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Recent achievements in kinetic modelling of stabilizer consumption and molar mass degradation in NC-based propellants

: Bohn, Manfred A.

University of Pardubice, Institute of Energetic Materials; University of Pardubice, Faculty of Chemical Technology:
17th Seminar on New Trends in Research of Energetic Materials 2014. Proceedings : April 9.-11.2014, Pardubice, Czech Republic
Pardubice: University of Pardubice, 2014
Seminar on New Trends in Research of Energetic Materials (NTREM) <17, 2014, Pardubice>
Fraunhofer ICT ()

With nitrocellulose stabilizers must be used to prevent it against the autocatalytic decomposition. However, the slow intrinsic decomposition cannot be prevented and therefore a slow degradation happens always, which leads to stabilizer consumption and the scissioning of the NC chains. The last effect means molar mass degradation of NC. Both phenomena are irresolvable coupled. In splitting-off the NO2 radical from the nitrate ester groups RCO-NO2 the remaining radical function written as RC-O stabilizes itself by molecular rearrangement of the anhyroglucopyranose ring and by splitting-off stable small molecular species. This means molar mass degradation happens by destruction of chain units and not be mere bond scissioning between chain units. This is in accordance with the well observed phenomena of mass loss with NC..
To model the stabilizer consumption correctly, the production of NOx by NC has to be regarded as well as the bimolecular reaction between NOx and the stabilizer. The preceding production of NOx from NC is explicitly included, because the concentration of it is not constant with time. The elaboration of this relatively simple reaction scheme ends up in an equation for the bimolecular stabilizer reaction. The model contains both reaction rate constants of the two reactions of the scheme, which is a general and necessary condition for correct modelling of stabilizer consumption.
Molar mass decrease of NC caused by random chain scission is describable with models based on the averaged characterizing parameters of the molar mass distribution (MMD) functions. The correct distribution function in terms of reaction kinetics is the one which counts the numbers (or mols) of polymers in the molar mass fraction M+dM. This one is named polymer number related MMD, in short hn and the first moment (center of gravity or arithmetic mean or expectation value) of it is the number averaged mean molar mass Mn. Based on this quantity Mn models of random chain scission and random chain scission with chain recombination are discussed.