Eliminating porosity via reformulation of a benzoxazine–epoxy resin transfer molding resin

Abstract

Use of benzoxazine resins in composites is limited by volatile-induced porosity, which degrades the thermomechanical properties of the product. In the present study, we demonstrate how to eliminate cure-induced volatilization and volatile-induced defects in benzoxazine composite laminates, using a chemistry-based approach. Like most resins formulated for high-temperature service, benzoxazine and benzoxazine–epoxy blends generally include solvent additives. Consequently, composite parts produced with such resins exhibit higher levels of cure-induced volatile release, often leading to porosity in the final manufactured part. Here, we develop a method to eliminate porosity by analyzing volatile release and the effects of residual solvent in a pre-commercial benzoxazine–epoxy system designed for liquid molding by resin transfer molding. Utilizing thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and dynamic mechanical analysis, we correlate the concentration of residual solvent remaining within the final manufactured part with the T_g, degradation temperature, and dynamic modulus. Lastly, a resin synthesis method is demonstrated that eliminates residual solvent in order to produce composite parts with optimal surface finish and thermomechanical properties. The report outlines a methodology for optimizing blended resin chemistry for production of high-quality composite parts.