Application of the systemic rheology to the in situ follow-up of viscosity evolution with reaction time in the synthesis of urethane prepolymers

For the first time, the in situ follow-up of the viscosity evolution with reaction time during the synthesis of isocyanate-terminated urethane prepolymers has been done. For that, the systemic rheology has been applied with the purpose of mimic the unit operations of adhesive processing, particularly the mixing and chemical reaction. Several behaviors were revealed with the methodology applied. For all the synthesis prepared at 100 degrees C, we observed that the viscosity evolution tended toward a plateau, i.e., the viscosity reached constant values. However, for all the synthesis made at 60 degrees C, the viscosity plateau was not at all observed and the viscosity remarkably increased during the studied reaction time. The viscosity decreased with an increase of: (a) molecular weight of the PPG or PPG blends, and (b) NCO/OH ratio. We also found that the latent pre-reaction time increased and the PPG's reactivity decreased when the molecular weight of PPG was increased. Moreover, at low NCO/OH ratio, the viscosity quickly increased when the amount of PPG-400 related to the PPG-2000 was high. Surprisingly, this behavior was not observed at high NCO/OH ratio.