New Developments in Composite Propellants Catalysis

J of Applied Polymer Sci

2021

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 pregel stage governed by viscous behavior, is modeled through the Arrhenius rheokinetic model. This thermoplastic elastomer undergoes gelation, a transition that is analyzed 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 modeled 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.

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

J of Applied Polymer Sci

2021

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

2021

Macro Materials & Eng

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.

Catalytic effects over formation of functional thermoplastic elastomers for rocket propellants

Polymers for Advanced Techs

2021

Rheometry was the main method to characterize the curing process of binders made of functional polyurethanes (PUs). The macroglycols characterization by means of additional techniques, such as nuclear magnetic resonance, size exclusion chromatography, and differential scanning calorimetry, provided further information for the chemorheological description. Materials were based on Butacene ((ferrocenylbutyl) dimethylsilane grafted to hydroxyl-terminated polybutadiene (HTPB)), used in the solid propulsion field. First, the flow parameters for the uncured reactive mixtures of Butacene and four different diisocyanates were analyzed via viscometry and these were markedly influenced by the chemical structure of the curing agents. Analyzing the rheokinetic constant values of the pre-gel stage for Butacene-and HTPB-reactive systems, relevant catalysis caused by the ferrocene moiety was detected when aliphatic reactants were used, such as isophorone diisocyanate or 1,6-hexamethylene diisocyanate (IPDI and HMDI, respectively). No catalytic effect was found for 2,4-toluene diisocyanate (2,4-TDI) or even for 4,4 0 -methylenediphenyl diisocyanate (4,4 0 -MDI). Finally, the use of dynamic rheology was useful to evaluate the critical points during gelation process, where the reactivity of curing agents was associated with the achievement of elastic properties. Both techniques agreed the reactivity order of curing agents with Butacene, which is 4,4 0 -MDI > HMDI > > 2,4-TDI ≥ IPDI.The knowledge of the structure-reactivity relationship and, moreover, the kinetics of the urethane network formation for these metallo-PUs is paramount in manufacturing processes for advanced thermoplastic elastomer applications.