Fully Wood-Based Transparent Plates with High Strength, Flame Self-Extinction, and Anisotropic Thermal Conduction

Glasses and transparent plastics are used as daylighting face materials in buildings and vehicles. However, these transparent materials are mechanically inferior to other structural members such as metals, concretes, and fiber-reinforced plastics, and have no contribution to the load-bearing capacities of whole structures incorporating them. Herein, we report clearly transparent and mechanically strong plate materials (haze 8%, strength 256 MPa) comprising solely wood-derived cellulose nanofibers (CNFs) with carboxy functionality. These plates have a millimeter-thick laminated structure and are united by self-adhesive forces of the CNFs. Their strength is comparable to those of light-weight structural materials, such as aluminum alloys and glass fiber-reinforced plastics, while the CNF plates are even lighter. The plates feature excellent flame self-extinguishing properties due to the carboxylate structure of CNFs and further show anisotropic thermal conduction between the in-plane and out-of-plane directions. A strategy for overcoming the high water absorption, a major issue of CNF materials, through the counterion design of carboxy groups is also demonstrated.