Reaction kinetic handling of complex reaction behaviour in ageing of CTPB-based composite rocket propellants

In general, ageing reactions in energetic materials are complex and are not deconvolutable with typical thermo-analytical evaluations. High-temperature analysis of solid CRP (composite rocket propellant) does not provide the essential reaction data needed to predict ageing and in-service periods at in-service temperatures, because the decomposition of AP and the related reactions are the main effects observed with DSC and TGA. In contrast, the in-service time of assets using CRP is related to the safety and functional reliability of the system, two features that are often controlled by changes in mechanical properties of CRP, and these are in turn caused by entirely different reactions than the decomposition of AP, namely oxidation and other changes of the binder. Among the examples of complex reaction behaviour is the curring of CTPB (carboxyl-terminated polybutadiene) with aziridine-type curing agents, such as MAPO (tris[1-(2-methylaziridinyl)] phosphine oxide). MAPO shows a special behavior: alongside the main curing reaction, it shows also a splitting of the binder network by N-P bond breakage inside its own chemical frame. CTPB binders have interesting properties, in that, contrary to HTPB binder, the adhesion be-tween solid particles and binder is supported by more polar molecular sites and is therefore intrinsically stronger. Therefore, CTPB binder systems are still in application with solid CRP. To handle this complex reaction behaviour, suitable information must be available. With rubber systems, the Young modulus (elastic modulus in tension) is proportional to cross-link density and can often be correlated with the strength for slow strain rates, where non-equilibrium vis-cous stress is not significant. This creates a useful link between chemical modelling and me-chanical strength analysis. The cross-link density is a state property of the binder. By correlating it to propellant modulus and strength, the latter also become state properties of the propellant, and the kinetic modelling can then be used not only to compare different thermal histories, for example service life vs. qualification test profiles, but also to make quantitative predictions on mechanical integrity of the specimen under key design loads. The Young modulus is easily extracted from stress-strain curves. Because of proportionality with the chemical properties (cross-link density), it can be used directly in kinetic modelling. Reaction kinetic models are established and applied to the data.