Three-phase syntactic foams: structure-property relationships

This study addresses a unique problem that occurs in composite materials containing hollow reinforcements, that is the density-filler content relationship. Unlike a traditional solid reinforcement, a hollow reinforcement can be manufactured with a variety of densities. Subsequently, when fabricating composite materials with a particular density requirement, a large variation in volume percent of that reinforcing phase can occur. Hollow reinforcements under consideration are carbon microballoons (CMBs) of various densities determined by both tap density and pycnometry. Our approach is to study several different densities and volume percents of microballoons while maintaining a constant volume percent (8.5%) of the polymer binder phase. The resulting syntactic foams are three-phase materials consisting of binder, microballoon (MB), and interstitial void phases. The volume of the MB and binder phase is measured by helium pycnometry. The complementary volume of the interstitial void phase will depend on the volume of microballoons in the billet. Mechanical characterization is done by compression and flexure testing and results are discussed to highlight structure-property relationships. Results show that, in addition to bulk density of the foam, the packing arrangement of the CMBs is an important factor in the mechanical behavior of the foam and is shown to be an important design criterion. C 2006 Springer Science + Business Media, Inc. 1. Introduction A syntactic foam is a composite material containing hollow particles dispersed in a binder phase. This field of materials began to mature in the late 1960s and early 1970s. The largest application was and still is in marine and submarine sectors. Others are core materials in sandwich structures and aerospace applications [1]. The sizes of particles can range from nanometer to millimeter. When the hollow particles are in the micrometersize range they are commonly referred to asmicroballoons (MBs). Many types of hollow particles have been used in syntactic foams, however, glass MBs are the most prevalent in research and applications. There are two basic types of syntactic foams [1–3], three- and two-phase. A three-phase syntactic foam contains hollow particles and binder phase but not a sufficient binder volume to fill the interstitial positions between par- ∗Author to whom all correspondence should be addressed. ticles. The three phases being: MB (wall material and contained void), binder, and interstitial void. A two-phase material contains hollow particles and enough matrix material to fill these interstices. In this case, since the volume in-between theMBsis filled with material, the only phases are MB and matrix. The most common syntactic foams are two-phase materials; for example, glass MBs in an epoxy matrix. This study, however, focuses on a different type of hollow particles, carbon microballoons (CMBs) as a component in a three-phase structure. Three-phase foams are of interest because of the very lowdensity and therefore high specific properties and the ability to tailor foam density and MB and binder volume percent. Carbon has several properties that make it more appealing than glass in certain applications, specifically higher thermal and electrical conductivity and most alluring is the lower density. Carbon is 0022-2461