), and epoxy In order to form the ?ber-resin-microsphere compos cresol novolacs (such as those sold under the trade ite of the present invention having the desired density marks ECN 1235, ECN 1273, ECN 1280 and ECN 1299 and coef?cient of thermal expansion, each of the three by Ciba Products Company). components must be selected so that the resulting com The particular epoxy resins preferred in the practice bination thereof provides a mixture amenable to being of the present invention are polyglycidyl aromatic cast into the desired con?guration, as well as providing amines, i.e. N-glycidyl amino compounds prepared by a ?nal product having the required structural and physi reacting a halohydrin such as epichlorohydrin with an cal properties. Acceptable mixtures must have a viscos amine. Examples of the most preferred polyglycidyl ity that produces an accurate, void-free casting with aromatic amines include diglycidylaniline, diglycidyl uniform material properties. In addition, the proportion orthotoluidine, tetraglycidyl ether of methylene diani of ?ber in the composite must provide the required line and tetraglycidyl metaxylene diamine, or mixtures thermal expansion, strength, and stiffness properties.
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