Yahaya Mohammed PhD
The Effect of Moisture on High Temperature Ageing of Extruded Double Base Propellant
Nitrocellulose (NC) based propellants are commonly used in military munitions and cartridge/propellant actuated devices (CAD/PAD). These propellants are generally classified as nitrate esters and are known for their tendency to chemically degrade over time. This chemical instability has been attributed to the relatively weak O-N bond has been shown to be responsible for this instability responsible for this bond on is the primary reason for this chemical instability. There are two possible reaction pathways for nitrate ester decomposition and these are hydrolysis and thermolysis. Thermolysis involves a thermally induced cleavage of the -O-NO2 bonds or with the release of NOx radicals. Hydrolysis on the other hand, involves the reaction of NOx with water to form acids and hydroxyl radicals. While thermolysis is thought to dominate at higher temperatures, hydrolysis has been described as the dominant mechanism at lower temperatures. The free radicals generated due to hydrolysis or thermolysis are highly reactive and will attack the nitrocellulose polymer, producing more radicals and eventually leading to a highly exothermic autocatalytic reaction. Although chemical stabilizers are added to nitrate ester propellants during manufacture to mop-up the NOx radicals, the stabilizers are eventually consumed.
In determination of the stability of propellants, several techniques are used, depending on what mechanism is investigated. In most cases, a combination of techniques is required. In this paper, the influence of moisture on high temperature (80 – 100 °C) ageing of an extruded double base (EDB) propellant was examined using heat flow calorimetry (HFC). Propellant samples were preconditioned at relative humidity values of 0, 25, 50, 75, and 95% relative humidity (RH) before ageing in the HFC. The result showed that a direct correlation between humidity levels and heat flow, and it showed that moisture affects the heat flow values and shape of the HFC curve especially at early conversion and at the onset of autocatalysis. This is significant because it gives an indication that moisture influences the decomposition of NC propellants even at higher temperatures.