The effect of resin chemistry on the curing behavior and chemorheology of unsaturated polyester resins

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Abstract
The effect of the structure of unsaturated polyester resin on its curing and rheological behavior during isothermal cure has been investigated, using three different grades of resin. In the investigation, the structure of the resins was determined, using nuclear magnetic resonance spectrometry (NMR), together with chemical analysis. Both a differential scanning calorimeter (DSC) and an infrared (IR) spectrometer were used to determine the curing kinetics, and a cone‐and‐plate rheometer was used to determine the variation of rheological properties during isothermal cure. On the basis of the experimental study, we have concluded: (1) at the same styrene‐to‐fumarate mole ratio, the resin having isophthalates cures slower than the one having none; (2) everything else being equal, the resin having a high styrene‐to‐fumarate mole ratio cures faster than the resin having a low one; (3) the higher the concentration of initiator, the faster a resin cures. It has been found that a resin that cures faster does not necessarily achieve a higher final degree of cure than one that cures slowly. We have found that a mechanistic kinetic model developed in our previous investigation is very useful for investigating the reactivity of unsaturated polyester resin, by determining the rate constant and activation energy of the propagation reaction. On the basis of rheological investigation, we have concluded that both tη∞ determined from steady shearing flow measurement and ttan δ = 1 determined from oscillatory shearing flow measurement may be used as a measure of gel time.