Characterization of butacene based composite propellants by loss factor curves determined with DMA

Butacene® binder, ferrocenyl groups chemically bonded to HTPB backbone, leads to high burn rates BR for Al-AP based composite propellants (BR > 20 mm/s at 7 MPa ), and low pressure exponents (n < 0.5) which gives more flexibility to the rocket designer. The key molecular level characteristic of such elastomeric binder systems of composite rocket propellants is the glass-to-rubber transition region, which is mainly defined by the molecular mobility of the components in the polymeric network during the transition from energy to entropy elasticity with respect to temperature. Analyzing molecular rearrangement regions or binder mobility fractions related to the glass-rubber transition of such composite propellants during storage is important in terms of in inservice time estimations. This feature has a strong dependence on temperature and is detectable by dynamic mechanical analysis (DMA) whereby obtaining the loss factor (tand= loss module / storage module) curves. Butacene® and HTPB based binders alone and two propellant formulations (HTPB/Butacene®/AP(bi-modal)/Al) were prepared and analyzed unsing the loss factor curves obtained by torsion DMA. A special modelling with so-named EMG (exponentionally modified Gauss) distributions was used to define and quantify sub-transition regions in the loss factor curve. For comparison with Butacene based propellants, also a HTPB/AP/Al composite propellant formulation was produced and analysed.