Polyurethane binder systems based on hydroxyl-terminated polybutadiene (HTPB) possess several superior properties such as superior adhesion, high solid-loading capacity, outstanding mechanical performance, etc. They have been widely used in coatings and adhesives as well as in medical and military industries. The cure reaction between hydroxyl-terminated polybutadiene (HTPB) and diisocyanates plays a key role in the properties of final products as well as the adjustment of process parameters. FT-IR spectroscopy is applied to investigate the kinetics of the curing reaction of HTPB and isophorone diisocyanate (IPDI) in the presence of a low toxic and low viscosity catalyst, stannous isooctoate (TECH). The concentrations of the isocyanate groups (NCO) characterized by FT-IR during the cure reaction with respect to time were recorded at different temperatures and at constant stoichiometric ratio R_n[NCO]/n[OH] = 1.0. The kinetic parameters, i.e., activation energy (E_a), pre-exponential factor (A), activation enthalpy (ΔH) and activation entropy (ΔS) were determined. In addition, the curing process and mechanism of the HTPB-IPDI reaction are discussed.

As the hydroxyl-terminated polybutadiene (HTPB)-based binder system is widely applied in many industries, the curing process plays an important role in the final properties of the resulting product containing such a binder system. This study used a viscometer to measure viscosity buildup in the curing process of the binder system with various curing agents under isothermal conditions. Key parameters such as rheological reaction rate constant (k_ƞ) and pot life of different were measured and calculated. The rheological reaction rate constants of the HTPB-based binder systems included 0.0423 min⁻¹ (MDI), 0.0049 min⁻¹ (HDI-trimer) and 0.0014 min⁻¹ (HMDI). The pot lives of the HTPB-MDI, HTPB-TDI, and HTPB-HDI-trimer were 0.6 h, 3.6 h and 8.1 h, respectively. One interesting finding is that HTPB-HDI-trimer binder, which had the long pot life, exhibited an accelerated trend in the viscosity buildup in the late phase. This feature is of great significance to improving the final properties of the products generated in propellant manufacturing and other fields. The cause of this phenomenon and the curing process of HTPB-HDI-trimer binder system were analyzed and discussed in the present study.

In this research, rheokinetics is used to study the curing reaction of hydroxyl terminated polybutadiene (HTPB) and trimer of hexamethylene-1,6-diisocyanate (HDI-trimer) with 1,4-diazabicyclo[2.2.2]octane (DABCO) as catalyst under different catalyst mass fraction. The results show that the pot life of the system depends on the catalyst mass fraction in the binder system. Furthermore, with increased catalyst mass fraction, the fitting diagram obtained by plotting ln (viscosity) versus curing time shows a better linear relationship. Therefore, the amount of catalyst required to achieve a certain pot life can be calculated through the formula. It is worth mentioning that the applicable pot life equations are proposed in the paper. From the equations, we find that under isothermal curing conditions at 35 °C, when the mass fraction of DABCO was 0.216 wt.%, the pot life of the HTPB/HDI-trimer binder system reaches 4 h.