Curing Behavior of Thick-Sectioned RTM Composites

Abstract

The successful manufacture of thick-sectioned composites is challenging, 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 lowered as much as 50% from those suggested for thin parts. Differential Scanning Calorimetry (DSC) experiments of a vinyl-ester resin system at these lower temperatures revealed a significant dependence on temperature for the maximum extent of cure. If the resin is cured isothermally at 55°C, the final conversion of the resin was found to only reach 70%. When the maximum extent of cure parameter was incorporated into an empirical autocatalytic kinetic model, it was found to significantly improve the description of the cure kinetics. Inhibitors, added to the resin to improve shelf-life, disappear rapidly at higher cure temperatures but can double the time required to cure a thick composite processed at 55°C. A zeroth order kinetic relationship was developed to estimate the amount of inhibitor in the system during the resin's cure. The inhibitor relationship and the improved kinetic model were used in a finite difference cure simulation to successfully predict the thermal gradients during cure of a 2.54 cm thick composite manufactured by resin transfer molding (RTM).

Resin transfer molding of natural fiber reinforced composites: cure simulation

“…As discussed before important temperature and degree of cure gradients were predicted. Michaud et al [18] presented a similar study and predicted the thermal gradients during cure of thick-sectioned composites. Semling et al [19] proposed an algorithm to calculate the optimal process temperature for a RTM system.…”

“…With the assumptions made above the temperature solutions are based on Fourier's heat conduction equation for one-dimensional, transient heat transfer and an internal heat generation term [16][17][18]:…”

“…In the early stage of the RTM process, once the mold starts heating up, the variations of temperature observed in the resin would therefore only be created by conduction. In the literature it was found that most inhibitors exhibited zeroth order disappearance [18]. The change in relative inhibitor concentration is therefore given by:…”

Multiscale Modeling of Residual Stress Development in Continuous Fiber-Reinforced Unidirectional Thick Thermoset Composites

“…Typically, these models involve three aspects: first, the coupled solution of cure kinetics and heat transfer equations to obtain the temperature and degree of cure profiles in the part; this is especially important for thick composites [8][9][10]; second, the development of models for resin, laminate, and composite property evolution with cure and temperature; and third, the solution of the structural mechanics equations to estimate the induced strains and residual stresses in the part. Models for composite property evolution during cure can be based on direct experimental measurements on the composite itself during cure [11][12][13][14].…”

“…Cure process models provide a valuable tool to understand the effect of composite microstructure, part geometry, and processing parameters on the development of residual stresses; such models also provide a relatively inexpensive virtual means of optimizing cure processing to minimize the residual stresses. Process models for composite cure are available that address residual stress development at the part level [6,[8][9][10][11][12][13][14][15][16][17][18] or at the "meso" levels involving the microstructure or fiber-matrix interface [19][20][21][22][23]. However, a systematic framework for bridging the information at the two scales has not been demonstrated; such bridging of information is critical for obtaining realistic estimates of residual stresses at the fiber matrix interface and for arriving at an understanding of the efficacy of reinforcement.…”