Role of accelerator in curing of epoxy-anhydride pressure impregnant

In the present paper, Differential Scanning Calorimetry (DSC) has been applied to bring out the role of zinc naphthenate accelerator in the curing of epoxy-anhydride pressure impregnant used for impregnation of electrical machines. Effect of concentration of zinc naphthenate on curing behavior of bisphenol-A epoxy resin and methylhexahydrophthalic anhydride hardener has been studied by obtaining dynamic DSC thermograms for uncatalysed, and catalysed system with various concentration of zinc naphthenate. The investigations reveal that the un-catalyzed purified epoxyanhydride mix exhibits an endothermic peak at temperature around 260 °C in place of cure exothermic peak in the DSC curve. The analysis of DSC and TGA data of individual resin, hardener and their mix indicate that the endothermic peak is attributed to the evaporation of resin and hardener implying that the hardener does not react with resin in absence of accelerator. The addition of zinc naphthenate accelerator at concentration of 1 pph (parts of zinc naphthenate per hundred parts of resin- hardener mix) causes two exothermic peaks in the DSC thermogram. The position and area under these peaks vary with the accelerator content resulting in the variation in the processing parameters and the performance properties of the cured material. The first peak from esterification reaction appears between 90 and 200 °C and second peak from etherification reaction appears in the temperature range of 180 to 330 °C. The area under the first exothermic peak appearing at low temperature increases with increase of accelerator content and predominates at concentration of ≥ 1.5 pph. The area under the second exothermic peak appearing at high temperature increases with the decrease of accelerator content and predominates at concentration of ≤ 0.5 pph. The accelerator concentration of 2 pph provides maximum reactivity.