Vinyl ester resins are well known for their versatility as a composite matrix. With the development of a promising room temperature molding technology, vacuum‐assisted resin transfer molding, e.g. Seemann Composite Resin Infusion Molding Process (SCRIMP), the processability of vinyl ester resins at low temperatures has attracted considerable attention from the composite industry. The objective of this paper is to provide a better understanding of the reaction kinetics of this resin system at low temperatures. In this study, a differential scanning calorimeter (DSC) coupled with a Fourier transform infrared (FTIR) spectrometer was employed to measure the reaction profile of a vinyl ester resin with different promoter and styrene contents. A kinetic model based on the free radical co‐polymerization mechanism was developed for simulating the reaction rates and conversions of styrene vinyl and vinyl ester vinylene groups. The model parameters were determined from several FTIR experiments under isothermal conditions. This model, in conjunction with heat transfer analysis, was able to successfully predict the temperature profiles during curing in two SCRIMP molding cases based on groove type resin distribution system.
ABSTRACT:Vinyl ester resin is a major thermoset polymer used in low-temperature composite manufacturing processes such as the Seemann composite resin infusion-molding process (SCRIMP). Volume shrinkage and residual styrene are important concerns for composites produced in such processes. A low-shrinkage additive (LSA) is a typical agent added to control the volume shrinkage of vinyl ester resins during molding. In this study, the effects of LSA content and the temperature profile (the temperature gradient and peak temperature) on the volume shrinkage control of a vinyl ester resin were investigated. The reaction kinetics of the resin system were also studied. We achieved good volume shrinkage control if we raised the curing temperature slowly to allow sufficient time for phase separation and if the curing temperature reached a high value after phase separation to allow microvoid formation. On the basis of experimental results, we designed an improved SCRIMP to increase resin conversion, reduce resin shrinkage, and produce composites with better properties.
Many composite manufacturing methods are room temperature processes, which require the resins to be cured at low temperatures. Examples are Seemann Composite Resin Infusion Molding Process (SCRIMP)—a vacuum assisted resin transfer molding (VARTM) process, hand lay‐up and spray‐up. Vinyl ester resins have been widely used in this type of processes because of their versatility as a composite matrix. Low temperature polymerization between styrene and vinyl ester tends to be complex because of the presence of different curing agents. This paper reports on (1) the effects of a promoter (cobalt naphthenate) on the initiatin at low temperatures, and (2) the effects fo a retarder (2,4‐pentanedione) and an inhibitor (1,4‐benzoquinone) on the pot life, gel time, and cure kinetics. A differntial scanning calorimeter (DSC) and a Fourier transform infrared (FTIR) spectrometer were used to investigate the reaction kinetics of vinyl ester resins. The influence of the retarders and inhibitors on the rheological changes of the resin during curing was also studied using a Rheometris Dynamic Analyzer (RDA).
A major concern in low temperature composite manufacturing processes is how to design and control the mold filling and curing time. Inhibitors or retarders are often used to prevent premature gelation and provide a sufficiently long time to fill the mold completely. However, the addition of these chemical species tends to result in a low mold curing rate and a low final resin conversion. In this study, a chelating agent 2,4-pentanedione [2,4-P) was used to manipulate resin gelation and curing. This agent is known to affect the catalytic activity of the promoters (i.e. metal compounds such as cobalt carboxylates) in the decomposition of initiators. It can function as either a retarder or a co-promoter in the co-polymerization of styrene/polyester and styrene/vinyl ester resins depending on the acidity of the resin system. Based on this observation, an improved room temperature vacuumassisted resin transfer molding process was designed. This design allows 2.4-P to serve first as a retarder during mold filling to achieve a long gel time, it then as a co-promoter during curing to increase the curing rate. The 2.4-P also increases the resin conversion as the acidity of the resin increases.
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