Monitoring Unsaturated Polyester Resins for SMC: Testing the Reactivity of the Magnesias Used in the Curing of the Resins.

It is important to know and control the thickening process of unsaturated polyester resins, in order to realize prepregs with the SMC (Sheet Molding Compounds) process. Magnesium oxide MgO is the thickening agent mostly used but its reaction mechanisms are still not fully understood. The authors have characterized some magnesium oxides with various macro- and microstructures. The main reactivity parameters of MgO were established: surface aspect, size and distribution of particles, carbonation and hydration, previous impasting. It was found that an acid-base reaction seems to be a good model of the reaction's first step, which leads to a gel formation. As a consequence, an easy and quick test of acid attack is suggested for estimating the thickening power of MgO.