Investigating Thermal Decomposition Kinetics and Thermodynamic Parameters of Hydroxyl-Terminated Polybutadiene-based Energetic Composite

Abstract

Hydroxyl-Terminated Polybutadiene (HTPB)-based energetic compositions have been developed for enhanced blast energetic composite, composite rocket propellant formulations, metal cutting, demolition, welding and explosive reactive armour in civil and military applications. The types and choice of curing agents are crucial in enhancing the mechanical and structural integrity of the binder. To understand the stability and safety of energetic composites for potential applications, it is necessary to understand the thermal decomposition kinetics and thermodynamic parameters clearly. The main objective is to study the decomposition kinetic and thermodynamic parameters of energetic composites cured by different curing agents. A series of energetic composites based on HMX (1,3,5,7-tetranitro-1,3,5,7- tetrazocane) and HTPB-based binder system cured with various curing agents were prepared by the cast cured method. The curatives, namely MDI (4,4’-methylene diphenyl diisocyanate), IPDI (isophorone diisocyanate), TDI (toluene dissocyanate) and TMDI (2,2,4-trimethylhexamethylene diisocyanate) were used. The thermal analysis method was employed to investigate the thermal decomposition characteristics, which are closely associated with the thermal stability and safety considerations during handling, processing, and storage. The kinetic parameters for thermal decomposition reactions were studied by employing the Flynn-Wall-Ozawa method. The thermodynamic parameters of the activation enthalpy, activation Gibbs energy free and activation entropy of all energetic composites were also determined by the theory of activated complex. The thermogravimetric results show that the thermal stability is almost similar for all composites cured with the different types of curing agents. The average activation energy of the energetic composites cured with IPDI, MDI, TMDI and TDI was 207.5, 237.3, 243.3 and 187.6 kJ/mol, respectively. The thermodynamic parameters for the thermal decomposition process show that they are generally thermodynamically stable and non-spontaneous. Scanning electron microscope (SEM) micrographs of all the samples clearly indicate that HMX crystals are well embedded in the polymer matrices. The thermogravimetric results show that the thermal stability and thermal decomposition behaviour do not change significantly by varying the type of the curing agent in the HTPB-based binder. The SEM micrographs of all the samples clearly indicate that HMX crystals are well embedded in the polymer matrices. The averaged activation energy for the HMX/HTPB/MDI, HMX/HTPB/IPDI, HMX/HTPB/TDI and HMX/HTPB/TMDI samples obtained from FO method was 237.3, 207.5, 187.6 and 243.3 kJ/mol, respectively. The thermodynamic parameters including the activation enthalpy, activation Gibbs free energy and activation entropy for the thermal decomposition process show that they are generally thermodynamically stable and non-spathaceous. The thermal stability of all energetic composites is almost constant. The activation energy of the prepared energetic composites is significantly varied with varying the type of curing agents in the HTPB-based binder system. The thermodynamic parameters indicate that composites possess superior stability and thermal safety. The SEM micrographs indicate that HMX crystals of prepared composites are embedded in the polymer matrix.