Synthesis of TPEG, and curing and characterization of polymer matrices based on TPEG

Polyethylene glycol copolymer (TPEG) was synthesized and cured to investigate its possible use as a binder in rocket propellants. Ten TPEG syntheses and one synthesis of a copolyether from Velvetol H250 (a polyether polyol from 1,3-polypropane) and polytetrahydrofuran (polyTHF) were conducted, in addition to 15 curing reactions. The synthetic products were analyzed by nuclear magnetic resonance (NMR) spectroscopy, rheology and differential scanning calorimetry (DSC) to obtain the molar mass and the PEG/polyTHF ratio, the viscosity, and the glass transition temperature (Tg), respectively. The curing was monitored with Fourier transform infrared spectroscopy (FTIR) and rheology, and the products were analyzed by Shore A, dynamic mechanical analysis (DMA) and DSC. Some samples were subjected to thermogravimetric analysis (TGA).

To synthesize TPEG, the temperature and pressure must be controlled. Evaporated THF was collected and used as a measure of how far the reaction should proceed at 400 mbar. The temperature should be 130 °C, but it was difficult to monitor in the up-scaled reactions, as there was no thermometer placed in the reaction mixture. For a standard reaction batch, 6 mL of THF was collected, compared to 64 mL (instead of 48 mL) for an up-scaled (x8) synthesis, to achieve a satisfactory ratio PEG/polyTHF and molar mass. Decantation was the easiest work-up procedure due to its simplicity and the fact that it involves few steps. The reaction mixture was cooled down until phase separation. The water phase was decanted off, followed by addition of base and THF to the remaining polymer phase. The resulting mixture was filtered and THF evaporated.

The Tg values of the synthetic TPEG products were between -80.6 °C and -73.7 °C. This is close to Tg for standard TPEG (-75.7 °C). The Tg of the product based on Velvetol H250 was -82.1 °C. No correlation was found between the polymers’ viscosities and their molar masses. The viscosities varied between 35 mPa·s and 1.1 Pa·s.

The curing reactions were conducted at 60 °C with dibutyltin dilaurate (DBTDL) as catalyst. The curing agent was either Desmodur N100 (N100) or isophorone diisocyanate (IPDI), and for IPDI trimethylol propane (TMP) was used as crosslinker. TMP dissolved in all prepolymers at 60 °C. The cured samples had Tg between -76 °C and -65 °C. TGA showed that no samples started to decompose below 189 °C. Samples based on the prepolymer hydroxyl terminated polybutadiene (HTPB) decomposed in two steps – the others in one.

HTPB samples were harder than the others, and the ones containing synthesized TPEG were the softest. The curing system IPDI/TMP gave harder products than N100, except for the HTPB samples, proving that TMP acts as a crosslinker. Getting reliable Shore A results was difficult due to air bubbles in the samples. This may have two reasons. Air might be captured in the samples during mixing of the liquid reactants or the bubbles could be CO2 from a reaction of isocyanate with water. Evacuation at 60 °C proved to reduce the presence of bubbles significantly, and future samples should therefore be evacuated prior to curing. Rheology data showed that N100 samples cured much faster than the IPDI samples. If N100 should be used, the amount of catalyst has to be reduced. Catalyzed reactions had complex kinetics.

In conclusion, TPEG cured with IPDI or N100 appears promising for use in rocket propellants.