Synergistic effect of nano-additives and tackifier resins on hybrid rocket fuel performance

Abstract

Conventional paraffin based hybrid rocket fuels have inherently weak mechanical performance. This paper aims to investigate potential mechanical toughening achieved by the co-addition of tackifier resins and different particles as additives to increase the mechanical performance without effecting the fuel flow characteristics. A styrenated terpene tackifier resin and 4 different particles are considered as additives to considered base fuel formulations(BF) (containing dominantly macro-crystalline wax). Integrated particles are classified as i) nano-silica powder (spherical) ii) carbon black (spherical) iii) activated carbon (random shape, high surface area) and iv) multi-walled carbon nanotubes (tubular, high aspect ratio). An initial screening effort is performed to fuel blends with 0.1 wt.% particle concentration and 10 wt.% tackifier resin. Mode I fracture toughness and work of fracture of the obtained fuel samples are measured by notched four point bending tests. Parallelly, melt viscosity of the samples are recorded in order to approximate the fuel regression rate. Particle/fuel interactions are portrayed by differential calorimetry (DSC) analysis. Particle type screening efforts suggested that MWCNTs are best suitable candidates due to their ”pin” like structure that provides higher deformability and fracture toughness to fuel formulations. Then a weight fraction screening is performed to to fuel blends with three different MWCNT concentrations (0.01, 0.1 and 0.25 wt.%) in order to identify the optimum amount of MWCNT to be considered. Results suggested that when used at very low amounts 0.01 wt.%, the effectiveness and processability of MWCNTs increases. Furthermore, when co-added with tackifier resins, a synergistic ”adhere” and ”pin” toughening strategy can be obtained that significantly increases the fracture toughness of fuel blends with no increase in melt viscosity. Being cost effective, easy to process and superior such formulations are found to increase the resistance of hybrid rocket fuel grains against mechanical failures during operation.