Isothermal curing by dynamic mechanical analysis of three epoxy resin systems:: Gelation and vitrification

Times to gelation (t(gel)) and times to vitrification (t(vit)) during isothermal curing for the epoxy systems diglycidyl ether of bisphenol A (DGEBA)/1,3-bisaminomethylcyclohexane (1,3-BAC), tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM)/4-4'-diaminodiphenylsulfone (DDS), and TGDDM/epoxy novolac (EPN)/DDS were measured at different curing temperatures. This article reports on a method to determine t(gel) and t(vit) by dynamic mechanical analysis (DMA). Gelation was determined at the onset of the storage modulus or by the peak of the loss factor. Vitrification was defined as the curve of the storage modulus as the curve reached a constant level (endset) in DMA tests. The experimental values obtained for t(gel) and t(vit) were compared with values obtained by other experimental methods and with theoretical values (t(gel)'s) or indirect determinations (t(vit)'s). From kinetic analysis by differential scanning calorimetry, conversions corresponding to gelation were obtained for the three systems; this yielded a constant value for each system that was higher than theoretical value. Values of the apparent activation energies of the DGEBA/1,3-BAC, TGDDM/DDS, and TGDDM/EPN/DDS epoxy systems were obtained from plots of t(gel)'s against reciprocal temperatures. They were 53.2, 58.2, and 46.5 kJ/mol, respectively. (C) 2002 John Wiley & Sons, Inc.