Multi-scale study of the adhesion between flax fibers and biobased thermoset matrices

• A double analysis of the thermoset biocomposites fiber–matrix adhesion was performed.
• Debonding tests allowed to estimate the interface properties at the microscopic scale.
• Tensile and in plane share experiments were conducted on macroscopic specimens.
• A strong sensitivity of the IFSS to the in-plane shear strength was highlighted.

The environmental impact of composite materials made with a thermoset matrix can be reduced in two ways. First, glass fibers can be replaced by natural fibers. Second, petrochemical components from the matrix can be replaced by biobased renewable equivalents. The quality of the interface between the matrix and the fibers has a strong influence on the composite mechanical properties. In this study, tensile performances of flax fibers and commercially partly biobased epoxy and polyester matrices have been investigated and corresponding unidirectional composites were elaborated. Their mechanical performances are in accordance with fiber and matrices properties, taking into account fiber dispersion. Then, at the microscopic scale, the debonding test was used; a great adhesion between flax fiber and thermoset matrices was highlighted. Finally, tensile tests on ±45° laminates were carried out to create an in-plane shear at the macroscopic scale. Interestingly, the results obtained at the macroscopic scale are well correlated to the ones given by the debonding test at the microscopic scale.