Thermosetting Resins Gelation, Vitrification, and Viscoelasticity

The gd point is normally used to distinguish this process into two stages. Growth and branching of the polymer chains occur before the gel point, when the reactive system is soluble and fusible. However, after the gel time an infinite network of polymer chains appears and develops, such that the reactive system loses its solubility and fusibility as a consequence, the final reactions take place in the solid state. 

The gel time has been determined according to different criteria as [10]  

1) The criterion of maximum peak in tand, based on the point where there is a maximum difference between the elastic and viscous behaviors of the system. 

2) The criterion corresponding to the crossover between the G and G curves at this point, the system presents not only an elastic but also viscous behavior, storing a similar amount of energy to the energy that is dissipated. 

3) The criterion of the tangent line to the G curve this point corresponds to the crossing between the baseline (G = 0) and the tangent drawn at the G curve when G reaches a value dose to lOOkPa s. 

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