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.degree. 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 dianhydride 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 dianhydride 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 microspheres, they lack the wide range of utility that would be afforded by microspheres.