Understanding Explosive Sensitivity with Effective Trigger Linkage Kinetics

Abstract

We present a simple linear model for ranking the drop weight impact sensitivity of organic explosives that is based explicitly on chemical kinetics. The model is parameterized to specific heats of explosion, Q, and Arrhenius kinetics for the onset of chemical reactions that are obtained from gas-phase Born-Oppenheimer molecular dynamics simulations for a chemically diverse set of 24 molecules. Reactive molecular dynamics simulations sample all possible decomposition pathways of the molecules with the appropriate probabilities to provide an effective reaction barrier. In addition, the calculations of effective trigger linkage kinetics can be accomplished without prior physical intuition of the most likely decomposition pathways. We found that the specific heat of explosion tends to reduce the effective barrier for decomposition in accordance with the Bell-Evans-Polanyi principle, which accounts naturally for the well-known correlations between explosive performance and sensitivity. Our model indicates that sensitive explosives derive their properties from a combination of weak trigger linkages that react at relatively low temperatures and large specific heats of explosion that further reduce the effective activation energy.

Modeling Photolytic Decomposition of Energetically Functionalized Dodecanes

“…For instance, lead azide (Pb­(N 3 ) 2 ) has a high thermal stability, but is also highly sensitive to impact and friction insults and will even explosively decompose before it melts . This observation could potentially be explained by the abnormally high pre-exponential factor computed from Arrhenius plots of MD kinetic calculations, even though this explanation seems insufficient for our calculations. In the MD kinetic calculations, the maximum energy probed was 0.2 eV, while our lowest energy was 4 eV.…”

Identifying the Molecular Properties that Drive Explosive Sensitivity in a Series of Nitrate Esters

“…66 While the chemistry and trigger linkage kinetics of the four PETN derivatives are clearly very similar, our drop weight impact testing reveals a precipitous increase in H 50 when nitrate esters are replaced by hydroxyl groups. In a recent publication, 32 we proposed that drop weight impact sensitivity depends jointly on the kinetics of trigger linkage rupture and the enthalpy of explosion via…”

“…We obtained a value of about γ ≈ 1.6 g/kcal, but we found that γ is somewhat system-dependent. Equation , which depends only on gas-phase thermochemistry, accounts for at best 70% of the variation in ln­( H 50 ), and we attribute the remaining 30% of the variation to condensed phase properties and chemistry that are not captured by the model . The intrinsic limitations associated with eq and the small numbers of families of explosive molecules that are suitable for parametrization of γ lead inevitably to significant uncertainties in the values of γ.…”

“…The correlation between explosive performance and sensitivity is well-known since, in general, the most powerful explosives are also the most sensitive. Both the body of work from Zeman 12 , 30 , 31 and recent studies by our team 32 find that the Bell-Evans–Polanyi principle 33 , 34 provides the best description of the dependence between sensitivity and performance, through the specific heat of explosion, Q . Understanding sensitivity trends in an explosive system in which the energy release is changed while maintaining the trigger linkage chemistry is difficult, because modifications to molecular structure often change a range of physical properties, such as melting point, density, and crystal packing, all of which are thought play a role in determining handling sensitivity.…”

“…While the chemistry and trigger linkage kinetics of the four PETN derivatives are clearly very similar, our drop weight impact testing reveals a precipitous increase in H 50 when nitrate esters are replaced by hydroxyl groups. In a recent publication, 32 we proposed that drop weight impact sensitivity depends jointly on the kinetics of trigger linkage rupture and the enthalpy of explosion via where α is a positive constant. It follows from eq 2 and the observation that this set of derivatives have similar normalized rates, activation enthalpies, E a , and pre-exponential factors, Â , that the variation of H 50 across the series should follow where the superscript “0” denotes those parameters for a reference molecule in the series of derivatives, and γ is a constant.…”