Combustion enhancement and agglomeration inhibition of HTPB/AP/Al propellants using Al@AP combined with a hydrazo-triazine derivative

In this work, the combustion enhancement and agglomeration inhibition of HTPB-based composite propellants have been achieved by incorporation of Al@AP composites containing a nitrogen-rich compound 4,4′,6,6′-tetra(hydrazino)hydrazo-1,3,5-triazine (THHT) as a burning rate modifier. The thermal reactivity and decomposition behavior of Al@AP/THHT composites were studied using DSC/TGA/FT-IR techniques. These composites showed a greater heat release with lower decomposition peak temperature. The propellant formulations could be well tuned by adjusting the mutual distance between Al/AP and THHT crystals. The results suggest that the inclusion of Al@AP/THHT in solid propellants would lead to enhanced energy release, with the value of explosion heat increased from 5632.0 J g^-1 to 5971 J g^-1 (+6.0 %), the ignition delay time decreased from 105 ms to 79 ms (-24.8 %) and the flame radiation intensity maximum raised from 2199 to 1988 (+10.6 %). The modified propellants are featured with a chemical reaction-dependent flame propagation rate at lower pressure range (1∼10 MPa) and a pressure-controlled process over 10 MPa. The burning rate of propellant at 1 MPa can be increased from 6.1 mm s^-1 to 7.0 mm s^-1 (+14.8 %) by the inclusion of Al@AP/THHT. The pressure exponent of modified propellants was reduced from 0.48 to 0.33 in pressure range 1∼15 MPa, which is mainly attributed to the activation of Al@AP at lower pressure. Furthermore, the inhibition of Al agglomeration was found once it was coated with AP. The characterizations on condensed combustion products (CCPs) demonstrated an enhancement in combustion efficiency due to an increased content of Al₂O₃, accompanied with a remarkable reduction of mean size from 14.0 μm to 1.3 μm (-90.7 %) and a rise in the proportion of nano-/micron-sized particles. The overall effects of Al@AP interfacial control and precise catalysis of THHT on the energy release and combustion of propellants were identified and the primitive combustion mechanisms are proposed.