Mechanism And Kinetics Of Paraffin-induced Thermal Decomposition Of HMX

Polymer bonded explosive is a typical composite material composed of explosive,binder,desensitization agent and stabilizer,which is widely used in aerospace and military fields.Due to the complex composition of polymer bonded explosives and the complex mechanism of influence of different functional auxiliaries on explosive properties,safe and effective design and supervision methods have not yet been formed.Various properties of polymer bonded explosives,such as detonation,aging and safety properties,are controlled by regulating the types and contents of functional auxiliaries.Under various performance parameters,the probability of accidents is high when ammunition is subjected to external thermal stimulation,so we usually pay the most attention to the thermal safety performance of composite explosive,especially the slow cook performance.However,little is known about the mechanism by which functional auxiliaries affect the thermal properties of explosives,which makes it difficult to evaluate and explain the mechanism of improving the slow cook off properties of composite explosives.In this paper,the influence of paraffin wax on the thermal decomposition mechanism and kinetics of HMX was studied,which provided theoretical understanding and data input for the design and selection of paraffin/HMX based explosives.The main work of this paper is as follows:(1)The influence of paraffin on the abnormal phenomena and mechanism of thermal decomposition of HMX was evaluated by thermal analysis technology,combined with crystal morphology analysis,molecular dynamics simulation,kinetic analysis and gas product analysis.During the initial decomposition,paraffin infiltrates the HMX crystals’ surface,reduces the energy barrier of chemical bond dissociation,and induces the decomposition of molecules on HMX crystals,resulting in a lower initial decomposition temperature.Paraffin consumes the active gas produced by HMX with further thermal decomposition and inhibits the dramatic increase in HMX thermal decomposition rate.In decomposition kinetic,this effect is manifested as that paraffin inhibits the transition from an n order reaction to an autocatalytic reaction.(2)The thermal decomposition kinetics of paraffin/HMX mixture with different particle sizes was studied by thermal analysis kinetics,and a three-step reaction independent kinetic model was obtained.From the kinetic mechanism,the thermal decomposition behavior of paraffin/HMX mixture was deduced and reasoned.The results show that the first n order reaction has a great influence on the thermal decomposition of paraffin/HMX.When the dominant role of n order reaction increases,the autocatalysis of the second and third steps will decrease.At the same time,the smaller the size of HMX,the stronger the dominant role of the n order reaction in the first step,the weaker the autocatalysis of the second and third steps.The thermal safety of paraffin /HMX mixtures was evaluated based on kinetic data.Based on the kinetic data,the effects of different sizes of HMX on the thermal safety of paraffin/HMX mixture were evaluated.The results showed that the smaller the size of HMX,the more unstable the thermal safety of paraffin /HMX mixture.The simulation results under slow cook conditions showed that the ignition position of the paraffin /HMX mixture was all on the surface,and the response temperature decreased with the decrease of HMX particle size.(3)The effect of different paraffin content on the kinetics of paraffin/HMX mixture was studied by thermal analysis kinetics under adiabatic condition.Unlike the linear heating procedure,the adiabatic paraffin/HMX mixture exhibited only one step decomposition reaction,and the first n order reaction did not bring the mixture into self-sustaining decomposition.At the same time,with the decrease of paraffin content,the decomposition of HMX was dominant,and the n order reaction of paraffin was weakened.The thermal safety of paraffin /HMX mixtures was evaluated based on adiabatic kinetic data.The results showed that the thermal safety of paraffin HMX was more and more stable with the decrease of paraffin content.