Nitroglycerine is incorporated into propellant formuiations as an energetic plasticiser. It is well established that because nitroglycerine is highly mobile it can readily migrate from the propellant grain into the rocket motor liner or evaporate into the conduit. Clearly this has an effect on the chemical and mechanical properties of the propellant. The diffusion of nitroglycerine through a propellant and its subsequent evaporation can be measured by a technique such as dynamic vapour sorption (DVS) whereas sandwich tests can be used to isolate the diffusion behaviour of nitroglycerine. Sandwich tests involve placing discs of propellant either side of an inert material and measuring the amount of nitroglycerine which migrates out of the propellant. The data from these techniques can then be used to model the process of nitroglycerine migration and evaporation. However, before this can be fully understood it is necessary to measure the vapour pressure of the nitroglycerine above the propellant as this affects the diffusion and more specifically, the evaporation step. There have been numerous studies over the years which have involved measuring the vapour pressure of neat nitroglycerine using a variety of techniques but few have concentrated on quantifying the vapour pressure of nitroglycerine above specific propellants. This paper reports some initial work which aimed to measure the vapour pressure of nitroglycerine above a complex hybrid propellant using a headspace sampling technique coupled with analysis by gas chromatography with an electron capture detector. The relevance of the vapour pressure data to the modelling of nitroglycerine diffusion and evaporation from a particular propellant will also be discussed.
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