Thermal stability of a eutectic mixture of bis(2,2-dinitropropyl) acetal and formal: Part B. Degradation mechanisms under water and high humidity environments

Abstract

As a continuation of our effort to understand degradation mechanisms of a eutectic mixture of bis(2,2-dinitropropyl)acetal (BDNPA) and bis(2,2dinitropropyl)formal (BDNPF) (referred to as NP) under various environmental conditions, we investigated the thermal stability of NP under water and 74% relative humidity (RH) environments at temperatures below 70 °C. Based on a comprehensive characterization of samples aged over a period of two years, we conclude that in the presence of water the reaction pathways of the NP degradation are different from those observed in air or under nitrogen atmosphere. We found that the physical state of water molecules plays an important role as it determines the ability of oxygen to participate in the NP aging process. Based on the results obtained in Parts A and B of these studies, we conclude that the rate of NP degradation increases in the order: nitrogen < water < air < water vapor + air.

Comprehensive Data Analysis and Analytic Method Development for PBX9501 Material (FY2019 Annual Report of Aging and Lifetimes Program)

“…Conversely, when the NP container is constantly exposed to a dry environment, such as that of Los Alamos (<50 RH%), the NP tends to lose this trace amount of water, and shows degradation at ambient temperature [20]. This observation might also explain why the NP degraded at 38°C and below within 24 months in our previous study, where the free volume was large and the headspace composition was periodically reset during the sampling [18,19]. In this study, the composition does not freely exchange with its surroundings and, except for the water peaks, the NIR spectra remain almost the same for 36 months at temperatures below 55°C.…”

“…This procedure resulted in a change in the concentrations of reactants and products in the headspace, stimulating further degradation of NP. This earlier work indicated the degradation of NP, even at temperatures below 38°C via changes in color and the FTIR spectra, but not in the NMR spectra [18,19]. This was attributed to the change in the concentrations of reactants and products in the headspace and its large free volume.…”

“…Although temperature control is relatively easy to achieve, mimicking headspace conditions to simulate weapon applications is more difficult. In our earlier work, the aging containers had a large free volume (>30x the NP volume) [18,19]. This was not representative of weapon conditions, where the systems are sealed with very little free volume.…”

“…The stability of various classes of polynitro compounds has been investigated previously. However, relatively few studies have focused on understanding the decomposition pathways of BDNPA/F at temperatures below 70°C [12,18,19,[22][23][24][25][26]. Also, although numerous studies have focused on the aging behavior of the PBX9501 [12,13,16,[27][28][29][30][31][32], few have focused on the NP alone.…”

“…Therefore, the thermolysis of NP is often treated as a chaos degradation. The detailed degradation mechanisms and potential reactive intermediates at moderated temperatures can be found in references [1,18,19,23,25,[33][34].…”