CURING BEHAVIOR OF THICK-SECTIONED RTM COMPOSITES

The successful manufacture of thick-sectioned composites is challengin g, since the highly exothermic nature of thermoset resins and limited temperature control make it difficult to avoid detrimental thermal and cure gradients within the composite. In order to make quality parts, it has been found experimentally that cure temperatures must be lowere d as much as 50% from those suggested for thin parts. Differential Sca nning Calorimetry (DSC) experiments of a vinyl-ester resin system at t hese lower temperatures revealed a significant dependence on temperatu re for the maximum extent of cure. If the resin is cured isothermally at 55 degrees C, the final conversion of the resin was found to only r each 70%. When the maximum extent of cure parameter was incorporated i nto an empirical autocatalytic kinetic model, it was found to signific antly improve the description of the cure kinetics. Inhibitors, added to the resin to improve shelf-life, disappear rapidly at higher cure t emperatures but can double the time required to cure a thick composite processed at 55 degrees C. A zeroth order kinetic relationship was de veloped to estimate the amount of inhibitor in the system during the r esin's cure. The inhibitor relationship and the improved kinetic model were used in a finite difference cure simulation to successfully pred ict the thermal gradients during cure of a 2.54 cm thick composite man ufactured by resin transfer molding (RTM).