Chemorheology of a Highly Filled Epoxy Compound

Based on measurements of the dynamic viscosity, a strategy is proposed to find an expression that relates the viscosity of an epoxy compound during curing, to the temperature, shear rate, and degree of reaction. It appeared that the dynamic viscosity is a unique function of the effective shear rate (i.e. the product of frequency and strain) over a wide range of frequencies and strains after being corrected for the temperature. The effective shear rate dependence of the viscosity is described with a power law with an exponent that depends on the conversion. The effect of temperature is described with an Arrhenius-type equation with conversion dependent parameters. Differential scanning calorimetry is applied to determine the kinetic equation that is used, in combination with the thermal history, to obtain the conversion during the rheological measurements. The description of the viscosity gives a good prediction of the measured viscosity in the region between melting and gelation of the compound. The theories proposed in the literature to detect the gel point from dynamic experiments are examined. It is found that neither the G -G crossover, investigated by e.g. Tung and Dynes (1), nor the frequency-independence of tan( ), described by Winter (2, 3), can be used to determine the gel point of the given material. In contrast, the curves of G against conversion for measurements performed at equal strain but with different thermal histories appear to converge at a conversion of 18%, which agrees with the gel point determined through extraction experiments.