Investigation into the Decomposition Pathways of an Acetal-Based Plasticizer

Abstract

Until now, it has been assumed that the primary decomposition pathway for the liquid plasticizer bis(2,2-dinitropropyl)acetal and bis(2,2-dinitropropyl)formal (BDNPA/F) was nitrous acid elimination (NAE). An ultrahigh-performance liquid chromatography (UHPLC) coupled to quadrupole time-of-flight mass spectrometry (QTOF) methodology was developed to discover and identify the degradation products of BDNPA/F. No evidence of NAE was found. However, two other degradation pathways were found: (1) hydrolysis of the acetal/formal functional group and (2) radical-based homolysis of the C–N bond, followed by hydrogen atom abstraction. Hydrolysis of BDNPA/F proceeds by the formation of 2,2-dinitropropanol (DNPOH) and 2,2-dinitropropyl hemiacetal/hemiformal, which further decompose into DNPOH and ethanal/methanal, respectively. Hydrolysis is the dominant decomposition pathway in all samples; however, at higher temperatures, C–N homolysis becomes more significant. Also, the solid PBX 9501 has different ratios of decomposition products than the liquid BDNPA/F due to the slower rate of diffusion through solids than liquids.

Roles of HNOxand Carboxylic Acids in the Thermal Stability of Nitroplasticizer

“…When the HNO 3 concentration reaches the threshold of 1.35 mM, which is capable of stabilizing the protonated form of acetal/formal oxygen in NP, NP can undergo nucleophilic addition of water to first form 2,2-dinitropropanol (DNPOH, 3b ) and an oxonium ion, which then produce the hemiacetal/hemiformal (Scheme , 2b ) in the latter water addition (Scheme ). − Moreover, the localized clusters or microdroplets of produced water can promote an interfacial nitration reaction in the presence of HNO 3 and volatiles (e.g., NO 2 + ), − which accelerates PBNA depletion. This early-stage process is described as a partial hydrolysis of NP.…”

“…Therefore, the NP is projected to fully decompose into DNPOH ( 4b ) and aldehydes (Scheme ). Surrounded by HNO 3 and water, aldehydes can be oxidized to organic acids (e.g., acetic and formic acids, 5b ). − In a highly acidic environment, the hemiacetal/hemiformal is expected to only exist as a transient intermediate. Additionally, CO 2 and CO can be converted into oxalic acid: (1) a coupling product of CO 2 that is promoted by the energy storage of C–C bond formation (2CO 2 + 2e – → C 2 O 4 2– ); (2) a dialkyl oxalate product of the reaction between an alcohol (i.e., DNPOH) and CO, which is then hydrolyzed into oxalic acid, as shown in Scheme …”

Development of a Gas Chromatography with High-Resolution Time-of-Flight Mass Spectrometry Methodology for BDNPA/F

“…Analysis of thermally labile compounds such as explosives and energetic materials is often performed using liquid chromatography (LC) as opposed to gas chromatography (GC) to avoid any undesirable thermal decomposition in the inlet or analytical column. − This is especially true when evaluating decomposition products formed through thermal aging of energetic materials. ,, However, a drawback of LC is the difficulty in analyzing small molecules with higher volatilities and the lower separation efficiency compared to GC. While LC still appears to be the main choice of technique for the analysis of energetics, the use of GC has been becoming more significant since the introduction of programmed temperature vaporization (PTV) inlets. − One of the first studies for analyzing energetic materials by GC-MS using a temperature-programmable inlet was performed in 1996 by Yinon who was able to analyze thermally labile energetic samples without compound degradation in the inlet .…”

“…BDNPA/F is a key component in plastic-bonded explosive (PBX) 9501, which is composed of 94.9 wt % of the explosive (1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)), 2.5 wt % of a polymeric binder (Pearlstick 5703), 0.1 wt % of a radical stabilizer (Irganox 1010), and 2.5 wt % of BDNPA/F. Aging products of BDNPA/F have been investigated using LC coupled to quadrupole time-of-flight mass spectrometry (QTOF), which identified the decomposition pathways of hydrolysis of the acetal/formal functional group and homolysis of the nitro functional group . While LC-QTOF can provide insights into the aging of BDNPA/F as well as the variations in manufacturing processes, it has been hypothesized that these initial aging products can continue to degrade into smaller compounds, which are best analyzed via GC.…”

“… 1 − 6 This is especially true when evaluating decomposition products formed through thermal aging of energetic materials. 5 , 7 , 8 However, a drawback of LC is the difficulty in analyzing small molecules with higher volatilities and the lower separation efficiency compared to GC. While LC still appears to be the main choice of technique for the analysis of energetics, the use of GC has been becoming more significant since the introduction of programmed temperature vaporization (PTV) inlets.…”