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Tracking of the curing of binders for composite rocket propellants by heat flow microcalorimetry and pressure-temperature sensors

: Mußbach, Günter; Bohn, M.A.

Fraunhofer-Institut für Chemische Technologie -ICT-, Pfinztal:
Energetic Materials. Characterization and Modeling of Ignition Process, Reaction Behavior and Performance : 44th International Annual Conference of ICT, June 25 - 28, 2013, Karlsruhe, Germany; Proceedings
Pfinztal: Fraunhofer ICT, 2013
Fraunhofer-Institut für Chemische Technologie (International Annual Conference) <44, 2013, Karlsruhe>
Conference Paper
Fraunhofer ICT ()

Ageing studies on composite rocket propellants (CRP), which are based on HTPB (hydroxyl-terminated polybutadiene) binder systems, are often affected by the post-curing reaction. To obtain satisfying mechanical properties, the so called equivalent ratio NCO/OH for the curing reaction is set normally to values lower than 1. This means, there are more hydroxyl groups available from the binder (HTPB) than isocyanate groups from the curing agent are present in the mix. Nevertheless a complete conversion of the isocyanate groups during the curing reaction is hindered by the reduction of the mobility or in other words the increasing viscosity due to the curing progress. The knowledge of the curing reaction kinetics is important for ageing studies, for the assumptions of the stress-free temperature value and not at least for assessing the post-curing reaction. The value of stress-free temperature is also influenced by ageing effects. Post-curing increases the cross-link density at storage temperature, resulting in a new network part, which overlays and counteracts to the one of the main network formed at curing temperature. This effect should be minimized and the investigation of the time-temperature behavior of the curing reaction is used to determine optimal curing times. In order to separate the influence of filler particles on the curing reaction and to get basic information about the reaction, several formulations have been used: binder formulations with and without plasticizer as well as with and without curing catalyst, besides the curing of complete propellant formulations. The curing processes were followed by heat flow microcalorimetry using the heat of reaction as indicator and by pressure measurements with pressure-temperature transducers in closed pressure cells using the volume shrinkage of the material during curing as indicator.