Papers as functional green materials

Paper is constituted of natural fibers and represents a perfect example of structural multifunctional materials. Indeed, its fibrous structure is engineered to fit the different end use properties: both optical and mechanical properties are usually required. These requirements may lead to contradictory needs in terms of structure. The influence of the structure on the physical properties is classically tackled based on standard methods such as the estimation of the porosity. However, this macroscopic property is not sufficient in terms of optimization of the fibrous network. For example, fluid transport has to be controlled either in the bulk of the material or only at its surface in the case of health or printing applications. Consequently, the characterization at the macro-level of the structure has to be complemented with an experimental measurement at the fiber level. The X-ray synchrotron micro-tomography, an imaging technique, is based on X-ray transmission. It allows the structure to be analyzed in 3D. It was carried in a large instrument (ESRF, France). The characterization of samples containing different recycled fibers was carried out. In particular, the influence of the number of cycles of drying-pulping is studied. Both qualitative and quantitative characterizations are obtained. The use of recycled fibers may also be included in the elaboration of materials, taking into account the modification of the fibers in terms of morphology and mechanical properties, essentially flexibility. Mechanical properties (tensile and deformation) constitute the main examples of the analysis showing the effect of the recycling of natural fibers: the decrease in mechanical resistance of the fibrous network is explained in terms of the increase of the global porosity, essentially in the bulk of the materials. The profile of porosity in the thickness direction is found to be essential to understand the evolution of physical properties.