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Stability and characterization of GAP bonded ADN-propellant - the problem of gas formation during curing with isocyanates and characteristics of the glass-rubber transition

: Bohn, Manfred A.

Volltext urn:nbn:de:0011-n-3498667 (801 KByte PDF)
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Erstellt am: 31.7.2015

Fraunhofer-Institut für Chemische Technologie -ICT-, Pfinztal:
Energetic materials - performance, safety and system applications : 46th International Annual Conference of the Fraunhofer ICT, June 23 - 26, 2015, Karlsruhe, Germany
Pfinztal: Fraunhofer ICT, 2015
ISSN: 2194-4903
Fraunhofer-Institut für Chemische Technologie (International Annual Conference) <46, 2015, Karlsruhe>
Konferenzbeitrag, Elektronische Publikation
Fraunhofer ICT ()

During manufacturing of a GAP-ADN formulation in the framework of HISP - program *) a porous structure in the formulation was encountered. The pores are caused by gas formation during curing of the propellant mix. This is very disadvantages with respect to mechanical properties and burning behaviour. The conditions of pore formation and the possible causes should be clarified in order to avoid or at least to reduce the pore formation below a critical extent. The origin of the gas formation was assumed to be found in AND itself or a reaction between ADN and the isocyanate DesmodurTM E305, which was used to form the polyurethane based binder in the formulation. Gas evolution at several temperatures, 50°C, 60°C and 70°C as function of measurement time was applied to characterize the gas evolution rates. Besides DesmodurTM E305 also the isocyanate IPDI (isophorone diisocyanate) was included in this investigation. The gas evolution was followed with two methods: by IR absorption of the concentration increase of N2O (only with E305 because of to high vapour pressure of IPDI), a decomposition product of ADN and by total gas evolution using the vacuum stability apparatus equipped with pressure transducers. With IPDI also the anomalous decomposition behaviour of ADN was observed. The obtained results suggest that the reaction of ADN with isocyanate is the cause of the gas formation and that the control of temperature during propellant preparation, the mixing and especially the curing, plays a key role to avoid the formation of bubbles and pores in the cured formulation.
The so-named HISP 32 propellant (60 mass-% bimodal ADN (208μm and 55μm), 16 mass-% Al (18μm), 24 mass-% GAP diol binder with combined isocyanate and BPS curing) was further investigated to characterize its glass-rubber transition. For this dynamic mechanical analysis (DMA) in torsion mode was employed and the loss factor curve (tanδ = G’’/G’) was analysed in terms of binder parts with different reorientational behaviour during the transition from glassy to rubbery state. The shape of the loss factor is composed of two transitions: the one at lower temperatures originates from the undisturbed binder part and the one on the higher temperature side is caused by mobility restrictions exerted on the binder by the fillers, here ADN and Al powder. The perspective is that the different binder parts are differently sensitive to external loads as ageing and strain rate.
*) HISP = High Performance Solid Propellants for In-space Propulsion.
HISP has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 262099)