Characterisation of Expancel thermoplastic microspheres and their influence on the mechanical properties of all-polymer syntactic foams

Due to their highly favourable thermal, mechanical, and acoustic properties in extreme environments, syntactic foams have emerged as a popular material choice for a broad variety of applications. They are made by infiltrating a polymeric matrix, in either a glassy or polymeric state, with hollow microspheres made from a wide range of materials. In particular, hollow plastic microspheres, such as Expancel made by Nouryon have recently emerged as an important filler medium, with the resulting all-polymer composites taking on excellent damage tolerance properties, strong recoverability under large strains, and very favourable energy dissipation characteristics. There is however, a near-complete absence of statistical information on the diameter and shell-thickness distributions of these microspheres. In this work, using X-Ray computed tomography, focused ion beam, and scanning electron microscopy, we report on these quantities and observe the spatial distribution of microspheres within a syntactic foam. We then employ this data to predict the effective stress-strain response of the foams at small strains, using both analytical micromechanical methods and computational finite element methods, where the latter involves the construction of appropriate representative volume elements. We find excellent agreement between the predictions of the effective Young's modulus and Poisson ratio from the computational and theoretical methods and good agreement between these predictions and experimental results for the macroscopic response.