Epoxy resin cure III: Boron trifluoride catalysts

Abstract Boron trifluoride forms complexes with nitrogen‐and oxygen‐containing Lewis bases. These acid–base complexes are sometimes added to epoxy resins to accelerate their cure at elevated temperature (120°C–160°C). Several possible processing concerns with the use of boron trifluoride complexes have been investigated. It was shown that each complex can be readily identified by its distinct fluorine‐19 NMR spectrum. All of the boron trifluoride complexes are converted to fluoroboric acid under normal epoxy resin cure conditions. The fluoroboric acid is the true catalyst for epoxy resins. Experimental results show that fluoroboric acid does not catalyze the polymerization of an epoxy resin at room temperature. Finally, it was shown that fluoroboric acid is not hydrolyzed to hydrofluoric acid.