Influence of isothermal ageing on the mechanical properties of HTPB-bonded composite rocket propellants expressed as master curves of torsion DMA measurements

Composite rocket propellants (CRP) have an elastomer as binder for the solid fillers, which serve as oxidizer and fuel. Elastomers are not linear elastic in behaviour in a way as metals or glasses. Their behaviour as function of temperature is named viscoelastic, to in-corporate the feature of the glass-to-rubber transition in going from low temperatures to high temperatures and the so named relaxation phenomenon, which invokes considerable internal flow characteristics. The distinctive property of the glass-to-rubber or vice versa rubber-to-glass transition is a dissipative energetic process, which causes the molecular rearrangements from more 'ordered' configuration to more 'disordered' configuration or vice versa. The more ordered configuration appears at low temperatures and is called 'energy elastic' as metals and glasses are. The disordered configuration appears beyond the glass-to-rubber transition and is called 'entropy elastic'. The modules of viscoelastic materials are dependent on temperature and of the deformation rate. In case of thermo-rheological simple materials the effects of time and temperature on the material can be linearly superposed without loss in information. Time and temperature act 'orthogonal' on the material. This connection is also the base of the time-temperature-shift theorem ac-cording to Williams, Landel and Ferry. In other words a distinct correlation between modulus and temperature on one side and modulus and deformation rate on other side exists. From measurements at low temperatures the behaviour of the modules can be assigned to high deformation rates. One can establish so-named master curves to predict the material response at other load situations. Some artificially aged CRP were investigated by torsion DMA [1], [2]. The temperature range was from about -100°C to +40°C and higher with sinusoidal deformation at four frequencies in the range 0.1 to 56 Hz. The material proper-ties storage shear modulus G', loss shear modulus G'' and loss factor tan(d) obtained by torsion DMA behave with increasing deformation rate according to the viscoelasticity of the material. The ageing has distinct influence especially evident in the loss factor curves. The master curves and shift factors established from these data show the ageing influence in transformed way, which can give some insides to better understand the behaviour of the materials on the molecular base.