An automatic ultrasonic velocity measuring system was utilized to investigate an isothermal curing process of allyl-ester, a new thermosetting resin composed of an allyl-oligomer and a curing agent. The logarithmic curing time vs sound velocity curves showed apparent bends at curing temperatures below 140°C, while a monotonous relation above 150°C.The analysis based on a reaction rate equation suggested that such ultrasonic behavior resulted from the differentmechanism of gelation and/or vitrification, which became dominant depending on the curing temperature.
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