Ageing Effects on the Structure and Chemistry of Gun Propellants

Abstract. Gun propellants mainly consist of nitrate esters, such as nitrocellulose and nitroglycerin, which naturally degrade over time, even under controlled storage conditions. The degradation reaction rate can, however, be mitigated with stabilizers based on aromatic amines (e.g., diphenylamine) or urea derivatives (e.g., centralites). Various standard test procedures are employed to evaluate propellant stability and to predict service life. These methods can provide a snapshot of the propellant condition at a given time or, through accelerated ageing protocols, simulate long-term ageing to estimate remaining lifespan. This work aims to investigate the structural, morphological, and chemical changes associated with ageing in gun propellants of different calibres. The analysed materials include 20 mm, 30 mm, 12.7 mm, and 7.62 mm gun propellants used by the Portuguese Armed Forces. The ageing process followed the principles outlined in AOP-48 (2008), with thermal exposition performed by storing the gun propellants in a controlled oven environment. To assess ageing-related changes, optical microscopy was used to observe surface and internal features; mercury porosimetry to evaluate porosity; helium pycnometry for density measurements; elemental analysis to investigate the chemical nature of mass losses, and high-performance liquid chromatography to quantify stabilizer content. The results reveal distinct differences between gun propellants at various ageing stages, primarily reflected by a progressive loss of stabilizer content. In addition, localized light-coloured regions are observed to be exposed through the grey metallic coating, highlighting structural heterogeneities associated with ageing.