Flame-retardant and mechanically strong transparent papers produced via synergistic chemical‒mechanical loosening of wood pulp

Wood pulp papers are sustainable structural materials that can be formed through a rapid, energy-efficient manufacturing process. However, conventional papers exhibit mechanical weakness and optical opacity, limiting their applications. These drawbacks arise from the high porosity of the papers or sheet materials of networked pulp fibers. Herein, we report the fabrication of mechanically strong transparent papers with a low porosity of less than 20% utilizing a traditional paper making process, without vacuum filtration. Prior to paper fabrication, the rigid cell wall structure of the pulp fibers was loosened via chemical oxidation and subsequent mechanical beating. The resulting papers exhibited excellent mechanical and optical properties comparable to those of cellulose nanofiber sheets. The tensile stiffness and strength of the papers reached approximately 22 GPa and 270 MPa, respectively, and their haze decreased to approximately 33%. These paper properties were tunable by varying the number of beatings. The papers can also self-extinguish flames, making them the first light-permeable sheets made from wood pulp fibers with this function. We further demonstrated that the low water resistance, which is the major issue with traditional papers, can be greatly improved by the use of conventional wet chemistry in the field of paper science. These transparent pulp papers with high mechanical strength and flame retardant ability will open new avenues for the development of sustainable structural materials.