COMPATIBILITY OF FLAME RETARDANTS IN PROPELLANTS, PART II - INFLUENCE OFOXYGEN ON DECOMPOSITION REACTIONS

摘要

Flame retardants are added to propellants to prevent muzzle flashes which sometimes appear after firing. This muzzle flash is unwanted as it is a danger for the soldiers and it increases the detectability of the weapon's position. To prevent this muzzle flash potassium salts are added to the propellant. They reduce this unwanted effect nearly independently from their corresponding anion. Therefore it seems to be practical to introduce potassium salts with a positive oxygen balance to increase the oxygen balance and improve the susceptibility to ignition of the propellant. Potassium nitrate appears to be the ideal flame retardant from this point of view. In the first part of this study we compared the effect of four different flame retardants (potassium nitrate, potassium sulphate, sodium oxalate and potassium hydrogen tartrate) on the ageing behaviour of propellants, measured by microcalorimetry, vacuum stability and stabilizer depletion on a total of 75 different experimental propellant samples. It turned out that potassium nitrate is less compatible with nitrate esters than other potassium salts, especially when nitroglycerin is present. This is due to its oxidizing properties. Therefore it doesn't seem to be practical to use potassium nitrate in double base propellants stabilized with 2-NO_2-DPA, TPA or DPA. The effect of oxygen on the ageing behaviour was studied by varying the air/propellant ratio tested in microcalorimetry, followed by stabilizer depletion analysis. This oxygen effect has recently been studied in other propellants as well. The major influence is visible when the propellants are stabilized with DPA, the smallest effects were observed on propellants stabilized with centralite I and akardite II. The present work shows the combination of oxidative effects on propellant decomposition. The first (inherent) oxidative effect derives from the liberation of nitrous oxides from the propellant itself (NO_2 is reduced to NO by oxidising Nc). The second oxidative effect comes from oxygen itself. The oxidation of propellants by atmospheric oxygen is always the first step in the decomposition process. An accumulation of oxidative potential by potassium nitrate and oxygen should be therefore much more severe than the presence of only oxygen or of only potassium nitrate. But the incompatibility of KN with nitrocellulose/nitroglycerin is not of oxidative nature, as an accumulation of oxidative decomposition is not observed when oxygen and KN are present at the same time.