The kinetics of cure and network formation

Although the cure of an epoxy resin involves reaction between epoxy and hardener reactive groups, as discussed in chapter 2, the full characterization of the cure process involves many other factors. During cure, a liquid or fluid resin-hardener mixture is converted to a solid (chapter 1, section 1.1 and Figure 1.1). Critical features are gelation and the onset of vitrification. The latter effect occurs as the glass transition temperature, T g , approaches the cure temperature, T c . The determination and alternative definitions of T g will be considered later (section 3.2). However, it should be noted that the glass transition temperature is a function of the extent of reaction, X e , \({X_e} = \frac{{{E_o} - {E_{({t_c})}}}}{{{E_o}}}\) with E o and \({E_{({t_c})}}\) being the concentrations of epoxy groups present initially and at cure time te and has been used as a measure of the extent of reaction (Wisanrakkit and Gillham, 1990). When the difference between the cure temperature and the glass transition temperature becomes ‘small’, that is, ΔT = T c - T g (X e ) is small, curing reactions become diffusion controlled because molecular mobility is rapidly reduced as ΔT → 0. Cure may continue very slowly even when ΔT becomes negative. Then the glass transition temperature exceeds the cure temperature.