Chemorheology and Kinetics of High‐Performance Polyurethane Binders Based on HMDI

Abstract

Aliphatic diisocyanates, such as 1,6‐hexamethylene diisocyanate (HMDI), are preferred curing agents for the formation of polyurethanes (PUs) in applications where resistance to abrasion or degradation by ultraviolet light takes precedence. Aside from the final properties, the curing agent plays a key role in the bulk manufacturing of such materials, and it mainly affects the polymerization kinetics and their rheology. The copolymerization of HMDI and a metallo‐prepolymer derivative from hydroxyl‐terminated polybutadiene (HTPB) is studied under isothermal conditions (50–80 °C). This study is carried out by means of an indirect method, using both rotational viscometry and dynamic rheometry. At the beginning of the process, the viscosity growth fit well to a first‐order kinetic model. Afterward, the reactive system passes through gelation, from which only rheology is allowed for the investigation of the entire polymerization process. This transition is analyzed in depth together with predictions from percolation theory. The conversion degree is determined from rheological measurements, and then an autocatalytic kinetic model is applied to describe the overall process. Finally, an isoconversional method allows the evolution of activation energy to be studied. This analysis merits attention for the development of high‐performance binders that are of great interest in aerospace propulsion technology.

Rheological kinetics of ferrocenylsilane functionalized polyurethanes based on 4,4'‐methylenediphenyl diisocyanate for advanced energetic materials

“…It can be claimed that the rheological behavior observed by both techniques for butacene‐4,4'‐MDI can be described with the same model over the temperature range covered. This result was also found for systems based on this polyol and IPDI, HMDI or 2,4‐TDI; corroborating the independence of the model applied with the diisocyanate under study 23,25,26 . Nevertheless, the chemoviscosity of these asymmetric isocyanates reacting with butacene evolved in two stages when oligomers were formed due to the different reactivities of the NCO groups that they contain.…”

“…To conclude the viscosity analysis, the scaling theory proposes describe the viscosity evolution of a gelling system. As shown for similar metallo‐PUs in the pregel region, 23,25,26 when the complex viscosity data were plotted as the Equation () states, a curve was obtained. Close to gelation, the tangent of the curve would be 0.1.…”

“…Additionally, 4,4'‐methylenediphenyl diisocyanate (4,4'‐MDI) is a widely used reactant in the PU industry. Its structure (symmetry, flexibility, and aromaticity; see Scheme 1) is expected to influence the curing of the binders compared with IPDI, 2,4‐TDI, and 1,6‐hexamethylene diisocyanate (HMDI) studies of which were conducted previously by our group 22–26 …”

Catalytic effects over formation of functional thermoplastic elastomers for rocket propellants

“…These are symmetric reactants with equivalent NCO groups. From the kinetic studies in the literature, 18,19,22,23 the reactivity of the functions that form the monourethane and the diurethane do not display any difference. For this reason, the same trend can be expected when these curing agents react with OH groups of Butacene.…”

“…In our previous works, the copolymerization of Butacene with IPDI, 2,4-TDI, HMDI and 4,4 0 -MDI was assessed by rotational viscometry under different isothermal conditions within 50 and 80 C. [16][17][18][19] The application of the first-order chemorheological equation, shown in Equation ( 2), was successful to describe the viscous evolution for the pre-gel region of the polymerization for such systems. In short, the viscosity (η(t) T ) follows first-order kinetics with time (t) under the given isothermal conditions (T), where η 0 represents the viscosity of the uncured reagent mixture and k η is the rate constant of the process evolution.…”