Degradation of complex hybrid propellants

Abstract

The ageing behaviour of a complex hybrid propellant (known as Composition A) which contains a nitroglycerine (NG) and polyethylene glycol (PEG) based binder has been investigated. The aims were to ascertain if this propellant degrades differently in an open or sealed environment, if the decomposition mechanisms are driven by the production of water in the propellant and whether this compound accumulates over time and if the presence of ammonium perchlorate (AP) is important. A further aspiration was to establish which analytical techniques are suitable for characterising these sorts of materials. The research has identified a number of analytical techniques that can be used to characterise changes to such propellants upon ageing. Furthermore, other experimental methods which have not proved to be so useful are briefly discussed. It has also been demonstrated that one of the key ageing mechanisms is the oxidation of polyethylene glycol (PEG), which ultimately leads to the softening of the propellant. When there is no AP present in the compositions with a NG / PEG binder, ageing proceeds rapidly. However, water does not appear to accumulate unless liquefaction occurs as all of the cross- links between the PEG chains have broken. At temperatures below 90 °C, when AP is present it has a beneficial effect as it initially hinders the oxidation of PEG and the hydrolysis of NG. In turn, acidic compounds do not form quickly and so the rates of decomposition of NG / PEG based propellants with AP are much slower compared with those without AP at such temperatures. At elevated temperatures though, for example above 90 °C, compositions containing AP decompose very quickly. This is because the degradation of NG advances rapidly and the resultant products react with perchloric acid, which ultimately leads to the ignition of the propellant.