Search

Composites were prepared by impregnating commercial nonwoven and unidirectional flax fibers mats, with a mimosa tannin/hexamine resin without addition of NaOH as it was described in previous papers and with improved results. The influence of various parameters was observed: the curing cycle including temperature, time, pressure, the moisture content, and the number of fiber mats the composites were made of. A new two-step method was investigated: full drying of the pre-impregnated mats for storage first and then rehydratation just before pressing. The composites obtained gave good modulus of elasticity and tensile strength in traction as well as a good resistance to water swelling for composites prepared with 50% matrix resin/50% natural fibers. Best results appear to be obtained using a slow curing at low temperature (130 °C for 35 min) with moisture content of 20% on dry material. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]