Characterisation and modelling of the curing reaction of HTPB with isocyanates by reaction heat determined by heat flow microcalorimetry and by volume shrinkage determined with a pressure curing cell

Composite rocket propellants have often three-dimensional polyurethane elastomers as binders. One of the high performance binders is HTPB (hydroxyl-terminated polybutadiene) cross-linked to an elastomer with an isocyanate in polyaddition reaction. It is essential for the mechanical properties of the final rocket propellant that cross-linking or curing has reached a high degree of conversion during the heat treatment of the initially liquid propellant mix. To assess the degree of curing is especially important, when evaluating and assessing the ageing. Therefore, one should understand the kinetics of the curing reaction in order to know, when residual curing during in-service time does not interfere with loss in strain capacity by ageing. Some binder and propellant mixes were investigated (1) via the reaction heat followed by heat flow calorimetry and (2) by volume shrinkage caused by cross-linking, at several temperatures in the range of 50°C to 80°C. The shrinkage of the volume Vg of the reaction mixture was followed in a closed pressure curing cell (PCC) by measurements of the system pressure pS. A kinetic model based on the bimolecular reaction between OH-terminated pre-polymer and poly-isocyanate was developed, which describes the heat generation rates dQ/dt and the volume change rates dVg/dt or pressure change rates dpS/dt. The equivalent ratio between OH-groups and NCO-groups is considered in the model. However, the application of the model is not straightforward in spite of the basic simplicity of the model. The reason is that the reaction of second order between two components demands the knowledge of the real start point in time of the reaction, as well as the start concentrations of the reactants. Further on, there are many handling procedures as kneading with the isocyanate, taking out the samples from the batch, transportation of samples to the laboratory, filling of measurement cells and finally the preparation of the measurement itself. During all these steps, the reaction is already in progress and a certain conversion is achieved already up to the time from where the measurement of heat generation rate or volume shrinkage is usable for evaluation. Therefore, a special modelling procedure was developed to take into consideration of all these time-temperature sections with their pre-reactions with regard to the main part of the curing reaction in the microcalorimeter or in the PCC. By this procedure, the heat flow curves dQ/dt and the pressure rate curves dpS/dt or the system pressure course pS = f(t) can be at first reconstructed and secondly described. With measurements at several temperatures, the Arrhenius parameters are obtained and conversion curves can be established. By this, the curing reactions can be assessed and suitable curing times can be defined. Further, the molar heat of reaction and the molar volume of reaction for one molar conversion of NCO were determined. The effect of curing catalysts are easily recognizable.