| abstract |
A non-elastomeric epoxy having a mol wt less than about 10,000 and curing agent therefor ("epoxy-curing-agent"), both of which (epoxy/epoxy-curing-agent) are liquid while at curing temperatures, provide a continuous liquid phase in which a polyimide (PI) generally, and a bisimide (BI) in particular, may be homogeneously dispersed to form a fluidized dispersion. In one embodiment, the less viscous epoxy/epoxy-curing-agent is the dispersive medium (continuous phase) and the PI the dispersed or discontinuous phase. In the more preferred embodiment, the epoxy/epoxy-curing-agent and PI are miscible, forming a single liquid phase in which the PI is homogeneously dispersed. In each embodiment, the epoxy is essentially unreactive with the PI. Upon curing the epoxy in a first stage, to form a polymeric epoxy matrix (also referred to as "cured epoxy resin" herein) at a temperature below 150 DEG C., which is below that at which the PI cures, the cured epoxy matrix provides a stable three dimensional framework in which the PI is homogeneeously dispersed. Subsequently, in a second stage, the PI is cured, optionally with a curing agent for the PI ("PI-curing-agent"), at a sufficiently high temperature to crosslink the PI, but below that at which the cured epoxy matrix is degraded. The cured resin blend so obtained has an essentially continuous (more than 5000 hr) service temperature above 176 DEG C. (350 DEG F.). For the PI to cure, it must be present in an amount sufficient to ensure molecular proximity of the PI molecules, or PI/PI-curing-agent molecules, sufficient for interaction therebetween. The cured resin blend is preferably post-cured in a composite at a temperature above 176 DEG C. but below that at which the cured epoxy matrix loses 50% of its physical and mechanical properties. |