Enhancing mechanical performance of cellulose materials with designed structural complexity

There is an urgent need for sustainable alternatives to oil-based packaging materials with appropriate mechanical performance. Cellulose fiber materials provide high strength at relatively low density. However, physical limits lie much beyond conventional planar random fiber networks obtained with water forming. In analogy to macroscopic structures such as buildings and bridges, the material efficiency can be greatly improved by intelligent design of the multi-scale structure. We have developed new solutions by taking advantage of the structure tailoring enabled by foam forming technology. The unique characteristics of foam rheology free oneself from conventional planar webs to complex 3D structures. The formed fibre materials include mm-scale structural features that greatly improve material efficiency. We have implemented design thinking into developing structural and perceptual attributes by introducing strategic design already at early stages of the development work. This approach allows holistic understanding of inherent capacities of cellulose fiber structures and facilitates the development of applied solutions.The preparation of fiber materials was based on foam moulding using sophisticated 3D-printed moulds. The design method was iterative prototyping with several cycles including sample preparation from varied fiber raw materials and mechanical testing. The improved mechanical strength came from locally increased material density within a designed geometry, providing a high number of inter-fiber joints. Similar strength as for planar random fiber network was achieved at much lower effective density due to empty regions of a structure. Multi-scale structural characteristics came not only from structural geometry but also from the choice of fibers ranging from chemi-thermomechanical (CTMP) pulp to mixtures of highly refined hardwood kraft pulp and softwood TMP-reject shives. In compression tests, the strength could be varied by a factor of three by changing the furnish composition. Intriguingly, geometrical changes gave rise to an almost tenfold variation in strength properties at roughly equal effective density levels.