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In this work we challenge some earlier theoretical ideas on the strength of lightweight fiber materials by analyzing an extensive set of foam-formed fiber networks. The experimental samples included various different material densities and different types of natural and regenerated cellulose fibers. Characterization of the samples was performed by macroscopic mechanical testing, supported by simultaneous high-speed imaging of local deformations inside a fiber network. The imaging showed extremely heterogeneous deformation behavior inside a sample, with both rapidly proceeding deformation fronts and comparatively still regions. Moreover, image correlation analysis revealed frequent local fiber dislocations throughout the compression cycle, not only for low or moderate compressive strains. A new buckling theory including a statistical distribution of free-span lengths is proposed and tested against the experimental data. The theory predicts universal ratios between stresses at different compression levels for low-density random fiber networks. The mean ratio of stresses at 50% and 10% compression levels measured over 57 different trial points, 5.42 ± 0.43, agrees very well with the theoretical value of 5.374. Moreover, the model predicts well the effect of material density, and can be used in developing the properties of lightweight materials in novel applications.

A new sound absorbing material made from 100% softwood fibres by means of a foam-forming technique is introduced. In foam forming, a wet foam is created by mechanically mixing water, fibres and a surfactant. The air bubbles keep the wet fibres separate, and a highly porous fibre network is formed during drying. The sound absorption of foam-formed structures was measured by means of an impedance tube. The results showed that foam-formed softwood materials possessed a competitive sound absorption coefficient compared to different types of commercial sound absorber materials. The material is based on 100% softwood fibres without added binders and is semi-rigid and does not completely recover from compression. Improvement in the strength properties of softwood material can be obtained by using starch or cellulose microfibrils. The material could be used in indoor applications, for example, in replacing mineral wool acoustic ceiling panels or polyester non-woven materials in office acoustics products.

In the study, the effect of refining and fractionation of wood fibres on fibre morphology and biocomposite properties was determined. Kraft pine pulp and thermomechanical pulp were selected for the fibre treatments. Effects of physical treatment on fibre morphology were analysed with a fibre analyser and microscopy techniques. For the composites, polylactic acid was used as a polymer matrix. Composites were produced by melt processing to a fibre content of 30 wt-%, and the mechanical properties of the injection moulded biocomposites were investigated. In general, thermomechanical pulp fibres improved the mechanical properties of polylactic acid more than pine pulp fibres did. Two different fibre fractions with distinct fibre properties were obtained by fractionation, and the use of a long fibre fraction provided improved mechanical properties for composites. The refining of pine pulp led to clear fibrillation of the fibre surface, but it had a negative effect on the mechanical properties of biocomposites.