Much work has been devoted to the toughening of thermosetting resins. Generally, the approach taken has been to add a rubber modifier to a system forming an insoluble second phase to inhibit crack propagation. Another technique involves blending various resins together to obtain the best properties of each, e.g. thermal capabilities and toughness, but this approach rarely yields the optimum properties of each of the resins used, only the optimum compromise of properties. The above approaches necessitate trade-offs. In order to obtain toughness in a cured resin, another property must frequently be sacrificed or reduced, such as chemical resistance or thermal properties. As long as the trade-off is not detrimental to the final application, it is acceptable even though not desirable.
Epoxy resins are typical of such thermosetting resins. Thus, epoxy resins are deficient in properties such as elongation to failure and ability to bear load when damaged. Attempts to modify the resins have involved use of the above noted approaches, particularly rubber modification.
Additionally, the morphology of a toughened epoxy resin system containing a modifier beyond its solubility limit in the thermoset often results in a phase inversion microstructure. The thermoplastic phase in this instance is very susceptible to undesirable solvent crazing. When the thermoplastic content is reduced in an attempt to minimize the crazing, the mechanical properties suffer, particularly the resin fracture toughness.
As previously noted, thermoplastic resins have been added to epoxy resins. For example, U.S. Pat. Nos. 3,715,252, 3,737,352, 3,784,433, 3,796,624, 4,073,670 and 4,131,502 disclose the possible combination of epoxy resins and non heat-curable thermoplastic components including poly(vinylacetal), nylon, neoprene rubber, acrylonitrile rubber, phenoxy resins, polysulfones and .alpha.-olefin copolymers. More specifically, U.S. Pat. No. 3,530,087 discloses adhesive compositions containing an epoxide resin and a thermoplastic polysulfone resin. U.S. Pat. No. 3,641,195 discloses curable compositions comprising an epoxide resin dispersed in a thermoplastic copolymer of an .alpha.-olefin hydrocarbon with an .alpha.-olefin which is an ether or ester of a carboxylic acid. U.S. Pat. No. 4,117,038 discloses epoxy resin adhesives containing a polyglycidyl compound, an acrylonitrile-butadiene-styrene graft polymer and a copolymer of ethylene, acrylic acid and acrylate. Despite the general thermoplastic nature of these materials, they have not provided a broad pattern of increased performance characteristics.
Blends of polyimides and epoxy resins are also known to those skilled in the art. For example, U.S. Pat. No. 4,237,262 discloses compositions containing at least one aliphatic epoxy resin and at least one curable polyimide prepolymer formed from an aliphatic bismaleimide, an aromatic polyamine and an aromatic bismaleimide. German Offenlegungsschrift 2,900,560 likewise discloses mixtures of polyamide-polyimide resins and epoxy compounds. Japanese Kokai 79/129,097 discloses reaction products of an unsaturated bisamide, a phenolic compound and an epoxy compound. Japanese Kokai 79/142,298 discloses epoxy resins derived from bis(4-carboxyphthalimido) compounds and polyepoxides. These systems are distinct.
Europe 99,338 discloses the incorporation of a thermoplastic polyimide into an epoxy resin system which provides a substantially discontinuous phase within the continuous phase of the epoxy resin. As a result, toughness, mechanical and thermal properties are enhanced without significant adverse effect on other performance characteristics. Finally, Europe 274,899 discloses the concept of distributing resin particles as a separate phase throughout prepregs prepared from fiber-reinforced thermosetting resins. The thermosetting resins include epoxy and maleimide resins, among others, while the resin particles are thermoplastic or thermosetting resins or mixtures thereof. Polyamides, polyethersulfone and polysulfone are identified as particularly suitable for use as the resin particle.
It is, accordingly, the primary object of the invention to improve the performance characteristics, particularly toughness, of various thermosetting resin systems without substantially diminishing existing desired properties thereof.
It is a further object to accomplish said improvement in toughness, mechanical and thermal properties by incorporating both a dissolved and a dispersed thermoplastic phase into the thermosetting resin system.
Various other objects and advantages of this invention will become apparent from the following description thereof.