Evaluation of thermal stability in ammonium nitrate systems modified with boric acid and urea for safety assessment

Ammonium nitrate (AN) is a widely used compound in fertilizers and energetic materials due to its high nitrogen content and strong oxidizing properties. However, its limited thermal stability can create significant safety concerns, particularly when additives alter its decomposition pathway. In this work, AN-based systems containing urea (U) and boric acid (BA) were investigated under conditions favoring either predominantly endothermic or exothermic decomposition. Differential scanning calorimetry and differential thermal analysis coupled with thermogravimetry and mass spectrometry were used to simulate appropriate conditions. Measurements first covered the pure components and their binary mixtures, then were extended to a full experimental design that covered 18 ternary AN-U-BA compositions in order to capture possible synergistic and antagonistic effects. The results show that U and BA often lower the overall heat evolved during decomposition, whereas urea-rich mixtures can promote unexpectedly rapid, strongly exothermic decomposition and thus increase hazard. Conversely, specific low-concentration systems showed signs of stabilization. Overall, the study demonstrates that evaluating AN-based systems over a range of thermal conditions is necessary to resolve the nuanced influence of each additive. Results support safer formulation of AN-containing materials and improve understanding of their decomposition mechanisms in the context of process safety and thermal risk assessment.