Production and characterization of epoxy syntactic foams highly loaded with thermoplastic microballoons

Glass microballoon syntactic foams consisting of 60–70？vol% hollow glass microballoons and epoxy resin matrix have gained considerable attention in recent years due to their unique combination of mechanical properties and low density, with applications in the naval and aerospace industries. An important limitation of these materials is the volume fraction ceiling (～0.74) and subsequent density limit (0.36？g/cm3). Utilizing thermoplastic microballoons, syntactic foams were produced with densities as low as 0.067？g/cm3, achieved by developing a method that produces epoxy/microballoon compositions comprising an unusually high volume fraction of microballoons (0.75–0.95). The resulting morphology features microballoons which, having expanded in a restricted volume, are deformed into irregular shapes that efficiently pack together and are encapsulated by a thin coating of epoxy. The compressive yield strength, tensile strength and initial modulus of these highly loaded syntactic foams exhibit a non-linear decrease with increasing microballoon volume fraction to values typical of highly porous polymers, but display a high degree of recovery, or rebound, from large compressive strain compared with glass microballoon syntactic foams