Relationship between viscoelastic properties and gelation in thermosetting systems

A method is described for determining from dynamic viscoelastic data the gel time of thermosetting resins. Gelation of nine systems including epoxies, acetylene‐terminated sulfone, and silicones was correlated with the crossover of the dynamic storage G ′ and loss G ″ moduli measured during isothermal curing. The effect of measurement frequency on gel time is discussed and related to the standard gel test. A lower temperature limit for the valid aplication of this technique was found to be at T > T gg + 30°C, where T gg is the glass transition of the system at the gel point.