Karine Charlet

Mechanical Properties of Flax Fibers and of the Derived Unidirectional Composites

Flax fibers were used to process unidirectional composites by two different methods. Their mechanical properties obtained by tensile testing are discussed with respect to the properties of the fibers and those of the matrix (unsatured polyester). The similarity of the tensile curves of the composites and of the elementary fibers is attributed to the good adhesion of the fibers with the matrix. Moreover, as there is almost a linear evolution of the composite properties with the fiber volume fraction, these properties are used to estimate those of the real reinforcement material, that is, the flax bundles: the calculations lead to a fiber strength of 500-800 MPa and a fiber modulus of roughly 30 GPa, which is half the values obtained by tensile testing elementary fibers. These data may be helpful when trying to model the deformation behavior of flax fiber-reinforced composites.

Direct fluorination applied to wood flour used as a reinforcement for polymers

Direct fluorination was applied to wood flour in order to improve its compatibility with polymers and thus enhance the properties of wood-polymer composites. Fourier-transform infrared spectra and (19)F solid-state nuclear magnetic resonance results underlined a successful covalent grafting of fluorine atoms onto the wood chemical structure. No physical damage of the wood particles was observed during scanning electron microscopy analysis. The thermal behaviour of the wood flour was also studied by thermogravimetric analysis. The hydrophilic property changes of wood flour were examined by evaluating the water content and the rate of water uptake of samples under different relative humidity conditions. A decrease in the wood flour water content was noted after fluorination. All these studies tend to prove the efficiency of this treatment on wood hydrophilia.

Direct insights on flax fiber structure by focused ion beam microscopy.

In this article, it is shown that focused ion beam (FIB) systems can be used to study the inner structure of flax fibers, the use of which as a reinforcing material in polymer composites still draws much interest from multiple disciplines. This technique requires none of the specific preparations necessary for scanning electron microscopy or transmission electron microscopy studies. Irradiation experiments performed on FIB prepared cross sections with very low Ga+ ion beam currents revealed the softer material components of fibers. Thus, it confirmed the presence of pectin-rich layers at the interfaces between the fibers of a bundle, but also allowed the precise localization of such layers within the secondary cell wall. Furthermore, it suggested new insights on the transition modes between the sublayers of the secondary cell wall.

Fluorination as an Effective Way to Reduce Natural Fibers Hydrophilicity

In order to get composite materials with high mechanical properties, the quality of the interface between the fibres and the matrix has to be good enough to enable the load transfer. In the case of wood polymer composites, made of hydrophilic wood particles and of a generally non-polar polymer, the lack of natural compatibility between the constituents hinders the load transfer. Aiming at decreasing the gap of polarity between wood fibres and polymer matrices, fluorination has been applied to wood. This treatment is known to be very efficient to make more hydrophobic materials without requiring solvent or high temperature. After the optimization of the treatment parameters so as to get a high level of fluorine grafting without burning the particles, the hygroscopic and thermal behaviors of the fluorinated wood flour have been evaluated and compared to the non-treated flour. For that purpose, several analyses were carried out: FT-IR spectroscopy, 19F solid-state NMR spectroscopy, SEM, contact angle measurements, TGA. The fluorine based treatment was shown to decrease notably the capacity of the wood particles to absorb water without damaging their surfaces. Lastly, at the composite scale, the wood fluorination was shown to strongly reduce its hydrophilicity and to largely enhance its tensile and flexural properties. This is directly linked with the improvement of the compatibility between the treated (and thus, less hydrophilic) wood particles and the polymer matrix, as also proved by X-ray tomography.

Interfaces within flax fibre bundle: Experimental characterization and numerical modelling

In natural fibre based-composites, the interfaces within the fibre bundles are not as well described as those between fibres and classical polymer matrices. To compensate for this lack of information, which is however of great importance when trying to model the composite behaviour, morphological and mechanical characterization of the middle lamella present between flax fibres has been carried out. The interfacial strength of the pectic cement was found to be very low compared with that of the fibre/matrix interface. Assuming this fibre/fibre interface to be a cohesive zone material and using the experimental results obtained on pairs of fibres, a model has been developed at the scale of the fibre and applied to study numerically the tensile properties of bundles. The results of the simulations were in agreement with the experimental data obtained on bundles, both leading to a mean bundle strength of 500 MPa.