Delignified Wood–Polymer Interpenetrating Composites Exceeding the Rule of Mixtures

Wood is increasingly considered in sustainable structural materials development due to its hierarchical structure, including an oriented reinforcing cellulose phase combined with carbon capturing and renewability. Top-down manufacturing techniques can provide direct access to this hierarchical cellulose scaffold for use in new functional materials. For high-performance load-bearing wood-based materials, the volume content of the reinforcing phase needs to be increased to much higher fiber volume contents (FVCs). This has been achieved by structure-retaining delignification followed by densification. The obtained matrix-free materials possess high tensile stiffness due to preservation of hierarchical fiber alignment; however, they demonstrate low mechanical properties in bending and cannot be used in moist conditions due to their propensity for water absorption. In order to address these two challenges, an interpenetrating wood polymer phase composite is developed using a delignified wood scaffold as a continuous reinforcing phase and epoxy resin as the interconnected matrix phase. We utilize the continuous flow channels in delignified wood for vacuum-assisted matrix infiltration in a condition of open continuous porosity in the wood scaffold. Prior to matrix curing, the material is densified in order to increase the FVC, decrease porosity, and reduce density variations in the wood scaffold. Due to the compressibility of delignified cellulose fibers, interpenetrating phase composites (IPCs) with very high FVCs of up to 80% could be produced, leading to exceptionally high tensile stiffness and strength of up to 70 GPa and 600 MPa. The obtained stiffness values far exceed the upper limit of the rule of mixtures due to an enhanced stress transfer through mechanically interlocked fiber–fiber interfaces combined with the stiffness providing matrix phase that further aids stress transfer between neighboring wood cells via their pits. This new approach paves the way for an efficient production of high-performance sustainable materials that can be used as alternative for glass fiber reinforced composites or natural fiber composites.