Renewable and biodegradable alternatives to fossil-based materials are essential as concerns over depleting finite resources and the pollution of our ecosystems are growing. Abundant, highly anisotropic, and mechanically strong cellulose nanofibrils (CNF) are attractive building blocks for the fabrication of high-performance biobased materials that can compete with their conventional fossil-based counterparts. This thesis presents potential solutions to key challenges in the production and properties of CNF and CNF-based materials, such as low moisture resistance and energy-intense processing, by using the physicochemical properties of tannins. The benchmarking of CNF to improve energy-efficient production was investigated and the ability of plant-derived tannins to precipitate proteins, react with nucleophiles when oxidized, and coordinate to metal ions was exploited to produce multifunctional films and foams that were inspired by Nature or traditional processes.

Wet strong, antioxidant, and UV-blocking CNF-based films were produced by mimicking the traditional process of leather tanning. Oxidized CNF were grafted with gelatin that was precipitated with a water-soluble tannin. The polyphenolic tannin provided the films with good radical scavenging properties and efficient blocking of light in the UV-B/UV-C range. The insoluble gelatin–tannin complexes conferred upon the material wet mechanical properties that were comparable to the dry mechanical performance of fossil-based packaging films. So far, there is no universally accepted approach to account for how the swelling of a hygroscopic CNF-based film influences its mechanical properties in humid or wet conditions. Here, a best practice for determining and reporting wet strength is suggested.

Inspired by the sclerotization of insect cuticle, a scalable route towards moisture-resilient, strong, and thermally insulating CNF-based foams was developed. The CNF were modified with a polyamine, ice-templated, treated with an oxidized tannin, solvent-exchanged to ethanol, and evaporatively dried. The cross-linked structure had a high compressive modulus and a thermal conductivity close to that of air, even at high relative humidities.

A method to produce micron-sized patterns on CNF films based on the traditional Bògòlanfini dyeing technique is presented. The films were pre-impregnated with a tannin and patterned using microcontact printing with a metal-salt-soaked stamp. The line and dot patterns were analyzed and their colors were tuned by changing the metal ion in the printing ink or the pH.

The final part of the thesis describes a novel approach to assess the degree of CNF fibrillation during energy-efficient grinding by analyzing the structure and properties of anisotropic foams. The optimal energy input during fiber disintegration that produced CNF foams with the best mechanical and thermal insulation properties, as well as the highest CNF and foam cell wall orientation, was identified.