Study of the Consumption of the Curing Agent on the Liner Surface by Infrared Spectroscopy in Reflection Mode

Monitoring the reaction between the isocyanate group (NCO) of the curing agent 2,4-toluene diisocyanate and the hydroxyl group (OH) of hydroxyl-terminated polybutadiene is important for the rocket engine loading process, since the reduction of NCO content on the adhesive surface (liner) directly affects adhesion to the propellant. Traditionally, the tack point, which represents the optimal time window for rocket motor loading, is determined based on the operator’s tactile perception. Viscosimetry is employed to monitor adhesive curing and subsequently assess tack; however, as this method evaluates the reaction within the bulk material, adhesion control remains predominantly empirical when chemical reference parameters are preferred. This study proposes a methodology based on infrared spectroscopy (IR) in universal attenuated total reflection mode, evaluating four stoichiometric ratios R(NCO/OH): R = 1.00 and, with excess NCO, R = 1.05, 1.10, and 1.15. The ratio R = 1.05 showed the best performance, with a methodological error of 4%. The main contribution of this work is a fast and precise IR surface analysis that enhances process control during solid rocket motor loading.