Delay Mechanism of β→δ Phase Transition of Cyclotetramethylene Tetranitramine in Polymer Bonded Explosive Formulations by Heat Conduction Obstacle

Abstract

The β→δ phase transition (PT) of cyclotetramethylene tetranitramine (HMX) plays an important role in the safety of explosives when they are exposed to heat. In this work the effect of HMX content on the PT of HMX in Polymer Bonded Explosive (PBX) is reported. Using in situ X‐ray diffraction (XRD) test combined with quantitative analysis, the dependence of the PT temperature and dynamic behavior of HMX in PBX formulations [HMX+triaminotrinitrobenzene (TATB)+Olefin] on the content of HMX is investigated. The results show that the β→δ PT temperature gradually increases with the reduction of HMX content and with the increase of TATB content. Additives of TATB and olefin in high concentration can form compact coatings on the HMX crystals. Such coatings can delay the nucleations of δ‐HMX by inhibiting the volumetric expansion during PT. Moreever, these coatings separate the HMX crystals and build up a heat conduction obstacle. As a result the growths of δ‐HMX are also suppressed by the coatings. In contrast, fewer additives lead to larger free surface area of HMX, which accelerates the PT.

Core@Double-Shell Structured Energetic Composites with Reduced Sensitivity and Enhanced Mechanical Properties

“…A possible reason for this phenomenon might be attributed to the formation of perfect core−shell structure, which can delay the nucleations of δ-HMX and bring a heat conduction obstacle in solid−solid phase transition. 34 The polymorphic transition of HMX in HMX/TATB and HMX@TATB was inhibited by further PDA coating. The polymorphic transition temperature reached up to 212.8 °C for HMX@TATB@PDA-9h.…”

“…Compared with HMX/TATB, the suppression of a polymorphic transition was more obvious in the presence of core–shell structure, with the polymorphic transition temperature shifted to 201.4 °C. A possible reason for this phenomenon might be attributed to the formation of perfect core–shell structure, which can delay the nucleations of δ-HMX and bring a heat conduction obstacle in solid–solid phase transition . The polymorphic transition of HMX in HMX/TATB and HMX@TATB was inhibited by further PDA coating.…”

Core-Shell Structured HMX@Polydopamine Energetic Microspheres: Synergistically Enhanced Mechanical, Thermal, and Safety Performances

“…So far, coating approaches have been mainly developed to tune the phase transition temperature of the explosives [12]. Additives of TATB and olefin in high concentration can form compact coatings on the HMX crystals to delay the nucleations of δ-HMX and build up a heat conduction obstacle, leading to a higher temperature required for the β-δ phase transition [13]. The phase transition of HMX explosives can be greatly improved after core-shell coating of melamine-formaldehyde resin, with the appreciable increment of more than 16 °C [7].…”

“…However, the high concentration of shell materials is necessary to completely coat explosive crystals. In contrast, fewer additives lead to larger free surface area of HMX, which accelerates the phase transition [13]. Furthermore, solid–solid phase transition of explosive crystals depends on the chemical interaction between explosives and binders, which may promote or delay the phase transition [14].…”

Kinetics for Inhibited Polymorphic Transition of HMX Crystal after Strong Surface Confinement

“…First, in the case of polymorphic transition inhibition, surface coating was demonstrated as an effective strategy to suppress undesired polymorphic transition. − Nevertheless, the highly efficient inhibition of polymorphic transition of polymorphic energetic materials still faces challenges. Typical difficulty is what kind of surface coating is the most efficient in suppressing the polymorphic transition by even using minimum coating.…”