DOE OSTI2014
Syntactic foams are an important category of composite materials that have abundant applications in a wide variety of fields. The bulk phase of syntactic foams is a three-part epoxy resin formulation that consists of a base resin, a curative (curing agent) and a modifier (diluent and/or accelerator) [12]. These thermoset materials [12] are used frequently for their thermal stability [9], low moisture absorption and high compressive strength [10]. The characteristic feature of a syntactic foam is a network of beads that forms pores within the epoxy matrix [3]. In this review, hollow glass beads (known as glass microballoons) are considered, however, solid beads or microballoons made from materials such as ceramic, polymer or metal can also be used [3M, Peter]. The network of hollow beads forms a closed-cell foam; the term closed-cell comes from the fact that the microspheres used in the resin matrix are completely closed and filled with gas (termed hollow). In contrast, the microspheres used in open-cell foams are either not completely closed or broken so that matrix material can fill the spheres [11]. Although closed foams have been found to possess higher densities than open cell foams, their rigid structures give them superior mechanical properties [12]. Past research has extensively studied the effects that changing the volume fraction of microballoons to epoxy will have on the resulting syntactic foam [3,4,9]. In addition, published literature also explores how the microballoon wall thickness affects the final product [4,9,10]. Findings detail that indeed both the mechanical and some thermal properties of syntactic foams can be tailored to a specific application by varying either the volume fraction or the wall thickness of the microballoons used [10]. The major trends in syntactic foam research show that microballoon volume fraction has an inversely proportionate relationship to dynamic properties, while microballoon wall thickness is proportional to those same properties [3,4,9,10]. The glass transition temperature has a proportional relationship to the volume fraction of microballoons used, however, there is limited research that supports correlations between other thermal variables and microballoons specifications. In fact, very little experimental data exists to relate thermal conductivity and volume fraction or wall thickness of microballoons [5]. This review proposes that thermal conductivity should be a topic of interest for future researchers because of how frequently syntactic foams are used in insulating applications. This paper will explore three aspects pertaining to epoxy resin syntactic foams with glass microballoons: the immense range of applications that syntactic foams are used for, the materials and fabrication techniques most commonly used, and lastly the results from characterization of syntactic foams with varying microballoon volume fractions and wall thicknesses. In addition to varying microballoon parameters, it is also possible to change the base, accelerator and curing agent used in the epoxy formulation. For simplicity, this paper will focus on a very common combination of materials produced by the Dow Chemical Company®.