The Combined Effect of Ambient Conditions and Diluting Salt on the Degradation of Picric Acid: An In Situ DRIFT Study

Abstract

An unexpected promoting effect of KBr, used as a diluting salt, on the degradation of picric acid (PA) was observed during in situ diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy experiments performed here under accelerated ageing conditions—at 80 °C and under an inert or oxidative atmosphere. While the formation of potassium picrate was excluded, this promoting effect—which is undesired as it masks the possible effects of test conditions on the ageing process of the material—was assumed to favor a first step of the decomposition mechanism of PA, which involves the inter- or intramolecular transfer of hydrogen to the nitro group, and possibly proceeds up to the formation of an amino group. An alternative diluting salt, ZnSe, which is much less commonly used in infrared spectroscopy than KBr, was then proposed in order to avoid misleading interpretation of the results. ZnSe was found to act as a truly inert diluting salt, preventing the promoting effect of KBr. The much more chemically inert nature (towards PA) of ZnSe compared to KBr was also confirmed, at much higher temperatures than DRIFT experiments, by dynamic differential scanning calorimetry (DSC) runs carried out on pure PA (i.e., PA without salt) and PA/salt (ZnSe or KBr) solid mixtures.

Shelf Life Prediction of Picric Acid via Model-Based Kinetic Analysis of Its Thermal Decomposition

“…The kinetics predicted a negligible conversion of substrate also confirmed by the HPLC analysis of this sample. This is consistent with the almost overlap of the DSC thermograms recorded under dynamic conditions for fresh (i.e., unaged) and naturally aged material [19]. The good agreement between predictions and experiments shown in Table 5 and Figure 8 is a first proof of the reliability of the kinetics adopted for simulating the effect of thermal aging for PA.…”

An Alternative Approach for Predicting the Shelf Life of Energetic Materials

“…Predictions were found to be in good agreement with experimental data; Figure 2 shows the comparison under high temperature isothermal conditions. For naturally aged PA, a negligible conversion was found-this was consistent with the almost overlap of the dynamic DSC thermograms recorded for a sample of this material and a sample of fresh (i.e., unaged) PA [15] (Figure 3). The good agreement between predictions and experimental aging data proves the reliability of the kinetics extracted for PA, also supporting the extension of the adopted model-based approach to the prediction of the shelf life of EMs that, like PA, decompose in an autocatalytic manner.…”

“…(From Ref. [15]). proves the reliability of the kinetics extracted for PA, also supporting the extension of the adopted model-based approach to the prediction of the shelf life of EMs that, like PA, decompose in an autocatalytic manner.…”

“…Figure 3.Dynamic DSC thermograms (i.e., specific heat power, q, versus temperature curves) recorded for fresh and naturally aged PA (heating rate of 20 • C min −1 ). (From Ref [15]…”