A new treatment to reduce wood 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 and polar 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 waterproof Kraft paper and more hydrophobic carbon fibres, without using solvent or high temperature. The first step of this study was to optimize the treatment parameters (time, temperature, atmosphere…) so as to get a high level of fluorine grafting without damaging the particle surface. Several analyses were carried out to determine them (Fourier transform infrared spectroscopy, 19F solid-state nuclear magnetic resonance spectroscopy, scanning electron microscopy…). Then, the hygroscopic and thermal behaviours of the fluorinated wood flour have been evaluated and compared to the non-treated wood flour, using for example contact angle measurements and thermogravimetric analysis. The fluorine based treatment was shown to decrease notably the capacity of the wood particles to absorb water without damaging their surfaces. Lastly, the mechanical and hygroscopic properties of the composites made with fluorinated wood and polyester matrix were determined. The results revealed that the wood fluorination strongly reduced the hydrophilicity of the composite, and largely enhanced 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.