Effect of Resin Formulation on Thermoset Viscoelasticity

Abstract

Viscoelastic relaxation curves of thermoset resins may change considerably for relatively small changes in properties of the initial monomer mixture or final conversion level. In order to be able to predict the effect of such changes, a model is proposed which relates the properties of the initial monomer mixture such as the functionality, mixing ratio, and conversion level to changes in the relaxation curves. It basically consists of two parts. Firstly, the crosslink density is calculated based on the exact composition of the monomer mixture and, secondly, the effect of this crosslink density on the position of the glass transition and the rubbery modulus was calculated. The model was tested with a series of Novolac epoxies in which the functionality, mixing ratio, and conversion level were varied systematically. It turned out that changes in the relaxation curves due to variations in conversion level could be predicted quite accurately from the shape and shift factor of the fully cured mastercurve. The agreement between relaxation curve predictions for the series with changing functionality and mixing ratio was only moderate, which was ascribed to errors in the prediction of correct values for the crosslink density.