Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-09-09
2001-05-22
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S054000, C521S056000, C521S184000, C521S185000
Reexamination Certificate
active
06235803
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to polyimides. It relates in particular to shaped articles composed of aromatic polyimides, especially hollow microspheres.
2. Description of the Related Art
High performance polyimides are presently employed in a number of applications, for example in joining metals to metals, and in joining metals to composite structures in the aerospace industry. In addition, polyimides are rapidly finding new uses as foam insulation in cryogenic applications and as structural foam, having increased structural stiffness without large weight increases. Foams of various densities and thermal and mechanical properties are now being required for future reusable launch vehicles, maritime ships, and aircraft. Polyimide foam materials have a number of beneficial attributes in these applications, such as high temperature and solvent resistance, flame resistance, low smoke generation, high modulus and chemical and hot water resistance.
U.S. Pat. Nos. 5,147,966 and 5,478,916 disclose polyimides that can be melt processed into various useful forms such as coatings, adhesives, composite matrix resins and films. These polyimides are prepared from various diamines and dianhydrides in various solvents. The use of monoanhydrides as endcapping agents is also disclosed in these patents to control the molecular weight of the polymers and, in turn, to make them easier to process in molten form. The use of ethers to make polyimide adhesives was disclosed in U.S. Pat. No. 4,065,345, which demonstrates another method to produce polyimide resin systems.
FIG. 1
shows the method employed by these patents to produce polyimides.
U.S. Pat. No. 3,483,144 discloses a process for making polyimide foam by ball milling a mixture of monomers and heating the mixture to 300° C. In all cases, the foams produced by this patent are the result of dianhydrides or tetraacids being dissolved by a diamine upon melting. The ensuing reaction produces water and thus foams the molten material.
FIG. 2
illustrates the process to make foam by this patent.
The state-of-the-art technology for making polyimide foams as disclosed in U.S. Pat. Nos. 5,298,531, 5,122,546, 5,077,318, and 4,900,761 utilizes solutions of diamines and dianhydrides derivatives in a low molecular weight alkyl alcohol solvent. Polyimide precursor solutions and powders therefrom are then processed into foams through the expulsion of water and alcohol (R—OH) during the thermal imidization process. In these cases the alcohol solvent reacts initially with the dianhydrides to form a covalently bonded specie referred to as a dialkylester-diacid (DADA) before the aromatic diamine is added. The aforementioned patents also illustrate the use of blowing agents to aid in the foaming process. The blowing agents utilized by these patents serve as a separate entity and usually result in a foam that has residual blowing agent within its cell walls.
FIG. 3
demonstrates the state-of-the-art in this foam preparation technology.
Howsoever useful, these related art foam products and processes do not provide for all that is required in present and future applications. In particular, they do not provide for efficient and efficacious repair of foam insulation already in place, for example on aircraft, spacecraft, and maritime ships. Moreover, they do not readily and effectively afford the preparation of syntactic foam structures, the demand for which continues to increase because of their beneficial attributes.
U.S. Pat. Nos. 4,407,980; 4,425,441; and 4,433,068 disclose the preparation of macro balloons made from a mixture of polyimide and polyimide-amide polymers, the particle size thereof being at least 0.5-10 mm. Because these structures are not pure polyimide products, they are found wanting in respect of thermal stability and non-flammability. Because these structures are macrospheres, they lack the wide range of utility that would be afforded by microspheres.
SUMMARY OF THE INVENTION
It is accordingly a primary object of the present invention to provide what is lacking in the related art, viz., a foam-like product which can provide for efficient and efficacious repair of foam insulation already in place, for example on aircraft, spacecraft, and maritime ships, and which readily and effectively affords the preparation of syntactic foam structures. It is a related object of the present invention to provide a polyimide foam-like product which has all of the attributes characteristic of polyimides, and which in addition can supply what has been found wanting in the related art. It is another related object of the present invention to provide a syntactic foam structure which has properties and characteristics which meet present and future demands.
These objects and their attending benefits are achieved and the disadvantages of the related art are avoided, by the present invention. In one aspect thereof, the present invention is a multiplicity of hollow polyimide microspheres i.e., shaped articles, each of which has an aromatic polyimide shell, a hollow interior, and an essentially spherical structure with a particle size between about 100 and about
1500 &mgr;m.
In another aspect thereof, the present invention is a syntactic foam, which is made up of a multiplicity of shaped articles according to the present invention, which are bonded together by a matrix resin to form an integral, composite structure having a density according to ASTM D-3574A of between about 3 and about 30 pounds/ft
3
and a compression strength according to ASTM D-3574C of between about 100 and about 1400 pounds/in
2
.
REFERENCES:
patent: 4407980 (1983-10-01), Gagliani et al.
patent: 4425441 (1984-01-01), Gagliani et al.
patent: 4433068 (1984-02-01), Gagliani et al.
Echigo Yoshiaki
Kaneshiro Hisayasu
St. Clair Terry L.
Weiser Erik S.
Foelak Morton
Hawkins Hillary W.
The United States of America as represented by the Administrator
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