Showing a rheological effect of gelation on epoxy-amine systems

The effect of gelation was studied on the rheological curves, during hardening of a resin subjected to sinusoidal shearing between parallel plates. The epoxy-amine system was chosen for the moderate value of its maximum glass transition temperature Tg, ∞ which means that it is possible to avoid vitrification by operating at T > Tg, ∞. The shape of the variation curve of the storage modulus G' does not depend on temperature; G' obeys a single percolation law as a function of the reduced variable t/tgel. On the other hand, gelation leads to a fall in the growth rate of the loss modulus G” after the gel point. This effect increases with temperature and can become considerable at very high temperatures: in this case a minimum of G” between the gel point and the rubbery plateau can be observed. We have verified that this effect is indeed a rheological effect of gelation as it begins practically at the gel point determined on the storage modulus curves. Several qualitative conclusions are drawn, and the nature of this effect is discussed.