Evaluating mold growth in tannin-resin and flax fiber biocomposites

Abstract

Flax fiber is an industrially available, renewable product with attractive technical properties for the composite industry such as density, mechanical strength and sound/heat insulation. Applications are developing quickly for biocomposites based on these fibers. However, unlike plastics, there is no standard used to assess its degree of contamination by microorganisms. Thus, the aims of this work were to detect and quantify molds in samples of nonwoven fibrous materials from industry and high resin content natural matrix-natural fiber biocomposites. In a first attempt, we tried to evaluate the fungal development on the nonwoven materials as defined in the ISO 846 standard. Despite the materials having been inoculated with molds, no fungal mycelium could be detected by this way. To validate this observation, we developed a qualitative approach based on the specific aniline blue method to stain the mycelium present in the material. This allowed us to detect the mold occurrence in the material and to evaluate the colonization extent on the nonwoven fibrous materials incubated under different conditions. The biocide efficiency of a boric acid pretreatment of flax fibers used in the manufacturing process of nonwoven fibrous materials and high resin content natural matrix-natural fiber biocomposites was determined by grading the mold development using the microscope-based method that we developed. The results showed clearly that in optimal mold growth conditions, (Aw>0.6, 26°C) the colonization of the material was hampered by the boric acid pretreatment of the flax fibers. To get semi-quantitative data of the colonization extent, we used a molecular method. DNA was extracted from the different materials and the ribosomal region of the fungal DNA was amplified by PCR allowing the detection and the quantification of fungal colonization in the biosourced material. Results confirmed the microscopy-based method. Work is currently in progress to determine risk classes of mold colonization in wood-derived and fibrous-based composite materials.