Roles of HNOX and Carboxylic Acids in the Thermal Stability of Nitroplasticizer

In the thermal aging of nitroplasticizer (NP), the produced nitrous acid (HONO) can decompose into reactive nitro-oxide species and nitric acid (HNO3). These volatile species are prone to cause cascaded deterioration of NP and give rise to various acidic constituents. To gain insight on the early stage of NP degradation, an adequate method for measuring changes in the concentrations of HONO, HNO3, and related acidic species is imperative. The typical assessment of acidity in nonaqueous solutions (i.e., acid number) cannot differentiate acidic species and thus presents difficulty in the measurement of HONO and HNO3 at a micromolar concentration level. Using liquid-liquid extraction and ion chromatography (IC), we developed a fast and unambiguous analytical method to accurately determine the concentration of HONO, HNO3, acetic/formic acids, and oxalic acid in aged NP samples. Given by the overlay analysis results of liquid chromatography coupled with quadrupole time-of-flight mass spectrometry and IC, the prominent increase of produced HONO after the depletion of antioxidants is the primary cause of HNO3 formation in the late stage of NP degradation, which results in the acid-catalyzed hydrolysis of NP into 2,2-dinitropropanol and acetic/formic acids. Our study has demonstrated that the aging temperature plays a crucial role in accelerating the formation and decomposition of HONO, which consequently increases the acidity of aged NP samples and hence accelerates the hydrolyzation of NP. Therefore, to prevent NP from undergoing rapid degradation, we suggest that the concentration of HNO3 should be maintained below 1.35 mM and the temperature under 38 degrees C.

Automated summary

During the thermal aging of nitroplasticizer (NP), the formation of nitrous acid (HONO) leads to the decomposition of HONO into reactive nitro-oxide species and nitric acid (HNO3), causing cascaded deterioration of NP and resulting in acidic constituents. It is important to develop an analytical method to measure the concentrations of HONO, HNO3, and related acidic species accurately. By using liquid-liquid extraction and ion chromatography (IC), we established a fast and unambiguous method to determine the concentrations of HONO, HNO3, acetic/formic acids, and oxalic acid in aged NP samples. Our study demonstrates that the aging temperature is crucial in accelerating the formation and decomposition of HONO, thereby increasing the acidity of aged NP samples and accelerating the hydrolysis of NP.