Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Stratified or layered articles
Utility Patent
1999-02-10
2001-01-02
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Stratified or layered articles
C264S122000
Utility Patent
active
06168736
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to structural materials having low densities, and, more particularly, to a nonmetallic, low-density, syntactic foam material and to methods of preparing same.
BACKGROUND OF THE INVENTION
Low-density nonmetallic materials with good mechanical properties have found utility in various applications including aerospace applications. For example, radomes that house radar antennas are generally required to have sufficient strength to withstand aerodynamic forces. Nevertheless, the material used to construct the radome cannot interfere with the signals transmitted therethrough. In another example, stiffened sandwich core structures are formed by placing a core of material between spaced-apart face sheets. The core spaces the stiff face sheets apart and stabilizes the face sheets so that the structure has a high flexural rigidity.
In these applications, it is often desired that the materials of construction have as low a density as possible while achieving the required mechanical properties. The low density materials provide many advantages over higher density materials. For example, the low density materials are lighter weight which leads to reduced fuel consumption in aircraft uses. Additionally, low density and weight may also be desirable in specialized non-aircraft applications such as rotating machinery to reduce inertia forces.
Structural metals of relatively low density such as aluminum have been widely used for many decades. More recently, structural nonmetallic materials such as graphite/epoxy composite materials have been developed and have entered service. Foamed polymeric materials have also found many uses. The latter class of materials have densities even less than the polymeric materials from which they are constituted, since a portion of the interior of the material is made up of distributed voids that are produced in the material.
One example of these polymeric foam materials are syntactic foams. Syntactic foams are typically formed of microballoons which are distributed throughout a polymeric matrix. Often, these syntactic foam materials are formed of curable polymeric materials and can include other additives or materials such as reinforcing fibers.
U.S. Pat. No. 4,595,623 to DuPont et al. describes syntactic foams formed from liquid thermosetting resins which can be used to produce lightweight structures for spacecraft applications. Nevertheless, the use of liquid resin materials in polymeric foam materials has some disadvantages. For example, liquid resin materials when used to form these structures do not produce highly uniform distributions of the solid materials in the foam such as fibers. Specifically, the fibers will follow fluid flow lines during the formation of the foam structures. As a result, fibers in the syntactic foam will align in the flow direction during mixing and mold filling resulting in an anisotropic foam structure.
SUMMARY OF THE INVENTION
The present invention provides a method of preparing a syntactic foam material which can be used to prepare low density structures for various applications including aerospace applications. The syntactic foams have a uniform distribution of microballoons throughout the resin matrix. Furthermore, fibers used in the syntactic foams maintain a random orientation and uniform distribution during the cure process to maintain good material performance. Moreover, the syntactic foam material of the present invention has desirable electrical characteristics (e.g., RF signal absorption) for aerospace applications. There is also great flexibility in the processing of the syntactic foam material which allows for control of the final density and mechanical properties of the material thus allowing the material to be tailored for specific applications.
The method of preparing a structural, low-density article according to the invention comprises mixing a plurality of microballoons and a finely divided solid thermosetting resin such as an epoxy resin. The mixture can also include short length fibers which form a network in the resin matrix prior to curing. The mixture is then heated to a processing temperature for a time sufficient for the thermosetting resin to flow and to wet the microballoons and fibers. The thermosetting resin is thereafter cured to form a syntactic foam material having highly uniform structure and properties. The thermosetting resin can be cured at moderate temperatures so that high-temperature mold materials are not required. The thermosetting resin can also be cured in more than one step to provide the syntactic foam of the invention. For example, it may be desirable for thicker molds to alternate increasing the temperature of the mold and maintaining the mixture at a substantially constant temperature. The mixture can also be compressed during curing if higher density foams are desired. The resulting syntactic foam material can be prepared in a semifinished form and machined into articles, or the material can be formed to its final shape.
In the method of the invention, a dry resin mixture comprising the thermosetting resin, microballoons, and optionally fibers, can be placed into a mold prior to heating. For example, the dry resin mixture can be placed in a mold having a controllable final volume and the volume can be controlled during heating to attain a final preselected volume. In this particular situation, the density of the material can be controlled for a fixed amount of microballoons and thermosetting resin, by reducing the final volume of the mold. Alternatively, the volume of a fixed shape mold can be determined and the dry resin mixture weighed out to fill the volume to meet a specific target density. In either case, the mold can be further lined with a dry fabric having an intimately contacting thermosetting dry resin coated on the fabric and distributed throughout the fabric and the thermosetting resin can be cocured with the thermosetting resin of the dry resin mixture to form the syntactic foam article. Advantageously, the skin can be integrated into the syntactic foam material without the preparation of a separate liquid resin prepreg prior to adding the dry resin material to the mold.
The present invention also includes structural, low-density syntactic foam articles having highly uniform properties and comprising a cured epoxy thermosetting resin matrix and a plurality of uniformly distributed microballoons embedded within the resin matrix. The article preferably further includes a plurality of uniformly distributed and randomly oriented short length fibers also embedded within the resin matrix thereby providing a mechanical network for the material prior to curing. Additionally, the fibers can be used to reduce the bulk density of the mixed components allowing fabrication of lower density articles. The fibers can also be selected to impart certain electrical properties to the foam material, e.g., RF signal absorption. The material can also contain air in interstices to controllably reduce the density of the material. Thus, the properties can be manipulated by selectively and controllably retaining air within interstices of the material during manufacture and by controlling fiber volume.
Other features and advantages of the present invention will be apparent to one skilled in the art upon reading the detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
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Bridges Donald J.
Harrison Edward S.
Melquist James L.
Alston & Bird LLP
McDonnell Douglas Corporation
Theisen Mary Lynn
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