JOURNAL OF COMPOSITE MATERIALS Article Characterization of mixture epoxy syntactic foams highly loaded with thermoplastic and glass microballoons Kerrick R Dando 1,2 , William M Cross 3 , Marc J Robinson 4 and David R Salem 1,2,3 Abstract Syntactic foams comprising glass or thermoplastic microballoons have gained considerable attention in recent years due to mechanical and thermal properties that are advantageous for naval and aerospace applications. This work reports a method for producing syntactic foams with unusually high-volume fraction microballoon loadings (>0.74) and its util- ization for the creation of ‘‘hybrid’’ epoxy resin-based syntactic foams comprising various mixtures of glass and thermo- plastic microballoons. Microstructural analyses using X-ray micro-computed tomography provided non-destructive quantitative characterization of microballoon packing, confirming the high loading levels suggested by density measure- ments. By systematically varying the glass/thermoplastic microballoon ratio, it was shown that a range of mechanical properties can be engineered into these lightweight materials. The peak impact force of these syntactic foams can be significantly reduced (30% reduction) through combining glass and thermoplastic microballoons in a ratio where the thermoplastic microballoons are the dominant fraction but not the sole microballoon component. Keywords Syntactic foams, mechanical properties, polymer-matrix composites, impact behavior Introduction Recent work has shown that expandable thermoplastic microballoons can be used to enable the production of syntactic foams with microballoon volume fractions (V mb ) of up to 0.95 and densities as low as 0.07 g/cc. 1 This discovery eliminated the pre-existing limitation set by glass microballoon epoxy syntactic foams having a density ceiling of 0.35 g/cc due to packing limitations of the rigid, polydisperse glass spheres (0.7 V mb ). 2,3 While the density of thermoplastic microballoon syn- tactic foams is reduced with the incorporation of large volume fractions of microballoons, mechanical proper- ties decrease in a corresponding manner, owing to the large amount of purposely placed voids within the composite. In an attempt to counteract the diminished mechan- ical properties in highly loaded (0.9 V mb ) thermoplastic microballoon epoxy syntactic foams, nano-additives such as carbon nanofibers (CNF) or halloysite nano- tubes (HNT) were incorporated into the composite microstructure. 4 It was observed that compressive strength and modulus enhancements as large as 180% and 250%, respectively, can be achieved with a 0.25 wt.% addition of CNF and increases of 165% and 244%, respectively, can be achieved with a 0.5 wt.% addition of HNT. Tensile strength and modu- lus enhancements as large as 110% and 165%, respect- ively, were observed with a 0.125 wt.% addition of CNF and increases of 133% and 173%, respectively, were achieved with a 0.125 wt.% addition of HNT. 4 1 Composites and Polymer Engineering (CAPE) Laboratory, South Dakota School of Mines & Technology, USA 2 Nanoscience and Nanoengineering Program, South Dakota School of Mines & Technology, USA 3 Department of Materials and Metallurgical Engineering, South Dakota School of Mines & Technology, USA 4 Department of Civil and Environmental Engineering, South Dakota School of Mines & Technology, USA Corresponding author: David R Salem, Composites and Polymer Engineering (CAPE) Laboratory, South Dakota School of Mines & Technology, 501 East Saint Joseph St., Rapid City, SD 57701, USA. Email: david.salem@sdsmt.edu Journal of Composite Materials 0(0) 1–13 ! The Author(s) 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0021998318810782 journals.sagepub.com/home/jcm