Nickel acetylacetonate as decomposition catalyst for HTPB-based fuels: Regression rate enhancement effects

The slow regression rate induced by the difficulty of pyrolysis has limited the practical application of hydroxyl-terminated polybutadiene (HTPB)-based fuels for hybrid rocket propulsion. A possible strategy is the use of suitable transition metal elements promoting polymer matrix thermal decomposition of fuels. This paper investigates the effects of nickel acetylacetonate, Ni(acac) 2 , on the thermal stability and combustion of HTPB-based fuel formulations. The presented experimental results show that the addition of Ni(acac) 2 can intensely decrease the thermal stability of HTPB thus enhancing the solid fuel regression rate even at a small additive mass fraction: under an oxygen mass flux of 50 kg/m 2 s, HTPB  + 5 wt% Ni(acac) 2 shows a 25.5% increase over the non-loaded baseline. Kinetics analyses reveal that the catalytic effect is mainly induced by the Ni 2+ in Ni(acac) 2 at the early stage of decomposition, and by the NiO produced from the oxidative decomposition of Ni(acac) 2 in the fuel final degradation stage. On the other hand, the addition of Ni(acac) 2 decreases the combustion heat of HTPB-based fuels significantly and implies the accumulation of its decomposition products (much metal Ni, moderate elemental C, and a little NiO.) at the fuel regressing surface. Eventually, when the content of Ni(acac) 2 exceeds 5 wt%, the growth of regression rate slows down rapidly, and a performance detriment occurs at 40 wt%. This study verifies the catalytic effect of Ni(acac) 2 on polymer matrix for HTPB based fuels showing the attractive regression rate performance of this additive.