Compatibility and thermal decomposition kinetics between HMX and some polyester-based polyurethanes

A polyester (PE)-based polyurethane (PU) is widely used as a polymeric binder with an energetic compound especially more energetic octhydro-1,3,5,7 tetranitro-1,3,5,7 tetrazocine (HMX) in many energetic composite formulation. Compatibility between PE-based PU containing various curatives and HMX was studied through vacuum stability test, thermogravimetry and differential scanning calorimetry methods. PUs were obtained from the reaction of the prepolymer PE with various diisocyanates as curatives especially 4,4 ' methylenediphenyl diisocyanate (MDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI) and 2,2,4 trimethylhexamethylene diisocyanate (TMDI), and these PUs were designated by PE/MDI, PE/IPDI, PE/TDI and PE/TMDI, respectively. The compatibility was judged through standardization agreement (STANAG) 4147. The results showed that the binary mixture of HMX with various PU, namely PE/MDI, PE/IPDI, PE/TDI and PE/TMDI, possessed a good compatibility. Kinetics of the binary mixture of HMX and PU was studied at three heating rates by means of the Ozawa and Kissinger methods. The calculated activation energy values of the binary mixture of HMX with PE/MDI, PE/IPDI, PE/TDI and PE/TMDI from the Kissinger method were found to be 483.9, 504.7, 502.2 and 485.3 kJ mol(-1), respectively. For comparison, the values of neat PE and HMX were also calculated and were 118.2 and 299.3 kJ mol(-1), respectively. The kinetic parameters were strongly depended on the type of the curative groups in the PE-based PU. Similar results were obtained from the Ozawa and ASTM E698 methods. The activation energy values calculated from the ASTM E698 method for the HMX/PE/MDI, HMX/PE/IPDI, HMX/PE/TDI and HMX/PE/TMDI samples were 204-215, 219-235, 145-157 and 172-246 kJ mol(-1) in the low conversion of 0.05-0.15, respectively. Subsequently, these were demonstrated by a remarkable deviation in the activation energy in a range of 247-474, 235-499, 166-578 and 245-528 kJ mol(-1) at the conversion (0.15-0.95), respectively.