1H, 19F and 11B nuclear magnetic resonance characterization of BF 3: amine catalysts used in the cure of C fibre-epoxy prepregs

Abstract

1H, 19F and 11B nuclear magnetic resonance studies are reported which characterize the complexes of boron trifluoride with monoethylamine and with piperidine, BF 3:NH 2C 2H 5 and BF 3:NHC 5H 10, respectively. These complexes are used as catalysts for the cure of high performance C fibre-epoxy composities from prepregs. The chemical compositions of commercial BF 3:amine complexes are variable and contain BF 4 − and BF 3(OH) − salts together with other unidentified highly reactive species. The BF 3:amine complexes, which are susceptible to hydrolysis, also partially convert to the BF 4 − salt (i.e. BF 4 −NH 3 +C 2H 5) 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 BF 3:NH 2C 2H 5 or BF 3:NHC 5H 10 species together with their BF 4 − salts and a variety of boron-fluorine or carbon-fluorine prepreg species. Considerable variation in the relative quantities of BF 3:amine to its BF 4 − 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 BF 4 − salt is the predominant catalytic species, acting as a cationic catalyst for the prepreg cure reactions. During the early stages of cure the BF 3:amine catalysts convert to the BF 4 − salts in the presence of epoxides, whereas the BF 3-prepreg species are susceptible to catalytic deactivation and immobilization.