Enhancing mechanical performance of lightweight fibre structures with foam forming technology

The success of traditional water forming is based on the ability of water to simultaneously transfer cellulose fibres and join them together. The drawback of the method is flocculation, which is worsened by long fibres and high consistency. The common technical solution is to create turbulence in the head box of a paper machine to break the flocs. As a result, the fibre network structure is formed with a random deposition process, which leads to a lognormal distribution of pore sizes. If wet foam is used instead as the fibre carrier phase, the flocculation can be prevented without turbulence. This opens up the possibility to use much wider raw material space including structural units of different scales, such as very long natural or regenerated fibres, short wood fibres, fines and polymers. Moreover, the foam-fibre interaction provides a tool to tailor not only structural homogeneity but also micro-porous structure. In particular, material properties can be extended beyond the earlier limits of cellulose products in terms of density and mechanical performance. We have taken advantage of these new possibilities and enhanced the elasticity of thick cellulose materials under compressive loads by utilizing multi-scale structural features of the fibre network. The improved strain recovery improves the suitability of these natural materials in various applications, e.g. as padding for furniture, panels, shoes, pillows and mattresses, or as insulation materials. Ideally the material would exhibit sponge-like spring back behaviour after repeated compression cycles. Our hypothesis is that this can be achieved by controlling local stresses so that plastic strains within the fibre network can be largely avoided. This requires sufficiently open pore space where fibres can bend without rigid contacts with other parts of the network. Such space is provided by a significant proportion of very large pores formed as traces of the foam bubbles. On the other hand, sufficient strength is achieved by optimally combining fibres and fines of different length-scales. Elasticity is improved by added polymers accumulating at fibre joints and helping the network structure to expand back to the initial size after compression. We used natural rubber as the polymer additive as it is known to link effectively with cellulose. In this way, we have achieved over 95% recovery of the network of viscose fibres and wood fibre fines after 70% initial compression at 50% relative humidity.