Catalytic effects over formation of functional thermoplastic elastomers for rocket propellants

Abstract

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 analysed via viscometry and these were markedly influenced by the chemical structure of the curing agents. Analysing 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’-methylenediphenyl diisocyanate (4,4’-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’-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.