Densified cellulose materials – superior performance by retaining the hierarchical structure of wood
M. Frey, K. Casdorff, D. Widner, T. Keplinger, I. Burgert
Wood Materials Science, Institute for Building Materials, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich, Switzerland;
Applied Wood Materials, Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland;
The excellent material properties of wood originate from a sophisticated hierarchical structure and a pronounced fibre directionality. Both beneficial features of the biomaterial are usually lost, when wood is disintegrated to develop nanocellulosic materials. These materials can by far more easily be functionalized than wood, but assembly processes to reach application relevant dimensions are still a limiting factor. Here we present an alternative approach for the production of a cellulose material, in which the hierarchical structure and the fibre directionality are retained. The material concept is based on the delignification of wood and a subsequent densification step. The obtained material is of higher homogeneity compared to wood and possesses highly desirable material properties, such as high strength and stiffness. A further advantage of the material is that the cellulose scaffold is easily deformable in wet state. Therefore, elements of different shapes and geometries, even with high curvatures, keeping the fibres perfectly oriented, can be achieved by non-cutting processes. Additionally, density gradients within one sample can be created by varying the local pressure.
A specific focus of the presentation is laid on the densification process, which combines compression and lateral shear. The obtained densified scaffolds show a homogeneous folding and stacking of the fibres on a microscopic level and AFM imaging revealed a different densification behavior at the cell wall level when comparing densification with and without lateral shear. Moreover, the cellulose materials were infiltrated with different matrix systems and the infiltration behavior was analyzed. Infiltration or functionalization of the cellulose scaffolds offers the possibility to produce bio-inspired fibre-reinforced composites with high performance based on the renewable resource wood.