Novel method for curing of epoxy resins

Epoxy-based resins are nowadays used in numerous High Tech applications
ranging from electrical insulation and stabilization in electrical engines and
generators, aeronautic and nautical parts to chemical anchor bolts and
repairing applications. Curing of bulk specimens is usually carried out by
energy consuming heating of the whole part and therefore limits the
applications to temperature insensitive materials. Limited pot life of such
formulations containing amine- or anhydride-based catalysts is another
disadvantage.
Frontal polymerization initiated by a stimulus of light or heat at the surface
is a technique in which the polymerization heat is used for the cleavage of
a thermal initiator which can reinitiate the polymerization reaction in
adjacent regions. This effect leads to a moving localized reaction zone which
is referred as reaction front.
The newly developed system for RICFP consists of a so called C-C labile
thermal initiator, a common onium-based cationic photoinitiator and a
matrix based on bisphenol-A diglycidylether or any other epoxy resins
like hexanedioldiglycidylether, cyclohexanedimethanoldiglycidylether or
cyclohexanepoxide based systems.
The generation of the curing catalyst from the cationic photoinitiators is
accomplished by irradiation with UV-light to start the polymerization. The
thermal initiator is activated by the heat of polymerization. The formed
radicals decompose the cationic initiator in regions which cannot be reached
by UV-light leading to the frontal polymerization.
The described system has no negative impact of the classical thermomechanical
and electrical properties of the materials, even improvements in some
cases were observed. Also inorganic fillers display no problems for the curing
mechanism.