Investigating the transfer of toughness from rubber modified bulk epoxy polymers to syntactic foams

Syntactic foams are lightweight, high specific strength materials used in the aerospace and naval 10 industries. Their utility is limited by their brittleness. The epoxy polymer matrix in an epoxy/hollow 11 glass microsphere (GMS) syntactic foam was modified using carboxyl-terminated butadiene-acrylonitrile 12 (CTBN) rubber with the aim to increase fracture toughness. The microstructure and fracture properties 13 were investigated, and compared to CTBN modified bulk epoxy polymers. The formation of complex CTBN microstructures was responsible for the increase in fracture energy, from 193 J/m2 14 for the unmodified syntactic foam, to 296 J/m2 15 at 12 wt% CTBN modification. However, this increase is much smaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m2 16 to 1112 J/m2 17 was measured for the same CTBN concentration. There is little toughness transfer from the 18 bulk epoxy polymers to the syntactic foams, attributable to small interstitial regions between the GMS, 19 restricting plastic zone size. A statistical approach to the analytical modelling of fracture energy in the 20 bulk epoxy polymers highlights the importance of considering the underlying distribution of rubber 21 particle and void sizes. The increase in fracture energy achieved in this work can increase the overall 22 usefulness of syntactic foams in structural applications