Interfacial properties of hemp fiber/epoxy system measured by microdroplet test: Effect of relative humidity

Abstract The hygro-mechanical behavior of single hemp fiber/epoxy interface was investigated through its characterization at the microscopic scale. Microdroplet debonding tests were developed in controlled humidity chambers to study the interfacial properties and adhesion mechanism of hemp/epoxy systems in in-use environmental conditions (from 9% to 98% relative humidity). The increase in environment relative humidity and therefore moisture content in the fiber/matrix system engenders a non-monotonic evolution of interfacial shear strength (IFSS) and post-debonding strength. Capillary adhesion illustrated with interfacial meniscus and attractive capillary force contribute to interfacial bond strength until RH = 50%. Increasing RH above 50%, corresponding to a specific amount of water molecules in the system, provokes a reduction of interactions between the hydroxyl groups on the fiber surface and the resin. Debonding behavior analysis associated to fiber/matrix interface fracture observation highlights a modification of the stress state at the interface when increasing relative humidity. The residual hygroscopic stresses generated during storage at various humidity are shown to be the major contributor of the optimal interface performances at 50% relative humidity (RH). The sensitivity to moisture of natural fibers could therefore be beneficial for improving biocomposite interface properties.