In order to explore the reinforcing capabilities of cellulose nanofibrils, composites containing high contents of cellulose nanofibrils were prepared through a combination of water-assisted mixing and compression moulding, the components being a cellulose nanofibril suspension and an aqueous dispersion of the polyolefin copolymer poly(ethylene-co-acrylic acid). The composite samples had dry cellulose nanofibril contents from 10 to 70 vol%. Computed tomography revealed well dispersed cellulose fibril/fibres in the polymer matrix. The highest content of 70 vol% cellulose nanofibrils increased the strength and stiffness of the composites by factors of 3.5 and 21, respectively, while maintaining an elongation at break of about 5%. The strength and strain-at-break of cellulose nanofibril composites were superior to the pulp composites at cellulose contents greater than 20 vol%. The stiffness of the composites reinforced with cellulose nanofibrils was not higher than for that of composites reinforced with cellulose pulp fibres.

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The possibility to change the fibre network structure of paperboard by using the needle punching method was studied. Needling was performed on the middle layer made from unbleached softwood kraft pulp at a dry content of 20%. The needled structure was studied by several microscopy methods, which showed that the fibres were redistributed mainly in-plane. Denser regions of the fibre network structure were observed close to the needle penetration sites. The local density was estimated to be 25% higher than that of the surrounding unaffected structure.