Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry Analysis of Eutectic Bis(2,2-dinitropropyl) Acetal/Formal Degradation Profile: Nontargeted Identification of Antioxidant Derivatives

Abstract

In the eutectic mixture of bis(2,2-dinitropropyl) acetal (BDNPA) and bis(2,2-dinitropropyl) formal (BDNPF), also known as nitroplasticizer (NP), n -phenyl-β-naphthylamine (PBNA), an antioxidant, is used to improve the long-term storage of NP. PBNA scavenges nitrogen oxides (e.g., NO x radicals) that are evolved from NP decomposition, hence slowing down the degradation of NP. Yet, little is known about the associated chemical reaction between NP and PBNA. Herein, using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF), we thoroughly characterize nitrated PBNA derivatives with up to five NO 2 moieties in terms of retention time, mass verification, fragmentation pattern, and correlation with NP degradation. The propagation of PBNA nitration is found to depend on the temperature and acidity of the NP system and can be utilized as an indirect, yet reliable, means of determining the extent of NP degradation. At low temperatures (<55 °C), we find that the scavenging efficiency of PBNA is nullified when three NO 2 moieties are added to PBNA. Hence, the dinitro derivative can be used as a reliable indicator for the onset of hydrolytic NP degradation. At elevated temperatures (≥55 °C) and especially in the dry environment, the trace amount of water in the condensed NP (<700 ppm) is essentially removed, which accelerates the production of reactive species (e.g., HONO, HNO 3 and NO x ). In return, the increased acidity due to HNO 3 formation catalyzes the hydrolysis of NP and PBNA nitro derivatives into 2,2-dinitropropanol (DNPOH) and nitrophenol/dinitrophenol, respectively.

Roles of HNOxand Carboxylic Acids in the Thermal Stability of Nitroplasticizer

“…Concentrations of BDNPA/F and DNPOH were determined using calibration ranges of 90–240 and 2–80 ppm, respectively. The detailed analytical method is described in our previous works. , A Thermo Nicolet iS50 FTIR spectrometer was used to conduct the FTIR measurement in the attenuated total reflectance mode between 4000 and 450 cm –1 . A diamond crystal was used.…”

“…As shown in Figures 4 and 5, in the late stage, the concentrations of all degraded species are greatly increased, on average by 30-fold in HONO, 10-fold in HNO 3 , 2) the formation of tetra-nitro derivatives is catalyzed by high acidity and high temperatures. 15 2.3.3. Decomposition of HNO 3 .…”

“…To address the effect of HONO decomposition on NP degradation, a nitration pathway of the antioxidant n -phenyl-β-naphthylamine (PBNA) established in the previous study must be revisited . As proposed in Scheme , the reactions between PBNA ( 1a ), NP ( 1b ), and the key volatile products (NO 2 • , NO 2 + , and HNO 3 ) can generate a suite of degraded products from NP.…”

“…There are three degradation routes which progress synchronously and dynamically as a function of time: (a) PBNA nitration, (b) HONO formation and decomposition, and (c) NP hydrolysis (detailed mechanism as shown in Scheme ). In the early stage, since the produced NO 2 • from HONO decomposition is scavenged by PBNA, the formation of HNO 3 (i.e., reaction ) is minimized. , As the number of its NO 2 moieties increases, the scavenging efficiency of PBNA decreases. Among the identified PBNA nitro derivatives, dinitro-PBNA ( 2a ) is the last derivative that remains effective in scavenging NO 2 • , whereas trinitro-PBNA ( 3a ) is the nullified (less effective) form of this antioxidant. , Consequently, HNO 3 starts to form at a slow rate.…”

“…In the early stage, since the produced NO 2 • from HONO decomposition is scavenged by PBNA, the formation of HNO 3 (i.e., reaction ) is minimized. , As the number of its NO 2 moieties increases, the scavenging efficiency of PBNA decreases. Among the identified PBNA nitro derivatives, dinitro-PBNA ( 2a ) is the last derivative that remains effective in scavenging NO 2 • , whereas trinitro-PBNA ( 3a ) is the nullified (less effective) form of this antioxidant. , Consequently, HNO 3 starts to form at a slow rate. At low acidity and due to the electron density being drawn away by the nitro groups, the protonation of acetal/formal oxygen in NP is destabilized (Scheme ).…”