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

Abstract

4,4’-Methylenediphenyl diisocyanate (4,4’-MDI) is a symmetric aromatic isocyanate commonly used as a curing agent in the production of polyurethanes (PUs). The chemorheology and kinetics of its reaction with a metallocenic-prepolymer derivative from hydroxyl-terminated polybutadiene (HTPB) is studied in bulk and under isothermal conditions at 50-80 °C by means of rheological measurements. The viscosity of the initial part of the PU formation, the pre-gel stage governed by viscous behaviour, is modelled through the Arrhenius rheokinetic model. This thermoplastic elastomer undergoes gelation, a transition that is analysed in depth together with predictions according to percolation theory. The gel point (tgel) is determined from the intersection in tan δ versus curing time for different shear frequencies. From the viscoelastic properties, like the elastic storage modulus (G´), the conversion degree is determined, and the entire polymerization process is modelled through the Kamal-Sourour and Sato kinetic expressions. Significant variation in the reaction orders and the activation energies might reveal a change in the process mechanism, depending on the temperature. This work demonstrates that an indirect method makes it possible to gain relevant knowledge about the chemistry of these thermoplastic PUs during curing, which is essential for their manufacturing. This study merits attention for the development of a new generation of high-performance binders with great potential in aerospace propulsion.