Effect of curing agents and plasticizers on the loss factor curves of HTPB-binders quantified by modelling

One of the key molecular level characteristic of elastomeric binder systems of composite rocket propellants is the glass-rubber transition region, which is mainly defined by the molecular mobility of the components in the polymeric network during phase transition from energy to entropy elasticity with respect to temperature. This feature has a strong dependence on temperature and is detectable by dynamic mechanical analysis (DMA) whereby obtaining the loss factor (tand= loss module divided by storage module) curves. HTPB R45HT based binders with four different types of isocyanates (IPDI, HDI, DE305 and 6HMDI), with and without the common plasticizer DOA (dioctyl adipate), were analyzed using loss factor curves obtained by torsion DMA. Exponentially modified Gauss distribution functions (EMG) were used to model the effect of the isocyanates on the molecular mobility (MM) of the polymeric networks. Developments of loss factor curves as a function of temperature showed distinct differences in the intensity of loss factor curves with different isocyanates in the following order HDI > IPDI >> 6HMDI > DE305. A relationship between MM observed by loss factor curves with tensile test was developed to support the effect of the isocyanates on the binder network. Additional series of HTPB R45HT+IPDI binders were formulated with four different plasticizers (DOA, DOS, DOZ and IDP) to make further analysis of the effect of plasticizer types. Loss factor curves with different plasticizers had showed not such pronounced differences compared with the differences obtained with the isocyanate types. Mainly, the four plasticizers showed the same level of intensity in loss factor curves and maximum peak temperature Tg, except IDP, which provided with the lowest glass-rubber transition temperature. Molecules, which can give more free volume to polymeric network due to fewer hindrances, in other words which restrict less the mobility of binder molecules, give the lowest Tg as in the case of IDP with HTPB R45HT+IPDI binder networks.