Formation and characterization of polymer matrices

In this work binder materials for solid rocket propellants have been studied. A total of 37 polyurethane gum stocks were prepared, where parameters like prepolymer, curing agent, curing catalyst and temperature were varied. The cured samples were analyzed by thermoanalysis and hardness measurements. In addition, the kinetics of the curing reaction was investigated, mainly by FTIR. Most of the polymer samples were based on the prepolymer HTPB. When cured with IPDI at 60 °C, the curing rate constant was smaller for HTPB than for the diol EP1900, but significantly higher than for 1,4-butane-diol. The curing rate constant was higher for HTPB than for Terathane 2000 when cured with N100. Among the curing agents, DDI reacted faster than IPDI with HTPB, and even faster than N100. When the curing catalyst triphenyl bismuth was included into HTPB/IPDI mixtures, a strong increase in the curing rate was observed. However, the energy of activation was unaltered, indicating that the higher curing rate constant was due to an increase in the pre-exponential factor in the Arrhenius’ equation. Addition of 100 ppm of the curing catalyst DBTDL to HTPB/IPDI mixtures did also lead to an increase in the rate constant, but to a lesser extent than the effect of including 0.03 or 0.1 weight percent of triphenyl bismuth to the mixtures. The formation of crosslinks was demonstrated by rheology for one of the samples, whereas thermogravimetric analysis of a selection of cured HTPB gum stocks showed that they were thermally stable at least up to 250–300 °C, where a two-step decomposition started. The amount of crosslinks could be indirectly determined by hardness analysis (Shore A). In general, samples based on HTPB, which has a functionality of more than two, exhibited higher hardness values than samples based on diols. In the same way, prepolymers cured by the trifunctional isocyanate N100 were harder than samples cured by one of the difunctional curing agents DDI or IPDI. By varying the ratio between HTPB and IPDI it was found that the highest Shore A values for this system were obtained when c/p was close to unity. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) were applied to determine the glass transition temperatures (Tg) of the polymer matrices. DSC is a quicker method than DMA, but in some cases the DSC results were difficult to interpret as the energy involved in this transition is quite low. The Tg values of the HTPB samples were approximately 10 K lower than equivalent EP1900 and Terathane gum stocks. When HTPB was copolymerized with diols, Tg was measured to be nearly the same as for cured HTPB samples. Tg was lowered by 14 K when the plasticizer DOS was added. These results show how the processing properties and characteristics of polyurethane based binders may be controlled by choice of starting materials, relative compositions, and temperature.