Cure-dependent microstructures and their effect on elastic properties of interlayer toughened thermoset composites

To shorten the cure cycle for thermoset prepreg composites, we can either increase the heating rate or the curing temperature. However, this raises the question whether modifying the cure cycle affects the laminate microstructure and physical and mechanical properties. Current state-of-the-art aerospace prepreg composites incorporate thermoplastic particles into the interlayer to increase delamination and impact resistance, which creates a more complicated composition and laminate microstructure. This research investigates the effect of curing conditions on the microstructure and elastic properties of an interlayer toughened prepreg system, Toray's T800SC/3900-2B. Laminates were processed to the same degree of cure using cure cycles with different heating rates. Optical microscopy showed that microstructural characteristics such as the interlayer thickness, particle shape, particle volume fraction and inter-particle distance are dependent on curing conditions. It was found that the glass transition temperature of the toughening particle, the resin viscosity and the elastic deformation of the fibre bed are important for the cure-dependent microstructural evolution at the pre-gelation stage. The out-of-plane lamina shear modulus (G13) dependence on temperature of fully cured laminates was measured with Dynamic Mechanical Analysis (DMA). The lamina G13 decreases with increasing cure cycle heating rate at 150–200 °C, where toughening particles are rubbery and the matrix is glassy. A micromechanical model demonstrates that the reduction of lamina G13 at higher heating rates results from a decrease in the interlayer shear modulus. Interlayer microstructural features such as particle volume fraction, particle aspect ratio and inter-particle contact contribute to the cure path dependence of the interlayer shear modulus. This study provides insight into the fundamental processing behavior of interlayer toughened prepregs and the influence on elastic properties.