Radiofrequency (27.12 MHz) Processing of Thermosetting DGEBA Epoxy Resins

Abstract

Radiofrequencies (27.12 MHz) or RF are used to activate the curing reaction of a DGEBA epoxy resin in presence of 4,41 -diaminodiphenylmethane as cross-linking agent. The article is divided into four parts. The introduction is devoted to the presentation of the principle of the RF activation of the processing of the epoxy prepolymers. Afterwards, in the experimental part, the applied electromagnetic technology is described, and the polymerization experiments are carefully detailed. The samples to be irradiated are placed inside an applicator of condenser type, which consists in two parallel steel plates used as electrodes. When the plates are put at an alternating electrical potential of frequency of 27.12 MHz, the phenomenon of dipolar relaxation inside the chemical mixture is activated. Thus, a part of the initial electrical energy is converted into mechanical energy, which is itself converted into heat because of internal friction. Consequently, the temperature of the samples grows and, as soon as their thermal level is sufficiently high, the reaction of curing works. An extended parametrical study of this reaction, carried out at given electrical voltage or dielectric loss, is described. The determination of the glassy transition temperature of all final products shows that no structural difference is detectable between the epoxy networks issued from the activation processes using radiofrequencies or classical heating inside an oven, that is to say there is no specific effect of the electromagnetic field on the structure of the crosslinked epoxy materials.