Parametric modelling of complex ageing behaviour of tensile modulus in CTPB-bonded propellant

Composite rocket propellants with CTPB-binder (carboxyl terminated polybutadiene) are in use besides the ones based on HTPB-binder (hydroxyl-terminated polybutadiene). HTPB is cross-linked via polyisocyanates to polyurethane network. CTPB binders are cross-linked to a rubber network with substances containing aziridine (aza-cyclopropane) groups or epoxide (oxirane, oxa-cyclopropane) groups. In both cases polyaddition reactions occur. A system consisting of CTPB + MAPO is considered here. MAPO is called tris[1-(2-methylaziridinyl)] phosphane oxide. The curing part of this molecule are three aziridine groups. Curing with MAPO works well, but problems can occur. The main drawback is the splitting of the P-N-bond, because it is suscep-tible to hydrolytic cleavage. Samples of a CTPB-MAPO propellant with AP and plasticizer were aged at 45°C for nearly 3 years and investigated by uniaxial tensile measurements. The tensile modulus (Young modulus, E-modulus) showed a complex behavior: first increasing, then de-creasing and further decreasing with lower rate. With rubber systems, the tensile modulus E and shear modulus G areproportional to cross-link density. The task was to develop a kinetic description with the available measurement data. The increase and following decrease of E modulus are caused by post-curing and binder backbone splitting. The decrease can be distin-guished in two parts, where the second one is seen to originate by diffusion-control of the water migrating to the reaction sites in the propellant. The kinetic modelling is performed here with so-called parametric descriptions. Post-curing and splitting of the binder could be modeled with combined reactions of second order using power time terms in the rate equations.