High Density Biocomposite from Natural Fibers and Tannin Resin

Abstract Successful experiments on high, medium and low density natural fiber reinforced composites made from tannin/hexamine resin and flax–hemp fibers have recently been carried out. This study focused on the high density composites. In this work composite panels have been made of three different tannin extracts: mimosa, sulfited mimosa and quebracho, each with their own properties. The panels were tested for modulus in elasticity (MOE) in bending, and for swelling in cold water, following the French standard NF EN 622-2. It appeared that a strong correlation exists between the density and the mechanical performances of the composites. Water, which is necessary for hardening resin, can, when in excess cause a strong drop in the mechanical performances by increasing the porosity in a composite structure. In the course of carrying out this work, some manufacturing problems were found: cracks appeared during the pressing of large panels resulting in the need for a specific pressing cycle; and deformations occurred during the cooling phase due to internal stresses. Experimental procedures were developed and tested to reduce these problems.