Relief of Cure Stress in Prepreg Composites with Engineered Voids: A Solution to Warping in Composite Phenolic Resin/Fiberglass Laminates

The warp of a phenolic resin/fiberglass honeycomb composite panel is investigated using void and volatile analysis, and a solution to geometric warp is proposed and tested. The surface void content in semicured phenolic polymer composite facesheets was found to be an indicator for the average warp value of a fully cured composite laminate panel. Several potential sources in the resin formulation for producing voids in the phenolic resin/fiberglass composite sheets were investigated. Ultimately, high surface tension and polymer incompatible cosolvents are correlated to the population of voids. It is proposed that these voids act as sites for the relief of internal stresses that develop in the phenolic matrix during the final cure of the part. Void-inducing additives are added at the formulation stage which phase separate from the polymer matrix in partially cured fiberglass/phenolic composite sheets, and upon evaporation, lead to voids that prevent warping in honeycomb composite panels.