Simplified Modeling of the Dynamic Gel Time and/or the Dynamic Gel Temperature for a Thermosetting Resin or Composite

Abstract

The simple analytical model developed in this study has been found to give a successful engineering estimate of when gel will occur during the cure cycle of a thermosetting resin. This simple analytical model provides a quick result that appears to be equivalent to similar results obtained with much more complicated computer programs. The replacement of the Arrhenius activation energy with the reaction release energy in the relationship between the isothermal temperature and the time to reach gel significantly contributed to the simple analytical solution that was able to be developed in this study. Resins used in both autoclave processing and Vacuum Assisted Resin Transfer Molding (VARTM) processing were evaluated in this study. It was found that the calculated Dynamic Gel Temperatures were much higher and the calculated Dynamic Gel Times were much longer for the autoclave resins than for the VARTM resins at the same heating rate. The VARTM resins also appeared to be more exothermic on a relative scale than the autoclave resins. Several multiple step cure cycles were evaluated for two Hercules resins. In general the agreement between the measured and calculated Dynamic Gel Temperatures was very good. In addition, two cure cycles for Hexcel F-263 composites were calculated and autoclave evaluated for dynamic gel time and the resulting mechanical properties. For Cure Cycle I the autoclave pressure was not applied until after dynamic gel had been reached. For Cure Cycle II the dynamic gel was achieved after the pressure was introduced into the autoclave. The short beam shear strength Cure Cycle I was nearly one-half the strength of the composite generated with Cure Cycle II. In addition, the void content of the composite for Cure Cycle I was approximately four times higher than the composite for Cure Cycle II. This study has clearly shown that if the time-temperature history of a cure cycle is known, then the dynamic gel analysis developed in this study should be able to predict when a particular resin formulation will gel. The importance of the correct prediction of when the gel time occurs in a cure cycle was also clearly shown.