In-source analysis of radiation curing of unsaturated polyester resin by charging current technique

Abstract

The dielectric and conductive properties of unsaturated polyester (UP) resins undergo significant changes in the course of crosslinking reaction. In-source radiation curing of UP resins was studied by combining dielectric, thermal and charging current analyses. Dielectric permittivity (by stepwise changing of frequency) and charging current were measured in the course of cobalt-60 γ-irradiation at constant temperatures. The exothermic reaction could be monitored by differential thermal analysis (DTA), while the observed change of dielectric permittivity can show how the dielectric relaxation time distribution shifted through the experimental window represented by reaction temperature and by frequency range of dielectric spectrometer. The decrease in dielectric permittivity and in charging current may be mainly associated with the partial immobilization of permanent carbonyl and hydroxyl dipoles attached to the oligoester chain. The measurements of charging properties of UP resins during the curing seems very promising due to the ease with which the relaxation parameters and essential information on the molecular interactions and mobility can be derived. The electrical properties of UP resins are mainly determined by inter- and intramolecular hydrogen bonds. The shape of the charging current spectra during the curing of UP resins showed a heterogeneous nature of vitrification process. The results show that two relaxations occur: the first one is attributed to the increase in viscosity of liquid phase as oligoester chains are crosslinked prior to the gelation, and the second one to the further crosslinking in the gel phase, resulting in the vitrification of the system. The characteristic times to gelation and vitrification of UP resins in the course of radiation isothermal curing are decreasing functions of reaction temperature.