The effect of lignin on the reactivity of natural fibres towards molecular fluorine
Résumé
In this study, a range of natural fibres and their main components have been treated by direct fluorination using F2 gas. Fourier-Transform infrared spectroscopy, 19F solid state Nuclear Magnetic Resonance and tensiometry results underline the substitution of OH groups by fluorine atoms and thus the formation of CF covalent bonds. Fluorination induces a significant decrease in the hydrophilic properties in the case of lignocellulosic materials. The relationship between the chemical composition of the natural fibres and their reactivity during fluorination has been established. Indeed, on the one hand, the affinity for F2 is improved by the presence of lignin, leading to a better conversion of COH into CF bonds. On the other hand, a high cellulose content tends to inhibit the reaction. As a consequence, the cellulose and lignin contents of wood materials can be determined thanks to their reactivity during fluorination. However, for high-content cellulose materials, estimating the cellulose and lignin contents by this method is more difficult because of their low reactivity during fluorination. In this case, a thermal assisted fluorination is necessary to increase reactivity and thus classify samples according to their cellulose content.