1H, 19F and 11B nuclear magnetic resonance characterization of BF3: amine catalysts used in the cure of C fibre-epoxy prepregs

¹H, ¹⁹F and ¹¹B nuclear magnetic resonance studies are reported which characterize the complexes of boron trifluoride with monoethylamine and with piperidine, BF₃:NH₂C₂H₅ and BF₃:NHC₅H₁₀, respectively. These complexes are used as catalysts for the cure of high performance C fibre-epoxy composities from prepregs. The chemical compositions of commercial BF₃:amine complexes are variable and contain BF₄^? and BF₃(OH)^? salts together with other unidentified highly reactive species. The BF₃:amine complexes, which are susceptible to hydrolysis, also partially convert to the BF₄^? salt (i.e. BF₄^?NH₃⁺C₂H₅) upon heating. This salt formation is accelerated in dimethyl sulphoxide solution and in the presence of the epoxides that are present in commercial prepregs. Commercial C fibre-epoxy prepregs are shown to contain either BF₃:NH₂C₂H₅ or BF₃:NHC₅H₁₀ species together with their BF₄^? salts and a variety of boron-fluorine or carbon-fluorine prepreg species. Considerable variation in the relative quantities of BF₃:amine to its BF₄^? salt was observed from prepreg lot to lot, which will cause variable viscosity-time-temperature prepreg cure profiles. It is concluded that the chemically stable and mobile BF₄^? salt is the predominant catalytic species, acting as a cationic catalyst for the prepreg cure reactions. During the early stages of cure the BF₃:amine catalysts convert to the BF₄^? salts in the presence of epoxides, whereas the BF₃-prepreg species are susceptible to catalytic deactivation and immobilization.